Deep-Sea Research, Vol. 34, No. 12, pp. 2019-2035, 1987. 0198-0149/87 $3.00 + 0.00 Printed in Great Britain. ~) 1987 Pergamon Journals Ltd. INSTRUMENTS AND METHODS = An in situ microprofiling instrument for measuring interfacial pore water gradients: methods and oxygen profiles from the North Pacific Ocean CLARE E. REIMERS* (Received 17 February 1987; in revised form 12 June 1987; accepted 22 June 1987) Abstract--An in situ microprofiling instrument has been developed to study the composition of pore waters at the interface of deep-ocean sediments. The device lowers and operates microelec- trodes. It has been deployed by free vehicle, at four North Pacific Ocean sites spanning water depths of 1275-5790 m. Oxygen profiles with a vertical resolution of 1 mm, are reported from these deployments. Benthic oxygen fluxes calculated from the in situ gradients range from 0.04 + 0.02 to 0.14 + 0.04 Ixmol crn-2 day-t. Microelectrode oxygen measurements in box-cored sediments never precisely dupficated the in situ distributions, especially near the sediment-water interface, where definition of the gradient is most critical to flux determinations. INTRODUCTION AN important sampling problem in deep-sea research is resolving relatively steep concentration gradients of pore water solutes near the sediment-water interface. Benthic fluxes can be calculated from these gradients, and fluxes provide a measure of reminerali- zation rates for organic and inorganic sedimentary constituents. In all previous deep-sea studies, the methods for delineating interstitial chemical profiles have depended on coring or in situ extraction devices to collect discrete pore water samples (MANHEIM, 1966; SAYLES et al., 1976; BARNES et al., 1979; JAHNKE et al., 1982; BENDER et al., 1985; and many others). Such samples, when analysed, have revealed general stoichiometric relationships between many dissolved species; however, they have not produced the precise, water-to-sediment distributions needed to predict benthic fluxes. The main reason for this shortcoming is that in situ samplers and corers disturb, in varying degrees, natural boundary conditions, the surface sediment, and infauna when emplaced. The sampled solutions are also homogenates drawn or extracted from depth intervals rather than representing specific closely spaced points. Other artifacts may be produced through gas exchange between the sample and laboratory air, contamination of pore waters before analysis, or, in the case of cored samples, because temperature and pressure changes alter the concentrations of the interstitial solutes (BISCHOFF et al., 1970; FANNING and PILSON, 1971; SAYLES et al., 1976; MURRAY et al., 1980; FROELICH et al., 1983). In 1980 Revsbech et al. published the first results of a new technique that represents a markedly different and potentially artifact-free approach for studying pore water * Scripps Institution of Oceanography, A-015, La Jolla, CA 92093, U.S.A. 2019