210pb– ra– th systematics in very low sedimentation rate...

210Pb–226Ra–230Th systematics in very lowsedimentation rate sediments from the MendeleevRidge (Arctic Ocean)1,2

Christelle Not, Claude Hillaire-Marcel, Bassam Ghaleb, Leonid Polyak, andDennis Darby

Abstract: U-series isotope behaviour in subsurface sediment of the Arctic Ocean is investigated based on high resolu-tion measurements of natural radionuclides (210Pb, 226Ra, 230Th) and a few analyses of anthropogenic 137Cs in cores col-lected during the 2005 Healy-Oden Trans-Arctic Expedition (HOTRAX). Cores from the Mendeleev Ridge, representingdistinct bathymetric settings, are analyzed in more detail as a means to assess the dating potential of such radionuclidesat sites characterized by very low sedimentation rates (*3 mm ka–1). The sediment consists of variable proportions offine-grained carbonates, clays, and ice-rafted debris and shows excesses in 210Pb (210Pbxs) over parent 226Ra content,down to *1 cm below core top. This 210Pbxs distribution is due to shallow mixing by benthic organisms and (or) dif-fusion from the sediment–water interface, as also indicated by 137Cs activities. From *1 to 7 cm downcore, 210Pb ac-tivities closely follow 226Ra activities. Below 7 cm downcore, 226Ra activities are controlled by variable excesses inparent 230Th (230Thxs) resulting from its scavenging in the overlying water column. 226Ra diffusion is observed towardsthe water column occuring from the upper *7 cm of sediment below the seafloor (with a flux of *0.043 disintegra-tions per minute (dpm) cm–2 a–1) and deeper in the sediment below 230Thxs peaks but with lesser fluxes. Both coresshow identical 210Pb profiles despite their 1 km bathymetric difference. This suggests negligible 230Th and 210Pb scav-enging below water depths of *1.6 km, i.e., the bathymetry of the shallower core. In such settings where sedimenta-tion rates are very low and vertical particle rain is the major sediment source, estimates of the actual 210Pbxs requireprecise knowledge of the 226Ra-supported fraction, which is controlled by 230Thxs, Ra diffusion, and thus sedimentationrates and porosity.

Resume : Le comportement des isotopes des series de l’uranium dans les sediments de sub-surface de l’ocean Arctiqueest etudie en se basant sur des mesures a haute resolution de radionucleides naturels (210Pb, 226Ra, 230Th) et de quelquesanalyses de 137Cs anthropique dans des carottes prelevees durant l’expedition HOTRAX « Healy-Oden Trans-Arctic Ex-pedition ». Deux carottes de la ride Mendeleev, representant deux environnements bathymetriques distincts, sont analy-sees plus en detail afin d’evaluer le potentiel de datation de tels radionucleides a des sites caracterises par de tresfaibles taux de sedimentation (*3 mm ka–1). Les sediments comprennent des proportions variables de carbonates agrain fin, d’argiles et de debris transportes par les glaces, et ils montrent des excedents de 210Pb (210Pbxs), par rapportau contenu parent de 226Ra, jusqu’a *1 cm sous le sommet de la carotte. Cette distribution du 210Pbxs est due au mel-ange, a faible profondeur, par des organismes benthiques et (ou) la diffusion a partir de l’interface sediment–eau; la pro-fondeur du melange est aussi indiquee par les activites de 137Cs. De *1 a 7 cm du haut de la carotte, les activites210Pb suivent de pres les activites du 226Ra. Plus bas dans la carotte, les activites du 226Ra sont controlees par des excesvariables du 230Th (230Thxs) parent, qui decoule de son piegeage dans la colonne d’eau sus-jacente. La diffusion du226Ra vers la colonne d’eau est observee dans les *7 cm superieurs de sediments sous le plancher oceanique (avec unflux d’environ 0,043 dpm cm–2 an–1) et plus profondement dans les sediments au niveau des pics de 230Thxs, mais avecdes flux moindres. Les deux carottes montrent des profils de 210Pb identiques malgre leur difference bathymetrique d’un(1) km. Cela suggere un scavenging negligeable du 230Th et du 210Pb a des profondeurs d’eau inferieures a *1,6 km,soit la bathymetrie de la carotte la moins profonde. Dans de tels milieux ou les taux de sedimentation sont tres faibleset que la pluie verticale de particules constitue la principale source de sediments, les estimations du 210Pbxs reel exigent

Received 9 April 2008. Accepted 11 September 2008. Published on the NRC Research Press Web site at cjes.nrc.ca on 10 December2008.

Paper handled by Associate Editor P. Hollings.

C. Not, C. Hillaire-Marcel,3 and B. Ghaleb. GEOTOP, Universite du Quebec a Montreal, C.P. 8888, Montreal, QC H3C 3P8, Canada.L. Polyak. Byrd Polar Research Center, Ohio State University, Columbus, OH 43210, USA.D. Darby. Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Ave, Norfolk, VA 23529,USA.1This article is one of a series of papers published in this Special Issue on the theme Polar Climate Stability Network.2GEOTOP Publication 2008-0031.3Corresponding author (e-mail: [email protected]).

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une connaissance precise de la fraction soutenue par le 226Ra, laquelle est controlee par le 230Thxs, la diffusion du Ra etdonc par les taux de sedimentation et la porosite.

[Traduit par la Redaction]

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IntroductionThe Holocene interval in the central Arctic Ocean ridges

is relatively well documented by radiocarbon chronologies(Darby et al. 1997; Polyak et al. 2004), suggesting in partic-ular low mean sedimentation rates (0.3–1.5 cm ka–1 andmuch less in the last glacial except during deglaciationevents). However, information on these rates in the most re-cent past, i.e., within the last few thousand years (Dyke etal. 1996), is rare and often disputable. In a similar fashion,chronostratigraphies of Pleistocene sediments beyond 14Ctime control (>40 ka) are still poorly constrained (Backmanet al. 2004). During glacial periods, exemplified by the lastglaciation, deposition rates display a large variability, reach-ing up to tens of cm ka–1 during episodes of massive icebergdischarge but decreasing occasionally to very low values oreven to hiatuses of up to several thousand years, particularlyduring the last glacial maximum in the Amerasian Basin(Darby et al. 2006). This drop in deposition rates, coupledwith an absence of faunal remnants, indicates that a verythick or perennially snow-covered pack-ice existed duringthe last glacial maxima, suppressing both biological produc-tivity and lithic deposition. Most late Cenozoic sedimentaryrecords from the Arctic Ocean’s interior yield unconstrainedage models leading to contrasting paleoceanographic inter-pretations depending upon the stratigraphic approaches used(Poore et al. 1994; Jakobsson et al. 2000, 2001). In this con-text, investigations on the behaviour of U series and short-lived isotopes, particularly on 210Pb, can help document sed-imentation rates, as well as benthic mixing processes.

