Uranium isotope evidence for an expansion of anoxia in terminal Ediacaran oceans

Abstract

Anoxic and iron-rich oceanic conditions prevailed throughout most of the Archean and Proterozoic (4000 to c.540 million years ago, Ma), but the oceans are hypothesised to have become progressively oxygen-rich during the Ediacaran-Cambrian transition interval, coincident with the rise of animal life. We utilise the uranium isotope ratio of seawater (238 U/ 235 U; reformulated as  238 U), an effective tracer of oceanic redox conditions, as a proxy for changes in the global proportion of anoxic seafloor. We present a new δ 238 U dataset for carbonate rocks from the Lower Nama Group, Namibia, deposited in a shelf ramp succession during the terminal Neoproterozoic (~550 to ~547 Ma). These data have persistently low δ 238 U (average =-0.81 ± 0.06‰) compared with the signature of modern day seawater. Such low δ 238 U are consistent with enhanced U drawdown from the water column under anoxic conditions, and the preferential export of 'heavy' 238 U to sediments following U(VI)-U(IV) reduction. Placing our results into a steady state ocean box model suggests at least a third of the global seafloor was covered by anoxic bottom waters compared with only 0.3% in today's oxygenated oceans. Comparison with δ 238 U from older sediments deposited in other basins further supports an expansion of anoxic bottom waters towards the end of the Ediacaran. Our data are consistent with an emerging picture of a dominantly anoxic Ediacaran ocean punctuated by brief ocean oxygenation events. In the Nama Group, the transition towards globally widespread anoxic conditions postdates the first appearance of both skeletal metazoans and softbodied fauna of the Nama Assemblage. This suggests that the global expansion of anoxia did not coincide with the decline of the Ediacaran biota, or drive the biotic turnover between the White Sea and Nama Assemblages. The impact of this global redox change on metazoan ecosystems is unclear, since the expansion of anoxia, if contained mainly within deeper waters, may not have impinged significantly upon continental shelves that host the majority of biodiversity.

Global marine redox changes drove the rise and fall of the Ediacara biota

“…Therefore, an episode of pervasive ocean oxygenation across the SE may have been an extrinsic driver either for the emergence of the Ediacara biota during the Avalon assemblage or its diversification in the White Sea assemblage. The subsequent shift to extensive anoxic conditions during the terminal Ediacaran Period coincides with the decline and extinction of the Ediacara biota (Tostevin et al, ; Wei et al, ; Zhang, Xiao, et al, ). Thus, although genetic, environmental, and ecological factors may have played a role in shaping the evolutionary history of early animals, our data suggest that the rise and fall of the Ediacara biota is, on the first order, coupled with the wax and wane of global ocean oxygenation.…”

“…551 − 541 Ma) and in the early Cambrian (during the Cambrian Age 2 at ca. 525 Ma) with a temporary transition to more oxygenated conditions at the Ediacaran−Cambrian boundary (Tostevin et al, ; Wei et al, ; Zhang, Xiao, et al, ). Therefore, this and previous studies confirm that the oceanic redox evolution from the Neoproterozoic to the Paleozoic was not a history of simple and unidirectional oxygenation, but one with rapid perturbations in the relative proportions of anoxic versus oxic waters (Figure ) (Johnston et al, ; Sahoo et al, ; Wood et al, ; Zhang, Xiao, et al, ).…”

“…Compilation of Ediacaran–Cambrian δ 238 U data. Data sources: This study (light blue); Kendall et al, 2015 (black); Zhang, Xiao, et al, 2018 (orange); Wei et al, 2018 (dark green); Tostevin et al, 2019 (light green) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Recent studies provide evidence that an episode of extensive marine anoxia during the terminal Ediacaran Period may have contributed to the decline of the Ediacara biota (Tostevin et al, ; Wei et al, ; Zhang, Xiao, et al, ). However, the cause of the rise of the Ediacara biota during the middle Ediacaran Period remains a subject of intensive debate.…”

“…The U isotope system ( 238 U/ 235 U, denoted as δ 238 U) measured in carbonate sedimentary rocks is a more direct probe of global ocean redox conditions and can be used to place quantitative constraints on the extent of global redox changes (Brennecka, Herrmann, Algeo, & Anbar, ; Clarkson et al, ; Elrick et al, ; Lau et al, ; Tostevin et al, ; Wei et al, ; Zhang, Algeo, Cui, et al, ; Zhang, Algeo, Romaniello, et al, ; Zhang, Romaniello, et al, ; Zhang, Xiao, et al, ). The power of U isotopes as a global proxy derives from the fact that in the modern ocean U has a long residence time, ~500 kyr (Dunk, Mills, & Jenkins, ), and hence, δ 238 U is uniform in the open ocean (e.g., Tissot & Dauphas, ).…”