Origin of the organic matter in the Late Quaternary sediments of the eastern margin of Japan Sea

Abstract

This study was carried out on sediment samples collected by piston-coring in two areas of the eastern margin of the Japan Sea. One area is located at open sea conditions in the Oki Trough, offshore Kanazawa city, and the other is located in the enclosed bay conditions of the Joetsu Basin, offshore Joetsu city. Using these samples it was possible to differentiate the source of the organic matter in the sediments of Holocene and late Pleistocene time on the basis of δ13Corg and TOC/TN ratios coupled with palynofacies analysis. The Holocene sediments are characterized by high TOC and TN contents, low TOC/TN ratio, and heavier Corg values, which indicate a predominant marine organic matter production, probably due to warming and inflow of warm ocean currents and coastal currents along the East China Sea. These currents carried abundant phytoplankton from the Pacific Ocean as a result of the sea level rise. Occurrence of particulate organic matter shows abundant primary productivity during the Holocene under marine conditions. On the other hand, the LGM sediments are characterized by low TOC and TN contents, high TOC/TN ratio, and lighter δ13Corg signatures, which are characteristic of terrestrial organic matter, probably due to seaward migration of shorelines and strong input of freshwater with terrestrial materials. This terrestrial influence decreased gradually from the LGM to the Holocene because of the sea level rise and consequent increase in the marine organic matter.