In the ocean, 210Pb originates from the decay of 226Ra dis-solved in the water column and from the decay of its gas-eous daughter, 222Rn, in the atmosphere. In the first case,226Ra diffuses from the sediments, thus maintaining a moreor less steady-state 226Ra concentration in the water column.This makes the production rate of 210Pb in the water columnpredictable (Cochran 1992). The residence time of 210Pbwith respect to scavenging is brief at the ocean’s surface(on the order of 1 year), whereas it reaches 30–100 years inthe deeper ocean (Cochran et al. 1990). Since intermediatedecay products between 22Rn and 210Pb depict short half-lives by comparison with the average residence time ofaerosols, it is usually assumed that essentially all atoms of222Rn are transformed into 210Pb in the atmosphere. This at-mospheric 210Pb is then scavenged by precipitation and dryfallout. On a global scale, 210Pb delivery from the atmos-phere to the ocean surface is known to be latitude dependent(Turekian et al. 1977).

In the Arctic Ocean, investigations on the behavior of nat-ural (210Pb, 226Ra, 7Be) and anthropogenic (137Cs, 239Pu,240Pu) radionuclides, in coastal and shelf regions in particu-lar, provide information on sea-ice transit time, sea-ice inter-actions with surface seawater, and surface sediment(Baskaran and Naidu 1995; Baskaran et al. 1996, 2000; Bas-

karan 2005). A few studies have also addressed natural ra-dionuclide behavior in the water column (Smith and Ellis1995) and helped document shelf–basin interactions (Smithet al. 2003) as well as infaunal benthos ecology (Clough etal. 1997; Smith et al. 2003). In addition to the high scaveng-ing efficiency, 210Pb deficits measured in the Arctic Oceansuggest that additional processes, such as ‘‘boundary scav-enging’’, contribute to the removal of 210Pb (Smith et al.2003). To understand 210Pb behavior in the Arctic environ-ment, a thorough investigation of 210Pb parent isotopes(222Rn, 226Ra) is necessary. The relatively low concentra-tions of 222Rn in northern latitudes suggest that the atmos-pheric flux of its longer lived decay product, 210Pb, mayalso be reduced in this environment (Weiss and Naidu1986). Permafrost can suppress exhalation of gaseous 222Rn,the 210Pb parent from soils, resulting in a lower 210Pb inputto Arctic sediment (Hermanson 1990). In addition, radonloss from the ocean surface is about two orders of magni-tude lower than from soils (Baskaran and Naidu 1995).

Despite the importance of the Arctic Ocean sedimentaryarchives with respect to the understanding of ocean–climatedynamics, investigations on the behaviour of such isotopesin sediments from deep Arctic basins and ridges are scarce,essentially due to the lack of records available for such stud-ies. Recent major coring expeditions such as the 2005Healy-Oden Trans-Arctic Expedition (HOTRAX) yielded anew and important series of sedimentary records from suchsites (Darby et al. 2005b; Moran et al. 2006; Stein 2007).We conducted a systematic study of the distributions ofshort-lived and U-series radionuclides in several of theHOTRAX cores to help constrain sedimentation rates andthe stratigraphy of core-top sediments. This study focuseson core HLY0503-11 raised from a depth of 2570 m on theMendeleev Ridge (Fig. 1), for which analyses of both short-lived (210Pb, 137Cs) and parent isotope (226Ra, 230Th) wereperformed. In this particular setting (i.e., far from slopeprocesses and deep current influence), vertical fluxes fromthe overlying water column are dominant. We demonstratethat both the low sedimentary fluxes and dominant verticalparticulate rain result in a downcore 210Pb distribution essen-tially controlled by the behaviour of parent 226Ra and 230Th,independently of bathymetry of the site below a given waterdepth.

Materials and methods

Core locationSediment multicores (MC) were collected in the summer

of 2005 during the HOTRAX expedition (Darby et al.2005a). In this study, we present 210Pb measurements onnine of these cores selected along the trans-Arctic route ofthe coring vessel US Coast Guard Cutter (USCGC) Healy(Fig. 1). They represent various settings with respect to thebathymetry and ice-cover (from shelf to deep basin or ridge;from perennial-ice margin to permanent ice cover; Table 1).

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Special attention is paid to two cores from the northernMendeleev Ridge, collected *22 nautical miles apart, repre-senting two distinct bathymetric domains (*1600 and*2600 m, respectively), in which exhaustive geochemicalinvestigations were carried out (HLY0503-11MC andHLY0503-12MC, henceforth cores MC-11 and MC-12, re-spectively; Table 1; Fig. 1). The Mendeleev Ridge extendsfrom the East Siberian shelf to where it joins the AlphaRidge (Fig. 1). The Mendeleev Ridge is characterized by afairly uniform sedimentary layer that thickens to the south.As with most of the central Arctic far from coastal influen-ces, sedimentation is dominated by particulate fluxes strictly

linked to ice-rafted debris (IRD), plus some unknown butminor amount of advected sediment in the form of nephe-loid transport or turbidity flows above the bottom (Fahl andNothig 2007; Hwang et al. 2008), and to a very minor extentto particles linked to productivity in the ice and water columnand eolian particles (Darby et al. 1989). This property makesthis site ideal for investigating the behavior of U-series iso-topes (210Pb, 226Ra, 230Th).

Sedimentological analysesMulticores were sampled at half-centimetre intervals.

The sediment was dried and ground in an agate mortar.

Fig. 1. Location of multicores from the HOTRAX 2005 expedition used in this study. Bathymetry from International Bathymetric Chart ofthe Arctic Ocean (IBCAO; Jakobsson et al. 2008). Circles indicate lead analyses and squares indicate lead, radium, and thorium analyses.

