Progressive formation of authigenic carbonate with depth in siliciclastic marine sediments including substantial formation in sediments experiencing methanogenesis

Authigenic carbonate (AC) forms in siliciclastic marine sediments in part as a result of the degradation of organic matter and methane. Degradation pathways vary considerably and each impact the chemical evolution of sediments and porewater differently. The relative importance of these reactions in contributing to carbonate authigenesis are poorly understood, especially from a global perspective. Modern porewater geochemical data and authigenic carbonate carbon isotope compositions (delta C-13(ac)) of globally distributed marine sediment sites allow direct assessment of the relative importance of diagenetic pathways. Common correlations between bulk delta(13)C(ac )and depth reveal that AC tends to form progressively by multiple reaction pathways, rather than within a particular sediment horizon. A general lack of shallow AC within sediments 1) containing porewater sulfate and 2) exhibiting decreasing dissolved inorganic carbon (DIC) delta C-13 with depth suggest that organotrophic sulfate reduction does not promote significant authigenesis. Instead, the anaerobic oxidation of methane (AOM) may (in part) account for most AC expressing C-13-depleted isotope compositions, including those with delta C-13 values between -25 (the approximate marine organic matter value) and 0 parts per thousand VPDB. Widespread increases in delta Ca-13, with depth in conjunction with values that exceed seawater DIC compositions indicate precipitation in sediments exhibiting methanogenesis, likely aided by contemporaneous marine silicate weathering. Deeper AC formation may occur in sediments experiencing thermal decarboxylation. When data from all sites are considered collectively, sediments experiencing AOM and methanogenesis emerge as the most significant in yielding AC. The relative importance of authigenesis pathways controls potential impacts of AC deposition on marine carbon budgets and provides insight into the geochemical signatures exhibited by ancient carbonate concretions.