Joshua C. Koch

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.

Role of ground ice dynamics and ecological feedbacks in recent ice wedge degradation and stabilization

Ground ice is abundant in the upper permafrost throughout the Arctic and fundamentally affects terrain responses to climate warming. Ice wedges, which form near the surface and are the dominant type of massive ice in the Arctic, are particularly vulnerable to warming. Yet processes controlling ice wedge degradation and stabilization are poorly understood. Here we quantified ice wedge volume and degradation rates, compared ground ice characteristics and thermal regimes across a sequence of five degradation and stabilization stages and evaluated biophysical feedbacks controlling permafrost stability near Prudhoe Bay, Alaska. Mean ice wedge volume in the top 3 m of permafrost was 21%. Imagery from 1949 to 2012 showed thermokarst extent (area of water-filled troughs) was relatively small from 1949 (0.9%) to 1988 (1.5%), abruptly increased by 2004 (6.3%) and increased slightly by 2012 (7.5%). Mean annual surface temperatures varied by 4.9°C among degradation and stabilization stages and by 9.9°C from polygon center to deep lake bottom. Mean thicknesses of the active layer, ice-poor transient layer, ice-rich intermediate layer, thermokarst cave ice, and wedge ice varied substantially among stages. In early stages, thaw settlement caused water to impound in thermokarst troughs, creating positive feedbacks that increased net radiation, soil heat flux, and soil temperatures. Plant growth and organic matter accumulation in the degraded troughs provided negative feedbacks that allowed ground ice to aggrade and heave the surface, thus reducing surface water depth and soil temperatures in later stages. The ground ice dynamics and ecological feedbacks greatly complicate efforts to assess permafrost responses to climate change.

Life in the Main Channel: Long-Term Hydrologic Control of Microbial Mat Abundance in McMurdo Dry Valley Streams, Antarctica

Given alterations in global hydrologic regime, we examine the role of hydrology in regulating stream microbial mat abundance in the McMurdo Dry Valleys, Antarctica. Here, perennial mats persist as a desiccated crust until revived by summer streamflow, which varies inter-annually, and has increased since the 1990s. We predicted high flows to scour mats, and intra-seasonal drying to slow growth. Responses were hypothesized to differ based on mat location within streams, along with geomorphology, which may promote (high coverage) or discourage (low coverage) accrual. We compared hydrologic trends with the biomass of green and orange mats, which grow in the channel, and black mats growing at stream margins for 16 diverse stream transects over two decades. We found mat biomass collectively decreased during first decade coinciding with low flows, and increased following elevated discharges. Green mat biomass showed the greatest correlations with hydrology and was stimulated by discharge in high coverage transects, but negatively correlated in low coverage due to habitat scour. In contrast, orange mat biomass was negatively related to flow in high coverage transects, but positively correlated in low coverage because of side-channel expansion. Black mats were weakly correlated with all hydrologic variables regardless of coverage. Lastly, model selection indicated the best combination of predictive hydrologic variables for biomass differed between mat types, but also high and low coverage transects. These results demonstrate the importance of geomorphology and species composition to modeling primary production, and will be useful in predicting ecological responses of benthic habitats to altered hydrologic regimes.

Runoff sources and flow paths in a partially burned, upland boreal catchment underlain by permafrost

Boreal soils in permafrost regions contain vast quantities of frozen organic material that is released to terrestrial and aquatic environments via subsurface flow paths as permafrost thaws. Longer flow paths may allow chemical reduction of solutes, nutrients, and contaminants, with implications for greenhouse gas emissions and aqueous export. Predicting boreal catchment runoff is complicated by soil heterogeneities related to variability in active layer thickness, soil type, fire history, and preferential flow potential. By coupling measurements of permeability, infiltration potential, and water chemistry with a stream chemistry end-member mixing model, we tested the hypothesis that organic soils and burned slopes are the primary sources of runoff, and that runoff from burned soils is greater due to increased hydraulic connectivity. Organic soils were more permeable than mineral soils, and 25% of infiltration moved laterally upon reaching the organic-mineral soil boundary on unburned hillslopes. A large portion of the remaining water infiltrated into deeper, less permeable soils. In contrast, burned hillslopes displayed poorly defined soil horizons, allowing rapid, mineral-rich runoff through preferential pathways at various depths. On the catchment scale, mineral/organic runoff ratios averaged 1.6 and were as high as 5.2 for an individual storm. Our results suggest that burned soils are the dominant source of water and solutes reaching the stream in summer, whereas unburned soils may provide longer term storage and residence times necessary for production of anaerobic compounds. These results are relevant to predicting how boreal catchment drainage networks and stream export will evolve given continued warming and altered fire regimes.

