DBPapers
DOI: 10.5593/SGEM2014/B32/S15.078

ESTUARINE AND SHALLOW SHELF ENVIRONMENTAL CHANGES RECONSTRUCTED BY GEOCHEMISTRY OF BOTTOM SEDIMENTS IN AMUR BAY (JAPAN SEA) OVER THE LAST 200 YEARS

I. Kalugin, A. Аstakhov, K. Аksentov, A. Darin, V. Babich
Wednesday 1 October 2014 by Libadmin2014

References: 14th International Multidisciplinary Scientific GeoConference SGEM 2014, www.sgem.org, SGEM2014 Conference Proceedings, ISBN 978-619-7105-14-8 / ISSN 1314-2704, June 19-25, 2014, Book 3, Vol. 2, 593-600 pp

ABSTRACT
Amur Bay, Sea of Japan is one of the most studied areas in relation to seasonal anoxia in the estuarine-shelf basins, induced by the water inflow increase (Razdolnaya-Suifun River) during the summer monsoon. Eutrophication is associated with stratification of the water column due to warm fresh water entry, and also to synchronous increase of the phytoplankton productivity in July and August. To identify the causes of anoxia, sediment layers accumulated over the past 200 years near the mouth of the Razdolnaya River in the Amur Bay were tested. A new approach to the study of sediment records has appeared due to an automatic technique for scanning microanalysis along cores in situ. X-ray fluorescence analysis on synchrotron radiation (XRF SR) in the Institute of Nuclear Physics (Novosibirsk, Russia) was used to determine more than 35 elements in freshly opened cores and in solid preparates with steps of 0.1-1 mm. Tracing of sediment composition along the river-sea profile showed a reduction of carbonate-organophilic elements (Ca, Br, and Sr) closer to the river mouth. Respectively, high values of Fe, Rb, Y, Th and Rb/Sr ratio represent the flooding accumulation of clay material and hydroxides in that shallow freshened zone. High Br content, Mo/Mn and Rb/Sr and low Mn and Ca are observed within the axial part of the bay with seasonally anoxic bottom waters. The results suggest the increase of seasonal anoxia in bottom waters north of the Amur Bay and the Razdolnaya estuary in dry summer periods. Yearly drops of Br content mark extremal floods during typhoon events 1-3 times per decade. Zn, Ni, Cu, as well as Hg, most probably indicate the degree of technogenic contamination in sediments, together with an increase in eutrophication and organic matter content for the last 50 years. High-resolution studies of the element composition show alternating periods of marine or more dilute waters. The subdivision into layers was based on statistical models (cluster analysis, discriminant analysis), whilst the chronological framework was established using the 210Pb and 137Сs technique.

Keywords: Estuary, micro XRF, geochemical records, seasonal anoxia, time series