Search

Recent research has suggested that East Asia has experienced a prevailing monsoon climate since the Eocene. However, there is little knowledge about the development of the East Asian monsoon system before the Miocene, particularly in southern China, due to a lack of well‐dated continuous sediment records. Here, we present new magnetic proxy records from International Ocean Discovery Program Site U1501 in the northern South China Sea. We conducted rock magnetic experiments, made scanning electron microscopy observations and performed diffuse reflectance spectrum analysis on the late Eocene‐early Oligocene core sediments. The magnetic signal of the sediments is dominated by detrital magnetite and titanomagnetite formed during the silicate weathering and erosion processes, which were used to infer the evolution of summer monsoon precipitation in southern China. Our results along with geochemical and clay mineral data from Site U1501 strongly indicate that the East Asian summer monsoon generally weakened across the Eocene‐Oligocene transition. This change was linked to coeval global cooling rather than tectonic processes. Prior to the Eocene‐Oligocene transition, the summer monsoon intensified, likely due to the latest Eocene warming event caused by the enhancement of the Atlantic Meridional Overturning Circulation, which may have preconditioned the Earth system for the greenhouse‐to‐icehouse transition. Plain Language Summary: To date, there is little knowledge regarding the long‐term evolution of monsoon climate in South China before the Miocene due to a lack of well‐dated continuous sediment records. The recent International Ocean Discovery Program Expedition 368 retrieved continuous late Eocene‐early Oligocene sediments at Site U1501 within the northern South China Sea, thus providing an opportunity to fill this gap. In this study, we investigate the evolution of the East Asian summer monsoon in South China during the late Eocene‐early Oligocene through a comprehensive analysis of the magnetic properties of the sediments. The results strongly indicate that the summer monsoon generally weakened across the Eocene‐Oligocene transition, which was linked to coeval global cooling. A short‐term strengthening of the summer monsoon prior to the Eocene‐Oligocene transition was also recorded. This change is attributed to the warming caused by the latest Eocene strengthening of the Atlantic Meridional Overturning Circulation, which may have preconditioned the Earth system state before causing a transition into an icehouse Earth. Key Points: The environmental magnetic record in the northern South China Sea indicates monsoon evolution during the late Eocene to early OligoceneThe East Asian summer monsoon weakened across the Eocene‐Oligocene transitionGlobal cooling drove the long‐term weakening of East Asian summer monsoon [ABSTRACT FROM AUTHOR]

Human–environment interactions across historical periods in China have been well reconstructed over the past decade; however, few studies have focused on southern China, particularly Hainan Island. This study investigated two sediment cores (cores X1 and X2) collected from the continental shelf off Hainan Island in the northern South China Sea. Combined with geochemical compositions (Al, Ni, Cu, Zn, Cd, and Pb), Sr-Nd isotopes, grain size, and AMS 14C ages, our results showed that Cd and Cu in core X1 and Cd and Pb in core X2 have been mainly influenced by anthropogenic activities since ~4.0 kyr BP. The enrichment factors showed obvious increases starting at ~4.0 kyr BP and were in good accordance with the appearance of abundant archeological sites on Hainan Island. Significant climatic changes (i.e. decreases in the East Asian summer monsoon and increases in El Niño–Southern Oscillation activities) at ~4.0 kyr BP likely promoted more intense, localized human activities, which in turn enhanced heavy metal accumulation in the continental shelf sediments. These results provide new insights into the history of human activities on Hainan Island and indicate that the geochemical compositions of continental shelf sediments have the potential for broadening our knowledge of the human activities and cultural development in tropical areas. [ABSTRACT FROM AUTHOR]

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: 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 3Ma is from Taiwan, and not from Pearl River as previously thought. Before 3Ma, 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 ~3Ma 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. [ABSTRACT FROM AUTHOR]

