Quaternary Science Reviews

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1 Quaternary Science Reviews 29 (2010) 3430e3441 Contents lists available at ScienceDirect Quaternary Science Reviews journal homepage: Quaternary Arctic Ocean sea ice variations and radiocarbon reservoir age corrections Daniela Hanslik a, *, Martin Jakobsson a, Jan Backman a, Svante Björck b, Emma Sellén a, Matt O Regan a, Eliana Fornaciari c, Göran Skog d a Department of Geological Sciences, Stockholm University, Stockholm, Sweden b Department of Earth and Ecosystem Sciences, Division of Geology, Quaternary Sciences, Lund University, Sweden c Department of Geosciences, University of Padova, Italy d Department of Earth and Ecosystem Sciences, Division of Geology, Radiocarbon Dating Laboratory, Lund University, Sweden article info abstract Article history: Received 1 December 2009 Received in revised form 30 April 2010 Accepted 3 June 2010 A short sediment core from a local depression forming an intra basin on the Lomonosov Ridge, was retrieved during the Healy-Oden Trans-Arctic Expedition 2005 (HOTRAX). It contains a record of the Marine Isotope Stages (MIS) 1e3 showing exceptionally high abundances of calcareous microfossils during parts of MIS 3. Based on radiocarbon dating, linear sedimentation rates of 7e9 cm/ka persist during the last deglaciation. The Last Glacial Maximum (LGM) is partly characterized by a hiatus. Planktic foraminiferal abundance variations of Neogloboquadrina pachyderma sinistral and calcareous nannofossils reflect changes in Arctic Ocean summer sea ice coverage and probably inflow of subpolar North Atlantic water. Calibration of the radiocarbon ages, using modeled reservoir corrections from previous studies and the microfossil abundance record of the studied core, results in marine reservoir ages of 1400 years or more, at least during the last deglaciation. Paired benthiceplanktic radiocarbon dated foraminiferal samples indicate a slow decrease in age difference between surface and bottom waters from the Lateglacial to the Holocene, suggesting circulation and ventilation changes. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction The Arctic Ocean is unique among Earth s five major oceans being relatively small and virtually landlocked with a single restricted deep-water connection to other oceans. It is also a recipient for huge amounts of river run-off from the large surrounding catchment areas. Its geographic position result in long and dark winters, favorable for the development of seasonal and perennial sea ice. The sea ice and its albedo effect play a key role in a series of processes influencing oceanography and climate. For example, sea ice reduces the heat and moisture exchange between the ocean and atmosphere, enhances surface water stratification, causes deep-water formation from brine rejection during freezing, and influences primary production and its seasonal length (Aagaard, 1981, Fig. 5; Li et al., 2005; Ellingsen et al., 2008). There is still limited knowledge about sea ice distribution and its variability in the geological past. Modern satellite observations, which started in 1979, show negative linear trends in sea ice extent for every month from 1979 to 2006 (Serreze et al., 2007), and after * Corresponding author. address: daniela.hanslik@geo.su.se (D. Hanslik) a decline of 10.2% per decade at the end of the summer melt season in September (Comiso et al., 2008). Climate change models predict an amplification of future temperature change and warming in Polar Regions (Johannessen et al., 2004; IPCC, 2007). To increase our knowledge about past sea ice fluctuations and to address whether Arctic perennial sea ice was persistent since the last glacial period and during the nearest previous late Pleistocene interglacials and interstadials, we need to study highly resolved sediment archives. There is a lack of such records from the central Arctic Ocean as recovered Holocene to Lateglacial sediment sequences rarely exceed 20 cm (Backman et al., 2004). Here we present a study of a short sediment core from the central Lomonosov Ridge in the Arctic Ocean in which the preservation of calcareous micro- and nannofossils is exceptionally good. The Marine Isotope Stage (MIS) 1 time interval, divided into a Lateglacial period 11.7e14 ka BP starting with Termination I (LR04, Lisiecki and Raymo, 2005) after the last cold stage and the Holocene 0e11.7 ka BP, documents distinct abundance fluctuations of both planktic and benthic foraminifera, which are likely related to sea ice cover and surface water circulation changes. In addition, we address the issue of marine radiocarbon reservoir ages, which may be up to 1400 years during the Lateglacial /$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.quascirev

2 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e Material and methods 2.1. Coring One of the areas cored during the HOTRAX expedition (Darby et al., 2005) is located on the central Lomonosov Ridge at about 88 N (Fig. 1). The region is characterized by a >1000 m deep perched basin (Björk et al., 2007). This local basin is hereafter referred to as the Intra Basin. Three cores were retrieved from the approximately 2500 m deep Intra Basin using a jumbo piston corer and a multi corer. The trigger weight core from site HLY at N E, and 2598 m water depth (Fig. 1), was chosen for a detailed microfossil study and the jumbo piston core for correlation to other nearby locations. The multi cores from the HLY site contain undisturbed fine grained surface sediments, suggesting a relatively highly resolved Holocene to Lateglacial sediment sequence and low energy depositional environment. This is in contrast to other cores form the Intra Basin that were clearly influenced by currents as demonstrated by high resolution chirp sonar profiles (Björk et al., 2007) Sample preparation and grain size measurements Samples were taken from the jumbo piston corer HLY JPC and its trigger weight core HLY TC (hereafter referred to as 