14 Experimental assessment of community metabolism in the subsurface

Faculty Opinions recommendation of Subseafloor life and its biogeochemical impacts.

“…Balancing diffusive transport with reaction rate parameterized in this way leads to the expectation that catabolic reaction rate decreases exponentially with sediment depth. This relationship between catabolic rate and diffusive distance is consistent with the generally observed strong and initially rapid decline in organic oxidation rate with sediment depth 17 . It is also consistent with the generally strong correlation between organic burial efficiency and sedimentation rate 65,75 .…”

“…1). Rates of organic-fueled activity are highest in young, near-seafloor sediment and decline rapidly with increasing depth below seafloor 17 (Fig. 2).…”

“…Organic oxidation rate is generally highest at the seafloor and decreases with sediment depth, declining rapidly near the seafloor and increasingly slowly with greater distance from the seafloor. This relationship is typically modeled as a power-law function 17,75,76 . The basic reaction series can be simplified as follows:…”

“…These models are variations on Berner's 1964 G Model, which assumed organic oxidation rate to be proportional to organic matter concentration 76 . More recent models assume that bulk organic oxidation rate declines with sediment depth and age because increasingly recalcitrant pools of organic matter are successively depleted 17 . This assumption is consistent with laboratory experiments that show sequential utilization of different sedimentary organic carbon pools by aerobic heterotrophs 80 .…”

“…Despite this consistency, reliance on relative degradability to explain organic oxidation rates is problematic for the following reasons. First, the molecular composition of these organic pools is never identified by studies that assume them to model organic oxidation rates 17,76 . Second, the degradability of an organic-matter pool, such as kerogen (the fraction of organic matter insoluble to organic solvents) in deeply buried sediment, is not inherent to the organic molecules that compose that pool, but depends on the environment in which the organic pool is present 81 .…”