Microscale δ³⁴S heterogeneities in cold seep barite record variable methane flux off the Lofoten-Veståralen Continental Margin, Norway

Authigenic carbonate and barite crusts were analyzed from recently discovered cold seeps on the Lofoten-Vesterålen (LV) continental slope, northern Norway. Carbonate phases in these crusts are methane-derived Mg-calcite and aragonite. Scanning electron microscopy (SEM) was used to petrographically characterize cold seep crusts, and secondary ion mass spectrometry (SIMS) was used to measure the microscale sulfur isotope composition (δ³⁴S values) of authigenic barite. The mean δ³⁴S_BaSO4 value from SIMS spot analyses is 70‰ (n = 303), significantly elevated with respect to seawater sulfate (∼21‰). The δ³⁴S values can vary more than 40‰ within individual barite aggregates (< 250 μm) and more than 50‰ within cm-scale samples. δ³⁴S_BaSO4 values in layered barite aggregates are most variable parallel to growth axes, with minimal variability in the perpendicular direction; fluctuations in δ³⁴S_BaSO4 values along growth axes are inferred to record temporal changes in sulfate distillation during barite precipitation. In layered barite aggregates, δ³⁴S_BaSO4 values frequently approach ∼90‰, but at these high δ³⁴S values, barite dissolution features become increasingly prevalent and may reflect an upper limit for porewater ³⁴S_SO4 enrichment while maintaining barite saturation in this system. We suggest the primary forcing affecting sulfate distillation is varying activity of anaerobic oxidation of methane coupled to sulfate reduction (AOM-SR) due to temporal changes in cold seep methane flux. These findings provide the first semi-continuous geologic proxy for paleo-methane flux on the Lofoten-Vesterålen continental margin and suggest methane advection rates varied considerably over the course of carbonate-barite crust formation. In addition to systematic microscale changes in δ³⁴S_BaSO4 values, periodic intra-aggregate dissolution features indicate a dynamic seepage environment with two or more periods of enhanced methane flux in recent geological history.