中国科学院南京地质古生物研究所机构知识库

The hydrography of the South China Sea (SCS) is characterized by a south-north (S-N) thermocline gradient in the upper water column from the northern margin of the western Pacific warm Pool (WPWP) to the sea area largely controlled by the East Asian monsoon. Here we examine the records of planktonic foraminifers from Ocean Drilling Program (ODP) Sites 1143 and 1146 in the southern and northern parts of the SCS, respectively, that identify three stages of evolution of the S-N hydrographic gradient in the SCS since the middle Miocene: first, the S-N thermocline gradient possibly occurred in the SCS for the first time during the period 11.5 similar to 10.6 Ma, reflected by an opposite change in the relative abundance of deep-dwelling planktonic foraminiferal species in the south and north; next, the S-N thermocline gradient weakened or even disappeared during the period 10.6-4.0 Ma, indicated by similar changes in the relative abundance of deep-dwelling species in the south and north; last, the S-N thermocline gradient substantially increased from about 4.0 similar to 3.2 Ma, marked by a major increase in abundance of deep-dwelling species in the northern SCS and a decrease in the southern SCS. Based on the results of oceanic and coupled ocean-atmosphere model experiments and previous studies on planktonic foraminifers in the Pacific Ocean, it is inferred that the initial WPWP was probably formed during the period of 11.5 similar to 10.6 Ma in response to the closure of the Indonesian seaway; The WPWP then weakened or became extremely unstable, before developing its present expression about 4.0-3.2 Ma, induced by the emergence of the Isthmus of Panama. Our observations and model experiments support the argument that the stages in evolution of the WPWP are linked to tectonic changes in ocean gateways. (c) 2005 Elsevier B.V. All rights reserved.