Sulfate-driven anaerobic oxidation of methane as the origin of extremely 13C-depleted calcite in the Doushantuo cap carbonates in South China

Abstract:

The cap carbonate in Doushantuo Formation (ca. 635 Ma) has been extensively studied for Earth systems change following the Marinoan ‘snowball Earth’ glaciation. An important feature of this cap carbonate is the local occurrence of extremely negative δ¹³C_carb values (down to –50‰) from dark-colored calcite cements. These calcites have been interpreted as carbonate cements precipitated from cold methane seeps or as hydrothermally induced diagenetic carbonates. To test these contrasting interpretations, we mechanically separated the calcite cements from host dolostones and analyzed stable isotope compositions of pyrite, carbonate-associated sulfate (CAS), and organic carbon in both components of the Doushantuo cap carbonate in the Yangtze Gorges area, South China. The data show that δ³⁴S_pyrtie of extremely ¹³C-depleted calcite (22.8–73.9‰) are up to 34‰ higher than those of the dolomite (14.7–39.9‰). Similarly, δ³⁴S_CAS of calcite (37.1–80.1‰) are up to 40‰ higher than those of the dolomite (24.5–41.5‰). The δ¹⁸O_CAS of calcite (12.9–22.2‰; VSMOW) are also systematically higher than those of dolomite (13.3–16.8‰; VSMOW). In contrast, δ¹³C_org of calcite cements (-27.2 ‰ to -46.1‰) are lower than those of the dolostones (-26.5‰ to -31.7‰). In addition, there is a strong positive correlation between δ³⁴S_CAS and δ¹⁸O_CAS and a negative correlation between δ¹³C_org and δ³⁴S_pyrtie of the calcite (Figure 1). The data demonstrated convincingly that the ¹³C-depleted calcites were formed in a environment facilitated by sulfate-driven anaeorobic oxidation of methane (AOM). The co-occurrence of unusually low δ¹³C_org and high δ³⁴S values requires presence of active flow of both methane and sulfate, a condition not far away from conducive seawater sulfate supply.

Figure 1: Cross plots of δ³⁴S_CAS vs. δ¹⁸O_CAS and δ¹³C_org vs δ³⁴S_pyrtie in host dolomite and in the extremely ¹³C-depleted calcite cements.