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Universidade de São Paulo
2014-08-23
Enhanced primary productivity and
magnetotactic bacterial production
in response to middle Eocene warming in the
Neo-Tethys Ocean Palaeogeography, Palaeoclimatology,
Palaeoecology, v. 414, p. 31-45,
2014http://www.producao.usp.br/handle/BDPI/48818
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Enhanced primary productivity and magnetotactic bacterial
productionin response to middle Eocene warming in the Neo-Tethys
Ocean
Jairo F. Savian a,⁎, Luigi Jovane b, Fabrizio Frontalini c,
Ricardo I.F. Trindade d, Rodolfo Coccioni c,Steven M. Bohaty e,
Paul A. Wilson e, Fabio Florindo f, Andrew P. Roberts g,Rita
Catanzariti h, Francesco Iacoviello b
a Departamento de Geologia, Instituto de Geociências,
Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves,
9500, 91501-970 Porto Alegre, Brazilb Departamento de Oceanografia
Física, Instituto Oceanográfico, Universidade de São Paulo, Praça
do Oceanográfico, 191, 05508-120 São Paulo, Brazilc Dipartimento di
Scienze della Terra, della Vita e dell'Ambiente, Università degli
Studi di Urbino “Carlo Bo”, Campus Scientifico, Località
Crocicchia, 61029 Urbino, Italyd Departamento de Geofísica,
Instituto de Astronomia, Geofísica e Ciências Atmosféricas,
Universidade de São Paulo, Rua do Matão, 1226, 05508-090 São Paulo,
Brazile Ocean and Earth Science, University of Southampton,
National Oceanography Centre, Southampton SO14 3ZH, UKf Istituto
Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605,
00143 Rome, Italyg Research School of Earth Sciences, The
Australian National University, Canberra ACT 0200, Australiah
Istituto di Geoscienze e Georisorse CNR, 56124 Pisa, Italy
a b s t r a c ta r t i c l e i n f o
Article history:Received 23 August 2013Received in revised form
5 August 2014Accepted 9 August 2014Available online 23 August
2014
Keywords:PaleoproductivityMECOPutative magnetofossilsMonte
CagneroItaly
Earth's climate experienced awarming event known as theMiddle
Eocene Climatic Optimum (MECO) at ~40Ma,which was an abrupt
reversal of a long-term Eocene cooling trend. This event is
characterized in the deepSouthern, Atlantic, Pacific and
IndianOceans by a distinct negative δ18O excursion over 500 kyr.We
report resultsof high-resolution paleontological, geochemical, and
rock magnetic investigations of the Neo-Tethyan MonteCagnero (MCA)
section (northeastern Apennines, Italy), which can be correlated on
the basis of magneto- andbiostratigraphic results to the MECO event
recorded in deep-sea sections. In the MCA section, an interval
witha relative increase in eutrophic nannofossil taxa (and
decreased abundances of oligotrophic taxa) spans theculmination of
the MECO warming and its aftermath and coincides with a positive
carbon isotope excursion,and a peak in magnetite and
hematite/goethite concentration. The magnetite peak reflects the
appearance ofputative magnetofossils, while the hematite/goethite
apex is attributed to an enhanced detrital mineralcontribution,
likely as aeolian dust transported from the continent adjacent to
the Neo-Tethys Ocean during adrier, more seasonal climate during
the peak MECO warming. Based on our new geochemical,
paleontologicaland magnetic records, the MECO warming peak and its
immediate aftermath are interpreted as a period ofhigh primary
productivity. Sea-surface iron fertilization is inferred to have
stimulated high phytoplanktonproductivity, increasing organic
carbon export to the seafloor and promoting enhanced
biomineralization ofmagnetotactic bacteria, which are preserved as
putative magnetofossils during the warmest periods of theMECO event
in the MCA section. Together with previous studies, our work
reinforces the connection betweenhyperthermal climatic events and
the occurrence (or increased abundance) of putative magnetofossils
in thesedimentary record.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
The early part of the Cenozoic Era was characterized by
greenhouseconditions through the early Eocene, followed by a ~17
Myr-longcooling trend (e.g. Zachos et al., 2008). This long-term
cooling trendwas interrupted by the Middle Eocene Climatic Optimum
(MECO) — awarming event that peaked at ~40 Ma (base of Chron
C18n.2n) (e.g.Bohaty and Zachos, 2003; Jovane et al., 2007; Bohaty
et al., 2009). TheMECO event has been recognized from multiple
sites in the Southern,
Atlantic, Pacific, Indian, and Tethyan Oceans (Fig. 1a). It was
first identi-fied in foraminiferal stable isotope records from the
Atlantic and Indiansectors of the Southern Ocean (Barrera and
Huber, 1993; Bohaty andZachos, 2003) and the hallmark of the event
is a distinct negative δ18Oexcursion that spanned 500 kyr (Bohaty
et al., 2009). The end of theevent wasmarked by a prominent
negative shift in benthic foraminifer-al δ13C of up to ~1.0‰ (e.g.
Bohaty et al., 2009; Edgar et al., 2010). Thelong-lasting δ18O
excursion, with a b100 kyr warming peak (MECOwarming), has been
interpreted to indicate a ~4–6 °C temperature in-crease of both
surface and intermediate deep waters (Bohaty et al.,2009; Edgar et
al., 2010). Organic molecular paleothermometry in thesouthwest
Pacific revealed absolute sea surface temperatures of 24 °C
Palaeogeography, Palaeoclimatology, Palaeoecology 414 (2014)
32–45
⁎ Corresponding author. Tel.: +55 51 33086364; fax: +55 51
33086337.E-mail addresses: [email protected],
[email protected] (J.F. Savian).
http://dx.doi.org/10.1016/j.palaeo.2014.08.0090031-0182/© 2014
Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology
j ourna l homepage: www.e lsev ie r .com/ locate /pa laeo
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to 26 °C just below the onset of MECO, and MECO peak
temperaturesexceeding 28 °C (Bijl et al., 2010). The temperature
increasecorresponded to a concomitant pCO2 increase by a factor of
2 to 3 (Bijlet al., 2010). An atmospheric pCO2 rise has also been
inferred at othersites by changes in deep ocean chemistry as
revealed by the net declinein carbonate accumulation that reflects
widespread calcite compensa-tion depth (CCD) shoaling (Bohaty et
al., 2009). The abrupt pCO2 in-crease during the MECO event has
been tentatively ascribed tomassive decarbonation during subduction
of TethyanOceanpelagic car-bonates under Asia as India drifted
northward (Bohaty and Zachos,2003; Bijl et al., 2010).
The Contessa Highway (CHW) section in Central Italy represents
thefirst outcrop section of marine sediments in which MECO was
documented (Fig. 1a, b; Jovane et al., 2007). More recently, the
MECOevent has also been recognized at the Alano di Piave section,
northeastItaly (Fig. 1a; Luciani et al., 2010). In the Alano
section the MECOevent is followed by deposition of two organic-rich
intervals (ORG1and ORG2; Spofforth et al., 2010), which are thought
to representrapid organic carbon burial contemporaneous with the
global pCO2drawdown during the post-MECO recovery interval.
