Stephanie A. Ewing

Geomorphologic evidence for the late Pliocene onset of hyperaridity in the Atacama Desert

The Atacama Desert has experienced a long and protracted period of hyperaridity that has resulted in what may be the most unusual biome on Earth, but the duration of this aridity is poorly constrained. We reconstructed aspects of the fluvial and geochemical history of this region using integrated landscape features (alluvial fans, hillslope soils, soil chemistry, river profiles) in the southern portion of the present desert. Topographic reconstructions of a large watershed (11,000 km 2 ) show deep incision and sediment removal between the late Miocene and the end of the Pliocene, and modest to negligible incision in post-Pliocene times. These changes in incision suggest an ∼50–280× reduction in river discharge, which should reflect corresponding changes in precipitation. Changes in the nature of hillslope soils in the Atacama Desert indicate that in the Pliocene or earlier, hillslopes were mantled with silicate-derived soil. This mantle was stripped off and locally deposited as alluvial fans (late Pliocene to early Pleistocene) that now block or otherwise cause a rearrangement of Pliocene and earlier river channels. Finally, the hillslopes have largely accreted a soil mantle of dust and salt since the apparent late Pliocene stripping, suggesting a decline in annual precipitation of at least 125 mm yr -1 or more (mean annual precipitation [MAP] is now -1 ). Embedded in the long post-Pliocene era of salt accumulation, there are a variety of features suggesting overland flow on hillslopes (rills, striped gravel deposits, piping, and water spouts) and large, infrequent storms that infiltrated gentle alluvial fans (due to the depth of salt-rich horizons). Despite evidence for episodes that punctuate the hyperaridity, the magnitude and duration of these pluvial events have been insufficient to remove the regional accumulations of sulfate, chloride, and nitrate. The late Pliocene cessation of many fluvial features is coincident with recent research on the tropical Pacific, which shows that the Pacific was in a permanent El Nino state until ca. 2.2 Ma, at which time sea-surface temperatures offshore of South America declined greatly relative to those of the western Pacific, in turn setting up the present El Nino–Southern Oscillation (ENSO) climate system. These observations indicate that the latest period of aridity has been prolonged and largely continuous, and it appears to have occurred in step with the onset of the ENSO climate system, beginning ∼2 m.y. ago.

Effect of permafrost thaw on CO2 and CH4 exchange in a western Alaska peatland chronosequence

Permafrost soils store over half of global soil carbon (C), and northern frozen peatlands store about 10% of global permafrost C. With thaw, inundation of high latitude lowland peatlands typically increases the surface-atmosphere flux of methane (CH4), a potent greenhouse gas. To examine the effects of lowland permafrost thaw over millennial timescales, we measured carbon dioxide (CO2) and CH4 exchange along sites that constitute a ?1000 yr thaw chronosequence of thermokarst collapse bogs and adjacent fen locations at Innoko Flats Wildlife Refuge in western Alaska. Peak CH4 exchange in July (123???71 mg CH4?C m?2 d?1) was observed in features that have been thawed for 30 to 70 (<100) yr, where soils were warmer than at more recently thawed sites (14 to 21 yr; emitting 1.37???0.67 mg CH4?C m?2 d?1 in July) and had shallower water tables than at older sites (200 to 1400 yr; emitting 6.55???2.23 mg CH4?C m?2 d?1 in July). Carbon lost via CH4 efflux during the growing season at these intermediate age sites was 8% of uptake by net ecosystem exchange. Our results provide evidence that CH4 emissions following lowland permafrost thaw are enhanced over decadal time scales, but limited over millennia. Over larger spatial scales, adjacent fen systems may contribute sustained CH4 emission, CO2 uptake, and DOC export. We argue that over timescales of decades to centuries, thaw features in high-latitude lowland peatlands, particularly those developed on poorly drained mineral substrates, are a key locus of elevated CH4 emission to the atmosphere that must be considered for a complete understanding of high latitude CH4 dynamics.

