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Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea

Wanli Chen Xiaoxia Huang Shiguo Wu Gang Liu Haotian Wei Jiaqing Wu

Wanli Chen, Xiaoxia Huang, Shiguo Wu, Gang Liu, Haotian Wei, Jiaqing Wu. Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1713-6
Citation: Wanli Chen, Xiaoxia Huang, Shiguo Wu, Gang Liu, Haotian Wei, Jiaqing Wu. Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1713-6

doi: 10.1007/s13131-021-1713-6

Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea

Funds: The National Natural Science Foundation of China-Guangdong joint Foundation under contract No. U1701245; the Hainan Provincial Natural Science Foundation of China under contract No. 418QN306; the Land and Georesource Bureau of Hainan Province under contract No. SQ2016KJHZ0027; the Pioneer Hundred Talents Program under contract No. Y910091001; the Guangzhou Marine Geological Survey Project under contract No. GZH201400210.
More Information
  • Figure  1.  Location map of the Xisha Islands in the northern part of the South China Sea (a) and multibeam bathymetry map of the Xisha Islands (b). The bathymetry map in a has been generated by GMT software, and the data source is from https://www.ngdc.noaa.gov/mgg/bathymetry. In b, the black dot marks the Yongxing Island, and the SSZK1 borehole was drilled on the south of the Yongxing Island, Xuande Atoll.

    Figure  2.  Cross-section through the Xisha Platform showing the distribution of limestones and dolostones (a, adapted from Wang et al. (2018)), and integrated stratigraphic succession of Neogene rocks on the Xisha Islands showing the distribution of dolostones and limestones as well as diagenetic zones (b, adapted from Li et al. (2018). The orange line in Fig. 1 indicates the location of the cross-section in a. In b, the stratigraphic reflectors (T20, T30, T40, T50, T60) were interpreted on seismic profiles shown by Wu et al. (2014).

    Figure  3.  Integrated data of late Quaternary strata of the Core SSZK1: (1) Chronostratigraphy based on ages come from U-Th dating results of the Well SSZK1 which propose the uppermost aeolianite facies formed after MIS 5; (2) facies; (3) variations in abundance of coral, red algae, green algae, benthic foraminifera and planktonic foraminifera; (4) vertical variations of carbonate and oxygen isotope; (5) correlation coefficients of carbonate and oxygen isotope vary in shallow-facies-dominated and deep-facies-dominated interlayer; and (6) mineralogical compositions of samples are concluded from analytical result of six samples and from the previous work in the nearby wells XK-1 and XC-1 in the Xisha Archipelago (Liu et al., 1997, 2019). ARAG-aragonite, HMC- high-Mg calcite, LMC- low-Mg calcite.

    Figure  4.  Coral reef facies. a. Core photo of coral framestone. 1. Favia, 22.56–22.64 m; b. Plane-polarized photomicrograph of coral framestone, 23.70 m; and c. Plane-polarized photomicrograph of coral-debris dominated packstone, formed in the interstices between coral colonies. 1. Coral fragments, 2. red algal fragments, 3. Amphistegina lobifera, and 4. micritic envelopes, 34.15 m.

    Figure  5.  Coral-algal reef facies. a. Plane-polarized photomicrograph of algae encrusting coral. 1. Porites, 2. Lithophyllum sp, 29.90 m; and b. Plane-polarized photomicrograph of algae encrusting coral. 1. Coral, 2. Lithophyllum sp, and 3. Echinoid, 47.40 m.

    Figure  6.  Reef cap facies. a. Core photo of iron oxide-stained coral framestone; 1. Karst cave, 27.40–27.57 m. b. Plane-polarized photomicrograph of coral framestone showing intense leaching of bioclasts, at least leached coral fragments are distinguished from their cortical coatings, formed by original intergranular porosity occlusion; 1. Porites, and 2. coral moldic pores, 28.93 m. c. Plane-polarized photomicrograph of coral framestone with coral moldic pores, 37.05 m.

    Figure  7.  Inner bank facies, outer bank facies and aeolinite facies. a. Plane-polarized photomicrograph of grainstone in inner bank facies, exhibiting subangular and poorly-sored bioclasts; 1. Amphistegina lessonii, 2. Globorotalia truncatulinoides, and 3. Halimeda, 23.98 m. b. Plane-polarized photomicrograph of packstone in inner bank facies, exhibiting subangular and poorly-sored bioclasts; 1. Cibicidoides subhaidingerii (Parr) and 2. red algae, 32.70 m. c. Cross-polarized photomicrograph of packstone in inner facies, showing the occurrence of quartz, 46.35 m. d. Plane-polarized photomicrograph of grainstone in outer bank facies, showing planktonic foraminifera-rich bioclastic fragments; 1. planktonic foraminifera, 20.20 m.

    Figure  8.  Plane-polarized photomicrographs of products of meteoric diagenesis. a. Neomorphosed coral skeleton showing sparry calcite enclosed by microcrystalline calcite, 34.55 m. b. Micrite envelopes(yellow arrow), 33.20 m. c. Dissolved bioclasts, leaving molds with micrite envelope (yellow arrow), 42.5 m. d. Grains are connected by meniscus calcite cements causing round pore (yellow arrow), 32.9 m. e. Isopachous fibrous calcite fringe around grains; dissolution also occur on these early generated fibrous cements (yellow arrow), 25.14 m. f. Isopachous dogtooth calcite cements (yellow arrow), surrounding bioclasts, 47 m.

    Figure  9.  Cluster diagram for variables using correlation coefficient. The coefficients larger than 0.5 among these major elements means a link among the major components.

    Figure  10.  Vertical profile of facies and major elements variations, for symbols see legend in Fig. 3. The different colors of line titles mean different clusters of major elements.

    Figure  11.  Continental crust normalized major element composition (average value) for the reef facies-association, inner facies and aeolianite facies. Normalized values are from Taylor and McLennan (1985).

    Figure  12.  Dominant diagenetic environment at the uppermost 60 m depth intervals of the SSZK1 (a), XK-1 (b) and XC-1 (c) cores based on stable isotopic and mineralogical data. The previous work on XK-1 core is from Liu et al. (2019) and XC-1core is from Liu et al. (1997). HMC: high-Mg calcite. LMC: low-Mg calcite.

    Figure  13.  The gradual decrease of S, NaO and Sr content associated with stable isotopic values in different stage, for symbols see legend in Fig. 3. The dash line marks the ladder reduction of the average S, NaO and Sr concentration in different depth intervals.

    A1.  X-ray diffraction pattern of the samples. Ar means aragonite; LH means low-Mg and high-Mg calcite; and L means low-Mg calcite.

    Table  1.   Results of X-ray diffraction (XRD) analyses in samples

    Depth/mAragonite/%High-Mg calcite/%Low-Mg calcite/%
    2.0054.723.521.8
    18.3976.4023.6
    27.8043.6056.4
    31.2000100.0
    43.8000100.0
    53.6500100.0
    下载: 导出CSV

    Table  2.   Descriptive statistics of major elements analysis results in reef facies-association, inner bank facies and aeolianite facies in comparison with the continental crust

    Al2O3CaOFe2O3MgOK2OSiO2Na2OTiO2MnOP2O5S
    Reef facies-associationAverage0.01954.8500.0160.5220.0090.3550.1510.0020.0030.0440.224
    Max0.14455.5610.1020.9800.0342.1150.5120.0060.0050.1830.006
    Min0.00153.7950.0030.2270.0040.1160.0310.0010.0010.0080.053
    Inner bank faciesAverage0.05054.1640.0390.5610.0191.1830.1870.0030.0030.0630.342
    Max0.30055.3800.1660.8890.0949.7360.6670.0080.0050.1830.014
    Min0.01049.0540.0090.2980.0070.1390.0750.0010.0010.0080.063
    Aeolianite faciesAverage0.04651.7980.0391.9730.0150.2580.4940.0020.0030.1090.340
    Max0.40052.8970.2192.6340.0571.7640.6330.0130.0061.7700.165
    Min0.01649.9010.0090.7250.0100.0810.3630.0010.0020.0350.266
    Continental crust15.0405.3906.1703.6702.58061.7103.1800.6700.0900.170
    下载: 导出CSV

