Assessing global-scale organic matter reactivity patterns in marine sediments using a lognormal reactive continuum model

Abstract

Abstract. Organic matter (OM) degradation in marine sediments is largely controlled by its reactivity and profoundly affects the global carbon cycle. Yet, there is currently no general framework that can constrain OM reactivity on a global scale. In this study, we propose a reactive continuum model based on a lognormal distribution (l-RCM), where OM reactivity is fully described by parameters μ (the mean reactivity of the initial OM bulk mixture) and σ (the variance of OM components around the mean reactivity). We use the l-RCM to inversely determine μ and σ at 123 sites across the global ocean. The results show that the apparent OM reactivity (〈k〉=μ⋅exp⁡(σ2/2)) decreases with decreasing sedimentation rate (ω) and that OM reactivity is more than 3 orders of magnitude higher in shelf than in abyssal regions. Despite the general global trends, higher than expected OM reactivity is observed in certain ocean regions characterized by great water depth or pronounced oxygen minimum zones, such as the eastern–western coastal equatorial Pacific and the Arabian Sea, emphasizing the complex control of the depositional environment (e.g., OM flux, oxygen content in the water column) on benthic OM reactivity. Notably, the l-RCM can also highlight the variability in OM reactivity in these regions. Based on inverse modeling results in our dataset, we establish the significant statistical relationships between 〈k〉 and ω and further map the global OM reactivity distribution. The novelty of this study lies in its unifying view but also in contributing a new framework that allows predicting OM reactivity in data-poor areas based on readily available (or more easily obtainable) information. Such a framework is currently lacking and limits our abilities to constrain OM reactivity in global biogeochemical or Earth system models.

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).…”

Iron Promotes the Retention of Terrigenous Dissolved Organic Matter in Subtidal Permeable Sediments

“…Coastal sediments receive a mixture of organic matter (OM) from various marine, terrestrial (e.g., river and groundwater), and anthropogenic sources. − The proportion of OM originating from different sources is controlled by regional geological settings, rates of erosion and weathering, physical transport and mixing, biotic/abiotic transformation, and decomposition processes along the land–ocean transition zone. , Moving away from the coasts, terrigenous DOM continuously declines in abundance and is replaced by marine DOM, which is mainly derived from plankton and comparatively more biodegradable. ,, OM degradation in permeable sediments is facilitated by oxygen supply via porewater advection. ,, As redox conditions change with sediment depth, OM remineralization continues with a shift in terminal electron acceptors from oxygen to nitrate, manganese and iron oxides, and sulfatealthough at lower rates for anaerobic compared to aerobic degradation. − The degradation of OM is expected to follow an intrinsic reactivity continuum, with less bioavailable, less saturated, and more aromatic compounds enriched along the redox gradients. − In subtidal permeable sediments, porewater advection can cause frequent variations of redox conditions and mass exchanges, ,, thus disrupting the horizontal redox zonation and degradation pathways. , …”