3.1 The Central Basins
 The halocline layers of the central Arctic Ocean have a clear signature of brine from sea ice (negative sea ice melt) but with different depth distributions in the Eurasian and Canadian Basins (Figure 2). In the Amundsen and Makarov Basins the maximum brine contribution is found at about 50 m (Figure 2a) while in the Canada Basin it is spread over the range of 50–250 m (Figure 2b) depending on station position. A similar pattern in the brine distribution was found by Yamamoto-Kawai et al.  in a thorough evaluation of the freshwater sources of the central Arctic Ocean using both TA and δ18O. The observed difference in the depth extent reflects the vertical salinity distribution of the basins, as is seen in a plot of fraction versus salinity (Figures 2c and 2d) where the maximum brine contribution is centered around a salinity of ~33 in both regions. However, the difference is that in the Canada Basin the maximum brine content is associated with a maximum in nutrients (here exemplified by silicate), which is not the case in the Amundsen and Makarov Basins (Figures 2e and 2f).
Figure 2. Distribution of sea ice melt water in the upper 500 m of (a) the Amundsen and Makarov Basins and (b) the Canada Basin, and as a function of salinity for the full depth in (c) the Amundsen and Makarov Basins and (d) the Canada Basin, as well as silicate concentration versus salinity for the full depth in (e) the Amundsen and Makarov Basins and (f) the Canada Basin.
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 The origin of the signature of the upper halocline, centered around a salinity of 33, was early on suggested to originate from sea ice brine produced over the continental shelves that mixes with underlying water to form a cold, saline water that is advected into the basins of the Arctic Ocean [Aagaard et al., 1981; Melling and Lewis, 1982; Cavalieri and Martin, 1994]. This formation mechanism agrees with the high nutrient content observed is a result of organic matter mineralization at the sediment surface on the shelves with the decay products added to the cold, saline bottom water [e.g., Kinney et al., 1970; Jones and Anderson, 1986; Moore and Smith, 1986].
 One question would be why the high nutrient content is confined to the Canada Basin and not found in the Amundsen and Makarov Basins during the 2005 Beringia cruise. Its extent has earlier been observed also in the Makarov Basin and as far over to the Atlantic side as the Lomonosov Ridge [e.g., Moore et al., 1983]. Consequently the extension of the nutrient maximum varies with time and is likely determined by the Beaufort Gyre that can vary in magnitude depending on the atmospheric pressure pattern [e.g., Proshutinsky et al., 2002; Proshutinsky et al., 2009]. The reason why the waters in the Atlantic sector do not contain this high nutrient water is due to the much lower nutrient concentrations in the inflowing Atlantic water in combination with little or no outflow of high nutrient brine-enriched water from the Barents and Kara Seas. The Barents Sea has high productivity [Sakshaug, 2004] but it is relatively deep (~200 m), resulting in less organic matter entering the sediment compared to the shallow Chukchi and East Siberian seas on the Pacific side of the Arctic Ocean. High-salinity brine-enriched waters have been observed in the eastern Barents Sea [Midttun, 1985], but no observation of a nutrient-rich water flowing out to the Arctic Ocean through the St. Anna Trough has been reported. However, these areas have been sparsely sampled and the details of this outflow are still open for speculation.
 That the water in the approximate upper 100 m in the Eurasian Basin has a brine signature, but no elevated nutrients, is likely a result of local sea ice formation. Rudels et al.  suggest winter homogenization of the upper ~100 m by brine addition from sea ice formation within the Eurasian Basin. In our summer observation the surface water (S < ~32) is impacted by sea ice melt water giving a lower brine fraction close to the surface and the sea ice melt fraction profile has a minimum at about 50 m (Figure 2a).
