ENHANCED METHANE CYCLING IN THE EARLY PALEOZOIC OCEAN

Summary Microbial production and consumption of methane represent critical components of Earth’s carbon cycle and climate system. Methane cycling feedbacks on climate may have been important during the Paleozoic Era, with anoxic deep waters and low sulphate promoting greater fermentative recycling of sedimentary organic matter and an enhanced methane cycle. We find that the typical Phanerozoic marine average for the 3-methylhopane index of 1–3% is substantially and consistently exceeded in a large suite of Ordovician and Silurian marine rocks from Laurentia and Baltica, often by an order of magnitude. Although other bacterial sources are possible for 3-methylhopanoids (3-MeH), in many Paleozoic marine rocks and oils these were largely derived from microaerophilic methanotrophic bacteria. Strong support for this methanotrophic bacterial source assignment comes from compound-specific δ13C analysis on individual hopane compounds. The Ordovician and Silurian hopanes yield some of the most 13C-depleted signatures found for Phanerozoic marine hopanes, outside of unusual methane seep settings. The protracted decrease in 3-MeH abundance through the Silurian-Devonian transition may be tracking the progressive oxygenation of the deep global ocean. Here, we test this idea by expanding the existing organic geochemical dataset on Ordovician and Silurian rocks with biogeochemical modeling of the Ordovician Earth system.