Paper No. 4
Presentation Time: 1:50 PM


JOHNSON, Clark, Department of Geoscience, University of Wisconsin-Madison, NASA Astrobiology Institute, 1215 W. Dayton St, Madison, WI 53706 and VAN KRANENDONK, Martin J., School of Biological, Earth and Environmental Sciences, University of New South Wales, Kensington, NSW, 2052, Australia,

The continental crust remains an important record of biogeochemical cycles through Earth history. Molecular phylogeny constrains relative ages of microbial metabolisms, and t stable isotope variations of elements cycled by life (C, N, O, S, Fe, etc.) can constrain both the occurrence, as well as the “footprint” that these metabolisms had in surface environments of ancient Earth. A long-standing question is the role of crust-mantle-atmosphere evolution in either driving, or responding to, changes in the biosphere. Smoothly evolving crustal growth models are difficult to reconcile with the punctuated record of atmosphere evolution, global glaciations, iron formation deposition, and stable isotope variations in the rock record. Recent work on O-Hf isotope variations in detrital zircons have highlighted periods of extensive crust formation and crustal reworking, beginning at ~3 Ga, and including peaks in reworking rates at ~2.7-2.6 Ga, ~1.9-1.7 Ga, and 1.2-1.1 Ga corresponding to changes in continental configuration and orogenic activity. The first major peak in crustal reworking correlates with changes in C and Fe isotope variations that indicate a significant footprint for methanotrophy and methanogenesis, as well as dissimilatory iron reduction, followed by extensive iron formation deposition. There is increasing evidence for free oxygen in the photic zone of the ocean during this first peak in crustal reworking, reflecting evolution of oxygenic photosynthesis prior to the rise in free atmospheric oxygen. The rise of atmospheric oxygen is directly correlated with a lull in magmatic activity (2.4-2.2 Ga) and the C isotope record in marine carbonates, accompanied by an increase in the footprint of dissimilatory sulfate reduction but contraction of dissimilatory iron reduction, as reflected in S-Fe isotope variations. The second peak in crustal reworking is correlated with a renewal of iron formation deposition but with markedly different stable isotope compositions than earlier periods, as well as a return in the C isotope compositions of marine carbonates to typical values. These observations suggest a strong, causative link between changes in mantle-crust formation processes and changes in the evolution of life on early Earth. Further research will focus on unraveling these connections.