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.沿海及海洋沉积物富硫酸盐区域内发生的甲基营养型产甲烷作用（methylotrophic methanogenesis），在功能上与厌氧甲烷氧化（anaerobic methane oxidation, AOM）紧密关联，二者共同构成了一处隐秘的甲烷循环。本研究针对美国加利福尼亚南部卡皮特里亚盐沼保护区（Carpinteria Salt Marsh Reserve, CSMR）内一条海陆样带的4个采样点开展工作，其中包含2个半咸水站点、1个海洋站点及1个高盐站点，所获数据聚焦于该区域的此类隐秘甲烷循环过程。研究针对表层沉积物（0~20 cm深度）开展了地球化学剖面分析，并结合放射性示踪培养实验，实验分别采用³⁵S标记硫酸盐、¹⁴C标记一甲胺以及¹⁴C标记甲烷作为示踪剂。 所有采样点的沉积物孔隙水中甲烷浓度普遍较低（3~28 μM），仅海洋站点例外：其孔隙水甲烷浓度随深度增加而升高，最高可达665 μM。所有站点的全深度剖面中均检出了以一甲胺为底物的甲烷生成过程，其估算速率范围为每立方厘米沉积物每日亚纳摩尔至纳摩尔量级。本研究通过¹⁴C标记甲烷示踪剂对厌氧甲烷氧化（AOM）进行定量，结果显示各站点的AOM均与甲基营养型产甲烷作用共存，其活性水平介于0.03~19.4 nmol·cm⁻³·d⁻¹之间。 孔隙水化学分析结果显示，所有采样点的硫酸盐与溶解性铁浓度均处于较高水平。硫酸盐浓度范围为9~91 mM，始终足以支撑硫酸盐还原过程，该过程的活性水平介于1.5~2506 nmol·cm⁻³·d⁻¹之间。硫化物与Fe(II)的垂直剖面特征表明，该海陆样带存在明显的地球化学分异：半咸水站点以铁还原作用为主导，而海洋与高盐站点则以硫酸盐还原作用为主导。厌氧甲烷氧化（AOM）的活性分布与硫酸盐还原及Fe(II)富集区域高度重合，这表明在所有采样点位中，甲烷氧化均可分别与硫酸盐还原及铁还原过程形成耦合。