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Quaternary Science Reviews 82 (2013) 133e144

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Quaternary Science Reviews

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A Pb isotope tracer of ocean-ice sheet interaction: the record from theNE Atlantic during the Last Glacial/Interglacial cycle

Kirsty C. Crocket a,*, Gavin L. Foster b,1, Derek Vance b,2, David A. Richards a,Martyn Tranter c

aBristol Isotope Group, School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, United KingdombBristol Isotope Group, Department of Earth Sciences, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, United KingdomcBristol Glaciology Centre, School of Geographical Sciences, University of Bristol, University Road, Bristol BS8 1SS, United Kingdom

a r t i c l e i n f o

Article history:Received 21 May 2013Received in revised form17 October 2013Accepted 17 October 2013Available online 9 November 2013

Keywords:Pb isotopesBritisheIrish Ice Sheet (BIIS)Last Glacial MaximumGlacial/interglacial cyclesChemical weatheringAuthigenic and pre-formed FeMnoxyhydroxides

* Corresponding author. Now at: BiogeochemistryAssociation for Marine Science, Scottish Marine Instand Bute, United Kingdom.

E-mail address: Kirsty.Crocket@sams.ac.uk (K.C. Cr1 Now at: Ocean & Earth Science, National Oceano

University of Southampton Waterfront Campus, SouKingdom.

2 Now at: Institute of Geochemistry and Petrology, DETH Zürich, NW D81.4, Clausiusstrasse 25, 8092 Züric

0277-3791/$ e see front matter � 2013 Natural Envirhttp://dx.doi.org/10.1016/j.quascirev.2013.10.020

a b s t r a c t

Ice sheet-ocean interactions are both a response to climate forcing and a source of climate feedback,releasing freshwater to the surface ocean and influencing climate and atmospheric CO2 throughchanges in ocean circulation. Documenting the outcomes of these interactions for recent glacial cyclesis important given current and future scenarios of polar ice retreat. However, this is currentlyhampered by lack of accurate constraints on ice sheet development and demise. Marine sedimentaryPb isotope records have potential to investigate these aspects of ice sheet feedbacks at high temporalresolution because of the sensitivity of the Pb isotope composition to continental weathering intensityand solute flux. Here we present a Pb isotope record sourced from the FeMn oxyhydroxide fraction inmarine sediments from ODP Site 980 on Feni Drift (2168 mbsl, Rockall Trough, NE Atlantic), spanningthe last 43 ka. The location of Site 980 at the northern edge of the BritisheIrish Ice Sheet (BIIS) makesit well-placed to monitor changes in BIIS development as it responded to migration of the Polar Frontduring the Last Glacial/Interglacial cycle. The data reveal millennial-scale cyclicity in Pb isotopecomposition, reminiscent of DansgaardeOeschger cycles, from the start of the record until Heinrichevent 2 (43e24 ka), characterised by extreme shifts to radiogenic compositions (i.e. variation in206Pb/204Pb from w18.9 to 20.5). The period 24e17.5 ka is also characterised by exceptionally radio-genic and highly variable Pb isotope compositions, associated with the rapid and repeated expansionand collapse of the BIIS. The presence of such radiogenic Pb isotope compositions during periods ofmaximum ice sheet activity support interpretation of the subglacial environment as an activeweathering environment, contributing to biogeochemical cycles through the transport vectors ofmeltwater release and debris-laden ice calving.

� 2013 Natural Environment Research Council. Published by Elsevier Ltd. All rights reserved.

1. Introduction

Ice sheet-ocean dynamics are an important internal feedback inthe climate system with the potential to generate abrupt climate

and Earth Science, Scottishitute, Oban PA37 1QA, Argyll

ocket).graphy Centre Southampton,thampton SO14 3ZH, United

epartment of Earth Sciences,h, Switzerland.

onment Research Council. Publish

change. A number of rapid climate events during the Last Glacialcycle have been attributed to substantial freshwater fluxes releasedby ice calving and meltwater events that impacted on ocean circu-lation (e.g. Clark et al., 2001; McManus et al., 2004; Kleiven et al.,2008). In this regard, the NW European Ice Sheets (NWEIS:BritisheIrish Ice Sheet, BIIS; Fennoscandian Ice Sheet, FIS; IcelandicIce Sheet, IIS; Fig. 1) occupy a critical position in the North Atlanticregion, acting as a sensitive monitor of changes in the Polar Front(Scourse et al., 2009) and both responding to and influencingMeridional Overturning Circulation (MOC) through storage andsupply of freshwater to the surrounding surface ocean (e.g. Hall et al.,2006; Peck et al., 2006; Haapaniemi et al., 2010). To gain furtherinsight on how ice sheets have responded to and, in turn, influencedclimate requires accurate time constraints on ice sheet development

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Fig. 1. The location of ODP Site 980 on Feni Drift, Rockall Trough, in the North East Atlantic, and the North West European Ice Sheets (NWEIS): BritisheIrish Ice Sheet (BIIS), IcelandIce Sheet (IIS), Fennoscandian Ice Sheet (FIS). The Greenland Ice Sheet (GIS) is labelled. The dotted line denotes the ice margin of the NWEIS during the LGM, based on estimatesfrom the literature (Svendsen et al., 2004; Sejrup et al., 2005; Bradwell et al., 2008; Scourse et al., 2009; Ballantyne, 2010; Bigg et al., 2010; Dunlop et al., 2010).

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and demise. These are proving difficult to establish, particularly assome sections of the NWEIS margin extended onto the continentalshelf, thus erasing terrestrial ice-sheet records (e.g. Sejrup et al.,2005; Bradwell et al., 2008; Dunlop et al., 2010). Integrated ap-proaches to establishing ice sheet histories that incorporate on- andoff-shore evidence are providing a more comprehensive picture ofBIIS evolution (e.g. Clark et al., 2012a; Clark et al., 2012b). Ascer-taining the ice sheet histories also remains an important goal fortesting the accuracy of ice sheet only models (e.g. Hubbard et al.,2009; Bigg et al., 2010) and intermediate complexity ocean-atmosphere-sea ice-land climate models (e.g. Fluckiger et al., 2006;Kageyama et al., 2010; Otto-Bliesner and Brady, 2010).

