Fractionation of Sr and Hf isotopes by mineral sorting in Cascadia Basin terrigenous sediments

Chemical Geology 382 (2014) 67–82

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Chemical Geology

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Fractionation of Sr and Hf isotopes by mineral sorting in Cascadia Basinterrigenous sediments

Marion Carpentier a,⁎,1, Dominique Weis a, Catherine Chauvel b,c

a Pacific Centre for Isotopic and Geochemical Research, Department of Earth and Ocean Sciences, University of British Columbia, 6339 Stores Road, Vancouver, British Columbia V6T 1Z4, Canadab Univ. Grenoble Alpes, ISTerre, F-38041 Grenoble, Francec CNRS, ISTerre, F-38041 Grenoble, France

⁎ Corresponding author.E-mail address: [email protected]

1 Present address: LaboratoireMagmas etVolcans, CNRSClermont-Ferrand, France.

http://dx.doi.org/10.1016/j.chemgeo.2014.05.0280009-2541/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 24 February 2014Received in revised form 21 May 2014Accepted 23 May 2014Available online 2 June 2014

Editor: K. Mezger

Keywords:Oceanic sedimentsCascadia BasinSr–Nd–Pb–Hf-isotopesSediment provenanceMineralogical sorting

Oceanic sediments deposited on continental margins consist mainly of erosion products of the nearby exposedcontinental areas. Detrital input usually dominates their geochemical budget, and the composition of these sed-iments should record potential changes in their continental sources. However, alongmargins, mineral sorting as-sociatedwith transport and sedimentary processes induces significant chemical and isotopic fractionation over afew tens of kilometers. The study of margin sediments should help to quantify the extent of modification of thecontinental terrigenous supply when it reaches deep oceans.Reported Sr, Nd, Hf and Pb isotopic compositions of fifty-seven sediments from the northernmost part of theCascade forearc (Ocean Drilling Program, ODP, Sites 888 and 1027) suggest the involvement of two dominantend-members coming from the nearby Canadian Cordillera. Erosion products of the depleted, western part ofthe Cordillera dominate the detrital input, while the eastern enriched terranes of the Cordillera contribute only10 to 28% of the input. There is no marked change of provenance of sediments during the last 3.5 Myr andthey all appear unaffected by glacial–interglacial climate cycles. The average isotopic compositions of the twosites are slightly different, but are both dominated by continental signature; these values can be used in futurestudies to identify any subducted sediment contribution to the Cascades Arc.On a finer scale, there are differences in the isotopic signature between samples dominated by clayminerals andthose with coarser lithologies. For a given Nd isotopic composition, fine sediments have more radiogenic Sr andHf isotope ratios than sands, andwe interpret the difference as resulting frommineral sorting during transport ofthe particles. Fine sediments concentrate minerals with radiogenic Sr and Hf such as clays and micas, whilecoarse-grained detritus carry the unradiogenic mineral component of a given source rock through plagioclase–epidote and zircon. ODP Site 1027 is located 100 km further away from continent than ODP Site 888 and containsmore clay. As a consequence, it has significantly more radiogenic Sr and Hf bulk composition than ODP Site 888.Similar differences in isotopic signatures related to the distance to continent certainly occur in other areas in theword, and will account for a large part of differences known between continental sources and deep-seasediments.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Vast amounts of detrital material carried by rivers are delivered tothe oceans, where hydrodynamic processes tend to concentrate coarseand heavy minerals close to continental margins while fine particulatesare preferentially transported towards abyssal plains. As a consequence,margin environments represent critical locations where large chemicaland isotopic fractionation due to mineral sorting can occur within thedelivered continental material (e.g. Patchett et al., 1984; Eisenhauer

r (M. Carpentier)., Université Blaise Pascal, 63000

et al., 1999; Carpentier et al., 2009; Chauvel et al., 2009; Vervoortet al., 2011; Garçon et al., 2013, 2014).

In the Cascadia Basin that lies along the northwestern continentalmargin of North America (Fig. 1), high sedimentation rates (from 8 to180 cm/kyr, Su et al., 2000; Knudson and Hendy, 2009) preclude anyconcentration of hydrogenous and phosphate-rich material, and conti-nental material dominates the trace element budget (Prytulak et al.,2006; Carpentier et al, 2013). As a consequence, the sediment isotopiccompositions reflect those of the detrital fraction, and can be used totrace source regions as well as to evaluate the role of mineral sortingin modifying isotopic signatures.

The Cascadia Basin has been the locus of a few drilling campaigns. Inparticular, during Ocean Drilling Program (ODP) Legs 146 and 168, sev-eral sites were drilled between Vancouver Island and Juan de Fuca

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Fig. 1. a) Simplified geological map of the Northwestern American Cordillerawith watershed boundaries of the Fraser and Columbia Rivers, adapted from Reed et al. (2004) and Kiyokawaand Yokoyama (2009). The offshore locations of the two sampled sites (ODP 146 Site 888 and ODP 168 Site 1027) aswell as the location of DSDP Site 174 (Astoria fan) studied by Prytulaket al. (2006) and IODPSite 1301 studied byKiyokawa and Yokoyama (2009) are also shown.Numberswritten inwhite boxes are the averageNd isotopic compositions (given as εNd) of thedifferent terranes. References are given in Fig. 1b caption, except for Columbia Riverflood basalts forwhich average εNdwas calculatedusing precompiled file fromGEOROC (http://georoc.mpch-mainz.gwdg.de/georoc/). b) Distribution histogram of εNd of the rocks forming the different belts of the Canadian Cordillera. Nd isotopic compositions are fromGreene et al. (2009)for Vancouver Island (Wrangellia Terranes from the Insular belt), Cui and Russell (1995a), Friedman et al. (1995) andMahoney et al. (2009) for the CoastMountain Belt, from Samson et al.(1989), Smith and Lambert (1995), Smith et al. (1995) and Patchett and Gehrels (1998) for igneous and sedimentary rocks belonging to the Intermontane Belt and data for Cascades Arcnorth of 44°N are from Halliday et al. (1983), Leeman et al. (1990, 2004), Tepper et al. (1993), Tepper (1996), Bacon et al. (1997), Conrey et al. (2001), Schmidt et al. (2008), Jicha et al.(2009),Mullen andWeis (2013) andMullen andMcCallum (2014). For the eastern part of theCordillera, Nd isotope ratios are from Frost andO'Nions (1984), Burwash et al., (1988), Ghoshand lambert (1989) and Patchett and Gehrels (1998) for sedimentary rocks from Belt–Purcell Supergroup and Omineca belts and from Brandon and Lambert (1993, 1994), Brandon andSmith, (1994) and Driver et al. (2000) for the intrusive rocks.

68 M. Carpentier et al. / Chemical Geology 382 (2014) 67–82

Ridge. Among the available cores in this area,we focus our study on sed-iments recovered at ODP Sites 888 (48°10.01′N, 126°39.79′W, 2516 mwater depth) and 1027 (47°45.41′N, 127°43.85′W, 2657 m waterdepth) (Fig. 1). The two sites are located at different distances fromthe continent (120 km for ODP Site 888 and 220 km for ODP Site1027) and represent ideal targets to investigate the effect of sedimenta-ry sorting on the composition of sediments. The study of the elementalcomposition of the same set of samples (Carpentier et al., 2013) showedthat 1)major and trace element compositions of these sediments reflecttheir derivation from relatively juvenile and unweathered terranes inthe Canadian Cordillera, 2) ODP Site 888 and 1027 sediments sharecommon continental sources, and their bulk compositions are compara-ble, 3) variability of major and trace element concentrations is mainlycontrolled by mineral sorting processes.

Here we present Sr, Nd, Hf and Pb isotopic compositions of fifty-seven of these well-characterized sediment samples. We compare thenew isotopic data to published data on Cordillera rocks and provide fur-ther constraints on the geographical provenance of sediments and theirevolution over the 3.5 Ma of sedimentary deposition. We also explorethe effect ofmineral sorting processes onNd–Hf andNd–Sr isotopic sys-tems and highlight some significant decoupling as a function of distancefrom the continent.

2. Geological setting and sampling

In the northeastern part of the Pacific Ocean, the Cascadia Basin liesbetween the active Juan de Fuca Ridge and the Cascadia subductionzone (Fig. 1). The young (b7Ma) Juan de Fuca oceanic plate is currentlybeing subducted below the western boundary of the North Americacontinent at a rate of about 4.2 cm/year (Westbrook et al., 1994), andthis generates the volcanism present along the west coast of North

America. Sedimentation in the Cascadia Basin is dominated by largeterrigenous input owing to proximity to the continent, and is largely in-fluenced by fluvial discharge coming from the Fraser and ColumbiaRiver systems (Fig. 1). Deposition rate is generally high in the area,with a marked decrease westward. While it can reach up to 1.8 m/kyrat ODP Site 888 (Knudson and Hendy, 2009), it varies between 8 and74 cm/kyr at ODP Site 1027 (Fig. 1). As a consequence, the thicknessof the sedimentary cover displays a strong west–east gradient, andis much greater close to the coastline where it reaches 2500 m atODP Site 888 despite the young age (~6 Ma) of the oceanic floor(Westbrook et al., 1994).

3. Study site and sampling

We studied sedimentary sections recovered at two different loca-tions, ODP Sites 888 and 1027 (Fig. 1). ODP Site 888 was drilled about120 km from the Vancouver Island coast and only the top 567 m ofthe 2.5 km-thick sedimentary cover was cored, corresponding to a600 kyr record (Westbrook et al., 1994) (Fig. 2). At ODP Site 1027, locat-ed 220 km from the coast and about 100 km south west of ODP Site 888(Fig. 1), a 606m-thick sedimentary sequence overlying the 3.58Maoce-anic basement was drilled (Davis et al., 1997) (Fig. 2). At ODP Site 1027,the oldest sediments deposited shortly after the formation of oceaniccrust formed relatively far from the continent (Fig. 1). Sedimentary col-umns drilled at both sites are shown in Fig. 2. Detailed lithologies wereprovided by Westbrook et al. (1994) and Davis et al.(1997) whileCarpentier et al. (2013) reported elemental concentrations on thesame sample set.

Briefly, ODP Site 888 sediments consist of turbidites (Fig. 2). At ODPSite 1027, recovered sediments are hemipelagic silty claywith interbed-ded turbidites and the lowermost unit is a composite of igneous rocks

http://georoc.mpch-mainz.gwdg.de/georoc/

http://georoc.mpch-mainz.gwdg.de/georoc/

Glacial

Interglacial

Glacial

Interglacial

?

