Ordovician cyanobacterial calcification: A marine fossil proxy for atmospheric CO2

Ordovician atmospheric CO2 level was an important influence on climate and life. However, modelled estimates of Ordovician CO2 differ widely and, in contrast to much of the subsequent Phanerozoic, they lack fossil proxy constraints. In vivo cyanobacterial sheath calcification, promoted by carbon dioxide concentrating mechanisms (CCMs), creates distinctive microfossils. These provide a direct ecophysiological link to ambient concentrations of CO2 and, to a lesser extent, O2. Cyanobacteria do not calcify at high CO2 concentrations. Experiments show that CCMs can be induced in present-day cyanobacteria at CO2 levels below ∼10 times present atmospheric level (PAL) and that this promotes sheath calcification. We compiled a global database of cyanobacterial calcification in marine environments throughout the Ordovician (485-443 Myr ago) and compared it with modelled estimates of atmospheric CO2 and O2. These data show that many genera of marine calcified cyanobacteria reappeared from Cambrian Series 2 or first appeared globally during the late middle to late Upper Ordovician (late Darriwilian to late Katian) in carbonate platform facies, resulting in a previously unrecognized ten-fold increase in global diversity. Such a large increase in calcification suggests widespread induction of CCM expression in cyanobacteria, consistent with sustained decline in external CO2 concentrations below ∼10× PAL, together with increase in dissolved O2 level, during the late Darriwilian, Sandbian and Katian stages, ∼460-445 Myr ago. These cyanobacterial calcification fossil proxy data provide a first order constraint on modelled estimates of atmospheric CO2 for the Ordovician. Estimated CO2 outputs from COPSE and recent GEOCARB-based models are broadly consistent with this cyanobacterial calcification proxy for the Ordovician. In view of the long history of cyanobacteria, the calcified cyanobacterial proxy offers potential to assist interpretation of CO2 deeper into geological time.