Regional and Global Patterns of Apparent Organic Matter Reactivity in Marine Sediments

Abstract

Organic matter (OM) degradation in marine sediments is fundamental to understanding and constraining global biogeochemical cycling, whereby OM reactivity is at its core. Here, we use benthic diffusive oxygen uptake (DOU) rates as a proxy for OM reactivity. We apply an analytical diagenetic model to inversely determine OM reactivities in marine sediments (i.e., Reactive Continuum Model parameters a and ν) using datasets of global DOU, surface sediment OM contents, and seafloor boundary conditions. Simulated oxygen depth profiles show good agreement with observations, increasing confidence in our reactivity estimates. Inversely‐determined reactivities vary over orders of magnitude between individual sites ranging from high(k=0.252 year−1) for sediments in the Polar region to extremely low (k=7.96 ⋅ 10−5 year−1) in South Pacific. Our findings highlight the heterogeneity of OM reactivities, revealing regional patterns that broadly agree with observations and prior assessments. In general, high benthic reactivity can be linked to limited pelagic OM degradation favored by either a rapid vertical or lateral OM transport to the sediment or environmental factors, such as low oxygen concentrations or low temperature, slowing pelagic OM degradation. Finally, we develop a set of transfer functions that allow estimating OM reactivity as a function of DOU, organic matter content and water depths, and use one to derive the first global maps of benthic OM reactivity based on two global DOU maps. Despite the inherent observational biases in the datasets, our results provide a good first‐order estimate of the apparent benthic OM reactivity agreeing with our current mechanistic understanding and observations.

Organic matter reaction kinetics in bioturbated sediments

“…Accordingly, Berner's one-G model based on a single, sitespecific, depth-independent reactivity constant can be applied to sediments receiving extensively pre-processed organic matter (with high 𝑎 values) as well as sediments that are well-mixed (irrespective of initial reactivities). Our results also show that organic matter reactivity constants show large differences among sedimentary settings (i.e., the type of organic matter delivered and residence time of particles in the mixed layer), consistent with field observations (Emerson & Hedges, 1988;Middelburg, 1989;Arndt et al, 2013;Pika et al, 2023;Xu et al, 2023).…”

“…To explore the importance of the nature of the incoming organic matter, simulations with two initial reactivity constants 𝑎 will be presented: 𝑎 = 10 yrs and 𝑎 = 10,000 yrs for input of labile and refractory organic matter, respectively. This range of initial reactivity values 𝑎 covers most sediments (Pika et al, 2023) and this combination of mixing intensities and organic matter reactivities is consistent with the adopted mixed-layer depth (𝑧 0 ≈ 4.6√…”

“…Moreover, each sediment layer now contains a whole assemblage of particles with a broad distribution of particle ages (Meile & Van Cappellen 2005;Kuderer, 2022;Rooze et al, 2023). This shift in time scale and particle transport mode has major consequences for biogenic particles arriving at the seafloor characterized by a range of reactivities such as organic matter (Middelburg 1989, Boudreau & Ruddick 1991, Arndt et al 2013Pika et al, 2023;Rothman, 2024). Recently settled organic particles show a large range (orders of magnitude) in their susceptibility to degradation.…”