The Ordovician long-term global cooling trend is a key feature of the early Paleozoic climate that started in the late Cambrian (~500 Myrs ago), and proceeded rapidly during the Early Ordovician before tropical sea-surface temperatures reached a plateau during the Middle to Late Ordovician (~440 Myrs ago), and subsequently stabilized at modern-like levels. The striking feature of this climatic transition is its magnitude, with tropical sea-surface temperatures plummeting by around 10°C from ~40°C to 30°C until the Hirnantian glacial event between 445-444 Ma (Fig .1a). This cooling marks the transition from a greenhouse state to the first major continental glaciation of the last 540 million years. This end-Ordovician glaciation coincided with a mass extinction, the first of the Big Five extinctions, and the only mass extinction that occurred during icehouse conditions.
Because of the sparsity of data and the large spread of temperature estimates, the cause(s) of the long-term Ordovician cooling remains poorly understood. There have been several hypotheses proposed to explain this long-term cooling, including increasing continental weatherability to increase carbon uptake, or decreasing the amplitude of the solid Earth's carbon degassing.
The importance and contribution of each of these factors remain to be quantified in an integrated numerical framework since these hypotheses have been proposed in isolation using different methods and tools. In their new study, Marcilly and collaborators simulated the long-term Ordovician climate response to changes in paleogeography and degassing using the climate-carbon model GEOCLIM.
The new simulations indicate that continental weathering increased gradually during the Ordovician due to continental rearrangement (Fig. 2), while plate tectonic degassing decreased. Although this would have contributed to the global cooling, it cannot explain its magnitude (Fig. 1a).
Degassing estimates used in this study are the highest reconstructed for the Early Ordovician, but also the ones that exhibited the largest drop until the end-Ordovician. While this drop is significant, it is not sufficient to reproduce the temperature proxy values available for the period (Fig. 1a). The degassing required to reach proxy-derived temperatures for the Early Ordovician was calculated using an inverse modeling approach and showed values significantly higher than modern values (Fig. 1b). Moreover, to simulate the observed Ordovician cooling trend, the solid Earth degassing would have to be reduced to modern values within 30 Myrs, implying a sudden change in Earth's tectonic activity.
Contact information: – Chloé M. Marcilly, c.f.m.marcilly@geo.uio.no
Publication details: Marcilly, C. M., Maffre, P., Le Hir, G., Pohl, A., Fluteau, F., Goddéris, Y., ... & Torsvik, T. H. (2022). Understanding the early Paleozoic carbon cycle balance and climate change from modelling. Earth and Planetary Science Letters, 594, 117717. https://doi.org/10.1016/j.epsl.2022.117717
Funding for this research came from the Research Council of Norway (RCN), through its Centres of Excellence funding scheme, project 223272 (CEED).
