Advances in Polar Science  2011, Vol. 22 Issue (4): 205-214

  The article information

XIAO Wenshen, WANG Rujian, CHENG Xinrong
Stable oxygen and carbon isotopes from the planktonic foraminifera Neogloboquadrina pachyderma in the Western Arctic surface sediments: Implications for water mass distribution
Advances in Polar Science, 2011, 22(4): 205-214
10.3724/SP.J.1085.2011.00205

Article history

Received: August 1, 2011
Accepted: October 10, 2011
Stable oxygen and carbon isotopes from the planktonic foraminifera Neogloboquadrina pachyderma in the Western Arctic surface sediments: Implications for water mass distribution
XIAO Wenshen , WANG Rujian, CHENG Xinrong     
State key laboratory of Marine Geology, Tongji University, Shanghai 200092, China
Received August 1, 2011; accepted October 10, 2011
Corresponding author: XIAO Wenshen(email: wxiao@tongji.edu.cn)
Abstract: Neogloboquadrina pachyderma is the most abundant planktonic foraminifera species found in the modern polar oceans. The δ18O and δ13C of N. pachyderma from the Western Arctic Ocean sediments were analyzed to reveal the implications of the proxies to environmental changes. The δ18O from N. pachyderma in the Chukchi Sea reflect the water mass distribution in this area. Heavier δ18O values were found along the Anadyr Current (AC) and lighter values in the central and eastern Chukchi Sea. These may reflect the freshwater signal from the Alaska Coastal Current (ACC) and Bering Sea Shelf Water (BSSW). The light δ18O signature in the high Arctic basin comes from the freshwater stored in the Arctic surface layer. The δ13C distribution pattern in the Chukchi Sea is also influenced by the current system. High primary productivity along the AC results in heavy δ13C. The relatively low primary productivity and the freshwater component from the BSSW and ACC may be the reason for this light δ13C signal in the central and eastern Chukchi Sea. Our data reveal the importance of well ventilated Paciflc Water through the Chukchi Sea into the Arctic Ocean.
Keywords: δ18O    δ13C    Neogloboquadrina pachyderma    Western Arctic Ocean    

0 IntroductionThe Arctic Ocean is surrounded by continents and the largest shelf areas on earth. It is connected to the At-lantic and Pacific Oceans via narrow Fram Strait and the shallow Bering Strait,respectively. The Arctic Ocean is strongly influenced by riverine input,which is responsi-ble for~10% of total global river runoff[1, 2]. Seasonal sea ice variation modulates the heat balance of the Arc-tic Ocean through the sea-ice albedo effect[3]. Along with global warming,the fast melting of sea ice amplifies the change in global climate[4]. The wider open water terri-tory could be an important area as a future global carbon sink[5],although some recent studies suggest less amount of CO2 drawdown in the Arctic Ocean than expected[6]. It is closely related to biological productivity,water prop-erties and structure (e. g.,freshwater input,ventilation,stratification) in the Arctic Ocean.

Neoglohoquadrina pachyderma(formerly known as N. pachyderma sinistral form[7]) is the dominant plantonic foraminifera species in the high latitude oceans. The sta-ble oxygen and carbon isotopes (δ18O and δ13C) of this species have become important tools for reconstructing Arctic surface water properties[8]. They have previously been used as proxies for surface water properties/ocean circulation[9, 10, 11, 12, 13],sea ice formation[14] and melt water events[15, 16]. However,those studies are mainly focused on the Eastern Arctic Ocean[9] and data is still very lim-ited from the western side of the Arctic Ocean.

In this study,we analyzed the δ18 O and δ13 C of N. pachyderma from the surface sediments of the West-ern Arctic Ocean recovered during the first and second Chinese National Arctic Research Expeditions. The sed-iments are from the Chukchi Sea,the Beaufort Sea and the Canadian Basin,covering a wide area of the West-ern Arctic Ocean. Because N. pachyderma are likely to calcify their tests during the summer months[17],we dis-cuss the relationship between the δ18 O and δ13 C of N. pachyderma with summer water properties. We then use these relationships to examine any implications from this proxy for paleoceanographic reconstructions in the West-ern Arctic Ocean.