Palaeo-seawater Pb isotope records offer the possibility ofinvestigating both the timing and nature of ice-sheet ocean inter-action and the effects of weathering regime change on solute fluxassociated with glacial/interglacial climate (Foster and Vance,2006). Due to the overwhelming influence of anthropogenic Pb,knowledge about the processes that drive change in the Pb isotopecomposition of weathered products is based largely on laboratoryexperiments and soils (Erel et al., 1994; Harlavan et al., 1998;Harlavan and Erel, 2002). These have demonstrated an inverserelationship between the age of debris weathered and the Pbisotope composition released (Harlavan et al., 1998). Investigationof chemical weathering rates in catchments of variously ageddebris has also demonstrated that the highest solute fluxes areassociated with weathering of the youngest (glacial) debris, and arebest described by a power law relationship (Taylor and Blum,1995).

The link between palaeo-seawater Pb isotope compositions andclimate-mediated inputs of weathered and eroded continentalmaterial was first demonstrated by correlative changes in ferro-manganese (FeMn) crust Pb isotope records and the global marined18O record (Christensen et al., 1997). Recognition of the funda-mental effect on seawater Pb isotope composition from inputs ofglaciated landmasses (von Blanckenburg and Nagler, 2001) hassince been demonstrated in many locations and timescales. Forexample, the rise in radiogenic Pb isotope composition in FeMncrust records from the North Atlantic during the Quaternary(Burton et al., 1997; Reynolds et al., 1999; Foster and Vance, 2006),

and in the increased radiogenic Pb component from the authigenicFeMn oxyhydroxide fraction in marine sediments over the shortertimescale of the last deglaciation (Gutjahr et al., 2009; Kurzweilet al., 2010; Crocket et al., 2012).

Ice sheets also influence climate through their effect on themagnitude and composition of continent-ocean solute fluxes.Glacial comminution and selective mineral dissolution in poten-tially acidic and anoxic subglacial environments (Wadham et al.,2010) leads to solute profiles emerging from ice sheet marginsthat are compositionally distinct to equivalent, non-glacial profiles(Anderson et al., 1997; Raiswell et al., 2006; Statham et al., 2008;Föllmi et al., 2009). Intense iceberg calving and meltwater releasealong glacial margins results in efficient export to the surface oceanof solutes that would ordinarily be sequestered in estuarine andcontinental shelf environments during riverine transport (e.g.Turekian,1977), for example Fe (Raiswell et al., 2006; Statham et al.,2008) and Pb (Crocket et al., 2012).

Here we present a high resolution Pb isotope record extractedfrom the authigenic FeMn oxyhydroxide component of marinesediments from ODP Site 980 (Feni Drift, NE Atlantic) spanning thelast 43 ka. The results from similar studies in the NW Atlanticdemonstrated how the isotope composition of Pb in the labilefraction of marine sediments responded to increased ice calvingand meltwater during Heinrich events associated with the demiseof the Laurentide Ice Sheet (Gutjahr et al., 2009; Kurzweil et al.,2010; Crocket et al., 2012). In this study, we examine if this rela-tionship holds true at other locations, by looking at the response ofthe marine sedimentary Pb isotope composition in the FeMn oxy-hydroxide fraction to activity of the BIIS. On the basis of theseprevious studies and because of the proximal location of Site 980 tothe British Isles, we interpret the Pb isotope data as being pre-dominantly controlled by the influx of continental Pb supplied tothe NE Atlantic by interglacial riverine runoff and glacial/deglacialice calving and meltwater discharges from the BIIS. The location ofSite 980 makes it well placed to contribute to the ongoing debateabout the timing of BIIS activity, for which the advance and retreatfrom the Atlantic margin remains poorly constrained (Knutz et al.,2007; Chiverrell and Thomas, 2010; Clark et al., 2012a).

Fig. 2. The Pb isotope record of the FeMn oxyhydroxide fraction from Site 980: (a) theNGRIP d18O record (Andersen et al., 2004); (b) the benthic and planktonic d18O recordsfrom Site 980 (McManus et al., 1999); (c) the 206Pb/204Pb, data. The grey bars denotethe approximate timing of the Younger Dryas (YD) and Heinrich events 1e4 (H1eH4;Hemming, 2004). The Pb isotope errors (listed in Appendix Table A.1) are smaller thanthe actual data points.

Fig. 3. Superimposition of the 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of theFeMn oxyhydroxide fraction to highlight differences between them. Vertical alignmentbetween each ratio is arbitrarily based on the most radiogenic data point in all threeratios at 21.3 ka.

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2. Materials and methods

Sediments from ODP Site 980 (Feni Drift, Rockall Trough;55�290N, 14�420W, 2168 mbsl; Fig. 1) were analysed for the Pbisotope composition in the authigenic FeMn oxyhydroxide anddetrital fractions and bulk sediment. The samples span the last43 ka and a core depth of 0.05e8.13 m composite depth (mcd). ODPSite 980 offers an expanded, high resolution sedimentary record ofalternating nannofossil ooze and dark grey clay with nannofossils(Jansen et al., 1996). The sediments are well characterised and havepublished d18O and d13C records (McManus et al., 1999; Floweret al., 2000). The age model used in this study is based on theorbitally tuned benthic d18O chronology of McManus et al. (1999)that was graphically correlated to existing deep-sea chronologies(Martinson et al., 1987; Shackleton et al., 1990). The age of samplesin this study was determined by linear interpolation within thisd18O chronology, which has a resolution ofw1.2 ka. Given the shortresidence time of Pb in the deep ocean (e.g. w30 years in the deepNorth Atlantic; Henderson and Maier-Reimer, 2002), and thetemporal resolution (w450 years) and timescales in this study,changes in the Pb isotope record are therefore equated directly withthe timing of events delivering Pb to the ocean.

Pb was extracted from the hydrous authigenic FeMn oxy-hydroxide fraction of 102 marine sediment samples (average 70mgdried sediment per sample), of which 17 were analysed for the Srisotope composition, closely following the technique of Gutjahret al. (2007). The exact technique, including chromatographicseparation of Pb and Sr, and the isotopic measurements, aredescribed in detail elsewhere (Crocket et al., 2012). The residualdetrital fractionwas further processed for Pb (16 samples) and Sr (7samples) isotope compositions. After extraction of the FeMn oxy-hydroxide fraction, the sediments were reacted in 50mMHHþ 15%acetic acid for a further 24 h to remove all trace of FeMn oxy-hydroxides, and then in aqua regia to destroy organic matter. Afterreaction the aqua regia was dried down, and the remaining sedi-ment was digested in concentrated HFeHNO3 over a 3 day period inclosed Teflon vials. Bulk digestion of 16 samples was carried out forPb isotope composition only. The samples were first reacted with6 ml 1 MMgCl2 for 1.5 h, and thereafter the bulk digestion followedthe same protocol as the detrital digestion. The measured blankwas�0.09 ng for the FeMn oxyhydroxide fraction and 0.3 ng for thedetrital and bulk sediment fractions. In all cases, the Pb blankconstituted <1& of sample Pb mass, obviating blank correction tothe data. As a means of verifying potential artefacts associated witha FeMn oxyhydroxide matrix and to provide a measure of externalprecision, a 50 ng Pb aliquot of Nod-A1 was processed per set ofcolumn chemistry (i.e. 6 samples, 1 Nod-A1, 1 blank) and measuredin sequence with samples by MCeICP-MS.

