Age and weathering rate of sediments in small catchments:The role of hillslope erosion

Uranium-series (U-series) isotopes in river material can be used to determine quantitative time constraints on the transfer of erosion products from source to sink. In this study, we investigate the U-series isotope composition of river-borne material in small catchments of Puerto Rico and southeastern Australia in order to improve our understanding of (i) the controls on the U-series isotope composition of river-borne material and (ii) how erosion products acquire their geochemical characteristics. In both regions, thorium isotopes track the origin of sediment and dissolved loads. Stream solutes are mainly derived from the deepest part of the weathering profile, whereas stream sediments originate from much shallower horizons, even in landslide-dominated Puerto Rican catchments. This suggests that in environments where thick weathering profiles have developed, solutes and sediments have distinct origins. The U-series isotope composition of stream sediments was modelled to infer a weathering age, i.e. the average time elapsed since the sediment\u27s minerals have started weathering. In southeastern Australia, the weathering age of stream sediments ranges between 346 ± 12 kyr and 1.78 ± 0.16 Myr, similar to values inferred from weathering profiles in the same catchment. Old weathering ages likely reflect the shallow origin of sediments mobilised via near-surface soil transport, the main mechanism of erosion in this catchment. Contrastingly, in Puerto Rico weathering ages are much younger, ranging from 5.1 ± 0.1 to 19.4 ± 0.4 kyr, reflecting that sediments are derived from less weathered, deeper saprolite, mobilised by landslides. Weathering ages of stream sediments are used to infer catchment-wide, mineral-specific weathering rates that are one to two orders of magnitude faster for Puerto Rico than for southeastern Australia. Thus, the type of erosion (near-surface soil transport vs. landslide) also affects the weathering rate of river sediments, because their weathering ages determine the potential for further weathering during sediment transport and storage in alluvial plains

The impact of vegetation on fractionation of rare earth elements(REE) during water–rock interaction

Previous studies on waters of a streamlet in the Vosges mountains (eastern France) have shown that Sr and rare earth elements (REE) principally originate from apatite dissolution during weathering. However, stream water REE patterns normalized to apatite are still depleted in light REE (LREE, La–Sm) pointing to the presence of an additional LREE depleting process. Speciation calculations indicate that complexation cannot explain this additional LREE depletion. In contrast, vegetation samples are strongly enriched in LREE compared to water and their Sr and Nd isotopic compositions are comparable with those of apatite and waters. Thus, the preferential LREE uptake by the plants at the root–water–soil (apatite) interface might lead to an additional LREE depletion of the waters in the forested catchment. Mass balance calculations indicate that the yearly LREE uptake by vegetation is comparable with the LREE export by the streamlet and, therefore, might be an important factor controlling the LREE depletion in river waters

U–Th and ¹⁰Be constraints on sediment recycling in proglacial settings, Lago Buenos Aires, Patagonia

International audienceThe estimation of sediment transfer times remains a challenge to our understanding of sediment budgets and the relationships between erosion and climate. Uranium (U) and thorium (Th) isotope disequilibria offer a means of more robustly constraining sediment transfer times. Here, we present new uranium and tho-rium disequilibrium data for a series of nested moraines around Lago Buenos Aires in Argentine Patagonia. The glacial chronology for the area is constrained using in situ cosmogenic 10 Be analysis of glacial outwash. Sediment transfer times within the periglacial domain were estimated by comparing the deposition ages of moraines to the theoretical age of sediment production, i.e., the comminution age inferred from U disequilibrium data and recoil loss factor estimates. Our data show first that the classical comminution age approach must include weathering processes accounted for by measuring Th disequilibrium. Second, our combined data suggest that the pre-deposition history of the moraine sediments is not negligible, as evidenced by the large disequilibrium of the youngest moraines despite the equilibrium of the corresponding glacial flour. Monte Carlo simulations suggest that weathering was more intense before the deposition of the moraines and that the transfer time of the fine sediments to the moraines was on the order of 100-200 kyr. Long transfer times could result from a combination of long sediment residence times in the proglacial lake (recurrence time of a glacial cycle) and the remobilization of sediments from moraines deposited during previous glacial cycles. 10 Be data suggest that some glacial cycles are absent from the preserved moraine record (seemingly every second cycle), supporting a model of reworking moraines and/or fluctuations in the extent of glacial advances. The chronological pattern is consistent with the U-Th disequilibrium data and the 100-200 kyr transfer time. This long transfer time raises the question of the proportion of freshly eroded sediments that escape (or not) the proglacial environments during glacial periods

Is subsurface geophysics as seismic and acoustic investigations a rescue to groundwater flow inversion?

