Several lines of geological and geochemical evidence indicate that the level of atmospheric oxygen was extremely low before 2.45 billion years (Gyr) ago, and that it had reached considerable levels by 2.22 Gyr ago. Here we present evidence that the rise of atmospheric oxygen had occurred by 2.32 Gyr ago. We found that syngenetic pyrite is present in organic-rich shales of the 2.32-Gyr-old Rooihoogte and Timeball Hill formations, South Africa. The range of the isotopic composition of sulphur in this pyrite is large and shows no evidence of mass-independent fractionation, indicating that atmospheric oxygen was present at significant levels (that is, greater than 10(-5) times that of the present atmospheric level) during the deposition of these units. The presence of rounded pebbles of sideritic iron formation at the base of the Rooihoogte Formation and an extensive and thick ironstone layer consisting of haematitic pisolites and oölites in the upper Timeball Hill Formation indicate that atmospheric oxygen rose significantly, perhaps for the first time, during the deposition of the Rooihoogte and Timeball Hill formations. These units were deposited between what are probably the second and third of the three Palaeoproterozoic glacial events.

Geochemical, microbiological, and molecular analyses of alkaline saline groundwater at 2.8 kilometers depth in Archaean metabasalt revealed a microbial biome dominated by a single phylotype affiliated with thermophilic sulfate reducers belonging to Firmicutes . These sulfate reducers were sustained by geologically produced sulfate and hydrogen at concentrations sufficient to maintain activities for millions of years with no apparent reliance on photosynthetically derived substrates.

Nature © Macmillan Publishers Ltd 19988 with occasional peaks up to 17. Furthermore, the peak in charcoal abundance inferred by Verardo and Ruddiman 17 at the Last Glacial Maximum corresponds to samples with a very small range of C/N ratios between 8 and 12, suggesting that these samples are predominantly composed of marine-derived carbon. Isolated peaks in C/N ratio up to 18 in some samples may correspond to events similar to those identified in ODP-668B.Second, the flux rates of elemental carbon calculated by Verardo and Ruddiman 17 range as high as 50,000 g cm −1 kyr −1 , which is two orders of magnitude higher than calculated for a range of recent, Quaternary and Tertiary marine sediments by other researchers. Smith et al. 16 calculated an average pre-industrial flux rate for Pacific and Atlantic sediments of 100 g cm −1 kyr −1 , whereas Herring 11 calculated Quaternary fluxes of 3-600 g cm −1 kyr −1 and Tertiary fluxes of 0.3-10 g cm −1 kyr −1 for Pacific Ocean sediments remote from coastal regions. These values agree well with the OREC fluxes calculated for ODP-668B (1-400 g cm −1 kyr −1 ), despite the fact that the data were generated by different techniques.A sample of Antarctic marine sediment previously shown by Bird and Gröcke 28 to contain Ͻ1% elemental carbon was analysed in triplicate using the Verardo and Ruddiman 17 technique. The results indicate that only 49 Ϯ 5% of the carbon in this sample was removed by in situ nitric acid oxidation, implying an elemental carbon content of around 50%. It is possible that, whereas some organic carbon is oxidized directly to CO 2 , a considerable quantity of more refractory organic carbon is only partly oxidized and solubilized by the treatment but not removed, leading to an overestimate of 'charcoal' abundance using the Verardo and Ruddiman technique.The OREC results from our study suggest that during the transition from interglacial to glacial mode the regional climate may have been destabilized and highly variable, leading to the buildup of fuel loads during wet periods followed by intense biomass burning in subsequent dry periods. In addition, biomass burning may be one mechanism by which high terrestrial organic carbon stocks built up during interglacials are shed at the onset of the subsequent glaciation.The most recent peak in OREC abundance is unique in the past million years, in that it has occurred during an interglacial period. Anthropogenic biomass burning is considered the likely cause of this peak. Given that the current conditions of interglacial aridity in the region are also considered by some to be unusual [5][6][7]22 , further consideration should be given to the possibility that a range of human activities in sub-Saharan Africa may have resulted in regional modifications to 'natural' vegetation patterns and climate. Ⅺ

Email alerting services cite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribe to subscribe to Geology www.gsapubs.org/subscriptions/ click Permission request to contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society. positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

Email alerting servicescite this article to receive free e-mail alerts when new articles www.gsapubs.org/cgi/alerts click Subscribeto subscribe to Geology www.gsapubs.org/subscriptions/ click Permission requestto contact GSA http://www.geosociety.org/pubs/copyrt.htm#gsa click viewpoint. Opinions presented in this publication do not reflect official positions of the Society.positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political article's full citation. GSA provides this and other forums for the presentation of diverse opinions and articles on their own or their organization's Web site providing the posting includes a reference to the science. This file may not be posted to any Web site, but authors may post the abstracts only of their unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make GSA, employment. Individual scientists are hereby granted permission, without fees or further requests to

Microbial communities responsible for methane cycling in mud volcanoes onshore are poorly characterized. This study analysed bubbling fluids and cored sediments retrieved from a mud volcano in eastern Taiwan. The pore water profiles revealed that methane concentrations generally increased with depth and changed dramatically at different depth intervals at different sites. The methane concentrations were inversely correlated with Fe(2+)/Mn(2+) concentrations and δ(13)C values of methane, marking iron/manganese-methane transition zones in the sediment cores. Archaeal communities were dominated by ANME-2a members and methylotrophic methanogens, whereas bacterial communities consisted primarily of Proteobacteria, Firmicutes and Bacteroidetes. The 16S rRNA gene copy numbers of ANME-2a and Desulfuromonas/Pelobacter populations varied by two to three orders of magnitude along the profile and exhibited a pattern comparable with those of Fe(2+) and δ(13)C values of methane. These lines of evidence suggest a coupling between anaerobic methanotrophy and metal reduction in the metal-methane transition zones under sulfate-deficient conditions, a metabolic scheme contrasting with that observed in marine cold seeps. Anaerobic methanotrophs proliferate by removing methane produced from in situ methanogenesis and originating from the deep source. Methane finally emitted into the atmosphere is quantitatively and isotopically altered by various microbial processes compartmentalized at different depth intervals.

No abstract

Left-lateral motion along the Ailao Shan±Red River (ASRR) Shear Zone has been widely advocated to be the result of the collision between the Indian and Eurasian plates and to account for sea-¯oor spreading in the South China Sea. Our new 40 Ar/ 39 Ar data on the south-easternmost outcrop of the Day Nui Con Voi metamorphic massif, northern Vietnam, suggest that the exhumation of metamorphic massif by shearing along the ASRR zone began H27 Ma and lasted until H22 Ma. A perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of H6 cm y À1. Such a good correlation also indicates that the onset of the leftlateral movement of the shear zone may have occurred later than H27.5 Ma. This conclusion is consistent with our previous interpretation that collision-induced southeastward extrusion of Indochina along the ASRR Shear Zone postdates the opening of the South China Sea, and that extrusion tectonics in SE China may not be responsible for the opening of the South China Sea. #