1 Oceanographic settings 1.1 Ocean currentsThe Chukchi Sea is a marginal sea in the Western Arc-tic Ocean,with an average water depth of~50 m. It connects the Pacific Ocean via the Bering Strait,and is strongly influenced by the Pacific Ocean[18]. Three wa-ter masses flow through the Bering Strait from the Pa-cific Ocean. From west to east they are the nutrient rich Anadyr Current (AC),characterized by relatively high salinity and low temperature,the Bering Sea Shelf Wa-ter (BSSW) and the warm and fresh Alaska Coastal Cur-rent (ACC) (Figure 1). From the northwest,the Siberian Coastal Current (SCC) flows into the Chukchi Sea via the Long Strait[19, 20, 21]. The Canadian Basin is dominated by the clockwise circulating Beaufort Gyre. The Atlantic water enters the Arctic through Fram Strait and the Ba-rents Sea and sinks to about 200 m,becoming Arctic Intermediate Water that circulates anti-clockwise[22, 23]. The Arctic Ocean also receives large amount of fresh wa-ter from the rivers in the surrounding continents. For example,approximately 307 km3•a-1 of freshwater and 106 t·a-1 of sediment from the Mackenzie River are trans-ported to the Beaufort Sea[1, 2].

Figure 1 Site map and oceanographic setting[18,19,20,21,22,23] of the Western Arctic Ocean (see also Table 1 ). AC: Anadyr Cur- rent, BSSW: Bering Sea Shelf Water, ACC: Alaska Coastal Current, SCC: Siberian Coastal Current, CP: Chukchi Plateau, NR: Northwind Ridge, MR: Mendeleev Ridge, HC: Herald Canyon. The dashed line denotes the subsurface At- lantic water.
Table 1 Location and water depth of the surface sediment samples from the Western Arctic Ocean[24,25]
Site Longitude Latitude Water depth/m Coring device
R03A 169°0'0〃W 68°0'0〃N 51 B
R06 169°0'0〃W 69°29'43〃N 53 B
R11 169°39'54〃W 72°0'50〃N 55 G
R15A 168°59' 26〃W 73°59' 53〃N 175 B
C11 167°59'3〃W 71°39'51〃N 48 B
C13 165°59'51〃W 71°36'51〃N 38 B
C15 164°0'46〃W 71°34'45〃N 43 B
C16 163°0'52〃W 71°32'51〃N 57 M
C17 162°2'1〃W 71°29'21〃N 57 B
P11 169°59'37〃W 75°0'24〃N 167 B
P21 167°21' 38〃W 77°22Z44〃N 562 B
P22 164°55'59〃W 77°23'43〃N 320 B
P23 162°31'5〃W 77°31'40〃N 2200 B
P24 158°43'16〃W 77°48'38〃N 1890 M
P27 156°0'22〃W 75°29'33〃N 3050 M
CNIS7 149°6'55〃W 78°23'14〃N 3850 B
IS10 151°50'49〃W 79°17' 36〃N 3800 B
B11 156° 19'54〃W 73°59' 42〃N 3500 G
B77 152° 22'28〃W 77° 31'10〃N 3800 B
B78 147°1'41〃W 78°28'43〃N 3850 G
B80A 146°44'16〃 W 80°13' 25〃N 3750 M
M01 169°0'46〃W 77°17'56〃N 1456 B
M03 171°55' 52〃W 76°32'13〃N 2300 B
M07A 171°56/35〃W 75°0'3〃N 388 B
S11 159°0'14〃W 72°29' 24〃N 40 B
S16 157°9'50〃W 73°35'28〃N 2800 B
S26 152°40'0〃W 73°0'0〃N 3000 B
P6700 169°58' 38〃W 67°0'17〃N 47 B
P7100 169°59'28〃W 70°59'17〃N 40 B
P7230 168°38'10〃W 72°29'33〃N 54 B
P5 157°21'4〃W 73°27' 12〃N 2600 M
P7 161°7'17〃W 75°4'55〃N 1700 M
Notes: B = box corer, M = multi corer and G = gravity corer.
1.2 Temperature and salinity distribution in the Western Arctic OceanAccording to the hydrological survey during the second Chinese National Arctic Research Expedition in 2003 (Figure 2),the surface water (i.e.,0 m) in the east Chukchi Sea along the Alaska coast is 2℃-3℃ higher than the west side,north of 71°N. Additionally the wa-ter temperature gradually becomes uniform at high lati-tudes decreasing from 0C to~-1.5℃ towards north,as a transitional area from the open water to the sea ice co-vered water. At 50 m depth,the difference in water temperature between east and west Chuckchi Sea re-duces,still showing a cooling trend northward. North of 71°N the water temperature is around -1℃-1.5℃ whereas in the Beaufort Sea it is 0.5℃ to 1C higher than the Chukchi Plateau and Northwind Ridge area. Due to the shallow Chukchi Sea shelf area,the hydrological sites for water deeper than 100 m are north of the Chukchi Slope. At 100 m,the Chukchi Plateau,Northwind Ridge area and the Cana-dian Basin are uniform in water temperature (-1℃——-1.5℃). However,the Beaufort Sea is still warmer than~ the other regions. At 150 m,the area of 74°N to 75°N,170°W is-0.5℃-1℃,warmer than the surroun- ding area by~0.5℃. From the Canadian Basin towards the Beaufort Sea,the temperature increases from -1.5℃ to~-1℃ in the coastal area. Compared to 150 m,at 200 m depth the water temperature at the continental slope area is 0.5℃ to 1C colder,but in the central basin area it is warmer by about 0.5℃. These features indicate the influence of the intrusion of warm Atlantic water at subsurface depth.