18JPC and 18TC respectively) by sub-sampling 2 cm thick slices from u-channels previously retrieved for paleomagnetic measurements. Additional 1 cm thick slices were taken for radiocarbon measurements from a parallel u-channel of 18TC. All sample depths are recorded by the sampling midpoints. The micro- and nannofossils results presented here as well as sediment fine-fraction (<63 mm) are from the upper meter of 18TC. Core 18JPC was sampled at every 10 cm down to 6 m core depth for foraminifera analysis. After freeze drying, about 3 g of sediment was re-suspended overnight on a shaker-table at a speed of 180 r/min. De-ionized water saturated with pure calcium carbonate (Pro Analysi) was used as the suspension liquid to prevent carbonate dissolution. The samples were put in an ultrasonic bath for 4 s before sieving at 63 mm. The dried >63 mm residuals were dry-sieved and planktic and benthic foraminifera were counted in the 63e100, 100e125, 125e150 and >150 mm subfractions. Numbers are expressed as specimens per gram of dry bulk sediment. Calcareous nannofossils were prepared from raw sediment using standard methods (Haq and Lohmann, 1976). The fine-fraction (<63 mm) remaining after wet sieving was analyzed for grain size distribution with a Sedigraph Initially, the fine-fraction was allowed to settle for three days, after which the calcium carbonate saturated water was removed. Subsequently the settled material was re-suspended with calgon (NaPO 3 ) using an ultra-sonic bath and a tube vortex and agitated with an ultra-sonic probe before being analyzed with the Sedigraph. The Sedigraph method is described in Bianchi et al. (1999). Fig. 1. Bathymetric map of the Arctic Ocean (Jakobsson et al., 2008) with the study area marked, and a detailed map inserted showing the location of core HLY TC in the Intra Basin (IB) on the central Lomonosov Ridge. Also shown the location of ACEX, core 96/12-1pc, PS2185 and PS 2177 on the Lomonosov Ridge, PS2166 on the Gakkel Ridge, PS2195 in the Amundsen Basin and B-15 in the Chukchi Sea.

3 3432 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e3441 The data obtained from the Sedigraph were corrected for the removal of coarse fraction through sieving and the mean size fraction of the sortable silt (10e63 mm) was calculated following McCave et al. (1995). All cores retrieved during the HOTRAX expedition were logged using a Geotek Multi-Sensor Core Logger. The core logger measured bulk density, p-wave velocity and magnetic susceptibility. In this study we use the bulk density of cores 18TC and 18JPC for core correlation purposes Isotopes Stable oxygen and carbon isotope analyses were performed on 23 planktic foraminiferal samples in the >150 mm size fraction. Only four-chambered, encrusted quadrate specimens of Neogloboquadrina pachyderma (sinistral) were picked in order to avoid possible ecological biases of different morphotypes (Healy- Williams, 1992), and of secondary calcite crusts (Volkmann and Mensch, 2001). Samples were measured by standard techniques (Duplessy, 1978) with a Finnigan MAT 252 mass spectrometer connected to an automated Carbo-Kiel device. Carbon and oxygen isotopes were calibrated to the Vienna Pee Dee Belemnite standard (VPDB) and converted to conventional delta notation (d 13 C and d 18 O) (Coplen, 1996). Analytical precision is better than 0.1& for both d 13 C and d 18 O Radiocarbon Sixteen planktic foraminifera samples (N. pachyderma) and six benthic foraminifera samples (Cibicides wuellerstorfi and mixed species samples) were radiocarbon dated with the single stage accelerator mass spectrometer (SSAMS) at Lund University Radiocarbon Dating Laboratory, using sample volumes of 50e1000 mg C. Calibration of the 14 C ages was performed using OxCal 4.0 with the Marine04 calibration data set (Hughen et al., 2004). Results were rounded to the nearest 10 years and expressed with a 2 sigma standard deviation (2 SD) (Table 1). The 14 C ages from the benthic foraminifera were not calibrated as they are only used to address deep-water ventilation at the coring site and not to construct the age model. Two of the planktic 14 C-dated samples yielded ages older than the extent of the OxCal calibration data set (29.7 and 31.2 cm). The Fairbanks0107 calibration curve (Fairbanks et al., 2005) ( was used for these two samples and the results are presented with 1SD(Table 1). 3. Results 3.1. Foraminifera The planktic and benthic foraminiferal assemblages in core 18TC are consistently well preserved and only rarely show signs of dissolution. The planktic assemblages are monospecifically composed of N. pachyderma with >95% of the left coiling variety. The highest abundances in the >125 mm size fraction occur in the lower part of the core, between 51.7 and 61.7 cm, reaching a maximum of w3600 spec./g at 57.7 cm (Fig. 2B). The lowest abundances occur at 23.7 cm and 35.7e37.7 cm levels, with about 80e180 spec./g. The benthic foraminifera have peak abundances at 57.7 cm, with 60 spec./g. While benthic foraminiferal assemblages were not identified, it was noted that only calcareous species are present. None of the marker species often used for identifying MIS 5 and 7, Bulimina aculeata, Epistominella exigua and Pullenia bulloides (Backman et al., 2004; Polyak et al., 2004; Nørgaard-Pedersen et al., 2007a), were found in 18TC or in the examined samples of 18JPC. Both planktic and benthic foraminifera show a reoccurring pattern of five consecutive peaks separated by four troughs. The proportion of the number of planktic foraminifera of both plankticebenthic specimens per gram is close to constant throughout the core. This suggests that there is no significant influence of carbonate dissolution in the foraminifera rich intervals Calcareous nannofossils Compositions and abundances of biostratigraphically useful calcareous nannofossil taxa in core 18TC are presented in Fig. 2D, E. The observed taxa are all described from extra-arctic oceans, implying that those observed in core 18TC are dependent on water