ALBERT

Description: Free or hydrate-bound gas in the seafloor has been of scientific, ecologic and economic interest for many years because it predominantly contains high concentrations of low-molecular-weight hydrocarbons. A prerequisite of accurate quantifications of gases in sediments is to preserve pressure and temperature close to the in situ conditions during recovery. Here we introduce two new sediment coring devices that allow for the recovery of near-surface gas- and gas-hydrate-bearing sediments and subsequent investigations using several different techniques such as visualisation by computerized tomography, quantitative degassing, and sediment and porewater analyses. The first coring tool, the Multiple Autoclave Corer (MAC), resembles a standard multiple corer in terms of applications, size and core length of about 55 cm. The second tool, the Dynamic Autoclave Piston Corer (DAPC), is similar to a piston corer in application and size and enables one to take cores of up to 2.5 m length. Both focus on the investigation of near-surface sediments, which are most strongly affected by changes in bottom-water temperature and hydrostatic pressure, which in turn influence continental slope stability. Some results from recent offshore applications show the potential of these tools.

Description: Local curvature characterizes every point of a surface and measures its deviation from a plane, locally. One application of local curvature measures within the field of image and geometry processing is object segmentation. Here, we present and evaluate a novel algorithm based on the fundamental forms to calculate the curvature on surfaces of objects discretized with respect to a regular three-dimensional grid. Thus, our new algorithm is applicable to voxel data, which are created e.g. from computed tomography (CT). Existing algorithms for binary data used the Gauss map, rather than fundamental forms. For the calculation of the fundamental forms, derivatives of a surface in tangent directions in every point of the surface have to be computed. Since the surfaces exist on grids with restricted resolution, these derivatives have to be discretized. In the presented method, this is realized by projecting the tangent plane onto the discrete object surface. The most important parameter of the proposed algorithm is the size of the chosen window for the calculation of the gradient. The size of this window has to be selected according to object size as well as with respect to distances between objects. In our experiments, an algorithm based on the Gauss map provided inconsistent values for simple test objects, whereas our method provides consistent values. We report quantitative results on various test geometries, compare our method to two algorithms working on gray value data and demonstrate the practical applicability of our novel algorithm to CT-reconstructions of Greenlandic firn.

Description: Ice cores are powerful archives for reconstructing volcanism as they contain both soluble (i.e. aerosols) and insoluble (i.e. tephra) products of volcanic eruptions and for more recent periods have high-precision annually resolved chronologies. The identification and geochemical analysis of cryptotephra in these cores can provide their volcanic source and latitude of injection, complementing records of sulphur injections from volcanic eruptions developed using continuous flow ice-core analysis. Here, we aim to improve the volcanic record for the Southern Hemisphere using a sampling strategy for cryptotephra identification based on coeval deposition of sulphate and microparticles in ice cores from the interior of East Antarctica covering the Mid-to Late Holocene. In total, 15 cryptotephras and one visible horizon were identified and geochemically characterised. Through comparisons to proximal deposits a range of possible sources were isolated for these horizons including the South Sandwich Islands, South Shetland Islands, Victoria Land (Antarctica) and South America. This new tephra framework contributes to the volcanic history of the region by extending the known geographical range of tephra deposition for previously identified events and providing a potential indication of phases of eruptive activity from key sources. Using the tephra-based source attributions and comparison of the timing of the events to a database of sulphur injections from Holocene volcanic eruptions it is possible to refine injection latitudes for some events, which can lead to improved estimates of their radiative forcing potential. The relatively low magnitude of the volcanic stratospheric sulphur injections related to the events in the tephra framework indicates they would have had a limited impact on Southern Hemisphere climate. Further work is required to improve source attributions for some events and/or to determine the magnitude of sulphur injections for individual events during years when coeval eruptions occurred. One limitation of the framework is the dominance of cryptotephra from regional volcanic sources and a lack of tephra from tropical sources, which hampers the refinement of eruption parameters for these large magnitude and often climate-impacting eruptions. This issue could be explored further through increased sampling of these events and/or development of additional analytical techniques for the identification and robust geochemical analysis of glass tephra shards less than 5 μm in diameter. Such investigations could be coupled with model experiments to determine the likelihood that past tropical eruptions deposited glass tephra shards over Antarctica and the potential size range and geographical spread of deposition.