A carbon isotope challenge to the snowball Earth

Abstract

The snowball Earth hypothesis postulates that the planet was entirely covered by ice for millions of years in the Neoproterozoic era, in a self-enhanced glaciation caused by the high albedo of the ice-covered planet. In a hard-snowball picture, the subsequent rapid unfreezing resulted from an ultra-greenhouse event attributed to the buildup of volcanic carbon dioxide (CO(2)) during glaciation. High partial pressures of atmospheric CO(2) (pCO2; from 20,000 to 90,000 p.p.m.v.) in the aftermath of the Marinoan glaciation (∼635 Myr ago) have been inferred from both boron and triple oxygen isotopes. These pCO2 values are 50 to 225 times higher than present-day levels. Here, we re-evaluate these estimates using paired carbon isotopic data for carbonate layers that cap Neoproterozoic glacial deposits and are considered to record post-glacial sea level rise. The new data reported here for Brazilian cap carbonates, together with previous ones for time-equivalent units, provide estimates lower than 3,200 p.p.m.v.--and possibly as low as the current value of ∼400 p.p.m.v. Our new constraint, and our re-interpretation of the boron and triple oxygen isotope data, provide a completely different picture of the late Neoproterozoic environment, with low atmospheric concentrations of carbon dioxide and oxygen that are inconsistent with a hard-snowball Earth.

A cold, hard look at ancient oxygen

“…Fresh isotopic work (5,6) promised to lift this veil, offering what seemed to be direct evidence for extremely elevated CO 2 levels following the proposed Marinoan snowball interval 635 million years ago. This conclusion was recently turned on its head, and more moderate CO 2 levels inferred from the same evidence (7). In PNAS, Cao and Bao (8) clarify the reasons for these conflicting interpretations and provide mechanistic support for a high-CO 2 postsnowball atmosphere.…”

Multiproxy constraints on alteration and primary compositions of Ediacaran deep-water carbonate rocks, Yangtze Platform, South China

“…Rapid glacial meltdown would lead to a global transgression and super-greenhouse warming would result in intense continental weathering, producing an enormous alkalinity and nutrient flux into the oceans and a lowering in ocean pH (Hoffman et al, 1998;Kasemann et al, 2005). Recent work by Sansjofre et al (2011) postulated much lower pCO 2 levels for the Ediacaran, challenging the super greenhouse rapid meltdown hypothesis of previous authors. Evidence from trace metal data (Lyons et al, 2014 and references therein) indicates anoxic (and locally euxinic) conditions in most parts of the middle to late Neoproterozoic ocean.…”

Interpreting carbonate and organic carbon isotope covariance in the sedimentary record

“…Whether or not those ancient deposits were exposed to the same degree of freshwater alteration as Clino is still a matter of debate 28 . In many cases, negative d 13 C carb excursions have been interpreted to be pristine records of global carbon cycling 15,[19][20][21]56,57 , because sedimentological evidence of subaerial exposure was not observed 26,58 . However, subaerial exposure surfaces can be cryptic in the rock record, and other workers have interpreted the same geochemical changes to be diagenetic in origin 28,59 .…”