Multiple negative molybdenum isotope excursions in the Doushantuo Formation (South China) fingerprint complex redox-related processes in the Ediacaran Nanhua Basin
Introduction
The tempo of marine oxygenation during the Ediacaran Period (635–542 million years ago, or Ma) is debated. It is generally accepted that the shallow ocean was oxygenated throughout the Ediacaran (Lowenstein et al., 2013, and references therein). However, two predominant viewpoints exist for the O2 contents of the deeper waters: (1) always anoxic (Johnston et al., 2013, Sperling et al., 2015) or (2) subject to episodic ocean oxygenation events (OOEs [Fike et al., 2006, McFadden et al., 2008, Kendall et al., 2015, Sahoo et al., 2012, Sahoo et al., 2016]). Resolution of this debate is important to understanding what role – if any – O2 levels in Ediacaran oceans played in controlling the dynamics of early animal evolution (Knoll, 2011, Lenton et al., 2014).
The most commonly invoked evidence for Ediacaran OOEs comes in the form of redox-sensitive element (RSE; e.g., V, Mo, Re, and U) enrichments in shales – foremost from the Doushantuo Formation of South China (Sahoo et al., 2012, Sahoo et al., 2016). In the modern ocean, widespread oxygenation supports large seawater reservoirs of RSE, which enables strong sedimentary RSE accumulation in the anoxic organic-rich marine sediments that cover a small percentage of the ocean floor (e.g., Scott et al., 2008, Sahoo et al., 2012, Partin et al., 2013, Sheen et al., 2018). Intuitively, ancient black shales deposited in the primarily anoxic Precambrian global ocean (Reinhard et al., 2013) and also during episodes of extensive global ocean anoxia during the Cambrian (Gill et al., 2011, Owens et al., 2016) have much lower RSE abundances because widespread burial in anoxic sediments depleted RSE seawater reservoirs. The transition away from a predominantly anoxic Precambrian ocean and toward a well-oxygenated one more similar to today’s is expected to have led to first-order increases in RSE seawater reservoirs and sedimentary RSE accumulation. The geochemical fingerprints of at least the initial, likely transient phases of this transition seem to have been captured in black shales from the Ediacaran Doushantuo Formation (Scott et al., 2008, Sahoo et al., 2012, Sahoo et al., 2016).
Some RSE trends in the Doushantuo Formation are peculiar, causing some researchers to question their straightforward link to Ediacaran ocean oxygenation (e.g., Miller et al., 2017). For example, some RSEs are enriched in Doushantuo shales to levels comparable to those found in only the most RSE-enriched Phanerozoic shales (V in particular, which reaches wt. % abundances [Sahoo et al., 2016]). Furthermore, during the ca. 580 Ma OOE recorded in Doushantuo shales, some RSEs are not enriched at all (e.g., Mo [Sahoo et al., 2016], although pyrite from these shales is enriched in Mo [Gregory et al., 2017]). Lastly, the widespread ocean oxygenation implied by the episodes of RSE enrichment in the Doushantuo Formation does not seem to be supported by some geochemical compilations (e.g., a recent compilation of the Fe speciation record [Sperling et al., 2015]).
Using the Mo isotope paleoredox proxy, we provide new perspective on the sedimentary RSE record from South China. The Mo isotope composition of organic-rich marine shales can be an effective way to track redox changes in Earth’s ancient oceans (see a recent review by Kendall et al., 2017). For example, sedimentary rocks deposited under anoxic and sulfidic (hereafter referred to as euxinic) conditions in restricted basins can sometimes capture the coeval seawater δ98Mo (e.g., in deep portions of the Black Sea [Neubert et al., 2008], Kyllaren fjord [Noordmann et al., 2015], and Lake Rogoznica [Bura-Nakić et al., 2018]). Transfer of the seawater δ98Mo to marine sediments is possible in these settings because nearly all Mo in marine bottom waters can be transferred to underlying sediments. The Mo isotope composition of seawater is a useful parameter because it is thought to be a direct consequence of the relative distribution of oxic versus euxinic conditions on the seafloor (Barling et al., 2001, Arnold et al., 2004). For these reasons, the primary application of the Mo isotope paleoredox proxy to date has been as a tool for estimating global marine redox conditions using ancient sedimentary rocks originally deposited under euxinic conditions (Kendall et al., 2017).
