Stable oxygen and carbon isotopes in planktonic foraminifera Neogloboquadrina pachyderma in the Arctic Ocean: An overview of published and new surface-sediment data

    Stable oxygen and carbon isotopes in planktonic foraminifera Neogloboquadrina pachyderma in the Arctic Ocean: an overview of published and new surface-sediment data Wenshen Xiao, Rujian Wang, Leonid Polyak, Anatolii Astakhov, Xinrong Cheng PII: DOI: Reference:

S0025-3227(14)00086-3 doi: 10.1016/j.margeo.2014.03.024 MARGO 5085

To appear in:

Marine Geology

Received date: Revised date: Accepted date:

30 May 2013 25 March 2014 27 March 2014

Please cite this article as: Xiao, Wenshen, Wang, Rujian, Polyak, Leonid, Astakhov, Anatolii, Cheng, Xinrong, Stable oxygen and carbon isotopes in planktonic foraminifera Neogloboquadrina pachyderma in the Arctic Ocean: an overview of published and new surface-sediment data, Marine Geology (2014), doi: 10.1016/j.margeo.2014.03.024

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ACCEPTED MANUSCRIPT Stable oxygen and carbon isotopes in planktonic foraminifera Neogloboquadrina pachyderma in the Arctic Ocean: an overview of

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published and new surface-sediment data

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Wenshen Xiao 1, Rujian Wang 1, Leonid Polyak 2, Anatolii Astakhov 3, Xinrong Cheng 1

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1. State key laboratory of Marine Geology, Tongji University, Shanghai 200092, China 2. Byrd Polar Research Center, Ohio State University, Columbus, OH 43210, United

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States

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3. V. I. Il’ichev Pacific Oceanological Institute, Russian Academy of Sciences,

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Vladivostok 600041, Russia

[Corresponding author] E-mail: [email protected]

Abstract

A summary study on oxygen and carbon stable isotopes in planktonic foraminifera from Arctic Ocean floor appeared in Marine Geology 20 years ago (Spielhagen and Erlenkeuser, 1994). We revisit this topic with a wealth of new data focused on the western (Amerasian) Arctic, a spotlight in the current climatic and oceanic change. Neogloboquadrina pachyderma, the most abundant polar planktonic foraminiferal species, is an important tool for reconstructing surface/subsurface water changes in the 1

ACCEPTED MANUSCRIPT Arctic. Our new data on N. pachyderma (150-250 m) from the surface sediments show δ18O and δ13C values of <1.5‰ and 0.8-1.5‰, respectively, in the ice covered Canada

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Basin, and 2-3.5‰ and 0.6-0.9‰, respectively, at the shelf break. Combined data from

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the western and eastern Arctic indicate that planktonic δ18O and δ13C are both influenced

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by complex water sources, water column structure, and foraminiferal depth habitat. The observed distribution of N. pachyderma stable isotopes confirms a shallow habitat of this

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species in the sea-ice covered central Arctic, especially in the Canada Basin, probably in relation to the shallow chlorophyll maximum. The associated light N. pachyderma δ18O

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reflects the long-term storage of fresh water. At the shelf break, a deeper dwelling of N. pachyderma with heavier δ18O is supported by a more extensive photic zone and nutrient

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availability. Air-sea exchange plays an important role in δ13C distribution and is

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consistent with heavy N. pachyderma δ13C in the perennially ice-covered central Arctic

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Ocean. Light δ13C composition at the shelf break is additionally influenced by shelf bottom waters enriched in isotopically light, remineralized terrestrial carbon. Distribution of foraminiferal δ13C on the Chukchi Shelf reflects primary production patterns in the area.

Keywords:

Arctic

Ocean,

seafloor

sediments,

planktonic

foraminifers,

Neogloboquadrina pachyderma, δ18O, δ13C

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Introduction

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ACCEPTED MANUSCRIPT The stable oxygen and carbon isotopes (δ18O and δ13C) of calcite secreted by organisms such as foraminifera are widely used for studying paleo-sea water temperature

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and salinity (δ18O), or oceanic circulation and carbon cycle (δ13C) (e.g., Rohling and

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Cooke, 1999; Ravelo and Hillaire-Marcel, 2007). Neogloboquadrina pachyderma (formerly referred to N. pachyderma sinistral coiling form, Darling et al., 2006) is the

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dominant planktonic foraminifera species in polar oceans, often comprising more than

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90% of the planktonic foraminifera assemblage in the Arctic ocean (Volkmann, 2000; Eynaud et al., 2011). N. pachyderma calcifies with a nearly consistent offset of 0.8-1‰

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from equilibrium conditions with ambient waters, where growth and encrustation of tests co-occurs with sinking along the pycnocline (Kohfeld et al., 1996; Ortiz et al., 1996;

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Bauch et al., 1997). Stable oxygen and carbon isotopes in N. pachyderma tests have become important tools for reconstructing Arctic paleoenvironment such as circulation

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(e.g., Stein et al., 1994; Polyak et al., 2004; Adler et al., 2009), sea ice formation (Hillaire-Marcel and de Vernal, 2008), and glacier melt water events (Lubinski et al., 2001; Knies and Vogt, 2003; Spielhagen et al., 2004).

Due to heavy sea ice cover in the Arctic Ocean, investigations of the N. pachyderma ecology and distribution in the water column have been scarce (Kohfeld et al., 1996; Carstens et al., 1997; Bauch et al., 2000; Volkmann, 2000; Volkmann and Mensch, 2001), which limits our ability to interpret paleoenvironmental implications of its stable-isotopic data in sedimentary records. This deficiency can be compensated to some extent by investigating N. pachyderma in surface sediments of the Arctic Ocean floor as an analogue for geological records. A summary study on the distribution of N. pachyderma

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ACCEPTED MANUSCRIPT stable isotopes in the Arctic surface sediments was published 20 years ago (Spielhagen and Erlenkeuser, 1994) and has been widely used in Arctic paleoceanographic research

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since then. The paper showed a strong overall decrease in N. pachyderma δ18O from the

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Fram Strait towards the central Arctic Ocean along with generally high δ13C values in the high Arctic. The interpretation suggested a good relationship between N. pachyderma

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δ18O and sea water salinity at the inferred habitat depths, and argued for the influence of

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Atlantic water in the distribution of stable-isotope composition in the western Eurasian Basin, and inferred for a shallower habitat reflecting lower salinities towards the north

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and east. However, the work was mainly focused on the Eurasian part of the Arctic Ocean due to very limited sample coverage and large uncertainties with sediment age in

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the heavily ice covered Amerasian Arctic, which bears profoundly different oceanographic settings. This shortcoming considerably limits the interpretation of the

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distribution of N. pachyderma stable isotopes and their application to paleo-records.

