DID ATMOSPHERIC CARBON DIOXIDE REALLY DECLINE IN THE LATE PALEOZOIC AS SUGGESTED BY CARBON CYCLE MODELS?

The hypothesized decline in atmospheric carbon dioxide concentration (pCO₂) during the Late Paleozoic Era (approximately 420 to 300 million years ago) remains unresolved, despite the predictions of seminal geologic pCO₂ modeling studies such as the GEOCARB model (Berner, 1992; Berner & Kothavala, 2004). These models suggest a pCO₂ drawdown driven by biological innovations on land, particularly the evolution of deep soil-forming roots that enhanced the sensitivity of silicate weathering, thereby reducing pCO₂. However, empirical verification of this modeled decline has been contentious. While some proxy studies, such as those utilizing paleosol carbonate proxies by Mora et al., 1996, support the modeled stepwise decrease in pCO₂, others do not (Witkowski et al., 2018). Moreover, recent modeling studies by D’Antonio et al., 2020 argue against a change in silicate weathering during this period, citing carbon budget mass balance constraints.

To address this conundrum, we propose an alternative approach. If a decline in pCO₂ occurred during the Late Paleozoic due to enhanced soil formation (as suggested by the GEOCARB model), it should correspond to a significant increase in terrestrial chemical weathering observable in the rock record. We aim to reconstruct terrestrial silicate weathering through this period by analyzing a high-quality proxy of chemical weathering, i.e. Lithium isotopes preserved in brachiopods, at a high stratigraphic resolution. This reconstruction will allow us to determine whether the hypothesized pCO₂ model(s) such as GEOCARB, that base pCO₂ drawdown during this period on enhanced chemical weathering, need reevaluation (no change or depletion in Lithium isotopes over this record) or if plant root evolution did result in enhanced rates of silicate weathering, resulting in lower pCO₂ during the Late Paleozoic (enriched Lithium isotopes over this record).

This research is crucial in understanding the geological carbon cycle as the Late Paleozoic is thought to represent a state-shift in pCO₂, from a world of elevated CO₂ to the relatively low CO₂ realm we inhabit today. In addition, this work will shed light on the hypothesized role of organisms (trees with deep roots) in shaping the Earth's physical structure (particularly the atmospheric composition), during its past.