Comment on "A persistent oxygen anomaly reveals the fate of spilled methane in the deep Gulf of Mexico".

Kessler et al. (Reports, 21 January 2011, p. 312) reported that methane released from the 2010 Deepwater Horizon blowout, approximately 40% of the total hydrocarbon discharge, was consumed quantitatively by methanotrophic bacteria in Gulf of Mexico deep waters over a 4-month period. We find the evidence explicitly linking observed oxygen anomalies to methane consumption ambiguous and extension of these observations to hydrate-derived methane climate forcing premature.

Archaeal diversity in ODP legacy borehole 892b and associated seawater and sediments of the Cascadia Margin.

The Cascadia Margin is a region of active accretionary tectonics characterized by high methane flux accompanied by the formation of sedimentary gas hydrates, carbonate nodules, and carbonate pavements. Several sediment cores have been obtained from this region by the Ocean Drilling Project (ODP), and in some cases the boreholes have been sealed off, serving as sites for long-term observatories. We characterized geochemical parameters and diversity of Archaea in one such "legacy" borehole, ODP site 892b, as well as in bottom water immediately above the borehole and in two nearby sediments. The methane concentrations in the samples varied over five orders of magnitude, from approximately 25 to 35 nM in the bottom water to approximately 1.4mM in one of the sediment samples. Despite these differences, the Archaeal community in all samples was dominated by gene sequences related to the methanogenic Archaea, a finding that correlates with studies of other environments characterized by high methane flux. The archaeal phylotype richness in borehole ODP 892b was limited to two phylotypes; one specifically related to Methanosaeta spp., the other to the anaerobic methane oxidizing ANME-1 group. Although some similar groups were observed in nearby sediment and seawater samples, their archaeal phylotype richness was significantly higher than in the borehole. The possible presence of a dynamic microbial community in the Cascadia Margin sub-surface and its potential roles in methanogenesis, anaerobic oxidation of methane, and authigenic precipitation of carbonate in the Cascadia Margin are discussed.

Seawater sulfur isotope fluctuations in the Cretaceous.

The exogenic sulfur cycle is tightly coupled with the carbon and oxygen cycles, and therefore a central component of Earth's biogeochemistry. Here we present a high-resolution record of the sulfur isotopic composition of seawater sulfate for the Cretaceous. The general enrichment of isotopically light sulfur that prevailed during the Cretaceous may have been due to increased volcanic and hydrothermal activity. Two excursions toward isotopically lighter sulfur represent periods of lower rates of pyrite burial, implying a shift in the location of organic carbon burial to terrestrial or open-ocean settings. The concurrent changes in seawater sulfur and inorganic carbon isotopic compositions imply short-term variability in atmospheric oxygen partial pressure.