Early Holocene water budget of the Nakuru-Elmenteita basin, Central Kenya Rift

The Nakuru-Elmenteita basin in the Central Kenya Rift, contains two shallow, alkaline lakes, Lake Nakuru (1770m above sea level) and Lake Elmenteita (1786m). Ancient shorelines and lake sediments at 1940m suggest that these two lakes formed a single large and deep lake as a result of a wetter climate during the early Holocene. Here, we used a hydrological model to compare the precipitation-evaporation balance during the early Holocene to today. Assuming that the Nakuru-Elmenteita basin was hydrologically closed, as it is today, the most likely climate scenario includes a 45% increase in mean-annual precipitation, a 0.5°C decrease in air temperature, and an increase of 9% in cloud coverage from the modern values. Compared to the modeling results from other East African lake basins, this dramatic increase in precipitation seems to be unrealistic. Therefore, we propose a significant flow of water from the early Holocene Lake Naivasha in the south towards the Nakuru-Elmenteita basin to compensate the extremely negative hydrological budget of this basin. Since we did not find any field evidence for a surface connection, as often proposed during the last 70years, the hydrological deficit of the Nakuru-Elmenteita basin could have also been compensated by a subsurface water exchang

Topographic stress and rock fracture: a two-dimensional numerical model for arbitrary topography and preliminary comparison with borehole observations

Theoretical calculations indicate that elastic stresses induced by surface topography may be large enough in some landscapes to fracture rocks, which in turn could influence slope stability, erosion rates, and bedrock hydrologic properties. These calculations typically have involved idealized topographic profiles, with few direct comparisons of predicted topographic stresses and observed fractures at specific field sites. We use a numerical model to calculate the stresses induced by measured topographic profiles and compare the calculated stress field with fractures observed in shallow boreholes. The model uses a boundary element method to calculate the stress distribution beneath an arbitrary topographic profile in the presence of ambient tectonic stress. When applied to a topographic profile across the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania, the model predicts where shear fractures would occur based on a Mohr–Coulomb criterion, with considerable differences in profiles of stresses with depth beneath ridgetops and valley floors. We calculate the minimum cohesion required to prevent shear failure, C[subscript min], as a proxy for the potential for fracturing or reactivation of existing fractures. We compare depth profiles of C[subscript min] with structural analyses of image logs from four boreholes located on the valley floor, and find that fracture abundance declines sharply with depth in the uppermost 15 m of the bedrock, consistent with the modeled profile of C[subscript min]. In contrast, C[subscript min] increases with depth at comparable depths below ridgetops, suggesting that ridgetop fracture abundance patterns may differ if topographic stresses are indeed important. Thus, the present results are consistent with the hypothesis that topography can influence subsurface rock fracture patterns and provide a basis for further observational tests.United States. Army Research Office (Award W911NF-14-1-0037)United States. Dept. of Energy (Award DE-FG01-97ER14760

Timing and patterns of debris flow deposition on Shepherd and Symmes Creek fans, Owens Valley, California, deduced from cosmogenic 10Be

Debris-flow fans on the western side of Owens Valley, California, show differences in their depths of fan head incision, and thus preserve significantly different surface records of sedimentation over glacial-interglacial cycles. We mapped fan lobes on two fans (Symmes and Shepherd Creek) based on the geometry of the deposits using field observations and high-resolution Airborne Laser Swath Mapping (ALSM) data, and established an absolute fan lobe chronology by using cosmogenic radionuclide exposure dating of large debris-flow boulders. While both fans and their associated catchments were subject to similar tectonic and base level conditions, the Shepherd Creek catchment was significantly glaciated while that of Symmes Creek experienced only minor glaciation. Differences in the depth of fan head incision have led to cosmogenic surface age chronologies that differ in the length of the preserved depositional records. Symmes Creek fan preserves evidence of exclusively Holocene deposition with cosmogenic 10Be ages ranging from 8 to 3 ka. In contrast, the Shepherd Creek fan surface was formed by late Pleistocene and Holocene debris-flow activity, with major deposition between 86-74, 33-15, and 11-3 ka. These age constraints on the depositional timing in Owens Valley show that debris-flow deposition in Owens Valley occurred during both glacial and interglacial periods, but may have been enhanced during marine isotope stages 4 and 2. The striking differences in the surface record of debris-flow deposition on adjacent fans have implications for the use of fan surfaces as paleoenvironmental recorders, and for the preservation of debris-flow deposits in the stratigraphic record

