1. Introduction
The tripartite subdivision of the Neoproterozoic Era (ICS Timescale 2015, Cohen et al. Reference Cohen, Finney, Gibbard and Fan2015a) reflects the time intervals characterized by global environmental changes related to plate tectonics, climate fluctuations, ocean geochemistry and redox state (Johnston et al. Reference Johnston, Poulton, Dehler, Porter, Husson, Canfield and Knoll2010; Van Kranendonk et al. Reference Van Kranendonk, Altermann, Beard, Hoffmsan, Johnson, Kasting, Melezhik, Nuyman, Papineau, Pirajno, Gradstein, Ogg, Schmitz and Ogg2012; Lyons, Reinhard & Planavsky, Reference Lyons, Reinhard and Planavsky2014; Planavsky et al. Reference Planavsky, Tarhan, Bellefroid, Evans, Reinhard, Love, Lyons, Polly, Head and Fox2015). The radically changing natural environments shaped the ecosystems and stimulated evolutionary modifications of the highest magnitude in the marine realm. The divergence of eukaryotic protists and finally multicellular organisms including animals (Narbonne, Reference Narbonne2005; Knoll et al. Reference Knoll, Javaux, Hewitt and Cohen2006; Porter, Reference Porter, Xiao and Kaufman2006; Butterfield, Reference Butterfield2011; Knoll, Reference Knoll2014) might have been triggered, along with genetic mutations, by the development of progressively oxygenated marine basins, which existed in warm to temperate climatic zones (Li, Evens & Halverson, Reference Li, Evens and Halverson2013; Spence, Le Heron & Fairchild, Reference Spence, Le Heron and Fairchild2016; Turner & Bekker, Reference Turner and Bekker2016). The oxidation of marine basins could have also been influenced by the evolution of increasingly complex eukaryotes (Lenton et al. Reference Lenton, Boyle, Poulton, Shields-Zhou and Butterfield2014), but also eukaryote evolution could have been more independent of oxygen concentration (Butterfield, Reference Butterfield2009; Milles et al. Reference Milles, Watson, Goldblatt, Boyle and Lenton2011; Sperling et al. Reference Sperling, Frieder, Raman, Girguis, Levin and Knoll2013). The oxygen level rose owing to the steady-state photosynthetic production of free oxygen (Falkowski & Raven, Reference Falkowski and Raven2007; Jackson, Reference Jackson2015; Schirrmeister, Gugger & Donoghue, Reference Schirrmeister, Gugger and Donoghue2015), and cyanobacteria and red and green algae thrived at the time (Schopf, Reference Schopf, Schopf and Klein1992; Butterfield, Reference Butterfield2000; Sergeev, Reference Sergeev2006; Moczydłowska, Reference Moczydłowska2008a, Reference Moczydłowska2016; Love et al. Reference Love, Grosjean, Stalvies, Fike, Grotzinger, Bradley, Kelly, Bhatia, Meredith, Snape, Bowring, Condon and Summons2009; Moczydłowska et al. Reference Moczydłowska, Landing, Zang and Palacios2011; Tang et al. Reference Tang, Pang, Xiao, Yuan, Ou and Wan2013; Xiao et al. Reference Xiao, Shen, Tang, Kaufman, Yuan, Li and Qian2014a,b). Oxygen started to accumulate in the atmosphere–hydrosphere system after the Great Oxidation Event (c. 2.3 Ga; Holland, Reference Holland2002; Bekker et al. Reference Bekker, Holland, Wang, Rumble III, Stein, Hannah, Coetzee and Beukes2004) or possibly earlier (Lyons, Reinhard & Planavsky, Reference Lyons, Reinhard and Planavsky2014; Jackson, Reference Jackson2015; Lalonde & Konhauser, Reference Lalonde and Konhauser2015). The ocean redox state fluctuated but oxygen content increased in Neoproterozoic time (Halverson et al. Reference Halverson, Wade, Hurtgen and Barovich2010; Planavsky et al. Reference Planavsky, Tarhan, Bellefroid, Evans, Reinhard, Love, Lyons, Polly, Head and Fox2015; Sahoo et al. Reference Sahoo, Planavsky, Jiang, Kendall, Owens, Wang, Shi, Anbar and Lyons2016). Although low atmospheric oxygen concentrations might have prevailed during mid Proterozoic time (Lyons, Reinhard & Planavsky, Reference Lyons, Reinhard and Planavsky2014; Planavsky et al. Reference Planavsky, Reinhard, Wang, Thomson, Mcgoldrick, Rainbird, Johnson, Fischer and Lyons2014; Li et al. Reference Li, Planavsky, Love, Reinhard, Hardisty, Feng, Bates, Huang, Zhang, Chu and Lyons2015), oxygen concentrations might have been higher (Cox et al. Reference Cox, Jarrett, Edwards, Crockford, Halverson, Collins, Poirier and Li2016; Mukherjee & Large, Reference Mukherjee and Large2016; Tang et al. Reference Tang, Shi, Wang and Jiang2016; Zhang et al. Reference Zhang, Wang, Wang, Bjerrum, Hammarlund, Costa, Connelly, Zhang, Su. and Canfield2016), including a ventilated or relatively well-oxygenated surface ocean with oxygen oases or oxygen whiffs (Anbar et al. Reference Anbar, Duan, Lyons, Arnold, Kendall, Creaser, Kaufman, Gordon, Scott, Garvin and Buick2007; Kaufman, Corsetti & Varni, Reference Kaufman, Corsetti and Varni2007; Poulton & Canfield, Reference Poulton and Canfield2011; Partin et al. Reference Partin, Bekker, Planavsky, Scott, Gill, Li, Podkovyrov, Maslov, Konhauser, Lalonde, Love, Poulton and Lyons2013) possibly allowing the deep ocean to remain anoxic and sulfidic (Canfield, Reference Canfield1998; Anbar & Knoll, Reference Anbar and Knoll2002).
