Rare earth elements and carbon isotope geochemistry of the Doushantuo Formation in South China: Implication for middle Ediacaran shallow marine redox conditions

Abstract

The middle Ediacaran Shuram excursion, the largest negative δ 13 C carb excursion in Earth history, has been interpreted as indirect evidence for episodic oxidation and remineralization of deep ocean DOC (dissolved organic carbon). It has been hypothesized that such oxidation event may have occurred when anoxic DOC-laden deep water was brought to shallow shelves during oceanic upwelling, which is expected to cause localized anoxia in shallow environments. To test this prediction, we systematically analyzed rare earth elements (REE) and δ 13 C carb of the upper Doushantuo Formation carbonates in the Yangtze Gorges area of South China, which were deposited in an inner shelf environment and record a large negative δ 13 C carb excursion correlated to the Shuram event. The REE data show a significant positive shift in Ce/Ce* values, synchronous with a pronounced negative δ 13 C carb shift. This positive Ce/Ce* shift is interpreted to represent an oceanic anoxia event in shallow shelf environments, which may have been caused by the upwelling or impingement of oxygen-depleted and 12 C-enriched deep water onto shelves. This anoxia event coincides with a sharp decline in the abundance and diversity of Ediacaran acanthomorphic acritarchs, raising the possibility that these two geobiological events may be causally related. Carbon and sulfur isotope data indicate that deep oceans were episodically oxygenated and deep ocean DOC progressively remineralized during the Ediacaran Period [1,2]. These remineralization events likely involved upwelling of anoxic DOC-laden water to shallow shelves, impacting on the shallow-water redox conditions and biological evolution. Indeed, the Ediacaran fossil record shows a significant biotic turnover in shallow shelf environments during the middle Ediacaran Period, with the early Ediacaran biota characterized by globally distributed acanthomorphic acritarchs [3], and the late Ediacaran ones by classical Ediacara megafossils [4]. Previous researchers have argued that this turnover may have been driven by a middle Ediacaran glaciation event [5], predation pressure from macrophagous metazoans [3], or the oxygenation of continental shelves that released the ecological impetus for the development of acanthomorphic resting stages [6]. Integrated paleobiological and geochemical data from the Doushantuo Formation in the Yangtze Gorges area suggest that this turnover is coincident with a pronounced negative δ 13 C carb excursion that is correlated with the Shuram excursion [7], and thus may be related to shallow water anoxia resulting from upwelling of deep marine waters. The Doushantuo Formation (635-551 Ma in age, including four lithostratigraphic members I-IV in ascending order; Figure 1) in the Yangtze Gorges area was considered to

Was the Ediacaran Shuram Excursion a globally synchronized early diagenetic event? Insights from methane-derived authigenic carbonates in the uppermost Doushantuo Formation, South China

“…Other data are from Zhou et al (2012). Petrographic studies suggest that samples with high Mg/Ca and Mn/Sr ratios are altered during late dolomitization.…”

Trace element and REE geochemistry of the Ediacaran Doushantuo Formation from Fanjingshan area, northeast Guizhou province, China

“…Y distribution patterns at the top of lower Doushantuo Formation show a positive shift in dCe values, synchronous with a pronounced negative d 13-C carb shift (Zhou et al 2012;Xiong et al in press). This positive Ce anomaly represents an oceanic anoxia event in shallow shelf environments, and might have resulted from an upwelling or impingement of oxygen-depleted and 12 Cenriched deep water into shelves (Zhou et al 2012). Positive Eu anomalies of NASC-normalized REE ?…”

“…Previous studies reveal that rare earth elements (REEs) in carbonate rocks might gain insights into the depositional environmental system such as seawater or freshwater sources (Y/Ho ratios, e.g., Frimmel 2009;Zhao et al 2009;Allwood et al 2010;Yan et al 2010;Nagarajan et al 2011;Meyer et al 2012), oceanic palaeo-redox conditions (Ce anomaly, e.g., Yang et al 1999;Azmy et al 2009;Frimmel 2009;Madhavaraju and Lee 2009;Ge et al 2010;Yan et al 2010;Ling et al 2011;Nagarajan et al 2011;Huang et al 2012;Meyer et al 2012;Zhou et al 2012), hydrothermal vents (Eu anomaly, e.g., Frimmel 2009;Allwood et al 2010;Nagarajan et al 2011;Huang et al 2012;Meyer et al 2012), and paleogeography and depositional models (e.g., Kamber and Webb 2001;Kamber et al 2004). Meanwhile, minor and trace elements during the deposition of carbonate rocks could also provide information about the redox conditions of bottom waters (e.g., Azmy et al 2009;Huang et al 2009Huang et al , 2012Madhavaraju and Lee 2009;Ge et al 2010;Nagendra et al 2011;Meyer et al 2012;Palomares et al 2012), volcanic activities (Sial et al 2010), and palaeoproductivity of surface seawater (Font et al 2006;Nédélec et al 2007).…”

Acanthomorphic acritarchs of the Doushantuo Formation from an upper slope section in northwestern Hunan Province, South China, with implications for early–middle Ediacaran biostratigraphy

“…It has been further speculated that environmental changes (e.g., surface water anoxia, final oxidation of the global ocean) as recorded in EN3 drove the extinction of DPAs (Cohen et al, 2009;Jiang et al, 2007;Zhou et al, 2012). However, the preservation of DPAs is strongly influenced by taphonomic processes and the full stratigraphic range of DPAs may have been underestimated.…”