TILTING AT SNOWBALLS
Paul F. Hoffman and Adam C. Maloof Department of Earth & Planetary Sciences Harvard University, Cambridge, MA, USA 02138 January 30, 2001
ABSTRACT
Ten arguments were recently presented in The Australian Geologist [1] against the ‘snowball Earth’ scenario for equatorial glaciation, a scenario that explains many previously enigmatic features of Neoproterozoic Earth history such as post-glacial cap carbonates, large shifts in stable isotope ratios of seawater proxies, and extensive oxide-facies iron formations. We show that the counter arguments are based on incomplete understanding of the ‘snowball Earth’ scenario, in particular the continuously changing conditions that must occur in response to rising atmospheric CO2 levels during the glacial period. There is no conflict with geological observations when this and other factors are considered. We also suggest that evidence central to the theory of high orbital obliquity for Proterozoic low-latitude glaciation may be open to alternative interpretation.
Introduction
None has done more to focus attention on the Proterozoic glacial record than George Williams and Phillip Schmidt, and to this end we welcome their ten-point discussion [1] of the ‘snowball Earth’ hypothesis. However, many of their points derive from a false or incomplete portrayal of the hypothesis, and their “rebuttal” of it is therefore less “formidable” than they imagine. That aspects of the snowball Earth scenario are misunderstood is not surprising. The hypothesis itself [2,3,4] is comparatively recent and its literature is terse. An ice-covered globe and its ultra-greenhouse aftermath invoke very different worlds from any familiar one and much remains to be learned about how such worlds would work. Even the parents of the hypothesis don’t know exactly how the child will turn out. In defense of the snowball Earth scenario as best we understand it, we respond to each of Williams and Schmidt’s [1] ten points in order, using their headings (with question marks added or deleted). We also suggest a possible alternative explanation, independent of seasonality, for the occurrence of polygonal sand wedges near the equator. That observation and its conventional seasonal interpretation lies at the heart of the hypothesis preferred by Williams and Schmidt [1,5], a high obliquity of the ecliptic, or ‘Big Tilt’ for short. 2
1. Vigorous hydrological cycle shows no global freeze-up?
When we described a snowball Earth as having “continental ice cover that was thin and patchy because of the virtual elimination of the hydrologic cycle” [3], we were referring to the situation immediately after the ice-albedo runaway. From then on, the frigid climate will slowly ameliorate due to the steady increase in atmospheric CO2 (from volcanic outgassing in the absence of sinks for carbon). As temperatures rise (most rapidly early on because of non-linear CO2-greenhouse forcing [6]), the ablation of sea ice in the tropics and subtropics will feed a weak hydrological cycle [7]. Wally Broecker and Richard Alley pointed out to us that even the very low rates of snow accumulation at higher elevations (due to the lapse rate) and higher latitudes that would result from such a hydrological cycle should, over the course of millions of years, result in dynamic, wet-based glaciers. Ablation rates are high where air currents descend, as in the Antarctic dry valleys (~30 cm yr-1 [8]). An equal accumulation rate would produce 3,000 m of ice in 10,000 years. Given that a Proterozoic ‘snowball Earth’ would last for millions of years [6,3], dynamic glaciers would have existed in coastal areas with strong sunlight and significant topography in all but the initial stages of a snowball Earth. This is consistent with the presence of Proterozoic glacial deposits, locally >1000 m thick, with faceted and striated clasts of extrabasinal derivation [9].
