Sediment geochemistry and microbial metabolic activity data on cryptic methane cycling in the Carpinteria Salt Marsh Reserve, California from sampling in July of 2019

Methylotrophic methanogenesis occurring within sulfate-rich zones of coastal and marine sediments is functionally linked to anaerobic methane oxidation (AOM), together constituting a cryptic methane cycle. This investigation presents data on such cryptic methane cycling across a land-to-sea transect comprising four sampling sites within the Carpinteria Salt Marsh Reserve (CSMR) in southern California, USA: two brackish, one marine, and one hypersaline. Surface sediments (upper 20 cm) were examined using geochemical profiling and radiotracer incubation experiments employing 35S-labeled sulfate, 14C-labeled monomethylamine, and 14C-labeled methane.

Methane concentrations in sediment porewater were generally low (3–28 µM) across all sites, except at the marine station, where levels increased with depth, reaching up to 665 µM. Methane production from monomethylamine was detected throughout the depth profiles at all stations, with estimated rates ranging from sub-nanomolar to nanomolar per cubic centimeter of sediment per day. AOM, quantified via 14C-CH₄ tracer, co-occurred with methylotrophic methanogenesis at each station, exhibiting activity levels between 0.03 and 19.4 nmol cm⁻³ d⁻¹.

Porewater chemistry revealed elevated concentrations of sulfate and dissolved iron across all sites. Sulfate levels (9–91 mM) remained sufficiently high to support sulfate reduction, which showed activity levels from 1.5 to 2,506 nmol cm⁻³ d⁻¹. Vertical profiles of sulfide and Fe(II) suggested a geochemical shift along the transect—from iron-dominated reduction at the brackish stations to sulfate-dominated reduction at the marine and hypersaline stations. AOM activity overlapped with zones of sulfate reduction and Fe(II) enrichment, indicating that methane oxidation may be coupled to both sulfate and iron reduction across all sampling locations.