mass transport into the Arctic Basin. The Fram Strait is the major gateway for such transport, which was marginally affected by glacialeinterglacial scale changes in sea-level, in contrast to the Bering Strait. The net inflow of Atlantic water to the Arctic Ocean, however, is likely affected by glacialeinterglacial changes as approximately one third of the inflow is at present taking the route over the Barents Sea (Mauritzen, 1996), which was occupied by ice sheets during major Pleistocene glaciations, including the last two glacial cycles (Svendsen et al., 2004). All calcareous nannofossil bearing samples contain Emiliania huxleyi, indicating an age younger than MIS 8 (Thierstein et al., 1977). The use of calcareous nannofossil assemblages after MIS 8 for biochronological purposes in Arctic Ocean settings is based on pattern recognition in terms of composition and abundance changes (Gard and Backman, 1990; Gard, 1993). In core 18TC, calcareous nannofossils are restricted to the upper 65 cm. The 60 cm and 55 cm levels contain Gephyrocapsa muellerae, E. huxleyi and very small (3 mm) Gephyrocapsa spp. This assemblage is dominated by G. muellerae, indicating an age younger than MIS 7 (Backman et al., 2009). As calcareous nannofossils are rare or absent during MIS 6 in the Arctic Ocean (Backman et al., 2009), the dominance of G. muellerae in samples also holding E. huxleyi implies an age equivalent to or younger than MIS 5. The 45 cm level shows the highest abundances of calcareous nannofossils so far observed from any central Arctic Ocean sediment core, with up to 3000 spec./mm 2 (Fig. 2E). This number is approximately one order of magnitude higher compared to the abundances derived from neighboring cores on the Lomonosov Ridge (Jakobsson et al., 2000; Backman et al., 2009), using identical smear-slide preparation and census data collection techniques. The 45 cm sample contains a near 50/50 ratio between G. muellerae and E. huxleyi, together with common specimens of the large E. huxleyi morphotype, making up nearly 10% of all specimens belonging to this taxon. This conspicuous compositional and abundance pattern could indicate MIS 5.1 as the oldest possible age (Backman et al., 2009), although the fluctuations in abundance among the E. huxleyi plexus and between these forms and Gephyrocapsa spp. still remain ambiguous in terms of Arctic Ocean biostratigraphy and therefore should be used with caution in the MIS 5e3 interval. A single sample at 42 cm holds abundant Thoracosphaera spp., a calcareous dinocyst genus. This sample still shows a dominance of G. muellerae over E. huxleyi. The abundance relationship between these two taxa changes between 40 and 35 cm. E. huxleyi is dominant in the latter sample and remains so throughout the 5e35 cm interval (Fig. 2D). The 35 cm level is within the radiocarbon age range, indicating middle MIS 3. This abundance crossover in MIS 3 time between G. muellerae and E. huxleyi is consistent with observations from two other Arctic Ocean cores (Backman et al., 2009). Calcareous nannofossil biostratigraphy thus suggests an age range from MIS 5 to MIS 3 in the 60e40 cm interval in core 18TC. An indication that this 20 cm interval belongs entirely to MIS

4 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e Table 1 Conventional 14 C ages (half-life 5568 years) calibrated with Marine04 data set (Hughen et al., 2004) in OxCal 4.0 with 2 sigma standard deviation (2 SD) for four different marine reservoir ages. Mid depth (cm) 14 C-dated species Lab no. 14 C age years BP SD DR ¼ 0 cal. age years BP 2SD DR ¼ 300 cal. age years BP 2SD DR ¼ 650 cal. age years BP 2SD DR ¼ 1000 cal. age years BP 0.9 N. pachyderma LuS Mix benthic LuS N. pachyderma LuS C. wuellerstorfi LuS N. pachyderma LuS Mix benthic LuS N. pachyderma LuS , , , , N. pachyderma LuS , , , , , N. pachyderma LuS , , , , , N. pachyderma LuS N. pachyderma LuS , , , , , N. pachyderma LuS , , , , , Mix benthic LuS , N. pachyderma LuS , ,437 a ,096 a , ,272 a N. pachyderma LuS , ,873 a ,512 a , ,618 a N. pachyderma LuS , N. pachyderma LuS , N. pachyderma LuS 7351 >42, C. wuellerstorfi LuS 7350 >44, N. pachyderma LuS 7352 >42, N. pachyderma LuS 7354 >42, C. wuellerstorfi LuS , a Calibration with Fairbanks0107 (Fairbanks et al., 2005) and 1 SD. 2SD 3, is provided by the absence of the benthic foraminifer B. aculeata in all foraminiferal bearing samples, which, if present, would imply a MIS 5 age (Backman et al., 2004, Fig. 5). The Holocene section of core 18TC is marked by increased abundances of Coccolithus pelagicus, together with dominant E. huxleyi and fewer G. muellerae. The increase in abundance of C. pelagicus is typical for Holocene sediments in the Arctic Ocean (Gard and Backman, 1990; Gard, 1993) Regional stratigraphic correlation While the overall age assignments from the calcareous nannofossil stratigraphy remain somewhat ambiguous, the placement of MIS 5 below the upper 65 cm of microfossil rich sediment is consistent with stratigraphic correlations between 18TC/JPC and regional cores for which published age models exist. These include core 96/12-1pc (Jakobsson et al., 2001) and the composite section from the Arctic Coring Expedition (ACEX) (O Regan et al., 2008) (Fig. 3). Based on stratigraphic correlation, the upper 65 cm of 18TC correspond to the upper 40 cm of the ACEX record, which falls within MIS 3. Similarly, well defined variations in bulk density and coarse fraction content associated with MIS 6 and the substages of MIS 5 occur deeper in 18JPC. Stratigraphic tie points thus suggest that MIS 5 lies between 230 and 280 cm in core 18JPC Oxygen and carbon isotopes The d 18 O values for N. pachyderma (s) range between 1.8 and 3.1 & (Fig. 2C). Four samples show heavier values than 2.5& where planktic foraminiferal abundances are low (9.7, 21.7, 39.7 and 63.7 cm). The lightest values occur between 23.7 and 29.7 cm (1.6&). These d 18 O values are within the same amplitude range as observed in other cores from the Lomonosov Ridge as well as