Modeling the early Paleozoic long-term climatic trend.
Résumé
The early Paleozoic climate has been described as warm and equable. However, recent data based on conodont oxygen isotopic composition reveal a large, long, cooling trend through the Ordovician, followed by an abrupt cooling during the Late Ordovician glaciation. This long-term climate change is associated with a major radiation in the Earth life history. Nonetheless, the driving mechanisms for this cooling trend remain unknown. Carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change. However, the impact of the plate motion context has not been explored yet, although it might have a major impact on atmospheric CO2 levels. Simulations with a climate model coupled to a biogeochemical model (GEOCLIM) show that the atmospheric CO2 decreased from more than 20 PAL (~5600 ppmv) in the Furongian down to approximately 10 PAL (~2800 ppmv) in the Llandovery before rising again in the Early Devonian. We suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large CO2 drawdown that led to the onset of cooler to glacial conditions from the Middle Ordovician to the Llandovery. The weathering of fresh volcanic rocks is itself responsible for 33% of the Late Ordovician atmospheric CO2 decrease; the rest being related to the continent motion through the intertropical convergence zone (ITCZ). Mean annual continental temperature falls by 3°C in the Early Ordovician, reaching 13.5°C during the glacial interval, and rises to 16°C in the Early Devonian.