2. No extreme fall of sea level rebuts global glaciation?
If every continent in the world were covered by ice sheets of comparable mean thickness to those in Greenland and Antarctica, then the absolute fall in sea level after isostatic adjustment would be on the order of 900 m. (Isostasy would be maintained assuming the ice caps grew slowly.) George Williams has repeatedly pointed to the fact that no evidence for extreme sea- level fall has been observed geologically. This is a false test because it is not the absolute but the relative sea-level fall that leaves its mark geologically. The late Pleistocene rise in sea level can be tracked because we have ocean islands and continental margins that were never depressed by ice caps. But if all the continents were burdened by ice the thickness of Greenland or Antarctica, then their mean isostatic depression would be on the order of 750 m, offsetting much of the glacio-eustatic fall in sea level. In detail, the relative sea-level change at continental margins would vary spatially on the length-scale of lithospheric flexure that must accommodate the shift in water load from the oceanic to the continental areas. In addition, any tectonic subsidence (or uplift) that occurred during a prolonged snowball event would change the net relative sea-level fall. The bottom line is that the fall in relative sea level to be expected in a snowball Earth is probably in the range of 0-300 m, consistent with observations [10]. More extreme falls are still possible locally (Fig. 1). The deepest stratigraphic incisement reportedly associated with Neoproterozoic glaciation is located not far outside Adelaide and ironically was discounted as being too extreme (~500 m) to be eustatic in origin [11]. Regardless of the truth in this example, the idea that global glaciation should leave evidence of extreme falls in relative sea level is untrue.
3. Tidal rhythmites show an open ocean?
We are baffled as to why the presence of tidal rhythmites demands an open ocean [1]. Tidal forces act on ice as they do on water. A global sea-ice shell with a mean thickness of only 1 km 3
(and only ~10 m at the equator [7]) would offer negligible resistance to flexure of tidal amplitudes (meters) and wavelengths (20,000 km at the equator). Cracks would form at ice grounding lines (E.J. Gaidos, personal communication, 2000). Tidal channels and deltas might develop, for example, where the underside of a floating ice shelf hugs the crest of an abandoned ‘surge moraine’ (see below)[12]. In any event, patches of open water at low latitudes were assumed to exist in the original snowball Earth hypothesis [2] and the Adelaidean coast could well have opened before the main ocean near the end of a snowball event. Aeolian sandstones of Marinoan age on the Stuart shelf show unimodal offshore-directed paleowinds (from present northwest) [13]. If these winds were descending from an inland ice plateau, adiabatic heating would make them strongly ablative at lower elevations, causing the coastal sea ice to thin and locally disappear. Open coastal waters would lead to increased precipitation and more dynamic glaciers.
4. Wave action indicates an unfrozen sea?
Williams [14] has described submarine slumps with superimposed storm-wave structures in the Marinoan glacial succession in South Australia. Storm-wave activity requires open water. The unanswered question is at what point in the glacial cycle did the storm-wave structures form? The entire Marinoan succession in the area described is less than 150 m thick and could have been deposited towards the end of the glacial period. Areas of open water must surely have developed at low latitudes around the time of deglaciation. According to the snowball hypothesis, high atmospheric CO2 and reverse ice-albedo feedback would drive deglaciation violently. Strong warming in low-albedo areas of open water and steep temperature gradients at ice margins would make for very stormy weather. Slumps and storm-wave structures are to be expected near the end of a snowball event, and are commonly observed in Neoproterozoic post- glacial ‘cap’ carbonates.
