Environmental Geochemistry Records In Corals Of South China Sea And Global Change

This study focused on stable isotopic and trace elements ratios proxies(δ¹¹B,Rare earth elements,Sr/Ca andMg/Ca) in the modern and fossil corals from the South China Sea(SCS) which were picked from Leizhou Peninsula,Hong Kong,Hainan, Xisha and Nansha Islands.These proxies provide the history of sea surface temperature,pH and sea level change of the SCS spanning the last two centuries and part time of the Holocene.Using these geochmisty records in the corals,we investigated the variations,trends,cycles,amplitudes and shift events of the climate-environment change of the SCS which are related with ocean-atmosphere interactions,Asian monsoon,ENSO and anthropogenic forcing.We used positive thermal ionization mass spectrometry(PTIMS) to generate high precisionε¹¹B records in Porites corals of the mid-late Holocene from the SCS.The paleo-pH records of the SCS,reconstructed from theβ¹¹B data,were not stable as previously thought but showed a gradual increase from the Holocene thermal optimal and a sharp decrease to modern values.The latter is likely caused by the large amount of anthropogenic CO₂ emissions since the Industrial Revolution but variations of atmospheric pCO₂ cannot explain the pH change of the SCS before the Industrial Revolution.We suggest that variations of monsoon intensity and sea level during the mid-late Holocene may have driven the sea surface pH increase from the mid to late Holocene.Results of this study indicate that the impact of anthropogenic atmospheric CO₂ emissions may have reversed the natural pH trend in the SCS since the mid-Holocene.Annual Rare earth element(REE) records in two corals from Hainan and Hong Kong display an apparent declining trend.Both records are significantly correlated with sea level.This could be interpreted by sea level rise which is accompanied by intrusion of seawater into the estuaries.However,the trend of Er/Nd in the coral near Pearl River is not convincing and there is no good relation between Er/Nd and total REEs as found for the coral near Wanquan River of Hainan.Unlike Wanquan River who is famous for its tourism and nature landscape,the Pearl River is significantly distorted by serious human activities such as the construction of dams/reservoirs and sediments mining of the riverbed.The sediment load of the Pearl River decreased steadily which may also contribute to the decline trend of REE in this coral.Coastal corals like Hainan are better candidates to investigate the possible effect of changes of sea level on seawater/coral REE because of small river discharge and less anthropogenic disruptionss.Using the linear regression equtation between total REEs of Hainan coral and sea level,5-yr interval relative sea level series from 1856-1995 has been established.The reconstuted sea level shows a increasing trend since 1856 with an average rising rate of 0.5mm/a.The sea level change of Hainan at west Pacific and San Francisco at east Pacific is antiphased which may be related with ENSO.The decadal variations of two 120-yr monthly Sr/Ca records from Hainan at north Pacific and Rarotonga at south Pacific appear to synchronous.This hemispheric symmetry suggests that tropical forcing may be an important factor in at least some of the decadal variability observed in the Pacifc Ocean.Harmonic spectrum of the Hainan coral Sr/Ca record reveals 34.6-year,24.2-year and 8.3-year cycles.The 24.2-year cycle appears as a dominant cycle for both Hainan and Rarotonga,revealing a decadal SST change in the Pacific Ocean.This oscillation has been previously identified as a robust,recurring pattern of ocean-atmosphere climate variability in the North PacificEast of Hainan Island is located within the one of the upwelling centers of this region.The reconstructed summer SST by the coral Sr/Ca ratio from the upwelling area is significant lower and shows a different trend in the 20th century compared to non-upwelling areas.We firstly established a long term index for summer coastal upwelling intensity off northern shelf of the SCS by the coral-SST differences between upwelling areas and non-upwelling areas.The results show a rapid 20th-century increase in coastal upwelling at the northern SCS which is related to sharp globe warming and maybe partly resulted from stronger and more frequent El Ni(?)o events in the tropical ocean.At the same time,we found that the difference of winter SSTs between the two sites(Xisha and Hainan) is significantly negatively correlated with the instrumental WMV(winter monsoon velocity).This negative correlation may be related to the intrusion of the warm Kuroshio Current into the SCS through the Luzon Strait promoted by the strong northeastern monsoon winds in the winter.Using the relationship between our coralline data and observed WMV,we generated a record of WMVcs from 1906 to 1998.The WMVc in the 20th century shows significant interannual and decadal variability with a trend of persistent decline in the whole 20th century.The WMVc has decreased significantly by about 30%from the early to the late of 20th century due to globe warimg.In addition,an obvious decline shift of WMV around 1976 can be seen in both instrumental and proxy records and it coincides with many other Pacific records.This shift is likely to correspond to a Pacific-wide change in the Pacific Decadal Oscillation occurring at the same time.