In order to achieve a broad coverage of the entire Arctic Ocean and to account for the water-column data on N. pachyderma and hydrographic compilations generated during the last 20 years, we investigate a large data set of N. pachyderma δ18O and δ13C in surface sediments from the Amerasian Arctic including the Canada Basin and the Chukchi margin (Fig. 1), where sea ice retreat and associated climate and biotic changes are especially pronounced today (e.g. Stroeve et al., 2011). The new data are integrated with the previous studies for a comprehensive overview of the N. pachyderma stable isotope distribution in the Arctic Ocean. Data are analyzed for their relationship to

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ACCEPTED MANUSCRIPT properties of the upper water column and interpreted for (paleo)environmental

Arctic Ocean hydrography

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2.

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implications.

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The Arctic surface circulation system that controls the distribution of sea ice mainly

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consists of the Transpolar Drift (TD) from the Siberian shelf through the Eurasian Basin to the Fram Strait, and the clockwise circulating Beaufort Gyre (BG) in the Amerasian

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Basin (Fig. 1). Pacific water enters the Arctic Ocean via the shallow Bering Strait and Chukchi Sea with an average water depth of ~50 m. Three water masses can be divided

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in the Pacific inflow; from west to east they are the nutrient rich Anadyr Current (AC) with relatively high salinity and low temperature, the Bering Sea Shelf Water (BSSW),

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and the warm, freshened Alaska Coastal Current (ACC) (Weingartner et al., 1998; 2005; Weingartner, 2001; Woodgate et al., 2005) (Fig. 1). The Atlantic water enters the Arctic Ocean through Fram Strait and the Barents Sea and sinks to depths below 200 m, forming the Arctic Intermediate Water that circulates anti-clockwise around the Arctic Ocean basins (Coachman and Barnes, 1963; Anderson et al., 1994; Jones, 2001; Woodgate et al., 2007). In the surface layer the Arctic Ocean receives large amount of riverine water (Aagaard et al., 1981; Holmes et al., 2002; Dittmar and Kattner, 2003). Due to the large storage of fresh water in the Beaufort Gyre, the halocline is deeper in the Amerasian than Eurasian Arctic (Fig. 2). Sea ice covers the entire Arctic Ocean in winter extending to the northern Bering Sea, and retreating to the Chukchi shelf break in summer (Parkinson and Cavalieri, 2008). Sea ice affects the surface/subsurface waters by forming brines during

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ACCEPTED MANUSCRIPT its freeze-up in cold season and by the input of melt water in summer (Bauch et al., 2011a). Sea ice also serves as an important control on the distribution of biological

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production in the Arctic Ocean (Wang et al., 2005; Carmack et al., 2006; Grebmeier et al.,

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Arctic N. pachyderma ecology and isotope studies

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2006).

Studies of N. pachyderma ecology and habitats in the Arctic have been limited and

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mainly focused on the Eurasian part. It has been shown that N. pachyderma in the northern polar regions calcifies at variable depths that range from the mixed surface layer

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to a few hundred meters (Kohfeld et al., 1996; Bauch et al., 1997). In the Fram Strait its depth distribution suggests a preference of the Atlantic water underlying the cold polar

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surface water between 50 and 200 m (Kohfeld et al., 1996; Carsten et al., 1997). At the shelf break of the Laptev and Barents seas the maximum abundance of living N. pachyderma was found between 50 and 100 m depth (Volkmann, 2000). In the Nansen Basin south of 83 °N, it was found to prefer waters below the pycnocline at ~100 m, whereas, further north maximum abundances occurred in the upper 50 m (Carstens and Wefer, 1992). Bauch et al. (1997) has shown that N. pachyderma habitat in the Nansen Basin may change from ~150 m in the south to ~80 m in the north, but the calcification depth varies between 100 and 200 m. In the Nordic Seas it calcifies between 70 and 250 m off Norway, closer to the sea surface (20-50 m) in the Arctic water domain of the western Nordic Seas, and at 70-130 m yet further west in the East Greenland Current (Simstich et al., 2003). In the Northeast Water Polynya highest abundance was found in

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ACCEPTED MANUSCRIPT the surface 20-80 m (Kohfeld et al., 1996). Scarce investigations in the western Arctic also suggest N. pachyderma habitat preference of 0-100 m in the Canadian Archipelago

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and Baffin Bay (Stehman, 1972; Vilks, 1970; 1975). The controls on N. pachyderma

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habitats in the Arctic are poorly understood. In some studies it was regarded as a pycnoline species (Hilbrecht, 1997; Hillaire-Marcel et al., 2004), while other

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investigations confirmed its affinity to chlorophyll maximum associated with elevated

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food supply (e.g. Fairbanks and Wieber, 1980; Kohfeld et al., 1996; Schiebel et al., 2001).

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With a limited knowledge of its ecology, variations in N. pachyderma stable-isotopic signature are incompletely understood. Based on the investigation of its stable isotopes

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from 139 surface sediment samples, mainly from the Eurasian Arctic Ocean, Spielhagen

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and Erlenkeuser (1994) inferred salinity as the main control of N. pachyderma δ18O, in

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agreement with the plankton-tow study at the Laptev Sea shelf break (Volkmann and Mensch, 2001). About 1‰ δ18O offset from the equilibrium calcite of the ambient water was found in plankton net samples off the East Greenland, in the Nansen Basin, and at Laptev Sea shelf break (Kohfeld et al., 1996; Bauch et al., 1997; 2000; Volkmann and Mensch, 2001). This offset is attributed to the vital effect, and a concomitant 1-2‰ δ13C offset may be exacerbated by the Suess effect from isotopically light anthropogenic CO2 input (Bauch et al., 2000; Volkmann and Mensch, 2001). According to the habitat difference between foraminiferal species Turborotalita quinqueloba and N. pachyderma in the Nordic Seas, Simstich et al. (2003) suggested using their δ18O difference as an indicator of surface water stratification. N. pachyderma δ18O depletions in Arctic paleoceanographic records have been interpreted by various authors as freshwater input

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ACCEPTED MANUSCRIPT events (Lubinski et al., 2001; Polyak et al.. 2004; Spielhagen et al., 2004; Adler et al., 2009). Light excursions of δ18O have also been interpreted as potential indicators of sea

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ice production by way of rejection of isotopically light brines (Hillaire-Marcel and de

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Vernal, 2008). Clarifying the controls on N. pachyderma isotopic signature is needed for

Materials and methods

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a comprehensive interpretation of paleoceanographic environments in polar regions.

