Understanding the early Paleozoic carbon cycle balance and climate change from modelling

Abstract

The Ordovician global cooling trend observed by several temperature proxies, which coincides with one of the most significant evolutionary diversifications on Earth, is yet to be fully understood. This study presents new simulations of pCO2 and surface temperatures using a spatially resolved climate-carbon cycle Earth system model fed with refined continental reconstructions and new estimates of solid Earth degassing. First, in order to quantify the respective roles of paleogeography and degassing, the impact of continental drift alone is investigated. This is done by calculating its imprint on continental weathering rates throughout the Ordovician under a constant pCO2, and by testing different topography inputs. Secondly, the sensitivity of the Earth's climate to paleogeography and carbon degassing changes is investigated by coupling the climate and carbon cycles in the GEOCLIM model. Based on our experiments we show that, although Early Ordovician high temperatures can be replicated within error margins, our new constraints cannot explain the intense cooling over the Mid to Late Ordovician, even if a progressive enhancement in Earth surface weatherability is taken into account. By using GEOCLIM in an inverse modelling approach, we calculate that the theoretical degassing necessary to reach proxy-derived temperatures for the Early Ordovician is three-to-five times higher compared to modern values. Further, in order to simulate the following Ordovician cooling trend, the solid Earth degassing must be reduced to modern-day values in only 30 Myrs. We conclude that, if accepting the veracity of the high Early Ordovician temperatures, alternative sources but also sinks of carbon must be considered to explain the climatic shift over the period.