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The South China Sea: Middle to Late Miocene Proto-Warm Pool Development and the East Asian Monsoon

Nathan, Stephen A.1; Leckie, R. Mark 2
1 Office of the Massachusetts State Geologist, University of Massachusetts, Amherst, MA.
2 Department of Geosciences, University of Massachusetts, Amherst, MA.

During the middle to late Miocene, changes in tectonics and sea level profoundly impacted both global climate and ocean circulation. This study uses planktic foraminifera to examine the interplay between the constriction of the Indonesian Seaway, the uplift of the Himalaya-Tibetan Plateau, and changes in sea level, and how they irrevocably affected the South China Sea (SCS) and the western equatorial Pacific for the interval ~13 - 6 Ma. The SCS records the onset of a proto-warm pool after ~12 Ma, as also observed in the western equatorial Pacific [at Ontong Java Plateau (OJP), Ocean Drilling Program (ODP) Site 806]. The eustatic changes that contributed to the initiation of a proto-warm pool may have transformed the SCS into a semi-enclosed basin, or a basin with greatly restricted flows across the Sunda Shelf. This configuration of the SCS may have lasted until sea level began to rise after ~9.6 Ma or later, as noted by a decrease in the δ18O gradient between thermocline and mixed layer taxa. At ~8.5 Ma, an abrupt increase in the relative abundance of thermocline taxa and changes in the carbon and oxygen isotope gradients may signal increased productivity or increased seasonality associated with intensification of the East Asian monsoon. For the northern SCS (ODP Site 1146) the non-warm pool state may have lasted until the end of the record (~5.6 Ma) while in the southern SCS (ODP Site 1143) the record is less conclusive. Both sites display low planktic foraminiferal δ18O values relative to the western equatorial Pacific (OJP), suggesting that the SCS was significantly influenced by fluvial input.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009