Is methane venting at the seafloor recorded by δ¹³C of benthic foraminifera shells?

Abstract

[1] The isotopic composition of the dissolved inorganic carbon (DIC) collected at sites of active methane discharge on Hydrate Ridge, Oregon, reveals anaerobic methane oxidation mediated by bacteria, with d 13 C DIC reaching values as low as À48% in the upper 4 cm of the sediment. In spite of the high sulfide levels in the discharging fluids, living specimens of the benthic foraminifera Uvigerina peregrina are abundant in the vents, probably owing to the rich bacterial food source. Although pore water d 13 C DIC is extremely low (À6 to À48%), the d 13 C values of living (Rose Bengal stained) foraminifera shells collected from active methane seeps are not significantly lower than those observed in nonventing pelagic sediments, and are within the range expected from local organic matter decomposition (0 to À4%). The apparent d 13 C disequilibrium between biogenic calcite and DIC suggests that at seep localities, foraminifera calcify mostly during periods when there is little methane discharge or during intermittent episodes of seawater flow into the sediments. The isotopic composition and Mg/ Ca ratios of fossil (unstained) foraminifera recovered at carbonate-rich sites on the northern Hydrate Ridge reveals overprinting of the biogenic record by inorganic calcite with high Mg/Ca and anomalously low d 13 C values. Thus overprinting of the original isotopic composition of foraminifera by overgrowths or recrystallization at or below the sediment surface, rather than primary calcification in contact with 13 C depleted DIC, can explain extreme 13 C depletion observed in fossil foraminifera recovered from sites of active methane discharge.

Evidence for large methane releases to the atmosphere from deep-sea gas-hydrate dissociation during the last glacial episode

“…The possible presence of diagenetic calcite that might not be visible in scanning electron microscope investigation has been evaluated further by determining Mg͞Ca on two deep-dwelling foraminifera species: Neogloboquadrina dutertrei and Pulleniatina obliquiloculata. Mg͞Ca in foraminiferal tests has been shown to be a sensitive marker of diagenetic alteration, with values up to 150-250 mmol͞mol depending on the amount of authigenic calcite (25,26). The paucity of G. ruber across the negative carbon excursion prevented trace metal analysis on this species.…”

Characteristics of meiofauna in extreme marine ecosystems: a review

“…However, in some studies, strong disequilibrium has not been noted between the δ 13 C of living foraminiferal tests from cold seep zones and the expected very low δ 13 C DIC of hydrate-gas fluid (e.g. Sen Gupta and Aharon 1994; Torres et al 2003;Bernhard et al 2010). Therefore, it has been suggested that foraminiferans may calcify during periods of low methane discharge or during intermittent episodes of seawater flow into sediments (Torres et al 2003).…”

“…Living benthic foraminifera from modern cold seeps have been investigated in a number of studies (e.g. Sen Gupta and Aharon 1994;Kitazato 1996;Sen Gupta et al 1997Rathburn et al 2000Rathburn et al , 2003Bernhard et al 2001Bernhard et al , 2010Torres et al 2003Torres et al , 2010Martin et al 2004Martin et al , 2010Heinz et al 2005;Panieri 2006;Mackensen et al 2006;Lobegeier and Sen Gupta 2008;Fontanier et al 2014b). This work suggests that foraminiferal species observed in cold seep areas are not endemic and may be recruited from adjacent non-seep zones (e.g.…”

Evidence against clathrate‐derived methane release to Santa Barbara Basin surface waters?

“…Torres et al [2003] recently reported that fossil planktonic and benthic foraminifera collected from modern methane seeps off Oregon exhibit extremely depleted d 13 C values (typically between À4% and À10%) similar to those in the Santa Barbara Basin record. However, anomalously high Mg/Ca ratios in the shells of these specimens (the partition coefficient for Mg in foraminiferal calcite is orders of magnitude lower than that for inorganically precipitated calcite) indicate they are contaminated with authigenic overgrowths or calcite replacement that is undetectable by visual microscopy [Torres et al, 2003]. Today the Santa Barbara Basin sediment column is supersaturated with carbonate fluorapatite and calcite below $2 cm and dispersed calcite precipitation occurs throughout the sediment column [Reimers et al, 1996].…”

“…Carbon isotope analyses on organic matter specifically from surface water marine organisms may be an even better constraint. Trace metal (Mg/Ca) screening of planktonic and benthic foraminiferal specimens could be used to determine the extent of authigenic calcite before d 13 C analyses [Torres et al, 2003]. In this way, specimens found to have no authigenic contamination would be considered to record a primary environmental signal.…”