Syndepositional diagenetic control of molybdenum isotope variations in carbonate sediments from the Bahamas

© 2016 Elsevier B.V. Molybdenum (Mo) isotope variations recorded in black shales provide important constraints on marine paleoredox conditions. However, suitable shales are not ubiquitous in the geologic record. Moreover, reliable reconstruction of Mo isotope records from shales requires deposition from a water column containing very high concentrations of sulfide-a condition which is both rare and difficult to verify with certainty when examining preserved sediments. The utility of Mo isotopic records could be improved if reconstructions were possible using alternative lithologies, such as marine carbonates, which are more abundant in the geologic record.Here, we focus on the role of early diagenesis in determining the Mo isotopic composition preserved in shallow-water carbonate sediments from four push cores collected in different shallow-water depositional environments in the Bahamas. In contrast with carbonate primary precipitates, which generally contain 10 ppm Mo). The extent of this authigenic enrichment appears to be driven by high concentrations of hydrogen sulfide in the porewaters. In cores with the least authigenic Mo enrichment and lowest pore water sulfide, Mo isotopes are ~1-1.2‰ lighter than seawater, while cores with greater Mo enrichments and higher pore water sulfide approach seawater Mo isotope values (2.2-2.5‰), even under oxic bottom water conditions. However, the sensitivity of bulk carbonate δ98Mo to syndepositional diagenetic conditions potentially complicates interpretation of a carbonate Mo isotope paleoredox proxy. Robust reconstruction of seawater Mo isotopic composition from carbonates will thus require the ability to place constraints on early diagenetic conditions of pore waters at the time of deposition. We show that in order to record seawater Mo isotope values, carbonate pore waters must contain 50-100 μM H2Saq, which is achieved only in organic- and sulfide-rich carbonate sediments

Biological effects on uranium isotope fractionation (\u3csup\u3e238\u3c/sup\u3eU/\u3csup\u3e235\u3c/sup\u3eU) in primary biogenic carbonates

© 2018 Elsevier Ltd Determining whether U isotopes are fractionated during incorporation into biogenic carbonates could help to refine the application of 238U/235U in CaCO3 as a robust paleoredox proxy. Recent laboratory experiments have demonstrated that heavy uranium (U) isotopes were preferentially incorporated into abiotic aragonite, with an isotope fractionation of ∼0.10‰ (238U/235U). In contrast, no detectable U isotope fractionation has been observed in most natural primary biogenic carbonates, but the typical measurement precision of these studies was ±0.10‰ and so could not resolve a fractionation of the magnitude observed in the laboratory. To resolve this issue, we have developed a high precision 238U/235U method (±0.02‰ 2 SD) and utilized it to investigate 238U/235U in primary biogenic carbonates including scleractinian corals, calcareous green and red algae, echinoderms, and mollusks, as well as ooids from the Bahamas, Gulf of California, and French Polynesia. Our results reveal that many primary biogenic carbonates indeed fractionate U isotopes during U incorporation, and that this fractionation is in the same direction as observed in abiotic CaCO3 coprecipitation experiments. However, the magnitude of isotope fractionation in biogenic carbonates is often smaller than that predicted by abiotic CaCO3 coprecipitation experiments (0.00–0.09‰ vs. 0.11 ± 0.02‰), suggesting that one or more processes suppress U isotope fractionation during U incorporation into biogenic carbonates. We propose that closed-system behavior due to the isolation of the local calcificiation sites from ambient seawater, and/or kinetic/disequilibrium isotope fractionation caused by carbonate growth kinetics, explains this observation. Our results indicate that U isotope fractionation between biogenic carbonates and seawater might help to constrain U partition coefficients, carbonate growth rates, or seawater chemistry during coprecipitation