Rapid runoff via shallow throughflow and deeper preferential flow in a boreal catchment underlain by frozen silt (Alaska, USA)

In high-latitude catchments where permafrost is present, runoff dynamics are complicated by seasonal active-layer thaw, which may cause a change in the dominant flowpaths as water increasingly contacts mineral soils of low hydraulic conductivity. A 2-year study, conducted in an upland catchment in Alaska (USA) underlain by frozen, well-sorted eolian silt, examined changes in infiltration and runoff with thaw. It was hypothesized that rapid runoff would be maintained by flow through shallow soils during the early summer and deeper preferential flow later in the summer. Seasonal changes in soil moisture, infiltration, and runoff magnitude, location, and chemistry suggest that transport is rapid, even when soils are thawed to their maximum extent. Between June and September, a shift occurred in the location of runoff, consistent with subsurface preferential flow in steep and wet areas. Uranium isotopes suggest that late summer runoff erodes permafrost, indicating that substantial rapid flow may occur along the frozen boundary. Together, throughflow and deep preferential flow may limit upland boreal catchment water and solute storage, and subsequently biogeochemical cycling on seasonal to annual timescales. Deep preferential flow may be important for stream incision, network drainage development, and the release of ancient carbon to ecosystems.RésuméDans les bassins versants de haute latitude où le permafrost est présent, les dynamiques de lessivage et d’écoulement sont compliquées par le dégel saisonnier des couches productives, qui peut causer une modification des chenaux principaux si l’eau entre progressivement en contact avec des sols minéraux à conductivité hydraulique basse. Une étude de deux ans, menée sur un bassin versant de hautes terres en Alaska (USA) sous-jacent à silt éolien bien calibré gelé, a examiné les changements d’infiltration et d’écoulement avec la dégel. On a pris comme hypothèse qu’un écoulement rapide serait soutenu par un flux à travers des sols peu épais au cours du début de l’été et par un flux préférentiel profond plus tard durant l’été. Les variations saisonnières d’eau du sol, infiltration et intensité du ruissellement, localisation et chimie, suggèrent que le transport est rapide, même au maximum d’extension du dégel. Entre juin et septembre, l’emplacement de l’écoulement change, en rapport avec l’écoulement préférentiel de subsurface, dans les zones en pentes et humides. Des isotopes de l’uranium suggèrent que l’écoulement d’été tardif érode le permafrost, indiquant qu’un écoulement rapide substantiel peut savoir lieu le long de la limite gelée. Simultanément, l’écoulement superficiel et l’écoulement préférentiel profond peuvent limiter le bassin versant du plateau boréal et l’emmagasinement de soluté, et subséquemment le cycle biochimique aux échelles saisonnière à annuelle. L’écoulement profond préférentiel peut être important pour la coupure du flot, le développement du réseau de drainage et la restitution de carbone ancien à l’écosystème.ResumenEn las cuencas de altas latitudes donde el permafrost está presente, la dinámica de lixiviación y escurrimiento se complican por el deshielo estacional de la capa activa, que puede causar un cambio en las trayectorias dominantes del flujo de agua cada vez más en contacto con suelos minerales de baja conductividad