5. Strongly seasonal climate confutes a frozen-over Earth?
The existence at low latitude of periglacial sand wedges and varves (if truly seasonal) is the observation at the core of the Big Tilt hypothesis. High orbital obliquity is the simplest way to enhance seasonality close to the equator [15]. The existence of sand-wedge polygons in Neoproterozoic glacial successions of unknown paleolatitude [16] presents no problem, but those on display in the Cattle Grid open pit in South Australia [17] formed at ~8˚ latitude [18] (or ~16˚ if the geomagnetic field had a 25% octupole component). The wedges are far too deep (many 2.5 m) to result from diurnal temperature fluctuations. Does the snowball hypothesis provide for any alternative explanation? Glaciers that surge periodically could cause large, multi-annual, temperature fluctuations in periglacial soils (Fig. 2). Many glaciers in cold, arid regions are polythermal (‘subpolar’). Their thin, cold-based extremities undergo ablation, but their upper parts become thick and wet-based because of net accumulation [19]. This is an inherently unstable condition—the glacier’s toes are frozen to the bed and create a barrier to the flow of incoming ice from the accumulation zone. A positive feedback results where the surface gradient of the glacier increases, leading to increased precipitation rates in the accumulation zone. As the accumulation zone thickens, more and more subglacial meltwater is produced that cannot drain through the frozen ablation zone. Subglacial water pressures build and the surface gradient grows until the toe-hold releases and the glacier 4 surges. During surges, ice evacuates the upper reaches of the glacier and flows down into a proglacial repository bounded by a ‘surge moraine’. Surges last for 1-3 years, during which time the ice line (or the ice grounding line of an ice shelf) may advance tens of kilometers [19]. Periglacial soils that were formerly exposed to cold air, or covered only by the thin, cold- based toe of the glacier, are abruptly insulated beneath a thick, warm blanket of wet-based ice. A frozen bed resists shearing when a soft, wet-based glacier flows over it, but may fracture as a result of pore-fluid overpressures. Later, as it thaws, the bed may shear if the surge continues or deform diapirically if the surge is complete. During the quiescent phase, which lasts for decades to centuries, the toe of the glacier stagnates and ablates. The mass loss near the toe and mass gain in the upper part of the glacier results in an increase in the overall glacier gradient, which continues until the next surge cycle is initiated. As the surge ice ablates away during the long quiescent phase, the underlying soils cool and contract. Polygonal cracks open up and, where the ice has ablated away, may be filled by wind-blown sand. Soil temperatures change over the course of a surge cycle from near the melting point after the surge to near the ambient air temperature when the surge ice has ablated away. The cycle repeats as long as the climatic conditions responsible for the non-steady ice-dynamic persists. The width of the sand wedges in the Cattle Grid open pit [17] would require 10s to 100s of millenia to form by this mechanism if surge cycles are 10-100 years in duration [19]. In applying this idea to the Marinoan sand wedges [17], we bear in mind that they are overlain by breccia or by aeolian sandstone lacking glacial deposits [13]. During surges, the wet-based ice must have carried most debris basinward to points east of the Cattle Grid open pit, where thick glacial diamictites occur in equivalent strata [20]. The cold-based ice of the quiescent stage would not have caused significant erosion or carried much debris [21]. Glacial surge cycles are consistent with the observed multiple sand-wedge horizons (see below), the horizontal truncation of sand wedges [17], the diapiric interjection of Cattle Grid Breccia and aeolian sandstone [13], and the unimodal, offshore-directed paleowinds [13]. A seasonal origin for varve-like argillites with dropstones in the glaciomarine Gowganda Formation in Ontario has been questioned [22]. Alternatively, they could represent the distal effects of glacial surge cycles having no bearing on low-latitude seasonality.
6. Glacial cycles deny prolonged freezing?
Williams and Schmidt [1] point out that Proterozoic glacial successions commonly display evidence of repeated glacial advances and retreats. Proterozoic glacial cycles in extra-tropical successions have been attributed to Milankovitch-type orbital forcing [23] but a snowball Earth may not respond strongly to orbital forcing, particularly when atmospheric CO2 levels are high. In any event, orbital forcing is weakest at the equator. Given the multi-million-year time scale of a snowball event [3], the four examples cited by Williams and Schmidt [1] are likely to have different origins. The two glacial advance-retreat cycles in the Sturtian of South Australia could be related to third-order eustatic changes or to extensional tectonism in the region [24]. The multiple glacial-periglacial cycles of the Port Askaig Formation in Scotland [25] could represent glacier surge cycles (see above). There are four generations of periglacial sand wedges in the Cattle Grid open pit in South Australia [1,17]. The first two generations are separated by Cattle Grid Breccia and the last three by periglacial aeolian sand sheets. This indicates occasional preservation of glacial and aeolian material during surge and quiescent stages, respectively, within a regime of mainly non-depositional surge cycles. The three glacigenic formations in the 5
Huronian succession (Ramsay Lake, Bruce, Gowganda) are separated by many 100s of meters of strata in which there is no evidence of glaciation (see below). We consider them to represent discrete episodes of glaciation rather than advance and retreat cycles of a pan-Huronian ice sheet.
