Abstract
Abstract. Constraining the mechanisms controlling 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 detailed quantitative understanding of what controls OM reactivity in marine sediments and, consequently, a general framework that would allow model parametrization in data-poor areas. To fill this gap, we quantify apparent OM reactivity (i.e. OM degradation rate constants) by extracting reactive continuum model (RCM) parameters (a and v, which define the shape and scale of OM reactivity profiles, respectively) from observed benthic organic carbon and sulfate dynamics across 14 contrasting depositional settings distributed over five distinct benthic provinces. We further complement the newly derived parameter set with a compilation of 37 previously published RCM a and v estimates to explore large-scale trends in OM reactivity. Our analysis shows that the large-scale variability in apparent OM reactivity is largely driven by differences in parameter a (10−3–107) with a high frequency of values in the range 100–104 years. In contrast, and in broad agreement with previous findings, inversely determined v values fall within a narrow range (0.1–0.2). 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 traditionally proposed, single environmental controls (e.g. water depth, sedimentation rate, OM fluxes). Thus, we caution against the simplifying use of a single environmental control for predicting apparent OM reactivity beyond a specific local environmental context (i.e. well-defined geographic scale). Additionally, model results 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. Furthermore, model results show that apparent OM reactivity exerts a key control on the relative significance of OM degradation pathways, the redox zonation of the sediment, and rates of anaerobic oxidation of methane. In summary, our large-scale assessment (i) further supports the notion of apparent OM reactivity as a dynamic ecosystem property, (ii) consolidates the distributions of RCM parameters, and (iii) provides quantitative constraints on how OM reactivity governs benthic biogeochemical cycling and exchange. Therefore, it provides important global constraints on the most plausible range of RCM parameters a and v and largely alleviates the difficulty of determining OM reactivity in RCM by constraining it to only one variable, i.e. the parameter a. It thus represents an important advance for model parameterization in data-poor areas.
Highlights
Organic matter (OM) buried in marine sediments represents the largest reactive reservoir of reduced carbon on Earth (e.g. Hedges, 1992)
The integrated data–model analysis yielded a comprehensive picture of OM reactivity parameters (a, v, and k(z)), as well as OM degradation dynamics and benthic–pelagic coupling for our large-scale compilation of depositional environments (Table 6)
Our results show that sulfate reduction is the main oxidative pathway in regions characterized by high apparent OM reactivity (a < 102 years) and intermediate OM fluxes (JOM,in ∼ 10−4–10−3 mol C cm−2 yr−1), e.g. Arabian Sea region (Cowie, 2005; Luff et al, 2000) and Arkona Basin and Aarhus Bay (Dale et al, 2008b; Jørgensen et al, 2019a; Mogollón et al, 2012)
Summary
Organic matter (OM) buried in marine sediments represents the largest reactive reservoir of reduced carbon on Earth (e.g. Hedges, 1992). Since OM production slightly exceeds degradation, a small fraction of the photosynthetically produced organic carbon escapes heterotrophic degradation and is buried in sediments (Berner, 2003; Burdige, 2006) This relatively small imbalance connects the short- and longterm carbon cycles, controls the long-term evolution of atmospheric CO2, and has enabled the accumulation of oxygen in the atmosphere (Berner, 2003; Hedges, 1992). We currently lack a general framework that allows quantification of the reactivity of OM in a given environmental context This knowledge gap seriously compromises our ability to constrain OM reactivity in both space and time (Arndt et al, 2013; Lessin et al, 2018). The current lack of such a general framework limits our ability to quantify pathways and rates of biogeochemical processes over different temporal and spatial scales and, predict the impact and feedbacks of past, present, and future climate change on global biogeochemical cycles and climate (Hülse et al, 2018)
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