During the Tonian Period (1000 – c. 720 Ma), the supercontinent Rodinia was fragmented and rifted along newly formed continental margins creating seaways with active circulation, mixing water masses and increased input of mineral nutrients from the weathering of continental crust (Halverson et al. Reference Halverson, Wade, Hurtgen and Barovich2010; Li, Evens & Halverson, Reference Li, Evens and Halverson2013; Spence, Le Heron & Fairchild, Reference Spence, Le Heron and Fairchild2016). The subsequent collapse of many ecosystems during the Cryogenian Period (c. 720–635 Ma) due to severe ice ages (Hoffman & Schrag, Reference Hoffman and Schrag2002; Eyles & Januszczak, Reference Eyles and Januszczak2007; Allen & Etienne, Reference Allen and Etienne2008; Arnaud, Halverson & Shields-Zhou, Reference Arnaud, Halverson and Shields-Zhou2011) caused the extinction of the majority of biotas (Knoll, Reference Knoll1994; Vidal, Reference Vidal and Bengtson1994; Vidal & Moczydłowska-Vidal, Reference Vidal and Moczydłowska-Vidal1997). However, this extinction process or reduction in diversity might have been initiated before the onset of the Sturtian glaciation, thus in late Tonian time, due to eutrophication (Nagy et al. Reference Nagy, Porter, Dehler and Shen2009) or other as yet unclear factors (Riedman et al. Reference Riedman, Porter, Halverson, Hurtgen and Junium2014). Despite the catastrophic Cryogenian environmental conditions, some lineages and discrete cyanobacterial and algal taxa survived the ice ages and even in the meantime originated (Papillomembrana), as well as ciliates and foraminifera, during the interglacial cycle(s), as evident from the fossil record in the pre-, inter- and post-Cryogenian successions (Corsetti, Awramik & Pierce, Reference Corsetti, Awramik and Pierce2003; Moczydłowska Reference Moczydłowska2008a,Reference Moczydłowskab; Nagy et al. Reference Nagy, Porter, Dehler and Shen2009; Bosak et al. Reference Bosak, Macdonald, Lahr and Matys2011, Reference Bosak, Lahr, Pruss, Macdonald, Gooday, Dalton and Matys2012; Riedman et al. Reference Riedman, Porter, Halverson, Hurtgen and Junium2014; Cohen et al. Reference Cohen, Macdonald, Pruss, Matys and Bosak2015b; Corsetti, Reference Corsetti2015; Ye et al. Reference Ye, Tong, Xiao, Zhu, An, Tian and Hu2015). The recovery of ecosystems following de-glaciation and sea-level rise in the Ediacaran Period (635–541 Ma) paved the way for the exponential radiation of phytoplankton, the rise of multicellular organisms of the Ediacara-type and the bilaterian animals of modern phyla (Grey, Reference Grey2005, Reference Grey, Fedonkin, Gehling, Grey, Narbonne and Vickers-Rich2007; Narbonne, Reference Narbonne2005; Moczydłowska & Nagovitsin, Reference Moczydłowska and Nagovitsin2012; Narbonne, Xiao & Shields, Reference Narbonne, Xiao, Shields, Gradstein, Ogg, Schmitz and Ogg2012; Liu et al. Reference Liu, Xiao, Yin, Chen, Zhou and Li2014; Xiao et al. Reference Xiao, Zhou, Liu, Wang and Yuan2014b).
The environmental and evolutionary history of the Tonian Period is renowned for the development of marine habitats that sustained robust planktonic and benthic communities, and induced further life expansion as is shown by the high diversification of auto- and heterotrophic protists (Vidal & Moczydłowska-Vidal, Reference Vidal and Moczydłowska-Vidal1997; Knoll et al. Reference Knoll, Javaux, Hewitt and Cohen2006; Porter, Reference Porter, Xiao and Kaufman2006; Sergeev, Reference Sergeev2006; Cohen & Knoll, Reference Cohen and Knoll2012; Cohen & Macdonald, Reference Cohen and Macdonald2015). However, several microfossil taxa (Pterospermopsimorpha, Valeria, Tasmanites, Schizofusa; certain Leiosphaeridia) have persisted since the Mesoproterozoic Era (Yan & Liu, Reference Yan and Liu1993; Lamb et al. Reference Lamb, Awramik, Chapman and Zhu2009; Moczydłowska et al. Reference Moczydłowska, Landing, Zang and Palacios2011; Agić, Moczydłowska & Yin, Reference Agić, Moczydłowska and Yin2015). The Tonian diversification is well recorded in the Visingsö Group of the Lake Vättern Basin (Fig. 1) and in numerous successions worldwide, such as the Vadsø, Tanafjord and Hedmark groups in Norway, and successions in Russia (the southern Urals and Siberia), the USA (the Chuar Group in Arizona, Uinta Mount Group in Utah and the Pahrump Group in California) and Canada (the Fifteenmile and Harper groups in Yukon) (Vidal, Reference Vidal1976; Vidal & Ford, Reference Vidal and Ford1985; Jankauskas, Mikhailova & German, Reference Jankauskas, Mikhailova and German1989; Horodyski, Reference Horodyski1993; Vidal & Moczydłowska, Reference Vidal and Moczydłowska1995; Porter, Reference Porter, Xiao and Kaufman2006; Nagy et al. Reference Nagy, Porter, Dehler and Shen2009; Cohen & Knoll, Reference Cohen and Knoll2012; Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014; Porter & Riedman, Reference Porter and Riedman2016). The Visingsö Group contains a diverse assemblage of cyanobacteria, stromatolites, organic-walled microfossils (OWM) and vase-shaped microfossils (VSM) (Vidal, Reference Vidal1972, Reference Vidal1976; Martí Mus & Moczydłowska, Reference Martí Mus and Moczydłowska2000; Agić, Moczydłowska & Willman, Reference Agić, Moczydłowska and Willman2015; Loron, Reference Loron2016). In addition, newly recovered mineralized scale microfossils (MSM; Fig. 2a, b, d; ongoing study) resemble the type known from the Tonian Fifteenmile Group in Yukon, Canada (former Tindir Group; Allison & Hilgert, Reference Allison and Hilgert1986; Macdonald et al. Reference Macdonald, Smith, Strauss, Cox, Halverson, Roots, MacFarlane, Weston and Relf2011; Cohen & Knoll, Reference Cohen and Knoll2012).
Figure 1. (a) Map of Baltoscandia showing tectonostratigraphic domains of the Caledonides and the Fennoscandian Shield basement with the distribution of the remnant Proterozoic sediments. (b) Extension of the Visingsö Group along Lake Vättern. (c) Lithologic succession with position of studied samples and distribution of microfossils. Modified from Vidal (Reference Vidal1982) and Lundmark & Lamminen (Reference Lundmark and Lamminen2016). Abbreviations: SF – Sveconorwegian deformation front; MZ – Mylonite Zone; OWM – organic-walled microfossils; VSM – vase-shaped microfossils; MSM – mineralized scale microfossils; S – stromatolites; fm. – formation.
Figure 2. New record of vase-shaped microfossils (b, c) and newly discovered mineralized scale microfossils (a, b, d) preserved in phosphatic nodules in the upper formation of the Visingsö Group. (a) Paleomegasquama arctoa, Slide 6, K-35-2. (b) Cycliocyrillium torquata (specimen on left side), and Bicorniculum brochum (specimen on right side), Slide 6, V40-3. (c) Melanocyrillium hexadiadema (upper specimen), Slide 7, M10-4. (d) Archeoxybaphon polykeramoides, Slide 7, J24-2. Scale bar equal to 15 μm in (a), 30 μm in (b), 50 μm in (c), 20 μm in (d). Collection PMU-V72G14, Slides 6–7. England Finder Coordinates provided for each specimen.
Until now, the age of the Visingsö Group was estimated palaeontologically to between 800 and 700 Ma (Vidal & Moczydłowska, Reference Vidal and Moczydłowska1995). Our dating of detrital zircons provides a maximum depositional age of c. 886 Ma and, together with the biochronology of common microfossil taxa established in the Chuar and Mount Harper groups at a minimum age of c. 740 Ma (Dehler et al. Reference Dehler, Elrick, Bloch, Crossey, Karstrom and Des Marais2005; Nagy et al. Reference Nagy, Porter, Dehler and Shen2009; Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014), is consistent with a Tonian age for the Visingsö Group.
