Tropospheric O2 Research Articles

The role of depositional environment and chemical composition on the triple oxygen isotope ratios of ferromanganese precipitates and their endmember components

Ferromanganese precipitates - crusts, nodules, and hydrothermal deposits – typically exhibit negative Δ'17O values. These deviations are thought to reflect partial incorporation of dissolved atmospheric O2, a component with strongly negative Δ'17O values, seawater oxygen, and deep sea O2 into Mn-oxide minerals, in combination with the other major mineralogical endmember components (Fe-oxyhydroxide and silicate minerals), and kinetic processes such as oxidation at mineral surfaces and biological respiration. Our understanding of the triple oxygen isotope systematics of these materials are limited, however; in the current body of literature, triple oxygen isotope data has only been generated for marine hydrogenetic (i.e., metals accrete directly from seawater) Fe-Mn crusts and Mn nodules. This study evaluates the underlying chemical and environmental factors contributing to the bulk triple oxygen isotopic composition of Fe-Mn precipitates of different genetic types (i.e., hydrothermal, hydrogenetic, mixed-type) from a variety of formation environments (e.g., volcanic arc, pull-apart basin, hotspot, freshwater lake). To improve our understanding of the triple oxygen isotope systematics of natural Fe-Mn precipitates and better constrain the endmember components, we isolate and measure close approximations of the Mn-oxide, Fe-oxyhydroxide, and silicate fractions. Hydrothermal Fe-Mn deposits show greater variation in δ'18O and Δ'17O values (6.0 to 29.6 ‰ and −0.286 to 0.007 ‰, respectively) than hydrogenetic Fe-Mn crusts (5.1 to 11.9 ‰ and −0.200 to −0.086 ‰, respectively). Lacustrine Mn nodule δ'18O and Δ'17O values (5.1 to 8.8 ‰ and −0.230 to −0.090 ‰, respectively) are similar to marine hydrogenetic Fe-Mn crusts, primarily reflecting tropospheric O2. The triple oxygen isotope ratios of nearly pure Mn-oxide samples cluster into distinct groups based on oxygen source, and mass-balance mixing models suggest a significant proportion of incorporated oxygen was photosynthetic in origin. The layered Fe-Mn precipitates in this study show irregular to rhythmic patterns of δ'18O and Δ'17O values depending on formation environment that mostly do not correlate with Mn, Fe, and Si contents. Shale-normalized rare earth element and yttrium (REYSN) patterns and anomalies (CeSN/CeSN*, EuSN/EuSN*, and YSN/HoSN), which can provide insight on formation conditions (e.g., redox, oxygen fugacity, and REY speciation of fluids), are compared with the triple oxygen isotope data. The REYSN anomalies increase from the bottom to the top of the profile of the layered marine sample, suggesting conditions became more oxic over time. Although the triple oxygen isotope ratios correlate with the Eu anomaly in most Fe-Mn precipitates, hydrothermal input can limit their utility as indicators of oxygen fugacity. The triple oxygen isotope ratios of Fe-Mn precipitates reflect complex processes and mixing of multiple sources. Constraining these oxygen sources will assist future studies in interpreting paleoenvironmental conditions (e.g., the saturation state and triple oxygen isotope ratios of dissolved O2 in bottom water) at the time of Fe-Mn precipitate formation.

The triple oxygen isotope composition of marine sulfate and 130 million years of microbial control

The triple oxygen isotope composition (Δ'17O) of sulfate minerals is widely used to constrain ancient atmospheric pO2/pCO2 and rates of gross primary production. The utility of this tool is based on a model that sulfate oxygen carries an isotope fingerprint of tropospheric O2 incorporated through oxidative weathering of reduced sulfur minerals, particularly pyrite. Work to date has targeted Proterozoic environments (2.5 billion to 0.542 billion years ago) where large isotope anomalies persist; younger timescale records, which would ground ancient environmental interpretation in what we know from modern Earth, are lacking. Here we present a high-resolution record of the [Formula: see text]O and Δ'17O in marine sulfate for the last 130 million years of Earth history. This record carries a Δ'17O close to 0o, suggesting that the marine sulfate reservoir is under strict control by biogeochemical cycling (namely, microbial sulfate reduction), as these reactions follow mass-dependent fractionation. We identify no discernible contribution from atmospheric oxygen on this timescale. We interpret a steady fractional contribution of microbial sulfur cycling (terrestrial and marine) over the last 100 million years, even as global weathering rates are thought to vary considerably.

