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Proceedings of the National Academy of Sciences of the United States of America | 2016

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

Kimberly V. Lau; Kate Maher; Demir Altiner; Brian M. Kelley; Lee R. Kump; Daniel J. Lehrmann; Juan Carlos Silva-Tamayo; K. L. Weaver; Meiyi Yu; Jonathan L. Payne

Significance The end-Permian mass extinction not only decimated taxonomic diversity but also disrupted the functioning of global ecosystems and the stability of biogeochemical cycles. Explaining the 5-million-year delay between the mass extinction and Earth system recovery remains a fundamental challenge in both the Earth and biological sciences. We use coupled records of uranium concentrations and isotopic compositions to constrain global marine redox conditions across the end-Permian extinction horizon and through the subsequent 17 million years of Earth system recovery. Our finding that the trajectory of biological and biogeochemical recovery corresponds to variations in an ocean characterized by extensive, shallow marine anoxia provides, to our knowledge, the first unified explanation for these observations. 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 238U/235U isotopic compositions (δ238U) 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 δ238U 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 shelves—global 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.


Geological Society of America Bulletin | 2014

Rise and Fall of Late Pleistocene Pluvial Lakes in Response to Reduced Evaporation and Precipitation: Evidence from Lake Surprise, California

Daniel E. Ibarra; Anne E. Egger; K. L. Weaver; Caroline R. Harris; Kate Maher

Widespread late Pleistocene lake systems of the Basin and Range Province indicate substantially greater moisture availability during glacial periods relative to modern times, but the climatic factors that drive changes in lake levels are poorly constrained. To better constrain these climatic forcing factors, we present a new lacustrine paleoclimate record and precipitation estimates for Lake Surprise, a closed basin lake in northeastern California. We combine a detailed analysis of lake hydrography and constitutive relationships describing the water balance to determine the influence of precipitation, evaporation, temperature, and seasonal insolation on past lake levels. At its maximum extent, during the last deglaciation, Lake Surprise covered 1366 km 2 (36%) of the terminally draining Surprise Valley watershed. Using paired radiocarbon and 230 Th-U analyses, we dated shoreline tufa deposits from wave-cut lake terraces in Surprise Valley, California, to determine the hydrography of the most recent lake cycle. This new lake hydrograph places the highest lake level 176 m above the present-day playa at 15.19 ± 0.18 calibrated ka ( 14 C age). This significantly postdates the Last Glacial Maximum (LGM), when Lake Surprise stood at only moderate levels, 65–99 m above modern playa, similar to nearby Lake Lahontan. To evaluate the climatic factors associated with lake-level changes, we use an oxygen isotope mass balance model combined with an analysis of predictions from the Paleoclimate Model Intercomparison Project 3 (PMIP3) climate model ensemble. Our isotope mass balance model predicts minimal precipitation increases of only 2%–18% during the LGM relative to modern, compared to an ∼75% increase in precipitation during the 15.19 ka highstand. LGM PMIP3 climate model simulations corroborate these findings, simulating an average precipitation increase of only 6.5% relative to modern, accompanied by a 28% decrease in total evaporation driven by a 7 °C decrease in mean annual temperature. LGM PMIP3 climate model simulations also suggest a seasonal decoupling of runoff and precipitation, with peak runoff shifting to the late spring–early summer from the late winter–early spring. Our coupled analyses suggest that moderate lake levels during the LGM were a result of reduced evaporation driven by reduced summer insolation and temperatures, not by increased precipitation. Reduced evaporation primed Basin and Range lake systems, particularly smaller, isolated basins such as Surprise Valley, to respond rapidly to increased precipitation during late-Heinrich Stadial 1 (HS1). Post-LGM highstands were potentially driven by increased rainfall during HS1 brought by latitudinally extensive and strengthened midlatitude westerly storm tracks, the effects of which are recorded in the region9s lacustrine and glacial records. These results suggest that seasonal insolation and reduced temperatures have been underinvestigated as long-term drivers of moisture availability in the western United States.


