PNNL

Abstract

Methylotrophic methanogenesis in the sulfate reduction zone of wetland and marine sediment has recently been coupled to anaerobic oxidation of methane (AOM), now referred to as the cryptic methane cycle. In this study we present evidence of cryptic methane cycling activity within the sulfate-reducing zone, along a land-ocean transect, consisting of four stations; two brackish, one marine, and one hypersaline, within the Carpinteria Salt Marsh Reserve (CSMR), Carpinteria, CA, USA. The top 20 cm of sediment collected along the transect was subjected to geochemical and molecular analysis, in vitro methanogenesis batch incubations, and radiotracer incubations using 35S-SO2-4, 14C-mono-methylamine, and 14C-CH4 to find evidence of cryptic methane cycling. Results showed that subsurface porewater salinity increased with increasing sediment depth in the two brackish stations while remaining close to saline or hypersaline in the other stations, suggesting complex subsurface hydrology across the CSMR. Methane concentrations were consistently low (3 to 28 µM) except at the marine station, which showed increasing methane with increasing sediment depth (max 665 µM). In vitro methanogenesis batch incubations showed no linear build-up of methane over time, except with sediment from deeper intervals at the marine station, suggesting a process that is limiting methane production in the sediment. AOM rates from direct measurements with 14C-CH4 were low in the two brackish and hypersaline stations (0.03 to 0.66 nmol cm-3 d-1) while at the marine station AOM rates increased with increasing sediment depth (Max 19.4 nmol cm-3 d-1). Total organic carbon (TOC) and total organic nitrogen (TON) were highest in the sediment intervals near the surface at the two brackish stations and hypersaline station, while at the marine station TOC and TON were higher towards the bottom of the core (