"Quiet" interval

Sand-rich interval

Unit IA:Sand-Silt turbidites

and hemi-

pelagic mud

Unit IB:Silt

turbidites and

hemi-pelagic

mud

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pelagic mudstone

Unit III: basalt & mudstone

Basalt

0

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e 10

27 B

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e 88

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bedded clayey

silts and sands

Unit II:massive

sand with

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silt

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Unit IIIB:silt with

interbeds of sand

and gravel

87Sr/86Sr 143Nd/144Nd 206Pb/204Pb176Hf/177Hf

GL

OS

S II

<0.6

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Ave

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water depth = 2656 m

0.074

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quartz content increases

18.6 18.8 19.0 19.4 19.6 19.819.20.2822 0.2826 0.28300.5120 0.5124 0.51280.704 0.708 0.712

18.6 18.8 19.0 19.4 19.6 19.819.20.2822 0.2826 0.28300.5120 0.5124 0.51280.704 0.708 0.712

2 3 4 5 6

2 3 4 5 6

(47)

(56)

(29)

(19)

(74)

(16)

(10)

(10)

(187)

(69)

Age in Ma - (sedimentation rate) in cm/kyr

Fig. 2. Lithostratigraphic column drilled at ODP Sites 888 and 1027, afterWestbrook et al. (1994) andDavis et al. (1997) respectively, and variations of SiO2/Al2O3 (Carpentier et al., 2013),Sr–Nd–Hf–Pb isotope ratios with depth. Analytical errors are smaller than symbol sizes. Ages and sedimentation rates given along the column drilled at ODP Site 1027 are from Su et al.(2000). At ODP Site 888, location of last glacial/interglacial intervals, ages and sedimentation rates along the top 250 m of the core are from Knudson and Hendy (2009). Revised globalsubducting sediment (GLOSS II) Sr, Nd and Pb isotopic compositions are from Plank (2014) and Hf isotopic compositions for average subducted oceanic sediments are from Chauvelet al. (2008).

69M. Carpentier et al. / Chemical Geology 382 (2014) 67–82

and carbonate-rich claystone (Fig. 2). Samples, provided by the Inte-grated Ocean Drilling Program's (IODP) Gulf Coast Repository, areabout 1 cm thick. Seventeen representative samples were selectedalong ODP Site 888 core. With an average sedimentation rate of94.5 cm/kyr, our sampling corresponds to 17 periods of ≈11 yearstaken every 35 kyr. At ODP Site 1027, thirty-seven samples were select-ed;with an average sedimentation rate of 16.9 cm/kyr, one sample rep-resents at time-span of ≈59 years taken every 97 kyr.

At both sites, the proportion of biogenic silica is extremely low(b1%, Westbrook et al., 1994; Davis et al., 1997). This is also the casefor biogenic carbonate, less than 1% for the vast majority of samples,

with the exception of the bottom unit of ODP Site 1027, wherenannofossils and foraminifers can represent more than 50% of the sedi-ments (Davis et al., 1997). Chemical compositions of studied sedimentsmainly reflect those of their continental sources because of negligiblehydrogenous or biogenic input (Carpentier et al., 2013). Only three sam-ples located in the lowermost part of ODP Site 1027 contain highamounts of biogenic Ca and therefore are not representative of theircontinental source. Since the main source of silica in the sediments iscontinental, SiO2/Al2O3 can be used as proxy of their grain-size (e.g.Bouchez et al., 2011), as quartz-rich sands have higher SiO2/Al2O3

than fine, clay-dominated sediments. At ODP Site 1027, most samples


have uniformly low SiO2/Al2O3 ratios, with a sand-rich interval between39 and 110 m depth (Fig. 2). At ODP Site 888 high SiO2/Al2O3, quartz-rich sands are common along the top 450 m of the sedimentary section(Fig. 2).

4. Analytical methods

Sediments from ODP Sites 888 and 1027 were hand-crushed in anagate mortar. Digestion and preparation of samples were carried outin clean rooms at the Pacific Centre for Isotopic and GeochemicalResearch (PCIGR, Vancouver). For each sample, isotopic compositionsof Sr, Nd, Hf and Pb, as well as trace element concentrations(Carpentier et al., 2013), were measured on a single aliquot powder ofabout 100 mg. Samples were dissolved in a HF–HNO3–HClO4 mixturein high-pressure PTFE bombs for 7 days at 190 °C, then dried on ahotplate and taken up in 6 N HCl and re-bombed for 24 h at 190 °C.Since chromatographic extraction procedures used to isolate Sr, Ndand Pb are similar to those given in Weis et al. (2005, 2006, 2007) andin Connelly et al. (2006) for Hf, they are only briefly described here.Leadwas the first element isolated using two successive passes throughcolumns filled with anionic Bio-Rad AG1-X8 resin. In these columns, Sr,Nd andHfwithmatrixwere first collectedwith 0.5MHBr, while Pbwasnext eluted using 6 N HCl. The dried HBr fractions were then loaded oncolumns filled with cationic AG50W-X8 in order to isolate three frac-tions containing Hf, Sr and Nd, respectively. Neodymium and Hf frac-tions were further purified using one pass on Ln-Spec resin for Nd andone pass on TODGA resin for Hf (Connelly et al., 2006). Total procedureblanks (n= 8) have 80, 350, 140 and 15 pg of Pb, Sr, Nd and Hf respec-tively, which is negligible relative to the amount of Pb (~1 μg), Sr(~27 μg), Nd (~1.6 μg) and Hf (~0.27 μg) processed.

Strontium and Nd isotope ratios were measured by TIMS (ThermoFinnigan Triton) in static mode with relay matrix rotation andwere normalized for mass fractionation to 86Sr/88Sr = 0.1194 and146Nd/144Nd = 0.7219. The Sr SRM 987 and Nd La Jolla standardswere run regularly and yielded an 87Sr/86Sr of 0.710239 ± 18 (2σ,n = 22) and a 143Nd/144Nd of 0.511854± 14 (2σ, n= 29) respectively.Hafniumand Pb isotopic compositionsweremeasured in staticmode ona MC-ICP-MS (Nu Instruments Ltd., Nu 021). 176Hf/177Hf was normal-ized for mass fractionation relative to 179Hf/177Hf = 0.7325, and for Pbisotope ratio measurements, we used the natural thallium additiontechnique (White et al., 2000). The NBS 981 Pb standard was runevery second sample and yielded mean values of 206Pb/204Pb =16.9425 ± 27 (2σ, n = 93), 207Pb/204Pb = 15.4991 ± 30 and208Pb/204Pb = 36.7196 ± 88. Using the sample-standard bracketingtechnique (White et al., 2000), Pb isotope ratios were corrected tothe NBS 981 TIMS triple spike values recommended by Galer andAbouchami (1998). The JMC 475 Hf standard was also run every secondsample and yielded 176Hf/177Hf = 0.282161 ± 14 (2σ, n = 88), thedaily average of standard analyses was used to normalize the Hf isotoperatios of samples to the JMC 475 176Hf/177Hf= 0.282160 (Vervoort andBlichert-Toft, 1999). More information is given in the footnote ofTable 1. In order to assess accuracy and external reproducibility, eightsamples were duplicated and international reference materials(BHVO-2, AGV-2, RGM-1, and G-2) were processed and analyzedthroughout the course of this study. Results are reported at the bottomof Table 1 and are in good agreement with previous published isotopicdata on these reference materials (e.g. Weis et al., 2005, 2006, 2007;Chauvel et al., 2011).

5. Results

Sr–Nd–Hf–Pb isotopic compositions of individual samples are re-ported in Table 1 and average compositions of lithological units ofODP Sites 888 and 1027 together with their bulk compositions are sum-marized in Table 2.

5.1. Range of isotopic compositions and variation with time

Variations of isotopic compositions of ODP Site 888 and 1027 sedi-ments as a function of depth are shown in Fig. 2. If both sites are consid-ered, 87Sr/86Sr ratios vary from 0.70480 to 0.71157 (Table 1 and Fig. 2),within the range of previously published data on nine samples fromODP Site 1027 (Chan et al., 2006). Neodymium isotope ratios vary be-tween 0.51216 and 0.51270, with the least radiogenic values observedin ODP Site 888. Similarly, 176Hf/177Hf ratios vary between 0.28211and 0.28298, with the lowest values in ODP Site 888 sediments(Table 1 and Fig. 2). Finally, Pb isotope ratios vary between 18.68 and19.68 for 206Pb/204Pb and between 38.42 and 39.45 for 208Pb/204Pb(Table 1). Samples with the least radiogenic isotopic ratios belong tothe deepest strata drilled at Site 1027, just above the oceanic basement(Fig. 2), while the most radiogenic ratios are observed in a sedimentfrom ODP Site 888 (Unit II, 220m depth, Fig. 2). The range of Pb isotoperatios reported here is roughly similar to what was measured in IODPSite 1301 sediments (drilled close to Site 1027, Noguchi et al., 2008)and in ODP Site 856, 857 and 1035 sediments drilled at the north endof the Juan de Fuca ridge (Bjerkgard et al., 2000; Cousens et al., 2002).

Throughout the studied 600 kyr section of ODP Site 888, isotopiccompositions do not change systematically with depth (Fig. 2). Similar-ly, at Site 1027, no clear variations occur with depth, but the presence ofa “quiet” interval between 255 and 410 m, with very little isotopic var-iations, can be observed. This interval lasts about 460 kyr (between1.23 Ma and 0.76 Ma, Su et al. (2000)) (Fig. 2), and is directly followedby a period characterized by high amplitude isotopic variations(Fig. 2). We emphasize that there is no direct relationship between sed-imentation rate and isotopic variability. Indeed, sedimentation rate(given in Su et al., 2000) drastically changed during the “quiet” intervalwith no obvious impact on isotopic signatures (Fig. 2).

5.2. Bulk composition of ODP Sites 888 and 1027

At each site, we calculated average isotopic compositions of eachlithological unit (Table 2). These compositions, combined with bulktrace-element compositions (Carpentier et al., 2013), provide useful in-formation to quantify sediment contribution to the source of themagmas erupted along the Northern Cascades Arc. At ODP Site 888,there are no sharp differences over the three units (Table 2). In contrast,at ODP Site 1027, the deepest unit (Unit III) hasmarkedly less radiogen-ic average Pb isotope ratios (206Pb/204Pb of 18.80) compared to theother units of this site with 206Pb/204Pb ranging from 19.06 to 19.08(see Table 2). Some trace element features of ODP Site 1027 sedimentsfrom Unit III such as Ba and Pb enrichment and negative Ce anomaly(Carpentier et al., 2013) suggest the presence of a hydrothermal compo-nent that could account for the unradiogenic Pb ratios (Cousens et al.,1995; Abouchami et al., 1997; Ling et al., 1997).WhenUnit III (volumet-rically minor, Table 2) is excluded, only slight differences exist betweenthe Pb average compositions of the other ODP Site 1027 units (Table 2).