hidráulica. Un estudio de dos años de duración, realizado en una cuenca alta de Alaska (EEUU) sustentada por limos eólicos congelados, bien ordenados, examinó los cambios en la infiltración y el escurrimiento con el deshielo. La hipótesis fue que el escurrimiento rápido podría ser mantenido por el flujo a través de suelos someros a principios del verano y por el flujo preferencial más profundo después del verano. Los cambios estacionales en la humedad del suelo, infiltración, la magnitud del escurrimiento, la ubicación y la química sugieren que el transporte es rápido, incluso cuando los suelos están descongelados en su máxima extensión. Entre junio y septiembre se produjo un cambio en la ubicación del escurrimiento, consistente con el flujo subsuperficial preferencial de zonas escarpadas y húmedas. Los isótopos de uranio sugieren que a finales del verano el escurrimiento erosiona el permafrost, lo que indica que un sustancial flujo rápido puede ocurrir a lo largo del límite congelado. Conjuntamente, el flujo horizontal somero y el flujo profundo preferencial pueden limitar el agua de la cuenca boreal alta y el almacenamiento de solutos, y posteriormente el ciclo biogeoquímico en escalas de tiempo estacionales a anuales. El flujo profundo preferencial puede ser importante para la incisión corriente, para el desarrollo de la red de drenaje, y para la liberación de carbono antiguo a los ecosistemas摘要高纬度流域内的冻土永久存在,季节性活动层消融导致的淋滤及径流的动力学复杂,会引起由于水与低水力传导系数的矿质土壤接触增加而发生的主导流径的变化。美国阿拉斯加州山地流域下伏分选好的风成冻土,在该区对融水的渗透及径流变化监测两年。假定快速流可通过夏季初的浅层土壤以及夏季晚期的深层优先流维持。土壤水分、渗透量、径流量、位置以及化学结果的季节性变化表明即使土壤解冻到最大程度,径流也是迅速的。六月到九月,径流的位置存在转变,与陡湿区地下优先流相一致。铀同位素显示晚夏的径流消融了永久冻土,表明大量快速流在冻土边界发生。同时,浅部径流及深部优先流限制了山地北面流域中水及溶质的存储以及随后的季节及多年时间尺度上的生物地球化学循环。深部优先流对于河流切割、排水网络发育、以及古代碳向生态系统中的排放是有意义的。ResumoNas bacias localizadas a altas latitudes onde ocorre permafrost, a dinâmica de lixiviação e de escoamento é dificultada pelo degelo sazonal da camada ativa, o qual pode causar uma mudança nos sentidos de fluxo dominantes enquanto a água aumenta o contacto com solos minerais de baixa condutividade hidráulica. Um estudo de dois anos realizado na cabeceira de uma bacia hidrográfica no Alasca (EUA) coberta por material congelado, siltes eólicos bem calibrados, observou variações na infiltração e no escoamento durante o degelo. Admitiu-se a hipótese que o escoamento rápido seria mantido pelo fluxo através dos solos superficiais durante o princípio do verão e que o fluxo preferencial mais profundo ocorreria no final do verão. As variações sazonais na grandeza da humidade do solo, da infiltração e do escoamento, na localização e no quimismo sugerem que o transporte é rápido, mesmo quando os solos sofrem descongelação na sua máxima extensão. Entre junho e setembro ocorreu um deslocamento da posição do escoamento, consistente com fluxo subsuperficial preferencial em áreas declivosas e húmidas. Os isótopos de urânio sugerem que o escoamento do fim do verão erode o permafrost, indicando que este fluxo rápido substancial pode ocorrer ao longo da fronteira de congelação. O escoamento subsuperficial e o fluxo preferencial profundo podem em conjunto limitar o armazenamento de água e de solutos nas bacias superiores em zonas boreais e, subsequentemente, o ciclo biogeoquímico às escalas sazonal a anual. O fluxo preferencial profundo pode ser importante para a incisão das linhas de água, o desenvolvimento da rede de drenagem e a libertação do carbono antigo para os ecossistemas.

