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Accumulation of freshwater in the permanent ice zone of the Canada Basin during summer 2008
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Jinlu TONG1, Min CHEN1, *, Weifeng YANG1, 2, Run ZHANG1, Hong PAN1, Minfang ZHENG1, Yusheng QIU1, 2, Wangjiang HU1, Jian ZENG1
Acta Oceanologica Sinica | 2017, 36(8) : 101 - 108
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Acta Oceanologica Sinica | 2017, 36(8): 101-108
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Accumulation of freshwater in the permanent ice zone of the Canada Basin during summer 2008
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Jinlu TONG1, Min CHEN1, *, Weifeng YANG1, 2, Run ZHANG1, Hong PAN1, Minfang ZHENG1, Yusheng QIU1, 2, Wangjiang HU1, Jian ZENG1
Affiliations
  • 1 College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China
  • 2 State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361102, China
Published: 2017-08-01 doi: 10.1007/s13131-017-1023-1
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A combination of δ18O and salinity data was employed to explore the freshwater balance in the Canada Basin in summer 2008. The Arctic river water and Pacific river water were quantitatively distinguished by using different saline end-members. The fractions of total river water, including the Arctic and Pacific river water, were high in the upper 50 m and decreased with depth as well as increasing latitude. In contrast, the fraction of Pacific river water increased gradually with depth but decreased toward north. The inventory of total river water in the Canada Basin was higher than other arctic seas, indicating that Canada Basin was a main storage region for river water in the Arctic Ocean. The fraction of Arctic river water was higher than Pacific river water in the upper 50 m while the opposite was true below 50 m. As a result, the inventories of Pacific river water were higher than those of Arctic river water, demonstrating that the Pacific inflow through the Bering Strait is the main source of freshwater in the Canada Basin. Both the river water and sea-ice melted water in the permanent ice zone were more abundant than those in the region with sea-ice just melted. The fractions of total river water, Arctic river water, Pacific river water increased northward to the north of 82°N, indicating an additional source of river water in the permanent ice zone of the northern Canada Basin. A possible reason for the extra river water in the permanent ice zone is the lateral advection of shelf waters by the Trans-Polar Drift. The penetration depth of sea-ice melted waters was less than 30 m in the southern Canada Basin, while it extended to 125 m in the northern Canada Basin. The inventory of sea-ice melted water suggested that sea-ice melted waters were also accumulated in the permanent ice zone, attributing to the trap of earlier melted waters in the permanent ice zone via the Beaufort Gyre.

18O  /  freshwater  /  river water  /  sea ice melted water  /  Canada Basin
Jinlu TONG, Min CHEN, Weifeng YANG, Run ZHANG, Hong PAN, Minfang ZHENG, Yusheng QIU, Wangjiang HU, Jian ZENG. Accumulation of freshwater in the permanent ice zone of the Canada Basin during summer 2008[J]. Acta Oceanologica Sinica, 2017 , 36 (8) : 101 -108 . DOI: 10.1007/s13131-017-1023-1
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1 Introduction
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Freshwater plays a dynamic role in global climate by affecting air-ice-ocean heat exchange in the Arctic Ocean (Carmack, 2000). It maintains strong stratification that inhibits heat transfer from deep ocean to the surface, and consequently has a dramatic effect on sea ice cover, even the Arctic climate system (Shimada et al., 2006). Freshwater affects not only circulations in the Arctic Ocean but also affects convective processes in the North Atlantic (Aagaard and Carmack, 1989; Proshutinsky et al., 2002). In the Arctic Ocean, the largest amount of freshwater, including sea-ice melted water, river water and low-salinity Pacific water flowed through the Bering Strait (Steele et al., 2004), is stored in the Beaufort Gyre (BG) in the Canadian Basin. The quantification of freshwater components, including their source, distribution and pathway, is crucial for understanding the fate of pollutants, nutrients, and marine processes in the Arctic Ocean (Cai et al., 2010; Yamamoto-Kawai et al., 2006, 2008).
