Silica in Bering Sea Deep and Bottom Water

Dynamics of the Bering Sea • 1999 285 CHAPTER 13 Silica in Bering Sea Deep and Bottom Water Lawrence K. Coachman School of Oceanography, University of Washington, Seattle, Washington Terry E. Whitledge Marine Science Institute, University of Texas, Port Aransas, Texas John J. Goering Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, Alaska Abstract Neither the enormously high concentrations of silicic acid, higher than in any other ocean basin, nor the source, rate of supply, and flushing of the deep and bottom waters of the Bering Sea basins have been adequately explained. In this paper these questions are examined using the few available data, and a model describing the silicate distribution is proposed. The source for Bering Sea bottom water is North Pacific water from ~3,500- 4,000 m depths, which enters through the westernmost pass in the Aleutian- Commander island arc (Kamchatka Strait) with high silicate concentrations, and then circulates into the other basins. The bottom water slowly dis- places the deep water upward; at the same time silicic acid concentrations are increased by regeneration both within the water columns and from the bottom. Model results suggest bottom regeneration rates are about 4- 5 times faster than those within the water columns, and that total resi- dence times for the deep water are about 250-300 years. The deep Bering Sea acts like an “appendix” to the North Pacific Ocean—it may be an important location to monitor certain aspects of both climate change and an- thropogenic pollution. Introduction Silicate concentrations of deep Bering Sea basin water exceed 230 µM, values higher than in any other basin of the World Ocean and nearly 40% T.E. Whitledge is currently at Institute of Marine Science, University of Alaska Fairbanks, Fairbanks, AK. 286 Coachman et al. — Silica in Bering Sea Deep and Bottom Water greater than at the same depths in the adjacent North Pacific. The Bering Sea has aptly been dubbed “the sea of silica” (Tsunogai et al. 1979). Whence these uniquely high values? There appear three viable possibilities: (1) production over the deep basins of siliceous phytoplankton is extraordi- nary, such that the high concentrations of dissolved silicate are produced by dissolution at depth of an extra-large “rain” of diatoms and radiolaria; (2) flushing of deep basin waters is inordinately slow, so that high concentrations are long-time accumulations of dissolution products from more ordinary production levels (could deep Bering Sea water be the oldest water in the World Ocean?); and (3) regeneration rates are extraordinarily high, particularly at the sediment-water interface. Tsunogai et al. (1979) estimated that the regeneration rate in the deep Bering Sea is 4-5 times greater than that in the deep North Pacific. Perhaps some combination of these is likely. The rate of renewal of the deep water in the Bering Sea must be an important factor, but the source of Bering Sea bottom water and its rate of supply and basin flushing have never been described. Likewise, no measurements have ever been made of particulate silica, biogenic silica production, or dissolution rates of silica in the water columns of the deep basin. It is the purpose of this paper to examine, using the few available data, the question of bottom water formation—sources, rates of supply, and flushing—and then to present a preliminary model explaining the enormous concentrations of silica in the deep and bottom water of the Bering Sea which can serve as the basis for future research. The deep Bering Sea (depths of 3,900-4,000 m) is not “at the greatest depths” considered as one basin, but three (Fig. 1). The westernmost Kam- chatka Basin is connected with the North Pacific at depths to 4,000 m via Kamchatka Strait. Shirshov Ridge extends south from Cape Olyutorsk almost to the Aleutian Islands separating Kamchatka Basin from those to the east. There is a small connection with an approximate 25 km-wide gap northwest of Attu Island with depths to about 3,700 m. Geologically, Shir- shov Ridge connects with Bowers Ridge which arcs northeastward and then south to the Aleutian Islands, nearly isolating small Bowers Basin from the large and deep Central Basin. The best bathymetric chart (Heezen and Tharp 1975) suggests that at abyssal depths, 3,900-4,000 m, only the deepest 200 m of these three basins is actually isolated by sills, but the direct connection for water exchange between them below ~3,000 m is narrow and tortuous. The deep Bowers and Southeast basins (it is sometimes convenient to consider the large Central Basin in two parts, North Central and Southeast, though there is no topographic separation) are almost completely isolated from the North Pacific by the