High-resolution record of export production in the eastern ...apaytan/publications/2013_Articles/Erhardt... · production in the eastern equatorial Pacific across the Eocene-Oligocene

High-resolution record of exportproduction in the eastern equatorial Pacific across the Eocene-Oligocene transitionand relationships to global climatic records

Andrea M. Erhardt,1,2 Heiko Pälike,3 and Adina Paytan2

Received 18 May 2012; revised 29 October 2012; accepted 30 October 2012.

[1] Understanding changes in export production through time provides insight into theresponse of the biological pump to global climate change, particularly during periods of rapidclimate change. In this study we consider what role changes in export production may havehad on carbon sequestration and how this may have contributed to the onset of the Eocene-Oligocene transition (EOT). In addition, we consider if these export production variations aredominantly controlled by orbitally driven climate variability. To accomplish these objectives,we report changes in export production in the Eastern Equatorial Pacific (EEP) from SiteU1333 across the EOT reconstructed from a high-resolution record of marine bariteaccumulation rates (BAR). BAR fluctuations suggest synchronous declines in exportproduction associated with the two-step increases in oxygen isotopes that define thetransition. The reduction in productivity across the EOT suggests that the biological pumpdid not contribute to carbon sequestration and the cooling over this transition. We also reporta previously undocumented peak in EEP export productivity before the EOT onset. Thispeak is consistent with export production proxies from the Southern Ocean, potentiallyimplying a global driver for this precursor event. We propose that this enhanced exportproduction and the associated carbon sequestration in the late Eocene may have contributedto the pCO2 drawdown at the onset of Antarctic glaciation.

Citation: Erhardt, A. M, H. Palike, and A. Paytan (2013), High-resolution record of export production in the easternequatorial Pacific across the Eocene-Oligocene transition and relationships to global climatic records, Paleoceanography,28, doi:10.1029/2012PA002347

1. Introduction

[2] The Eocene Oligocene transition (EOT) representsa time where a dramatic change from “greenhouse” to“icehouse” conditions occurred [Zachos et al., 2001; Crameret al., 2009] with the onset of sustained Antarctic glaciationas evidenced by the shifts in benthic foraminifera oxygenisotope records [Miller et al., 1987; Miller et al., 2005;Zachos et al., 2001; Cramer et al., 2009]. The oxygen iso-tope record, along with Mg/Ca data, also indicates coolerdeep water and surface water temperatures during the earlyOligocene compared to the late Eocene [Coxall et al., 2005;Lear et al., 2008; Bohaty et al., 2012]. Additional changes

including deepening of the calcite compensation depth (CCD)[Coxall et al., 2005; Rea and Lyle, 2005; Lyle et al., 2005;Lyle et al., 2008], changes in ocean circulation [Huber andNof, 2006], aridification of continents [Dupont-Nivet et al.,2007], changes in weathering intensity [Ravizza and Peucker-Ehrenbrink, 2003], and major ecosystem shifts [Nilsen et al.,2003; Falkowski et al., 2004; Coxall et al., 2005; Zachosand Kump, 2005; Merico et al., 2008]. Changes in oceanproductivity [Diester-Haass, 1995;Diester-Haass and Zahn,2001; Diester-Haass and Zachos, 2003; Schumacher andLazarus, 2004; Anderson and Delaney, 2005; Coxall andWilson, 2011], export production [Diester-Haass and Zahn,1996; Salamy and Zachos, 1999; Latimer and Filippelli,2002; Griffith et al., 2010] and the organic to inorganic car-bon burial ratio [Griffith et al., 2010] have also been observedat this time interval.[3] Oxygen and carbon isotope records in foraminifera

and high-resolution orbitally tuned stratigraphy indicate thatthe transition from Eocene greenhouse conditions to Oligo-cene icehouse conditions occurred as two distinct steps[Coxall et al., 2005; Katz et al., 2008]. The first major iso-topic shift, a ~0.6% d18O positive excursion, started at34Ma (EOT-1), and took place over ~200 kyr [Katz et al.,2008; Coxall and Wilson, 2011]. The second major shift,

1Department of Geological and Environmental Sciences, StanfordUniversity, Stanford, California, USA.

2Institute of Marine Sciences, University of California, Santa Cruz,California, USA.

3School of Ocean and Earth Science, University of Southampton,Southampton, UK.

Corresponding author: A. Erhardt, Department of Geological and Envi-ronmental Sciences, Stanford University, Stanford, CA 94305, USA.([email protected])

©2012. American Geophysical Union. All Rights Reserved.0883–8305/13/2012PA002347

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PALEOCEANOGRAPHY, VOL. 28, 1–13, doi:10.1029/2012PA002347, 2013

Oi-1 at 33.7Ma, represents a ~0.9% d18O increase over<60kyr [Coxall et al., 2005; Coxall and Wilson, 2011]. The~300 kyr between these events show minor changes ind18O, including a potential smaller excursion (EOT-2)~100 kyr before the onset of Oi-1 [Katz et al., 2008]. Thistwo-step behavior is evident in the isotope records of deep-water sediments at locations across the globe [DunkleyJones et al., 2008; Katz et al., 2008] and continental shelfenvironments [Katz et al., 2008] and is also observed inCCD records [Coxall et al., 2005; Coxall and Wilson, 2011].[4] The first isotopic shift, EOT-1, was driven primarily by

temperature declines with little ice growth [Lear et al., 2008;Katz et al., 2008] culminating in a ~20m sea level drop[Houben et al., 2012]. The second shift, Oi-1, represents rapidand lasting ice growth in Antarctica [Zachos et al., 1999;Zachos et al., 2001; Ivany et al., 2006; Coxall and Pearson,2007] and a sea level drop of 50–60m. It has been suggestedthat the sea level drop exposed fresh rocks, the erosion ofwhich reduced pCO2 providing a positive feedback to thecooling [Merico et al., 2008]. In addition, orbital parameterswere favorable at this time for ice formation [Coxall et al.,2005; Merico et al., 2008].[5] The above observations, as well as a 1% positive excur-

sion in benthic d13C [Coxall et al., 2005; Coxall and Wilson,2011] and a possible decrease in atmospheric pCO2 [Pearsonet al., 2009; see also Pagani et al., 2011] at this time intervalsuggest that the carbon system played an important role inthe transition. Indeed, modeling studies have proposed CO2

