Sedimentary Records Of Continental Shelf Weathering And Organic Carbon Burial In The Northern South China Sea Since The Last Glacial Period

The continental margin basin is the main sedimentary sink of the products of continental weathering and erosion,and it is a natural laboratory to reveal the process and mechanism of weathering-carbon burial.This study is based on the analysis of AMS¹⁴C dating,terrigenous material accumulation rate,clay mineral composition and crystallographic characteristics,grain size parameters,elemental geochemical parameters,total organic carbon-total nitrogen content and stable carbon isotopes in TWS-1 core located in Taixinan Basin of the northeastern South China Sea and ZK20core located in Pearl River Estuary.The sediment source,silicate weathering history and the history and force mechanism of terrigenous organic carbon burial in the Taixinan Basin since 23 ka BP during the last glacial period were revealed.Based on the comprehensive analysis of the Sr-Nd isotopic composition,clay mineral composition,and the buried ancient river channels and submarine canyons observed in the shallow stratum profile,Taiwan was considered as the main source area of the TWS-1 core during the last glacial period and Holocene.Based on the analysis of the weathering proxy records such as K/Al,kaolinite/illite and Rb/Sr,the chemical weathering history of the sediments in TWS-1 core since 23 ka BP was reconstructed.They all show that the chemical weathering intensity experienced by the sediments in the TWS-1 core during the last glacial period is significantly higher than that in Holocene,which imply that the unconsolidated continental shelf sediments weathered again during glacial sea level low-stands.During the last glacial period,the sea level fell up to 120 m compared to the present,and the continental shelf was widely exposed.The northern South China Sea is located in the tropics.It has favorable silicate weathering conditions such as moderate temperature,suitable precipitation and sufficient duration of silicate chemical weathering during the glacial periods,which makes the unconsolidated sediments on the exposed continental shelf weathered again.Thus,it is proved that the glacial tropical continental shelf weathering is an important neglected mechanism during the glacial-interglacial carbon cycle.Comparison with potential source end-elements shows that Taiwan Island was the main source of terrigenous organic carbon to the study site,contributing 58%of the total.The terrigenous materials were mainly transported through submarine canyon channels and shelf rivers during the sea level low-stands.The reconstructed terrigenous organic carbon flux shows two peaks of about 0.16 g/cm²/ky during the early last deglaciation(19-13 ka BP)and 0.09 g/cm²/ky during the middle Holocene(7-4 ka BP).The comprehensive analysis showed that the two peaks were controlled by the enhanced weathering and erosion of the continental shelf during the glacial sea level low-stands and the intensified erosion in ancient Taiwan Island driven by precipitation during the Holocene maximum monsoon period,respectively.Therefore,we believe that the burial of organic carbon in continental margin driven by sea level change and monsoon during the glacial-interglacial cycle may have considerable effects on the global carbon cycle and the evolution of atmospheric CO₂ concentration.