Biological diversity and resource plunder in the geological record: casual correlations or causal relationships?

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The two largest pools of carbon on Earth, carbonates and organic matter (kerogens, preserved primarily in shales), are both contained in the lithosphere (1). Both pools contain signatures of the two stable carbon isotopes, 13C and 12C; however, the processes responsible for the resulting isotopic fractionation differ. In the precipitation of calcite (the primary preserved form of carbonate in marine sediments), isotopic fractionation is dictated by thermodynamics; however, the isotopic discrimination is relatively small (2, 3). Hence, to first approximation, the 13C/12C composition of carbonates reflects the availability and isotopic signature of the source carbon (sea water). In contrast, the precipitation of organic carbon is dictated by kinetics; that is, when inorganic carbon is not limiting, the fixation of inorganic carbon by the enzyme ribulose 1,5 bis-phosphate carboxylase/oxygenase (RubisCO) leads to a ≈25–30‰ discrimination against the heavier isotope in the source carbon (4). As CO2 becomes limiting, the fractionation decreases. As all photosynthetic organisms contain RubisCO, and photosynthesis is by far the major route of entry of inorganic carbon into the organic realm, photosynthetic organisms potentially influence both the total pool of inorganic carbon and its isotopic distribution at any instance in time in Earth's history. The isotopic difference, ɛtoc, between the inorganic and organic carbon pools is, in principle, a semiquantitative proxy of the total inorganic carbon in the ocean–atmosphere system. In this issue of PNAS, Rothman (5) demonstrates that, as marine animal and terrestrial plant diversity increased in the latter part of the Phanaerozoic, ɛtoc values decreased, and the changes are significantly correlated. Using a model of marine phytoplankton isotope fractionation (6), Rothman suggests the relationship between ɛtoc and the diversity of both marine animals and terrestrial plants in the fossil record is causal and inverted the …