Search

The 2–7-year periodicities recorded in fossil coral records have been widely used to identify paleo-El Niño events. However, the reliability of this approach in the South China Sea (SCS) has not been assessed in detail. Therefore, this paper presents monthly resolution geochemical records covering the period 1978–2015 obtained from a Porites coral recovered from the SCS to test the reliability of this method. The results suggest that the SCS coral reliably recorded local seawater conditions and the super El Niño events that occurred over the past 3 decades, but does not appear to have been sensitive enough to record all the other El Niños. In detail, the Sr/Ca series distinctly documents only the two super El Niños of 1997–1998 and 2014–2016 as obvious low values, but does not match the Oceanic Niño Index well. The super El Niño of 1982–1983 was identified by the growth hiatus caused by the coral bleaching and subsequent death of the coral. Three distinct stepwise variations occur in the δ13C series that are coincident with the three super El Niños, which may be related to a substantial decline in endosymbiotic zooxanthellae density caused by the increase in temperature during an El Niño or the selective utilization of different zooxanthellaes that was required to survive in the extreme environment. The increase in rainfall and temperatures over the SCS during El Niños counteracts the effects on seawater δ18O (δ18Osw) and salinity; consequently, coral Δδ18O series can be used as a proxy for δ18Osw and salinity, but are not appropriate for identifying El Niño activity. The findings presented here suggest that the method to identify paleo-El Niño activity based on the 2–7-year periodicities preserved in the SCS coral records might not be reliable, because the SCS is on the edge of El Niño anomalies due to its great distance from the central equatorial Pacific and the imprints of weak and medium strength El Niño events may not be recorded by the corals there. [ABSTRACT FROM AUTHOR]

Anthropogenic impacts on weathering processes in the late Holocene have been widely demonstrated in recent studies, but the mechanism by which enhanced chemical weathering is linked to human activity remains unclear. In this study, we present a comprehensive reconstruction of sediment provenance, chemical weathering, vegetation type and evidence of human activity in core ZK20 in the estuary of the Pearl River during the last 7 ka to investigate the interactions of weathering, climate, and human activity. Evidence from major elements and clay minerals indicates an abrupt increase in chemical weathering intensity since ∼3.0 cal ka BP. This result cannot be explained by changes in provenance, climate or other natural factors but agrees well with the stronger human activity, confirming the significance of anthropogenic impacts on silicate weathering. The nearly synchronous increase in C4 plant abundance since 3.0 ka BP, as reflected by the sediment records from the upper reach and river mouth of the Pearl River, implies that the intensified weathering signal was mainly derived from contemporaneous minerals alteration (less than millennium timescale) rather than reworked old soils. We propose that the varied weathering conditions in response to agricultural activity and vegetation change intensified the alteration degree of minerals. The farming-induced biogeochemical effects, such as the releasing of acids, regulation of hydrology and uptake of cations, played a significant role in intensifying chemical weathering. This study highlights the importance of farming-related biogeochemical effects, which have been largely neglected in previous studies, in the process of chemical weathering. • We present high-resolution Holocene records of weathering, vegetation and human activity at the Pearl River Basin. • Increasing anthropogenic impact on chemical weathering in the Pearl River Basin occurred since ∼3.0 ka BP. • Farming-related biogeochemical effects might intensify chemical weathering since the late Holocene. [ABSTRACT FROM AUTHOR]