Environmental Geochemistry Records In Black Corals Of The South China Sea

In this work, using Scanning Electron Microscope (SEM), high resolution (-2.5 μn) Synchrotron Radiation Micro X-ray Fluorescence (SR μ-XRF) and Isotope Ratio Mass Spectrometry (IR-MS), we analyzed black corals (Cirrhipathes spp.) collected from the South China Sea (SCS) for variation characteristics of radial growth rate, stable isotope and the iron (Fe) profile. The response of radial growth rate of black corals to the environmental factors (e.g., ENSO, sea surface temperature (SST) and precipitation) was observed, and the feasibility of using element Fe profile across the skeleton to reestablish the dust deposition history in Xisha Islands was discussed. The influence of human activities on the sea carbon reservoirs and water quality conditions was detected by the stable isotope (δ13C,δ1SN) in black coral skeletons.Main conclusions are as follows:1) Radial growth rate was compared with meteorological data to investigate its relationships with SST, precipitation and ENSO event in the Xisha Islands. Coral radial growth rate presented a significant correlation with SST (r=0.519, P<0.01, n=111) and ENSO Index (r=0.0492, P<0.01, n=61). A negative correlation was observed between coral radial growth rate and precipitation (r=-0.34, n=32). In addition, radial growth rate shared similar period with SST and ENSO (3.09 years and 2.77 years, respectively). No evident similar period was found between coral radial growth rate and precipitation. This implied that black coral growth rate had quick respond to the ENSO event.2) The dust from northern China could be deposited in the SCS under the effect of East Asian Winter Monsoon (EAWM). The annual variation of Fe concentration across the black coral skeleton was measured with high resolution synchrotron radiation micro X-ray fluorescence (SR μ-XRF) analysis under a resolution of 2.5μm. Significant positive correlations were observed between the annual variation of Fe in JYI-1 sample and dust storm frequency in the northwest China (Minqin and Hetian) during 1954- 2007 (r=0.647, P<0.01, n=54; r=0.625, P<0.01, n=54, respectively). Moreover, the annual variation of Fe in JYI-1 sample presented coordinated variation with EAWM Index, indicating that black corals were sensitive to the variation of dust deposition in Xisha Islands, where dust may be carried from the northwest of China.3) The δ13C in the skeleton of LHT (near shore) and JYI-1 (off shore) sample significantly decreased over the past century, and δ13C in LHT sample decreased more rapidly than that in JYI-1 sample, suggesting that the anthropogenic atmospheric CO2 emissions (known as the origin of the "Suess effect") may have substantial effect on the natural seawater in the SCS. In addition, the 815N in JYI-1 sample (off shore) collected in the Xisha Islands presented no significant change throughout the coral’s lifetime (～0.525%o), reflecting less anthropogenic pollution and a relatively pristine condition in the Xisha Islands. In contrast,δ15N in the skeleton of LHT sample (which was near Sanya City) was on the rise since 1950s, and experienced considerable increase since 1980s (-0.518%o during 1980-2000) in particular, reflecting an increasing number of sewage discharged into Sanya River and ultimately transported into seawater driven by blooming economic activities since liberation.