Ca and Sr isotope constraints on the formation of the Marinoan cap dolostones

Highlights

• A dramatic negative ${\delta }^{44}$Ca excursion (0.6‰) is found in the Marinoan cap dolostone.

• Coupled Ca and Sr isotope variations indicate a mixing of meltwater and seawater.

• Vast amounts of Ca and HCO₃ are attributed to large meltwater/runoff and new exposed mineral.

Abstract

Neoproterozoic cap dolostones, which ubiquitously overlie Marinoan glacial diamictites, may record marine and climatic paleo-environmental conditions at the termination of the largest glacial epoch in Earth's history. Many geochemical indices have been used to interpret cap dolostone formation in the context of extreme climate change in the aftermath of the Marinoan glaciation. However, there are significant debates about whether these geochemical data represent global signals or regional sedimentary and/or diagenetic processes. Here we analyzed cap dolostones from three different continental cratons for their Ca-isotope, Sr-isotope and trace element compositions in order to obtain new insights into formation of the Marinoan cap dolostone and post-Marinoan paleo-environmental conditions (<∼635 Ma). In three globally separated sections from South China (Yangtze Gorges area), North China (Northwest Tarim) and northwest Namibia, a similarly large negative ${\delta }^{44}$Ca excursion (∼0.6‰), coupled to a positive ⁸⁷Sr/⁸⁶Sr excursion, is recorded in the lower part of these cap dolostone successions. In the context of a relatively short duration for Marinoan cap dolostone (∼10⁴ year timescale), we propose that the preservation of both the large negative ${\delta }^{44}$Ca excursion and the positive ⁸⁷Sr/⁸⁶Sr excursion in three widely-separated stratigraphic sections was not caused by globally uniform changes in isotopic compositions of the whole marine Ca and Sr reservoir. Instead, these excursions may have been caused by the addition of terrestrial meltwater and later deglacial runoff, carrying large amounts of Ca and Sr sourced from continental weathering into shallow seawater. A combined diagenetic-mixing model is used to track the coupled ${\delta }^{44}$Ca and ⁸⁷Sr/⁸⁶Sr variations in the cap dolostones. Large inputs of terrestrial meltwater and deglacial runoff under high CO₂ atmospheric conditions in the aftermath of Marinoan glaciation is likely to have supplied abundant Ca, Mg and bicarbonate to shallow shelf seawater, helping facilitate cap dolostone deposition and contributing to the recorded negative Ca isotope excursion.

Introduction

The onset of the Ediacaran Period is characterized by widespread, thin cap dolostone successions which abruptly overlay the glacial diamictite and have been suggested to have formed under the warming of Earth's climate in the aftermath of the Marinoan ‘Snowball Earth’ glaciation (Hoffman et al., 1998, Hoffman et al., 2017). These Marinoan cap dolostone units are characterized by buff-colored, micro-peloidal or microcrystalline dolomite, and show characteristic sedimentary structures including grading, ripples, teepee structures and sheet cracks (e.g., Jiang et al., 2003, Hoffman, 2011). Moreover, the cap dolostones generally preserve microbialite or stromatolite bioherms and/or ‘tubestone’ structures, barite and carbonate seafloor crystal fans (see Hoffman et al., 2017 for a review), and are overlain by dolomitic shale or carbonate (limestone or dolostone). Despite the variable stratigraphic thicknesses of cap carbonates in different sections (Hoffman et al., 2007), there is a consensus that cap dolostone deposition occurred during a major transgression in response to ice melting following the end of the Marinoan glaciation. However, the formation mechanism of the mysterious cap dolostone, which is ubiquitously distributed all over the world, is stilled debated. Based on stratigraphic, sedimentological and geochemical analyses of the cap dolostones, various hypotheses have been proposed for the formation of the cap dolostones, including: rapid deglaciation from a ‘Snowball Earth’ state (Hoffman et al., 1998, Higgins and Schrag, 2003), destabilization of methane hydrates (Jiang et al., 2003), a meltwater ‘plumeworld’ (Shields, 2005) or a condensed sequence from a multiphase transgression (Kennedy and Christie-Blick, 2011).

In addition to the debates on genesis of the Marinoan cap dolostone, there are also some doubts as to its timescale and temporal evolution. A short timeframe of several thousand years for cap dolostone formation was initially proposed based on the ‘Snowball Earth’ model (Hoffman et al., 1998, Higgins and Schrag, 2003). Conversely, much longer timescales of 0.1 Myr to 1 Myr have been suggested based on the preservation of multiple magnetic reversals (Font et al., 2010). Recent modeling or geochemical studies derived an intermediate timescale (∼10⁴ years) for cap dolostone deposition based on carbon cycle and energy models or the chemical composition of post-glacial barite fans (Ridgewell et al., 2003; Crockford et al., 2016, Yang et al., 2017). Moreover, there have been suggestions that the cap dolostone successions in marine basins are diachronous across different paleo-depths or within the sedimentary cross-section, with deep marine settings likely becoming ice-free and forming carbonate earlier than shelf settings (Hoffman et al., 2007, Rose and Maloof, 2010). Meanwhile, semi-diachronous conditions (i.e. diachronous basal cap dolostone and isochronous upper cap dolostone deposition) have been proposed by the plumeworld model, where a meltwater lens precipitated cap dolostones (Shields, 2005). Independent of these models, there is a correlation between the maximum thickness of a cap carbonate succession (sometimes including carbonate units stratigraphically above the cap dolostone) and its paleo-latitude, which could suggest longer durations of carbonate precipitation with earlier onset of ice melting in equatorial regions (Hoffman and Li, 2009) and a potential diachroneity for the basal cap dolostone.

Many stratigraphic and geochemical indices (e.g., C, Sr, Ca, S, ${\mathrm{\Delta }}^{17}$O isotope systems) have been used to track the global environmental change in the aftermath of the Marinoan glaciation (e.g., Jiang et al., 2003, Yoshioka et al., 2003, Kasemann et al., 2014, Crockford et al., 2016). However, geochemical signatures of carbonates were likely susceptible to regional differences in sedimentary and diagenetic conditions across the global marine environment. The lack of systematic research on the behavior of various isotope systems during regional precipitation and diagenetic processes of the cap dolostone compromises the use of these indices to reconstruct the global deglacial atmospheric–oceanic environment. In order to better constrain the genesis of the Marinoan cap dolostone and associated environmental conditions, we studied the evolution of multiple geochemical indices (Ca and radiogenic Sr isotopes, trace element concentrations and ratios) for three widely separated sections, by considering the effects of sedimentary environment and early diagenesis on these proxies in the cap dolostone successions. Ca and radiogenic Sr isotopes of Marinoan cap dolostones from sections in South China, Northwest Namibia and Northwest Tarim consistently exhibit a co-varying trend in the lower cap dolostone successions, which can be well explained with a new diagenetic-mixing model. Based on this model, a novel scenario is proposed for cap dolostone formation, and the marine environment in the aftermath of the Marinoan glaciation.