12. Geochemistry of Pliocene Sediments from ODP Site 1143 (Southern South China Sea)1

Rolf Wehausen,2 Jun Tian,3 Hans-Jürgen Brumsack,2 Xinrong Cheng,3 and Pinxian Wang3


Pliocene core intervals from Ocean Drilling Program Site 1143 located in the southern part of the South China Sea were measured for their major and minor element composition at ~2-k.y. resolution. Changes in sediment composition parallel glacial-interglacial changes as indicated by the corresponding oxygen isotope record of benthic foraminifers. Carbonate contents are lower during glacial stages and higher during interglacial stages. This is attributed to dilution by a higher contribution of terrigenous detrital matter during glacial intervals and to enhanced biological productivity during interglacials as indicated by enrichments in barium. The terrigenous detrital matter fraction shows small but distinct changes in composition. These variations may be explained by multiple processes, including changes in provenance and weathering caused by monsoonal climate variability and sea level fluctuations. Gradual or long-term changes in sediment composition also occur during the investigated interval. There is a shift toward lower TiO2 (carbonate-free basis; cfb) values at around 2.9 Ma and an increase in Al2O3 (cfb) variations at around 2.55 Ma. These changes can be explained in both cases by an increasing contribution from the Mekong River, delivering material with low TiO2 (cfb)/Al2O3 (cfb) ratios, followed by a gradual increase in weathering, causing higher Al2O3 (cfb) (probably reflecting kaolinite) contents. Both may be the result of an enhanced summer monsoon. The manganese and phosphate geochemistry of Site 1143 sediments seem to be strongly dominated by biological productivity and a high burial flux of carbonate, which acts as an adsorbing agent and/or substrate. During strong interglacials and corresponding times of enhanced productivity, Mn may be remobilized from continental margin sediments deposited within an oxygen minimum zone and subsequently exported into the pelagic region, where it oxidizes and settles to form Mn-enriched layers in the sediment.

1Wehausen, R., Tian, J., Brumsack, H.-J., Xinrong, C., and Wang, P., 2003. Geochemistry of Pliocene sediments from ODP Site 1143 (southern South China Sea). In Prell, W.L., Wang, P., Blum, P., Rea, D.K., and Clemens, S.C. (Eds.), Proc. ODP, Sci. Results, 184 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/184_SR/201/201.htm>. [Cited YYYY-MM-DD]

2Institut für Chemie und Biologie des Meeres (ICBM), Carl-von-Ossietzky-Universität, D-26111 Oldenburg, Germany. Correspondence author: wehausen@icbm.de

3Key Laboratory of Marine Geology, MOE, Tongji University, Shanghai, 200092 People’s Republic of China.

Initial receipt: 8 August 2001
Acceptance: 26 June 2002
Web publication: 6 February 2003
Ms 184SR-201