Search

The interactions between Earth surface processes, climate and tectonics determine the landscape in mountain regions. The Japanese Alps uplifted throughout the Quaternary and now reach elevations of up to 3,000 m. However, quantifying relief changes in response to tectonic activity, magmatism and Late Quaternary glaciation is challenging due to the young age of the Japanese Alps and the difficulty of measuring surface processes at the timescale of glacial-interglacial cycles. Here, we use ultra-low temperature thermochronometers based on the luminescence of feldspar minerals and the electron spin resonance (ESR) of quartz minerals, in combination with inverse modelling to derive rock cooling rates and exhumation rates histories at 104-106 years timescales. We focus on the Tateyama region in the Hida range of the Japanese Alps, which was glaciated during the late Quaternary period. In total, 19 new samples were analyzed by luminescence and ESR thermochronometry. While most luminescence signals have already reached their upper dating limit, ESR signals (Al and Ti centres) yielded ESR ages of between 0.5-0.9 Ma. In general, thermal stability is lower for the Al centre compared to that of the Ti centre, but both centres constrain similar exhumation rates. Inversions reveal rock cooling rates on the order of 30-80 °C/Ma, which can be inverted to erosion rates of

High-resolution topographic modeling has become more accessible due to the development of structure from motion (SfM)-image-matching algorithms in digital photogrammetry. Large archival databases of historical aerial photographs are available in university, public, and government libraries, commonly as paper copies. The photographs can be in poor condition (i.e., deformed by humidity, scratched, or annotated). In addition, the negatives, as well as metadata, may be missing. Processing such photographs using classic stereo-photogrammetry is difficult and, in many instances, impossible. SfM can be applied to these photosets to access the valuable archive of geomorphic changes over the past century. In this paper, we illustrate the utility of the SfM technique using 568 digitized vertical aerial photographs of Mount Meager volcano, located in southwestern British Columbia, Canada. We use the aerial photographs, which span the period from 1947 to 2006, to track glaciers and glacier–landslide interactions on the volcano. Over this period, glaciers have thinned and retreated, interrupted by minor advances in the 1960s and 1970s. Landslides are frequent on the volcano and contribute to debris cover on the glaciers affecting the ablation process. SfM processing of the aerial photographs allowed us to unlock geomorphic information and reconstruct landscape change that would otherwise have been impossible. The results from SfM provide a visually effective way of presenting landscape change to a broad public audience, as a form of virtual geoheritage. The approach can thus be broadly applied in scientific and professional practices for improving land planning and hazard management.

Melt from debris-covered glaciers represents a regionally important freshwater source, especially in high-relief settings as found in central Asia, Alaska, and South America. Sub-debris melt is traditionally predicted from surface energy balance models that determine heat conduction through the supraglacial debris layer. Convection is rarely addressed, despite the porous nature of debris. Here we provide the first constraints on convection in supraglacial debris, through the development of a novel method to calculate individual conductive and nonconductive heat flux components from debris temperature profile data. This method was applied to data from Kennicott Glacier, Alaska, spanning two weeks in the summer of 2011 and two months in the summer of 2020. Both heat flux components exhibit diurnal cycles, the amplitude of which is coupled to atmospheric conditions. Mean diurnal nonconductive heat flux peaks at up to 43% the value of conductive heat flux, indicating that failure to account for it may lead to an incorrect representation of melt rates and their drivers. We interpret this heat flux to be dominated by latent heat as debris moisture content changes on a diurnal cycle. A sharp afternoon drop-off in nonconductive heat flux is observed at shallow depths as debris dries. We expect these processes to be relevant for other debris-covered glaciers. Debris properties such as porosity and tortuosity may play a large role in modulating it. Based on the present analysis, we recommend further study of convection in supraglacial debris for glaciers across the globe with different debris properties.