Environmental Science & Technology | 2016

Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit

Anirban Basu; John N. Christensen; Paul W. Reimus; Jeffrey M. Heikoop; Ardyth M. Simmons; Giday WoldeGabriel; Kate Maher; K. L. Weaver; James T. Clay; Donald J. DePaolo

We use uranium (U) isotope ratios to detect and quantify the extent of natural U reduction in groundwater across a roll front redox gradient. Our study was conducted at the Smith Ranch-Highland in situ recovery (ISR) U mine in eastern Wyoming, USA, where economic U deposits occur in the Paleocene Fort Union formation. To evaluate the fate of aqueous U in and adjacent to the ore body, we investigated the chemical composition and isotope ratios of groundwater samples from the roll-front type ore body and surrounding monitoring wells of a previously mined area. The (238)U/(235)U of groundwater varies by approximately 3‰ and is correlated with U concentrations. Fluid samples down-gradient of the ore zone are the most depleted in (238)U and have the lowest U concentrations. Activity ratios of (234)U/(238)U are ∼5.5 up-gradient of the ore zone, ∼1.0 in the ore zone, and between 2.3 and 3.7 in the down-gradient monitoring wells. High-precision measurements of (234)U/(238)U and (238)U/(235)U allow for development of a conceptual model that evaluates both the migration of U from the ore body and the extent of natural attenuation due to reduction. We find that the premining migration of U down-gradient of the delineated ore body is minimal along eight transects due to reduction in or adjacent to the ore body, whereas two other transects show little or no sign of reduction in the down-gradient region. These results suggest that characterization of U isotopic ratios at the mine planning stage, in conjunction with routine geochemical analyses, can be used to identify where more or less postmining remediation will be necessary.


Geochemistry Geophysics Geosystems | 2017

Uranium isotope evidence for an expansion of marine anoxia during the end‐Triassic extinction

Adam B. Jost; Aviv Bachan; Bas van de Schootbrugge; Kimberly V. Lau; K. L. Weaver; Kate Maher; Jonathan L. Payne

The end-Triassic extinction coincided with an increase in marine black shale deposition and biomarkers for photic zone euxinia, suggesting that anoxia played a role in suppressing marine biodiversity. However, global changes in ocean anoxia are difficult to quantify using proxies for local anoxia. Uranium isotopes (δ238U) in CaCO3 sediments deposited under locally well-oxygenated bottom waters can passively track seawater δ238U, which is sensitive to the global areal extent of seafloor anoxia due to preferential reduction of 238U(VI) relative to 235U(VI) in anoxic marine sediments. We measured δ238U in shallow-marine limestones from two stratigraphic sections in the Lombardy Basin, northern Italy, spanning over 400 m. We observe a ∼0.7‰ negative excursion in δ238U beginning in the lowermost Jurassic, coeval with the onset of the initial negative δ13C excursion and persisting for the duration of subsequent high δ13C values in the lower-middle Hettangian stage. The δ238U excursion cannot be realistically explained by local mixing of uranium in primary marine carbonate and reduced authigenic uranium. Based on output from a forward model of the uranium cycle, the excursion is consistent with a 40–100-fold increase in the extent of anoxic deposition occurring worldwide. Additionally, relatively constant uranium concentrations point toward increased uranium delivery to the oceans from continental weathering, which is consistent with weathering-induced eutrophication following the rapid increase in pCO2 during emplacement of the Central Atlantic Magmatic Province. The relative timing and duration of the excursion in δ238U implies that anoxia could have delayed biotic recovery well into the Hettangian stage.


Geochimica et Cosmochimica Acta | 2016

A spatially resolved surface kinetic model for forsterite dissolution

Kate Maher; Natalie Johnson; Ariel Jackson; Laura N. Lammers; Abe B. Torchinsky; K. L. Weaver; Dennis K. Bird; Gordon E. Brown


Archive | 2008

Regional isotopic patterns in granitic rocks of southern Tibet and evolution of crustal structure during the Indo-Asian collision

Donald J. DePaolo; K. L. Weaver; Xuanxue Mo; Zhongdan Zhao; Timothy Harrison


Archive | 2006

Neodymium and strontium isotopic composition of the Strand Fiord Basalts: Cretaceous Volcanism in the Canadian Arctic

K. L. Weaver; John A. Tarduno; Donald J. DePaolo


Geochemistry Geophysics Geosystems | 2017

Uranium isotope evidence for an expansion of marine anoxia during the end-Triassic extinction: END-TRIASSIC URANIUM ISOTOPES

Adam B. Jost; Aviv Bachan; Bas van de Schootbrugge; Kimberly V. Lau; K. L. Weaver; Kate Maher; Jonathan L. Payne


Archive | 2007

Isotopic Exchange in Igneous Zircons

K. L. Weaver; Donald J. DePaolo


Archive | 2005

Sr-isotopic evolution of the mantle

K. L. Weaver; Donald J. DePaolo

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Anne E. Egger

Central Washington University

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Ardyth M. Simmons

Los Alamos National Laboratory

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