Comparison of Ediacaran platform and slope δ²³⁸U records in South China: Implications for global-ocean oxygenation and the origin of the Shuram Excursion

Abstract

The Ediacaran Shuram negative carbon isotope excursion (SE) records major paleoceanographic changes during the late Neoproterozoic, possibly linked to a global oceanic oxygenation event, yet its cause(s) remain uncertain. Earlier studies of the upper Ediacaran Doushantuo Formation in South China based on local redox proxies have documented strong spatial redox heterogeneity along shelf-to-basin transects, but variations of δ²³⁸U (a global redox proxy) have not yet been examined in deep-water SE carbonates. In this study, we examined δ²³⁸U variations through the SE in the upper slope Siduping section. Similar to platform SE sections, Siduping exhibits a shift toward higher δ²³⁸U values correlative with the peak of the SE (i.e., maximum negative δ¹³C_carb), confirming inferences of global ocean oxygenation during the SE. This raises an apparent paradox, because a global negative carbon isotope excursion implies net oxidant consumption, requiring an ocean-based oxygenation mechanism. We hypothesize that an increase in the efficiency of phosphorus burial due to a plankton-driven shift from dominantly dissolved organic matter (DOM) cycling to greater particulate organic matter (POM) export depleted the ocean of nutrient phosphorus. By producing a steep redox gradient close to the sediment-water interface, we suggest that ocean oxygenation also triggered a globally simultaneous diagenetic event in which isotopically light δ¹³C_carb was precipitated in authigenic carbonate minerals. This scenario can account for δ²³⁸U differences between shallow-water and deep-water carbonates, which reflect precipitation of relatively larger amounts of authigenic carbonate minerals in shallow-water settings, generating both a larger negative δ¹³C_carb shift and a larger early diagenetic δ²³⁸U offset.

Introduction

Globally distributed mid-Ediacaran carbonate rocks record the largest negative δ¹³C_carb excursion in Earth history, known as the “Shuram excursion” (SE; Burns and Matter, 1993, Grotzinger et al., 2011). The minimum values of seawater are ca. −12‰ and well below the mantle δ¹³C of ca. −5‰, requiring massive inputs of isotopically light carbon to the Earth surface system during the SE. Three major sources of isotopically light carbon have been proposed. First, some studies have argued for secondary alteration scenarios involving either meteoric waters or burial diagenetic fluids, based mainly on observations of positive correlations between δ¹³C_carb and δ¹⁸O_carb (Knauth and Kennedy, 2009, Derry, 2010, Swart and Kennedy, 2012). However, secondary alteration is inherently a local process, which is difficult to reconcile with the global nature of the SE (Grotzinger et al., 2011). Second, global-scale precipitation of ¹³C-depleted early authigenic carbonate minerals below the water-sediment interface has been proposed to account for the SE (Schrag et al., 2013, Cui et al., 2017). Third, the SE may record a global excess of organic carbon oxidation over burial, leading to strongly ¹³C-depleted dissolved inorganic carbon (DIC) (Rothman et al., 2003, Fike et al., 2006; Jiang et al., 2007; McFadden et al., 2008), although the quantity of oxidants required far exceeds what would have been available in the mid-Ediacaran ocean-atmosphere system (Bristow and Kennedy, 2008). A complementary hypothesis that provides an additional oxidant source is weathering of evaporite deposits and reburial of evaporite sulfur as pyrite (Shields et al., 2019). To avoid the problem of an insufficient quantity of oxidants and to account for the heterogeneity of SE δ¹³C_carb records, a model with spatially and temporally variable oxidation of the DOC reservoir was proposed by Li et al. (2017).

The SE is suspected of having a link to the evolution of early animals. The initial diversification of large, mobile, and morphologically complex animals occurred approximately concurrently with the SE (Xiao and Laflamme, 2009, Xiao et al., 2016), and this diversification has been linked to major changes in global-marine redox landscape during the SE (Fike et al., 2006, McFadden et al., 2008). However, the extent of global-ocean redox changes through this critical interval remains in debate. For example, carbon and sulfur isotope data from Fike et al. (2006) and McFadden et al. (2008) suggested widespread oxygenation of the deep ocean during the SE. However, Fe speciation and sulfur isotope data from Canfield et al., 2008, Johnston et al., 2013 and Li et al. (2010) supported a redox-stratified and ferrous-iron-enriched deep ocean, with only limited oxidation of shelf settings during the SE (Shi et al., 2018).

Testing the major hypotheses for the origin of the SE and their implications for contemporaneous ocean-redox and biological evolution will require quantitative data regarding global-marine redox changes during this critical interval. A recent study applied the carbonate uranium isotope proxy to place quantitative constraints on global-marine redox changes based on mid-Ediacaran shallow-water sections from South China, Siberia, and the western United States, concluding that oceanic dissolved oxygen rose to near-modern levels during the SE (Zhang et al., 2019a). However, carbonates in shallow- and deep-marine settings can experience substantially different syndepositional and post-depositional diagenetic effects, potentially leading to fundamental differences in how δ¹³C_carb and δ²³⁸U signals are recorded. In this study, we present new U isotope data from the deep-water Siduping section and compare these results with U isotope records from a coeval shallow carbonate platform section (Jiulongwan) to evaluate their relative fidelity in recording contemporaneous global-ocean redox signals. We then propose a new hypothesis for the cause of abrupt oceanic oxygenation during the SE, based on the appearance of a more efficient ‘biological pump’, and we introduce a biogeochemical model to show how this mechanism could have radically lowered marine P concentrations and thus oxygenated the late Ediacaran ocean. Finally, we consider the implications of our hypothesis for the origin of the SE.