Gondwana Research 36 (2016) 94–110

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GR Focus Review Field and laboratory tests for recognition of Ediacaran paleosols

Gregory J. Retallack ⁎

Department of Geological Sciences, University of Oregon, Eugene, OR 974031, USA article info abstract

Article history: Not a single paleosol had been described from rocks of Ediacaran age until 2011, but 354 Ediacaran paleosols have Received 18 January 2016 been described by 20 different authors since then. Some of these newly recognized paleosols have proven Received in revised form 4 May 2016 controversial, so this paper reviews 20 distinct tests to determine whether a particular Ediacaran bed could be Accepted 7 May 2016 a paleosol, or not. One problem has been that Ediacaran paleosols are not precisely like modern soils because Available online 13 May 2016 they lack root traces, a diagnostic feature of Silurian and geologically younger paleosols. The principal problem Handling Editor: M. Santosh for recognition of some Ediacaran paleosols is the occurrence in them of megafossils assumed to have been marine, although most of these fossils remain problematic for both biological and ecological affinities. Not all Keywords: the tests discussed here are diagnostic of paleosols, some are ranked permissive or persuasive. Permissive Paleoclimate conditions for paleosols include ripple marks, hummocky bedding, pyritic limestones, acritarchs or thalloid Petrography fossils, low strontium isotopic ratios, high δ26Mg ratios, and red color. Persuasive tests include loessites, Geochemistry tsunamites, desert playa minerals, low boron content, high δ10B isotopic ratios, high carbon/sulfur ratios, and Stable isotopes very low total/reactive iron ratios. Diagnostic tests include matrix-supported lapilli or crystal tuff parent Vendobiont materials, ice wedges and other cryoturbation, sepic birefringence fabrics, evaporitic sand crystals, and negative Ediacaran geochemical strain and mass transfer, and highly correlated δ13C and δ18O. Like other geological periods, the Ediacaran is known from a variety of marine and non-marine paleoenvironments © 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Contents

1. Introduction...... 95 2. Ripplemarks(permissive)...... 95 3. Hummocky cross-stratification(permissive)...... 96 4. Pyriticlimestones(permissive)...... 97 5. Acritarchsandthalloidfossils(permissive)...... 97 6. Strontiumisotopes(permissive)...... 97 7. Magnesiumisotopes(permissive)...... 97 8. Redcolor(permissive)...... 97 9. Loessites(persuasive)...... 98 10. Tsunamites(persuasive)...... 99 11. Desertplayaminerals(persuasive)...... 99 12. Boroncontent(persuasive)...... 99 13. Boronisotopes(persuasive)...... 99 14. Carbon/sulfurratios(persuasive)...... 100 15. Totalandreactiveironratios(persuasive)...... 101 16. Lapilliandcrystaltuffs(diagnostic)...... 101 17. Icedeformation(diagnostic)...... 101 18. Birefringencefabrics(diagnostic)...... 102 19. Evaporiticsandcrystals(diagnostic)...... 102 20. Geochemicaltauanalysis(diagnostic)...... 103 21. Carbonandoxygenisotopes(diagnostic)...... 104 22. Conclusions...... 105

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http://dx.doi.org/10.1016/j.gr.2016.05.001 1342-937X/© 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. G.J. Retallack / Gondwana Research 36 (2016) 94–110 95

Acknowledgments...... 106 References...... 107

1. Introduction the Ediacaran “savannah hypothesis” (Budd and Jensen, 2016), but could these epigrams have been descriptive rather than metaphorical? Discovery of Ediacaran paleosols has been controversial (Retallack, Were the strange quilted fossils of the Ediacaran a variety of worms, 2013a; Xiao et al., 2013), because some paleosols are closely associated or were they sessile osmotrophic and photosymbiotic organisms like with Ediacaran fossils long considered marine invertebrates (Tarhan fungi and lichens (bottom versus top row of Fig. 2)? Ontogenetic series et al., 2015a) or marine algae (Xiao et al., 2013). Thus, some described of fossils are evidence of growth by addition of quilts to the narrow end Ediacaran paleosols have been dismissed without giving reasons (Antcliffe and Brasier, 2008; Gold et al., 2015), but was the larger (C. Liu et al., 2014; Liu et al., 2015), and other reviews have judged all segment at the other end a holdfast or a head? Cephalization and advent evidence for described Ediacaran paleosols inconclusive (Callow et al., of the “noösphere” in the sense of Pierre Teilhard de Chardin has been 2013; Tarhan et al., 2015a). Before the evolution of trees and large argued by regarding the big end as a neurologically differentiated burrowing animals, Ediacaran paleosols may indeed have had some head (McMenamin, 2001). By the other point of view, however, the peculiarities (Retallack, 2012, 2013a), but there is a need to agree a big end was a holdfast or growth initial of an extinct clade of priori on tests of whether a particular bed is marine or non-marine, a Vendobionta, and segments were added by a terminal meristem, like paleosol or not. This review outlines 20 distinct tests for the interpreta- addition of leaves to a tree (Seilacher, 2013). tion of individual Ediacaran beds as non-marine, or as paleosols. These Were these problematic fossils the evolutionary stirrings of the various tests are triaged here into categories based on strength of Cambrian explosion of marine animals (Erwin et al., 2011; Droser and evidence. First are permissive tests that allow for interpretation of Gehling, 2015)? Or did they merely facilitate that adaptive radiation paleosols but do not require a paleosol interpretation. Second are by providing shelter and resources (Retallack, 2013a; Budd and persuasive tests that favor a paleosol interpretation, but inconclusively, Jensen, 2016)? Paleosol evidence for terrestrial habitats applies only to because of rare exceptions. Third are diagnostic tests that require a the distinctive quilted vendobionts (Fig. 2), and not yet to calcareous paleosol interpretation. My aim is to outline tools for future assessment, shells, such as Cloudina, nor chitinous tubes, such as Corumbella,found rather than adjudicate past disputes. For opposing interpretations of the in undisputed gray marine facies (Warren et al., 2011, 2012; Pacheco South Australian Ediacara Member (Retallack, 2013a; Xiao et al., 2013), et al., 2015). Also favoring a terrestrial interpretation of vendobionts this review offers 8 additional lines of evidence not yet attempted, are discovery of Ediacaran fossils demonstrating nutrient extraction among a total of 20 outlined here. from substrates (Antcliffe et al., 2015), satellite vegetative propagules Ediacaran non-marine facies have been recognized for some time in (Mitchell et al., 2015), limited interactions between fossils (Mitchell, South Australia (Mawson and Segnit, 1949) and Mali (Bertrand-Sarfati 2015), and substrate-specific assemblages (Retallack, 2016), more and Moussine-Pouchkine, 1983). Surprisingly, no individual paleosols like terrestrial vegetation than marine animal communities. Such were described until 2011, but since then 354 paleosols have been sophisticated inferences are colored by interpretation of the substrates described by 20 different authors (Retallack, 2011a-b; Retallack, 2012, and geological context of the fossils, which is the main issue addressed 2013a-c; 2014a-d; Retallack, 2016; Maslov and Grazhdankin, 2011; here. Are there, or are there not, Ediacaran paleosols, and how can they Kolesnikov et al., 2012, 2015; Retallack et al., 2014; Marconato et al., be recognized? 2014; Liivamägi et al., 2014; Vircava et al., 2015). This review summa- rizes this past research on distinguishing marine and non-marine Ediacaran rocks, as well as recognizing individual Ediacaran paleosols. 2. Ripple marks (permissive) At stake with this work is whether the Ediacaran Period has only a record of ancient marine life, as in traditional reconstructions (Fig. 1a), A simple test proposed by Xiao (2013) is to consider ripple marks or do some of these fossils represent freshwater (Fig. 1c), or fully evidence of active sedimentary paleoenvironments (Fig. 1a), and thus terrestrial (Fig. 1c) ecosystems? These strange ecosystems have been a bed unaltered by soil formation. This would be true if the ripple portrayed as the “Garden of Ediacara” (McMenamin, 1986)andinspired marks were fresh, and lacked evidence of later alteration such as wind

