Extensive marine anoxia during the terminal Ediacaran Period

Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).The terminal Ediacaran Period witnessed the decline of the Ediacara biota (which may have included many stemgroup animals). To test whether oceanic anoxia might have played a role in this evolutionary event, we measured U isotope compositions (d238U) in sedimentary carbonates from the Dengying Formation of South China to obtain new constraints on the extent of global redox change during the terminal Ediacaran. We found the most negative carbonate d238U values yet reported (−0.95 per mil), which were reproduced in two widely spaced coeval sections spanning the terminal Ediacaran Period (551 to 541 million years ago). Mass balance modeling indicates an episode of extensive oceanic anoxia, during which anoxia covered >21% of the seafloor and most U entering the oceans was removed into sediments below anoxic waters. The results suggest that an expansion of oceanic anoxia and temporal-spatial redox heterogeneity, independent of other environmental and ecological factors, may have contributed to the decline of the Ediacara biota and may have also stimulated animal motility.NASA Exobiology Program || (no. NNX13AJ71G) NSF Frontiers in Earth System Dynamics program || (award EAR-1338810) NASA grant || (no. NNX15AL27G) Natural Sciences and Engineering Research Council of Canada Discovery Grant || (RGPIN-435930). American Association of Petroleum Geologists Grants-In-Aid Program Explorers Club Washington Group Exploration Field Research Grant Carnegie Institution for Scienc

Stratigraphic and Geochemical Investigation of the Mesoproterozoic Atar and El Mreiti Groups, Mauritania: Insights into Carbon Cycling and Ocean Redox Stratification in a Low Oxygen World

The protracted oxygenation of Earth’s surface environments played a critical role in biospheric evolution during the Proterozoic eon. Initial oxygenation began ~2.3 Ga during the Great Oxidation Event, yet Earth’s oceans did not become fully oxygenated until at least the end of the Neoproterozoic—coincident with the first appearance of metazoans in the fossil record. Patterns of environmental change and evolutionary innovation are more complex and less certain, however, in the prolonged period between these two oxygenation thresholds. The late Mesoproterozoic (1.3 to 1.0 Ga) was marked by increasing biospheric oxygen—evidenced by increased carbon isotopic variability and an increase in marine sulfate concentrations—and an increase in diversity among early eukaryotes. Eukaryotic diversification occurred largely in nearshore environments, yet redox proxy investigations of shallow water Mesoproterzoic strata have been limited, leaving us unable to directly examine the coupled evolution of life and environment in the Mesoproterozoic. In this study, I investigate the geochemical record of epicratonic and pericratonic strata of the 1.1 Ga Atar/El Mreiti Group, Mauritania, which were deposited in an epeiric sea during sea level highstand. In Chapter I, I explore carbon isotopic heterogeneities across the epeiric sea, and use trace element substitution in carbonate to relate isotopic heterogeneity to chemically distinct water masses between onshore and offshore environments. In Chapter II, I explore the shale-based redox proxy record (iron speciation, pyrite sulfur isotopes, and trace metal concentrations) of Atar/El Mreiti Group strata. Results suggest that euxinia—at least within substrate pore fluids—was common across the epeiric sea, and that the chemocline intersected the seafloor deep in the craton interior. In Chapter III, I explore the affect of nearshore euxinia on trace metal delivery to the global ocean. Model simulations suggest that expanded epeiric seas in the late Mesoproterozoic could have effectively crashed the oceanic Mo reservoir, with potentially disastrous consequences for early eukaryotes with high biochemical demands for Mo. Ultimately, this study is the first to directly examine redox conditions in late Mesoproterozoic epeiric seas, and provides rare insight into the chemical workings of the global ocean during a critical interval in Earth’s biogeochemical evolution

Uranium isotope dataset from the Lower Mississippian Sunbury Shale, Appalachian Basin

Uranium isotope data from the Lower Mississippian Sunbury Shale, Appalachian Basin, US

Geochemical and Hydrographic Evolution of the Late Devonian Appalachian Seaway: Linking Sedimentation, Redox, and Salinity Across Time and Space

Abstract Continental interiors were flooded by epeiric seas during many intervals of the geologic past. Few modern analogs exist for these environments, however, and basic variables such as redox, salinity, and restriction are difficult to reconstruct in deep time. Despite these challenges, constraining epeiric watermass properties is critical because much of our preserved and accessible sedimentary record was deposited in such settings. Here, we present a four‐dimensional reconstruction of watermass evolution in the Late Devonian Appalachian Seaway of North America. We use combined proxies for sediment supply, paleosalinity, paleoredox, and basin hydrography in six cores through the Upper Devonian Cleveland Shale deposited across a paleo‐depth transect. Cyclic, coupled changes in sedimentation, redox, and salinity are recorded in environments near the Catskill Delta. Additionally, a pronounced salinity gradient was present from low‐brackish conditions near the delta to fully marine conditions in the basin interior, with a lower‐salinity mixing zone recorded across the Cumberland Sill. We also identified two broad sequences—the lower and upper Cleveland Shale—each of which shows distinct watermass signatures. The lower Cleveland Shale records a redox gradient with euxinia only present along the Cumberland Sill, whereas the upper Cleveland Shale records intensification of euxinia (potentially in the photic zone) at all six sites, which may be coincident with the Hangenberg extinction event. Ultimately, this study identifies pronounced epeiric watermass gradients over short timescales (millennia) and distances (hundreds of km or less), highlighting the need for interpreting the geochemistry of epicontinental deposits in the context of basin hydrography and paleosalinity