Sedimentological perspectives on climatic, atmospheric and environmental change in the Neoproterozoic Era

Abstract

AbstractSedimentological insights underpin many of the important recent advances in understanding of Earth system behaviour in the Neoproterozoic Era. This article reviews three main areas: (i) chemical proxies and their preservation, with emphasis on carbonate facies; (ii) glacial and post‐glacial facies, including their age constraints; (iii) sedimentary evidence for biotic innovations and responses. Chemostratigraphy plays an important role in ordering Neoproterozoic events and defining disturbances to the carbon cycle. There is increasing attention being paid to assessing the role of diagenetic origination or modification of chemostratigraphic signals. Alongside this, new criteria for identifying primary dolomite and precursor metastable phases such as ikaite have been developed. In respect of oxygenation, geochemical proxies substantiate the concept of a Neoproterozoic Oxygenation Event as a very gradual transition, the ocean being at any one time a heterogeneous assemblage of ferruginous, sulphidic and oxic conditions, with some evidence of increasing deep‐sea oxidation through the Ediacaran Period. Techniques such as Fe‐speciation need to be supplemented with proxies sensitive to suboxic conditions. More generally, it is predicted that petrographically constrained microanalytical studies will also become more important in reconstructing palaeoenvironmental conditions. The global distribution of Neoproterozoic glacial deposits combined with palaeomagnetic evidence supports the concept of panglaciations in which ice sheets reached sea‐level in the tropics. Advances in radiometric dating have demonstrated the synchronous onset of global (Sturtian) glaciation at 717 Ma and the demise of a second (Marinoan) glaciation at 635 Ma, and plausibly indicate long durations for each (55 Myr for Sturtian and 5 to 15 Myr for Marinoan). However, a compilation of radiometric dates indicates ambiguities indicating the need for further improvements to the radiometric and Sr‐isotope database to understand events within the Sturtian time frame, the timing of onset of Marinoan glaciations, and the age and synchroneity of individual negative δ13C anomalies. Sturtian deposits are typically thick, rift‐related successions containing a range of environments influenced or dominated by dynamic glaciers, as well as ice‐free marine intervals. Marinoan glacial deposits, by contrast, tend to be thin and continental. During the latter interval, oxygen isotope systematics of sulphate demonstrate that atmospheric CO2 was high, as predicted by Snowball Earth theory, and that sedimentation was influenced by orbital forcing. The Sturtian record, by comparison, needs to be searched for evidence of cold‐climate hiatuses on the one hand and orbital forcing on the other. Cap carbonate formation appears to have coincided with rising sea‐levels following panglaciations. Snowball theory considers that they formed rapidly in the postglacial greenhouse, but an alternative model of slower formation with clastic sediment starvation during transgression may prove to be consistent with new data and models showing extensive glacier terminations on land. For all facies, but especially caps, the use of microanalytical techniques and holistic studies of petrogenesis are future priorities. There are important discrepancies between molecular clock predictions of early metazoan origination and the hard evidence from sedi‐mentary records which largely depend on local exceptional preservation by early diagenesis. A variety of life survived panglaciation, and there is little evidence that glaciations directly caused oxygenation or stimulated evolution.