As the climate warms, increases in glacier melt and altered glacier dynamics will result in changes to the dynamics of the Greenland Ice Sheet, impacting the sediment delivery in these rivers. In turn, examining the processes by which proglacial rivers transport sediment delivered by the ice sheet has important implications for the delivery of sediment to the oceans. Applying current knowledge of sediment transport from glacially-fed catchments in alpine regions is difficult, given several pronounced differences compared to glacially-fed catchments in the Arctic. These differences include elevated water discharge and reduced amplitude in diurnal variations of water discharge. Thus, it is imperative that we understand the differences in sediment dynamics between these two regions and evaluate the processes responsible for sediment transport between the ice sheet and ocean. To pursue this understanding, we installed seismic stations to measure bedload transport near the terminus of Russell Glacier during the summer of 2021. We convert the seismic signal from these stations to a bedload transport rate by evaluating several environmental variables, including the transported grain size and ground properties near the river. One station was close to the glacier, whilst the other is 1.5 km downstream. The distance between the stations allows us to evaluate the timing of proglacial sediment transport and deposition. Additionally, the operation of the instruments from early June through mid-August allows us to evaluate seasonal characteristics in sediment discharge. Lastly, we compare these results to the current knowledge of sediment transport from glacierized catchments in mountain regions.

The Japanese Alps uplifted throughout the Quaternary and reach elevations of up to 3,000 m today. However, understanding the interaction between rates of Earth surface processes, tectonics and climate is challenging, partly due to the difficulties of measuring changes in the rates of Earth surface processes at the timescale of glacial-interglacial cycles. In particular, the youth of the Japanese Alps has made measurement of their exhumation histories complicated. To help resolve this issue, we investigate the potential of ultra-low temperature thermochronometers based on the luminescence and electron spin resonance (ESR) of feldspar and quartz minerals, respectively. We focus on the Tateyama region in the Hida range, which was glaciated during the late Quaternary period. In total, eight samples were analyzed by luminescence and ESR thermochronometry. While most luminescence signals have already reached their upper dating limit, ESR signals do constrain exhumation rates.We measured the ESR dose response and thermal decay properties of all samples, specifically targeting the Al and Ti centres. In general, thermal stability is higher for the Ti centre than the Al centre, resulting in ESR ages of between 0.5-0.9 Ma, although the natural intensity of the Ti centre is close to or above the upper dating limit. In contrast, the Al signal still grows with time and is suitable for determining finite exhumation rates. Initial inversions reveal rock cooling rates on the order of 80 °C/Ma, which can be inverted to preliminarily rates of rock exhumation of

In mountainous environments, steep hillslopes tend to erode rapidly. These hillslopes, where present above glaciers, trundle rocks onto glacier surfaces below them. The loose rocks (debris) are subsequently transported along with glacial ice as it moves down valley. Debris can be so abundant that it produces continuous blankets across glaciers. Where debris cover is extensive it reduces ice melt and lowers the surface slope of glaciers. This feeds back to reduce basal sliding, thus impacting bedrock erosion. The erosion of bedrock produces loose sediment that is transported down valley along with the ice and by subglacial water flow.Here, using a coupled numerical model, we tie three realms of sediment transport (supraglacial, englacial, and subglacial) with feedbacks between ice dynamics and surface melt. The model runs in 2D (x, z) using the shallow-ice approximation and a simple formulation for melt under debris. Our simulations use glaciological parameters meant to loosely represent glaciers in the Khumbu region of Nepal. We present numerical experiments to reveal the first order effects of debris cover on erosion and landscape evolution.