Significantpaleoredox, foraminifera and calcareous nannofossil
assemblagechanges have been documented in the same section that
point to ashift towardmore eutrophic waters and a lowering of
oxygen availabilityat the end of MECO during deposition of
organic-rich beds (Luciani et al.,2010; Spofforth et al., 2010;
Toffanin et al., 2011). Deposition of organic-rich sediments after
the transient warming event has been associated
a
b c
Fig. 1. (a) Paleogeographic reconstruction at 40Ma and global
distribution of sites fromwhich theMECO event has been studied. The
sites include: Kerguelen Plateau (ODP Leg 119, Sites738, 744; ODP
Leg 120, Site 748),Weddell Sea (ODP Leg 113, Sites 689, 690),
Angola Basin (DSDP Leg73, Site 523), Islas Orcadas Rise (ODP
Leg114, Site 702), BlakeNose (ODP Leg 171, Site1051), ShatskyRise
(ODPLeg198, Site 1209), Equatorial Pacific (ODPLeg199, Sites 1218,
1219; IODPLegs 320–321, SitesU1331–U1333),MascarenePlateau
(ODPLeg115, Sites 709, 711),Walvis Ridge (ODP Leg 208, Site 1263),
Demerara Rise (ODP Leg 207, Sites 1258, 1260), East Tasmanian
Plateau (ODP Leg 189, Site 1172), Labrador Sea (ODP Leg 105, Site
647) and theonshore Monte Cagnero (MCA), Contessa Highway (CHW),
Alano, Kršteňany, and Seymour Island sections. The reconstruction
was made with web-based software available at
http://www.odsn.de/odsn/services/paleomap/paleomap.html. (b)
Location of the MCA and CHW sections. (c) Simplified geological map
of the MCA section (Lat. 43°38′50″N, Long. 12°28′05″E, 727 m above
sea level) with formation names and boundaries indicated (Coccioni
et al., 2013).
33J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
http://www.odsn.de/odsn/services/paleomap/paleomap.htmlhttp://www.odsn.de/odsn/services/paleomap/paleomap.html
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with enhanced delivery of terrestrial material that would have
bothincreased nutrient availability to the sea surface and
stimulated primaryproductivity (Spofforth et al., 2010).
Inmarine environments, changes in primary productivity are
directlyrelated to the distribution and availability of nutrients
and can be trackedby several proxies. One proxy is the relative
abundance of magnetofossilsin pelagic sediments (e.g. Hesse, 1994;
Tarduno, 1994; Tarduno andWilkison, 1996; Lean and McCave, 1998;
Yamazaki and Kawahata,1998; Kopp and Kirschvink, 2008; Yamazaki,
2008, 2009; Roberts et al.,2011; Chang et al., 2012; Larrasoaña et
al., 2012; Yamazaki, 2012).Magnetofossils are the inorganic remains
of magnetotactic bacteria,which intracellularly biomineralize
magnetically non-interacting singledomain crystals, composed of
magnetite, griegite, or both, which arearranged in chains within
the cell (Bazylinski and Frankel, 2003; Faivreand Schüler, 2008;
Kopp and Kirschvink, 2008; Moskowitz et al., 2008).These chains of
nanocrystals (40–300 nm) are used by themagnetotacticbacteria to
orient themselves relative to Earth's magnetic field(Blakemore,
1975) to find optimal living conditions in strongly chemi-cally
stratified aquatic environments (Bazylinski and Frankel,
2004).Magnetosomes can be preserved in sediments and in some cases
mayaccount for 20–60% of their bulk magnetization (Egli, 2004;
Housenand Moskowitz, 2006; Roberts et al., 2011). Although
magnetotacticbacteria are found in various marine environments
(e.g. Petersenet al., 1986; Vali et al., 1987; McNeill, 1990;
Petermann and Bleil,1993; Hesse, 1994; Housen and Moskowitz, 2006;
Jovane et al., 2012),detection of fossil magnetosomes can be
complicated by their mixturewith other magnetic minerals, or
withmagnetic particles with differentmagnetic domain structures.
Nevertheless, magnetofossils have beenreported from ancient pelagic
marine environments of different ages,such as the
Cretaceous–Paleogene boundary (e.g. Abrajevitch andKodama, 2009),
the Paleocene–Eocene boundary (e.g. Chang et al.,2012; Larrasoaña
et al., 2012), the Eocene–Oligocene (e.g. Robertset al., 2011,
2012; Yamazaki et al., 2013), the Oligocene–Miocene(Channell et
al., 2013; Florindo et al., 2013; Ohneiser et al., 2013), andthe
Pliocene (Yamazaki, 2009; Yamazaki and Ikehara, 2012).Magnetosome
abundance in sediments is strongly controlled by theavailability of
particulate iron and organic carbon flux to the seafloor(Roberts et
al., 2011), which can be related to climate, includinghyperthermal
conditions (e.g. Schumann et al., 2008; Chang et al.,2012;
Larrasoaña et al., 2012).
Here we present a high-resolution environmental
magnetic,micropaleontological and stable isotopic investigation of
a westernTethyan pelagic marine section at Monte Cagnero (MCA),
Italy, whichwas deposited toward the end of the middle Eocene.
Environmentalmagnetic data are used to detect magnetofossil
variations across theMECO event and just after the warming event.
These results, combinedwith paleoecological data from nannofossils
and foraminifera, are usedto assess paleoenvironmental scenarios
associated with the MECOevent in the Neo-Tethys realm.
2. Geological setting andmagneto- andbio-stratigraphic
framework
A continuous Paleogene sedimentary record is preserved in
theScaglia limestone, which includes pelagic limestones and
marlylimestones of the Umbria–Marche succession, central Italy. The
MCAsection is exposed on the southeastern slope of Monte Cagnero
(Lat.43°38′50″N, Long. 12°28′05″E, 727 m above sea level) near the
townof Urbania, northeastern Apennines, Italy (Fig. 1a, b). Because
of itscompleteness and stratigraphic continuity, the MCA section is
animportant pelagic sedimentary succession for studying Eocene
andOligocene climatic events (Coccioni et al., 2008; Hyland et al.,
2009;Jovane et al., 2013; Fig. 1c). Following Coccioni et al.
(2008), the metersystem in the MCA section was established with
meter level 100 asthe stratigraphic equivalent to meter level 0 of
the Global BoundaryStratotype Section and Point (GSSP) for the
Eocene/Oligocene boundaryatMassignano (Premoli Silva and Jenkins,
1993).We focus on the lower
part of the section, from 58 to 72meters stratigraphic level
(msl) (Fig. 2).This 14-m-thick interval of the MCA section belongs
to the ScagliaVariegata Formation and consists of bundles of
limestone-marl couplets.From foraminifera assemblages, a lower
bathyal (1000–2000 m) waterdepth was inferred for the depositional
environment of the MCA sectionduring the middle Eocene (Guerrera et
al., 1988; Parisi et al., 1988),which gradually shoaled to
mid-bathyal (800–1000 m) and upperbathyal (400–600 m) water depths
in the late Eocene and earlyOligocene, respectively.
High-resolution magneto- and bio-stratigraphic calibration of
theMCA section was carried out by Coccioni et al. (2008), Hyland et
al.(2009), and Jovane et al. (2013). The studied interval spans
from themiddle–upper part of Chron C18r to the lowermost part of
C18r.1rwith an average sedimentation rate of ~8.57 m/Myr following
thegeomagnetic polarity timescale (GPTS) of Cande and Kent
(1995)(confirmed by Ogg, 2012). The primary nannofossil and
foraminiferalbiostratigraphic events identified are (from bottom to
top) the:(l) lowest occurrence (LO) of Orbulinoides beckmanni at
63.2 msl;(2) highest occurrence (HO) of O. beckmanni at 65.5 msl;
and (3) HOof Chiasmolithus solitus at 70.0 msl (Jovane et al.,
2013). It is worthnoting, however, that recognition of the LO of O.
beckmanni can besubjective because this taxon originates from
Globigerinatheka euganea,and some transitional forms of problematic
taxonomic assignmentoccur from 62.8 to 63.2 msl. Additionally,
identification of the HO of O.beckmanni is hampered by its scarce
abundance, moderate preservationof planktic (P) and benthic (B)
foraminifera and some problematicspecimen identifications up to
66.60 msl. Nevertheless, the MCAbiostratigraphic record in the
58–72 msl interval is likely continuousand is interpreted to span
planktonic foraminiferal Zones P12 to P14of Berggren et al. (1995)
and Zones E11 to E13 of Wade et al. (2011),and calcareous
nannofossil Zones NP16 to NP17 of Martini (1971) andCP14a to CP14b
of Okada and Bukry (1980).
