Tegu Chen

Interannual variation of rare earth element abundances in corals from northern coast of the South China Sea and its relation with sea-level change and human activities

Here we present interannual rare earth element (REE) records spanning the last two decades of the 20th century in two living Porites corals, collected from Longwan Bay, close to the estuarine zones off Wanquan River of Hainan Island and Hong Kong off the Pearl River Delta of Guangdong Province in the northern South China Sea. The results show that both coral REE contents (0.5-40 ng g⁻¹ in Longwan Bay and 2-250 ng g⁻¹ in Hong Kong for La-Lu) are characterized with a declining trend, which are significantly negative correlated with regional sea-level rise (9.4 mm a⁻¹) from 1981 to 1996 in Longwan Bay, 13.7 mm a⁻¹ from 1991 to 2001 in Hong Kong). The REE features are proposed to be resulted from seawater intrusion into the estuaries in response to contemporary sea-level rise. However, the tendency for the coral Er/Nd time series at Hong Kong site is absent and there is no significant relation between Er/Nd and total REEs as found for the coral at Longwan Bay site. The observations are likely attributed to changes of the water discharge and sediment load of Pearl River, which have been significantly affected by intense human activities, such as the construction of dams/reservoirs and riverbed sediment mining, in past decades. The riverine sediment load/discharge ratio of the Pearl River decreased sharply with a rate of 0.02 kg m⁻³ a⁻¹, which could make significant contribution to the declining trend of coral REE. We propose that coastal corals in Longwan Bay and similar unexplored sites with little influences of river discharge and anthropogenic disruption are ideal candidates to investigate the influence of sea-level change on seawater/coral REE.

The high-resolution climate recorded in the delta O-18 of Porites lutea from the Nansha Islands of China

APorites lutea core from Yongshu Reef of Nansha Islands covering 50 years growth history was analyzed for oxygen isotopic composition with monthly and seasonally resolution. The calibration of the δ18O with the instrumental temperature indicated that the coral δ18O is a good indicator for sea surface temperature (SST) and air temperature (t). It can be used to reconstruct the SST and air temperature of the Yongshu Reef sea area. In addition, the coral δ18O provides signatures for the intensity of the East Asia monsoon and it is a record for the activities of El Nino events. With the calibrated SST and air temperature formulas, the most recent fifty years SST and air temperature were reconstructed based on the coral δ18O, thus back up the understanding of the climate of Nansha Islands to 1950, far beyond the limit of the instrumental recording since September 1988. It was found that, in general, increasing 1°C air temperature results in 0.24%0 decrease in skeletal δ18O.

Alpha‐cellulose δ13C variation in mangrove tree rings correlates well with annual sea level trend between 1982 and 1999

A pilot study of tree rings in a modern mangrove tree (Rhizophora apiculata) from Leizhou Peninsula, northern South China Sea shows that ( 1) the tree-rings are annual; ( 2) the ring widths decrease; and ( 3) their alpha-cellulose delta(13)C values increase from 1982 to 1999 AD, consistent with the trends of annual sea level, salinity and sea surface temperatures in the same period. We propose that such changes were caused by increasingly longer duration of waterlogging in response to sea-level rise. If this is the case, alpha-cellulose delta(13)C in mangrove tree rings can be used as a potential indicator of past sea level fluctuations.

A prediction of sea level rising amplitude in 2030 and defensive countermeasures in the Zhujiang delta

By analysing comprehensively the factors such as theoretical sea level, abnormal fluctuation of sea level, rising amplitude of floodtide water level and land deformation, a rising amplitude of 22-33 cm of relative sea level by the year of 2030 is forecasted in the Zhujiang Delta. From the forecasted amplitude of 30 cm, using hydraulic and hydrometric models the scope influenced by sea level rise is delimited and the reappearing periods of floodtide water level, protecting ability of embankments, design parameters of water conservancy project, drain waterlogging, saline intrusion, resource of mud beach and comprehensive prevention measures are discussed.

Testing coral paleothermometers （B/Ca, Mg/Ca, Sr/Ca, U/Ca and δ18O） under impacts of large riverine runoff

Sea surface temperature (SST) proxies including B/Ca, Mg/Ca, Sr/Ca, U/Ca and δ18O were analyzed in the skeleton of a Porites coral collected from the Zhujiang River (Pearl River) Estuary (ZRE). These geochemical proxies are influenced by river runoff and this area of the northern South China Sea is strongly affected by seasonal freshwater floods. We assessed the robustness of each SST proxy through comparison with the local instrumental SST. Coral Sr/Ca shows the highest correlation with SST variations (r2=0.59), suggesting Sr/Ca is the most robust SST proxy. In contrast, coral δ18O (r2=0.46), B/Ca (r2=0.43) and U/Ca (r2=0.41) ratios were only moderately correlated with SST variations, suggesting that they are disturbed by some other factors in addition to SST. The poor correlation (r2=0.27) between SST and Mg/Ca indicates that Mg/Ca in coral skeletons is not a simple function of SST variations. This may ultimately limit the use of Mg/Ca as a coral paleothermometer.

Reconstruction of sea surface temperature series in the last 220 years by use of reef corals in Xisha waters, South China Sea

On the basis of construction of the growth rate thermometers by use ofPorites Iutea from the northern part of the South China Sea and the hindcast sea surface temperature (SST) in the last 100 years, we reconstructed the SST series in the past 220 years in the Xisha waters and analyzed the characteristics of the climatic change of that period. The results show that the low and high SST changes in the last 220 years in the Xisha waters appeared in the stage and the quasi-periodic variation within the century scale. The range of annual SST variation in the interage was increasing. The SST values were slightly rising during the last 100 years.

Cold tolerance of subtropical Porites lutea from the northern South China Sea

Marginal scleractinian corals growing at their latitudinal limits should be quite sensitive to variations in winter sea surface temperatures (SSTs). An extreme cold event occurring in early 2008 offered a unique opportunity to examine the effect of cold-water anomalies on Porites lutea corals and their physiological tolerance and acclimation in the subtropical northern South China Sea (NSCS). Besides in-situ observation, a subsequent aquarium-based experiment was designed for reproducing the chilling process and a 50-year-long Sr/Ca ratio profile from two P. lutea skeletal slabs was analyzed for reconstructed the historical annual minimum SSTs which ceased Porites calcification. The 2008 low-temperature anomaly caused the minimum daily mean SSTs dropped below 13°C in the Daya Bay. The stress symptoms displayed by local P. lutea colonies included polyp retraction, reduced coloration and pale, but none showed tissue sloughing. The ability of P. lutea to survive implied its tolerance of extreme low temperatures. Here we suggest a model on the tolerance of high-latitude Porites under low-temperature stresses, which is when SSTs drop below 18°C, Porites corals contract their tentacles (losing heterotrophic capability), then cease calcification (reducing energy consumption), and meanwhile maintain relatively high levels of zooxanthellae density (sustaining host’s life via photosynthetic capacity of symbiotic zooxanthellae). This study revealed remarkable acclimatization of P. lutea corals to low temperature extremes. This acclimatization is beneficial for Porites corals in the NSCS to expand their living ranges towards the higher-latitude areas and have the potential to be the incipient reef former.