In the present study we report hydrological (T and S) and hydrochemical data obtained during a Russian Trans-Arctic cruise in 2000 onboard the Hydrographic Vessel (HV) Nikolay Kolomeytsev, and describe the top layer of the sediment (a typical sample was taken from the upper 0–5 cm layer of bottom sediment) and the distribution of the organic carbon (δ13Corg) and nitrogen (δ15Norg) isotope ratios. Using both historical water data and data from our cruise, we divide the ESS into two specific areas: the Western area, influenced strongly by Lena River input, and the Eastern area, under direct influence of Pacific-derived water. We also used the stable δ13Corg and δ15Norg isotopes to detect the sediment geochemical boundary (or “geochemical FZ”) between Pacific “marine-derived sediments” and “terrestrial derived sediments” which can be considered to reflect the long-term (on a scale of 102 years) position of the most westward extension of Pacific water. These are among the first reliable hydrological and geochemical data reported for the ESS from the Dmitry Laptev Strait to the Long Strait, and they reveal novel insights about interaction between Pacific water and local shelf water.
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 The continental shelf of the East Siberian Sea (ESS) is the widest and shallowest in the World Ocean, yet it is the least explored [Stein and Macdonald, 2003]. The wide shelf acts as an important region for production and processing of organic matter before the material is transported into the Arctic Ocean. Areas of enhanced gradients were discovered and termed “fronts” and/or “frontal zones” (FZs) [Nikiforov and Shpaikher, 1980; McLaughlin et al., 1996]. It has been surmised that a branch of Pacific source waters flows west into the ESS [Jones et al., 1998]. But the question of how far westward the Pacific – derived water may travel over the ESS shelf has never been discussed. Likewise, the eastward transport and fate of river runoff has been little studied.
 According to Antonov  and Nikiforov and Shpaikher , a major plume of riverine fresh water usually transits the East Siberian Sea eastward within the coastal zone. Warming causes thawing of the permafrost, which underlies a substantial fraction of the Arctic; this process could accelerate river discharge [Savelieva et al., 2000] and carbon losses from soils. The role of the ESS coastal zone in transport and fate of freshwater and terrestrial organic carbon has not been discussed sufficiently. Measurements of hydrochemical variables, especially of nutrients, were mostly of low quality and made using a variety of different techniques, impossible to replicate now. Thus we focus our attention on the coastal zone, where processes of interaction between the local shelf waters (influenced strongly by fluvial and coastal erosion input) and Pacific-derived waters are most pronounced.
 An important effect on distribution of Atlantic and Pacific waters in the upper Arctic Ocean could be caused by a change in river runoff associated with a shift from the Zn-mode to the Az-mode of circulation when the Trans-Arctic Current (which transports fresh water northward from the Asian shelf) is intensified and shifted toward Siberia [Proshutinsky and Johnson, 1997; Nikiforov and Shpaikher, 1980]. It has been found that this switch of atmospheric circulation regimes (from Az- to Zn-mode) causes a significant inter-annual shift in position of the Trans-Arctic Current, and its correlation (0.78) with hydrological conditions (anomalies of the temperature [T] and salinity [S]) in the ESS is quite high [Shpaikher and Yankina, 1969]. Thus, we can consider the variability in position of the FZ between the “Pacific waters” and “local shelf waters” as an indicator of regional climate change as reflected in alternations of the Zn and Az circulation regimes.
2. Study Area and Data
 In September 2000 the HV Nikolay Kolomeytsev was the first research vessel to enter the ESS from the west heading east, and to use modern equipment for gathering hydrological and hydrochemical data. Our studies were performed in the near-shore zone (open water between the coast and drifting ice) of the ESS. In total, 35 oceanographic stations were conducted, comprising the first CTD survey through the ESS from the west to the east. The present study also accomplished the first measurement of stable carbon and nitrogen isotopes (13C and 15N) in the surface organic matter of the bottom sediments in the shallow ESS. Previously the distribution of δ13Corg and δ15Norg in the bottom sediment had been studied only in the most eastern part of the ESS [Naidu et al., 2000].