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Particle-size analyses were performed as follows: Na-hexametaphosphate was added to deflocculate the sedimentby tumble mixing for 3 h in deionized water. Sampleswere treated in an ultrasound bath for 10 min followed byrotated agitation for 3 min. Sediments were sieved (2 mm)then disaggregated in an ultrasonic bath for 90 s prior totheir analysis. The disaggregated sediment samples wereanalyzed with a laser-diffraction particle-size analyzer(LS13320, Beckman-Coulter). Only particle sizes between0.04 and 2000 mm were analyzed. Laser calibration wasverified before and after analyses using three standards(Latron 300-0.3 mm, G15-15 mm, and GB500-500 mm).Analyses of 6–14 spectra are necessary to obtain enoughstability on the spectrum to compare results. The mean ofthe last two spectra is weighted to 100% to minimize mi-nor statistical deviations. The particle-size data and granu-lometric statistics are then processed with the Gradistatprogram (Blott and Pye 2001).

Organic and inorganic carbon contents (OC and IC, re-spectively) were measured with a Carlo ErbaTM elementalanalyzer. A first aliquot is dried and analyzed as evolvedCO2 for its total carbon content. A second aliquot is acidi-fied twice with HCl (1 N) to dissolve carbonates and thenanalyzed in a similar fashion for its residual carbon contentconsidered to represent OC. IC was then calculated by bal-ancing the difference between the two measurements. ICand OC are both expressed in dry weight percent (dw%) oftotal sediment and represent the mean of two analyses. Un-certainties (±1s), as determined from replicate measure-ments of standard materials, average ±0.06% for IC(i.e., ±0.6% for its expression as CaCO3 equivalent).Although fast, this method may result in biases for IC cal-culations when HCl (1 N) hydrolysable minerals (e.g.,chlorite) or compounds (e.g., Fe and Mn oxyhydroxydes)are present in the sediment (e.g., Leventhal and Taylor1990; Raiswell et al. 1994), thus resulting in higher appa-rent OC contents in the second acidified sample aliquot andin a subsequent underestimation of the IC content. In thepresent case, the sediment shows low OC values in compar-ison with the IC except in the upper 7 cm (Table 3). Someunderestimation of the IC content thus seems likely, notablynear core top. Semi-quantitative X-ray diffraction measure-ments of mineralogical assemblages (Not et al. 2007) in-deed suggest a slightly higher carbonate content in thesediment, particularly in the upper few centimetres, andalso indicate that it consists, in average, of *75% dolomiteand *25% calcite.

Radioactive isotope measurements

Lead-210 analysesLead-210 (half life t1/2 = 22.6 year) activities were meas-

ured on bulk sediment samples, dried, and ground in anagate mortar. Measurements were made by a counting ofthe activity of 210Pb-daughter isotope, 210Po (t1/2 =138.4 days, a = 5.30 MeV). Measurements were performedabout 200 days following sampling. A few replicate meas-urements were carried out *1 year later to verify theachievement of equilibrium between the two isotopes in thestudy samples. All replicate data were identical to the firstset of measurements within counting errors. Chemical ex-traction and counting efficiencies were determined using a209Po spike. Chemical procedures for 210Po extraction fol-lowed Baskaran and Naidu (1995) (i.e., with a classicalHCl–HNO3

––HF treatment), and samples were deposited ona silver disk (Flynn 1968). The 209Po and 210Po activitieswere measured in a silicon surface barrier a spectrometer(EGG and ORTEC type 576A). Uncertainties were esti-mated as 1s standard deviation for counting statistics(*2%–4% of the value obtained).

Cesium-137 analysesCesium-137 (t1/2 = 30.17 years) was measured on

*1 cm3, bulk, dried, and ground subsamples by g-ray spec-trometry at 661.6 keV (g ray yield = 85.0%), using a low-background high-purity Ge well detector (Canberra). IAEA-300 sediment was used to calibrate the yield of the detector.Uncertainties were estimated for counting errors.

Radium-226 analysesAliquots of geochemical subsamples were weighed and

spiked with a 228Ra solution processed for dissolution,chemical separation, and purification of Ra through MnO2precipitation following Ghaleb et al. (2004). Data acquisi-tion used a Daly-ion counter by peak jumping mode on aVG sector-54 mass spectrometer. Analytical uncertainty rep-resents two standard errors and was *1% for the majorityof the samples.

Thorium isotope analysesThorium isotopes (230Th and 232Th) were extracted and

measured following conventional techniques (Lally 1982)by a spectrometry after separation on an anion exchangeresin (AG1-X8, 200–400 mesh), purification of thorium us-ing the anion exchange resin. The thorium was then depos-

Table 1. Location and water depth of the HOTRAX multicores used in this study.

Station Water depth (m) Latitude Longitude SettingHLY0501-04 462 72841.56’N 157827.13’W Upper slopeHLY0503-08 2791 79835.65’N 172827.53’W RidgeHLY0503-11 2570 83807.73’N 174841.57’W RidgeHLY0503-12 1586 83817.80’N 171854.99’W RidgeHLY0503-14 1874 84818.48’N 149805.90’W RidgeHLY0503-18 2654 88826.23’N 146840.99’E RidgeHLY0503-NP 4224 89859.33’N 158801.30’E BasinHLY0503-21 2015 86839.59’N 056856.21’E RidgeHLY0503-22 783 80828.70’N 007843.88’E Plateau


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Table 2. Data from cores MC-11 and MC-12 used in the present study.

MC-11 MC-12

Depth incore (cm)

Cinorga

(dw%)

14C age(years)b

137Csc

(dpm g–1)

210Pbc

(dpm g–1)

226Rad

(dpm g–1)

230Thc

(dpm g–1)

210Pbc

(dpm g–1)Cinorg

a

(dw%)0.25 2.97 8 525±35e 0.15±0.04 22.42±0.58 4.91±0.02 25.37±0.23 26.70±0.72 3.550.75 0 9.96±0.28 9.46±0.27 2.371.25 2.63 0 8.68±0.28 7.39±0.10 20.94±0.07 8.02±0.25 2.151.75 8.24±0.27 9.08±0.30 2.112.25 2.36 9.66±0.31 8.58±0.11 29.42±0.09 9.57±0.31 2.062.75 9.46±0.30 9.96±0.33 2.193.25 2.77 9.71±0.18 9.24±0.47 37.54±0.33 9.42±0.29 2.283.75 10.36±0.19 10.30±0.31 2.774.25 2.67 32 630±510 11.94±0.24 9.66±0.11 13.05±0.05 12.90±0.59 3.464.75 11.40±0.20 11.20±0.30 3.855.25 2.54 11.17±0.18 10.35±0.11 19.30±0.08 12.30±0.31 4.535.75 12.52±0.24 12.90±0.38 5.186.25 2.09 13.95±0.42 16.97±0.14 17.15±0.19 13.70±0.46 5.346.75 14.61±0.48 12.90±0.40 4.807.25 1.87 14.03±0.45 13.82±0.12 9.29±0.05 12.90±0.39 4.267.75 12.04±0.44 12.20±0.38 3.628.25 1.13 27 010±250f 10.53±0.35 4.95±0.04 9.84±0.31 2.638.75 7.72±0.24 7.68±0.27 2.019.25 0.52 5.99±0.20 2.90±0.08 5.56±0.17 0.939.75 5.25±0.18 4.11±0.15 0.34