When bulk compositions of both sites are compared, ODP Site888 has generally more enriched isotopic compositions (more radio-genic Sr and Pb isotope ratios, together with lower 143Nd/144Nd and176Hf/177Hf) than bulk ODP Site 1027 (Table 2). Both Sites have loweraverage Sr isotopic compositions than the recently revised Globalsubducting sediment estimate, GLOSS II (Plank, 2014, see Fig. 2),which is an excellent representation of the composition of worldwideoceanic sediments currently facing active margins. Consistently, theNd bulk isotopic compositions of both sites are more radiogenic thanGLOSS II (see Fig. 2). No Hf isotopic data is provided for GLOSS II butour bulk ODPSite 888 has lower 176Hf/177Hf than the value recommend-ed for average subducted oceanic sediments by Chauvel et al. (2008),while bulk ODP Site 1027 has slightly higher 176Hf/177Hf. Finally, bulkPb isotopic compositions of ODP Site 888 and 1027 sediments are signif-icantly more radiogenic than GLOSS II (see Fig. 2).

Table 1Sr–Nd–Hf–Pb isotopic compositions of ODP Site 888 and 1027 sediments. DD stands for duplicate dissolution and DR for duplicate run; uncertainties reported are in-run errors. Sr andNd isotope ratios are normalized to 87Sr/86Sr = 0.710248 and 143Nd/144Nd = 0.511858 for the SRM 987 and La Jolla standards, respectively (Weis et al., 2006). Hf isotope ratiosare given relative to 176Hf/177Hf = 0.282160 for the JMC 475 standard (Vervoort and Blichert-Toft, 1999) and Pb isotope ratios relative to NBS 981 values of 206Pb/204Pb = 16.9405,207Pb/204Pb = 15.4963 and 208Pb/204Pb = 36.7219 (Galer and Abouchami, 1998). Reproducibility calculated using complete duplicate analyses of 8 samples is 0.000013 (19 ppm, 2σ),0.000027 (54 ppm) and 0.000026 (90 ppm) on the measured 87Sr/86Sr, 143Nd/144Nd and 176Hf/177Hf ratios respectively. For Pb isotopes, the 2σ external reproducibility is 0.0073(380 ppm), 0.0017 (108 ppm) and 0.0108 (277 ppm) for 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios respectively.

Depth(m)

Lith.unit

87Sr/86Sr 2 σm143Nd/144Nd

2 σm176Hf/177Hf

2 σm206Pb/204Pb

2 σm207Pb/204Pb

2 σm208Pb/204Pb

2 σm

Leg ODP 146, Site 888:888B 1 H 2 106–107 2.5 I 0.708180 0.000008 0.512444 0.000006 0.282792 0.000004 19.0541 0.0009 15.6482 0.0008 38.9557 0.0020888B 1 H 2 106–107 DR 0.282800 0.000004 19.0560 0.0008 15.6498 0.0007 38.9603 0.0025888B 4 H 2 100–101 27.0 I 0.705633 0.000007 0.512535 0.000007 0.282691 0.000006 19.2977 0.0012 15.6554 0.0010 38.8704 0.0026888B 7 H 2 104–105 55.5 I 0.705917 0.000007 0.512604 0.000015 0.282870 0.000003 19.0548 0.0013 15.6318 0.0007 38.7849 0.0024888B 13 H 3 40–41 112.0 I 0.706375 0.000008 0.512360 0.000007 0.282546 0.000005 19.2264 0.0009 15.6632 0.0010 38.9409 0.0027888B 13 H 3 40–41 DR 0.282552 0.000005888B 16 H 2 103–104 139.6 I 0.709702 0.000007 0.512290 0.000010 0.282767 0.000004 19.1904 0.0012 15.6745 0.0011 39.0546 0.0030888B 16 H 2 103–104 DR 0.282768 0.000005 19.1908 0.0008 15.6743 0.0008 39.0547 0.0022888B 19 X 2 101–102 168.1 I 0.709187 0.000007 0.512236 0.000006 0.282607 0.000006 19.2423 0.0006 15.6803 0.0006 39.1141 0.0017888B 19 X 2 101–102 DD 0.709190 0.000008 0.512234 0.000012 0.282624 0.000004 19.2311 0.0009 15.6791 0.0008 39.0910 0.0021888B 25 H 2 105–106 220.0 II 0.709245 0.000008 0.511949 0.000007 0.282107 0.000006 19.6751 0.0010 15.7220 0.0009 39.4455 0.0023888B 26 H 2 100–101 229.4 II 0.710482 0.000009 0.512195 0.000006 0.282633 0.000007 19.2387 0.0010 15.6830 0.0009 39.1252 0.0025888B 29 H 2 103–104 254.3 II 0.705910 0.000007 0.512483 0.000010 0.282714 0.000005 19.1925 0.0006 15.6508 0.0005 38.8603 0.0014888B 29 H 2 103–104 DR 0.282706 0.000005888B 32 H 3 24–25 278.2 II 0.705541 0.000007 0.512482 0.000012 0.282786 0.000005 19.0960 0.0009 15.6490 0.0007 38.8663 0.0017888B 36 H 3 41–42 311.6 II 0.706652 0.000008 0.512287 0.000006 0.282485 0.000004 19.2236 0.0008 15.6661 0.0007 39.0370 0.0019888B 36 H 3 41–42 DR 19.2233 0.0010 15.6657 0.0009 39.0363 0.0024888B 40 H 3 102–103 351.1 II 0.705762 0.000008 0.512352 0.000009 0.282619 0.000005 19.1767 0.0009 15.6517 0.0009 38.9301 0.0018888B 44 X 1 44–45 385.9 II 0.708486 0.000007 0.512305 0.000007 0.282697 0.000005 18.9787 0.0008 15.6571 0.0007 38.8674 0.0018888B 56 X 1 21–22 487.5 III 0.711542 0.000008 0.512215 0.000006 0.282651 0.000006 19.0124 0.0008 15.6641 0.0007 38.9599 0.0019888B 56 X 1 21–22 DD 0.711560 0.000010 0.512198 0.000012 0.282656 0.000005 19.0077 0.0007 15.6620 0.0007 38.9534 0.0018888B 57 X 2 46–47 498.2 III 0.706820 0.000007 0.512467 0.000011 0.282780 0.000007 19.1581 0.0010 15.6571 0.0009 38.9230 0.0023888B 62 X 2 45–46 533.4 III 0.705657 0.000007 0.512697 0.000007 0.282941 0.000005 19.0178 0.0010 15.6215 0.0009 38.6811 0.0018888B 62 X 2 45–46 DD 0.705647 0.000006 0.512679 0.000006 0.282938 0.000005 19.0175 0.0009 15.6225 0.0008 38.6854 0.0026888B 65 X 2 41–42 560.0 III 0.705243 0.000008 0.512659 0.000008 0.282897 0.000006 19.0398 0.0011 15.6235 0.0009 38.7416 0.0025

Leg ODP 168, Site 1027:1027B 3 H 4 25–26 18.5 IA 0.704797 0.000008 0.512509 0.000008 0.282878 0.000005 19.1235 0.0008 15.6548 0.0007 38.9219 0.00191027B 5 H 5 40–41 39.1 IA 0.705545 0.000008 0.512435 0.000007 0.282634 0.000006 19.1476 0.0009 15.6512 0.0008 38.8716 0.00211027B 6 H 3 42–43 45.6 IA 0.708084 0.000008 0.512398 0.000007 0.282762 0.000005 19.1607 0.0006 15.6610 0.0005 38.9573 0.00171027B 7 H 5 40–41 56.1 IA 0.704880 0.000008 0.512543 0.000008 0.282752 0.000005 19.0843 0.0011 15.6344 0.0009 38.7889 0.00251027B 9 H 5 40–41 77.0 IA 0.704928 0.000010 0.512580 0.000006 0.282733 0.000007 19.1079 0.0007 15.6333 0.0007 38.7546 0.00201027B 9 H 5 40–41 DD 0.704910 0.000010 0.512557 0.000007 0.282749 0.000004 19.1099 0.0008 15.6328 0.0007 38.7448 0.00191027B 10 H 5 49–50 86.7 IA 0.704846 0.000006 0.512605 0.000007 0.282676 0.000006 19.1481 0.0011 15.6358 0.0010 38.7678 0.00281027B 13 X CC 13–14 107.2 IA 0.705674 0.000007 0.512509 0.000007 0.282710 0.000013 19.3873 0.0012 15.6801 0.0013 38.9818 0.00281027B 13 X CC 13–14 DD 0.705680 0.000007 0.512536 0.000006 0.282769 0.000005 19.3730 0.0010 15.6804 0.0010 38.9755 0.00251027B 17 X 1 40–41 145.9 IA 0.709312 0.000007 0.512420 0.000006 0.282789 0.000005 18.9177 0.0007 15.6455 0.0006 38.7959 0.00171027B 19 X 1 40–41 165.1 IA 0.707277 0.000007 0.512651 0.000014 0.282870 0.000005 19.0465 0.0010 15.6357 0.0009 38.8130 0.00361027B 20 X CC 12–13 174.4 IA 0.707655 0.000008 0.512544 0.000008 0.282916 0.000005 19.0994 0.0007 15.6459 0.0006 38.8820 0.00161027B 22 X 4 40–41 198.4 IB 0.710844 0.000008 0.512333 0.000007 0.282786 0.000005 19.0725 0.0009 15.6581 0.0006 39.0046 0.00191027B 23 X 1 42–43 203.6 IB 0.706174 0.000008 0.512637 0.000005 0.282942 0.000005 19.1554 0.0010 15.6471 0.0008 38.8311 0.00221027B 25 X 1 116–117 223.6 IB 0.706143 0.000009 0.512594 0.000008 0.282916 0.000004 19.0554 0.0008 15.6383 0.0008 38.7865 0.00211027B 25 X 1 116–117 DR 0.282919 0.000005 19.0551 0.0010 15.6377 0.0009 38.7853 0.00191027B 26 X 1 47–48 232.5 IB 0.711570 0.000009 0.512160 0.000006 0.282641 0.000005 19.2427 0.0008 15.6846 0.0006 39.1920 0.00181027B 28 X 1 45–46 251.8 IB 0.708405 0.000009 0.512424 0.000007 0.282855 0.000005 19.1195 0.0010 15.6588 0.0010 38.9615 0.00321027B 29 X 1 44–45 261.3 IB 0.705673 0.000008 0.512688 0.000006 0.282972 0.000005 19.0065 0.0007 15.6200 0.0006 38.6777 0.00151027B 29 X 1 44–45 DR 0.705651 0.000008 0.282979 0.0000051027B 32 X 1 48–49 290.3 IB 0.706459 0.000008 0.512633 0.000007 0.282940 0.000006 19.0520 0.0009 15.6314 0.0008 38.7855 0.00211027B 34 X 2 41–42 311.0 IB 0.706589 0.000008 0.282963 0.000005 19.0218 0.0007 15.6292 0.0007 38.7578 0.00161027B 35 X 1 40–41 319.1 IB 0.706750 0.000010 0.512624 0.000006 0.282951 0.000005 19.0145 0.0009 15.6282 0.0007 38.7654 0.00201027B 35 X 1 40–41 DR 19.0160 0.0007 15.6299 0.0006 38.7695 0.00171027B 37 X 1 42–43 338.4 IB 0.705171 0.000007 0.512695 0.000006 0.282963 0.000005 18.9782 0.0007 15.6135 0.0006 38.6488 0.00171027B 38 X 2 40–41 349.6 IB 0.706628 0.000007 0.512613 0.000007 0.282948 0.000005 19.0087 0.0007 15.6298 0.0007 38.7814 0.00221027B 40 X 1 40–41 367.4 IB 0.707843 0.000009 0.512774 0.000008 0.283015 0.000004 19.0052 0.0009 15.6065 0.0008 38.5660 0.00191027B 41 X 1 42–43 377.0 IB 0.705534 0.000007 0.512697 0.000009 0.282943 0.000004 18.9945 0.0009 15.6212 0.0009 38.6663 0.00231027B 41 X 1 42–43 DD 0.705540 0.000007 0.512758 0.000008 0.282973 0.000004 18.9928 0.0009 15.6197 0.0008 38.6620 0.00221027B 44 X 1 41–42 405.8 IB 0.705416 0.000007 0.512707 0.000007 0.282965 0.000004 19.0390 0.0012 15.6172 0.0010 38.6805 0.00241027B 45 X 2 44–45 416.9 IB 0.705132 0.000008 0.512744 0.000008 0.282983 0.000004 18.9994 0.0009 15.6135 0.0007 38.6354 0.00211027B 45 X 2 44–45 DR 0.282986 0.000005 18.9995 0.0008 15.6137 0.0007 38.6370 0.00201027B 46 X 1 43–44 425.0 IB 0.708129 0.000008 0.512453 0.000006 0.282841 0.000005 19.0208 0.0006 15.6426 0.0007 38.8658 0.00161027B 49 X 1 47–48 454.0 IB 0.708085 0.000007 0.512447 0.000007 0.282821 0.000005 19.0881 0.0011 15.6500 0.0007 38.9258 0.00181027B 51 X 1 41–42 473.0 II 0.706995 0.000009 0.512431 0.000007 0.282731 0.000005 19.1459 0.0008 15.6553 0.0007 38.9392 0.00191027B 51 X 1 41–42 DR 0.282726 0.000005 19.1451 0.0010 15.6538 0.0009 38.9363 0.00231027B 52 X 1 41–42 482.6 II 0.708516 0.000008 0.512281 0.000008 0.282693 0.000004 19.1277 0.0007 15.6610 0.0007 39.0354 0.00191027B 52 X 1 41–42 DD 0.708529 0.000007 0.512273 0.000009 0.282705 0.000006 19.1257 0.0013 15.6600 0.0012 39.0368 0.00211027B 54 X 1 40–41 501.7 II 0.709919 0.000007 0.512393 0.000007 0.282793 0.000005 19.0769 0.0007 15.6444 0.0006 38.9751 0.00171027B 56 X 2 133–134 523.4 II 0.708959 0.000010 0.512412 0.000006 0.282817 0.000010 18.9542 0.0010 15.6355 0.0008 38.8392 0.00231027B 57 X 1 40–41 530.7 II 0.706867 0.000009 0.512645 0.000006 0.282928 0.000006 18.9906 0.0008 15.6193 0.0007 38.6958 0.0018

