Study Area and Sample Collection

Abstract

Global warming significantly impacts on ecosystems and has drawn increasing attention in recent years. However, how climate changes affect lives and the environment remains unclear. Seabirds move across aquatic/terrestrial ecosystem boundaries and their fossil remains have been widely used for studying ecological responses to climate change. Five sediment cores from Ganquan (GQ), Guangjin (GJ3), Jinqing (JQ), Jinyin (JY2) and Chenhang (CH) islands of the Xisha archipelago, South China Sea, were collected during field investigations. Well-preserved seabird remains were observed in these sediment cores. Via analysis of several natural and anthropogenic radionuclides in the ornithogenic sediments, 210Pb dating has been proven to be effective for coral sand sediments in the Xisha Islands. The average supply rate of 210Pb was 126 Bq m−2 a−1, very close to the flux of northern hemisphere average (125 Bq m−2 a−1). Radiocarbon analysis of ancient bones showed that the most ancient sample of these cores dated back to approximately 2,000 years. Geochemical characteristics of four profiles GQ, GJ3, JQ and JY2 were analyzed. The results suggest that the source materials of ornithogenic sediments have changed gradually from a two-component (coral sand, guano) mixture to a three-component (coral sand, guano and humus) mixture, likely indicating the slow development of vegetation following seabird occupation. Cu, Cd, Zn, P, As, Se and Ba were identified as a group of avian bio-elements. A 2,200-year record of seabird population on Ganquan Island was further reconstructed by avian-bioelements and reflectance spectroscopy. A cool climate during the LIA seems more favorable to seabirds on the Xisha Islands. Relative low sea surface temperature (SST), stronger monsoons and weaker ENSO activity during the LIA might result in increase in marine nutrient mixing and high primary productivity. This provided high food availability and thus supported more seabirds. Moreover, a change in nitrogen/carbon isotope compositions in the collagen of seabird bones suggested that seabirds might dive deeper or travel further to catch prey, due to intra/inter-competitions. The abrupt decline of seabird population in recent times is probably attributed to human activity. The level of Hg in eggshells and the bulk ornithogenic sediments was determined,. Eggshell Hg and Hg flux well-recorded past human activity. We reconstructed a 400-year record of black carbon (BC) deposition flux from profiles GJ3, JQ and JY2. In the recent 30 years, the BC flux displayed decreasing trend, very likely due to a change of energy structure and development of pollution control techniques. Preliminary analysis of DNA in ancient bird droppings suggests that the guano contains quite a low level of ancient DNA, due to possible severe degradation.