Diagenesis of hydrocarbon-seep carbonates: Common patterns, divergent pathways and conceptual pitfalls

Abstract

Authigenic carbonates formed at marine hydrocarbon seeps are an important component of carbon cycling at continental margins. Ancient hydrocarbon-seep deposits serve as archives of the evolution of chemosynthesis-based ecosystems, past pore-water chemistry, and the tectonic architecture and geological history of their host sedimentary basins. However, while modern and ancient seep deposits share many similarities, they also show notable differences in their petrological and geochemical characteristics. The insufficient understanding of the origin of these differences limits our ability to address fundamental questions about the palaeoecology of seep-specialised communities, the geological evolution of seep-hosting plate margins, and secular changes in the composition of the seeping fluids and/or seawater. Here, we critically evaluate the carbon, oxygen and strontium isotope signatures, as well as selected minor and trace element systematics of ancient seep carbonates. We identify distinctive, recurring patterns and discuss them in the context of current understanding of the controls on carbonate authigenesis and alteration in marine pore waters. The precipitation of seep carbonates mostly from sediment pore waters at the transition between the sulphate reduction and methanogenic zones favours the preservation of primary textural and geochemical characteristics. However, in this anoxic, spatially and temporarily complex setting, the concentrations of many minor and trace elements, including the refractory lithophile elements, are subject to increased mobility during fluid-rock interactions, mineral transformations and organic matter decomposition. This mobility complicates their traditional use as indicators of post-depositional alteration, laboratory artefacts, or original mineralogies. The high concentrations of strontium with non-marine ⁸⁷Sr/⁸⁶Sr ratios that may be introduced with the seeping fluids offer insights into the fluid sources and basement characteristics, but also limit the utility of Sr isotope stratigraphy for dating seep deposits. A negative correlation between the δ¹³C and δ¹⁸O values, commonly observed in microcrystalline phases, most likely reflects the continuous cementation of seep carbonates during burial in the methanogenic zone. This process is driven by methanogenesis-associated aluminosilicate weathering, releasing ¹³C-enriched and ¹⁸O-depleted fluids. Another common pattern, the narrow clustering of strongly negative δ¹⁸O signals, appears to record selective δ¹⁸O resetting during fluid-dominated post-depositional alteration. The observed increased δ¹³C values of seep carbonates during parts of the Paleozoic and Mesozoic may be associated with the secular changes in the dissolved inorganic carbon concentration of seawater, while the role of other controls remains unclear. Our work provides a timely synthesis of the constraints on recognizing the primary and post-depositional features of ancient seep deposits, offering a framework for interpreting their geochemical systematics in terms of specific diagenetic processes.