3. Results

All Pb isotope ratios of the FeMn oxyhydroxide fraction fromODP Site 980 (Fig. 2, Tables A.1 and A.2) show exceptionally largeand frequent changes over the last 43 ka. Between 24 and 43 ka,radiogenic excursions (e.g. 206Pb/204Pb > 19.5) with a millennial-scale cyclicity are apparent, reminiscent of DansgaardeOeschger(DeO) cycles in the Greenland ice core d18O records, although thismillennial scale variation appears more subdued between 24 kaand 30 ka. The largest excursions to sustained radiogenic Pb valuesoccur between 17.6 ka and 24 ka, bracketed by Heinrich events (H)1 and 2. These are followed by a rapid decrease over 2 ka to lessradiogenic compositions during the deglaciation. The Holoceneshows the least variation, trending to a maximum value at 7.3 ka(19.16 206Pb/204Pb). The mid-Holocene shows a steady decline,punctuated by a small radiogenic excursion at 3.2 ka (19.32

206Pb/204Pb), followed by a drop to the least radiogenic Pb isotoperatios in the record between 2.7 ka and 1.2 ka (w18.92 206Pb/204Pb).The other Pb isotope ratios almost exactly track the changes in the206Pb/204Pb (Fig. 3).

Some of the samples between 17.5 ka and 24.0 ka containedexceptionally high Pb concentrations in the measured fractions(Fig. 4). For example at 18.4 ka, the FeMn oxyhydroxide fractioncontains 1708 mg/g of dried (bulk) sediment, and the detrital frac-tion contains 146 mg/g of dried sediment. Expected concentrationslie in the range ofw10e20 mg/g for the detrital fraction (Taylor andMcLennan, 1985), andw0.1e6 mg/g of dried sediment for the FeMnoxyhydroxide fraction (Gutjahr et al., 2007), which indeed isobserved in the majority of samples in this study. Examination ofthe anomalous samples by scanning electron microscopyconfirmed the presence of plumbiferous FeMn micronodules,which are assumed to be the source of the exceptionally high Pbconcentrations in the detrital fraction as a result of non-quantitative removal of the micronodules during the leachingstep. This radiogenic subset of samples demonstrates poor

Fig. 4. (a) The 206Pb/204Pb of the FeMn oxyhydroxide and detrital fractions, and bulksediments. (b) The Pb concentrations (mg/g bulk sediment) of the same fractions as inpanel (a). Appendix Tables A.1 and A.3.

Fig. 5. The 87Sr/86Sr vs. [Pb/Sr] of the FeMn oxyhydroxide and detrital fractions of theSite 980 sediments (concentrations are in g/g; Appendix Table A.6). The Site 980 FeMnoxyhydroxide and detrital fraction data are from this study, and the average marineFeMn crust data are from a compilation in Hein et al. (1999, and references therein; thegrey box represents 1 SD of the FeMn crust data). The marine FeMn crust data areplotted at the 87Sr/86Sr value of modern seawater (0.70917; Henderson et al., 1994). Forthe sake of clarity, the 980 FeMn oxyhydroxide data with exceptionally high [Pb/Sr]have been omitted (i.e. 3 values of 45, 46 and 506). Their range of 87Sr/86Sr is 0.7095e0.7111 (Appendix Table A.4).

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reproducibility in leaching replicates (Figs. A.1 and A.2), most likelyas a consequence of incomplete homogenisation of the FeMnmicronodules within the dried and ground sediment aliquot.

FeMn micronodules are normally interpreted as aggregatedauthigenic FeMn oxyhydroxides either directly precipitated fromseawater or formed through early diagenesis of authigenic FeMnoxyhydroxides at the sediment/seawater interface (Dymond et al.,1984). A hydrothermal source is also possible, but the exception-ally radiogenic Pb isotope compositions argue against such anorigin (see Mertz and Haase, 1997; for typical Pb isotope compo-sitions of high latitude Mid-Atlantic Ridge volcanic rocks). A fourthoption is precipitation of FeMn micronodules at redox boundariesin the subglacial environment, and subsequent transport by icerafting or meltwater. The correspondingly radiogenic Pb isotopecompositions support this option, explored in more detail in Sec-tion 4.1.3.

The 87Sr/86Sr ratios of the FeMn oxyhydroxide fraction (Fig. 5discussed in Section 4.1.1, Table A.4) are generally more radio-genic than seawater 87Sr/86Sr (0.70917; Henderson et al., 1994),varying between 0.70888 and 0.71214, whereas the detrital fractionhas a mean value of w0.725 � 0.002. Among the Sr samples, twofrom the Holocene show anomalously high Sr concentration in theFeMn oxyhydroxide fraction (e.g. 158 mg/g and 94 mg/g) comparedto an average Sr concentration of the remaining samples(3.4 � 5.3 mg/g 2 SE; n ¼ 31).

4. Discussion

4.1. Significance of the FeMn oxyhydroxide Pb isotope record andimplications for its interpretation

Validation of Pb isotope records by comparison to modern Pbisotope compositions is not possible because anthropogenic Pbcontamination has obscured natural seawater Pb isotope compo-sitions (Schaule and Patterson, 1981). Although no other

palaeoseawater Pb isotope records of suitable temporal resolutionhave been published for this region, surface scrapings of hydroge-nous FeMn crusts in the NE Atlantic, thought to closely approximatethe pre-industrial Pb isotopic composition of deep water, have206Pb/204Pb ratios of w19.1 (Henderson and Maier-Reimer, 2002),which are very similar to the Late Holocene values we present here(Fig. 2). A full discussion of more indirect ways to assess the fidelityand significance of FeMn oxyhydroxide Pb isotope records has beenrecently published in Crocket et al. (2012). Here we provide asummary of that discussion with specific reference to the Feni Driftrecord.

