ALBERT

Beschreibung: Basaltic andesites of the Ongeluk Formation, Transvaal Supergroup in Griqualand West constitute part of a large flood volcanic province that extruded at ca. 2.22 Ga onto the partly submerged Kaapvaal Craton. Pods and short veins of megaquartz, with albite, epidote and traces of calcite and Cu-Ni-Co sulphides are closely associated with beds and pods of jasper and chert that are products of low-temperature seafloor alteration. Characterization of the megaquartz pods and veins yields evidence for an origin in a ''passive'' seafloor alteration regime. Fluid inclusion studies suggest the involvement of two aqueous fluid end members, one NaCl-dominated with salinity similar to modern seawater, the other Ca-dominated and with distinctly elevated salinity. The chemical composition of both fluids is akin to that of seawater, modified in its composition by interaction with the volcanic host rock. The low salinity fluid appears only little affected by fluid-rock interaction processes; the composition of the more saline Ca-rich fluid is more distinctly modified. The chemical composition of the two fluids has important implications for our understanding of the composition of ocean water during the Paleoproterozoic Era. Cl/Br ratios, widely regarded as being conservative in hydrothermal solutions, are significantly below those of present-day seawater, but remarkably similar to that predicted for Archean seawater. This observation suggests that Paleoproterozoic seawater was still buffered by vent fluids, and lacked sufficient organic matter to fractionate Cl from Br.

Beschreibung: River-borne quartz carries a cosmogenic nuclide memory that is a function of the catchment-wide erosion rate. This record may be preserved in fluvial deposits such as river terraces. If the age of a terrace is independently known and transport time in the river system is relatively short, then the upstream erosion rate at the time of terrace deposition can be determined. We have used cosmogenic nuclides to date river terraces in the lower Meuse catchment, the Netherlands, and to obtain a 1.3 Ma record of paleoerosion rates in a 104-km2 drainage basin comprising the Ardennes Mountains. Paleoerosion rates were uniform within the range of 25–35 mm/ka from 1.3 to 0.7 Ma. After 0.7 Ma, erosion rates have increased progressively to Late Pleistocene values of around 80 mm/ka. Around 0.7 Ma, both climatic and tectonic boundary conditions changed. The amplitude and duration of climate cycles increased significantly, resulting in long periods of sustained low temperatures in the Meuse catchment. In addition, an episode of magmatic underplating and mafic volcanism in the nearby Eifel caused up to 250 m of surface uplift in the Meuse catchment. The main streams in the region have responded to the perturbation at 0.7 Ma within a few 105 yr. Our data indicate that the catchment-wide response time is much longer. Further investigations are required to attribute the observed increase in paleoerosion rates to one or the other mechanism discussed.

Beschreibung: Ocean isotope tracers of intermediate residence time are becoming of increasing importance in studies of paleo-ocean circulation, global weathering fluxes, and trace pollutant dispersion in ocean basins. While analytical models exist that predict the way in which conservative tracers are dispersed in ocean gyres, the lateral movement of reactive tracers has not yet received much attention. Here we present time-dependent numerical simulations of the lateral mixing process (advection-diffusion-reaction) of reactive tracers with different residence times in gyres with varying Péclet number, diffusivity, western boundary intensification, and source geometry. Our main conclusions are that in our model gyre (1) homogenization of tracer concentrations and also of isotopic ratios in those cases where the isotopes have different pathways (e.g., atmospherically deposited cosmogenic 10Be, continent-derived 9Be depends largely on the residence time and eddy diffusivity and isonly a weak function of Péclet number; (2) western intensification influences homogenization only in the case of short residence times and large Péclet numbers; (3) homogenization of isotopic ratios of continent-sourced tracers (e.g., 206Pb/204Pb, 143Nd/144Nd) is favored by high water velocities and hence shallow ocean levels; (4) boundary scavenging can result in significant lateral redistribution of output fluxes for even very short residence time tracers (e.g., 230Th) if Péclet numbers are high; (5) the width of source and scavenging regions exerts a strong control over the actual tracer concentrations, but the spatial distribution and degree of tracer homogenization is not affected. The space dependence of input and scavenging is applicable to several reactive isotope tracers such as Be, Pb, Nd, Hf, Th, or Pa. Relative uniformity in their isotope ratios has been observed by mapping of these tracers in individual ocean basins (von Blanckenburg and Igel, 1999). Such uniformity would not be necessarily expected since the isotopes concerned enter through entirely different pathways. The simulations presented here demonstrate that homogenization of such short residence time tracers in ocean gyres is a feasible mechanism that can account for these observations.

