A new colorimetric DET technique for determining mm-resolution sulfide porewater distributions and allowing improved interpretation of iron(II) co-distributions

Abstract

Measurement of sulfide in pore waters is critical for understanding biogeochemical processes, especially within coastal sediments. Here we report the development of a new colorimetric DET (diffusive equilibration in thin films) technique for determining mm-resolution, two-dimensional sulfide distributions in sediment pore waters. This colorimetric sulfide DET method was based on the standard spectrophotometric methylene blue assay, but modified to allow quantitation of sulfide by computer imaging densitometry. The method detection and effective upper measurement limits of the optimised technique were 3.7 and 1000 μmol L^-1, respectively. The optimised sulfide DET method was combined with the colorimetric iron(II) DET method to obtain co-distributions in coastal seagrass (Zostera muelleri) colonised sediment under light and dark conditions. In the dark, seagrass sediments were more reduced than in the light, with large areas being dominated by high porewater sulfide concentrations. These co-distributions were compared with those obtained using the previously described DET-DGT (diffusive gradients in thin films) method for measuring iron(II) and sulfide co-distributions. There was less overlap of iron(II) and sulfide distributions using the sulfide DET as the two DET methods are influenced most by the later hours of deployment, whereas the sulfide-DGT measurement integrates concentrations over the whole deployment period. Overlap was most apparent in very dynamic sediment zones, such as burrow wall sediments.

Keywords: Computer imaging densitometry; Diffusive equilibration in thin films; Diffusive gradients in thin films; Environmental variability; High-resolution distributions; Sediment biogeochemistry.