The implications of the Visingsö microfossil record set within a geochronologic framework are significant for reconstructing Neoproterozoic evolutionary events and ecological processes. These are the origin and divergence of phytoplanktonic (many OWM) and heterotrophic (VSM) protists, MSM of uncertain but likely algal affinity (Cohen et al. Reference Cohen, Schopf, Butterfield, Kudryavtsev and Macdonald2011) and benthic bacteria forming microbial mats and stromatolites. Their passive dispersal (cysts of algal phytoplankton and heterotrophs) or active migration (motile vegetative cells of phytoplankton and motile heterotrophic protists) in the marine realm was facilitated by the global hydrological cycle and patterns of current circulation changing at given time intervals throughout the Neoproterozoic period.
2. Geological setting
The Mesoproterozoic Sveconorwegian belt exposed in southwestern Scandinavia (Fig. 1a) represents deeply eroded continental crust reworked during Sveconorwegian (1.14–0.90 Ga) orogenesis (Möller et al. Reference Möller, Andersson, Dyck and Lundin2015 and references therein). The latter is associated with the construction of Rodinia (Pease et al. Reference Pease, Daly, Elming, Kumpulainen, Moczydłowska, Puchkov, Roberts, Saintot and Stephenson2008; Bingen, Belousova & Griffin, Reference Bingen, Belousova and Griffin2011). Post-orogenic relaxation and gravitational collapse led to uplift and cooling at c. 900 Ma (Viola et al. Reference Viola, Henderson, Bingen and Hendriks2011). Scandinavia, as part of Baltica, gradually rifted from Rodinia between c. 850 and 630 Ma with concomitant marine transgression (Li, Evens & Halverson, Reference Li, Evens and Halverson2013). This paper focuses on the Tonian (1000 – c. 720 Ma) depositional history of Fennoscandia.
During Sveconorwegian orogenesis, southeastward imbrication and displacement of the crust occurred. The eastern limit of the orogen is defined by the Sveconorwegian deformation front (SF), a steeply dipping zone of high strain that marks the limit of Sveconorwegian ductile deformation and metamorphism (Möller et al. Reference Möller, Andersson, Dyck and Lundin2015; Fig. 1a). To the east of the SF, igneous and metamorphic rocks of the 2.0–1.75 Ga Svecokarelian orogen and 1.86–1.66 Ga plutonic and volcanic rocks of the Transcandinavian Igneous Belt (TIB) are unaffected by Sveconorwegian ductile deformation or metamorphism (Stephens et al. Reference Stephens, Ripa, Lundström, Persson, Bergman, Ahl, Wahlgren, Persson and Wickström2009). The orogen-parallel 0.97–0.95 Ga Blekinge-Dalarna dolerite dyke swarm intrudes along and east of the SF (Söderlund et al. Reference Söderlund, Isachsen, Bylund, Heaman, Patchett, Vervoort and Andersson2005) and documents the last known magmatic activity along the SF. The SF is a long-lived crustal-scale feature. It was active as early as c. 1200 Ma during the early phase of Sveconorwegian orogenesis, and was reactivated later (c. 950 Ma) during uplift of the Eastern Segment (e.g. Viola et al. Reference Viola, Henderson, Bingen and Hendriks2011). This was followed by the formation of the ‘proto-Vättern graben’ with deposition of the Visingsö Group in Neoproterozoic time (Vidal & Moczydłowska, Reference Vidal and Moczydłowska1995).
Late to post-Sveconorwegian sediments are not preserved within the Sveconorwegian belt. Post-orogenic sediments interpreted to reflect rift- and passive-margin settings associated with the break-up of Rodinia were deposited in the Sveconorwegian hinterland (Pease et al. Reference Pease, Daly, Elming, Kumpulainen, Moczydłowska, Puchkov, Roberts, Saintot and Stephenson2008). These are now preserved within the nappes (Hedmark Group) and parautochthonous successions (Vadsø and Tanafjord groups) of the Caledonian orogen (Bingen, Belousova & Griffin, Reference Bingen, Belousova and Griffin2011). Along the SF, erosional remnants of these sediments, e.g. the Visingsö Group, the Amesåkra Group, the Dala Sandstone and successions in the Sparagmite Basin, unconformably overlie TIB granitoid basement (Bingen, Belousova & Griffin, Reference Bingen, Belousova and Griffin2011; Lundmark & Lamminen, Reference Lundmark and Lamminen2016; Fig. 1a, b).
3. Visingsö Group succession, previous work and sampling
The Visingsö Group is exposed along Lake Vättern and on Visingsö Island. The Group consists of terrigenous clastic rocks with minor carbonates deposited on TIB-related rocks of various ages (Vidal, Reference Vidal1974, Reference Vidal1976, Reference Vidal1982, Reference Vidal, Persson, Brunn and Vidal1985; Larson & Nørgaard-Pedersen, Reference Larsen and Nørgaard-Pedersen1988; Ulmius, Andersson & Möller, Reference Ulmius, Andersson and Möller2015; Fig. 1b). The succession is also known from 15 boreholes penetrating various portions of the strata. The Visingsö strata are unmetamorphosed and undeformed except for local normal faults with low dips of 5–25° (Vidal, Reference Vidal1976; Morad & Al-Aasm, Reference Morad and Al-Aasm1994). The Visingsö Group is c. 1426 m thick and comprises lower, middle and upper formations (informal nomenclature; Fig. 1c). The lower formation consists of quartzofeldspathic sandstone interbedded with shale and arkosic sandstone (over 400 m in thickness), and represents a progradational fluvial-deltaic environment. The boundary between the lower and middle formations is gradational from quartzitic sandstone coarsening into feldspathic sandstone, respectively. The middle formation comprises feldspathic sandstone and conglomerate succeeded by alternating sandstone, mudstone and shale (at least 446 m), deposited in a pro-delta setting characterized by occasional delta lobes prograding into shallow marine environments. The boundary with the upper formation is sharp at the top of quartz sandstone (middle formation) and the base of laminated mudstone (upper formation). The upper formation consists of alternating shale, mudstone and fine-grained sandstone, and dolomitic limestone with stromatolites (580 m thick). Deposition occurred in a shallow marine, tidally influenced mud flat environment with distinct intervals of subtidal and intertidal sedimentation.
Geological relationships indicate that the Visingsö Group sediments are younger than c. 946 Ma, the age of dolerite dykes that cut the granitoid basement upon which the Group is deposited (Söderlund et al. Reference Söderlund, Isachsen, Bylund, Heaman, Patchett, Vervoort and Andersson2005). Earlier isotopic studies of the Visingsö Group include K–Ar detrital mica ages (1060–985 Ma; Magnusson, Reference Magnusson1960) and Rb–Sr ages on clay and whole-rock fractions of shale from the upper formation (703–663 Ma; Bonhomme & Welin, Reference Bonhomme and Welin1983), ages now interpreted to reflect the time of crystallization and diagenesis, respectively.
