Improved efficiency of the biological pump as a trigger for the Late Ordovician glaciation

The first of the 'Big Five' Phanerozoic mass extinctions occurred in tandem with an episode of glaciation during the Hirnantian Age of the Late Ordovician. The mechanism or change in the carbon cycle that promoted this glaciation, thereby resulting in the extinction, is still debated. Here we report new, coupled nitrogen isotope analyses of bulk sediments and chlorophyll degradation products (porphyrins) from the Vinini Creek section (Vinini Formation, Nevada, USA) to show that eukaryotes increasingly dominated marine export production in the lead-up to the Hirnantian extinction. We then use these findings to evaluate changes in the carbon cycle by incorporating them into a biogeochemical model in which production is increased in response to an elevated phosphorus inventory, potentially caused by enhanced continental weathering in response to the activity of land plants and/or an episode of volcanism. The results suggest that expanded eukaryotic algal production may have increased the community average cell size, leading to higher export efficiency during the Late Katian. The coincidence of this community shift with a large-scale marine transgression increased organic carbon burial, drawing down CO₂ and triggering the Hirnantian glaciation. This episode may mark an early Palaeozoic strengthening of the biological pump, which, for a short while, may have made eukaryotic algae indirect killers.