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4.1. Materials

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The new materials used in this study include 149 surface sediment samples (0-2 cm) collected by box, multi and gravity cores from the western Arctic Ocean and northern

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Bering Sea during the I-IV Chinese National Arctic Research expeditions (CHINARE IIV: 1999, 2003, 2008, 2010) by R/V Xuelong (CAAA, 2000; Zhang, 2004; Zhang, 2009; Yu, 2011), and 28 box core samples mainly from the western Chukchi shelf collected during the 2012 cruise of the Russian-American Long-term Census of the Arctic (RUSALCA-2012) by R/V Professor Khromov. This data set is combined with data compiled for the Amerasian Arctic based on collections by the R/V Hudson (1960), the US Geological Survey (1992-1993), USCGC Healy (2004-2005), and the LOMROG2007 Expedition and partially published data (Poore et al., 1999; Hillaire-Marcel et al., 2004; Polyak et al., 2004; 2009; Adler et al., 2009). The resulting data set (suppl. Table 1) was further integrated with published data from the Eurasian Arctic (Spielhagen and Erlenkeuser, 1994) (Fig. 1).

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4.2. Methods

The samples were dried at 40 °C in the oven. About 10 g of the dry samples were

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wet-sieved through a 63 μm mesh and dried. Foraminifera were separated from the >63

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through 150 and 250 μm mesh successively.

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μm fraction and counted under microscope. The >63 μm fraction was then dry sieved

Between 20 and 25 specimens of the planktonic foraminifera N. pachyderma were

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picked from the 150 to 250 μm size fraction. Among a total of 177 samples, only 66 CHINARE samples contained enough N. pachyderma tests for isotope analysis due to

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low abundances in the Chukchi Shelf sediments (normally less than 10 specimens/g and dominated by benthic species) resulting from the dilution by biosiliceous and

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terrigeneous material (Walsh et al., 1989; Polyak et al., 2009) and carbonate dissolution (Chierici and Fransson, 2009). Foraminiferal tests were crushed and cleaned by ultrasonic agitation. Stable isotopes of N. pachyderma δ18O and δ13C were analyzed using a Finnigan MAT 252 mass spectrometer for CHINARE I-II samples and MAT 253 for CHINARE III-IV samples in the State Key Laboratory of Marine Geology, Tongji University, China. The isotope results are reported to the PDB standard. Analytical uncertainties with MAT 252 for δ18O and δ13C are ± 0.08‰ and ± 0.06‰, respectively; and ± 0.07‰ and ± 0.04‰ with MAT 253. All other stable-isotope data used in the paper were generated using similar sampling and measurement techniques (Spielhagen and Erlenkeuser, 1994; Poore et al., 1999; Hillaire-Marcel et al., 2004; Polyak et al., 2004; 2009; Adler et al., 2009).

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ACCEPTED MANUSCRIPT For better age constraint, AMS 14C dating on N. pachyderma tests was performed on 23 deep water samples across the study area. These data were compiled altogether with

Results

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earlier generated data, both published and unpublished (suppl. Table 2, Fig. 3).

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5.1. Age of surface sediments

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Sedimentation rates vary greatly from 0.5-1cm/kyr in the central Arctic basin to >10 cm/kyr on the continental margins, depending primarily on distance from the coasts and

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sea-ice conditions (e.g., Stein, 2008; Polyak et al., 2009). Thus, the sampled top 1 or 2 cm of sediment can represent deposits of a relatively wide age range, from recent centuries at

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the margin to several thousand years in the basin. This pattern is confirmed by the distribution of 14C ages showing an age range within the Holocene, with the oldest ages, and thus lowest sedimentation rates in the center of the Canada Basin (Fig. 3). Although with possible bioturbation on the shelf area, considerably younger ages are known to characterize the Chukchi shelf and upper slope, confirmed by both

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Pb and

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C dating

(Viscosi-Shirley et al., 2003; Keigwin et al., 2006; Darby et al., 2009). This distribution shows that our data likely represents average Holocene (mostly late Holocene) N. pachyderma stable-isotopic composition. Assuming a relatively low variability in oceanic δ18O and δ13C in the Holocene, we consider the existing data set suitable for approximating recent environments.

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ACCEPTED MANUSCRIPT 5.2. Stable isotopes of N. pachyderma

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The integration of new and published data allow us to generate a comprehensive picture of N. pachyderma stable isotope distribution in the Arctic Ocean (Fig. 4). On the

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Chukchi Sea shelf and in the adjacent northern Bering Sea, N. pachyderma δ18O values

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generally range between 1.5 and 2 ‰. At the Chukchi shelf break, heavy δ18O values of

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2-3.5‰ are recorded. In the Beaufort Sea, extremely light δ18O values (~1‰) were obtained by plankton tow samples close to the Mackenzie River, with heavier values of

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~2.5‰ in the Beaufort slope area. Light δ18O (<1.5‰) occur on the Chukchi Plateau, Northwind Ridge and in the Canada Basin. Further north in the central Arctic Ocean,

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Makarov Basin, Lomonosov Ridge, δ18O values range between 1.6 and 2.3‰, averaged

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at around 1.9‰. In the Eurasian Arctic, the δ18O values increase from the central basin

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towards the continental margin of Barents and Laptev Seas from ~1.9 to 3.4‰.

