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Carbon and oxygen isotope stratigraphy of Late Pleistocene carbonate deposits at Shidao Island, Xisha Islands, China
... Previous studies of the Xisha carbonate platforms have been limited to analysis of shallow boreholes and examination of modern carbonate deposits on the islands (He and Zhang, 1986;Zhao et al., 2011). However, in the course of rapidly advancing deep-water hydrocarbon exploration, both commercial and academic seismic surveys have been conducted in deep-water settings of the northern South China Sea. ...
Newly acquired seismic data allow improved understanding of the architecture and evolution of isolated carbonate platforms on the continental slope of the northern South China Sea. The Xisha carbonate platforms initiated on a basement high, the Xisha Uplift, in the early Miocene and have remained active to the present. Their distribution is limited to pre-existing localized, fault-bounded blocks within the Xisha Uplift so individual platforms were small in size at the beginning of the Miocene. However, during the middle Miocene, the platform carbonate factories flourished across an extensive area with 55,900 km2. The platforms began to backstep in response to a relative sea-level rise in the late Miocene. Platform-edge reefs, patch reefs, pinnacle reefs, atoll reefs and horseshoe reefs, all developed on various platforms. The distribution of platform carbonates shrank significantly during Pliocene-Quaternary time to isolated carbonate platforms, represented today by Xuande Atoll and Yongle Atoll. Tectonics and eustasy were the two main controls on platform development. Tectonics controlled both the initial topography for reef growth and the distribution of platforms, including those that survived the drowning event associated with the late Miocene rapid relative sea-level rise. Eustasy controlled high-frequency carbonate sequence development.
... Various dating methods have been applied to the aeolianites of Shidao Island, such as radiocarbon ( 14 C), Electron Spin Resonance, uranium series age methods, and oxygen isotopes (Chen, 1978;Lu et al., 1979;Zhu, 1981;Qin and Zhu, 1982;Chen and Jiao, 1982;Lv et al., 1986;He et al., 1986;Ye et al., 1987;He et al., 1988;Ye et al., 1990) (Fig.10). However, most of the ages are questionable because they are influenced easily by late diagenesis. ...
The aeolianite deposits on Shidao Island of the Xisha Islands, the South China Sea, contain five stages of aeolian biocalcarenites
and four paleosols. The aeolian biocalcarenites consist of two sedimentary facies: dune and interdune deposits. In the dunes,
large-scale festoon cross-bedding, humped cross-bedding and high-angle foreset bedding are well developed, and in the interdunes,
large-scale flat-bedding and low-angle wedge shaped cross-bedding are well developed. The sedimentary structures and lamella
features indicate that the aeolian deposits are driven mainly by the northeast monsoon. The aeolian biocalcarenite and paleosols
may reflect the arid and humid climates of the East Asian monsoon, respectively. By comparison with the stalagmite oxygen
isotope climosequence of Hulu Cave, Nanjing, we inferred that the aeolianite formed in the last glacial stage, and the paleosols
were formed during relatively long-term warm events.