Potential extraneous influences on the Pb isotope compositionsof marine authigenic FeMn oxyhydroxide include: (i) post-burial Pbcontributions from the detrital and/or other non-authigenic frac-tions in the sediments, (ii) temporal variations in geographicalsource region, and (iii) a terrigenous (“pre-formed”) as opposed tomarine origin of FeMn oxyhydroxides. Temporal variations incontinent-ocean transport efficiency of this “pre-formed” compo-nent may also influence the record. These are discussed in turnbelow.

4.1.1. Detrital contaminationThe concentrations of Pb vs. FeþMn for the FeMn oxyhydroxide

fraction of Site 980 samples are plotted in Fig. 6, with comparison tothe trends observed in bulk samples of FeMn crusts A1 and P1(Axelsson et al., 2002) and a Central Pacific crust (Bau et al., 1996;Abouchami et al., 1997). The trend defined by the detrital fractionmeasured in this study is also shown (line and shaded area). Themajority of leached data plot on or above the trends defined byFeMn crusts, with little deviation towards the detrital trend. Amongthe leached data, those samples containing FeMn micronoduleshave significantly elevated Pb compared to the detrital fraction,although exhibiting [Fe þ Mn] <1500 similar to the other samples.Regardless, it is clear that all these FeMn oxyhydroxide data have[Pb]/[Fe þ Mn] ratios that are consistent with derivation from auniquely marine FeMn oxyhydroxide precipitate, with no signifi-cant indication of detrital contamination.

A second test of potential detrital contamination uses the87Sr/86Sr composition of the FeMn oxyhydroxide fraction. This has

Fig. 6. The concentrations of [Pb] and combined [Fe þ Mn] from the FeMn oxy-hydroxide fraction of Feni Drift sediments in mg leached Pb or Fe þ Mn per gram ofdried sediment (this study). The lines denote the [Pb]/[Fe þ Mn] ratios of bulk FeMncrust samples from the North Atlantic (Nod-A1), Pacific (Nod-P1), the Central Pacific(Bau et al., 1996; Abouchami et al., 1997; Axelsson et al., 2002), and the detrital fractionof the Feni Drift sediments (see Appendix Table A.3). The grey field on the detritalfraction represents the 2SD envelope. For the sake of clarity, the FeMn oxyhydroxidedata with exceptionally high Pb concentrations have been omitted (i.e. w1800 mg/g).Their range of [Fe þ Mn] does not exceed 1500 mg/g (Appendix Table A.5).

Fig. 7. The compositional distribution (206Pb/204Pb vs. 3Nd) of marine sediments in theNorth Atlantic: the carbonate-free, <63 mm fraction of bulk marine sediments (Bensonet al., 2003; Farmer et al., 2003), and the carbonate-free, clay-sized (<2 mm) fraction(Fagel et al., 2004). The FeMn oxyhydroxide fraction in sediments from Site 980 arerepresented by open circles: Pb isotope data from this study, Nd isotope data fromCrocket et al. (2011). The core site in Benson et al. (2003) is HU87-9 in the NE LabradorSea (62.5�N, 59.4�W; 1447 m water depth), and in Fagel et al. (2004) is MD99-2227 onthe SW Greenland Rise (58.2�N, 48.4�W, 3460 m water depth). The study by Farmeret al. (2003) features 16 core locations across the North Atlantic and Greenalnd-Iceland-Norwegian Seas, from Hudson Bay to the Barents Sea.

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often been used to evaluate the marine origin of the FeMn oxy-hydroxide fraction through comparison to the seawater 87Sr/86Srcomposition (e.g. Rutberg et al., 2000), which is essentiallyinvariant over the timescales of this study (0.70917; Hendersonet al., 1994). The 87Sr/86Sr in the FeMn oxyhydroxide fraction hereare generally more radiogenic than seawater, with a maximumdifference to seawater 87Sr/86Sr of þ0.00297 (Fig. 5; Table A.4), butdo not support leaching of Sr from the more radiogenic detritalfraction (mean value of w0.725 87Sr/86Sr). Mass balance calcula-tions based on Sr data are used to determine the amount of detritalcontamination of the FeMn oxyhydroxide fraction, as detailed inGutjahr et al. (2007). Similar to that study we find relatively minoramounts of detrital Pb are being leached by our protocol, of theorder of 0.5& (Fig. A.3, Table A.7). Given the detrital Pb isotopiccomposition, this level of detrital contamination is not significant.

4.1.2. Variation in the Pb isotope record: geographical vs.weathering intensity

Early work on palaeoceanographic Pb isotope records postu-lated changes in geographical source region as the cause of varia-tion in authigenic Pb isotope composition (see Frank, 2002; for areview), however this cannot be the case for the Pb isotope data inthis study for two good reasons. Firstly, none of the surroundingcontinental landmasses have bulk rock Pb isotope compositionsthat approximate those in the FeMn oxyhydroxide fraction (Bensonet al., 2003; Farmer et al., 2003; Fagel et al., 2004). For example,Fig. 7 shows the 206Pb/204Pb vs. 3Nd of the silt and clay sized frac-tions of carbonate-free, bulk marine sediments, which representthe compositional range of potential sediment sources in the NorthAtlantic region. Although the corresponding Nd isotope data fromthe leached fraction of Site 980 sediments (Crocket et al., 2011) fallwithin the range of North Atlantic marine sediments, much of thePb isotope data are largely too radiogenic to be sourced from anycircum-North Atlantic source region. Secondly, when plotted inPbePb space the data show a trend consistent with one dominantsource (Fig. A.4). If these data represented multiple sources actingat different times, the trend would be expected to show morevariability.

Rather than variations in geographic source region, the increasein radiogenic Pb in the glacial North Atlantic reflects a fundamentalchange in theway the continents are weathered (von Blanckenburgand Nagler, 2001). Glaciation of old cratonic landmasses encour-ages incongruent weathering and the preferential release ofradiogenic Pb from accessory phases during the early stages ofchemical weathering (e.g. Harlavan et al., 1998). Subsequentoceanic Pb isotope records have been interpreted within thisframework and used to understand variability in this process as afunction of regolith age, with implications for associated variabilityin cation flux (e.g. Taylor and Blum, 1995), as a function of ice sheetcover and other climatic variables of Quaternary glaciations (Fosterand Vance, 2006; Gutjahr et al., 2009; Kurzweil et al., 2010; Crocketet al., 2012).

