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

摘要

High-precision major and minor element records of Pliocene core intervals from Ocean Drilling Program (ODP) 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) shows 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 enrichment, seem to reflect variations in winter monsoon intensity. A stronger winter monsoon may have increased nutrient availability via dust input and/or upwelling phenomena. According to our geochemical proxy records, summer and winter monsoons are approximately 180° out-of-phase. This may imply forcing of the winter monsoon through Southern Hemisphere summer insolation maxima. The K/Si ratio shows a linear correlation with the La90(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 reflectances 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 Himalayan-Tibetan Complex and its consequences regarding land-sea heating and atmospheric circulation.