δ18O and salinity (S) are usefully conservative tracers for calculating fractions of river water and sea-ice melted water, as river water is highly depleted in δ18O relative to sea-ice melted water and saline water. Since its first successful application in the Fram Stait and Eurasian Basin (Östlund and Hut, 1984), δ18O-S method has been widely used in the Arctic seas. One of the most important findings is that a great amount of river runoff is stored in the interior Arctic Ocean, especially in the southern Canada Basin (Macdonald et al., 2002; Chen et al., 2003; Yamamoto-Kawai et al., 2008; Jones et al., 2008; Newton et al., 2013). It has been proposed that the variation in freshwater accumulated in the Canada Basin is generally modulated by strength of the wind-driven Beaufort Gyre (Giles et al., 2012). A significant amount of freshwater is accumulated during anticyclonic regime and released to the North Atlantic during cyclonic regime (Proshutinsky et al., 2002, 2009). More recently, Morison et al. (2012) suggested that the freshening of the Canada Basin during 1990s–2008 should be ascribed to the runoff pathway variation modulated by the Arctic Oscillation (AO) or the changes of wind-stress curl. Based on the δ18O and other geochemical evidences, it has been confirmed that the Pacific inflow is the main source of freshwater below ~50 m in the Canada Basin (Jones et al., 2008; Yamamoto-Kawai et al., 2008). On the other hand, the southern Canada Basin was reported to be a significant net producer of sea ice (Melling and Moore, 1995; Chen et al., 2003), as well as the shelf and slope (Bauch et al., 1995, 2011; Macdonald et al., 1995; Melling and Moore, 1995; Anderson et al., 2013). Sea ice formation was accompanied by the formation of the upper halocline water in the Canada Basin (Bauch et al., 2011; Anderson et al., 2013). Macdonald et al. (2002) reported net ice formation from the continental margin to 200 km offshore, but net sea-ice melted waters under permanent pack in the southern Canada Basin. Yamamoto-Kawai et al. (2005) found that net ice melting occurred along the surface flow of water from the Pacific and the Atlantic, while net ice formation resided in the central Arctic Ocean. Because of the perennial sea ice cover in the northern Canada Basin, the freshwater source and budget have been rarely studied. The purpose of this study is to investigate the source of freshwater in the Canada Basin, especially freshwater accumulation in the permanent ice zone, which is covered by sea ice all year.
In this study, δ18O-salinity approach under two different saline end-members was used to determine the fractions of total river water, Arctic river water, Pacific river water, and sea-ice melted water. The spatial distributions of total river water, Arctic river water, Pacific river water and sea-ice melted water were examined to reveal their source and pathways in the Canada Basin, especialy in the northern Canada Basin. Lastly, the mechanism of accumulation for freshwater in the permanent ice zone was discussed.
2 Materials and methods
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2.1 Sampling
Seawater samples were collected using 10 L Niskin bottles mounted on a rosette together with SBE-911plus conductivity-temperature-depth (CTD) sensors in the western Arctic Ocean, aboard the R/V Xuelong during August 10–29, 2008. A total of 18 stations were surveyed, covering a region among 71.9–85.4°N, 154.7–143.5°W. Most stations were located in the Canada Basin with water depths greater than 2 000 m, and two stations (S22, S23) were situated on the Beaufort slope (Fig. 1). The north-south section begins north of Barrow, Alaska, at 71.9°N, 154.7°W, in approximately 100 m water depth over the Alaskan continental shelf, and ends off the Alpha-Mendeleyev Ridge (AMR), at 85.4°N, 147.5°W (Fig. 1). Seawater samples for 18O measurement were filled bubble-free into 50 mL polypropylene bottles and sealed until analysis in land laboratory.
2.2 Seawater 18O analysis
Once return, seawater 18O was determined by CO2-H2O equilibration method (Horita et al., 1989; Bourg et al., 2001). Briefly, 18O in seawater was equilibrated with CO2 at 25°C for 24 h, and detected with a Finnigan Deltaplus XP isotopic ratio mass spectrometer. A series of working standards (with δ18O values of –8.71‰, –5.63‰, 0.34‰, 0.39‰, respectively) pre-calibrated against Vienna Standard Mean Ocean Water (VSMOW) were used for determining 18O abundance. These standards were inserted during measurement at an interval of about ten samples. Each sample was measured repeatedly for ten times.