Uranium isotope fractionation during coprecipitation with aragonite and calcite

© 2016 Elsevier Ltd. Natural variations in 238U/235U of marine calcium carbonates might provide a useful way of constraining redox conditions of ancient environments. In order to evaluate the reliability of this proxy, we conducted aragonite and calcite coprecipitation experiments at pH ~7.5 and ~8.5 to study possible U isotope fractionation during incorporation into these minerals.Small but significant U isotope fractionation was observed in aragonite experiments at pH ~8.5, with heavier U isotopes preferentially enriched in the solid phase. 238U/235U of dissolved U in these experiments can be fit by Rayleigh fractionation curves with fractionation factors of 1.00007 + 0.00002/-0.00003, 1.00005 ± 0.00001, and 1.00003 ± 0.00001. In contrast, no resolvable U isotope fractionation was observed in an aragonite experiment at pH ~7.5 or in calcite experiments at either pH. Equilibrium isotope fractionation among different aqueous U species is the most likely explanation for these findings. Certain charged U species are preferentially incorporated into calcium carbonate relative to the uncharged U species Ca2UO2(CO3)3(aq), which we hypothesize has a lighter equilibrium U isotope composition than most of the charged species. According to this hypothesis, the magnitude of U isotope fractionation should scale with the fraction of dissolved U that is present as Ca2UO2(CO3)3(aq). This expectation is confirmed by equilibrium speciation modeling of our experiments. Theoretical calculation of the U isotope fractionation factors between different U species could further test this hypothesis and our proposed fractionation mechanism.These findings suggest that U isotope variations in ancient carbonates could be controlled by changes in the aqueous speciation of seawater U, particularly changes in seawater pH, PCO2, Ca2+, or Mg2+ concentrations. In general, these effects are likely to be small (\u3c0.13‰), but are nevertheless potentially significant because of the small natural range of variation of 238U/235U

Shale heavy metal isotope records of low environmental O2 between two Archean Oxidation Events

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ostrander, C. M., Kendall, B., Gordon, G. W., Nielsen, S. G., Zheng, W., & Anbar, A. D. Shale heavy metal isotope records of low environmental O2 between two Archean Oxidation Events. Frontiers in Earth Science, 10, (2022): 833609, https://doi.org/10.3389/feart.2022.833609.Evidence of molecular oxygen (O2) accumulation at Earth’s surface during the Archean (4.0–2.5 billion years ago, or Ga) seems to increase in its abundance and compelling nature toward the end of the eon, during the runup to the Great Oxidation Event. Yet, many details of this late-Archean O2 story remain under-constrained, such as the extent, tempo, and location of O2 accumulation. Here, we present a detailed Fe, Tl, and U isotope study of shales from a continuous sedimentary sequence deposited between ∼2.6 and ∼2.5 Ga and recovered from the Pilbara Craton of Western Australia (the Wittenoom and Mt. Sylvia formations preserved in drill core ABDP9). We find a progressive decrease in bulk-shale Fe isotope compositions moving up core (as low as δ56Fe = –0.78 ± 0.08‰; 2SD) accompanied by invariant authigenic Tl isotope compositions (average ε205TlA = –2.0 ± 0.6; 2SD) and bulk-shale U isotope compositions (average δ238U = –0.30 ± 0.05‰; 2SD) that are both not appreciably different from crustal rocks or bulk silicate Earth. While there are multiple possible interpretations of the decreasing δ56Fe values, many, to include the most compelling, invoke strictly anaerobic processes. The invariant and near-crustal ε205TlA and δ238U values point even more strongly to this interpretation, requiring reducing to only mildly oxidizing conditions over ten-million-year timescales in the late-Archean. For the atmosphere, our results permit either homogenous and low O2 partial pressures (between 10−6.3 and 10−6 present atmospheric level) or heterogeneous and spatially restricted O2 accumulation nearest the sites of O2 production. For the ocean, our results permit minimal penetration of O2 in marine sediments over large areas of the seafloor, at most sufficient for the burial of Fe oxide minerals but insufficient for the burial of Mn oxide minerals. The persistently low background O2 levels implied by our dataset between ∼2.6 and ∼2.5 Ga contrast with the timeframes immediately before and after, where strong evidence is presented for transient Archean Oxidation Events. Viewed in this broader context, our data support the emerging narrative that Earth’s initial oxygenation was a dynamic process that unfolded in fits-and-starts over many hundreds-of-millions of years.This work was supported financially by the NSF Frontiers in Earth System Dynamics program award NSF EAR-1338810 (AA), a Woods Hole Oceanographic Institution Postdoctoral Scholarship (CO), a NSERC Discovery Grant (RGPIN-435930) and the Canada Research Chair program (BK), and a NASA Exobiology award 80NSSC20K0615 (SN)

Mercury abundance and isotopic composition indicate subaerial volcanism prior to the end-Archean “whiff” of oxygen

