Clumped isotope geochemistry of island carbonates in the South China Sea: Implications for early diagenesis and dolomitization

Highlights

• Reef carbonate diagenesis is constrained by clumped isotope thermometry.

• Diagenetic alteration in Meiji Reef occurred within the water depth of <200 m.

• Massive dolomite in Meiji Reef is likely formed from evaporative seawater.

Abstract

Studying the diagenesis of island carbonates can be hampered by the difficulty of accurately constraining the mineral formation temperature. The carbonate clumped isotope composition (measured as the Δ₄₇ value) is based on the temperature-dependent binding of ¹³C to ¹⁸O, and offers a promising means of calculating the formation temperature. In this study, we report the Δ₄₇ values of island carbonates from a ~1 km-long drill core from the Meiji Reef in the South China Sea, which are integrated with petrography, carbon (δ¹³C) and oxygen isotope (δ¹⁸O) compositions, and trace element contents to constrain the diagenetic environment. We find that most of the digenetic alteration of the initial carbonates likely occurred above a water depth of ~200 m below sea level at temperatures between ~15 and 28 °C, and that the fluids involved during the formation of the diagenetic calcites and dolomites have similar δ¹⁸O values to modern seawater. Considering the estimated secular variation in seawater δ¹⁸O, the dolomite likely formed from an evaporative fluid source via a brine reflux mechanism and with a high convection rate during the late Miocene. Although isotopic compositions of the reef carbonates were overprinted by early diagenetic alteration, the Δ₄₇ values can reflect the temperature of their depositional environment if the diagenesis occurred at depths similar to the depositional depth. These results can help refine the existing dolomitization model of tropical islands and increase the utility of Δ₄₇ values of diagenetic reef carbonates in shallow burial settings as valuable records of the shallow water and climate conditions.

Introduction

Natural massive dolomites are commonly formed at temperature typical of Earth's surface (Land, 1980). However, dolomite with perfect cation ordering is difficult to precipitate inorganically under laboratory conditions at room temperature (Land, 1998). This discrepancy between field and laboratory observations has limited our ability to understand the formation mechanism of dolomite. Although many dolomitization models (e.g., brine reflux, mixing zone, and thermal convection) have been proposed to resolve the so-called “dolomite problem” (Budd, 1997; Machel, 2004; Machel and Mountjoy, 1986; Warren, 2000), the basic formation mechanism of natural dolomites is mainly hydrologically controlled and often site-specific. Studies of carbonate diagenesis provide clues to the environmental controls on diagenetic alteration that occurs after the deposition of carbonates, and provide a general framework for resolving the outstanding issues regarding the origin of dolomite (Bathurst, 1975), which have attracted the interest of geochemists for many decades (Land, 1980; Mazzullo, 1992; Swart, 2015).

Insights into the formation mechanisms of diagenetic carbonates can be gleaned from the properties of the fluids in which the dolomitization or diagenetic reprecipitation occurred. Conventionally, oxygen isotope compositions (δ¹⁸O) of diagenetic carbonates have been used to ascertain the origin of fluids, which is important for evaluating the nature of the diagenetic environment (Swart, 2015; Vahrenkamp and Swart, 1994). However, a robust estimation of the δ¹⁸O value of a diagenetic fluid uses the equilibrium fractionation factor (between carbonate minerals and water) and the mineral's formation temperature (O'Neil et al., 1969). Temperature can be derived using fluid inclusion, but such inclusions are not always present in a sample and their compositions can be altered after formation (Goldstein, 1994). The recently developed carbonate clumped isotope geochemical technique offers a means to independently estimate the formation temperature of carbonate minerals (Eiler, 2007, Eiler, 2011; Ghosh et al., 2006). The carbonate clumped isotope composition (expressed as the Δ₄₇ value) is a measure of the internal ordering of ¹³C-¹⁸O bonds within the carbonate lattice. The equilibrium state of the ¹³C-¹⁸O bond ordering is a function of temperature, and can be well preserved after carbonate precipitation (Schauble et al., 2006). This property makes the Δ₄₇ value of carbonates a valuable geothermometer, which has been widely applied to reconstruct paleo-temperature (Eiler, 2011; Huntington and Lechler, 2015). Combined with δ¹⁸O thermometry, Δ₄₇ thermometry can be used to derive the δ¹⁸O value of diagenetic fluids with high precision (Huntington et al., 2011; Murray and Swart, 2017; Swart et al., 2016).

