Graham A. Shields-Zhou

The case for a Neoproterozoic Oxygenation Event: Geochemical evidence and biological consequences

The Neoproterozoic era marked a turning point in the development of the modern earth system. The irreversible environmental changes of that time were rooted in tectonic upheavals that drove chain reactions between the oceans, atmosphere, climate, and life. Key biological innovations took place amid carbon cycle instability that pushed climate to unprecedented extremes and resulted in the ventilation of the deep ocean. Despite a dearth of supporting evidence, it is commonly presumed that a rise in oxygen triggered the evolution of animals. Although geochemical evidence for oxygenation is now convincing, our understanding of the Neoproterozoic earth system and of early animal evolution has changed apace, revealing an altogether more complicated picture in which the spread of anoxia played an important role. The challenge to future researchers lies in unraveling the complex entanglement of earth system changes during this pivotal episode in Earth’s history.

The Geological Record of Neoproterozoic Glaciations

In recent years, interest in Neoproterozoic glaciations has grown as their pivotal role in Earth system evolution has become increasingly clear. One of the main goals of the IGCP Project No. 512 was to produce a synthesis of newly available information on Neoproterozoic successions worldwide similar in format to Hambrey & Harlands (1981) Earths pre-Pleistocene Glacial Record. This Memoir therefore consists of a series of overview chapters followed by site-specific chapters. The overview chapters cover key topics including the history of research on Neoproterozoic glaciations, identification of glacial deposits, chemostratigraphic techniques and datasets, palaeomagnetism, biostratigraphy, geochronology and climate modelling. The site specific chapters for 60 successions worldwide include reviews of the history of research on these rocks and up-to-date syntheses of the structural framework, tectonic setting, palaeomagnetic and geochronological constraints, physical, biological, and chemical stratigraphy, and descriptions of the glaciogenic and associated strata, including economic deposits.

Chapter 4 Chemostratigraphy and the Neoproterozoic glaciations

Abstract Although the pre-glacial Proterozoic isotopic record is poorly constrained, it is apparent that the chemical and isotopic composition of the oceans began to change during the early to mid-Neoproterozoic and experienced considerable fluctuations alongside climatic instability during much of the subsequent Cryogenian and Ediacaran periods. The earliest known large negative δ13C excursion appears to post-date 811 Ma and fluctuations became progressively more extreme, culminating in the late-Ediacaran ‘Shuram–Wonoka’ anomaly. The negative excursions are commonly associated with pre-glacial and post-glacial times, while extremely high δ13C values are characteristic of strata between glaciations. The precise causal mechanism for these excursions is subject to debate. Seawater 87Sr/86Sr rose during the Neoproterozoic, with abrupt increases following deglaciation consistent with enhanced weathering rates. Reported marine sulphate and pyrite δ34S data exhibit marked variation through this interval, although the changes are not always consistent within or between sedimentary successions of equivalent age. Iron-speciation studies indicate that much of this variation was caused by fluctuating and low sulphate concentrations in seawater, which at times led to the build-up of ferruginous conditions in the ocean. The application of chemostratigraphy to understanding and correlating the Neoproterozoic glaciations evokes considerable controversy, and many questions persist regarding the reliability and calibration of the δ13C, 87Sr/86Sr and δ34S record. Nevertheless, the individual glaciations appear to be characterized by distinct combined chemostratigraphic signatures, in large part due to the generally increasing strontium isotope composition of seawater through the Neoproterozoic Era.

Nitrogen cycle feedbacks as a control on euxinia in the mid-Proterozoic ocean

Geochemical evidence invokes anoxic deep oceans until the terminal Neoproterozoic ~0.55 Ma, despite oxygenation of Earths atmosphere nearly 2 Gyr earlier. Marine sediments from the intervening period suggest predominantly ferruginous (anoxic Fe(II)-rich) waters, interspersed with euxinia (anoxic H(2)S-rich conditions) along productive continental margins. Today, sustained biotic H(2)S production requires NO(3)(-) depletion because denitrifiers outcompete sulphate reducers. Thus, euxinia is rare, only occurring concurrently with (steady state) organic carbon availability when N(2)-fixers dominate the production in the photic zone. Here we use a simple box model of a generic Proterozoic coastal upwelling zone to show how these feedbacks caused the mid-Proterozoic ocean to exhibit a spatial/temporal separation between two states: photic zone NO(3)(-) with denitrification in lower anoxic waters, and N(2)-fixation-driven production overlying euxinia. Interchange between these states likely explains the varying H(2)S concentration implied by existing data, which persisted until the Neoproterozoic oxygenation event gave rise to modern marine biogeochemistry.