from the Chukchi Sea (Nørgaard-Pedersen et al., 1998; Hillaire-Marcel et al., 2004). Oxygen isotopes measured on N. pachyderma in the 100e150 mm fraction from the Chukchi Sea are similar in range to our data set from the Lomonosov Ridge, but a slight offset is observed between 6 and 11 cal ka BP (Fig. 4A). Fluctuations in Pleistocene stable carbon isotopes of planktic foraminifera from the Arctic Ocean have been interpreted in terms of productivity (Stein et al., 1994), suggesting similar behavior of the d 13 C amplitude changes in extra-arctic regions and the Arctic Ocean. The influence of meltwater and the strong stratification must also be taken into account when interpreting carbon isotope records derived from Arctic sediments (Spielhagen and Erlenkeuser, 1994; Bauch et al., 2000; Spielhagen et al., 2004). In core 18TC, the d 13 C curve shows two coherent intervals with values ranging between 0.5& and 0.9&, from 0.9 to 9.7 cm and from 37.7 to 63.7 cm, respectively. Values in the intervening interval between 9.7 and 37.7 cm show a range from 0.1& to 0.4& (Fig. 2C). A change of 0.5& towards lighter values occurs between 37.7 and 33.7 cm. Similar negative changes from other Arctic cores are interpreted in terms of meltwater input which is low in 13 C (Nørgaard-Pedersen et al., 1998). The amplitude range of the carbon isotope values in core 18TC is similar to values observed in the late Pleistocene and Holocene interval from the Chukchi Sea (Hillaire-Marcel et al., 2004). They indicated that d 13 C values for N. pachyderma are size dependent, with larger specimens showing progressively heavier carbon isotopes. Our carbon isotope data from the Lomonosov Ridge were acquired from specimens >150 mm and most specimens analyzed were >200 mm. The middle to late Holocene d 13 C amplitude range in this size fraction corresponds well to the core top (Holocene) values obtained from the 100e150 mm size fraction in the Chukchi Sea (Fig. 4A; Hillaire-Marcel et al., 2004, Fig. 5). A study of sinistrally coiled N. pachyderma from water column samples, however, suggests that the d 13 C in this taxon represents an integration of the uppermost 200 m of the water column (Kohfeld et al., 1996). Although the details of the d 13 C relationships in sinistrally coiled N. pachyderma are not yet fully understood, as pointed out by Hillaire-Marcel et al. (2004), the distinct stratigraphic distribution of the stable carbon isotope values in core 18TC makes it reasonable to interpret them in terms of productivity, the two intervals showing values in excess of 0.4& would then represent times of increased productivity when the summer sea ice cover was broken up permitting primary productivity in relatively open waters. Based

5 3434 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e3441 Fig. 2. Summary of microfossil (PF ¼ planktic foraminifera; BF ¼ benthic foraminifera) and calcareous nannofossil results against sediment depth (cm) with a suggested age model in Marin Isotope Stages (MIS) based on radiocarbon dates and nannofossil assemblage. A. Finite uncalibrated planktic (filled squares) and benthic (open squares) 14 C ages ka before present (BP) (se also Table 1). B. Abundance of planktic and benthic foraminifera (>125 mm) per gram sediment and samples with uncalibrated planktic radiocarbon ka BP. C. Stable oxygen (black line) and carbon (gray line) isotopes measured on N. pachyderma (s) >150 mm; gray horizontal lines indicating interval with low d 13 C values. D. Relative abundance of E. huxleyi, Gephyrocapsa spp. and C. pelagicus. E. Total abundance of number of coccolith specimens per 1.24 mm 2. F. Coarse fraction (>63 mm) in percent dry weight. on the carbon isotope record these two intervals could thus correspond to interglacial or interstadial conditions, separated by an interval with less productivity under more severe sea ice conditions. Similar patterns have been discussed by Nørgaard- Pedersen et al. (2003) Grain size The coarse fraction (>63 mm) is mostly below 5% in core 18TC, the exception being four samples with values between 7 and 12% of the dry weight sediment (Fig. 2F). Shell fragments and sand (27.7 cm), small pebbles (29.7 cm), bryozoan fragments (39.7 cm), and terrigenous sand (43.7 cm) explain the increase of coarse fraction material in these four samples. In some settings, the mean size of the sortable silt fraction can be used as a proxy for near-bottom paleocurrent velocity as it varies independently of sediment supply in current-sorted sediments with higher values representing higher current flow speeds (McCave et al., 1995). Grain size studies on modern sea ice sediment from the Arctic Ocean all report a bias towards an increased silt

6 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e Fig. 3. Upper two panels show coarse fraction and bulk density contents of core 18TC/JPC. Tie points for correlation with cores 96/12-1pc and ACEX are indicated by dashed lines. The lowermost panel shows stacked bulk density records using tie points to place 96/12-1pc and 18TC/JPC on the ACEX depth scale (referred to as the revised meters composite depth (rmcd)). The stacked records clearly illustrate the strong correlation between downhole/core physical properties among these cores. MIS 5 and MIS 6 stage boundaries in the ACEX record (O Regan et al., 2008) are shown for reference. Fig. 4. Isotopic records of N. pachyderma sinistral (Npl) plotted against time in e A. different size fractions from the Chukchi Sea. Redrawn from Hillaire-Marcel et al., 2004; B. four locations in the Eurasian Basin (PS2177 and PS2185 from the Lomonosov Ridge, PS2195 from the Amundsen Basin, and PS2166 from the Gakkel Ridge). Redrawn from Nørgaard- Pedersen et al., 1998 (data from Isotopic results from core 18TC are plotted in gray in all panels. The 14 C ages of core 18TC are calibrated with a marine reservoir age of 650 years in order to make them comparable with the Chukchi Sea record, and 400 years for the Eurasian Basin, as applied in the original papers.