    Table  3.   The correlations of main elements in SSZK1 carbonate profile

    Al2O3CaOFe2O3MgOK2OSiO2Na2OTiO2MnOP2O5SSr
    Al2O31
    CaO–0.341
    Fe2O30.57–0.431
    MgO0.08–0.880.171
    K2O0.79–0.480.680.091
    SiO20.52–0.210.530.150.821
    Na2O0.15–0.880.260.750.26–0.071
    TiO20.90–0.240.520.020.760.500.061
    MnO0.15–0.210.300.230.230.140.090.131
    P2O50.63–0.300.220.190.420.160.160.580.021
    S0.07–0.870.200.820.14–0.190.98–0.010.090.111
    Sr0.09–0.750.210.610.15–0.170.930.02–0.020.100.921
    Note: Correlation coefficients larger than 0.5 (P<0.01, n=138) are marked by bold numbers.
    下载: 导出CSV

    Table  4.   230Th dating results for corals from core SSZK1, Yongxing Island

    Depth/m[U]/ppm234U/238U234U/238U (initial)230Th/238U230Th/232ThAge (a BP)Error (2σ)
    7.210.8161.1471.1490.047 359.44 239246
    13.472.4501.1411.1440.082 17 090.38 114190
    18.392.4901.1261.1530.53901 178.869 795274
    24.302.9461.1211.1550.63002 886.688 094349
    30.111.0281.1101.2141.0100709.8236 5242 131
    42.520.8911.1041.2601.0830710.0329 1845 601
    44.891.2791.0971.2941.10201 456.1392 3449 658
    55.921.0001.0871.3371.10701 227.7479 63619 388
    Note: The unreliable dating results are marked by bold numbers.
    下载: 导出CSV

    A1.   Detection limit of the analytical method

    ElementAnalytical methodDetection limit
    CaOXRF0.032 2 (10–2)
    MgOXRF0.003 (10–2)
    Al2O3XRF0.002 7 (10–2)
    Fe2O3XRF0.002 7 (10–2)
    Na2OXRF0.040 2 (10–2)
    SiO2XRF0.004 (10–2)
    K2OXRF0.002 2 (10–2)
    P2O5XRF0.000 9 (10–2)
    MnOXRF0.002 3 (10–4)
    TiO2XRF0.001 (10–2)
    SXRF0.000 6 (10–2)
    SrICP-MS1.0000 (10–6)
    下载: 导出CSV
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Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea

doi: 10.1007/s13131-021-1713-6
    基金项目:  The National Natural Science Foundation of China-Guangdong joint Foundation under contract No. U1701245; the Hainan Provincial Natural Science Foundation of China under contract No. 418QN306; the Land and Georesource Bureau of Hainan Province under contract No. SQ2016KJHZ0027; the Pioneer Hundred Talents Program under contract No. Y910091001; the Guangzhou Marine Geological Survey Project under contract No. GZH201400210.
    通讯作者: E-mail: huangxx@idsse.ac.cnswu@idsse.ac.cn

English Abstract

Wanli Chen, Xiaoxia Huang, Shiguo Wu, Gang Liu, Haotian Wei, Jiaqing Wu. Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1713-6
Citation: Wanli Chen, Xiaoxia Huang, Shiguo Wu, Gang Liu, Haotian Wei, Jiaqing Wu. Facies character and geochemical signature in the late Quaternary meteoric diagenetic carbonate succession, Xisha Islands, South China Sea[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-021-1713-6
    • The studies of the Quaternary shallow-water carbonates in meteoric diagenetic environments, generally with karstic features, have provided insights into sequence stratigraphy, paleoenvironmental interpretation and the prediction of hydrocarbon reservoirs (Allan and Matthews, 1982; Hajikazemi et al., 2010; Holail, 1999; Jian et al., 1997; Melim et al., 2002; Quinn, 1991; Sherman et al., 1999). A meteoric diagenetic model of Pleistocene shallow-water carbonates, called as the “Barbados model”, has been established to describe the isotope patterns associated with thorough meteoric diagenesis of carbonates in which aragonite was completely converted to low-Mg calcite (Allan and Matthews, 1982). The isotope patterns summarized by the Barbados model are useful for identifying different diagenetic environments. The coexistence of unstable aragonite and high-Mg calcite (HMC), as well as stable low-Mg calcite (LMC), occurs in the Barbados boreholes, showing the signature of incomplete meteoric diagenesis and mineralogical stabilization. Research on stable carbon and oxygen isotopes, together with mineralogy of suites of rocks, has been considered as the most effective way to decipher the freshwater diagenesis of carbonates for over 50 years (Allan and Matthews, 1982; Melim et al., 2002). Isotopic data from bulk rock has proven rather effective for studying meteoric diagenesis in shallow-water carbonates (Dickson, 1985; Liu et al., 1997; Melim, 1996; Melim et al., 2002; Swart, 2015). The geochemical behavior of shallow-water carbonates reflect changes in the environmental conditions (Dang et al., 2019; Jiang et al., 2019; McGregor and Gagan, 2003; Saller and Moore, 1991; Zhao et al., 2019). Meteoric alteration of the carbonate chemistry takes place because the new precipitating phases are in equilibrium with the chemistry of the ambient meteoric water, which is commonly depleted in elements such as sodium (Na), strontium (Sr) and sulfate (SO4) as compared to seawater (Budd and Land, 1990; Gill et al., 2008). Meteoric diagenesis has prevented carbonates to be used as paleoproxies for ancient seawater chemistry (Fan et al., 2020; Swart and Eberli, 2005).

      The development and evolution of atolls in the Xisha archipelago, South China Sea, are closely linked to eustatic sea-level fluctuations (Shao et al., 2017a). Meteoric diagenesis generally takes place on the subaerially exposed carbonate platform during the sea-level lowstands. These atolls, with repeated emergence during the sea-level lowstands, are great locations to gain insight into freshwater diagenesis. Previous studies of drilled cores mainly focused on stratigraphy, fossil components, dolomite rock, elemental geochemistry and the carbonate platform evolution of the Xisha Islands (Fan et al., 2020; Ma et al., 2018; Shao et al., 2017b; Wang et al., 2018a, b; Wu et al., 2019; Xu et al., 2019). Only a few publications concentrated on meteoric diagenesis (Liu et al., 1997, 2019). Liu et al. (1997) has made a nice progress on the meteoric diagenesis from the Holocene to Pleistocene carbonate interval (0–179 m) in the Well XC-1. Meteoric and marine diagenesis was studied through a petrographic, cathodoluminescence and geochemical study of the Well XK-1 by Liu et al. (2019). However, the various degrees of meteoric diagenesis are poorly discussed.

      In this study, a recently drilled core (SSZK1) from the Yongxing Island is applied to the investigation of the degree of diagenetic alteration in the Pleistocene shallow-water carbonates. We firstly perform a detailed facies analysis through core observations and laboratory analyses, aiming to uncover water depth change of the environment and establish a basic framework for the reef evolution. Then we explore the relationship between the various degrees of diagenetic alteration and reef evolution. Facies and its relationship with geochemical signature of the shallow-water carbonates are analyzed. This study provides a new record of meteoric diagenesis at the Xisha Islands, and discovers that the different stable isotopic patterns exist between the complete and incomplete meteoric diagenesis. Our study demonstrates that the meteoric diagenesis of the shallow-water carbonates can be well reflected by whole-rock mineralogy and geochemistry.