Fig. 1. Alternative interpretations of Ediacaran ecosystems as shallow marine tropical invertebrates (a), aquatic microbial colonies (b), and as terrestrial lichens (c); a, classical diorama of Ediacaran ecosystems of South Australia from Museum of the Earth, near Ithaca, New York; b, jelly balls of the cyanobacterium Nostoc pruniforme near the River Liene near Hannover Germany (by and with permission of Christian Fischer); c, foliose lichen (Xanthoparmelia reptans) on bare red soil between red mallee (Eucalyptus socialis) and porcupine grass (Triodia scariosa) near Damara station, near Balranald, N.S.W., Australia. 96 G.J. Retallack / Gondwana Research 36 (2016) 94–110

Fig. 2. The biological affinities of these Ediacaran fossils are problematic. Is the big end a head of an animal, or is it a holdfast of a sessile creature?

deflation, biological soil crusts, or bedding disruption comparable with 3. Hummocky cross-stratification (permissive) that of roots or rhizines. Root traces are the most common guide to pedogenic modification Alluvial, lacustrine, estuarine, deltaic, and shallow marine environ- of ripple marks and other sedimentary structures in Phanerozoic ments are difficult to distinguish in sparsely fossiliferous early Paleozoic paleosols formed over short time spans (for example, Sikahk of and Precambrian sandstones, because cross-bedding and lamination are Retallack, 1994;FrazerofRetallack, 1999;SayaykofRetallack et al., similar in marine and non-marine settings (Selley, 1970; Bristow et al., 2000). Root traces are unknown in paleosols older than Silurian 2009; Pandey and Bahadur, 2009; Callow et al., 2013). For example, (Retallack, 2015a), but rhizine-like structures of millimetric scale, Gehling and Droser (2013) consider “mass flow sandstones” adistinc- detectable by hand lens in the field, and confirmed in petrographic tive submarine canyon facies, but comparable massive sandstones are thin sections, are known in Ordovician (Retallack, 2015b), Cambrian found in the Triassic fluvial Hawkesbury Sandstone (Conaghan, 1980) (Retallack, 2011c), Ediacaran (Retallack, 2011a), and Paleoproterozoic and the modern Brahmaputra River (Coleman, 1969). Another example paleosols (Retallack et al., 2013). Especially obvious are drab-haloed is paleovalleys, paleochannels, and disconformities which are mappable microbial filament traces (ichnogenus Prasinema) in red paleosols for 100 km along strike in the Rawnsley Quartzite of South Australia, (Retallack, 2011c). and especially common toward the Gawler Craton to the west, but Ediacaran fossils on the rippled surface illustrated by Xiao considered submarine by Gehling (2000) and subaerial by Retallack (2013) are on a background of irregularly furrowed “old elephant (2013a). Whether submarine canyons were excavated under the sea skin” texture (ichnospecies Rivularites repertus), considered a non- or by rivers during low sea level has been a long-running debate in marine trace fossil because of its complex healed and gaping boththeHoloceneandEdiacaran(Retallack et al., 2014). cracks, pustules, and pinnacles, comparable with modern desert Hummocky cross-stratification has been considered an exception, crusts (Retallack, 2012). Aquatic microbial mats in contrast show produced by storm wave interference in shallow subtidal sands (Dott flexuous, non-brittle undulation, and detached and folded mat and Bourgeois, 1982). The original Eocene examples are associated fragments (Noffke, 2010), not yet reported from fossiliferous with marine fossils, including surf clams and brittle stars (Retallack, Ediacaran slabs (Seilacher, 2007). Also of laminated and flexuous 2016) in beach ridge sandstones interbedded with intertidal facies of a form are microbial mats of the intertidal zone forming on organic large delta (Dott, 1966). However, numerous examples of hummocky and sulfidic sediments, which also are regarded as Sulfaquept and cross-stratification have been documented from modern and ancient Sulfaquent soils (Retallack, 2013b). lakes (Martel and Gibling, 1993). One claim of hummocky cross-stratifi- The form of the ripple marks is also relevant, because the cation in deep marine sediments (Mulder et al., 2009) was probably on ripple marks figured by Xiao (2013) are oscillation ripples formed a continental shelf, and not such deep water (Higgs, 2011). in very shallow standing water of lakes and intertidal lagoons, Nevertheless, occurrences of hummocky cross-stratification in rather than open ocean (Retallack, 2014a). Ripple marks etched sequences with Ediacaran fossils have been considered evidence that across their crests by wind deflation or mud cracking of clay they were marine (Callow et al., 2013; Antcliffe and Hancy, 2013; drapes are also evidence of subaerial exposure (Vortisch and Tarhan et al., 2015a). Hummocky cross-stratification has been docu- Lindström, 1980; Hocking, 1991), known from the Ediacaran peri- mented from the Ediacaran Mistaken Point Formation of the Bonavista od (Mapstone and McIlroy, 2006; Pandey and Bahadur, 2009; Peninsula in Newfoundland (Retallack, 2014c). Other cases of Ediacaran Kolesnikov et al., 2012). Sediment ridges downwind from hummocky cross-stratification are unclear. An example from Red obstacles (setulfs) are other examples of surface exposure also Range, South Australia (Gehling, 2000), has a truncated top, can be known from sandstones of Ediacaran age (Sarkar et al., 2011). seen in only two dimensions, and does not contain fossils. Examples Climbing translatent cross-bedding is yet another form of ripple from Nilpena, South Australia (Tarhan et al., 2015a), are trains of mark formed only by wind in terrestrial settings (Hunter, 1977), isolated sets only 4 cm thick, like washed out megaripples, unlike and also known from the Ediacaran (Retallack, 2011a). Using rip- thicker (12–20 cm) cosets (usually 1–2 m thick) of hummocky bedding ple marks as a test of a paleosol requires careful study of the mor- (Dott and Bourgeois, 1982; Retallack, 2016). Use of hummocky cross- phology of the sedimentary structures as well as their petrography stratification as evidence of marine conditions is mistaken, because in thin section. they permit a lacustrine interpretation (Martel and Gibling, 1993), but G.J. Retallack / Gondwana Research 36 (2016) 94–110 97 careful attention to three dimensional morphology and petrography is decay of 87Rb to 87Sr, but marine limestones over the same interval of needed (Higgs, 2011). time show 87Sr/86Sr ratios as high as 0.710 due to increased abundance of granites and intensified weathering on land (Capo et al., 1998; Veizer 4. Pyritic limestones (permissive) et al., 1999; Shields and Veizer, 2002). Weathering can raise whole-soil 87Sr/86Sr ratios to as high as 0.729 in soils on granite, with even higher Non-marine Ediacaran habitats have been considered ruled out for values of 0.795 in clayey fractions (Blum and Erel, 1997). Ratios of vendobiont fossils because they are sometimes found in laminated, 87Sr/86Sr in Pleistocene basaltic soils range from mantle values of black, calcareous, and pyritic shales (Grazhdankin et al., 2008; Chen 0.703 to as high as 0.720 (Stewart et al., 2001). Variation in87Sr/86Sr et al., 2014). This is an inconclusive test because the fossils may have ratios is thus a crude metric of granite-soil versus basalt-marine been washed out into marine facies, like fossil plants in marine shales carbonates: values below about 0.709 are found in indisputable marine of Eocene age in Italy (Tang, 2002) and of Triassic age in New Zealand carbonate of well preserved sea shells (Veizer et al., 1999; Prokoph et al., (Retallack, 1985). Ediacaran vendobiont fronds of North Carolina 2008), but higher values reflect both soil alteration and admixture of are highly effaced and lack attachment stalks of comparable fossils, as soil material. indications that they were probably transported (McMenamin and Basal Ediacaran carbonate of the Nuccaleena Formation in South Weaver, 2002). Australia and the Ol Formation in Mongolia have likely granite-soil Another alternative is that vendobionts in growth position in pyritic values (0.709–0.715 and 0.708–0.709, respectively), which have been black shales and limestones represent intertidal vegetation comparable interpreted as evidence of a glacial meltwater plume of fresh water with Phanerozoic salt marsh and mangal (Retallack, 2016). The covering the world ocean for an implausible 8000 years (C. Liu et al., intertidal zone is a sedimentary environment with distinctive sedimen- 2014). An alternative explanation is that these high values were created tary structures including flaser and linsen bedding (Selley, 1970), but by weathering in paleosols previously described from the same strati- also supports soils variously identified as Thionic Fluvisols and graphic level of the Nuccaleena Formation (Retallack, 2011a). Strontium Sulfaquepts (Retallack, 2013b). In addition to root traces, such paleosols isotopic analysis of other Ediacaran limestones and nodules could be have relict bedding and nodules of calcite, pyrite, or phosphate in an revealing tests of the influence of continental weathering. organized horizon (Bg horizon) near the surface (Retallack and Dilcher, 2012). Intertidal soils develop with increased size of nodules 7. Magnesium isotopes (permissive) in the subsurface, organic matter in the surface, and disruption of beddingbyrootsandfilaments, also apparent from sequences of Soil clays are enriched in isotopically heavy magnesium (as much as peritidal paleosols (Driese et al., 1992; Williams and Krause, 2000; +0.6‰ ô26Mg) compared with their isotopically lighter soil parent Retallack and Dilcher, 2012; Retallack, 2013b). Comparable paleosols material (Brenot et al., 2008; Opfergelt et al., 2012). Chinese terrestrial with Ediacaran vendobiont fossils are known from Newfoundland silicate loess has a value of −0.602‰ ô26Mg, but marine limestones (Retallack, 2016)andChina(Retallack, 2014a). In contrast, marine and foraminifera range from −0.109 to −5.279‰ ô26Mg, and Ediacaran facies of North Carolina (McMenamin and Weaver, 2002) speleothems range from −2.594 to −4.840‰ ô26Mg (Young and and France (Retallack, 2012) lack organic, nodular and non-laminated Galy, 2004). The overlap of marine limestones and non-marine horizons. speleothems may be inherited from marine carbonate cave walls but is unlikely to confound use of ô26Mg as a paleosalinity proxy because 5. Acritarchs and thalloid fossils (permissive) of the distinct petrography and form of speleothems (Railsback et al., 2002). Organic walled acritarch microfossils and thalloid, branching, carbo- Secular variation in marine carbonate ô26Mg is not well known back naceous compression fossils are additional Ediacaran fossils commonly in geological time (Tipper et al., 2006), but within modern marine range compared with marine phytoplankton and sea weeds, respectively, are values of −2.22 to −1.43‰ ô26Mg for Ediacaran carbonates of and thus taken as indications of marine habitats (Moczydłowska et al., South Australia, Mongolia, and Namibia (C. Liu et al., 2014; Kasemann 2011; Xiao et al., 2013). Both, however, are problematic in biological et al., 2014). However, there are marked excursions from common affinities and habitats. Many Ediacaran acritarchs have been found in depleted values to the most enriched values (−1.43 to −1.92‰ Mesoproterozoic lake beds (Strother et al., 2011) and there is evidence ô26Mg) attributed to freshwater input because of high values of from clay mineralogy for freshwater deposition of the upper 87Sr/86Sr (0.708–0.715) in the same samples (C. Liu et al., 2014). Doushantou Formation (Bristow et al., 2009; but see also Huang et al., Paleosols have been found at that same stratigraphic level in South 2013)yielding“sea weeds” described by Xiao et al. (2002). Some Australia (Retallack, 2011a), so the observed enrichment in ô26Mg Ediacaran acritarchs are more like fungal spores (Retallack, 2015c)or may reflect that soil formation. Comparable analyses of ô26Mg through animal cysts (Cohen et al., 2009) than phytoplankton (Moczydłowska short sections of suspected paleosols could be applied to other Ediacar- et al., 2011). Branching Ediacaran thalli do have a thin texture and an carbonates. A larger compilation of ô26Mg may also prove that narrow flexuous stalks of organisms living underwater (Xiao et al., discriminating values between marine and non-marine carbonates 2002), but are as similar to freshwater algae, such as the green alga were slightly different in the Ediacaran than today, as for87Sr/86Sr and Ulva (formerly “Enteromorpha”) intestinalis and red alga Compsopogon δ18O(Veizer et al., 1999). caeruleus (Pentecost, 1984), as to marine algae, such as the green alga Ulva clathrata and red alga Pterocladia capillacea (Graham and Wilcox, 8. Red color (permissive) 2000). Determining the biological affinities of such nondescript carbonaceous compressions requires exceptional cellular preservation Red color is a useful first alert that paleosols may be present in a rock or reproductive structures but may not be helpful because single genera sequence (Fig. 3a–b) because red beds are commonly sequences such as Ulva include both marine and freshwater species (Pentecost, of well-drained paleosols (Retallack, 1994; Retallack et al., 2000). 1984; Graham and Wilcox, 2000). However, drab-colored paleosols of coal measures and estuarine se- quences represent waterlogged paleoenvironments (Retallack, 1999; 6. Strontium isotopes (permissive) Retallack and Dilcher, 2012; Retallack, 2015a-b). Conversely, most marine rocks are grey with organic matter, but there also are marine, The ratio of stable isotopes 87Sr/86Sr in basaltic rocks most represen- red, chemical sediments, such as oolitic iron ores (Brett et al., 1998), tative of Earth's mantle shows limited variation from 3800 million years abyssal red clays (Gleason et al., 2002), ammonitico rosso, and red ago to the present of 0.700 rising to a current 0.703 due radiometric radiolarian cherts (Mamet and Preat, 2006). Red color in all these 98 G.J. Retallack / Gondwana Research 36 (2016) 94–110

