EVIDENCE FOR MESOPROTEROZOIC LIFE ON LAND AND ITS MODERN COUNTERPART IN ARID SOILS

We have identified a suite of sedimentary structures, preserved on desiccation surfaces of sandstone/siltstone sequences within floodplain deposits of the Mesoproterozoic (~1200 Ma) Dripping Spring Quartzite, Apache Group, Arizona. These features show remarkable similarity to analogous structures formed by photosynthetic communities that comprise biological soil crusts on alluvial fan and floodplain surfaces of the Sonoran Desert today. This suggests that the ancient Dripping Spring examples were similarly biologically formed. While the intermontane alluvial fan setting of the modern Sonoran desert crusts differs in detail from the coastal alluvial plain setting inferred for the Dripping Spring, we believe the fine-grained, iron-rich sediments of the modern and Mesoproterozoic examples are united by deposition in interchannel flood plain environments, subject to periodic desiccation. Some of the desiccated surfaces in the Dripping Spring indeed contain microbial fossil casts. Additionally, well-preserved, spherical cellular microfossils were discovered within fine-grained roll-ups derived from mud-cracked surfaces. We combined thin section petrography with laser Raman spectroscopy to confirm that these microfossils are both organic-walled and indigenous to the rock. Collectively, the evidence suggests that microbial communities were present on exposed land surfaces under conditions of prolonged desiccation during the Mesoproterozoic. These observations, along with other evidence (e.g. light carbon isotopes, microfossil-like structures, fluvial microbialites) reported previously from similar sedimentary environments in the Precambrian, suggests that continental landmasses could have been colonized by microbes as early as the Archean