3. Materials and methods
Two-hundred-sixty-five bulk rock samples were collected at ~5
cmstratigraphic intervals from the MCA section, corresponding to
atemporal spacing of ~6 kyr between samples. High-resolution
calciumcarbonate, stable isotopic, rock magnetic and
micropaleontologicalanalyses were performed. Carbonate contents
were measured at theNational Oceanography Centre, Southampton
(NOCS). One-hundred-twenty-five bulk rock samples were reduced to
fine powder in anagate mortar and their CaCO3 content was obtained
using a Dietrich-Frühling calcimeter thatmeasures the CO2
volumeproduced by completedissolution of pre-weighed samples (300 ±
1 mg each) in 10% vol. HCl.Total carbonate contents (wt.% CaCO3)
were computed with a precisionof 1% taking into account pressure
and temperature of the laboratoryenvironment, the amount of bulk
sample used, and the volume of CO2in the calcimeter. Standards of
pure calcium carbonate (e.g. CarraraMarble) were measured every ten
samples to ensure proper calibration(Appendix A). For
coarse-fraction analyses, weighed freeze-driedsamples were soaked
in deionized distilled water, and were washedthrough 63 μm sieves.
The N63 μm fraction residue was collected, dried,and weighed. The
coarse fraction is defined as the weight percent ratioof the N63 μm
size fraction to the weight of the bulk sample (~100 g foreach
sample) (Broecker and Clark, 1999, 2001). This index has
beenlargely used as an indicator of carbonate dissolution (e.g.
Hancock andDickens, 2005; Colosimo et al., 2006; Leon-Rodriguez and
Dickens,2010; Luciani et al., 2010).
Semi-quantitative mineralogy of the bulk fraction was
determinedon 6 representative samples distributed along the section
(58.15,63.25, 63.50, 64.90, 65.10, and 69.05 msl) using powder
X-raydiffraction (PXRD) analysis. For bulk analyses, an aliquot of
about 1 gwas crushed and pulverized in a mortar. About 15 mg was
used forPXRD analysis made on an automated Olympus® BTX
diffractometer,using Co-K radiation, operated at 30 kV and 0.326
mA, over the range
34 J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
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58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
40 50 60 70 80 90 100
Str
atig
raph
ic th
ickn
ess
(msl
)
CaCO3
(%)
0 2 4 6 8 10 12
Coarse Fraction (%)
1.4 1.6 1.8 2 2.213C
(%o, VPDB)
-2 -1.5 -1
18O (%o, VPDB)
0 1 2 3 4 5 6 7
Susceptibility(x10 m3/kg)
0 2.4 4.8 7.2 9.6
IRM900 mT(x10 -5 Am2/kg)
0 0.2 0.4 0.6 0.8 1
S-ratio
(1) (2) (3) (4) (5) (6) (7)
40.1
3040
.145
P12
E11
P13
P14
E12
E13
NP
16C
P14
aN
P17
CP
14b
1
2
3
4
5
a
b
c
d
e
f
g
h
imarlcalcareous marl
lithology:
marly limestonelimestone
C18
rC
18n
-8
0 10 20 30 40
Susceptibility CFB (x10-8 m3/kg)
0 1 2 3 4 5
ARM (x10
-6 Am2/kg)
0 6 12 18 24 30
ARM CFB (x10-6 Am2/kg)
0 2 4 6 8
IRM900mT CFB (x10-5 Am2/kg)
0 0.5 1 1.5 2
HIRM (x10 -5 Am2/kg)
0 6 12 18(x10-6 Am2/kg)
HIRM CFBδ
δ
Fig. 2. Changes in CaCO3 content, coarse fraction, δ13C and δ18O
in bulk sediments, low-fieldmagnetic susceptibility (χ), and
rockmagnetic properties (anhysteretic remanentmagnetization, ARM;
isothermal remanentmagnetization, IRM at 900mT;hard isothermal
remanentmagnetization, HIRM; S-ratio300) across the 14-m-thick
studiedMCA section. χ, ARM, IRM900mT, andHIRM300mTwere also
calculated on a carbonate-free basis (CFB, dashed lines).
Themagnetobiostratigraphy is from Jovaneet al. (2013). Numerical
ages (1) are fromCande andKent (1995) (star) andOgg (2012)
(diamond). Biostratigraphy is based on the planktonic foraminiferal
Zones of (2) Berggren et al. (1995) and (3)Wade et al. (2011) and
calcareous nannoplanktonZones of (4) Martini (1971) and (5) Okada
and Bukry (1980). The areas shaded in yellow, dark yellow and green
highlight the intervals (2–4) that represent the MECO, MECO warming
peak and the post-MECO periods.
35J.F.Savian
etal./Palaeogeography,Palaeoclimatology,Palaeoecology
414(2014)
32–45
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5–55° of 2θ, with a step size of 0.05° and 100 exposures, for an
analysistime of 22min.Mineral identification and analysis was
carried out usingthe XPowder software (Version 2010.01.15 PRO),
which uses the PDF-2International Centre for Diffraction Data
(ICDD) database. In order tominimize subjective influences, the
baseline was determined automat-ically with XPowder defaults. X-ray
diffraction identification criteriawere based on the indications of
Biscaye (1965), Brown and Brindley(1980), and Moore and Reynolds
(1997). For quantitative analysis, theReference Intensity Ratio
(RIR) method, which is based upon scalingall diffraction data to
the diffraction of standard reference materials,was used (Chung,
1974). The Xpowder software automatically com-putes an amorphous
phase that is characterized by all the phases notconsidered in the
quantitative analysis. Minerals were identified bytheir basal
reflection at 3.85 and 3.03 Å (calcite), 4.26 and 3.34 Å(quartz),
15–16 Å (smectite), and 14.2, 7, and 4.72 Å (chlorite).
Oxygen and carbon isotope analyses were conducted using VGOptima
and VG Prism dual-inlet isotope ratio mass spectrometersat NOCS.
One-hundred-twenty-five samples were reacted in acommon acid bath
at 90 °C using an automated carbonatepreparation system with a
carousel device. NBS-19, Atlantis II, and anin-house Carrara Marble
standard were included in all sample runs.All values are reported
in standard delta notation (δ) in parts per mil(‰) relative to VPDB
(Vienna Pee Dee Belemnite), and analyticalprecision is estimated at
0.06‰ (1σ) for δ13C and 0.08‰ (1σ) for δ18O(Appendix A).
Paleomagnetic and environmental magnetic measurements
werecarried out at NOCS and at the University of São Paulo (USP).
Themagnetic remanence of 253 unoriented block samples
(5-cmresolution) was measured using a three-axis 2-G Enterprises
cryogenicmagnetometer (model 755R), housed in a magnetically
shielded roomat NOCS.
Low-field magnetic susceptibility (χ) was measured with
aKappabridge KLY-3 (AGICO) magnetic susceptibility meter. All
datawere normalized by mass, due to the irregular sample volumes.
Ananhysteretic remanent magnetization (ARM) was imparted in a100 mT
alternating field (AF) with a direct current bias field of0.05 mT.