4.1.3. Variation in the Pb isotope record: a marine vs. “pre-formed”origin of FeMn oxyhydroxides

The discussion thus far has established that (i) the dominantprocess controlling the Pb isotope composition in the FeMn oxy-hydroxide fraction is incongruent release of Pb during continentalweathering, and (ii) the Pb isotope composition of the FeMn oxy-hydroxide fraction is not significantly influenced by post-depositional leaching of the detrital fraction. However, the originof the FeMn oxyhydroxide fraction itself requires clarification. Theubiquitous nature of FeMn oxyhydroxides in low temperaturecontinental and marine environments raises the distinct possibilitythat FeMn oxyhydroxides in marine sediments may not representpurely marine precipitates. Given the close proximity to the BIIS,FeMn oxyhydroxides may have been transported by ice and melt-water to Site 980 and therefore some proportion of the marinesedimentary FeMn oxyhydroxides may carry a terrigenous or “pre-formed” Pb isotope signature.

Following the same line of argument presented in Crocket et al.(2012), if the FeMn oxyhydroxides were sourced from a terrestrialenvironment, the 87Sr/86Sr should reflect that of the weatheringfluids from which they precipitated. In glacial weathering envi-ronments, this is likely to be radiogenic due to preferential releaseof radiogenic Sr from, for example, biotite interlayers (Blum and

Fig. 8. Occurrence of IRD in the Rockall Trough during MIS 2 and 3: (a) the NGRIP d18Orecord in & (Andersen et al., 2004); (b) the FeMn oxyhydroxide 206Pb/204Pb recordfrom Site 980 (this study); (c) the record of IRD (>250 mm grains g�1) from core MD95-2006 on Barra Fan (Knutz et al., 2001; Dickson et al., 2008; Peters et al., 2008); (d) therecord of BIIS IRD per gram of sediment from core MD01-2461 in the Porcupine Bight(Peck et al., 2007); (e) the record of % IRD from Site 980 (McManus et al., 1999). Thegrey bars denote the approximate timing of the Younger Dryas (YD) and Heinrichevents 1e4 (H1eH4; Hemming, 2004).

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Erel, 1997), a phenomenon that has been observed both in the field(Hagedorn and Hasholt, 2004) and in the laboratory (Erel et al.,2004). Fig. 5 shows comparison of the 87Sr/86Sr of the leacheddata to the [Pb/Sr] of both the detrital fraction and FeMn crusts,which represent a purely marine precipitate. In the case of detritalcontamination, the leached FeMn oxyhydroxide data would beexpected to form a mixing trend between the FeMn crust anddetrital compositions. This is not apparent, as has been demon-strated in Section 4.1.1.

However, although the majority of the FeMn oxyhydroxide dataplot within the zone defined by average FeMn crusts, the leachateswith the most radiogenic 87Sr/86Sr compositions also have thehighest [Pb/Sr] ratios and define a trend away from the FeMn crusts,with no relation to the detrital fraction. This is better explained bymixing between a predominantly marine component and an end-member with radiogenic Sr and high [Pb/Sr]. A reasonable expla-nation for this is that the end-member represents terrestrialweathering fluids, whose composition was captured during pre-cipitation of FeMn oxyhydroxides in a terrestrial environmentbefore transportation to the ocean. The trends observed here, tohigher [Pb/Sr] with increasingly radiogenic 87Sr/86Sr, are alsoevident in a similar study from the NW Atlantic (Crocket et al.,2012). As yet, it is not clear why pre-formed FeMn oxyhydroxidesshould have high [Pb/Sr].

This explanation also provides insights into the origin of thosesamples with the highest [Pb/Sr] (i.e. a range of 45e506 [Pb/Sr])and the most radiogenic Pb (20.5e21.1 206Pb/204Pb), i.e. those fromthe sediment horizon containing FeMn micronodules. If themicronodules represent the composition of the terrestrial weath-ering fluid end-member, then their dissolution during leachingwould push the leachates towards these extremes in Pb isotopecomposition and [Pb/Sr].

4.1.4. Continent-ocean transport of PbThe extent towhich the continental Pb isotope signal is captured

by the marine sedimentary archive depends on the continent-ocean transport efficiency of Pb, where variation in this efficiencymay result in disparate Pb isotope signals preserved in marinesediments corresponding to different climate and geographicregimes.

Whether under glacial or interglacial climate conditions, themost likely carriers of Pb to the oceans are FeMn oxyhydroxides,with which Pb has a high propensity to coprecipitate (Erel et al.,1991; Erel and Morgan, 1992; Tessier et al., 1996; Taillefert et al.,2000). In fact, Erel and Morgan (1992) demonstrated highlycorrelated, but low, concentrations of Fe and Pb in unpollutedfreshwaters. Under interglacial conditions, riverine delivery favoursPb sequestration in estuaries and ocean margins with only a smallfraction reaching the ocean (Turekian, 1977). Removal occursmostly through preferential complexing of dissolved Pb by Fe inestuarine systems in the low salinity mixing zone (Wen et al., 1999;Waeles et al., 2008; Wen et al., 2008), and the near quantitativeremoval of Fe (and hence Pb) from solution during estuarinemixingthrough flocculation and deposition (e.g. Boyle et al., 1977;Sholkovitz, 1978; Wu and Luther, 1996). Further removal of dis-solved Pb occurs in coastal oceans as a consequence of the highparticle flux and the particle reactive nature of dissolved Pb(Biscaye et al., 1988; Nozaki et al., 1997).

By comparison to riverine delivery, the transport of FeMnoxyhydroxide phases by ice rafting and meltwater discharge mayresult in a higher Pb flux to the open ocean as a result of cir-cumventing the estuarine barrier, although this has yet to bedemonstrated. Whether Pb is then released into the surface oceanor transported to the seafloor depends on the solubility of IRD-bound FeMn oxyhydroxides. Some degree of (or total) FeMn

oxyhydroxide solubilisation is likely given the reactive nature ofthese nanoparticulate phases in the surface ocean (Sherman,2005; Schroth et al., 2009; Raiswell, 2011; Smith, 2011), and theimportance of Fe as a bioactive trace metal. This statement issupported by observations of elevated dissolved Fe concentrations(Smith et al., 2007) and primary productivity (Smith et al., 2007;Schwarz and Schodlok, 2009; Geibert et al., 2010; Cefarelli et al.,2011) in the immediate vicinity of drifting icebergs in the Wed-dell Sea. As discussed in the previous section, comparison to the87Sr/86Sr and [Pb/Sr] of FeMn crusts, a purely marine precipitate,demonstrates that the chemical composition of most of theleachates is predominantly marine in origin. However, thoseleachates with exceptionally high [Pb/Sr] and radiogenic Sr (andPb) isotope compositions most likely represent a mix betweenmarine and terrigenous FeMn oxyhydroxides.