    • The South China Sea (SCS) is the biggest marginal sea in East Asia, covering an area of about 3 500 000 km2 across over 20° of northern latitude from the equator (Fig. 1a). The Xisha Islands (Archipelago) are located northwest of the SCS (Fig. 1b), comprising more than 40 islets, cays and reefs. Previous studies has utilized seismic profiles to uncover the thick carbonate succession, which is composed of the unconformity-bounded Sanya Formation (Lower Miocene), Meishan Formation (Middle Miocene), Huangliu Formation (Upper Miocene), Yinggehai Formation (Pliocene) and Ledong Formation (Quaternary) (Wu et al., 2014; Ma et al., 2011). Since the early Miocene, the region has entered a tectonic phase of post-rifting, characterized by thermal subsidence (Sun et al., 2015, 2016). The development of the Xisha carbonate platforms was obviously impacted by the interplay of tectonic background, sea level, terrestrial input, antecedent topography and ocean currents (Fan et al., 2020; Jiang et al., 2019; Shao et al., 2017a; Wu et al., 2019, 2020; Xu et al., 2015). Due to the drowning during the late Miocene, some flat-topped carbonate platforms were isolated from the adjacent continent and survived during the sea level rise in the Quaternary (Huang et al., 2020; Wu et al., 2014, 2016). The modern reef flats lie on the resulting carbonate platform, reaching depths of 1 000 m over surrounding deep-water terrane (Fig. 1b). The physical oceanography in the Xisha Archipelago is mainly influenced by the Asian monsoon in the modern time (Hu et al., 2000). The East Asian monsoon wind blows from southwest during the summer and from the northeast during the winter (Hu et al., 2000). The Xisha Islands receive between 1 300 mm and 2 000 mm of rainfall each year, and the annual average surface temperature of the seawater is 22°C to 30°C, with a near-surface salinity of 33.14‰ to 34.24‰ (Shao et al., 2017a).

      Figure 1.  Location map of the Xisha Islands in the northern part of the South China Sea (a) and multibeam bathymetry map of the Xisha Islands (b). The bathymetry map in a has been generated by GMT software, and the data source is from https://www.ngdc.noaa.gov/mgg/bathymetry. In b, the black dot marks the Yongxing Island, and the SSZK1 borehole was drilled on the south of the Yongxing Island, Xuande Atoll.

      A total of six wells were drilled at the Xisha Islands, including the wells XK-1, XY-1, XY-2, XS-1, XC-1 and CK-2. These wells have provided a great opportunity to study the regional paleooceanographic events and the global climatic and ecological changes in the long-term coral reef platform development at the Xisha Islands (Fan et al., 2020; Liu et al., 1997; Jiang et al., 2019; Shao et al., 2017a, 2017b; Shi et al., 2002; Wang et al., 2018a; Wu et al., 2019; Yi et al., 2018). Three of these wells (XY-1, XK-1, CK-2) penetrated the entire Cenozoic carbonate succession (Fig. 2), revealing that the Tertiary carbonates are composed largely of dolostones with minor limestones while the Pleistocene carbonates mostly contain limestones with minor dolomite in the low part (Wang et al., 2018a). Identified fossil components include corals, algae, foraminifera, bryozoans, and echinoderms, and their debris in the carbonate succession (Li et al., 2018; Shao et al., 2017a, b; Wu et al., 2019).

      Figure 2.  Cross-section through the Xisha Platform showing the distribution of limestones and dolostones (a, adapted from Wang et al. (2018)), and integrated stratigraphic succession of Neogene rocks on the Xisha Islands showing the distribution of dolostones and limestones as well as diagenetic zones (b, adapted from Li et al. (2018). The orange line in Fig. 1 indicates the location of the cross-section in a. In b, the stratigraphic reflectors (T20, T30, T40, T50, T60) were interpreted on seismic profiles shown by Wu et al. (2014).

      Previous examinations have shown that the Pleistocene-Holocene limestones drilled by the wells XC-1 and XK-1 are evidently influenced by diagenetic alteration, resulting in variations of mineralogical components in the vertical direction (Liu et al., 1997, 1998, 2019). These studies show that the different diagenetic zones were recognized in the wells XK-1 and XC-1, including meteoric diagenetic zone, marine diagenetic zone and burial diagenetic zone (Fig. 2b). The relationship between geochemical behaviors and mineralogical components of these cores was also summarized in these literatures. For instance, it was documented that the less stable HMC occurs in the uppermost loose aeolianite sediment (Liu et al., 1997, 2019), and HMC is completely consumed in the consolidated reef limestones underlying the loose aeolianite sediments, leading to the significant decrease in Mg content (Liu et al., 1997). The complete precipitation of stable LMC was thought to be marked by rather flat δ13C and δ18O curves and the precipitation of dolomite was considered to result in the elevated δ13C and δ18O values (Liu et al., 1997, 2019). However, due to the low-resolution sampling strategy of these studies, the geochemical characteristics of the shallow-buried meteoric diagenetic carbonates and its relationship with reef evolution are poorly studied.

    • The borehole SSZK1 (55.92 m) was drilled on the Yongxing Island of the SCS (Fig. 1) and it was continuously cored with an average recovery of 70.5%. Eight corals were selected from the core SSZK1 for U/Th dating. Separation of uranium and thorium from the carbonate matrix were completed by using Eichrom UTEVA resin following the published method of Liao et al. (2018). Uranium and thorium isotope ratios were measured on a Neptune Plus Multiple-collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) at Laboratory for uranium chronology analysis, Institute of Geology and Geophysics, Chinese Academy of Sciences. The secular equilibrium standard (HU-1, uranium ore solution) was used to calibrate the measurement. According to the 230Th/232Th and 234U/238U ratios, ages (2σ standard deviation) were calculated using the U-half-lives and Th half-lives published by Cheng et al. (2000). The 37.53 m limestone underlying the aeolianite sediment was studied through a preliminary core observation and thin section microscopic observation. A total of 140 samples (sampling interval of 20–50 cm) were observed for the identification of fossils, skeletal assemblages and sedimentary fabrics. The abundance of coral, red algae, benthic foraminifera and planktonic foraminifera was semi-quantitatively determined on thin-section using polarized light microscopy. Among the fossils, generally, coral and red algae were identified to genus level, and foraminifera to species level. Deposition textured-based classification scheme of carbonates was performed following the classifications of Dunham (1962) and Embry III and Klovan (1971) (Dunham, 1962; Embry III and Klovan, 1971). Sample texture, lithology and taxonomic composition were comprehensively applied to identify facies and explain their depositional environments. Six samples were powderized and relative amounts of carbonate minerals measured by X-ray diffraction (XRD: Rigaku D/max-IIIa X diffractometer) at University of Science and Technology of China in Hefei. The mineralogy of the bulk samples from the nearby cores XC-1 and XK-1 was documented by Liu et al. (1997) and Liu et al. (2019) respectively and mineralogical compositions of samples of the SSZK1 core are consistent with their results.

      A total of 279 samples were collected at 20 cm intervals for stable isotope analyses. The δ18O and δ13C were measured from the bulk rock in the Institute of Geology and Geophysics of the Chinese Academy of Sciences. Analysis of δ13C and δ18O was carried out on a Finnigan MAT-253 stable isotope mass spectrometer that attached to a Fairbanks carbonate preparation device. Powdered samples (approximately 20 mg) were reacted with 100% H3PO4 at 75°C in an automated carbonate device to extract CO2. Isotopic ratios were reported in the per millilitre (%) convention and normalized to the Vienna Pehe Dee Belemnite (VPDB) using the GBW04405 standard (δ13C=0.57‰, δ18O=–8.49‰). Multiple measurements (n=20) of this standard yielded a standard deviation of 0.04% for δ13C, and 0.09% for δ18O. In addition, approximately 20% of all the samples were measured repeatedly and guarantee that the differences between double measurements were within the range of analytical errors.

