Changes of eco-environment in the Xisha Islands in response to climate change and human activity over the past 2000 years
過去 2000 年西沙群島生態環境對氣候變化和人類活動的響應
Student thesis: Doctoral Thesis
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. The past is the key to the future, and study of the past is essential for predicting the future. 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 elemental, isotopic and biological geochemistry analyses, the history of the islands has been examined and major points of the present study are summarized as follows. 1. Establishment of chronologies for the five sediment cores Via analysis of several natural and anthropogenic radionuclides (210Pb, 226Ra and 137Cs) in the ornithogenic sediments, 210Pb dating has been proven to be effective for coral sand sediments in the Xisha Islands. The Constant Initial Concentration (CIC) model and the Constant Rate of Supply (CRS) model were applied for age calculation. 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). The main source of radionuclides in the sediments is atmospheric precipitation, and the organic matter derived from plant and produced by nutrient-rich guano could further enhance them. Chronologies of the five sediments cores were further established by both 210Pb dating and radiocarbon analysis of ancient bones. Age models showed that the most ancient sample of these cores dated back to approximately 2000 years. 2. Geochemical characteristics of the bulk sediments and implications Geochemical characteristics of four profiles GQ, GJ3, JQ and JY2 were analyzed. We noted clear differences in the distribution of organic matter (OM) and elements above and below the respective critical depths of 11 cm (GQ), 16 cm (GJ3), 9 cm (JQ) and 12 cm (JY2) in the four profiles, revealing an apparent change in sediment source materials. Our 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. Seabird activities appear central to terrestrial production and soil development on the islands. 3. Seabird population records over the past 2000 years and possible causes for seabird population changes Using avian-bioelements and reflectance spectroscopy, seabird population records of the islands over the past 2000 years were reconstructed. The results show that seabird occupation history of these tropical islets follows a rather similar pattern. Seabird populations peaked during the Little Ice Age (LIA, 1400-1850 AD). A cool climate during the LIA seems more favorable to seabirds on the Xisha Islands of the South China Sea. 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. 4. Cadmium in guano as an indicator of historical maritime productivity, and reconstruction of marine productivity over the last millennium Cadmium in ancient guano particles was analyzed, and has been identified as an indicator of maritime productivity in the open sea around the Xisha archipelago. The reconstructed maritime production record suggests that the Xisha area had a relatively high productivity and supported more seabirds during the LIA. The East Asian winter monsoon plays a principal role in biomass production in the South China Sea. The intensified winter monsoon during the LIA promoted deep seawater mixing and thus enhanced nutrient cycling. Terrestrial dust along with the cold/dry winter monsoon from Asian continent to the South China Sea may also have promoted maritime productivity. Solar irradiation variability is perhaps the ultimate controlling factor in seabird population on the atolls on the centennial to millennial scales. 5. Evidence of past human activity The level of Hg in eggshells of GJ3 was examined, and was identified as a marker for past Hg deposition in marine environment. Deposition flux of anthropogenic Hg over the past 700 years was also reconstructed from bulk sediments of GJ3, JY2 and JQ. Eggshell Hg and Hg flux well recorded past human activity, including enhanced metallurgy activity in Ming Dynasty of China, exploitation on the Europe/America and World War II. Our records suggest that Hg production center had gradually shifted from Europe and America to Asia since 1970s. We reconstructed a 400-year record of black carbon (BC) deposition flux from profiles GJ3, JQ and JY2. The BC flux remained low before the onset of 20th century; it started to increase from approximately 1900 AD, and peaked around the 1970s. 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. 6. DNA isolation from ancient guano samples 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.
- Paracel Islands, Effect of human beings on, Environmental conditions, Climatic changes, Nature