Anbar
Ariel D.
Anbar
Ariel D.
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ArticleShale heavy metal isotope records of low environmental O2 between two Archean Oxidation Events(Frontiers Media, 2022-04-26) Ostrander, Chadlin ; Kendall, Brian ; Gordon, Gwyneth W. ; Nielsen, Sune G. ; Zheng, Wang ; Anbar, Ariel D.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.
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ArticleTechnical comment on "Reexamination of 2.5-Ga 'whiff' of oxygen interval points to anoxic ocean before GOE"(American Association for the Advancement of Science, 2023-03-03) Anbar, Ariel D. ; Buick, Roger ; Gordon, Gwyneth W. ; Johnson, Aleisha C. ; Kendall, Brian ; Lyons, Timothy W. ; Ostrander, Chadlin M. ; Planavsky, Noah J. ; Reinhard, Christopher T. ; Stüeken, Eva E.Many lines of inorganic geochemical evidence suggest transient "whiffs" of environmental oxygenation before the Great Oxidation Event (GOE). Slotznickassert that analyses of paleoredox proxies in the Mount McRae Shale, Western Australia, were misinterpreted and hence that environmental Olevels were persistently negligible before the GOE. We find these arguments logically flawed and factually incomplete.
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ArticleRecurrent photic zone euxinia limited ocean oxygenation and animal evolution during the Ediacaran.(Nature Research, 2023-07-03) Zheng, Wang ; Zhou, Anwen ; Sahoo, Swapan K. ; Nolan, Morrison R. ; Ostrander, Chadlin M. ; Sun, Ruoyu ; Anbar, Ariel D. ; Xiao, Shuhai ; Chen, JiubinThe Ediacaran Period (~635–539 Ma) is marked by the emergence and diversification of complex metazoans linked to ocean redox changes, but the processes and mechanism of the redox evolution in the Ediacaran ocean are intensely debated. Here we use mercury isotope compositions from multiple black shale sections of the Doushantuo Formation in South China to reconstruct Ediacaran oceanic redox conditions. Mercury isotopes show compelling evidence for recurrent and spatially dynamic photic zone euxinia (PZE) on the continental margin of South China during time intervals coincident with previously identified ocean oxygenation events. We suggest that PZE was driven by increased availability of sulfate and nutrients from a transiently oxygenated ocean, but PZE may have also initiated negative feedbacks that inhibited oxygen production by promoting anoxygenic photosynthesis and limiting the habitable space for eukaryotes, hence abating the long-term rise of oxygen and restricting the Ediacaran expansion of macroscopic oxygen-demanding animals.
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ArticleReconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth(American Association for the Advancement of Science, 2021-09-29) Johnson, Aleisha C. ; Ostrander, Chadlin M. ; Romaniello, Stephen J. ; Reinhard, Christopher T. ; Greaney, Allison T. ; Lyons, Timothy W. ; Anbar, Ariel D.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.
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ArticleUp in smoke: most aerosolized Fe from biomass burning does not derive from foliage(American Geophysical Union, 2023-08-25) Tegler, Logan A. ; Sherry, Alyssa M. ; Herckes, Pierre ; Romaniello, Stephen J. ; Anbar, Ariel D.Iron (Fe) is a limiting micronutrient in many marine ecosystems. The lack of sufficient Fe can stunt marine productivity and limit carbon sequestration from the atmosphere to the ocean. Recent studies suggest that biomass burning represents an important Fe source to the marine environment because pyrogenic particles have enhanced solubility after atmospheric processing. We examined foliage representative of four distinct biomes subject to frequent burning events, including boreal/temporal forests, humid tropical, arid tropical, and grassland. We burned these samples in the absence of soil to isolate the Fe from the fine particle (PM2.5) fraction that is derived directly from the burning foliage. We find that <1.5% of the Fe in plant matter is aerosolized throughout the burn in the fine fraction. We estimate that between 2% and 9% of the Fe released from biomass burning can be attributed to the fine fraction of the foliage itself, and <50% from the foliage overall. Most of the Fe aerosolized during biomass burning is accounted for by soil-suspended particles.