Evolution and development of Miocene “island dolostones” on Xisha Islands, South China Sea

On Xisha Islands, located in the South China Sea, the Neogene succession includes the unconformity-bounded Huangliu Formation that is 210.5 m thick in well CK-2 and formed almost entirely of dolostones. The diverse biota in the Huangliu Formation, which includes corals, algae, bivalves and foraminifera, indicates that the original carbonate sediments accumulated in water that was < 30 m deep. The dolostones are formed of various mixtures of low- and high-calcium calcian dolomite with limpid dolomite lining the walls of many cavities. The O-18 and C-13 stable isotopes suggest that dolomitization was mediated by slightly modified seawater. The Sr-87/Sr-86 ratios from the dolostones suggest that dolomitization took place similar to 9.4 and 2.3 Ma ago, with the age of dolomitization becoming progressively younger towards the top of the formation. "Island dolostones" like these, found on many islands throughout the Pacific Ocean and the Caribbean Sea, have commonly been linked to eustatic changes in sea-level with dolomitization taking place during lowstands, highstands, or transgressive phases. Data from the Huangliu Formation in well CK-2 suggests that dolomitization was associated with (semi-) continuous transgressive conditions that were controlled by the interaction of tectonic subsidence and eustatic changes in sea level

Controlling factors and environmental significance of BIT and delta C-13 of sedimentary GDGTs from the Pearl River Estuary, China over recent decades

The Pearl River Delta region has become one of the fastest developing regions in China in recent decades. As a result, large volumes of nutrients and carbon have been delivered to the Pearl River Estuary (PRE), causing eutrophication over the past century. In this study, four sediment cores were collected, they were analyzed for branched and isoprenoid tetraether lipids, total organic carbon content (TOC), total carbon to nitrogen (TOC/TON) ratio and delta C-13 values of crenarchaeol-derived biphytanes (delta(13)(CBP-Cren)) after ether cleavage of isoprenoid glycerol dialkyl glycerol tetraether lipids (iGDGT's). The purpose of this study is to analyze the controlling factors and environmental significance of the BIT index (a paleoproxy based on concentrations of branched and isoprenoid tetraether lipids) and delta C-13(Bp-Cren), values, and then to explore the possible responses of the BIT index and delta C-13(Bp-Cren) values to increasingly eutrophication (i.e., frequent outbreaks of algal blooms) occurring near the PRE in recent decades. The BIT index exhibits a decreasing trend in cores off the PRE (A8 and A9), indicating there was an increasing relative contribution of marine-derived organic carbon to the TOC content in the sediment. The delta C-13 values of all four core records exhibit decreasing trends toward the present day, exhibiting the influence of the Suess effect. However, the rates of delta C-13(Bp-Cren) reduction are lower in cores A8 and A9 that are close to the PRE, than those in cores that are more offshore (E501 and LE01, east of Hainan Island). In particular, the decrease in delta C-13(Bp-Cren) with time in cores A8 and A9 effectively stopped after the 1970s, but continued to occur in cores collected from east of Hainan Island. This may be mainly caused by the reduced riverine input of GDGT's and increasing eutrophication. The relative proportion of riverine Thaumarchaeota (with more negative delta C-13 values) decreased off the PRE, leading to the delta C-13(Bp-Cren) values of cores here being more positive. Furthermore, the declining trend of delta C-13(Bp-Cren) values (in response to the Suess effect) in cores A8 and A9 off the PRE may be partially offset by increasing water productivity. This study indicates that the BIT index and delta C-13(Bp-Cren) values in estuarine sediments were affected by variation in both riverine input of GDGT's and surface water productivity. As a result, the BIT index and delta C-13(Bp-Cren) in estuarine sediments may represent potential proxies for past changes in eutrophication in estuaries

Carbon isotopic disequilibrium between seawater and air in the coastal Northern South China Sea over the past century

Six coastal sediment cores collected from the Northern South China Sea were dated by Pb and analyzed for stable carbon and oxygen isotope composition of planktonic foraminifer Globigerinoides ruber (δ C and δ O ). Three of these cores were located east of Hainan Island and the other three off the Pearl River Estuary (PRE). Surface seawater δ O and dissolved inorganic carbon (DIC) δ C were identified as the dominant factors controlling downcore variations of δ O and δ C , respectively. Results of δ C were then used to study surface water δ C and its relation to δ C of atmospheric CO (δ C ) over the past century. Downcore records showed rather constant δ C in cores off Hainan Island, but moderate decreases of δ C , at rates between -0.006‰ and -0.009‰ per year, in cores off the PRE. Isotopic disequilibrium between δ C and δ C was observed, with δ C apparently higher than expected at equilibrium with δ C except at the site closest to the PRE. The relatively steady δ C values east of Hainan Island were explained by balanced vertical mixing and biological pump, whereas the moderate δ C decreases with time off the PRE were attributable to fluvial input of terrestrial carbon

Coexistence of negative photoconductivity and hysteresis in semiconducting graphene

