The Guaymas Basin spreading center, at 2000 m depth in the Gulf of California, is overlain by a thick sedimentary cover. Across the basin, localized temperature anomalies, with active methane venting and seep fauna exist in response to magma emplacement into sediments. These sites evolve over thousands of years as magma freezes into doleritic sills and the system cools. Although several cool sites resembling cold seeps have been characterized, the hydrothermally active stage of an off-axis site was lacking good examples. Here, we present a multidisciplinary characterization of Ringvent, an ~1 km wide circular mound where hydrothermal activity persists ~28 km northwest of the spreading center. Ringvent provides a new type of intermediate-stage hydrothermal system where off-axis hydrothermal activity has attenuated since its formation, but remains evident in thermal anomalies, hydrothermal biota coexisting with seep fauna, and porewater biogeochemical signatures indicative of hydrothermal circulation. Due to their broad potential distribution, small size and limited life span, such sites are hard to find and characterize, but they provide critical missing links to understand the complex evolution of hydrothermal systems.

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Redox stratification, especially hypolimnetic anoxia resulting from eutrophication, and salinization resulting from application of salts for road deicing is investigated in three kettle lakes in southwest Michigan. Two of the lakes (Asylum and Woods Lakes) are located in urban Kalamazoo, Michigan, and the third (Brewster Lake) is located in rural Hastings, Michigan. In summer, the water columns of all three lakes are distinctly redox stratified, with anoxic hypolimnia and significant accumulation of reduced solutes (e.g., Mn(II), Fe(II), ammonia) in the lake bottom waters. Extremely elevated conductivity, chloride, sodium, and potassium levels are observed in the urban Asylum and Woods Lakes compared to the rural Brewster Lake, presumably due to runoff of road salt deicers applied in the surrounding watershed. These significant changes in water quality are of concern because they may detrimentally impact lake mixing, biodiversity, and ecosystem function in the urban lakes.

Microbial methanogenesis is a known source of methane in marine environment, but the factors affecting the production of methane from different pathways remain largely unconstrained. We investigated the effects of pressure, methane concentration, temperature, and sulfate reduction activity on the conversion of methanogenic substrates to methane using radiotracers in nearshore and offshore surface sediments in the northern Gulf of Mexico. The transformation of bicarbonate to methane increased with methane concentrations under quasi in situ conditions of deep‐sea sediment, potentially reflecting, to some degree, enhancement of the enzymatic back reaction under high‐methane conditions. In coastal sediments, a positive effect of both increased methane concentration and inhibition of sulfate reduction by molybdate on 14CH4 accumulation suggests that anaerobic oxidation of methane and methanogenesis occur concurrently. In contrast, methane production from methanol was suppressed by increased methane concentration likely due to the reduced energy yield of methanogenesis at elevated methane concentrations. Temperature could limit hydrogenotrophic and acetoclastic methanogenesis under in situ conditions, as higher rates were observed at 40°C in nearshore sediments. Inhibitor experiments with 2‐bromoethanesulfonate and molybdate indicated that sulfate‐reducing bacteria preferentially utilized acetate, as well as H2. Methanol was a noncompetitive substrate for methanogens in the deep‐sea sediments but was competitively cycled in coastal sediments. An authentic noncompetitive substrate, methylamine, was channeled predominantly to methane production under all conditions tested. The different response of methanogenic activity to substrate availability, metabolic competition, temperature, and pressure between sites suggest variable environmental controls on the methane production in coastal vs. deep‐sea surface sediments.

We present a complete description of the depth distribution of marine snow in Orca Basin (Gulf of Mexico), from sea surface through the pycnocline to within 10 m of the seafloor. Orca Basin is an intriguing location for studying marine snow because of its unique geological and hydrographic setting: the deepest ~200 m of the basin are filled with anoxic hypersaline brine. A typical deep ocean profile of marine snow distribution was observed from the sea surface to the pycnocline, namely a surface maximum in total particle number and midwater minimum. However, instead of a nepheloid (particle-rich) layer positioned near the seabed, the nepheloid layer in the Orca Basin was positioned atop the brine. Within the brine, the total particle volume increased by a factor of 2–3 while the total particle number decreased, indicating accumulation and aggregation of material in the brine. From these observations we infer increased residence time and retention of material within the brine, which agrees well with laboratory results showing a 2.2–3.5-fold reduction in settling speed of laboratory-generated marine snow below the seawater-brine interface. Similarly, dissolved organic carbon concentration in the brine correlated positively with measured colored dissolved organic matter (r2 = 0.92, n = 15), with both variables following total particle volume inversely through the pycnocline. These data indicate the release of dissolved organic carbon concomitant with loss in total particle volume and increase in particle numbers at the brine-seawater interface, highlighting the importance of the Orca Basin as a carbon sink.