Isotopic Fractionation of Oxygen During Sediment Respiration

Abstract

The apparent community respiration fractionation factor for deep-sea sediments has a predicted value of 9%0 (Bender, 1990). This value is based on a model of sediment oxygen consumption that satisfies the assumption of first order kinetics with respect to carbon (i.e. a pore water oxygen profile which decreases exponentially with depth below the sediment-water interface). This implies that the organic matter oxidation rate is carbon limited. Bender suggests the geometry of the sediments (a partially-closed system) prevents the expression of the full microbial respiration fractionation which has been determined to be 19.1%o (cz = 0.980) for freeliving bacteria (Kiddon et al., 1993). The apparent fractionation in sediments (9%0 or et = 0.991) has a square-root relation to the fractionation factor for bacterial cultures as a result of the assumption of an exponential oxygen decrease in the sediments. Brandes and Devol (1996) have presented oxygen isotope data for benthic chamber deployments in Puget Sound, Washington. Their results indicate an apparent sedimentary respiration fraction factor of 3%0 (~ = 0.997). The Puget Sound sediments are sites of rapid organic matter diagenesis and complete oxygen consumption within the top centimeter of the sediments. Brandes and Devol argued that the apparent fractionation is dramatically reduced from Bender's value of 9%0 (or from the free-living bacterial fractionation of 18%o) due to complete oxygen removal and due to the diffusive limitation present in a sedimentary system. Their model parameterizes intense diagenesis in discrete sediment micro-sites wherein oxygen is completely consumed. The apparent fractionation is small because oxygen concentrations go to zero at the micro-site. They suggest this small fractionation arises as a result of oxidant limitation. When the diffusive oxygen flux is insufficient to balance the respiratory demand all oxygen molecules are consumed and there is no distinction between the heavy and the light isotopes.