Geochemistry of the surface sediments of the Sulu and South China Seas

Abstract

The chemical and mineralogical composition of the surface sediments of the Sulu Sea, a deep, hydrographically-restricted basin in the Indonesian archipelago, and the neighbouring area of the South China Sea have been examined. The aim of the study was to characterize the geochemistry of the sediments of the isolated, oxygen-deficient basin and to compare them with sediments at equivalent water depths outside the basin, as a prelude to a further study of the response of the composition of the sediments to oceanographic changes, including possible anoxia, during the last glacial maximum. The concentrations of most of the chemical constituents in the surface sediments along four transects change markedly with water depth in the Sulu Sea (SS) and the South China Sea (SCS). Thus, CaCO 3 decreases from around 60% by weight in the shallowest samples (800–1000 m) to around a few percent in the deepest sample at 4203 m in the SCS, where the CCD lies at around 4000 m. In the SS, however, the CCD lies much deeper, so that the CaCO 3 contents are still approximately 40% at 4500 m depth. Organic C also decreases with depth in both basins, from values of around 1–2% in the shallowest samples to less than 1% in the deepest samples. The ratios of Si, Mg, Na, K, Co, Cr, Cu, Ni, Pb, Rb, Y, Zr and Zn to Al all decrease with increasing water depth because of the change in mineralogy and sediment texture with water depth. The P Al is higher in shallower water in both areas because of the higher abundance of detrital phosphorite or phosphatized limestone in these sediments. The Sr Al ratio is also higher in shallower water, reflecting the supply of aragonitic shell debris from the shallow banks and shelves around the basin. The Ba Al ratio has a maximum at 2000 m depth in the SCS, due to higher primary production in overlying surface waters, whereas it is uniformly low in the SS. Manganese is also enriched at intermediate water depths in both the SS and the SCS because of the presence of oxyhydroxides, and this also produces a slight enrichment in Mo in the same samples because of their association in this authigenic phase. The concentration of I and the I C organic ratio decreases with water depth in both areas due to the higher abundance of fresh, marine organic matter at the shallow water sites, which scavenges proportionally more I from sea water. Finally, the δ 13C organic values tend to decrease irregularly with increasing water depth probably reflecting a higher contribution of terrestrial organic matter in deeper water. A discriminant analysis of the full data set reveals that, compared with the SCS samples, the oxygen-deficient SS sediments have higher CaCO 3 contents, lower N and S contents and lower I C organic , Ba Al , Fe Al , K Al , Pb Al , Rb Al , Ti Al , V Al , Y Al and Zn Al ratios. These contrasts are brought about by the different depths of the CCD, differences in the nature of the organic matter and different types of aluminosilicate detritus in the two areas. Although the I C organic ratios are slightly but significantly lower in the SS, which may be due to the low dissolved oxygen contents of the waters of this basin, the Mn and Mo contents and other minor elements that respond significantly to ambient bottom water oxygen levels are not significantly different in the SS and the SCS. Moreover, the organic C contents are also indistinguishable in the two areas; hence, the low bottom water oxygen levels in the SS do not lead to higher organic carbon contents in the surface sediments. The composition of sediments in contact with waters having even lower oxygen concentrations should be evaluated in order to conduct a more definitive test of the effect of low oxygen conditions on carbon preservation. Comparison of glacial-interglacial sediment sections from this area must take particular care to separate the effects of source variations on those potentially caused by low oxygen levels.