Stable isotopic composition of oxygen in seawater was reported as delta value (δ) given in per mill:
$\delta {}^{{\rm{18}}}{\rm{O}} = \left[ {\frac{{{{({}^{{\rm{18}}}{\rm{O/}}{}^{{\rm{16}}}{\rm{O}})}_{{\rm{sample}}}}}}{{{{({}^{{\rm{18}}}{\rm{O/}}{}^{{\rm{16}}}{\rm{O)}}}_{\rm{VSMOW}}}_{{\rm{}}}}} - 1} \right] \times 1\,000,$
where the subscript “sample” and “VSMOW” denoted the measured seawater and the Vienna Standard Mean Ocean Water, respectively. The reproducibility for seawater δ18O measurement was within 0.025‰.
2.3 Freshwater component calculation
The fractions (%) of each freshwater component can be derived from mass balance calculation as follows:
${f_{\rm{s}}} + {f_{\rm{r}}} + {f_{{\rm{SIM}}}} = 1,$
${f_{\rm{s}}} \times {S_{\rm{s}}} + {f_{\rm{r}}} \times {S_{\rm{r}}} + {f_{{\rm{SIM}}}} \times {S_{{\rm{SIM}}}} = {S_{\rm{M}}},$
${f_{\rm{s}}} \times {\delta ^{18}}{{\rm{O}}_{\rm{s}}} + {f_{\rm{r}}} \times {\delta ^{18}}{{\rm{O}}_{\rm{r}}} + {f_{{\rm{SIM}}}} \times {\delta ^{18}}{{\rm{O}}_{{\rm{SIM}}}} = {\delta ^{18}}{{\rm{O}}_{\rm{M}}},$
where fs, fr, fSIM are fractions (%) of the saline water, river water and sea-ice melted water, respectively. SM and δ18OM respresent the measured salinity and δ18O values in seawater sample respectively. The inventories of river water (Ir, m) and sea-ice melted water (ISIM, m) were calculated from a trapezoidal integration over the depth at which the river water fraction declined to zero.
The end-member values of δ18O and salinity assessed for the Atlantic Water, river water and sea-ice melted water were listed in Table 1. The values of δ18O and salinity for the river water were adopted as –20‰ and 0, respectively (Cooper et al., 2005; Yamamoto-Kawai et al., 2008). As for sea-ice melted water, values –2.0‰ and 4 were chosen as the end-member values of δ18O and salinity (Macdonald et al., 2002). In order to discern the contribution of Arctic river water and Pacific river water, both the winter Bering Sea Water and the Atlantic Water were assigned as the end-member saline water in this study. When the Atlantic Water (AW, S=35, δ18O=0.3‰; Ekwurzel et al., 2001) was adopted as the end-member, the fraction of total river water (frtotal) can be calculated, representing the sum of the Arctic river water and the Pacific river water. When the winter Bering Sea Water (wBSW, S=33.1, δ18O=–1.1‰; Macdonald et al., 2002) was adopted, the calculated fraction of river water (frArctic) represents the contribution solely from the Arctic river runoff. The difference between frtotal and frArctic stands for the contribution of Pacific river water (frPacific). In this calculation, the calculated fractions of river water will be biased when the end-member value of δ18O in the Arctic rivers is not equal to that of the Pacific rivers. Fortunately, the end-member value of δ18O in the Arctic rivers was not significantly different to that of the Pacific rivers. The end-member δ18O values in the Arctic rivers were reported as (–20±2)‰ (Östlund and Hut, 1984; Bauch et al., 1995, 2011; Ekwurzel et al., 2001; Macdonald et al., 2002; Cooper et al., 2005, 2008; Yamamoto-Kawai et al., 2008, 2009), while those of the Pacific rivers (most from Yukon River) ranged from –17‰ to –23‰ with an average of (–19.5±4)‰ (Coplen and Kendall, 2000; Grebmeier et al., 1990; Cooper et al., 1997, 2005). The sensitivity analysis indicated that uncertainties of the calculated fractions of the river water and sea-ice melted water were mostly sensitive to the end-member value of δ18O for the river water. However, the calculated fractions of the river water and sea-ice melted water were biased only less than 2% when the end-member value of δ18O varied from –20‰ to –19.5‰ (Tong et al., 2014; Pan et al., 2015).