Rock walls in high-alpine glacial environments are becoming increasingly unstable due to climate warming. This instability increases the erosion of headwalls above glaciers modifying glacial surface debris cover and mass balance and, thus, affecting the response of glaciers to climate change. As debris is deposited on glaciers, it is passively transported downglacier forming medial moraines where two glaciers join.We assess headwall erosion by systematic downglacier-debris sampling of medial moraines and by computing headwall erosion rates from their 10Be-cosmogenic nuclide concentrations. Around Pigne d’Arolla in Switzerland, we collected a total of 39 downglacier medial moraine debris samples from five adjacent glaciers. We explicitly chose medial moraines with source headwalls that vary in size, orientation and morphology, to investigate how different debris source area characteristics may express themselves in medial moraine cosmogenic nuclide concentrations. At the same time, the downglacier-debris sampling enables us to derive headwall erosion rate estimates through time, as medial moraine deposits tend to be older downglacier.Preliminary results reveal systematic differences in 10Be concentrations for the studied glaciers. At Glacier d’Otemma, Glacier du Brenay, and Glacier de Cheilon 10Be concentrations average at 17x103, 31x103, and 4x103 atoms g-1, respectively. Downglacier 10Be concentrations at Glacier d’Otemma vary systematically and headwall erosion rates tend to increase towards the present. At both Glacier du Brenay and Glacier de Cheilon downglacier 10Be concentrations are more uniform, suggesting that headwall erosion rates did not evolve significantly through time. Results for Glacier de Tsijiore Nouve and Glacier de Pièce will follow soon. In addition, samples at Glacier d’Otemma were collected along two parallel medial moraines sourced by different but adjacent headwalls. Yet, their downglacier 10Be concentrations deviate and our analyses suggest that at Glacier d’Otemma both differences in headwall orientation and headwall deglaciation history may account for the deviation of the two medial moraine records. For all five glaciers, we currently explore how lithology, slope angles, exposition, deglaciation, and elevation vary between the debris source areas and how differences therein could result in the observed differences in 10Be concentrations.

Publisher's version (útgefin grein), Strong similarities in Holocene climate reconstructions derived from multiple proxies (BSi, TOC – total organic carbon, δ13C, C∕N, MS – magnetic susceptibility, δ15N) preserved in sediments from both glacial and non-glacial lakes across Iceland indicate a relatively warm early to mid Holocene from 10 to 6 ka, overprinted with cold excursions presumably related to meltwater impact on North Atlantic circulation until 7.9 ka. Sediment in lakes from glacial catchments indicates their catchments were ice-free during this interval. Statistical treatment of the high-resolution multi-proxy paleoclimate lake records shows that despite great variability in catchment characteristics, the sediment records document more or less synchronous abrupt, cold departures as opposed to the smoothly decreasing trend in Northern Hemisphere summer insolation. Although all lake records document a decline in summer temperature through the Holocene consistent with the regular decline in summer insolation, the onset of significant summer cooling occurs ∼5 ka at high-elevation interior sites but is variably later at sites closer to the coast, suggesting that proximity to the sea may modulate the impact from decreasing summer insolation. The timing of glacier inception during the mid Holocene is determined by the descent of the equilibrium line altitude (ELA), which is dominated by the evolution of summer temperature as summer insolation declined as well as changes in sea surface temperature for coastal glacial systems. The glacial response to the ELA decline is also highly dependent on the local topography. The initial ∼5 ka nucleation of Langjökull in the highlands of Iceland defines the onset of neoglaciation in Iceland. Subsequently, a stepwise expansion of both Langjökull and northeast Vatnajökull occurred between 4.5 and 4.0 ka, with a second abrupt expansion ∼3 ka. Due to its coastal setting and lower topographic threshold, the initial appearance of Drangajökull in the NW of Iceland was delayed until ∼2.3 ka. All lake records reflect abrupt summer temperature and catchment disturbance at ∼4.5 ka, statistically indistinguishable from the global 4.2 ka event, and a second widespread abrupt disturbance at 3.0 ka, similar to the stepwise expansion of Langjökull and northeast Vatnajökull. Both are intervals characterized by large explosive volcanism and tephra distribution in Iceland resulting in intensified local soil erosion. The most widespread increase in glacier advance, landscape instability, and soil erosion occurred shortly after 2 ka, likely due to a complex combination of increased impact from volcanic tephra deposition, cooling climate, and increased sea ice off the coast of Iceland. All lake records indicate a strong decline in temperature ∼1.5 ka, which culminated during the Little Ice Age (1250–1850 CE) when the glaciers reached their maximum Holocene dimensions., This work was supported primarily by the Icelandic Center for Research through grants awarded to Áslaug Geirsdóttir and Gifford H. Miller (no. 130775051 and Grant of Excellence no. 141573052) and several grants awarded to Áslaug Geirsdóttir from the UI Research Fund. We thank Sædís Ólafsdóttir, Celene Blair, Sydney Gunnarson, Sarah Crump, Thorsteinn Jónsson, and Sveinbjörn Steinthorsson, who all contributed to this work by taking part in field work, laboratory analyses, and/or discussion. Thorough and constructive reviews from two anonymous reviewers have substantially improved the paper.