In the northern Bering Sea shelf, southern and northeastern Chukchi Sea, N. pachyderma δ13C values range between 0.8 and 1.1‰; whereas in the central Chukchi Sea near the Herald Shoal, they can be as low as 0.4-0.5‰. In the Chukchi Borderland area, δ13C varies between 0.6 and 0.9‰. Amerasian Arctic further north, including the Mendeleev and Alpha Ridges, and Canada and Makarov Basins, is characterized by heavy δ13C (0.8-1.5‰). Values of 0.4-0.7‰ are observed in the Beaufort Sea close to the Mackenzie River, and in the outer Beaufort Sea, they can be as low as near or even below 0‰. At the Lomonosov Ridge and in the Eurasian Basin, δ13C values vary between 0.75 and 0.95‰. Light values (<0.2‰) occur in the northeastern Fram Strait and on the

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ACCEPTED MANUSCRIPT Yermak Plateau. On the northern Barents Sea shelf, δ13C values are around 0.5‰, and on

Discussion

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6.1. N. pachyderma δ18O in Arctic surface sediments

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the Laptev Sea continental margin they increase seawards from ~0.4‰ to 0.7‰.

Planktonic foraminiferal δ18O documents the isotopic composition of the ambient

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seawater during calcite precipitation. It has been related to changes in water temperature and salinity (e.g., Shackleton, 1974; Ravelo and Hillaire-Marcel, 2007). According to the

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equilibrium calcite δ18O and temperature relationship at low temperature range

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(Shackleton, 1974, Equation 1), modified from O’Neil et al. (1969) (Equation 2), a

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change of 1 °C in water temperature roughly corresponds to a 0.25‰ change in the δ18O of the foraminiferal calcite.

(1) T=16.9-4.0(δ18Oc - δ18Ow)

(2) T=16.9-4.38(δ18Oc - δ18Ow)+0.1(δ18Oc - δ18Ow)2

Where T is water temperature (°C), δ18Oc is δ18O in calcite of foraminifera test (‰, PDB scale), and δ18Ow is δ18O of water (‰, SMOW scale).

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ACCEPTED MANUSCRIPT Investigations from the Eurasian Arctic suggest that N. pachyderma δ18O is depleted by about -1‰ from equilibrium values at all depths (Kohfeld et al., 1996; Bauch et al.,

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1997). This systematic offset will not bias the general distribution pattern.

Since Arctic N. pachyderma likely calcify their tests during summer months (Carsten

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and Wefer, 1992), we compare its δ18O with the summer (July to September)

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environmental properties. The instrumental hydrographic data obtained in recent decades may not provide accurate correlation to the sediments representing centuries to thousands

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of years, especially in the central Arctic with extremely low sedimentation rates (Polyak et al., 2009). Nevertheless, the observational hydrographic data is a useful reference base

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for understanding the general features of the foraminiferal isotopic composition (e.g., Hillaire-Marcel et al., 2004). To avoid involving the most recent dramatic changes in the

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Arctic climate, we used the 2005 World Ocean Atlas (Locarnini et al., 2006; Antonov et al., 2006) to extract water temperature and salinity at the core sites.

The gradient in summer sea surface temperature (SST) across the area characterized by N. pachyderma isotope data (Fig. 4) is about 4 °C from the northern Bering Sea to the Chukchi Sea, and about 8°C further to the sea ice margin at the Chukchi shelf break (Fig. 2). The central and eastern parts of the Chukchi Shelf are mostly under the influence of Bering Sea Shelf Water and Alaska Coastal Current, with stronger temperature gradient than the Anadyr Current to the west. Similarly, a SST gradient of about 3°C is observed from the northeastern Fram Strait to the southern Nansen Basin in the Eurasian Basin, where the warm Atlantic water enters the Arctic Ocean. Thus, an increase of 1-2‰ in

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ACCEPTED MANUSCRIPT equilibrium calcite δ18O is expected to reflect the temperature gradient from the ice free northern Bering/Nordic Seas to the Sea ice margin. Further in the high Arctic basin, SST

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is rather uniform at around the freezing point. The latitudinal temperature gradient decreases with depth. The overall SST pattern clearly differs from that of our N.

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pachyderma δ18O data, suggesting that temperature is not the main controlling factor of

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its δ18O distribution in the Arctic Ocean.

Based on data mainly from the Eurasian Arctic (Spielhagen and Erlenkeuser, 1994)

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the N. pachyderma δ18O has been proposed to reflect salinity changes depending on the sea-water isotopic composition (δ18Ow) in different water masses. The Arctic Ocean,

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particularly the surface layer, consists of various water masses bearing different δ18Ow signature. The Pacific water from the Bering Strait, Atlantic water, vapor and

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precipitation, river runoff, and sea ice melt water, carry δ18Ow signature of ~-1‰, ~0‰, 10 to -30‰, ~-20‰, and ~-2‰, respectively (Östlund and Hut, 1984; Melling and Moore, 1995; Eicken et al., 2002; Cooper et al., 2005; 2008; Yamamoto-Kawai et al., 2008; 2010). The primary sea-water isotopic composition can be altered by a number of processes such as a long-term storage of fresh water in the Beaufort Gyre of the Amerasian Basin. In addition, isotopically light brines alter the sea water δ18O through depth during sea ice formation (Hillaire-Marcel et al., 2008; Bauch et al., 2009; 2011b). The mixture of different water sources complicates the δ18O recorded in N. pachyderma.

To gain better insights into the controls on δ18O distribution, we analyze the relationships of δ18Ow with salinity (Fig. 5) and surface-sediment N. pachyderma δ18O

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ACCEPTED MANUSCRIPT (Fig. 6) by several water-column intervals from the surface to 200 m depth, the inferred range of N. pachyderma dwelling in the Arctic (Carstens and Wefer, 1992; Carstens et al.,

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1997; Bauch et al., 1997). The δ18Ow vs. salinity relationship shows that, at the same

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depth, lighter δ18Ow and lower salinities occur in the Amerasian Arctic. This pattern reflects a larger storage of fresh water in the Beaufort Gyre from the Pacific inflow and

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river runoff, and thus a deeper halocline (Macdonald et al., 2002; Guay et al., 2009;

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Yamamoto-Kawai et al., 2008; 2010). The Beaufort Sea represents the lowest salinity and most negative δ18Ow as a result of fresh water input from the Mackenzie River. The Fram

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Strait and the Barents Sea represent the opposite end member with the high salinity / heavy δ18Ow influenced by the Atlantic inflow. The δ18Ow vs. salinity graphs (Fig. 5)

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show a scattering pattern for the upper 50 m, indicating multiple water sources with

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different δ18Ow signatures. The strongest δ18Ow vs. salinity relationship characterizes

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water depths of 100 m and 150 m (R2>0.8), showing a better mixing of water sources. Depths below ~200 m are occupied by the Atlantic water with mostly consistent high salinities and δ18Ow composition of >0 ‰ (e.g., Yamamoto-Kawai et al., 2008; 2010).