3 Results
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3.1 Temperature
Temperature in the upper 200 m along the transect showed clearly a south-north decreasing pattern (Fig. 2a). To the south of 82°N, temperature ranged from –1.51°C to –0.66°C at depth of 50–150 m, while temperature was relatively low (–1.51°C to –1.13°C) in north of 82°N. The depth of the coldest water in the southern basin (~200 m) was deeper than that in the northern basin (~150 m). Temperature at depth of 300–600 m was greater than 0.3°C with little variability, probably indicating the warming of the Arctic Intermediate Water (Carmack et al., 1995; Zhao et al., 2005).
3.2 Salinity
Salinity in the upper 50 m to south of 82°N (24.8–31.7) was lower than those to the north of 82°N (24.8–32.4) (Fig. 2b), implying the influence of sea-ice melted water and river water. Salinity increased northward in the southern region while slightly decreased with increasing latitude in the northern basin. Salinity increased with depth to about 34.2 at 200–300 m. Two haloclines were observed at 50–150 m and 200–300 m, corresponding to the Upper Halocline Water (core salinity 33.1) and the Lower Halocline Water (core salinity 34.2) (Jones and Anderson, 1986; Rudels et al., 1996), respectively. The waters with salinity higher than 34.2 below 200 m was called the Arctic Intermediate Water (Jones et al., 1998; Chen et al., 2003; Shi et al., 2005). Salinity at depth of 300–600 m ranged from 34.6 to 34.9 along the transect.
3.3 δ18O
Surface δ18O values ranged from –4.01‰ to –1.04‰ with an average of –2.98‰, showing an increase from 71.9°N to 82°N and then a slightly decrease northward (Fig. 2c). In the upper 200 m water column, δ18O values generally increased downward but with a more rapid increase in north of 82°N. The lowest δ18O values (–4.01‰ to –3.07‰) were observed in surface water. The δ18O values to the south of 82°N (–4.01‰ to –1.68‰) were lower than those to the north of 82°N (–2.88 to –1.59‰) in the upper 50 m water column. δ18O values below 150 m reached around 0.3‰ in the north of 82°N, illustrating the characteristic of the Atlantic Water, but this signal was not observed untill 200 m in south of 82°N. Obviously, the isoline with δ18O value of zero was shoaled toward north, indicating much stronger influence of freshwater in the southern Canada basin.
δ18O values in the upper 50 m water column mostly lie to the left of the two end-member conservative mixing line of Pacific Water and river water (Fig. 3), suggesting a notable influence of the sea-ice melted water. In contrast, δ18O values at depths of 50–200 m (with salinities of about 33.1) stand to the right of the mixing line (Fig. 3), indicating an effect of brine water. The freezing and melting of sea ice resulted in a large variability in salinity but relatively small variability in seawater δ18O values. The fractionation factor of oxygen isotopes during either freezing or melting of sea ice was generally within 2‰ (Melling and Moore, 1995; Eicken et al., 2002; Macdonald et al., 2002; Pfirman et al., 2004). Most of seawater δ18O at depths of 200–600 m were greater than zero and close to the characteristic value (0.3‰) of the Atlantic Water (Ekwurzel et al., 2001). Interestingly, δ18O value of the winter Bering Sea Water fall on the conservative mixing line of the Atlantic Water and river water (Fig. 3), suggesting that the winter Bering Sea Water can be expressed as a linear mixture of the Atlantic Water and river water on the δ18O-S diagram but with no contribution of Arctic river water. We thus suggest that water mass in our study area can be regarded as a mixture of the river water (including Arctic river water and Pacific river water), sea-ice melted water and Atlantic Water.