Understanding the history of the response of coral reefs to past climate changes can provide valuable information for predicting the future response of modern reefs. However, dating such ancient biotic carbonate is still challenging because of its sensitivity to diagenetic alteration processes, scarcity of well-preserved fossils, and low magnetic mineral content. There have been a long debates about the origin and evolutionary history of coral reefs in the northern South China Sea, mainly due to the lack of direct and reliable age constraints. This provides us with a good opportunity to verify the practicability of different dating approaches, especially the strontium (Sr) isotope analysis of bulk carbonate. Here, we retrieved a 972.55-m-long core from the Xisha Islands to provide a credible chronologic constraint on the carbonate platform evolution. The lithostratigraphy, strontium isotope stratigraphy, and magnetostratigraphy were analyzed throughout the whole reef sequence. The lithostratigraphic results show that the 873.55 m reef sequence developed on an ancient volcaniclastic basement and experienced multiple evolutionary phases. The 87Sr/86Sr results of all 100 bulk carbonate samples vary from 0.708506 to 0.709168 and show a monotonic increase with decreasing depth, except for a few outliers. Trace-element criteria and stable isotope (δ18O and δ13C) methods were applied to these bulk carbonate samples, and results imply that the primary or near-primary seawater 87Sr/86Sr values were likely preserved, although different degrees of diagenetic alteration occurred. In addition, the paleomagnetic results indicate 10 normal polarity and eight reversed polarity magnetozones. Based on the 87Sr/86Sr ratios of the selected 58 samples and paleomagnetic reconstruction of polarity reversals, the bottom of the reef sequence is dated to 19.6 Ma, and the observed polarity chronozones extend from chron C6 (19.722−18.748 Ma) at 866.60 m to present at the top. Based on the new data, we propose a new chronologic framework for the evolutionary history of the reef islands, where: (1) the reefs initiated in the early Miocene (19.6 Ma) and were drowned until 16.26 Ma; (2) during 16.26−10.66 Ma, lagoon to lagoon slope environments prevailed; (3) the lagoon environment progressively transformed into a reef crest environment from 10.66 to 4.36 Ma and 4.36 to 1.59 Ma; and (4) the reef started to be drowned again during 1.59−0 Ma. Our study provides a new and reliable chronologic constraint on the general evolutionary history of the reef islands in the northern South China Sea. Furthermore, the 87Sr/86Sr results from bulk carbonate indicate that strontium isotope stratigraphy is a powerful dating tool only when rigid sample selection, sequential leaching procedures, and strict trace-element and isotopic criteria are applied.
In order to explore the developmental process of carbonate platform in the South China Sea (SCS) and its control factors, the organic molecular compound index method were described in this study. The branched isoprenoid tetraether (BIT) index is the ratio between the branched glycerol dialkyl glycerol tetraethers (bGDGTs) from terrigenous supply and isoprenoid GDGTs (iGDGTs) from marine crenarchaeol comprising the majority of denominator, which serves as a proxy for provenance discrimination of the organic matter and related sedimentary environments in paleoenvironmental research. The BIT curve of well Xike-1 is characterized by a unique “high-low-high” feature up section, indicating the effects of sea-level variations and carbonate platform development since Middle Miocene in the SCS. Strata with reef-beach facies were developed within carbonate platforms of Xisha Islands influenced by decrease in global as well as regional sea-level during the Late Miocene, with increased BIT values due to plenty of precipitation. Between Late Miocene and Pliocene when sea-level was higher, carbonate platforms were better formed in shallow marine environment with low BIT values. Carbonate platforms of Xisha Islands were periodically exposed in Quaternary as a result of sea-level decline over glacial cycles, leading to the second increase in BIT. Our results demonstrate that carbonate platforms of Xisha Islands have been affected by regional sea-level variations, reflecting a coupling effect on the SCS by global sea-level fluctuations and regional tectonic subductions.
The oxygen stable isotopes in carbonate sediment of well Xike-1 cannot be used to calibrate strata ages due to the destructive effect of recrystallization and dolomitization, while the paired δ13C records can be used to calibrate Xike-1 strata age since 200 ka, because of the similar patterns with those of the South China Sea and other oceans. Accordingly, the 0-50 m sediment of the well corresponds to the oxygen isotope stages (MIS) 1-7, with the MIS 1/2 boundary (14 ka) at 5 m, the MIS 2/3 boundary (29 ka) at 11.70 m, the MIS 3/4 boundary (57 ka) at 13.90 m, the MIS 4/5 boundary (71 ka) at 16.8 m, the MIS 5/6 boundary (129 ka) at 23.8 m and the MIS 6/7 boundary at 35.65 m respectively. Moreover, the δ13C records exhibit low values in glacial periods and high values in interglacial periods. The U-Th dating of coral from 25.21 m is 131.062±2.320 ka. It is concluded from the dating that there are no sediments younger than 5 ka existing on the Shidao Island of Xisha, and the transition from the interglacial to glacial periods with downward movement of global sea-level lead to the exposition and erosion of the local carbonate platform, which indicates that the abrupt changes of the δ13C records of the carbonate sediments from Xike-1 are responses to global climate variations.