Fig. 3. Field appearance of paleosols in South Australia (a–b) and Newfoundland (c–e); a, “old elephant skin” (Rivularites repertus) showing sutured radial growth, crack fills and ridge impressions, effaced discoid fossils (white circles) and fossil impressions (Hallidaya brueri in positive relief to left, and Rugoconites enigmaticus in negative relief to right), on sole of sandstone slab from Ediacara Member of Rawnsley Quartzite in Crisp Gorge, South Australia; b, named pedotypes of paleosols (labeled) and fossil levels (arrows) from Ediacara Member of Rawnsley Quartzite in Brachina Gorge, South Australia, with hammer for scale (white circle); c, “old elephant skin” (Rivularites repertus) showing sutured cracks, pustules, and fossil impressions (Fractifusus misrai, in positive relief) on surface E at 10.9 m in section of Mistaken Point Formation of Retallack (2014c), Mistaken Point, Newfoundland; d, Acis palaeosol (sharp top indicated by arrow) in outcrop at 7.3 m in section of Mistaken Point Formation by Retallack (2014c) at Mistaken Point, Newfoundland; e, Maglona palaeosol (sharp top indicated by arrows) in polished slab from 22.5 m in section of Retallack (2014c), Mistaken Point (specimen number NFM G-167 in The Rooms, St Johns).

