ALBERT

Description: 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.

Description: 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.

Description: Cosmogenic nuclides, measured in quartz from recent river bedload, provide a novel tool to quantify catchment-wide erosion rates at geologically meaningful time scales. Here we present an analysis of the geomorphological evolution of the 350 km2 Wutach catchment in the uplands of the south-west German Black Forest. The robustness of the method is demonstrated by the fact that, although the area was affected by river capture at 18 kyr BP, the formed gorge is so narrow that spatially averaged erosion rates were not resolvably perturbed. However, because cosmogenic nuclides preserve an erosion memory of several thousand years, the only perturbation introduced was detected in the minor areas that have been subject to the last maximum glaciation. In unglaciated areas, an important relationship between lithology and erosion can by quantified: sandstone lithologies erode at 12–18 mm kyr-1, granite lithologies at 35–47 mm kyr-1 and limestone lithologies (as deduced from river load gauging) at 70–90 mm kyr-1.

Description: Geological reconstructions and general circulation models suggest that the onset of both Northern Hemisphere glaciations, 2.7 Myr ago, and convection of Labrador SeaWater (LSW) were caused by the closure of the Panama Gateway ∼ 4.5 Myr ago. Time series data that have been obtained from studies of ferromanganese crusts from the northwestern Atlantic suggest that radiogenic isotopes of intermediate ocean residence time (Pb and Nd) can serve as suitable tracersto reconstruct these events. However, it has been unclear until now as to whether the changes that have been observed in isotope composition at this time are the result of increased thermohaline circulation or due to the effects of increased glacial weathering. In this paper we adopt a box model approach to demonstrate that the shifts in radiogenic isotope compositions are unlikely to be due to changes in convection in LSW but can be explained in terms of increases of erosion levels due to the glaciations of Greenland and Canada. Furthermore, we provide experimental evidence for the incongruent release of a labile fraction of strongly radiogenic Pb and nonradiogenic Nd from continental detritus eroding into the Labrador Sea. This can be attributed to the glacial weathering of old continents and accounts for the paradox that one of the areas of the world most deficient in radiogenic Pb should provide such a rich supply of radiogenic Pb to the oceans. An important general Conclusion is that the compositions of radiogenic isotopes in seawater are not always a reflection of their continental sources. Perhaps more importantly, the transition from chemical weathering to mechanical erosion is likely to result in significant variations in radiogenic tracers in seawater.

Description: Some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep, humid, tropical highlands of Sri Lanka. The total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites, bedrock from mountain crests, and bedrock from inselbergs. Denudation rates are in the range of 5-30 t km-2 yr-1(2-11 mm ky-1). These rates average denudation over the last 50-250 ky. Weathering exports in rivers draining the mountainous Central Highlands show that silicate weathering rates are also low, varying from 5 to 20 t km-2 yr-1 today (2-7 mm ky-1), but they represent a significant fraction of the total denudation. All these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief, temperatures, and precipitation. We speculate that the high relief in Sri Lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 Ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup. It is possible that throughout this history, hillslopes, where not exposing bare bedrock, were protected by thick weathered profiles. Such clay-rich layers would inhibit silicate weathering by shielding bedrock from weathering agents. In the absence of landscape rejuvenation, physical erosion rates are low, and fresh mineral surfaces are not being supplied. The observation that wet, steep, tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long-term controls of atmospheric CO2 budgets: High temperature and precipitation, which are much invoked though controversial agents for silicate dissolution and CO2 drawdown, become ineffective in promoting weathering in areas that are not tectonically active.

Description: Cover beds, widespread on hillslopes of temperate climate zones, represent layers of allochthonous material laterally transported by periglacial processes during the Late Pleistocene. Two soil sections comprised of cover beds from the Bavarian Forest, SE Germany, have been analysed for in situ-produced cosmogenic 10Be. Major changes in the nuclide concentration agree well with soil section boundaries defined by field observations and grain size analyses. Numeric modeling of these cosmogenic nuclide sections demonstrates that simple continuous erosion and regolith mixing models fail to explain the measured nuclide concentrations. Instead, the measured data can be best described by modeling an admixture of material such as loess or reworked allochthonous material, which have different nuclide concentrations. A comparison of cosmogenic nuclide concentrations from the two cover bed sections with concentrations from river bedload sediments of the Regen catchment reveals that cover bed formation might affect the result of basin-wide erosion rate determinations based on cosmogenic nuclides. Nuclide concentration of river bedload is potentially a binary mixture produced by (1) spatial erosion of the soil surfaces; and (2) spatially nonuniform incision into deep cover bed layers that contributes sediment low in nuclide concentration.