skip to main content

Explore Research Products in the NSF-PAR

Title: Triple oxygen isotope constraints on atmospheric O 2 and biological productivity during the mid-Proterozoic
Reconstructing the history of biological productivity and atmospheric oxygen partial pressure ( p O 2 ) is a fundamental goal of geobiology. Recently, the mass-independent fractionation of oxygen isotopes (O-MIF) has been used as a tool for estimating p O 2 and productivity during the Proterozoic. O-MIF, reported as Δ′ 17 O, is produced during the formation of ozone and destroyed by isotopic exchange with water by biological and chemical processes. Atmospheric O-MIF can be preserved in the geologic record when pyrite (FeS 2 ) is oxidized during weathering, and the sulfur is redeposited as sulfate. Here, sedimentary sulfates from the ∼1.4-Ga Sibley Formation are reanalyzed using a detailed one-dimensional photochemical model that includes physical constraints on air–sea gas exchange. Previous analyses of these data concluded that p O 2 at that time was <1% PAL (times the present atmospheric level). Our model shows that the upper limit on p O 2 is essentially unconstrained by these data. Indeed, p O 2 levels below 0.8% PAL are possible only if atmospheric methane was more abundant than today (so that p CO 2 could have been lower) or if the Sibley O-MIF data were diluted by reprocessing before the sulfates were deposited. Our model also shows that, contrary to previous assertions, marine productivity cannot be reliably constrained by the O-MIF data because the exchange of molecular oxygen (O 2 ) between the atmosphere and surface ocean is controlled more by air–sea gas transfer rates than by biological productivity. Improved estimates of p CO 2 and/or improved proxies for Δ′ 17 O of atmospheric O 2 would allow tighter constraints to be placed on mid-Proterozoic p O 2 .  more » « less
Award ID(s):
1920523
NSF-PAR ID:
10323184
Author(s) / Creator(s):
; ; ; ; ; ; ;
Date Published:
Journal Name:
Proceedings of the National Academy of Sciences
Volume:
118
Issue:
51
ISSN:
0027-8424
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; ; ; ; ; ; ( , Journal of Geophysical Research: Atmospheres)
    Abstract

    We present airborne observations of the vertical gradient of atmospheric oxygen (δ(O2/N2)) and carbon dioxide (CO2) through the atmospheric boundary layer (BL) over the Drake Passage region of the Southern Ocean, during the O2/N2Ratio and CO2Airborne Southern Ocean Study, from 15 January to 29 February 2016. Gradients were predominately anticorrelated, with excesses ofδ(O2/N2) and depletions of CO2found within the boundary layer, relative to a mean reference height of 1.7 km. Through analysis of the molar ratio of the gradients (GR), the behavior of other trace gases measured in situ, and modeling experiments with the Community Earth System Model, we found that the main driver of gradients was air‐sea exchange of O2and CO2driven by biological processes, more so than solubility effects. An exception to this was in the eastern Drake Passage, where positive GRs were occasionally observed, likely due to the dominance of thermal forcing on the air‐sea flux of both species. GRs were more spatially consistent than the magnitudes of the gradients, suggesting that GRs can provide integrated process constraints over broad spatial scales. Based on the model simulation within a domain bounded by 45°S, 75°S, 100°W, and 45°W, we show that the sampling density of the campaign was such that the observed mean GR (± standard error), −4.0± 0.8 mol O2per mol CO2, was a reasonable proxy for both the mean GR and the mean molar ratio of air‐sea fluxes of O2and CO2during the O2/N2Ratio and CO2Airborne Southern Ocean Study.

     
    more » « less
  2. ; ; ; ; ; ; ( , Geobiology)
    Abstract

    The potent greenhouse gas nitrous oxide (N2O) may have been an important constituent of Earth's atmosphere during Proterozoic (~2.5–0.5 Ga). Here, we tested the hypothesis that chemodenitrification, the rapid reduction of nitric oxide by ferrous iron, would have enhanced the flux of N2O from ferruginous Proterozoic seas. We empirically derived a rate law,, and measured an isotopic site preference of +16‰ for the reaction. Using this empirical rate law, and integrating across an oceanwide oxycline, we found that lownM NOand μM‐lowmMFe2+concentrations could have sustained a sea‐air flux of 100–200 Tg N2O–N year−1, if N2fixation rates were near‐modern and all fixed N2was emitted as N2O. A 1D photochemical model was used to obtain steady‐state atmospheric N2O concentrations as a function of sea‐air N2O flux across the wide range of possiblepO2values (0.001–1PAL). At 100–200 Tg N2O–N year−1and >0.1PALO2, this model yielded low‐ppmv N2O, which would produce several degrees of greenhouse warming at 1.6 ppmvCH4and 320 ppmvCO2. These results suggest that enhanced N2O production in ferruginous seawater via a previously unconsidered chemodenitrification pathway may have helped to fill a Proterozoic “greenhouse gap,” reconciling an ice‐free Mesoproterozoic Earth with a less luminous early Sun. A particularly notable result was that high N2O fluxes at intermediate O2concentrations (0.01–0.1PAL) would have enhanced ozone screening of solarUVradiation. Due to rapid photolysis in the absence of an ozone shield, N2O is unlikely to have been an important greenhouse gas if Mesoproterozoic O2was 0.001PAL. At low O2, N2O might have played a more important role as life's primary terminal electron acceptor during the transition from an anoxic to oxic surface Earth, and correspondingly, from anaerobic to aerobic metabolisms.