10.25 0.64 4.49±0.20 2.70±0.03 3.12±0.12 0.2810.75 4.09±0.16 2.89±0.11 0.0711.25 0.88 3.93±0.16 3.14±0.03 2.49±0.10 0.0611.75 3.78±0.17 2.41±0.09 0.0712.25 0.87 33 550±540 4.06±0.12 3.37±0.06 2.35±0.11 0.0812.75 4.12±0.15 1.99±0.05 2.61±0.14 0.0613.25 0.49 4.00±0.16 2.34±0.05 2.20±0.09 0.0614.25 0.16 3.11±0.12 2.28±0.05 2.08±0.09 0.1515.25 0.12 3.17±0.12 6.95±0.12 2.34±0.09 0.2816.25 0.55 4.73±0.19 8.28±0.08 2.54±0.10 0.2417.25 0.61 5.62±0.15 5.11±0.10 5.46±0.09 0.5318.25 3.42±0.11 3.95±0.08 5.92±0.11 0.5619.25 0.24 2.73±0.09 1.89±0.05 2.62±0.07 0.0920.25 0.21 2.28±0.07 1.45±0.03 2.44±0.07 0.8221.25 0.19 2.13±0.69 1.03±0.04 2.15±0.08 0.4522.25 0.02 1.90±0.08 1.35±0.04 1.99±0.05 0.0823.25 0.03 1.68±0.07 1.18±0.04 1.90±0.07 0.0024.25 0.01 1.89±0.07 1.28±0.04 1.93±0.07 0.0025.25 0.01 1.93±0.07 5.10±0.09 1.83±0.11 0.0026.25 0.02 3.44±0.11 5.41±0.07 2.07±0.08 0.0327.25 0.92 5.07±0.15 5.22±0.09 2.72±0.07 0.6428.25 1.81 5.29±0.18 5.94±0.10 5.64±0.10 1.4429.25 1.84 5.52±0.15 4.47±0.07 6.11±0.16 1.6130.25 1.89 5.87±0.16 3.96±0.04 4.72±0.12 1.6531.25 1.70 5.82±0.17 2.84±0.03 5.30±0.16 1.5332.25 1.68 4.86±0.13 2.55±0.04 5.56±0.16 1.5033.25 2.04 4.97±0.12 2.64±0.07 5.66±0.16 1.7734.25 1.91 — 4.78±0.14 1.3835.25 2.03 3.90±0.1136.25 2.06 3.06±0.1037.25 2.27 2.68±0.1138.25 3.47 1.79±0.07

Note: dpm g–1, disintegrations per minute and per gram.a±1s *0.05%.bUncorrected Libby’ years from AMS measurements on assemblages of Neogloboquadrina pachyderma (l).c±1s.d±2s.eAge of mixed layer (cf. text).fAge used to estimate a minimum sedimentation rate of *3 mm ka–1 (see text).

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ited on steel disks by electrodeposition (Lally 1982). Activ-ities were measured using a spectrometry (EGG and OR-TEC type 576A). 228Th-spike solution was used to calculatechemical extraction and counting efficiencies (natural 228Thwas assumed to be in secular equilibrium to 232Th). Analyti-cal uncertainties were estimated from counting statistics to*3% for all Th isotopes. All other isotopes activities re-ported in Tables 2–4 are expressed in disintegrations per mi-nute and per gram of dry bulk sediment (dpm g–1).

Results and discussion

Lead-210 in Arctic Ocean sedimentsThe nine cores from the trans-Arctic transect depict two

patterns of 210Pb distribution in subsurface sediments(Figs. 2, 3). In the first pattern (Fig. 2A), 210Pb activities de-crease sharply in the first 1–2 cm then show a reverse in-creasing trend deeper in the sediment. In the second pattern(Fig. 2B), 210Pb activities decrease more steadily to deeperdepths (*4–5 cm). In the very first 0.5 cm of sediment,210Pb activities of both patterns differ by almost one orderof magnitude (i.e., between 9 and 72 dmp g–1). These twobroad patterns of 210Pb distribution thus respond to distinctsedimentological settings and overall sedimentation rates.The first one seems exclusive to ridges in the central ArcticOcean, i.e., at sites below permanent sea-ice cover. The210Pb distribution in core MC-14 (Fig. 2C) corresponds tothe first pattern although the near-surface maximum is miss-ing due to the loss of *1 cm of core-top sediment when thiscore tube was extruded from the multicorer. The second pat-tern corresponds to cores from much shallower water depths,where particulate organic carbon fluxes and mixing depthsare higher. This is quite consistent with previous studies on210Pb in the Arctic (Clough et al. 1997).

