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

Abstract

Sedimentological 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 d 13 C 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 CO 2 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 (2016) 63, 253-306 doi: 10.1111/sed.12261 STATE OF THE SCIENCE model o...

The Late Cryogenian Warm Interval, NE Svalbard: Chemostratigraphy and genesis

“…Available evidence suggests anoxic oceans (largely ferruginous but locally euxinic) beneath an oxygenated surface mixed layer (Och & Shields-Zhou, 2012;Tahata et al, 2015;Sperling et al, 2015). This model, based on redox proxies, is consistent with petrographic evidence from the LCWI in South Australia, where primary dolomite cements in a reef complex with significant vertical relief record gradients in Fe and Mn , 2015.…”

“…Chronological and chemostratigraphic constraints on the mid-Cryogenian in general, although improving (Halverson et al, 2010;Rooney et al, 2014Rooney et al, , 2015, are currently more fluid than the base and top of the Cryogenian (Spence et al, 2016). A tuffaceous bed just above the Sturtian glaciation in South China has yielded a U-Pb age of 663 ± 4 Ma (Zhou et al, 2004).…”

Basinal setting and origin of thick (1·8 km) mass‐flow dominated Grand Conglomérat diamictites, Kamoa, Democratic Republic of Congo: Resolving climate and tectonic controls during Neoproterozoic glaciations

“…Li et al . () argued from palaeomagnetic evidence, and entirely counter to the geological record, that Neoproterozoic glacially‐influenced deposits accumulated on ‘stable platforms’, others have suggested that globally‐correlative, orbitally‐forced ice advance and retreat cycles might ultimately be identified from such deposits (Condon et al ., ; Leather et al ., ; Arnaud, ; Rieu et al ., ; Hu et al ., ; Le Heron et al ., ; Benn et al ., ; Busfield & Le Heron, ; Spence et al ., ). These latter ideas are worthy of further investigation but need to be reconciled with what is known of the very active tectonic setting of many Neoproterozoic glacially‐influenced basin fills, which are overwhelmingly deep water mass flow dominated and rift‐related (Western North America, Greenland, Scotland and Scandinavia; Eyles & Januszczak, , ).…”

New perspectives on the Luoquan Glaciation (Ediacaran‐Cambrian) of North China

“…Most focus has been on the Sturtian ( ca 717–660 Ma) and the Marinoan ( ca 651–635 Ma) glaciations (Evans & Raub, ; Fairchild & Kennedy, ; Hoffman & Li, ; Hoffman et al., , ; Li et al., ; Rooney, Strauss, Brandon, & Macdonald, ; Zhao & Zheng, ; Zhou et al., ). Many workers consider these to be discrete global Snowball Earth glaciations, although poor global age control equally allows them to be viewed as diachronous (Spence, Le Heron, & Fairchild, ). Two other glaciations (Kaigas, ca 757–741 Ma, and Gaskiers, ca 590–582 Ma) are also recognized but considered to represent non‐Snowball Earth conditions (Gao et al., ; Li et al., ; Pu et al., ).…”

“…The Gaskiers glaciation of north east North America is an example of an increasingly tightly constrained glaciation, likely ≤340 Ka in duration, which was posited to have occurred <9.5 Myr prior to the advent of the Ediacaran biota (Pu et al., ). In spite of the emerging view that the Gaskiers glaciation is of regional, rather than global extent (Pu et al., ; Spence et al., ), glacial deposits of broadly similar age are recognized on other continents (Li et al., ). In China, these include the deposits of the Tarim Craton in the north‐west, and those of the North China Craton (NCC).…”