PRODUCTIVITY, NUTRIENTS AND RIVER DISCHARGE IN THE OKHOTSK SEA, A SUBARCTIC BASIN INTERACTING WITH LOW-LATITUDE CLIMATE DETERMINANTS

Today, the paleoceanography and paleoproductivity in the Okhotsk Sea is governed by exceptional climatic boundary conditions. Amongst those are the southernmost extent of sea-ice cover within the Northern Hemisphere’s world ocean and a particularly high, mostly biosiliceous, productivity. In addition, highly variable discharge by the river Amur transports large amounts of freshwater and terrestrial organic matter into the basin. Major intention of this study is to reconstruct the impact of this freshwater discharge on the sedimentary and oceanographic settings and the interplay with rapid natural climatic changes. We use sediment surface samples along the continental margin of Sakhalin, presumably following the main pathway of the freshwater flow. Additional samples from selected areas like the Derugin Basin and transects across the central Okhotsk Sea towards the Kamchatka continental margin enlarge our dataset and act as reference stations for pelagic conditions. Accumulation rates and contents of biogenic opal, TOC/TC, nitrogen as well as C/N ratios and N-15 isotope ratios serve as proxies for spatial distribution patterns of riverine freshwater discharge, biogenic productivity and nutrient utilization processes. Large portions of terrigenous-siliciclastic and organic matter, predominantly in silt and clay size fractions, enter the investigated area by the freshwater flux out of the Amur estuary. This flux is largely controlled by the amount of precipitation reaching the watershed with the progradation of the East Asian summer monsoon. We observe a highly variable nutrient utilization and deposition of particulate organic matter at the different locations. High accumulation rates of biogenic opal and TOC correlate well with the main flow direction of freshwater along the continental margin of E-Sakhalin. However, especially in deeper parts of the basin, high supply of organic matter from a variety of sources plays a crucial role in the buildup of phytodetritus layers and oxygen minimum zones or even dysoxic conditions in the past. Finally, intercomparison of our different records permits to evaluate the significance and soundness of single datasets and helps to test proxies so far unused in this region against well-established records.