Non-Conventional Isotopes (Boron And Calcium) And Elemental Ratios In Scleractinian Coral Skeleton:New Constraints On Biological Activity

Corals are unique marine organisms who possess the ability to convert aqueous seawater ions into calcium carbonate(CaCO3)mineral aragonite at a tremendous rate in the tropics.The calcification of corals is sensitive to a number of environmental conditions including light,temperature,nutrient supply,seawater pH and turbidity.This not only makes corals highly susceptible to the climate change,but also makes themselves ideal archives of recording past environmental variability.In the paleoceanography and paleoclimate studies,scleractinian coral skeleton geochemistry offers important tools to reconstruct environmental changes,such as elemental ratios(e.g.Sr/Ca,Mg/Ca,etc.)and light isotope fractionation(e.g.?13C??180).Following the recent rapid development of mass spectrometry techniques,non-traditional isotope proxies have draw new attentions to their applications on coral-based paleo-environmental studies.The present study focuses on the chemical composition of Porites coral skeletons using elemental concentration and ratios,as well as non-traditional isotopes,to evaluate their applicability as geochemical proxies and to constrain the biological effects and elements biogeochemical cycles.The dataset for our coral skeleton chemistry contains both modern and fossil Porites corals from different locations including Hainan Island(northern South China Sea)and Kimberley region(northwest Australia,Indian Ocean).Moreover,analysed material was also extended to seawater which was used to investigate the short-term variability of trace metals in surface ocean.First,the short-term variability of selected trace elements was investigated in order to elucide their variation patterns and the underlying controlling mechanism.The concentrations of dissolved trace elements(V,As,Se,W,and Y),dissolved oxygen(DO),and carbonate system parameters were measured over diurnal cycles on the Luhuitou fringing reef,located in the northern South China Sea.We show that the concentrations of these selected trace elements exhibit diurnal changes in the Luhuitou fringing reef,in step with the daily variability of dissolved oxygen(DO)and carbonate system parameters(i.e.DIC and pH).The prominent diurnal variations in DO and carbonate system parameters indicate that biological activities,especially photosynthesis and respiration,are the primary controls on reef water chemistry.Given that the measured trace elements V,W,and Y co-varied significantly with seawater pH and DIC,it implys that these elemens may participate in the community photosynthesis and respiration.In contrast,As and Se demonstrated divergent diurnal to semi-diurnal behaviors,and their concentrations were not correlated with carbonate system parameters,suggesting that they may be involved in the other biogeochemical processes,for instance the assimilation of nutrients.These findings suggest that on the daily timescale,biological activities within the reef environment exert important influences on the variability of dissolved trace elements in the reef water.To further study the long-term behavior of trace elements in surface seawater,coral skeleton was therefore used as an archive of the trace elements variability,as these elements can be incorporated into coral's aragonite lattice in proportion to their concentrations in ambient seawater with slightly modulations by temperature and vital effects.Here we report a 159-year record of trace metal concentrations(Mn,Cu,and V)from a,Porites coral from the northern South China Sea(SCS),and discuss how oceanic and climatic processes control variations in Mn,Cu,and V concentrations in this region.Our results show that trace metal concentrations in the coral skeleton demonstrate decadal to interdecadal fluctuations,and that their variations are controlled by different mechanisms.The input of Mn to reef water is partly controlled by the Pacific Decadal Oscillation(PDO),which controls precipitation and river runoff.Surface water concentrations of the nutrient-like element Cu are controlled by summer upwelling to the east of Hainan Island.The concentrations of V show complex interrelationships,and are linked to riverine input prior to the 1990 and to upwelling after the 1990.Our results imply that in the northern SCS,ocean-atmosphere climate fluctuations,such as the PDO and the East Asian Summer Monsoon(EASM),are important factors that influence long-term variability of Mn,Cu,and V concentrations in seawater,by controlling precipitation-related river runoff and the strength of upwelling systems.Trace elements in coral skeleton not only can be used to track their historical variations in seawater,but also provide a reliable tool to reconstruct the sea surface temperature(SST)variability.Coral skeletal Sr/Ca ratios are the most widely used temperature proxy especially in tropical oceans,helping to generate multi-century reconstruction of SST,which allow us to study past climatic variability and track temperature variation through geological timeframe.Nonetheless,the robustness of Sr/Ca proxy is often limited by its site-and species-specific relationships with temperature,as it suffers to varying degree from "vital effects" such as biologically mediated Rayleigh fractionation that can vary between species as well as colonies of the same species.Recently,an alternative temperature tracer Li/Mg has been developed to better constrain the temperature signal preserved during their incorporation into coral skeletons,since it can potentially overcome the influences from both vital and kinetic effects on Rayleigh fractionation.The generally more widely applicable Li/Mg-temperature relationship