Authigenic carbonates from methane seeps of the northern Congo fan: Microbial formation mechanism

Using iron speciation in authigenic carbonates from hydrocarbon seeps to trace variable redox conditions

“…This inference is consistent with the relatively high Fe T and Fe auth contents and the observation of abundant pyrite by optical microscopy (cf. Feng et al, 2010;Haas et al, 2010;Pierre and Fouquet, 2007). According to the well documented behavior of Fe in marine sediments, the high content of authigenic Fe mainly represented by pyrite suggests that sulfidic conditions prevailed during formation of the Congo Fan carbonates (Fig.…”

“…1). All carbonates have been shown to represent marine hydrocarbonseep deposits (Hass et al, 2010;Feng et al, 2010Feng et al, , 2009Peckmann et al, 2001). The Congo Fan carbonates analyzed here were collected by TV-guided grabs from the sites Hydrate Hole and Diapir Field during RV METEOR Cruise M56.…”

“…The extremely negative d 13 C values of the authigenic carbonates indicate that the carbon was derived from oxidation of biogenic methane (Feng et al, 2010;Haas et al, 2010;Pierre and Fouquet, 2007). The isomer shifts and quadruple splitting values of Congo Fan carbonates are close to those of pyrite, suggesting that the authigenic Fe in the carbonates is indeed mainly represented by pyrite.…”

“…The Congo Fan carbonates are mainly composed of aragonite and high-Mg calcite, associated with abundant pyrite framboids (Feng et al, 2010;Haas et al, 2010;Pierre and Fouquet, 2007). The extremely negative d 13 C values of the authigenic carbonates indicate that the carbon was derived from oxidation of biogenic methane (Feng et al, 2010;Haas et al, 2010;Pierre and Fouquet, 2007).…”

Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

“…Therefore the possibility of contamination requires careful assessment before the observed REE + Y compositions of the Doushantuo cap carbonates can be reliably used to deduce chemical peculiarities of precipitating waters and depositional environments. In order to minimize contamination, several approaches were taken in this study: 1) materials that are associated with later-stage mineral cements filling in fractures, sheet cracks, and cavities were avoided; 2) samples without evidence of recrystallization and metamorphism by examining thin sections under crossed-polarized light were used; 3) mm-sized chips free of visible later-stage minerals after crushing were carefully handpicked using an optical microscope; and 4) gentle acid was employed to dissolve collected samples and acid-insoluble residues were discarded; we emphasize that the same dissolution experiments as ours were performed for dissolving marine carbonates, which yielded primary REE + Y distributions (Feng et al, 2010;Birgel et al, 2011).…”