Search

Abstract: Changes in paleoenvironments over the last 17,500yr have been documented by a high-resolution clay mineralogy and grain–size records of Core KNG5 from the northern slope of the South China Sea. Our results indicate that clay minerals are mainly from the Pearl River from 17,500 to12,500calyr BP, and the South China Sea modern current system began to form since 12,500calyr BP, as a result, Taiwan turns to be the major contributor of clay minerals after 12,500calyr BP. Two grain-size populations with high variability through time were identified in the 13–28μm and 1–2.2μm grain-size intervals. The 1–2.2μm grain-size population are mainly controlled by provenance supply and current transport. The 13–28μm grain-size fraction could be controlled mainly by the sea-level change. The 1–2.2μm grain-size population record demonstrates that East Asian Summer Monsoon intensity generally follows changes in insolation and that the response is similar for a large area of China and other northern low-latitude records, implying the globality of the monsoon evolution since Holocene. The anomalous environmental conditions in the northern South China Sea may imply intensified ENSO activity during the late Holocene. [Copyright &y& Elsevier]

Abstract: 128 samples from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea were analyzed for grain size, clay minerals, biogenic opal content and quartz in order to reconstruct changes in East Asian monsoon climate since 8.5 Ma. An abrupt change of terrigenous mass accumulation rate (MAR), clay mineral assemblage, median grain size and biogenic opal MAR about 5.2 Ma suggests that between 8.5–5.2 Ma the source of terrigenous sediment was mainly in the region of surface uplift and basaltic volcanism in southern Vietnam. A simple model of East Asian summer monsoon evolution was based on the clay/feldspar ratio, kaolinite/chlorite ratio and biogenic opal MAR. The summer monsoon has two periods of maximum strength at 8.5–7.6 Ma and 7.1–6.2 Ma. Subsequently, there was a relatively stable period at 6.2–3.5 Ma, continued intensification about 3.5–2.5 Ma, and gradually weakening after 2.5 Ma. Since 1 Ma the monsoon has intensified, with remarkable high-frequency and amplitude variability. Simultaneous increase in sedimentation rates at ODP Sites 1143, 1146 and 1148, as well as in MAR of terrigenous materials, quartz, feldspar and clay minerals at ODP Site 1143 at 3.5–2.5 Ma, may be the erosional response to both global climatic deterioration and the strengthening of the East Asian summer monsoon after about 3–4 Ma. [Copyright &y& Elsevier]

Changing weathering intensity, sediment transport, and provenance variations over the past 22.0 ka BP have been investigated by high-resolution clay mineralogy, grain-size and stable oxygen isotopes of planktonic foraminifera records along core CG2 recovered from the continental slope of the Sunda Shelf (southern South China Sea). Our results indicated that the reworking of older sediments outcropping on the Sunda Shelf exerted a great influence on the sediment supply during the last glacial and most of the last deglacial, modulated by sea level and monsoon rainfall changes. During the last 9.0 ka BP, relative increased kaolinite and heavier δ 18 O seawater values might reflect the higher influence of the tropical Indonesian Islands sources due to the reopen of southern straits, implying the formation of modern oceanic circulation and depositional patterns. High sediment fluxes in core CG2 during Heinrich stadial 1 might be a synthetic result of the intensified monsoon rainfall originated from the southward shift of the Intertropical Convergence Zone and the proximal location of the study core before the flooding of the Sunda Shelf. Fluctuations in smectite/(illite + chlorite) ratios correlated well with monsoon intensity, and periods of strong monsoon rainfall (lighter δ 18 O seawater values) were associated with an intensification of erosion of pre-existing, more weathered materials on the Sunda Shelf. Finally, we concluded that sediment composition and mineralogy in the southern South China Sea slope were controlled by varying degrees of reworking on the Sunda Shelf, as well as climatically modulated sediment supply from the Mekong River and southern tropical islands over the last 22.0 ka BP. [ABSTRACT FROM AUTHOR]

We present a multi-proxy geochemical analysis of core KNG5 retrieved from the northern South China Sea slope spanning the last 13.0 ka BP to assess the response of weathering and erosion processes in the Southwestern Taiwan River basin to climate change. A provenance analysis indicates that the core sediments were primarily derived from the Southwestern Taiwan River, and can be used to reconstruct the erosion and/or weathering history of this river basin. The chemical index of alteration, the plagioclase index of alteration, Rb/Sr, Al/Ti, principle component 1, and K/Al combined with terrigenous mass accumulation rates and the proportion of clay exhibit sharp excursions from 12.0 to 5.5 ka, peaking at 10.0 ka, which suggests strong chemical weathering and physical erosion during the Early-Mid Holocene, the period of the maximum summer monsoon. Abrupt falling weathering trends during 5.2 and 3.5 ka BP correlate with a weaker monsoon. After 3.5 ka BP, a stable lower weathering trend to the present day was evident, indicating the deposition of less weathered sediments in the slope when the monsoon rainfall was weakening. A good correlation between our proxy records of chemical weathering and variations of the monsoon intensity in South China emphasizes that variation in monsoon intensity is most likely the primary drive of the intensity of weathering and erosion in Southwestern Taiwan River sediments over the last 13.0 ka BP. Finally, we concluded that geochemical parameters can be used as reliable proxies for climate variation on a millennial-centennial time scale in such areas as Taiwan, where weathering-limited regimes exist. [ABSTRACT FROM AUTHOR]

Abstract: 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 100ka, 30ka, 28ka, 25ka, and 22ka, 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. [Copyright &y& Elsevier]