An isothermal remanent magnetization (IRM) was impartedin a direct
field of 900 mT (IRM900mT) and was demagnetized in back-fields of
100 mT (−IRM100mT) and 300 mT (− IRM300mT). From thesemeasurements,
we calculated the S-ratio (S300mT = [− IRM300mT /IRM900mT]) and
“hard” isothermal remanent magnetization(HIRM300mT = [IRM900mT +
IRM300mT] / 2), in order to investigate thecoercivity of magnetic
minerals. Hysteresis loops and first-orderreversal curve (FORC)
diagrams were analyzed for eight samples fromthe MCA section
spanning the MECO interval and from intervalsimmediately before and
after thewarming event. FORCsweremeasuredwith an averaging time of
200 ms, a smoothing factor (SF) of 4, andusing the input parameters
of Egli et al. (2010) (Hc1 = 0 mT, Hc2 =110 mT; Hu1 = −15 mT, Hu2 =
+15 mT; δH= 0.63 mT). For sampleswith natural remanent
magnetization above 10−6 Am2/kg we per-formed only one FORC run
using a vibrating sample magnetometer(Princeton Measurements Corp.,
PMC) at NOCS, whereas for weaksamples we performed multiple runs
with a PMC alternating gradientmagnetometer to increase the
signal-noise ratio of the FORC distribu-tion (Heslop and Roberts,
2012). FORC diagrams derived from singleruns were calculated using
the MATLAB routine of Egli et al. (2010).FORC diagrams obtained
with multiple runs were stacked andcalculated using the MATLAB
routine of Heslop and Roberts (2012).Magnetic mineralogy was
further investigated at USP for selectedsamples through the
acquisition of an IRMandmeasurement of thermo-magnetic curves. IRM
acquisition curves were obtained for six sampleswith a 2-G
Enterprises pulsemagnetizer and a cryogenicmagnetometer(model
755UC). IRM acquisition curves were analyzed with
cumulativelog-Gaussian (CLG) functions using the software of
Kruiver et al. (2001).The CLG function is described by three
parameters (SIRM, B1/2 and dis-persion parameter, DP) that
characterize magnetic minerals
(Robertson and France, 1994; Kruiver et al., 2001).
Thermomagneticcurves up to 700 °C were obtained using a KLY-4S
AGICOmagnetic susceptibility meter with high-temperature
attachmentat USP to determine the Curie or Néel temperatures of
magneticminerals.
For calcareous nannofossil analyses, samples were prepared
fromunprocessedmaterial as simple smear slides using standard
preparationmethods (Bown and Young, 1998) at the Istituto di
Geoscienze eGeorisorse CNR di Pisa (Italy). Smear slides were
studied under a LeitzLaborlux 12 Pol light microscope both under
crossed nicols and trans-mitted light at a magnification of 1250×.
Most nannoplankton specieswere identified according to the taxonomy
of Perch − Nielsen (1985)except for sphenoliths and Dictyococcites
that were classified followingFornaciari et al. (2010) and
Reticulofenestra umbilicus that was definedfollowing the taxonomic
criteria adopted by Backman and Hermelin(1986), ascribing to this
species all specimens N14 μm and groupingas Reticulofenestra spp.
all specimens b14 μm. Assemblages were stud-ied following
quantitative countingmethods based on at least 300 spec-imens
(Appendix B). Following Toffanin et al. (2011), the
relativeabundances of species belonging to the genus
Sphenolithuswere deter-mined by counting 100 specimens. Rare taxa,
such as the genusHelicosphaera and species belonging to the genus
Chiasmolithus werecounted in a prefixed area of 10 mm2 (three–four
transects). The stan-dard calcareous nannofossil zonations of
Martini (1971) and Okadaand Bukry (1980) are widely used for low-
and middle-latitude Eo-cene–Oligocene (E–O) biostratigraphic
studies, and were used in thispaper. To infer probable temperature
and trophic variations of surfacewaters, most calcareous
nannofossils were, when possible, allocatedinto groups of
environmental affinities, largely following Haq andLohmann (1976),
Aubry (1992), Gardin and Monechi (1998),Bralower (2002a,b),
Tremolada and Bralower (2004), Persico and Villa(2004), Gibbs et
al. (2006), Villa et al. (2008), Raffi et al. (2009)and Agnini et
al. (2011). Based on this literature, the following environ-mental
groups were used: eutrophic taxa (Dictyococcites
bisectus,Dictyococcites scrippsae, Reticulofenestra daviesii), and
oligotrophictaxa (Cribrocentrum reticulatum, Ericsonia spp.,
Sphenolithus spp.,Zygrhablithus bijugatus) (Appendix B).
For foraminiferal analyses, samples were treated at
theUniversità degli Studi di Urbino, following the cold
acetolysetechnique of Lirer (2000), by sieving through a 63 μm mesh
anddrying at 50 °C. The cold acetolysis method enabled extractionof
generally easily identifiable foraminifera even from
induratedlimestones. This technique offered the possibility of
accurate tax-onomic determination and detailed foraminiferal
assemblageanalysis. For planktonic foraminifera, all samples were
studiedfor biostratigraphy and quantitative analysis was performed
ona subset of 101 samples. The residues were studied with a
binoc-ular stereomicroscope to characterize assemblages and to
identi-fy biostratigraphic marker species. A representative split
of atleast 300 specimens was picked from the N63 μm
fraction,mounted on micro-slides for permanent record and
identificationpurposes, and classified following the taxonomic
criteria ofBerggren and Pearson (2005). The planktonic
foraminiferal zona-tions of Berggren et al. (1995) and Wade et al.
(2011) werefollowed. Following Hancock and Dickens (2005), the
fragmenta-tion index (FI) was calculated using at least 300
specimens and in-cluding the whole test, fragments and dissolved
tests to estimatecarbonate dissolution effects (Appendix C). For
benthic foraminifera,a quantitative study of sixty-four selected
samples was performed. Arepresentative split of the N90 μm fraction
was used to pick approx-imately 300 specimens. The sample–split
weight used to pick ben-thic foraminifera was determined so that
the foraminiferal density(FD), expressed as the number of
foraminifera per gram of dry sedi-ment, could be calculated. The
planktonic to planktonic and benthic(P/P + B) ratio, expressed as a
percentage, and the percentages ofagglutinants were also calculated
(Appendices C and D).
36 J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
-
4. Results
4.1. Micropaleontology
Benthic foraminiferal assemblages are generally diverse
andwell-preserved throughout the studied MCA section, except
at63.2–64.0 msl where evidence of partial dissolution is
observed(Fig. 3). The assemblages are dominated by
calcareous-hyaline formswith variable percentages of agglutinants
that are more abundant atthe base of the section and at 63.2–64 msl
(Fig. 3). The P/(P + B) ratiofluctuates throughout the sequence,
with the lowest values at63.2–64 msl where both the highest FD
value and percentage fragmen-tation are documented (Fig. 3). The
63.2–65.5 msl interval is thereforecharacterized by pronounced
paleoecological and paleoenvironmentalchanges. The increase of
benthic to planktonic foraminifera and ofagglutinated forms that
are less prone to dissolution might reflectshallowing of the
lysocline. In addition, increased FD values mightsuggest greater
nutrient availability at the seafloor probably related toincreased
detrital mineral influx associated with intensified hydrologicaland
weathering cycles. In two intervals eutrophic taxa increase
inabundance relative to oligotrophic taxa: 63.5–65.5 msl and
67.0–70.2 msl(Appendix B). Eutrophic and oligotrophic percentages
vary throughoutthe section, but within the background level of
variation.
4.2. Stable isotopes
Bulk δ13C and δ18O values average 1.8‰ and −1.4‰,
respectively,through the studied MCA section (Figs. 2c, d). δ13C
data havebackground values of ~1.8‰ and are characterized by an
increasefrom ~1.6 to 2.1‰ at 63.2–65.5 msl (~40.2–40.1 Ma), with a
peakvalue of 2.1‰ at 63.35 msl (~40.1 Ma) (Fig. 2c). The peak value
isfollowed by a smoothly decreasing trend up to 66.34 msl (~39.8
Ma).Bulk δ18O varies from−0.2‰ to−3.3‰ and is noisy (Fig. 2d).
Similarlynoisy δ18O in the correlative CHW section was interpreted
as resultingfrom burial diagenesis and/or meteoric water diagenesis
(Jovane et al.,2007). It is well known that bulk carbonate δ13C is
more robust to
diagenetic alteration than δ18O because the carbon content of
fluidsis often too low compared to that of carbonate rocks to
modifysignificantly the carbonate carbon isotopic composition (e.g.
Veizerand Hoefs, 1976).
