TERRESTRIAL GEOBIOLOGY AND BIOSPHERE-ATMOSPHERE INTERACTIONS IN THE LATE MESOPROTEROZOIC

The ~1.1 Ga Midcontinental Rift (MCR) provides a unique window into terrestrial geobiology and biosphere-atmosphere interactions. The North Shore Volcanic Group (Minnesota) and Copper Harbor Conglomerate (Michigan) both preserve microbialites and unaltered organic carbon in floodplain facies and the lacustrine Nonesuch Formation (Michigan, Wisconsin) preserves organic carbon, degraded biomarkers, and acritarchs. Previous work on MCR paleosols indicates low atmospheric CO₂ levels (<10x PAL), which is supported by new CO₂ estimates based upon C isotope discrimination by cyanobacteria in Copper Harbor Conglomerate stromatolites. Using those CO₂ levels to drive an AOGCM climate simulation for the Mesoproterozoic, relatively high levels of a secondary greenhouse gas (either CH₄ or N₂O) are needed to maintain the observed equable climate conditions, but oceans at that time were likely limited in their CH₄ production capacity. As a result, it was recently suggested that wetlands may have been able to support high enough CH₄ levels to avoid a climate paradox. To that, we add the possibility of significant CH₄ flux from lakes. Nonesuch Formation δ¹³C values are ~-8‰ relative to floodplain values and are negative enough that only a methanogenic source is geologically plausible. One consequence of relatively high atmospheric CH₄ levels is that they could only be maintained if O₂ levels were also low. This conclusion is supported by ongoing work on Cr isotopes from MCR paleosols; they largely record positive values relative to the crustal inventory, inconsistent with strongly oxidative weathering. Early workers on MCR paleosols suggested that Fe was retained during weathering based upon simple Fe³⁺/Fe²⁺ ratios, but failed to account for volume change during weathering. Re-evaluation of paleosol Fe inventories to account for volume change indicates instead that there was significant Fe loss. Both types of data are consistent with low atmospheric O₂, intermediate between pre-GOE levels and the rising levels of the Neoproterozoic and Phanerozoic. At the same time, δ¹³C depth profiles through many of the paleosols exhibit Rayleigh fractionation consistent with in situ oxidation of organic matter, suggesting that low atmospheric O₂ levels did not significantly impact terrestrial microbial productivity.