cases comes from ferric hydroxides (such as goethite) and oxides (such of Ediacaran rocks in South Australia (Retallack, 2012, 2013a; Retallack as hematite) formed either in sediment-starved oceans (Brett et al., et al., 2014). 1998; Gleason et al., 2002; Mamet and Preat, 2006) or by oxidative The alternative of modern weathering of Ediacaran fossils in the weathering in well-drained soils geologically younger than the Great Flinders Ranges of South Australia has been revived by 234U/238U Oxidation Event of 2.4 Ga (Lyons et al., 2013; Retallack, 2014b). Oxida- alpha recoil dating of ferric oxides as less than 2 Ma in age (Tarhan tive weathering can also occur in outcrop (Tarhan et al., 2015a), so the et al., 2015b). This technique of dating cannot analyze geologically timing of ferruginization is critical. older hematite in the rock, which is dead for 234U. Furthermore, the Ediacaran red beds of South Australia have been a testing ground for 234U/238U analyses were done on fossil specimens in cover sandstones these ideas since Mawson and Segnit (1949) marshaled field evidence to the putative paleosols, deposited by floods or storms, rather than of centimeter-scale alternating red-green lamination unrelated to on fossil substrates. The study of Tarhan et al. (2015b) was inconclusive, modern outcrop as evidence for Precambrian oxidation. These color but there is a need for additional paleomagnetic and radiometric dating variations were not due to copper mineralization, which is easily of ferric oxides associated with Ediacaran fossils. detected in this region from sulfide ores and brecciated gossans (Lambert et al., 1987). To this evidence has now been added observa- 9. Loessites (persuasive) tions of red clasts redeposited in drab sandstones, unweathered black and gray shales, and limestones interbedded with red beds in outcrop, Loess is a distinctive kind of glacially derived, eolian silt, and loessite red color of the same beds at depths of 71–91 m below drab beds in the equivalent consolidated siltstone, but both are difficult to recognize drill cores, greater chemical weathering of red than gray siltstones, without petrographic study. Many Precambrian “claystones” examined and greenschist metamorphic grade of Ediacaran red siltstones in thin section have turned out to be loessites, with more than 60% (Retallack, 2012, 2013a; Retallack et al., 2014). Another indication of angular grains of silt size (0.002–0.063 mm or 5–6 Φ) as determined original oxidation comes from clast, reversal and fold tests of hematite by grain measurements (Mawson and Segnit, 1949; Edwards, 1979; used for paleomagnetic stratigraphy of Ediacaran rocks of South Retallack, 2011a, 2012, 2013a, 2016). Lack of weathering, grain Australia (Schmidt and Williams, 2010, 2013). The expectation of red- angularity, low clay content, and associated paleosols, climbing dening in outcrop by deep lateritic Mesozoic or Cenozoic weathering translatent cross-stratification, and wind ripples are especially charac- (Gehling, 2000; Mapstone and McIlroy, 2006)wouldbe teristic of both Quaternary loess (Swineford and Frye, 1951; Kemp unmetamorphosed kaolinite clays, boehmite, and gibbsite (Retallack, et al., 2004) and Ediacaran loessites (Retallack, 2011a, 2013a, 2015a), 2010), and these minerals were not detected by x-ray diffraction studies Loess grains blown out to sea remain identifiable in thin section, but G.J. Retallack / Gondwana Research 36 (2016) 94–110 99 are diluted by clay and microfossils (Gleason et al., 2002). A case has the minerals of marine evaporites include sylvite [KCl], carnallite been made that loess may have been more common in Early Paleozoic [KMgCl3·6(H2O)], langbeinite [K2Mg2(SO4)3], polyhalite marine siltstones before the land was stabilized by plants (Dalrymple [K2Ca2Mg(SO4)4·2H2O], and kainite [KMg(SO4)Cl·3H2O]. Gypsum et al., 1985), but some of the red siltstones considered marine for that [CaSO4·2H2O], anhydrite [CaSO4], and halite [NaCl] are common and study are now considered rich in paleosols (Retallack, 2015a-b). In- often dominant in both marine and non-marine evaporates (Warren, fluxes of eolian dust from modern deserts are large in some modern ma- 2006). rine sediments (Nakai et al., 1993), so evaluating this claim for the Ediacaran to early Cambrian marine evaporites with sylvite, Ediacaran will need a wider inventory of desert versus non-desert Edi- carnallite, kainite, and polyhalite are widespread from Oman acaran paleosols than currently available. (Schoenherr et al., 2010; Smith, 2012), through Pakistan (Kovalevych et al., 2006) to India (Cozzi et al., 2012; Davis et al., 2014). Ediacaran 10. Tsunamites (persuasive) non-marine evaporite minerals also are known in the form of glauberite in the Hongchunping Formation of China (Meng et al., 2011), and Tsunamites, formed by coseimic tsunamis (Atwater et al., 1992), palygorskite in the Téniagouri Group of Mauretania (Shields et al., and tempestites, formed by storms (Seilacher, 1982), are event beds 2007). Saponite and palygorskite from the Ediacaran Doushantou For- formed in shallow coastal waters and floodplains, whereas turbidites mation of China have been taken as evidence of an alkaline lake are event beds of lakes and oceans (Bouma, 1962). Turbidites have (Bristow et al., 2009; Hohl et al., 2015) and can be recognized despite long been recognized in Ediacaran fossiliferous beds (Misra, 1971), partial recrystallization due to deep burial (Derkowski et al., 2013). On but tempestites and tsunamites were only recently applied to interpre- the other hand, the Doushantou Formation has been considered marine tation of Ediacaran event beds (Retallack, 2014c, 2016). Turbidites vary because it contains fossil acritarchs (see Section 2 above), and the sapo- enormously in thickness and facies sequences but are normally graded nite considered detrital from soils of a short-lived mafic–ultramaficvol- beds with grain size diminishing upwards. Sedimentary structures canic source terrane (Huang et al., 2013). Further x-ray diffraction associated with turbidites include flute casts, claystone breccias, ripple studies of Ediacaran rocks are needed to identify additional mineral spe- marks, and intervening thick shales (Talling et al., 2012). Undisputed cies, because these are rare minerals requiring a deliberate search. Many Ediacaran turbidites are known from the Poudingue de Granville of of the evaporite minerals are very soluble in formation waters and out- France (Retallack, 2012) and the Mall Bay and Briscal Formation of crop and may be replaced by calcite, barite, or quartz, thus detailed crys- Newfoundland (Retallack, 2013c, 2014c). Tempestites in contrast tallographic analysis is needed of acicular crystals in Ediacaran rocks are sandy, either normally or reverse graded, and have associated (Kennedy, 1996; Jiang et al., 2006). hummocky cross-stratification, claystone breccias, and enclosed fossils (Seilacher, 1982). Ediacaran tempestites have been identified in the 12. Boron content (persuasive) Mistaken Point Formation of Newfoundland (Retallack, 2014c, 2016). Tsunamites may have a tempestite-like form offshore but are best Boron content of rivers is low (0.2 ppm) compared with marine known within Holocene tidal flats, marshes, and coastal plains in water (5 ppm or greater) and evaporating tidal pools (as much as the Pacific Northwestern U.S.A. (Atwater and Hemphill-Haley, 1997) 88 ppm), so that salinity (S in ‰) of water can be predicted from its and southern Chile (Atwater et al., 1992; Cisternas et al., 2005), where boron content (B in ppm) by the following equation: S =2.97B + they can often be attributed to known subduction zone earthquake 20.3, with some confidence (r2 = 0.92, s.e. = ±13‰,n=34).In events by radiocarbon dating of the remaining “ghost forests.” addition to these data, Frederickson and Reynolds (1960) also provided Tsunamites are clean, white, planar bedded, sand beds, 5–20 cm thick analyses from shales showing that paleosalinity could be estimated within sequences of peat and claystone including stumps and leaf litter from boron content of the illite fraction of clays (but not kaolinite, nor of buried coastal soils. Comparable thin, planar to wavy bedded sands smectite) as a proxy for paleosalinity variations between different also buried the agricultural fields and paved streets around Sendai, successive geological formations. Shaw and Bugry (1966) supported Japan, from the Tohoku-oki tsunami of March 11, 2011 (Szczuciński the method with additional shale analyses, suggesting about 50 ppm et al., 2012). Soils buried by tsunamites are thin and weakly developed, as the maximum for freshwater unmetamorphosed illites, or about 10 often with features of coastal waterlogging such as pyrite nodules, be- times the cutoff for modern waters. Furthermore, high-grade metamor- cause their ecosystems are destroyed by tsunamis on time scales of phism capable of segregating boron into tourmaline veins may lower about 600 years (Goldfinger et al., 2003). Comparable sandstones with the boron content of shales (Shaw and Bugry, 1966). The order of both sharp bottoms and tops, interbedded with ungraded, mottled magnitude more boron in illites than in modern waters may also reflect paleosols have recently been recognized in the Mistaken Point Forma- burial illitization (Lerman, 1966; Moldovanyi et al., 1993; Williams tion of Newfoundland (Fig. 3d–e). Exposed surfaces of the paleosols et al., 2001). Metamorphism to greenschist facies allowed detection of host assemblages of fossils (Fig. 3c). A tsunamite interpretation is com- both marine and non-marine intervals in Paleoproterozoic sequences patible with evidence from ash geochemistry, granitic basement, and of South Africa, because original differences in boron content were regional structure for a forearc basin tectonic setting above an active magnified by regional burial alteration (Eriksson et al., 1996). Similarly, subduction zone during the Ediacaran in Newfoundland (Retallack, Ediacaran rocks have greenschist (Retallack, 2012, 2013a) or prehnite 2014c, 2016). pumpellyite facies (Retallack, 2014c) metamorphic grade. Surprisingly, Ediacaran gray shales of Estonia, very similar in outward appearance to 11. Desert playa minerals (persuasive) fossiliferous Ediacaran shales of the Russian White Sea (Grazhdankin, 2004), have low boron contents considered indicative of freshwater Desert playas are claimed by sedimentology as playa lakes (Pirrus, 1992). Comparable boron analyses of other Ediacaran illitic (Selley, 1970), and also by soil science as Solonchak and Natrargid shales are needed. soils (Retallack, 2001). Such desert evaporitic soils have a distinctive suite of minerals (Warren, 2006), including nahcolite [NaHCO3], 13. Boron isotopes (persuasive) trona [Na3(CO3)(HCO3)·2H2O], bloedite [Na2Mg(SO4)2·4H2O], 11 borax [Na2B4O7·10H2OorNa2{B4O5(OH)4}·8H2O], epsomite Boron isotopic ratios (δ B) in marine carbonate are proxies for [MgSO4·7H2O], gaylussite [Na2Ca(CO3)2·5H2O], glauberite pH of the ocean, and in turn, for the ancient levels of atmospheric [Na2Ca(SO4)2], mirabilite [Na2SO4·10H2O], thenardite [Na2SO4], CO2, which forms carbonic acid in proportion to its concentration searlesite [Na2SO4], palygorskite [(Mg,Al)2Si4O10(OH)·4(H2O)], and (Pearson and Palmer, 2000). The method is compromised by saponite [Ca0.25(Mg,Fe)3((Si,Al)4O10)(OH)2·n(H2O)]. In contrast, paleotemperature and by fluvial inputs of boron which can cause 100 G.J. Retallack / Gondwana Research 36 (2016) 94–110 large local variation in relatively more stable marine isotopic 14. Carbon/sulfur ratios (persuasive) ratios (Royer et al., 2001). These problems are mitigated by selecting planktic foraminifera in central Pacific Ocean cores, A useful proxy for distinguishing marine shales from terrestrial which show a rise to 25‰ δ11B taken as 8.3 pH for 300 ppm paleosols is the ratio of carbon to sulfur (Berner and Raiswell, 11 CO2 atmospheres of the Neogene from 22 ‰ δ B taken as 1984; Raiswell and Berner, 1986) because marine shales become 7.4 pH for 3600 ppm atmospheric CO2 during the Paleocene, a euxinic by becoming strongly pyritic. Similar processes elevate 0.9 pH unit change over 60 million years (Pearson and Palmer, the C/S ratio of intertidal soils of salt marshes and mangroves, 2000). Well-preserved Ediacaran carbonates assumed to have which are organic and dysaerobic but bathed in sulfate-rich ma- been marine have yielded dramatic fluctuations in δ11B over very rine water (Altschuler et al., 1983). Marine rocks are sulfur rich short sections: from − 2.2‰ δ11B taken as 7.8 pH to +14.8‰ and carbon poor (C/S b 2.6), but freshwater sediments and soils δ11B taken as 9.3 pH over only 4 m (Ohnemüller et al., 2014). are carbon rich and sulfur poor (C/S N 2.8: Johnston et al., 2012). Paleomagnetic considerations have been taken as evidence that The ocean may have been very low in dissolved sulfate during this 4 m represents only about 8000 years (C. Liu et al., 2014). the Ediacaran (Canfield et al., 2007, 2010; Canfield and Farquhar, Such an ocean-sterilizing geochemical change is unlikely. There 2009)andArchean(Foriel et al., 2004; Crowe et al., 2014; is another explanation from comparable Australian “cap- Zhelezinskaia et al., 2014). Compilations of marine shale analyses carbonates” with paleosols, for which dramatic pH changes over show that the discriminatory C/S ratio was lower further back in sort intervals are unremarkable (Retallack, 2011a). Paleokarst time, about 0.68 for the Early Silurian to Middle Ordovician, 0.56 paleosols have also been noted in the Chinese sections studied for the Early Ordovician, and 0.50 for the Cambrian (Raiswell by Ohnemüller et al. (2014), but detailed documentation of and Berner, 1986). Extrapolating backward to the base of the Edi- those paleokarsts remains to be completed (Yuan et al., 2005; acaran (635 Ma) from these three values give an Ediacaran dis- Retallack, 2014a). Lowering of pH and δ11B by weathering in criminant C/S ratio of 0.48 ± 0.03, and extrapolating back to paleosols and paleokarsts should be considered in future studies 635 Ma from all the Phanerozoic data of Raiswell and Berner of Ediacaran carbonate δ11B. (1986) gives a value of 0.81 ± 0.40. These low values may reflect

