Predicting the reactivity of trawl-disturbed sediment carbon from measurable seabed properties

<jats:p>Mobile bottom-contact fishing gear disturbs a large area of the continental shelf each year. How much carbon that releases depends on how fast the disturbed material is mineralised. Assessments represent that with a first-order rate constant, and because its value is unmeasured they sample it across five orders of magnitude, which makes it their dominant uncertainty. Here the constant is derived instead of assumed. Organic matter arriving at the seabed is described as a reactivity continuum, each component is transported through an accreting and bioturbated bed, and the mass-weighted rate constant of the layer a gear removes follows in closed form. Two independent numerical solutions confirm it. Sweeping the parameters over shelf ranges then shows where the answer comes from. With the arriving material held fixed the rate constant varies 39-fold; letting it vary widens the spread to four orders of magnitude. Four fifths of that wider spread is carried by the age of the arriving material and one thousandth by the depth the gear cuts to. No gear reaches fresh-deposit reactivity at any cutting depth, because reactive components are consumed before they can accumulate: the sediment surface is already 1.4 to 56 times less reactive than the material landing on it. Rapidly accumulating, weakly mixed beds give rate constants about thirty times higher than slowly accumulating, well-mixed beds. Measurement effort should therefore go to the material settling from the water column rather than to the disturbance, but only pays once that material is constrained to within a factor of eight, which no published regional inversion achieves.</jats:p>