SOILS, PLANTS, AND THE EVOLUTION OF THE INORGANIC CARBON CYCLE ON LAND

Much of what is speculated about the long-term evolution of terrestrial biogeochemistry on the Earth is inferred from marine sedimentary records – i.e., from the sink for continental weathering rather than the source. This has been supplemented by low-resolution direct records of terrestrial biogeochemistry from paleosols (fossil soils), in particular focused on redox geochemistry as a means of reconstructing atmospheric oxygen levels. Herein, to address both of these current problems (a “sink only” view of biogeochemistry and low resolution of “source” geochemical changes), we compile ~3000 geochemical analyses from paleosols spanning the past 3 Ga to look at first-order changes in Earth surface environments with an emphasis on the bioavailability of P, Fe, and Ca. P delivery from the continents is a key control on marine productivity, Fe is a key enzymatic cofactor, and Ca is both biologically important as well as a critical ingredient in inorganic C burial. From this compilation, we note three key features in the record: 1) P distribution in soils has been comparable to modern environments for the past 3 Ga, compatible with a terrestrial biosphere throughout that time; 2) Fe chemistry has followed a three-stage evolution wherein Fe was leached during pedogenesis until the Ordovician, indicating that high atmospheric oxygen levels were reached relatively recently in Earth history; and 3) Ca distribution in soils fundamentally changed after the evolution of land plants. This last point is supported by the lack of pedogenic carbonates until the Silurian. Mechanistically, this innovation of a terrestrial inorganic C sink may have been partially responsible for the drawdown of CO₂ in the Paleozoic and the global shift toward cooler climate conditions. Collectively, these records indicate that a putative “late Precambrian greening” of the land inferred from marine records is incorrect both because there was life on land billions of years prior and because fundamental changes to the C cycle did not occur until an inorganic C burial pathway had evolved on land.