Solute release mechanisms from confined sediment cores in stirred benthic chambers and flume flows

Abstract

Interfacial solute fluxes from sediment cores enclosed in stirred chambers were compared with solute fluxes from a reference core inserted In a flume simulating in situ conditions with unidirectional boundary-layer flow. Utilizing Rhodamine-WT dye as conservative solute, and quantifying the hydrodynamic driving forces by hot film anemometry and a highly sensitive wet/wet d~fferential pressure transducer, the pathways of solutes in chamber-and flume-confined cores are ~dentified. The critical parameter limiting confined-core flux techniques is the sediment permeability, with equivalency between experimental techniques and natural, smooth bottoms found for impermeable sediments only, where bottom stress and Schmidt number act as the controlling parameters permitting use of diffusive laws. In contrast, pressure gradients, induced by edge and sidewall effects in cores under unidirectional flow and by circular flow patterns in stirred chambers, being ineffective for impermeable sediments, generate non-negligible advective porewater flows in permeable sediments. For fine sandy sediment (4 = 2.2; permeability K = 15 Darcy, D), stirred-chamber circular flow enhanced solute release up to 6-fold over that from the reference core exposed to 2-dimensional flow, with intruding supernatant water reaching up to 8-fold deeper ~n t o the porespace. Consequences of these advective exchange processes include release of anoxic porewater and increased oxygen uptake in permeable cores, with shifts in redox isolines affecting biological and geochemical processes in the sediment. Results suggest that chamber fluxes are both site-and device-dependent and cannot be automatically equated with In situ fluxes.