Reducing microenvironments promote incorporation of magnesium ions into authigenic carbonate forming at methane seeps: Constraints for dolomite formation

Abstract

In part composed of Mg calcite and dolomite with a nearly continuous spectrum of MgCO 3 contents, carbonates forming at marine methane seeps are ideal candidates to study the formation of early diagenetic dolomite at surface conditions. Laboratory experiments, modelling and the co-variation of mineralogical and geochemical attributes of seep carbonates suggest that sulphidelocally released from sulphate-driven anaerobic oxidation of methanedrives catalytic dolomite formation at seeps. Direct comparison of the concentration of dissolved sulphide with the MgCO 3 content of seep carbonate could test this hypothesis. Although the concentration of sulphide during precipitation of carbonate cannot be determined, sedimentary fabrics probably had an effect on local sulphide concentration. Carbonates from the seabed of the Shenhu seepage area of the South China Sea reveal longitudinal, winding and branched fabrics with a width of 400 to 700 μm, interpreted to represent burrows piercing semi-consolidated sediment. The abandoned burrows were infilled with fine-grained sediment, which was subsequently cemented by microcrystalline dolomite and Mg calcite. The degree of cation ordering of dolomite in burrow infills and host sediment is similar, indicating virtually coeval dolomite formation. The Mg/Ca mole ratios of carbonate minerals are lower in burrow infills than in the surrounding sedimentary matrix. Similarly, δ 13 C carbonate values tend to be higher for burrow infills (from −41 to −33‰) than for the matrix (from −43 to −38‰), suggesting stronger seawater influence on pore waters in the burrows. More reducing microenvironments of the sedimentary matrix supposedly came along with higher concentrations of dissolved sulphide, which allowed more Mg 2+ ions to enter the crystal lattice and led to more catalytic dolomite formation in the host sediment. The distribution of MgCO 3 contents and δ 13 C values in the authigenic Shenhu carbonates reinforces the hypothesis that sulphide release by sulphate-driven anaerobic oxidation of methane is a key factor in the formation of dolomite at seeps.

Role of bottom water chemistry in the formation of fibrous magnesium calcite at methane seeps in the Black Sea

“…At seeps, SD‐AOM produces large amounts of dissolved 13 C‐depleted carbonate species and dissolved sulphide. The former, together with marine dissolved inorganic carbon with a δ 13 C value of around zero, and Ca and Mg ions from seawater, enables the precipitation of carbonate minerals (Peckmann & Thiel, 2004), while high levels of the latter promote the formation of Mg calcite or dolomite (Lu et al ., 2018, 2021; Tong et al ., 2019). Under strong SD‐AOM conditions, higher levels of 13 C‐depleted carbonate species and dissolved sulphide result in more negative δ 13 C values and higher MgCO 3 contents of the authigenic carbonate minerals.…”

From biogenic silica and organic matter to authigenic clays and dolomite: Insights from Messinian (upper Miocene) sediments of the Northern Mediterranean

“…Vasconcelos et al ., 1995; Sanz‐Montero et al ., 2009; Bontognali et al ., 2010). At high sulphide levels, the kinetic inhibition of dolomite formation under surface conditions is overcome, and dolomite forms by sulphide catalysis and subsequent ordering during early diagenesis (Zhang et al ., 2012; Lu et al ., 2021). The hydrogen sulphide released by bacterial sulphate reduction also reacts with iron to form pyrite, a mineral common in the studied lithologies (e.g.…”

Protracted post-glacial hydrocarbon seepage in the Barents Sea revealed by U–Th dating of seep carbonates

“…Aragonite is the dominant carbonate phase, except for the magnesium-calcite rich micrite in sample HH1029 (Table 2). It is not unusual for seep carbonates to mainly comprise aragonite and partly include Mg-calcite (e.g., Greinert et al, 2001;Crémière et al, 2018;Lu et al, 2021). At hydrocarbon seeps aragonite formation is likely favoured over calcite by relatively high sulfate concentrations (Burton, 1993).…”

“…In contrast, during episodes of weak seepage with low gas flux, AOM was likely mediated deeper in the sediment at relatively lower pore water sulfate concentrations, and Mg-calcite precipitation was favoured (e.g., Greinert et al, 2001). Increased magnesium incorporation into the calcite lattice is plausible under low flux conditions, when abundant AOM-produced hydrogen sulfide (HS − ) supports de-hydration of Mg 2+ ions, thus increasing the pore fluid Mg 2+ /Ca 2+ ratio (Zhang et al, 2012;Lu et al, 2021). At the same time, the increased alkalinity resulting from AOM-produced bicarbonate (HCO 3 − ) also supports carbonate precipitation.…”