We examine three samples from the Visingsö Group at Lake Vättern (Fig. 1b). The lower formation sample (V15-Lem) was collected from the NE wall at the entrance to the Lemunda Quarry, and consists of white-yellowish, medium-grained, weakly consolidated quartz arenite with faint thin bedding. Two samples of the middle formation were collected from the Visingsö 1 borehole on Visingsö Island (Fig. 1b) at depths of 137.50–140.10 m (V15-10) and 120.40–120.95 m (V15-9). They are medium-grained quartzofeldspathic sandstone.
4. Age and provenance of the Visingsö Group from detrital zircons
4.a. Analytical methods
Zircons were separated from 1–2 kg of sample using conventional water table and heavy liquid mineral separation techniques. Approximately 200 zircon grains with various morphologies, sizes and colours were hand-picked onto double-sided tape, cast into epoxy resin, sectioned and polished. A deliberate effort was made to select all zircon colours, sizes, morphologies, etc., during picking. Scanning electron microscope (SEM) and cathodoluminescence (CL) images of the zircons were used to identify textures and select analytical locations; these were obtained using a FEI SEM at the Department of Geological Sciences, Stockholm University. Analytical methods follow those described in Zhang, Roberts & Pease (Reference Zhang, Roberts and Pease2015). Further details of the analytical method are provided in the online Supplementary Material available at http://journals.cambridge.org/geo.
4.b. Analytical results
Our analytical results are summarized below and in Figure 4. The data and inverse concordia diagrams, as well as a more detailed discussion of sediment provenance, are also presented in the online Supplementary Material available at http://journals.cambridge.org/geo (Table S2, and Figures S1, S2 and S3). Errors are reported at the 2-sigma level. For zircon ages younger than 1.2 Ga the 206Pb–238U ages were used in the final analysis and for ages older than 1.2 Ga the 207Pb–206Pb ages were used in the final analysis. Analyses with high common Pb as well as those with > 10% discordance or > 10% uncertainties were excluded from the final data synthesis. Concordia diagrams and probability density distribution plots were made using ISOPLOT/Ex 4.15 (Ludwig, Reference Ludwig2012).
V15-Lemunda. The lower formation of the Visingsö Group. Zircon from this quartz arenite reflects a diverse detrital assemblage of grains, i.e. aspect ratios of 1:1 to 1:5, a variety of colours, with and without inclusions, and a range of CL textures from igneous oscillatory zoning to uniformly bleached zones indicating secondary fluid migration. The grains are generally low in U (500 ppm or lower in 93% of the crystals) with diverse Th/U ratios (0.14–2.1). Seventy per cent of the analyses meet the quality assessment criteria (117/154), yielding a continuous spread of ages from 1850–887 Ma (online Supplementary Material available at http://journals.cambridge.org/geo, Fig. S1). Neoproterozoic peaks at c. 1026, 945 and 900 Ma dominate the age spectra, while lesser peaks occur at c. 1600, 1446, 1268 and 1100 Ma (peak ages typically ± 25 Ma). A weighted mean of the youngest four analyses = 886±9 Ma (MSWD = 0.81, Prob = 0.49) and provides a conservative maximum age for the sediment.
V15-10. The middle formation of the Visingsö Group. Zircon from this quartz-arkosic sandstone, similar to V15-Lem, reflects a diverse detrital assemblage of grains with the addition of CL-dark rim overgrowths on most grains. The grains have moderate U concentrations with 77% between 100 and 700 ppm, 13% < 100 ppm and 10% > 1000 ppm. Modern lead-loss is apparent in the 238U/206Pb versus 207Pb/206Pb concordia diagram, in accord with high-U grains and metamictization. Th/U ratios are diverse (0.08–2.4). Sixty per cent of the analyses meet the quality assessment criteria (98/165), yielding a continuous spread of ages from 1913–990 Ma (online Supplementary Material available at http://journals.cambridge.org/geo, Fig. S2). Palaeoproterozoic to Mesoproterozoic peaks dominate the age spectra, namely c. 1751, 1625 and 1450 Ma, while a spread of ages occurs at c. 1300–990 Ma with a minor peak at c. 992 Ma. A weighted mean of the youngest three analyses = 997±13 Ma (MSWD = 0.38, Prob = 0.68) and provides the maximum age of the sediment.
V15-9. The middle formation of the Visingsö Group. Zircon from this quartz-arkosic sandstone also reflects a diverse detrital assemblage of grains with the addition of CL-dark rim overgrowths on most grains. The grains range from 32–1152 ppm U, with 71% between 100 and 500 ppm, and 5% > 1000 ppm. Modern lead-loss is also apparent in the 238U/206Pb versus 207Pb/206Pb concordia diagram. Th/U ratios are diverse (0.04–3.6). Seventy-five per cent of the analyses meet the quality assessment criteria (132/168) and yield a continuous spread of ages from 1878–1043 Ma (online Supplementary Material available at http://journals.cambridge.org/geo, Fig. S3). The dominant peaks in the detrital spectra are Mesoproterozoic in age, namely c. 1640, 1580 and 1439 Ma, while older (c. 1780 Ma) and younger ages (c. 1260–1045 Ma) are minor contributors to the age spectra. A weighted mean of the two youngest analyses = 1050±15 Ma (MSWD = 0.57, Prob = 0.45) and provides a maximum age for the sediment.
Our new LA-ICP-MS U–Pb detrital zircon data from the three Visingsö samples provide maximum depositional ages for the middle formation of ≤ 1050±15 Ma (2σ; V15-9) and ≤ 997±13 Ma (2σ; V15-10), and for the lower formation of ≤ 886±9 Ma (2σ; V15-Lem) (Fig. 4). The youngest age, obtained from the lower formation, represents the best estimate of the maximum depositional age for the Visingsö Group at c. 886 Ma.
4.c. Provenance
The provenance of the Visingsö zircons is consistent with derivation from the igneous, metamorphic and recycled sedimentary rocks known to be exposed in the region at the time of deposition, i.e. Svecokarelian rocks (c. 2.0–1.75 Ga), TIB (c. 1.86–1.66 Ga plutonic and volcanic rocks), metamorphic and igneous rocks associated with the Gothian (1.66–1.52 Ga), Hallandian (c. 1.47–1.38 Ga) and Sveconorwegian (c. 1.14–0.90 Ga) orogens (Möller et al. Reference Möller, Andersson, Dyck and Lundin2015; Lundmark & Lamminen, Reference Lundmark and Lamminen2016), as well as swarms of 1.6–0.95 Ga dolerite dykes (Söderlund et al. Reference Söderlund, Isachsen, Bylund, Heaman, Patchett, Vervoort and Andersson2005) that intrude the Fennoscandian basement. In addition, the Meso- to Neoproterozoic sediments now only locally preserved across the shield (e.g. Morad & Al-Aasm, Reference Morad and Al-Aasm1994; Bingen, Belousova & Griffin, Reference Bingen, Belousova and Griffin2011; Lundmark & Lamminen, Reference Lundmark and Lamminen2016; Fig. 1a), sources within the Sveconorwegian belt that include a far-travelled Laurentian component and sources from Fennoscandia east of the belt (Bingen, Belousova & Griffin, Reference Bingen, Belousova and Griffin2011) are also potential contributors. Thus, we regard the Visingsö Group as predominantly regionally derived. That the stratigraphically lowest sample contains the youngest zircons suggests either (i) increasingly older rocks were being unroofed and eroded, or (ii) an expanding depositional system covered younger, more locally derived material with older, more distal material. We are unable to distinguish between these two scenarios.