Fig. 4. Geochemical indices of freshwater C/S ratios and FeHT/FeTOT for the Ediacaran, Mistaken Point Formation of Newfoundland. Limiting values of these ratios are for modern sediments fromBerner and Raiswell (1984); Raiswell and Berner (1986),andKu et al. (2008), and Mistalen Point data are from Canfield et al. (2007). G.J. Retallack / Gondwana Research 36 (2016) 94–110 101 sulfur poor oceans but could also reflect carbon poor streams and subaerial volcanism. A key difference between tuffs of mixed grain lakes. size deposited in water is grading by grain size, with the largest grains Whether the discriminant value for the Ediacaran is similar to at the bottom settling first though still water. On land, however, lapilli modern (“freshwater” in Fig. 4), or as extrapolated above from Berner and crystal tuffs may be ungraded or inverse graded because of a and Raiswell (1984: similar to “euxinic” of Fig. 4), short-term variation significant component of base surge (Fisher and Schminke, 1984; Cas in Ediacaran C/S ratios is striking (Fig. 4). The values bounce around and Wright, 1987). between modern freshwater and modern marine values over only a Avarietyofdefinitively terrestial volcaniclastic deposits have found few meters, and it is more likely that this represents change in sea associated with Ediacaran fossils in the volcaniclastic Mistaken Point level than short-term freshenings of the world ocean (Retallack, and Drook Formations of Newfoundland: spindle bombs, gas escape 2013c, 2016). Future studies should consider finer stratigraphic structures, block and ash flows, and lapilli and crystal tuffs (Retallack, sampling around paleosols in sections like that of Fig. 4. 2014c). The Newfoundland lapilli tuffs are especially diagnostic of deposition on land not only because the lapilli and scoria fragments are ungraded (Fig. 5) but because non-calcareous lapilli are unstable 15. Total and reactive iron ratios (persuasive) and disperse in water (Reimer, 1983). Marine volcaniclastic deposits of Ediacaran age with normally graded and clast supported tuffs and Another geochemical proxy useful for distinguishing marine and tuffaceous rocks also are known, notably the Poudingue de Granville non-marine rocks is the ratio of highly reactive iron (Fe , mainly pyrite, HR of France (Retallack, 2012), the Mall Bay and Briscal Formations of magnetite, and hematite) over total iron (Fe , including iron within TOT Newfoundland (Retallack, 2014c), and the Floyd Church Formation of clays and other minerals. This ratio is a reflection of massive pyrite North Carolina (Retallack, 2014a). Fabrics and textures of volcanic cementation of marine euxinic shales (Canfield et al., 2007), which tuffs and tuffaceous sediments could be more widely employed in greatly exceeds the formation of goethite and hematite during forma- deciphering Ediacaran paleoenvironments. tion of most soils (Ku et al., 2008). Exceptions include pyrite in intertidal soils (Altschuler et al., 1983) and hematite in lateritic soils (Retallack, 2010), but these kinds of paleosols are distinct (Retallack, 2010, 17. Ice deformation (diagnostic) 2013b). It is uncertain how far back in time such relationships hold because the Precambrian ocean may have had much more chemically Ice wedges are the best known of a variety of ice deformation struc- reduced iron (Fe2+) than the modern ocean, and thus even more tures diagnostic of frigid soils. Ice wedges have passive or layered fill massive pyritization than now (Lyons et al., 2013). It is also possible and form in maritime frigid climates like that of coastal Alaska, but that lower Ediacaran biological productivity of the ocean left sea floors sand wedges have vertically deformed fill and form in continental frigid with lower organic carbon, and more oxidizing than modern sea floors climates that that of Antarctica (Williams, 1986). Ice deformation (Canfield et al., 2007). features also include frost boils, periglacial involutions, thufur mounds, Tests for this proxy are sequences in which marine and non-marine pingos, moraines, frost heave, freeze–thaw banding, and stone jacking, Fe /Fe ratios alternate, because whole ocean changes in iron HR TOT spanning a variety of scales and rock grain sizes (Washburn, 1980). content on short time scales are less likely than marine–non-marine Such features are not widely appreciated compared with similar facies alternation (Retallack, 2013c, 2016). One such test of Fig 4 has sedimentary structures such as clastic dikes, ball and pillow structure, boundaries calibrated to modern paleoenvironmental values (Ku et al., tepee structures, debris flows and coarse lamination (Gehling, 2000), 2008), and perhaps the Ediacaran sea floor was more oxidized and so that past studies of sedimentary structures deserve reexamination less organic than modern sea floors. However, it is unlikely that from the perspective of ice deformation. Ediacaran sea floors were more highly oxidized than well drained Many periglacial structures have been found in Ediacaran rocks, modern soils, as these data show (Fig. 4). Comprehensive inventories including ice wedges (Cameron and Jeanne, 1976; Cameron, 1979), of marine and terrestrial Fe /Fe back into the Ediacaran are needed HR TOT thufur mounds (Retallack, 2011a, 2012), and moraines (Retallack, to fully evaluate the usefulness of this proxy, as for other geochemical 2013c ). Intrastratal deformation identified as “ball and pillow structure” proxies (Veizer et al., 1999). by Gehling (2000) has several features not found in simple foundering of thixotropic sand: four overlapping episodes of upward-directed 16. Lapilli and crystal tuffs (diagnostic) liquefaction, progressively obscured by ferruginization between failures, and successively onlapped by continuing sedimentation, Volcaniclastic rocks formed on land and in water are distinct because which later failed (Fig. 6a–b). These features are comparable with of the different viscosity and density of water and air (White et al., periglacial involutions from episodic melting of discontinuous 2003), and similar physical differences may allow discrimination permafrost (Retallack, 2012). The surface expression of this structure between deep versus shallow marine tuffs (Busby, 2005). Large volume, would have been frost boils, with puffs of uplifted soil surrounded by hot, ash flow tuffs may overwhelm these differences, but thin tuffs and a network of uninterrupted soil and vegetation (Walker et al., 2011). lavas show differences between land and water (Table 1). Pillow basalts Plan structures like this are also known from the Ediacaran, such as a and hyaloclastic breccias are indicative of marine volcanism, but lapillli large slab excavated from the quarry of Fig. 6c–d with smooth, barren tuffs, crystal tuffs, aerodynamic bombs, and gas escape tubes of patches surrounded by sunken alleys. This slab on display in the

