This paper reviews both the recent and longer-term （Holocene） ecological history of coral reefs in the South China Sea （SCS）. （1） Local ecological monitoring since the 1960s shows that the coral ...reefs in the South China Sea have declined dramatically, reflecting the rapid decrease of living coral cover and the great loss of symbiotic zooxanthellae. Collectively, this has led to a significant decrease of annual CaCO3 production. Heavy anthropogenic activities and global warming are recognized as major triggers of the observed coral reef degradation. Observations show that the modern coral reefs in the SCS are a source of at- mospheric CO2 in summer. （2） Coral reefs of the SCS have been widely used to reveal longer-term environmental variations, including Holocene high-resolution sea surface temperature （SST） and abrupt climate events, millennial-scale E1 Nifio varia- tions, millennial- and centennial-scale sea level oscillations, strong and cyclic storm activities, East Asian monsoon intensities, variation in seawater pH, and recent seawater pollution. （3） Coral reefs of the southern SCS have experienced repeated epi- sodes of bleaching over the last 200 years due to high SST and intense E1 Nino events; coral reefs of the northern SCS suffered high levels of mortality during several abrupt winter cold-water bleaching events during the middle Holocene warm period. On average, recovery after the middle Holocene cold-bleaching took 20-30 years; recovery following other middle Holocene en- vironmental stresses took approximately 10-20 years. Such findings have significantly contributed to the understanding of the present ecological pressures faced by the coral reefs in the SCS, the histories of Holocene climate/environment changes, and the long-term models of coral reef responses to various past environmental changes.
Fishery activities are an important source of microplastic pollution in coastal areas but have received little attention. The Beibu Gulf, a traditional fishing ground of China and the China-Indo ...Peninsula, was selected in this study, and the focus was on the impacts of fishery activities on the horizontal distribution of microplastics in sediment. The results showed that the dominant contaminants (polypropylene fibers and polyethylene fibers) might originate from the abrasion of fishing gear and contributed to 61.6% of the total abundance of microplastics in surface sediment. The abundance of polypropylene fibers and polyethylene fibers exhibited a strong correlation (R 2 = 0.8586, p = 0.015) with values of fishery yields of different districts, which highlighted the effects of different fishery activities on microplastic contamination in marine sediment. Microplastics could be “hidden” in deep sediment to a depth of 60 cm. The estimated storage of microplastics in deep sediment (185 tons) was 5 times that in surface sediment. The assessment of microplastic storage worldwide might be underestimated because most previous studies only examined surface sediment. The abundance distribution and size distribution of microplastics in the sediment core suggested long-term burial of microplastics in deep sediment. Bioturbation might be responsible for the vertical transport of microplastics, leading to “fresh microplastics” preservation in “old sediment”.
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•Ten OPEs were detected in organisms of a coral reef foodweb in the South China Sea.•Composition profiles of OPEs were species-specific among different organisms.•Biomagnification of ...TPTP in the pelagic foodweb was found for the first time.•A significantly linear correlation was found between TMFs and latitudes.
Environmental exposure to organophosphate esters (OPEs) continues to be a concern. Little is known about their bioaccumulation and trophodynamics, especially in tropical food webs. This study collected seawater and fifteen types of organism from a tropical ecosystem, South China Sea, to investigate the species-specific compositional, bioaccumulation, and trophic transfer of OPEs. The total concentrations of 11 target OPEs (ng/g dw) in the organisms decreased with the increase of their trophic levels in the order: phytoplankton (922) > zooplankton (660) > oysters (309) > crabs (225) > coral tissues (202) > fishes (58.2). The composition profiles (relative abundances) of OPEs were different among the species of organisms, which is likely affected by metabolism and the physicochemical property of OPEs. The trophic biomagnification of tripentyl phosphate (TPTP) in the pelagic food web was unexpected and requires further investigation. The trophic magnification factors (TMFs) of OPEs were generally lower in this tropical aquatic food web than in temperate and frigid aquatic food web. Our analysis suggests that there is a significant positive linear correlation between latitude and TMF. Intakes of OPEs through the consumption of the seafood involved in this work does not pose health risk to adults.
