Cruise Report R.V. Littorina, Cruise No.: L20-10

25.05. – 04.06.2020 Kiel – Heiligenhafen – Stohl – Schönhagen Cruise L10-20 was carried out in the framework of the project ‘Morphologische Projektionen Ostseeküste 2100’ (Morphological projections Baltic Sea Coast; financed by MELUND). The purpose of this cruise was to collect information on seafloor topography and sedimentology and shallow sub-seafloor stratigraphy of the morphodynamically active coastal zone of the focus areas Heiligenhafen,Stohl and Schönhagen. All three areas feature erosional cliff coasts acting as a sediment sources during high energy wave events. One of the aims of the project is to identify sediment transport pathways from the cliffs to the nearshore zone and to quantify the sediment budget in the coastal transport cells. This cruise served as a baseline study to be complemented during repetitive future cruises to the focus areas

Sediment reworking mechanisms in shelf seas : In situ observations from the southeastern North Sea

The seafloor in shallow coastal and shelf seas is a highly dynamic system providing plenty of natural resources and important ecosystem services. Driven by energetic physical forcing, biological activity and intense anthropogenic use, its mobile sediments are constantly overturned. The reworking processes mediate the exchange of abundant organic matter and pollutants across the benthic interface. Coherent in situ measurements of forcing and morphodynamic response help to identify relevant reworking mechanisms, to outline their spatial and temporal scales, and to quantify their impacts on the benthic ecosystem. New high-resolution observation techniques enable also the assessment of small-scale processes that, due to their ubiquity, are yet of shelf-wide importance. This thesis explores three mechanisms in the southeastern North Sea: The morphology and dynamics of small scale-bedforms, the relation of physical and biogenic sediment reworking by bedform migration and bioturbation, and the effect of submarine fluid expulsion. The analysis of these processes reveals new links between physical and biological drivers and illustrates their complex interactions at the benthic interface

Dynamics of Stone Habitats in Coastal Waters of the Southwestern Baltic Sea (Hohwacht Bay)

Cobbles and boulders on the seafloor are of high ecological value in their function as habitats for a variety of benthic species, contributing to biodiversity and productivity in marine environments. We investigate the origin, physical shape, and structure of habitat-forming cobbles and boulders and reflect on their dynamics in coastal environments of the southwestern Baltic Sea. Stone habitats are not limited to lag deposits and cannot be sufficiently described as static environments, as different dynamic processes lead to changes within the physical habitat structure and create new habitats in spatially disparate areas. Dynamic processes such as (a) ongoing exposure of cobbles and boulders from glacial till, (b) continuous overturning of cobbles, and (c) the migration of cobbles need to be considered. A distinction between allochthonous and autochthonous habitats is suggested. The genesis of sediment types indicates that stone habitats are restricted to their source (glacial till), but hydrodynamic processes induce a redistribution of individual cobbles, leading to the development of new coastal habitats. Thus, coastal stone habitats need to be regarded as dynamic and are changing on a large bandwidth of timescales. In general, wave-induced processes changing the physical structure of these habitats do not occur separately but rather act simultaneously, leading to a dynamic type of habitat

The influence of forcing schemes on the diffusion properties in pseudopotential-based Lattice Boltzmann models for multicomponent flows

In dem Vortrag wird gezeigt, wie die Wahl eines Forcing Schemas (bspw. von Shan & Doolen (1995) oder He & Doolen (1998)) über die Chapman-Enskog Entwicklung zu einem vom Forcing abhängigen Diffusionskoeffizienten für Mehrphasenströmungen führt. Der kritische Wert von G wird danach bestimmt und bekannte in der Literatur ad hoc verwendete Beziehungen abgeleitet (Huan et al. (2007)), sowie für tau=1 der bisher nicht erklärbare Shift um den Faktor 2 abgeleitet. Simulationen zeigen die exzellente Übereinstimmung des Diffusionskoeffizienten bspw. mit dem Fick’schen Gesetz für unterschiedliche Werte von G.The talk presents the influence of the choice of forcing schemes in pseudopotential-based LBM methods for multi-phase flows (e.g. Shan & Doolen (1995) or He & Doolen (1998)) on the diffusion coefficient, obtained via Chapman-Enskog analysis. The critical value of G is obtained und known relations used in the literature are subsequently derived based on this theory (e.g. Huan et al. (2007)). The often observed shift at tau=1 for the critical value of G can be directly derived. Simulation show excellent agreement between the diffusion coefficient and Fick’s law for various values of G

Regulation of benthic oxygen fluxes in permeable sediments of the coastal ocean

Large areas of the oceanic shelf are composed of sandy sediments through which reactive solutes are transported via porewater advection fueling active microbial communities. The advective oxygen transport in permeable sands of the North Sea was investigated under in situ conditions using a new benthic observatory to assess the dynamic interaction of hydrodynamics, sediment morphodynamics, and oxygen penetration depth. During 16 deployments, concurrent measurement of current velocity, sediment topography, and porewater oxygen concentration were carried out. In all cases the oxyclines were found at depths of 1–6 cm, correlating with the topography of stationary and migrating bedforms (ripples). Different conditions in terms of bottom water currents and bedform migration led to fluctuating oxygen penetration depths and, hence, highly variable redox conditions in up to 2.5 cm thick layers beneath the surface. Volumetric oxygen consumption rates of surface sediments were measured on board in flow-through reactors. Bedform migration was found to reduce consumption rates by up to 50%, presumably caused by the washout of organic carbon that is otherwise trapped in the pore space of the sediment. Based on the observations we found oxygen penetration depths to be largely controlled by oxygen consumption rates, grain size, and current velocity. These controlling variables are summarized by an adapted Damköhler number which allows for prediction of oxygen penetretion depths based on a simple scaling law. By integrating the oxygen consumption rates over the oxygen penetration depth, oxygen fluxes of 8–34 mmol m−2 d−1 were estimate