 For the present study nutrients, total inorganic carbon (TIC), and particulate material (PM) were measured in water samples taken from Niskin bottles. Total nitrogen (TN), organic carbon (OC), C/N molar ratio and stable isotopic composition data were obtained from surface sediments sampled by Van-Veen grab. The same sediment measurements were conducted in the ESS (44 oceanographic stations) during the First Russia-US Cruise in 2003 onboard the Ivan Kireev (http://www.iarc.uaf.edu/expeditions/ess_cruise/east_siberian_cruise2.php). Thus far, 79 surface sediment samples from the ESS have been analyzed for 13C and 15N isotope ratios. Isohaline 24.5 psu has been suggested as a marker for the boundary of surface shelf waters diluted by riverine runoff [Nikiforov and Shpaikher, 1980; Antonov, 1968]. All available historical summertime T and S data (about 7,000 oceanographic stations performed in the East-Siberian and Laptev seas in 1932–2000) were used to define the mean multi-year FZ position (Figure 1).
3. Results and Discussion
 A notable characteristic of the ESS is an extremely large gradient of hydrological and biogeochemical parameters from Long Strait/Wrangell Island to the New Siberian Islands, that corresponds to geographically critical contrasts in the Arctic system where the Pacific and local shelf waters interact over the shelf (Figure 2). The T, S, and hydrochemical data (Table 1 and Figure 2) show that over the shallow shelf winds mixed the water column from the top to the bottom in the Western part (area) of the ESS (roughly between 140 E and 160 E). In September 2000, a narrow FZ located roughly near 160 E separated the local shelf waters and the Pacific-derived waters. Note that in September 2000 a strong westward current (with speed up to 1 knot) was detected in Long Strait. In 2000 the highest concentration of multi-year ice was measured in the most eastern part of the ESS and Long Strait. That may indicate an increase in the Az-mode of general ocean circulation. Note that in September 2003 inflow of Pacific water was bounded by 172 E (http://www.iarc.uaf.edu/expeditions/ess_cruise/east_siberian_cruise2.php); this may indicate a resurgence of the Zn-mode.
Table 1. Water and Sediment Characteristics of the Western and Eastern Areas of the East-Siberian Sea
Near bottom water
Psammite fraction, %
Aleurite fraction, %
Pelite fraction, %
Organic carbon, %
Molar ratio C/N
Contribution of terrestrial organic carbon, CTOM (%)
 We define the “Western hydrological area” as the mixed freshened ESS shelf water located west of the FZ position. The long term average distribution of T and S indicates an average position of the FZ roughly near 160 E (Figure 1), which agrees well with the position of the FZ obtained in September 2000. The Western hydrological area is characterized also by a high concentration of terrigenous PM containing significant amounts of OC introduced mainly by coastal erosion. Degradation of the eroded OC causes a decrease in values of pH and dissolved oxygen, while pCO2 and TIC are increased [Semiletov, 1999].
 The Eastern hydrological area is located east of the FZ. It is under the direct influence of Pacific-derived waters that are modified over the Chukchi Sea shelf [Nikiforov and Shpaikher, 1980; Walsh et al., 1989]. The data from 2000 shown in Table 1 (and Figures 1–2) demonstrate that the freshened local shelf waters (Western hydrological area) are much warmer than the Pacific source water (Eastern hydrological zone). This is an interesting feature because although in general the Pacific waters are a source of heat for the Arctic Ocean as a whole, this is not true for the ESS which is the iciest of the Siberian Seas. That means that the Pacific water loses a major portion of its thermal capacity over the Chukchi Sea shelf, and the warming effect of Siberian rivers is enough to keep the local shelf waters warmer than the modified Pacific water. A crude evaluation shows that even the Kolyma summertime discharge (about 67 cubic km) contains enough heat energy to melt about 43 cubic km of sea ice [Shpaikher, 1963]. The thermal effect from the Lena may be one order of magnitude greater, because the volume of the Lena discharge is one order of magnitude higher than that of the Kolyma [Savelieva et al., 2000; Semiletov et al., 2000]. The “warm plume” from the Lena may be clearly seen in Figures 1–2. Historical observations [Antonov, 1968; Nikiforov and Shpaikher, 1980] also demonstrate that in summer, a major portion of the Lena discharge is always transported towards the Long Strait.