Thick Cenozoic dolostone successions found on many isolated carbonate islands have commonly been attributed to “dolomitization events”, whereby time-restricted dolomitization processes repeatedly ...affected the original limestone successions. Any model invoked to explain dolomitization of these successions depends on accurate dating of the “events” because that may allow correlation with other processes such as sea level oscillations or climate change. The 87Sr/86Sr ratios available from island dolostones throughout the world, which have been used to date dolomitization under the premise that seawater mediated its formation, indicate that most of these island dolostones developed between the late Miocene and late Pleistocene (~10 to <0.5 Ma). Analysis of available 87Sr/86Sr ratios (n = 348) shows that (semi-)continuous “time-transgressive” dolomitization processes linked to tectonic subsidence and sea level changes underpinned the development of island dolostones. Islands that experienced rapid subsidence generally have much thicker dolostone successions than those islands that experienced minimal subsidence and/or uplift. It is readily apparent that the tectonic history of an island must be factored into any model developed to explain the evolution of the dolostones. Further refinement of the “time-transgressive dolomitization” genesis model of island dolostones will require more 87Sr/86Sr ratios from the thick dolostone successions found on islands that experienced substantial tectonic subsidence.
The strontium‐to‐calcium (Sr/Ca) ratio of coral is widely used as a paleothermometer for tropical sea surface temperature (SST) at subannual resolutions. The fidelity of Sr/Ca‐derived SST estimates ...largely depends on the properties of the monthly age model selected. Here, we develop a revised monthly age model for coral Sr/Ca estimation from the northern South China Sea based on specific tie points and evaluate the impacts of different age models on SST reconstruction. Our results indicate that our revised monthly age model reduces conversion errors by up to nearly 20%–30% relative to previously reported values, thereby improving the precision of seasonal SST estimates and enabling better paleoclimate interpretations, such as El Niño–Southern Oscillation variability. The revised monthly age model provides a generalized approach for developing more robust seasonal resolution Sr/Ca–SST reconstructions in other regions.
Key Points
We develop a revised monthly age model for coral Sr/Ca‐derived sea surface temperature estimates
The new age model reduces errors by up to 20%–30% relative to previously reported
The new age model improves the accuracy of the South China Sea climate and El Niño–Southern Oscillation reconstructions
As global warming threatens their existence, there are widespread concerns regarding the persistence of lime-sand islands and the future of mid-ocean atoll nations. To investigate how climate change ...has affected lime-sand islands, changes in vegetation and other characteristics of such islands in the South China Sea (SCS) were investigated from 1989 to 2019 using 67 satellite images. First, boundaries of the lime-sand islands and their vegetation were extracted using an active contour extraction procedure called the gradient vector flow snake model. Afterwards, the spatial extents were estimated by enclosing the extracted boundaries, and the digital shoreline analysis system was used to calculate beach widths. Finally, area growth rates and speeds were used to evaluate areal changes in the lime-sand islands and their respective vegetation. Based on the estimated area growth rates, area growth speeds, and beach widths, the lime-sand islands in SCS eroded over the past three decades whereas their vegetation expanded. Further analysis suggested that direct inundation caused by sea-level rise might not be clearly identified from the satellite images. However, other climate change-related factors were most likely responsible for the observed island erosions. These factors included higher wave energy, stronger typhoon intensity and destructiveness, and accelerated coral reef degradation. In addition, the observed expansion of vegetation on the lime-sand islands was likely due to the increase in precipitation in a warming world. The results show that 1) the lime-sand islands will continue to erode whereas vegetation will continue to expand; 2) As vegetation growth is significantly inhibited by salt water when it is adjacent to the ocean, vegetation areas on lime-sand islands may start to continuously decline. Overall, this study is the first to quantitatively examine changes in SCS lime-sand islands due to global warming.
