Oceanographic Records of Microorganisms in the Surface Sediments from the Bering and Chukchi Seas

Based on the quantitative study of calcium (foraminifera and ostracoda) and siliceous (diatom, radiolarian and sponge spicule) microorganisms as well as the content of calcite in 41 surface sediments of the Chukchi and Bering Seas, we find that planktonic foraminifera are almost absent in the surface sediments, which could be attributed to the low surface primary productivity and strong carbonate dissolution in the studied area. On the contrary, benthic foraminifera are widely observed in most surface sediments. It has been revealed that the distributions of benthic foraminifera and siliceous microorganisms are mainly controlled by the sedimentary types, surface primary productivity, carbonate dissolution and water masses. On the shelf of Chukchi Sea the surface primary productivity is low due to the influence of sea ice, low sea surface temperature and salinity. As a consequence, benthic foraminiferal abundance and diversity are low, radiolarian disappear and only few ostracoda are counted, while diatoms and sponge spicules are abundant in the surface sediments. The surface primary productivity is much higher in the surface sediments on the slope of Bering Sea than that on the shelf of Chukchi Sea. Benthic foraminiferal abudance on the slope of Bering Sea is one order higher than that in the shelf of Chukchi Sea, and higher diversity is also observed in Bering Sea. Similar to calcium microorganisms, siliceous microorganisms (diatom, radiolarian and sponge spicule) are more abundant in Bering Sea than that in Chukchi Sea. According to the abundances of benthic foraminifera and siliceous microorganisms, as well as the ratio of benthic foraminiferal fauna of agglutinated tests and the contents of calcite changing with the water depth in the Bering Sea, carbonate lysocline and compensation depth (CCD) are probably located at the water depths of 2000 m and 3800 m respectively.