RECONSTRUCTING CRYOGENIAN MARINE OXYGEN LEVELS FROM LIMESTONE SUCCESSIONS IN MONGOLIA
Nicole A. O'Keefe1, Kimberly V. Lau2, Francis A. Macdonald3, Jonathan L. Payne2.
1Saint Mary's College of California, Moraga, CA, 2Stanford University, Stanford, CA, 3Harvard University, Cambridge, MA.
Variation in oxygen availability has been an important control on the evolution of life over geological time. Earth’s atmosphere first became oxygenated 2.4 Gya, but the nature and trajectory of the rise to present-day levels and its relationship to the origin and early evolution of animals remain poorly constrained. In the Cryogenian Period (720 to 635 Ma), the Earth was nearly covered in glaciers during the Sturtian and the Marinoan Snowball Earth events. The earliest fossil evidence of animals also occurs during this interval. To assess marine oxygen concentrations during Cryogenian time, we analyzed trace metal concentrations, uranium(U), molybdenum(Mo), and vanadium (V), in limestone collected from 2 stratigraphic sections in Mongolia, representing different facies belts. By measuring trace metal concentrations from the 2 locations, we are better able to differentiate between chemical alteration and variations in Cryogenian seawater chemistry. Above the Sturtian glacial deposits, U, Mo, and V are elevated and then decrease coincident with a large negative δ13C excursion. These data indicate a post-Sturtian shift toward higher oxygenation levels in the oceans, followed by a return to anoxic conditions coincident with a large perturbation to the carbon cycle. This temporary oxygenation may provide a link between the expansion of habitable seafloor and the rise of complex metazoans.