5. Age of the Visingsö Group from diagnostic assemblages
An approximate relative age of c. 800–700 Ma for the Visingsö Group has been previously inferred by correlating diagnostic assemblages of OWM, VSM and stromatolites with successions that have an established chronostratigraphy (Vidal & Moczydłowska, Reference Vidal and Moczydłowska1995; Martí Mus & Moczydłowska, Reference Martí Mus and Moczydłowska2000). Such assemblages are known from the Hedmark, Vadsø and Tanafjord groups in the Caledonides of Norway, the Thule and Eleonore Bay groups of Greenland, the Chuar, Uinta Mountain and Pahrump groups in the western USA, the Little Dal, Mount Harper and Fifteenmile groups in Canada, and others in Siberia, the Urals and Svalbard (Vidal, Reference Vidal1976; Vidal & Ford, Reference Vidal and Ford1985; Horodyski, Reference Horodyski1993; Vidal & Moczydłowska-Vidal, Reference Vidal and Moczydłowska-Vidal1997; Porter & Knoll, Reference Porter and Knoll2000; Porter, Reference Porter, Xiao and Kaufman2006; Nagy et al. Reference Nagy, Porter, Dehler and Shen2009; Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014). Recent datings of the successions in the western USA and Canada provide more accurate age constraints (Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014). The VSM and certain OWM taxa in the upper formation of the Visingsö Group (Figs 2, 3; list of species in the online Supplementary Material available at http://journals.cambridge.org/geo), which co-occur in the Kwagunt Formation (the upper Chuar Group) and the Callison Lake dolostone (informal unit in the lower Mount Harper Group), provide a biochronological minimum age for the Visingsö Group of c. 740 Ma (see Section 6 discussion below). Thus, the Visingsö Group is now robustly constrained to < 886–740 Ma.
Figure 3. New record of organic-walled microfossils from the upper formation of the Visingsö Group, (a–i) light transmitted and (j, k) scanning electron micrographs. (a) Squamosphaera colonialica, V14-66-4-(J44). (b) Synsphaeridium sp., V14-14-3-(F30-3). (c) Valeria lophostriata, V14-14-3-(J28). (d) Simia annulare, V14-14-3-(P24-4). (e) Pterospermopsimorpha pileiformis, V14-79-4-(M37-4). (f) Leiosphaeridia ternata, V14-14-3-(L25-3). (g) Leiosphaeridia sp., V14-36-5-(S44). (h, i) Lanulatisphaera laufeldii, V14-66-4-(C40-3); V14-66-4-(U39-1). (j, k) Cerebrosphaera globosa (Ogurtsova & Sergeev, Reference Ogurtsova and Sergeev1989) Sergeev & Schopf, Reference Sergeev and Schopf2010; (j) V14-80-4-L57; (k) V14-52-1-04. Scale bars equal 20 μm for light transmitted micrographs. Collection PMU-Visingsö.2014 (V14- followed by the sample and slide numbers, and England Finder Coordinates).
Figure 4. Probability density distribution plots of detrital zircons and their ages from the Visingsö Group sandstones in stratigraphic order. The Sveconorwegian (0.90–1.14 Ga), Hallandian (1.38–1.47 Ga), Gothian (1.52-1.66 Ga) and TIB (1.66–1.86 Ga) sources are indicated (grey bars).
6. Discussion and evolutionary implications
The Tonian Visingsö Group documents a diverse microbial association of prokaryotic cyanobacteria and eukaryotic OWM, VSM (Vidal, Reference Vidal1972, Reference Vidal1976; Martí Mus & Moczydłowska, Reference Martí Mus and Moczydłowska2000; Agić, Moczydłowska & Willman, Reference Agić, Moczydłowska and Willman2015; Loron, Reference Loron2016) and new MSM that are partly identified. The MSM could represent biomineralizing green algae (Cohen et al. Reference Cohen, Schopf, Butterfield, Kudryavtsev and Macdonald2011; Cohen & Knoll, Reference Cohen and Knoll2012) adding another dimension to the complex ecosystem and development of biomineralization. A new record of OWM, including Cerebrosphaera, Valeria, Schizofusa, Simia, Tasmanites and Pterospermopsimorpha among other taxa (Fig. 3; online Supplementary Material available at http://journals.cambridge.org/geo), strengthen the ranges of potential species for Neoproterozoic biostratigraphy. These taxa are recognized as possible members of green algal lineages of Prasinophyceae and Chlorophyceae (Grey, Reference Grey2005; Lamb et al. Reference Lamb, Awramik, Chapman and Zhu2009; Moczydłowska et al. Reference Moczydłowska, Landing, Zang and Palacios2011; Moczydłowska, Reference Moczydłowska2016; Agić, Moczydłowska & Willman, Reference Agić, Moczydłowska and Willman2015; Loron, Reference Loron2016), but many other OWM taxa remain unidentified phylogenetically. Geochronologically better understood, and now constrained by isotopic dating, the Visingsö microbiota will contribute to reconstructing the relationships among early eukaryotes (Knoll, Reference Knoll2014) by further reconciling the fossil record with molecular clock estimates.
The recognition and identification of OWM, VSM and MSM microfossils allows us to make biochronologic correlations with the Chuar, lower Mount Harper and the upper Fifteenmile groups. The Visingsö microfossils, both uni- and multicellular, are well preserved, abundant and consist of established as well as new species. Some have features that support their various protistan affinities (ongoing study). The OWM in the Visingsö Group were originally described by Vidal (Reference Vidal1976) from all formations and additional records derive from the middle and upper formations (Agić, Moczydłowska & Willman, Reference Agić, Moczydłowska and Willman2015; Loron, Reference Loron2016; ongoing study; Fig. 3; online Supplementary Material available at http://journals.cambridge.org/geo). The assemblage consists of 20 species recognized by distinct morphology (surface sculpture, excystment structure, wall perforation) and bodyplan (sphere-in-sphere, internal body). Several new species, including those with spinous ornamentation, await formal description. A great variety of spheroidal specimens displaying a wide range of vesicle size and wall thickness, which are attributed by some authors to different species of Leiosphaeridia (crassa, jacutica, minutissima and tenuissima), are left under open nomenclature as Leiosphaeridia spp. Their quantity is enormous (thousands of specimens), yet they lack objective morphologic features and overlap in dimensions to make identification reliable. The cyanobacterial coccoidal and filamentous microfossils preserved as solitary specimens, colonies and fragmentary bacterial mats are attributed to seven genera with more numerous species. In total, the OWM record is among the highest diversity recognized in a single Tonian-age stratigraphic unit. This diversity is of the same taxonomic magnitude as in the Chuar Group assemblage accounting for some 32 OWM species (Nagy et al. Reference Nagy, Porter, Dehler and Shen2009; Porter & Riedman, Reference Porter and Riedman2016), and many are in common. Thus, we correlate the middle and upper formations of the Visingsö Group with the Chuar Group. The lower formation of the Visingsö Group consists of spheroidal and cyanobacterial species that are not age-diagnostic.