Many glaciers in High Mountain Asia are experiencing the debris-cover anomaly. The Kennicott Glacier, a large Alaskan Glacier, is also thinning most rapidly under debris cover. This contradiction has been explained by melt hotspots, such as ice cliffs, streams, or ponds scattered within the debris cover or by declining ice flow in time. We collected abundant in situ measurements of debris thickness, sub-debris melt, and ice cliff backwasting, allowing for extrapolation across the debris-covered tongue. A newly developed automatic ice cliff delineation method is the first to use only optical satellite imagery. The adaptive binary threshold method accurately estimates ice cliff coverage even where ice cliffs are small and debris color varies. We also develop additional remotely-sensed datasets of ice dynamical variables, other melt hot spots, and glacier thinning.Kennicott Glacier exhibits the highest fractional area of ice cliffs (11.7 %) documented to date. Ice cliffs contribute 26 % of total melt across the glacier tongue. Although the relative importance of ice cliffs to area-average melt is significant, the absolute area-averaged melt is dominated by debris. At Kennicott Glacier, glacier-wide melt rates are not maximized in the zone of maximum thinning. Declining ice discharge through time therefore explains the rapid thinning. Through this study, Kennicott Glacier is the first glacier in Alaska, and the largest glacier globally, where melt across its debris-covered tongue has been rigorously quantified.We also carefully explore the relationship between debris, melt hotspots, ice dynamics, and thinning across the debris-covered tongue. In doing so we reveal a chain of linked processes that can explain the striking patterns expressed on the debris-covered tongue of Kennicott Glacier.

Debris-covered glaciers are fed from steep bedrock hillslopes that tower above the ice. These headwalls are eroded by rockfalls and rock avalanches, mobilizing fractured bedrock, which is subsequently deposited on the ice surface along the sides of valley glaciers and transported downglacier on and in the ice. Where glaciers join, marginal debris merges to form medial moraines. Due to the conveyor-belt-nature of glacier ablation zones, debris tends to be older downglacier and, for typical Alpine glaciers, single deposits may persist on the glacier surface for hundreds to a few thousand years.Recent observations in high-alpine glacial environments suggest that rock walls are increasingly destabilized due to climate warming. An increase in headwall erosion and debris deposition onto glacier surfaces will modify glacial mass balances, as surface debris cover alters the rate at which underlying ice melts. Consequently, we expect that the response of debris-covered glaciers to climate change is likely also related to the response of headwalls to climate change.In this context, we quantify headwall retreat rates by measuring the concentration of in situ-produced cosmogenic 10Be in debris samples collected from a partly debris-covered Swiss valley glacier. By systematic downglacier-sampling of two parallel medial moraines, we aim to assess changes in headwall erosion through time for small and delineated source areas. Our results indicate that indeed, nuclide concentrations along the medial moraines vary with time: downglacier and further back in time deposits have higher nuclide concentrations, whereas upglacier and more recently deposits have lower concentrations. Currently, we explore possible processes which could account for 10Be concentration changes through time, other than changes in erosion rates. These include the sensitivity of 10Be concentrations to supraglacial transport time and to temporal and spatial changes in nuclide production rates on the deglaciating headwalls. First analyses reveal, however, that neither the additional accumulation of 10Be during transport nor changes in source area production rates associated with the uncovering of formerly ice covered headwall parts alone can account for the observed trend.

