ALBERT

Description: Investigations at a Late Weichselian freshwater basin in northwestern Jutland, Denmark, yielded a fairly rich assemblage of vertebrate remains, mostly bones and teeth of small mammals. The remains are primarily allochthonous and the bones have been subjected to different taphonomic pathways and agents. AMS 14C-dates on terrestrial organic remains provided ages of Middle to Late Allerød time. Identifications revealed the first fossil record in Scandinavia of Rana arvalis, Sorex minutus, Ochotona cf. pusilla, Microtus gregalis, Microtus oeconomus, and Sicista cf. betulinu. Spermophilus cf. major and Desmana moschata, previously found only once and twice respectively, were retrieved, and Sorex araneus and Arvicola terrestris were recovered for the first time beyond the Atlantic chronozone. Ecologically, the Nørre Lyngby small mammal fauna can be characterized by its very high and almost equal proportions of boreal forest and steppe elements followed by a relatively high proportion of tundra elements. The fossil species share a modern area of sympatry north of the Caspian Sea from the river Volga in the west to the southern and western slopes of the Urals. If, however, the large Allerød mammals are added, the fauna is without modern analogues. The Nørre Lyngby fauna can be seen as a last expansion of the North European glacial fauna. Provided that an absolute chronology and a differentiated sea-level curve for the area can be established, the Nørre Lyngby fauna could become important for studies in mammalian dispersal and migration rates.

Description: A new radiolarian-based transfer function for sea surface temperature (SST) estimations has been developed from 23 taxa and taxa groups in 53 surface sediment samples recovered between 35° and 72°S in the Atlantic sector of the Southern Ocean. For the selection of taxa and taxa groups ecological information from water column studies was considered. The transfer function allows the estimation of austral summer SST (December-March) ranging between -1 and 18°C with a standard error of estimate of 1.2°C. SST estimates from selected late Pleistocene squences were sucessfully compared with independend paleotemperature estimates derived from a diatom transfer function. This shows that radiolarians provide an excellent tool for paleotemperature reconstructions in Pleistocene sediments of the Southern Ocean.

Description: Microstructural investigations of ocean crust samples provide a complementary approach to both marine surveys and laboratory experiments. The recovery of relatively undeformed diabases from Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) Hole 504B provides a first opportunity to examine a reference section of microstructural features that influence strain localization at depths of 2 km in the ocean crust. Syn- and post-crystallization features in plagioclase and augite crystals have been examined by optical microscopy and secondary and backscattered electron imaging. These features show a strong influence of modal composition and primary textures on early sites of strain localization. Thermal cracking and subsequent alteration intensities and distribution are strongly phase dependent. A consistently higher intragranular fracture density is observed in augite crystals relative to plagioclase. The impact of alteration on the mechanical response of diabases is likely to depend on the primary textural characteristics. Even where extensive augite alteration occurs, the rock remains supported by a framework of weakly altered plagioclase crystals. The Hole 504B diabases from Leg 140 provide a valuable comparison for future studies of more deformed sections likely to be encountered at depth. Advances in constraining the detailed rheology of the ocean crust at spreading centers would benefit from experimental deformation of texturally diverse diabase and gabbro samples.

Description: Dolerite samples, recovered in the dike complex between 1589 and 1991 meters below seafloor in Hole 504B during Legs 137/140, have been altered by hydrothermal fluids as shown by the intense replacement of igneous minerals by secondary amphiboles, chlorites, albite, and anorthite. These rocks display a negative correlation between alteration extent and density. This is interpreted in terms of the development of secondary light minerals and an increase in porosity. For hydrogen, secondary hydroxyl groups (amphibole) dominate over igneous volatiles and control the dD(whole-rock) (-53 per mil to -40 per mil mean = -44 per mil), the D/H fractionation between amphibole (actinolite) and water being about -44 per mil ± 3 per mil at 350°C assuming the fluid has a sea water D/H ratio (dD(fluid) = 0 per mil). The d18O(whole-rock) values, ranging from 4.7 per mil to 2.7 per mil (mean = 4.1 per mil), are negatively correlated with the abundance of secondary amphiboles, chlorites, and albite (alteration percent) determined by point counting. The comparison of halos related to veins or patches with less altered adjacent parts shows that the process of alteration produces an increase in the amphibole content related to a decrease in the d18O(halo/patch). The halos/patches from the deepest samples in Hole 504B have the lowest d18O(whole-rock) values. This feature can be explained by the progressively decreasing proportion of albite with depth in the dike section. According to Taylor's model, the depletion in 18O of these dolerites (relative to fresh magmatic values: d18O(whole-rock) ~5.7 per mil) is explained by seawater-rock interaction in which the seawater/rock ratio is higher than 0.15 in atomic proportion (~0.3 in mass). Consequently, we deduce that large quantities of seawater-derived fluids have circulated deep in the lower part of the sheeted dike complex of Hole 504B. The d18O profile in Hole 504B shows a net disequilibrium in the 18O exchange between seawater and the oceanic crust in favor of enrichment in 18O in the seawater. This can be explained by a particular strong expression of high-temperature hydrothermal interaction with seawater-derived fluids at shallow depths (i.e., in Layer 2).