The Geological Record of Neoproterozoic ice ages

The IUGS- and UNESCO-funded International Geoscience Programme Project #512 (Neoproterozoic Ice Ages) was conceived to contribute towards a global synthesis of current geological data on the number, duration, extent, causes and consequences of glacial episodes during the Neoproterozoic Era. IGCP 512 attracted more than 200 scientists from over 30 countries, many of whom provided their regional and specialist expertise on Neoproterozoic successions around the world to the realization of this volume. IGCP 512 focused on integrating various aspects of Neoproterozoic geology: geochronology, geochemistry, sedimentary geology, biostratigraphy, palaeomagnetism and economic geology. At its inaugural meeting on 27 August 2005 during the International Association of Sedimentology conference on glacial processes and products in Aberystwyth, Wales, IGCP 512 members decided to produce a volume that summarized existing data sets in a form similar to Earths Pre-Pleistocene Glacial Record by Hambrey & Harland (1981). An enormous amount of work has been carried out in the 12 years since the publication of Hoffman et al. s (1998) paper on the Snowball Earth hypothesis for Neoproterozoic glaciation (Fairchild & Kennedy 2007). The Snowball Earth hypothesis and, more generally, Neoproterozoic climate, have been the topic of numerous special volumes, special sessions, a dedicated conference in Ascona (Switzerland) in 2006 (Shields 2006), and numerous documentaries. Motivated by this intense worldwide interest in the Neoproterozoic glaciations and an exploding body of research into the topic, this volume synthesizes the state-of-the-art in this now highly multidisciplinary research field. It is intended to facilitate the integration of data sets, inspire new research projects, and inform ongoing work into the definition and subdivision of the Neoproterozoic timescale, including selection of the Global Stratotype Section and Point (GSSP) for the base of the Cryogenian Period. Despite such lofty aims, any book such as this cannot claim to be complete, and there are …

Biogeochemistry: Toxic Cambrian oceans

Isotopic evidence from carbon and sulphur points to the spread of anoxia and toxic sulphide as the chief culprits in at least one of a series of crises for marine ecosystems during the nascent stages of early animal evolution. See Letter p.80 It has been suggested that the Cambrian ocean was oxygen deficient, but physical evidence for widespread anoxia has been lacking. Gill et al. present sulphur isotope data from Cambrian rocks at six different locations around the world and find a positive sulphur isotope excursion in phase with a large excursion in the marine carbon isotope record, which is thought to be indicative of a global carbon cycle perturbation at the time. A prolonged period of anoxia during the Cambrian may explain the previously enigmatic peculiarities seen in the fossil record.

Chapter 11 The record of Neoproterozoic glaciation in the Taoudéni Basin, NW Africa

Abstract The Taoudéni Basin covers over 1 000 000 km2 of the West African Craton, bounded by Pan-African orogenic belts. Four supergroups separated by craton-scale unconformities are recognized, with Neoproterozoic glaciogenic deposits occurring at the base of Supergroup 2. The Jbéliat Group occurs along a continuous, 1300-km-long, narrow belt from the Adrar region of Mauritania to the eastern limit of the Hank in Algeria and comprises thin glacial drift capped widely by periglacial polygonal structures, with more complex glacial sequences preserved in palaeo-depressions. A thicker, variously marine and continental glaciogenic succession can be found in southern parts, while fully marine, glacially influenced successions are only known from the extreme SW of the basin. The ‘triad’ sequence of diamictites overlain by barite-bearing ‘cap’ dolostones and then by green shales and/or bedded cherts (silexites) is ubiquitous and has long been used to correlate the Supergroup 1/2 boundary across the basin and into the surrounding orogenic belts. The bedded cherts commonly show a volcanic influence and are cemented by early marine calcite at their base at Adrar, Mauritania. Although fossil-based age constraints are scarce and ambiguous, regional tectonic events indicate that ‘triad’ deposition occurred between the Bassaride (665–655 Ma) and Dahomeyide (610–580 Ma) orogens. Recent U–Pb zircon studies of ignimbrite tuffs provide a minimum age for the glaciation of c. 600 Ma. Correlation of supergroup 2 glacial deposits with the c. 635 Ma end-Cryogenian (‘Marinoan’) glaciation is likely and is supported by limited carbon and strontium isotope data. Barite is commonly found within the cap carbonate and may relate to methane seepage and/or unusual oceanographic conditions after deglaciation. Several studies have attributed sequence complexity within the post-glacial succession to isostatic reequilibration. The Taoudéni Basin represents a rare Neoproterozoic example of terrestrial tillites and associated periglacial facies.