(continued on next page)


Table 1 (continued)

Depth(m)

Lith.unit

87Sr/86Sr 2 σm143Nd/144Nd

2 σm176Hf/177Hf

2 σm206Pb/204Pb

2 σm207Pb/204Pb

2 σm208Pb/204Pb

2 σm

1027B 57 X 1 40–41 DR 0.706848 0.0000081027B 58 X 1 40–41 540.3 II 0.706176 0.000008 0.512579 0.000008 0.282908 0.000004 19.0237 0.0009 15.6271 0.0007 38.7432 0.00201027B 60 X 1 42–43 559.5 II 0.709937 0.000007 0.512526 0.000007 0.282891 0.000007 19.0611 0.0007 15.6358 0.0007 38.8527 0.00191027C 2 R 2 49–50 595.5 III 0.705042 0.000008 0.512794 0.000008 0.283015 0.000004 19.0123 0.0008 15.6019 0.0007 38.5480 0.00191027C 2 R 7 40–41 602.9 III 0.709154 0.000008 0.512476 0.000007 0.282873 0.000010 18.8670 0.0009 15.6331 0.0009 38.7996 0.00191027C 3 R 2 96–97 606.1 III 0.709152 0.000007 0.512518 0.000008 0.282839 0.000005 18.6847 0.0009 15.5726 0.0009 38.4210 0.0024

International rock standards:G-2 0.709775 0.000008 0.512224 0.000011 0.282506 0.000004 18.4040 0.0009 15.6363 0.0008 38.9006 0.0021AGV-2 0.704001 0.000008 0.512786 0.000006 0.282970 0.000005 18.8740 0.0008 15.6192 0.0007 38.5509 0.0019RGM-1 0.704186 0.000008 0.512806 0.000006 0.283018 0.000010 19.0052 0.0007 15.6318 0.0007 38.7006 0.0019RGM-1 DD 0.704203 0.000009 0.512795 0.000008 0.283016 0.000007 18.9991 0.0011 15.6285 0.0010 38.6896 0.0027BHVO-2 0.703486 0.000009 0.513003 0.000007 0.283102 0.000005 18.6390 0.0008 15.5342 0.0007 38.2392 0.0017


6. Discussion

Compared to the whole spectrum of oceanic sediments (biogenic,hydrothermal–hydrogenous and terrigenous, Plank and Langmuir,1998; Plank, 2014), ODP Site 888 and 1027 sediments belong to thegroup dominated by detrital continental material. These two sites arelocated very close to continental sources, where large input of detritalmaterial carried by rivers and very high sedimentation rates (from 8to 180 cm/kyr, Su et al., 2000; Knudson and Hendy, 2009) prevent theconcentration of hydrogenous material. In this tectonic setting, sedi-ment is formed by mixing of detrital material with a biogenic compo-nent. In the case of ODP Sites 888 and 1027, the proportion of biogenicmaterial is particularly low, with the exception of the bottom 180 mof ODP Site 1027 core where calcium carbonate can reach 45% in a fewsamples (Carpentier et al., 2013). In any case, because biogenic silicaand calcium carbonates are very poor in most trace elements (e.g.Plank and Langmuir, 1998; Olivier and Boyet, 2006), the Nd, Hf and Pbisotopic compositions of the sediments correspond to the compositionof the detrital material. Only two samples from Unit III of ODP Site1027 (at 606.1 and 602.9mdepth) probably have their Pb isotopic com-positions dominated by a contribution from Pb-rich hydrothermalfluids. Strontium isotopes are, in contrast, highly sensitive to the pres-ence of biogenic carbonates owing to the high input of seawater-derived Sr associated with calcium carbonate. The three carbonate-rich samples belonging to the lowermost part of ODP Site 1027 section

Table 2Average isotopic compositions of lithological units of ODP Sites 888 and 1027, and bulk composcontents (Carpentier et al., 2013) and isotopic compositions of discrete samples. Then, for each sto calculate the site bulk composition (Plank et al., 2007). The relativemass proportion of each uWestbrook et al. (1994) and fromDavis et al. (1997) forODP Sites 888 and1027 respectively. AtUnit III (Davis et al., 1997). The basaltic component was excluded from calculation of both the

Site 888

Unit I Unit II Unit III Bulk

Thickness (m) 184 273 110 567Wet bulk density 1.84 1.96 2.08 1.94Water (%) 31.2 24.9 21.0 26.2Dry bulk density 1.26 1.47 1.64 1.44Mass proportion (%) 29 49 22 100Number of samples ppm 6 7 4 17Sr 320 313 309 314Nd 20.3 19.6 21.0 20.1Hf 4.10 3.99 3.66 3.95Pb 11.1 11.5 14.5 12.0SiO2/Al2O3 3.91 4.88 3.60 4.2887Sr/86Sr 0.707445 0.707307 0.707106 0.70730143Nd/144Nd 0.512397 0.512234 0.512478 0.51233176Hf/177Hf 0.282701 0.282552 0.282814 0.28265206Pb/204Pb 19.1766 19.1795 19.0459 19.1430207Pb/204Pb 15.6619 15.6683 15.6494 15.6616208Pb/204Pb 38.9779 39.0030 38.8745 38.9620ΔεHf 2.86 2.54 4.42 2.88Δ87Sr/86Sr 12.4 0.5 14.1 7.1

(at 523.4, 602.9 and 606.1 m depth) have high Sr contents (Carpentieret al., 2013) and their Sr isotopic compositions are probably influencedby Pacific seawater Sr (87Sr/86Sr ~ 0.709, Ling et al., 1997). With the ex-ception of these three samples, the isotopic compositions of ODP Site888 and 1027 sediments should reflect the composition of the detritalfraction, and can be used to constrain continental source areas and tem-poral evolution over the last 3.5 Myr.

In the first part of the discussion, we use mainly Nd isotopes to tracethe origin of the sediments. The Nd isotopic system is a robust proxy totrace sources (e.g. Clift et al., 2002) and large-scale Nd isotopic varia-tions reflect mixing of material eroded from terranes with contrastingsignatures. In the second part of the discussion, we examine the causesof fine-scale variations of Sr andHf isotope ratios at constant Nd isotopiccompositions, and attribute them to mineral sorting effects.

6.1. Sediment provenance

The relationship between Nd, Sr and Pb isotopes (Figs. 3–5) suggeststhat the diversity of isotopic compositions present in sediments of ODPSites 888 and 1027 can essentially be explained by a binary mixture oftwo end-members: the first one shows depleted characteristics(143Nd/144Nd N 0.5128, 87Sr/86Sr b 0.704 and 206Pb/204Pb b 18.9) andthe second one has more crustal values (143Nd/144Nd b 0.5118,87Sr/86Sr N 0.712 and 206Pb/204Pb N 19.7). ODP Sites 888 and 1027 arelocated in the Northern part of the Cascadia Basin, bordered at this

ition of each Site. For a given unit, average composition is calculated using Sr, Nd, Hf and Pbite, relativemass proportions and average compositions of the different units are combinednit depends on its thickness and its dry bulk density.Water contents and density are fromODP Site 1027, Unit III is 37 m thick, but basaltic layers represent 38% of the total volumeofaverage composition of Unit III and the bulk composition of the sedimentary section.

