Abstract:

Authigenic carbonate minerals are important proxies for the methane seepage and its periodical variation. In order to identify carbonate mineral easily and quickly, the authors discuss whether the end-members (which are aragonite, high-Mg calcite, biogenic calcite and detrital individually) model presented by previous researchers is applicable, and then calculate the authigenic carbonates contents according to the Sr/Ca and Mg/Ca ratios of the bulk sediment of two cores individually in Shenhu area in the northern South China Sea. The results show that there are authigenic carbonate minerals in different depths, which are also proved by the results of XRD and the appearance of authigenic high-Mg calcite and aragonite crystals in the sediments. The appearance of the authigenic carbonate minerals show that the methane seepages has been existing in ancient time and nowadays. The vertical variances based on the sedimentary geochemical proxies implicate that the methane seepages had multi-stages variations. So the end-members model presented by previous researchers can be used in the northern South China Sea. The end-members model is a good method to gain the high-resolution authigenic carbonate minerals profiles, consequently to identify methane seepages recorded in marine sediments, and potential to trace methane seepages and to explore gas hydrate in extent area.

Key words: methane seepages, carbonate, Sr/Ca, Mg/Ca, geochemistry, northern South China Sea