Figure 2 Water temperature (℃) and salinity (‰) at dif- ferent depths in the Western Arctic Ocean, surveyed during the second Chinese National Arctic Research Expedition. The black dots denote the hydrological sites from this study.

The salinity of the Western Arctic show that the surface water salinity in the Chukchi Sea is low in the east and high in the west (Figure 2). It manifests the properties of the ACC and the AC and the high latitude Arctic basin is characterized by lower salinities due to high sea ice cover. The lowest salinity (~28‰) occurs in the central Canadian Basin. At 50 m,the salinity in the Canadian Basin (~31.6‰) is still lower than in the Chukchi Sea and the continental slope area (~32.5‰). The salinity of the Beaufort Sea is ~31 ‰. At 100 m,except in the Beaufort Sea area (<32‰),the salinity in the Western Arctic is ~32.7‰. At 150 m,the salinity of Beaufort Sea area (~32‰) is slightly lower than other ar-eas characterized by salinities of 33% to 34% . The sali-nity distribution pattern at 200 m is similar to that at 150 m,decreasing from ^34.5% in the basin area to-wards the Beaufort Sea area to~33‰. The reversed salinity distribution pattern through depth indicates the influence of Atlantic water at intermediate and subsur-face depths.

A clear characteristic appears in the Alaskan coastal area around 70°N. At all depths,the water temperature in this area is higher,and the salinity is lower than the surrounding ocean. This indicates the influence of warm,fresh water from the land (e.g.,the Mackenzie River). The surface water in the high latitude Arctic is cha-racterized by cold fresh melt water,and the subsurface water is influenced by warm saline Atlantic water.

2 Materials and methodsWe collected 32 surface sediment samples (0-2 cm) from box cores,multi-cores and gravity cores recovered du-ring the first and second Chinese National Arctic Re-search Expeditions (Table 1). Those samples were taken from the Chukchi Sea,the Beaufort Sea and the Cana-dian Basin,covering the latitudes 67° to 80°N,and longi- tude 146° to 172° W in the Western Arctic Ocean[24, 25].

The Chukchi Sea surface sediments were dark gray to black silty clays and rich in organic carbon due to high biological productivity and high sedimentation rates at these sites[24, 25]. However,there were low abundances of the planktonic foraminifera N. pachyderma in the sam-ples. This is likely due to the dilution of other materials in this area,or alternatively the shallow water depth of the Chukchi Sea may not be a favorable environment for this species. In the deep sea area of the Chukchi Plateau,the Northwind Ridge,the Beaufort Sea and the Canadian Basin,the surface sediments are mainly brownish mud,and rich in N. pachyderma.

