Johnston David T.

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Johnston
First Name
David T.
ORCID
0000-0002-2487-1084

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  • Article
    Revised microbial and photochemical triple-oxygen isotope effects improve marine gross oxygen production estimates
    (Oxford University Press, 2022-10-12) Sutherland, Kevin M. ; Johnston, David T. ; Hemingway, Jordon D. ; Wankel, Scott D. ; Ward, Collin P.
    AbstractThe biogeochemical fluxes that cycle oxygen (O2) play a critical role in regulating Earth’s climate and habitability. Triple-oxygen isotope (TOI) compositions of marine dissolved O2 are considered a robust tool for tracing oxygen cycling and quantifying gross photosynthetic O2 production. This method assumes that photosynthesis, microbial respiration, and gas exchange with the atmosphere are the primary influences on dissolved O2 content, and that they have predictable, fixed isotope effects. Despite its widespread use, there are major elements of this approach that remain uncharacterized, including the TOI dynamics of respiration by marine heterotrophic bacteria and abiotic O2 sinks such as the photochemical oxidation of dissolved organic carbon (DOC). Here, we report the TOI fractionation for O2 utilization by two model marine heterotrophs and by abiotic photo-oxidation of representative terrestrial and coastal marine DOC. We demonstrate that TOI slopes associated with these processes span a significant range of the mass-dependent domain (λ = 0.499 to 0.521). A sensitivity analysis reveals that even under moderate productivity and photo-oxidation scenarios, true gross oxygen production may deviate from previous estimates by more than 20% in either direction. By considering a broader suite of oxygen cycle reactions, our findings challenge current gross oxygen production estimates and highlight several paths forward to better understanding the marine oxygen and carbon cycles.
  • Preprint
    An evaluation of sedimentary molybdenum and iron as proxies for pore fluid paleoredox conditions
    ( 2018-04) Hardisty, Dalton S. ; Lyons, Timothy W. ; Riedinger, Natascha ; Isson, Terry T. ; Owens, Jeremy D. ; Aller, Robert C. ; Rye, Danny ; Planavsky, Noah J. ; Reinhard, Christopher T. ; Gill, Benjamin C. ; Masterson, Andrew L. ; Asael, Dan ; Johnston, David T.
    Iron speciation and trace metal proxies are commonly applied together in efforts to identify anoxic settings marked by the presence of free sulfide (euxinia) or dissolved iron (ferruginous) in the water column. Here, we use a literature compilation from modern localities to provide a new empirical evaluation of coupled Fe speciation and Mo concentrations as a proxy for pore water sulfide accumulation at non-euxinic localities. We also present new Fe speciation, Mo concentration, and S isotope data from the Friends of Anoxic Mud (FOAM) site in Long Island Sound, which is marked by pore water sulfide accumulation of up to 3 mM beneath oxygen-containing bottom waters. For the operationally defined Fe speciation scheme, ‘highly reactive’ Fe (FeHR) is the sum of pyritized Fe (Fepy) and Fe dominantly present in oxide phases that is available to react with pore water sulfide to form pyrite. Observations from FOAM and elsewhere confirm that Fepy/FeHR from non-euxinic sites is a generally reliable indicator of pore fluid redox, particularly the presence of pore water sulfide. Molybdenum (Mo) concentration data for anoxic continental margin sediments underlying oxic waters but with sulfidic pore fluids typically show authigenic Mo enrichments (2-25 ppm) that are elevated relative to the upper crust (1-2 ppm). However, compilations of Mo concentrations comparing sediments with and without sulfidic pore fluids underlying oxic and low oxygen (non-euxinic) water columns expose non-unique ranges for each, exposing false positives and false negatives. False positives are most frequently found in sediments from low oxygen water columns (for example, Peru Margin), where Mo concentration ranges can also overlap with values commonly found in modern euxinic settings. FOAM represents an example of a false negative, where, despite elevated pore water sulfide concentrations and evidence for active Fe and Mn redox cycling in FOAM sediments, sedimentary Mo concentrations show a homogenous vertical profile across 50 cm depth at 1-2 ppm. A diagenetic model for Mo provides evidence that muted authigenic enrichments are derived from elevated sedimentation rates. Consideration of a range of additional parameters, most prominently pore water Mo concentration, can replicate the ranges of most sedimentary Mo concentrations observed in modern non-euxinic settings. Together, the modern Mo and Fe data compilations and diagenetic model provide a framework for identifying paleo-pore water sulfide accumulation in ancient settings and linked processes regulating seawater Mo and sulfate concentrations and delivery to sediments. Among other utilities, identifying ancient accumulation of sulfide in pore waters, particularly beneath oxic bottom waters, constrains the likelihood that those settings could have hosted organisms and ecosystems with thiotrophy at their foundations.
