Methane-derived authigenic carbonates from modern and paleoseeps on the Cascadia margin: Mechanisms of formation and diagenetic signals

Authigenic carbonate precipitation occurs within marine sediments where sulfate-dependent anaerobic methane oxidation occurs. Geochemical and isotopic analyses of authigenic carbonates are commonly used as indicators of carbon sources and environmental conditions present during carbonate formation, but burial diagenesis and recrystallization can overprint these signals. Plane polarized light (PPL) and cathodoluminescent (CL) petrography allows for detailed characterization of carbonate phases and their subsequent alteration. Petrographic, isotopic, and geochemical characteristics of modern offshore authigenic carbonates from central and northern Cascadia are compared with Oligocene–Pliocene fossil seep carbonates uplifted on the Olympic Peninsula. Coupled analyses show the value and complexity of separating primary vs. secondary signals with relevance to understanding fluid-burial history in methane seep provinces on tectonically active convergent margins. The modern, offshore, near-seafloor diagenetic environment (S. Hydrate Ridge and Barkley Canyon) is dominated by acicular and microcrystalline aragonite and high-Mg calcite (HMC, {\textgreater} 12 mol {%} Mg). PPL and CL data illustrate that aragonite and HMC phases recrystallize to intermediate-Mg calcite (IMC, 5–12 mol{%} Mg) during burial and diagenesis and eventually to low-Mg calcite (LMC, 18O attributable to elevated fluid temperatures during recrystallization.