Using in situ diffusion-based samplers to investigate iron, sulfide and trace metal biogeochemistry in heterogeneous coastal sediments

Sediments are complex environments that play an important role in the cycling of carbon, nutrients, and metals. Sediment biogeochemical processes are driven by microbial respiration and produce three major zones: the oxic zone, dominated by aerobic respiration and the presence of O2; the sub-oxic zone, dominated by anaerobic respiration and mildly reducing conditions with the presence of the reduced products of electron acceptors (NO2 -, N2, Fe(II), Mn(II)); and the anoxic zone, dominated by strongly reducing conditions, such as sulfide in coastal sediments. The inherent vertical distribution of these biogeochemical zones is dramatically modified by the presence of burrowing macro- and micro-invertebrates and benthic plants, introducing considerable heterogeneity. Therefore, in metabolically active heterogeneous sediment, the distance between the oxic and anoxic sulfidic layers may be as short as a few millimetres with differing biogeochemical zones occurring at the same depth, but in laterally separated locations. Iron(II) and sulfide distributions are useful indicators for determining the redox zonation and understanding the dominant biogeochemical processes occurring within the sediment. The diffusive equilibration in thin films (DET) and the diffusive gradients in thin films (DGT) methods are in situ passive sampling methods. Combined colorimetric DET-DGT methods allow simultaneous measurement of iron(II) and sulfide in sediment pore waters and provide twodimensional (2D) distributions at high (mm) spatial resolution. In this thesis, the combined DETDGT techniques were applied in a range of coastal zone sediment habitats with high replication to investigate the influence of sediment heterogeneity on describing ‘typical’ biogeochemical distributions of iron(II) and sulfide. In this study, the degree of sediment heterogeneity was analysed quantitatively using texture analysis methods; gray level co-occurrence matrix (GLCM) with the use of 2D iron(II) and sulfide distributions. The degree of sediment heterogeneity was also compared among different coastal zone sediment habitats. These measures confirmed that the benthic habitats were highly heterogeneous but were not able to provide much further distinction over qualitative analysis. The simplistic view of sediment biogeochemistry considered in conventional sediment pore water analysis is challenged by the complex heterogeneity of coastal zone sediment habitats. Traditional pore water sampling techniques require the removal and processing of sediment cores by slicing the collected cores at 0.5 cm or greater resolution and extraction of the pore waters by centrifugation or squeezing. This method mixes the pore water solutes and thus the measured concentrations represent an average of the sampled volume, at best. Therefore, these conventional pore water sampling and analysis methods simply cannot distinguish concentration changes over the small distances between oxic and anoxic microniches in the sediments. This thesis compares the capabilities and limitations of in situ passive sampling techniques (DET and DGT) and ex situ conventional sediment core sampling followed by centrifugation to measure iron(II), metal(loid)s, and sulfide concentrations in sediment pore waters. Ex situ conventional pore water analysis methods were not capable of reliably measuring redox– active pore water solutes in heterogeneous sediments due to the mixing of porewater solutes during the extraction process and substantially underestimated both iron(II) and sulfide concentrations in comparison to the DET-DGT method. The DGT measured metal(loid) concentrations were generally lower than the metal(loid) concentrations obtained by centrifugation extracted pore waters (except Fe and Ni) due to two reasons: 1) The non-labile metal fraction in sediment pore water, which is not measured by DGT, and 2) limited resupply of metal(loid)s from the solid phase to the solution phase. However, metal(loid) mobilisation/sequestration mechanisms were informed well by high-resolution (2 mm) DGT measurements. Mobilisation of metal(loid)s within aquatic sediments is associated with the biogeochemical processes of major redox active species present in marine sediments; iron(II), manganese(II) and sulfide. However, relating mobilisation of particular metal(loid)s to specific processes has been a challenge due to the limits of conventional methods. Simultaneous measurements of iron(II), metal(loid)s, and sulfide concentrations in sediment pore water at the same spatial location was facilitated by the use of triple layer combined DET-DGT probes. Mobilisation of metal(loid)s through the reductive mobilisation of Fe and Mn hydr(oxides) in the sub-oxic zone and sequestration of divalent metals with sulfide in anoxic deeper sediments were identified. The application of iron(II) DET and sulfide DGT methods in this and previous studies showed substantial overlaps between iron(II) and sulfide distributions. The simultaneous DET and DGT measurements may not be contemporary as DGT is a time-integrated technique and DET is an equilibration method that responds to the most recent conditions. A novel colorimetric 2D highresolution DET technique was developed for the measurement of dissolved sulfide distributions in sediment pore waters. This sulfide DET method limits the temporal averaging of sulfide measurements found with the sulfide DGT method and facilitates the contemporary measurement of sulfide co-distributions with other solutes such as iron(II). Field application of this method showed that the overlap between DET iron(II) and DET sulfide distributions was significantly lower than the average overlap between DET iron(II) and DGT sulfide.