Ferromanganese crusts as recorders of marine dissolved oxygen

The distinct triple oxygen isotope composition of tropospheric O2 relative to seawater is the result of biogeochemical reactions (e.g. primary productivity, respiration), exchange with the stratosphere, and the relative size of different oxygen-containing reservoirs, namely O2, O3, and CO2. This difference in isotopic composition gives tropospheric O2 utility as a record of biogeochemical and atmospheric processes and may also be used for determining where in the rock record isotopic fingerprints of tropospheric oxygen may be preserved. The isotopic record of tropospheric oxygen in previous studies is largely limited to analyses of gas trapped in continental glaciers and a patchwork of other proxies, most notably the triple oxygen signature of sulfate. Here we show the uppermost layers of hydrogenetic, deep-ocean ferromanganese crusts from each of the major ocean basins have a triple oxygen isotope composition consistent with the direct incorporation of dissolved oxygen. The range of δ18O and Δ′17O in ferromanganese crusts suggests the Mn oxide endmember contains a near 50:50 mixture of oxygen from water and dissolved O2. Our data indicate this signal also persists into older layers of the crusts, potentially preserving near 75 million years of the oxygen isotopic composition of the lower troposphere and subsequent deep-ocean respiration. Our analysis of oxygen isotope values, bulk chemistry, and estimated local dissolved oxygen for crust top samples reveals that variations in bulk chemistry ultimately exhibit more influence on the oxygen mass balance than changes in dissolved oxygen, presenting a challenge for unambiguous determination of local dissolved oxygen. Although analytical challenges remain, these widespread, layered deposits of ferromanganese crust may offer a viable path for future interrogation of the history or relative history of the oxygen cycle of the troposphere and deep ocean millions of years into the past.

Measurement of 18 O18 O and 17 O18 O in atmospheric O2 using the 253 Ultra mass spectrometer and applications to stratospheric and tropospheric air samples.

RationaleThe doubly substituted isotopologues (e.g., 18O18O, 17O18O) in atmospheric O2 are potential tracers for ozone photochemistry and atmospheric temperatures. Their low abundances and isobaric interference are the major analytical challenges. The 253 Ultra high‐resolution stable isotope ratio mass spectrometer is suitable for resolving isobaric interferences.MethodsO2 from air is purified using gas chromatography on a packed column filled with molecular sieve 5 Å and cooled to −78°C. The δ17O, δ18O, Δ17O, Δ35 and Δ36 values are measured on the extracted O2 with the 253 Ultra at medium mass resolution (M/ΔM ~10000) using Faraday detectors for the singly substituted isotopologues and ion counters for the doubly substituted isotopologues.ResultsInterferences from isobars, mainly 35Cl for 17O18O and H35Cl and 36Ar for 18O18O, are sufficiently resolved to enable high‐precision determination of Δ35 and Δ36. The Δ35 and Δ36 values of O2 after photochemical isotope equilibration at −63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ35 and Δ36 values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O2 differ from those expected at equilibrium.ConclusionsThe 253 Ultra allows interference‐free clumped isotope measurements of O2 at medium mass resolution. The Δ35 and Δ36 signatures in atmospheric O2 are mainly governed by O3 photochemistry, temperature and atmospheric transport. Tropospheric O2 is isotopically well mixed and retains a significant stratospheric signature.

