Proximity to ice fields and lake depth as modulators of paleoclimate records: a regional study from southwest Yukon,Canada

Pronounced climate warming during the past century has been well documented in high-latitude regions. Nonetheless, considerable heterogeneity exists in northern climate trends. We examined the roles of cryospheric landscape and lake depth in modulating the rate and magnitude of local climate responses through a paleolimnological study of lakes from southwest Yukon, Canada. By sampling lakes at varying distances from the Wrangell-St. Elias ice fields, we hypothesized that, for lakes of similar maximum depth, sites closest to the ice fields would be relatively complacent in terms of their chironomid and diatom assemblage changes over the past ~200 years. This hypothesis is based on the moderating effect of the glaciers on local climate, which would be most pronounced in the lakes nearest to the ice fields. However, given the known ecological differences between deep and shallow lakes, we further predicted that, for a given distance from the ice fields, a sediment record from a shallower lake would show the greatest change in stratigraphic subfossil assemblages. Because of the complicated shape of the ice fields, we applied the longitude for each site (which decreases from west to east) to approximate the proximity of our study lakes to the ice fields. Consistent with our predictions, we observed a space-transgressive pattern in the chironomid assemblage turnover that was associated with their proximity to the ice fields (r = ?0.75, P = 0.034, n = 8) across lakes of similar depth (mean maximum depth ± 1, SE = 18.1 ± 2.6 m). Considering a broader network of lakes that represented a greater range in maximum depth (4.9–29 m), we found that differences in subfossil chironomid assemblages between the modern and ca. AD 1800 sediment layers were strongly related to lake depth (r = ?0.77, P < 0.001, n = 15), but failed to detect a significant relationship with latitude or longitude (i.e. our proxy for proximity to the ice fields). Similarly, our comparative high-resolution analyses of two lakes with distinct lake morphometries, but similar proximities to the ice fields, demonstrated the predicted contrasting pattern: we observed pronounced post-1880 changes in the biotic assemblages in the shallow lake and a muted and delayed response (i.e. ~1970s) in the deeper lake. Our findings confirm that cryospheric landscape features can strongly modulate regional climate. Furthermore, our work shows that investigators need to be conscious of how climate change affects the structure and functioning of lakes of different typologies, which influences the way in which paleoclimate signals are recorded and interpreted.     

Using Multiple-Point Geostatistics for Tracer Test Modeling in a Clay-Drape Environment with Spatially Variable Conductivity and Sorption Coefficient

This study investigates the effect of fine-scale clay drapes on tracer transport. A tracer test was performed in a sandbar deposit consisting of cross-bedded sandy units intercalated with many fine-scale clay drapes. The heterogeneous spatial distribution of the clay drapes causes a spatially variable hydraulic conductivity and sorption coefficient. A fluorescent tracer (sodium naphthionate) was injected in two injection wells and ground water was sampled and analyzed from five pumping wells. To determine (1) whether the fine-scale clay drapes have a significant effect on the measured concentrations and (2) whether application of multiple-point geostatistics can improve interpretation of tracer tests in media with complex geological heterogeneity, this tracer test is analyzed with a local three-dimensional ground-water flow and transport model in which fine-scale sedimentary heterogeneity is modeled using multiple-point geostatistics. To reduce memory needs and calculation time for the multiple-point geostatistical simulation step, this study uses the technique of direct multiple-point geostatistical simulation of edge properties. Instead of simulating pixel values, model cell edge properties indicating the presence of irregularly shaped surfaces are simulated using multiple-point geostatistical simulations. Results of a sensitivity analysis show under which conditions clay drapes have a significant effect on the concentration distribution. Calibration of the model against measured concentrations from the tracer tests reduces the uncertainty on the clay-drape parameters. The calibrated model shows which features of the breakthrough curves can be attributed to the geological heterogeneity of the aquifer and which features are caused by other processes.     

