CARBON ISOTOPE RATIOS OF FOSSIL TOOTH DENTINE PRESERVED IN SOILS: A PROXY FOR WATER AVAILABILITY, FOR IDENTIFYING PROCESSES OF ORGANIC MATTER DECOMPOSITION & FOR CHARACTERIZING CO2 & CH4 EMISSIONS IN THE PAST

An important characteristic of soils is water availability, as water (1) plays a fundamental role as a chemical reactant in a number of biochemical reactions associated with their formation (e.g. chemical weathering, photosynthesis), and (2) it plays a fundamental role as a physical barrier to the movement/diffusion of other chemical reactants into soils – in particular atmospheric O₂ – necessary for other reactions to take place (e.g. oxidation of Fe and organic matter). Water availability in soils is reflected by the position of the water table, which is the boundary between lower parts of soils where pore spaces are filled with water, and upper parts of soils where they are not.

To study the position of the water table and water availability in ancient soil horizons, it is necessary to rely on proxies. Here a new proxy is described, one that first takes advantage of water’s role as a physical barrier to diffusion of O₂ into a soil and thus in determining the biogeochemical processes by which organic matter reacts to form CO₂: if O₂ is present, it can react with organic matter to form CO₂ and H₂O (i.e. oxidation), or in the absence of O₂ organic matter can react to form CH₄ and CO₂ (i.e. methanogenesis). Critically, each of these processes imparts a unique carbon isotope signature on soil CO₂. The second part of the proxy takes advantage of the fact that soil CO₂ can be incorporated into other materials present in soils, in particular bioapatite in fossil tooth dentine, such that carbon isotope ratio of this dentine can serve as a proxy for the occurrence of oxidation vs methanogensis in paleosols and thus the type of gases emitted from them, and in turn for water availability.

As an illustration of how this proxy can be applied, data from a suite of late Cretaceous and early Eocene fossil localities are used to investigate CH₄ emissions to the atmosphere and forest structure during these time periods. It is found that proxy evidence for CH₄ emissions match those of GCM predictions in some cases but not others and that the ‘isotopic canopy effect’ may have two forms depending on the position of the water table.