Life associated with a 2.76 Ga ephemeral pond?: Evidence from Mount Roe #2paleosol

Dark sericitic material at and near the top of the 2.765 +/- 0.01 Ga Mount Roe #2 paleosol in Western Australia contains 0.05-0.10 wt% organic carbon with delta(13)C values between -33 parts per thousand and -51 parts per tho usand PDB (Peedee belemnite). Such negative isotopic values strongly indica te that methanotrophs once inhabited this material. The textures and the ch emical composition of the dark sericitic material indicate that the methano trophs lived in or at the edges of ephemeral ponds, that these ponds became desiccated, and that heavy rains transported the material to its present s ites. The discovery of methanotrophs associated with the Mount Roe #2 paleo sol may extend their geologic record on land by at least 1.5 b.y. Methanotr ophy in this setting is consistent with the notion that atmospheric methane levels were greater than or equal to 20 mu atm during the Late Archean. Th e radiative forcing due to such high atmospheric methane levels could have compensated for the faint younger sun and helped to prevent massive glaciat ion during the Late Archean.