In order to compare the N. pachyderma δ18O and δ18Ow (Fig. 6), we excluded the temperature factor following the approach of Spielhagen and Erlenkeuser (1994): the N. pachyderma δ18O data were normalized to the temperature of thermally homogenous Arctic surface waters of -1°C (δ18Onorm) by equation 1 (Shackleton, 1974). The temperature data used are from World Ocean Atlas 2005 (Locarnini et al., 2006). The δ18Onorm vs. δ18Ow relationships vary between different Arctic Ocean regions (Fig. 6), apparently resulting from the local mixtures of various water sources. The best δ18Onorm

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ACCEPTED MANUSCRIPT vs. δ18Ow relationship characterizes the subsurface waters at 30-50 m (R2>0.4), especially in the central Arctic Ocean (Amerasian and Eurasian Basins). In deeper layers, the

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relationship weakens as δ18Ow values at most sites become heavier, but the δ18Onorm

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values remain little changed due to a very small temperature gradient (Fig. 6). This pattern suggests a mostly shallow dwelling of N. pachyderma in the central Arctic Ocean,

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consistent with in situ investigations in the Nansen Basin (Carstens and Wefer, 1992). A

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recent plankton tow study at the boundary between the present Transpolar Drift and Beaufort Gyre in the Makarov Basin (88.4 °N, 177.6 °W; Ding et al., 2014) indicates

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maximum abundance of N. pachyderma test (>150 µm) at 50-100 m, at the depth of the halocline with a strong gradient of δ18Ow. The δ18O values measured in these foraminifera

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(1.68-2.68‰, averaged 2.27 ‰) are similar to that in the core-top data from this area. Although this investigation characterizes just one site, it clearly exemplifies the influence

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of halocline on N. pachyderma habitat and isotopic signature. The gradually decrease of N. pachyderma δ18O from the Eurasian towards Amerasian central Arctic (Figs. 4, 6) may be related to the deepening of the halocline (Fig. 2) and resultant foraminiferal dwelling in surface waters with lower δ18Ow.

In the marginal seas the relationship between δ18Onorm and δ18Ow (salinity) is weak (Fig. 6). Although N. pachyderma δ18O values in the central Chukchi Sea are similar to those in the northern Bering Sea (Fig. 4), they do not show a clear trend in relation to temperature or salinity gradient. This may reflect the complexity of water masses in the Chukchi Sea including the cold and saline Anadyr Current in the west, and the warmer, relatively fresh Alaska Coastal Current in the east.

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A common feature at the shelf break of the Beaufort, Chukchi, Laptev and Barents

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Seas, as well as in the Fram Strait, is the N. pachyderma δ18O decrease towards the central Arctic Ocean (Fig. 4). Spielhagen and Erlenkeuser (1994) attributed the heavy

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δ18O tongue at the Barents continental margin, where N. pachyderma dwells at relatively

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large (50-200 m) depths (Carstens and Wefer, 1992; Volkmann, 2000) to the

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comparatively high salinity of subsurface Atlantic water. Similarly, the decrease of N. pachyderma δ18O towards the central basin off the continental margin of the Laptev,

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Chukchi and Beaufort Seas (Fig. 4) may be related to the shoaling of its habitat, as suggested by Spielhagen and Erlenkeuser (1994) for the Laptev Sea. Thus, heavy δ18O

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values may suggest a relatively deep dwelling of N. pachyderma and the associated

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influence of high-salinity subsurface waters, be it a halocline or Atlantic water.

The deeper dwelling of N. pachyderma at or near the Arctic shelf break can be explained by the proximity of the summer sea ice margin (Fig. 1). As found at the sea ice margin off eastern Greenland and in the Fram Strait (Kohfeld et al., 1996; Carstens et al., 1997), maximum N. pachyderma abundances occur at or below the depth of the chlorophyll maximum, associated with high food supply (e.g. diatoms, Hemleben et al., 1989). The chlorophyll maximum on the shallow Arctic shelf is normally in the upper 50 m (Ardyna et al., 2013), but can have a wide depth range unconstrained by bathymetry at the shelf break due to favorable light conditions and nutrient availability near the sea ice margin. An additional source of nutrients in deeper layers (50-100 m) at the shelf break results from remineralization of nutrients, especially nitrates (Garcia et al., 2006;

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ACCEPTED MANUSCRIPT Anderson et al., 2013), which limits primary production in the Arctic Ocean (Rysgaard et al., 1999; Ardyna et al., 2011). In the Canadian Arctic, the subsurface chlorophyll

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maxima was shown to be closely associated with the nitracline (Martin et al., 2010; 2013).

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These nutrients may sustain phytoplankton in the lower euphotic zone as a food source for N. pachyderma. In contrast to the Arctic Ocean periphery, under the perennial sea ice,

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a shallower chlorophyll maximum is limited to the upper ~50 m (McLaughlin and

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Carmack, 2010; Arrigo et al., 2011; Griffith et al., 2012). This setting is consistent with the shallow dwelling of N. pachyderma in the central Arctic inferred from the relatively

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good linear correlation of δ18Onorm vs. δ18Ow at depth of 30 and 50 m (Fig. 6). In addition to changes in the habitat depth, the shoaling of Atlantic water along the Eurasian

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continental margin may alter the δ18Ow towards heavier values.

