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International audience; The development of fiber optic (FO) Distributed Temperature Sensing (DTS) as a tool for hydrogeological measurements with high spatial and temporal resolution has shown potential for characterizing aquifer heterogeneity, which remains a challenge and is needed to predict contaminant transport. Recent studies have shown that groundwater fluxes can be quantified along a vertical profile in granular aquifers by inverting the thermal responses from active heat tracer tests using FO cables. Here, we further investigate the use of active FO-DTS methods and the resulting high resolution profiles for granular aquifer characterization by comparing results with cone penetration tests (CPT), which provide indications of hydrofacies. A multiscale characterization and active heat tracer experiments were performed in a well-studied heterogeneous deltaic aquifer located north of Quebec City, Canada.Four active FO-DTS heat tracer experiments were conducted by deploying fiber optic cables by direct push at locations with a previous CPT. Interpretation of thermal responses from the active FO-DTS experiments was done with analytical solutions for heat transport, providing independent and accurate estimates of thermal properties and fluxes every 25 centimeters. The resulting profiles of fluxes from DTS measurements correlate well with the response obtained with CPT. Furthermore, as a previous study established a relationship between CPT response and different aquifer materials and their hydraulic properties, the resulting flux profiles from active FO-DTS can be used to obtain a stratigraphy of the different hydrofacies.Active FO-DTS experiments can thus provide a qualitative or quantitative proxy for hydraulic conductivity and allow the recognition of hydrofacies at a sub-metric vertical scale. At the aquifer scale, the total flux estimated from FO-DTS measurements can also be compared and used as constraints for fluxes obtained from a numerical model. Overall, this study shows that not only does FO-DTS provide coherent results with other characterization methods, but it also adds the key measurement of groundwater flux with great accuracy that cannot be easily obtained by other means. FO-DTS has thus the potential to become a significant addition to existing characterization methods for granular aquifers.

The Yukon–Alaska Highway corridor in southern Yukon is subject to geohazards ranging from landslides to floods and earthquakes on faults in the St. Elias Mountains and Shakwak Valley. Here we discuss the late Holocene seismic history of the Denali fault, located at the eastern front of the St. Elias Mountains and one of only a few known seismically active terrestrial faults in Canada. Holocene faulting is indicated by scarps and mounds on late Pleistocene drift and by tectonically deformed Pleistocene and Holocene sediments. Previous work on trenches excavated against the fault scarp near the Duke River reveals paleoseismic sediment disturbance dated to ∼300–1,200, 1,200–1,900, and 3,000 years ago. Re-excavation of the trenches indicates a fourth event dated to 6,000 years ago. The trenches are interpreted to show a negative flower structure produced by extension of sediments by dextral strike-slip fault movement. Nearby Crescent Lake is ponded against the fault scarp. Sediment cores reveal four abrupt sediment and diatom changes reflecting seismic shaking at ∼1,200–1,900, 1,900–5,900, 5,900–6,200, and 6,500–6,800 years ago. At the Duke River, the fault offsets sediments, including two White River tephra layers (∼1,900 and 1,200 years old). Late Pleistocene outwash gravel and overlying Holocene aeolian sediments show in cross section a positive flower structure indicative of post-glacial contraction of the sediments by dextral strike-slip movement. Based on the number of events reflecting ∼M6, we estimate the average recurrence of large earthquakes on the Yukon part of the Denali fault to be about 1,300 years in the past 6,500–6,800 years.

Ongoing climate change focuses attention on the Arctic cryosphere’s responses to past and future climate states. Although it is now recognized the Arctic Ocean Basin was covered by ice sheets and their associated floating ice shelves several times during the Late Pleistocene, the timing and extent of these polar ice sheets remain uncertain. Here we relate a relict barrier-island system on the Beaufort Sea coast of northern Alaska to the isostatic effects of a previously unrecognized ice shelf grounded on the adjacent continental shelf. A new suite of optically stimulated luminescence dates show that this barrier system formed during one or more marine transgressions occurring late in Marine Isotope Stage 5 (MIS 5) between 113 ka and 71 ka. Because these transgressions occurred after the warmest part of the last interglacial (ca. 123 ka) and did not coincide with the global eustatic sea-level maximum during MIS 5e, this indicates Arctic ice sheets developed out-of-phase with lower-latitude sectors of the Laurentide and Fennoscandian ice sheets. We speculate that Arctic ice sheets began development during full interglacial conditions when abundant moisture penetrated to high latitudes, and low summer insolation favored glacier growth. These ice sheets reached their full extents at interglacial-glacial transitions, then wasted away at the heights of mid-latitude glaciations because of moisture limitations.

