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 13C. Coral skeletal δ13C 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 δ13C at present. This kind of research is useful for us to obtain environmental and climatic information from coral skeletal δ13C. 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 δ13C and δ18O were analyzed. Besides, seawater was collected during an oil spill in 28 January 2018, and was used for δ13CDIC (δ13C values of DIC) analysis. The purpose is to explore the potential impact of oil spills on δ13CDIC and coral δ13C. δ13CDIC value during oil spill was significantly lower than that of normal seawater (Δδ13C≈-2.38‰). Coral δ13C of both two coral colonies showed lower values since 2008 (average value of δ13C before and after 2008 change from −2.04 ± 0.48‰ to −3.33 ± 0.55‰ ‰ and −1.77 ± 0.49‰ to −2.88 ± 0.54‰ of W1 and W3 respectively), especially, several abrupt negative shifts in 2008, 2009, 2011, 2012 and 2014 when oil spills occurred (extent of δ13C variation were W1：−3.08‰, −1.82‰, −1.82‰, −1.53‰, −1.37‰; W3：−3.27‰, −1.8‰, −1.91‰, −1.21‰, −1.13‰ respectively). Our results clearly show that degradation of oil by microorganisms when oil spill occurs should render a decrease in the 13C/12C ratio of the seawater dissolved inorganic carbon (DIC) which is the ultimate carbon source of coral calcification.