changes as a driver for Antarctic glaciation [Deconto et al.,2008; Liu et al., 2009; Tigchelaar et al., 2010]. However,the exact mechanisms linking specific C cycle responses andfeedbacks to the observed climatic changes are still enigmaticand several processes and links have been suggested. These in-clude changes in silicate weathering [Ravizza and Peucker-Ehrenbrink, 2003] and river inputs [Salamy and Zachos,1999; Rea and Lyle, 2005; Merico et al., 2008], changes indeposition loci of carbonates [Coxall et al., 2005; Mericoet al., 2008], changes in ocean ventilation [Miller et al.,2009], changes in the terrestrial biosphere [Salamy andZachos, 1999], and changes in ecosystem structure and ma-rine organic matter export and burial [Ravizza and Paquay,2008; Miller et al., 2009]. Enhanced productivity, and thepotential associated increases in export production, has alsobeen postulated as a carbon sequestration mechanism acrossthis transition [Scher and Martin, 2006, 2008; Pearson et al.,2009; Coxall and Wilson, 2011].[6] When considering these changes within the larger

context of the Late Eocene, we see additional events of chang-ing ocean chemistry and export productivity. Carbonate accu-mulation events (CAEs) represent periods of intermittentlyhigher rates of carbonate accumulation resulting from changesin the CCD [Lyle et al., 2005]. These events, while smaller andnot permanent as the changes during the EOT, also illustratethe climatically dynamic nature of this period.[7] Changes in ocean productivity, ecosystem structure, and

export production can precipitate from changes in tempera-ture, CO2, nutrient availability, and nutrient stoichiometryrelated to climate conditions, atmospheric composition,and climatically induced circulation changes [Sarmiento andToggweiler, 1984; Broecker and Henderson, 1998]. In turn,changes in upwelling and export production can serve as feed-backs to the climate system by increasing the efflux or

sequestration of C, respectively [Archer et al., 2000; Diester-Haass and Zahn, 2001; Takahashi et al., 2009]. Many studieshave documented the response of the oceanic biosphere tochanges across the EOT; changes in primary production, exportproduction, and ecosystem structure have been documented butmost records are not of high resolution and interpretations basedon different proxies are inconsistent.[8] The purpose of this study is to determine what, if any,

role changes in export production may have played inthe drawdown of carbon and the potential contribution ofincreased export production to the climate changes observedat the EOT. In addition, we examine if changes in exportproduction are driven by orbitally based climate cycles orif other drivers may be contributing to these changes. Tomore closely relate climate changes reflected in the oxygenisotope record and marine export production records in theEastern Equatorial Pacific (EEP), we obtain BAR recordsover ~8 million years bounding the EOT (39.59-31.78Ma)with high-resolution sampling between 35 and 32Ma toemphasis changes during the cooling and glaciation events(EOT-1 and Oi-1).

2. Methods

2.1. Sample Location and Collection

[9] Samples from IODP Site U1333, Expedition 320 PacificEquatorial Age Transect [Pälike et al., 2009] were used forthis study. This site (10�310N, 138�250W, 4853m modern wa-ter depth) (Figure 1) was at a paleo latitude of ~3�N during theEOT, placing it at the edge of the present day equatorial high-productivity zone. Three holes were drilled at this location,allowing for a record with no significant gaps or hiatuses.Samples used for this study are listed in Table S1 in the supple-mental materials.

2.2. Barite Separation

[10] Marine barite (BaSO4), a common, albeit minor,component of marine sediments, is abundant in sedimentsunderlying areas of high productivity [e.g., Church, 1970;Paytan and Griffith, 2007]. A strong correlation betweenBAR and organic C export has been observed in core top sedi-ments [Paytan et al., 1996; Eagle et al., 2003]. Assuming theprocesses that govern barite formation and preservationremained similar through time, changes in BAR reflect varia-tions in export production. Indeed, BAR and excess Ba havebeen widely used to reconstruct changes in export productionthrough time [Schmitz, 1987; Rutsch et al., 1995; Paytan et al.,1996; Dean et al., 1997; Nürnberg et al., 1997; Bonn et al.,1998; Bains et al., 2000; Martinez-Ruiz et al., 2003; Averytand Paytan, 2004; Jaccard et al., 2005; Olivarez Lyle andLyle, 2006; Griffith et al., 2010].[11] Approximately 20 grams of dry, homogenized

sediment was used for barite separation. A sequential leachingprocedure [Paytan et al., 1996] was employed to remove allsedimentary fractions except barite and a small amount ofother resistant minerals. Barite recovery for this process istypically better than 90% [Eagle et al., 2003] and the consis-tency of stratigraphically closely spaced samples reinforcesthe vigor of this method. All samples were screened using ascanning electron microscope (SEM) with energy dispersiveX-ray spectroscopy (EDAX) to verify sample purity, with allbut three samples in this study being >99% pure barite.

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We analyzed 250 samples spanning a period of 7.8 Myrbetween 31.78 and 39.59Ma, with an average sample spacingof 20–50 kyr for most of the record and a resolution of5–10 kyr during the EOT.

2.3. Age Models and Accumulation Rates

[12] The generation of barite accumulation rates from rawpercent barite data is achieved by multiplying barite contentsby mass accumulation rates. Accumulation rate fluxes in turnrequire linear sedimentation rates and dry bulk densities asinput parameters. Thus a carefully constructed age model isimportant, as it determines the linear sedimentation rates.While awaiting a pending astronomical age calibration for SiteU1333, we have applied the following strategy to generate twoage model versions for U1333. First, we apply a paleomag-netic age model to Site U1333 based on shipboard paleomag-netic reversals and corresponding ages that were developed aspart of IODP Expeditions 320 and 321 [Pälike et al., 2010].These paleomagnetic reversal ages are based on those derivedfrom ODP Sites 1218 and 1219 by astronomical tuning andstratigraphic correlation [Pälike et al., 2006] but are linearlyinterpolated between magnetic reversals. We put physicalproperty data from Sites 1218 and U1333 on a common agemodel by using the detailed stratigraphic site-to-site correla-tion developed by Westerhold et al. [2012]. The work ofWesterhold et al. [2012] places Site U1333 and Site 1218 ona common depth scale, allowing for comparison between thetwo records at decimeter resolution. In addition, we applythe detailed orbital age calibration previously provided for Site1218 [Coxall et al., 2005; Coxall and Wilson, 2011; Pälikeet al., 2006] to Site U1333 using this correlation, allowingfor comparison of a lower-resolution paleomagnetic agemodel and one that is orbitally calibrated for the late Eoceneand Oligocene. Accumulation rates were calculated usinglinear sedimentation rates based on the above age modelsand shipboard measured dry bulk densities (Table S1). Thissedimentation rate was smoothed between major climateshifts to minimize the potential of sharp sedimentationrate changes generating artificial peaks in the BAR record.Figure S1 in the supplemental materials shows a comparisonof linear sedimentation rates, barite content, and barite accu-mulation rates for these two age model versions. Since BAR

is a direct function of linear sedimentation rates and thus thechosen age model (as well as bulk densities), it is importantto evaluate the potential influence of age model choice. Wenote that barite content (which is independent from the agemodel chosen) tends to be higher when sedimentation ratesare lower and vice versa. This observation suggests that theresulting BAR variations are not amplified or muted bychanges in the sedimentation rate used.