Sedimentological properties and sedimentation rates inthe Mendeleev Ridge cores

MC-11 and MC-12 are 38.5 and 34.5 cm in length, re-spectively. The sediment is characterized by alternatingbrown – dark-brown and yellowish-brown layers. It showsan alternation of fine carbonate-rich layers with coarsergrain units. Accelerator mass spectrometry (AMS) 14C ageswere obtained on low-abundance assemblages of Neoglobo-quadrina pachyderma (left-coiled) in core MC-11 at depthsof 0.25, 4.25, 8.25, and 12.25 cm (Table 3). Although theages at 4.25 and 12.25 cm are near 14C dating limits withrespect to deep-sea core carbonates, they may indicate thepresence of shells reworked from marine isotope stage(MIS) 3 or older possibly carried to the site through ice raft-ing. Nonetheless, the 14C age of *27 ka at 8.25 cm pro-vides a lower limit of the mean sedimentation rate for theupper 8 cm of sediment. It corresponds to a calibrated ageof about 33 ka, using the calibration model of Fairbanks etal. (2005). Potential errors in estimating the calibrated agenotably due to uncertainties about reservoir age correctionfor Arctic Ocean surface waters during the late MIS 3should not be of major importance here. This date results ina sedimentation rate of *3.1 mm ka–1. Another approachconsists of using the 14C age of *8.5 ka at 0.5 cm (i.e.,*9.5 calibrated ka; cf. Reimer et al. 2004) in the mixed

layer (Table 3) to estimate the most recent sedimentationrate. Using a simple model based on a constant 14C-carrierflux to the sea floor and a homogeneous-mixed layer (e.g.,Berger and Johnson 1978), the age at the core top (T) canbe expressed as follows:

!1" T # 1=lln !1$ %lX=S&"

where l is the decay constant of 14C (1.21 ' 10–4 a–1), S isthe sedimentation rate, and X is the mixed layer thickness.

As for most low sedimentation rate sites investigated inthis study, the mixed layer thickness for 14C-carrier materialshould be at least equal to the depth penetration of 210Pbxs,here X & 10 mm (see the following and Fig. 2), assumingthat this excess is due to mechanical mixing more than dif-fusion. This approach yields a sedimentation rate of about*1 mm ka–1 or significantly less than the linear radiocarboncalculation of *3 mm ka–1 or more. This means that theflux of 14C carriers cannot be considered constant or themixed layer thickness for these 14C carriers is much higherthan that for 210Pb (or both). Using the 3 mm ka–1 estimatefrom previous for S, a value of *50 mm could be calcu-lated for the theoretical thickness of the 14C-carrier mixedlayer in this second hypothesis (e.g., see Boudreau and Im-boden 1987 for deep burrowing impact on mixing models).Since foraminifer shells at a depth of 4.25 cm yielded andage of *32 ka, one can discard the second hypothesis. Themixing of ‘‘old’’ material with recent shells seems indeedmore likely. Note that using eq. [1] with an S value of3 mm ka–1 and the 10 mm thickness for the mixed layer X,a constant 14C-carrier flux would have resulted in a 14C ageof *3 ka instead of the calibrated age of 9.5 ka. Neverthe-less, 230Th–231Pa analyses in MC-11 (Not et al. 2007) indi-cate that MIS 5 sediments are present below 25 cm in thiscore, thus suggesting a mean sedimentation rate in the 2–3 mm ka–1 range. Similarly, Sellen et al. (2007) reported10Be evidence for sedimentation rates of *2 mm ka–1 inMC-14, a nearby core showing sedimentological and geo-chemical features almost identical to those of MC-11(Fig. 2). Thus, we will retain the estimate of *3 mm ka–1

for the mean sedimentation rate of MC-11 and, as will beseen in the following, for MC-12 as well. Consequently, theexcess 210Pb seen in core tops of low sedimentation ratesites (Fig. 2A) illustrates either mechanical mixing by bio-turbations and (or) chemical diffusion as documented byCourcelles (1998) in very low sedimentation rate lacustrinesequences.

A few 137Cs measurements were performed as a means todocument the distribution in the sediment of the post-1950A.D. thermonuclear signal (Table 4). 137Cs activity is ob-served down to 1 cm in cores (Fig. 2A) and down to 4 cmin core MC-22 (Fig. 2B). In the second case, mechanicalmixing down to this 4 cm seems probable. In the first case,both diffusion and mixing could also explain the 137C distri-bution in the upper centimetre of sediment. Other studies ofsedimentation rates in the Arctic report values ranging from10 to 0.2 mm ka–1 based on different proxies, such as 14C(Ku and Broecker 1965; Darby et al. 1997, 2006; Polyak etal. 2007), 230Th (Ku and Broecker 1965; Huh et al. 1997),paleomagnetism (Jones 1987), and micropaleontology (Aksuand Mudie 1985). However, recent reviews propose that the


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key issue to constrain Arctic sedimentation rates is not onlythe choice of the proxy but the significant variability of sed-imentation rates across the Arctic Ocean with the lowestrates characteristic of the central areas of the Amerasian ba-sin, such as the Alpha and northern Mendeleev ridges, asalso illustrated here (Backman et al. 2004; Sellen et al.2008). Nonetheless, if the distribution of the 210Pbxs in suchsettings cannot be used to estimate recent sedimentationrates, it may be used as a proxy for the downcore distribu-tion of 226Ra–230Th parent isotopes and thus as a strati-graphic tool, as discussed in the following section.

210Pb–226Ra systematics in MC-11The excess 210Pb (210Pbxs), i.e., the fraction exceeding

parent-isotope activity, has been determined in coreHLY0503-11MC by subtracting 226Ra activity (availabledown to 7.5 cm) from the total 210Pb activity. This excess isonly seen in the top centimetre of the sediment (Fig. 3). Be-low, the 210Pb seems to be nearly all supported by 226Ra,which shows a decreasing trend from *7 cm to the coretop in response to an upward diffusive gradient. Below*7 cm, both isotopes seem to closely follow their parent230Th activity except for some diffusion of 226Ra below

230Th peaks (Fig. 3). Worth of mention is the fact that 230This in excess relative to its parent uranium in the MC-11 se-quence (Not et al. 2007), thus representing the 230Th fractionscavenged by sinking particles when deposited but for theradioactive decay since their deposition. One sample at21.25 cm below sediment surface (dbsf) shows a minimum230Th activity (*1 dpm g–1), possibly representative of thedetrital heritage of 230Th and its daughter products.

Nonetheless, the 210Pb activity that is strictly representa-tive of 210Pb fluxes from the overlying water column candiffer slightly from the Pbxs previously estimated by sub-tracting 226Ra activity from that of total 210Pb in the core-top samples. Two processes may account for such a slightdifference: (1) the diffusive 226Ra may not be totally in ra-dioactive equilibrium with its daughter 210Pb (i.e., producedin situ; cf. Cochran 1992); and (2) some diffusion of the in-termediate gaseous 222Rn isotope could also result in a slightimbalance between 226Ra and its daughter 210Pb. This diffu-sion of 226Ra is generally thought to have little impact onestimates of the supported fraction of 210Pb (Key et al.1979; Imboden and Stiller 1982; Baskaran and Naidu 1995).However, the effect of 226Ra diffusion is not necessarilynegligible. Deschamps (1997) has illustrated large deficits

Table 3. 210Pb (in dpm g–1) in core-top sediments from Fig. 1 (see Table 2 for MC-11 and MC-12).