3.4 Components of river water
The fractions of total river water (frtotal) and Arctic river water (frArctic) both decreased with depth and increasing latitude, but slightly increased northward to north of 82°N (Figs 4a and b). The largest frtotal (17.4%–20.1%) and frArctic (11.7%–14.8%) were both observed in surface waters in the southern Canada Basin. Two contour lines of frtotal (8% and 0%) both shoaled toward north, and showed different patterns at the boundary of 82°N as well as frArctic (5% and 0%). To south of 82°N, frtotal decreased with depth from 20.1% in surface water to 0% at about 300 m. frArctic also decreased with depth from 14.8% in surface water to 0% at about 200 m. The frtotal was almost zero below 300 m, while frArctic was close to zero below 200 m, indicating the penetration depth of the total river water (300 m) was deeper than the Arctic river water (200 m) in the southern Canada Basin. frtotal and frArctic showed an opposite spatial characteristic to the north of 82°N with a slightly increase with increasing latitude. The depth of two contour lines of frtotal (8% and 0%) increased northward, corresponding to an increased penetration depth of total river water from ~200 m to ~300 m. Similarly, the penetration depth of Arctic river water was also increased northward from ~100 m to ~125 m. Thus, the total river water and Arctic river water were more abundant toward high latitudes in the northern Canada Basin.
The fractions of Pacific river water (frPacific) gradually increased with depth till the lower halocline, and then sharply decreased to zero below (Fig. 4c). The frPacific was lower than frArctic at depth of 0–50 m, while the opposite was true at deeper layers (Figs 4b and c), indicating that the Pacific inflow was the main source of river water below 50 m depth in the Canada Basin. The distribution of frPacific also showed different characteristics at the boundary of 82°N. The frPacific values increased with depth from 5.2% in surface water to 8.9% at about 200 m, and then sharply decreased to 0%–3.2% at depth of 300 m in the southern Canada Basin. Below 300 m, there was no Pacific river water, indicating that the penetration depth to the south of 82°N was about 300 m. Comparably, frPacific increased slightly with depth from 5.6% in surface water to 7.1% at about 100 m, and then decreased rapidly to 0%–3.8% at 125–200 m to the north of 82°N. Obviously, the penetration depth of frPacific in the northern Canada Basin was shallow to 200 m.
The inventories of three river water components (Irtotal, IrArctic and IrPacific) in the water column were listed in Table 2. Because Irtotal represented the total river water imported into the Canada Basin, it was higher than IrPacific. Irtotal, IrArctic and IrPacific were (19.1±4.0) m, (8.3±2.8) m and (10.8±2.1) m, respectively, in the southern Canada Basin. By comparison, such values decreased to (10.3±1.7) m, (4.5±0.9) m and (5.9±1.0) m, respectively, in the northern Canada Basin. It is clearly shown that more abundant river water was stored in the southern Canada Basin, probably due to the Ekman convergence of Beaufort Gyre (Proshutinsky et al., 2002, 2009).
The inventory of total river water in the Canada Basin was higher than previous estimates in the Makarov Basin (11.5 m), the Amundsen Basin (9.6 m) and the Nansen Basin (2.9 m) (Bauch et al., 1995), but close to that in the Canadian Basin (13 m; Bauch et al., 1995), indicating that the Canada Basin was a main storage region of river water in the Arctic Ocean, despite most of the river discharge was located in the eastern Arctic. This discrepancy may be related to the change of atmospheric pressure fields to cyclonicity (Proshutinsky and Johnson, 1997; Thompson and Wallace, 1998), and the resultant altered pathway of major Russian rivers spreading onto the East Siberian Shelf instead of flowing directly into the Trans-Polar Drift (TPD) (Semiletov et al., 2000; Ekwurzel et al., 2001; Guay et al., 2001; Johnson and Polyakov, 2001). Such forcing would undoubtedly promote the accumulation of river water in the Canada Basin (Proshutinsky et al., 2002; Shimada et al., 2006).
