Authigenic carbonates as archives of past gas hydrates: Definition, identification and research directions

Gas hydrates are widespread under the seafloor along continental margins, where high-pressure and low-temperature conditions ensure their stability. They represent a significant potential energy resource; however, their exploita- tion poses serious environmental and climatic risks, including submarine slope failures, tsunamis, climate warming, ocean acidification and oxygen depletion. Many hyperthermal events and associated biotic crises have been linked to large-scale destabilisation of gas hydrates. These assumptions are based primarily on circumstantial evidences, as distinctive hydrate-related rocks have not been documented in sedimentary successions coeval to these events. Despite this, gas hydrates must have existed since at least the Neopro- terozoic, suggesting that their geological record is underrepresented, and their role in Earth history remains elusive. Recognising vestiges of past gas hydrates is, therefore, essential for the understanding of their impact on ancient geosys- tems and their potential contribution to near-future climate change. This review aims to enhance the identification of past gas hydrate occurrences in marine settings by: (1) outlining their distribution, occurrence, dynamics and associated geohazards in modern environments; (2) summarising current per- spectives on their role in Earth history; (3) reviewing their sedimentological and geochemical vestiges in the sedimentary record. Particular emphasis is posed on the most reliable archive of past gas hydrates: hydrate-derived authi- genic carbonates; (4) clarifying existing confusion in terminology and propos- ing a comprehensive classification of gas hydrate-associated rocks; and (5) identifying the most promising research directions and methods in this field.