Testing the snowball Earth hypothesis for the Ediacaran Alexei V. Ivanov1, Anatoly M. Mazukabzov1, Arkady M. Stanevich1, Stanislav V. Palesskiy2, and Olga A. Kozmenko2 1
Institute of the Earth’s Crust, Siberian Branch, Russian Academy of Sciences, Lermontov Street 128, Irkutsk 664033, Russia Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Academician Koptyug Boulevard 3, Novosibirsk 630090, Russia
2
suitable site to test if the Ediacaran glaciation reached the snowball stage via analysis of PGE concentrations in sediments.
ABSTRACT Ediacaran Siberia was at tropical paleolatitudes when the glacigenic strata of the Goloustnaya Formation (Baikal Group, Siberia) were deposited at sea level. The presence of such deposits (at tropical latitudes) is at the core of the snowball Earth hypothesis, which is generally accepted for the previous Cryogenian glaciations. To test this hypothesis for the Ediacaran Period, we determined concentrations of platinum group elements (PGE) in the transitional unit between glacigenic conglomerates and postglacial cap carbonates of the Goloustnaya Formation. We speculate that if oceans were completely covered by ice during the glaciation, the ice prevented accumulation of PGE-rich cosmic dust and micrometeorites during that period, i.e., the snowball Earth stage. Such particles would have accumulated rapidly on the ocean floor at the ice-melting event, providing a geochemical signal; however, unlike the previous Cryogenian glaciations, this signal is at a background level, and we conclude that either the Ediacaran glaciation did not reach the snowball stage, or it was of very short duration. INTRODUCTION The Cryogenian Period of the Neoproterozoic was so named because of severe glaciations during that time, i.e., the Sturtian, ca. 716–711 Ma (Macdonald et al., 2010), and the Marinoan, which ended ca. 635 Ma (Condon et al., 2005). Both glaciations are suggested as indicating snowball Earth conditions. According to the snowball hypothesis, the continents and oceans were completely covered by ice; the ice melted rapidly and Earth returned to warm conditions (Hoffman et al., 1998). Glacigenic units (diamictites and conglomerates) overlain by cap carbonates (dolomites) provide evidence of these conditions (Hoffman et al., 1998). The glaciations of the Ediacaran Period ca. 582 Ma and 571 Ma are known as the Gaskiers (Bowring et al., 2003) and the Fauquier (Hebert et al., 2010), respectively; however, they are more enigmatic, and snowball conditions for them are questioned (e.g., Passchier and Erukanure, 2010; Smith, 2009). These glaciations are of particular importance, however, because of a potential link between the extreme climate conditions, the oxygenation of the oceans, and the evolution of metazoans (Grey et al., 2003; Fike et al., 2006; Gostin et al., 2010, 2011; Och and Shields-Zhou, 2012). If the oceans were completely frozen, cosmic dust and micrometeorites with high platinum group element (PGE) concentrations and chondrite-like PGE patterns would have accumulated on the ice during the prolonged stage of snowball Earth, and would have been deposited rapidly as a constituent of the sedimentary layer on the ocean floor when the ice melted. Such a model was successfully tested for the Sturtian and the Marinoan glaciations (Bodiselitsch et al., 2005), but never applied for other glaciations. Modeling of glacial flow at snowball conditions suggests that a dust layer
GEOLOGIC BACKGROUND Few sequences of Ediacaran age have been studied; some of them are located in Siberia, Russia (Smith, 2009) (inset, Fig. 2). A classic sequence from southeastern Siberia that belongs to the so-called Neoproterozoic Baikal Group has been shown to be Ediacarian in age (Kuznetsov et al., 2013) and not Cryogenian, as previously believed (Sovetov, 2011). Rocks of the Baikal Group crop out at the northwestern shore of Lake Baikal; there are three formations (from bottom to top), the Goloustnaya, Uluntui, and Kachergat (Fig. 2). The carbonates of the Uluntui Formation were dated by the Pb-Pb isochron method as 560 ± 30 Ma and by Sr chemostratigraphy as between 550 and 580 Ma (Kuznetsov et al., 2013). New radioisotopic age constraints make regional correlations to the >1700-km-long southern boundary of the Siberian Craton (Sovetov, 2011; Stanevich et al., 2007); the age data are in agreement with the time frame suggested for the evolution of acritarchs. For example, an analogue of the Uluntui Formation northeast of this cratonic boundary is the Ura Formation, which has rich Pertatataka-type acritarch assemblages (Golubkova et al., 2010); the ages for these acritarch