3.2 The East Siberian and Western Chukchi Seas
 High primary production together with elevated concentrations of nutrients in bottom waters have been reported from the East Siberian and Laptev seas [e.g., Codispoti and Richards, 1968; Olsson and Anderson, 1997] and the Chukchi Sea [e.g., Walsh et al., 1989; Cota et al., 1996; Codispoti et al., 2005; Lepore et al., 2007]. The data collected in 2008 reveal that the high nutrient concentrations are associated with brine enrichment with generally increasing levels toward the bottom in both the East Siberian Sea and the Herald Valley in the Chukchi Sea (Figure 3). In Figure 3 the nutrients are represented by silicate with a similar pattern also seen for phosphate and nitrate. The elevated nutrient concentrations are coupled to a decrease in oxygen concentration and pH values, strongly supporting mineralization of organic matter as the process behind these signatures.
Figure 3. Silicate and sea ice melt fraction profiles from (a and b) the Herald Valley and (c and d) the East Siberian Sea.
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 The minimum oxygen concentration observed was close to 100 µmol/kg but nevertheless there was a substantial deficit in nitrate relative to phosphate as expressed by N** values as low as –25. N** equals 0.87[NO3] – 16[·PO4] + 2.9, which is a signature of loss in nitrate as a result of denitrification and/or anammox. One possible explanation for these low values could be that ammonium is formed during mineralization but has not yet oxidized to nitrate. Ammonium was determined for some samples, including those of low N**, but adding ammonium to the N** expression only had a minor effect. Hence, it is likely that mineralization occurred at oxygen concentrations significantly lower than those observed in the water column. Such conditions are likely to be found in the surface sediments, making them a plausible environment for mineralization.
 That the mineralization signature is strongest close to the bottom and linked to brine enrichment lends further support to the hypothesis that mineralization occurs in surface sediments with the decay products released to the bottom water. This bottom water has had its salinity increased by addition of brine from sea ice formation, which was also observed in some waters with salinities < 30, close to the coast in the East Siberian Sea. This low-salinity, brine-enriched water was not observed in the outer parts of the shelf or the shelf slope, at least not with the same chemical signature. Hence, the waters of the shallow inner shelf are most likely mixed throughout the water column before exiting the East Siberian Sea.
3.3 The East Siberian Shelf Break
 High nutrient concentrations are found at the East Siberian shelf break in the depth range of 50 to 200 m (Figure 4). This nutrient-rich water also includes ~5% of brine, but unfortunately only a few samples for δ18O were collected and no detailed profile is available. However, it is obvious that the surface water does not include brine, but rather a sea ice melt signal. Also, the deepest sample collected, > 1000 m, has a sea ice fraction of about zero. This is expected because it is of Atlantic water origin. Considering that the boundary current along this shelf break comes dominantly from the west [Woodgate et al., 2001] these data strongly suggest that nutrient-rich water is leaving the East Siberian Sea to contribute to the central Arctic Ocean halocline.
Figure 4. Profiles at the continental margin of the East Siberian Sea of silicate concentration (left) and sea ice melt water fraction (right) computed from δ18O and salinity. Station locations are shown in Figure 1 right.
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 A closer inspection of the salinity-nutrient property of this nutrient-rich water (Figure 5) reveal that high silicate concentrations are found in a larger salinity range (Si concentrations >30 µmol/L between 32.5 and 34.5 salinities) compared to past observations in the central Arctic Ocean, but similar to what Nishino et al.  observed in 2004 in the same region. Nishino et al.  had a higher vertical sampling resolution and observed silicate maxima at S ≈ 32.5 and at S ≈ 34 relative to the concentration found in between these salinities and a minimum in N** at S ≈ 32.5. The 2008 data show minima of N** at salinities between 32 and 33 depending on station, except for one sample with a minimum at higher salinity (Figure 5). This data point with the lowest N** and highest silicate concentration also has the lowest oxygen concentration and lowest pH value. Hence, it is consistent with extensive mineralization, likely also in low oxygen environment, but because salinity was not determined directly on the water collected we cannot be sure that the CTD salinity reading is matching that of the water sample.
 A closer inspection of the silicate concentration and pH levels along a depth section from the Indigirka river estuary to the shelf slope (Figure 6) reveals that the data point with the maximum silicate concentration at a depth of about 150, close to the bottom, is accompanied by high concentrations at the same depth but farther off shore. The same goes for other constituents, like the pH minimum, which supports that the data close to the bottom at this depth has a signature of organic matter mineralization.
