Holocene Climate Change And Human Activity From An Archive Of Continental Shelf Sediments In The Northern South China Sea

As a major component of the Asian summer monsoon system,the East Asian summer monsoon?EASM?controls the variations of hydrological and ecological environment.The variability in the EASM has a significant effect on economic activity and society stability within its influence area.In the context of global warming,future change in the EASM is a matter of great concern to both the academic community and the general public.Understanding the nature of the variability of the Asian monsoon on various time-scales is critical for our understanding of present climatic conditions and for predicting future climate change.The historical evolution of the EASM has been widely studied using various geological archives,including loess–paleosol sequences,marine and lacustrine sediments,peat deposits,and stalagmites.These studies have mainly focused on the EASM variations in north China,and high-resolution and well-dated paleoclimatic records in southern China are scarce.This limitation hinders comparisons with high-resolution records in other monsoonal regions.In addition,spatial patterns on various time-scales in the EASM region and its underlying forcing mechanism remain debated.On orbital timescales,some researchers inferred a broadly synchronous climatic evolution throughout the Holocene,but others suggested an asynchronous evolution of the EASM in different monsoon regions.Spatial patterns in centennial-scale variations of the EASM during the Holocene are poorly constrained due to the lack of high-resolution records with detailed chronologies,especially in southern China.During the past 2000 years,the paleomonsoon inferred from various geological archives in monsoonal China display a general decrease in monsoon precipitation.However,several researchers suggested a long-term increase in the EASM.Given that human influences on the natural environment have become increasingly important,enhanced human activity would mask the natural climatic fluctuations in paleoenvironmental archives.Here,high-resolution multi-proxy records with a robust chronology of a marine sediment core derived from the northern inner shelf of the SCS are presented,spanning the past 7500 years.To reconstruct various elements of monsoon evolution,such as land-use and mineral resource extraction during the Holocene,records of chemical weathering proxies?CIA and Al/K?were generated along with the concentration and isotopic composition of organic carbon(TOC and?¹³C_org),terrigenous influx indexes?Al,Ti and Fe?,magnetic parameters(?_lf and?_fd),and elemental concentrations?Cu and Pb?throughout the sediment core.The interactions between the Holocene climate variability,human activity,and environmental change were investigated.A comprehensive comparison of the climatic proxy data of the YJ Core with other high-resolution moisture and temperature records,as inferred from multiple paleoclimate records in different monsoonal regions,reveals an overall synchronous decrease in monsoon strength during the interval of 7500–2000 cal yr BP.These orbital synchronous decrease in the EASM during this interval is mainly controlled or modulated by orbitally induced insolation forcing,which may also be driven by changes in the position of the ITCZ and ENSO activity.To achieve a robust understanding of centennial-scale oscillations,the data were first standardized.The standardization treatment was performed on the proxy data from the YJ Core as well as comparison data,including stalagmite?¹⁸O data from Dongge and Heshang Caves,precipitation reconstructions from Gonghai Lake,?¹³C_27-31 data from Xiaolongwan Lake,and total solar irradiance?TSI?data.Spatial variability of the EASM on centennial timescales can be explored by analysing these datasets.Six distinct intervals of decreased terrigenous influx and chemical weathering,as inferred from standardized values of Al contents and Al/K ratios,were detected and attributed to a weaker EASM.These weak summer monsoon events have been revealed in other regional proxy records.Therefore,the concurrence of weak EASM events in different monsoonal regions probably indicates that the variations in the EASM were broadly synchronous on centennial timescales across the Asian monsoon region.Interestingly,comparison between total solar irradiance?TSI?and these monsoon records reveal a good coherency between centennial-scale monsoon variability and solar activity.In general,weak EASM events consistently correspond to intervals of weak solar activity,which suggests that changes in solar activity trigger the spatial synchronisation of the EASM on these timescales.To further identify the possible relationship between the EASM and solar activity,the power spectrum of these time series was determined and cross-spectral analyses were carried out.The spectral analysis demonstrates that the EASM and solar activity share some general cyclicity patterns.We therefore suggest that solar activity is a fundamental driving force for the spatial synchronisation of the EASM on centennial timescales across the monsoon region.During the past 2000 years,the EASM intensity inferred from various geological archives in monsoonal China display a general decrease.Conversely,the intensity of chemical weathering,fluvial discharge,and terrigenous influx in the YJ Core increased rapidly over the past 2000 years.There appears to be a decoupling between the present records and the general trends of temperature and precipitation,suggesting that climate change alone cannot be responsible for the variations in chemical weathering,fluvial discharge,and terrigenous influx during the past 2000 years.In addition,the enhanced chemical weathering,fluvial discharge,and terrigenous influx during the past 2000 years also coincide with an increase in both magnetic susceptibility(?_lf)and frequency-dependent magnetic susceptibility(?_fd),as well as an increase in sedimentary Cu and Pb concentrations.This interval also saw an increase in the population of Guangdong Province.Therefore,it is inferred that an increase in soil erosion and the use of metal tools were stimulated by a growing human population and the expansion of agricultural and mining practices.The present results suggest that enhanced human activity during the past 2000 years has overwhelmed the natural climatic controls on the environment and landscape of the Ling-Nan region.