      Elemental geochemistry analyses of core samples were carried out in the Institute of Marine Geology Survey, Bureau of Hainan Marine Geology Survey. The major element and trace element analyses of bulk samples were performed on 138 samples (approximately at 40 cm intervals) and were determined by an X-ray fluorescence (XRF: ZSX Primus II) and inductively coupled plasma emission spectrometer (ICP-MS: X–SERIES II) respectively. The Chinese National Standard GBW07130 was used to determine the detection limit of the analytical method (Table A1). For consistency, one repeated sample and one blank sample were analyzed for each 20 samples. Every sample was measured more than five times and calibrated with the GB/T 20260.8—2006, GB/T 14506.28—2010 and DZG 93-05 standard. Precision was better than 2%.

    • Based on the differences in lithology, components and microscopic textures, Wu et al. (2019) recognized six facies in the neighbouring core XK-1, including coral reef, coral-algal reef, reef cap, inner bank, outer bank, and aeolianite. These facies types are also discovered in this newly described core (Fig. 3). The graphic log of the measured core is shown as Fig. 3.

      Figure 3.  Integrated data of late Quaternary strata of the Core SSZK1: (1) Chronostratigraphy based on ages come from U-Th dating results of the Well SSZK1 which propose the uppermost aeolianite facies formed after MIS 5; (2) facies; (3) variations in abundance of coral, red algae, green algae, benthic foraminifera and planktonic foraminifera; (4) vertical variations of carbonate and oxygen isotope; (5) correlation coefficients of carbonate and oxygen isotope vary in shallow-facies-dominated and deep-facies-dominated interlayer; and (6) mineralogical compositions of samples are concluded from analytical result of six samples and from the previous work in the nearby wells XK-1 and XC-1 in the Xisha Archipelago (Liu et al., 1997, 2019). ARAG-aragonite, HMC- high-Mg calcite, LMC- low-Mg calcite.

    • The coral facies are commonly observed in the Core SSZK1 profile (Fig. 3). It consists of coral framestone together with minor packstone and wackstone. The coral colonies exhibit massive morphologies, containing Porites, Turbinaria, Favia (Figs 4a and b). Generally, the wackstone and packstone matrix appear in the interstices between coral colonies and the pores within the coral individuals. The wackstone and packstone matrix are mainly composed of bioclastic debris, including abundant coral fragments and frequent to rare benthic foraminifera, red algae, rare green algae, echinoids and bivalves (Fig. 4c). Benthic foraminifera presented here are shallowest-living Amphistegina lobifera of the genus Amphistegina, which indicates a shallow and euphotic environment.

      Figure 4.  Coral reef facies. a. Core photo of coral framestone. 1. Favia, 22.56–22.64 m; b. Plane-polarized photomicrograph of coral framestone, 23.70 m; and c. Plane-polarized photomicrograph of coral-debris dominated packstone, formed in the interstices between coral colonies. 1. Coral fragments, 2. red algal fragments, 3. Amphistegina lobifera, and 4. micritic envelopes, 34.15 m.

    • This facie mostly occurs in three intervals (47.40 m to 47.00 m, 30.82 m to 29.70 m and 20.80 m to 20.43 m), comprising red algae and coral frame- to bindstone, red algae bindstone, floatstone and wackstone. Porites with coralline algae crusts are observed in red algal and coral frame- to bindstone (Fig. 5). Common to abundant coral debris is found, indicating a shallow environment. Red algae are mainly represented by specimens of corallinales such as Lithophyllum sp. Accessory components are debris of some foraminifera and echinoid (Fig. 5b). Foraminifera are dominated by Amphistegina lessonii which belong to the species that generally live in upper-to-intermediate photic environment, slightly deeper than Amphistegina lobifera occurring in coral reef facies. Therefore, the facies are thought to develop at a relatively shallow condition, but deeper than coral reef facies.

      Figure 5.  Coral-algal reef facies. a. Plane-polarized photomicrograph of algae encrusting coral. 1. Porites, 2. Lithophyllum sp, 29.90 m; and b. Plane-polarized photomicrograph of algae encrusting coral. 1. Coral, 2. Lithophyllum sp, and 3. Echinoid, 47.40 m.

    • This facie is featured with evident iron-oxide staining and karst cave at 20.00 m to 18.39 m, 29.70 m to 27.35 m and 37.80 m to 33.89 m. A number of karst features are observed in these carbonate layers of reef cap facies. Small caves with a range from 1 cm to 2 cm in width and 5 cm to 10 cm in length were found perpendicular to the karst layers. The fillings in those karst spaces are dominated by carbonaceous mud, along with other mixed seepage materials, such as carbonate sand and calcite. The small- and medium-sized caves are mostly horizontal (Fig. 6a). This facie consists of framestone, grainstone, packstone and minor wackstone. The biological debris contains abundant to frequent coral and frequent to rare red algae and benthic foraminifera. Reef cap facie is formed in the subaerial settings during the sea-level lowstands.

      Figure 6.  Reef cap facies. a. Core photo of iron oxide-stained coral framestone; 1. Karst cave, 27.40–27.57 m. b. Plane-polarized photomicrograph of coral framestone showing intense leaching of bioclasts, at least leached coral fragments are distinguished from their cortical coatings, formed by original intergranular porosity occlusion; 1. Porites, and 2. coral moldic pores, 28.93 m. c. Plane-polarized photomicrograph of coral framestone with coral moldic pores, 37.05 m.

    • This facie is mainly found at depths between 54.60 m and 50.18 m, 46.90 m and 42.5 m, 32.80 m and 30.82 m, as well as between 24.90 m and 23.10 m. It comprises grainstone, packstone and wackstone. Biological fragments contain abundant to frequent red algae, abundant to frequent green algae, abundant to rare benthic foraminifera, frequent to rare coral, rare planktonic foraminifera, some echinoids, bryozoans and bivalves. Bioclastic debris is angular to subangular and mostly poorly-sorted (Figs 7a and b). Detrital material is frequently observed in this facie (Fig. 7c). Benthic foraminifera mainly consisting of Amphistegina lessonii and Cibicidoides subhaidingerii and rare planktonic foraminifera are observed in this facies. The observed specialties of benthic foraminifera Cibicidoides subhaidingerii generally live in deep-water settings, such as lagoon and inner shelf at warm temperature (Lei and Li, 2016; Weinmann et al., 2019). Red algae are mainly represented by specimens of Hapalidiales. The Hapalidiales thrives more in deep water while the corallinales mostly live in slightly shallower water (Coletti et al., 2018; Kroeger et al., 2006). The green algae are represented by specimens of Halimeda which is the most abundant organic constituent of the lagoon (Perry et al., 2016; Webster et al., 2009). According to the different preferred habitats of both benthic foraminifera and algae assemblages, the facies are thus considered to be formed in a relatively deep environment.

      Figure 7.  Inner bank facies, outer bank facies and aeolinite facies. a. Plane-polarized photomicrograph of grainstone in inner bank facies, exhibiting subangular and poorly-sored bioclasts; 1. Amphistegina lessonii, 2. Globorotalia truncatulinoides, and 3. Halimeda, 23.98 m. b. Plane-polarized photomicrograph of packstone in inner bank facies, exhibiting subangular and poorly-sored bioclasts; 1. Cibicidoides subhaidingerii (Parr) and 2. red algae, 32.70 m. c. Cross-polarized photomicrograph of packstone in inner facies, showing the occurrence of quartz, 46.35 m. d. Plane-polarized photomicrograph of grainstone in outer bank facies, showing planktonic foraminifera-rich bioclastic fragments; 1. planktonic foraminifera, 20.20 m.