Site 1027

Unit IA Unit IB Unit II Unit III Bulk

184 283 102 23 5921.81 1.8 2 2.04 1.85

31.4 30.9 21.3 19.1 28.91.24 1.24 1.57 1.65 1.32

29 45 21 5 10010 17 7 3 37

351 300 321 719 34018.9 20.7 20.4 18.6 20.03.61 3.02 3.07 1.71 3.14

12.4 12.4 12.4 15.1 12.63.95 3.12 3.19 3.47 3.38

4 0.706015 0.707010 0.708109 0.708477 0.7070737 0.512516 0.512562 0.512464 0.512602 0.5125300 0.282769 0.282901 0.282819 0.282938 0.282841

19.0817 19.0604 19.0630 18.7969 19.051715.6483 15.6385 15.6407 15.5926 15.639138.8501 38.8261 38.8796 38.5340 38.82681.68 4.96 5.26 5.05 3.795.7 18.6 23.3 35.8 17.2

87Sr/86Sr

143 N

d/14

4 Nd

0.5112

0.5114

0.5116

0.5118

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0.5134

0.700 0.710 0.720 0.730 0.740 0.750 0.760

0.5114

0.5118

0.5122

0.5126

0.5130

0.5134

0.70 0.72 0.74 0.76 0.78 0.80 0.82 0.84

Depleted Western terranes

Fraser River hyperbola(Cameron and Hattori, 1997)

ODP Site 1027

ODP Site 888

Th

is w

ork

Pu

blis

hed

dat

a Depleted Western terranes

Vancouver Island

Cascades Arc

Coast Mountain Belt

Intermontane Belt

Enriched Eastern terranes

Omineca and Belt-Purcell Belts (metamorphic and sedimentary)

Omineca and Belt-PurcellBelts (intrusive)

Fraser River suspended matter

Fig. 3.Nd andSr isotopic compositions of sediments fromODP Sites 888 and1027 compared to those of their potential source terranes. Compositions of “depleted” source terranes are fromGreene et al. (2009) for Vancouver Island, Cui and Russell (1995a), Friedman et al. (1995) andMahoney et al. (2009) for the Coast plutonic complex, from Samson et al. (1989), Smith andLambert (1995) and Smith et al. (1995) for igneous and sedimentary rocks belonging to the Intermontane Belt, data for Cascades Arc north of 44°N are fromHalliday et al. (1983), Leemanet al. (1990, 2004), Tepper et al. (1993), Tepper (1996), Bacon et al. (1997), Conrey et al. (2001), Schmidt et al. (2008), Jicha et al. (2009) andMullen andWeis (2013). For the “enriched”terranes from eastern British Columbia, Sr and Nd data are fromGhosh and Lambert (1989) for sedimentary rocks from Belt–Purcell Supergroup and Omineca belts and from Brandon andLambert (1993, 1994), Brandon and Smith, (1994) andDriver et al. (2000) for the Belt–Purcell Supergroup andOmineca belt intrusive rocks. The inset shows Sr–Nd isotopic compositionsof suspendedmatter of the Fraser River and its tributaries, collected in late July 1993 (Cameron and Hattori, 1997). The curve shows binarymixing between an enriched end-member, thesuspendedmatter from the Fraser River at Fitzwilliam(extreme east of thedrainage) and a depleted end-member, the suspendedmatter fromChilcotin River (draining in thewestern partof the Cordillera, see Cameron and Hattori, 1997 for more detail).

37.5

38.0

38.5

39.0

39.5

40.0

40.5

41.0

18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5206Pb/204Pb

208 P

b/20

4 Pb

ODP Site 1027

ODP Site 888

This work

Published data

Vancouver Island

Cascades Arc

Coast Mountain Belt

Intermontane Belt

Omineca and Belt-Purcell Belts (intrusive)

Astoria fan

Hydro-thermal sediment

(HT)

HT

Fig. 4. 208Pb/204Pb vs. 206Pb/204Pb diagram showing results obtained for ODP Sites 888 and 1027 sediments. DSDP Site 174 sediment Pb isotopic compositions are given in Prytulak et al.(2006). Pb isotope ratios are fromGreene et al. (2009) for Vancouver Island, fromCui and Russell (1995a) andWetmore andDucea (2011) for theCoast plutonic complex, fromChurch andTilton (1973), Church (1976), Halliday et al. (1983), Leeman et al. (1990, 2004), Bacon et al. (1997), Conrey et al. (2001), Jicha et al. (2009) andMullen andWeis (2013) for Cascades lavaserupted north of 44°N, from Smith and Lambert (1995) for igneous rocks from the Intermontane Belt and from Brandon and Lambert (1993, 1994) and Brandon and Smith (1994) for theBelt–Purcell Supergroup and Omineca belt intrusive rocks.


Astoria fan trend

ODP Site 888 and 1027 trend

0.5112

0.5114

0.5116

0.5118

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0.5134

18.0 18.5 19.0 19.5 20.0 20.5 21.0 21.5206Pb/204Pb

143 N

d/14

4 Nd

ODP Site 1027

ODP Site 888

This work

Published data

Vancouver Island

Cascades Arc

Coast Mountain Belt

Intermontane Belt

Omineca and Belt-Purcell Belts (intrusive)

Astoria fan

Hydro-thermal

sediments

Fig. 5. Nd and Pb isotopic compositions of ODP Site 888 and 1027 sediments and their potential source terranes. Data sources are as follows: Vancouver Island (Greene et al., 2009);Cascades Arc north of 44°N (Halliday et al., 1983; Leeman et al., 1990; Bacon et al., 1997; Conrey et al., 2001; Leeman et al., 2004; Jicha et al., 2009; Mullen andWeis, 2013); Coast plutoniccomplex (Cui and Russell, 1995a); Intermontane Belt (Smith and Lambert, 1995); Belt–Purcell Supergroup and Omineca belt intrusive rocks (Brandon and Lambert, 1993, 1994; Brandonand Smith, 1994). Astoria fan sediment compositions (DSDP Site 174, Prytulak et al., 2006) are shown for comparison.


latitude by the Canadian Cordillera. Thismountain range, drained by theFraser River and secondary coastal rivers (Fig. 1), presents high reliefand many summits with elevations higher than 2000 m located only afew tens of kilometers from the Pacific Ocean, and is the most likelysource of sediment.

6.1.1. Potential source terranesThe Canadian Cordillera is made of distinct belts, of different ages

and nature (Fig. 1). Thewestern belts have depletedNd isotopic compo-sitions, while the eastern belts have more enriched compositions(Fig. 1). On Vancouver Island in the west, the Insular Belt, a Triassic ac-creted oceanic plateau (Jones et al., 1977), is characterized by radiogenic143Nd/144Nd (εNd ≈ 7.1 ± 3.3, see Fig. 1) together with low 87Sr/86Sr(Greene et al., 2009) that plot at the depleted end of Site 888and 1027 sediment Sr–Nd isotopic array (Fig. 3). Further east, thesubduction-related granitoids from the Coast Mountain Belt (Fig. 1)also have radiogenic Nd isotope ratios associated with low 87Sr/86Sr,206–208Pb/204Pb (εNd ≈ 5.0 ± 2.3, see Figs. 1, 3–5) (Cui and Russell,1995a; Friedman et al., 1995; Mahoney et al., 2009; Wetmore andDucea, 2011). In addition, the Coast Mountain Belt is a prominentmountain range, lying along the Pacific coast and is much probablyone of the major sources of sediments in the Northern Cascades Basin(Underwood et al., 2005; Kiyokawa and Yokoyama, 2009; Carpentieret al., 2013). Recent volcanic formations in the northern part of theCascades Arc might also contribute to the sedimentary supply. Their ra-diogenic Nd isotopic compositions (εNd ≈ 6.1 ± 2.2, see Fig. 1, Hallidayet al., 1983; Leeman et al., 1990; Tepper et al., 1993; Tepper, 1996;Bacon et al., 1997; Conrey et al., 2001; Leeman et al., 2004; Schmidtet al., 2008; Jicha et al., 2009; Mullen and Weis, 2013; Mullen andMcCallum, 2014), and low 87Sr/86Sr all plot at the depleted end of thearray defined by sediments from Sites 888 and 1027 in Fig. 3. TheIntermontane belt located east of the Coast Mountain Belt also hasoceanic affinities (Monger et al., 1982) and is mainly made of maficigneous rocks with radiogenic 143Nd/144Nd ratios and low 87Sr/86Sr(εNd ≈ 4.8 ± 5.9, see Figs. 1 and 3, Samson et al., 1989; Smith andLambert, 1995; Smith et al., 1995; Patchett and Gehrels, 1998). Even ifa few rocks from these belts have radiogenic 206–208Pb/204Pb (Fig. 4),

they are all characterized by depleted Nd and Sr isotopic compositions(Figs. 3 and 5) that preclude them to account for the whole range ofcompositions of ODP Site 888 and 1027 sediment.

In contrast to the depleted isotopic signatures of the proximalsources, the source regions located farther inland have more enrichedisotopic signatures (Fig. 1). The sedimentary andmetamorphic Ominecaand Belt–Purcell Supergroup belts (Fig. 1) have strongly negative εNd(≈−16.3 ± 11.6, see Fig. 1), suggesting dominant input from old con-tinental crust (Frost andO′Nions, 1984; Burwash et al., 1988; Ghosh andLambert, 1989; Patchett and Gehrels, 1998). They plot at the enrichedend of the Sr–Nd array defined by ODP Site 888 and 1027 sediments(Fig. 3). These two belts are intruded by younger plutons volumetricallyminor but for which more extensive isotopic dataset exists (Figs. 3–5,Brandon and Lambert, 1993, 1994; Brandon and Smith, 1994; Driveret al., 2000). The Omineca and Belt–Purcell intrusive rocks haveunradiogenic Nd isotopes (εNd ≈ −8.1 ± 9.9, Fig. 1) and elevated87Sr/86Sr, 206–208Pb/204Pb ratios (Figs. 3–5).

6.1.2. Source mixingFig. 3 shows that mixing of “depleted” detritus coming from the

western part of the Canadian Cordillera with “enriched” detritus deriv-ing from the easternmost belts of the cordillera might account for therange of Sr and Nd isotopic compositions displayed by sediments fromODP Sites 888 and 1027. Not only do Sr and Nd isotopes define a rela-tively good correlation, but this is also the case for Pb–Pb and Nd–Pbin binary isotopic spaces Figs. (4-5). Given the large isotopic diversityof the enriched end-members, such tight correlations require very thor-ough homogenization of the end-members before mixing occurs. Thegood linear relationship between La/Yb and 143Nd/144Nd ratios of sedi-ment from ODP Sites 888 and 1027 (Fig. 6) further suggests that thetwo dominant components have compositions that remained mostlyunchanged over the ~3.5 Myr period of deposition (Westbrook et al.,1994; Davis et al., 1997).