Beschreibung: Hydrogenetic ferromanganese crusts incorporate elements from ambient seawater during their growth on seamounts. By analysing Nd, Pb and Be isotope profiles within crusts it is possible to reconstruct seawater tracer histories. Depth profiles of 10Be/9Be ratios in three Pacific ferromanganese crusts have been used to obtain growth rates which are between 1.4 and 3.8 mm/Ma. Nd and Pb isotopes provide intact records of isotopic variations in Pacific seawater over the last 20 Ma or more. There were only small changes in Pb isotope composition in the last 20 Ma. This indicates a constant Pb composition for the erosional sources and suggests further that erosional Nd inputs may have been uniform too. εNd values vary considerably with time and most probably reflect changes in ocean circulation. The Epsilon(tief:Nd) values of the crusts not only vary as a function of age but also as a function of water depth. From 25 to 0 Ma, crust VAl3/2 from 4.8 km water depth has a similar pattern of εNd variation to the two shallower crusts from 1.8 and 2.3 km, but about 1.0 to 1.5 units more negative. This suggests that εNd stratification in Pacific seawater, as demonstrated for the present day, has been maintained for at least 20 Ma. Each crust shows a decrease in εNd from 3-5 Ma to the present, which is interpreted in terms of an increase in the NADW component present in the Pacific. From 10 to 3-5 Ma ago the crusts show an increase in εNd. This suggests a decreasing role for a deep water source with εNd less than circum-Pacific sources. In this regard the Panamanian gateway restriction from ~ 10 Ma with final closure at 3-5 Ma may have played an important role in reducing access of Atlantic-derived Nd to the Pacific.

Beschreibung: Knickpoints are a reliable indicator of stream channel incision. How these features impact a drainage basin as a whole as they migrate upstream however is less clear. To address this question, we have studied drainage basins in the Swiss Mitteland that have differing histories with respect to knickpoints and knickpoint migration. The Grosse Fontanne is a bedrock stream that drains the Molasse conglomerates of the Napf region. The system has been described as an actively incising rejuvenated landscape. Incised valleys and knickpoints are present on the trunk stream and many tributaries. Seven basinaveraged erosion rates varying from 330 to 640 mm/ky, derived from cosmogenic nuclides, were calculated for the watershed. The highest rates were recorded for the trunk stream near the confluence with the Kleine Emme River, while the lowest rates are found in small headwater basins. Of the samples, three were collected upstream of visible knickpoints, and two were collected downstream of visible knickpoints. The two remaining samples were collected on tributaries that had no visible knickpoints. The upstream erosion rates are lower, 350 mm/ky vs. 560 mm/ky, than the downstream erosion rates. The watersheds with no visible knickpoints are similar to the downstream samples, eroding at 500 mm/ky, suggesting that the knickpoint has already propagated through these catchments. The basins with active or presumed relict knickpoints also tend to have higher average relief than the basins which the knickpoints have not yet reached. The timing of initial formation, based on the volume of incision and a calculated erosion rate for the areas experiencing rapid downcutting, appears to be post-glacial. The cause of the knickpoint formation appears to be stream piracy by the Kleine Emme.

Beschreibung: The fossil record of the subsurface biosphere is sparse. Results obtained on subsurface filamentous fabrics (SFF) from 〉225 paleosubsurface sites in volcanics, oxidized ores, and paleokarst of subrecent to Proterozoic age are presented. SFF are mineral encrustations on filamentous or fibrous substrates that formed in subsurface environments. SFF occur in association with low-temperature aqueous mineral assemblages and consist of tubular, micron-thick (median 1.6 micron) filaments in high spatial density, which occur as irregular masses, matted fabrics, and vertically draped features that resemble stalactites. Micron-sized filamentous centers rule out a stalactitic origin. Morphometric analysis of SFF filamentous forms demonstrates that their shape more closely resembles microbial filaments than fibrous minerals. Abiogenic filament-like forms are considered unlikely precursors of most SFF, because abiogenic forms differ in the distribution of widths and have a lower degree of curvature and a lower number of direction changes. Elemental analyses of SFF show depletion in immobile elements (e.g., Al, Th) and a systematic enrichment in As and Sb, which demonstrates a relation to environments with high flows of water. Sulfur isotopic analyses are consistent with a biological origin of a SFF sample from a Mississippi Valley–Type deposit, which is consistent with data in the literature. Fe isotopes in SFF and active analogue systems, however, allow no discrimination between biogenic and abiogenic origins. The origin of most SFF is explained as permineralized remains of microbial filaments that possibly record rapid growth during phases of high water flow that released chemical energy. It is possible that some SFF formed due to encrustation of mineral fibers. SFF share similarities with Microcodium from soil environments. SFF are a logical target in the search for past life on Mars. The macroscopic nature of many SFF allows for their relatively easy in situ recognition and targeting for more detailed microstructural and geochemical analysis.

Beschreibung: A 450-year spring/summer flood layer time series at seasonal resolution has been established from the varved sediment record of Lake Ammersee (Southern Germany) applying a novel methodological approach. The main results are (1) a precise chronology obtained by microscopic varve counting, (2) the identification of detrital layers representing flood2 triggered fluxes of catchment material into the lake, and (3) the recognition of the seasonality of these flood layers from their micro-stratigraphic position within a varve. Tracing flood layers in a proximal and a distal core and correlating them applying the precise chronology provided information on the depositional processes. Comparing the seasonal flood layer record with daily runoff data of the inflowing River Ammer for the period from 1926 to 1999 allowed defining an approximate threshold in flood magnitude above which the formation of flood layers becomes very likely. Moreover, it was for the first time possible to estimate the ‘completeness’ of the flood layer time series and to recognize that mainly floods in spring and summer representing the main flood seasons in this region are well preserved in the sediment archive. Their frequency distribution over the entire 450-year time series is not stationary, but reveals maxima for colder periods of the Little Ice Age when solar activity was reduced. The observed spring/summer flood layer frequency further shows similar trends as the occurrence of flood-prone weather regimes since AD 1881, probably suggesting a causal link between solar variability and changes in mid-latitude atmospheric circulation patterns.