Revising key parameters for long-term carbon cycle models

CO₂ is the most important greenhouse gas in the Earth’s atmosphere and has fluctuated considerably over geological time. However, proxies for past CO₂ concentrations have large uncertainties and are mostly limited to Devonian and younger times. Consequently, CO₂ modelling plays a key role in reconstructing past climate fluctuations. Facing the limitations with the current CO₂ models, we aim to refine two important forcings for CO₂ levels over the Phanerozoic, namely carbon degassing and silicate weathering.

Silicate weathering and carbonate deposition is widely recognized as a primary sink of carbon on geological timescales and is largely influenced by changes in climate, which in turn is linked to changes in paleogeography. The role of paleogeography on silicate weathering fluxes has been the focus of several studies in recent years. Their aims were mostly to constrain climatic parameters such as temperature and precipitation affecting weathering rates through time. However, constraining the availability of exposed land is crucial in assessing the theoretical amount of weathering on geological time scales. Associated with changes in climatic zones, the fluctuation of sea-level is critical for defining the amount of land exposed to weathering. The current reconstructions used inmodels tend to overestimate the amount of exposed land to weathering at periods with high sea levels. Through the construction of continental flooding maps, we constrain the effective land area undergoing silicate weathering for the past 520 million years. Our maps not only reflect sea-level fluctuations but also contain climate-sensitive indicators such as coal (since the Early Devonian) and evaporites to evaluate climate gradients and potential weatherablity through time. This is particularly important after the Pangea supercontinent formed but also for some time after its break-up.

Whilst silicate weathering is an important CO₂ sink, volcanic carbon degassing is a major source but one of the least constrained climate forcing parameters. There is no clear consensus on the history of degassing through geological time as there are no direct proxies for reconstructing carbon degassing, but various proxy methods have been postulated. We propose new estimates of plate tectonic degassing for the Phanerozoic using both subduction flux from full-plate models and zircon age distribution from arcs (arc-activity) as proxies.

The effect of revised modelling parameters for weathering and degassing was tested in the well-known long-term models GEOCARBSULF and COPSE. They revealed the high influence of degassing on CO₂ levels using those models, highlighting the need for enhanced research in this direction. The use of arc-activity as a proxy for carbon degassing leads to interesting responses in the Mesozoic and brings model estimates closer to CO₂  proxy values. However, from simulations using simultaneously the revised input parameters (i.e weathering and degassing) large model-proxy discrepancies remain and notably for the Triassic and Jurassic.