2.4. Spectral Analysis

[13] To determine the temporal relationships between theBAR record and global orbitally controlled climaticchanges, the BAR record was analyzed for orbital periodic-ity. In addition, this record was compared for coherencyand phase relationships with magnetic susceptibility anddensity records from the same core, which have been sug-gested to vary with orbital frequency [Hovan et al., 1989;Barthes et al., 1999]. We also compare the record to timeequivalent d18O and d13C records from Site 1218. Site1218 was used because analysis of benthic foraminifera datafrom Site U1333 is not yet completed. To deconstruct time-dependent variability in spectral characteristics, we appliedcross-spectral wavelet analysis techniques [Torrence andCompo, 1998; Maraun et al., 2007]. This method requiresresampling of the BAR record onto the higher-resolutiond18O record from Site 1218.[14] Throughout the entire record, sample spacing allows

us to identify potential 405 kyr cycles (or any longer timeframe periodicity), and this observation should be robusteven with an age model based on paleomagnetic reversals.The concentration of sampling in the time period surround-ing the EOT (5–20 kyr spacing from 32 to 36Ma) alsoenables resolving the possibility for 96 and 126 kyr and po-tentially 41 kyr cycles at select time periods (i.e., the earliestOligocene). Higher-resolution cycles currently cannot beresolved at the sampling resolution. As a result, the focusof the analysis is on eccentricity cycles and their relationto BAR.

3. Results

[15] Our data is generally consistent with previous observa-tions of BAR in the EEP [Griffith et al., 2010, Figure 2]

0˚ 0˚

34 Ma Reconstruction

30° S

30° N

Site U1333 Site 1218Site 574

180° 150° W 120° W 90° W 60° W150° E

Figure 1. Location map of the three cores discussed in this study plotted on a 34Ma reconstruction ofplate tectonics. Sample locations based on the work of Moore and Kamikuri [2012]. Plate reconstructionsadapted from Ocean Drilling Stratigraphic Network (ODSN) Paleomap project (http://www.odsn.de/odsn/services/paleomap/paleomap.html).

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showing overall higher BAR in the late Eocene (39.59–33.8Ma average 1.0mg cm-2 kyr-1) compared to the earlyOligocene (33.8–31.78Ma average 0.49mgcm-2 kyr-1). How-ever, our higher-resolution data provides considerably moredetail and reveals previously unrecognized features (Figures 2and 3), specifically a close correspondence between BAR andexisting proxy records across the E/O transition is observed.[16] Between 39.59 and 35Ma the BAR record oscillates

between 0.17 and 1.9mgcm-2 kyr-1. Just prior to the EOT,between 35 and 34.2Ma, BAR fluctuate between 0.6 and1.3mg cm-2 kyr-1 with an average of 0.9mgcm-2 kyr-1. How-ever, at 34.20Ma, a relatively rapid increase in BAR takingplace over ~90 kyr is observed. This increase peaks at34.11Ma (e.g., prior to EOT-1) with a BAR of 2.08mg cm-2

kyr-1 (double pre-excursion). The high BAR is main-tained for ~38 kyr before it declines over 75 kyr and returnsto pre-excursion values of ~0.88mg cm-2 kyr-1. This BARevent from the start of the increase to return to pre-peak valuesspans ~200 kyr and terminates during EOT-1 (34.0Ma). Asmall increase in BAR from 0.9 to 1.23mgcm-2 kyr-1 is ob-served right after EOT-1 and throughout the 300 kyr betweenEOT-1 and Oi-1 BAR fluctuates between 0.51 and 1.36mgcm-2 kyr-1. A pronounced drop to 0.06mgcm-2 kyr-1 is seenat Oi-1 (33.7Ma) after which BAR is maintained at lowvalues (0.05-0.48mgcm-2 kyr-1) until 33.44Ma. BAR slightlyincreases to 0.90mg cm-2 kyr-1 at Oi-1b, declining to 0.07mgcm-2 kyr-1 by 33.2Ma. The BAR remains lower in the earlyOligocene compared to the late Eocene, averaging 0.58mgcm-2 kyr-1, about half the average late Eocene BAR. TheseBAR values correspond to export production rates varyingbetween a low of 13 gC m-2 y-1 in the early Oligocene toand a peak of 16 gC m-2 y-1 during the late Eocene based onpresent day core top calibrations obtained from this region[Paytan et al., 1996; Eagle et al., 2003] (Figure 2).[17] Wavelet analysis of the BAR data (Figures 4a and 5a)

allows identification of orbital signals, particularly at the~100 kyr and ~405 kyr orbital eccentricity periods, but alsoat the obliquity (~41 kyr) period where the sample spacing ishigh enough (e.g., the earliest Oligocene). Throughout therecord, the BAR shows power at the 405 kyr long-term

eccentricity cycle, along with intermittent power in the 96and 126 kyr shorter-term eccentricity cycles during periodsof sufficient sampling frequency (Figure 4a). These shorter-term eccentricity cycles are observed between 38.2 and39.5Ma, 35.7 and 37Ma, and 32 and 34.5Ma. Obliquity(41 kyr) cycles are also alluded to during these intervals,though with less power than the longer period cycles. Overall,the 405-kyr cycle is the dominant driver throughout the recordin both strength and continuity. In addition, the periodicityextends into longer period cycles (e.g., ~750 kyr) not directlyrelated to defined orbital cycles.[18] While these trends, in particular the eccentricity cycles,

are strongest during time periods of highest sample resolution,i.e., 33–35Ma, they are present throughout the record. Thisleads us to conclude that sampling resolution is not drivingtheir presence or absence during this period. Since the sam-pling resolution during the lower resolution sections of thecore averages 75 kyr while the high-resolution sections aver-age 5–10 kyr, the 405 kyr cycles are significant throughoutthe record while cycles with periods of 41 kyr are only cap-tured during high sampling resolution intervals.[19] The 405 kyr and longer cycles are also seen in the bulk

density and magnetic susceptibility records reported by Pälikeet al. [2009] (Figure 4), though these signals are not as domi-nant as in the BAR records. The bulk density and magneticsusceptibility records can be used as proxies for carbonatecontent [Ortiz et al., 1999], and the comparison with theBAR allows us to identify relationships with carbonate contentand compensation depth and related changes in the carboncycle. In addition, since these records are all from the sameset of cores, a direct comparison can be made for the entiredata set without the correlation problems inherent in a com-parison between samples from different locations.[20] Overall trends and the coherency between BAR and

the bulk density and magnetic susceptibility records wereevaluated. When considering these data sets temporally, wesee relatively stable values for density and magnetic suscep-tibility from ~38Ma to ~34Ma with the two sharp changes atEOT-1 and Oi-1 clearly captured. In addition, both bulk den-sity and magnetic susceptibility are coherent with the BAR