4.3. Environmental magnetism
Low-field magnetic susceptibility (χ) along the studied MCA
sectionvaries between 0.57 × 10−8 and 6.42 × 10−8 m3/kg (Fig. 2e),
whereasCaCO3 contents vary between 47% and 93% (Fig. 2a). Two
narrowintervals (63.30–63.90 msl and 64.15–65.30 msl) have lower
CaCO3contents and higher χ values (Fig. 2a, e). CaCO3 and χ have a
significantnegative correlation (r=−0.72), i.e. lower CaCO3 is
related to higher χvalues, which correspond to peak abundances of
paramagnetic (e.g.clays) and ferrimagnetic minerals. Six
representative samples fromdepths of 58.15, 63.25, 63.50, 64.90,
65.10, and 69.05 msl have similarXRD patterns, with slight
differences between samples. The mainphases present, in order of
abundance, are: calcite (Cal), quartz (Qtz),smectite (Sm) and
chlorite (Chl) (Fig. 4). Calcite contents vary betweena minimum of
61.3% (63.50msl; Fig. 4c) and 77.2% (58.15 msl; Fig. 4a);samples
from 58.15 to 69.05 msl (Fig. 4a, f) have relatively higheramounts
of calcite (77.2% and 71.8, respectively) compared to samplesfrom
63.25, 63.50, 64.90 and 65.10 msl (61.5%, 61.3%, 65.9% and
66.1%,respectively, Fig. 4b, c, d, e). These results are compatible
with thesmaller amounts of CaCO3 in the same interval (Fig. 2a).
Quartz valueshave an opposite trend, with higher values (4.6%) in
sample63.50 msl, and lower amounts (2.7%) in sample 58.15 msl.
Smectiteand chlorite have similar trends, with lower values (8.3%,
10.3% and5.7%, 7.9%, respectively) in samples 58.15 msl and 69.05
msl. Incontrast, higher percentages of smectite and chlorite are
found insamples 63.25, 63.50, 64.90 and 65.10 msl. The amorphous
phase alsohas a comparable trend to that of quartz, smectite and
chlorite, withhigher contents in samples from 63.25, 63.50, 64.90
and 65.10 msl.
To account for dilution effects by the carbonatematrix,we
calculatedmagnetic parameters (χ, ARM, IRM900mT and HIRM300mT) on
acarbonate-free basis (CFB, dashed lines in Fig. 2e–h). To achieve
this,
limestone
NP
16N
P17
CP
14b
CP
14a
P12
P14
E13
E11C
18r
C18
n
58
60
62
66
68
70
72
71
69
67
65
61
59
P13
E12Bar
toni
an
Lithology:calcareous marl
marly limestonemarl
(6)(4)(5)(2) (3)(1) (7) 0 10 20 30
Faunal Density(x100)
0 10 20 30 40
Agglutinants(%)
100 98 96 94 92
P/(P+B)(%)
64
63
10 20 40 60 80
Fragmentation Index(%)
1
2
3
4
5
Str
atig
raph
ic th
ickn
ess
(msl
)
40.1
3040
.145
Fig. 3. Changes in selected benthic and planktonic foraminiferal
parameters [P / (P + B) ratio], foraminiferal density (FD),
agglutinants, and fragmentation index across the studied
MCAsection. Magnetobiochronostratigraphy and shaded areas are in
Fig. 2.
37J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
-
we normalized the magnetic parameters by (100 wt.% CaCO3).
Allmagnetic parameters that depend on the concentration of
ferrimagneticminerals are well correlated before and after this
normalization(Fig. 2e–h). For example, the ARM and ARM CFB,
IRM900mT andIRM900mT CFB, and HIRM300mT and HIRM300mT CFB have
coherentpeaks at 63.30–63.90 msl and 64.15–65.30 msl, as mentioned
above,but also a less pronounced peak at 66.6–68.8 msl.
The relative concentration of different magnetic minerals across
thestudied interval can be inferred from the S-ratio300mT and
HIRM300mT(Bloemendal et al., 1992; Liu et al., 2007). The large
IRM300mT andS-ratio300mT fluctuations indicate variable proportions
of low andhigh-coercivity magnetic minerals, i.e. magnetite
(predominant whenS-ratio is near 1) and hematite/goethite (present
in significantconcentrations when HIRM is high and S-ratio is close
to 0). Thepresence of mixtures of magnetic minerals with
contrasting coercivitiesis corroborated by stepwise IRM acquisition
curves. IRM acquisitioncurves were measured at fields up to 1 T for
six representative samplesalong theMCA section (Fig. 5a; Table 1)
and components due to differentmagnetic minerals were fitted using
CLG functions (Fig. 5b–d) (Kruiver
et al., 2001; Heslop et al., 2002). Three magnetic components
wereidentified (Table 1). They correspond to a low-coercivity
component(B1/2 = 16 mT, DP = 0.40–0.45), a medium-coercivity
component(B1/2 = 50–70 mT, DP= 0.30–0.40) and a high-coercivity
component(B1/2 = 250–479 mT, DP = 0.24–0.30). Following Roberts et
al. (2011,2012), component 1 (lowest coercivity component in Table
1) isinterpreted as related to relatively coarse-grained magnetite
of detritalorigin. However, we cannot discount that component 1
represents thecoarse end of a distribution of fine-grained, largely
superparamagneticparticles, perhaps produced in situ by
dissimilatory iron-reducingbacteria (e.g. Egli, 2004). The
magnetofossil component is correlatedto themedium-coercivity
component 2 (Table 1) with coercivity valuesaround 40 mT and a
small DP (~0.3), similar to the IRM signal ofmagnetotactic bacteria
found in other studies (e.g. Egli, 2004; Chenet al., 2007; Jovane
et al., 2012; Roberts et al., 2012). Component 3 isinterpreted to
represent a high coercivity hematite fraction, but canalso include
some fraction of goethite. Inside the magnetic mineralconcentration
peaks, the medium-coercivity component usuallydominates the IRM
signal, with contributions varying between 31%
Sm ChlQtzCal AmorphousLEGEND
Rel
ativ
e In
tens
ity
Cal
Cal
CalCal
CalSm Chl Qtz CalChl Qtz
MCA-58.158.3
5.72.7
77.2
6.1
Rel
ativ
e In
tens
ity
Cal
Cal
Cal Cal CalSm Chl QtzCalChl
Qtz
MCA-63.2514.3
11.2
3.8
61.6
9.1
Rel
ativ
e In
tens
ity
Cal
Cal
CalCal
CalSm Chl Qtz
CalChl Qtz
MCA-63.5014.4
11.0
4.6
61.3
8.7
Rel
ativ
e In
tens
ityCal
Cal
CalCal Cal
SmChl Qtz CalChl Qtz
MCA-64.9014.1
8.5
3.7
65.9
7.8
0 10 20 30 40 50
Rel
ativ
e In
tens
ity
Cal
Cal
CalCal
CalSmChl
QtzCalChl Qtz
MCA-65.1012.9
9.4
3.7
66.1
7.9
Rel
ativ
e In
tens
ity
Cal
Cal
CalCal
CalSmChl Qtz CalChl Qtz
MCA-69.0510.3
7.9
3.2
71.8
6.8
a b
c d
e f
Degrees 2θ 0 10 20 30 40 50
Degrees 2θ
0 10 20 30 40 50
Degrees 2θ 0 10 20 30 40 50
Degrees 2θ
0 10 20 30 40 50
Degrees 2θ 0 10 20 30 40 50
Degrees 2θ
Fig. 4. Representative XRD diffractograms for six representative
samples (a) before theMECO, (b, c) during theMECOwarming peak, (d,
e) during the post-MECO period, and (f) after theMECO event. Pie
charts contain the semi-quantitative summaries for each sample.
38 J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
-
and 52%. The low-coercivity component varies from 5% to 17%. It
occursinside the magnetic concentration peaks but is absent (or
below thedetection level) outside these stratigraphic levels. In
these samples(MCA-67.55, MCA-68.60), the high-coercivity component
can reachup to 82% of the total IRM.