Depth(cm) MC-04 MC-08 MC-14 MC-18 MC-NP MC-21 MC-22

0.25 18.37±0.58 65.77±1.37 9.02±0.27 72.63±1.43 43.62±0.82 14.47±0.30 42.89±0.890.75 18.33±0.43 24.69±0.52 9.74±0.30 25.86±0.52 50.57±0.82 6.66±0.16 35.81±0.661.25 18.92±0.68 11.30±0.30 7.66±0.28 9.04±0.25 12.92±0.32 2.38±0.08 28.90±0.631.75 18.55±0.52 11.78±0.28 8.48±0.25 7.35±0.19 6.26±0.14 3.07±0.08 26.91±0.462.25 18.59±0.67 12.66±0.30 9.28±0.29 6.85±0.19 6.97±0.18 2.13±0.07 23.02±0.372.75 19.50±0.58 13.99±0.33 9.36±0.24 7.45±0.22 7.98±0.20 2.32±0.09 20.76±0.453.25 — 13.09±0.32 9.93±0.30 6.46±0.28 8.42±0.19 2.30±0.07 14.63±0.293.75 18.23±0.42 13.99±0.32 10.22±0.28 7.70±0.31 8.79±0.19 2.52±0.08 7.70±0.174.25 18.53±0.48 12.89±0.33 13.37±0.39 7.69±0.19 9.45±0.19 2.39±0.08 10.46±0.244.75 17.06±0.43 12.52±0.30 12.45±0.38 8.03±0.29 8.90±0.19 2.39±0.07 6.61±0.155.25 15.20±0.44 10.05±0.22 12.33±0.37 8.17±0.31 9.44±0.20 2.40±0.07 6.89±0.165.75 14.94±0.34 9.51±0.25 11.18±0.38 8.38±0.34 9.96±0.21 2.51±0.08 5.19±0.126.25 13.40±0.31 13.95±0.42 10.09±0.19 9.83±0.34 10.02±0.24 — 4.85±0.126.75 11.75±0.28 6.87±0.23 9.31±0.16 10.82±0.42 10.03±0.27 — 5.00±0.147.25 11.65±0.29 8.84±0.34 11.06±0.28 10.34±0.43 10.41±0.32 — 4.99±0.167.75 10.06±0.27 7.37±0.26 7.74±0.19 10.51±0.40 11.71±0.40 — 5.90±0.188.25 9.11±0.24 4.73±0.18 5.31±0.13 10.33±0.39 11.64±0.41 — 6.76±0.178.75 — 6.86±0.33 4.05±0.11 11.11±0.35 11.92±0.39 — 7.46±0.229.25 8.09±0.16 7.18±0.29 2.88±0.10 11.57±0.35 11.92±0.39 — 9.66±0.289.75 8.08±0.17 6.40±0.23 2.60±0.07 12.33±0.40 12.47±0.38 — 11.23±0.27

10.25 7.81±0.17 7.24±0.27 2.36±0.06 11.61±0.41 12.36±0.37 — 11.68±0.2710.75 5.58±0.16 7.27±0.26 2.05±0.06 12.43±0.45 12.15±0.40 — 10.56±0.2711.25 7.26±0.19 6.73±0.24 2.02±0.06 12.73±0.42 11.91±0.43 — 8.34±0.2211.75 9.64±0.21 8.08±0.32 — 6.50±0.21 11.68±0.41 — 7.03±0.26

Table 4. 137Cs measurements (in dpm g–1) in a few core tops.

Depth (cm) MC-18 MC-22 MC-11 MC-120.25 0.71±0.19 — 0.15±0.04 0.30±0.060.75 0.15±0.09 — 0 01.25 0 — 0 —1.75 — 0.85±0.15 — —

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(*40%) in 228Ra, but excesses in daughter 228Th (versustheir parent 232Th) in fine particles sedimenting in the mod-ern St. Lawrence estuary linked to processes occurring dur-ing their water transport and earlier terrestrial evolution insoils. Assuming a similar behavior for 226Ra in the Mende-leev Ridge sediment, 226Ra diffusion during particle trans-port would result in some excess of the ‘‘inherited’’ fractionof 210Pb versus its parent ‘‘detrital’’ 226Ra fraction. In an op-posite way, 226Ra fluxes resulting from the excess 230Th incore-top sediments may result in some daughter 210Pb deficitlinked to the time interval it needs to reach equilibrium withits parent 226Ra. Fortunately, here, the very low sedimenta-tion rate but high residence time of particles in the uppersedimentary column (*3 ka, as previously estimated)

should carry back these potential disequilibria to nearly sec-ular equilibrium conditions at the 210Pb time scale, as dis-cussed in the following.

There is no correlation between water depth and 210Pbdistribution in surface sediment and downcore. MC-11 andMC-12 raised from depths of 2570 and 1586 m, respec-tively, present similar 210Pbxs distributions (Fig. 4), suggest-ing that the particle flux is identical at these two sites. Inearlier studies of 210Pbxs distribution in Arctic sedimentcores (Clough et al. 1997; Huh et al. 1997; Smith et al.2003), the reverse trend at a few centimetres below the sur-face has been reported in several cases; Smith et al. (2003)explained it using a bioturbational mixing, whereas Cloughet al. (1997) presented it most likely as a result of 226Ra in-

Fig. 2. 210Pb activities in multicores: (A) and (B) represent the two distribution patterns observed; (C) illustrates the profiles from all coresfollowing the (A) pattern.


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growth. As illustrated in Fig. 4, this pattern is closely con-trolled by the subsurface diffusion gradient of 226Ra pro-duced by the excess in parent 230Th that characterizes themost recent sediment from core tops.

226Ra diffusion at MC-11 siteThe 226Ra distribution in MC-11 typically corresponds to

a diffusion profile in Arctic Ocean sediments previouslynoted (Baskaran and Naidu 1995). This diffusion patterncombines diffusion of 226Ra ejected from particles by alpharecoil and diffusion of 226Ra from adsorbed 230Th on grains.Unfortunately, without pore-water 226Ra data, direct diffu-sion rates cannot be calculated. Cochran (1980) proposed amethod to estimate 226Ra fluxes from deep sediments using226Ra activities of sediment. This method is based on a boxmodel where the 226Ra concentration is maintained in themixed core-top layer by balancing its sources (production of226Ra by 230Th + 226Ra in freshly deposited sediment) andlosses (radioactive decay of 226Ra + burial).