    • The outer bank facie only occurs below the uppermost reef cap facies in the whole section. Outer bank facie is also observed below the uppermost reef cap facies in the carbonate succession of the core XK-1 (Wu et al., 2019). It consists of bioclastic grainstone and packstone. More planktonic foraminifera are found in this facie (Fig. 7d), suggesting that this facie was formed in deeper water than the inner bank facies. Furthermore, more Hapalidiales but no Corallinales are observed and indicate deep-water setting (Coletti et al., 2018; Kroeger et al., 2006). A generally better sorting in biological fragments than inner bank facies is observed due to the longer transportation distance that the sediment has experienced to reach the outer bank facies (Wu et al., 2019).

    • This facie was encountered in the interval of uppermost 18.39 m of the core (Fig. 3), which corresponds to the uppermost 21.00 m aeolianite facies encountered by the well XK-1 (Wu et al., 2019). The aeolianite compositions are mainly composed of loose bioclastic grainstone, with fragments mostly of abundant to rare benthic foraminifera, common to rare red algae, planktonic foraminifera, which were found in the core XK-1 (Li et al., 2018; Wu et al., 2019).

    • The shallow-water carbonates show evidence of having undergone various diagenetic processes such as neomorphism, micritization, cementation and dissolution (Melim et al., 2002). The term “neomorphism” refers to all transformations of minerals into either polymorphs or crystalline structures which are structurally identical to the original ones (Boggs, 2009). Neomorphism mainly occurs in reef association-facies. The fragments of coral, gastropod, mollusk shells, consisting of aragonite, and algae, echinoderms and some foraminifera, composing of HMC, have been altered or recrystallized to LMC. The size of the calcite crystals is determined by the degree of neomorphism. The central part of coral skeleton contains coarser calcite crystals than the fringe, suggesting a higher degree of neomorphism of central part of coral skeleton (Fig. 8a). Micritization of crustose coralline algal cellular skeletons is obvious. The micritization is mostly found in coral-algae facies and inner bank facies. Repeated algal microborings and subsequent filling of the microborings by micritic precipitates contribute to the 2–10 μm thick micrite envelopes (Figs 8b and c). Aragonite or calcite cements, are extensively observed in the Core SSZK1. There is a wide variety of cement morphologies, including meniscus, pendant, isopachous fibrous and isopachous dogtooth cements (Figs 8df). Dissolution features are well developed, especially in reef cap facies. Some biological grains have been considerably dissolved, leaving molds determined by calcite cements (Fig. 8c). Dissolution also occurs on these early generated cements (Fig. 8d).

      Figure 8.  Plane-polarized photomicrographs of products of meteoric diagenesis. a. Neomorphosed coral skeleton showing sparry calcite enclosed by microcrystalline calcite, 34.55 m. b. Micrite envelopes(yellow arrow), 33.20 m. c. Dissolved bioclasts, leaving molds with micrite envelope (yellow arrow), 42.5 m. d. Grains are connected by meniscus calcite cements causing round pore (yellow arrow), 32.9 m. e. Isopachous fibrous calcite fringe around grains; dissolution also occur on these early generated fibrous cements (yellow arrow), 25.14 m. f. Isopachous dogtooth calcite cements (yellow arrow), surrounding bioclasts, 47 m.

    • The δ18O values of bulk rock range from –10‰ to –1.9‰, with an average of –6.47‰. The δ13C values change from –7.2‰ to 5.2‰, and the average is 1.15‰. The mineralogical data for the six samples from the Core SSZK1 shows the vertical variation of the aragonite, HMC and LMC concentrations (Table 1). Based on variation in the distribution of aragonite, HMC and LMC, the 100 m shallow carbonate succession in the cores XC-1 and XK-1 has been divided into three units, namely Unit I (generally larger than 30 m), Unit II and Unit III (uppermost aeolianite facies)(Fig. 3). The rocks in Unit I display high amount of LMC, almost lacking aragonite; The rocks from Unit II contain high amount of LMC and more aragonite than Unit I. The rocks of Unit III in uppermost aeolianite facies are mainly composed of aragonite, HMC and minor LMC (Liu et al., 1997, 1998, 2019). The results of X-ray diffraction analysis on six samples show that the boundaries of the Unit I, Unit II and Unit III are 31.02 m and 18.39 m, respectively (Table 1). The X-ray diffraction pattern of the six samples is shown in Fig. S1. The showing curves of δ18O and δ13C display different characteristics of the three units (Fig. 3). The δ18O and δ13C values from Unit I are both low and fluctuate wildly while the values of both δ18O and δ13C from Unit II have very high-frequency and high-amplitude fluctuations. The δ18O and δ13C values at the depth interval between 7.2 m and 11.5 m are more negative than the underlying strata. The values of δ18O and δ13C from Unit III have relatively low-amplitude fluctuations and are comparatively high.

      Table 1.  Results of X-ray diffraction (XRD) analyses in samples

      Depth/mAragonite/%High-Mg calcite/%Low-Mg calcite/%
      2.0054.723.521.8
      18.3976.4023.6
      27.8043.6056.4
      31.2000100.0
      43.8000100.0
      53.6500100.0

      Positive correlations exist between the δ18O and δ13C for the bulk rock (Fig. 3). The correlation coefficients of δ18O and δ13C in the Unit I are lower than 0.5, highest in the Unit II and relatively high in Unit III. Further, in Unit II, the correlation coefficients are relatively small in the interval of inner bank facies and large in the interval of reef facies-association. From Unit II to Unit III, correlation coefficients decrease in the overall trend with the increase of depth. The correlation coefficients are slightly smaller in the interval of inner bank facies than the lowmost reef facies-association in Unit III.

    • The contents of major elements in different facies are listed in Table 2. The content of CaO in the section of Core SSZK1 is significantly higher than the average of continental crust (Taylor and McClenna, 1985). Correlation-coefficient matrices between element pairs for the entire dataset were determined in order to examine potential relationships and any controls on the chemical composition of the sediments. Table 3 shows the correlation coefficients among the elements’ concentrations. According to the correlation coefficients, these major elements can be divided into four clusters (Fig. 9).

      Table 2.  Descriptive statistics of major elements analysis results in reef facies-association, inner bank facies and aeolianite facies in comparison with the continental crust

      Al2O3CaOFe2O3MgOK2OSiO2Na2OTiO2MnOP2O5S
      Reef facies-associationAverage0.01954.8500.0160.5220.0090.3550.1510.0020.0030.0440.224
      Max0.14455.5610.1020.9800.0342.1150.5120.0060.0050.1830.006
      Min0.00153.7950.0030.2270.0040.1160.0310.0010.0010.0080.053
      Inner bank faciesAverage0.05054.1640.0390.5610.0191.1830.1870.0030.0030.0630.342
      Max0.30055.3800.1660.8890.0949.7360.6670.0080.0050.1830.014
      Min0.01049.0540.0090.2980.0070.1390.0750.0010.0010.0080.063
      Aeolianite faciesAverage0.04651.7980.0391.9730.0150.2580.4940.0020.0030.1090.340
      Max0.40052.8970.2192.6340.0571.7640.6330.0130.0061.7700.165
      Min0.01649.9010.0090.7250.0100.0810.3630.0010.0020.0350.266
      Continental crust15.0405.3906.1703.6702.58061.7103.1800.6700.0900.170

      Table 3.  The correlations of main elements in SSZK1 carbonate profile

      Al2O3CaOFe2O3MgOK2OSiO2Na2OTiO2MnOP2O5SSr
      Al2O31
      CaO–0.341
      Fe2O30.57–0.431
      MgO0.08–0.880.171
      K2O0.79–0.480.680.091
      SiO20.52–0.210.530.150.821
      Na2O0.15–0.880.260.750.26–0.071
      TiO20.90–0.240.520.020.760.500.061
      MnO0.15–0.210.300.230.230.140.090.131
      P2O50.63–0.300.220.190.420.160.160.580.021
      S0.07–0.870.200.820.14–0.190.98–0.010.090.111
      Sr0.09–0.750.210.610.15–0.170.930.02–0.020.100.921
      Note: Correlation coefficients larger than 0.5 (P<0.01, n=138) are marked by bold numbers.

