Analyses of pollen, plant macrofossils (seeds, fruits, wood and mosses), molluscs, diatoms and vertebrate (mainly fish) remains allowed a detailed reconstruction of a middle-Holocene alluvial forest and its associated hydrological conditions. The use of multiple proxies resulted in a taxonomically more detailed and environmentally more comprehensive understanding of terrestrial as well as aquatic habitats. The results demonstrate possible biases in palaeoecological reconstructions of alluvial and estuarine environments drawn from single proxies. Many locally occurring woody taxa were underrepresented or remained undetected by pollen analyses. Seeds and fruits also proved to be inadequate to detect several locally important taxa, such as Ulmus and Hedera helix. Apparently brackish conditions inferred from diatoms, pollen and other microfossils conflicted strikingly with the evidence from molluscs, fish bones and botanical macroremains which suggest a freshwater environment. Brackish sediment (and the microfossil indicators) is likely to have been deposited during spring tides or storm surges, when estuarine waters penetrated more inland than usual. Despite the reworking and deposition of estuarine and saltmarsh sediment well above the tidal node at such events, local salinity levels largely remained unaffected.
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The seabed of the North Sea is covered with ammunition dating back from World Wars I and II. With increasing human interference (e.g. fisheries, aggregate extraction, harbor related activities), it forms a threat to the safety at sea. In this study, test mines were deployed on a sandy seabed for 3 months to investigate mine burial processes as a function of hydrodynamic and meteorological conditions. The mine experiment was conducted in a shallow (9 m), macrotidal environment characterized by highly turbid waters (yearly and depth-averaged suspended particulate matter concentration of 100 mg/l). Results showed some variability of the overall mine burial, which corresponded with scouring processes induced by a (sub-) tidal forcing mechanism. The main burial events however were linked to storm-related scouring processes, and subsequent mine roll into the resulting pit. Two storms affecting the mines during the 3-month experiment resulted in enduring increases in burial volume to 60% and 80%, respectively. More cyclic and ephemeral burial and exposure events appear to be linked to the local hydrodynamic regime. During slack tides, suspended sediment settles on the seabed, increasing the burial volume. In between slack tides, sediment is resuspended, decreasing the burial volume. The temporal pattern of this never reported burial mechanism, as measured optically, mimics the cyclicity of the suspended sediment concentration as recorded by ultrasonic signals at a nearby benthic observatory. Given the similarity in response signals at the two sites, we hypothesize that the formation of high-concentrated mud suspensions (HCMS) is a mechanism causing short-term burial and exposure of mines. This short-term burial and exposure increase the chance that mines are ‘missed’ during tracking surveys. Test mines contribute to our understanding of the settling and erosion of HCMS, and thus shed a light on generic sedimentary processes.
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