3.5 Component of sea-ice melted water
fSIM-wBSW was a little higher than fSIM-AW (1.5%–1.9%), due to that the mixing line of RR and wBSW slightly lied to the right of the mixing line of RR and AW (Fig. 3). However, fSIM-wBSW and fSIM-AW showed the same spatial pattern in the Canada Basin (Figs 5a and b), and thus only the fSIM-AW was used for discussion in this study.
fSIM-AW showed different patterns with the boundary of 82°N (Fig. 5a). The maximum fSIM-AW was observed at depth of 0 m (2.5%–16.8%), indicating an intense sea ice melting in surface water. fSIM-AW decreased rapidly with increasing depth (–3.9%–16.8%) with the penetration depth less than 30 m in the southern Canada Basin. By contrast, the penetration depth of sea-ice melted water (0.2%–12.2%) was more than 50 m to the north of 82°N. A weak signal of sea-ice melted water (~1%) was even observed at depth of 100–125 m at Stas B82, B83 and B84, reflecting abundant sea-ice melted waters accumulated in the northern Canada Basin.
The minimum core of fSIM-AW extended northward from the depth of 50 m on the slope to the depth of 200 m at 82°N (Fig. 5a), corresponding to the maximum layer of frPacific (Fig. 4c). The negative fSIM-AW at these layers indicated that saline brines released during sea ice formation involved in the formation of Upper Halocline Layer in the Canada Basin, similar to the results of previous sampling campaign during summer 1999 (Chen et al., 2003). Interestingly, negative values of fSIM-AW were also found below 50 m at the three high-latitude basin stations (B84A, B85 and B85A; Fig. 5a), indicating the influence of brines advected from shelf regions. Similarly, Bauch et al. (2011) found a pronounced layer influenced by brines released during sea-ice formation is present at depths of 30–50 m over the Lomonosov Ridge, which was attributed to advection from shelf regions.
4 Discussion
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4.1 Pacific inflow is a major source of freshwater in the Canada Basin
IrPacific is designated by “Pacific river water” which represented the river runoff contributed by Pacific inflow. The IrPacific ranged from 4.6 m to 13.5 m, with an average of (8.4±2.8) m in the Canada Basin, while the IrArctic was 3.3–12.7 m, with an average of (6.7±2.8) m. In addition, the inventory of Pacific river water was always higher than Arctic river water, except for Stas S24 and S25 (Fig. 6). The contribution of Pacific river water to total river water (IrPacific/Irtotal) ranged from 51.2% to 68.3% except for Stas S24 and S25 (42.2% and 48.5%) in the Canada Basin. They both demonstrated that the Pacific inflow was the major source of river water in the Canada Basin. Considering the averaged inventory of sea-ice melted water was only –0.8 m, it thus confirms that the Pacific river water was the major contributor of freshwater in the Canada Basin in summer 2008. Aagaard and Carmack (1989) estimated the freshwater flux through the Bering Strait as 1 670 km3/a based on an annual velocity of water transport (0.8 Sv) and salinity (32.5), which was about 1/3 of the total freshwater input to the Arctic. Woodgate and Aagaard (2005) revised the freshwater flux through the Bering Strait from 1 670 km3/a to 2 500 km3/a, implying more freshwater was transported to the Arctic Ocean by the Pacific inflow. Woodgate et al. (2006) reported that the increase of Pacific river water (~800 km3) in the Canada Basin from 2001 to 2004 was about 1/4 of annual Arctic river runoff. Besides, the mooring data indicated that the Bering Strait throughflow increased ~50% from 2001 (~0.7 Sv) to 2011 (~1.1 Sv), and the freshwater flux variability exceeds variability in other Arctic freshwater sources (Woodgate et al., 2012). Obviously, the Pacific inflow is a major source of freshwater in the Canada Basin.