Table 1 Features of land, shallow, and deep water pyroclastic rocks.

Subaerial eruption onto land Subaerial eruption into water Shallow (N1 km) subaqueous eruption Deep (N1 km) subaqueous eruption

Spindle, ribbon bombs Spindle, ribbon bombs Hyaloclastite breccia Silicic spatter and glass Reticulite and pumice Pumice Quench-fragmented pumice Quench-fragmented pumice Strained lithophysae Massive-graded tuff Perlitic fractures Perlitic fractures Ungraded tuff Graded tuff Graded tuff Graded tuff Block and ash flow Tuff turbidites Fine-ash-rich ignimbrite Fine-ash-poor ignimbrite Blocky silicic peperite Blocky silicic peperite Blocky silicic peperite Globular silicic peperite Welded ash flow tuff Welded ash flow tuff Welded ash flow tuff Fluidal silicic lava flows

Note: from Fisher and Schminke (1984); Cas and Wright (1987); White et al. (2003); Busby (2005). 102 G.J. Retallack / Gondwana Research 36 (2016) 94–110

Fig. 5. Matrix-supported lapilli tuffs cannot accumulate under water (Table 1), but here is an example from 8 m in measured section of Mistaken Point FormationbyRetallack (2016),near Catalina, Newfoundland: a, polished slab (The Rooms St Johns, specimen NFM G152); b, petrographic thin section showing scoria fragments and partly recrystallized lapilli (Condon Collection, University of Oregon specimen R3932).

