Transgressive geochemical records of two cores taken from the East China Sea: a preliminary study

Abstract

Geochemical and sedimentological analyses were undertaken of two piston cores of sediments from the East China Sea, with a view to tracing paleoceanographic environmental changes. Organic matter and carbonate contents of the sediments proved quite different in the upper and lower parts of the two cores. The most striking characteristics are a gradual increase upward in both carbonate and total organic carbon (TOC) content above 100 cm in Core 19 and above 150 cm in Core 20. In addition, the δ13C of organic material shows a similar gradual increase upward. These changes in TOC and carbonate, along with parallel changes in the sedimentology, clearly reflect shifts in the boundaries of the sedimentary environments. The sedimentological and geochemical signals of the two cores allow the sedimentary column to be divided into three lithostratigraphic units: Units I, II, and III, corresponding to a highstand stage, a transgressive stage, and a lowstand stage, respectively. Within Unit I, the TOC and CaCO3 contents and the grain size show slight variations that gradually increase upward, probably reflecting a stable modern oceanographic environment. Within Unit II the geochemical signals are characterized by steep and abrupt fluctuations typical of a transgressive stage. In Unit III the geochemical signals are stable but exhibit a different pattern to Unit I. Unit III appears to correspond to a lowstand stage, given the large amount of terrigenous organic matter that it contains.