Seasonal and interannual variability of coccolithophore flux in the northern South China Sea

摘要

Coccolithophorids are key player in the marine biological pump and marine carbon cycle, as their production and community-structure are crucial for export and sequestration of carbon from the atmosphere to the deep sea with important implications to climatic trends. Variations in the species composition of coccolithophore communities largely reflect environmental changes and are therefore fundamental for palaeoceanographic reconstructions. The South China Sea (SCS) is an ideal area to study the response of coccolithophores to environmental change because of the remarkable seasonal and interannual variations of the monsoonal climate and the hydrography. However, to date only limited studies on the temporal changes of coccolithophores in the modern SCS have been reported. In the present work, coccolithophores in the northern SCS were investigated using time-series sediment traps during 2009-2010 and 2011-2012, Extinct coccoliths which had their last occurrence in the Miocene (e.g. Triquetrorhabdulus longus, Redculofenestra floridanus), Pliocene (e.g. Reticulofenestra pseudoumbilica, Discoaster auras), and Pleistocene (e.g. Pseudoemiliania locunosa, Discoaster variabilis) were a frequent component of the coccoliths throughout all the seasons and provided, for the first time, a strong micropaleontological evidence for lateral advective transport in the deep SCS. The source of the fossil coccoliths most likely were the reworked Pleistocene sands which cover the outer shelf and upper slope to the west and south of the Dongsha Islands between 20 m and 600 m water depth. These sediments contain limestone fragments with foraminiferal assemblages of Miocene to Pliocene age, and, to the north and south of the islands, Miocene strata are exposed on the sea floor. Mesoscale eddies, both cyclonic and anticyclonic, were probably the main agent for resuspending and transporting the coccoliths as they propagated westwards alongslope from the Dongsha area to the mooring site. Extant coccolithophore were composed of 31 taxa with Florisphaera profunda, Gephyrocapsa oceanica, and Emlliania huxleyi contributing 91.4% and 83.8% of the annual coccolithophore export flux in 2009-2010 and 2011-2012, respectively; F. profunda was the predominant species. Enhanced fluxes of extant coccoliths occurred in the summer of 2009/2010, spring 2010, autumn 2011 and winter 2011/2012, but varied in phase with the extinct species. Therefore, lateral advection of extant taxa may have also taken place but the extent to which this may have masked primary signals from in-situ coccolithophore production remains open. The low fluxes of coccoliths in the winter of 2009/2010 in association with relatively reduced wind strength, higher SST and a shallower mixed layer might compared to those of 2011/2012 have been driven by the weak El Nino event, which affected the northern SCS during that season.