The sediments were dried at 50 0C,and wet-sieved through a 63 μm mesh. The >63 μm fraction was then dried and sieved through a 154 μm mesh. 20-25 spec-imens of the planktonic foraminifera Neogloboquadrina pachyderma were picked from the 154 to 250 μm size fraction. The shells were cleaned by ultrasonic agitation. The stable isotopes of δ18O and δ13C were analyzed using a Finnigan MAT 252 mass spectrometer. The results are expressed to the PDB standard. The standard errors of the measurements were δ0.08‰ for δ18O and δ0.06‰ for δ13C. All the sample preparation and analyses were carried out in the State Key Laboratory of Marine Geol-ogy,Tongji University,China.

3 ResultsThe δ18O distribution pattern from N. pachyderma in the Western Arctic Ocean can be divided into three ar-eas (Figure 3):

Figure 3 The δ18O and δ13C(‰) from N. pachyderma taken from surface sediments in the Western Arctic Ocean. Am- biguous heavy δ18O values at sites B11 and P5 are highlighted in yellow.

(a) The Chukchi continental shelf area had the~ heaviest values up to 4.6‰. This site is lacated close to the Bering Strait. In the central Chukchi sea the δ18O was relatively lighter (<2.0%).

(b) In the Chukchi Sea continental slope area,va-lues were up to 2% to 3.5% from the area between 71.5° and 73°N.

(c) Lighter values of <2% were found in the Chukchi Plateau,Northwind Ridge and Canadian Basin area.

The most depleted δ13C from N. pachyderma were found in the central/east Chukchi Sea with values rang-ing between 0.4% and 0.6%. In the west and north Chukchi Sea,and in the sea ice covered Canadian Basin,the Chukchi Plateau and the Northwind Ridge area,rel-atively heavier δ13C from N. pachyderma were observed ranging from 0.8% to 1.1%.

4 Discussion 4.1 Age of sedimentsThe age of the sediments is a key issue in the interpre-tation of our data. The sedimentation rates vary in dif-ferent areas,thus even the top 2 cm of sediments can represent deposition from different ages. However,if the sediments are from the Holocene,the variation of the iso-tope is minimal[9]. The Holocene sedimentation rate in the central Arctic basin is between 0.5 and 1 cm•(ka)-1,and increases towards the continental margins to 5 to >10 cm•(ka)-1[26]. The Chukchi Sea is characterized by a high sedimentation rate due to the high input of terri- geneous material and bioproductivity responding to sea-sonal open water and high nutrient supply from the Pa-cific Ocean[27—29]. In the area of Chukchi Plateau,North- wind Ridge and Canadian Basin,most of the surface sediments are characterized by brownish color and rela-tively high abundance in N. pachyderma,also suggesting Holocene deposition. Additional AMS14 C dating of core M03 (Figure 1) yields an age of 7-8 ka BP from the top 2 cm[30]. Although more datings from different regions are needed,the ages of the surface sediments in our study are likely to be within the Holocene.

4.2 Isotopic signatures of N. pachyderma as pa- leoceanographic proxies in the Arctic OceanN.pachyderma is a typical pycnocline planktonic foraminifera species. In the Arctic Ocean,the N. pachy-derma calcifies at variable depths that range from the mixed surface layer down to a few hundred meters[8, 10, 31]. The maximum abundances of N. pachyderma are associ-ated with the chlorophyll maximum in the surface from ~20 to 80 m[8]. It is generally thought to inhabit the water column between 50 and 200 m[8, 17],although this varies regionally. At the Fram Strait,the depth distribu-tion of N. pachyderma suggests a preference of the At-lantic water underlying the cold polar surface water be-tween 50 and 200 m[32]. In the outer Laptev and Barents Seas,the maximum abundance of living N. pachyderma was between 50 and 100 m depth[12]. In the Nansen Basin,a latitudinal variation south of 83°N was found. The data suggest that N. pachyderma prefers water be-low the pycnocline at ~100 m. North of 83°N maximum abundance occurred in the upper 50 m[17]. Other investi-gations show that the habitat of N. pachyderma changes from ~150 m in the south to ~80 m in the north,but the calcification depth varies between 100 and 200 m[10].