Island dolomites, which are widely distributed in tropical oceans, are often geologically young and typically have not been subjected to deep burial diagenesis (Budd, 1997; Ren and Jones, 2018). Therefore, studying their formation mechanisms in the context of carbonate diagenesis is a straightforward and promising way to resolve the issue of dolomite formation. The formation temperature of many island dolomites in the tropical ocean could reflect the surface ocean or near-surface ocean temperature. This has been tested by studies using carbonate Δ₄₇ thermometry in the Great Bahama Bank, which show that dolomites and adjacent calcites reflect the near-surface temperature conditions, despite experiencing shallow burial alteration at depths of <1 km (Murray et al., 2021; Murray and Swart, 2017; Winkelstern and Lohmann, 2016). Therefore, dolomitization and diagenetically reprecipitated calcite formation likely took place near the Earth's surface, and the resulting minerals were not overprinted isotopically by later alteration during shallow burial. However, carbonate reprecipitation usually occurs within pore fluid during burial. In this case, the clumped isotopes should be reset and thereby reflect the ambient burial temperature, such as cold bottom seawater or warm geothermal heat (Staudigel and Swart, 2019; Stolper et al., 2018; Winkelstern and Lohmann, 2016) rather than the initial depositional temperature. It thus remains unclear to what degree the carbonate clumped isotope temperature reflects the conditions of diagenetic alteration during burial at depths of <1 km. This issue should be clarified by examining more cases of island carbonates to enable the reliable use of clumped isotopes of island carbonates for sea surface temperature reconstructions.

Island dolomites have different degrees of cation ordering and are assumed to form from the replacement of pre-existing calcites and the influence of magnesium-rich fluids through dissolution-reprecipitation reactions (Budd, 1997). If these reactions take place under near-surface conditions (e.g., low burial depths), then dolomite and diagenetic calcite may form or be altered under similar temperature conditions. Previous studies have shown low Δ₄₇ variability due to cation ordering, but they were mainly based on synthetic dolomites (Bonifacie et al., 2017; Müller et al., 2019; Winkelstern et al., 2016). The relationship between Δ₄₇ variability and cation ordering for natural dolomites remains unclear. Consequently, analyzing the Δ₄₇ value of island carbonates provides an opportunity to further clarify this relationship.

It is generally accepted that Δ₄₇ fractionation during acid digestion of carbonate is temperature-dependent (Guo et al., 2009). However, there is an ongoing debate regarding whether the Δ₄₇ fractionation of dolomite is similar to that of calcite (Bonifacie et al., 2017; Defliese et al., 2015; Müller et al., 2017; Murray et al., 2016). The situation is more complicated if acid fractionation is also dependent on the absolute Δ₄₇ value of carbonate samples (Swart et al., 2019). It is difficult to synthesize inorganic dolomite with high cation ordering under the conditions of the Earth surface (e.g., temperature and pressure); thus, analyzing natural dolomite and calcite with similar Δ₄₇ values and formation temperatures may help resolve the debate.

In this study, clumped isotope compositions of island carbonates from a core (NK−1) recovered from the Meiji Reef in the Nansha Islands in the South China Sea (SCS) were used to explore the nature of diagenesis and the origin of dolomite in the carbonate islands. We integrated our clumped isotope data with optical petrography, carbon and oxygen isotope compositions, and elemental contents to constrain the diagenetic environment, thereby revealing the behaviour of clumped isotope variability during shallow burial.