Feedback between mantle and biological cycles over the Earth's history

Details Feedback between mantle and biological cycles over the Earth's history Feedback between mantle and biological cycles over the Earth's history

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.v51f..01s&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #V51F-01

Mathematics

Logic

0300 Atmospheric Composition And Structure, 0400 Biogeosciences, 1030 Geochemical Cycles (0330), 8413 Subduction Zone Processes (1031, 3060, 3613, 8170), 8425 Effusive Volcanism

Scientific paper

The Earth condensed from silicate gas after the Moon-forming impact. H2 was a major component in the atmosphere until the surface cooled enough that the oceans condensed and the interior cooled enough that basalt covered the surface. During this hot epoch, about as much H2 as is now found in the ocean escaped from the atmosphere to space. Once basalt became the dominant rock-type in ocean basins, as it is now, outgassing and formation of carbonates buffered CO2; reaction of water with ferrous iron in serpentinites and hydrous volcanism produced H2. Some CH4 may have formed abiotically. Escape of hydrogen to space buffered the total H2 + 0.5 CH4 so it matched the flux from the interior. The situation following the advent of methanogens was similar except that they converted most of the H2 to methane to their thermodynamic limit of ~10 ppm. The flux of H2 from the mantle was small enough (<10e12 moles per year) that it did not greatly affect the redox state of the mantle. Less than 1% of the degassed CO2 ever became organic carbon before it was buried as carbonate. The air contained crudely 4500 ppm CO2, 100 ppm CH4, and 10 ppm H2. A transition with the evolution of (anoxygenic) photosynthesis occurred probably before 3.8 Ga. Biological methane became a significant species that released up to 5e12 moles per year to space. Carbon passed through photosynthesis and decay many times before it was buried. Marine sulfate was reduced to sulfide in shallow oceanic crust. Arc volcanoes thereafter were enriched in S relative to MORB. The net effect was to oxidize the Earth's crust, rather than to measurably change the redox state of the mantle. Two more transitions followed. At ~2.4 Ga, there was enough sulfate in the ocean that it formed anhydrite in oceanic crust that later redissolved. The oxygen and sulfur fluxes to the mantle then became independent of sulfate concentrations in the ocean. O2 in the air prevented further CH4 escape. At ~1.8 Ga, the O2 concentration in the ocean was enough to oxidize vented sulfide at hydrothermal vents. The cycles thereafter were similar to today. However a change is about to occur, as massive pelagic CaCO3 deposits will soon arrive at subduction zones.

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