4.2 Accumulation of river water and sea-ice melted water in permanent ice zone
As described in Section 3.4, fr (frtotal, frArctic and frPacific) slightly increased toward the north pole from 82°N and the penetration depth of fr was deeper in the northern Canada Basin, indicating that river water, including the Arctic river water and the Pacific river water, was more abundant in the permanent ice zone than those in the region with sea-ice just melted in the northern Canada Basin. On the basis of geographic position, stations to the north of 84°N (B84, B84A, B85, and B85A) were located around the Alpha-Mendeleyev Ridge (Fig. 1), the Irtotal, IrArctic and IrPacific at these stations were 7.9–10.2 m, 3.3–4.3 m and 4.6–6.6 m, with an average of (9.0±1.2) m, (3.7±0.5) m and (5.3±1.1) m, respectively (Fig. 6). By comparison, Irtotal, IrArctic and IrPacific at Stas B82 and B83 were 11.2–12.4 m, 4.8–5.9 m and 6.4–6.5 m, with an average of (11.7±0.6) m, (5.2±0.6) m and (6.5±0.1) m, respectively (Fig. 6). Obviously, Irtotal, IrArctic and IrPacific to north of 84°N increased by 2.7 m, 1.5 m and 1.2 m, respectively, similar to observation by Newton et al. (2013). Both the increase of IrArctic and IrPacific to the north of 84°N suggested accumulation of the Arctic river water and Pacific river water in the permanent ice zone in the northern Canada Basin.
Two mechanisms may be responsible for the accumulation of IrArctic and IrPacific in the northern Canada Basin. First, the higher Arctic river water around the Alpha-Mendeleyev Ridge may result from topographically trapped river waters from the Trans-Polar Drift (TPD) during their transport from the Siberia to the Fram Strait. TPD originated from the continental shelves containing abundant river water (Newton et al., 2008). The flow of TPD was driven by the steric height gradient between the Siberian-Alaskan shelves and the North Atlantic, and the prevailing wind stress between the Canadian and Eurasian Basins. Less energy input is required at these features to balance vorticity changes as the thickness of isopycnal layers increases, and thus the trans-Arctic transport tends to follow the flanks of the ridges as it crosses the central basin (Holloway and Wang, 2009; Newton et al., 2008). The topographically trapped plumes of runoff-rich shelf waters will result in the accumulation of river water over the Alpha-Mendeleyev Ridge. Previous studies suggested that one branch of the Pacific inflow was incorporated into TPD at the north of Herald Valley (Steele et al., 2004). This branch provided the additional Pacific river water for the northern Canada Basin. Second, the AO index increased from –0.38 to 0.18 during 2005–2008 (data from NOAA), indicating that surface circulation became more anticyclonic in the Arctic Ocean. This change will result in more coastal water to be carried across shelf to the eastern Makarov Basin and Chukchi borderland regions, and further incorporated into the BG (Morison et al., 2012). Meanwhile, a branch of the Pacific inflow located at northwest of Herald Valley would incorporate into the BG (Pickart et al., 2010). The modified waters flowing along the Alpha-Mendeleyev Ridge increased the IrArctic and IrPacific in the northern Canada Basin.
The inventories of sea-ice melted water (ISIM-AW) ranged from –9.1 m to 8.5 m, with an average of –0.7 m in the Canada Basin (Fig. 7), which was higher than previous reported values (Macdonald et al., 2002; Chen et al., 2003; Yamamoto-Kaiwai et al., 2008; Newton et al., 2013). It seemed that the sea-ice melted water in the Canada Basin increased over the past few years, comparable to the observation by Yamamoto-Kawai et al. (2009). To south of 82°N, most ISIM-AW values were negative except at Sta. B33, indicating a net ice formation in the southern Canada Basin. Values of ISIM-AW were more negative at Stas S22, S24 and S25 (Fig. 7), suggesting the Beaufort slope is a major region for sea ice formation, which was consistent with previous results (Macdonald et al., 2002; Bauch et al., 2005). Surprisingly, inventory of sea-ice melted water at Sta. B33 was as large as 3.3 m, showing a significant sea ice melting. The penetration depth of sea-ice melted water at this station (50 m) was also deeper than those in the southern Canada Basin (30 m) (Fig. 5). However, the reason for significant sea ice melting at Sta. B33 was not clear and further studies are needed.
The inventory of sea-ice melted water to the north of 82°N ((4.0±2.8) m) was much higher than those in the southern Canada Basin (–(3.1±3.6) m, Fig. 7), indicating that a large quantity of sea-ice melted water accumulated in the permanent ice zone during our investigation. This phenomenon was similar to previous observation in the southern Canada Basin (Macdonald et al., 2002; Tong et al., 2014), although the locations of sea ice melted water accumulation shifted northward. The accumulation of sea ice melted water in the high latitude regions resulted in increase of the penetration depth of sea-ice melted water to more than 50 m in the permanent ice zone (Fig. 5).