On this basis, we associate increases in the radiogenic Pb isotopecomposition and, to a lesser extent, the Pb concentration as timeswhen the ice sheet was active at the marine margin, directingsubglacially weathered products into the open Northeast Atlanticby eithermeltwater or ice-rafting. Equally, times of unradiogenic Pbare associated with periods when the ice margin was inactive orhad receded from the marine margin, shutting off the source ofradiogenic Pb to the open ocean. Comparison between the Pbisotope record and neighbouring IRD records (Fig. 8) shows goodcorrelation, although it should be noted thatmarine Pb isotope datamay not be recording the same processes as IRD counts, since

K.C. Crocket et al. / Quaternary Science Reviews 82 (2013) 133e144 139

meltwater events are also likely to play a role and are not neces-sarily represented by IRD records.

4.2. Interpretation of the Pb isotope record from ODP Site 980

4.2.1. The HoloceneThe Holocene records for each Pb isotope ratio (206Pb/204Pb,

207Pb/204Pb and 208Pb/204Pb; Fig. 9) are very similar, recording agradual rise in the radiogenic content until the Holocene ClimaticOptimum (HCO) atw7.5 ka, followed by an initially slow then rapiddecline. These trends are best explained in terms of regionalclimate warming in the NE Atlantic (Seppa et al., 2005), promotingchemical weathering of freshly exposed glacial debris (Erel et al.,1994; Taylor and Blum, 1995; Harlavan et al., 1998; Harlavan andErel, 2002). Given the association between elevated radiogenic Pbrelease and high cation fluxes from young soils (Taylor and Blum,1995), the rise in radiogenic Pb in the early Holocene also pointsto an increase in the alkalinity and solute fluxes to the ocean.

The gradual cooling in the NE Atlantic region after the HCO,estimated to have started at 6.5 ka and progressed with increasingseverity (Oppo et al., 2003), is reflected in increased sea-salt Na fluxin the GISP2 record (O’Brien et al., 1995) and % hematite stainedquartz in the neighbouring core VM23-81 to Site 980 (Bond et al.,2001). The Pb isotope ratios also show a gradual decline, moststrongly pronounced in 206Pb/204Pb, starting at w7 ka until 3.4 ka.The solitary radiogenic sample at 3.2 ka may be related to a severeclimate cooling event centred on 2.7 ka (Hall et al., 2004). In anycase, the Pb isotope composition of this data point is different to thetrend described by the rest of the Holocene Pb data (Fig. 10), andsuggests a change in process or source.

The Holocene Pb isotope compositions in PbePb space (Fig. 10)show a temporal trend consistent with laboratory leaching exper-iments carried out on pulverised granite to mimic natural chemicalweathering (Harlavan and Erel, 2002), where the end-point Pbisotope ratio approaches whole rock compositions. The decrease atw2.5 ka to the most unradiogenic composition in the record (18.9206Pb/204Pb) is rapid and large, and could be due to anthropogenicPb contamination, particularly as the youngest sample is located atan actual core depth of 5 cm. Assuming an anthropogenic

Fig. 9. The Feni Drift Pb isotope records during the Holocene: (a) the NGRIP d18O record (Anchange (Seppa et al., 2005); (c) the 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of the FeMof the cold periods inferred from IRD peaks is indicated by the shaded bars (Bond et al., 19

206Pb/204Pb composition of 18.4 from early Pb ore smelting activ-ities in the British Isles starting at w1000 years BC (Le Roux et al.,2004), a w30% anthropogenic contribution to the natural back-ground Pb flux (w19.1 206Pb/204Pb; Fig. 10) would be sufficient togenerate the observed decrease to 18.9 (206Pb/204Pb). The timing ofearly anthropogenic Pb pollution, however, is poorly constrained inour record since downward migration of anthropogenic Pb due tobioturbation and bio-irrigation cannot be ruled out.

In summary, these Holocene Pb data suggest that, in the absenceof marine-marginal ice sheets (excluding the 3.2 ka data point), Pbisotope compositions at Site 980 are clearly responding to co-related changes in climate, chemical weathering flux and runoffthat have occurred in UK and Ireland over the last 12 ka or so.

4.2.2. The BritisheIrish Ice Sheet (BIIS)The Feni Drift glacial Pb data are very different to the Holocene

data, being characterised by extreme radiogenic Pb isotope com-positions and very large, rapid fluctuations, particularly during theperiod 24e18 ka (Figs. 2 and 3). The nature of the record suggests aradically different flux and/or source of Pb to Site 980 during thiscold interval. The short residence time of Pb in seawater, estimatedat w30 years in the deep North Atlantic (Henderson and Maier-Reimer, 2002) and w3 years in the surface ocean (Weinstein andMoran, 2005), means the most likely sources of Pb are proximallandmasses. For Site 980 this is the British Isles and, for the glacialperiod in particular, the NW sector of the BIIS (Fig. 1).

The BIIS was a highly mobile, metastable ice sheet, located at thesouth-westerly extreme of glaciated Europe and therefore sensitiveto changes in the position of the polar front and associated heat andmoisture transport by the North Atlantic Current (Knutz et al.,2002; Rasmussen and Thomsen, 2008; Scourse et al., 2009). Thehistory of BIIS during marine isotope stages (MIS) 2 and 3 is char-acterised by several cycles of advance and retreat of the ice margin,at times to the Atlantic continental shelf edge, with intense icestreaming as the ice sheet expanded over marine shelf sediments(Knutz et al., 2001; McCabe et al., 2005; Sejrup et al., 2005; Knutzet al., 2007). Placing temporal and spatial constraints on thewaxing and waning of the BIIS margin has proven difficult (Sejrupet al., 2005; Bradwell et al., 2008; Dunlop et al., 2010) and resulted

dersen et al., 2004); (b) the pollen-based reconstruction of Scandinavian temperaturen oxyhydroxide fraction from ODP Site 980 (this study; Appendix Table A.1). The timing97). See text for discussion. HCO refers to the Holocene Climatic Optimum.

Fig. 10. (a) Plot of the 208Pb/207Pb vs. 206Pb/207Pb isotope ratios over the interval 0 to12 ka. (b) The 208Pb/207Pb and 206Pb/207Pb compositions of anthropogenic Pb preservedin the Lindow peat bog (Manchester, UK; Le Roux et al., 2004) over the period w1e3 ka BP, and how a w30% contribution could account for the decrease in radiogenic Pbobserved in the youngest samples from Site 980.