The VSM in the upper formation of the Visingsö Group are recorded in unmetamorphosed phosphate nodules embedded in organic-rich mudstone and shale (Knoll & Vidal, Reference Knoll and Vidal1980; Martí Mus & Moczydłowska, Reference Martí Mus and Moczydłowska2000; unpub. data). The phosphate nodules are composed of francolite, a cryptocrystalline phosphate. These were precipitated early in diagenesis in suboxic to sulfate-reduction zones within decimetres to metres of burial below the sediment–water interface (Morad & Al-Aasm, Reference Morad and Al-Aasm1994) on tidal mud flats (Larson & Nørgaard-Pedersen, Reference Larsen and Nørgaard-Pedersen1988). Francolite precipitation was microbially mediated and microbial mats occur as patches and thin discontinuous laminae in the nodules and host mudstone. VSM are abundant, with up to several hundred specimens present in a single petrographic thin-section, and mostly observed in longitudinal or slightly transversal sections. No perpendicular-to-the-long-axis sections or sections through the oral part of the tests are seen in thin-section.
The VSM preserved as three-dimensional organic-walled tests and extracted by acid maceration are known only from the Eleonore Bay Group of East Greenland (Vidal, Reference Vidal1979), the Kwagunt Formation of the Chuar Group type locality (Bloeser, Reference Bloeser1985; Porter & Knoll, Reference Porter and Knoll2000; Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003) and from the Tien Shan Mountains in Kyrgyzstan (Jankauskas, Mikhailova & German, Reference Jankauskas, Mikhailova and German1989). Mostly they are preserved as permineralized casts and moulds (Martí Mus & Moczydłowska, Reference Martí Mus and Moczydłowska2000; Porter & Knoll, Reference Porter and Knoll2000; Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003; Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014). The Visingsö specimens have not yet been isolated from the rock matrix or nodules, and are observed as casts and moulds replicated by precipitation of francolite, quartz and berthierine (Martí Mus & Moczydłowska, Reference Martí Mus and Moczydłowska2000). Therefore, their identification is limited to the overall habit and symmetry of the tests observed in thin-section without oral opening ornamentation and shape. We identified Melanocyrillium hexadiadema Bloeser, Reference Bloeser1985 (Fig. 2c, upper specimen) by distinguishing in a longitudinal section test flexure marking the oral termination (neck-like region) and an invaginated aperture between broad indentation, though the transversal section of the hexagonal aperture was not seen. Synonymy, based on comparable thin-sections of the species, includes specimens illustrated by Bloeser (Reference Bloeser1985, fig. 7:14; identical to our specimens), Porter, Meinsterfeld & Knoll (Reference Porter, Meinsterfeld and Knoll2003, fig. 4:11) and Strauss et al. (Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014, fig. 2A). A specimen with an apertural margin with a minimal short collar and flushing into the test wall (Fig. 2b) is similar to a thin-section illustration by Porter, Meinsterfeld & Knoll (Reference Porter, Meinsterfeld and Knoll2003, fig. 6:21) and attributed to Cycliocyrillium torquata Porter, Meisterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003. The species is also recognized in the assemblage studied by Martí Mus & Moczydłowska (Reference Martí Mus and Moczydłowska2000, fig. 3A; see Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003), alongside Cycliocyrillium simplex Porter, Meisterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003 (Martí Mus & Moczydłowska, Reference Martí Mus and Moczydłowska2000, figs 6A, C–F, 7A–C, E–F). The latter species is recognized by a bulbous outline of the test with a simple aperture that is relatively narrow in relation to the test width and without any marginal thickening, as seen in the SEM image and the thin-section illustration by Porter, Meinsterfeld & Knoll (Reference Porter, Meinsterfeld and Knoll2003, fig. 6:9). These authors also suggested this species might be present in the Visingsö assemblage studied by Knoll & Vidal (Reference Knoll and Vidal1980, fig. 1D–G), as well as Trigonocyrillium horodyski (Bloeser, Reference Bloeser1985) Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003 and T. fimbriatum (Bloeser, Reference Bloeser1985) Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003, although the two latter species were without reference to individual specimens or collection. T. fimbriatum has been documented only by SEM images in the type collection (Bloeser, Reference Bloeser1985), but an elongate test with oral fringe seen in the longitudinal view (Bloeser, Reference Bloeser1985, figs 10:21, 41, 71, 11:3) is very similar to the specimen illustrated by Martí Mus & Moczydłowska (Reference Martí Mus and Moczydłowska2000, fig. 2D). This makes a record of five common geographically distributed VSM species in the upper formation of the Visingsö Group among 12 species known in total from the upper Kwagunt Formation (Porter & Knoll, Reference Porter and Knoll2000; Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003). Three species (M. hexadiadema, C. simplex and C. torquata) also co-occur in the assemblage of eight species recorded in the Callison Lake dolostone of the lower Mount Harper Group, Yukon, Canada (Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014). Allison & Awramik (Reference Allison and Awramik1989) reported an older stratigraphic record of VSM in this area (the Tindir Creek, Yukon) from the upper Tindir Group (now the upper Fifteenmile Group; Macdonald et al. Reference Macdonald, Cohen, Dudás and Schrag2010a,b, Reference Macdonald, Smith, Strauss, Cox, Halverson, Roots, MacFarlane, Weston and Relf2011; Cohen & Knoll, Reference Cohen and Knoll2012) that underlies the Callison Lake dolostone and additionally some 670 m thick succession belonging to the Craggy Dolostone Formation. They document VSM Melanocyrillium sp. and new species Hyalocyrillium clardyi Allison in Allison & Awramik, Reference Allison and Awramik1989, along with MSM. The latter taxon was recognized by Allison & Awramik (Reference Allison and Awramik1989) as being similar to VSM from the Visingsö Group (and successions in Greenland, Brazil and Saudi Arabia) but differing from those in the Chuar Group described by Bloeser (Reference Bloeser1985) by having a thicker wall. This morphologic or taphonomic difference is insignificant and H. clardyi belongs to the VSM, thus proving the co-occurrence of VSM with MSM (Allison & Awramik, Reference Allison and Awramik1989; Cohen & Knoll, Reference Cohen and Knoll2012).
