Dynamic fluid flow and chemical fluxes associated with a seafloor gas hydrate deposit on the northern Gulf of Mexico slope

Changing redox conditions at cold seeps as revealed by authigenic carbonates from Alaminos Canyon, northern Gulf of Mexico

“…Observations at modern seeps revealed a high degree of spatial and temporal variability of fluid flow (e.g. Solomon et al, 2008). The dynamics of fluid flow, thus, provide the means to explain changing redox conditions archived in seep limestones.…”

“…Resultant changes from anoxic to oxic conditions were held responsible for the contradiction of methane-derived carbonates apparently partly forming under oxic conditions (Feng et al, 2009b). Interestingly, recent studies suggested that the downward flux of oxic seawater into the otherwise anoxic sediment at some seeps is significant (Solomon et al, 2008). It seems feasible that an increase of this downward flux or, alternatively, a decrease in seepage activity may account for an intermittent change to oxic conditions in the shallow sediments at seeps.…”

“…chemical characteristics of seepage-influenced sediments have been found to be highly variable and complex (Tryon and Brown, 2004;Solomon et al, 2008), representing possible steering factors that determine the composition of prokaryotic communities at seeps.…”

Authigenic minerals from the Paola Ridge (southern Tyrrhenian Sea): Evidences of episodic methane seepage

“…Nevertheless, it has been observed that the presence/absence of a negative Ce anomaly in authigenic methaneimprinted carbonates can be due to mineral formation in high pore water alkalinity where organic matter occurs (Pourret et al, 2008;Kim et al, 2012) to the intermittent oxygenation of sediments (Birgel et al, 2011;Hu et al, 2014) and can be also affected by leaching procedure (Zhang et al, 2015). Discontinuous fluids discharge and downward flow of sea water during periods of reduced seepage can produce carbonates with a particular geochemical signature and disguise the original Ce anomaly (Solomon et al, 2008;Kim et al, 2012;Hu et al, 2014) resulting in a Ce/Ce* ratio close to 1. The lack of a Ce anomaly, typical of both limestone and siderite at the Paola Ridge (Tables 2 and 3) can be thus explained by discontinuity of methane seepage and sustained conditions of high alkalinity and high organic matter content.…”

“…This, in turns, increases the pore water alkalinity and favors the precipitation of authigenic carbonates. It has been proved that the discontinuous seepage of methane may influence the overall geochemical composition of the authigenic carbonates affecting both stable isotope composition of carbon and oxygen (Birgel et al, 2011;Hu et al, 2014) and REE distribution (Solomon et al, 2008;Kim et al, 2012;Hu et al, 2014).…”