     
    more » « less
  3. ; ; ; ; ; ; ; ; ( , Global Biogeochemical Cycles)
    Abstract

    To examine seasonal and regional variabilities in metabolic status and the coupling of net community production (NCP) and air‐sea CO2fluxes in the western Arctic Ocean, we collected underway measurements of surface O2/Ar and partial pressure of CO2(pCO2) in the summers of 2016 and 2018. With a box‐model, we demonstrate that accounting for local sea ice history (in addition to wind history) is important in estimating NCP from biological oxygen saturation (Δ(O2/Ar)) in polar regions. Incorporating this sea ice history correction, we found that most of the western Arctic exhibited positive Δ(O2/Ar) and negativepCO2saturation, Δ(pCO2), indicative of net autotrophy but with the relationship between the two parameters varying regionally. In the heavy ice‐covered areas, where air‐sea gas exchange was suppressed, even minor NCP resulted in relatively high Δ(O2/Ar) and lowpCO2in water due to limited gas exchange. Within the marginal ice zone, NCP and CO2flux magnitudes were strongly inversely correlated, suggesting an air to sea CO2flux induced primarily by biological CO2removal from surface waters. Within ice‐free waters, the coupling of NCP and CO2flux varied according to nutrient supply. In the oligotrophic Canada Basin, NCP and CO2flux were both small, controlled mainly by air‐sea gas exchange. On the nutrient‐rich Chukchi Shelf, NCP was strong, resulting in great O2release and CO2uptake. This regional overview of NCP and CO2flux in the western Arctic Ocean, in its various stages of ice‐melt and nutrient status, provides useful insight into the possible biogeochemical evolution of rapidly changing polar oceans.

     
    more » « less
  4. ; ( , Paleoceanography and Paleoclimatology)
    Abstract

    All else equal, if the ocean's “biological [carbon] pump” strengthens, the dissolved oxygen (O2) content of the ocean interior declines. Confidence is now high that the ocean interior as a whole contained less oxygen during the ice ages. This is strong evidence that the ocean's biological pump stored more carbon in the ocean interior during the ice ages, providing the core of an explanation for the lower atmospheric carbon dioxide (CO2) concentrations of the ice ages. Vollmer et al. (2022,https://doi.org/10.1029/2021PA004339) combine proxies for the oxygen and nutrient content of bottom waters to show that the ocean nutrient reservoir was more completely harnessed by the biological pump during the Last Glacial Maximum, with an increase in the proportion of dissolved nutrients in the ocean interior that were “regenerated” (transported as sinking organic matter from the ocean surface to the interior) rather than “preformed” (transported to the interior as dissolved nutrients by ocean circulation). This points to changes in the Southern Ocean, the dominant source of preformed nutrients in the modern ocean, with an apparent additional contribution from a decline in the preformed nutrient content of North Atlantic‐formed interior water. Vollmer et al. also find a lack of LGM‐to‐Holocene difference in the preformed13C/12C ratio of dissolved inorganic carbon. This finding may allow future studies to resolve which of the proposed Southern Ocean mechanisms was most responsible for enhanced ocean CO2storage during the ice ages: (a) coupled changes in ocean circulation and biological productivity, or (b) physical limitations on air‐sea gas exchange.

     
    more » « less
  5. ; ; ; ; ; ( , Journal of Geophysical Research: Biogeosciences)
    Abstract

    The ratio of dissolved oxygen to argon in seawater is frequently employed to estimate rates of net community production (NCP) in the oceanic mixed layer. The in situ O2/Ar‐based method accounts for many physical factors that influence oxygen concentrations, permitting isolation of the biological oxygen signal produced by the balance of photosynthesis and respiration. However, this technique traditionally relies upon several assumptions when calculating the mixed‐layer O2/Ar budget, most notably the absence of vertical fluxes of O2/Ar and the principle that the air‐sea gas exchange of biological oxygen closely approximates net productivity rates. Employing a Lagrangian study design and leveraging data outputs from a regional physical oceanographic model, we conducted in situ measurements of O2/Ar in the California Current Ecosystem in spring 2016 and summer 2017 to evaluate these assumptions within a “worst‐case” field environment. Quantifying vertical fluxes, incorporating nonsteady state changes in O2/Ar, and comparing NCP estimates evaluated over several day versus longer timescales, we find differences in NCP metrics calculated over different time intervals to be considerable, also observing significant potential effects from vertical fluxes, particularly advection. Additionally, we observe strong diel variability in O2/Ar and NCP rates at multiple stations. Our results reemphasize the importance of accounting for vertical fluxes when interpreting O2/Ar‐derived NCP data and the potentially large effect of nonsteady state conditions on NCP evaluated over shorter timescales. In addition, diel cycles in surface O2/Ar can also bias interpretation of NCP data based on local productivity and the time of day when measurements were made.

     
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email