The new genus and species Hyalocyrillium clardyi (Allison & Awramik, Reference Allison and Awramik1989, fig. 10:10–11) is similar if not identical to Cycliocyrillium simplex (Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003, thin-section fig. 6:9) and the two taxa are considered conspecific. This synonymy implies that Hyalocyrillium Allison in Allison & Awramik, Reference Allison and Awramik1989 has taxonomic priority over Cycliocyrillium Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003, and its type species C. simplex is a junior synonym of the type species H. clardyi. Consequently, we recommend that C. torquata Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003 should be transferred to H. torquata (Porter, Meinsterfeld & Knoll, Reference Porter, Meinsterfeld and Knoll2003) new combination, although we do not formalize it in this paper.
The range of VSM in Laurentia was recognized within the time interval c. 780–740 Ma (Dehler, Reference Dehler2014; Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014) and, as aforementioned, they extend through a number of formations across the western margin of Laurentia from the Grand Canyon to the Yukon Territory. In the Yukon, the range of VSM through the Callison Lake dolomite (Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014) is in fact wider and extends into the upper Fifteenmile Group to above the isotopically dated layer at 811.5 Ma (Allison & Awramik, Reference Allison and Awramik1989; Macdonald et al. Reference Macdonald, Schmitz, Crowley, Roots, Jones, Maloof, Strauss, Cohen, Johnston and Schrag2010b; Cohen & Knoll, Reference Cohen and Knoll2012). This poses the need to (i) correlate the upper Fifteenmile Group with other successions containing VSM, and (ii) extend the VSM lower range to c. 788 Ma, consistent with the MSM range (see below).
MSM occur side-by-side with VSM (Fig. 2b) in the upper formation of the Visingsö Group at two stratigraphic levels (Fig. 1c) and they are of the type of scale-like microfossils known from the 811.5–739.9 Ma upper Fifteenmile Group of the Yukon Territory, Canada (Allison & Hilgert, Reference Allison and Hilgert1986; Macdonald et al. Reference Macdonald, Cohen, Dudás and Schrag2010a,b, Reference Macdonald, Smith, Strauss, Cox, Halverson, Roots, MacFarlane, Weston and Relf2011; Cohen et al. Reference Cohen, Schopf, Butterfield, Kudryavtsev and Macdonald2011; Cohen & Knoll, Reference Cohen and Knoll2012; Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014). Their discovery in the Visingsö succession for the first time documents their occurrence outside the type locality on Laurentia and is significant because their stratigraphic range is wider than previously recorded. This is evident from their vertical distribution within a rock succession of shale and mudstone c. 300 m thick (Fig. 1c) in comparison to the interval of 58 m of limestone containing MSM in the upper Fifteenmile Group (Cohen & Knoll, Reference Cohen and Knoll2012). Regardless of the different rates of sedimentation between the fine-grained siliciclastic versus carbonate rocks in the two localities, both accumulating in shallow subtidal environments (Larson & Nørgaard-Pedersen, Reference Larsen and Nørgaard-Pedersen1988; Macdonald et al. Reference Macdonald, Smith, Strauss, Cox, Halverson, Roots, MacFarlane, Weston and Relf2011, respectively), it appears that the MSM vertical range in the Visingsö Group involves a longer time span.
The Visingsö MSM are observed in thin-sections of phosphate nodules in shale and have not yet been successfully isolated from the host rock to see their three-dimensional morphology, thus making the identification preliminary. They are simple morphotypes, ellipsoidal in outline, smooth in appearance and not perforated, with sharply defined narrow marginal rims and central portion (Fig. 2a, b) or showing additionally one or two marks or holes in the centre (Fig. 2d). Their dimensions are 18–39 μm in length, with 1.5–6.0 μm wide marginal rims. The present specimens, by comparison with specimens observed in thin-sections of chert nodules but also with those isolated from the Fifteenmile Group limestone, are identified as Paleomegasquama arctoa Cohen & Knoll, Reference Cohen and Knoll2012 (Fig. 2a), Bicorniculum brochum Allison & Hilgert, Reference Allison and Hilgert1986 (Fig. 2b) and Archeoxybaphon polykeramoides (Allison & Hilgert, Reference Allison and Hilgert1986) emend. Cohen & Knoll, Reference Cohen and Knoll2012 (Fig. 2d). The specimen of P. arctoa (Fig. 2a) is an ellipsoidal scale, 21×30 μm in diameter, with a smooth surface and two distinct portions: a narrow marginal rim 1.5–2.3 μm in width and a large central portion. It resembles isolated Fifteenmile Group specimens of placolith form and is of their dimensions (Cohen & Knoll, Reference Cohen and Knoll2012, fig. 9.7–9.9), and if seen in section it would be identical to the specimen illustrated by Cohen & Knoll (Reference Cohen and Knoll2012, in fig. 9.8). The specimen of B. brochum (Fig. 2b) is an ellipsoidal scale, 30×39 μm in diameter, with two marginal rings: a narrow inner and a wider outer, together 6 μm in width, around a central ellipsoidal portion. It is similar to specimens in illustrated thin-sections by Allison & Hilgert (Reference Allison and Hilgert1986, figs 10.1, 10.2), although the tooth-like band is not visible clearly in our section. However, the higher dimensional proportion of the two rings to the small central portions of the scale is typical of the species and differs from other scale microfossils. The species A. polykeramoides is an elliptical scale, 18×25 μm in diameter, smooth without any visible pores, with a thin marginal rim and 1–2 central elongate markings or holes. Certain three-dimensionally preserved Fifteenmile Group specimens show elevated elements or holes in the central portion of the scale (Cohen & Knoll, Reference Cohen and Knoll2012, figs 3.1, 3.5), which if sectioned would appear similar to those in the Visingsö specimens (Fig. 2d).
The stratigraphic position of MSM in the type area of the Western Ogilvie Mountains, Yukon, in the Lower Tindir Group, upper shale informal unit, has been defined to be above the Bitter Springs C-isotopic anomaly stage, which is also recognized in the upper Fifteenmile Group in the Central Ogilvie Mountains above the horizon isotopically dated to 811.5 Ma (Macdonald et al. Reference Macdonald, Schmitz, Crowley, Roots, Jones, Maloof, Strauss, Cohen, Johnston and Schrag2010b). The MSM described in detail by Cohen & Knoll (Reference Cohen and Knoll2012) have been subsequently attributed to the upper Fifteenmile Group, and tentatively to its Craggy Dolomite Formation in the Mt Slipper section, where the fossiliferous strata are 58 m thick. This is the same lithostratigraphic unit as the ‘limestone unit of the upper Tindir Group’ in the Tindir Creek locality studied originally by Allison & Hilgert (Reference Allison and Hilgert1986).