Site U1333 BAR (mg cm-2kyr-2)

Site U1333 Export Productivity (mgC m-2day-1)13

Age

(M

a)

32

34

36

38

400

Site U1333 Magnetic Susceptibility (10-5 SI)50

1.2 1.81.61.4Site U1333 GRA Bulk Density

(g/cm3)

Oi-1 Glaciation

106 8

14 15 16

0 10 20 30 40 0 2 4

0 2 4 6 8Site 574 BAR (mg cm-2kyr-2)

Site 1218 BAR (mg cm-2 kyr-2)

CAE-4

CAE-5

CAE-6

CAE-7

2.521.510.5

EOT-1 Initial Glaciation

Figure 2. Comparison of Site U1333 BAR (this study, purple), magnetic susceptibility (blue), and GRAbulk density records (orange) [Pälike et al., 2009] is shown with BAR from Site 1218 and 574 (light pur-ple and pink) [Griffith et al., 2010]. The proposed export productivity for Site U1333 BAR is shown, cal-culated using the Paytan et al. [1996] core top empirical calibration. CAEs occurring during this interval[Lyle et al., 2005] and the two major glaciation events are noted.

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at the 405 kyr power between ~34.5Ma and 33.5Ma, afterthe Oi-1 boundary (Figure 4b). Throughout the reminderof the record, coherency is only observed intermittently atthe ~100 kyr shorter-term eccentricity cycles. The BAR andbulk density records are strongly in phase during all coherentperiods from 40Ma until 33.5Ma, while the BAR and mag-netic susceptibility show more variability (Figure 4c). Theinterval between 35 and 33.7Ma corresponds to the sectionwith higher data resolution in the BAR record and it is pos-sible that the limited coherence outside this interval is aconsequence of the lower sampling resolution. In addition,the high degree of coherence during this interval is likelytied to the predominance of the large stepwise shifts in bothrecords at the EOT-1 and Oi-1. The reduction in cyclicityin the magnetic susceptibility records outside of this timeperiod is another potential influence limiting coherency.[21] A strong relationship between BAR and the oxygen

and carbon isotopes records from site 1218 is observed at

both long and short time scales (Figure 3 and Figure 5).Overall, the major isotope shifts marking the EOT-1 andOi-1 are clearly observed in the barite record. The positiveshifts in the isotope records, associated with cooling andglaciations, correspond to reductions in BAR. This relation-ship is best illustrated at Oi-1, where the d18O undergoes a1% increase and BAR is reduced by 95% from ~1mg cm-2

kyr-1 to ~0.05mg cm-2 kyr-1.[22] A clear peak is observed in the BAR record prior

to the EOT, corresponding to a period of potentially largeshifts in pCO2. When compared to the Pearson et al.[2009] pCO2 reconstruction, the peak in pCO2 observedjust prior to the EOT, roughly corresponds to the rise and de-cline in BAR. The Pagani et al. [2011] record, however,does not show the pre-EOT pCO2 peak. The resolution ofthe pCO2 data sets is very coarse, and differences betweenthe two records may reflect the capture of smaller-scalechanges or age model differences. While the low-resolution

Age

(M

a)

Site 1218 18O (‰)

Site U1333 BAR (mg cm-2kyr-2) Site 1218 13C (‰)

Site U1333 Export Productivity (mgC m-2day-1)13

32

32.5

33

33.5

34

34.5

3500

Site 1218 Benthic Foram Accumulation Rate (cm2 ky-1)

pCO2 (ppm)Equatorial Pacific Records

Oi-1 Glaciation

Site1090 Ba Excess(umol cm-2 ky-1)

Site 1090 Opal Accumulation Rate(mmol cm-2kyr-1)

Site1090 Preact

Accumulation Rate(umol cm-2 ky-1)

Site 744 Opal Accumulation Rate(mmol cm-2kyr-1)

14 15 16

0.5 1 1.5

0 0.5 1 1.5 2

0 0.5 1 1.5 2 2.5 1000 2000 3000

500 700 900 1100 1300

0 5 10 15 20

0 2 4 6 8 101 2 3

0 10 20 30 40 50 60 70

0.2 0.3 0.40.1

Site 1090 P/Ti ratios(g/g)

Age

(M

a)

32

32.5

33

33.5

34

34.5

Southern Ocean Records

Oi-1bOi-1a

Oi-1 Glaciation

EOT-1 Initial Glaciation

Oi-1bOi-1a

EOT-1 Initial Glaciation

Figure 3. Compilation of productivity and climatic records for the Equatorial Pacific and SouthernOcean for the Late Eocene and Early Oligocene. Equatorial Site U1333 BAR (this study) is shown in pur-ple and Site 1218 BFAR, oxygen and carbon benthic foraminifera isotopic records [from Coxall andWilson, 2011] are shown in red, blue, and red, respectively. Southern Ocean Indian Sector Site 744 opalaccumulation rates is shown in orange [Salamy and Zachos, 1999] and Atlantic Sector Site 1090 opal,barium excess, reactive P [Anderson and Delaney, 2005], and P/Ti ratios [Latimer and Filippelli, 2002]are shown in green, brown, red, and pink, respectively. Estimates of pCO2 from boron isotopes in benthicforaminifera from Tanzania are shown in red (assuming constant carbonate ion concentration) and blue(using variable carbonate ion concentration) from Pearson et al. [2009]. Orange and green dots representestimates from Pagani et al. [2011] based on TEX86 and UK’

37 sea surface temperatures, respectively, fromSite 925 in the Equatorial Atlantic.

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nature of these two records does not allow for a direct com-parison to the shifts observed in the BAR record, it is clearthat the EOT is a time of large changes in global climateand the carbon cycle potentially impacting global and re-gional ocean productivity.[23] Since accurate calculation of accumulation rates is a

critical component to this record, BAR was calculated usingtwo age models, Site U1333 paleomagnetism ages and Site1218 tuned ages from Coxall and Wilson [2011] (supplemen-tal material). BAR calculated using each of these age modelsshow very similar features, with all significant peaks anddeclines represented. The absolute ages of the transitions areslightly different between the model, with the decline atEOT-1 beginning ~100 kyr later using the 1218 age modeland occurring over a longer time frame. The initiations ofthe export production peak preceding EOT-1, and thetransition at Oi-1, occur at the same time regardless of theage model and associated sedimentation rate used. Since eachindependent agemodel results in the same overall character forthe BAR record, we do not believe that age model relatederrors and uncertainties are driving the observed features.