Under steady state conditions,

!2" lRaAXThrX $ SA0

Rar # lRaAXRarX $ SAX

Rar

where X is the mixed layer thickness (here about 10 mm), Sis the sedimentation rate (here *3 mm ka–1), AX

Th is the to-tal 230Th activity in the mixed layer, AX

Ra is the total 226Raactivity in the mixed layer, A0

Ra is the activity of 226Ralinked to detrital sources (about 1 dpm g–1 here as estimatedabove), r is the sediment density (in g cm–3), and lRa is thedecay constant of 226Ra (in ka–1).

Here, the term SA0Rar is negligible versus lRaAX

ThrXwhen comparing the total 230Th activity near the core topwith the activity of its ‘‘detrital fraction’’ (*1 dpm g–1),seen as a proxy for A0

Ra (Fig. 3). Using this model, about60% of 226Ra losses from the mixed layer would occur atthis site through radioactive decay and about 40% throughburial. One useful information gained from this exercise isthat any initial disequilibrium between 210Pb, 226Ra, and230Th disequilibrium in the detrital fraction of the sedimentshould be largely reduced at burial stage, i.e., below themixed layer, as previously estimated from the age of themixed layer above.

Fig. 3. (A) 210Pb, 226Ra, and 230Th measurements in MC-11; (B–C) inorganic carbon content and mean particle size of bulk sediment sam-ple, respectively.

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As a consequence, in such low sedimentation rate sites,with shallow mixing (if any), a direct estimate of the 226Raflux can be obtained by the depth-integrated (0.5 cm integra-tion step) deficiency in 226Ra inventory relative to the 230Thinventory (see later in the text for details on inventory esti-mates) down to *7.5 cm. The obtained 226Ra flux is*0.043 dpm cm–2 a–1. This estimate of the 226Ra flux overthe Mendeleev Ridge is in the upper range of values re-ported previously from Pacific and Atlantic sediments (Co-chran 1980; Cochran and Krishnaswami 1980; Moore andDymond 1991; Moore 1999; Hancock et al. 2000). Cochran(1980) suggested an inverse and nonlinear correlation be-tween 226Ra fluxes from sediments and sedimentation rates.Our estimate (0.043 dpm cm–2 a–1) thus corresponds to val-ues expected in very low sedimentation rate areas.

210Pb behavior downcoreA striking feature is the similarity of 210Pb vertical distri-

bution in cores MC-11 and MC-12 despite their *1 kmbathymetric difference (Fig. 4). Below the high activity ofthe surface layer (25 dpm g–1 in the top 0.5 cm), there is(i) a decreasing trend of 210Pbxs from core top down to*1.5 cm, linked to its return to equilibrium with parent226Ra with likely some diffusion–mixing from thesediment–water interface; (ii) a reverse increasing trenddown to *7.5 cm following the upward diffusion gradient of

226Ra; and (iii) a fluctuating 210Pb distribution below, stillclosely following 226Ra distribution. Downcore, 226Ra followsthe 230Th distribution except for some diffusion below activ-ity peaks of this isotope, thus suggesting an overall patternprimarily governed by the distribution of residual excesses in230Th linked to its variable scavenging rates through time andits decay since deposition (Figs. 3, 4). 210Pb and 230Th distri-butions show two major peaks downcore, between 15 and20 cm and between 27 and 32 cm, with a slight decouplingbetween 210Pb distribution and 230Th profiles due to 226Ra dif-fusion away from the 230Th peaks.

The second step in 210Pb distribution, i.e., its increasefrom *1.5 to 7.5 cm here, is found in other Arctic sedi-ments (Clough et al. 1997) and was explained as the resultof growth of 226Ra in sediments. Our study provides sup-porting 226Ra data to validate this hypothesis of 226Ra in-growth from 230Thxs decay and further documents thediffusion pattern of 226Ra. 230Th distribution (and thus 210Pbdistribution) downcore closely follows the carbonate contentand inversely correlates with grain size (Fig. 3), thus point-ing to variable scavenging rates due to particle availabilityand size. Between peaks, we observe a larger particle sizeand a lower carbonate content, possibly linked to the mini-mum sedimentary supply during glacial periods, essentiallyrepresented by relatively coarse but scarce IRDs (Fig. 2; seealso Moran et al. 2006).

Lead-210 excess inventories in MC-11 and MC-12Inventories of 210Pbxs in deep-sea sediments are calculated

from measurements of 210Pb minus the supported fractionestimated ideally from parent-isotope activity (226Ra) usingthe equation:

!3" I #X

i

%riAixsDXi&

where I is the inventory of 210Pbxs (in dpm g–1), ri is the drybulk density (in g dry sediment per cm3 of wet sediment) ofthe ith depth interval, Ai

xs is the 210Pbxs activity (in dpm g–1)of the ith depth interval, and DXi is the thickness of ithdepth interval (in cm). 10Pb inventories of 12 and14 dpm cm–2 are measured in cores MC-11 and MC-12, re-spectively, and are within a range of values for the Arctic(Baskaran and Naidu 1995; Huh et al. 1997; Smith et al.2003). The small difference in 210Pbxs inventories betweenMC-11 and MC-12 is not significant, considering the poten-tial errors in sediment-depth measurements and in dry bulk-density estimates. This similarity in 210Pbxs inventories atthe two sites representing distinct water depths (*2.6 ver-sus *1.6 km) suggests that 210Pbxs inventories in Mende-leev Ridge surface sediments do not simply reflect theproduction of 210Pb from 226Ra and 222Rn decay in theiroverlying water column and losses by advection must occur.

210Pbxs in sediments includes fractions from both its at-mospheric fallout and production in the water column. Inthe following calculations, we use a flux of atmospheric210Pb of *0.08 dpm cm–2 a–1 measured near Point Barrowin Alaska. We expect some variability in 210Pb fluxes dueto its relatively large range in Arctic haze aerosols (from0.0134 to 0.0613 dpm m–3; see Moore and Smith 1986;Weiss and Naidu 1986; Bacon et al. 1989; Baskaran and

Fig. 4. 210Pb profiles in MC-11 and MC-12 raised from distinctwater depths (*2.6 and *1.6 km, respectively).