To quantify the proportion of the two main end-members, we con-centrate on the Nd isotopic system that is not significantly fractionatedduring erosion/transport processes, and is therefore a good proxy forsediment source tracking (Garçon et al., 2014 and reference therein).

0

10

20

30

40

50

60

0.5112 0.5116 0.5120 0.5124 0.5128 0.5132

La/Y

b

143Nd/144Nd

ODP Site 1027

ODP Site 888

This workPublished data

Vancouver Island

Cascades Arc

Coast MountainBelt

Intermontane Belt

Omineca and Belt-Purcell Belts(intrusive)

Astoria fan

Fig. 6. La/Yb vs. 143Nd/144Nd diagram with results obtained for sediments from ODP Sites 888 and 1027. Trace element concentrations of ODP Site 888 and 1027 sediments are given inCarpentier et al. (2013). Data for DSDP Site 174 sediments from the Astoria fan (Prytulak et al., 2006) are shown for comparison. Data sources for potential source terranes are as follows:Vancouver Island (Greene et al., 2009); Cascades Arc north of 44°N (Halliday et al., 1983; Leeman et al., 1990; Tepper et al., 1993; Tepper, 1996; Bacon et al., 1997; Conrey et al., 2001;Schmidt et al., 2008; Jicha et al., 2009; Mullen and Weis, 2013); Coast plutonic complex (Cui and Russell, 1995a, 1995b; Mahoney et al., 2009); Intermontane Belt (Smith and Lambert,1995); Belt–Purcell Supergroup and Omineca belt intrusive rocks (Brandon and Lambert, 1993, 1994; Brandon and Smith, 1994; Driver et al., 2000).


In addition, Nd isotopic compositions have been determined on a largeset of samples from the Canadian Cordillera covering the entire spec-trum of potential sources, in contrast to Pb and Sr (see Fig. 1b). Thehigh 143Nd/144Nd end-member corresponds to the average of themulti-ple depleted sources. Because the easternmost enriched terranes showmuch more variability, three different low 143Nd/144Nd end-memberswith variable Nd contents were used for modeling (see Fig. 7 captionfor details). Mixing calculations show that the most enriched ODP Site888 sample requires between 38 and 62% of the eastern enriched com-ponent in its source, depending on the chosen composition for this

0.5110

0.5112

0.5114

0.5116

0.5118

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

0 10 20 30 40 50 60 70 80

Bulk ODP Site 1027

Bulk ODP Site 888

62%38%

28%17%

16%10%

Proportion of Eastern enriched component

143 N

d/14

4 Nd

in s

edim

ent

a

8

Western depleted end-member

Eastern enricend-members

Fig. 7. a) Calculated Nd isotopic compositions of a sediment formed bymixture of detritus comicoming from the eastern enriched terranes of the Cordillera; b) distribution histogram of 143NdCanadian Cordillera (samedataset as in Fig. 1b). Also reported is the range of Nd isotopic compospart of thedrainage,where only Omineca andBelt–Purcell terranes are drained (Cameron andHcompositions in the enriched sediment end-members. Compositions of the end-members usedresponds to the average of the several depleted sources (2) enriched end-members, EE-1: Nd=45 ppm, 143Nd/144Nd= 0.511435. These three end-members cover the range of isotopic and cmost enriched terranes.

component (Fig. 7). In comparison, a contribution of 10 to 16% and 17to 28% from the eastern enriched source is required to fit the bulkODP Site 1027 and 888 sediments of 0.5125 and 0.5123 respectively(Fig. 7). To conclude, sediments from ODP Sites 888 and 1027are dominated by the proximal depleted components, with high143Nd/144Nd and low 87Sr/87Sr, 206–208Pb/204Pb. However some of themost enriched samples might require a contribution of about 40 to60% from the easternmost terranes of the Canadian Cordillera. The oc-currence of monazites as old as 1.8 Gyr in IODP Site 1301 sedimentsdrilled in the vicinity of ODP Site 1027 (Fig. 1) is another evidence for

90 100

(%)n

143Nd/ 144N

d

Site

s 88

8 &

1027

b

n

ODPSites 888

&1027

Eas

tern

Fra

ser

Riv

er s

uspe

nded

mat

terEE-1

EE-2

EE-3

hed (EE)

0.5110

0.5112

0.5114

0.5116

0.5118

0.5120

0.5122

0.5124

0.5126

0.5128

0.5130

0.5132

Western depletedterranes

Eastern enrichedterranes:

Intrusive

Metamorphic and sedimentary

Potential sources

120 100 80 60 40 20

20

ng from thewestern depleted terranes of the Canadian Cordillerawith enriched sediments/144Nd in ODP Site 888 and 1027 sediments and in rocks forming the different belts of theitions of the suspendedmatter of the Fraser River and its tributaries sampled in the easternattori, 1997). In panel a, different curves result fromvariable Nd concentration and isotopicfor calculations: (1) depleted end-member: Nd= 15 ppm, 143Nd/144Nd= 0.5129; it cor-35 ppm, 143Nd/144Nd= 0.5117, EE-2: Nd=40 ppm, 143Nd/144Nd= 0.51155, EE-3: Nd=hemical compositions reported for sedimentary and metamorphic rocks from the eastern-


detrital input deriving from the eastern Cordillera in the northernCascadia Basin (Kiyokawa and Yokoyama, 2009).

6.1.3. Influence of the Fraser River on sediment provenance patternsOn the basis of mineralogical similarities between sands from the

Fraser River and IODP Site 1301, the Fraser River has been proposed asthe main source of recent sediments in the northern part of CascadiaBasin (Kiyokawa and Yokoyama, 2009). Present-day Fraser watershedextends farther inland and the river and its tributaries drain the entirespectrum of the Canadian Cordillera belts (Fig. 1). A downstream de-crease of 87Sr/86Sr has been recognized in the Fraser River water(Cameron and Hattori, 1997; Voss et al., 2014). Suspended matterfrom the Fraser River (collected at different sites along the river in July1993) and its tributaries has variable Sr and Nd isotopic compositions(Fig. 3 inset) that define a remarkable hyperbola interpreted as a binarymixing curve by Cameron and Hattori (1997). As the Fraser River flowsdownstream, the high 87Sr/86Sr suspended matter (inherited from the“enriched” eastern catchments) mixes with low 87Sr/86Sr suspendedmatter derived from the depleted western belts of the Cordillera(Cameron and Hattori, 1997). Sediment from ODP Sites 888 and 1027plot on the same mixing array as the Fraser River suspended matter(Fig. 3 inset), but only towards the depleted end. This is consistentwith an overall, albeit variable, predominance of material derivedfrom the proximal depleted western terranes.

A geomorphologic study of the Fraser River basin reveals that theriver flowed southward only for the last 760 kyr, while previously itdrained northward (Andrews et al., 2012). Since sedimentation atODP Site 1027 started about 3.5Myr ago (Davis et al., 1997), the isotopiccompositions of the sediments in this site can be used to evaluatewhether the source changed significantly 760 kyr ago. Not only dothey not show any clear change through time (Fig. 2), but also thetight correlations between various isotopic compositions and betweenisotopic compositions and trace element ratios (Figs. 3–6) suggest theinvolvement of two dominant components with nearly constant

-50

-40

-30

-20

-10

0

10

20

30

-30 -25 -20 -15 -10

ODP Site 888

ODP Site 1027

Sands

Oceanic sedimen

sand-rich sediments

ε Hf

clay-rich sediments

This work Published data

Enrichedend-member

Fine

Coarse

Fig. 8.Nd and Hf isotopic compositions of ODP Site 888 and 1027 sediments. εHf and εNd valuesarray corresponds to εHf = 1.55 ∗ εNd+ 1.21 as defined by Vervoort et al. (2011). Regression linculated using the followingmethod: slope a is defined by a= σεHf / σεNdwhere σ is the standab =(εHf)average− a ∗ (εNd)average. The equation of the regression line on clay-rich sediment is εHit is εHf = 1.90 ∗ εNd+ 1.74 (r= 0.96). Ranges of potential Hf isotopic composition of the depleLiterature data: Astoria fan: Prytulak et al. (2006), Aleutians and Alaska forearc: Vervoort et al. (Flierdt et al. (2007), Carpentier et al. (2008, 2009) Chauvel et al. (2009) and Bayon et al. (2009

compositions through time. The 760 ka transition at ODP Site 1027marks the end of a “quiet” period that lasted about 460 kyr, and largerisotopic variability characterizes sediments deposited after 760 ka(Fig. 2). Nevertheless, this increased variability does not require amarked change in the source provenance. Even if the Fraser River ac-quired its present-day southward flow only 760 kyr ago (Andrewset al., 2012), the isotopic data of ODP Site 1027 sediments show that de-tritus coming from the eastern part of the Cordillera has beentransported to this part of the Cascadia Basin for at least 3.5 Myr.

6.1.4. Lack of evidence for glacial-interglacial changes in sediment supply oroutburst floods

Several glaciations occurred in the Canadian Cordillera duringthe last 3.5 Myr (Jennings et al., 2007), and the Cordilleran Ice Sheet ex-tended up to Vancouver Island during the last glacial maximum(~15,000 years BP, Clague and James, 2002). One might expect thatthe difference in glacier volume associated with the different climateconditions would influence the composition of sediments eroded fromthe continent and be recorded in the temporal sequences of ODP Sites888 and 1027. The precise locations of glacial and interglacial intervalsalong the two drill cores are poorly known but Knudson and Hendy(2009) recognized two pairs of glacial/interglacial intervals in the top250mof ODP Site 888 section (shown in Fig. 2). No clear and systematicisotopic differences exist between sediments deposited during glacial orinterglacial intervals (Fig. 2), suggesting that the relative proportion oferoded material coming from the western and eastern terranes wasnot drasticallymodified by the climate conditions. Similarly, the relativeuniformity of isotopic ratios in sedimentary section of ODP Site 1027suggests that climate change had no major influence on the composi-tion of material eroded over the last 3.5 Myr.

During the same time period, further south, periodic outburstflooding occurred at the mouth of the Columbia River: collapse of icedams released enormous amounts of water and sediments that was de-posited at various distances from the Columbia River mouth (Brunner

-5 0 5 10 15 20

ts

Terrestrial array

εNd

Depletedend-member

Fine

Coarse

Astoria fanAleutiansAlaska

-30

-20

-10

0

10

20

-20 -15 -10 -5 0 5 10

Regional variations

Published data

ΔεHf>0

ΔεHf<0

Terrestrial array

were calculated using the CHUR composition given by Bouvier et al. (2008). The terrestriales through sand-rich (SiO2/Al2O3 N 4) and clay-rich (SiO2/Al2O3 b 4) sediments were cal-

rd deviation of εHf and εNd calculated for a given group of samples, intercept b is defined byf= 1.17 ∗ εNd+ 4.98 (Pearson correlation coefficient r= 0.96), and on sand-rich sedimentted and enriched end-members (with Nd isotopic compositions given in Fig. 7) are shown.2011), other oceanic sediments: Vervoort et al. (1999, 2011), Vlastélic et al. (2005), van der).

