Affiliation:
1. Department of Geology and Geophysics, Woods Hole Oceanographic InstitutionWoods Hole, MA 02543, USA
2. Joint Program in Chemical Oceanography, Woods Hole Oceanographic Institution, and Massachusetts Institute of TechnologyCambridge, MA 02139, USA
Abstract
Earth's biogeochemical cycle of carbon delivers both limestones and organic materials to the crust. In numerous, biologically catalysed redox reactions, hydrogen, sulphur, iron, and oxygen serve prominently as electron donors and acceptors. The progress of these reactions can be reconstructed from records of variations in the abundance of
13
C in sedimentary carbonate minerals and organic materials. Because the crust is always receiving new CO
2
from the mantle and a portion of it is being reduced by photoautotrophs, the carbon cycle has continuously released oxidizing power. Most of it is represented by Fe
3+
that has accumulated in the crust or been returned to the mantle via subduction. Less than 3% of the estimated, integrated production of oxidizing power since 3.8 Gyr ago is represented by O
2
in the atmosphere and dissolved in seawater. The balance is represented by sulphate. The accumulation of oxidizing power can be estimated from budgets summarizing inputs of mantle carbon and rates of organic-carbon burial, but levels of O
2
are only weakly and indirectly coupled to those phenomena and thus to carbon-isotopic records. Elevated abundances of
13
C in carbonate minerals
ca
2.3 Gyr old, in particular, are here interpreted as indicating the importance of methanogenic bacteria in sediments rather than increased burial of organic carbon.
Subject
General Agricultural and Biological Sciences,General Biochemistry, Genetics and Molecular Biology
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