Pb-Pb age of earliest megascopic, eukaryotic alga bearing Rohtas Formation, Vindhyan Supergroup, India: implications for Precambrian atmospheric oxygen evolution

Abstract

Though considered to be the earliest known, well preserved megascopic body fossil of an eukaryotic alga, Grypania spiralis has not yet been dated directly and reliably in any of its four known locations in the world. We report a Pb-Pb isochron age of 1599±48 Ma for the Grypania bearing Rohtas Formation of the Vindhyan Supergroup, Central India. Besides being about 500 myears older than the previously inferred age for this formation, this Pb-Pb age together with close age constraints on the same fossil bearing horizons in Jixian, China and Montana, USA indicates that Grypania bloomed on a global scale between 1.6 and 1.5 Ga. Being an obligate aerobe, the appearance of a much older Grypania within one of the principal BIF deposits near Michigan, USA has been linked to a significant jump in atmospheric oxygen between 2200 and 1900 Ma inferred from geochemical evidence. The long gap of ~400 myears between the older and younger Grypania occurrences could be due to reduction of atmospheric oxygen below the Grypania survival threshold as a result of extensive BIFs between about 1950 and 1850 Ma.