EVIDENCE FOR MARINE METHANE PROCESSING AS A CHEMISTRY-CLIMATE FEEDBACK IN THE EARLY PALEOZOIC (Invited Presentation)

The lower Paleozoic greenhouse-ice house transition was a period of dramatic geochemical, climatic, and biotic instability. Many mechanisms have been invoked to explain the onset and cessation of glacial episodes, extinction drivers, and to constrain ocean chemistry changes; in any of these models, however, the frequent large shifts in climatic and redox conditions imply a high gain system impacted by feedbacks.

Lipid biomarker analysis of Ordovician-Silurian age rocks of Laurentia identified a consistent profile of exceptionally elevated abundances of 3beta-methylhopanes [3-MeH], particularly during warm greenhouse intervals (Rohrssen et al., 2013). Further investigation reveals that elevated 3-MeH, significantly higher than the Phanerozoic baseline range for organic-rich rocks and oils, are a common feature of Ordovician–Silurian rocks of Baltica as well. Precursors of these compounds in the modern day are most abundantly produced by methane oxidizing bacteria (MOB). Because MOB may incorporate methane-derived carbon, their hopanoids impart a ¹³C-depleted signature to total sedimentary hopanoids when very abundant. Compound-specific carbon isotope analyses of early Paleozoic hopanes reveal depletions in ¹³C, as expected for a mixture of methanotroph and non-methanotroph hopanoids, in comparison to marine hopanoids throughout the Phanerozoic. Based upon the high abundance of lipids associated with methanotrophy and results of compound-specific carbon isotope analysis, combined with biogeochemical modeling of the ocean-atmosphere system, we suggest that enhanced methane fluxes were pervasive in Ordovician-Silurian marine sediments and water columns in comparison with other Phanerozoic intervals.

We propose that the lower oxidant status of the early Paleozoic ocean promoted temperature-dependent diagenetic methane processing through the coupled effects of a strengthened methane source and a weakened sink. Our findings suggest that pervasive ocean-atmosphere methane cycling may have played a significant role in amplifying climate change in intervals of the lower Paleozoic.

Rohrssen, et al., 2013, Geology 41:127-130