ALBERT

Description: The Late Pliocene is thought to be characterized by the simultaneous intensification of both the East Asian winter monsoon (EAWM) and East Asian summer monsoon (EASM). However, the evolution of the EASM during the Pliocene remains still controversial and only little is known about the dynamics of the EASM during the Pliocene on orbital time scales. Here we use clay mineral assemblages in sediments from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS) to obtain proxy records of past changes in the EASM climate during the Pliocene. Provenance analysis suggests that illite, chlorite and kaolinite originated mainly from the Mekong River drainage area. Smectite was derived mainly from the Indonesian islands. The kaolinite/illite ratio and the chemical index of alteration (CIA) of siliciclastic sediments allowed us to reconstruct the history of chemical weathering and physical erosion of the Mekong River drainage area and thus, the evolution of the EASM during the Pliocene. Our clay minerals proxy data suggests a stronger EASM during the Early Pliocene than during the Late Pliocene. We propose that the long-term evolution of the EASM has been driven by global cooling rather than the uplift of the Tibetan Plateau. Spectral analysis of kaolinite/illite ratio displays a set of strong periodicities at 100 ka, 30 ka, 28 ka, 25 ka, and 22 ka, with no clear obliquity-related signal. Our study suggests that the Pliocene EASM intensity on orbital time scales is not only controlled by the Northern Hemisphere summer insolation, but also strongly influenced by equatorial Pacific ENSO-like ocean-atmosphere dynamics.

Description: Seismic profiles provide evidence that there has been strong transport by deep-water bottom currents and drift deposition on the northern slope of the South China Sea. Earlier geochemical studies suggest that the drift sediments originated primarily from Taiwan. However, the transport process, history and origin of the deep-water bottom deposition in the northern South China Sea, on both glacial-interglacial and tectonic time scales, remain unclear. Here, we show new high-resolution records of clay minerals, grain size and mass accumulation rate (MAR) of terrigenous materials from Ocean Drilling Program (ODP) Site 1144, together with trace element concentrations in siliciclastic sediments from ODP Site 1146. Combined with other published data, we find that the primary source for sediments at ODP Sites 1144-1148 since 3 Ma is from Taiwan, and not from Pearl River as previously thought. Before 3 Ma, however, sediment source to ODP Sites 1146 and 1148 was mainly from the Pearl River. Increased contribution of terrigenous supply from Taiwan to the northern South China Sea since ~ 3 Ma may be related to the formation of the Taiwan orogen and strengthening of deep-water bottom current transport in the northern South China Sea. Variations in clay mineralogy and sedimentology at ODP Site 1144, located on a sediment drift, shows strong glacial-interglacial cyclicity. This suggests that bottom current deposition is highly dependent on sea-level fluctuations, which control the terrigenous supply to the deep sea.