Tracing the oxygen isotope composition of the upper Earth\u2019s atmosphere using cosmic spherules

Molten I-type cosmic spherules formed by heating, oxidation and melting of extraterrestrial Fe,Ni metal alloys. The entire oxygen in these spherules sources from the atmosphere. Therefore, I-type cosmic spherules are suitable tracers for the isotopic composition of the upper atmosphere at altitudes between 80 and 115 km. Here we present data on I-type cosmic spherules collected in Antarctica. Their composition is compared with the composition of tropospheric O2. Our data suggest that the Earth's atmospheric O2 is isotopically homogenous up to the thermosphere. This makes fossil I-type micrometeorites ideal proxies for ancient atmospheric CO2 levels.

Rapid photochemical equilibration of isotope bond ordering in O2

AbstractThe abundances of 18O18O and 17O18O in the atmosphere were recently found to be enriched relative to the stochastic distribution of isotopes in O2. The enrichment is believed to arise from O(3P) + O2 isotope exchange reactions, which reorder the isotopes in O2 to a distribution that favors bonds between heavy isotopes. Theoretical predictions and laboratory experiments suggest that the reordered distribution of isotopes should reflect internal isotopic equilibrium, but a laboratory test of this hypothesis for the complete O2 isotopologue system has not yet been realized. Here we use a simple photochemical experiment that reorders the isotopes in O2 at temperatures between 200 K and 350 K. Using simultaneous measurements of five O2 isotopologues, we show that O(3P) + O2 reorders the isotopes in O2 to isotopic equilibrium. Furthermore, we use this scheme to calibrate measurements of isotopic ordering in samples of O2, obtaining Δ36 and Δ35 values within ±0.1‰. Measurements of atmospheric O2 sampled at the University of California, Los Angeles, from 2012 to 2014 have mean values of Δ36 = 1.97 ± 0.07‰ and Δ35 = 1.0 ± 0.1‰ (2 SE; n = 23), with no detectable long‐term trend. These measurements are consistent with values for air reported earlier, but with a threefold to fourfold improvement in precision. Together, the experiments and observations support the case that isotopic ordering in tropospheric O2 is altered by O(3P) + O2; however, they also suggest that tropospheric Δ36 and Δ35 values do not reflect complete isotopic equilibration in the troposphere. Isotopic ordering in atmospheric O2 likely reflects the decadal‐scale balance of stratospheric and tropospheric air masses modulated by variations in tropospheric photochemistry and convection.

Gas-phase tropospheric chemistry of 2,3,7,8-tetrafuorinated dibenzo-p-dioxin

Growing attention has been devoted to understanding the formation and destruction of polyfluorinated dibenzo-p-dioxins (PFDDs). High-accuracy molecular orbital calculations have been performed to investigate the tropospheric oxidation reaction of 2,3,7,8-tetrafuorinated dibenzo-p-dioxin (TFDD) initiated by OH radical, NO3 radical and O3. The rate constant of TFDD reaction triggered by the OH radical, NO3 radical and O3 is about 2.30×10−11cm3molecule−ls−l, 3.18×10−13cm3molecule−ls−l, and 3.30×10−19cm3molecule−ls−l, respectively. OH radical is the major gas phase tropospheric sink for TFDD. Once TFDD-OH intermediates are produced in the initial reactions, they can react with tropospheric O2 subsequently to generate peroxy radical isomers. The TFDD-OH-O2 can further react with tropospheric NO via isomerization or combination, resulting that the dioxin ring will be ruptured completely. This study can serve as a template for tropospheric degradation of the gaseous PFDDs, which is beneficial for assessing their tropospheric behaviors.

