High dependence of Ordovician ocean surface circulation on atmospheric CO2 levels

• We model Early, Middle and Late Ordovician ocean surface circulation.
• Our basemaps are fully compatible with the most recent palaeontological databases.
• Synthetic maps are produced to aid the interpretation of palaeobiogeographical data.
• Major patterns of modelled ocean surface circulation appear to be model-independent.
• Ordovician surface circulation is highly dependent on atmospheric CO2 levels.

Life on Earth radiated dramatically during the Ordovician Period (485–444 Ma) at all taxonomic levels during an evolutionary episode known as the ‘Great Ordovician Biodiversification Event’. Abundant scientific literature has documented the pattern of biodiversification for numerous organism groups. However, fully understanding this event also requires detailed knowledge about the spatial distribution of the organisms. The full potential of the palaeobiodiversity patterns remains under-explored due to the lack of knowledge about past ocean circulation. Surface circulation significantly conditions the dispersal of organisms, in creating preferential ocean migration pathways or, in contrast, by isolating specific regions and thus establishing major physical barriers. These uncertainties persist because neither direct observational data, nor proxy data independent from the fossil record, are available to reconstruct the ocean surface circulation. Here we present new maps of Ordovician ocean surface circulation based on simulations using the general circulation model FOAM. This is a coupled ocean-atmosphere general circulation model and represents a methodological improvement over previously published work. In addition, we use the most up-to-date palaeogeographical reconstructions to increase compatibility with recent palaeontological databases. In providing clear, synthetic maps of ocean surface circulation in addition to raw model outputs, we aim to facilitate data-model comparison, to assist in interpretation of palaeontological datasets, and to promote renewed discussion about Ordovician biogeography. Finally, we investigate the sensitivity of the circulation pattern to the atmospheric CO2 content, which remains poorly constrained in the Ordovician, potentially ranging from more than 15 times the preindustrial atmospheric CO2 level (PAL) to ~ 5 PAL. Maps are systematically constructed for a high (16 PAL), medium (8 PAL) and low (4 PAL) CO2 value. We show that the circulation pattern is much more sensitive to CO2 than previously suggested.