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Shaw 2001; Smith et al. 2003). Production rate of 210Pb inthe water column is dictated by the distribution of 226Ra,and the scavenging flux can be estimated from the disequili-brium between 226Ra and 210Pb in water column. Althoughwater column 226Ra and 210Pb data are not available at MC-11 and MC-12 core sites, data from nearby in the CanadaBasin are available (Moore and Smith 1986; Bacon et al.1989; Smith and Ellis 1995). Thus, assuming that the226Ra–210Pb disequilibrium relationship shows minimum var-iability across the basin, a theoretical scavenging rate of0.16 ' Z dpm cm–2 a–1 can be estimated in the water col-umn using a 0.16 value derived from the average 210Pb defi-ciency (relative to 226Ra), where Z is the water depth inkilometres (Huh et al. 1997). In this scenario, as illustratedin Table 5, the 210Pbxs inventory in MC-12 would represent128% of the total flux (from the atmosphere and the watercolumn), whereas in MC-11, it would represent only 75%of this flux. However, if these values do illustrate some210Pb missing at the deep site (MC-11) in comparison withthe shallower one (MC-12), they must be used carefully.For example, Masque et al. (2007) showed that sea ice inter-cepts the atmospheric flux of 210Pb and transports at leastsome of it out of the Arctic. Nonetheless, this imbalancewith vertical production rates implies some boundary scav-enging responsible for an export of dissolved 210Pb (Huh etal. 1997; Smith et al. 2003).

Thus, despite the bathymetric differences, 210Pbxs profilesin MC-11 and MC-12 are nearly similar, which allows us toconclude that the vertical particle fluxes are identical at thetwo sites and these particles do not scavenge additional210Pb at least below 1600 m. One possible explanation linkedto the scarcity of scavenging particles in such a setting (al-ready evoked by Bacon et al. 1989) would relate to the satu-ration of these particles in adsorbed heavy metals, like 210Pband 230Th. Studies on 222Rn escape rates in the northern polarregions highlight the fact that the 210Pb fallout at this highlatitude is fairly low. The average estimate of 222Rn escaperate is *0.3 atom cm–2 s–1 (Baskaran and Naidu 1995). Us-ing the sediment inventory of 210Pbxs and reported flux(flux = 210Pbxs inventory / mean life of 210Pb, 31.9 a), a222Rn escape rate of *0.22 atom cm–2 s–1 can be estimated,assuming that the sediment inventory contains all the 210Pbproduced in situ by 222Rn. This 222Rn escape rate over theMendeleev Ridge is similar to that obtained on the continen-tal shelf of the East Chukchi Sea (Baskaran and Naidu 1995).

ConclusionLead-210 distribution in the Arctic sedimentary sequences

investigated here show distinct profiles partly independent

of the water depth but closely linked to sedimentologicalsettings. For example, cores in ridge environments show ashallow mixing–diffusion depth (*1 cm). Analyses of pa-rent isotopes of 210Pb show that the 210Pb distribution belowthis layer is controlled by the 226Ra distribution. This isotopeshows a steep diffusive gradient towards the water columnin the top few centimetres of the sedimentary column, thena distribution governed by that of excesses in the parent230Th below except for some diffusion from abundancepeaks of 230Thxs. The diffusive flux of 226Ra towards thewater column estimated at 0.043 dpm cm–2 a–1 on the Men-deleev Ridge is consistent with the very low sedimentationrate estimated over the ridge (*3 m ka–1). In such environ-ments, the patterns of 210Pb distribution indicate that 210Pbcan be a proxy for the downcore distribution of parent226Ra and 230Th except for the slight decoupling of 226Raand 230Th at the core top and from 230Th peaks, downcoredue to 226Ra diffusion. Therefore, 210Pb can have a potentialstratigraphic use in correlating cores in such environmentswhere the usual stratigraphic tools have limited resolution orare unavailable. The 210Pbxs inventory of the shallower coreMC-12 indicates that the 210Pb present in the sediment prob-ably represents the sum of the atmospheric and water column210Pb fluxes. However, in the deeper water MC-11 core, analmost similar 210Pbxs inventory indicates a deficit of 210Pb incomparison with 210Pb fluxes from the atmosphere and watercolumn, thus there is probably export of part of the 210Pb inthe water column through boundary scavenging. However,the scarcity of particles to scavenge particle-reactive nuclidescan account for their possible saturation in adsorbed heavymetals. Thus, suspended and particulate water-column meas-urements are needed to better document such processes.

AcknowledgementsWe wish to acknowledge Captains Dan Oliver and Tho-

mas Arnell, the crews of US Coast Guard Cutters Healyand Oden, Martin Jakobsson and Anders Karlgvist, respec-tively, chief scientist and project director onboard theOden, for their support during the 2005 HOTRAX cruise.We are grateful to J.K. Cochran and one anonymous re-viewer for helpful suggestions. Thanks are also due to Dr.Sandrine Solignac for onboard sampling, Dr. Guillaume St-Onge for particle size analyses, and Agnieska Adamowiczfor carbon analyses. HOTRAX was funded by the US Na-tional Science Foundation’s Office of Polar Programs(OPP-0352395), the US Coast Guard, the Swedish PolarResearch Secretariat, and the Swedish Science Council.The present study is a contribution to the Polar ClimateStability Network program (Canadian Foundation for Cli-mate and Atmospheric Sciences). Support from Natural

Table 5. 210Pb budget, inventories, and fluxes.

CoreWaterdepth (m)

Inventory insediments(dpm cm–2)

Fluxa tosediments(dpm cm–2 a–1)

Scavenging ratein water column(dpm a–1)

Atmospheric flux(dpm cm–2 a–1)

Fractionb of totalflux accountablein sediments

HLY0503-11MC 2570 12 0.36 0.41 0.08 0.73HLY0503-12MC 1586 14 0.43 0.25 0.08 1.28

aFlux to sediments = inventory in sediment / mean life of 210Pb.bCalculated by dividing flux to sediments by the sum of atmospheric flux and water column scavenging flux.

Not et al. 1217

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Sciences and Engineering Research Council of Canada andthe Killam Foundation to CHM and Fonds Quebecois de laRecherche sur la Nature et les Technologies (GEOTOPgrant and a team grant to de Vernal et al.) is also acknowl-edged.

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210pb– ra– th systematics in very low sedimentation rate...

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