Hysteresis data (Fig. 6; Table 2), including the ratio of
saturationremanence to saturation magnetization (Mrs/Ms) and the
coercivity ofremanence to coercive force (Bcr/Bc), from MCA samples
lie within thepseudo-single domain (PSD) field of Day et al.
(1977). The presence ofhematite mixed with fine-grained magnetite
is indicated by thewasp-waisted shape of the loops (Roberts et al.,
1995). The mixture of
low- and high-coercivity phases does not affect significantly
the Dayplot because SD hematite has similar hysteresis ratios to
those of SDmagnetite (Roberts et al., 1995). The significant
departure of bulkhysteresis parameters from values expected for
uniaxial SD magnetitefor both theMCA and CHW samples (Fig. 6) can
be related to significantmixtures of non-SD detrital magnetite
grains (superparamagnetic, PSDand multi-domain grains) in the
studied samples (Dunlop, 2002;Roberts et al., 2012) as suggested by
the IRM acquisition curves.
FORC diagrams provide detailed information about
magneticinteractions and microcoercivity distributions (Roberts et
al., 2000).High-resolution FORC measurements following the
specifications of
0
0.2
0.4
0.6
0.8
1
0 200 400 600 800 1000
63.25
M/M
r
Field(mT)
63.5064.5565.1067.5568.60
a
1
2
3
4
5
6MCA - 63.25 msl
Raw dataComponent 1Component 2Component 3Sum of components
Syn
thet
ic IR
M (
x10
-4A
m2 /
kg)
1
2
3
4
5
6
1 1.5 2 2.5 3
MCA - 65.10 msl
1 1.5 2 2.5 3
0.5
1
1.5
2
2.5MCA - 68.60 msl
0.5
1
1.5
2
2.5
3
1 1.5 2 2.5 3
1
2
3
4
5
6
1
2
3
4
5
6
b c dRaw dataComponent 1Component 2Component 3Sum of
components
Raw dataComponent 2Component 3Sum of components
Gra
dien
t (x1
0-4
)
10Log Applied Field (mT) 10Log Applied Field (mT) 10Log Applied
Field (mT)
Fig. 5. (a) IRM acquisition curves for six representative
samples from the MCA section. (b–d) IRM unmixing analyses (Kruiver
et al., 2001; Heslop et al., 2002) for three representativesamples;
(b) and (c) represent samples collected inside themagneticmineral
concentration peaks and (d) fromoutside the peaks. Rawdata
(circles) and calculated IRMacquisition curvesare shown for two and
three fitted components after fitting of a spline function.
39J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
-
Egli et al. (2010) can be used to infer the presence of
biogenicmagnetite(e.g. Egli et al., 2010; Roberts et al., 2011,
2012; Jovane et al., 2012;Larrasoaña et al., 2012; Yamazaki, 2012;
Yamazaki and Ikehara, 2012).FORC distributions are nearly identical
in the high-magnetizationintervals between 63.2 and 65.5 msl (Figs.
2, 7). These FORC diagramshave a sharp horizontal ridge (Fig. 7) at
Hb = 0 that indicate negligiblemagnetic interactions and a
dominance of non-interacting SD particles(Roberts et al., 2000)
that is characteristic of intactmagnetosome chains(e.g. Egli et
al., 2010; Roberts et al., 2011; Jovane et al., 2012; Larrasoañaet
al., 2012; Roberts et al., 2012; Yamazaki, 2012; Yamazaki and
Ikehara,2012). The coercivity distribution has a broad peak between
3 and25 mT, with a maximum at 15–20 mT (Fig. 7) that falls within
therange expected for magnetite magnetosomes (Egli, 2004; Kopp
andKirschvink, 2008; Egli et al., 2010). Outside the 63.2–65.5 msl
interval,the magnetic signal of the samples is much weaker and no
meaningfulFORC distribution could be obtained from these samples
(Fig. 7a, h).Multiple runs were then performed for intervals with
low magnetiza-tion and also within the 66.6–68.8 msl peak.
Intervals with lowmagne-tization have no meaningful FORC
distributions even after stacking ofnine runs (Fig. 7j). On the
other hand, statistically significant FORC
results are estimated for the 66.6–68.8 msl interval after
stacking ofnine measurement runs; resulting distributions are
similar to thoseobserved within the major magnetic peaks (Fig.
7k).
To compare our results with those from a nearby section, we
alsoperformed FORC analyses on samples from the MECO interval at
theCHW section, between 135 and 139 msl, as defined by Jovane et
al.(2007). This interval is characterized by a peak in
magneticconcentration-dependent parameters (e.g. χ, ARM, IRM). A
horizontalridge due to non-interacting SD magnetite with
coercivities between10 and 50 mT is also evident in these samples
(Fig. 7i).
5. Discussion
5.1. The MECO event in the Neo-Tethys
Benthic and planktonic foraminiferal and calcareous
nannofossilassemblages in the MCA section across the Chron
C18r/C18n boundaryare affected by important faunal turnovers.
Together with rockmagnetic,geochemical and stable isotope data,
they enable subdivision of thestudied 14-m-thick section into five
discrete intervals (Figs. 2, 3, 8):
Table 1Parameters associated with components identified from IRM
analysis of six representative samples from the Monte Cagnero
section.
Sample Component 1 Component 2 Component 3 Component 1 Component
2 Component 3
SIRM (Am2/kg) SIRM (Am2/kg) SIRM (Am2/kg) Contribution (%)
Contribution (%) Contribution (%)
MCA-63.25 6.00E−05 3.30E−04 2.00E−04 10 56 34MCA-63.50 3.00E−05
2.00E−04 4.20E−04 5 31 65MCA-64.55 1.20E−04 3.20E−04 2.50E−04 17 46
36MCA-65.10 5.00E−05 3.20E−04 2.50E−04 8 52 40MCA-67.55 NA 1.40E−04
2.40E−04 NA 37 63MCA-68.60 NA 4.00E−05 1.80E−04 NA 18 82
B1/2 (mT) B1/2 (mT) B1/2 (mT) Contribution (%) Contribution (%)
Contribution (%)
MCA-63.25 15.8 63.1 478.6 10 56 34MCA-63.50 15.8 50.1 251.2 5 31
65MCA-64.55 15.8 63.1 446.7 17 46 36MCA-65.10 15.8 70.8 446.7 8 52
40MCA-67.55 NA 70.8 239.9 NA 37 63MCA-68.60 NA 70.8 251.12 NA 18
82
DP (log10 mT) DP (log10 mT) DP (log10 mT) Contribution (%)
Contribution (%) Contribution (%)
MCA-63.25 0.40 0.33 0.24 10 56 34MCA-63.50 0.40 0.30 0.30 5 31
65MCA-64.55 0.45 0.34 0.24 17 46 36MCA-65.10 0.45 0.31 0.24 8 52
40MCA-67.55 NA 0.40 0.30 NA 37 63MCA-68.60 NA 0.40 0.30 NA 18
82
0
0.1
0.2
0.3
0.4
0.5
0.6
0 1 2 3 4 5 6
Mrs
/Ms
Bcr/Bc
SD
PSD
MD-3 10-6
-2 10-6
-1 10-6
0
1 10-6
2 10-6
3 10-6
-1 -0.5 0 0.5 1
MCA-63.85 msl
Mom
ent (
Am
2 )
Field (T)
a b
Fig. 6. (a) Hysteresis loop for one representative sample
fromMCA pelagic carbonate samples. (b) Day plot (Day et al., 1977)
for nine representative samples from the MCA section. Thedata
fields represented in the Mrs/Ms versus Bcr/Bc diagram are for
single domain (SD), pseudo-single domain (PSD) and multi-domain
(MD) titanomagnetite particles.
40 J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
-
Interval 1 (58–61 m, ~370 kyr), Interval 2 (61–63.2 m, ~250
kyr),Interval 3 (63.2–64 m, ~70 kyr), Interval 4 (64–65.5 m, ~200
kyr) andInterval 5 (65.5–72 m, ~780 kyr). Following Bohaty et al.