7. Iron formations are unrelated to glaciation?
Williams and Schmidt [1] raise essentially three objections to the idea that a snowball Earth could account for the anomalous occurrence of extensive, oxide-facies, iron formations in glacial successions of Neoproterozoic age [2,26]. The first is that most Paleoproterozoic iron formations have no associated glacial deposits. The second is that the Marinoan glacial succession in Australia and arguably equivalent deposits elsewhere do not contain iron formation. The third is that some of the Neoproterozoic iron formations appear too early in the glacial succession to be the result of ocean mixing upon deglaciation, as originally postulated [2,26]. In order to evaluate these objections, we must consider what it takes to make iron formation. There are three requirements: (1) deep ocean anoxia permits extensive transport of dissolved ferrous iron, (2) low sulfur availability prevents the removal of dissolved iron in the form of pyrite, and (3) surface-water oxygenation drives precipitation of oxide-facies iron formation at the redox gradient. In the Paleoproterozoic, low levels of atmospheric oxygen would keep the deep ocean anoxic and severely limit the riverine sulfate input from crustal weathering [27]. The oxygen needed to precipitate iron formation was probably supplied by oxygenic photosynthesis in the surface ocean. Canfield [27] has proposed that the ~400 million-year lag between the initial rise in atmospheric oxygen by 2.25 Ga and the disappearance extensive oxide-facies iron formation around 1.85 Ga may reflect the time needed to increase the sulfur flux to the point where it exceeds the dissolved iron supply. Accordingly, the existence of Paleoproterozoic iron formations without glacial deposits (the first objection) has no bearing whatsoever on the snowball Earth hypothesis. The only extensive, oxide-facies iron formations younger than 1.85 Ga are those intimately associated with Neoproterozoic glacial deposits [26]. Although oxygen levels had risen by Neoproterozoic times [28], global or near-global sea-ice cover would severely limit gas exchange between the atmosphere and ocean. As submarine hydrothermal systems provide sinks for oxygen as well as sources of dissolved iron, the oceans on a snowball Earth would quickly become anoxic [2,26]. However, because of the very long residence time of sulfate in the ocean (tens of millions years at present), only a very prolonged interruption of riverine sulfate input would cause the iron flux to exceed the sulfide supply [29]. Therefore, the lack of iron formation in the Marinoan (the second objection) might signify a shorter event than the Sturtian, or alternatively that dissolved sulfur levels were higher than when the Sturtian glaciation began. The third objection [1] concerns the timing of iron formation within the glacial succession. We suggest that iron formation could be deposited on a snowball Earth before the glacial termination, provided that sea ice was thin enough (<20 m) to permit oxygenic photosynthesis [7]. This suggestion is consistent with paleomagnetic data showing that the largest Neoproterozoic iron formation, the Rapitan in northwestern Canada (Fig. 3), was deposited close to the equator [30]. The Rapitan iron formation is overlain by as much as 800 m of glacial diamictite (Shezal Formation), which indicates that the iron formation formed before the glacial termination unless the diamictite represents a discrete younger glaciation as proposed elsewhere in the North American Cordillera [31]. On the other hand, Neoproterozoic iron formations like 6 the those of the Urucum District in Brazil [32] and the Sturtian Braemar facies in South Australia [33] do occur near the top of glacial successions, consistent with the original snowball conjecture [2,26].