Figure 6. Depth sections of silicate and pHtot along a section from the Indigirka River estuary to the shelf slope; see insert for station locations. The stippled line in the silicate section illustrates salinity 33 and the thick solid line salinity 34. Figure drawn by Ocean Data View [Schlitzer, 2011].
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 From the section it is seen that bottom water all the way across the shelf and inshore of the 100 m isobath has high silicate and low pH values, although salinities >33 are only found at the shelf break. Hence, high-nutrient water with salinities > 33 has either left the shelf earlier in the season or it has left the shelf farther to the west. Alternatively it could have achieved its signature at the shelf break, but this is less likely because the negative N** for salinities up to ~34.5 (Figure 5) suggest NO3– as electron acceptor, i.e., mineralization at low oxygen concentrations. Low oxygen concentrations are more likely to be produced close to the sediment surface on the shelf than along the continental margin, where a boundary current exists that supplies highly oxygenated water from the Atlantic. This argues against the suggestion by Nishino et al.  that the silicate maximum at the high salinity (> 34) was produced by decomposition of opal-shelled organisms along the continental margin. Furthermore, all waters with silicate concentrations above 25 µmol/L found during the ISSS-08 expedition had a brine content of 4% or more (Figure 7). The brine signature is formed at the surface and in order to keep this signature it has to reach the bottom before being diluted by mixing with surrounding waters. Hence, it is not realistic that this water has reached its depth, ~150 m, without following the bottom from shallower regions of the shelf. Consequently it is unlikely that water of Atlantic origin is the source without modification on the shelf.
 Looking at the five stations located at the shelf break (bottom depth between 143 and 334 m) gives some more insight. An Θ-S plot (Figure 8) shows a distinct temperature minimum between a salinity of 32 and 33, surrounded by fairly linear mixing both toward lower and higher salinities. At a salinity of about 34.5, corresponding to a depth of about 150 m, there was a local temperature minimum indicating interleaving of a colder water mass. This water coincides with the deep, high salinity, silicate maximum (Figure 9d), and could give an indication of the source of this maximum. Nishino et al.  did not observe such a distinct T minimum at this salinity, but there was a clear bend in their T-S diagram. This difference is likely a result of their stations being positioned farther away from the shelf break where mixing would smooth these details. The outermost station during the ISSS-08 cruise showed the same smooth Θ-S curve as reported in Nishino et al. .
Figure 8. Temperature versus salinity for five stations at the East Siberian Sea shelf break, bottom depths between 143 and 334 m.
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Figure 9. Profiles of (a) temperature, (b) salinity, (c) oxygen, (d) silicate, (e) pHtot(15°C), and (f) N**, for the same five stations as in Figure 8.
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 In the depth range 50–100 m temperatures were close to freezing, salinities were between about 32.5 and 33.5 and the oxygen concentrations were around 300 µmol/kg, except for a few samples where the concentration was substantially lower (Figures 9a–9c). The silicate concentrations were elevated and the pHtot as well as N** values were lower compared to the surface water (Figures 9d–9f). The low pHtot is a sign of mineralization of organic matter, and the low N** is a sign of mineralization at suboxic conditions. However, the measured oxygen concentrations of about 300 µmol/kg in the water column demonstrates that the anoxic conditions must prevail in the sediments where the mineralization occurs and the decay products are released to the bottom water.
 At depths around 150 m on the shelf slope the oxygen concentrations and pHtot values have minima, and the silicate concentrations are high, clearly showing that mineralization of organic matter is important for the properties of these waters. However, the N** values are about the mean of the minimum values and those found at 300 m depth, and the same is true for temperature. Hence, assuming that the source water was at a freezing temperature (suggested by brine being present, Figure 7) the water at 150 m depth could be a one-to-one mixture of the source water and the water at 300 m depth. Such a mixture would give source water concentrations of oxygen between 0 and 100 µmol/kg, silicate between 70 and 110 µmol/L, pHtot between 7.0 and 7.4 and salinity ≈ 34.0. For oxygen, silicate and pHtot this is about what has been observed as extreme values on the shelf, making this a plausible mixing scenario. The same is true for phosphate and nitrate, where the source water concentrations would be about 2.2 and 18 µmol/L, respectively.