-6

-4

-2

0

2

4

6

8

10

2 3 4 5 6 7 8

-6

-4

-2

0

2

4

6

8

10

0.00 0.02 0.04 0.06 0.08 0.10 0.12

b

ΔεH

fΔε

Hf

SiO2/Al2O3

Nb/Zr

Fine-grained sediments

Coarse-grained, quartz &

zircon-rich sediments


a

ODP Site 888ODP Site 1027Sands

Coarse-grained, quartz &

zircon-rich sediments

Fig. 9. a) Vertical deviation of εHf from the Terrestrial array (ΔεHf) vs SiO2/Al2O3; b)ΔεHf vsNb/Zr for ODP Site 888 and 1027 sediments. SiO2, Al2O3, Nb and Zr concentrations aretaken from Carpentier et al. (2013).


et al., 1999; Normak and Reid, 2003 and references therein). Accordingto Prytulak et al. (2006), these periodic outburst floods were a key pro-cess in building the Astoria fan, at the mouth of the Columbia River, forthe last 2.5 Myr. The Astoria fan is located about 350 km south of thetwo studied sites and has been drilled at Deep Sea Drilling Project(DSDP) Site 174 (Fig. 1). The compositions of DSDP Site 174 sediments(Prytulak et al., 2006) lie along the same general arrays as ODPSite 888 and 1027 sediments but are displaced towards the enrichedend-member, with generally higher 206Pb/204Pb, La/Yb and lower143Nd/144Nd (Figs. 4–6). In Pb–Pb isotopic space (Fig. 4), DSDP Site174 sediments define a tight linear array that lies distinctly above ODPSite 888 and 1027 field, with higher 208Pb/204Pb at a given 206Pb/204Pb.The Nd–Pb isotope relationship also reveals subtle differences betweenODP Site 888–1027 and DSDP Site 174 sediments (Fig. 5). If both groupsof samples define linear arrays, the depleted component that contrib-utes to DSDP Site 174 sediments must have lower 206Pb/204Pb at agiven 143Nd/144Nd than ODP Site 888–1027 depleted component(Fig. 5). Prytulak et al. (2006) identified two main sources for theAstoria fan sediments: Columbia River basalts and Proterozoic sedimen-tary rocks of the Belt Supergroup located further inland from theWash-ington–Oregon coast (Fig. 1). While some similarity between theenriched components sampled by ODP Site 888–1027 and by DSDPSite 174 sediments is likely on the basis of geological and isotopic con-straints, the depleted components are clearly different (Fig. 5).

Slight but significant differences in the isotopic compositions ofDSDP Site 174 and ODP Site 888–1027 sediments seem to demonstratethat deposits linked to the catastrophic outburst floods of the Columbiariver did not exist further north and that their influence on the sedimen-tary budget in the areas of ODP Sites 1027 and 888 was inexistent. Theexamination of submarine geomorphology of Cascadia Basin and sedi-mentology at ODP Site 888 leads Knudson and Hendy (2009) to reacha similar conclusion.

6.2. Decoupling of isotopic systems due to sedimentary sorting

6.2.1. The established Nd–Hf decouplingIn Hf–Nd isotopic space (Fig. 8), sediments from ODP Sites 888 and

1027 are compared to otherworldwide oceanic sediments, andmore lo-cally to sediments from the Astoria fan (Prytulak et al., 2006) and theAleutians and Alaska forearc (Vervoort et al., 2011). Astoria fan sedi-ments have less radiogenic Hf and Nd isotope ratios than ODP Site 888and 1027 sediments, consistent with higher proportion of old materialin their source (see previous section). In contrast, Alaska and Aleutiansforearc sediment has compositions similar to those of themost depletedODP Site 888 and 1027 sediments (Fig. 8).

Sediments from ODP Sites 888 and 1027 lie along a relatively well-defined linear array in Hf–Nd isotopic space (Fig. 8), and first order var-iations along this trend reflect mixing of material deriving fromenriched and depleted sources. However, significant vertical scatter ex-ists (Fig. 8). Quartz-rich samples (defined by SiO2/Al2O3 N 4), referred assands in Fig. 8, plot close to or slightly below the Terrestrial array ofVervoort et al. (2011). Fine-grained, Al-rich samples define an arraywith a much shallower slope, above the Terrestrial array (Fig. 8). Thenegative correlation between the vertical deviation of εHf from the Ter-restrial array (ΔεHf, Fig. 8) and SiO2/Al2O3 (Fig. 9a) strongly suggeststhat the decoupling of Nd and Hf isotopes is due to mineral sorting.Sandy samples with high SiO2/Al2O3 also have the lowest Nb/Zr ratios,due to higher proportions of zircon in the mineral assemblage(Carpentier et al., 2013). This “zircon effect” (Patchett et al., 1984) alsodecouples Nd and Hf in the sedimentary system (Vervoort et al., 1999;van der Flierdt et al., 2007; Chauvel et al., 2008, 2009; Bayon et al.,2009; Carpentier et al., 2009; Chauvel et al., 2009; Vervoort et al.,2011; Garçon et al., 2013) and the positive correlation between Nb/Zrand ΔεHf (Fig. 9b) demonstrates that variations of εHf at constant εNdin ODP Site 888 and 1027 sediments reflect variable amounts of zirconwith relatively unradiogenic Hf. The observed bias between clay-rich

and sand-rich samples is explained by a higher amount of zircons insands as observed in macroscopic samples by Westbrook et al. (1994),Chamov and Murdmaa (1995) and Davis et al. (1997).

Average ΔεHf varies between the different lithological units drilledat ODP Sites 888 and 1027, and in both sites, units with the highestSiO2/Al2O3 have the lowest ΔεHf (Table 2). For instance, at ODP Site1027 the upper Unit 1 has the lowest average ΔεHf that might be attrib-uted to coarser detrital input as the Juan de Fuca Plate approaches thecontinent. This is in agreement with the much higher average sand /(clay + silt) ratio of Unit 1 (~0.9, after smear slide analyses given inDavis et al., 1997) compared to the average ratio of the entire ODPSite 1027 section (~0.2). In addition, although bulk sediment fromboth ODP Sites 888 and 1027 has a slightly positive vertical deviationfrom the Terrestrial array, ΔεHf is lower at ODP Site 888 (+2.9) than atODP Site 1027 (+3.8, Table 2). The lowerΔεHf of bulk ODP Site 888 sed-iment is associated with a high bulk sand / (clay + silt) ratio of 1.9(Westbrook et al., 1994). This is most probably due to higher proportionof zircon-rich sands in this site, which is closer to the continent (Fig. 1).

Thus, ΔεHf can be used as a proxy to evaluate how far the sedimentshave been transported from the coastline to the ocean floor in a givenarea. The decoupling of Nd and Hf isotope systematics is enhancedwhen the source material is old enough to create large differences inthe 176Hf/177Hf ratio between the fine clay-rich and the sandy zircon-rich fractions. As a consequence, variations of ΔεHf along a margin as afunction of the distance from the coastline are expected to bemore pro-nounced when the source material is old. For example, the sedimentsdeposited in front of the Lesser Antilles arc come from an old craton


(White et al., 1985; Carpentier et al., 2008; 2009) and large differencesexist between sediments deposited 480 km from the South Americancontinent at Barbados Island and those deposited 860 km from thecoast and drilled at DSDP Site 543. While Barbados sediments have aΔεHf of −1.7, the DSDP Site 543 sediments have a ΔεHf of +10.9; the12.6 units increase in ΔεHf corresponds to a distance of only 380 km be-tween the two sites (Carpentier et al., 2009). The ΔεHf increase of 0.9unit between ODP Sites 888 and 1027, offshore from the Cascades,over a distance of about 100 km is comparatively low because thesources are younger and the various mineral fractions did not havemuch time to develop in-growth isotopic differences.

Depleted end-memberFine

0.2814

0.2816

0.2818

0.2820

0.2822

0.2824

0.2826

0.2828

0.2830

0.2832

0.703 0.704 0.705 0.706 0.707

0.5114

0.5116

0.5118

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0.5122

0.5124

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0.5130

0.703 0.704 0.705 0.706 0.707

ODP Site 888

ODP Site 1027

Sands

Δ

176 H

f/177

Hf

143 N

d/14

4 Nd

87S

sand-rich se

Δ87Sr/86Sr<0

Coarse

Fig. 10. a) 176Hf/177Hf vs. 87Sr/86Sr for ODP Sites 888 and 1027 sediments. Regression lines thrsediment regression line is 176Hf/177Hf = −0.060808 ∗ 87Sr/86Sr + 0.325877 (r = −0.77),−0.87); b) 143Nd/144Nd vs. 87Sr/86Sr for Sites 888 and 1027 sediments. Equation of clay-ric−0.84), and for sand-rich sediment it is 143Nd/144Nd = −0.156321 ∗ 87Sr/86Sr + 0.622793 (rto the reference line and is defined by Δ87Sr/86Sr = (87Sr/86Srsample − 87Sr/86Srextrapolated) ∗ 10the reference line. Ranges of potential Sr isotopic composition of the depleted and enriched143Nd/144Nd, 176Hf/177Hf isotope ratios of Sites 888 and 1027 sediments presented in a 3-D plocarbonate-rich samples from depth 523.4, 602.9, and 606.1 m are not shown since their Sr isot

6.2.2. Sr–Nd and Sr–Hf are also decoupled in terrigenous sedimentsThe systematic difference between silica-rich and alumina-rich sed-

iments observed in the Nd–Hf isotopic space (Fig. 8) also exists in theSr–Hf isotopic space (Fig. 10a), where the studied sediments plotalong two different arrays. Al-rich sediments define a shallowerarray than that defined by sands. Sands are characterized by a lower176Hf/177Hf at a given 87Sr/86Sr than Al-rich samples (Fig. 10a). In addi-tion, the relationship between Nd and Sr isotopic compositions alsoreveals differences between the two types of lithology (Fig. 10b):clay-dominated sediments have systematically higher 87Sr/86Sr at agiven 143Nd/144Nd than the sandy samples. Fig. 10c and d offers two

Enrichedend-member

FineCoarse

0.708 0.709 0.710 0.711 0.712 0.713

0.708 0.709 0.710 0.711 0.712 0.713

87Sr/86Sr>0

r/86Sr

a

sand-rich sediments

b

clay-rich sediments

clay-rich sediments

diments

ough sand-rich and clay-rich sediments were calculated as in Fig. 8. Equation of clay-richand for sand-rich sediment it is 176Hf/177Hf = −0.165863 ∗ 87Sr/86Sr + 0.399727 (r =h sediment regression line is 143Nd/144Nd = −0.093260 ∗ 87Sr/86Sr + 0.578501 (r == −0.92). The latter one is taken as reference line. Δ87Sr/86Sr is the horizontal deviation,000. 87Sr/86Srextrapolated is calculated using 143Nd/144Nd of the sample and the equation ofend-members (with Nd isotopic compositions given in Fig. 7) are shown; c) 87Sr/86Sr,t; d) same as Fig. 9c but different perspective views are shown. The three ODP Site 1027opic compositions are influenced by seawater Sr.