K.C. Crocket et al. / Quaternary Science Reviews 82 (2013) 133e144140

in contradictory reconstructions (e.g. Bowen et al., 2002; Bradwellet al., 2008). However, progress has been made on the maximumareal extent by, for example, mapping of submarine terminal mo-raines along the Atlantic shelf margin (Bradwell et al., 2008), and onthe timing of ice calving from chronologically well-constrainedmarine records of IRD (e.g. Scourse et al., 2009, and referencestherein).

A number of different ice streamswere present along the NWEISmargin (Sejrup et al., 2005; Bradwell et al., 2008; Bigg et al., 2010).Of predominant interest to this study, due to its proximity to Site980, is the Barra ice stream emanating from ice drainage catch-ments between Northern Ireland (Dunlop et al., 2010; Cofaigh et al.,2012) and the Outer Hebrides (Howe et al., 2012) that depositedsediment in the very large Donegal and Barra Fan (Fig. 1). Debrisprovenance studies conducted on sediments from the northernRockall Trough confirm the BIIS as the dominant source of debris tothe area (Knutz et al., 2007; Peck et al., 2007; Scourse et al., 2009;Hibbert et al., 2010), and in particular the Barra ice Stream isidentified as the most likely source of debris to Site 980 (Knutzet al., 2001; Scourse et al., 2009).

We present the glacial Pb isotope record from Site 980 in thefollowing section. Rather than covering the history of the BIIS indetail, which has been adeptly summarised from a marineperspective by Scourse et al. (2009) and is beyond the scope of this

paper, we highlight agreements and discrepancies between inter-pretation of the Pb isotope data and the current knowledge of theBIIS history.

4.2.3. The glacial Pb isotope recordThe Pb isotope data show millennial-scale cyclicity from the

start of our record at 43 ka until 24 ka, demonstrating that the BIISunderwent intermittent surging or pulsing with ice streamsreaching the marine margin during this period. This millennial-scale cyclicity is similar to that observed in IRD records from theRockall Trough (Knutz et al., 2001;Wilson and Austin, 2002), whichhave been linked to the DO cyclicity in the Greenland ice core re-cords due to the strong response of the BIIS to the varying positionof the Polar Front (Scourse et al., 2009). Closer scrutiny of themismatches in timing between the radiogenic peaks and the DOcyclicity in the NGRIP d18O record (Fig. 2) is not possible due tolimited accuracy of our age model.

At 24e23 ka the Pb record shows a rapid increase in radiogeniccomposition, from 19.0 to 20.9 (206Pb/204Pb), suggesting a verysudden increase in delivery of glacigenic debris and meltwater tothe open ocean. This agrees with all the circum-BIIS IRD recordsexamined by Scourse et al. (2009), that themaximum activity at themarine margin took place at w24 ka, coincident with H2 (Fig. 8).This rise in radiogenic Pb at 24 ka also heralds the start of a 6 kainterval of sustained and intense ice streaming activity from theNW sector of the BIIS, reflected in the Pb record by rapidly varyingand highly radiogenic compositions. Interpretation of the IRD re-cords indicate rapid ice recession after H2, with a further phase ofreadvance at w22 ka (Scourse et al., 2009), and the main phase ofdeglaciation starting at w21.5 to 19 ka (Knutz et al., 2007). The Pbisotope record reflects this history with the largest pulse of radio-genic Pb during this period starting at 19 ka and lasting forw1.5 ka,with 206Pb/204Pb ratios in excess of 21 at w18 ka(207Pb/204Pb> 15.8, 208Pb/204Pb> 40.0). To generate such sustainedradiogenic compositions, within the limitations of the temporalresolution, requires high rates of ice streaming and meltwaterinput. Without these, the short residence time of Pb in the oceanwould result in rapid decay of the radiogenic Pb signal.

Between 17.6 ka and 16 ka, the Pb isotope composition shows aprecipitous drop from values of w21 to w19 (206Pb/204Pb), indi-cating the end of ice calving and meltwater delivery from icestreams along the NWAtlantic margin of the BIIS. The drop in BIIS-sourced IRD at Porcupine Bight occurs at largely the same time, at18e17 ka (Peck et al., 2007). This also signals the retreat of the icesheet margin above sea level (Clark et al., 2012a), thus ending de-livery of glacial debris and meltwater carrying radiogenic Pb to theocean.

Following the major phase of BIIS decay, the deglacial transitionperiod (w17 until the end of the YD) sees minor fluctuations in thePb isotope signal (i.e. 19.03 to 19.15 206Pb/204Pb). In particular, theonset of cool climatic conditions at the start of the YD is not asso-ciated with a substantial increase in radiogenic Pb. At 13.3 ka, aminimum 206Pb/204Pb value of 19.02 is similar to values at the endof late Holocene (notwithstanding the potentially anthropogeniccontamination of the youngest samples), followed by a rise to 19.13betweenw13.0 and 12.5 ka, returning to 19.04 by 12.1 kawith littlechange thereafter. The small rise in radiogenic Pb at this time, bycomparison to the glacial radiogenic excesses, suggests very littleproximal ocean-ice sheet activity (Ballantyne and Stone, 2012), andperhaps a more distal source of Pb altogether. Regional IRD recordsin the Rockall Trough and neighbouring Porcupine Bight present adifferent story, with episodic increases in terrigenous detrital ma-terial during this interval (Fig. 8; e.g. McManus et al., 1999; Knutzet al., 2007; Peck et al., 2007; Small et al., 2013b). Detailed exami-nation of the IRD, based on UePb geochronology of detrital zircons

Fig. 11. Comparison of FeMn oxyhydroxide Pb isotope records from Site 980 (Feni Drift,NE Atlantic) and U1302/3 (Orphan Knoll, NW Atlantic; Crocket et al., 2012), with theNGRIP d18O record (Andersen et al., 2004).

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and rutile, focuses on a Laurentide and/or Greenland Ice Sheetsource rather than a BIIS source (Small et al., 2013b), with export tothe NE Atlantic during intervals of lowered sea surface tempera-tures (Small et al., 2013a).

