The effect of diagenesis on rare earth element geochemistry of the Quaternary carbonates at an isolated coral atoll in the South China Sea
Introduction
Atmospheric oxygen levels have played a vital role in the evolution of life and ecosystems on Earth (Lenton et al., 2014). The oxygen level in the atmosphere has been one of fundamental questions relating to the occurrence and evolution of life in the geological past (Fike et al., 2006). Due to the intense mixing, oxygen levels in upper oceans are generally in equilibrium with those in the atmosphere (Ling et al., 2013). Studies of the seawater redox provide an important clue for atmospheric oxygen levels in geological past (Planavsky et al., 2014; Tostevin et al., 2016; Hood et al., 2018).
Various materials have been used to extract the paleoredox states of seawater, including animal fossils, biogenic apatites, carbonates and others (Shields and Stille, 2001; Nothdurft et al., 2004; Chen et al., 2015). However, shallow-marine carbonates were considered one of the most promising candidates for reconstructing ancient atmospheric oxygen levels because of the narrow depth distribution of most shallow-water bio-species (Hardisty et al., 2014; X.-M. Liu et al., 2019). Numerous proxies have been applied to trace the paleoredox state recorded by shallow-marine carbonates and discuss the oxygen variations in ancient atmosphere, such as sulfur and carbon isotopes to determine free O2 of water column (Fike et al., 2006), molybdenum isotopes to present oxygen contents related to oxidative weathering of the continents or seawater (Scott et al., 2008; Chen et al., 2015), chromium isotopes to reconstruct the fluctuations of atmospheric oxygenation and continental weathering (Rodler et al., 2016), iron speciation to distinguish oxic from anoxic ferruginous or euxinic conditions (Canfield et al., 2008), iodine contents to reflect the surface ocean redox (Hardisty et al., 2014, Hardisty et al., 2017), and some redox-sensitive elements to reproduce the ocean oxygenation (Sahoo et al., 2012). Rare earth elements and yttrium (REY) incorporated in bio-carbonates, such as coral (Sholkovitz and Shen, 1995) and microbialite (Webb and Kamber, 2000), were found to have seawater-like distribution patterns. In seawater, the behavior of cerium is sensitive to oxygen levels so the Ce anomaly is taken as a unique redox proxy (Elderfield and Greaves, 1982). The REY distribution pattern and the Ce anomaly in shallow-marine carbonates were widely used to study seawater chemistry and oxygen-level fluctuations in ancient surface ocean (e.g., Ling et al., 2013; Wallace et al., 2017).
However, mineral and chemical compositions of shallow-marine carbonates are vulnerable to various diagenetic alterations (Derry et al., 1992; Banner, 1995). Noticeable changes in the contents of Sr, Na, Mg and Ba could be found in diagenetic carbonates (Nothdurft et al., 2004; Webb et al., 2009; Li and Jones, 2014). And these elemental changes are always accompanied with δ18O and δ13C variations and carbonate mineral phase transformation (Li and Jones, 2013; Jiang et al., 2019; X.-M. Liu et al., 2019). In contrast with many elements, the distribution pattern of REY in marine carbonates is thought to behave relatively insensitive to diagenetic alterations (Banner et al., 1988; Shields and Webb, 2004). Webb et al. (2009) compared the REY distributions in original coral skeleton and in neomorphic calcite in Windley Key, Florida and concluded rare earth elements behave conservatively during meteoric diagenesis with little change in their distribution patterns. The study of Carboniferous carbonates from Burlington–Keokuk formation in Mississippi by Banner et al. (1988) found that the dolomitization with extensive water–rock interactions did not alter the primitive seawater REY pattern and the original Nd isotope composition. Similar conclusions were drawn in the studies on Neogene reef carbonates from the Great Bahamas Bank (X.-M. Liu et al., 2019) and on Pleistocene corals and matrices from Grand Cayman, British West Indies (Li and Jones, 2014).