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6.2. N. pachyderma δ13C in Arctic Ocean surface sediments

The δ13C of a foraminiferal shell reflects the isotopic composition of the dissolved inorganic carbon (DIC) in ambient sea water during calcification (e.g., Ravelo and Hillaire-Marcel, 2007). Foraminiferal δ13C is generally regarded as an indicator of ocean ventilation and/or biological productivity (Sarnthein et al., 1994). Heavy planktonic foraminiferal δ13C values are often ascribed to good ventilation conditions (e.g., Mulitza et al., 1999). The gas exchange at the air-sea interface introduces δ13C depleted atmospheric CO2 into the surface ocean, and the isotopic equilibrium at low temperatures results in high δ13C values of surface water (Emrich et al., 1970; Lynch-Stieglitz et al., 1995). Complete isotopic equilibrium between the surface ocean and atmosphere at 0 °C

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ACCEPTED MANUSCRIPT will result in the sea-water δ13CDIC values of about 2.5 and 4‰, taken δ13C of atmospheric CO2 as -7.8‰ for modern and -6.4‰ for preindustrial conditions,

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respectively (Bauch et al., 2000). However, the surface ocean carbon can not attain a

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complete isotopic equilibrium with the atmosphere since the surface ocean ΣCO2

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equilibrates 10 times faster than δ13C (Lynch-Stieglitz et al., 1995). Due to the permanent sea-ice cover, the central Arctic Ocean likely has a limited gas exchange with the

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atmosphere, air-sea gas exchange mainly occurs in the seasonally ice-free shelf areas, where depletion of surface water δ13C by excess uptake of CO2 plays an important role in

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carbon isotope equilibration. This process also affects the Arctic interior by the inflow of Atlantic water, and subsurface/deep waters derived from the shelf by brine injection, but

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their influence decreases from the source regions towards the central Arctic (Bauch et al., 2000; Mackensen, 2013). Moreover, the longer resident time under the sea ice in the

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central Arctic than on the shelves allows better isotopic equilibration, which draws δ13C towards heavier values. This setting agrees with the overall distribution pattern of N. pachyderma δ13C in the Arctic Ocean that shows heavier values in the central Arctic basin than in the marginal areas (Fig. 4).

Other than the air-sea exchange, in the shelf areas δ13C is affected by high riverine discharge and coastal erosion, where terrestrial-derived DIC has δ13CDIC values in the range of -5‰ to -10‰ (Spielhagen and Erlenkeuser, 1994; Galimov et al., 2006; Alling et al., 2012). Degradation of terrestrial organic carbon in the Laptev and East Siberian Seas produces DIC with δ13C composition of -7.2 to 1.6‰, with lightest values found close to the river mouths and heavier values in surface waters on the outer shelf (Alling et

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ACCEPTED MANUSCRIPT al., 2012). These numbers are well in agreement with δ13CDIC data from the Kara and Laptev Seas, ranging from -8 to 1.5‰ (Erlenkeuser et al., 2003; Bauch et al., 2004). The

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isotopically light DIC is fast consumed by carbon fixation during primary production on

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the shelves (Erlenkeuser et al., 1995; 2003; Alling et al., 2012). Lower δ13CDIC values occur below the halocline than in the surface water due to the degradation of terrestrial

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organic matter as well as the remineraliztion of depleted organic matter produced by local

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productivity (Erlenkeuser et al., 1995; Bauch et al., 2004; Alling et al., 2012). The brineenriched shelf bottom water is transported to the Arctic interior and mixes into the

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halocline as well as deep water (Bauch et al., 2011a; Mackensen, 2013). Since N. pachyderma may dwell at relatively large depths at the shelf break, the observed

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relatively light N. pachyderma δ13C in this area may reflect the depleted δ13CDIC from the

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shelf halocline water (Fig. 7).

In the Canada Basin heavy N. pachyderma δ13C (Fig. 4) also appears to be related to the DIC distribution, where nutrient repleted water of mostly Pacific origin dominates the subsurface layer (Yamamoto-Kawai et al., 2008). The maximum DIC concentration occurs in the Pacific Winter Water, mostly at 50-200 m, which accumulates DIC as it flows across the highly productive Chukchi Sea (Griffith et al., 2012). The δ13CDIC in this water is ~0.3-0.5‰, as compared to heavy δ13CDIC values of ~1.5-1.6‰ in the surface Polar Mixed Layer (Griffith et al., 2012). The shallow position of δ13CDIC maximum may also be supported by local biological production at the depth of chlorophyll maximum (Kohfeld et al., 1996), which has a shallow subsurface depth in the central Arctic due to the limited light availability (Rudels et al., 1991; McLaughlin and Carmack, 2010;

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ACCEPTED MANUSCRIPT Griffith et al., 2012). Observed heavy N. pachyderma δ13C values are thus consistent

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with its shallow dwelling in the central Arctic Ocean.

Carbon isotope studies show ~1‰ more depleted N. pachyderma δ13C in comparison

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with the δ13CDIC of ambient sea water both in the Arctic and Antarctic waters (Charles

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and Fairbanks, 1990; Kohfeld et al., 1996; Bauch et al., 2000). This disequilibrium has

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been attributed to temperature changes, carbonate ion concentrations, and foraminiferal diet (Kohfeld et al., 2000). Considering the disequilibrium effect, N. pachyderma δ13C

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data from the central Arctic Ocean (Fig. 4) are consistent with the composition of δ13CDIC

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in the shallow subsurface water of the Canada Basin above ~50 m (Griffith et al., 2012).

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On the Chukchi shelf, lighter N. pachyderma δ13C values occur in the

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central/northern areas than in the northeastern and southern Chukchi Sea. This pattern may reflect the distribution of biological productivity. Satellite observations (Wang et al., 2005) indicate extensive summer phytoplankton blooms in the southern Chukchi Sea and coastal area of the Beaufort Sea. The seasonal variation of ice cover is the dominant factor here as the ice-edge bloom follows the northward retreating marginal ice zone; whereas, in the central Arctic Ocean primary productivity is limited by the permanent sea ice cover. In the western Chukchi Sea, along the Anadyr Current, productivity is higher than further east due to the high nutrient content of the Anadyr water. Detailed in-situ observations of biomass distribution indicate that some areas in the northeast Chukchi Sea also have high productivity (Grebmeier et al., 2006, Mathis et al., 2009). This distribution is in agreement with the observed N. pachyderma δ13C in the Chukchi Sea,

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ACCEPTED MANUSCRIPT where the differences in δ13C values may result from the nutrient consumption and

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primary productivity patterns.