Funding: This study was supported by National Aeronautics and Space Administration Exobiology Grant NNX16AI37G (R.B.) and by the MacArthur Professorship (J.D.B.) at the University of Michigan. M.A.K. acknowledges support from an Agouron Institute postdoctoral fellowship.Earth’s early atmosphere witnessed multiple transient episodes of oxygenation before the Great Oxidation Event 2.4 billion years ago (Ga) [e.g., A. D. Anbar et al., Science 317, 1903–1906 (2007); M. C. Koehler, R. Buick, M. E. Barley, Precambrian Res. 320, 281–290 (2019)], but the triggers for these short-lived events are so far unknown. Here, we use mercury (Hg) abundance and stable isotope composition to investigate atmospheric evolution and its driving mechanisms across the well-studied “whiff” of O2 recorded in the ∼2.5-Ga Mt. McRae Shale from the Pilbara Craton in Western Australia [A. D. Anbar et al., Science 317, 1903–1906 (2007)]. Our data from the oxygenated interval show strong Hg enrichment paired with slightly negative Δ199Hg and near-zero Δ200Hg, suggestive of increased oxidative weathering. In contrast, slightly older beds, which were evidently deposited under an anoxic atmosphere in ferruginous waters [C. T. Reinhard, R. Raiswell, C. Scott, A. D. Anbar, T. W. Lyons, Science 326, 713–716 (2009)], show Hg enrichment coupled with positive Δ199Hg and slightly negative Δ200Hg values. This pattern is consistent with photochemical reactions associated with subaerial volcanism under intense UV radiation. Our results therefore suggest that the whiff of O2 was preceded by subaerial volcanism. The transient interval of O2 accumulation may thus have been triggered by diminished volcanic O2 sinks, followed by enhanced nutrient supply to the ocean from weathering of volcanic rocks causing increased biological productivity.PostprintPeer reviewe

Molybdenum evidence for expansive sulfidic water masses in ~750Ma oceans

The Ediacaran appearance of large animals, including motile bilaterians, is commonly hypothesized to reflect a physiologically enabling increase in atmospheric and oceanic oxygen abundances (pO2). To date, direct evidence for low oxygen in pre-Ediacaran oceans has focused on chemical signatures in the rock record that reflect conditions in local basins, but this approach is both biased to constrain only shallower basins and statistically limited when we seek to follow the evolution of mean ocean chemical state through time. Because the abundance and isotopic composition of molybdenum (Mo) in organic-rich euxinic sediments can vary in response to changes in global redox conditions, Mo geochemistry provides independent constraints on the global evolution of well-oxygenated environments. Here, we establish a theoretical framework to access global marine Mo cycle in the past from the abundance and isotope composition of ancient seawater. Further, we investigate the ~ 750 Ma Walcott Member of the Chuar Group, Grand Canyon, which accumulated in a rift basin with open connection to the ocean. Iron speciation data from upper Walcott shales indicate that local bottom waters were anoxic and sulfidic, consistent with their high organic content (up to 20 wt.%). Similar facies in Phanerozoic successions contain high concentrations of redox-sensitive metals, but in the Walcott Member, abundances of Mo and U, as well as Mo/TOC (~ 0.5 ppm/wt.%) are low. δ98Mo values also fall well below modern equivalents (0.99 ± 0.13‰ versus ~ 2.35‰ today). These signatures are consistent with model predictions where sulfidic waters cover ~ 1–4% of the global seafloor, corresponding to a ~ 20–80 fold increase compared to the modern ocean. Therefore, our results suggest globally expansive sulfidic water masses in mid-Neoproterozoic oceans, bridging a nearly 700 million-year gap in previous Mo data. We propose that anoxic and sulfidic (euxinic) conditions governed Mo cycling in the oceans even as ferruginous subsurface waters re-appeared 800–750 Ma, and we interpret this anoxic ocean state to reflect a markedly lower atmospheric and oceanic O2 level, consistent with the hypothesis that pO2 acted as an evolutionary barrier to the emergence of large motile bilaterian animals prior to the Ediacaran Period.Organismic and Evolutionary Biolog

Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Johnson, A. C., Ostrander, C. M., Romaniello, S. J., Reinhard, C. T., Greaney, A. T., Lyons, T. W., & Anbar, A. D. Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth. Science Advances, 7(40), (2021): eabj0108, https://doi.org/10.1126/sciadv.abj0108.Evidence continues to emerge for the production and low-level accumulation of molecular oxygen (O2) at Earth’s surface before the Great Oxidation Event. Quantifying this early O2 has proven difficult. Here, we use the distribution and isotopic composition of molybdenum in the ancient sedimentary record to quantify Archean Mo cycling, which allows us to calculate lower limits for atmospheric O2 partial pressures (PO2) and O2 production fluxes during the Archean. We consider two end-member scenarios. First, if O2 was evenly distributed throughout the atmosphere, then PO2 > 10–6.9 present atmospheric level was required for large periods of time during the Archean eon. Alternatively, if O2 accumulation was instead spatially restricted (e.g., occurring only near the sites of O2 production), then O2 production fluxes >0.01 Tmol O2/year were required. Archean O2 levels were vanishingly low according to our calculations but substantially above those predicted for an abiotic Earth system.We would like to thank our funding sources, including FESD “Dynamics of Earth System Oxygenation” (NSF EAR 1338810 to A.D.A.), NASA Earth and Space Science Fellowship awarded to A.C.J. (80NSSC17K0498), NSF EAR PF to A.C.J. (1952809), and WHOI Postdoctoral Fellowship to C.M.O. C.T.R. acknowledges support from the NASA Astrobiology Institute. We also acknowledge support from the Metal Utilization and Selection across Eons (MUSE) Interdisciplinary Consortium for Astrobiology Research, sponsored by the National Aeronautics and Space Administration Science Mission Directorate (19-ICAR19_2-0007)

Multiple episodes of extensive marine anoxia linked to global warming and continental weathering following the latest Permian mass extinction

Copyright © 2018 The Authors. Explaining the ∼5-million-year delay in marine biotic recovery following the latest Permian mass extinction, the largest biotic crisis of the Phanerozoic, is a fundamental challenge for both geological and biological sciences. Ocean redox perturbations may have played a critical role in this delayed recovery. However, the lack of quantitative constraints on the details of Early Triassic oceanic anoxia (for example, time, duration, and extent) leaves the links between oceanic conditions and the delayed biotic recovery ambiguous. We report high-resolution U-isotope (δ238U) data from carbonates of the uppermost Permian to lowermost Middle Triassic Zal section (Iran) to characterize the timing and global extent of ocean redox variation during the Early Triassic. Our δ238U record reveals multiple negative shifts during the Early Triassic. Isotope mass-balance modeling suggests that the global area of anoxic seafloor expanded substantially in the Early Triassic, peaking during the latest Permian to mid-Griesbachian, the late Griesbachian to mid-Dienerian, the Smithian-Spathian transition, and the Early/Middle Triassic transition. Comparisons of the U-, C-, and Sr-isotope records with a modeled seawater PO43- concentration curve for the Early Triassic suggest that elevated marine productivity and enhanced oceanic stratification were likely the immediate causes of expanded oceanic anoxia. The patterns of redox variation documented by the U-isotope record show a good first-order correspondence to peaks in ammonoid extinctions during the Early Triassic. Our results indicate that multiple oscillations in oceanic anoxia modulated the recovery of marine ecosystems following the latest Permian mass extinction

Devonian Rise in Atmospheric Oxygen Correlated to the Radiations of Terrestrial Plants and Large Predatory Fish

The evolution of Earth’s biota is intimately linked to the oxygenation of the oceans and atmosphere. We use the isotopic composition and concentration of molybdenum (Mo) in sedimentary rocks to explore this relationship. Our results indicate two episodes of global ocean oxygenation. The first coincides with the emergence of the Ediacaran fauna, including large, motile bilaterian animals, ca. 550-560 million year ago (Ma), reinforcing previous geochemical indications that Earth surface oxygenation facilitated this radiation. The second, perhaps larger, oxygenation took place around 400 Ma, well after the initial rise of animals and, therefore, suggesting that early metazoans evolved in a relatively low oxygen environment. This later oxygenation correlates with the diversification of vascular plants, which likely contributed to increased oxygenation through the enhanced burial of organic carbon in sediments. It also correlates with a pronounced radiation of large predatory fish, animals with high oxygen demand. We thereby couple the redox history of the atmosphere and oceans to major events in animal evolution.Earth and Planetary SciencesOrganismic and Evolutionary Biolog