In the North Atlantic Ocean,the heavier/larger specimens calcify towards the colder,saline layer,and are thus characterized by heavy isotopic compositions,and vice versa[33,34]. In contrast,in the Western Arc-tic Ocean,a reverse linear relationship between shell weight/size and the δ18O content has been observed[31]. This may reflect the increasing temperature gradient from the cold surface mixed layer to the top of the warm intermediate Atlantic waters (150-200 m) where large specimens calcify. Thus,according to the species habitat,its isotopic signature reflects the water properties of va-rious depths in different regions rather than simply sur-face water.

4.3 Implications of δ18O in N. pachydermaThe δ18O of planktonic foraminifera documents the δ18O of the seawater,and also changes in water temperature and salinity[35—36]. The distribution pattern of δ18O from N. pachyderma in the Western Arctic Ocean reflects the changes in the water environment.

According to the δ18O and water temperature rela-tionship,changes of 10C water temperature corresponds to a 0.26% change in the δ18O of the foraminifera[35]. But the relationship between δ18O and salinity varies in different areas. In the Norwegian Sea and Eastern Arctic Ocean,regression coefficients of 0.61% and 0.73% δ18O per‰ salinity have been reported,respectively[37, 38]. In the Eurasian Basin,a coefficient of 0.79 is calculated for the Arctic surface waters,thus a 1.00% change in seawater δ18O is equivalent to about a 1.27‰ salinity change[9]. Meanwhile,the δ18O is also influenced by the fresh water characterized by depleted δ18O. The circum- Arctic meteoric water (precipitation and river runoff) carries a δ18O signature of ~-20‰[39,40,41],and the sea ice melt water ~-2‰[42,43].

The water temperature in the shallow (~50 m) Chukchi Sea shows a strong gradient decreasing from the Bering Strait to the Chukchi Sea shelf margin at about 71° to 72°N (Figure 2). However,the latitudi-nal distribution pattern of δ18O from N. pachyderma does not totally reflect a temperature gradient. It ap-pears to follow the current flow and mirror the mixing of different water masses. Following the AC in the west-ern Chukchi Sea,the δ18O values from N. pachyderma decrease from 4.66%。(P6700) towards ~3.4%。(R11 & P7230) at the continental slope close to Herald Canyon. This is the main path of AC water entering the Arctic Ocean. Using the temperature factor (a 1℃ increase = a 0.26% change in δ18O),the δ18O difference could be explained by a 5C of water temperature change. This generally agrees with the surface temperature gradient between the core sites,and the salinity differences be-tween the sites are minor (Figure 2). The sites in the central Chukchi Sea are bathed in the BSSW and lie more to the east in ACC water,which carries a considerable amount of fresh water from the land[44]. Thus,the light δ18 O values in the central and northeast Chukchi Sea may bear the signal of the freshwater components in the currents. In the Chukchi Sea continental margin area,from the Herald Canyon eastward,the generally decrea-sing δ18O may be related to the mixing of the AC water with the BSSW and ACC Water. At the shallower site S11 (~159°W),the δ18O may still bear the signal of AC,which mirrors its eastward extension.