Weizhou Island, located in the northwestern South China Sea (SCS), is the largest volcanic island in China. Its eruption represents the most recent volcanic activity in and around the SCS. We ...determined whole-rock major- and trace-element contents and Hf–Sr–Nd–Pb isotope compositions of Quaternary basalts from Weizhou Island to provide insights into the nature of their mantle sources and formation processes. These basalts have SiO2 contents of 48.21 to 50.04 wt% and belong to the alkaline series. The basalts bear the signature of typical ocean-island basalt, being characterized by enrichments in large-ion lithophile and high-field-strength elements, clear differentiation of light rare-earth elements (REEs) from heavy REEs ((La/Yb)N = 15.19–19.29, mean of 16.82), and show no obvious Eu anomalies (Eu/Eu* = 0.98–1.17, mean of 1.04). Hf–Sr–Nd–Pb isotope compositions show that the mantle source of these basalts can be regarded as a mixture of a depleted MORB mantle source and enriched mantle 2 (EM2), with geochemical data supporting an origin of the EM2 end-member from the Hainan mantle plume. Combining the new results with previous findings, we propose that Cenozoic intraplate volcanism in the SCS and surrounding areas is related to the magmatic activity of a large igneous province and that this province is associated with the Hainan mantle plume.
•The basalts have OIB-like composition and Dupal Pb anomaly.•The basalts have isotopic compositions defining a DMM-EM2 array.•The EM2 end-member may be derived from the Hainan mantle plume.•There may be a Cenozoic large igneous province in the SCS.
•Random Forest algorithm and a stepwise intercalibration approach were combined.•Multi-filter and bathymetric images were composited to improve intercalibration.•Noise impact on the intercalibration ...was greatly eliminated.•Consistent time series night-time light data specialized for large-scale oceanic area was build.•Total light index was implied as an effective indicator of ocean fishery activities.
Human activities in the ocean have never been chronically and continuously investigated on a large scale. Night-time light (NTL) images collected by the Defense Meteorological Satellite Program/Operational Linescan System (DMSP/OLS) have been used as a proxy for monitoring the distribution and intensity of some human activities in the ocean from 1992 to 2013. However, systematic radiometric biases exist among the average visible-light DMSP/OLS NTL images (DMSPavg) derived from different satellites. Moreover, the high randomness of fishing vessel locations and the large amount of noise impede the intercalibration of DMSPavg. To address these issues, this study has developed a method for generating a series of consistent NTL images from 1992 to 2013 for a large-scale oceanic area. A composite image was first constructed by combining the original DMSPavg, median, and standard deviation filter images derived from the DMSPavg, and a bathymetry image. Thereafter, Random Forest (RF) algorithm was employed to classify the composite image into effective and noisy pixels. Finally, a stepwise intercalibration method was adopted to reduce the systematic radiometric biases in the denoised images. The experimental results showed that RF had an overall accuracy of 96% and a Kappa coefficient of 0.775. Furthermore, the intercalibration was shown to significantly reduce the systematic radiometric biases owing to the noises being effectively discarded by the RF. Specifically, the Sum Normalized Different Index (SNDI) of the images intercalibrated by the proposed method can reach 0.61, which is 68.2% less than that of the original DMSPavg. In addition, the correlation coefficients between the intercalibrated DMSPavg and fishery catches in the exclusive economic zones (EEZs) of Japan and Malaysia can reach 0.949 and 0.901, respectively, which are higher than other values, such as the one intercalibrated using the Pseudo-Invariant Features (PIFs) method. In summary, the proposed method has been proven to be effective and feasible for generating consistent time-series NTL data for a large-scale oceanic area, and the derived Total Light Index (TLI) is an effective indicator of ocean fishery activities for ocean ecosystem research and related applications.
