Preprints
https://doi.org/10.5194/bg-2020-435
https://doi.org/10.5194/bg-2020-435

  06 Jan 2021

06 Jan 2021

Review status: a revised version of this preprint was accepted for the journal BG and is expected to appear here in due course.

Advancing on large-scale trends of apparent organic matter reactivity in marine sediments and patterns of benthic carbon transformation

Felipe S. Freitas1,3,a, Philip A. Pika2,4,b, Sabine Kasten5,6,7, Bo B. Jørgensen8, Jens Rassmann9, Christophe Rabouille9, Shaun Thomas10,c, Henrik Sass10, Richard D. Pancost1,3, and Sandra Arndt4 Felipe S. Freitas et al.
  • 1Organic Geochemistry Unit, School of Earth Sciences & School of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom
  • 2BRIDGE, School of Geographical Sciences, University of Bristol, Bristol, BS8 1RL, United Kingdom
  • 3Cabot Institute for the Environment, University of Bristol, Bristol, BS8 1UH, United Kingdom
  • 4Biogeochemistry and Earth System Modeling, Geosciences, Environment and Society Department, Université Libre de Bruxelles, Brussels, CP160/03 1050, Belgium
  • 5Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Bremerhaven, 27570, Germany
  • 6Faculty of Geosciences, University of Bremen, 28359 Bremen, Germany
  • 7MARUM – Center for Marine Environmental Sciences, University of Bremen, 28359 Bremen, Germany
  • 8Section for Microbiology, Department of Biology, Aarhus University, 8000 Aarhus C, Denmark
  • 9Laboratoiredes Sciences du Climat et de l'Environnement, LSCE/IPSL, CEA-CNRS-UVSQ-Université Paris Saclay, 91198 Gif-sur-Yvette, France
  • 10School of Earth and Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, United Kingdom
  • acurrent address: School of Earth Sciences, University of Bristol, Bristol, BS8 1RJ, United Kingdom
  • bcurrent address: Department of Earth Sciences, VU University of Amsterdam, 1081 HV Amsterdam, the Netherlands
  • ccurrent address: RSR Ltd, Parc Ty Glas, Llanishen, Cardiff, CF14 5DU, United Kingdom

Abstract. Constraining the mechanisms that control organic matter (OM) reactivity and, thus, degradation, preservation and burial in marine sediments across spatial and temporal scales is key to understanding carbon cycling in the past, present, and future. However, we still lack a quantitative understanding of what controls OM reactivity in marine sediments and, as a result, how to constrain it in global models. To fill this gap, we quantify apparent OM reactivity (i.e., model-derived estimates) by extracting reactive continuum model parameters (a and v) from observed benthic organic carbon and sulfate dynamics across 14 contrasting depositional settings distributed over five distinct benthic provinces. Our analysis shows that the large-scale range in apparent OM reactivity is largely driven by the wide variability in parameter a (10−3 < a < 107) with a high frequency of values in the range 100 < a < 104 years. In contrast, inversely determined v-values fall within a narrow range (0.1 < v < 0.2). Results also show that the variability in parameter a and, thus, in apparent OM reactivity is a function of the whole depositional environment, rather than the traditionally proposed, single environmental controls (e.g., water depth, sedimentation rate, OM fluxes). Thus, we caution against the simplifying use of a single environmental predictor for apparent OM reactivity beyond a specific local environmental context. In addition, diagenetic model results also indicate that, while OM fluxes exert a dominant control on depth-integrated OM degradation rates across most depositional environments, apparent OM reactivity becomes a dominant control in depositional environments that receive exceptionally reactive OM. Model results also show that apparent OM reactivity largely controls the relative significance of OM degradation pathways, and thus the redox zonation of the sediment, as well as depth of the sulfate-methane transition zone and rates of anaerobic oxidation of methane. Consequently, apparent OM reactivity also determines uptake and consumption of benthic terminal electron acceptors and nutrient recycling fluxes across a wide range of different depositional environments. In summary, our large-scale assessment not only further support the notion of apparent OM reactivity as a dynamic ecosystem property and highlights its crucial role for benthic biogeochemical cycling and exchange, but it also provides the first quantitative constraint on the most plausible range of reactivity parameters a and v. It thus represents an important advance for model parameterization as it largely alleviates the difficulty of determining OM reactivity in such models by constraining it to only one variable, i.e. the parameter a.

Felipe S. Freitas et al.

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Status: closed

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Felipe S. Freitas et al.

Felipe S. Freitas et al.

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Short summary
It remains unclear what controls carbon burial in marine sediments. Thus, we combined model and data analyses to identify patterns of organic matter reactivity at the seafloor on a large-scale. We found large spatial variability on organic matter reactivity, which results from complex regional environmental factors. Based on that, we estimated rates of carbon and nutrient recycling within sediments. Our results are essential to improve predictions of future changes on carbon cycling and climate.
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