Archangelsk Museum was interpreted not as frost boils displacing fossil level, this highly deviatoric and localized stress regime produces a into alleys, but as evidence of movement of the fossils in discontinuous distinctive pattern of highly oriented, and thus high birefringence, clay, steps along the old-elephant-skin textured alleys (Ivantsov and outlining patches of less oriented clay and silt. There is a progressive Malakhovskaya, 2002; Leonov et al., 2007; Ivantsov, 2013). These alter- increase in pedogenic strain as soil developed, but the end result natives deserve additional study. (Fig. 8a) is strikingly different from laminated sedimentary parent Ice crystal casts have a fossil record back to the Ediacaran (Bandel materials of similar grain size (Fig. 8b). The terminology of sepic plasmic and Shinaq, 2003), usually as individual frost needles or complex frost fabrics outlined by Brewer (1976) is widely applied to describe birefrin- feathers arranged randomly in shale as usual for needles forming in or gence fabrics of both soils and paleosols. on muddy water with limited temperature contrast (Arakawa, 1955; Sepic plasmic fabrics have been widely recognized in Ediacaran Mason et al., 1963). Needle ice on land, however, is more complex sedimentary rocks as evidence of paleosols (Grazhdankin et al., 2012; with associated uplifted clods of soil, small pellets of earth, and needles Retallack, 2013c; Kolesnikov et al., 2015). Some of these paleosols are nucleating from hollow objects such as leaves, or radiating from the low in greenschist facies metamorphism, but sepic fabrics can with- margins of depressions (Fig. 7a). Terrestrial hoar ice forms frost flowers, stand such alteration. The normal second-order birefringence over frost palisades, frost hair, and frost shawls because ice is extruded from first order background of modern and unmetamorphosed sepic plasmic liquid reservoirs by supercooled air (Arakawa, 1955; Mason et al., 1963; fabric becomes third- or fourth-order birefringence over second-order Lawler, 1993; Matthews, 1999, Wagner and Mätzler, 2008). Experi- background high in the greenschist facies (Retallack, 1997; Retallack ments with fungi in wood show that fungi, such as Exidiopsis effusa and Krinsley, 1993). In searching for sepic fabrics, beware that thin (Basidiomycota, Auriculariales), promote frost hair by creating cham- sections must be cut vertical to regional bedding to find differences bers and isolating liquid reservoirs (Hofmann et al., 2015). Uplifted like those of Fig. 8. clods and pellets, and sprays of needles radiating from organic objects and from depressions can be seen in some Ediacaran fossil slabs 19. Evaporitic sand crystals (diagnostic) (Fig. 7b). The alternative explanation of backhoe-style rake-feeding of large sluglike organisms (Gehling et al., 2014) requires a proboscis Desert roses such as barite sand crystals, which are the state rock of over 3 cm long and does not explain the pellets and clods, nor extruded Oklahoma (London, 2008), are characteristic of soils and paleosols, form of needles. These alternatives also deserve additional testing. including sabkha sedimentary facies (Retallack, 1997, 2001). The sand crystal faces have rough surfaces and cloudy interior from many 18. Birefringence fabrics (diagnostic) included grains within the gypsum, barite, halite, or other evaporitic mineral cement. In contrast, water-saturated marine clays host clear Soil structures include familiar mud cracks and slickensides but also crystals of gypsum, whose force of crystallization has excluded a variety of clods (peds), modified surfaces (cutans), differentiated surrounding matrix (Warren, 2006; Ziegenbalg et al., 2010), as in the objects (nodules, desert roses), and microscopic structures (birefringence “chrysanthemum stones” of China (Zhao et al., 1998). fabrics). Wetting and drying, heating and cooling, and bioturbation has a Evaporitic sand crystals are the only line of evidence for Ediacaran progressively obliterating effect on preexisting sedimentary or other paleosols accepted in the extended critique of paleosols by Tarhan structures as soils develop (Retallack, 1997, 2001). At a microscopic et al. (2015a), who note that the best known Ediacaran examples are G.J. Retallack / Gondwana Research 36 (2016) 94–110 103

Fig. 6. Ediacaran frost boils in section from the Ediacara Member of the Rawnsley Quartzite in Brachina Gorge, South Australia (a–b) and the Yorgia–“Epibaion” horizon of the Erga Beds at Zimnie Gory, Russia (c–d). Data and images are from Retallack (2012) and Leonov et al. (2007).

silica pseudomorphs (Fig. 9). Evaporite minerals are soluble in water, 20. Geochemical tau analysis (diagnostic) including groundwater. Silica pseudomorphs of evaporite minerals are also a problem for many Archean evaporites, because determining the Tau analysis is named for a notation of Brimhall et al. (1992) but is original mineralogy may depend on crystallographic measurements, similar to geochemical mass balance analyses of soils and ores dating or traces of barium or calcium to distinguish barite from gypsum or back to the nineteenth century (Brewer, 1976). The idea is to assume anhydrite (Lowe and Worrell, 1999; Sugitani et al., 2003). Fortunately, some components (Zr or Ti) or minerals (zirconium or ilmenite) stable new methods of tomographic imaging and trace element analysis and in the parent material of a soil and calibrate enrichments or depletions mapping by electron microprobe enable identification. (tau, or mass transfer) relative to that baseline. For soils, this will

Fig. 7. Modern ice needles (a) compared with putative Ediacaran “scratch marks” (b). a, Ice needle molds within mud (upper left) and ice needles (lower right) within small dry puddles on a mountain trail at the ski resort of Postavaru, Romania; b, putative Ediacaran radular scratch marks, “Kimberichnus teruzzi,” of the supposed mollusk Kimberella quadrata (lower left), from Zimnie Gory locality, Zimnie Gory Formation, White Sea region, Russia. Image A is courtesy of Stefan Puscasu, and B is specimen 3493/5137 of the Paleontological Institute, Moscow courtesy of Dima Grazhdankin. 104 G.J. Retallack / Gondwana Research 36 (2016) 94–110

Fig. 8. Ediacaran sepic birefringence fabric (a) compared with non-sepic fabric (b) from the Patsy Hill Member of the Wonoka Formation in Brachina Gorge, South Australia: a, mosepic fabric from A horizon of Arru sandy clay loam paleosol at 6.5 m in measured section of Retallack et al. (2014); b, argillasepic plasmic fabric from above Vulda clay loam paleosol at 4 m in measured section of Retallack et al. (2014). Specimens R3480 (A) and R3474 (B) in Condon Collection, University of Oregon. generally be loss of alkalies (Na, K) and alkaline earths (Ca, Mg) and phosphorus in the paleosols, but not sediment (Fig. 11), which can be enrichment of aluminum (Al) as feldspar and rock fragments are taken as evidence that the soils hosted organisms producing ligands weathered to clay (Fig. 10). These weathering reactions and diffuse for that bioessential element (Neaman et al., 2005). cracking also bring about volume changes, and the same stable compo- nents and minerals can be used to calibrate volume changes (epsilon or 21. Carbon and oxygen isotopes (diagnostic) strain). Loss of volume (negative strain) is typical for soils, but gain in volume (positive strain) is typical for sediments (Retallack, 2015a-b). A characteristic isotopic signature of paleosol carbonate is highly In sedimentary event beds, ilmenite and zirconium and other heavy significant correlation of δ13C and δ18O(Fig. 12b: Huang et al., 2005; minerals are concentrated near the base of the unit, but in soils, these Ufnar et al., 2008). These correlations have been considered due to ki- minerals become more concentrated near the surface as other minerals netic evaporative selection for light isotopologues of CO2 in soils are lost to weathering. These techniques have been widely used to (Ufnar et al., 2008), but an alternative explanation is that they are due quantify weathering in paleosols ranging in age from Proterozoic to re- to selection of light isotopologues of CO2 by rubisco, the essential photo- cent (Brimhall et al., 1992; Mitchell and Sheldon, 2009; Sheldon and synthetic enzyme of many plants and microbes (Retallack, 2015b). This Tabor, 2009; Retallack, 2015a, 2015b). new view is based on observations of the same tight covariance seen in

Only a few Ediacaran paleosols have been examined by tau analysis, respired soil CO2 (Ehleringer and Cook, 1998; Ehleringer et al., 2000), and these show expected hydrolytic enrichment of alumina at the ex- and in plant cellulose (Barbour and Farquhar, 2000; Barbour et al., pense of cationic bases as feldspar and rock fragments are depleted 2002). Tight correlations are only observed in paleosols of small areas (Retallack, 2011b, 2012, 2013a, 2013c, Retallack et al., 2014). Such and particular geological formations, because the isotopic composition changes are distinct from Ediacaran event deposits (Retallack, 2016; of soil CO2 varies with vegetation and climate (Barbour et al., 2002; Retallack et al., 2014). These differences are apparent in depth functions, Ufnar et al., 2008; X. Liu et al., 2014). Other correlations, although less but more obvious in a plot of element mass transfer versus strain statistically significant (Fig. 12), are found in soil carbonate crusts (Fig. 11), where Ediacaran and Miocene paleosols are in the lower left (Knauth et al., 2003), and marine limestone altered by deep circulation quadrant (negative strain and mass transfer), but an Ediacaran event of meteoric water (Lohmann, 1988; Melim et al., 2004). In contrast, bed is in the upper right quadrant (positive strain and mass transfer). unaltered marine limestone shows no hint of correlation and are clus- Another intriguing result of this analysis is clear depletion of tered near zero for both δ13Candδ18O(Veizer et al., 1999). Yet another