should have accumulated preferentially within ice of tropical regions, and influenced the termination of the glaciations via albedo change (Abbot and Pierrehumbert, 2010; Li and Pierrehumbert, 2011) (although there may be pathways for ice to flow from the surface to the underlying ocean in some regions; Goodman, 2006). This is a reason to expect the formation of a recognizably thick deposit enriched in cosmogenic particles after the deglaciation event. The Acraman impactor (Australia), with an expected near-Gaskiers age, is also a possible source of PGE elements (Williams and Gostin, 2005; Gostin et al., 2011). Ediacaran Siberia was once tropical (Fig. 1) (Li et al., 2008); the area contains glacigenic, transitional, and warm climate strata, and is a
South China
Tarim Elatina Greater India
Acraman
North China Siberia (2) (2)
North China 550Ma
Kalahari
Siberia (1)
East Ant. Congo Sao Francisco Arabia Nubia
North China (1) Goloustnaya at ~580-570 Ma Baltica 550Ma
Lavrentia Siberia 550Ma
Sahara West Africa
Amazonia Balt
ica
Figure 1. Paleogeographic reconstructions for Ediacaran Period (Li et al., 2008). Positions of continents at 600 Ma and 550 Ma are shown in gray and white, respectively. Arrows mark directions of movement for those continents, which changed their position by more than 15° during 50 m.y. Siberia at 600 Ma has two possible options (Siberia 1 and 2). Triangles show localities of dated Ediacaran glacigenic deposits; star is locality of Ediacaran Acraman megaimpact.
GEOLOGY, July 2013; v. 41; no. 7; p. 787–790; Data Repository item 2013217
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doi:10.1130/G34345.1
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Published online 16 May 2013
© 2013 Geological America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. GEOLOGY 2013 | of www.gsapubs.org | July Society
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Early Cambrian Ediacaran to Early Cambrian ush Ushakovka Formation
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Goloustnaya Formation Early Proterozoic igneous and metamorphic rocks
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Goloustnaya site Ediacaran tillites Dated Uncertain age
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Conglomerates Gritstones and sandstones Diamictites Aleurites Argillites
Carbon-bearing argillites Dolomites, calcareous dolomites Arenaceous and argillaceous dolomites Limestones
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Arenaceous and argillaceous limestones 1.8-1.9 Ga Limestone-dolomite sediments with stromatolites Granites
GL-10-8 GL-10-1 GL-10-3,4 GL-10-7 560 + 30 Ma
5 km
Ediacaran Baikal Group
ikal
Ba Lake
Figure 2. Geological map of southern Siberian Craton, cross section (A-B) at Goloustnaya site, and generalized stratigraphic scheme for Ediacaran. Ages for Uluntui Formation and granites at basement are from Kuznetsov et al. (2013) and Mazukabzov et al. (2001), respectively. Inset is from Smith (2009).
assemblages in Australian localities are between 570 and 585 Ma (Grey and Willman, 2009). Moczydłowska and Nagovitsin (2012), however, believe that the Ura Formation could be early Ediacaran (ca. 635–580 Ma). The age of the underlying Goloustnaya Formation is inferred by correlations. This formation is not cut by the abundant mafic dikes of the southern Siberian Craton, the emplacement age of which was estimated by 40Ar/39Ar and Sm-Nd methods to be 758 ± 4 Ma and 743 ± 47 Ma, respectively (Sklyarov et al., 2003). Therefore, we infer that the Goloustnaya Formation is younger than the dikes. Glacigenic conglomerates of this formation contain abundant granite pebbles from Paleoproterozoic (1.8 Ga) bedrock, and rare pebbles and fragments of altered mafic rocks. Strontium and carbon isotope values were obtained for four samples of carbonates from the upper part of the Goloustnaya Formation (Kuznetsov et al., 2013): the carbon results indicated a δ13C rise from −2.6‰
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to +2.8‰ that could be attributed to any of the Neoproterozoic postglacial increases of heavy carbon recorded for the post-Sturtian (ca. 700 Ma), post-Marinoan (ca. 630 Ma), or post-Gaskiers (ca. 570 Ma) (Halverson et al., 2005). The 87Sr/86Sr values for the same samples vary between 0.711 and 0.715, too high for any Neoproterozoic periods (Kuznetsov et al., 2013); however, the 87Sr/86Sr values could be elevated due to secondary alteration. The entire Baikal Group and glacial conglomerates were used for regional correlations during geologic mapping at scales of 1:200,000 and 1:50,000; the Goloustnaya, Uluntui, and Kachergat Formations were considered as three different members of the same sedimentary sequence. Following this conventional scheme, and bearing in mind that the Goloustnaya conglomerates represent the latest glaciation in Precambrian Siberia, we accept that they formed during the Gaskiers glaciation (ca. 582 Ma) or the Fauquier (ca. 571 Ma) glaciation.