can then account for both intercolonial and interspecies differences in Li/Mg responses to temperature,covering a wide range of temperature from 0 ? to 30 ? across different inhabiting environments.However,application of Li/Mg temperature proxy in tropical corals is potentially limited by its reduced sensitivity at higher temperatures,suggesting that site-specific calibrations maybe necessary to improve the fidelity of temperature reconstructions.In this study,we analyse both Li/Mg and Sr/Ca ratios in a Porites coral living in the thermally extreme Kimberley region,northwest Australia,aiming to evaluate the application of Li/Mg in the thermally tropical environment.The site-specific temperature calibrations show that Sr/Ca and Li/Mg ratios are well correlated with temperature,and can generate reliable temperature reconstructions,confirming their fidelity in such high temperature extreme reef settings.Further,we show that combined Sr/Ca and Li/Mg multiproxy calibrations can improve the precision of annual SST reconstructions.By using this conmbined Sr/Ca and Li/Mg multiproxy calibrations,we reconstruct the past century SST variation for the nearshore Kimberley region.Estimates of the Sr/Ca and Li/Mg multiproxy-SST appear to show a long-term trend towards warmer temperature of recent decades,which agrees with the other coral-based temperature reconstructions in the Indian Ocean.Superimposed on this trend are significant interannual to decadal fluctuations,of which the amplitude is also enhanced after the 1970s.In addition,the positive temperature anomalies of the past?20 years which are as high as?1.5 ? compared to the 1961-1990 mean suggest an intensified thermal stress in Kimberley region under the global warming background.However,according to the results of boron isotopes and B/Ca ratios for the Kimberley coral,despite this intensified warming,we find 'normal' seasonal variability in the coral's CF carbonate chemistry similar to those in Porites from the Great Barrier and Ningaloo Reefs of Australia.Higher(?x2)dissolved inorganic carbon(DICcf)combined with up-regulation of pHcf to?8.5,leads to an elevated aragonite saturation state(?cf)level of?16 to?20.Importantly we find that recent warming has affected coral's ability to concentrate inorganic carbon,with DICcf showing decreased and subdued seasonal variability consistent with reduced calcification observed during the most recent 2012 to 2016 warming period.Thus,while up-regulation of pHcf is shown to be an essential pre-requisite for calcification,reduced supplies of metabolic DICcf can severely limit rates of calcification during periods of thermal stress and coral bleaching.On the annual timescale,the carbonate chemistry in coral calcifying fluid exhibits interannual to interdecadal variations.The DICcf shows subdued variability,except a significant shift occurring around the 1990s that DICcf increased with temperature.The relationship between DICcf and temperature is weak as a whole,with a correlation coefficient of 0.31(p<0.001;n= 96),significantly lower compared to that on seasonal timescale(r = 0.60;p<0.000001;n = 58).This suggests that thermal stress may attenuate the relationship between DICcf and temperature on the longer timescale.However,the DICcf and pHcf still remain well correlated with a significant correlation coefficient of-0.87(p<0.0000001,n= 96),which is consistent with the strong inter-relationship between DICcf and pHcf.The antithetical variations for DICcf and pHcf therefore result in relatively stable ?cf levels.This again suggests that within certain temperature range,corals can manipulate its internal carbonate chemistry to maintain a favorable calcifying environment when coping with increasing temperature.In addition to the modern coral calcifying fluid carbonate chemistry,we also measured boron isotopes for four different fossil Porites corals from the offshore East Hainan Island,northern South China Sea.These fossil corals cover the important climate periods for the past 1000 years,namely the Little Ice Age and the Medieval Warm Period.Compared with the modern coral results,we find coral internal calcifying fluid pHcf displays different values in these periods,with the highest pHcf value found in the Little Ice Age,and the lowest pHcf value in the Current Warm Period.The underlying mechanism controlling pHcf variations remain unclear,as currently we are short of the data for the variation of DICcf.Nevertheless,it is highly possible that the coral reponses to different climate pattern(e.g.temperature)in these three periods are the main factor controlling their internal carbonate chemistry changes.Finally,we also analyzed the Ca isotopic composition(?44/40Ca)in a Porites spp.coral from the Great Barrier Reef at monthly intervals for two consecutive years.It was found that variations in skeletal ?44/40Ca values over the 2-year period are slightly greater than the analytical precision of the measurements,although other coralline geochemical records(i.e.,?13C,?180,?11B and Sr/Ca ratios)show remarkable variations.To evaluate the potential of ?44/40Ca as a paleoclimate proxy in corals,we compared ?44/40Ca with other well-established indicators,and found that the ?44/40Ca values show little relationship to ?18O,Sr/Ca,??18O,or ?11B values,thus suggesting that the influence of seawater temperature,seawater pH and river inputs on skeletal?44/40Ca islimited or overwhelmed by other factors.However,the skeletal 844/40Ca values are significantly related to ?13C values(r = 0.46,p<0.05;n = 27),thus indicating that Ca isotopic fractionation in corals is subjected to vital effect.The strong influence of biogenic factors on ?44/40Ca in corals,limit the applicability of coral ?44/40Ca to paleoclimate reconstruction.