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Subsurface fluid flow in oceans and lakes affects bathymetric morphology, sediment distribution, and water composition. We present newly discovered giant lacustrine pockmarks in Lake Neuchatel (up to 160m diameter and 30m deep) that rank among the largest known pockmarks in lakes. Our multidisciplinary study reveals 60m of suspended sediment inside a pockmark. The sediment suspension is 2.6 degrees warmer and isotopically lighter in O-18(H2O) by 1.5 than the ambient lake water, documenting currently active fluid flow by karstic groundwater discharge from the Jura Mountain front into the Swiss Plateau hydrological system. Strikingly, the levees of the pockmarks comprise subsurface sediment mobilization deposits representing episodic phases of sediment expulsion during the past. They strongly resemble subsurface fluid flow features in the marine realm. Comparable processes are expected to also be relevant for other carbonate-dominated mountain front ranges, where karstic groundwater discharges into lacustrine or marine settings.

During the February response cruise to Axial Volcano, methane and hydrogen concentrations within the plume were significantly elevated above background. CH 4 / 3 He ratios were found to be 5-10 times greater than previously reported values for unsedimented ridges. An average δ 13 C value of -32.7±0.8‰ was determined for CH 4 concentrations > 10 nM and CH 4 /Mn ratios were determined to be 2.2. Hydrogen concentrations as high as 183 nM were measured; however most were < 10 nM. Microbial oxidation of methane was negligible (

Methane concentrations in the Beaufort Sea under the winter ice canopy offshore from northern Alaska are 3 to 28 times greater than they are in late summer when the ice is absent in a similar region offshore from northern Canada where methane is in approximate equilibrium with the atmosphere. These observations suggest that methane concentrates in the water under the sea-ice cover during winter and ventilates rapidly in late summer as the ice melts and retreats. Conditions similar to those on the Beaufort Sea shelf likely exist on the much larger Siberian shelf, making the Arctic Ocean margin a possible seasonal, high-latitude, marine source of about 0.1 Tg yr[sup [minus]1] atmospheric methane. The small addition of methane likely contributes to the late-summer increase in atmospheric methane that is observed each year particularly in the northern hemisphere. 32 refs., 2 figs.

[1] Methane reservoirs and seeps are an active component of the continental margin carbon budget and represent a poorly characterized pathway for reduced carbon cycling and methane input to the atmosphere. Active gas seeps from three shelf settings on the Cascadia Continental Margin off Oregon and Northern California contain nearly pure methane with a heavy carbon isotope composition (−29 to −35‰). An extensive study of the gas seep at Coquille Bank, Oregon, revealed a warm, buoyant pore fluid associated with the pockmark. As methane enters the water column above these seeps in a steady gas stream, a fraction escapes directly to the atmosphere while the balance dissolves into local seawater. Measured oxidation rates are too slow for significant local oxidation within the water column near the seep. Large mats of pink and white bacteria, including Beggiatoa spp. are found around the vent, demonstrating the activity of sulfide oxidizers in this ecosystem.