The isotopic composition of carbonate carbon (“delta 13C”) in the rock record is a central benchmark for global geochemical change, because it tracks the relative amount of organic and inorganic carbon flowing through the ocean and sediment systems, which connects it to oxygen, ocean alkalinity, temperature (via CO2), and numerous other processes. One of the most immediately obvious features of the delta-13-C record over the entire course of Earth’s history is that the amplitude and period of fluctuation declines over time. In this paper, myself and my colleagues Tais Dahl, Christian Bjerrum and Donald Canfield conduct a comparative study of different mathematical models seeking to explain the nature of such delta-13C fluctuations. We note that many of the more realistic explanations center on low oxygen levels in marine sediments – and we suggest, roughly speaking, that when moving animals evolved and ventilated such sediments, these fluctuations became less likely.
From a broader point of view, this fits in with a general principle first identified by Vladimir Vernadsky; that life is the source of qualitative change in the thermodynamic field of the biosphere. One interpretation of this principle is that when we observe a clear directional change in the geochemical record, a biological influence is the first thing we should check for.