Uncertainty remains regarding the lithostratigraphic attribution of MSM, because of recent re-mapping and re-assessment of rock successions in the Yukon Territory, and revision of their stratigraphic position and regional correlation based on isotopic dating and δ13C chemostratigraphy (Macdonald et al. Reference Macdonald, Cohen, Dudás and Schrag2010a,b, Reference Macdonald, Smith, Strauss, Cox, Halverson, Roots, MacFarlane, Weston and Relf2011; Macdonald & Roots, Reference Macdonald, Roots, MacFarlane, Weston and Blackburn2010; Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014). The chronostratigraphy of these units has also changed and although previously attributed to the Cryogenian (850–635 Ma) is now referred to the Tonian Period (1000 – c. 720 Ma), following the International Chronostratigraphic Chart 2015 (Cohen et al. Reference Cohen, Finney, Gibbard and Fan2015a).
The stratigraphic position of MSM, whether in the basal Craggy Dolostone or at the top of the Reefal Assemblage, is constrained by the age of the Bitter Springs Stage (BSS). The BSS has been recognized as a globally synchronous C-isotope negative anomaly (Halverson et al. Reference Halverson, Wade, Hurtgen and Barovich2010) and is constrained to the interval after 811.5 Ma and before 788.7 Ma, lasting c. 7–17 Ma (Macdonald et al. Reference Macdonald, Schmitz, Crowley, Roots, Jones, Maloof, Strauss, Cohen, Johnston and Schrag2010b; Swanson-Hysell et al. Reference Swanson-Hysell, Maloof, Condon, Jenkin, Alene, Tremblay, Tesema, Rooney and Haileab2015). MSM occur above the BSS, thus their maximum age is c. 788 Ma. The range of MSM in the Ogilvie Mountains type area is very short and equal to the depositional time of 58 m thick limestone that may be just a few million years calculated from the rate of deposition of the succession (c. 1000 m thick carbonate succession deposited within the time interval 811–740 Ma). A wider vertical range that is closer to the minimum age of MSM is recorded in the Visingsö Group.
MSM in the upper formation of the Visingsö Group co-occur with more diverse VSM taxa known from the upper Kwagunt Formation and the Callison Lake dolostone (including M. hexadiadema) and are understood to record their upper stratigraphic range and minimum age. This is inferred from the present correlation of the upper formation of the Visingsö Group with these formations and constrained by the minimum age at 740 Ma of the Callison Lake dolostone (Strauss et al. Reference Strauss, Rooney, Macdonald, Brandon and Knoll2014). The MSM lower range and maximum age is recognized in the upper Fifteenmile Group and it coincides also with the earliest occurrence of VSM. The MSM upper range and minimum age are recorded in the upper formation of the Visingsö Group together with those of the VSM and indicates the time span of both microfossil groups at c. 788–740 Ma.
Tonian marine ecosystems were dominated, as seen in the fossil record by their taxonomic diversity and relative abundance (Porter, Reference Porter, Xiao and Kaufman2006; Nagy et al. Reference Nagy, Porter, Dehler and Shen2009; Cohen & Macdonald, Reference Cohen and Macdonald2015; Tang et al. Reference Tang, Pang, Yuan, Wan and Xiao2015), and enhanced by the new record from the Visingsö Group (represented by thousands of specimens; unpub. data), by photosynthesizing cyanobacteria and algae, and less frequently occurring heterotrophic protists, and some protists of uncertain origin (Butterfield, Reference Butterfield2000; Porter, Reference Porter, Xiao and Kaufman2006; Sergeev, Reference Sergeev2006; Cohen & Macfadden Reference Cohen and Macdonald2015; Porter & Riedman, Reference Porter and Riedman2016). Shallow marine habitats must have been relatively well oxygenated to sustain planktonic and benthic autotrophs, allowing them to fulfil their metabolic and life cycle requirements for sexual reproduction, as known from modern analogues (see discussion by Moczydłowska, Reference Moczydłowska2008a, Reference Moczydłowska2016). Relatively well-oxygenated ocean surface waters or at least oxygenated local basins in such a state are supported by geochemical studies (Jackson, Reference Jackson2015; Lalonde & Konhauser, Reference Lalonde and Konhauser2015; Turner & Bekker, Reference Turner and Bekker2016; Spence, Le Heron & Fairchild, Reference Spence, Le Heron and Fairchild2016) and this is in agreement with the presence of a microbiota of inferred algal affinities that were reproducing sexually in the Visingsö Group at the time, and in contemporaneous successions. Progressive evolution of phytoplankton in the Tonian Period, evident by comparison with the Mesoproterozoic record (Yan & Liu, Reference Yan and Liu1993; Javaux, Knoll & Walter, Reference Javaux, Knoll and Walter2004; Lamb et al. Reference Lamb, Awramik, Chapman and Zhu2009; Agić, Moczydłowska & Yin, Reference Agić, Moczydłowska and Yin2015; Sergeev et al. Reference Sergeev, Knoll, Vorobeva and Sergeeva2016), contributed to steady oxygenation of surface waters by the release of free oxygen, increased the production of net organic matter at the base of the food web and supported heterotrophic consumers – all related to the process of photosynthesis. The integrated environmental and evolutionary development with a positive feedback in a sustainable biosphere is first observed in the Tonian Period.
7. Conclusions
A Tonian age for the Visingsö Group is well defined by combining the maximum age of deposition from U–Pb dating of detrital zircons with the minimum age from biochronologic correlation of the Visingsö Group with the Chuar and the lower Mount Harper groups. This restricts its age to ≤ 886–740 Ma, and furthermore restricts its middle and upper formations to c. 788–740 Ma. These ages can be extrapolated to successions containing similar assemblages in the Caledonides, Greenland, southern Urals and elsewhere.
We report the presence of a diverse assemblage of OWM and several species of VSM, as well as the recovery of MSM similar to those from the Tonian upper Fifteenmile Group, Yukon, Canada, and for the first time outside Laurentia. We infer the time range of VSM and MSM at c. 788–740 Ma, which is constrained by isotopic datings of strata recording their lowermost and uppermost co-occurrence.
Geochronological constraint on the Visingsö microfossil assemblage is significant for revealing the time sequence of evolutionary events and divergence of auto- and heterotrophic protist lineages and for tracing their passive dispersal and active migration between the palaeocontinents. The presence of cosmopolitan taxa indicates a free connection with a global ocean and circulation of surface currents allowing biotic expansion along contiguous continental margins.
The evolution of marine ecosystems comprising similar biotas during the Tonian Period along newly opening marine basins on the margins of Baltica (Visingsö, Hedmark, Vadsø, Tanafjord and Barents Sea successions) and Laurentia (Chuar, Uinta Mountain, Pahrump, Little Dal, Mount Harper and Fifteenmile successions) established the first truly global and diverse eukaryotic protistan biosphere.
Acknowledgements
Our research was supported by Swedish Research Council (Vetenskåpsrådet) project grants Nr 621-2012-1669 to MM and Nr 621-2014-4375 to VP. The Geological Survey of Sweden (SGU) is kindly acknowledged for the access to the Visingsö 1 drillcore. The work of LW was conducted with the kind permission of the SGU Director. We thank the reviewers, Kathleen Grey and one anonymous reviewer, and the editor Mark Allen for their useful comments on the manuscript and the editorial work.
Supplementary material
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