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

Abstract

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 ³⁴S 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 ³⁴S 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 ³⁴S 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 ³⁴S and the bulk benthic foraminiferal δ ¹³C supports this interpretation, whereas the foraminiferal δ ¹⁸O-δ ³⁴S 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 ³⁴S as a novel tool to reconstruct the flux of CH ₄ emissions in geological records and to indirectly date fossil seeps.