Stable isotopic evidence for methane seeps in Neoproterozoic postglacial cap carbonates

Abstract

The Earth's most severe glaciations are thought to have occurred about 600 million years ago, in the late Neoproterozoic era. A puzzling feature of glacial deposits from this interval is that they are overlain by 1-5-m-thick 'cap carbonates' (particulate deep-water marine carbonate rocks) associated with a prominent negative carbon isotope excursion. Cap carbonates have been controversially ascribed to the aftermath of almost complete shutdown of the ocean ecosystems for millions of years during such ice ages--the 'snowball Earth' hypothesis. Conversely, it has also been suggested that these carbonate rocks were the result of destabilization of methane hydrates during deglaciation and concomitant flooding of continental shelves and interior basins. The most compelling criticism of the latter 'methane hydrate' hypothesis has been the apparent lack of extreme isotopic variation in cap carbonates inferred locally to be associated with methane seeps. Here we report carbon isotopic and petrographic data from a Neoproterozoic postglacial cap carbonate in south China that provide direct evidence for methane-influenced processes during deglaciation. This evidence lends strong support to the hypothesis that methane hydrate destabilization contributed to the enigmatic cap carbonate deposition and strongly negative carbon isotopic anomalies following Neoproterozoic ice ages. This explanation requires less extreme environmental disturbance than that implied by the snowball Earth hypothesis.

First discovery and characterizations of late Cretaceous seep carbonates from Xigaze in Tibet, China

“…A controversial one is the cap carbonate at the bottom of Doushantuo Formation in the Three Gorges area. This carbonate exhibits  13 C value lower than 40‰, but the origin of the carbonate is still controversial [29][30][31][32]. In addition, Zhou et al [33] proposed that the rhodochrosites in the Datangpo Formation of the Nanhua Series in northeastern Guizhou were seep carbonates, but the formation of these deposits has always been a controversial topic.…”

Geochemical constraints on the origin of Doushantuo cap carbonates in the Yangtze Gorges area, South China

“…The base of the Doushantuo Formation is comprised of the cap carbonate succession, in sharp contact with the underlying Nantuo diamictite. The Doushantuo cap carbonate succession displays a distinct suite of sedimentary structures and textures including cemented breccias, sheet cracks, tepee-shaped structures, stromatactis-like cavities, peloids, and local barite fans, and negative carbon isotope excursion of about −5‰ (Jiang et al, 2003a(Jiang et al, , 2006Lin et al, 2011;Peng et al, 2011), similar to those observed in equivalent cap carbonates worldwide (Kennedy, 1996;Hoffman et al, 1998;James et al, 2001;Frimmel and Folling, 2004;Shields et al, 2007b;Shen et al, 2008). Additionally, U-Pb Zircon dates indicate that the cap carbonates were deposited at 635 Ma (Condon et al, 2005).…”

“…One of the most intriguing is the Marinoan glaciation that extended into equatorial latitudes (Evans, 2000;Hoffman and Schrag, 2002;Hoffman and Li, 2009). Most Marinoan glacial deposits are directly overlain by "cap carbonates" with peculiar sedimentary features and negative carbon isotope signatures (Kennedy, 1996;Hoffman et al, 1998;James et al, 2001;Hoffman and Schrag, 2002;Jiang et al, 2003a;Frimmel and Folling, 2004;Shields et al, 2007a, b;Zhou and Xiao, 2007;Shen et al, 2008), implying severe and rapid climatic changes which are thought to serve as an 'environmental filter' for biological evolution (Hoffman et al, 1998;Runnegar, 2000;Hoffman and Schrag, 2002). Strata above these postglacial cap carbonates contain the Earth's earliest multi-cellular organisms interpreted as early Metazoans (Xiao et al, 2002;Xiao, 2004;Yin et al, 2007;McFadden et al, 2008;Yuan et al, 2011).…”

Oceanic redox conditions in the Late Mesoproterozoic recorded in the upper Vazante Group carbonates of São Francisco Basin, Brazil: Evidence from stable isotopes and REEs

“…The association of the reducing conditions with the preglacial negative ␦ 13 C carb is consistent with possible release of methane from organic-rich sediments on continental margins (e.g., Halverson et al, 2002;Schrag et al, 2002). However, the lack of a supporting sedimentological evidence (e.g., Jiang et al, 2003) in the Vazante carbonates, such as tube-like structures through which gas might have escaped (e.g., Schrag, 2002, Wang et al, 2008), might argue against the methane release scenario. Alternatively, the input of 12 C-enriched CO 2 in shelf environment might have been enhanced by sea level drop, caused by early regression preceding full glaciations, which brought some of the oxygen-restricted lower shelf water close to deep shelf settings rich in organic matter.…”