The loss of mass‐independent fractionation in sulfur due to a Palaeoproterozoic collapse of atmospheric methane

ABSTRACTWe use a 1‐D numerical model to study the atmospheric photochemistry of oxygen, methane, and sulfur after the advent of oxygenic photosynthesis. We assume that mass‐independent fractionation (MIF) of sulfur isotopes – characteristic of the Archean – was best preserved in sediments when insoluble elemental sulfur (S8) was an important product of atmospheric photochemistry. Efficient S8 production requires three things: (i) very low levels of tropospheric O2; (ii) a source of sulfur gases to the atmosphere at least as large as the volcanic SO2 source today; and (iii) a sufficiently high abundance of methane or other reduced gas. All three requirements must be met. We suggest that the disappearance of a strong MIF sulfur signature at the beginning of the Proterozoic is better explained by the collapse of atmospheric methane, rather than by a failure of volcanism or the rise of oxygen. The photochemical models are consistent in demanding that methane decline before O2 can rise (although they are silent as to how quickly), and the collapse of a methane greenhouse effect is consistent with the onset of major ice ages immediately following the disappearance of MIF sulfur. We attribute the decline of methane to the growth of the oceanic sulfate pool as indicated by the widening envelope of mass‐dependent sulfur fractionation through the Archean. We find that a given level of biological forcing can support either oxic or anoxic atmospheres, and that the transition between the anoxic state and the oxic state is inhibited by high levels of atmospheric methane. Transition from an oxygen‐poor to an oxygen‐rich atmosphere occurs most easily when methane levels are low, which suggests that the collapse of methane not only caused the end of MIF S and major ice ages, but it may also have enabled the rise of O2. In this story the early Proterozoic ice ages were ended by the establishment of a stable oxic atmosphere, which protected a renewed methane greenhouse with an ozone shield.

Triple-isotope composition of atmospheric oxygen as a tracer of biosphere productivity

Oxygen has three naturally occurring isotopes, of mass numbers 16, 17 and 18. Their ratio in atmospheric O2 depends primarily on the isotopic composition of photosynthetically produced O2 from terrestrial and aquatic plants1,2,3, and on isotopic fractionation due to respiration4. These processes fractionate isotopes in a mass-dependent way, such that 17O enrichment would be approximately half of the 18O enrichment relative to 16O. But some photochemical reactions in the stratosphere give rise to a mass-independent isotope fractionation, producing approximately equal 17O and 18O enrichments in stratospheric ozone5 and carbon dioxide6,7, and consequently driving an atmospheric O2 isotope anomaly. Here we present an experimentally based estimate of the size of the 17O/16O anomaly in tropospheric O2, and argue that it largely reflects the influences of biospheric cycling and stratospheric photochemical processes. We propose that because the biosphere removes the isotopically anomalous stratosphere-derived O2 by respiration, and replaces it with isotopically ‘normal’ oxygen by photosynthesis, the magnitude of the tropospheric 17O anomaly can be used as a tracer of global biosphere production. We use measurements of the triple-isotope composition of O2 trapped in bubbles in polar ice to estimate global biosphere productivity at various times over the past 82,000 years. In a second application, we use the isotopic signature of oxygen dissolved in aquatic systems to estimate gross primary production on broad time and space scales.

Analysis of the origins and implications of the18O content of stratospheric water vapor

Factors influencing the18O content of stratospheric H2O are reviewed in order to provide a theoretical framework for the interpretation of measurements of this quantity, which are now becoming available. Depletions in18O of 5–10% in stratospheric H2O are expected based on the known correlation between that of D and18O in tropospheric H2O and observed measurements of large (typically 50%) depletions of D in stratospheric H2O. H2O formed in the stratosphere as a result of oxidation of CH4 can be expected to reflect primarily the18O content of stratospheric O2, which is the same as that of tropospheric O2 (slightly enhanced with respect to standard mean ocean water). Thus, a reduction in the18O depletion is expected with increasing altitude, but not a large enhancement in18O in upper stratospheric H2O as found in recent far infrared measurements. The observed large enhancement of18O in stratospheric O3 is not expected to be reflected in stratospheric H2O. Necessary laboratory data for the improved quantification of these effects are reviewed.