Two localities stand out as having extremely light N. pachyderma δ13C values (close

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to or lower than 0 ‰): in the eastern Fram Strait and in the northern Beaufort Sea (Fig. 4).

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Low δ13C in the eastern Fram Strait were interpreted to be related to the intrusion of

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Atlantic water with a light δ13C signature (Spielhagen and Erlenkeuser, 1994). The observed N. pachyderma δ13C is still considerably lighter than the average Holocene

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values of ~0.4-0.5‰ in the Nordic Seas (Bauch et al., 2001; Nørgaard-Pedersen et al., 2003). Light δ13C values between -0.3 and 0.2‰ measured in N. pachyderma from

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plankton tows collected from the southern Nansen Basin along the Atlantic inflow and Arctic halocline, are significantly lower than the core-top values representing pre-

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industrial conditions in the same area (Bauch et al., 2000). These low δ13C values are attributed to the addition of anthropogenic CO2 in the surface ocean, known as the Suess Effect. The Suess Effect may also be the primary reason for the low core-top N. pachyderma δ13C values in the eastern Fram Strait (Fig. 4), where ice margin proximity results in high sedimentation rates, and, thus young (industrial times) surface sediment ages (Werner et al., 2011). Besides, meltwater from the Svalbard glaciers with depleted δ13CDIC similar to the atmospheric values, might also contribute to the light N. pachyderma δ13C in this region, while the associated stratification prevents ventilation of subsurface water (Bauch and Weinelt, 1997; Nørgaard-Pedersen et al., 2003; Spielhagen et al., 2004). The other case of anomalously light N. pachyderma δ13C values (-0.2 to 0.06‰) in the core tops from the deep northern Beaufort Sea is not well understood.

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ACCEPTED MANUSCRIPT These values are much lower than those from plankton net samples at the adjacent sites closer to the Mackenzie River delta (0.6 to 0.78‰) representing modern conditions

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(1970’s) and also affected by the Suess Effect. One possibility is that the light values are

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related to the δ13C-depleted shelf halocline water, similar to the Laptev shelf break. Another explanation may involve the influence of meltwater from the glaciers of the

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resolve the isotopic distribution in this area.

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Canadian Arctic, similar to that off west Svalbard. Further investigation is needed to

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6.3. Implications for N. pachyderma habitat in the Arctic Ocean

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Stable isotope results analyzed in the context of up-to-date hydrographic data provide insights into the habitat of N. pachyderma in the Arctic Ocean. The schematic

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representation of the inferred habitat pattern in the Arctic and associated stable-isotope signature is shown in Fig. 7. On the shelf the habitat is constrained to shallow depths by the topography; at the shelf break it follows the deeper nutrient maximum; in the perennially ice-covered central Arctic Ocean it migrates to the shallow depth corresponding to the chlorophyll maximum; towards the North Atlantic, it gradually shifts deeper to the subsurface Atlantic water.

This picture refers to N. pachyderma populations of 150-250 μm size that predominantly contain adult, encrusted tests. Limited data available for other size fractions indicate more complexity related to their vertical differentiation in the water column. Hillaire-Marcel et al. (2004) found an inverse relationship between N.

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ACCEPTED MANUSCRIPT pachyderma test size/weight and δ18O in the western Arctic Ocean, contrasting with the positive relationship in the North Atlantic. The accompanying δ13C pattern is "normal" in

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both regions, with heavier values in larger tests. The light δ18O composition of large

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Arctic specimens was attributed to a temperature increase from the surface to the Atlantic layer, and was thus used as an indicator of a deep dwelling of adult N. pachyderma

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(Hillaire-Marcel et al., 2004). However, this explanation does not account for heavier

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δ18Ow in the Atlantic water that compensates a temperature-driven decrease in foraminiferal δ18O and results in a weak N. pachyderma δ18Onorm vs. δ18Ow relationship in

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deep layers (Fig. 6). Further, the notion of a deep habitat is not supported by the δ13C distribution showing lighter values with depths increasing to the Atlantic layer (Griffith et

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al., 2012). Another explanation to be considered relates to the contribution of brines to the depth of adult N. pachyderma habitat in the central Arctic, estimated at ~30-50 m (Fig.

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6; Carstens and Wefer, 1992; Volkmann, 2000). These brines, formed during sea ice growth on the Arctic shelves, carry remineralized nutrients and light δ13CDIC to the upper halocline (Griffith et al., 2012; Anderson et al., 2013); however, their effect on the δ18Ow is small, as indicated by observations of increasig δ18Ow with depth at the shelf break (Bauch et al., 2011a).

We infer that the reversed N. pachyderma δ18O vs. test size relationship in the central Arctic Ocean is due to the calcification of large specimens at shallower depths than the small tests, unlike the pattern in the North Atlantic (Hillaire-Marcel et al., 2004) but similar to that in the stratified summer northwest Pacific (Kuroyanagi et al., 2011). Plankton tow data from the Nansen Basin also show that surface water is relatively

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ACCEPTED MANUSCRIPT enriched in N. pachyderma >160 µm, while frequency of smaller tests increases with depth (Carstens and Wefer, 1992). This differentiation may be related to the feeding of

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more mobile adult foraminifers in the shallow chlorophyll maximum, while juveniles might utilize bacteria on remineralized organic matter in the deeper nutricline resulting in

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light δ13C but heavy δ18O values. In addition to the hydrographic structure, the isotopic

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gradient between large and small specimens in the North Atlantic is affected by seasonal

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processes as large tests settle earlier in the season, thus recording lower temperature (heavier δ18O) than small tests (Jonkers et al., 2013). In contrast, in the central Arctic

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Ocean seasonality is unlikely to play a significant role in calcification variability because bioproduction is mostly limited to the short summer period concomitant with receding

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sea ice and increasing light availability.

