Coral δ¹⁸O, δ¹³C, Sr/Ca, and coral-derived d18O seawater from Hainan Island, South China Sea

Hainan Island in the South China Sea (SCS) is located in an important region of East Asia under the influence of the shifting East Asian monsoon system. Hermatypic corals living in the waters surrounding Hainan have the potential to record the changing sea surface conditions. Skeletal materials from a recently collected Porites lutea coral growing off the east coast of Hainan were analyzed for Sr/Ca, d18O, and d13C values as proxies of climate. The coral-based sea surface temperature (SST) reconstructions are more robust in winter than in summer. The reconstructions indicate an increasing secular warming trend in summer SST while the winter SST remained constant. This seasonal difference resulted in an increased annual SST range over the 109-year reconstruction period. A pronounced secular depletion trend in our coral d13C over the twentieth century was observed and is similar to the rate of d13C decrease in atmospheric CO2. This consistent isotopic depletion towards lower coral d13C values is reflecting the ongoing accelerated oceanic CO2 uptake due to rising anthropogenic CO2. In addition to the detected interannual variability in both reconstructed SST and d18O of seawater (d18Osw) between 3-7 years indicating the influence of the El Niño/Southern Oscillation (ENSO), significant interdecadal to multidecadal variability (~20-25 and ~32 years) were also found. The known Interdecadal Pacific Oscillation (IPO) phase shift of 1945 resulted in the freshening of the SCS that may be due to intensified monsoon conditions. The coherence of our tropical results to an extratropical record suggests large-scale tropical-extratropical teleconnection over multi-decadal timescales in response to IPO phase changes that is strongly influenced by ENSO.