8. Controversial carbonates and isotopic data
Post-glacial ‘cap carbonates’ (Fig. 4) are unique to Proterozoic glacial terminations and it was in Australia that their vast extent [34] and characteristic low 13C/12C ratios [35] were first recognized. In the snowball Earth scenario, cap carbonates result from the high flux of alkalinity generated by intense carbonate and silicate weathering in the transient high-CO2 aftermath [3]. To the degree that cap-carbonate dominates the global carbon burial flux, the dissolved carbon pool in the ocean becomes depleted in 13C, even if normal levels of organic productivity have been restored [4]. During global glaciation, collapse in organic production would cause the dissolved carbon 13 12 pool to evolve toward the low C/ C ratios of submarine hydrothermal CO2 input [36], but this trend would be buffered by dissolution of pre-glacial carbonate in response to the drop in pH 13 12 87 86 caused by the same CO2 input. The high C/ C and Sr/ Sr ratios reported for carbonates from within glacial diamictites in Namibia and Australia [37] are consistent with a detrital origin, with derivation from underlying pre-glacial carbonates. Acceptance of these values as representing the isotopic composition of the glacial ocean must await confirmation that the carbonate is primary, not detrital in origin. We would be remiss not to complete the account [1] of the three Huronian glaciations. The second glacial (Bruce) does have a cap limestone (Espanola), which is the only mappable carbonate unit in the entire 15-km thick Huronian sedimentary wedge [38] and has a low 13C/12C ratio like other cap carbonates [39]. The first glacial (Ramsay Lake) has no cap carbonate in its small area of preservation, but the third glacial (Gowganda) has by far the largest preserved extent (2.5 x 106 km2) and still no cap carbonate. Instead it is overlain by a thick blanket of fluviatile Lorrain quartzite. The unusually aluminous nature of the Lorrain has long been attributed to intense, in situ alteration of detrital feldspar under conditions analogous to those in which bauxites and laterites form in tropical soils today [40]. This unusual pairing of glacial diamictite and aluminous quartzite occurs in correlative strata on three different North American cratons (Superior, Wyoming, Hearne) and was interpreted as indicating a dramatic climate change following glaciation [40]. Most of the lower Lorrain quartzite is non-marine, precluding a cap carbonate, but the high-CO2 aftermath in the snowball scenario provides a ready explanation for the intense chemical weathering regime following the Gowganda glaciation.
9. Where are all the volcanics?
No “cataclysmic” volcanism is required to melt a snowball Earth, contrary to the intimation of Williams and Schmidt [1]. High levels of atmospheric CO2 will result from normal volcanic activity because a snowball Earth would have no sinks for carbon, provided there is no net accumulation of CO2 ice at the poles [2,3,4,6]. Subaerial volcanic ash would collect in the ice in certain areas over the course of a snowball event. (Dark particles are heated by solar radiation and sink into the ice, minimizing their effect on the ice albedo.) In Namibia, tuffaceous layers up to several meters thick occur sandwiched between glacial diamictites and their respective cap carbonates at various locations [41]. Unfortunately, as with most other ash layers in the same 7 successions, primary zircons are lacking. An area that is not close enough to silicic volcanoes to receive air-borne zircons during normal times would not do so during a snowball event.
