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...
A negative shift in the calcium isotopic composition of marine carbonate rocks spanning the end-Permian extinction horizon in South China has been used to argue for an ocean acidification event coincident with mass extinction. This interpretation has proven controversial, both because the excursion has not been demonstrated across multiple, widely separated localities, and because modeling results of coupled carbon and calcium isotope records illustrate that calcium cycle imbalances alone cannot account for the full magnitude of the isotope excursion. Here, we further test potential controls on the Permian-Triassic calcium isotope record by measuring calcium isotope ratios from shallow-marine carbonate successions spanning the Permian-Triassic boundary in Turkey, Italy, and Oman. All measured sections display negative shifts in δ 44/40 Ca of up to 0.6‰. Consistency in the direction, magnitude, and timing of the calcium isotope excursion across these widely separated localities implies a primary and global δ 44/40 Ca signature. Based on the results of a coupled box model of the geological carbon and calcium cycles, we interpret the excursion to reflect a series of consequences arising from volcanic CO 2 release, including a temporary decrease in seawater δ 44/40 Ca due to short-lived ocean acidification and a more protracted increase in calcium isotope fractionation associated with a shift toward more primary aragonite in the sediment and, potentially, subsequently elevated carbonate saturation states caused by the persistence of elevated CO 2 delivery from volcanism. Locally, changing balances between aragonite and calcite production are sufficient to account for the calcium isotope excursions, but this effect alone does not explain the globally observed negative excursion in the δ 13 C values of carbonate sediments and organic matter as well. Only a carbon release event and related geochemical consequences are consistent both with calcium and carbon isotope data. The carbon release scenario can also account for oxygen isotope evidence for dramatic and protracted global warming as well as paleontological evidence for the preferential extinction of marine animals most susceptible to acidification, warming, and anoxia.
34The geological calcium cycle is linked to the geological carbon cycle through the 35 weathering and burial of carbonate rocks. As a result, calcium (Ca)
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