Anomalous Ba/Ca signals associated with low temperature stresses in Porites corals from Daya Bay, northern South China Sea.

Barium to calcium (Ba/Ca) ratio in corals has been considered as a useful geochemical proxy for upwelling, river flood and other oceanic processes. However, recent studies indicated that additional environmental or biological factors can influence the incorporation of Ba into coral skeletons. In this study, Ba/Ca ratios of two Porites corals collected from Daya Bay, northern South China Sea were analyzed. Ba/Ca signals in the two corals were 'anomalous' in comparison with Ba behaviors seen in other near-shore corals influenced by upwelling or riverine runoff. Our Ba/Ca profiles displayed similar and remarkable patterns characterized by low and randomly fluctuating background signals periodically interrupted by sharp and large synchronous peaks, clearly indicating an environmental forcing. Further analysis indicated that the Ba/Ca profiles were not correlated with previously claimed environmental factors such as precipitation, coastal upwelling, anthropogenic activities or phytoplankton blooms in other areas. The maxima of Ba/Ca appeared to occur in the period of Sr/Ca maxima, coinciding with the winter minimum temperatures, which suggests that the anomalous high Ba/Ca signals were related to winter-time low sea surface temperature. We speculated that the Ba/Ca peaks in corals of the Daya Bay were most likely the results of enrichment of Ba-rich particles in their skeletons when coral polyps retracted under the stresses of anomalous winter low temperatures. In this case, Ba/Ca ratio in relatively high-latitude corals can be a potential proxy for tracing the low temperature stress.

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.

[Latest forty two years' sea surface temperature change of Weizhou Island and its influence on coral reef ecosystem].

Weizhou Island (109 degrees 00'-109 degrees 15'E, 21 degrees 00'-21 degrees 10'N) locates in the north coral reef distribution belt of the South China Sea. In such relatively high latitude area, sea surface temperature (SST) is an important factor affecting the development of coral reef ecosystem. Measurements of the latest 42 years' SST of Weizhou Island showed that with 3-4 years and 7-8 years fluctuating cycles, the SST almost had a synchronous increase with global warming, especially since the later 1980s. Overall, the coral reef of Weizhou Island could benefit from the SST warming, especially the cold months SST increase, because it's the north margin of tropical zone. But, the warming of the warmest month may be a thermal stress on the development of coral reef. It reaches the upper limits of coral reef development, and leads the coral reef bleaching. Mankind activities, including wide scale building, oil gas factory near the island, traveling, fishing and breeding, may be also the potential stresses on limiting the coral reef development. The combination of summer warming and mankind stress is possibly to destroy the coral reef ecosystem of Weizhou Island. Based on a detailed field investigation, the modern coral reef distribution map around Weizhou Island was described.