7 3436 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e3441 sized fraction (Clark and Hanson, 1983), which complicates simple interpretations of sortable silt size in Arctic sediments. The mean grain size of sortable silt ranges from 19.7 to 31.2 mm in core 18TC (Fig. 5). In general, the variations are within 2 mm difference between peaks and troughs, which is significantly higher than the precision of the method (Bianchi et al., 1999). The most prominent features in the mean sortable silt record are two large peaks centered at 21.7 and 27.7 cm (Fig. 5). The prominent mean sortable silt peak at 27.7 cm corresponds to an increase in foraminiferal abundance and a corresponding increase in both the coarse fraction content and the weight % of sortable silt. Conversely, the peak in mean sortable silt size at 21.7 cm corresponds to a decrease in the weight % of sortable silt, coarse fraction content and planktic foraminiferal abundance. Thus the observed size variations in the two sortable silt peaks appear to be linked to very different depositional patterns. Where the mean size of the sortable silt increases and the weight % of the sortable silt decreases, one may infer that this is a current controlled signal, where sortable silt is being winnowed away leaving a preferential enrichment of coarser silt. Where the coarse fraction content, mean size of the sortable silt and the weight % of the sortable silt increase together (as is seen through most of the record) this likely reflects variations in sea ice and iceberg input Radiocarbon age model In general, the 14 C ages follow a trend down to 31.7 cm with a succession of finite ages, followed by infinite ages between 33.7 and 57.7 cm (Fig. 2, Table 1). As the Lund SSAMS system provides reliable dates of samples down to 200 mg of carbon, the 50 mg Cin sample 57.7 cm was likely too small and a contamination with 1 mg modern material will result in a radiocarbon age of ca 32,000 BP for an infinitely old sample. Benthic to planktic age differences were calculated from the uncalibrated 14 C ages in four samples between 0.9 and 27.7 cm, three of which show older benthic ages (Fig. 6A). The age difference between the samples taken at 11.2 cm (planktic) and 11.7 cm (benthic) does not suggest an older benthic age implying that the bottom waters should be younger than the surface waters. The other three benthiceplanktic pairs show a succession from a w1200-year age difference at 27.7 cm to w700 years at 7.7 cm and finally w250 years difference in the 0.9 cm sample. Similar age differences are demonstrated for the modern Arctic Ocean by Schlosser et al. (1997): w250 years in the Eurasian Basin and w450 years in the Canadian Basin. However, the limited number of samples available for plankticebenthic age comparison together with effects of low sedimentation rates, bioturbation and the use of mixed in- and epifaunal species may bias the age comparison results. We applied four different approaches for reservoir correction (Table 1). Approaches 2e4 below are based on a model by Butzin et al. (2005), providing more explicit values for the study area at different water depths in different simulations. Specific reservoir age values were provided by Martin Butzin (personal communication, September 2008). 1. The global model ocean reservoir age as implemented in Marine04 with reservoir age changes between 300 and 500 years due to shifts in atmospheric 14 C production from 0e10.5 cal ka BP. The reservoir age is kept constant beyond 10.5 ka BP at years (Hughen et al., 2004). No regional correction (DR ¼ 0) was used. 2. A marine reservoir age of 700 years (DR ¼ 300), based on Butzin s present day simulation for the 50e100 m water mass layer. Fig. 5. Mean sortable silt (SS) calculated from the 10e63 mm fraction (thick line), weight % sortable silt (blue dashed line), coarse weight fraction (weight % >63 mm) (red dashed line), planktic foraminifera (PF) per gram sediment of the 63e100 mm size fraction (open squares) and >125 mm (closed squares). The shaded box indicates a current influenced signal (pink) and the light blue shaded box an example showing an enhanced sea ice input signal. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 3. A marine reservoir age of 1050 years (DR ¼ 650) based on Butzin s basic glacial simulation with GLAMAP sea surface temperatures. This should give the best representation of the Lateglacial time interval (Martin Butzin personal communication, September 2008), or the end of the Last Termination dated to 14.7e11.7 ka cal BP (Björck et al., 1998; Walker et al., 2009). 4. A doubling of the assumed present day marine reservoir age from 700 years to 1400 years (DR ¼ 1000). This value is a result from the Butzin et al. (2005) present day model for the surface layer (0e50 m) when sea ice inhibits airesea exchange of radiocarbon, when only limited mixing occurs immediately below the sea ice cover. To determine which DR values are most appropriate for calibration, we use the Younger Dryas (cold event ca 11.7e12.8 ka cal BP, Muscheler et al., 2008; Walker et al., 2009), which will correspond to different depth intervals depending on which of the four 14 C calibration approaches we chose.

