Nanodiamonds and the Usselo layer

514 Abstracts / Quaternary International 279-280 (2012) 462–565 be an alternative carbon source to plant-derived material for invertebrates in the sediment and water column of lakes, that can lead to very depleted d 13C values in invertebrate tissues, including their chitinous remains that preserve well in lake sediments. In studied lakes in Sweden and Siberia, d 13C values of remains of several invertebrate taxa in surface sediments were negatively correlated with diffusive methane fluxes, suggesting the incorporation of 13C-depleted methane-derived carbon. Strong and significant correlations are observed between diffusive methane fluxes and d 13C of Chironomini and Daphnia remains. Similar correlations were also observed within two Swedish lakes, in which surface sediment cores were obtained along transects with increasing water depth. d 13C values of Chironomini and Daphnia were lower in sections of the lake basin in which the sediments had higher methane release rates. These correlations suggest that d 13C in remains of several invertebrates, especially Chironomini and Daphnia can be indicative of methane availability in lakes. In a Siberian sediment record, d 13C values of the remains of Chironomini and Daphnia showed relatively large negative d 13C shifts in warm periods (between AD 850-1150 and since 1970) compared with d 13C values of bulk sediment and predominantly plant-feeding taxa. These results suggest increased methane availability in the studied lake during warm periods and demonstrate the potential to use taxon-specific d 13C analysis of invertebrate remains for qualitative reconstructions of methane availability in lakes. NANODIAMONDS AND THE USSELO LAYER Annelies van Hoesel. Utrecht University, Faculty of Geosciences, Netherlands E-mail address: hoesel@geo.uu.nl Nanodiamonds make up one of the important lines of evidence for the controversial hypothesis that an extraterrestrial impact took place at the onset of the Younger Dryas. These nanodiamonds have been found in the Allerød-Younger Dryas boundary layer or ‘black mat’ in North America, a section of the Usselo palaeosol in Belgium and in samples from the Greenland ice sheet. Nanodiamonds are known to occur in association with known impact events and within meteorites. However, the use of nanodiamonds as diagnostic evidence of an extraterrestrial impact is still debated. Concerning the Allerød-Younger Dryas boundary layer it has been suggested that the nanodiamonds accumulated over time from meteoritic rain or possibly formed during intense forest fires. In addition, it has been claimed that the nano-crystalline carbon in the North American black mat is graphene and not diamond. We have sampled the previously investigated Usselo layer in detail at two classic locations in the Netherlands, Aalsterhut (near Geldrop) and Lutterzand. Several individual charcoal particles of the Aalsterhut Usselo layer have been AMS dated to assess the variability in age in the Usselo layer at this location. Samples are analysed for the occurrence of nanodiamonds using electron microscopy. In addition, samples from modern wildfires and controlled heating experiments will be analysed for nanodiamonds to investigate possible non-impact related origins of the nanodiamonds. In the samples from the Usselo layer at Aalsterhut, we have found nano-crystalline carbon aggregates with selected area electron diffraction patterns similar to nanodiamond. MODELLING THE OPENING OF THE DOVER STRAIT: RETROGRADING PRO-GLACIAL LAKE SPILLAGE Annelies van Hoesel. Utrecht University, Faculty of Geosciences, Netherlands E-mail address: hoesel@geo.uu.nl Before the Eemian interglacial, a ‘landbridge’ linked Britain and continental Europe. The drainage divide separating north and south was approximately 30 m above interglacial sea-level over the Weald-Artois Chalk anticline. During the Middle Pleistocene the landbridge eroded, resulting in the Dover Strait seaway. The favoured theory explaining this erosion holds spillage from two great ice marginal lakes, one during the Anglian and one during the Saalian glacial maximum responsible for most of the removal. The first lake covered the Southern North Sea and most of the Netherlands and parts of Flanders. The second lake was smaller in area and most likely was restricted to the Southern North Sea only. We explored the hypothesis that erosion of the land bridge occurred as two episodes of 5000-15,000 years of steady spillway scouring and deepening during ice