Fig. 9. Evaporite pseudomorphs in outcrop (a–b) and petrographhic thin section (c) from the Ediacara Member of the Rawnsley Quartziite in South Australia: a, “peachstone nodules” weathered from By horizon of Inga paleosol near Arkarua; b–c, rosettes of gypsum sand crystals in Inga paleosol in Brachina Gorge after Retallack (2012). G.J. Retallack / Gondwana Research 36 (2016) 94–110 105

Fig. 10. Petrographic and geochemical analysis paleosol and sedimentary event bed compared: Yaldati paleosol from the Ediacara Member of the Rawnsley Quartzite in Brachina Gorge, South Australia (above, after Retallack, 2012), and a tempestite in the Mistaken Point Formation at Mistaken Point Formation, Newfoundland (below, after Retallack, 2014a). pattern of near constant δ18O but highly varied δ13C(Fig. 12f) is created expectations defined in advance, before results from petrographic or by microbial methanogenesis in carbonate of marine methane cold geochemical studies. Early results from some of these laboratory tests seeps (Aiello et al., 2001; Peckmann et al., 2002), and siderite of wetland have been discussed above, but their wider application is needed. palaeosols (Ludvigson et al., 1998, 2013). Conventional reconstructions of Ediacaran ecosystems (Fig. 1a) Soil-diagnostic correlation of δ13Candδ18O has been found in depict a variety of motile organisms on ripple-marked sea floors. Little numerous Early Paleozoic and Ediacaran paleosols (Fig. 12), in addition of this vision has survived subsequent scrutiny, as even proponents of to Ediacaran carbonate in the Fauquier Formation of Virginia (Hebert a marine habitat emphasize microbial crusts, sessile habit, stiff fronds, et al., 2010), and the Shibantan Member of the Dengying Formation of and non-swimming discoids (Gehling and Droser, 2013; Antcliffe China (Duda et al., 2014). Ediacaran δ18O values may be 10‰ lighter et al., 2015; Budd and Jensen, 2016). Furthermore, some Ediacaran than modern, due to changes in ocean crust hydrothermal regime fossils are of undisputed marine origin, such as shells of Cloudina in (Kasting et al., 2006). This effect is shown in Fig. 12e by analyses of thrombolitic limestones (Grotzinger et al., 2000), and the chitinous the Early Cambrian, Ajax Limestone of South Australia known to be tubes of Corumbella and Sinotubulus in laminated black shales (Liu marine because of its trilobites and abundant archaeocyathids (Surge et al., 2008; Warren et al., 2011, 2012). Nor are marine versus non- et al., 1997). The Juniata and Palmerton paleosols of this compilation marine the only choices: the Aspidella–Heimalora association of have been metamorphosed to greenschist facies (Retallack, 2015a-b) Newfoundland has been proposed as an intertidal community without destroying the relationships. The main way to obscure these (Retallack, 2016). For the enigmatic quilted forms usually assigned to relationships is to pool pedogenic carbonate from different formations vendobionts, opinion remains divided on which are marine (Xiao (Retallack, 2012). et al., 2013;A.G.Liu et al., 2015) or non-marine (Kolesnikov et al., 2012; Retallack, 2013a, 2016). 22. Conclusions Ediacaran paleosols were not identical with Phanerozoic paleosols because they lack large root traces and thorough bioturbation, and Two general kinds of tests have emerged from this survey: (1) field this may be why they escaped detection until recently. However, structures and (2) laboratory tests. Field structures are more controver- interpretation of Ediacaran paleoenvironments has emphasized that sial because they have not escaped notice and already have been they were so different from modern environments that uniformitarian interpreted from a perspective of marine geology (Callow et al., 2013; principles are inappropriate (Canfield et al., 2007; Ohnemüller et al., Tarhan et al., 2015a). Laboratory analyses constitute a blind test, with 2014; Callow et al., 2013). Thus, we have Ediacaran seafloors more

Fig. 11. Tau analysis of paleosols (Lal and Yaldati) and event bed (tempestite) compared: Yaldati silty clay loam is from the Ediacaran Ediacara Member of the Rawnsley Quartzite in Brachina Gorge, South Australia (after Retallack, 2012); Lal clay is from the Miocene Dhok Pathan Formation near Dhurnal, Pakistan (after Retallack, 1991); tempestite is from the Ediacaran Mistaken Point Formation at Mistaken Point, Newfoundland (after Retallack, 2016). 106 G.J. Retallack / Gondwana Research 36 (2016) 94–110

Fig. 12. Covariation of carbon and oxygen isotopic composition of carbonate in paleosols compared with other settings: a, Ordovician paleosols of Pennsylvania (Retallack, 2015b)and Australia (Retallack, 2009), compared with Ediacaran and Cambrian paleosols of Australia (Retallack et al., 2014) and Silurian paleosols of Pennsylvania (Retallack, 2015a); b, soil nodules (above Woodhouse lava flow, near Flagstaff, Arizona, from Knauth et al., 2003; and in Yuanmou Basin, Yunnan, China, from Huang et al., 2005); c, soil crusts on basalt (Sentinel Volcanic Field, Arizona, from Knauth et al., 2003); d, Quaternary marine limestone altered diagenetically by meteoric water (Key Largo, Florida, from Lohmann, 1988, and Clino Island, Bahamas, from Melim et al., 2004); e, Holocene (open circles) and Ordovician (open squares) unweathered marine limestones (Veizer et al., 1999) and Early Cambrian (closed circles), Ajax Lime- stone, South Australia (Surge et al., 1997); f, marine methane cold seep carbonate (Miocene, Santa Cruz Formation, Santa Cruz, California, from Aiello et al., 2001). Slope of linear regression (m) and coefficients of determination (R2) show than carbon and oxygen isotopic composition is significantly correlated in soils and paleosols.

highly oxidized than modern soils and Ediacaran oceans with less the extent to which they have been applied to Ediacaran sulfate that a modern Swiss lake (Canfield et al., 2007), and excursions paleoenvironments. These are necessary future avenues of re- of more than 1 pH unit in the ocean over only 8000 years (Ohnemüller search because it is no longer reasonable to continue assuming et al., 2012). These unlikely anomalies are better addressed by the that all Ediacaran sediments were marine. paleosol hypothesis, in which sulfates and pH can be highly variable over short distances (Retallack, 2011a, 2016). Some differences between Ediacaran and modern environments will survive future Acknowledgments scrutiny, but comparisons with modern experimental and field studies should remain as a reality check. Useful discussion is acknowledged from Alex Liu, Emily Mitch- Paleosols described here as evidence against the long- ell, Lidya Tarhan, Nathan Sheldon, Marc La Flamme, Paul Strother, cherished view that vendobionts were marine animals have been Jay Kaufman, Guy Narbonne, Nora Noffke, and Shuhai Xiao. Chris- ignored (C. Liu et al., 2014;A.G.Liu et al., 2015), or disputed tian Fischer and Dima Grazhdankin offered permission to reprint (Callow et al., 2013; Tarhan et al., 2015a), but sufficient evidence images. South Australian fieldwork was under permit from Flin- is now available to show that the Ediacaran, like other geological ders Ranges National Park thanks to Kate Lloyd, Pauline Coulthard, periods, had an array of marine and non-marine Arthur Coulthard, Ken Anderson, and Darren Crawford, and paleoenvironments, The purpose of this review is less to arbitrate assisted by Christine Metzger and Jim Gehling. Fieldwork under disputes, than to show a path forward with studies that can ad- permit from Parks and Natural Areas Division of Newfoundland dress the problem of recognizing Ediacaran paleosols, as well as and Labrador (director Siân French) was conducted with supervi- lacustrine, desert, periglacial, and intertidal environments. The sion and assistance of. Richard Thomas. Detailed review by Ilze 20 tests outlined here vary considerably in effectiveness and in Vircava greatly improved the manuscript. G.J. Retallack / Gondwana Research 36 (2016) 94–110 107

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