Oil spill recorded by skeletal δ13C of Porites corals in Weizhou Island, Beibu Gulf, Northern South China Sea

Oil, is one kind of natural product of biological origin which is depleted in C-13. Coral skeletal delta C-13 is an important archive of environmental and climatic change. However, few studies have been conducted on the effect of oil spill on the coral skeletal delta C-13 at present. This kind of research is useful for us to obtain environmental and climatic information from coral skeletal delta C-13. In this study, two continuously growing Porites corals (W1 and W3), aged 22 and 31 years (1994-2015 and 1985-2015), respectively, were collected from the fringing reef of Weizhou Island in October 2015, coral skeletal delta C-13 and delta O-18 were analyzed. Besides, seawater was collected during an oil spill in 28 January 2018, and was used for delta(CDIC)-C-13 (delta C-13 values of DIC) analysis. The purpose is to explore the potential impact of oil spills on delta(CDIC)-C-13 and coral delta C-13. delta(CDIC)-C-13 value during oil spill was significantly lower than that of normal seawater (Delta delta C-13 approximate to -2.38 parts per thousand). Coral delta C-13 of both two coral colonies showed lower values since 2008 (average value of delta C-13 before and after 2008 change from -2.04 +/- 0.48 parts per thousand to -3.33 +/- 0.55 parts per thousand parts per thousand and -1.77 +/- 0.49 parts per thousand to -2.88 +/- 0.54 parts per thousand of W1 and W3 respectively), especially, several abrupt negative shifts in 2008, 2009, 2011, 2012 and 2014 when oil spills occurred (extent of delta 13C variation were W:-3.08 parts per thousand, -1.82 parts per thousand, -1.82 parts per thousand, -1.53 parts per thousand, -1.37 parts per thousand; W3:-3.27 parts per thousand, -1.8 parts per thousand, -1.91 parts per thousand, -1.21 parts per thousand, -1.13 parts per thousand respectively). Our results clearly show that degradation of oil by microorganisms when oil spill occurs should render a decrease in the C-13/C-12 ratio of the seawater dissolved inorganic carbon (DIC) which is the ultimate carbon source of coral calcification