James Kasting

(PSU, Evan Pugh Professor, Department of Geosciences and Meteorology)

Mass-independent fractionation of oxygen isotopes during ozone formation and its implications for Proterozoic O2 levels.

When Feb 12, 2020
from 03:30 pm to 04:30 pm
Where 112 Walker Building, John Cahir Auditorium
Contact Name Steven Feldstein
Contact email
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James Kasting

Ozone, O3, forms more rapidly when one of its end O atoms is 17O or 18O instead of the normal isotope, 16O. Quantum chemists refer to this phenomenon as the ‘h effect’. This mass-independent fractionation pattern, denoted as D17O, is distinct from the normal mass-dependent fractionation that arises because of the difference in mass between the three isotopes. The positive D17O signal is passed on to CO2 via O3 photolysis and production of O(1D), followed by isotopic exchange between O(1D) and CO2. Residual atmospheric O2 develops negative D17O because of mass balance. Today, the atmospheric O2:CO2 ratio is large, so the D17O signal in O2 is small. But in the Mid-Proterozoic eon (1.8-0.8 b.y. ago), CO2 levels were higher and O2 was lower, so D17O in O2 was large enough to be recorded in sulfate minerals produced from oxidation of pyrite, FeS2. Implications of these data for Mid-Proterozoic atmospheric O2 levels will be discussed.

P. Crockford et al., Nature (2018)