Impacts of Revised Geologic Forcing Factors on the Phanerozoic Carbon Cycle

Abstract

Traditionally, biogeochemical models are forced by the 87Sr/86Sr, δ13C and/or δ34S isotopic records. This study builds on existing work to present a forwards model for the entire Phanerozoic that aims instead to predict these isotopic variations, as well as the key environmental variables of global temperature, pCO2, pO2 and ocean sulphate concentration.

The main geologic forcings; tectonic uplift, degassing, and basaltic area, of the COPSE biogeochemical model are revised following recent updates to the geologic record. In addition, a simple weatherability function is developed to account for the latitudinal location of large igneous province basalt. Further revisions are made to the model seafloor weathering and phosphorus weathering fluxes.

A simplified strontium isotope system is adopted, which successfully reproduces the broad-scale Phanerozoic 87Sr/86Sr record, providing a test of the model predictions. The model also successfully predicts proxy data for both δ13Ccarb and δ34S.

The updated model predictions of temperature and pCO2 are improved for the Early-Palaeozoic, which is characterised by a gradual cooling trend. However the duration of the Permo-Carboniferous glaciation is underestimated. This is attributed to an arbitrary reduction in the C:P burial ratio of plant biomass to account for the evolution of lignolytic organisms, although their impact has been questioned in recent research.

Revision of the geologic forcings leads to an improved prediction of Phanerozoic pO2 that shows better agreement with other biogeochemical models in the Mesozoic, a period in which the original COPSE model probably overestimated the atmospheric oxygen concentration.

Following the initial model updates, the complexity of the model is increased through the creation of a dynamic land area forcing, accounting for the incomplete preservation of large igneous provinces, and coupling the seafloor weathering function to both temperature and spreading rates. Whilst having clear impacts upon modelled temperature, pCO2 and pO2, such updates invalidate the 87Sr/86Sr prediction. This is addressed by increasing the complexity of the strontium isotope system through the inclusion of rubidium decay and a dynamic sediment strontium subsystem, however the 87Sr/86Sr prediction remains invalidated for large parts of the Phanerozoic. As such, a simplified strontium isotope system is seen to offer the best approximation of the Phanerozoic record.