Coherence of river and ocean conditions along the US West Coast during storms

The majority of water and sediment discharge from the small, mountainous watersheds of the US West Coast occurs during and immediately following winter storms. The physical conditions (waves, currents, and winds) within and acting upon the proximal coastal ocean during these winter storms strongly influence dispersal patterns. We examined this river–ocean temporal coherence for four coastal river–shelf systems of the US West Coast (Umpqua, Eel, Salinas, and Santa Clara) to evaluate whether specific ocean conditions occur during floods that may influence coastal dispersal of sediment. Eleven years of corresponding river discharge, wind, and wave data were obtained for each river–shelf system from USGS and NOAA historical records, and each record was evaluated for seasonal and event-based patterns. Because near-bed shear stresses due to waves influence sediment resuspension and transport, we used spectral wave data to compute and evaluate wave-generated bottom-orbital velocities. The highest values of wave energy and discharge for all four systems were consistently observed between October 15 and March 15, and there were strong latitudinal patterns observed in these data with lower discharge and wave energies in the southernmost systems. During floods we observed patterns of river–ocean coherence that differed from the overall seasonal patterns. For example, downwelling winds generally prevailed during floods in the northern two systems (Umpqua and Eel), whereas winds in the southern systems (Salinas and Santa Clara) were generally downwelling before peak discharge and upwelling after peak discharge. Winds not associated with floods were generally upwelling on all four river–shelf systems. Although there are seasonal variations in river–ocean coherence, waves generally led floods in the three northern systems, while they lagged floods in the Santa Clara. Combined, these observations suggest that there are consistent river–ocean coherence patterns along the US West Coast during winter storms and that these patterns vary substantially with latitude. These results should assist with future evaluations of flood plume formation and sediment fate along this coast.     

Geology and tectonics of Neoproterozoic salt diapirs and salt sheets in the eastern Willouran Ranges,South Australia

Allochthonous salt structures and associated primary and secondary minibasins are exposed in Neoproterozoic strata of the eastern Willouran Ranges, South Australia. Detailed geologic mapping using high‐quality airborne hyperspectral remote‐sensing data and satellite imagery, combined with a qualitative structural restoration, are used to elucidate the evolution of this complex, long‐lived (>250 Myr) salt system. Field observations and interpretations at a resolution unobtainable from seismic or well data provide a means to test published models of allochthonous salt emplacement and associated salt‐sediment interaction derived from subsurface data in the northern Gulf of Mexico. Salt diapirs and sheets are represented by megabreccias of nonevaporite lithologies that were originally interbedded with evaporites that have been dissolved and/or altered. Passive diapirism began shortly after deposition of the Callanna Group layered evaporite sequence. A primary basin containing an expulsion‐rollover structure and megaflap is flanked by two vertical diapirs. Salt flowed laterally from the diapirs to form a complex, multi‐level canopy, now partly welded, containing an encapsulated minibasin and capped by suprasalt basins. Salt and minibasin geometries were modified during the Late Cambrian–Ordovician Delamerian Orogeny (ca. 500 Ma). Small‐scale structures such as subsalt shear zones, fractured or mixed ‘rubble zones’ and thrust imbricates are absent beneath allochthonous salt and welds in the eastern Willouran Ranges. Instead, either undeformed strata or halokinetic drape folds that include preserved diapir roof strata are found directly below the transition from steep diapirs to salt sheets. Allochthonous salt first broke through the diapir roofs and then flowed laterally, resulting in variable preservation of the subsalt drape folds. Lateral salt emplacement was presumably on roof‐edge thrusts or, because of the shallow depositional environment, via open‐toed advance or extrusive advance, but without associated subsalt deformation.     