Multidecadal increases in the Yukon River Basin of chemical fluxes as indicators of changing flowpaths, groundwater, and permafrost

The Yukon River Basin, underlain by discontinuous permafrost, has experienced a warming climate over the last century that has altered air temperature, precipitation, and permafrost. We investigated a water chemistry database from 1982 to 2014 for the Yukon River and its major tributary, the Tanana River. Significant increases of Ca, Mg, and Na annual flux were found in both rivers. Additionally, SO4 and P annual flux increased in the Yukon River. No annual trends were observed for dissolved organic carbon (DOC) from 2001 to 2014. In the Yukon River, Mg and SO4 flux increased throughout the year, while some of the most positive trends for Ca, Mg, Na, SO4, and P flux occurred during the fall and winter months. Both rivers exhibited positive monthly DOC flux trends for summer (Yukon River) and winter (Tanana River). These trends suggest increased active layer expansion, weathering, and sulfide oxidation due to permafrost degradation throughout the Yukon River Basin.

Landscape effects of wildfire on permafrost distribution in interior Alaska derived from remote sensing

Climate change coupled with an intensifying wildfire regime is becoming an important driver of permafrost loss and ecosystem change in the northern boreal forest. There is a growing need to understand the effects of fire on the spatial distribution of permafrost and its associated ecological consequences. We focus on the effects of fire a decade after disturbance in a rocky upland landscape in the interior Alaskan boreal forest. Our main objectives were to (1) map near-surface permafrost distribution and drainage classes and (2) analyze the controls over landscape-scale patterns of post-fire permafrost degradation. Relationships among remote sensing variables and field-based data on soil properties (temperature, moisture, organic layer thickness) and vegetation (plant community composition) were analyzed using correlation, regression, and ordination analyses. The remote sensing data we considered included spectral indices from optical datasets (Landsat 7 Enhanced Thematic Mapper Plus (ETM+) and Landsat 8 Operational Land Imager (OLI)), the principal components of a time series of radar backscatter (Advanced Land Observing Satellite—Phased Array type L-band Synthetic Aperture Radar (ALOS-PALSAR)), and topographic variables from a Light Detection and Ranging (LiDAR)-derived digital elevation model (DEM). We found strong empirical relationships between the normalized difference infrared index (NDII) and post-fire vegetation, soil moisture, and soil temperature, enabling us to indirectly map permafrost status and drainage class using regression-based models. The thickness of the insulating surface organic layer after fire, a measure of burn severity, was an important control over the extent of permafrost degradation. According to our classifications, 90% of the area considered to have experienced high severity burn (using the difference normalized burn ratio (dNBR)) lacked permafrost after fire. Permafrost thaw, in turn, likely increased drainage and resulted in drier surface soils. Burn severity also influenced plant community composition, which was tightly linked to soil temperature and moisture. Overall, interactions between burn severity, topography, and vegetation appear to control the distribution of near-surface permafrost and associated drainage conditions after disturbance.

Review: groundwater in Alaska (USA)