 In some years, the FZ position may be shifted eastward to a significant extent. For example, in September 2004 the ESS surface salinity (in the 0–5 m layer) from Kolyma Bay (near 160 E) to Long Strait (roughly near 178 E) varied within 20–24 psu; no hydrological FZ was found in the ESS near shore zone in September 2004 (I. Semiletov, unpublished data, 2004). Such a change in the salinity regime may indicate development of the Zn circulation regime. Note that in winter 1956, when the Zn-mode was strongly developed [Shpaikher and Yankina, 1969], the intensity of the Siberian Coastal Current (SCC) was increased and caused a salinity anomaly in the southwestern Chukchi Sea which was detected as far away as the Bering Strait.
 According to Proshutinsky , the Az circulation regime dominated in the 20th century. Therefore, the long-term average position of the FZ (Figure 1) mainly reflects a distribution of Pacific-origin and local shelf waters that is roughly typical for the Az-mode.
 Two areas with different geochemical regimes may be defined in the ESS. The Western geochemical area extends from the Dmitry Laptev Strait in the west to Cape Chukochiy (roughly near 172 E) in the east, where it is bounded roughly by the isoline of δ13Corg = −24.5‰ and δ15Norg = 7 (Figure 3); this agrees with the observed abrupt (5–10 times) eastward decrease in PM concentration (Table 1). This portion of the ESS is strongly influenced by PM transport induced by coastal erosion and fresh water transport from the Lena runoff. Highest rates of coastal erosion were detected between the Lena and Kolyma rivers where coastal Ice-complex contained ice-wedges (up to 50–70% by volume) is widely distributed and easily damaged by warming [Romanovskii et al., 2000; Semiletov, 1999]. The organic matter in the sediment is generally a mixture between marine and terrigenous material that is characterized by a range of C/N ratios between 6 and 15: typically the lower values characterize marine organic material, and the higher values characterize the terrigenous material [Stein and Macdonald, 2003]. Distribution of δ15Norg also shows an eastward change from its “typical terrestrial” values (less than 6 [Walsh et al., 1989]) to the “typical marine” values (more than 8.) The boundary between the Western (mean C/N = 10.2) and Eastern (mean C/N = 7.6) geochemical areas is delineated by C/N ratios = 7–8, which agrees with the observed drastic eastward decrease in PM concentration and high gradients of δ13Corg and δ15Norg (Figure 3 and Table 1). The C/N ratio is negatively correlated with δ13C (−0.73) and δ15N (r = −0.77). Note that plume of relatively heavy δ13Corg < −24.5‰ was found in the northern part of the study area (Figure 3) that may be associated with increased marine signal there.
 The spatial trends of δ13Corg in the bottom sediment can be used to quantitatively estimate the contribution of terrestrial organic matter (CTOM) to the ESS sediment west and east of the geochemical FZ. Following Walsh et al. , the amount of OC derived from terrestrial end-member δ13Cter (terrestrial C) can be calculated from the data as
where “o”, “mar”, and “ter” refer to the δ13C values of observed, marine, and terrestrial sediment. If we take the two end members to be a δ13Cter of −27, typical of higher plants, and a δ13Cmar of −21, typical of phytoplankton [Walsh et al., 1989; Naidu et al., 2000], then the CTOM values for the Western geochemical area and the Eastern area (bounded by CTOM = 70%) become 86% and 52% of terrestrial OC, respectively (Table 1). Calculations indicate that there is a significant amount of terrestrial OC stored within sediments, especially in the near shore zone most strongly influenced by coastal erosion: between the Dmitry Laptev Strait and the Kolyma mouth, the OC is almost all of terrestrial origin (from 81% to 100%, mean CTOM = 93%). In contrast, the sediments underneath transformed Pacific – origin water (the Eastern area) are almost half of marine origin.