(2009), thetotal duration of the MECO event is estimated to be ~500
kyr with awarming maxima at the end of the event. During the
warming peak, ashoaling of the CCDbyup to 500–1500mhas been
reported in the Pacific,Atlantic and Indian oceans where a total
loss of carbonate is reported atsites with paleodepths below ~3000
m (Bohaty et al., 2009; Pälikeet al., 2012). Interval 3 of the MCA
section is characterized by the lowestCaCO3 contents in the studied
section and the highest bulk carbonate δ13Cvalues. It also has the
lowest concentrations of the coarsest fraction,which is used as a
proxy for carbonate dissolution (e.g. Hancock andDickens, 2005;
Colosimo et al., 2006; Leon-Rodriguez and Dickens,2010; Luciani et
al., 2010). Dissolution during this interval is alsosupported by
increased values of the planktonic foraminiferal FI,
greaterrelative abundances of agglutinated foraminifera that are
less prone todissolution, and a clear decrease of the P/P + B ratio
(Fig. 3). Theincrease in dissolution likely contributes to the
higher concentrationsof magnetic particles in this interval
indicated bymagnetic susceptibility,ARM (andARMCFB), IRM900mT (and
IRM900mT CFB), andHIRM300mT (andHIRM300mT CFB) (Fig. 2). In
contrast, there is no evidence for carbonatedissolution upsection
in Intervals 4 and 5. Interval 3 falls within the low-ermost part
of Chron C18n, therefore, it can be
magnetostratigraphicallycorrelated with peak MECO warming in marine
sediment cores (Bohatyet al., 2009), and in the Tethyan CHW (Jovane
et al., 2007) and Alano sec-tions (Luciani et al., 2010; Spofforth
et al., 2010; Toffanin et al., 2011).Thus, observed changes in
paleoenvironmental proxies provide cluesthat point to seafloor
CaCO3 dissolution and lysocline shallowing at theMCA site during
the MECO warming. Interval 3 is immediately followedby an
intervalwith lower CaCO3 contents associatedwith highermagnet-ic
susceptibility values. On the basis of available magneto- and
bio-stratigraphic results, Interval 4 correlates well with the
organic-richORG1 unit identified at the Alano section (Luciani et
al., 2010; Spofforthet al., 2010). These organic-rich layers are
thought to represent rapidorganic carbon burial events at the end
of the MECO event, probablyinduced by enhanced delivery of
terrestrial material to the ocean(Spofforth et al., 2010).
5.2. Ocean iron fertilization and magnetotactic bacterial
abundance duringthe MECO event
Detrital magnetic minerals can be delivered to the deep sea
throughdifferent transportation pathways, including ice-rafting,
mass flows,hemipelagic sediment plumes or wind (e.g. Evans and
Heller, 2003;Liu et al., 2012). Fine-grained terrigenous inputs
that reached thedeep, carbonate-dominated MCA site were most likely
transported bysuspended plumes or wind. Both processes result in
similar grain-sizedistributions and mineralogy (Rea, 1994).
Hemipelagic sediment hasbeen reported in modern environments to
distances of 500 km fromthe coast and possibly as far as 900 km
(Rea, 1994 and references there-in). Such distances are within the
expected range of the MCA sectionfrom the paleoshoreline of the
Neo-Tethys. Nevertheless, processes
that control hemipelagic and aeolian inputs to the deep sea tend
to beout of phase;wet conditions (and enhanced runoff) in the
source regionwould favour hemipelagic fluxes, whereas drier source
region climateswould enhance aeolian dust fluxes. Enhanced aeolian
supply relativeto detrital fluxes into theNeo-Tethys has been
reported on the southernTethyanmargin (central Egypt) as a result
of drier, likelymore seasonal,climatic conditions during the
Paleocene–Eocene Thermal Maximum(PETM) (Schulte et al., 2011).
Similar observations have been madefor the PETM in the Bighorn
Basin, Wyoming (Wing et al., 2005; Krausand Riggins, 2007; Smith et
al., 2009), East Africa (Handley et al.,2012) and the
southernKerguelenPlateau, IndianOcean, on theAntarcticmargin
(Larrasoaña et al., 2012).
Environmental magnetic parameters can serve as sensitive
aeoliandust proxies, given that hematite forms in oxidizing,
dehydrating desertenvironments (Larrasoaña et al., 2003; Liu et
al., 2012 and referencestherein). Hematite concentrations can be
easily tracked using the“hard” IRM on a carbonate free basis
(HIRM900mT CFB). In the MCAsection, HIRM900mT CFB has two
pronounced peaks in Intervals 3 and4 (Fig. 8c), which coincide with
the MECO peak warming and thepost-MECO recovery. We interpret the
increased hematite concentra-tions in Intervals 3 and 4 to be
related to the presence of an enhancedaeolian dust component
similar to scenarios proposed for other Eocenesites (Roberts et
al., 2011; Larrasoaña et al., 2012). For example, rapidcontinental
aridification has been recorded by lithofacies and pollenrecords
from the Xining Basin dated at 40.0 Ma (Bosboom et al.,2014). The
onset of continental aridification and obliquity-dominatedclimate
cyclicity coincides with the MECO-peak and continuesafterward into
the post-MECO cooling.
Aeolian dust is a potential source for iron fertilization of the
ocean(e.g. Maher et al., 2010; Roberts et al., 2011; Larrasoaña et
al., 2012;Liu et al., 2012). Roberts et al. (2011) reported
enhanced iron supplyby aeolian dust in Eocene sediments (ODP Hole
738B) of the southernKerguelen Plateau (IndianOcean),where amarked
increase in hematiteconcentrations coincided with a switch from
oligotrophic to eutrophicconditions. The interval with higher
hematite concentrations is alsocoincident with increased
magnetofossil abundances. Roberts et al.(2011) argued that iron
input and increased organic carbon deliveryto the seafloor were the
main factors that controlled the abundance ofmagnetotactic
bacterial populations. Likewise, in the MCA section themagnetic
proxies classically used to trace low-coercivity magnetite(ARM CFB)
and high-coercivity hematite/goethite (HIRM900mT CFB)co-vary, which
indicates a concomitant increase in concentration ofboth magnetic
minerals in Intervals 3 and 4 (Figs. 2 and 8b, c). Asignificant
fraction of the magnetite in these intervals is due to
non-interacting SD particles, as indicated by a FORC central ridge
signature(Fig. 7), which we attribute to putative magnetofossils.
The sameintervals are characterized by a relative increase in
eutrophic calcareousnannofossil taxa (Fig. 8d). Eutrophication
could have been stimulatedby sea surface iron fertilization
produced by addition of iron-richaeolian dust to surface waters.
This promoted enhanced organic carbonexport to deeper waters and
burial on the seafloor, which stimulatedincreased magnetotactic
biomineralization (e.g., Villa et al., 2014). Theiron needed for
biomineralization by magnetotactic bacteria is likelyto have been
provided by diagenetic iron reduction that released Fe3+
from the most reactive iron-bearing minerals, including hydrous
ferricoxide and lepidocrocite (Poulton et al., 2004). Magnetite and
hematiteare more resistant to dissolution (Yamazaki et al., 2003;
Poulton et al.,2004; Garming et al., 2005; Roberts et al., 2011)
and, therefore, survivedthis mild iron reduction that occurred
under iron-reducing but notanoxic conditions. Thus, simultaneous
delivery of enhanced organiccarbon, reactive iron-bearing aeolian
dust particles and non-reactiveaeolian hematite particles to the
seafloor would have released theexisting limitation on key
nutrients (carbon and iron) for an existing,but small, population
ofmagnetotactic bacteria to produce the observedmagnetic
signatures. An increase in ARMCFB and HIRM900mT CFB is alsoobserved
outside the MECO interval, but with lower intensity, at the
Table 2Measured hysteresis parameters for studied sediments from
the MCA and CHW sections.
