Paper No. 130-5
Presentation Time: 10:00 AM
SILICON AND NITROGEN CYCLING IN RESPONSE TO BIOGENIC OPAL PRODUCTIVITY DURING THE LAST 600 KA IN THE BERING SEA (IODP EXP. 323 SITE U1343)
Multidisciplinary paleoceanographic analyses (biogenic opal, diatom silicon isotope (δ30Siopal), diatom-bound nitrogen isotope (δ15Ndb), and nitrogen isotope (δ15N) of bulk sediments) were conducted for Site U1343 of IODP Exp. 323, in order to investigate the degree of nutrient utilization in response to changes of diatom productivity in the slope area of the Bering Sea. An age model by the oxygen isotope stratigraphy of benthic foraminifera for Site U1343 presents a record of 600 ka for the studied interval. High δ30Siopal values (+0.7‰ to +1.3‰) during interglacial periods were attributed to increased diatom production under nutrient-replete conditions, which would have resulted in higher silicic acid utilization in the surface water. In contrast, low δ30Siopal values (+0.2‰ to +1.1‰) during glacial periods were caused by reduced diatom production due to extensive sea-ice cover, leading to lower silicic acid utilization in the surface water. This indicates that a large portion of silicic acid in the surface water was removed by increased diatom productivity under abundant silicic acid condition, whereas less silicic acid was drawn down by decreased diatom productivity under depleted silicic acid condition. Covariance between δ15N of bulk sediments and diatom-bound δ15Ndb values at Site U1343 confirms that δ15N can be used as a utilization proxy in the Bering Sea. The δ15N values generally vary synchronously with diatom productivity during the last 600 ka, which is different from the results of the southern Bering Sea, the Okhotsk Sea, and the subarctic northwestern Pacific. This suggests that nitrate utilization in the slope area of the Bering Sea is controlled mainly by sea-ice rather than surface water stratification. High rate of nitrate utilization was attributed to increased diatom productivity under more open-ocean condition with abundant nitrate in the surface water during warm periods. In contrast, low rate of nitrate utilization was ascribed to suppressed diatom productivity by extensive sea-ice development during cold periods. Therefore, cycling of silicic acid and nitrate is closely related to degree of diatom productivity in the slope area of the Bering Sea during the last 600 ka.