Abstract: We here reconstruct the past change of the East Asian monsoon since 20 Ma using samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea based on a multi-proxy approach including a monomineralic quartz isolation procedure, identification of clay minerals by X-ray Diffraction (XRD) and grain-size analysis of isolated terrigenous materials. Terrigenous supply to ODP Site 1146 was dominated by changes in the strength of multiple sources and transport processes. Grain-size data modeled by an end-member modeling algorithm indicate that eolian dust from the arid Asian inland and fluvial input have contributed on average 20% and 80% of total terrigenous material to ODP Site 1146, respectively. Specifically, about 40–53% of the total (quartz+feldspar) and only 6–11% of the total clay is related to eolian supply at the study site. Detailed analysis of the sedimentary environment, and clay minerals combined with previous studies shows that smectite originates mainly from Luzon, kaolinite from the Pearl River and illite and chlorite from the Pearl River, Taiwan and/or the Yangtze River. The proportion and mass accumulation rate (MAR) of the coarsest end-member EM1 (interpreted as eolian dust), ratios of (illite+chlorite)/smectite, (quartz+feldspar)% and mean grain-size of terrigenous materials at ODP Site 1146 were adopted as proxies for East Asian monsoon evolution. The consistent variation of these independent proxies since 20 Ma shows three profound shifts in the intensity of East Asian winter monsoon relative to summer monsoon, as well as aridity of the Asian continent, occurred at ∼15 Ma, ∼8 Ma and the youngest at about 3 Ma. In comparison, the summer monsoon intensified contemporaneously with the winter monsoon at 3 Ma. The phased uplift of the Himalaya–Tibetan plateau may have played a significant role in strengthening the Asian monsoon at ∼15 Ma, 8 Ma and 3 Ma. [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]

Silicate weathering plays a significant role in regulating the carbon cycle and global climate by consuming atmospheric CO 2 over geological timescales. However, the importance of this mechanism in relation to paleoclimate changes remains controversial. A major obstacle to progress is the lack of comprehensive and detailed silicate weathering records preceding the Neogene. Here, we present a long-term and continuous record of major and trace elemental compositions of clay-sized (<2 μm) siliciclastic sediment fractions from the International Ocean Discovery Program (IODP) Site U1501 in the northern South China Sea to reconstruct the evolutionary history of silicate weathering since 30 Ma and assess its links to tectonic/climate changes. The weathering proxies of the chemical index of alteration (CIA) and K/Al ratio indicate that the silicate weathering intensity of these sediments has had a decreasing trend over the past 30 Myr, which is coupled with paleoclimate proxies, deep-sea δ18O values and black carbon δ13C values, suggesting that global cooling and regional drying are the main driving factors for weathering intensity evolution. Furthermore, this study also provides reconstructions of the silicate weathering fluxes since the early Oligocene, highlighting that the CO 2 consumption fluxes caused by silicate weathering have been mainly controlled by physical erosion fluxes rather than by chemical weathering intensity over geological timescales. Consequently, our findings indicate that with increased physical erosion flux and resultant elevated silicate weathering flux, carbon output modulates the carbon cycle, contributing to global cooling and ice sheet development through enhanced CO 2 consumption. • We present long-term records of silicate weathering in South China since 30 Ma. • Silicate weathering intensity has been weakened in response to cooling and drying climate. • Silicate weathering flux is controlled by physical erosion flux over geological timescales. • Silicate weathering as a CO 2 sink regulates the carbon cycle and global climate. [ABSTRACT FROM AUTHOR]

The emergence and expansion of terrestrial C 4 vegetation represent the most striking evolutionary adaptation of ecosystems to climate change during the Cenozoic. However, the long-term history of terrestrial C 4 ecosystem expansion in East Asia remains poorly documented, and its links to tectonic and/or climate changes remain unclear due to the complexity of its forcing factors. Here, we reconstruct the long-term evolution of terrestrial C 3 –C 4 plants biomass in South China since the late Eocene using continuous records of the stable carbon isotopic compositions (δ13C) of black carbon extracted from sediments at International Ocean Discovery Program (IODP) Site U1501 in the northern South China Sea (SCS). This study reveals remarkable stepwise expansions of C 4 plants in South China at ∼20 Ma and 15 Ma with a gradual expansion trend at 7–4 Ma. We demonstrate that stepwise drying (long-term weakening of the East Asian summer monsoon and simultaneous strengthening of the East Asian winter monsoon), as well as the enhanced seasonality of precipitation, is responsible for the ecological evolution in South China since the early Miocene. Our black carbon records, combined with previous studies, indicate a coupled evolution of East Asian drying, vegetation shifts and global cooling. We suggest that the major Asian topography framework was established before the middle Miocene, and global cooling overwhelmed tectonic controls of the long-term evolution of the East Asian monsoon thereafter. This study highlights that both Tibetan uplift and global cooling have played a significant role in the long-term evolution of regional climate and vegetation. • Stepwise expansion of C 4 plant in South China occurred at 20 Ma, 15 Ma and 7–4 Ma. • Stepwise drying and enhanced seasonality of precipitation induced C 4 plant expansion in South China. • Both Tibetan uplift and global cooling played a significant role in evolution of East Asian monsoon. [ABSTRACT FROM AUTHOR]

