Abstract
Abstract. The kinetics of particulate organic carbon (POC) mineralization in marine surface sediments is not well constrained. This creates considerable uncertainties when benthic processes are considered in global biogeochemical or Earth system circulation models to simulate climate–ocean interactions and biogeochemical tracer distributions in the ocean. In an attempt to improve our understanding of the rate and depth distribution of organic carbon mineralization in bioturbated (0–20 cm) sediments at the global scale, we parameterized a 1-D diagenetic model that simulates the mineralization of three discrete POC pools (a multi-G model). The rate constants of the three reactive classes (highly reactive, reactive, refractory) are fixed and determined to be 70, 0.5 and ∼ 0.001 yr−1, respectively, based on the Martin curve model for pelagic POC degradation. In contrast to previous approaches, however, the reactivity of the organic material degraded in the seafloor is continuous with, and set by, the apparent reactivity of material sinking through the water column. Despite the simplifications of describing POC remineralization using G-type approaches, the model is able to simulate a global database (185 stations) of benthic oxygen and nitrate fluxes across the sediment–water interface in addition to porewater oxygen and nitrate distributions and organic carbon burial efficiencies. It is further consistent with degradation experiments using fresh phytoplankton reported in a previous study. We propose that an important yet mostly overlooked consideration in upscaling approaches is the proportion of the reactive POC classes reaching the seafloor in addition to their reactivity. The approach presented is applicable to both steady-state and non-steady state scenarios, and links POC degradation kinetics in sedimentary environments to water depth and the POC rain rate to the seafloor.
Highlights
Mineralization and burial of particulate organic carbon (POC) in marine sediments is a core component of the Earth’s carbon cycle, helping to regulate atmospheric oxygen and carbon dioxide levels over glacial timescales and longer (Berner and Canfield, 1989; Mackenzie et al, 2004)
Earth system models deal with the recycling of biogenic material in seafloor sediment in a variety of ways, from the very simple to the more complex yet computationally expensive coupling with 1-D vertically resolved or layered diagenetic models (e.g., Munhoven, 2007)
From the individual rain rates, carbon burial efficiency (CBE) (Eq 21), kapp(wd) and the three rate constants for POC degradation, the proportion of reactive-to-highly reactive POC was determined for all ocean depths (Fig. 2b)
Summary
Mineralization and burial of particulate organic carbon (POC) in marine sediments is a core component of the Earth’s carbon cycle, helping to regulate atmospheric oxygen and carbon dioxide levels over glacial timescales and longer (Berner and Canfield, 1989; Mackenzie et al, 2004). Earth system models deal with the recycling of biogenic material in seafloor sediment in a variety of ways, from the very simple (ignoring it completely) to the more complex yet computationally expensive coupling with 1-D vertically resolved or layered diagenetic models (e.g., Munhoven, 2007). Intermediate approaches make use of computationally efficient transfer functions (vertically integrated models) to simulate benthic feedbacks of the N, P, Fe and O2 cycles (Middelburg et al, 1997; Soetaert et al, 2000; Wallmann, 2010; Tschumi et al, 2011; Bohlen et al, 2012; Somes et al, 2013; Dale et al, 2015; Capet et al, 2016; Yang and Gruber, 2016).
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