10. Where are the high-latitude glacial deposits?
Paleomagnetic evidence of high-latitude glaciation is lacking because there is no paleomagnetic evidence for high-latitude continents in the late Neoproterozoic [42]. In fact, Kirschvink [2] postulated that a preponderance of low-latitude continents might be a condition for a snowball Earth because of the increased planetary albedo (non-vegetated land has an albedo three times that of water). The polar oceans would be frozen over in a snowball Earth but would leave no paleomagnetic record. Not only the presence of low-latitude continents but the absence of high-latitude ones would favor extreme glaciation because the negative feedback to consumption of atmospheric CO2 by silicate weathering would be largely eliminated [4]. When polar ice caps expand, high-latitude continents are frozen and their contribution to global weathering activity and hence CO2 consumption is removed. This relationship has been investigated in a number of climate modeling studies [43]. The possibility that a preponderance of low-latitude continents could account for the 13C enrichment (δ13C>4‰ PDB) observed in most late Neoproterozoic carbonates except for those bounding the glacial intervals is being investigated [44]. The end of high δ13C values and the last Neoproterozoic glaciation coincided approximately with the movement of Laurentia to high latitudes [30] and continents have existed at high-latitude throughout the Phanerozoic [45]. As the presence of high-latitude continents has been the norm in Earth history, their absence as prerequisite for a ‘snowball Earth’ could explain why the climate mode of repetitious global glaciations occurred only twice, near the beginning and near the end of the Proterozoic eon [3,46]. While additional geochronological data that might test the synchroneity of Proterozoic glaciations [1] would be welcome, the known coincidence of large negative δ13C anomalies with Neoproterozoic glaciation has already caused many to abandon the idea of diachronous glaciation [47,37]. This fulfills the expectation of pioneers workers [48,34], who were impressed by the abrupt stratigraphic confines of Proterozoic glacial formations (Fig. 3) and believed that they would form an ideal basis for inter- regional correlations.
The Proterozoic climatic paradox
There are two credible hypotheses for low-latitude Proterozoic glaciation. The snowball Earth scenario accounts for many salient features in addition to low-latitude glaciation (Table I). It is well grounded in the theory of runaway ice-albedo feedback [49] and is not disqualified by Williams and Schmidt’s ten-point “rebuttal” [1]. However, it does not produce strong equatorial seasonality. Conversely, a Big Tilt predicts strong semi-annual seasonality at the equator [15] but fails to explain other important features associated with Proterozoic glaciations (Table I). Moreover, a viable mechanism for abruptly reducing the tilt of the spin axis at the end of the Precambrian (or at any other time) has yet to be identified. The proposed ‘core-mantle friction’ mechanism [50] is inconsistent with Williams’ own tidal data [51] from the Elatina rhythmites [52]. An alternative ‘climate friction’ mechanism [53] relies on the waxing and waning of polar ice caps, which are a virtual impossibility with a highly tilted spin axis because land temperatures at the poles in summer would be near the boiling point [15]. 8
We agree with Williams and Schmidt [1] that the crux of the problem is the evidence for strong equatorial seasonality. This evidence amounts to the Neoproterozoic sand wedges in the Cattle Grid open pit [17] and the Paleoproterozoic varve-like argillites with dropstones in the glacigenic Gowganda Formation of Ontario, Canada [22]. Can a satisfactory alternative explanation for these features be found that does not involve strong seasonality? We suggest above that periglacial sand wedges and varve-like argillites might result from the surge cycle of polythermal glaciers [12,19]. In cold, arid climates, the surge cycle operates on time scales of 10-100 years [19] and is not dependent on seasonality. If this or any other non-seasonal explanation for equatorial sand wedges and varve-like argillites exists, then the superior explanatory power of the snowball scenario (Table I) would be uncompromised. We also agree with Williams and Schmidt [1] that Australian research helped identify the ‘Proterozoic climatic paradox’ and may ultimately lead to a paradigm shift in the Earth sciences. The first champion of the idea that the late Proterozoic ice age was global or nearly so was none other than Douglas Mawson [48], who should need no introduction to readers of The Australian Geologist. Schmidt and Williams [54] were the first to show conclusively that ice lines descended to sea level near the equator during the Marinoan glaciation. And it was George Williams who focussed attention on post-glacial cap carbonates and first documented their characteristic 12C enrichment [35], the explanations of which [3,4] are substantially responsible for the recent revival of interest in the snowball Earth hypothesis.
Acknowledgments
Our studies of Neoproterozoic Earth history are supported by the Earth System History and Polar Programs of the U.S. National Science Foundation, the NASA Astrobiology Institute, the Canadian Institute for Advanced Research, the Geological Survey of Namibia, Geological Survey of Morocco, Norwegian Polar Institute in Svalbard, and Harvard University.