Pb Isotopes as an Indicator of the Asian Contribution to Particulate Air Pollution in Urban California

During the last two decades, expanding industrial activity in east Asia has led to increased production of airborne pollutants that can be transported to North America. Previous efforts to detect this trans-Pacific pollution have relied upon remote sensing and remote sample locations. We tested whether Pb isotope ratios in airborne particles can be used to directly evaluate the Asian contribution to airborne particles of anthropogenic origin in western North America, using a time series of samples from a pair of sites upwind and downwind of the San Francisco Bay Area. Our results for airborne Pb at these sites indicate a median value of 29% Asian origin, based on mixing relations between distinct regional sample groups. This trans-Pacific Pb is present in small quantities but serves as a tracer for airborne particles within the growing Asian industrial plume. We then applied this analysis to archived samples from urban sites in central California. Taken together, our results suggest that the analysis of Pb isotopes can reveal the distribution of airborne particles affected by Asian industrial pollution at urban sites in northern California. Under suitable circumstances, this analysis can improve understanding of the global transport of pollution, independent of transport models.

Temperature, Heat Flux, and Reflectance of Common Subarctic Mosses and Lichens under Field Conditions: Might Changes to Community Composition Impact Climate-Relevant Surface Fluxes?

Abstract Bryophytes and lichens are ubiquitous in subarctic ecosystems, but their roles in controlling energy fluxes are rarely studied at the species level despite large, recent observed shifts in subarctic vegetation. We quantified the surface and subsurface temperatures and spectral reflectance of common moss and lichen species at field sites in Alaska and Sweden. We also used MODIS observations to determine if the removal of Cladonia spp. by reindeer overgrazing impacts land surface albedo and temperature. Radiometric surface temperature of a feather moss (Pleurozium schreberi) exceeded 50 °C on occasion when dry, up to 20 °C higher than co-located Sphagnum fuscum or C. rangiferina. Spectral reflectance of S. fuscum was on average higher than Polytrichum piliferum across the 350–1400 nm range, with substantial within-species variability. MODIS albedo was significantly higher on the Norwegian (relatively undisturbed) side versus the Finnish (disturbed) side of a border reindeer fence by an average of 1% during periods without snow cover. MODIS nighttime land surface temperatures were often significantly higher on the Norwegian side of the fence by an average of 0.7 °C despite higher albedo, likely due to poor conductance of heat to the subsurface as observed in C. rangiferina in the field. Changes to bryophyte and lichen community composition alter the surface energy balance, and future work must determine how to best incorporate these effects into Earth system models.

Long‐term anoxia and release of ancient, labile carbon upon thaw of Pleistocene permafrost

USGS through the Climate and Land Use Change Mission Area and the NRC postdoc program; Montana State University Vice President of Research and College of Agriculture; the Montana Agricultural Experiment Station; National Science Foundation (EAR 0630257)

Data from: An exploration for truth in soil parent materials in Hyalite Canyon, Montana [dataset]

Forested mountain soils are mapped and researched at coarser scales than agricultural areas. Additionally existing mass balance approaches have only recently begun to quantify soil chemical weathering referenced to two parent materials. Forests occupy soils on gneiss, sandstone, shale, limestone, and basaltic andesite in a lithosequence. Differences in the trace chemistry of immobile elements and their ratios were used to determine the influence of underlying bedrock and other parent materials on soils in southwest Montana. Differences between soils and underlying rock and soils and other candidate parent materials were used to determine the relative influence of both parent materials on each soil horizon. A conventional mass balance approach quantified total and elemental chemical weathering for both parent materials. The relative influences, as informed by trace elements and ratios, were used to scale mass fluxes from each parent material into a hybrid mass flux. We characterized one soil pit per lithology, sampled soil horizons, rock type, and collected five samples regionally as deposited loess. Rock samples (as underlying rock and colluvium as appropriate) and atmospheric deposition were considered candidate parent materials to these soils. Samples were analyzed for major and trace elements. The trace elements or ratios used determined the relative influence of underlying rock versus a second parent material. We developed a methods to assess the immobility of specific elements relative to underlying rock. Ratios deemed most immobile were used to determine fractional influences of rock and a second parent material. Most soils had stronger rock influence at depth and stronger influences of atmospheric deposition at the surface, except for soil on sandstone and basaltic andesite. Regardless, accounting for additions of parent materials aside from underlying rock reduced total mass losses in all soils. The biggest mass loss reductions occurred (~50%) in soils on shale and gneiss, whereas mass loss reductions in soil on basaltic were more modest (~15%) when accounting for atmospheric deposition as a second parent material.