Solution-processed graphene quantum dots (GQDs) possess a moderate bandgap, which make them a promising candidate for optoelectronics devices. However, negative photoconductivity (NPC) and hysteresis that happen in the photoelectric conversion process could be harmful to performance of the GQDs-based devices. So far, their origins and relations have remained elusive. Here, we investigate experimentally the origins of the NPC and hysteresis in GQDs. By comparing the hysteresis and photoconductance of GQDs under different relative humidity conditions, we are able to demonstrate that NPC and hysteresis coexist in GQDs and both are attributed to the carrier trapping effect of surface adsorbed moisture. We also demonstrate that GQDs could exhibit positive photoconductivity with three-order-of-magnitude reduction of hysteresis after a drying process and a subsequent encapsulation. Considering the pervasive moisture adsorption, our results may pave the way for a commercialization of semiconducting graphene-based and diverse solution-based optoelectronic devices

High‐Resolution Coral Records of Cadmium in Surface Seawater: Biogeochemical Cycling and a Novel Proxy for Winter Monsoon

Abstract Cadmium (Cd) geochemical cycle plays a significant role in the composition and function of the marine ecosystem. Skeletal cadmium‐to‐calcium (Cd/Ca) ratios in hermatypic corals have been applied to reconstruct the historical changes of oceanic and climatic processes, yet there was no systematic evaluation of this tracer's natural variability in high resolution over time. Here, we reported a coral skeletal Cd/Ca record in monthly resolution from 1999 to 2008 CE and reconstructed the history of Cd contents in surface seawater in the northern South China Sea. A significant seasonal variation (higher in the winter but lower in the summer) of Cd contents in surface seawater can be identified. We found that the seasonal variations in coral skeletal Cd/Ca ratios exhibited significant trends coupled with the surface wind speeds, indicating that strong winds had likely driven the vertical seawater mixing process and then induced the process of sediment remobilization on the shelf, which significantly increased Cd contents in surface seawater. The reduction in Cd contents in surface seawater due to biological processes might be masked by the impacts of surface winds. Importantly, we also observed that coral skeletal Cd/Ca records in the winter showed significant correlations with the winter monsoon index, highlighting the possibility as a new proxy of winter monsoon in the non‐upwelling shelf environments

Coral geochemical record of submarine groundwater discharge back to 1870 in the northern South China Sea

The importance of submarine groundwater discharge (SGD) is becoming increasingly recognized because of its potential significance as a source of dissolved species. To explore the probable coral geochemical signal of SGD and verify the validity of potential reliable proxies, multiple geochemical proxies over the last 137 years were identified from a Porites coral near a subterranean estuary in the northern South China Sea, where the SGD was reported to be the predominant flux of terrestrial waters to the coastal ocean. Results indicated that the SGD in the coastal zone was the dominant source of trace elements, especially REE and Ba, due to the various dissolution reactions occurring during groundwater flow in the karst terrain. The time- and frequency-domain comparison between the coral geochemical proxy and the local/regional precipitation indicated that coral REE/Ca ratios are predominantly impacted by the SGD associated with local precipitation, while coral Ba/Ca ratios are also affected by the primary productivity and allochthonous seawater Ba from surrounding areas. The REE signal from coral allows us to reconstruct the coastal surface seawater REE concentrations and the SGD rates on the coast of Sanya during 1870–2006. In a novel approach to developing a proxy for historic SGD to coastal waters, this study provides evidence that the coral REE/Ca record from the karst coast with large SGD has potentials to be a promising paleohydrological indicator

Effects of fertilization and clipping on carbon, nitrogen storage, and soil microbial activity in a natural grassland in southern China.

Grassland managements can affect carbon (C) and nitrogen (N) storage in grassland ecosystems with consequent feedbacks to climate change. We investigated the impacts of compound fertilization and clipping on grass biomass, plant and soil (0-20 cm depth) C, N storage, plant and soil C: N ratios, soil microbial activity and diversity, and C, N sequestration rates in grassland in situ in the National Dalaoling Forest Park of China beginning July, 2011. In July, 2012, the fertilization increased total biomass by 30.1%, plant C by 34.5%, plant N by 79.8%, soil C by 18.8% and soil N by 23.8% compared with the control, respectively. Whereas the clipping decreased total biomass, plant C and N, soil C and N by 24.9%, 30.3%, 39.3%, 18.5%, and 19.4%, respectively, when compared to the control. The plant C: N ratio was lower for the fertilization than for the control and the clipping treatments. The soil microbial activity and diversity indices were higher for the fertilization than for the control. The clipping generally exhibited a lower level of soil microbial activity and diversity compared to the control. The principal component analysis indicated that the soil microbial communities of the control, fertilization and clipping treatments formed three distinct groups. The plant C and N sequestration rates of the fertilization were significantly higher than the clipping treatment. Our results suggest that fertilization is an efficient management practice in improving the C and N storage of the grassland ecosystem via increasing the grass biomass and soil microbial activity and diversity