Glacial moraines represent one of the most spatially diverse climate archives on earth. Moraine dating and numerical modeling are used to effectively reconstruct past climate from mountain ranges at the global scale. But because moraines are often located downvalley from steep mountain headwalls, it is possible that debris-covered glaciers emplaced many moraines preserved in the landscape today.Before we can understand the effect of debris cover on the moraine recored we need to understand how debris modulates glacier response to climate change. To help address this need, we developed a numerical model that links feedbacks between mountain glaciers, climate change, hillslope erosion, and landscape evolution. Our model uses parameters meant to represent glaciers in the Khumbu region of Nepal, though the model physics are relevant for mountain glaciers elsewhere.We compare simulated debris-covered and debris-free glaciers and their length evolution. We explore the effect of climate-dependent hillslope erosion. We also allow temperature change to control frost cracking and permafrost in the headwall above simulated glaciers. Including these effects holds special implications for glacial evolution during deglaciation and the long-term evolution of mountain landscapes.Because debris cover suppresses melt, debris-covered glaciers can advance independent of climate change. When debris cover is present during cold periods, moraine emplacement can lag debris-free glacier moraine emplacement by hundreds of years. We develop a suite of tools to help determine whether individual moraines were formed by debris-covered glaciers. Our analyses also point to how we might interpret moraine ages and estimate past climate states from debris-perturbed settings.

Thick debris cover, greater than about 5 cm, insulates ice and reduces melt rates. Despite this melt-suppressing effect, glaciers often thin rapidly under thick debris cover. In High Mountain Asia, the European Alps, and Alaska many debris-covered and debris-free glacier tongues are thinning at similar rates (e.g., Kääb et al., 2012). This apparent paradox is known as the ‘debris-cover anomaly’ (Pellicciotti et al., 2015). Two mechanisms have been proposed to explain this behavior, which are not mutually exclusive. First, glacier thinning under thick debris is enhanced by melt hotspots (lakes, ice cliffs, and streams) within otherwise continuous debris cover. Second, the decline in ice flow from upglacier leads to thinning under thick debris (e.g., Vincent et al., 2016). We propose a new mechanism to explain why thinning amplifies under thick debris. It appears that debris cover—through its affect on the melt pattern—controls glacier geometry (i.e., patterns of ice thickness and surface slope). A characteristic debris-perturbed driving stress pattern results which in turn controls where dynamical thinning amplifies, often in the upper reaches of debris-covered tongues. Our explanation is supported with data from a suite of glaciers in the Himalaya and with simulations from a numerical debris-covered glacier model responding to climate change (Anderson and Anderson, 2016).In all numerical simulations, the zone of maximum glacier thinning initially occurs upglacier from the debris cover. This zone of maximum thinning then propagates downglacier into the debris-covered portion. We explain how this zone of maximum thinning can be spatially pinned and amplified at different locations relative to the terminus depending on debris thickness, bed slope, glacier size, and glacier topology. This seemingly paradoxical mechanism in which debris itself controls thinning under thick debris is further supported by an analysis of published thinning data from glaciers across High Mountain Asia.

Animal models of bone marrow have limited spatial and temporal resolution to observe biological events (intravasation and cellular egress) and are inadequate to dissect dynamic events at the niche level (100 microns). Utilizing microfluidic and stem cell technology, we present a 3D in vitro model of human bone marrow that contains perivascular and endosteal niches complete with dynamic, perfusable vascular networks. We demonstrate that our model can perform in vivo functions including maintenance and differentiation of CD34+ hematopoietic stem/progenitor cells (HSPC) for up to fourteen days, egress of myeloid progenitors, and expression of markers consistent with in vivo human bone marrow. The platform design enables the addition of tissue niches at a later timepoint to probe mechanisms such as tumor cell migration. Overall, we present a novel organ-on-a-chip platform that is capable of recapitulating the human bone marrow microenvironment to observe hematopoietic phenomena at high spatial and temporal resolution.

