Marine anoxia and delayed Earth system recovery after the end-Permian extinction

Abstract

Delayed Earth system recovery following the end-Permian mass extinction is often attributed to severe ocean anoxia. However, the extent and duration of Early Triassic anoxia remains poorly constrained. Here we use paired records of uranium concentrations ([U]) and 238 U/ 235 U isotopic compositions (δ 238 U) of Upper Permian−Upper Triassic marine limestones from China and Turkey to quantify variations in global seafloor redox conditions. We observe abrupt decreases in [U] and δ 238 U across the end-Permian extinction horizon, from ∼3 ppm and −0.15‰ to ∼0.3 ppm and −0.77‰, followed by a gradual return to preextinction values over the subsequent 5 million years. These trends imply a factor of 100 increase in the extent of seafloor anoxia and suggest the presence of a shallow oxygen minimum zone (OMZ) that inhibited the recovery of benthic animal diversity and marine ecosystem function. We hypothesize that in the Early Triassic oceans-characterized by prolonged shallow anoxia that may have impinged onto continental shelvesglobal biogeochemical cycles and marine ecosystem structure became more sensitive to variation in the position of the OMZ. Under this hypothesis, the Middle Triassic decline in bottom water anoxia, stabilization of biogeochemical cycles, and diversification of marine animals together reflect the development of a deeper and less extensive OMZ, which regulated Earth system recovery following the end-Permian catastrophe.paleoredox | uranium isotopes | biogeochemical cycling | carbon isotopes | Early TriassicT he end-Permian mass extinction-the most severe biotic crisis in the history of animal life-was followed by 5 million years of reduced biodiversity (1, 2), limited ecosystem complexity (3), and large perturbations in global biogeochemical cycling (4, 5). Ocean anoxia has long been invoked both as a cause of the extinction (6-8) and as a barrier to rediversification (9). Numerous lines of evidence demonstrate widespread anoxic conditions around the time of the end-Permian mass extinction (e.g., refs. 6 and 10-12). In contrast, the prevalence of anoxia during the 5-to 10-millionyear recovery interval remains poorly constrained (13,14).Reconstructing paleoredox conditions is challenging because some indicators of anoxia characterize only the local conditions of the overlying water column, whereas other indicators may be influenced by confounding factors, such as weathering rates on land. Here, we use paired measurements of [U] and δ 238 U in marine carbonate rocks to differentiate changes in weathering of U from variations in global marine redox conditions. Microbially mediated reduction of U(VI) to U(IV) under anoxic conditions at the sediment−water interface results in a substantial decrease in uranium solubility and a measureable change in 238 U/ 235 U (15-18). Because 238 U is preferentially reduced and immobilized relative to 235 U, the δ 238 U value of seawater U(VI) decreases as the areal extent of bottom water anoxia increases (Fig. S1). Consequently, a global increase in the extent of anoxi...

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

“…Marine carbonate sediments have been demonstrated to record the δ 238 U of seawater, subject to a 0.2‰–0.4‰ offset, which likely reflects incorporation of U(IV) into shallow sediments from anoxic porewaters (Chen et al, ; Romaniello, Herrmann, & Anbar, ; Tissot et al, ). Studies comparing the trends and absolute values of δ 238 U in coeval Permian–Triassic carbonate sediments from around the world have shown excellent agreement, demonstrating that carbonates may provide a robust record of variations in seawater δ 238 U (Brennecka et al, ; Elrick et al, ; Lau et al, ; Zhang, Algeo, Romaniello, et al, ; Zhang, Romaniello, et al, ).…”

“…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, ).…”

Reorganisation of Earth’s biogeochemical cycles briefly oxygenated the oceans 520 Myr ago

“…This is possible due to the long residence time and uniform δ 238 U of uranium in the modern ocean and predicted for the Cambrian ocean (Weyer et al, 2008;Dahl et al, 2014;Tissot and Dauphas, 2015). The δ 238 U proxy has been utilised to track past global ocean redox during three known oceanic anoxic events (Montoya-Pino et al, 2010;Brennecka et al, 2011;Dahl et al, 2014;Elrick et al, 2016;Lau et al, 2016), where anoxic water masses expanded over larger areas of the seafloor and caused negative δ 238 U excursions. Here, we use uranium isotopes to identify a transient global oxygenation episode during the radiation of animals in the Cambrian.…”

Paleoenvironmental reconstruction for termination stage of Superanoxia in Middle Triassic (Anisian) sedimentary sequence of the Mino belt, central Japan

“…Recent chemostratigraphic studies on redox conditions in a wide range of depositional settings (e.g. Grasby, Beauchamp, Embry, & Sanei, ; Huang, Chen, Wignall, & Zhao, ; Lau et al, ; Song et al, ; Tian et al, ; Wignall et al, ) have revealed that the three major anoxic events occurred in the Early Triassic (late Changhsingian–early Dienerian, Smithian–early Spathian and middle Spathian). In addition to the Early Triassic anoxia, the early Middle Triassic (Anisian) anoxic events are recognized and discussed in Japan and South China (Fujisaki et al, ; Huang et al, ; Kubo, Isozaki, & Matsuo, ; Song et al, ; Sugitani & Mimura, ; Wignall et al, ).…”