lake existence, slowly lowering and eroding the land bridge. This is an alternative for more-catastrophic ways of erosion, sustained for a few weeks only, involving lake drainage rather than steady spillage. We explore how capable steady spillage would be to erode a presumed landbridge during two glacial maxima epsiodes.We made a distributed landscape evolution model simulating progressive erosion in the study area. A palaeo-DEM of the region has been constructed and is used as input to the model. Discharge of the lake is calculated using the simulated runoff from the Hadley Center GCM and estimated glacier meltwater flux. The elevation of the lake spillway was probably around MSL during the time between the Anglian and Saalian. The model shows that this level is reached within 9,000 to 15,000 years. Erosion would need another 8,500 to 15,000 years during the Saalian to connect the spillway to the large icemargin rivers in the north. This provides mechanistic support for the idea that the Dover Strait opened due to gradual lake spillage predominanty, an explanation that we favour above more-catastrophic theories. LAST INTERGLACIAL (MIS 5E) SURFACE CIRCULATION IN THE NORTH ATLANTIC AND NORDIC SEAS, BASED ON DINOFLAGELLATE CYSTS Nicolas Van Nieuwenhove. IFM-GEOMAR, Germany E-mail address: nvannieuwenhove@ifm-geomar.de Sediments from the last interglacial (MIS 5e) were studied for their dinoflagellate cyst (dinocyst) content from cores across the North Atlantic and Nordic seas, in order to trace changes in the interaction between the warm water masses of the northward flowing Gulf Stream/North Atlantic Drift current system and the cold water masses of the East Greenland/ Labrador Current. Supported by stable isotope, IRD and planktic foraminiferal data, the qualitative and quantitative analysis of the dinocyst assemblages illustrates the stepwise development of interglacial conditions from late MIS 6 and Termination II towards a MIS 5e optimum and back to the colder MIS 5d stadial. It is shown that the development of the MIS 5e optimum occurred only late during MIS 5e in the eastern Nordic seas, as a consequence of a delayed intensification of the northernmost limb of the Gulf Stream/North Atlantic Drift. While prolonged deglacial processes presumably had hindered this northward protrusion of warm surface waters during early MIS 5e, its intensification during late MIS 5e in turn caused a reorganisation of the cold surface current system. DYNAMIC INTERACTION BETWEEN VEGETATION AND EROSION THROUGH HISTORICAL TIMES: EVIDENCE FROM THE AGRICULTURAL COMPLEX OF MELQUE (TOLEDO, SPAIN) Veerle Vanacker. Université Catholique de Louvain, Belgium E-mail address: veerle.vanacker@uclouvain.be In this paper, we present new data on temporal changes in erosion rates, and link these data with changes in vegetation and human occupation during ecohistorical periods. We work on the archaeological complex of Melque (Tajo basin, Toledo Province) as sedimentological and palynological analyses have provided excellent data for detailed palaeo-environmental reconstruction. Melque is a visigothic rural monastery and includes an agricultural complex with four small reservoirs that were constructed to irrigate crops. Seismic refraction measurements were realised in all four reservoirs to quantify accurately the volume of sediments that accumulated during historical times. Based on these results, two sediment profiles were selected and sampled for OSL dating, geochemical, mineralogical and cosmogenic nuclide analyses. In addition, four soil catenas were analysed in much detail. The variation in depth of the saprolite along these catenas was determined from seismic refraction measurements. Three soil pits were opened for each soil catena, and we quantified weathering intensities and soil production rates. Our data indicate that there exists an important variation of the cosmogenic nuclide concentrations with depth for the two sediment profiles. The 10Be concentrations that are measured in the sediment profiles are all significantly lower than the 10Be concentrations for the stream sediments. This indicates that erosion rates have been more important in the past. The soil production rates that are obtained for the twelve soil pits are all higher than the cosmogenic-derived erosion rates for the stream sediments. We can conclude that there exist large fluctuations in erosion rates through historical times for this archaeological complex. Our data on cosmogenic nuclide concentrations in soil and