The Northwest Africa (NWA) 5790 meteorite: A mesostasis‐rich nakhlite with little or no Martian aqueous alteration

Northwest Africa (NWA) 5790 is the most recently discovered member of the nakhlite group. Its mineralogy differs from the other nakhlites with a high abundance mesostasis (38.1 ± 3.6 vol%) and scarcity of olivine (4.0 ± 2.2 vol%). Furthermore, zoning of augite phenocrysts, and other petrographic and chemical characteristics suggest that NWA 5790 samples the chilled margin of its parent lava flow/sill. NWA 5790 contains calcite and rare clay minerals that are evidence for its exposure to liquid water. The calcite forms a cement to coatings of dust on the outer surface of the find and extends into the interior of the meteorite within veins. The presence of microbial remains within the coating confirms that the dust and its carbonate cement are terrestrial in origin, consistent with the carbon and oxygen isotope composition of the calcite. The clay minerals are finely crystalline and comprise ~0.003 vol% of the meteorite. δD values of the clay minerals range from ?212 ± 109‰ to ?96 ± 132‰, and cannot be used to distinguish between a terrestrial or Martian origin. As petrographic results are also not definitive, we conclude that secondary minerals produced by Martian groundwaters are at best very rare within NWA 5790. The meteorite has therefore sampled a region of the lava flow/sill with little or no exposure to the aqueous solutions that altered other nakhlites. This isolation could relate to the scarcity of olivine in NWA 5790 because dissolution of olivine in other nakhlites by Martian groundwaters enhanced their porosity and permeability, and provided solutes for secondary minerals.     

Implications of simultaneously mitigating and adapting to climate change: initial experiments using GCAM

Most research on future climate change discusses mitigation and impacts/adaptation separately. However, mitigation will have implications for impacts and adaptation. Similarly, impacts and adaptation will affect mitigation. This paper begins to explore these two veins of research simultaneously using an integrated assessment model. We begin by discussing the types of interactions one might expect by impact sector. Then, we develop a numerical experiment in the agriculture sector to illustrate the importance of considering mitigation, impacts, and adaptation at the same time. In our experiment, we find that climate change can reduce crop yields, resulting in an expansion of cropland to feed a growing population and a reduction in bioenergy production. These two effects, in combination, result in an increase in the cost of mitigation.     

Watershed-scale vegetation,water quantity,and water quality responses to wildfire in the southern Appalachian mountain region,United States

Wildfires are landscape scale disturbances that can significantly affect hydrologic processes such as runoff generation and sediment and nutrient transport to streams. In Fall 2016, multiple large drought-related wildfires burned forests across the southern Appalachian Mountains. Immediately after the fires, we identified and instrumented eight 28.4–344 ha watersheds (four burned and four unburned) to measure vegetation, soil, water quantity, and water quality responses over the following two years. Within burned watersheds, plots varied in burn severity with up to 100% tree mortality and soil O-horizon loss. Watershed scale high burn severity extent ranged from 5% to 65% of total watershed area. Water quantity and quality responses among burned watersheds were closely related to the high burn severity extent. Total water yield (Q) was up to 39% greater in burned watersheds than unburned reference watersheds. Total suspended solids (TSS) concentration during storm events were up to 168 times greater in samples collected from the most severely burned watershed than from a corresponding unburned reference watershed, suggesting that there was elevated risk of localized erosion and sedimentation of streams. NO₃-N concentration, export, and concentration dependence on streamflow were greater in burned watersheds and increased with increasing high burn severity extent. Mean NO₃-N concentration in the most severely burned watershed increased from 0.087 mg L⁻¹ in the first year to 0.363 mg L⁻¹ (+317%) in the second year. These results suggest that the 2016 wildfires degraded forest condition, increased Q, and had negative effects on water quality particularly during storm events.     

Creative democracy and the quiet politics of the everyday

John Dewey envisioned creative democracy as a process of agonistic engagement, and Bob Lake shares Dewey’s optimism for the possibilities of creative democracy. In this response, I suggest that scholars should look beyond the obvious moments of democratic political engagement, whether activism in the public square or in the occupied park, to pay attention to the quiet politics of the everyday, where everyday decisionmaking by individuals and communities can gradually, episodically, change dominant hegemonic norms and understandings, proviing new understandings for social change. I highlight several examples, including the Settlement house movement from the late 19th century and intentional neighboring from the twenty-first century, that illustrate the kind of daily work that brings together different social classes and ethnicities in a situation of sharing and working toward conditions of equality and new ways of living in the world.