4. Discussion

[24] One of the mechanisms proposed as contributing tothe cooling and glaciation observed at the EOT is an increase

in export production, sequestering C and reducing atmo-spheric pCO2 [Scher and Martin, 2006]. Previous studiesexploring productivity and export production across theEOT have utilized a variety of proxies including changesin diatom assemblages [Falkowski et al., 2004; Finkel etal., 2005; Finkel et al., 2007], benthic foraminifera accu-mulation rates (BFAR) [Coxall and Wilson, 2011], opalaccumulation rates [Salamy and Zachos, 1999], P/Ti ratios[Latimer and Filippelli, 2002], and barium-based proxiessuch as Ba/Ti ratios [Latimer and Filippelli, 2002] and ex-cess Ba [Anderson and Delaney, 2005]. These studies havegenerally shown productivity increases across the EOT inthe equatorial Atlantic [Diester-Haass and Zachos, 2003],South Atlantic [Hartlet al., 1995], NW Australia [Diester-Haass and Zahn, 2001], and in some continental marginlocations [Ravizza and Paquay, 2008]. In the SouthernOcean increased productivity at the EOT is evidenced by arapid diversification of diatoms [Falkowski et al., 2004;Finkel et al., 2005, 2007], high opal accumulation rates[Salamy and Zachos, 1999], high P/Ti and Ba/Ti ratios[Latimer and Filippelli, 2002], and high Ba excess [Andersonand Delaney, 2005]. The exact timing of productivity increasevaries depending on the site investigated and proxy used.For example, opal concentration increases only after Oi-1,while P/Ti and Ba excess increase gradually starting beforeEOT-1 [Latimer and Filippelli, 2002; Anderson and Delaney,2005] (Figure 3).

Figure 4. (a) Orbital frequencies for GRA bulk density, magnetic susceptibility, and BAR from SiteU1333. The measured values are plotted above each spectrum. Warmer colors correspond to enhancedpower in each spectrum, corresponding to different frequency periods. The records are plotted on thepaleomagetism age scale to eliminate any bias from orbital tuning. The major climatic steps outlined inthe text are denoted with vertical lines, with EOT-1 at 34.0Ma and Oi-1 at 33.7Ma. CAEs from Lyleet al. (2005) are mapped onto U1333 age scale. (b) Coherency comparisons for the GRA bulk densityversus BAR and magnetic susceptibility versus BAR. Warmer colors signify greater coherency, withsignificant regions outlined in black. (c) Phase relationships between GRA bulk density and BAR andMagnetic Susceptibility and BAR. Only regions with significant coherency are considered, with therelationship plotted as a time function of the period.

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[25] Postulated causes for the increase in productivityinclude increased wind driven upwelling and/or increasedinput of dissolved nutrients during initial glaciation througherosion or eolian deposition [Diester-Haass and Zahn, 1996;Salamy and Zachos, 1999; Latimer and Filippelli, 2002;Miller et al., 2009]. Our BAR record from the EEP suggestconsiderable fluctuations in export production over the sam-pled time interval, particularly between 35 and 32Ma. How-ever, in contrast to records from the Atlantic and SouthernOcean our record shows a general decline in export produc-tion across the EOT, consistent with other BAR records forthe Pacific [Griffith et al., 2010] (Figure 2).[26] In the present day ocean export production varies

considerably spatially throughout the ocean, and it is not sur-prising that trends in export production at different oceanbasins are decoupled. A potential mechanism to explain thisdecoupling calls for basin-specific changes in ocean circula-tion and mixing and the redistribution of the oceanic nutrientinventory among ocean basins rather than an overall globaldriver. Regional changes in ecosystem structure related tocirculation induced nutrient availability and temperaturechanges would also result in changes in export production.Specifically, the establishment of deep mixing in the SouthernOcean and formation of intermediate and deep water sourcesoriginating in the Southern Ocean at the EOT can resultin reorganization and redistribution of nutrient (and C) inven-tories and associated changes in export production may differspatially in the ocean.[27] It is important to note that the Pacific is the largest

ocean basin and the EEP in particular supports a significantproportion of export production (at present) and it is alsoresponsible for a large fraction of the CO2 efflux from theocean to the atmosphere [Chavez and Toggweiler, 1995;

Takahashi et al., 2009]; thus changes in export production inthis basin may have a proportionally larger global impact.Accordingly, any estimate of global C cycle responses andfeedbacks related to the EOT should take into account exportproduction changes in the Pacific basin.

4.1. The Late Eocene: Priming the System

[28] Export production at our site in the Late Eocene variedbetween 13 and 16 gCm-2 y-1, with an average of 14.4 gCm-2.These values are at the lower range of export production ob-served in the modern equatorial Pacific Ocean. One potentialexplanation for these lower values is that the upwelling andhigh productivity belt along the equator was shifted southwardof the equator at that time. This is consistent with the latitudinalgradient in BAR observed across the equator; BAR at SiteU1333 located at ~3�N is up to approximately threefold lowerthan at Site 1218, located at 2�N, and up to approximatelytenfold lower than at Site 574, located at paleo 1.3�S [Griffithet al., 2010; Moore and Kamikuri, 2012] (Figure 2).[29] BAR fluctuates between 0.5 and 1.5mg cm-2 kyr-1 in

the Late Eocene period until ~34.5Ma, with a tendency towardhigher BAR (increased export productivity) corresponding towarmer intervals as seen in the correlation with the oxygenisotope record (Figure 3). The oscillations in the BAR recordcorrespond to the long-term (405 kyr) eccentricity cycle,implying that climatic drivers are impacting export production(our sampling resolution from 40 to 35Ma is intermittently in-sufficient to capture higher-frequency cycles). In addition,long-term cycles at ~700 kyr are observed, though these arenot directly tied to a particular known orbital oscillation.[30] The pronounced rise in BAR (from 0.8 to 2.1mgcm-2

kyr-1) in the very latest Eocene, just before the onset ofEOT-1 (between 34.2 and 34Ma), suggests an increase in

Figure 5. (a) Orbital frequencies, (b) coherency, and (c) phase relationships for d18O and d13C from Site1218 [Coxall and Wilson, 2011] and BAR from Site U1333. Records were correlated and the Site U1333paleomagnetic age scale was used. The color relationships follow those in Figure 3.