Alpine ice varies from pure ice glaciers to partially debris-covered glaciers to rock glaciers, as defined by the degree of debris cover. In many low- to mid-latitude mountain ranges, the few bare ice glaciers that do exist in the present climate are small and are found where snow is focused by avalanches and where direct exposure to radiation is minimized. Instead, valley heads are more likely to be populated by rock glaciers, which can number in the hundreds. These rock-cloaked glaciers represent some of the most identifiable components of the cryosphere today in low- to mid-latitude settings, and the over-steepened snouts pose an often overlooked hazard to travel in alpine terrain. Geomorphically, rock glaciers serve as conveyor belts atop which rock is pulled away from the base of cliffs. In this work, we show how rock glaciers can be treated as an end-member case that is captured in numerical models of glaciers that include ice dynamics, debris dynamics, and the feedbacks between them. Specifically, we focus on the transition from debris-covered glaciers, where the modern equilibrium line altitude (ELA) intersects the topography, to rock glaciers, where the modern ELA lies above the topography. On debris-covered glaciers (i.e., glaciers with a partial rock mantle), rock delivered to the glacier from its headwall, or from sidewall debris swept into the glacier at tributary junctions, travels englacially to emerge below the ELA. There it accumulates on the surface and damps the rate of melt of underlying ice. This allows the termini of debris-covered glaciers to extend beyond debris-free counterparts, thereby decreasing the ratio of accumulation area to total area of the glacier (AAR). In contrast, rock glaciers (i.e., glaciers with a full rock mantle) occur where and when the environmental ELA rises above the topography. They require avalanches and rockfall from steep headwalls. The occurrence of rock glaciers reflects this dependence on avalanche sources because they are most common on lee sides of ridges and peaks where wind-blown snow enhances the strength of the avalanche source. To maintain positive mass balance, the avalanche cone developed in the winter must be sufficiently thick not to melt entirely in the summer, thus providing an ice accumulation area for the rock glacier. In the absence of rockfall, this would support a short cirque glacier. The presence of debris, however, facilitates the development of rock glaciers with lengths of hundreds of meters, thicknesses of tens of meters, and speeds of meters per year that are well described by numerical models. Numerical models are used to explore the alpine glacier response to its climate history. In warming climates, a debris-covered glacier can transform into a much shorter rock glacier, leaving in its wake a thinning ice-cored moraine. Rock glaciers will persist in landscapes well beyond debris-free counterparts because they have much longer response times to climate change. The headwaters of alpine basins with steep headwalls will therefore oscillate between glacier and rock glacier occupation over glacial-interglacial cycles, maintaining a means by which rock from the headwall can be conveyed away. This enhances the asymmetry of alpine ridgelines, with downwind valleys biting deeply into the range crests, as originally noted by G.K. Gilbert.

Many debris-covered glaciers have broadly similar debris thickness patterns: surface debris thickens and tends to transition from convex- to concave-up-down glacier. We explain this pattern using theory (analytical and numerical models) paired with empirical observations. Down glacier debris thickening results from the conveyor-belt-like nature of the glacier surface in the ablation zone (debris can typically only be added but not removed) and from the inevitable decline in ice surface velocity toward the terminus. Down-glacier thickening of debris leads to the reduction of sub-debris melt and debris emergence toward the terminus. Convex-up debris thickness patterns occur near the up-glacier end of debris covers where debris emergence dominates (ablation controlled). Concave-up debris thickness patterns occur toward glacier termini where declining surface velocities dominate (velocity controlled). A convex-concave debris thickness profile inevitably results from the transition between ablation-control and velocity-control down-glacier. Debris thickness patterns deviating from this longitudinal shape are most likely caused by changes in hillslope debris supply through time. By establishing this expected debris thickness pattern, the effects of climate change on debris cover can be better identified.

Drangajokull is a maritime ice cap located in northwest (Vestfirðir) Iceland. Drangajokull's evolution is therefore closely linked to atmospheric and ocean variability. In order to better constrain the Holocene climate and glacier history of Vestfirðir we model the past evolution of Drangajokull ice cap. Simulations from 10 ka to present are forced by general circulation model output, ice-core-based temperature reconstructions, and sea-surface temperature reconstructions. Based on these 10-thousand year simulations, Drangajokull did not persist through the Holocene. We estimate that air temperatures were 2.5–3.0 °C higher during the Holocene Thermal Maximum than the local 1960–1990 average. Simulations support Drangajokull's late Holocene inception between 2 and 1 ka, though intermittent ice likely occupied cirques as early as 2.6 ka. Drangajokull is primarily a Little Ice Age ice cap: it expanded between 1300 and 1750 CE, with the most rapid growth occurring between 1600 and 1750 CE. The maximum Holocene extent of Drangajokull occurred between 1700 and 1925 CE, despite the lowest late Holocene temperatures, occurring between 1650 and 1720 CE. Between 1700 and 1925 CE temperatures were likely 0.6–0.8 °C lower than the 1950–2015 reference temperature. The modern equilibrium line altitude (ELA) is bracketed by topographic thresholds: a 1 °C temperature increase from the modern ELA would eliminate the ice cap's accumulation area, while a reduction of 0.5 °C would lead to the rapid expansion of the ice cap across Vestfirðir. The proximity of Drangajokull to topographic thresholds may explain its late inception and rapid expansion during the Little Ice Age.