8 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e Fig. 6. A. Calibrated radiocarbon ages for DR ¼ 0 and the preferred model as described in the text DR ¼ 300/1000 with 2 sigma standard deviation showing changes in linear sedimentation rate. Gray bars on the left show benthic (B)eplanktic (P) pair age difference years. B. Planktic foraminiferal abundance (PF) plotted against calibrated age according to the same calibration models as above. Proposed cold (broad gray lines) and warm (thin gray lines) time intervals as deducted from foraminiferal abundance in the DR ¼ 300/1000 scenario. The calibration using a DR ¼ 1000 places the Younger Dryas at a prominent low in planktic and benthic foraminifera abundance (Fig. 7) and planktic mass accumulation rates (not presented here). The opposite occurs for the calibration using a DR ¼ 0. The middle of the Younger Dryas cold interval then coincides with a foraminiferal abundance peak (Fig. 7). It seems logical that a North Atlantic/Northern Hemisphere cold interval would lead to a lower production of foraminifera in the Arctic Ocean. We thus suggest that a DR of 1000 years seems more appropriate for this time interval. For the samples representing the post-younger Dryas period we apply a DR of 300 years following Butzin et al. (2005). Although a higher DR value could be applied for the interval between about 9.7 and 13.7 cm as this interval coincides with a foraminiferal abundance low similar to the Younger Dryas (Fig. 7). The radiocarbon based age model suggests that sediments deeper than 37 cm were deposited prior to the last glacial maximum, and that the interval between 37 and 65 cm occurs within MIS 3, consistent with both the absence of B. aculeata, the calcareous nannofossil biostratigraphy and the stratigraphic correlation with neighboring records. Linear sediment accumulation rates are 1.3e3.3 cm/ka with DR ¼ 300/1000 for the age calibration as described above (1.3e2.3 cm/ka with DR ¼ 0) in the upper 18 cm of the core. The rates increase to 7.0e9.4 cm/ka and 8.8e11.2 cm/ka, respectively, in the interval 18e28 cm representing the Lateglacial (Fig. 6A). We discarded the 14 C age reversal at 25.7 cm when calculating sedimentation rates. The accumulation rate is about four times higher in the Lateglacial/deglacial period than during the Holocene which could reflect a high freshwater input with sediments from decaying ice sheets and more summer sea ice melt resulting in increased sediment influx. 4. Discussion The radiocarbon dating results for core 18TC divide the core into two sections: the upper 37 cm with finite 14 C ages and the lower part between 37 and 65 cm with infinite 14 C ages. The uncalibrated 14 C ages in the upper 37 cm of the core show that the sediment was deposited between MIS 1 and MIS 3 (Lisiecki and Raymo, 2005). In comparison to previously studied cores in the central Arctic Ocean (Table 2) our 30 cm thick stratigraphic sequence of Holocene to Lateglacial age can be considered a high resolution record. Geographically close records from the Lomonosov Ridge crest contain Holocene to Lateglacial sequences of 5e20 cm (Gard, 1993; Jakobsson et al., 2000; Jakobsson et al., 2001; Phillips and Grantz, 2001; Spielhagen et al., 2004). In order to make detailed interpretations of paleoceanographic changes and correlate these to terrestrial and ice-core records, calibration with good estimates of the regional deviation of marine reservoir ages (DR) becomes important. The marine reservoir age for the central Arctic Ocean through the time span of radiocarbon dating remains unknown and

9 3438 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e3441 Fig. 7. Foraminiferal abundance in the upper 37 cm of core 18TC with calibration results for the four marine reservoir approaches described in the text; bold numbers surrounded by black boxes indicate the preferred age scenarios, gray boxes an alternative scenario for the early Holocene. Possible location for the Younger Dryas (Muscheler et al., 2008; Walker et al., 2009) according to foraminiferal abundances indicated in gray. the applied 14 C age correction and/or calibration vary greatly between different studies. Most commonly 400e450 years are used (Darby et al., 1997; Nørgaard-Pedersen et al., 1998; Poore et al., 1999; Darby et al., 2002; Darby, 2003; Nørgaard-Pedersen et al., 2003; Polyak et al., 2004; Spielhagen et al., 2004; Nørgaard- Pedersen et al., 2007b; Cronin et al., 2008; Adler et al., 2009) but some studies used higher reservoir ages of 550 years (Stein et al., 1994), 750 years (Hillaire-Marcel et al., 2004) or the Marine04 data set (Hughen et al., 2004) with a DR of 460 years (Darby et al., 2009). Most available DR values for the Arctic are derived in regions south of 81 N using mollusks found in coastal regions (Svalbard, Greenland, Canadian Arctic Archipelago, and Franz Josef Land) (Marine Reservoir Correction Database ( marine/), McNeely et al., 2006). Our core is located at 88 N in the central Arctic Ocean far from the northernmost corrected site and in a significantly different marine environment. Therefore, we investigated the effects of modeled reservoir ages based on Butzin et al. (2005) and compared these with the baseline Marine04 with DR ¼ 0 calibration results. From the Butzin et al. model scenarios we chose reservoir ages of 700 years (DR ¼ 300), 1050 years (DR ¼ 650), and 1400 years (DR ¼ 1000) according to different simulation runs at core 18TCs location (M. Butzin, pers com.). The Present day simulation yields a reservoir age as old as 1400 years in the uppermost water layer (0e50 m) when sea ice inhibits airesea gas exchange of 14 C and limited mixing occurs immediately below the sea ice cover, whereas the second model layer (50e100 m) has a reduced reservoir age of 700 years and