Groundwater in the US state of Alaska is critical to both humans and ecosystems. Interactions among physiography, ecology, geology, and current and past climate have largely determined the location and properties of aquifers as well as the timing and magnitude of fluxes to, from, and within the groundwater system. The climate ranges from maritime in the southern portion of the state to continental in the Interior, and arctic on the North Slope. During the Quaternary period, topography and rock type have combined with glacial and periglacial processes to develop the unconsolidated alluvial aquifers of Alaska and have resulted in highly heterogeneous hydrofacies. In addition, the long persistence of frozen ground, whether seasonal or permanent, greatly affects the distribution of aquifer recharge and discharge. Because of high runoff, a high proportion of groundwater use, and highly variable permeability controlled in part by permafrost and seasonally frozen ground, understanding groundwater/surface-water interactions and the effects of climate change is critical for understanding groundwater availability and the movement of natural and anthropogenic contaminants.RésuméL’eau souterraine dans l’état américain d’Alaska est essentielle à la fois pour les humains et pour les écosystèmes. Les interactions entre physiographie, écologie, géologie, climat passé et actuel, ont largement déterminé la localisation et les caractéristiques des aquifères comme d’ailleurs le rythme et l’amplitude des flux entrants, sortants et internes au système aquifère. Le climat s’échelonne du maritime dans la partie Sud au continental dans l’intérieur à l’arctique sur le Versant Nord. Durant l’ère quaternaire, topographie et nature des roches se sont combinées avec les mécanismes glaciaires et péri-glaciaires pour former les aquifères alluviaux non consolidés d’Alaska, d’où ont résulté des hydrofaciès extrêmement hétérogènes. De plus, la longue persistance d’un sol gelé, soit saisonnier soit permanent, affecte grandement la distribution de la recharge et de la décharge des aquifères. En raison d’une forte utilisation de l’eau de surface et de l ‘eau souterraine, et d’une perméabilité de la nappe hautement variable, contrôlée en partie par le permafrost et par le gel saisonnier, comprendre les interactions eau souterraine-eau de surface ainsi que les effets du changement climatique est crucial pour l’appréhension de la disponibilité en eau souterraine et du transfert des polluants naturels et anthropiques.ResumenEl agua subterránea en el estado de Alaska es crítica para los seres humanos y los ecosistemas. Las interacciones entre la fisiografía, ecología, geología y el clima actual y pasado han determinado en gran parte la ubicación y las propiedades de los flujos, así como el tiempo y magnitud de flujos hacia, desde y dentro del sistema de agua subterránea. El clima varía desde marítimo en la porción sur del estado a continental en el interior, y ártico en la ladera norte. Durante el período Cuaternario, la topografía y el tipo de roca se ha combinado con procesos glaciales y periglaciales para desarrollar los acuíferos aluviales no consolidados de Alaska y ha resultado en hidrofacies altamente heterogéneas. Además, la gran persistencia del terreno congelado, ya sea estacional o permanente, afecta en gran medida la distribución de la recarga y descarga del acuífero. Debido al alto escurrimiento y el uso del agua subterránea, y la permeabilidad altamente variable controlada en parte por el permafrost y estacionalmente por el terreno congelado, la comprensión de la interacción superficial – agua subterránea y los efectos del cambio climático es critico para el conocimiento de la disponibilidad de agua subterránea y los movimientos de contaminantes naturales y antropogénicos.摘要美国阿拉斯加州的地下水资源对于人类和生态系统都是至关重要的。自然地理条件、生态环境、地质条件和古往今来的气候条件之间的相互作用在很大程度上决定了含水层的位置和特性, 以及流入、流出和存在于地下水系统中的水流的流动时间和规模。州内的气候条件变化很大, 由南向北, 南部为海洋性气候, 中部为大陆性气候, 阿拉斯加北坡为北极气候。在第四纪期间, 地形和岩石的类型与冰期、间冰期过程相结合, 形成了阿拉斯加松散的冲积含水层, 导致了水相的高度非均质化。除此之外, 长期呈冷冻状态的土壤, 无论是季节性的还是永久性的, 都极大地影响了含水层源汇区的分布。由于径流量、地下水使用量很大, 且部分受永久性、季节性冻土控制的渗透性是高度变化的, 弄清地下水-地表水的相互作用和气候变化对其的影响, 对于了解地下水的可用性和自然、人为污染物的运移是非常重要的。ResumoA água subterrânea no estado norte-americano do Alasca é fundamental para os seres humanos e para os ecossistemas. Interações entre a fisiografia, a ecologia, a geologia e o clima atual e passado determinaram largamente a localização e as propriedades dos aquíferos bem como a temporização e a magnitude dos fluxos de, para e dentro do sistema de águas subterrâneas. O clima varia de marítimo na parcela sul do estado a continental no interior, e a ártico na Encosta Norte. Durante o período Quaternário, a topografia e o tipo de rochas combinaram-se com os processos glaciar e periglacial para desenvolver os aquíferos aluvionares não consolidados do Alasca, resultando em hidrofácies altamente heterogéneas. Além disso, o congelamento persistente do solo, seja sazonal ou permanente, afeta extremamente a distribuição da recarga e da descarga dos aquíferos. Devido ao elevado escoamento superficial e do uso da água subterrânea, e da variabilidade elevada da permeabilidade, controlada em parte pelo permafrost e pelo solo sazonalmente congelado, a compreensão das interações água subterrânea/água superficial e dos efeitos das mudanças climáticas é crítica para o conhecimento da disponibilidade de água subterrânea e do movimento de contaminantes naturais e antropogénicos.

Potential for real-time understanding of coupled hydrologic and biogeochemical processes in stream ecosystems: Future integration of telemetered data with process models for glacial meltwater streams

While continuous monitoring of stream flow and temperature has been common for some time, there is great potential to expand continuous monitoring to include water quality parameters such as nutrients, turbidity, oxygen and dissolved organic material. In many systems distinguishing between watershed and stream ecosystem controls can be challenging. The usefulness of such monitoring can be enhanced by application of quantitative models to interpret observed patterns in real time. Examples are discussed primarily from the glacial meltwater streams of the McMurdo Dry Valleys, Antarctica. Although the dry valley landscape is barren of plants, many streams harbor thriving cyanobacterial mats. Whereas a daily cycle of stream flow is controlled by the surface energy balance on the glaciers and the temporal pattern of solar exposure, the daily signal for biogeochemical processes controlling water quality is generated along the stream. These features result in an excellent outdoor laboratory for investigating fundamental ecosystem process and the development and validation of process based models. As part of the McMurdo Dry Valleys Long Term Ecological Research project, we have conducted field experiments and developed coupled biogeochemical transport models for the role of hyporheic exchange in controlling weathering reactions, microbial nitrogen cycling, and stream temperature regulation. We have adapted modeling approaches from sediment transport to understand mobilization of stream biomass with increasing flows. These models help to elucidate the role of in-stream processes in systems where watershed processes also contribute to observed patterns, and may serve as a test case for applying real-time stream ecosystem models. This article is protected by copyright. All rights reserved.