North of the Chukchi Sea continental margin,in an area of permanent sea ice cover[45],the water tempe-rature and salinity are uniform at various depths. It is only slightly fresher and warmer close to the Beau-fort Sea (Figure 2). The δ18O values are generally uniform decreasing from the continental margin to-wards the central Arctic basin (Figure 3). The wa-ter temperature and salinity changes from the shelf area to the high Arctic basin cannot explain the changes in δ18 O from N. pachyderma. This pattern is in agreement with the trend found in the Eas-tern Arctic Ocean[9]. The heaviest δ18O in the south-ern Nansen Basin is interpreted to reflect the inflow of~ Atlantic water at a habitat depth of 50-200 m (for N. pachyderma). The decreasing trend towards the central Arctic basin suggests a habitat change of N. pachyderma as it migrates from deeper layers to shallower fresh wa-ter depth with isotopically lighter δ18O[9]. Such habitat change was observed in the southern Nansen Basin[17]. However,a recent plankton tow investigation in the Makarov Basin (88.4°N,176.6° W) during the fourth Chi-nese National Arctic Research Expedition (summer 2010) is not in agreement with the habitat migration. The plankton tow showed that the maximum abundance of N. pachyderma occurred between 100 to 150 m (Wang R J,et al.,unpubl. data). Although this plankton tow is not necessarily representative of the entire Arctic basin,it may suggest that the light d18O signal actually comes from different water sources with a lighter δ18O signa-ture. The large amount of fresh water in the Arctic basin is the most likely reason for this. In the Cana-dian Basin,the Beaufort Gyre keeps fresh water from the Pacific Ocean (inflow from the Bering Strait) and river runoff,which dilutes the seawater δ18O in the sur-face ocean[41, 46, 47, 48]

Heavy values of δ18O from deep sites southeast of Northwind Ridge (e.g.,~3.4%。,B11 and P5,Figure 3) are ambiguous. They are much heavier than the adja-cent sites (1.35% and 2.1% at S16 and S26,respec-tively). The lithological description of the surface sed-iments B11 and P5 were grayish to dark grayish mud,which is different from the brownish mud in other deep sea surface sediment[24, 25]. The grayish sediments are possibly from a glacial/deglacial deposition characterized by heavy δ18O values[49]. Additional datings are needed for better age control of the sediments.

4.4 Implications of δ13C in N. pachydermaHeavy δ13C values are normally interpreted as good ven-tilation of surface waters[50]. In the central Arctic,the permanent sea ice cover prevents the gas exchange be-tween ocean and atmosphere,thus ventilation is very lim-ited. In contrast,ventilation mainly occurs in seasonally ice free areas,such as the shelf areas. However,our data show an inverse pattern that lighter δ13C values occur in the central Chukchi Sea whereas heavier δ13C values in the central Arctic. Thus,ventilation itself may not ex-plain the δ13C distribution pattern in the Western Arctic Ocean.

Besides ventilation,biological productivity plays a major role in the carbon isotopic fractionation and the d13C can also indicate nutrient consumption[51]. The car-bon assimilation by primary productivity and export to the deep ocean preferentially takes 12C,and thus 13C is enriched in the surface waters. This effect results in heavy δ13C in high primary productivity areas and light δ13C in low productivity area. Satellite observations[52] indicate that extensive phytoplankton blooms occur du-ring the summer in the Chukchi Sea and coastal Beau-fort Sea. The seasonal variation of ice cover is the domi-nant factor as the ice-edge blooms follows the northward retreating marginal ice zones and in the central Arctic basin primary productivity is limited due to the per-manent sea ice cover. In the west Chukchi Sea,along the AC,productivity is higher compared to the east Chukchi Sea,as a response of the high nutrient content of Anadyr water. More detailed in-situ observations of biomass distribution indicate other areas in the north-east Chukchi Sea also have high productivity[53]. This is also in agreement with the high opal and organic carbon content in the surface sediments from the corresponding high productivity areas[54]. Similar to the δ18O distri-bution,heavy δ13C values in the western Chukchi Sea correspond to the path of the AC,and in the northeast-ern Chukchi Sea with high bioproductivity. This area extends to the Chukchi Sea continental margin. In the central/eastern Chukchi Sea,sites with light δ13C val-ues are bathed in the BSSW and ACC. The differences in δ13C values between the central/eastern and west- ern/northeastern Chukchi Sea may result from the nu-trient consumption and primary productivity in the dif-ferent water masses.

The δ13C of planktonic foraminifera is assumed to record the δ13C signal of the surface water they live in. The ACC in the east Chukchi Sea carries considerable amount of fresh water. The riverine dissolved inorganic carbon (DIC) is usually depleted in 13C,with δ13C val-ues of-5‰-10‰[9,55]. We assume that the light δ13C in N. pachyderma from the central and east Chukchi Sea also reflect the fresh water signal from the ACC.