Understanding subsurface flow, especially in partly karstified rock formations mainly housing water through a few preferential pathways, is still challenging. This point is the consequence of the poor accessibility of the subsurface and lack of accurate depictions of water bearing bodies and distributions. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface.A 3-D seismic survey with shots and wave monitoring at the surface is carried out over a subsurface karstified reservoir located at the Hydrogeological Experimental Site (HES) of the University of Poitiers (France). Processing the 3-D data, in association with wave velocity calibration from vertical seismic profiles (VSP) recorded via geophones in wells, renders a 3-D velocity block. The velocity block is then converted into pseudo-porosity values revealing three high-porosity, presumably water-productive, layers, at depths of 35–40, 85–87, and 110–115 m.In addition, full wave acoustic logging (FWAL) can detect, close to wells, porous or open bodies that are too small for being captured by the spatial resolution of 3-D seismic images. A FWAL can also confirm or invalidate data from VSP recorded via hydrophones.The block of pseudo-porosities is compared to a different representation of the subsurface in the form of hydraulic conductivity distributions (or hydraulic diffusion) obtained by slug tests or by inversion of transient interference testing between wells. The inverted hydraulic conductivity maps do not match up the distribution of porous bodies identified by seismic data. This poses the question of guiding conventional inversions on the basis of a prior guess as the subsurface structure obtained via geophysical investigations

Is subsurface geophysics as seismic and acoustic investigations a rescue to groundwater flow inversion?

Understanding subsurface flow, especially in partly karstified rock formations mainly housing water through a few preferential pathways, is still challenging. This point is the consequence of the poor accessibility of the subsurface and lack of accurate depictions of water bearing bodies and distributions. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface.A 3-D seismic survey with shots and wave monitoring at the surface is carried out over a subsurface karstified reservoir located at the Hydrogeological Experimental Site (HES) of the University of Poitiers (France). Processing the 3-D data, in association with wave velocity calibration from vertical seismic profiles (VSP) recorded via geophones in wells, renders a 3-D velocity block. The velocity block is then converted into pseudo-porosity values revealing three high-porosity, presumably water-productive, layers, at depths of 35–40, 85–87, and 110–115 m.In addition, full wave acoustic logging (FWAL) can detect, close to wells, porous or open bodies that are too small for being captured by the spatial resolution of 3-D seismic images. A FWAL can also confirm or invalidate data from VSP recorded via hydrophones.The block of pseudo-porosities is compared to a different representation of the subsurface in the form of hydraulic conductivity distributions (or hydraulic diffusion) obtained by slug tests or by inversion of transient interference testing between wells. The inverted hydraulic conductivity maps do not match up the distribution of porous bodies identified by seismic data. This poses the question of guiding conventional inversions on the basis of a prior guess as the subsurface structure obtained via geophysical investigations

Regolith production and transport in the Susquehanna Shale Hills Critical Zone Observatory, Part 1: Insights from U-series isotopes

To investigate the timescales of regolith formation on hillslopes with contrasting topographic aspect, we measured U-series isotopes in regolith profiles from two hillslopes (north facing versus south facing) within the east-west trending Shale Hills catchment in Pennsylvania. This catchment is developed entirely on the Fe-rich, Silurian Rose Hill gray shale. Hillslopes exhibit a topographic asymmetry: The north-facing hillslope has an average slope gradient of ~20°, slightly steeper than the south-facing hillslope (~15°). The regolith samples display significant U-series disequilibrium resulting from shale weathering. Based on the U-series data, the rates of regolith production on the two ridgetops are indistinguishable (40 ± 22 versus 45 ± 12 m/Ma). However, when downslope positions are compared, the regolith profiles on the south-facing hillslope are characterized by faster regolith production rates (50 ± 15 to 52 ± 15 m/Ma) and shorter durations of chemical weathering (12 ± 3 to 16 ± 5 ka) than those on the north-facing hillslope (17 ± 14 to 18 ± 13 m/Ma and 39 ± 20 to 43 ± 20 ka). The south-facing hillslope is also characterized by faster chemical weathering rates inferred from major element chemistry, despite lower extents of chemical depletion. These results are consistent with the influence of aspect on regolith formation at Shale Hills; we hypothesize that aspect affects such variables as temperature, moisture content, and evapotranspiration in the regolith zone, causing faster chemical weathering and regolith formation rates on the south-facing side of the catchment. The difference in microclimate between these two hillslopes is inferred to have been especially significant during the periglacial period that occurred at Shale Hills at least ~15 ka before present. At that time, the erosion rates may also have been different from those observed today, perhaps denuding the south-facing hillslope of regolith but not quite stripping the north-facing hillslope. An analysis of hillslope evolution and response timescales with a linear mass transport model shows that the current landscape at Shale Hills is not in geomorphologic steady state (i.e., so-called dynamic equilibrium) but rather is likely still responding to the climate shift from the Holocene periglacial to the modern, temperate conditions