ERHARDT ET AL.: E-O TRANSITION PRODUCTIVITY

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export production in the EEP. This increase corresponds to atime when the P/Ti and excess Ba records of the SouthernOcean (proxies of nutrient availability and export production,respectively) also show an increase [Latimer and Filippelli,2002; Anderson and Delaney, 2005]. These concurrentincreases in export productivity at both the Southern Oceanand the EEP suggest a common, possibly global, drivingmechanism such as pCO2 or global temperature changes.[31] While the existing pCO2 record is data limited, the rise

in pCO2 observed by Pearson et al. [2009] is roughly concur-rent with the peak in BAR (Figure 3). However, this pCO2

peak is not observed in the Pagani et al. [2011] record until af-ter EOT-1 a time when BAR drops. Since both pCO2 recordsare of relatively low resolution, it is difficult to directly linkthese smaller-scale shifts directly to productivity changes.Overall, however, the greater export production observed inthe Eocene compared to the Oligocene in both the EEP BARrecord and the Site 1090 Southern Ocean proxies [Andersonand Delaney, 2005] may relate to the overall higher pCO2

and higher temperatures during this interval. It has been sug-gested that high pCO2 could increase primary production[Doney et al., 2009] and impact ecosystem structure [Fabryet al., 2008; Hall-Spencer et al., 2008] and the warmertemperatures associated with this high pCO2 would enhancebacterial activity and degradation of organic matter to formmarine barite.[32] The late Eocene export productivity peak is not cap-

tured in the benthic foraminifera accumulation record (BFAR)at nearby Site 1218. This discrepancy may result from selec-tive preservation of foraminifera due to a shallow CCD withCaCO3 preservation of less than 75%, influencing the BFAR[Coxall and Wilson, 2011]. In addition, while dissolution hor-izons at Site 1218 impede a complete comparison, the peak inBAR (and potentially pCO2) does not correspond to any pro-nounced changes in the benthic foraminifera oxygen isotoperecord from Site 1218 or in the global benthic isotope compi-lation [Zachos et al., 2008] during this time (Figure 3). If any-thing, a small increase in d18O (cooling) is seen despite thehigh pCO2, which would presumably induce a warming effect.This may indicate lower sensitivity of ocean temperatures tochanging pCO2 in the Late Eocene prior to Antarctic glacia-tion compared to the present. The carbon isotopic record atSite 1218 changes toward more negative values during thisinterval, though the change d13C does not start until BARhas already peaked and the fate of the d13C record followingthe BAR peak is obscured by the dissolution horizon. Thisrelationship may indicate that BAR in the EEP (and theSouthern Ocean) is responding to more global changes in thecarbon system that impact the d13C of the global ocean suchas circulation and upwelling intensity changes [Scher andMartin, 2008; Thomas et al., 2008; Cramer et al., 2009;Pusz et al., 2011].[33] The Late Eocene peak in BAR, in conjunction with

proxies from the Southern Ocean, is indicative of an increasein export production before the onset of EOT-1 and mayhave served as a feedback mechanism, reducing pCO2 andthus contributing to the conditions that precipitated the onsetof the Oligocene glaciation. The high export production inboth the EEP and the Southern Ocean together may havebeen sufficient to sequester enough C via the biologicalpump to be one of the many factors influencing pCO2.An increase in BAR from 0.8 to 2.1mg cm-2 kyr-1

(corresponding to an increase in burial of 2.2 gC m-2 d-1)persisting over 200 kyrs in the equatorial Pacific highproductivity region (area of 1.1� 1013m2) could accountfor the sequestration of ~2200 gigatons of carbon.

4.2. The Eocene Oligocene Transition and EarlyOligocene Glaciation

[34] Marine barite accumulation rates rapidly decline at theEOT and follow trends seen in the oxygen isotope record thatis used to define EOT-1 and Oi-1. The two sharp declines inBAR are concurrent with the changes in the isotope andCCD records, indicating that BAR is responding directly orindirectly to the same climatic events that impacted the isotopeand CCD records. The decline in export productivity at theEOT may be related to climatically induced changes in circu-lation at the onset of the Oligocene. Neodymium and C isotoperecords from the Southern Ocean indicate that with the onsetof glaciation, deep mixing, and increased ventilation havedeveloped [van de Flierdt et al., 2004; Via and Thomas,2006; Scher and Martin, 2008; Thomas et al., 2008; Crameret al., 2009; Pusz et al., 2011]. These circulation changesmay have resulted in higher productivity and nutrient con-sumption in the Southern Ocean and low nutrient watersarriving to the EEP. In addition, the lower temperatures ofsubsurface waters that originated from the Southern Oceanmay have increased stratification and reduced upwelling ratesin the EEP, decreasing the delivery of nutrients to the surfacewaters and decreasing productivity [Miller et al., 2009].Changes in nutrient stoichiometry may have also occurred in-ducing ecosystem changes impacting export production withrelatively little change in net primary production [Boyd andNewton, 1999]. Finally, the lower temperatures at intermediatedepths, where much of the organic matter is regenerated, mayhave lowered bacterial induced organic matter degradationand barite formation. This could increase the fraction of or-ganic matter arriving at the sediment water interface.[35] Indeed, model results suggest that the uptake of nutrients

in southern latitudes will limit available nutrient transport north,impacting productivity and export production at the EEP[Sarmiento et al., 2004]. This process is consistent with the ob-served latitudinal gradient in BAR (decreasing toward thenorth) [Griffith et al., 2010]. Maximum BAR during EOT-1 is0.2mg cm-2 kyr-1 at Site 1209 (paleo 15�N), ~2.5mgcm-2

kyr-1 for Site U1333 (paleo 3�N), 3.5mgcm-2 kyr-1 for Site1218 (paleo 2�N), and 6.5mgcm-2 kyr-1 for Site 574 (Paleo1.3�S) and, based on excess Ba, could be up to ~5mgcm-2

kyr-1 in the Southern Ocean (Site 1090) [Anderson andDelaney, 2005].[36] In contrast to the decrease in export production

implied by BAR during this transition, BFAR increases bytwofold to threefold, suggesting an increase in the food sup-ply to the sediment water interface [Coxall and Wilson,2011]. However, as the BFAR prior to the EOT-1 is not re-liable, it is hard to draw any conclusions about direction ofchange compared to the Late Eocene (the reported BFAR in-crease is in comparison to baseline Oligocene values afterOi-1). Moreover, the seeming lack of correspondence intrends between the records (e.g., BAR and BFAR) may notactually represent a disagreement among these proxies, sincethe proxies respond to different parameters. While BFAR cor-responds to food or organic C input to the sediment waterinterface BAR represents the amount of C exported out of