Holocene warming of alpine rockwalls decreased rockwall erosion rates

Alpine rockwall erosion studies suggest that deglaciated rockwalls in the European Alps are eroding slower today than at earlier times in the Holocene. Explanations for this have included a waned glacial debuttressing effect since the retreat of Last Glacial ice loads and the establishment of more moderate climates, but seldom have such explanations been robustly tested. We combine field data with modelling to reconstruct changes in rockwall erosion and associated climate drivers, in an alpine valley of the European Alps since the beginning of the Holocene. Paleo (Holocene to decadal-scale) erosion rates were calculated from talus accumulation below rockwalls, and compared with recent rates (2016–2019) measured from repeat laserscan surveys of the same rockwalls. We reconstructed the glacial retreat history in the valley, and modelled Holocene changes in permafrost distribution and frost cracking using calibrated rockwall temperature reconstructions. We found that rockwalls that have been free of glacier ice since ∼10 ka experienced higher Holocene-averaged erosion rates compared to recent erosion rates. Our modelling suggests this relates to periods of higher intensities of frost cracking and cycles of permafrost aggradation and degradation in the Holocene, relative to today. For a recently deglaciated high-elevation rockwall, erosion rates were 1–2 orders of magnitude greater than the lower-elevation sites, but decayed rapidly over time since deglaciation. A high, but rapidly decaying rate results from short-lived paraglacial adjustment, permafrost thaw and high frost cracking activity. Our findings suggest that periglacial activity strongly influences the rates and patterns of erosion of deglaciated alpine rockwalls. This helps to explain why paleo Holocene deglaciated rockwall erosion rates tend to exceed recent rates in the European Alps

Glacier-bed geomorphic processes and hydrologic conditions relevant to nuclear waste disposal

Characterizing glaciotectonic deformation, glacial erosion and sedimentation, and basal hydrologic conditions of ice sheets is vital for selecting sites for nuclear waste repositories at high latitudes. Glaciotectonic deformation is enhanced by excess pore pressures that commonly persist near ice sheet margins. Depths of such deformation can extend locally to a few tens of meters, with depths up to approximately 300 m in exceptional cases. Rates of glacial erosion are highly variable (0.05–15 mm a−1), but ratesa−1 are expected in tectonically quiescent regions. Total erosion probably not exceeding several tens of meters is expected during a glacial cycle, although locally erosion could be greater. Consolidation of glacial sediments that is less than expected from independent estimates of glacier thickness indicates that heads at the bases of past ice sheets were usually within 30% of the floatation value. This conclusion is reinforced by direct measurements of water pressure beneath portions of the West Antarctic ice sheet, which indicate average headsbed, despite thick ice at subfreezing temperatures. Therefore, in models of subglacial groundwater flow used to assess sites for nuclear waste repositories, a flux upper boundary condition based on water input from only basal melting will be far more uncertain than applying a hydraulic head at the upper boundary set equal to a large fraction of the floatation value

Lateral plucking as a mechanism for elongate erosional glacial bedforms : explaining megagrooves in Britain and Canada

Megagrooves are kilometre-scale linear topographic lows carved in bedrock, separated by ridges, typically in areas of largely devoid of till. They have been reported from several areas covered by Pleistocene glaciations, such as Canadian NW Territories, Michigan and NW Scotland. Here we report two previously undocumented megagroove fields from Ungava, Canada, and northern England, and present new analyses of the megagrooves from NW Scotland. This paper seeks to determine the nature of the lithological and structural controls on the occurrence and formation of megagrooves. Analysis of both geomorphological and bedrock properties shows that megagrooves are generally: a) confined to well stratified or layered bedrock, such as (meta)sedimentary rocks with closely spaced joints, and tend not to occur on massive rocks such as gneiss or granite, or thick-bedded sedimentary rocks; b) subparallel to palaeo-ice flow and the strike of the strata; and tend not to occur where palaeo-ice flow is at high angles to the strike of strata; c) produced by significant glacial erosion by sustained unidirectional ice flow. Detailed analysis of megagrooves in NW Scotland shows that neither glacio-fluvial erosion, nor differential abrasion was the dominant mechanism of formation. A mechanism, here termed ‘lateral plucking’, is suggested that involves block plucking on rock steps parallel to ice flow. Removal of joint-bounded blocks from such rock steps involves a component of rotation along a vertical axis. Block removal may be enhanced by a direct component of shear stress onto the vertical stoss sides. The lateral plucking mechanism results in horizontal erosion at right angles to the ice flow, and enhances the groove/ridge topography. Megagrooves are potentially useful as palaeo-ice flow indicators in areas devoid of till, and can thus complement the palaeo-ice stream datasets which are presently largely based on soft-sediment landform studies

Early Holocene water budget of the Nakuru-Elmenteita basin, Central Kenya Rift

ISSN:0921-2728ISSN:1573-041