It should be noted that the accumulation of sea ice melted water in the permanent ice zone observed during our sampling was contrary to the expectation that more sea-ice melted water would be retained in the southern Canada Basin, due to the earlier melting. We suggest that the lateral transportation of sea-ice melted water could be responsible for this. The sea-ice melted waters were easily suffered from wind fields and surface currents due to their shallow penetration depth. The penetration depths to the south of 82°N were less than 30 m. The early ice melted waters were transported and accumulated in the permanent ice zone by the anti-cyclonic Beaufort Gyre. A marked decline of sea ice in the Canada Basin over 2007 and 2008 (Perovich et al., 2011) will promote surface currents to transport the sea-ice melted waters to the high latitude regions. On the other hand, the presence of pack ice in the permanent ice zone will weaken the surface currents, and keep sea-ice melted waters reserved. Besides advection of the sea ice melted water, the melt pond or polynya existed widely in the high latitude regions may provide an additional effect. The open water in the melt pond or polynya could receive more solar radiation and promote the melting of sea ice.
5 Conclusions
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The freshwater components in the Canada Basin during 2008 summer were quantified by δ18O and salinity mass balance. The fractions of total river water and Arctic river water decreased with depth and latitude, while the fraction of Pacific river water increased with depth and decreased with latitude. The fractions of Pacific river water were higher than those of Arctic river water below 50 m. As a result, the inventories of Pacific river water were higher than those of Arctic river water in the Canada Basin. This demonstrated that the Pacific inflow through the Bering Strait is a main source of freshwater in the Canada Basin. The river water and sea ice melted water were abnormally accumulated to the north of 82°N in the Canada Basin. The lateral transport of freshwater was proposed to explain this phenomenon. Relatively high river water in the permanent ice zone, including Arctic river water and Pacific river water, was attributed to shelf waters trapped by Trans-Polar Drift or surface circulation. The accumulation of sea-ice melted waters in the permanent ice zone was attributed to the trap of the earlier melted waters by the enhanced Beaufort Gyre. The accumulation of freshwater in the permanent ice zone would profoundly exert influence on biogeochemistry in the Arctic Ocean, thus further studies are undoubtedly necessary to better understand its effects.
Funding
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  • The Chinese Polar Environment Comprehensive Investigation & Assessment Program under contract Nos CHINARE2017-03-04-03 and CHINARE2017-04-03-05; the Natural Science Foundation of China under contract No. 41125020.
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Year 2017 volume 36 Issue 8
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Article Info
doi: 10.1007/s13131-017-1023-1
  • Receive Date：2016-05-27
  • Online Date：2026-04-16
  • Published：2017-08-01
Article Data
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  • Received：2016-05-27
  • Accepted：2016-09-19
Funding
The Chinese Polar Environment Comprehensive Investigation & Assessment Program under contract Nos CHINARE2017-03-04-03 and CHINARE2017-04-03-05; the Natural Science Foundation of China under contract No. 41125020.
Affiliations
    1 College of Ocean and Earth Sciences, Xiamen University, Xiamen 361102, China
    2 State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361102, China

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表12种不同金属材料的力学参数
科
Family		 属数
Number of
genus		 种数
Number of
species		 占总种数比例
Percentage of
total species (%)		 属
Genus		 种数
Number of
species		 占总种数比例
Percentage of total
species (%)
鹅膏菌科Amanitaceae 2 11 5.26 鹅膏菌属 Amanita 10 4.78
小菇科 Mycenaceae 2 12 5.74 丝盖伞属 Inocybe 5 2.39
多孔菌科 Polyporaceae 8 14 6.70 蜡蘑属 Laccaria 5 2.39
红菇科 Russulaceae 3 23 11.00 小皮伞属 Marasmius 6 2.87
小菇属 Mycena 11 5.26
光柄菇属 Pluteus 5 2.39
红菇属 Russula 17 8.13
栓菌属 Trametes 5 2.39
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