Coral skeleton δ13C is a routinely measured indicator in modern insolation change research, but the knowledge of environmental and climatic signals recorded in its seasonality during geological time ...is sparse. In this study, we present eight Porites coral δ13C records from the mid‐late Holocene to the present from the northern South China Sea (NSCS). Compared with the main control factors for modern δ13C changes, coral δ13C seasonality in the NSCS since the mid‐Holocene shows a long‐term decreasing trend, which is consistent with the change trend as orbital‐induced (precession) insolation seasonal amplitude. By excluding other influencing factors, we speculate that the stronger coral δ13C seasonality (18.8%) over the mid‐Holocene than modern period is attributable to the metabolic effect, which predicts the stronger coral δ13C seasonality under stronger insolation seasonality. Our study has implications for coral δ13C seasonality as a potential indicator to record past insolation information under different climatic backgrounds.
Plain Language Summary
Numerous studies have documented the seasonal features of fast‐growing modern coral skeleton δ13C in response to insolation variation. The environmental and climatic signals recorded by coral δ13C seasonality in the geological period remain unclear owing to the relatively short length of observation records. Here we present monthly resolved δ13C records in seven fossil Porites corals (5.6–3.6 ka BP, before 1950 CE) and one modern Porites coral (1987–2001 CE) from the northern South China Sea (NSCS). Compared with the widely accepted prevailing control factors for modern coral δ13C, our results indicated that the stronger coral δ13C seasonality (18.8%) over the mid‐Holocene compared to the modern period is consistent with the orbital‐induced (precession) insolation changes, which indicate a declining seasonality from the mid‐Holocene to present. By excluding other influencing factors, we infer that a tendency toward higher δ13C seasonality during the mid‐Holocene was primarily driven by the metabolic fractionation caused by the enhanced insolation seasonality. This study highlighted coral δ13C seasonality as a potential indicator for recording past insolation information.
Key Points
The seasonal variation of modern coral skeleton δ13C in the northern South China Sea (NSCS) is primarily controlled by solar insolation
Coral δ13C seasonality in the NSCS since the mid‐Holocene shows a long‐term decreasing trend
The decrease of orbital‐induced (precession) insolation seasonal amplitude led to the declining trend of δ13C seasonality
To date, the decadal variability of the Global Monsoon (GM) has been mainly studied using instrumental data collected over the last 50 years, and further discussion has been hampered by the limited ...data length. Here, we present a coral δ18O record from the northern South China Sea, where the hydrology is related to the seasonal Intertropical Convergence Zone (ITCZ) migration, and we use this record to reconstruct the decadal migration of the ITCZ since 1851 A.D. Combining our record with a synthesis of monsoon records reveals an anti‐phase inter‐hemispheric variability of the GM over the last 150 years, indicating that the decadal‐scale hydrologic variability in the tropics is modulated by the meridional migration of the ITCZ. The plausible relationship observed between the decadal variability of the GM and the Atlantic Multidecadal Oscillation suggests that the decadal variability of tropical hydrological changes is likely linked to the climate perturbations in the North Atlantic.
Plain Language Summary
The prediction of monsoon variability in the coming decades will be of great importance for policy‐making and socio‐economic development. The Global Monsoon (GM) is the global‐scale seasonal atmospheric overturning associated with the migration of the Intertropical Convergence Zone (ITCZ). The variability and dynamics of the GM on decadal timescales are still unclear due to the limited length of the instrumental data (< 50 years). Here, we integrated a synthesis of the monsoon and the ITCZ records, including published records and our new coral record, and found that the GM varied in an anti‐phase inter‐hemispheric mode on decadal timescales associated with the ITCZ migration over the past 150 years, suggesting that the simultaneous intensification of the monsoons in both hemispheres since the ~1980s was unusual in a long‐term context. The linkage between the decadal variability of GM and the Atlantic Multidecadal Oscillation/Variability indicated that the North Atlantic has been driving the anti‐phase inter‐hemispheric variability of the GM on decadal timescales.
Key Points
The coral δ18O record from the northern South China Sea captures the migration of the regional ITCZ on decadal timescales
The Global Monsoon varied in an anti‐phase inter‐hemispheric mode on decadal timescales associated with the ITCZ migration since 1851 A.D
The decadal variability of the Global Monsoon was found to be linked to the Atlantic Multidecadal Oscillation
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