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Coastal lagoons with small catchment basins are highly sensitive to natural processes and anthropogenic activities. To figure out the environmental changes of a coastal lagoon and its contribution to carbon burial, two sediment cores were collected in Xincun Lagoon, southeastern Hainan Island and Pb activities, grain size parameters, total organic carbon (TOC), total nitrogen (TN), total inorganic carbon (TIC) and stable carbon isotopes (δC) were measured. The results show that in 1770-1815, the decreasing water exchange capacity with outer open water, probably caused by the shifting and narrowing of the tidal inlet, not only diminished the currents and fined the sediments in the lagoon, but also reduced the organic matter of marine sources. From 1815 to 1950, the sedimentary environment of Xincun Lagoon was frequently influenced by storm events. These extreme events resulted in the high fluctuation of sediment grain size and sorting, as well as the great variation in contributions of terrestrial (higher plants, soils) and marine sources (phytoplankton, algae, seagrass). The extremely high content of TIC, compared to TOC before 1950 could be attributed to the large-scale coverage of coral reefs. However, with the boost of seawater aquaculture activities after 1970, the health growth of coral species was severely threatened, and corresponding production and inorganic carbon burial flux reduced. The apparent enhanced inorganic carbon burial rate after 1990 might result from the concomitant carbonate debris produced by seawater aquaculture. This result is important for local government long-term coastal management and environmental planning. [ABSTRACT FROM AUTHOR]

The sulfur isotopic composition (δ34S) of Carbonate Associated Sulfate (CAS) is widely used to track changes in the isotopic composition of ancient seawater sulfate which signify the global sulfur cycle, Earth's surface redox evolution, and biological activity. However, our understanding of to what extent the isotopic composition of CAS records ambient seawater sulfate is primarily deduced from modern calcifying species. A few studies have examined modern bulk carbonate sediments which are important analogs for sedimentary carbonate rocks in the geologic record. To evaluate the fidelity of bulk carbonate sediments in recording ambient seawater δ34S values, we extracted CAS component and provided δ34S CAS results of five modern corals from the Hainan Island and 74 core-top unlithified carbonate sediments from the Nansha Islands in the South China Sea. Modern coral samples show a uniform δ34S CAS range from 21.5‰ to 21.7‰ close to previously published δ34S values for modern coral and seawater in other open oceans, consistent with globally homogeneous seawater δ34S compositions (i.e., δ34S sw = 21.2 ± 0.2‰). The δ34S CAS values of core-top carbonate sediments vary within a narrow range from 21.1‰ to 22.1‰, which is fractionated from the overlying seawater by 0.4‰ ± 0.2‰. We suggest that these irregularly positive but small S-isotope fractionations (<1‰) are potentially due to the varying proportions of various calcifying organism granules with species-dependent sulfur isotopic fractionation. Overall, our data indicate that the CAS of bulk carbonate sediments from the open ocean settings is a reliable archive of seawater δ34S composition, provided the isotopic compositions have not been altered by post-depositional diagenesis. • New and detailed δ34S CAS data of bulk carbonate sediments in the South China Sea. • Modern corals data confirms globally homogeneous δ34S sw composition. • Small positive δ34S fractionation (<1‰) from seawater has been shown in carbonates. • Calcifying organisms with species-dependent fractionation may lead to δ34S offset. • Bulk carbonates can be used as reliable archives for ancient seawater δ34S value. [ABSTRACT FROM AUTHOR]

This study analyzed the characteristics and the bearing capacity of water surge disasters in Boao Bay, a typical estuarine lagoon. The Danish Hydraulic Institute's (DHI's) MIKE 21 modeling suite was used to numerically simulate the water surges from flood, storm, and combined flood and storm processes. The results show that, in Boao Bay, the peak water surge level caused by the combination of flooding and storm surges is very similar to the level caused by flooding or storm surges alone. Additionally, the storm surge bearing capacity is much greater than the flood bearing capacity. When there is only one entrance, the bay's flood bearing capacity increases with the entrance width, but when the entrance width exceeds 550 m, the flood bearing capacity stabilizes rapidly. However, opening an additional entrance to the south can further increase the flood bearing capacity. Based on hydrometeorological forecasts, this study suggests that the risk of water surge disasters can be reduced by proactively modifying the shape of the entrance. [ABSTRACT FROM AUTHOR]