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Table I - Explanatory Power of the Big Tilt and Snowball Earth Hypotheses
Observation Big Tilt Snowball Earth
Low-latitude glacial deposits Yes Yes
Thick mixtites with faceted and striated clasts Yes Yes
No extreme fall in relative sea-level Yes(1) Yes
Tidal rhythmites Yes Yes
Wave-generated structures Yes Yes(2)
Equatorial varve-like argillites Yes Yes(3)
Equatorial sand-wedge polygons Yes Yes(3)
Equatorial glacial cycles Yes Yes(4)
Iron formation with dropstones No Yes
Cap carbonates No Yes
Carbon isotope anomalies No Yes
Sulfur isotope anomalies No Yes
High-latitude glacial deposits No Yes
Abrupt onset and termination of ice ages No Yes
Proterozoic hiatus in glaciation (2.2-0.8 Ga) No Yes
(1) Provided there were no high-latitude continents, where relative sea-level falls would be large. (2) Provided open water existed locally. (3) If not seasonal in origin (glacier surge cycles?). (4) If not caused by orbital forcing (glacier surge cycles?). 13
Figure Captions
Fig. 1. Incised paleovalley with 385 m of local relief associated with Neoproterozoic glaciation at Omutirapo, NW Namibia. Paleovalley cuts shelf carbonates (Ombombo subgroup) and is filled by glaciomarine diamictite (Chuos Formation). Inferred base-level fall is larger than the relative sea-level lowering expected if all continents were glaciated (see text), but may be accounted for by lithospheric flexure resulting from the transfer of water loads from ocean basins to continents, or by local tectonic uplift associated with crustal stretching and block rotation during prolonged glaciation.
Fig. 2. Surge cycle of a polythermal (‘subpolar’) glacier [19]. Thermal contraction of periglacial soils during each ablative quiescent stage causes polygonal cracks that are filled by wind- blown sand. Soils expand when covered by warm, wet-based ice during each surge stage. Polygonal sand wedges like those in the Cattle Grid open pit [17] might form without strong seasonality in a ‘snowball Earth’ as a result of glacier surge cycles.
Fig. 3. Banded iron-formation (Sayunei Formation) with dolomite dropstone associated with the Neoproterozoic Rapitan glaciation in the Mackenzie Mountains, NW Canada. All banded iron-formations younger than 1.85 Ga are associated with Neoproterozoic glaciations [26] and imply deep-water anoxia and low inputs of riverine sulfur during during glaciation, consistent with a ‘snowball Earth’ (see text). Iron formation is overlain by <800 m of diamictite (Shezal Formation), implying that it formed before the glacial termination. It may have been localized by oxygenic photosynthesis where sea ice was thin [7], consistent with paleomagnetic evidence for an equatorial setting [30]. Ocean sulfur levels may have been too high to permit oxide-facies iron formation during the equatorial Marinoan glaciation.
Fig. 4. Sharp contact between glaciomarine diamictite with abundant dropstones and post-glacial cap dolomite in the Neoproterozoic Tsumeb subgroup of northwestern Namibia [3]. Diamictite is composed exclusively of carbonate debris and is sharply confined stratigraphically between non-glacial carbonate-dominated successions >1000 m thick. High-obliquity hypothesis [5] provides no explanation for such abrupt transitions between apparent warm-water and glacial climatic conditions.
Cover. Skiing Skippy, mascot of ‘snowball Earth’. Artist unknown. Road sign outside Blinman, central Flinders Range, South Australia. Rasthof Fm (cap carbonate)
65 m
Chuos Fm Ombombo - 3 (glacial diamictite) Ombombo - 2 450 m paleovalley Ombombo - 1 accumulation zone dynamic ice line QUIESCENT STAGE (10-100 yrs)
glacier ablation zone sand dune
wet base cold base polygons open
SURGE STAGE (1-6 yrs)
surge moraine
wet base polygons close
Hoffman & Maloof Fig. 2