Description: Biogenic silica (BSi) is a major component in marine geochemical cycles and a suitableproxy for paleoproductivity. The Southern Ocean plays a key role in the biogeochemicalcycle of silicon. To understand opal preservation mechanisms and to assessthe global biogenic silica cycle it is important to study the processes controlling BSidissolution kinetics. Common techniques for the assessment of dissolution kineticsare flow-through and batch experiments. In this study, information on BSi dissolutionkinetics will be considered in a regional context. For this purpose about 60 samplesfrom the Atlantic sector of the Southern Ocean were investigated. To identify spatialpatterns of a BSi-Dissolution-Index, the result from fitting the leaching curves, derivedby wet-alkaline-extraction of biogenic silica, gives an estimate of the reactivityof biogenic silica in sediments. To get detailed information on kinetics and solubilityof biogenic silica, continuously stirred flow-through experiments were performed.Use of flow through reactors allows quantification of dissolution rates and saturationconcentrations under well defined conditions. Dissolution rates of sediment samplesin stirred flow-through reactors were measured as a function of the degree of undersaturationby varying the silica acid concentrations or the flow rate of the inflow solution.Sediment samples were selected from different regions of the Southern Ocean, e.g.Weddel Sea, Scotia Sea, Polar Front Zone. The results of the BSi dissolution kineticsand detailed information on diatom assemblages and clay mineralogy are considered.The combination of results from laboratory measurements and regional distribution ofparameters affecting the benthic silica cycle can be used to decipher processes regulatingthe BSi burial. In addition a more detailed understanding of the dissolution of BSi in surface sediments within certain regions of the Southern Ocean can be provided.

Description: Rhizon soil moisture samplers were successfully used for multi-level pore water sampling from in-situ sediments and sediment cores. By this method, high resolution pore water profiles may be sampled with minimum disturbance of both, the sediment structure and possible flow fields. To investigate biogeochemical cycles, especially at the sediment/water interface, we present a newly developed Rhizon In Situ Sampler (RISS) as a non-destructive tool for in-situ pore water sampling. Field experiments, tracer studies and numerical modeling were combined to assess the suitability of Rhizons for pore water sampling. A combination of a benthic chamber with the RISS allows studies of benthic fluxes and pore water profiles at the exact same location with negligible effect on the incubated sediment/water interface. This allows improved modeling of transport and reaction processes. Long term deployments of the RISS and repetitive pore water sampling will support future studies of the seasonal variation of benthic processes.

Description: To investigate coastal biogeochemical cycles, especially at the sediment/water interface,improved sampling methods are necessary. For this purpose, we developed apore water in situ sampler with miniature sampling devices, so called Rhizons. Rhizonsoil moisture samplers have been used as sampling devices in unsaturated soilsfor the last ten years. In aquatic science they have been rarely used to extract porewater from sediments. This study presents a new developed Rhizon In Situ Sampler(RISS) as a non-destructive and inexpensive tool for in situ pore water sampling. Fieldexperiments, tracer studies and numerical modeling were combined to assess the suitabilityof Rhizons for pore water sampling. Our investigations show that the RISS isa very suitable alternative to classical methods for in situ sampling. Combined withan in situ benthic chamber system the RISS allows studies of benthic fluxes and porewater profiles at the same location with negligible effect on the incubated sedimentwater interface. This allows improved calculation and modeling of transport and reactionprocesses. Results of nutrient and freshwater input into surface water derivedby in situ sampling of tidal flat sediments of the Wadden Sea (Sahlenburg/Cuxhaven,Germany) are presented. Long term deployments of the RISS and repetitive pore watersampling at the same location might support future studies of seasonal variation ofbenthic processes in sediments of the coastal zone and open ocean.

Description: Rhizon samplers were originally designed as micro-tensiometers for soil science forseepage water sampling in the unsaturated zone some ten years ago. We have introducedthese samplers for the use in marine sediments and in saturated groundwaterenvironments for direct pore water sampling.Rhizons consist of a small microporous polymer tube (2.5mm diameter) supported bya stabilizing wire that is connected to a PVC-tube and a standard luer-lock connector.By attaching vacuum to this connector (syringe, vacuumtube or peristaltic pump)small volumes of pore water samples may be extracted from sediments without furthermaintenance. As a side effect of the 0.1 micron pore width of the polymer tube,the samples are automatically filtered. We successfully used these samplers for highresolution sampling from closed sediment cores. The Rhizons were inserted through3mm holes in the liner walls. By this method very high resolution pore water profilesamples may be taken without disturbance of the sediment structure. Since samplesare collected in directly attached syringes or vacuumtubes, contact with ambient oxygenis avoided for anoxic environments, thus eliminating both, the need for glove boxsampling and eliminating the need for cumbersome pressure filtration. Recently otherRhizon models with carbon fibre support and microporous tube diameters down to1mm have been presented.

Description: Sediment redistribution at the seafloor is a widespread process in marine environments. In the last years, increasing evidence has been found that sedimentary budgets for silica and other components may be strongly biased by sediment focussing. The natural radioisotope Thorium-230 provides a method for correcting fluxes into the sediment for these lateral inputs. The 230Th-normalization method has so far been applied to individual sediment cores and some longitudinal sections only, giving a first impression of the strong influence of sediment redistribution. However, the database of 230Th-corrected data was too scarce to calculate reliable spatial budgets for whole ocean basins.Here we present a comprehensive set (114 samples) of 230Th-corrected silica accumulation rate data for surface sediments in the South Atlantic and adjoining areas. The focussing-corrected budgets for silica are considerably lower than earlier estimates. The focussing-corrected accumulation rates, representing vertical fluxes into the sediment, will facilitate the comparison of core data to model results.