  • Article
    The Sedimentary Geochemistry and Paleoenvironments Project
    (Wiley, 2021-07-05) Farrell, Úna C. ; Samawi, Rifaat ; Anjanappa, Savitha ; Klykov, Roman ; Adeboye, Oyeleye O. ; Agic, Heda ; Ahm, Anne-Sofie C. ; Boag, Thomas H. ; Bowyer, Fred ; Brocks, Jochen J. ; Brunoir, Tessa N. ; Canfield, Donald E. ; Chen, Xiaoyan ; Cheng, Meng ; Clarkson, Matthew O. ; Cole, Devon B. ; Cordie, David R. ; Crockford, Peter W. ; Cui, Huan ; Dahl, Tais W. ; Mouro, Lucas D. ; Dewing, Keith ; Dornbos, Stephen Q. ; Drabon, Nadja ; Dumoulin, Julie A. ; Emmings, Joseph F. ; Endriga, Cecilia R. ; Fraser, Tiffani A. ; Gaines, Robert R. ; Gaschnig, Richard M. ; Gibson, Timothy M. ; Gilleaudeau, Geoffrey J. ; Gill, Benjamin C. ; Goldberg, Karin ; Guilbaud, Romain ; Halverson, Galen P. ; Hammarlund, Emma U. ; Hantsoo, Kalev G. ; Henderson, Miles A. ; Hodgskiss, Malcolm S. W. ; Horner, Tristan J. ; Husson, Jon M. ; Johnson, Benjamin ; Kabanov, Pavel ; Keller, C. Brenhin ; Kimmig, Julien ; Kipp, Michael A. ; Knoll, Andrew H. ; Kreitsmann, Timmu ; Kunzmann, Marcus ; Kurzweil, Florian ; LeRoy, Matthew A. ; Li, Chao ; Lipp, Alex G. ; Loydell, David K. ; Lu, Xinze ; Macdonald, Francis A. ; Magnall, Joseph M. ; Mänd, Kaarel ; Mehra, Akshay ; Melchin, Michael J. ; Miller, Austin J. ; Mills, N. Tanner ; Mwinde, Chiza N. ; O'Connell, Brennan ; Och, Lawrence M. ; Ossa Ossa, Frantz ; Pagès, Anais ; Paiste, Kärt ; Partin, Camille A. ; Peters, Shanan E. ; Petrov, Peter ; Playter, Tiffany L. ; Plaza-Torres, Stephanie ; Porter, Susannah M. ; Poulton, Simon W. ; Pruss, Sara ; Richoz, Sylvain ; Ritzer, Samantha R. ; Rooney, Alan D. ; Sahoo, Swapan K. ; Schoepfer, Shane D. ; Sclafani, Judith A. ; Shen, Yanan ; Shorttle, Oliver ; Slotznick, Sarah P. ; Smith, Emily F. ; Spinks, Sam ; Stockey, Richard G. ; Strauss, Justin V. ; Stüeken, Eva E. ; Tecklenburg, Sabrina ; Thomson, Danielle ; Tosca, Nicholas J. ; Uhlein, Gabriel J. ; Vizcaíno, Maoli N. ; Wang, Huajian ; White, Tristan ; Wilby, Philip R. ; Woltz, Christina R. ; Wood, Rachel A. ; Xiang, Lei ; Yurchenko, Inessa A. ; Zhang, Tianran ; Planavsky, Noah J. ; Lau, Kimberly V. ; Johnston, David T. ; Sperling, Erik A.
    Geobiology explores how Earth's system has changed over the course of geologic history and how living organisms on this planet are impacted by or are indeed causing these changes. For decades, geologists, paleontologists, and geochemists have generated data to investigate these topics. Foundational efforts in sedimentary geochemistry utilized spreadsheets for data storage and analysis, suitable for several thousand samples, but not practical or scalable for larger, more complex datasets. As results have accumulated, researchers have increasingly gravitated toward larger compilations and statistical tools. New data frameworks have become necessary to handle larger sample sets and encourage more sophisticated or even standardized statistical analyses. In this paper, we describe the Sedimentary Geochemistry and Paleoenvironments Project (SGP; Figure 1), which is an open, community-oriented, database-driven research consortium. The goals of SGP are to (1) create a relational database tailored to the needs of the deep-time (millions to billions of years) sedimentary geochemical research community, including assembling and curating published and associated unpublished data; (2) create a website where data can be retrieved in a flexible way; and (3) build a collaborative consortium where researchers are incentivized to contribute data by giving them priority access and the opportunity to work on exciting questions in group papers. Finally, and more idealistically, the goal was to establish a culture of modern data management and data analysis in sedimentary geochemistry. Relative to many other fields, the main emphasis in our field has been on instrument measurement of sedimentary geochemical data rather than data analysis (compared with fields like ecology, for instance, where the post-experiment ANOVA (analysis of variance) is customary). Thus, the longer-term goal was to build a collaborative environment where geobiologists and geologists can work and learn together to assess changes in geochemical signatures through Earth history.
  • Article
    Multiple sulfur isotope constraints on the modern sulfur cycle
    (Elsevier, 2014-04-16) Tostevin, Rosalie ; Turchyn, Alexandra V. ; Farquhar, James ; Johnston, David T. ; Eldridge, Daniel L. ; Bishop, James K. B. ; McIlvin, Matthew R.
    We present 28 multiple sulfur isotope measurements of seawater sulfate (δ34SSO4δ34SSO4 and Δ33SSO4Δ33SSO4) from the modern ocean over a range of water depths and sites along the eastern margin of the Pacific Ocean. The average measured δ34SSO4δ34SSO4 is 21.24‰ (±0.88‰,2σ±0.88‰,2σ) with a calculated Δ33SSO4Δ33SSO4 of +0.050‰+0.050‰ (±0.014‰,2σ±0.014‰,2σ). With these values, we use a box-model to place constraints on the gross fraction of pyrite burial in modern sediments. This model presents an improvement on previous estimates of the global pyrite burial flux because it does not rely on the assumed value of δ34Spyriteδ34Spyrite, which is poorly constrained, but instead uses new information about the relationship between δ34Sδ34S and δ33Sδ33S in global marine sulfate. Our calculations indicate that the pyrite burial flux from the modern ocean is between 10% and 45% of the total sulfur lost from the oceans, with a more probable range between 20% and 35%.