Aleutian-Commander island arc. The five major passages for water through the arc are listed in Table 1 and their locations indicated in Fig. 1. There is no sill in Kamchatka Strait separating Kamchatka Basin from the North Pacific in the far west. Sill depths in Near Strait are close to 2,000 m in two or three narrow (20-30 km wide) gaps due south of the Dynamics of the Bering Sea 287 Figure 1. The basins and separating ridges of the deep Bering Sea. Sills dividing Bowers Basin from Karagin and the Central basins rise only about 200 m above the deeps; no sill divides the North Central from the Southeast Basin. Letters indicate locations of the major passes through the Aleu- tian-Commander island arc (Table 1). Shown also are locations of deep- reaching hydrochemical stations from Argo (47; 1966), GEOSECS (1973), Hakuho Maru (6,7,9-11; 1975), and R/V Korolëv (110, GEOSECS; 1988). Shirshov Ridge; these actually connect directly with the southeast corner of Kamchatka Basin rather than with the basins to the east. Elsewhere in Near Strait, sill depths are about 1,100 m. Bowers Basin is open to the North Pacific via Buldir Pass with sill depth <700 m. Amchitka and Amukta passes connect with the Southeast Basin with sill depths <1,200 m in Amchitka Pass. In the only study to date discussing bottom water, Tsunogai et al. (1979) hypothesized the source to be deep water from the North Pacific. They proposed, very generally, that the deep North Pacific water enters through Kamchatka Strait and from there spreads eastward over the whole basin. They argued that the vertical distribution of oxygen supported their hypothesis (without stating how), and that increasing values of the deep silicate maxima from the Kamchatka Basin through the Central Basin rep- resent an upstream-downstream progression of deepwater flow. These arguments are not convincing, and there are other discrepancies in their 288 Coachman et al. — Silica in Bering Sea Deep and Bottom Water Table 1. Sill depth and section area of the five largest passes through the Aleutian-Commander island arc. Sill depth Pass (m, approx.) Area (km2) Kamchatka Strait (A) 4,000 335.3 Near Strait (B) <2,000 239.0 Amchitka (C) 1,155 45.7 Buldir (D) 640 28.0 Amukta (E) 430 19.3 From Favorite 1974. Letters show locations in Fig. 1. discussion which require confirmation. For example, they state that the primary site of silicate regeneration is the sediment-water interface even though most deep silicate profiles actually show maxima some hundreds of meters above bottom. There are few data from the deep Bering Sea. The total of NODC hydrographic stations from all basins with observations deeper than 3,000 m number less than 100. This paper uses the GEOSECS station of 1973 from the Southeast Basin (Park et al. 1975), 4 stations from Hakuho Maru KH- 75-4 in 1975 (Tsunogai et al. 1979), and 8 stations from the Akademik Korolëv in 1988 which provide deep-reaching high-quality hydrographic and hydrochemical data. Overview of Deep Hydrography Interbasin Differences Fewer than 100 stations extending >3,000 m have ever been taken in the Bering Sea. The data span many years (back to 1933) and were taken by many expeditions. Bottom water characteristics among the basins are very similar. In these data, salinities range between 34.62 and 34.68 ppt, and coldest potential temperatures are slightly below 1.3°C. To assess interbasin differences, the NODC data were organized into groups by location, as shown in Fig. 2. θ/S envelopes for the stations of each group were compared in Fig. 3 and encompass the deepest observations at each station (all those >3,300 m). Though there is no guarantee that the maximum salinity or minimum temperature is recorded for each location, there are sufficient stations in each group that we believe the envelopes show values close to the temperature extremum. The only difference among groups that can be inferred for salinity is that water in Kamchatka Strait is slightly more saline than anywhere within the deep Bering Sea basins, perhaps by 0.01-0.02 ppt. Otherwise salin- Dynamics of the Bering Sea 289 Figure 2. Groupings of deep-reaching NODC stations for which the θ/S envelopes are shown in Fig. 3. A possible circulation of bottom water interpreted from potential temperature is indicated by arrows. ity values of the historical data are too inaccurate to allow meaningful interpretation. In minimum temperature, however, there appear to be definite differences between basins, and these differences have an upstream-downstream trend. Coldest potential temperatures are in Kamchatka Strait (Fig. 3; A). Next warmest shown in Fig. 3 are groups B and C, about equidistant to the north and to the southeast from Kamchatka Strait.

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