New insights into large-scale trends of apparent organic matter reactivity in marine sediments and patterns of benthic carbon transformation
Authors: Freitas Felipe S. , Pika Philip A. , Kasten Sabine , Jørgensen Bo B. , Rassmann Jens , Rabouille Christophe , Thomas Shaun , Sass Henrik , Pancost Richard D. , Arndt Sandra 
Published: August 2021

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.