A new benthic aqueous flux meter for very low to moderate discharge rates

Significant quantities of fluids and dissolved geochemical components are e xpelled through the sediment surface in ocean margin and sedimented ridge e nvironments. Recently, significant interest has been generated in constrain ing hydrological processes in these environments, but direct measurement of fluid flow in the marine environment has proven to be difficult and many a spects of marine hydrogeology remain poorly understood. To address the need for a means to make a significant number of direct measurements in a wide range of low to moderate flow environments, we have developed a new type of benthic aqueous flux meter that is capable of measuring diffuse fluid flow through the sediment surface on the order of 0.1 mm yr(-1)-15 m yr(-1) whe n the flow is through sediments with permeabilities of less than 10(-8) cm( 2) (typical seafloor sediments). The instrument measures fluid flow by dete rmining the degree of dilution of a chemical tracer that is injected by an osmotic pump at a known rate into the fluids venting into or out of a colle ction chamber situated on the sea bed. The pump also withdraws a subsample of this tracer/fluid mix into sample coils allowing a serial record of the flow rates to be determined. Both upward and downward flow can be measured and, when flux rates are high enough to effectively flush the collecting ch amber, the instruments also act as geochemical samplers. Three years of lab oratory testing and field use have constrained the effects of (1) temperatu re, pressure, and deployment duration on osmotic pump performance, (2) disp ersion/diffusion in the sample coils, and (3) deflection of flow under a ra nge of sediment permeabilities. Recent deployments on the Kodiak and Cascad ia accretionary prisms document the range and capabilities of the instrumen t in the field. (C) 2001 Elsevier Science Ltd. All rights reserved.