Abstrtact

The evolution of photosynthesis strongly coupled of geological and biological processes. It left a record preserved even in the Earth’s mantle. Pre-photosynthetic niches, including methanogenesis and acetogenesis that reacted H₂ from serpentinite with atmospheric CO₂, were reliable but meager. Photolysis products and SO₂ and H₂ from surface volcanoes provided additional food and pre-selection to survive near the surface of the ocean. Acetogenetic organisms benefited from the ability to use sunlight. Their reaction is photocatalytic and light use was likely initially quite inefficient. Photosynthesis using sulfide and ferrous iron to make sulfide and ferric iron soon augmented the organism’s productivity. Eventually an organism passed a threshold where it could dispense with H₂. Its productivity increased by factors of thousands and it colonized the oceans over a period of years. The back reaction provided a full cycle with heterotrophs. This photosynthesiser is the ancestor of most bacterial clades, informally Photobacteria. Its descendants, Hydrobacteria and Terrabacteria, initially stayed in the ocean and colonized land. Terrabacteria required FeO for photosynthesis, which was locked in volcanic rocks and minerals. Soil bacteria benefited by weathering the minerals and eating the Fe₂O₃ and organic matter produced at the surface by photosynthesis. Metamorphosed black shales from Isua Greenland indicate that these marine and land ecosystems were in place by 3.8 billion years ago. Graphite and diamond inclusions in detrital zircons indicate even earlier photosynthesis. Land weathering products include the normal sandstone, shale, and carbonate rock classes. Low-Fe rocks, including granites from melted sediments, were inhospitable to Fe-based photosynthesis. Cyanobacteria evolved on land from this selective pressure. The air and shallow ocean became oxidized by 2.45 billion years ago. The deep ocean was suboxic by 1.85 billion years ago, following the mass extinction from the Sudbury impact. The Earth’s mantle sequesters a record of biological events. Some low hanging fruit: The oldest evidence of carbonate subduction leading to habitability is ~4.26 billion years ago. Magmas melted from the subducted material were emplaced in the Indian lithosphere at ~3.6 billion years ago. They remelted and intruded into the shallow crust 1.48 billion years ago (data in Upadhyay et al., Nature, 2009). Kimberlites carry biosignatures including secular increase in the U/Th ratio in zircon as the surface environment became oxidized (Zartman and Richardson, Chemical Geology, 2005). The concept that some diamonds are fossils is well known (Nisbet et al., Nature, 1994).  It should be possible to recognize in CO2-rich magmas the host carbonated oceanic crust (basalt, komatiite, serpentinite), the hydrothermal water chemistry (saline, hypersaline, fresh), and perhaps the ambient ocean temperature with sophisticated geochemistry. Thallium isotope anomalies in Hawaii basalts record the subduction of Mn-nodules and the oxidation of the deep ocean (Nielsen et al 2006).

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