Impact of Atmosphere-sea Exchange on the Isotopic Expression of Carbon Excursions: Observations and Modeling of OAE-1a

Negative carbon isotope excursions are a recurring phenomenon in earth history (e.g., Permo-Triassic boundary, Jurassic and Cretaceous oceanic anoxic events, and the Paleocene-Eocene Thermal Maximum) variously attributed to destabilization of methane clathrates, a decrease in primary productivity, intensified volcanism, and more recently to widespread peat fires. Each forcing mechanism invoked accounts for both the magnitude of the negative isotopic shift and the reservoir required to drive the shift as observed at one to several locales. Studies rarely consider the effect of latitudinal temperature changes on the excursion. Here, we explore the early Aptian oceanic anoxic event as an example of a negative isotopic shift whose magnitude varies with paleolatitude in terrestrial settings. It increases (from -2.0 to -8.2 ‰) with paleolatitude (5° to 33°N) and is greater than that expected for changes in plant C isotope discrimination driven by environmental stresses (~3 ‰). Conceptually, an isotopic shift of terrestrial vegetation across paleolatitudes represents a response to its forcing mechanism and temperature. A closed system carbon cycle model constructed of five reservoirs (atmosphere, vegetation, soil, and shallow and deep oceans), and five fluxes (productivity, respiration, litter fall, atmosphere-ocean exchange, and surface-deep ocean exchange) was employed is assessment of a negative isotopic shift at 2x pre-industrial atmospheric levels (P.A.L.) for pCO2 keeping all variables constant with the exception of temperature. The model was run at 5°C increments from 5° to 40°C to simulate the effect of temperature gradients on isotopic shifts at variable latitudes, with the appropriate temperature dependent fractionations for atmosphere - sea exchange. The magnitude of the negative isotopic shift at each temperature was calculated for both terrestrial and marine organic matter. In terrestrial vegetation it changed from -4 to -5.8 ‰ with decreasing temperature (from 40° to 5°C), and from -2.3 to -3.0 ‰ for marine organic matter. Increasing pCO2 to 4x P.A.L. offered a better fit of the modeled data and published terrestrial values. Literature values for δ13C of organic matter from marine settings record a narrower range for the negative isotopic shift (-1.6 to -3.8 ‰) versus paleolatitude because most studies have focused on the Tethyan region where proximal basin sites may record mixtures of both terrestrial and marine organic matter thereby diluting the signature of marine organic matter. The magnitude of the negative isotopic shift, recorded in terrestrial organic matter is enhanced at lower temperatures and buffered by higher carbonate solubility in seawater. Thus, caution should be exercised in the use of the magnitude of the negative isotope shift for limited range in paleolatitudes in mass balance equations and evidence of a global forcing mechanism. These results are broadly applicable for all negative carbon isotope shifts, and their ascribed forcing mechanisms whether in terrestrial and marine records, and across geological time.