A new method for the U–Th dating of a carbonate chimney deposited during the last glaciation in the northern Okinawa Trough, East China Sea

Highlights

• Carbonates chimney from metals-rich sediments can be precisely dated with U-series.

• Heavy liquids were used to separate subsamples of seep impure carbonate.

• Multiple-point HL regressions provide the more reliable chronology.

• U–Th ages between 28.8 and 20.1 ka point to sea-level low stands during the LGM.

Abstract

Authigenic carbonates from cold seep systems, such as chimney and crust carbonate deposits, are important archives of biogeochemical processes associated with the migration and seepage of methane-rich fluids and seepages on the seafloor. Here we use a new U–Th dating method to precisely determine the ages of the authigenic carbonates of a chimney in the Okinawa Trough (OT), East China Sea. The method uses a series of heavy liquids (HL, a mixture of tribromomethane and alcohol with a density of 2.71–2.80 g/cm³) to separate subsamples of carbonate contaminated by detrital material. The contents of relevant elements (Al, Rb, Zr, Nb, and rare earth elements) of the subsamples reveal that the separation of the mineral phase was successful. Five samples from along the radius of a well-structured chimney (PES-1) from station D2-5 in the OT were analyzed and yielded U–Th ages of 28.8 ± 0.4 ka, 26.1 ± 0.5 ka, 24.4 ± 0.7 ka, 22.5 ± 0.6 ka and 20.1 ± 0.6 ka, from the exterior to the interior of the chimney section, and AMS ¹⁴C dating of the same samples produced the similar results. The agreement between the ²³⁰Th/U and AMS ¹⁴C ages suggests that the application of heavy liquid methods in U–Th dating can be very successful, and it reduces the influence of detrital contamination. Therefore, the approach has a great potential space for studying the evolutionary history of authigenic carbonates in cold seep systems. Moreover, the new ²³⁰Th/U ages show that the carbonate chimney was formed during the Last Glacial Maximum (LGM), which corresponding to the large sea-level fall in the East China Sea. Together with information on biogeochemical processes, the results will provide valuable geochronological information for the future study of active fluid venting on continental margins.

Introduction

Cold seep carbonates are widely deposited on the seafloor at continental margins (Chen et al., 2005; Bayon et al., 2009, 2015; Han et al., 2014; Feng and Chen, 2015). Their formation depends on the sulfate-driven and metal-driven anaerobic oxidation of methane (AOM)-rich fluids in sediments (Crémière et al., 2016; Sun et al., 2019). In certain environments, seep carbonates can provide an excellent record of their growth conditions. In recent years, widespread authigenic carbonates such as crusts, chimneys and nodules have been found in the Okinawa Trough (OT) in the East China Sea. While the biogeochemical mechanisms involved in the process of carbonate precipitation are well understood (Glasby and Notsu, 2003; Peng et al., 2017; Li et al., 2018; Sun et al., 2019; Guan et al., 2019), the duration of carbonate formation is poorly known in these areas.

The potential of cold seep carbonate as a powerful geological information carrier depends on the accurate determination of its absolute age. It has been revealed that some major environmental changes in the ocean during the geological history can faithfully be recorded in cold seep carbonates (e.g., Teichert et al., 2003; Crémière et al., 2016; Smrzka et al., 2020). There are, however, limited technique to effectively date their ages. At present, most studies have shown that ²³⁰Th/U disequilibrium dating is the most ideal geochronological tool for dating inorganic and biogenic materials ranging in age from modern to ∼640 ka (e.g., Edwards et al., 1987; Paull et al., 1989; Lalou et al., 1992; Watanabe et al., 2008; Bayon et al., 2009, 2015; Feng et al., 2010; Ludwig et al., 2011; Wirsig et al., 2012; Crémière et al., 2016; Cheng et al., 2016; Hu et al., 2017). When an authigenic mineral is precipitated from an aqueous solution, it may contain U but essentially no Th. Assuming that the mineral acts as a closed system with respect to U and Th isotopes after its formation, the age (t) can be calculated from the amount of ²³⁰Th produced according to the radioactive decay equations (Ivanovich et al., 1992), and by measuring the present-day activity ratios of [²³⁰Th/²³⁸U] and [²³⁴U/²³⁸U] (herein, activity ratios are bracketed).

However, most of the cold seep carbonates, which are of interest for studying the timing of the active seepage of hydrocarbon-rich fluids and the accompanying paleoceanographic conditions, are impure deposits. The assumption of an initial ²³⁰Th-free system is usually not fulfilled for dirty carbonate samples, because they contain so-called non-in-situ-produced ²³⁰Th. A major source of this ²³⁰Th is allochthonous minerals which are physically mixed with the authigenic phases. Corrections for such detrital contamination can be made using various approaches. In earlier schemes, the procedures often involved the extraction of multiple subsamples from a single horizon, and each sample was dissolved using different digestion protocols in order to minimize the ²³⁰Th leachate from detrital minerals; they are referred to as the L/R (leachate-residue) (Ku and Liang, 1984; Ludwig and Paces, 2002) and the L/L (leachate-leachate) methods (Schwarcz and Latham, 1989; Alcaraz-Pelegrinaa and Martínez-Aguirre, 2005; Peng et al., 2014). However, in these correction schemes it is difficult to know whether and when the acid-leaching treatment preferentially dissolves one isotope over another, or if it does so, whether the degree of fractionation remains constant for a given coeval sample. To circumvent these difficulties, a total sample dissolution scheme (TSD) was developed (Bischoff and Fitzpatrick, 1991; Luo and Ku, 1991). Although they have been widely applied (Rowe and Maher, 2000; Ludwig and Paces, 2002; Candy et al., 2005; Ludwig et al., 2011), these approaches have seldom been applied to cold seep carbonates.

In the present study we adopted an improved approach by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) to date the cold seep carbonates. We would like to use these results to try to better understand the U–Th isotopic system of cold seep carbonates with the overall aim of obtaining more accurate ages for these deposits. The procedure for determining ²³⁰Th/U ages uses a multiple-point regression line applied to carbonate chimney samples from the OT. As an interesting experiment, we found that the heavy liquid (HL) separation could separate the powdered samples within a narrow range of densities. The multiple subsamples analyzed were separated by a series HL mixed of tribromomethane (same as Bromoform, CHBr₃) and anhydrous ethanol. This method yields the most reliable U–Th isotopic measurements of carbonate samples and greatly reduces the sample size needed. The methodology was tested using sediments and seawater recovered from near an active methane seep in the OT which were analyzed for variations in U and Th systematics. To verify the ²³⁰Th/U dates, the carbonate samples were dated by AMS ¹⁴C, and to further determine the Th sources and changes in redox conditions within diagenetic environments, the rare earth element distributions of several analyzed samples were also discussed.