The nature of the Eocene stratigraphy of the South China Sea (SCS) has been a mystery for decades. In the southern SCS, the Eocene system with marine deposits is often considered as representing part of the ‘Proto-South China Sea’ [ 1]. In the northern SCS, however, the Eocene is dominated by non-marine sequences of South China origin [ 2], probably also related to the Proto-SCS [ 3]. Then the question is what kind of SCS northern margin was in the Eocene: part of the South China continent or part of Proto-SCS or even an eastern extension of the shrinking Tethys Ocean? Furthermore, the significance of the Eocene issue is enhanced by the recent debates on the opening age of the modern SCS, whether in the Eocene or in the early Oligocene [ 4]. The existence of the marine Eocene in the northern SCS was implied by reworked Eocene nannofossils found in offshore oil exploration wells over 20 years ago [ 5] by Eocene foraminifers and nannofossils found in cutting samples from deep-water oil wells [ 6] and by Eocene marine deposits reported recently from Taiwan [ 7]. Until now, however, solid direct evidence of marine Eocene depositions in the northern SCS has been absent. Here, we report new discoveries of deep- and shallow-water marine Eocene sediments based on the results of the International Ocean Discovery Program (IODP) drilling in the northern margin of the SCS.

Marine sediment core MD012394 from the Vietnam coastal upwelling area in the western South China Sea was investigated in order to reconstruct the last Quaternary monsoon upwelling based on planktic foraminifer fauna assemblages and fauna-based sea surface temperature (SST) estimates. The age model of core MD012394 was constructed using oxygen isotope stratigraphy of the planktic foraminifer G. sacculifer, with 10 accelerator mass spectrometry (AMS) 14C dating of planktic foraminifers from the sediment samples. Our studies on the relative and absolute abundances of planktic foraminifer assemblages reveal eight dominant species in core MD012394: N. dutertrei + N. pachyderma (right coiling), G. ruber, G. glutinata, G. sacculifer, P. obliquiloculata, G. menardii + G. tumida, G. calida, and G. inflata. In a Q-mode factor analysis of the fauna abundance data, the fauna factors show variations that do not parallel the glacial/interglacial changes throughout the last 135 kyr. The relative abundance patterns of G. inflata and N. dutertrei (including N. pachyderma-R) are interpreted as hydrographic proxies for East Asian summer and winter monsoon, respectively, in the current study. We calculated the fluctuations in the SST using the Revised Analog Method (RAM) in MD012394 and found that the abundance changes of the summer monsoon upwelling indicator G. inflata were similar and nearly synchronous. This suggests that the summer monsoon-driven upwelling signal was strong near the local summer insolation maximum, which induced low SSTs, particular around ~11, 33, 59, and 83 kya. Our studies support the view that the strengths of both summer insolation and the East Asian summer monsoon have determined the relative abundance of planktic foraminifers and the SSTs in the western SCS during the last 135 kyr