GLENDONITE FORMATION AT METHANE SEEPS – BIOGEOCHEMICAL OR TEMPERATURE CONTROL

Authigenic carbonates are often the most prominent witnesses of fossil hydrocarbon seep locations. Few carbonate morphologies, however, show characteristics that are highly indicative of seeps like clathrites and chemoherms so that even new terminologies were developed. The mineralogy of methane-derived carbonates (aragonite, calcite, high-magnesium calcite, Ca-rich dolomite and dolomite) is not solely indicative of methane bearing settings but occur as well in environments where the remineralization of organic matter by microbial sulfate reduction induces carbonate mineral precipitation.

During the last years, a calcium carbonate mineral that is well known for its formation in near-freezing, organic- and orthophosphate-rich environments has also been found at hydrocarbon seeps: Ikaite, a calcium carbonate hexahydrate. Since ikaite is a metastable carbonate it transforms finally to calcite and may be preserved as a pseudomorph in the geologic record. These pseudomorphs, called glendonite, are known since the Proterozoic throughout Earth's history. Few studies so far were able to show unambiguously that their formation is related to hydrocarbon seeps like from the Early Jurassic and the Holocene. In both cases, the environmental conditions i.e. temperature were not supportive of the glendonite-precursor ikaite. This poses the question if methane seeps provide a biogeochemical environment that induces ikaite formation at higher temperatures as yet assumed.