Methanogenesis and clay minerals diagenesis during the formation of dolomite nodules from the Tortonian marls of southern Spain

Highlights

• The ¹³C-rich dolomite nodules of SE Spain are related to methanogenesis.

• Methanogenesis delivered ¹³C-poor CH₄ and ¹³C-rich CO₂ in pore solutions.

• Carbonate and clays weathering by CO₂ of fluids provided cations and bicarbonate.

Abstract

Dolomite nodules are widespread within the Tortonian marls of Fortuna and Lorca basins in southern Spain. They occur as large bodies of various forms (round, ovoid, tabular) that are parallel or secant relative to the stratification. They are massive and present sometimes internal conduits that are considered as drains used for the migration of fluids.

This study brings new results on the mineralogy and elemental and isotopic geochemistry of these dolomite nodules to better describe the processes that were occurring during their formation. The diagenetic reactions that have driven authigenic dolomite precipitation within the sediments involved on the one hand methanogenesis that produced ¹³C-poor CH₄ and ¹³C-rich CO₂, on the other hand carbonate and silicate weathering by the CO₂-rich solutions, that released respectively alkalinity and cations in pore solutions. Moreover, the distribution of the major elements (Si, Al, K, Na, Ca, Mg, Fe) in the marls and dolomite nodules indicates that these elements were redistributed within the sedimentary formation during the diagenetic reactions without external inputs by the circulating fluids. These observations thus confirm the link between authigenic dolomite formation and clay minerals diagenesis in sediments where methanogenesis was active.

Introduction

Authigenic carbonate nodules have been described both in exposed sedimentary successions and in marine and lacustrine sediments (e.g. Aiello, 2005, Campbell et al., 2002, Capozzi et al., 2012, Curtis and Coleman, 1986, Irwin et al., 1977, Kelts and McKenzie, 1982, Pierre and Rouchy, 2004). Numerous examples of methane derived authigenic carbonates, either with very low δ¹³C values or very high δ¹³C values associated to marine cold seeps or to organic/hydrocarbon-rich sediments, have been studied in the past two decades, providing an integrated description of the biogeochemical processes that are responsible for the precipitation of carbonates in these specific environments (e.g. Aloisi et al., 2000, Bayon et al., 2007, Bohrmann et al., 1998, Crémière et al., 2012, Peckmann et al., 2001, Pierre et al., 2012, Roberts et al., 2010). The authigenic carbonate nodules occur generally within marly to silty/sandy deposits where the carbonate cement growth is sometimes obviously linked to the presence of preferential drains (conduit, fault, sedimentary discontinuity) that are thought to have favoured the circulation of fluids. However, in many cases these carbonate nodules appear to be distributed randomly without any evidence of structural or sedimentary control for their occurrence.

Authigenic carbonate nodules display also considerable variations in size and morphology, from mm to pluri-meter bodies with various shapes (round, elongated, contorted) that may be distributed in layers or be secant to the stratification. The carbonate mineralogy comprises various phases consisting of calcium carbonate (aragonite, magnesian calcite), dolomite and siderite that may be present as complex mixtures in the same nodule (Pierre et al., 2014).

The formation of authigenic carbonate in an unconsolidated sediment requires inputs of both bicarbonate ions and cations from pore solutions with steady conditions during enough time to maintain carbonate over-saturation.

The link between authigenic carbonates and hydrocarbon-rich fluids is evidenced by the very negative δ¹³C values indicating that microbial oxidation of hydrocarbons (mostly methane) is the source of dissolved inorganic carbon (DIC) in the pore solutions (Claypool and Threlkeld, 1983). The anaerobic oxidation of methane (AOM) coupled with bacterial sulphate reduction $\left({{\text{SO}}_4}^{2-}+{\text{CH}}_4\to {{\text{HCO}}_3}^{-}+{\text{HS}}^{-}+{\text{H}}_2\text{O}\right)$ represents the most important reaction in sediments for producing both DIC and hydrogen sulphide in pore solutions (Boetius et al., 2000, Hinrichs et al., 2000, Orphan et al., 2001).

Another source of DIC may be provided by carbonate or silicate chemical weathering by CO₂-rich solutions (i.e. hydrolysis) also called “CO₂ buffering” by Claypool and Threlkeld (1983), as in anoxic diagenetic environments where methanogenesis is active (Claypool and Threlkeld, 1983, Wallmann et al., 2008). The term silicate weathering was firstly used by Wallmann et al. in 2008 and corresponds to the diagenetic process that changes the chemical and mineralogical composition of marine sediments and buffers the CO₂-rich pore fluids. During the methanogenesis reaction resulting from organic matter fermentation (2·CH₂O → CO₂ + CH₄) the large ¹³C fractionation produces ¹³C-depleted CH₄ (down to −110‰) and ¹³C-rich CO₂ (up to +15‰) (Rosenfeld and Silverman, 1959, Claypool and Kaplan, 1974). The silicate weathering by CO₂ produced by methanogenesis appears as a major diagenetic process because it produces large amounts of DIC in the pore solutions (characterized by very high δ¹³C values) and it releases cations (i.e. Ca²⁺, Mg²⁺, Fe²⁺) that are thus available for authigenic carbonate precipitation.

Overall, these processes (AOM, methanogenesis, silicate weathering) are the major sources of DIC in pore solutions of organic-rich sediments as in the Bering Sea where DIC reaches up to 74 mM i.e. 37 times the DIC concentration of seawater (Wehrmann et al., 2011).

This study compares mineralogy, elemental geochemistry and oxygen and carbon stable isotopes of carbonate, in selected samples of authigenic carbonate nodules and of unconsolidated surrounding sediments, from the Tortonian marls of southern Spain, to confirm or not the link between silicate weathering and the formation of authigenic carbonate in relation with methanogenesis.