Is there a conflict between the Neoproterozoic glacial deposits and the snowball Earth interpretation: an improved understanding with numerical modeling
Introduction
Glacial deposits of the Sturtian ice age (ca. 760–700 Ma, [1]) are recognized on all continents [2] and are associated with strong variations of the Sr and C isotopic composition of seawater [3] and with a massive deposition of carbonate rocks immediately following glaciation [4], [5]. Paleomagnetic and geological data from several of these deposits suggest they were formed at tropical latitudes [6], [7]. The snowball Earth scenario, in which the Earth would have been completely ice-covered for a few millions of years, provides a single explanation for these unusual features. Several climate models have been used to investigate whether a diverse set of climate forcings could have initiated low-latitude ice sheets [8], [9], [10], [11], [12], [13], [14]. The forcings tested were a solar luminosity decrease, variation of atmospheric CO2 concentration, and a change in obliquity to high values (greater than 60°). Hyde et al. [10] show with a coupled energy balance model/ice-sheet model that a snowball Earth solution can be obtained if solar luminosity is reduced by 6% and CO2 levels are near present-day values for the Varanger glacial interval (600 Ma). A further result of this work and others [13], [14] is that there exists another solution (e.g. soft snowball or open water solution) for a given CO2 value range (0.7–3 times present-day values, cf. [14]) in which an equatorial ice sheet could have coexisted with an equatorial belt of open water. The key point of this new solution stems from concern for the survival of eukaryotic life in such extreme and extended glaciations [15]. For the same interval, Chandler and Sohl [12] do not find the same CO2 levels to get into or out of a snowball Earth (e.g. [8]), but they note that as sea-ice extent increases, snow accumulation begins to decline. Thus, they point out the discrepancy between the need for active, wet-based continental ice sheets to produce glacial deposits and the near shutdown of the hydrological cycle simulated by climatic models once the Earth is entirely glaciated. Indeed, evidence for the existence of dynamic glaciers is seen in many places in the world and is indisputable [16], [17], [18], [19]. Many glaciogenic sediments contain faceted and striated clasts, some far-traveled, as well as deformation structures caused by glacial flow [20], [21], [22]. Several Neoproterozoic glacial sequences are thick (350–2000 m in the central Rocky Mountains [22]; sometimes greater than 1000 m in Death Valley [23]), corresponding to the transportation of massive amounts of material, while other sequences are thin and highly discontinuous [5]. These thick glacial deposits have then been interpreted as indicating a well-functioning hydrologic cycle in order to allow the development of large erosive ice sheets, therefore raising doubts about the existence of a global glaciation [18], [21], [22], [24]. These conclusions were made because of the common idea that the continental ice cover of a snowball Earth should be thin and patchy. Due to these numerous speculations, it is of key importance to analyze the response of the ice sheets in a snowball Earth world, in order to know if there is really a conflict between the observations of the Neoproterozoic and the snowball Earth hypothesis.
In this paper, we investigate these problems using numerical tools. Firstly, we have performed a set of climate simulations with the LMD (Laboratoire de Météorologie Dynamique) (atmospheric) general circulation model ((A)GCM) for the older glaciation using the most reliable paleogeographic reconstruction. Secondly, the LMD AGCM results are used to run the ice-sheet model (ISM) developed at the LGGE (Laboratoire de Glaciologie et de Géophysique de l’Environnement). By means of these experiments, we want to improve the knowledge of the glacial terrestrial regime in such extreme conditions. We will also discuss what may or may not be inferred from the observations of glaciogenic sediments on the climatic state of the Earth during Neoproterozoic time.
Section snippets
Description of the models
The LMD AGCM is used extensively to investigate present, future and past climates. Model-data comparisons for different geological periods show that this tool is able to successfully simulate the last glacial/interglacial cycle [25] as well as earlier climates [26], [27]. A 50 m slab ocean that accounts for the storage of heat in the mixed layer is used [28]. Sea-ice is simulated by a change in albedo from 0.1 to 0.6 if the temperature is below −2°C. The resolution is 4° in latitude, 5° in
GCM simulation of a global glaciation at 1×CO2
The simulation reaches equilibrium after 40 yr of integration, at which time the global temperature was dropped from 15°C to −40°C. The boundary conditions used are thus sufficient to plunge the Earth into a snowball state; this is consistent with other modeling studies [8], [10]. Over the continents located in the equatorial zone, maximum temperatures are never greater than −25°C (Fig. 1). The coldest temperatures are around −110°C and are found over the West Africa craton that is the most
What does the continental ice cover become in such a cold world?
The nature and the behavior of the terrestrial glacial regime are among the uncertainties concerning the snowball Earth model. On the one hand, Hoffman and Schrag [19] agree that just after the ice-albedo runaway, a low-latitude ice sheet would not have time to develop, but they point out that, as atmospheric CO2 increases, the mean temperature will slowly rise, thus allowing for the building of wet-based ice sheets. On the other hand, Condon et al. [21] found dropstone intervals in the Ghaub
Are some glacial deposits in conflict with the snowball Earth model?
One of the essential factors defining the capacity of an ice sheet to erode and transport glacial sediments is the temperature at the base of the ice sheet. Areas where the basal temperature is at the pressure melting point produce sliding and the sub-glacial sediments are also more deformable when lubricated with water. Conversely, the basement is protected if the ice is frozen to it. Also, almost all ice transport to the ice-sheet margin takes place within rapidly moving outlet glaciers in
Discussion and conclusion
The glacial deposits seen by Condon et al. [21] are postulated as belonging to the early phase of the snowball Earth, just after the climate instability that may have driven the ocean to become covered with sea-ice and the hydrologic cycle to be virtually shut off. Our experiment is designed to improve our understanding of this particular phase and of the implications of the sluggish snowball hydrologic cycle. Our ice-sheet model run is forced by the climate corresponding to the early phase of
Acknowledgments
We thank A. Green and M. Kageyama for help in improving the manuscript. We thank S. Charbit for help in designing the ISM experiments. We are very grateful to David Pollard and Joe Meert who made useful and constructive comments on the manuscript. This work was supported by the French program ECLIPSE. Computing was carried out at LSCE, supported by the CEA.[AC]
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