better represents the present day oceanographic and climatic settings. To illustrate the different interpretations we concentrate on DR ¼ 0 and DR ¼ 1000 calibrated ages and the resulting position of the Younger Dryas cold event, 11.7e12.8 ka BP (Muscheler et al., 2008; Walker et al., 2009). According to the DR ¼ 0 calibration the Younger Dryas would be between 15.7 and 17.4 cm, coincident with a peak in planktic foraminifera abundance (Fig. 7) and planktic accumulation rate (data not presented here). With the DR ¼ 1000 age scenario the Younger Dryas is placed between 20.0 and 25.4 cm, where a low in foraminiferal abundance occurs. By arguing that heavier summer sea ice prevailed during the Younger Dryas cold event, resulting in reduced primary production, it seems more reasonable that the Younger Dryas sediments occur between 19 and 26.5 cm. However, not only changes in climatic conditions may influence foraminiferal abundance fluctuations in analyzed sediment cores: other factors such as changes in sedimentation rates or bottom currents could potentially play a role. For simplicity, we assume that the apparent age of the surface water remains constant through most of the Holocene (DR ¼ 300) and the Lateglacial (DR ¼ 1000), even if it is very likely that the reservoir age fluctuated within these times. For instance, records from the Norwegian west coast show an increase from 400 to C years in the mid-younger Dryas (Bondevik et al., 2006), while in the North Atlantic deep sea cores there is a 1200e C years reservoir time at the end of Heinrich event 1 and 930e C years at the end of the Younger Dryas (Waelbroeck et al., 2001) and C years in the Norwegian Sea during the Last Deglaciation 15e11 cal ka BP (Björck et al., 2003). Variation through time is also shown in a model by Franke et al. (2008) ( reservoirage.uni-bremen.de). However, variable residence time of 14 C is indicated by the sequence of age difference changes in benthiceplanktic 14 C pairs through the upper part of the core, displaying a slow decrease from 1200 years in the Lateglacial to 700 Table 2 Comparison of Marine Isotope Stage (MIS) 1 sediment sequence thicknesses across the central Arctic Ocean. Location MIS 1 Reference sequence Nansen 20 cm Baumann (1990), Gard (1993) Basin Amundsen 26 cm Gard (1993) Basin Lomonosov Ridge 5e20 cm Gard (1993), Jakobsson et al. (2000), Phillips and Grantz (2001), Spielhagen et al. (2004) Makarov Basin 8e15 cm Gard (1993), Nowaczyk et al. (2001) Alpha Ridge 10e15 cm Aksu (1985), Herman (1974), Spielhagen et al. (2004) Mendeleev 10e26 cm Adler et al. (2009), Phillips and Grantz (2001) Ridge

10 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e years in the mid Holocene and 250 years in the late Holocene. This change in the surface to bottom water age difference may be an indication for circulation and/or ventilation changes driven by variations in the inflow of North Atlantic water and/or more/less deep-water formation through brine release by sea ice. Our finding of a w250 years age difference in the sediment surface sample is in accordance with Schlosser s et al. (1997) modern D 14 C measurements, showing an isolation time of approximately 250 years for Eurasian Basin bottom waters (w450 years for Canadian Basin deep water). Our final age model places the foraminiferal abundance high between 10 and 11.6 ka BP coinciding with the peak of precessiondriven summer insolation (Berger and Loutre, 1991). It is interesting to note that Kaufman et al. (2004) recognized that the warming after the last glacial period was time-transgressive across the western Arctic. While warm conditions prevailed during the insolation maxima in northwest North America, cool conditions dominated in northeast Canada where the Holocene Thermal Maximum was delayed by several thousand years. Our results suggest that the Arctic Ocean may have responded more or less directly to the increased insolation and, thus, in phase with northwest North American climate. After 10 ka BP the foraminiferal abundance decreases in core 18TC before increasing again to reach a peak at about 7 ka BP in the Holocene (Fig. 6). A comparable pattern can also be seen in some of the cores described by Nørgaard-Pedersen et al. (1998). If we assume that sea ice extent, coverage and thickness is related to the abundance or absence of microfossils in the sediment, this decrease suggests a return to colder conditions. This assumption is supported by the results of Kellogg (1980) who showed that high planktic foraminiferal abundances are associated with increased surface water productivity as a result of less ice and/or more open leads. Furthermore, a higher absolute abundance of living planktic foraminifera occurs in areas of the Fram Strait and the Nansen Basin having less sea ice or a lack of sea ice, or along a relatively stable sea ice margin (Carstens and Wefer, 1992; Carstens et al., 1997). Applying this concept on core 18TC, the foraminiferal peak at 57.7 cm, in combination with our age model, suggests more favorable conditions for primary production and a reduced sea ice cover during MIS 3 compared to the Holocene. This interpretation is consistent with the calcareous nannofossil data. The significant difference in nannofossil abundance between MIS 3 and MIS 1, 3000 versus 100 spec./1.24 mm 2, respectively, presumably reflects variable inflow of Atlantic waters to the central Arctic Ocean. The total number of both planktic and benthic foraminifera per gram sediment is also significantly lower in MIS 1 compared to MIS 3, which points towards different circulation regimes during the MIS 3 interstadials and the present interglacial. However, we cannot exclude that the higher abundances of foraminifera and calcareous nannofossils found in MIS 3 instead are caused by a