In the majority of modern marine settings, however, including some that are defined as euxinic, near-quantitative transfer of Mo from deep water to sediments does not occur and results in sedimentary δ98Mo that are isotopically lighter than the coeval seawater value (e.g., Arnold et al., 2004, Poulson et al., 2006, Poulson Brucker et al., 2009, Nägler et al., 2011, Noordmann et al., 2015). In these settings, incomplete transfer of Mo from seawater to sediments and the complexation of Mo with Fe oxide minerals (Goldberg et al., 2009, Goldberg et al., 2012), Mn oxide minerals (Wasylenki et al., 2008), and organic matter (King et al., 2018) – as well as the persistence of intermediate thiomolybdate species (Neubert et al., 2008) – results in the preferential retention of lighter-mass Mo isotopes in these sediments. Therefore, a case can be made that an important utility of the Mo isotope paleoproxy rests with tracking these processes – rather than, or in addition to, its value as a proxy tracking global seawater δ98Mo.
We have measured the Mo isotope compositions of the same shale samples from the Doushantuo Formation from South China that yielded the RSE evidence for OOEs (i.e., those analyzed in Sahoo et al., 2012, Sahoo et al., 2016). Redox-sensitive elements, in addition to their sensitivity to global marine redox conditions, are also sensitive to the complexation processes that affect sedimentary δ98Mo (e.g., Morford and Emerson, 1999, Morford et al., 2005, Tribovillard et al., 2006, Scholz et al., 2011, Scholz et al., 2013, Scholz et al., 2018). Therefore, by identifying these complexation processes using Mo isotopes, we can assess their possible contribution to the RSE patterns in the Doushantuo Formation.
Section snippets
The Doushantuo Formation from South China
We targeted shales of the Ediacaran Doushantuo Formation (∼635–560 Ma [Condon et al., 2005, An et al., 2015]) in multiple sections from the Yangtze platform in South China, a paleo-location referred to as the Nanhua Basin (Jiang et al., 2011) (Fig. 1). In order of increasing distance from the paleo-shoreline, we measured Mo isotope compositions of shales originally deposited on the continental slope of the Nanhua Basin from sites at Rongxi, Taoying, and Wuhe and of shales deposited deeper
Results
Molybdenum isotope compositions in shales of the Doushantuo Formation from all slope and basin sections across the purported OOEs (i.e., in Doushantuo Members II, III, and IV) are predominantly negative (as low as −2.24 ± 0.10‰; 2SD; in Member II of the Taoying section) (Fig. 3, Fig. 4). The heaviest measured shale δ98Mo during the OOEs is 1.32 ± 0.15‰ (2SD) and comes from Member IV of the Rongxi section. Maximum δ98Mo in shales deposited during the older OOEs are isotopically lighter:
Discussion
In the following section, we begin by first discussing local processes in the Ediacaran Nanhua Basin that most likely played a role in driving the observed negative Mo isotope compositions in the Doushantuo Formation (Section 4.1). We then discuss how these local processes likely also played some role in governing the RSE patterns found in shales of the Doushantuo Formation (Section 4.2). Lastly, we finish this section by discussing a combination of plausible scenarios that may account for the
Conclusions
Our new Mo isotope data help us argue that local controls in the Ediacaran Nanhua Basin played important roles in driving some of the geochemical trends in black shales of the Doushantuo Formation from South China. In particular, the transient development of an Mn oxide shuttle and changes in the extent of tetrathiomolybdate formation linked to sulfide availability are both supported by the extremely negative shale δ98Mo excursions reported here. Coeval RSE patterns and accompanying Fe
Acknowledgements
We would like to thank Wang Zheng for his help with instrumental analysis at Arizona State University. This research was supported financially by the NSF Frontiers in Earth System Dynamics program award NSF EAR-1338810 (C.M.O., T.W.L., and A.D.A.), the Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant RGPIN-435930 (B.K.), the Earth-Life Transitions Program of the U.S. National Science Foundation (T.W.L. and N.J.P.) and the NASA Astrobiology Institute under
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