10Be dating of moraines has greatly improved our ability to constrain the timing of past glaciations and thus past cold events. However, the spread in ages from a single moraine is often greater than would be expected from measurement uncertainty, making paleoclimatic interpretations equivocal. Here we present 28 new 10Be ages from ice-cored Neoglacial moraines on Baffin Island, Arctic Canada, and explore the processes at play in moraine formation and evolution through field observations and a numerical debris-covered glacier model. The insulating effect of debris cover modifies glacier lengths and results in the development of ice-cored moraines over multiple advances and thousands of years. Although ice cores can persist for several millennia, spatially variable ice core melt-out contributes to moraine degradation and boulder destabilization, making it likely that the 10Be clock is reset on moraine boulders in these settings. Thus, exposure ages from ice-cored moraines must be interpreted with caution. The oldest ages, after excluding samples with inheritance, provide the best estimates of initial moraine formation. Three Baffin Island moraines yield 10Be ages suggesting formation at 5.2, 4.6 and 3.5 ka, respectively, adding to a growing body of evidence for significant summer cooling millennia before the Little Ice Age.

The mass balance of many valley glaciers is enhanced by the presence of ice cliffs within otherwise continuous debris cover. We assess the effect of debris and ice cliffs on the thinning of Kennicott Glacier in three companion papers. In Part A we report in situ measurements from the debris-covered tongue. Here, in Part B, we develop a method to delineate ice cliffs using high-resolution imagery and use empirical relationships from Part A to produce distributed mass balance estimates. In Part C we describe feedbacks that contribute to rapid thinning under thick debris. Ice cliffs cover 11.7 % of the debris-covered tongue, the most of any glacier studied to date, and they contribute 19 % of total melt. Ice cliffs contribute an increasing percentage of melt the thicker the debris cover. In the lowest 4 km of the glacier, where debris thicknesses are greater than 20 cm, ice cliffs contribute 40 % of total melt. Surface lake coverage doubled between 1957 and 2009, but lakes do not occur across the full extent of the zone of maximum glacier thinning. Despite abundant ice cliffs and expanding surface lakes, average melt rates are suppressed by debris, the pattern of which appears to reflect the debris thickness-melt relationship (or Østrem’s curve). This suggests that, in addition to melt hotspots, the decline in ice discharge from upglacier is an important contributor to the thinning of Kennicott glacier under thick debris.

The mass balance of many Alaskan glaciers is perturbed by debris cover. Yet the effect of debris on glacier response to climate change in Alaska has largely been overlooked. In three companion papers we assess the role of debris, ice dynamics, and surface processes in thinning Kennicott Glacier. In Part A, we report in situ measurements from the glacier surface. In Part B, we develop a method to delineate ice cliffs using high-resolution imagery and produce distributed mass balance estimates. In Part C we explore feedbacks that contribute to glacier thinning. Here in Part A, we describe data collected in the summer of 2011. We measured debris thickness (109 locations), sub-debris melt (74), and ice cliff backwasting (60) data from the debris-covered tongue. We also measured air-temperature (3 locations) and internal-debris temperature (10). The mean debris thermal conductivity was 1.06 W (m C)−1, increasing non-linearly with debris thickness. Mean debris thicknesses increase toward the terminus and margin where surface velocities are low. Despite the relatively high air temperatures above thick debris, the melt-insulating effect of debris dominates. Sub-debris melt rates ranged from 6.5 cm d−1 where debris is thin to 1.25 cm d−1 where debris is thick near the terminus. Ice cliff backwasting rates varied from 3 to 14 cm d−1 with a mean of 7.1 cm d−1 and tended to increase as elevation declined and debris thickness increased. Ice cliff backwasting rates are similar to those measured on debris-covered glaciers in High Mountain Asia and the Alps.