      Figure 9.  Cluster diagram for variables using correlation coefficient. The coefficients larger than 0.5 among these major elements means a link among the major components.

      (1) The first group is CaO, whose content is highest among the major components. CaO shows a negative correlation with the other major components and hence is regarded as one isolated type of major components. The concentrations of CaO from the Unit I are relatively lower than those of the Units II and III (Fig. 10). CaO shows an obvious positive anomaly in all of the facies (Fig. 11).

      Figure 10.  Vertical profile of facies and major elements variations, for symbols see legend in Fig. 3. The different colors of line titles mean different clusters of major elements.

      Figure 11.  Continental crust normalized major element composition (average value) for the reef facies-association, inner facies and aeolianite facies. Normalized values are from Taylor and McLennan (1985).

      (2) The second group includes MgO, S and Na2O. The contents of MgO, S and Na2O are highest in the Unit III, medium in the Unit II and lowest in the Unit I (Fig. 10). Na2O and S content is relatively higher in inner bank facies especially in Unit II (Fig. 10). As the second most abundant composition, high-value MgO and Na2O anomalies occur in the observed carbonate facies (Fig. 11).

      (3) The third group contains Al2O3, TiO2, SiO2, K2O, Fe2O3 and P2O5. Some sharp increases in major components of this type are identified in 42.5–46.9 m, 30.82–32.80 m, 18.39–18.41 m and in the top of the core. Continental crust normalized SiO2 value show a more positive anomaly in inner facies (Fig. 11). Low-value Al2O3, TiO2, K2O and Fe2O3 anomalies occur in the observed carbonate facies (Fig. 11). P2O5 values show an obvious increase in the top of the core.

      (4) The fourth group is MnO. MnO shows poor or negative correlation with the other major elements. The distribution of this component fluctuates greatly in the vertical profile (Fig. 10).

      For trace elements, Sr values show positive correlation with the MgO, S and Na2O, which belong to the second group of the major elements (Table 3).

    • The uranium content of the fossil corals that were dated is between 0.816 ppm and 2.946 ppm (Column 2, Table 4), which is generally lower than that of modern scleractinian corals collected from the Pacific Ocean and Atlantic Ocean (Robinson et al., 2004; Swart and Hubbard, 1982; Wienberg et al., 2010). Four corals have initial 234U/238U ratios that exceeded the expected seawater concentration of 1.14±0.03 (Coyne et al., 2007; Ku et al., 1977), and were therefore considered unreliable. Sample with 230Th/232Th ratios less than 20 was deemed unreliable (Coyne et al., 2007; Ku et al., 1977). Only the four shallow corals complied with these criteria. The U-series dating result is (8114±190) a BP at the depth of 13.43 m, indicating the loose sediments was mainly deposited during the Holocene. The core interval of depth between 18.39 m and 24.3 m was deposited during the late substages of the Marine Isotope Stage 5 (MIS 5a and MIS b). The U-series dating for the Well XK-1 also shows the reef limestones (25.2–27.8 m) underlying the aeolianite sediments were deposited during the early substage of the Marine Isotope Stage 5 (MIS 5e) (Li et al., 2018). Dates obtained from the four deep-buried corals fail to meet these criteria are considered unreliable, though the dated ages are within the interglacial periods (MIS 7, 9, 11 and 13) (Fig. 3 and Table 4).

      Table 4.  230Th dating results for corals from core SSZK1, Yongxing Island

      Depth/m[U]/ppm234U/238U234U/238U (initial)230Th/238U230Th/232ThAge (a BP)Error (2σ)
      7.210.8161.1471.1490.047 359.44 239246
      13.472.4501.1411.1440.082 17 090.38 114190
      18.392.4901.1261.1530.53901 178.869 795274
      24.302.9461.1211.1550.63002 886.688 094349
      30.111.0281.1101.2141.0100709.8236 5242 131
      42.520.8911.1041.2601.0830710.0329 1845 601
      44.891.2791.0971.2941.10201 456.1392 3449 658
      55.921.0001.0871.3371.10701 227.7479 63619 388
      Note: The unreliable dating results are marked by bold numbers.
    • On the basis of petrographic, stable isotopic, elemental data and mineralogical data, as well as mineralogical data from previous work on the cores XC-1 and XK-1, we conduct analysis of facies and diagenesis of Pleistocene shallow-water carbonates. Facies analysis shows that reef facies-dominated location indicates shallow-water settings. Petrographic record shows vigorous meteoric diagenesis in the non-dolomitized Core SSZK1. Considerable karst vugs and moldic porosity in the core representative of intense leaching by meteoric waters are an essential characteristic of meteoric diagenesis. It is proposed that the most reasonable interpretation of these data is that the non-dolomitized Pleistocene limestones have been diagenetically altered by fresh waters (Hajikazemi et al., 2010; Lohmann, 1988). Facies conditions also play a significant role in the elemental concentration.

    • Carbonate facies has been considered as an ideal indicator of water depth in the past decades of years (Amirshahkarami and Karavan, 2015; Eberli, 2000; Tomás et al., 2013). The coral reef, coral-algal and reef cap are characterized by relatively shallow-water setting, belonging to a reef facies-association. The inner bank and outer bank share characteristic of relatively deep-water setting, combining into a bank facies-association. Mostly reef facies-associations are interpreted in the Core SSZK1, which is consistent with the thriving of phototrophic corals during the Late Pleistocene found by the Core XK-1 (Wu et al., 2019). The U-series dating ages suggest that the interglacial highstand intervals (MIS 1, 5, 7, 9, 11 and 13) are preserved throughout the Core SSZK1 due to the high pre-existing topography. However, the dating results for the MIS 7, 9, 11 and 13 are considered unreliable because the initial 234U/238U ratios are not in accord with the modern sea water value.

      Through facies analysis, approximately four cycles of shallow-water reef facies and deep-water bank facies are distinguished, including cycles I, II, III and IV (Fig. 3). Owing to the reef accretion, the existence of high topography resulted in the frequent forming of reef cap facies in this region. Three exposure horizons reflected by reef cap facies can be recognized as the boundaries between the cycles in the Core SSZK1. The topmost horizon should be correlated with the eustatic fall after the Marine Isotope Stage 5 (MIS 5) as a result of the U series dating results show the underlying reef close to this horizon deposited during the MIS 5. The two deep exposure horizons may correspond to the eustatic falls after the MIS 7 and 9 respectively, which are deduced from the undependable dating results (Fig. 3). The sub-aerial exposure surfaces were also recognized at 20–40 m in the Core XK-1 (Liu et al., 2019; Wu et al., 2019), supporting the view that the exposures were frequent during the late Pleistocene. Actually, exposure horizons have been more frequent since 1.25 Ma at the onset of the Middle Pleistocene Transition (MPT) when 100-ka eccentricity cycles dominated and the amplitude of the oscillations grew to approximately 130 m (Wu et al., 2019). The high topography associated with large-amplitude sea-level oscillations during the MPT played a significant role in the frequent occurrences of exposure horizons during the glacial periods. These cycles of carbonate successions mainly developed during the interglacial periods due to the high topography. The occurrence of bank-facies in this ancient high could indicate that a maximum flooding surface formed during the interglacial period so that the reef failed to keep up with sea-level rise (Fig. 3). The evolution of the four cycles of shallow-water reef facies and deep-water bank facies is driven by sea-level change during the Pleistocene climate transition.

