Cu And Zn Isotopic Features Of Porites Corals In The South China Sea And Their Potential Applications

Scleractinian corals are mainly distributed in the tropical shallow waters and are sensitive to climate changes.Geochemical compositions in their skeletons have been widely used to reconstruct variations in paleoceanography,climate,and environmental changes through time.Together with advances in mass spectrometry analysis technologies,more and more researchers have focused on the trace metal elements and their isotopes in coral skeletons,among which Copper（Cu）and Zinc（Zn）elements are both essential micronutrients for marine primary producers,and their contents and isotopic compositions have big potential for tracking and quantifying specific biological-physical-chemical processes in the ocean.To date,few studies on the isotopic compositions of Cu and Zn in coral biogenic carbonates have been carried out,and their isotope fractionation mechanisms remain unclear,which largely limits the utility of skeletal Cu and Zn isotope compositions to trace paleo-climatic and environmental changes in oceans.To determine whether the Cu and Zn isotope compositions in coral skeletons could be utilized as potential tracking indicators in the paleoceanography field,we studied the inter-annual variabilities in Cu and Zn contents and isotopic compositions of Porites corals collected from seven sites in South China Sea and Luzon Strait.We discussed the fractionation mechanisms of coral Cu and Zn isotopes in biogenic carbonates and their potential to study paleoclimatic and paleoenvironmental changes.The major findings are as follows:（1）Porites corals have highly variable inter-and intra-colonyδ⁶⁵Cu,and are isotopically lighter than seawater,which may be related to the kinetic fractionation of Cu isotopes during calcification processes.As such,δ⁶⁵Cu in Porites corals can be hardly used to trace marine climate and environmental changes.（2）Zn isotopes in Porites corals are isotopically heavier than those in seawater and the isotopic offsets are positive and relatively constant(Δ⁶⁶Zn_coral-sw≈0.10‰).The Zn isotopic ratios are mainly controlled by the equilibrium isotope fractionation,but independent to sea surface temperature or other environmental changes.Therefore,Zn isotopic composition in coral biogenic carbonates may be a useful indicator for that in seawater.（3）Scleractinian corals are a major component of marine carbonates and important sinks for Cu and Zn elements.Our new datasets of corals,particularly the coralδ⁶⁵Cu values,which are measured for the first time,could provide a better constrain on the budgets of modern oceanic Cu and Zn isotopes,and have the potential for deeper understanding of the modern marine Cu and Zn geochemical cycling.（4）Despite substantially increased anthropogenic Zn flux in the past decades in research region,no evidence of Zn pollutions are found in the spatiotemporal records ofδ⁶⁶Zn in our corals.Our study suggests that the pervasive lowδ⁶⁶Zn in upper oceans may be not an anthropogenic origin as previously thought.