Widespread seafloor anoxia during generation of the Ediacaran Shuram carbon isotope excursion

Abstract

Reconstructing the oxygenation history of Earth's oceans during the Ediacaran period (635 to 539 million years ago) has been challenging, and this has led to a polarizing debate about the environmental conditions that played host to the rise of animals. One focal point of this debate is the largest negative inorganic C‐isotope excursion recognized in the geologic record, the Shuram excursion, and whether this relic tracks the global‐scale oxygenation of Earth's deep oceans. To help inform this debate, we conducted a detailed geochemical investigation of two siliciclastic‐dominated successions from Oman deposited through the Shuram Formation. Iron speciation data from both successions indicate formation beneath an intermittently anoxic local water column. Authigenic thallium (Tl) isotopic compositions leached from both successions are indistinguishable from bulk upper continental crust (ε205TlA ≈ −2) and, by analogy with modern equivalents, likely representative of the ancient seawater ε205Tl value. A crustal seawater ε205Tl value requires limited manganese (Mn) oxide burial on the ancient seafloor, and by extension widely distributed anoxic sediment porewaters. This inference is supported by muted redox‐sensitive element enrichments (V, Mo, and U) and consistent with some combination of widespread (a) bottom water anoxia and (b) high sedimentary organic matter loading. Contrary to a classical hypothesis, our interpretations place the Shuram excursion, and any coeval animal evolutionary events, in a predominantly anoxic global ocean.

Deep‐water first occurrences of Ediacara biota prior to the Shuram carbon isotope excursion in the Wernecke Mountains, Yukon, Canada

“…As noted by Ostrander et al. (2023) and Rooney et al. (2020), an older age for the Shuram CIE means there is relatively little geochemical data from the actual time the White Sea biota flourished (at least based on published ages from siliciclastic successions in Russia (Martin et al., 2000)).…”

“…This interpretation is primarily based on the observation that sediments deposited under ferruginous conditions often show a similar and negligible fractionation in uranium isotopes from seawater compared to oxic sediments (Cole, Planavsky, et al., 2020). Thallium isotopes, another redox‐sensitive isotopic proxy, also do not show evidence for oxygenation either across the Shuram CIE or the Ediacaran more broadly (Ostrander et al., 2020, 2023; but see alternate interpretations of data from South China in Fan et al. (2020) and subsequent discussion in Ostrander et al.…”

“…This interpretation is primarily based on the observation that sediments deposited under ferruginous conditions often show a similar and negligible fractionation in uranium isotopes from seawater compared to oxic sediments . Thallium isotopes, another redox-sensitive isotopic proxy, also do not show evidence for oxygenation either across the Shuram CIE or the Ediacaran more broadly (Ostrander et al, 2020(Ostrander et al, , 2023; but see alternate interpretations of data from South China in Fan et al (2020) and subsequent discussion in Ostrander et al (2020)). Thus, although there may have been minor oxygenation in the middle Ediacaran (that could have still been biologically meaningful; Sperling, Knoll, & Girguis, 2015), current interpretations of geochemical data do not support widespread oxygenation.…”

Cryogenian and Ediacaran integrative stratigraphy, biotas, and paleogeographical evolution of the Qinghai-Tibetan Plateau and its surrounding areas

“…Supported by the U and Tl elemental data obtained from the corresponding strata of the DOUNCE interval, it has been suggested that the redox conditions of the seafloor approached present-day oxygen levels (Zhang et al, 2019;Fan et al, 2020). However, an alternative study utilizing iron speciation and Tl isotopic analysis in Oman and Canada has indicated that the global ocean during the DOUNCE event was mainly anoxic (Ostrander et al, 2023). In summary, although the redox condition of the Ediacaran Ocean exhibited regional heterogeneities as revealed by geochemical datasets of the multiple proxies and modeling studies, there is a discernible trend of increasing oxidation levels toward the middle and late Ediacaran times, and the continuous increasing of oxygenation is one of the crucial factors in the stimulated Ediacaran biotic innovation and radiation.…”

Physical constraints during Snowball Earth drive the evolution of multicellularity

“…However, recent work suggests that sponges, a likely morphology for the last common metazoan ancestor, can survive oxygen levels as low as those present during the Neoproterozoic era [10], suggesting that low oxygen levels may not have been a physical constraint preventing the emergence of multicellular eukaryotes. Furthermore, other work suggests that the evolution of more complex eukaryotes, including multicellular organisms, could have led to ocean oxygenation [11] (as opposed to the other way around), and we know that multicellular eukaryotes can cope with low oxygen given that it is likely that the sea floor was anoxic when the first undisputed metazoan fossils appeared in deep water [12][13][14]. If the appearance of multicellularity was not caused by changing oxygen levels, an alternative mechanism for why multicellular eukaryotes emerged during this period is needed.…”