A GLOBAL CHANGE IN δ¹³C OF ORGANIC MATTER DURING THE LATE ORDOVICIAN HIRNANTIAN: IMPLICATIONS FOR ATMOSPHERIC CO₂ LEVELS AND GLACIATION

The cause of the Late Ordovician ice age is uncertain, and a paradoxical association with high atmospheric CO₂ levels (pCO₂) has been widely discussed. However, a recent model that includes Sr isotope data as a proxy for weathering of terrestrial volcanic rocks (Berner, 2006) has lowered these estimates of pCO₂. Still, the precise link between pCO₂ and ice sheet growth remain poorly understood because of uncertainty in global time correlation of stratigraphic sections recording geochemical proxy evidence with those containing physical records of glaciation (sea level changes, tillites). Here we report high-resolution paired δ¹³C_carb and δ¹³C_org analyses from well-dated rock sequences in Estonia and Anticosti Island, Canada. These isotopic records, from two separate paleocontinents (Baltica and Laurentia), are consistent with the notion that pCO₂ levels increased during the Late Ordovician as ice sheets expanded and reduced rates of silicate weathering. However, the highest pCO₂ levels coincide with peak δ¹³C_carb excursion values and geologic evidence of deglaciation (interglacial). Warming and oceanic stratification, during peak δ¹³C_carb values, increased organic carbon burial in the deep ocean, which eventually contributed (together with silicate weathering) to a second draw down in pCO₂ levels, and a second episode of glaciation.