The Goloustnaya Formation site contains all members of the glacial to postglacial associations; the cap carbonates (dolostones) there conformably overlie an ~5-cm-thick layer of argillite, which overlies Early Proterozoic granites (Mazukabzov et al., 2001) (Figs. DR1a and DR1b in the GSA Data Repository1). The argillites represent a specific glacial formation enriched in eolian particles, typically formed today in proximal glaciomarine environments of the cold Antarctic continental shelf system (Hambrey and Glasser, 2012). Modeling suggests that during snowball conditions, the ice dynamics led to a concentration of ice-entrained dust particles in once-tropical regions (Abbot and Pierrehumbert, 2010; Li and Pierrehumbert, 2011). We interpret that at the Goloustnaya site, such a deposit (the argillite layer) formed upon the ice melt above a submarine bedrock granite cliff. Thus, diamictites and argillites at the Goloustnaya site represent spatially separated lithofacies near the grounding zone and at a proximal zone, respectively. At the Homuty site, the complete sequence, with cap carbonates on top of the argillite layer, and the argillite layer on top of the diamictites, is observed (Fig. DR1c). SAMPLES AND METHODS For PGE determinations, we sampled argillite layers at both the Goloustnaya and Homuty sites. Two samples of the argillite layer (~1 cm thick) from the Homuty site were crushed and powdered. A sample of argillite from the Goloustnaya site was sliced into seven subsamples because of its relatively large (~5 cm) thickness. In addition, we sampled and analyzed the matrix from conglomerates of the glacigenic unit at the Goloustnaya Formation stratigraphically below the cap carbonates, two samples of cap dolomites overlying the argillite layer, and a quartzite stratigraphically above the cap carbonates (Fig. 2). As a reference for an extraterrestrial source, we determined PGE concentrations in two aliquots of layer J from Cretaceous-Paleogene boundary (K-Pg) clay associated with the Chicxulub impact collected at Gams, Austria (Grachev, 2009). In addition to the PGE, we determined Re concentrations in our samples (Table 1). We used the isotope dilution technique with Carius tube decomposition (following Pearson and Woodland, 2000; see the Data Repository for details). RESULTS Figure 3 shows chondrite-normalized (Mc Donough and Sun, 1995) PGE and Re patterns for the K-Pg clay from Gams, Austria. It is 1 GSA Data Repository item 2013217, photos of the studied sampling sites, and description of the analytical procedure, including data on reference materials, is available online at www.geosociety.org/pubs /ft2013.htm, or on request from editing@geosociety .org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
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GEOLOGY
TABLE 1. SAMPLE PLATINUM GROUP ELEMENT AND Re CONCENTRATIONS Sample
Ru (ppb)
Pd (ppb)
Re (ppb)
Os (ppb)
Ir (ppb)
Pt (ppb)
0.093
0.044
0.010
0.032
0.011
0.110
0.040 0.091 0.035 0.017 0.018 0.280 0.420 0.083 0.140
0.362 0.369 0.297 0.307 0.273 0.253 0.205 0.300 0.250
0.016 0.010 0.012 0.018 0.011 0.014 0.010 0.063 0.018
0.030 0.016 0.014 0.015 0.031 0.011 0.021 0.046 0.029
0.020 0.030 0.026 0.018 0.035 0.052 0.050 0.055 0.022
0.494 0.474 0.496 0.376 0.360 0.621 0.390 0.820 0.680
0.022 0.046
0.053 0.320
0.038 0.097
0.034 0.026
0.014 0.007
0.180 0.650
0.022
0.046
0.055
0.039
0.026
0.091
0.022 0.003
1.23 1.38
Glacigenic conglomerate (matrix) GL-10–8 Argillites GL-10–1/1 GL-10–1/2 GL-10–1/3 GL-10–1/4 GL-10–1/5 GL-10–1/6 GL-10–1/7 SH-10–2 SH-10–3 Cap carbonates GL-10–3 GL-10–4 Quartzite GL-10–7
Cretaceous-Paleogene boundary clay J-1 J-2
11.86 4.11
12.20 6.33
characterized by Ir, Ru, Pt, and Pd concentrations of ~0.01–0.02 of chondritic values, which are equal to an ~1%–2% contribution of extraterrestrial material (chondritic in composition; Kyte, 1998) to the K-Pg impact layer at Gams. The analyzed K-Pg clay is characterized by a moderate loss of Os and significant loss of Re compared to the estimated amount of extraterrestrial material. This can be explained by the formation of volatile OsO4 and ReO4 in the impact cloud, and fractionation of Os and Re from the other PGEs. K-Pg boundary sediments worldwide are characterized by a larger range of PGE. Previously analyzed samples (Kyte et al., 1985; Lee et al., 2003) have higher concentrations of Ir, Ru, Pt, and Pd, and variable, if any,