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According to this interpretation, relatively light N. pachyderma δ18O values in the early Holocene sediments from the western Arctic Ocean (Hillaire-Marcel et al., 2004) probably reflect increased precipitation and river discharge rather than a stronger inflow of Atlantic water at that time. This case exemplifies the importance of a correct identification of planktonic foraminiferal habitats in the Arctic for paleoclimatic reconstructions. Plankton-tow studies of N. pachyderma habitats and calcification in the Arctic Ocean are needed to conclude on the nature of isotopic patterns of different test generations.

7. Summary and Conclusions

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ACCEPTED MANUSCRIPT Surface sediments samples from the Amerasian Arctic (Chukchi/Beaufort margin and Canada Basin) in combination with earlier published data have been investigated for

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stable oxygen and carbon isotopes in planktonic foraminifera N. pachyderma (150-250

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m) to reveal their (paleo)environmental implications.

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The δ18O of N. pachyderma is strongly affected by the mixing of different water

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sources, which bear different δ18Ow signals. Its relationship with sea-water δ18O, with the strongest correlation at 30-50 m depths, confirms a shallow dwelling of N. pachyderma

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under perennial sea ice in the central Arctic Ocean, probably related to the shallow depth of the chlorophyll maximum. Lighter δ18O values in the Amerasian than in the Eurasian

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Arctic are likely caused by a large amount of freshwater kept in the Beaufort Gyre,

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resulting in a deeper halocline. Relatively heavy N. pachyderma δ18O values at the shelf

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break are probably due to a deeper foraminiferal dwelling here related to a more extensive photic zone and nutrient availability. The reason for an inverse relationship between N. pachyderma δ18O and foraminiferal test size in the western Arctic Ocean (Hillaire-Marcel et al., 2004) remains not well understood. One possibility is that larger foraminifers dwell in shallower waters with lighter δ18O signature. More studies are needed to test this inference.

The pattern of heavier N. pachyderma δ13C in the perennially ice covered central Arctic Ocean than in seasonally ice free areas is generally consistent with the air-sea carbon isotopic exchange, probably combined with the Suess effect in high sedimentation rate areas and the influence of isotopically light terrestrial-derived DIC gradually

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ACCEPTED MANUSCRIPT decreasing from the shelf towards the central basin. In addition, heavy N. pachyderma δ13C in the Canada Basin may be related to its shallow dwelling above the lower

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halocline with light δ13CDIC. A deeper dwelling at the shelf break results in lighter δ13C reflecting the isotopically light brine enriched shelf bottom water. The distribution of

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δ13C in the Chukchi Sea is related to bioproductivity patterns resulting in heavier δ13C

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values in the southern and northeastern areas of the Chukchi Sea than in its central part.

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Acknowledgments

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This work was funded by the National Natural Science Foundation of China (41030859), the Chinese Special Project of Arctic Ocean Marine Geology Investigation (CHINARE

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2012–03–02), and Chinese IPY Program (2007–2009), and the US National Science Foundation award ARC-1304755 to L. Polyak. We thank the cruise members of the CHINARE I-IV and RUSALCA-2012 for collecting the sediment samples, which were then provided by the Polar Sediment Repository of Polar Research Institute of China and V. I. Il’ichev Pacific Oceanological Institute of Russia. We also thank the US Geological Survey and the Geological Survey of Canada for providing access to additional samples. Comments from two anonymous reviewers and editor were very helpful for improving the manuscript.

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Fig. 1. A. Oceanographic settings of the Arctic Ocean and the Chukchi Sea (Jones, 2001; Weingartner, 2001). BG: Beaufort Gyre; TD: Transpolar Drift; AW: Atlantic Water; AC: Anadyr Current; ACC: Alaska Coastal Current; SCC: Siberian Coastal Current. CS: Chukchi Sea; CP: Chukchi Plateau; NR: Northwind Ridge; MR: Mendeleev Ridge; AR: Alpha Ridge; LR: Lomonosov Ridge; NB: Nansen Basin; AB: Amundsen Basin; MB: Makarov Basin; CB: Canada Basin. Mackenzie River and Yukon River are denoted by arrows. White dashed line: September sea ice (Parkinson and Cavalieri, 2008). Gray dashed line: subsurface Atlantic Water. B. site map of Arctic Ocean surface sediments (see text and suppl. Table 1). Red dots: CHINARE I-IV; green dots: RUSALCA-2012; blue dots: other cores; black dots are from Spielhagen and Erlenkeuser, 1994.

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Fig. 2. Temperature and salinity structure of the upper Arctic water column along a transect across the northern Bering Sea, Chukchi Sea, Amerasian and Eurasian Arctic, and Fram Strait (white line). Temperature and salinity are from World Ocean Atlas 2005 described in Locarnini et al. (2006) and Antonov et al. (2006), respectively.

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Fig. 5. δ18Ow and salinity distribution during summer (July to September) at the core sites at multiple water depths. The δ18Ow and salinity data are from Schmidt et al. (1999); and the salinity data are from World Ocean Atlas 2005 (Antonov et al., (2006) due to its better spatial coverage and resolution in the Arctic Ocean, especially the Amerasian Basin, than the Schmidt et al. (1999) dataset, respectively (suppl. Table 3).

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Fig. 6. N. pachyderma δ18O and δ18Ow distribution at the core sites at multiple water depths. N. pachyderma δ18O values are normalized to -1°C taking Locarnini et al. (2006) summer (July to September) temperature at corresponding water depths (suppl. Table 3). The summer δ18Ow are from Schmidt et al. (1999). Symbols as in Fig. 5. Fig. 7. Schematic illustration of N. pachyderma habitat and isotopic signature in the Arctic Ocean (left to right): (i) on the shelf the habitat is limited by shallow bathymetry, characterized by variable δ18O from Pacific inflow (Chukchi Shelf) and river runoff, and variable δ13C from bioproduction and airsea exchange; (ii) near the shelf break it follows the deeper nutrient maximum in shelf halocline water and higher bioproduction near sea ice margin, with high δ18O and variable δ13C from remineralized terrestrial carbon; (iii) in the perennial ice covered central Arctic the habitat is closer to the surface with low δ18O and high δ13C; (iv) towards the Fram Strait, its habitat is gradually shifting away from the sea ice influenced shallow dwelling to the subsurface Atlantic water showing high δ18O and low δ13C. The light to dark blue color shows salinity changes with the halocline deepening towards Amerasian Arctic.

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