2 INTRODUCTIONHow the Earth's atmosphere and ocean transitioned from their early, essentially anoxic state to our familiar oxygen-rich world remains controversial. It is well documented that an oxygenation event at ~2400 Ma (Ma: million years) established a persistently oxic atmosphere and surface ocean, but deep ocean chemistry remains uncertain through the remainder of the Proterozoic Eon (Kah and Bartley, 2011).Geologists long posited that the widespread disappearance of banded iron formation at ~1800 Ma reflects oxygenation of the deep ocean (Holland, 1984); however, Canfield (1998) proposed that under relatively low atmospheric O 2 , the deep ocean would remain anoxic and indeed become euxinic, reflecting increased rates of bacterial sulfate reduction at depth. The nature of subsurface ocean chemistry is critical to our understanding of Earth surface history and biological evolution. noting that oscillations between ferruginous and euxinic conditions in basinal strata track sedimentary total organic carbon contents, which suggests that euxinia is most likely to develop when organic carbon delivery exceeds the delivery of electron acceptors that outcompete sulfate (e.g. nitrate, ferric iron; see also Planavsky et al., 2011;Sperling et al., 2013). Adding to this emerging heterogeneity, data from mineral assemblages suggest that despite widespread anoxia in oxygen minimum zones, dysoxia (oxygen present but at low levels) apparently persisted in the deepest parts of at least some mid-Proterozoic oceans (Slack et al., 2007; Chandler, 1992), we also document the composition of microfossils preserved in basinal Arlan shales. These data are then placed in the context of information from other basins to examine redox heterogeneity in Mesoproterozoic oceans. GEOLOGIC BACKGROUND Geology of the Ural Mountains and Volgo-Ural regionFor many years, Russian geologists discussed Meso-and early Neoproterozoic stratigraphy in terms of a Riphean stratotype located in the Bashkirian meganticlinorium, a large structure on the western slope of the southern Ural Mountains (Chumakov and Semikhatov, 1981; Keller and Chumakov, 1983). In the southern Urals, the lower (Fig. 1B).In the Volgo-Ural region to the west, sub-surface Riphean stratigraphy is known from core and geophysical data. According to new geological and geophysical data the correlative stratigraphy to the Burzyan Group in this region, the Kyrpy Group, is subdivided into the Sarapul, Prikamskii and Or'ebash subgroups, and its base has not been penetrated by drilling (Kozlov et al., 2009. The A perennial question in pre-Mesozoic paleoceanography concerns the water-depth of sediments deposited beneath storm wave-base --these basinal strata are almost 7 certainly not 'deep' in the oceanographic sense of an average ocean depth of four kilometers. While the only hard constraint on these strata is that they were deposited in water depths persistently greater than ~150 meters (as indicated by the lack of wavegenerated sedimentary structures), such strata are generally con...

Abstract:The Neoproterozoic arrival of animals fundamentally changed Earth's biological and geochemical trajectory. Since the early description of Ediacaran and Cambrian animal fossils, a vigorous debate has emerged about the drivers underpinning their seemingly rapid radiation.Some argue for predation and ecology as central to diversification, whereas others point to a changing chemical environment as the trigger. In both cases, questions of timing and feedbacks remain unresolved. Through these debates, the last fifty years of work has largely converged on the concept that a change in atmospheric oxygen levels, perhaps manifested indirectly as an oxygenation of the deep ocean, was causally linked to the initial diversification of large animals.What has largely been absent, but is provided in this study, is a multi-proxy stratigraphic test of this hypothesis. Here, we describe a coupled geochemical and paleontological investigation of Neoproterozoic sedimentary rocks from northern Russia. In detail, we provide iron speciation data, carbon and sulfur isotope compositions, and major element abundances from a predominantly siliciclastic succession (spanning > 1,000 meters) sampled by the Kel'tminskaya-1 drillcore. Our interpretation of these data is consistent with the hypothesis that the pO 2 threshold required for diversification of animals with high metabolic oxygen demands was crossed prior to or during the Ediacaran Period.Redox stabilization of shallow marine environments was, however, also critical and only occurred about 560 million years ago (Ma), when large motile bilaterians first enter the regional stratigraphic record. In contrast, neither fossils nor geochemistry lend support to the hypothesis that ecological interactions altered the course of evolution in the absence of environmental change. Together, the geochemical and paleontological records suggest a coordinated transition from low oxygen oceans sometime before the Marinoan (~635 Ma) ice age, through better oxygenated but still redox-unstable shelves of the early Ediacaran Period, to the fully and persistently oxygenated marine Johnston et al., Ediacaran Redox Stability 3 environments characteristic of later Ediacaran successions that preserve the first bilaterian macrofossils and trace fossils. INTRODUCTIONThe hypothesis that increased oxygen availability facilitated Ediacaran (635-542 Ma) metazoan evolution dates back more than half a century (Cloud and Drake, 1968; Nursall, 1959).This hypothesis posits that an increase in the oxygen content of shallow-marine environments was physiologically necessary for the emergence of large, highly energetic animals (Raff and Raff, 1970; Rhoads and Morse, 1971). Ecological and physiological observations place lower dissolved oxygen (DO) limits for ocean waters in which different types of animals can live (e.g., (Diaz and Rosenberg, 1995; Levin, 2003)). They further make predictions about body shape in early animals, based on diffusion length-scales for organisms that lack a circulatory system for bulk oxy...

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The Kel'tminskaya-1 borehole, drilled along the northeastern margin of the East European Platform (EEP), reveals some 3,600 m of Neoproterozoic sedimentary rocks, mostly confined to the subsurface. The upper 1,000 m of the drilled section correlates with late Ediacaran Redkino and Kotlin successions on the EEP, whereas the lowermost 2,000 m can be related to pre-Sturtian (Upper Riphean) deposits in the Ural Mountains. In between lies the Vychegda Formation, a 600 m siliciclastic succession that has no counterpart in classic EEP stratigraphy.Vychegda microfossils can be separated into three assemblages. The upper part of the formation contains large, profusely ornamented acritarchs broadly comparable to those of the Ediacaran Complex Acanthomorph Palynoflora, including species of the generaAlicesphaeridium, Asterocapsoides, CavaspinaandTanariumconfined to Ediacaran-aged assemblages elsewhere. Diverse large acanthomorphs are known from Ediacaran strata around the world, but have not previously been recognized from the EEP, an absence attributed to a hiatus between the glacial Laplandian (>635 Ma) and Redkino (mostly <555 Ma) successions. The large acanthomorphic acritarchs record eukaryotic organisms with resting stages in their life cycles and likely include egg or diapause cysts of early animals. In contrast, the lower Vychegda assemblage, found in the basal 10 m of the succession, contains taxa typical of earlier Neoproterozoic successions. The middle assemblage contains only simple filaments and spheroidal acritarchs.The most parsimonious interpretation of Vychegda biostratigraphy is that pre-Marinoan rocks in the basal part of the formation are separated by a cryptic unconformity from early and middle Ediacaran deposits above. This interpretation is consistent with data from China and Australia, which indicate that the major paleontological transition to diverse ECAP assemblages took place within the Ediacaran Period and not in association with the preceding ice age. Vychegda acritarch assemblages thus contribute to a biostratigraphic model for the initial Ediacaran boundary.

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