0.700

0.705

0.710

0.715

0.5118 0.5120 0.5122 0.5124 0.5126 0.5128 0.5130

0.2820

0.2822

0.2824

0.2826

0.2828

0.2830

0.2832

0.7040.706

0.7080.7100

0.7120.714

0.5120

0.5125

0.51300.2820

0.2822

0.2824

0.2826

0.2828

0.2830

0.2832

176 H

f/177

Hf

c

d

176 H

f/177

Hf

143Nd/144Nd

143Nd/ 144Nd87Sr/86Sr

87S

r/86

Sr

Fig. 10 (continued).


different perspective views of the 3-D relationship between 87Sr/86Sr,143Nd/144Nd and 176Hf/177Hf isotope ratios. On the first plot (Fig. 10c),all samples form an elongated array with only minimum scatter, sug-gesting that the data plot along a plane rather than an ellipsoid. The sec-ond plot (Fig. 10d) highlights this planar dispersion and furtherillustrates that sands and clay-rich samples form two fields that mostlyhave distinct Sr and Hf isotopic compositions.

Because sediments fromODP Sites 888 and 1027 aremixtures ofma-terials derived from different continental areas, most of the Sr isotopicvariability is due to the relative proportion of enriched versus depletedmaterial. However, variations of 87Sr/86Sr ratios at constant 143Nd/144Ndratio might be due to mineral sorting processes, as it is the case for theHf–Nd isotope decoupling. We calculated regression lines through thesands and the clay-dominated samples in the Nd–Sr isotopic space(see Fig. 10b). Because no equivalent of the Nd–Hf Terrestrial array ex-ists in Nd–Sr isotopic space, we used the sand regression line as a refer-ence line, and calculated a horizontal deviation (Δ87Sr/86Sr) for eachsample (see Fig. 10b caption for further explanation). A good positive

correlation exists between Rb/Sr and Δ87Sr/86Sr (Fig. 11a). The system-atic low Δ87Sr/86Sr and low Rb/Sr of silica-rich samples compared toalumina-rich sediments (Fig. 11a) suggest that variations of 87Sr/86Srat constant 143Nd/144Nd are probably linked to mineral sorting process-es. Recent studies in river environment have pointed out systematic dif-ferences in Sr isotope ratios between bedloads and suspended loads,with the latter having significantly more radiogenic 87Sr/86Sr than theformer (Bouchez et al., 2011; Garçon et al., 2014). Using chemical andisotopic data on separated mineral fractions, Garçon et al. (2014) inter-pret the Sr isotopic bias between bedloads and suspended loads (for un-changed 143Nd/144Nd) as resulting from the higher relative proportionof Sr-unradiogenic minerals (e.g. epidote) in bedloads and radiogenicminerals (e.g. micas) in suspended loads.

The relationship between SiO2/Al2O3 and Δ87Sr/86Sr is shown inFig. 11b. At low SiO2/Al2O3 (b4) a large vertical scatter exists, whilesands with high SiO2/Al2O3 and low Δ87Sr/86Sr plot at the bottomright corner of the diagram (Fig. 11b). Quartz, plagioclase, epidote, am-phibole and pyroxene are the main mineral species in recovered sands,

Increasing 87Sr/86Sr and mica & clay proportion

Quartz addition

Decreasing 87Sr/86Sr and increasing epidote & plag. proportion

-20

-10

0

10

20

30

40

50

2 3 4 5 6 7 8

Δ87S

r/86

Sr

ΔεHf

-20

-10

0

10

20

30

40

50

-6 -4 -2 0 2 4 6 8 10

-20

-10

0

10

20

30

40

50

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

Rb/Sr

SiO2/Al2O3

ODP Site 888ODP Site 1027Sands

Δ87S

r/86

Sr

Δ87S

r/86

Sr

a

b

c


Bulk Site888

Bulk Site 1027

coarse-grained

sediments

Coarse-grained

sediments


Fine-grained

sediments

Coarse-grained

sediments

Fig. 11. a) Δ87Sr/86Sr vs. Rb/Sr; b) Δ87Sr/86Sr vs. SiO2/Al2O3; c) Δ87Sr/86Sr vs. ΔεHf for ODPSite 888 and 1027 sediments. SiO2, Al2O3, Rb and Sr concentrations are taken fromCarpentier et al. (2013). Bulk compositions of each site are given in Table 2. As forFig. 10a–d, the three ODP Site 1027 carbonate-rich samples are not shown in panels a–c.There are two outliers in panel a: one sample from Site 1027 (367.4m depth) plots clearlyabove the main array, while the uppermost sample from Site 1027 (18.5 m depth) plotsbelow the main array.


as shown by the petrographic studies (Chamov and Murdmaa, 1995;Underwood and Hoke, 2000; Kiyokawa and Yokoyama, 2009). Incontrast, fine-grained samples are dominated by clay and mica(Underwood and Hoke, 2000). Among theminerals present in our sam-ples, mica, clay, epidote and plagioclase are themain contributors to theSr budget in sediments (following Garçon et al., 2014). Mica and clayusually have very radiogenic signatures, owing to their time-integrated high Rb/Sr while epidote and plagioclase are characterized

by comparatively low Sr isotope ratios (Garçon et al., 2014). Thus, vari-ations in the relative proportion of radiogenic minerals (mica and clay)and unradiogenic minerals (epidote–plagioclase) should be able to cre-ate a large spread of theΔ87Sr/86Sr in the resulting sediments (Fig. 11b).Preferential concentration of plagioclase and epidote in sands, togetherwith quartz generates a systematic association of low Δ87Sr/86Sr withhigh SiO2/Al2O3, even if quartz does not contribute to the Sr budget(Fig. 10b).

A scattered positive correlation is also observed between ΔεHf andΔ87Sr/86Sr (Fig. 11c), indicating that sediments that concentrate thehigh time-integrated Lu/Hf minerals of a source rock also concentrateits high time-integrated Rb/Sr minerals. For a given 143Nd/144Nd,alumina-rich fine-grained detritus have more radiogenic 176Hf/177Hfand 87Sr/86Sr than the coarse-grained, Si-rich fraction. Consequently,bulk ODP Site 1027 has higher Δ87Sr/86Sr (together with higher ΔεHf)than bulk ODP Site 888 (see Table 2 and Fig. 11b and c). This is due tothe fact that ODP Site 1027 is located farther from continental sourcesthan ODP Site 888 and received higher proportion of fine detritus char-acterized by high ΔεHf and Δ87Sr/86Sr. In summary, significant differ-ences in Sr and Hf isotopic signatures, and subsequent decouplingfrom Nd isotopes, occur along a relatively short distance since only~100 km currently separates ODP Site 888 from ODP Site 1027(Fig. 1). Sampling farther offshore, at larger distances from the coastlinewould probably reveal even larger differences between the Sr and Hfisotopic compositions of fine detritus and source material.

To conclude,mineralogical sortingprocesses during transport ofma-terial from continental shelf to deep oceans induces fractionation in iso-topic signatures. Sediments from ODP Sites 888 and 1027 haveundergone such fractionation even if the present-day distance betweenthe two sites and the coastline does not exceed 220 km. Far-traveled de-trituswill probably have Sr andHf isotopic compositions distinctlymoreradiogenic than that of their crustal sources. The compositional bias be-tween fine continental material reaching abyssal plains and its crustalprecursor should be particularly pronounced when the latter is old.

7. Conclusion

Sediments drilled at ODP Sites 888 and 1027 consist mainly of detritalmaterial derived from the Canadian Cordillera. The isotopic compositionof recovered hemipelagic muds and turbidites suggests not only the in-volvement of the same two dominant end-members at both sites, butalso that they have not changed over the last 3.5 Myr. Mixing of the ero-sion products of the depleted, western part of the Canadian Cordillerawith enriched detritus from the easternmost part of the Cordillera ac-counts for the range of isotopic compositions of sediment from ODPSites 888 and 1027. The proximal depleted component dominates,while the eastern enriched source contributes to 10–28% of the accumu-lated sedimentary material. Sources have not significantly changed overthe last 3.5Myr andwe see no effect of the alternation of glacial and inter-glacial periods on the erosion products of the Canadian Cordillera.

At a finer scale, we demonstrate that mineral sorting processes notonly decouple Hf and Nd isotopic signatures but also affect the Sr isoto-pic systematics in sedimentary materials. While fine sediments aremainly composed of clay and mica with high time-integrated Lu/Hfand Rb/Sr, coarse-grained sediments concentrate low time-integratedLu/Hf and Rb/Sr minerals such as zircon and plagioclase–epidote, re-spectively. Because continental sources of ODP Site 888 and 1027 sedi-ments are relatively young, differences in 87Sr/86Sr and 176Hf/177Hfbetween fine clay-rich and sandy fractions are small, albeit significant.Evenwith only 100 kmbetween the two sampling sites,mineral sortinghas produced different Sr andHf isotopic signatures that are reflected inthe bulk composition of ODP Sites 888 and 1027. Our study documentshow isotope fractionation occurs along a 200 km-long transect across acontinental margin. Such fractionation should be enhanced furtheraway towards the deeper ocean, and also if the crustal sources of detritalmaterial are older.


Acknowledgments

ODP Site 888 and 1027 samples were provided by the IntegratedOcean Drilling Program's (IODP) Gulf Coast Repository. Jane Barlingand Bruno Kieffer are thanked for their help in the lab and analytical as-sistance during the MC-ICP-MS and TIMS measurements. FrançoisChauvet, Baptiste Dazas and Emily Mullen are thanked for their contri-bution to the drawing of Fig. 1a and Fig. 10c–d. We thanked the editoras well as the two anonymous reviewers for their thorough and veryconstructive reviews that allowed for a much-improved manuscript.M. Carpentier acknowledges the partial financial support from theFrench Government Laboratory of Excellence initiative n°ANR-10-LABX-0006 while finishing the manuscript. NSERC Discovery Grant toD. Weis provided the funding that contributed to this research.

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