Some other studies, however, questioned whether REY in shallow marine carbonates behaved conservatively under all diagenetic conditions (Azmy et al., 2011). Frimmel (2009) studied ancient marine carbonate rocks in southwestern and central Africa and found their REY patterns did not display modern oxic seawater characteristics. The study by Nothdurft et al. (2004) pointed that the dolomitization of the late Devonian reef carbonates in Canning Basin, western Australia might have caused the alteration of initial seawater REY patterns. Hood et al. (2018) also asserted that the original REY patterns of carbonates can be overprinted during burial diagenesis and dolomitization. There remains considerable debate about the behavior of REY in shallow-marine carbonates during complex diagenetic processes. In addition, a small amount of terrigenous detrital contamination can also influence the original seawater REY signatures recorded in marine carbonates (Nothdurft et al., 2004; Ling et al., 2013).
In the southern South China Sea (SCS), coral reefs are widely distributed on the continental shelf and slope (Fig. 1) and some of reefs have been present since the late Oligocene (Kudrass et al., 1986; Steuer et al., 2014). In 2017, the Meiji Atoll in central Nansha Islands (9.92°N, 115.56°E) was drilled to the basement (Well NK1 in Fig. 1). This coral atoll develops on a volcanic cone which is about 500 m higher than the surrounding seafloor. According to the stratigraphic studies in Nansha Islands (Steuer et al., 2014), the Meiji Atoll has been isolated from surrounding lands since late Oligocene. Due to very limited terrigenous influence, the Quaternary reefal carbonates on Meiji Atoll provide an excellent opportunity to investigate the behavior of REY in response to various diagenetic processes. This study aims to (1) uncover the Quaternary diagenetic history on Meiji Atoll based on the petrography, mineral and isotopic compositions of this new well and (2) determine the effect of different diagenesis processes on the REY geochemistry of the Quaternary carbonates on this isolated atoll including the REY patterns and the Ce anomaly.
Section snippets
Geological setting
The SCS is the largest marginal sea in the western Pacific Ocean. The Nansha block in southern SCS (also known as Dangerous Grounds) is bound to the Sunda Block in the west and is separated from the Xisha Block by the oceanic SCS basin (Fig. 1). It drifted from mainland China during the seafloor spreading of SCS and collided with the Borneo–Palawan Islands during the Miocene (Taylor and Hayes, 1983). According to dredged rocks on Nansha block (Kudrass et al., 1986) and some petroleum wells on
Sampling
The Well NK1 drilled on the Meiji Island penetrates into the volcanic basement, and contains ~1000 m of shallow-water carbonate rocks. According to chronological results from paleomagnetic, biostratigraphic and strontium isotopic ages (Luo et al., 2021), Quaternary strata span the top 200 m of Well NK1 which are the target of this study. A total of 182 bulk carbonate samples were collected for elemental, mineralogical composition (by X-ray diffraction; XRD) and for carbon/oxygen isotopic
Lithology and depositional facies
The Quaternary depositional sequence of Well NK1 is summarized in Fig. 2a. All Holocene carbonates are unconsolidated, consisting of corals, coralline algae, foraminifera, bivalves, and calcareous nanofossils. These carbonates are mainly deposited in a reef bank environment according to lithological compositions. Based on AMS14C and U-Th results, the Pleistocene/Holocene boundary was established as lying at 20.5 m below the drilling surface (Fig. 2a). All Pleistocene carbonates in NK1 are
Diagenetic environment
Bio-clastics in Unit I are unconsolidated and primary aragonite and high-Mg calcite have not been altered. Microscopic observations show that fibrous microstructures of scleractinian coral skeletons are well preserved (Fig. 3g). The Sr concentrations of corals in this study are similar to most reported values for modern coral skeletons (6500–7623 ppm) (Liu et al., 2008). The δ13C and δ18O isotopes of this unit (Figs. 2c, 6a) are close to the values in planktonic foraminifera in southern SCS (δ13
Conclusion
The Quaternary reefal carbonates on an isolated island in southern South China Sea provide an opportunity to evaluate the effects of multiple phases of diagenesis on REY geochemistry of shallow-water carbonates. Two diagenetic environments, the meteoric diagenetic realm and the mixing water dolomitization, were established in the upper 200 m of Well NK1 on Meiji Atoll based on petrological observations, elemental and stable isotope analyses.
The weathering process can cause the enrichment of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank Editor Prof. Brian Jones and the anonymous reviewer for supplying supportive suggestions which help improve this manuscript. The study is supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA13010102), the Key Special Project for Introduced Talents Team of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) (GML2019ZD0206), the National Natural Science Foundation of China (41976063; 41976062 and 41676031) and the
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