LAUSR

Ikaite (CaCO3·6H2O) forms at near-freezing temperatures and its precipitation is favored by high alkalinity and high concentrations of dissolved phosphate. With increasing temperatures during early burial, ikaite transforms into its calcite pseudomorph referred to as glendonite. To further constrain the biogeochemical processes that impact the transformation of ikaite to glendonite, glendonites from Cenozoic strata of western Washington State, USA, were analyzed for their petrographic characteristics, stable isotope (C, O, S) patterns, and lipid biomarker inventories. Glendonites from the Humptulips, Pysht, Lincoln Creek, and Astoria Formations occur in strata that enclose abundant methane-seep deposits. Despite robust evidence for the anaerobic oxidation of methane (AOM) at these ancient seep sites, molecular signatures of this biogeochemical process were not found within glendonite. Glendonite was found to contain abundant, moderately 13C-depleted iso- and anteiso-fatty acids, compounds interpreted as biomarkers of sulfate-reducing bacteria in marine settings. The 34S-enrichment in carbonate-associated sulfate (δ34SCAS = 54.1 ‰) and the 34S-depletion of pyrite (δ34SCRS = 6.8–12.5 ‰) in glendonite samples confirm that bacterial sulfate reduction was a prominent process in the sedimentary environment during the transformation of ikaite to glendonite. Low δ13Cglendonite values, such as those of the Washington State glendonites (as low as −21‰), have previously been interpreted as signatures of methane-derived carbon; however, the admittedly small data set obtained from the Washington State glendonites is best explained with organoclastic sulfate reduction as the alkalinity engine driving carbonate precipitation. This surprising finding reveals that more comprehensive work is needed to decipher the biogeochemical processes that governed the transformation of ikaite to glendonite in ancient marine settings, including the relative contribution of organoclastic sulfate reduction and AOM.