condensation of specimens per unit of sediment due to generally lower clastic sedimentation rates. The d 13 C values are often used to track changes in freshwater or carbon input (Spielhagen and Erlenkeuser, 1994; Stein et al., 1994; Bauch, 1997; Nørgaard-Pedersen et al., 1998; Poore et al., 1999; Volkmann and Mensch, 2001; Polyak et al., 2004). During early MIS 3 and the later part of the Holocene d 13 C values are in the Arctic Ocean found to be in the same order (around 0.9&) as the modern values (Spielhagen and Erlenkeuser, 1994), whereas both the late MIS 3 and Lateglacial show lower values in core 18TC. Similar negative shifts in the carbon isotope curve ( 0.5& around 37 cm) can be found in other central Arctic Ocean sediment cores and are correlated by e.g. Nørgaard-Pedersen et al. (1998) to a meltwater event at isotope stage 3.31 (55.5 ka BP). The low points in the planktic foraminifera abundance curve in connection with increasing d 18 O values similarly reflect colder climate with higher salinity (Volkmann and Mensch, 2001), less inflow of freshwater and probably more sea ice. We have compared core 18TCs oxygen and carbon stable isotope values encompassing the last 13 ka from the Lomonosov Ridge with Hillaire-Marcel s et al. (2004) from the Chukchi region (Fig. 4A). That core 18TCs oxygen and carbon isotope curve is comparable to the Chukchi small size fraction (100e150 mm) implies that either the foraminifera at the Lomonosov Ridge site lived at a shallower water depth or that the Lomonosov pycnocline water had a similar isotopic composition as the Chukchi surface water. It also suggests that the warmer North Atlantic water inflow recorded in the Chukchi region as seen by the depletion in d 18 O in the >250 mm planktic foraminifera in the early Holocene did not reach core 18TCs location. While comparing core 18TC to other cores from the Lomonosov Ridge (PS2177 and PS2185), Amundsen Basin (PS2195) and Gakkel Ridge (PS2166) (Nørgaard-Pedersen et al., 1998), the oxygen isotope curve follows the Lomonosov Ridge cores well, but a larger spread can be seen between these cores and those from the Amundsen Basin and Gakkel Ridge at least between 6 and 14 ka BP (Fig. 4B). After 6 ka BP the curves more or less overlap, indicating more similar and uniform surface water properties. The age model(s) for core 18TC show some intervals with relatively high linear sedimentation rates, but also reoccurring episodes with no or extremely low sedimentation rates. The missing interval around the Last Glacial Maximum seems to be a regional phenomenon that has been observed in many central Arctic sediment cores (e.g. Darby et al., 1997; Nørgaard-Pedersen et al., 1998; Poore et al., 1999; Polyak et al., 2004, 2009). The foraminifera and nannofossil bearing sediments (0e65 cm), appears to be confined to the MIS 3 through MIS 1 interval. The high nannofossil abundance in MIS 3 is unique for a central Arctic Ocean setting, being roughly one magnitude higher compared to other cores from the Lomonosov Ridge region (Backman et al., 2009), which we cannot fully explain. However, the occurrence of nannofossils during MIS 3 might generally be related to open leads in the pack ice and Atlantic water influx at this time. That this interval is not of MIS 5 age is constrained by correlation of physical properties of core 18TC/JPC to other nearby cores and the absence of typical stage 5 benthic foraminifera indicators as E. exigua and B. aculeata (Backman et al., 2004; Nørgaard-Pedersen et al., 2007b). 5. Conclusions Jumbo piston core HLY JPC and its trigger core 18TC from the Intra Basin of the central Lomonosov Ridge are characterized by an approximately 65 cm thick MIS 1e3 sediment sequence. The linear sedimentation rates during the last deglaciation calculated for this coring site range between 7 and 9 cm/ka, while other cores retrieved from the central part of the Lomonosov Ridge crest commonly show sedimentation rates between 1 and 3 cm/ka. Our age model of the MIS 1e3 sequence is based on 22 radiocarbon dated foraminifera samples and nannofossil biostratigraphy. In addition to relatively high sedimentation rates, the HLY TC/ JPC cores from the Intra Basin are also characterized by exceptionally high abundances of calcareous micro- and nannofossils during parts of MIS 3. We suggest that the abundance fluctuations of calcareous planktic micro- and nannofossils throughout the studied MIS 1e3 sediment sequence represent variations of the Arctic Ocean summer sea ice cover and inflow of North Atlantic water. The stratigraphic position of the Younger Dryas cold interval is inferred from a prominent drop in foraminiferal abundance assuming that this is a sign of colder conditions and more summer sea ice. Together with radiocarbon dates of four benthiceplanktic

11 3440 D. Hanslik et al. / Quaternary Science Reviews 29 (2010) 3430e3441 foraminifera pairs, this suggests a marine reservoir age correction for radiocarbon ages of planktic foraminifera from the Intra Basin of around 1200 years for the early Holocene and 700 years after 11 ka. Furthermore, the paired benthiceplanktic radiocarbon ages show a trend of declining differences from early to late Holocene. We interpret this decreasing age difference between surface and bottom water masses to represent an increased bottom water ventilation related to a general circulation change. Acknowledgments Financial support was received from the Swedish Research council (VR), the Swedish Royal Academy of Sciences through a grant financed by the Knut and Alice Wallenberg Foundation, the Swedish Society for Anthropology and Geography (SSAG) and the Ymer-80 Foundation. Klara Hajnal is thanked for the stable isotope analyses and Åsa Wallin for grain size analyses. 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