International audience; Understanding the source to sink relationship through time between the Eastern Kunlun Shan, one of the major mountain belts in the northern Tibetan Plateau, and the actively deforming Qaidam Basin to the north has important implications for unravelling the growth history of the Tibetan Plateau. In this study, we first carried out a flexural loading modeling to estimate the topography load of the Qilian Shan and Eastern Kunlun Shan during the deposition of the Lulehe Fm. and compared this result with the basin shape which was obtained from shortening restoration and decompaction by using the drill core, seismic profile and isopach data. In addition, U-Pb dating of detrital zircons from 22 sandstone samples collected from four sections within the southwestern Qaidam Basin is combined with provenance analysis and new seismic profile interpretation to investigate the mountain building of the Eastern Kunlun Shan and its effects on the development of the Qaidam Basin. Flexural loading estimate suggests that the single Qilian Shan failed to produce a sufficient load to create the observed Qaidam basin during the deposition of Lulehe Fm. The topographic load generated by both the Eastern Kunlun Shan to the south and the Qilian Shan-Altyn Tagh Range to the north was responsible for the flexural subsidence of the Qaidam basin during the deposition of Lulehe Fm. The U-Pb age distributions of detrital grains from Lulehe Fm. strata are characterized by a major component of Paleozoic to late Proterozoic ages. Carbonate debris containing foraminifera have been recognized in the Lulehe Fm. conglomerate sequences. We suggest that the Eastern Kunlun Shan was exhumed during the deposition of Lulehe Fm. The southward onlaps strata (Lulehe Fm. to Shangganchaigou Fm.) observed on seismic profiles and the appearance of a Mesozoic component in the detrital zircon age spectra of Xiaganchaigou Fm. to Shangganchaigou Fm. strata indicate that the Qaidam Basin was widening southward during that early Cenozoic period. Well-developed growth strata and the increasing proportion of Mesozoic and Paleozoic U-Pb ages in detrital zircon grains from late Neogene strata demonstrate that the relief of the Eastern Kunlun Shan and Altyn Tagh Range increased, leading to isolation and narrowing of the Qaidam Basin, from Miocene to the present.

International audience; Organic-rich anoxic sediments at DOE’s Rifle, CO site contain relatively high concentrations of uranium. Based on sediment characteristics, we speculated that these ‘naturally reduced zones’ (NRZs) are common and accumulate uranium at similar contaminated sites across the upper Colorado River Basin (CRB). To test this hypothesis, we sampled NRZs at 4 additional sites along a 700 km north-south transect of the upper CRB: Grand Junction and Naturita, CO; Shiprock, NM; and Riverton, WY. This work confirmed our hypothesis and showed that NRZs are also important reservoirs for nutrients and biogeochemical critical elements (BCE), including C, N, S, and Fe. Sulfate-reducing conditions are required for uranium accumulation, suggesting at least a strong indirect control of sulfide on U(VI) reduction. Indeed, the nominal oxidation state of water-soluble soil organic carbon was found to be correlated to sediment sulfide concentration, suggesting that sulfide plays a major role in poising the redox conditions of NRZs regionally.To better understand molecular controls over uranium behavior in NRZs, we performed controlled microcosm experiments designed to mimic sulfate-reducing conditions in NRZs and to more clearly define the potential roles of organic functional groups as uranium binding sites. U(IV) was found to be dominantly associated with surfaces of particulate organic carbon and to exhibit local molecular structure consistent with sorbed complexes. An important implication of this finding is that U(IV) will be readily mobilized in the presence of complexing agents and oxidants. Such conditions are common in NRZ sediments, which experience large variations in saturation state and redox conditions throughout the annual winter (base flow) / summer (meltwater discharge) cycle within the upper CRB.In ensemble, these studies suggest new conceptual and process models for uranium and BCE biogeochemical behavior that are unprecedented in their regional scale, detail, and awareness of variable hydrologic conditions. By studying anoxic sediment systems from a regional perspective, we have gained important insights into the range and intensity of processes that are likely to be important at individual sites.