The benthic foraminiferal δ(34)S records flux and timing of paleo methane emissions

In modern environments, pore water geochemistry and modelling simulations allow the study of methane (CH(4)) sources and sinks at any geographic location. However, reconstructing CH(4) dynamics in geological records is challenging. Here, we show that the benthic foraminiferal δ(34)S can be used to reconstruct the flux (i.e., diffusive vs. advective) and timing of CH(4) emissions in fossil records. We measured the δ(34)S of Cassidulina neoteretis specimens from selected samples collected at Vestnesa Ridge, a methane cold seep site in the Arctic Ocean. Our results show lower benthic foraminiferal δ(34)S values (∼20‰) in the sample characterized by seawater conditions, whereas higher values (∼25–27‰) were measured in deeper samples as a consequence of the presence of past sulphate-methane transition zones. The correlation between δ(34)S and the bulk benthic foraminiferal δ(13)C supports this interpretation, whereas the foraminiferal δ(18)O-δ(34)S correlation indicates CH(4) advection at the studied site during the Early Holocene and the Younger-Dryas – post-Bølling. This study highlights the potential of the benthic foraminiferal δ(34)S as a novel tool to reconstruct the flux of CH(4) emissions in geological records and to indirectly date fossil seeps.