    • In the Xisha Islands, the Holo-Pleistocene shallow-water carbonates were frequently exposed when sea-level dropped, undergoing frequent meteoric diagenetic alterations caused by meteoric fluids. The difference of mineralogical and geochemical data among the units I, II and III demonstrates that the Holo-Pleistocene shallow-water limestone at the Xisha Islands has undergone different degrees of meteoric diagenetic alteration (Fig. 12). Formed by stable aragonite and HMC, the rocks of Unit III have suffered smaller degree of meteoric diagenetic alteration characterized by more positive δ18O and δ13C values with medium-frequency and medium-amplitude fluctuations. From Unit III to Unit II, HMC is almost missing, suggesting the increase in the degree of meteoric diagenesis. The abrupt decrease of MgO from Unit III to Unit II is consistent with the fact that HMC has almost been altered to LMC. The incompletely calcitized limestone in Unit II is featured with high-frequency and high-amplitude fluctuations of δ18O and δ13C values. The LMC influenced by meteoric fluids exhibits more negative δ18O values than those of aragonite (Liu et al., 1997, 2019). The alternative isotopic analysis on aragonite or LMC in Unit II is used to interpret the positive correlation between δ18O and δ13C values, as well as the dramatic large-scale change of values. These positive correlations between δ18O and δ13C values are weak in the inner bank facies probably due to the contributions of different organisms from the coral reef facies. Green algae growing in the inner bank yield more positive δ13C values than corals (Swart, 2015). From Unit II to I, aragonite appears almost completely altered to LMC, suggesting larger degree of meteoric diagenesis. The complete precipitation of stable LMC features less variable δ13C and more negative δ18O values, as well as poor correlations between δ18O and δ13C values.

      Figure 12.  Dominant diagenetic environment at the uppermost 60 m depth intervals of the SSZK1 (a), XK-1 (b) and XC-1 (c) cores based on stable isotopic and mineralogical data. The previous work on XK-1 core is from Liu et al. (2019) and XC-1core is from Liu et al. (1997). HMC: high-Mg calcite. LMC: low-Mg calcite.

      Various degrees of meteoric diagenesis characterized by the distinct stable isotopic patterns are mainly controlled by the duration of time in the meteoric diagenetic environment. The amount of time that limestone has been in the freshwater diagenetic environment has the largest impact on the degree of meteoric diagenesis. At the macro-scale, the tectonic and sea-level history of an area determines the length of exposure of a sediment body in the meteoric diagenetic environment. The meteoric diagenetic environment can be subdivided into: sub-aerial exposure surface, vadose zone and freshwater phreatic zone in atolls or bank-margin carbonates (Matthews and Frohlich, 1987; Saller and Moore, 1989). Three sub-aerial exposure surfaces with evident kastifications identified at 18.39 m, 27.35 m and 33.89 m of the SSZK1 core, which were considered as an indicator of wet climate (Hajikazemi et al., 2010). The aeolianite island sediments in Unit III suffered small degree of meteoric diagenetic alteration result from short residence time of meteoric diagenetic environment. In this initial phase, the shallow-water carbonates on the island were mainly under temporally and vertically extensive paleovadose environments (Fig. 12), which is also documented in the XK-1 core sharing the characteristics of meniscus and pendant calcite cements (Li et al., 2018; Liu et al., 2019). The incompletely calcitized limestone in Unit II experiences larger degree of meteoric diagenesis due to the longer residence time of meteoric diagenetic environment. Interestingly, the limestone shows high-amplitude and high-frequency fluctuations of δ18O and δ13C curves in this unit. The top of the meteoric lens closely follows sea level changes (Ayers and Vacher, 1986). Previous literatures have proposed that the freshwater lens has great dissolutional and diagenetic potential (Beach, 1995; Buchbinder and Friedman, 1980). The evidence suggests that the sea level has undergone more frequent oscillations than previously assumed during the MIS 5, which was with an elevation of 10–25 m below present sea level during MIS 5a-1, 5a-2, 5c-1, 5c-2, 5c-3 and 5e-1 (Radtke and Schellmann, 2005; Surić et al., 2009). The shallow-water carbonates of Unit II were at the elevation of approximately 10–25 m below present sea level during these periods and thus should be frequently influenced by freshwater lens. After that the Unit II would subside to its current burial depth. With frequently various mineral compositions, the shallow-water carbonates of Unit II were likely under a frequent alternation of paleo vadose and phreatic environments due to high-frequency sea-level oscillations during MIS 5 (Fig. 12), leading to high-amplitude and high-frequency change of δ18O and δ13C values. The morphology of calcite cements and δ13C values are different between vadose and phreatic zones. Pendant, meniscus calcite and isopachous fibrous calcite cements are found in Unit II (Fig. 8e), suggesting this zone was frequently influenced by both vertical and lateral fluid flow. Furthermore, previous study also documented that the two meteoric diagenetic zones are significantly different in terms of mineralogy and stable isotopic composition (Allan and Matthews, 1982; Swart, 2015). These finds suggest more complex mineralogical and stable isotopic records preserved in Unit II. The largest degree of meteoric diagenesis in Unit III is caused by greatest amount of time that limestone has been in the freshwater diagenetic environment. The vadose zone receives abundant contributions from the decay of CO2 from the respiration of roots, while the phreatic zone receives less CO2 and thus features less variable and slightly more positive δ13C values compared to the vadose zone (Swart, 2015). Less variable and more positive δ13C values relative to Unit II suggest that the dominant paleo phreatic diagenesis affected Unit I (Fig. 12). Isopachous dogtooth cements are common in this Unit (Fig. 8f), and have been used as an indicator of the freshwater phreatic zone (Liu et al., 2019).

      Hydrological condition is another controlling factor on the degree of meteoric diagenesis. The time interval required for both aragonite and HMC to completely alter to LMC within freshwater environments is controlled by the hydrological conditions of regions. For example, in the arid regime of Barbados, both aragonite and HMC have survived for up to 300 ka, whereas in the humid places, HMC is completely stabilized in less than 83 ka and aragonite in less than 200 ka (Harrison, 1975; Matthews, 1968). The HMC is completely stabilized in less than 70 ka observed from the SSZK1 core, suggesting the sufficient rainfall existed in the past. Young strata invaded by the large volume of meteoric water likely experiences more extensive mineralogical stabilization than old strata supplied with small amount of meteoric water. The δ18O and δ13C values at the depth interval of 7.2–11.5 m are more negative than the underlying old strata, suggesting the diagenetic alteration rate was high due to the high volume of freshwater input in this depth interval. This view has been supported by the study of meteoric diagenesis in aeolianite sediments of the Well XK-1 (Li et al., 2018).

    • Four groups of major components with negative or poor correlation can be distinguished, representing four elemental types with different sources (Fig. 9). However, these results are obtained from the bulk rock, making it very difficult for us to analyze the mixed sources of these elements. In this classification, Calcium is the most abundant element (Fig. 11). Isolated from the mainland by deep-water troughs since the early Cenozoic, the Xuande atoll was far away from siliciclastic sediment sources (Wu et al., 2014, 2016). The overall trend of variation in Calcium content is mainly influenced by meteoric diagenesis that altered HMC to LMC.

      The MgO has a strongly negative correlation with CaO as result of the same meteoric diagenesis process. NaO is positively correlated with MgO due to the obvious decrease from Unit III to II. It is worth noting that NaO and S concentration drop evidently from Unit III to Unit I (Fig. 10). A reasonable explanation is that old carbonates experienced repeated meteoric diagenesis, leading to larger depletion of Na and S. Alteration of carbonates by freshwater is generally characterized by the depletion in elements such as sodium (Na) and sulfate (SO4) compared to seawater (Gill et al., 2008).

      The third type of components is represented by detrital and nutrient compositions (Al2O3, TiO2, SiO2, K2O, Fe2O3 and P2O5). Al, Ti and K preferentially accumulate in detrital sediments and are generally not influenced by biogenic or diagenetic processes (Cantalejo and Pickering, 2014). The sharp increases in these elemental concentrations probably depend on the facies conditions or airborne dust input. For example, the detrital compositions (Al2O3, TiO2, SiO2, K2O and Fe2O3) were enriched in inner facies at 32.8–30.82 m and 46.9–42.5 m. In the wells XK1 and CK-2, the high concentration of detrital elements in lagoonal facies has been documented (Jiang et al., 2019; Shao et al., 2017a). Considering that the Yongxing Island is geographically isolated by the surrounding deep oceanic waters, the high concentration of detrital material in the aeolianites potentially come from atmospheric dust, which is similar to the behavior of detrital elements observed in the nearby Core XK-1 (Li et al., 2018). Nutrient components such as P2O5, with a high-value anomaly in the top of the profile, were mostly derived from the guano on the island.