ERHARDT ET AL.: E-O TRANSITION PRODUCTIVITY

8

the surface layer and oxidized in the “twilight” zone (between500 and 1500m depth). If bacterial activity, responsible for or-ganic matter degradation and barite formation in the water col-umn during particle sinking, is less effective (due to cooling),it is conceivable that more organic matter actually arrives atthe sediment water interface, resulting in a response of thebenthic foraminifera. Alternatively, it is possible that the poolof organic matter that accumulated in the sediment during thehigh export productivity period in the Late Eocene was ac-cessible to and supported the benthic foraminifera at least forsome time following the decrease in export. It has also beensuggested that the mode of transport of food to benthic fora-minifera and the degree of benthic-pelagic coupling haschanged at that time, complicating interpretation of the ob-served trends [Thomas and Gooday, 1996]. The low organicC accumulation rate in the sediment at that time is, however,consistent with low export production and low C burial[Pälike et al., 2009]. A general reduction in bulk sedimenta-tion rates [Schumacher and Lazarus, 2004; Vanden Bergand Jarrard, 2004] also supports the observation of littleto no increase in export production in the Equatorial andNorth Pacific across the EOT.[37] The low export production between 33.2Ma and

33.7Ma, spanning the period of cooling and glaciating, indi-cates that C sequestration by the oceanic biological pumpdid not contribute to lowering pCO2 at this time. This is con-sistent with the suggestion by Pagani et al. [2011] thatchanges in silicate weathering are required to explain the sus-tained reduction in pCO2 through the Oligocene.[38] After the EOT in the early Oligocene, low BAR

implies sustained reductions in export production comparedto the Eocene. Within this time period, the two glacial max-ima associated with Oi-1a and Oi-1b may be differentiatedand resolved in the BAR record, with Oi-1a representingthe lowest BAR recorded during this study interval. Whilethe variability increases after 33.2Ma, export productionremains low and does not return to the levels of the lateEocene. This change in the average rate of export productionbetween the Eocene and Oligocene is consistent with amajor long-lasting change in ocean biogeochemistry linkedto the climatically induced oceanographic changes. Thisincludes global temperature changes, changes in ocean cir-culation, and possibly changes in atmospheric greenhousegas concentrations as well.

4.3. Relationships Between BAR and Climate Records

[39] It is accepted that variations in the Earth’s orbit exerta profound influence on climate [Imbrie et al., 1992, 1993].These changes have been previously captured in records ofmagnetic susceptibility and dry bulk density [Zachos et al.,2001] and are clearly shown in oxygen isotopic records[Coxall et al., 2005]. The 405-kyr long-term eccentricitycycles predominate in carbon isotopic records due to thelong residence time of carbon in the oceans [Pälike et al.,2006] and commonly occur during times of unipolar glacia-tion in the pre-Pliocene [Woodruff and Savin, 1991; Flowerand Kennett, 1993]. The ~100 kyr cycles predominate dur-ing glacial periods in the Pleistocene, responding specifi-cally to Northern Hemisphere glaciation, and appear moresensitive to high-latitude variability [Ruddiman, 2006].These eccentricity cycles are known to modulate thehigher-frequency precessional cycles. While the effect of

these changes in solar insulation on climate should be rela-tively insignificant [Berger et al., 1992], nonlinear responsescan amplify the impact. These nonlinear responses can in-clude internal feedbacks to the carbon cycle, ice sheetgrowth, and circulation [Imbrie et al., 1993].[40] Obliquity cycles (41 kyr) are prominent in benthic d18O

records from the early Oligocene and early Miocene [Pisiaset al., 1985; Zachos et al., 1996], consistent with high-latitudecontrols on climate variability [Berger and Loutre, 1991; Paulet al., 2000]. These higher-resolution cycles, particularlyobliquity (41 kyr) and precession (20 kyr) cycles, also modu-late climate but are difficult to identify in paleoceanographicrecords without exceptionally high sedimentation rates andsampling frequencies not commonly found in pre-Pleistocenerecords. To determine the relationship between BAR and cli-mate forcing, we analyzed each data set for orbital frequencyand the relationships between BAR, magnetic susceptibility,bulk density, and time-matched oxygen and carbon isotopes(from Site 1218).[41] The 405 kyr eccentricity cycle is strongly observed in

the BAR record (both in the low-resolution 40–35Ma andhigher-resolution 35–32Ma parts of the record). This is con-sistent with the strength of the 405 kyr cycle observed throughthe Oligocene [Pälike et al., 2006]. The 126 and 96 kyrshorter-term eccentricity cycles are observed intermittentlyand are most clearly detected between 38.2 and 39.5Ma,35.7 and 37Ma, and 32 and 34.5Ma (Figure 4a). This strongorbital pacing indicates an overarching climatic control onthe BAR and thus export productivity. This is consistent withother records showing climatically driven BAR variability[Paytan et al., 1996; Paytan and Griffith, 2007] and the reli-ance of productivity on climate influenced circulation andnutrient supplies, water column temperature changes, andgreenhouse gas levels, though reliable records of the latterare not currently available to evaluate this. Finally, the vari-ability in the barite record (order of magnitude changes) isstronger and more prevalent than that observed in the mag-netic susceptibility and density records, which are bothcoarse proxies for carbonate weight percent. This indicatesthat barite is more sensitive than these records and is ableto record climatic changes at locations where CCD variationsare not observed.[42] Since the sampling frequency for magnetic susceptibil-

ity and density is high throughout the study interval, theserecords provide context for the BAR record, particularlybetween 34.5 and 40Ma where the BAR record is of lowerresolution. The dominant trends observed in the magneticsusceptibility and density records are the sharp changes atEOT-1 and Oi-1, along with the dramatic differences betweenthe Eocene and Oligocene sections (Figure 4). This is consis-tent with known changes in the CCD over this time period[Lyle et al., 2005]. The magnetic susceptibility and densityrecords also provide context for the high level of variabilityobserved in the BAR record during carbonate accumulationevent CAE-4. Swings in the BAR record may be driven bythe same forces controlling the oscillations in the carbonaterecord through this interval. This is illustrated by the high levelof coherence these records have during this time, particularlyin the 96 kyr, 126 kyr, and 405 kyr frequencies (Figure 4b).In addition, phase relationship analysis through this interval(Figure 4c) shows that these covarying changes between the

