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

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.) using data sets 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 center dot 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, OM 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 data sets, our results provide a good first-order estimate of the apparent benthic OM reactivity agreeing with our current mechanistic understanding and observations. The microbial degradation of organic matter (OM) in marine sediments is of considerable global importance for the nutrient and oxygen balance of the ocean. Simultaneously, OM burial in marine sediments represents an important sink for atmospheric CO2. How susceptible OM is to degradation (i.e., its reactivity) represents arguably the most important parameter in diagenetic models. However, this reactivity is not globally uniform and depends on many controlling factors, making it notoriously difficult to define and represents a significant source of uncertainty in model estimates. Using an analytical diagenetic model, we inversely calculate the OM reactivity using global data sets of dissolved oxygen fluxes between the sediment and the overlying seawater. The resulting OM reactivities vary substantially, with high reactivities in polar marine sediments and low reactivities in sediments underlying the South Pacific. We observe that high OM reactivities coincide with accelerated lateral and/or vertical OM transport, low water column oxygen concentration or low water temperatures, while low OM reactivities coincide with low productivity, slow transport and remoteness from the coast. Finally, we construct a transfer function to provide the first global maps of benthic OM reactivity consistent with our existing mechanistic knowledge and observations.