      As the fourth type of components, the significant oscillation in concentration shows that the MnO may have a complex origin (diagenetic products or atmospheric dust). This speculation is based on the mixing signal of different components and digenetic products containing in the bulk rock. Average concentrations of MnO in different facies are equal (Table 2; Fig. 11). Previous work has revealed that meteoric diagenesis contributes to the increase of Mn content (Derry et al., 1992; Kaufman et al., 1993; Kaufman and Knoll, 1995). High concentrations of Mn can give rise to the formation of cements with luminescent characteristics (Swart, 2015). However, the Mn content is larger in the uppermost aeolianite facies with less degree of meteoric diagenesis. The possible explanation is that high concentrations of Mn can be also derived from atmospheric dust, especially in aeolianite islands. It is similar to the islands in the Bahamas, which receives significant quantities of atmospheric dust resulting in high concentrations of Mn in the sediments (Swart et al., 2014).

    • The reef evolution in the Xisha Islands significantly suffered the combined impact of sea-level change, tectonic subsidence, paleoclimate and diagenesis (Liu et al., 1997, 2019,; Jiang et al., 2019; Li et al., 2018; Shao et al., 2017a; Wang et al., 2018a; Wu et al., 2019). The dating results show that the reef sequences were mainly deposited during the interglacial periods (MIS 5, MIS 7?, MIS9?, MIS 11?). Previous studies also documented the development of the reef sequences mainly occurred in the sea-level highstands whereas meteoric diagenesis widely occurred on the subaerially exposed islands with a well-developed freshwater vadose/phreatic system during the sea-level lowstands (Geel, 2000; Hajikazemi et al., 2010; Melim, 1996; Quinn, 1991). The interaction of the reef development and meteoric diagenesis moves in cycles. Three scenarios are used to describe the relationship between the meteoric diagenesis and reef evolution. Firstly, the interglacial reef sequence such as the cycle IV of reef limestone suffered an initial meteoric diagenesis when sea level dropped and is mainly composed of metastable minerals. Secondly, the exposed reef sediments would have been covered by sea water due to the subsequent transgression and subsidence, receiving new sediments on then. Lastly, when the sea level fell again, both newly-formed and initially-formed reef sediments would have suffered meteoric diagenesis and the older carbonate sequences such as the cycles I, II, III of reef limestone underwent higher degree of meteoric diagenesis, which has been discussed in Section 5.2. Previous studies have shown that the S and Na2O, Sr are depleted due to the meteoric diagenesis (Gill et al., 2008; Martin et al., 1986). The cumulative depletion of these elements can be applied to reflect the relationship between the reef revolution and various degree of the meteoric diagenesis. The evident reduction of average S, Na2O and Sr concentrations from younger to older reef sequences are likely caused by the cumulative elemental depletions in the old reef sequences experiencing larger degree of meteoric dissolution (Fig. 13).

      Figure 13.  The gradual decrease of S, NaO and Sr content associated with stable isotopic values in different stage, for symbols see legend in Fig. 3. The dash line marks the ladder reduction of the average S, NaO and Sr concentration in different depth intervals.

      The various degrees of meteoric dissolution between the reef sequences can be clearly shown because of the longstanding exposure of the carbonate platform. During the Late Pleistocene, the alteration of interglacial and glacial period was induced by 100-ka eccentricity (Raymo et al., 1997; Ruddiman et al., 1989; Sun et al., 2019). 100-ka period driven glacial exposure may be used to interpret the evident differences in elemental concentration and stable isotopic values between interglacial reef sequences. The retention of aragonite in the MIS 5 means that one Milankovitch 100-ka eccentricity period is not long enough for aragonite to fully transform to LMC. According to the duration of mineralogical stabilization for aragonite, two 100-ka cycles are basically sufficient for the complete calcitization, which was documented by the published papers (Harrison, 1975; Matthews, 1968). Therefore, the complete alteration of aragonite into LMC in cycle III probably occurred in the MIS 7, in accordance with U-Th dating.

      Our study shows that the increase in meteoric diagenetic degree causing the obvious reduction of elements such as Sr, Na and S, which are used to distinguish the boundaries of the reef sequences. However, it is potentially only available for incompletely meteoric diagenetic shallow-water carbonates ultimately transforming to stable calcite. The relatively flat part of the curve in Unit I demonstrates that the degree of meteoric diagenesis of already stabilized shallow-water carbonates would be hardly reflected by the stable isotopic and elemental data (Fig. 13), and thus the data are unavailable to distinguish the boundary of reef sequences.

    • The detailed research of facies and geochemical signature in the Holo-Pleistocene meteoric diagenetic shallow-water carbonates at Xisha Islands has demonstrated the following points.

      (1) Through facies analysis, approximately four cycles of shallow-water reef facies and deep-water bank facies were distinguished, including cycles I, II, III and IV. The evolution of the four cycles is driven by sea-level changes during the Pleistocene climate transition.

      (2) The petrographic, stable isotopic and elemental data reveals that a gradually increased degree of meteoric diagenesis from young strata (Unit III) to old strata (Unit I) exists. The lowermost Unit I of Pleistocene shallow-water carbonate has suffered almost complete freshwater diagenesis, whereas the overlying Unit II and Unit III has undergone incomplete meteoric diagenesis. Various degrees of meteoric diagenesis are controlled by the hydrological condition and the duration of time in the meteoric diagenetic environment.

      (3) High-frequency and high-amplitude fluctuations of stable isotopic values occur in the Unit II. The reason could be that shallow-water carbonates experienced a frequent alternation of paleo vadose and phreatic environments due to the high-frequency sea-level oscillations during MIS 5, which leads to more complex mineralogical composition. Our study also reveals that the carbon and oxygen stable isotope values of meteoric diagenetic limestone are greatly influenced by mineralogical composition and hence can be a perfect proxy to learn the meteoric diagenesis of limestone.

      (4) The gradually decreased Na2O, S and Sr concentration in older strata is caused by various degrees of meteoric diagenetic dissolutions. In addition to the meteoric diagenesis, elemental concentration is also influenced by facies conditions. The inner facies is suitable for the preservation of Na2O, S and detrital elements.

      (5) The boundaries of the reef sequences can be clearly distinguished by the evident reduction in average S, Na2O and Sr concentrations, as well as δ18O and δ13C values. However, the stable isotopic and elemental data from completely stabilized shallow-water carbonates are unavailable for the identification of the boundary.

    • We thank the Institute of Hainan Marine Geological Survey for their assistance gathering this data set throughout two cruises in 2015. Yanyan Zhao is thanked for helping during sample preparation and XRD analysis. Christian Betzler is acknowledged for his precious advices.

      Figure A1.  X-ray diffraction pattern of the samples. Ar means aragonite; LH means low-Mg and high-Mg calcite; and L means low-Mg calcite.

      Table A1.  Detection limit of the analytical method

      ElementAnalytical methodDetection limit
      CaOXRF0.032 2 (10–2)
      MgOXRF0.003 (10–2)
      Al2O3XRF0.002 7 (10–2)
      Fe2O3XRF0.002 7 (10–2)
      Na2OXRF0.040 2 (10–2)
      SiO2XRF0.004 (10–2)
      K2OXRF0.002 2 (10–2)
      P2O5XRF0.000 9 (10–2)
      MnOXRF0.002 3 (10–4)
      TiO2XRF0.001 (10–2)
      SXRF0.000 6 (10–2)
      SrICP-MS1.0000 (10–6)
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