10.06 7.39
13.33 13.76
depletion in Os and Re (Fig. 3). The horizontal distribution of elements from Ir to Pd on the chondrite-normalized plot can be used as a reference for extraterrestrial matter in sediments. If the argillite layer of the Goloustnaya Formation was deposited by ice-encapsulated particles enriched in cosmic dust and micrometeorites as a result of a snowball Earth meltback event in the Ediacaran Period, we would expect it to mimic the Ir to Pd flat pattern of the K-Pg impact-related sediments. This is not observed (Fig. 3). The argillites have a slope from low Os, Ir, and Ru chondrite-normalized concentrations to elevated Pt, Pd, and Re chondrite-normalized concentrations, similar to average continental crust (Peucker-Ehrenbrink and Jahn, 2001)
Sample/Chondrite
Figure 3. ChondriteArgillites “Homuty” (2) Average values (this study): 1 normalized (McDonough K-Pg clay (2) Cap carbonates (2) and Sun, 1995) platinum Terrigenic sediments (2) Argillites “Goloustnaya” (7) group element (PGE) and Re for averaged samples 0.1 from Goloustnaya and HoK-Pg boundary muty argillites, cap carbonates, and terrigenic rocks (quartzite and ma0.01 Ir anomaly for Sturtian trix of conglomerate). and Marinoan glaciation Numbers in parentheses meltback layers refer to number of sam0.001 ples used to calculate averages. For comparison: Continental Cretaceous-Paleogene (Kcrust Pg) clay at Gams, Austria 0.0001 (this study), KT boundary sediments worldwide (Kyte et al., 1985; Lee et al., PT-boundary 2003), Permian-Triassic 0.00001 (PT) boundary sediments Os Ir Ru Pt Pd Re (Brookfield et al., 2010), average continental crust (Peucker-Ehrenbrink and Jahn, 2001), and Ir-rich layer between glacigenic units and cap carbonates of Sturtian and Marinoan glaciations (Bodiselitsch et al., 2005).
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(Fig. 3). Such patterns in oceanic sediments (e.g., Permian-Triassic boundary sediments) are usually attributed to terrigenic sources (Brookfield et al., 2010). It is not only the distribution of PGE and Re that suggests a terrigenic source for the Goloustnaya argillites, but also the absolute concentrations of the elements. For example, the transitions between glacigenic units and the cap carbonates of the Sturtian and Marinoan glaciations recorded in Namibia, Africa, are marked by Ir concentrations that are ~10× greater (Fig. 3). The transitions from glacial to postglacial sediments for these two glaciations are different; a sharp increase of Ir concentrations to 1.2 ppb was observed for the climatic transition of the Marinoan event, and less pronounced and more dispersed Ir signals were recorded slightly above the transition for the Sturtian event (Bodiselitsch et al., 2005). The Marinoan Ir signal was used to constrain the duration of snowball Earth conditions to a “most likely” value of 12 Ma (Bodiselitsch et al., 2005). The Sturtian Ir signal could be explained by an ~1-m.y.-long snowball stage (Bodiselitsch et al., 2005). Our data for the studied Ediacaran glaciation suggest that it either did not reach the snowball stage or was of very short duration, insufficient for significant accumulation of cosmogenic dust in tropical regions due to the ice dynamics. However, other locations have to be tested to see if it is not an effect of inhomogeneous distribution of extraterrestrial material. The terrigenous concentrations of PGE and the distribution patterns for the studied samples also suggest that the Acraman mega-impact event did not coincide with the time of accumulation of the Goloustnaya sediments. DISCUSSION AND CONCLUSIONS During the Ediacaran, Siberia moved from north to south, but it remained in tropical paleolatitudes, regardless of the reconstruction used (Fig. 1) (Li et al., 2008). This means that a tropical latitude of glaciers is not necessarily evidence for snowball Earth conditions, or that snowball conditions could be extremely short in duration (