North of the Chukchi continental margin,in an area with intensive sea ice cover,the ventilation and biopro-ductivity is limited. The heavy values (0.8‰ to 1.1‰ ) in the Canadian Basin,the Chukchi Plateau and the Northwind Ridge area are in agreement with a former investigation in the central Arctic basin[9]. This was in-terpreted as the transportation of well ventilated water from the shelf areas to the central Arctic basin. The light δ13C at the Fram Strait and southern Nansen Basin were related to the intrusion of Atlantic water[9]. Although with high amount of riverine input,the Pacific source contributes the major part of fresh water in the Cana-dian Basin[48],which also points to the importance of the Pacific water to the Arctic Ocean. Obviously,one of the well ventilated water sources is from the Chukchi Sea with the inflow of Pacific water and the contribu-tion of Pacific carbon isotope signal,possibly primarily from the AC,being of great importance. Other poten-tial sources,such as the circum Arctic shelf area,are yet difficult to define,due to the lack of surface water DIC and N. pachyderma δ13C information from these areas.

4.5 Limitations of this studyIn this study we have tried to establish the relation-ship between the water properties and the d18O and δ13C from N. pachyderma. However,due to the lack of seawater δ18O and δ13C of DIC measurements,our analysis is difficult. Moreover,in response to the rapid climate warming in recent years,the meltwater in-put by sea ice and ice sheet melting has strongly in-creased,as well as the river runoff. All these con-tribute to changes in the structure of the water co-lumn and modify the bioproductivity regime of the Arctic Ocean. The δ18O and δ13C from N. pachyderma in the surface sediment does not document these recent environ-mental changes. Thus error may occur during the rele-vance analysis and we cannot quantify these at present.

5 ConclusionsThe stable isotopes of 518O and 513C from the plank-tonic foraminifera Neogloboquadrina pachyderma were analyzed from 32 surface sediments retrieved from the Western Arctic Ocean. The distribution of δ18O in the Chukchi Sea reflects different water masses entering from the Pacific Ocean. The depleted d18O signal in the cen-tral and eastern Chukchi Sea may be from the fresh water of the ACC and BSSW,whereas the heavier δ18O car-ries the signal of the AC. Depleted δ18O values from the Chukchi Plateau,Northwind Ridge and Canadian Basin may reflect the surface freshwater in the high latitudes Arctic basin.

The foraminiferal δ13C in the Chukchi Sea is also strongly related to different water masses in this region. Well ventilated Anadyr water and high bioproductivity result in heavier d13C values in the western Chukchi Sea. Lighter δ13 C in the central and eastern Chukchi Sea are related to the relatively lower bioproductivity and the freshwater component in the BSSW and ACC. Our data suggest that the Pacific Ocean water is one of the major components of the well ventilated water in the central Arctic Ocean.

Acknowledgments This work is funded by the National Ba-sic Research Program of China (Grant no. G2007CB815903),the National Natural Science Foundation of China (Grant nos. 41030859 and 40321603),the China Program for International Polar Year 2007—2008 and the China Geological Survey Project (Grant no. H[2011]01—14—04). This work was part of the project “First Chinese National Arctic Research Expedition” and “Sec-ond Chinese National Arctic Research Expedition” supported by the Ministry of Finance of China and organized by the Chinese Arctic and Antarctic Administration (CAA),SOA. The partic-ipants in the joint work are from the institutions PRIC,FIO,SIO,Tongji University,etc. We thank the cruise members of the first and second Chinese National Arctic Research Expedi-tions,samples taken by Chen Ronghua,Chen Zhenbo,Gao Aiguo,Chen Jianfang,etc. and provided by the Polar Sediment Repos-itory of Polar Research Institute of China (PRIC); Zhao Jin-ping,Jiao Yutian and Gao Guoping from the FIO and Data- sharing Network of Earth System Science-Polar Regional Center (http://www.chinare.org.cn) for providing the hydrological data. Samples information and data issued by the Resource-sharing Plat-form of Polar Samples (http://birds.chinare.org.cn) maintained by PRIC and Chinese National Arctic & Antarctic Data Center (CN- NADC). Data issued by the Data-sharing Platform of Ploar Science (http://www.chinare.org.cn) maintained by PRIC.

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