Holocene hydrological and sea surface temperature changes in the northern coast of the South China Sea

Abstract

In order to reconstruct the Holocene environmental history of a coastal site in the northern South China Sea, this study analysed the organic carbon isotope ratios (δ13Corg) and alkenone unsaturation ratios (UK′37) from a 36.5m-long sediment core drilled at seabed in the mouth region of the Pearl River estuary and generated a coupled hydrological and temperature record. This record reveals changes of monsoon-induced sediment discharge and sea surface temperature of the Holocene in four stages. In Stage I, the site was under fluvial conditions prior to postglacial marine transgression. Stage II saw an increase of sea surface temperature from c. 23.0°C to 27.0°C, associated with a strengthened summer monsoon from c. 10,350 to 8900cal. years BP. This was also a period of rapid sea-level rise and marine transgression, during which the sea inundated the palaeo-incised channel, i.e. the lower part of the T-shape accommodation space created by the rising sea. In these 1500years, fluvial discharge was strong and concentrated within the channel, and the high sedimentation rate (11.8mm/year) was very close to the rate of sea-level rise. In the subsequent 2000years (Stage III) sea level continued to rise and the sea flooded the broad seabed above the palaeo-incised channel, resulted in fluvial discharge spreading thinly across the wide accommodation space and a much reduced sedimentation rate (1.8mm/year). Sea surface temperature in this stage reached 27.3°C initially, but dropped sharply to 26.1°C towards c. 8200cal. years BP. The final stage covers the last 7000years, and the site was under a stable sea level. Sedimentation in this stage varied a little, but averaged at 1.8mm/year. Whilst fluvial discharge and sea surface temperature didn’t change much, two short periods of hydrological and temperature change were observed, which are related to the climatic cooling events of c. 4200cal. years ago and the Little Ice Age.