Orbitally forced ice sheet fluctuations during the Marinoan Snowball Earth glaciation

Abstract

Two global glaciations occurred during the Neoproterozoic. Snowball Earth theory posits that these were terminated after millions of years of frigidity when initial warming from rising atmospheric CO2 concentrations was amplified by the reduction of ice cover and hence a reduction in planetary albedo1, 2. This scenario implies that most of the geological record of ice cover was deposited in a brief period of melt-back3. However, deposits in low palaeo-latitudes show evidence of glacial?interglacial cycles4, 5, 6. Here we analyse the sedimentology and oxygen and sulphur isotopic signatures of Marinoan Snowball glaciation deposits from Svalbard, in the Norwegian High Arctic. The deposits preserve a record of oscillations in glacier extent and hydrologic conditions under uniformly high atmospheric CO2 concentrations. We use simulations from a coupled three-dimensional ice sheet and atmospheric general circulation model to show that such oscillations can be explained by orbital forcing in the late stages of a Snowball glaciation. The simulations suggest that while atmospheric CO2 concentrations were rising, but not yet at the threshold required for complete melt-back, the ice sheets would have been sensitive to orbital forcing. We conclude that a similar dynamic can potentially explain the complex successions observed at other localities. Document embargo 24/02/2016.Peer reviewe

The Late Cryogenian Warm Interval, NE Svalbard: Chemostratigraphy and genesis

“…Where most completely exposed, modal cycle thickness is 0.5 m. If the cycles are forced by precession, as is consistent with the low palaeolatitude, the preserved thickness of the interval between glaciations represents a time period of 6-8 Myr. Hambrey (1982), Halverson (2011) and Benn et al (2015). Glacigenic units are shown in red and the E3-E4 interval, the subject of this paper, is shown in bold.…”

“…Deposition of all these units was followed by subaerial exposure and evidence of periglacial conditions: decimetre-scale folds in unit C and extensive development of parallel fracture networks of previously cemented dolomite (Fairchild & Hambrey, 1984). Benn et al (2015) associate these phenomena with Snowball-type glacial conditions, inferring a multi-million year hiatus.…”

“…Hence precessional forcing is our preferred hypothesis for the E3 cycles. In the overlying Wilsonbreen Formation, Benn et al (2015) have shown through climate modelling that glacial advances and retreats in the latter stages of a Snowball Earth ice age are consistent with aridity-humidity shifts on precessional timescales. At 650 Ma, the four precessional frequencies, which today are 18.…”

“…The uppermost surface of E4 at the base of the Wilsonbreen Formation has been interpreted (Benn et al, 2015) as a multi-million year hiatus, coinciding with a deep-frozen Snowball Earth condition culminating in periglaciation and glacifluvial erosion, followed by ice advances and retreats in a terrestrial setting. This represents a late stage in the glaciation when PCO 2 had risen to high levels (Bao et al, 2009;Benn et al, 2015).…”

“…This represents a late stage in the glaciation when PCO 2 had risen to high levels (Bao et al, 2009;Benn et al, 2015).…”