The Pb isotope data from Feni Drift and Orphan Knoll (a similarstudy using the same leachable sediment fraction; Crocket et al.,2012; Fig. 11) show comparable magnitudes of radiogenic Pbisotope excursions (i.e. w21 206Pb/204Pb) but there is clearly a dif-ference between the two sites with regard to the timing and natureof the radiogenic excursions. Whereas the baseline in the Feni Drift

Fig. 12. The modelled BIIS data from Hubbard et al. (2009) compared to the Site 980 Pb(Hubbard et al., 2009); (b) the Pb isotope record from Site 980 (this study); (c) changes in

record is fairly typical of the NE Atlantic (w19.0 206Pb/204Pb), the Pbisotope composition in the Orphan Knoll record shows a broad risefromminimum values at the Last Glacial Maximum to a peak valueduring the Holocene Climatic Optimum. It is likely that these dif-ferences are driven by the same processes of ice sheet disintegra-tion but each record is a response to very different ice sheetbehaviours (e.g. Dyke et al., 2002; Scourse et al., 2009). The OrphanKnoll record reflects behaviour of the much larger Laurentide IceSheet, the main source of HEs (Hemming, 2004). By comparison,the more mobile and smaller BIIS had its main period of growthimmediately prior to its main phase of demise (Fig. 12).

4.2.4. Comparison to the BritIce modelA recent model of the BIIS, the BritIce model (Hubbard et al.,

2009), represents much of what is known about BIIS evolutionover the last 40 ka, both on land and at the marine margin. Themodel is forced by inputs from the NGRIP d18O record and con-strained by current knowledge on BIIS chronology and the relativesea level record (Hubbard et al., 2009). Although we compare thePb isotope data to the BritIcemodel, the comparison is not expectedto be exact (Fig. 12) because the Pb isotope record reflects inputsdominated by the Barra ice stream, whereas the BritIce model re-flects the activity of the whole ice sheet. In addition, the BritIcemodel is subject to limitations inherent to model design. Despitethis, the similarities between the Pb isotope and model records arestriking, and lend confidence to our interpretation of the glacial Pbisotope record as predominantly reflecting glacially weathered in-puts from the BIIS (Fig. 12).

Particular features of note are that the modelled data show aperiodicity in meltwater events that is similar to the millennial-scale cyclicity in the Pb isotope record. These are not, however,exactly matched to the timing of meltwater discharges. Asmentioned above, this discrepancy could be due to the Site 980 agemodel or the resolution of the Pb isotope data. The Pb and themodelled meltwater data also show excellent correlation during

isotope record (206Pb/204Pb): (a) accumulation, area and volume changes of the BIIScalving and melt of the BIIS (Hubbard et al., 2009).

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the period 24 to 17 ka. The model suggests that increases in melt-water start atw24 ka and end atw17.3 to 17.0 ka, representing thestart and end of the major phase of BIIS activity, and coincide withthe rapid rise and abrupt decrease in the radiogenic Pb componentat 24 ka and 17.6 ka respectively. The Pb isotope data show lesssimilarity to the modelled ice calving data, perhaps suggestingmeltwater is a more important vector for FeMn oxyhydroxides thanice rafting (Crocket et al., 2012).

4.3. Palaeoceanographic and palaeoclimatic implications

This study has shown that ice marginal glaciation and deglaci-ation increase the radiogenic isotope component of Pb reaching thecontinental shelf and open ocean, and most likely increase the Pbflux too, as a consequence of weathering of highly reactive glacialdebris and its dominant transport by ice rafting and meltwaterdischarge. The association between glacial debris, high weatheringintensity and radiogenic Pb isotope composition, in associationwith transport by ice rafting or meltwater discharge (Fig. 12), alsoimplies concomitant increases in fluxes of other elements, inparticular those that are normally sequestered in estuarine andshelf environments during riverine transport, e.g. Fe. This is sup-ported by independent investigation of the continental Fe flux tothe ocean from modern ice margins (Statham et al., 2008). Ele-ments for which FeMn oxyhydroxides act as a carrier phase shouldalso increase in their flux to the open ocean during intervals of icesheet activity. For example, enhanced phosphorous release hasbeen observed in Alpine glaciers (Föllmi et al., 2009), although notas yet in a marine setting. The combined increases in Fe and P, bothof which are biolimiting nutrients in the ocean, carry implicationsfor marine primary productivity. However, the trace metalcomposition of solute fluxes to the ocean from modern glaciatedmargins remains poorly constrained and even more so those fromformer ice sheets of the Last Glacial. Further study is required tocharacterise these, and evaluate their potential influence onseawater composition and biogeochemical cycles.

5. Conclusions

During the Holocene, both climate change and the availability ofglacial debris to weathering were the main drivers of change in thePb isotope record at Feni Drift. The correlation of changes in the Pbisotope record to early Holocene climate warming, and then post-7 ka cooling, reinforces interpretation of the Pb isotope record as aresponse to climate change. Equally the evolution of the isotopecomposition in PbePb space over a 12 ka period reproduces theresults from time-dependent laboratory leaching experiments,highlighting the importance of highly reactive glacial debris inreleasing radiogenic Pb during weathering. The rapid decrease inPb isotope composition of the youngest samples (<3 ka) maypossibly be due to anthropogenic Pb inputs related to ore smeltingin the British Isles or downward mixing through bioturbation ofmore recent anthropogenic Pb pollution.

Themajor difference between the Holocene and the LGM are theexceptionally radiogenic compositions (w21 206Pb/204Pb) thatshow strong temporal agreement to activity of the BIIS. This isdemonstrated by comparison to existing IRD records and to theBritIce model of BIIS evolution during the Last Glaciation, inparticular the timing of meltwater discharge.

Differences between glacial and interglacial debris weatheringand solute transport result in higher glacial Pb fluxes to the openocean of more radiogenic isotope composition. As a consequence ofweathering of glacially comminuted debris, other element fluxesare also expected to increase. To gauge the impact of deglaciation

on seawater composition and potentially on biogeochemical cyclesrequires further study to characterise glacial runoff composition.

Acknowledgements

We acknowledge Chris Coath for his expertise and dedication tomanaging the Bristol Isotope Group mass spectrometry lab, and IanBastow for creating the map in Fig. 1. Marcus Gutjahr providedhelpful insights during various discussions of the data. The manu-script was improved by comments from two anonymous reviewers.We thank Claude Hillaire-Marcel for his editorial handling of themanuscript. This research used samples provided by the OceanDrilling Program, which is sponsored by the U.S. National ScienceFoundation and participating countries under management of JointOceanographic Institutions, Inc. Funding for this research wasprovided by the Natural Environment Research Council, UK (grantnumbers NER/S/A/2005/13258 and NE/D00876X/2).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.quascirev.2013.10.020.

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