The late Quaternary paleoceanography and paleoenvironment of the South China Sea (SCS) have been well reconstructed over the last decade. In contrast, the provenance of the terrigenous sediments that have accumulated in the northwestern continental shelf mud area remains enigmatic. This study investigated the provenance of these sediments and the paleoenvironmental changes archived in Core X2 via the analysis of geochemical elements, grain size, and accelerator mass spectrometry (AMS) 14 C ages. Based on the upper continental crust (UCC)-normalized REE patterns and REE fractionation parameters, southwestern and western Taiwanese rivers and the Pearl River were identified as the main sources of the fine-grained sediment deposited in the northwestern shelf mud area off Hainan Island. This finding further confirms the long-distance transport (> 1000 km) of fine-grained sediment from Taiwanese rivers to the northern SCS shelf and slope. Obvious changes in the grain size and Chemical Index of Alteration (CIA) record occurred at approximately 4.0 cal kyr BP and were likely caused by increased Hainan Island inputs due to sea level changes. [ABSTRACT FROM AUTHOR]

Atmospheric CO 2 and global climate are closely coupled. Since 800 ka CO 2 concentrations have been up to 50% higher during interglacial compared to glacial periods. Because of its dependence on temperature, humidity, and erosion rates, chemical weathering of exposed silicate minerals was suggested to have dampened these cyclic variations of atmospheric composition. Cooler and drier conditions and lower non-glacial erosion rates suppressed in situ chemical weathering rates during glacial periods. However, using systematic variations in major element geochemistry, Sr–Nd isotopes and clay mineral records from Ocean Drilling Program Sites 1143 and 1144 in the South China Sea spanning the last 1.1 Ma, we show that sediment deposited during glacial periods was more weathered than sediment delivered during interglacials. We attribute this to subaerial exposure and weathering of unconsolidated shelf sediments during glacial sealevel lowstands. Our estimates suggest that enhanced silicate weathering of tropical shelf sediments exposed during glacial lowstands can account for ∼9% of the carbon dioxide removed from the atmosphere during the glacial and thus represent a significant part of the observed glacial–interglacial variation of ∼80 ppmv. As a result, if similar magnitudes can be identified in other tropical shelf-slope systems, the effects of increased sediment exposure and subsequent silicate weathering during lowstands could have potentially enhanced the drawdown of atmospheric CO 2 during cold stages of the Quaternary. This in turn would have caused an intensification of glacial cycles. [ABSTRACT FROM AUTHOR]

The Pearl River drains the South China Block and delivers large amounts of sediments to the northern South China Sea, and its development can be closely linked to the regional paleoenvironmental changes. However, the evolutionary history of the Pearl River has been poorly understood, mainly because of the lack of continuous and high-resolution sedimentary records over a long geological time. In this study, we present multiple new analytical results, including clay mineral assemblages and Sr-Nd isotopic compositions of the <2 μm siliciclastic sediments, from the latest deep sea drilling core in the northern South China Sea from International Ocean Discovery Program (IODP) Site U1501. We aim to reconstruct the evolutionary history of the Pearl River by tracing changes in clay mineral assemblages and Sr-Nd isotopic compositions. Provenance analysis indicates that the sediment source experienced a significant change between 32 and 30 Ma. Specifically, sediments prior to 32 Ma were derived from the southeastern Cathaysia coast, whereas sediments post to 30 Ma were mainly supplied by a proto-Pearl River. This provenance transition indicates that the modern-scale Pearl River has fully developed at least ∼30 Ma and began to contribute sediments to the northern South China Sea. We additionally compared the timings of major climatic and tectonic events in this region, and suggest that the topographic uplift of the SE Tibetan Plateau most likely drove the large-scale westward expansion of rivers drainage from the coast located in the Cathaysia Block to the interior of Yangtze Block. As a result of this fluvial drainage evolution followed by the development of Pearl River, geochemical and clay mineralogical characteristics of the deposits in the northern South China Sea correspondingly changed. • The first continuous record of South China erosion from the South China Sea since ∼35 Ma. • The provenance records reveal the formation of the modern-scale Pearl River at ∼30 Ma. • The uplift of Southeastern Tibetan Plateau promotes the formation of the Pearl River. [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]