ERHARDT ET AL.: E-O TRANSITION PRODUCTIVITY

9

density and BAR records are dominantly in-phase, implying acommon driver.[43] This enhanced coherence also appears during the

remaining CAEs. All three records are coherent at a 96-kyrfrequency during CAE-6 and appear in phase, while onlythe magnetic susceptibility and BAR results are coherentduring CAE-5. While the sampling resolution throughoutthe record is high enough to observe the 405-kyr cycles,the higher sedimentation rates during these CAE eventsmay contribute to the identification of these 96- and 126-kyr cycles. Finally, all three records are strongly coherentacross the two major climatic steps, EOT-1 and Oi-1. Thisis likely driven by the magnitude of the shifts in each record,combined with the enhanced sampling frequency and highsedimentation rates as the record enters the Oligocene. Phaserelationships allow us to observe that the shifts in these datasets are contemporaneous, with BAR responding to the largeclimatic changes. The close relationship between BAR andcarbonate proxies suggest that when carbonate is highBAR is also high, consistent with increases in productivitynot only preservation or dilution impacts.[44] Comparison of the BAR record of Site U1333 to the

benthic foraminifera isotopic records of Site 1218 [Coxalland Wilson, 2011] provides more direct climate relation-ships during the time surrounding the EOT (Figure 5). Whilehigh-resolution relationships are more precarious whencomparing cores of different locations, significant efforts incross-correlation [Westerhold et al., 2012] provide confi-dence in these comparisons. Each of these records showsstrong cyclicity at the 405-kyr frequency, especially acrossthe EOT (Figure 5a). As a result, these records are coherentat the 405-kyr frequency throughout much of the highestresolution sampling interval between 33.4 and 34.5Ma(Figure 5b). We observe some evidence of an increase inthe shorter-term eccentricity cycles (96 and 126 kyr) directlyafter Oi-1. This could indicate a change in climate sensitiv-ity, though it may also reflect the higher sedimentation ratesand resulting sampling resolution across this interval. Coher-ency is not significant outside of these periods, implying thatsmaller-scale variations in BAR and oxygen are not in sync.[45] Since the BAR and d18O records show strong statisti-

cally significant coherency, power, and an in-phase relation-ship at Oi-1 (Figures 5b and 5c), the BAR is likely dominantlycontrolled by the glacial onset indicated by the d18O record.Specifically, BAR sharply declines as d18O increases at theonset of glaciation. The comparison with the d13C also showscoherence at this time interval. Coxall et al. [2005] show thatthe d13C record lags the d18O record at the Oi-1 step. Thereforeit is not surprising to see that the BAR, which is in phase withd18O, is also out of phase with the d13C record at the shorter-term, and more sensitive, eccentricity and obliquity cycles.This suggests that the sharp decline in BAR at Oi-1 iscontrolled by glaciation-induced circulation changes at thistime as recorded in the d18O record and not by global C cycleperturbations such as changes in vegetation, organic C burial,or atmospheric pCO2. Finally, the data gap in the 1218 recordpreceding EOT-1 makes it difficult to definitively determineany relationships between BAR and the isotopic tracers duringthis gap period.[46] To consider the temporal relations between export

production and climate on a global scale, we compared BARat Site U1333 to productivity records from Site 1218 in the

EEP, Site 744 in the Indian sector of the Southern Ocean,and Site 1090 in the Atlantic sector of the Southern Ocean(Figure 3).While preservation issues limit comparisons of Site1218 BFAR and Site 744 opal accumulation rates to the timeperiod after Oi-1, Site 1090 provides a robust record through-out the study interval. We observe that multiple proxies,including Ba excess, opal accumulation rates, reactive P, andP/Ti ratios all suggest a rise in export production just beforethe onset of EOT-1. A gradual increase in export productioninitiates at 34.7Ma, approximately 500 kyr before the rise inthe Site U1333 BAR record. The increase intensifies with asharp rise in the Southern Ocean export production proxiesat 34.3Ma, nearly contemporaneous with the BAR rise at34.2Ma and within the potential error of comparing twouncorrelated cores with different age models. This supportsthe conclusion of a global change in ocean chemistry poten-tially related to changes in pCO2, increases in temperature,and/or circulation changes. This would provide more nutrientsto these high productivity regions occurring in the latestEocene. Overall, we see that the BAR record of export produc-tion at this time interval is supported by trends in other oceanbasins and may correlate to global climatic cycles.

5. Conclusions

[47] Overall, we see that changes in export productionproxies relate to changes in both climate and the carbon cycleacross the study interval. The BAR record is responding tomany of the major climatic shifts, particularly EOT-1 andOi-1, that occur during this time period. We observe a strong405-kyr long-term eccentricity cycle in the BAR record andintermittently observe shorter-term 125- and 96-kyr eccentric-ity cycles. This suggests that direct climatically driven changessuch as temperature and pCO2 and indirect changes such ascirculation have impacted nutrient delivery and ecosystemstructure in the EEP. Specifically, in the latest Eocene priorto EOT-1 a large peak in BAR is observed, consistent withSouthern Ocean export production records. We believe thatmultiple occurrences of this export production increase pointtoward global forcing such as changes in temperature and/orpCO2. We speculate that this peak in export productivitymay have played a role in the greater carbon cycle throughcarbon sequestration, assisting in the drawdown of CO2

thought to prime the system for EOT-1.[48] When considering the larger record, we see evidence

that regional circulation changes may have impacted nutrientfluxes and the resulting export production. We observe lowerBAR in the more northern Site U1333 (3�N) than at Site 574south of the Equator (1.3�S) throughout the record, implyinga south to north transport and utilization of nutrients. Inaddition, Southern Ocean proxies show a larger and earlierincrease in export production than the EEP. We propose thatthe well-documented circulation changes occurring across thistime period [Scher and Martin, 2008; Thomas et al., 2008;Cramer et al., 2009; Pusz et al., 2011] may have impactedthe regional transport of nutrients and influenced exportproduction during the EOT.[49] Overall, a combination of changes in both global

climate and regional processes appear to impact exportproduction in the EEP. Future work providing better constrainton the global pCO2 record may allow us to discern causal rela-tionships with export production, while additional records of

ERHARDT ET AL.: E-O TRANSITION PRODUCTIVITY

10

productivity and circulation in the Pacific Oceanmay highlightinterplay between these variables. Export production in theEEP appears to be sensitive to major climatic changes andcontinues to be an important tracer of ocean dynamics, partic-ularly during periods of rapid climate change.

[50] Acknowledgments. We would like to thank Ted Moore forconstructive discussion of the results and insights into potential interpretations.Samples for this project were provided by the Integrated Ocean DrillingProgram (IODP). This work was significantly advanced though discussion atthe Leg 320/321 postcruise meeting attended with travel funding providedby the Consortium for Ocean Leadership. Laboratory work was supported byNSF CAREER grant OCE-0449732 to A.P.

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