 The Quartz/Feldspar (Q/FS) ratios in the Western and Eastern areas are the same (Q/FS = 0.26), while the Q/FS ratios typical for the Lena solid discharge are 10 times higher; Q/FS ratios range between 2 and 2.3 [Serova and Gorbunova, 1997]. This evidence indicates a hitherto neglected direct influence of Lena transport of PM into the ESS. The PM data obtained in the Dmitry Laptev Strait where the river PM signal is negligible (I. Semiletov et al., unpublished data, 1999) show that “new production” is formed from the old terrigenous carbon with a typical terrestrial signal of δ13Cter < −26.5. Thus the river OM discharge has probably no direct influence on marine productivity, while coastal erosion and consequent degradation of “fresh” old terrestrial organics [Semiletov, 1999; Guo et al., 2004] play a significant role in biogeochemical processes especially in the Western area of the East-Siberian Sea where coastal retreat is highest.
 General differences between the oceanographic and geochemical regimes of the Western and Eastern areas are reflected in the size fraction distribution in the surface sediments. The mean psammite (sand) fraction increased more than twice from the Western area to the Eastern area, while the aleurite (silt) fraction decreased almost twice (Table 1). Percentage contribution of the pelite (clay) fraction (particle size = <0.01mm) is almost the same in both the Western and Eastern areas; this fraction is almost completely terrestrial in origin in the Western area, but exhibits a mix of marine and terrestrial origin in the Eastern area. The highest OC concentration (up to 1.9%) exists in the pelite fraction.
 Distribution of relatively “heavy” Pacific-origin δ13Corg isotopes and “light” δ15Norg isotopes in the sediments bounded by the geochemical FZ at near 172 E (Figure 3) agrees well with the distribution of high-salinity Pacific-derived waters and position of the hydrological FZ observed in September 2003 (http://www.iarc.uaf.edu/expeditions/ess_cruise/east_siberian_cruise2.php). The eastward shift (about 10 degrees of longitude) from the long-term average position of the FZ (near 162 E) to the FZ position obtained in September 2003 (near 172 E) may be associated with domination of the “cyclonic” regime of circulation when transport of the Lena diluted water (including Kolyma and Indigirka run-off) extends, in general, far eastward [Antonov, 1968; Proshutinsky and Johnson, 1997]. According to Aksenov , rates of sediment accumulation over the shallow ESS are less than 1 m throughout the Holocene (10,300 yrs), or <0.1 mm/year. Therefore our sediment samples taken from the upper 0–5 cm of sediment integrate the geochemical signal from the modern sediment with signals from sediments aged about 500 years or older. Thus we can argue that the Zn circulation regime dominated on a scale of hundreds of years, but additional sediment studies are required.
 We draw two major conclusions from the results of this study.
 1. Based on distribution of the hydrological and hydrochemical data, two areas were identified in the shallow ESS: a Western area that is influenced strongly by the fresh water flux and PM transport of the coastal eroded material (the solid Lena discharge signal is negligible), and an Eastern area that is under the influence of Pacific derived waters. From year to year, the longitude shift of the FZ between Western and Eastern areas may reach 10 degrees and more.
 2. The position of the multi-centennial geochemical (sediment) FZ is located about 10 E eastward from the multi-decadal position of the water FZ. Thus we can suggest that over the last 50–70 years the Az mode of circulation has dominated, whereas the Zn mode dominated over the last hundreds of years.
 We thank Syun Akasofu, Valentine Sergienko, Georgui Golitsyn, Gunter Weller, Victor Akulichev and Boris Levin for their support of our work in the Siberian Arctic. This work was partially supported by, the International Arctic Research Center (IARC), University of Alaska Fairbanks, the US National Science Foundation (grant OPP-0230455), the Russian Foundation for Basic Research (02-05-65258a, 04-05-79162κ, 04-05-64819a) and the Russian Federal Program World Ocean. The support of Headquarters of the Far–Eastern Branch (No. 04-1-07-012) of Russian Academy of Sciences (RAS), RAS Headquarters (Program#13, Direction#7), and Institute of Atmospheric Physics RAS is gratefully acknowledged. Candace O'Connor, Irina Pipko, Nina Savelieva, and Sveta Pugach made a valuable contribution to the manuscript.