 The halocline water at the slope section likely flows to the east in a similar pattern as the lower halocline and Atlantic Layer water does at the Laptev Sea [Dmitrenko et al., 2011, and references therein]. Considering that the high-nutrient and low-pH waters show the strongest signal close to the continental slope (Figure 6) it is unlikely that these waters have their source in any other region than between the Laptev Sea and the observation region. Furthermore, because no water with silicate levels above 10 µmol/kg was found at the shelf slope in the middle of the Laptev Sea during the summers of 2007 to 2009 [Dmitrenko et al., 2011] this will even limit the likely region of formation to the east of about 140°E. However, the question is if waters of the observed properties can be found anywhere on the shelf.
 The lowest oxygen and N** concentrations and pHtot values observed on the shelf were around a position of 73°N and 155°E (Figure 10), making this region a plausible source area for the N** minimum found at 50–100 m depth at the shelf break. At this site the salinities were lower than observed at the shelf break, ~31.5 compared to 32.5, and the temperatures were higher, –1.1 °C compared to –1.7 °C. This study was performed in mid-September, opening for substantial mixing after the ice breakup in June-July.
Figure 10. Bottom water properties of (a) salinity, (b) temperature, (c) oxygen, (d) silicate, (e) pHtot (at 17°C), and (f) N** in the East Siberian Sea and the western Laptev Sea. The red circle shows the region with biogeochemical signature of active mineralization of organic matter, while the blue circle shows the region with cold water of relatively high salinity and signature of some mineralization of organic matter. Figure drawn by Ocean Data View [Schlitzer, 2011].
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 For the deep silicate maximum even higher salinities are observed. Using idealized models Dethleff  computed volume and salinities of brine-induced shelf plumes in the flaw leads of the Laptev Sea. In the region north of the New Siberian Islands the computed production of water with salinities above 34 was more than 0.02 Sv, with about 80% of this production occurring in the winter and 20% in the fall. This is the region where the surface water is least impacted by the runoff from mainly the Lena River and thus also the region where the highest salinities are likely to be found. The ISSS-08 cruise covered a part of the region north of the New Siberian Islands and the bottom water had salinities over 33, temperatures down to –1.3 °C, pHtot (at 15 °C) under 7.6, and oxygen under 250 µmol/kg (Figure 10). Unfortunately, no nutrient data were available. The observations are not within the range of the required source water properties, but they were done in mid-September about half a year after the peak of winter conditions. Hence, there were ample time for the dense waters to leave the shelf and also an ice-free summer likely contributed to efficient mixing in these shallow waters of 35 to 65 m depth. Nevertheless, both oxygen and pHtot decreased from the surface to the bottom, pointing to a clear signature of mineralization of organic matter.
 From the few existing observations of brine-enriched dense shelf waters formed by polynyas it can be assumed that the high salinity bottom water layer formed outside the actual polynya area is rather thin. Observations in the Chukchi Sea [Aagaard et al., 1981] indicate a layer thickness of less than 5 m. The strong vertical salinity stratification in such layer would also suppress the vertical mixing. This can increase the concentration of the decay products in the water above the sediments compared to the summer situation if the decay rate is similar. Mooring observations outside the New Siberian Islands at the 1700 m isobaths [Woodgate et al. 2001] show mean currents between 2.0–2.7 cm s–1 in the upper 300 m and it is likely that the speed is somewhat higher further up the slope. Using an advection speed of 5 cm/s and the distance between the New Siberian Islands and the shelf slope section of 650 km gives an advective time scale of around 6 months. This fits well with the hypothesis that the water observed at the slope has been formed at this shelf region during the previous winter.