Sample ID Depth(msl)
Mr (Am2/kg) Ms (Am2/kg) Mr/Ms Bc(mT)
Bcr(mT)
Bcr/Bc
CAG-58.15 58.15 18.61E−09 44.89E−09 0.41 28.65 287.27
10.00CAG-64.55 64.55 394.32E−09 1.03E−06 0.37 27.47 72.34
2.63CAG-63.25 63.25 428.86E−09 1.83E−06 0.23 20.02 57.05
2.85CAG-64.90 64.90 497.47E−09 1.26E−06 0.39 30.42 73.96
2.43CAG-63.50 63.50 741.31E−09 2.01E−06 0.36 29.24 84.64
2.89CAG-65.10 65.10 545.76E−09 1.70E−06 0.31 28.79 79.39
2.76CAG-63.85 63.85 768.56E−09 2.70E−06 0.28 22.17 60.15
2.71CAG-66.30 66.30 46.93E−09 997.2E−09 0.04 5.04 9.45
1.87CAG-64.50 64.50 429.7E−09 1.08E−06 0.39 33.79 91.15 2.70
41J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
-
c) Monte Cagnero 63.25 msl
10 20 30 40 50 60 70 80 90 100
10505
10
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
e) Monte Cagnero 63.50 msl
20 40 60 80 100
10505
10
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
g) Monte Cagnero 64.50 msl
20 40 60 80 100
10505
10
- 0.
8
- 0.
6
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
a) Monte Cagnero 58.15 msl
10 20 30 40 50 60 70 80 90 100
10505
10
- 0.
8
- 0.
6
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
b) Monte Cagnero 64.55 msl
20 40 60 80 100
10505
10
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
d) Monte Cagnero 64.90 msl
20 40 60 80 100
10505
10
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
f) Monte Cagnero 65.10 msl
10 20 30 40 50 60 70 80 90 100
10505
10
- 0.
8
- 0.
6
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
h) Monte Cagnero 66.30 msl
10 20 30 40 50 60 70 80 90 100
10505
10-
1
- 0.
8
- 0.
6
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
i) Contessa Highway 137.15 msl
10 20 30 40 50 60 70 80 90 100
10505
10
- 0.
6
- 0.
4
- 0.
20
0.2
0.4
0.6
0.81
Hc [mT]
Hc [mT] Hc [mT]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hb
[mT
]
Hc [mT] Hc [mT]
Hc [mT] Hc [mT]
Hc [mT] Hc [mT]
Hc [mT] Hc [mT]
20 40 60 80 100
10505
10
00.20.40.60.8
20 40 60 80 100
10505
10
0.60.40.20.81
j) Monte Cagnero 65.70 msl
k) Monte Cagnero 67.55 msl
42 J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
-
66.6–68.8 msl peak. FORC distributions for this interval are
similar tothose observed across the MECO (Fig. 7) and also coincide
with anincrease in eutrophic taxa (Fig. 8d). In this case, the same
mechanismcan be advocated for the coeval increase in magnetofossils
andhigh-coercivity phases (Fig. 8b, c) and suggests the presence of
amagnetotactic bacteria assemblage throughout the whole
studiedinterval that bloomedwith increased supply of limiting
nutrients duringepisodic warming events.
6. Conclusions
An integrated high-resolution stable isotope, geochemical,
micropa-leontological and environmentalmagnetic analysis has been
carried outover a 14-m-thick interval of the Monte Cagnero section
(Umbria-Marche Basin), Italy, which corresponds to the 40.8–39.1 Ma
periodaround the Middle Eocene Climatic Optimum (MECO).
Magneticparameters indicate a concomitant increase of aeolian iron
supply in theform of hematite, and a higher abundance of magnetite
magnetofossilsproduced by magnetotactic bacteria as indicated by
FORC diagrams thatare typical of non-interacting SD magnetite
between 63.2 and 65.5 msl.This interval corresponds to the peak
MECO warming and its aftermath.Intervals with enhanced putative
magnetofossil concentrationscorrespond to those for which other
proxies systematically point to anincrease in primary productivity,
which was probably stimulated by
increased aeolian supply of iron to surface ocean waters. Such a
scenariohas been recently envisaged for the PETM event (e.g. Chang
et al., 2012;Larrasoaña et al., 2012), and we now confirm a similar
connectionbetween putative magnetofossil abundance and
paleoproductivitythrough the MECO event. It reinforces the
connection betweenhyperthermal climatic events and the occurrence
(or increasedabundance) of putative magnetofossils. Further work is
needed to assesswhether the preserved inorganic remains of
magnetotactic bacteria canprovide a useful paleoproductivity proxy
in ancient carbonate sediments.
Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.palaeo.2014.08.009.
Acknowledgements
We acknowledge financial support from the Marie Curie
Actions(FP7-PEOPLE-IEF-2008, proposal n. 236311). Jairo F. Savian
acknowl-edges the Brazilian CNPq (Process 201508/2009 5) for
funding a VisitingFellowship at the National Oceanography Centre
Southampton (NOCS),where the magnetic and isotopic measurements
were completed.Andrew Roberts acknowledges the Australian Research
Council forproviding funding through grant DP140104544. Luigi
Jovane andFrancesco Iacoviello acknowledges the Fundação de Amparo
à Pesquisado Estado de São Paulo (FAPESP) for financial support
grant numbers2011/22018-3 and 2012/18304-3, respectively.
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
40 50 60 70 80 90 100
Str
atig
raph
ic th
ickn
ess
(msl
)
CaCO3
(%)
(1) (2) (3) (4) (5) (6) (7)
40.1
3040
.145
P12
E11
P13
P14
E12
E13
NP
16C
P14
aN
P17
CP
14b
1
2
3
4
5
a b c
marlscalcareous marl
lithology:
marly limestonelimestone
C18
rC
18n
0 1 2 3 4 5
ARM (x10
-6 Am
2/kg)
0 6 12 18 24 30
magnetofossil production- +ARM CFB
(x10-6 Am2/kg)
0 0.5 1 1.5 2
HIRM (x10
-5 Am
2/kg)
0 6 12 18
eolian dust- +
(x10-6 Am2 /kg) HIRM CFB
0 10 20 30 40 50
Eutrophic species (%)
20 30 40 50 60
Oligotrophic species(%)
d
Fig. 8. Productivity proxies based on selected calcareous
nannofossil genera and magnetic parameters from theMCA section. (a)
CaCO3, (b) ARM and ARM CFB, (c) HIRM and HIRM CFB,and (d) eutrophic
and oligotrophic nannofossil taxa. The increase in eutrophic taxa
coincides with an increase in ARM and HIRM during the MECO
interval. These results suggest anincrease in iron fertilization
and high primary productivity during MECO.
Magnetobiochronostratigraphy and shaded areas are in Fig. 2.
Fig. 7. FORCdiagrams for ten samples from theMCA section (within
and outside theMECOevent) andone sample from theContessaHighway
section (during theMECOevent).Within theMECO event, FORC diagrams
have sharp ridges that are indicative of non-interacting SD
particles (Roberts et al., 2000; Egli et al., 2010). Samples 58.15
and 66.30 do not have the samebehaviour. FORC diagrams in (a) to
(i)weremeasuredwith a single run in a VSM,whereas those in (j) and
(k)were obtained after stacking ninemeasurement runs using amore
sensitiveAGM instrument. FORC diagrams obtained with multiple runs
were stacked and calculated using theMATLAB routine of Heslop and
Roberts (2012). The smoothing factor (Roberts et al.,2000) is 4 in
all cases.
43J.F. Savian et al. / Palaeogeography, Palaeoclimatology,
Palaeoecology 414 (2014) 32–45
http://dx.doi.org/10.1016/j.palaeo.2014.08.009http://dx.doi.org/10.1016/j.palaeo.2014.08.009
-
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