A recent island survey reveals that the Xuande Atoll and the Yongle Atoll in the Xisha Islands can be classified into one of two systems: the depleted atoll system and growth atoll system; the survey also indicates that the decreased area of several shoals is an unbearable burden for the Xisha Islands, of which the largest island area is 2.13 km2 and the minimum elevation is 1.4 m. According to a survey on the ecological characteristics of Halimeda in the Laolongtou breaker zone of Shidao Island in the Xisha Islands, the green and white living Halimeda are collected, the isotopic ages of 14C contained in the Halimeda are shown to be 27 years and 55 years, respectively, and carbonate mainly occurs in five types, i.e., luster, segment, sand, sand grain, and marl in the formation. The Halimeda segments mainly provide the carbonate sediments of long-term biogenic deposits in the reef environment and the annual productivity per area is 60-100 g/m2; the characteristics of the microstructure of the Halimeda are analyzed, the aragonite raphide carbonate is deposited and enriched in the cortexes, medullas and cysts, and the Halimeda generally contain major elements such as C, O, Ca, Cl, Mg, K, Na, S and Al, and are rich in trace elements such as tellurium (Te), rhodium (Rh) and strontium. It is believed that the Halimeda grow slowly, including the biotic community of reef corals in the reef areas, thus they possess an environmental remediation capacity, but it takes much time to remedy the environment, and it is necessary to make the law to protect the diversity and vulnerability of the Xisha marine ecology, the ecology of the reef community and the island environment in a scientific way. As indicated in the survey, under the background of global warming and sea-level rise, the discovery of large amounts of Halimeda in the Laolongtou sea area is significant for the natural increase of the depleted atoll system of the Xuande Atoll, while the Halimeda segments represent the primary form of the fossil Halimeda, of which the species can be identified and preserved in great numbers under geological conditions. The Miocene was discovered in large amounts in the Xichen-1 well, therefore the study on the characteristics and mechanism of Halimeda carbonate sediments plays a pivotal role in the formation and construction of organic reefs in the South China Sea as well as oil and gas exploration.
The Xisha carbonate platforms, which include the modern-day Xisha Atoll, occur upon the northern continental margin of the South China Sea (SCS). In this study, we identify the seismic characteristics about various sedimentary facies of the carbonate platform and different types of reef in this area, based on the 2-D seismic data, reefs show high-amplitue moundy continuous reflection at the top and weak chaotic reflections inside. Carbonate platforms show high-amplitude continuous reflection at the top and high reflection alternative with weak reflection. The above analysis provided information about the influence of fluctuations in relative sea level, tectonic movements, paleotopography and the development mechanism of the carbonate platform. Based on the seismic data and data from drillholes (Xichen-1, Xiyong-1), we propose a schematic sedimentary model of the Xisha carbonate platforms at the northern margin of the SCS and outline five stages of development for the carbonate platform. Its sedimentary evolution consists of Initial establishment stage, development stage, exposure stage, drowning stage and large atoll reefs development stage. We also propose that several phases tectonisms supply proper environment and structural position for carbonate platform development, they can also destroyed the exist platforms. Besides, eustasy change was also the main influential factor on the development of the platforms.
Oxygen and carbon isotopic compositions of diagenesis? coral samples from NY-1 core at the depth of 22 to 50 m, together with
mineral compositions of transitive coral samples of 17 to 18 m at the depth, were meas ured. The data exhibited that when
aragonite changed to calcite, its oxygen and carbon isotopic compositions dropped roughly on a linear trend. The linear trend
implies that the oscillation pattern of the origin oxygen and carbon isotopic ratios of the aragonite could probably be retained
after they diageneticly changed into calcite. Oxygen isotopic stratigraphy for the NY-1 core at the depth between 22 and 50
m was determined according to the δ8O ratios of the calcite coral. The oxygen stratigraphy provided an age of about 289 ka for 45 m of the NY-1 core, which agreed
with that by paleomagnetic stratigraphy.
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