Geochemistry of Pliocene sediments from ODP Site 1143 (southern South China Sea)

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 approximately 2 kyr resolution. Changes in sediment composition parallel glacial-interglacial changes as indicated by the corresponding oxygen isotope record of benthic foraminifera. Carbonate contents are lower during glacial stages and higher during interglacial stages which can be attributed to dilution by a higher contribution of terrigenous detrital matter and due to enhanced biological productivity as indicated by enrichments in barium. The terrigenous detrital matter fraction shows small but distinct changes in composition. Probably, these variations are to 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 do also occur during the investigated interval. For example, there is a shift towards lower TiO2,cfb (cfb = carbonate-free basis) values at around 2.9 Ma and an increase in Al2O3,cfb variations at around 2.55 Ma. These changes can be explained by an increasing contribution from the Mekong River, delivering material with low TiO2,cfb/Al2O3,cfb ratios and a subsequent 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 seems to be strongly dominated by biological productivity and burial flux of carbonate which acts as 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 is exported into the pelagic region where it oxidizes and settles to form Mn enriched layers in the sediment.
 

Astronomical forcing of the East Asian monsoon mirrored by the composition of Pliocene South China Sea sediments

High precision major- and minor element records of Pliocene core intervals from Ocean Drilling Program Site 1145 located in the northern part of the South China Sea were generated at a 5 cm (approximately 2 kyr) resolution. From the geochemical data distinct proxies, those indicating changes in detrital matter provenance as well as those indicating productivity variations, could be derived. Both the terrigenous input to the northern South China Sea (K/Si) and variations in productivity (Ba/Al) show a response to insolation-forced monsoon variability. While the fluvial input (K/Si) responds to changes in the summer monsoon, productivity increases, as documented by Ba enrichments, seem to reflect variations in winter monsoon-driven upwelling. According to our geochemical proxy records, summer and winter monsoon are 180° out-of-phase. This implies forcing of the winter monsoon through Southern Hemisphere summer insolation maxima. The K/Si ratio shows a linear correlation with the La93(1,0.5) Northern Hemisphere summer insolation record and has therefore been used to create an astronomical timescale for the investigated sediments of ODP Site 1145. Carbonate contents and corresponding color reflectance display 41 and 100 kyr cycles. We propose that these do not reflect glacial cycles, but are rather the result of an interference signal of dilution by fluvial and eolian terrigenous material and carbonate production (burial flux). A sudden onset of gradual changes in sediment composition occurs after 3.0 Ma. This is also documented by an increase in sedimentation rates and may be caused by enhanced fluvial input from the Pearl River due to intensified weathering rates on the Asian continent. This may be linked to tectonic processes around the Himalaya-Tibetan Complex and its consequences regarding land-sea heating and atmospheric circulation.