Abstract
The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO 2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~ 750 Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20 wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~ 0.5 ppm/wt.%) are low. δ 98 Mo values also fall well below modern equivalents (0.99 ± 0.13‰ versus ~ 2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1–4% of the global seafloor, corresponding to a ~ 20–80 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800–750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O 2 level, consistent with the hypothesis that pO 2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.
Highlights
Most evidence suggests that the atmosphere contained essentially no O2 prior to the so-called "Great Oxidation Event" (GOE) 2.45-2.3 Ga [1, 2]
We propose that anoxic and sulfidic conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800-750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O2 level, consistent with the hypothesis that pO2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran
Increased sulfide production led to the expansion of sulfide into broad regions of the global ocean [6], perhaps especially in oxygen-minimum zone (OMZ) settings [7]
Summary
Most evidence suggests that the atmosphere contained essentially no O2 prior to the so-called "Great Oxidation Event" (GOE) 2.45-2.3 Ga [1, 2]. A variety of chemical proxies signal a rise in atmospheric O2 concentrations at this time [2, 3], to levels which probably remained well below present concentrations [4, 5]. Current models propose higher atmospheric oxygen levels promoted increased continental weathering of sulfide to sulfate, increasing the flux of sulfate to the ocean and stimulating marine sulfide production by sulfate reduction in low oxygen bottom waters. Increased sulfide production led to the expansion of sulfide into broad regions of the global ocean [6], perhaps especially in oxygen-minimum zone (OMZ) settings [7]. Mo chemistry provides a potentially powerful way of evaluating the history of euxinic water masses, because Mo reactivity and Mo burial rate dramatically increase in the presence of H2S. Available analyses remain limited and no data have been reported for rocks deposited between 1400 and 663 Ma
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