INVESTIGATION OF BENTHIC FORAMINIFERAL “NON-TRADITIONAL” STABLE ISOTOPES TO RECONSTRUCT METHANE FLUXES IN SEDIMENTARY ENVIRONMENTS

Methane (CH₄) is an important greenhouse gas, with a global warming potential much higher than carbon dioxide (CO₂) on a short time scale. Even if the residence time of CH₄ in the atmosphere is relatively short (tens of years), one of the products of CH₄ oxidation is CO₂, a greenhouse gas with a much longer residence time in the atmosphere (tens to hundreds of years). CH₄ has been proposed as one of the trigger mechanisms for rapid global climate change today and in the geological past. With regards to the geological past, numerous studies proposed the benthic foraminiferal carbon isotope ratio (δ¹³C) as a tool to reconstruct the impact of marine CH₄ on rapid climate changes; however, the investigation of modern benthic foraminiferal δ¹³C have produced inconclusive results.

CH₄ has a distinctive hydrogen isotope (δD) and δ¹³C signature compared to seawater, and sulfate reduction, often coupled to CH₄ anaerobic oxidation in sediments, changes the sulfur isotope signature (δ³⁴S) of the remaining sulfate in porewater. Therefore, we hypothesize that the δD and δ³⁴S signature of infaunal benthic foraminiferal species can provide a complementary approach to δ¹³C to study CH₄dynamics in sedimentary environments.

Here, we present the preliminary results obtained analyzing Uvigerina peregrina δD and δ³⁴S from three different locations at Hydrate Ridge, offshore Oregon. Unfortunately, the lack of chemical data related to the moment of foraminiferal calcification makes it difficult to build a robust relationship among the U. peregrina stable isotopes and the CH₄ fluxes at the sampling sites. However, our results look very promising, as each site is characterized by a different δD and δ³⁴S signature. We emphasize that this study represents the first step in the development of new proxies (δD and δ³⁴S), which may complement the more traditional benthic foraminiferal δ¹³C values, to reconstruct marine CH₄ fluxes in the geological past.