In vivo whole-animal optical (bioluminescence and fluorescence) imaging of Staphylococcus aureus infections has provided the opportunity to noninvasively and longitudinally monitor the dynamics of the bacterial burden and ensuing host immune responses in live anesthetized animals. Herein, we describe several different mouse models of S. aureus skin infection, skin inflammation, incisional/excisional wound infections, as well as mouse and rabbit models of orthopedic implant infection, which utilized this imaging technology. These animal models and imaging methodologies provide insights into the pathogenesis of these infections and innate and adaptive immune responses, as well as the preclinical evaluation of diagnostic and treatment modalities. Noninvasive approaches to investigate host-pathogen interactions are extremely important as virulent community-acquired methicillin-resistant S. aureus strains (CA-MRSA) are spreading through the normal human population, becoming more antibiotic resistant and creating a serious threat to public health.

Staphylococcus aureus (S. aureus) infections, including methicillin resistant stains, are an enormous burden on the healthcare system. With incidence rates of S. aureus infection climbing annually, there is a demand for additional research in its pathogenicity. Animal models of infectious disease advance our understanding of the host-pathogen response and lead to the development of effective therapeutics. Neutrophils play a primary role in the innate immune response that controls S. aureus infections by forming an abscess to wall off the infection and facilitate bacterial clearance; the number of neutrophils that infiltrate an S. aureus skin infection often correlates with disease outcome. LysM-EGFP mice, which possess the enhanced green fluorescent protein (EGFP) inserted in the Lysozyme M (LysM) promoter region (expressed primarily by neutrophils), when used in conjunction with in vivo whole animal fluorescence imaging (FLI) provide a means of quantifying neutrophil emigration noninvasively and longitudinally into wounded skin. When combined with a bioluminescent S. aureus strain and sequential in vivo whole animal bioluminescent imaging (BLI), it is possible to longitudinally monitor both the neutrophil recruitment dynamics and in vivo bacterial burden at the site of infection in anesthetized mice from onset of infection to resolution or death. Mice are more resistant to a number of virulence factors produced by S. aureus that facilitate effective colonization and infection in humans. Immunodeficient mice provide a more sensitive animal model to examine persistent S. aureus infections and the ability of therapeutics to boost innate immune responses. Herein, we characterize responses in LysM-EGFP mice that have been bred to MyD88-deficient mice (LysM-EGFP×MyD88(−/−) mice) along with wild-type LysM-EGFP mice to investigate S. aureus skin wound infection. Multispectral simultaneous detection enabled study of neutrophil recruitment dynamics by using in vivo FLI, bacterial burden by using in vivo BLI, and wound healing longitudinally and noninvasively over time.

Staphylococcus aureus (S. aureus) infections, including methicillin resistant stains, are an enormous burden on the healthcare system. With incidence rates of S. aureus infection climbing annually, there is a demand for additional research in its pathogenicity. Animal models of infectious disease advance our understanding of the host-pathogen response and lead to the development of effective therapeutics. Neutrophils play a primary role in the innate immune response that controls S. aureus infections by forming an abscess to wall off the infection and facilitate bacterial clearance; the number of neutrophils that infiltrate an S. aureus skin infection often correlates with disease outcome. LysM-EGFP mice, which possess the enhanced green fluorescent protein (EGFP) inserted in the Lysozyme M (LysM) promoter region (expressed primarily by neutrophils), when used in conjunction with in vivo whole animal fluorescence imaging (FLI) provide a means of quantifying neutrophil emigration noninvasively and longitudinally into wounded skin. When combined with a bioluminescent S. aureus strain and sequential in vivo whole animal bioluminescent imaging (BLI), it is possible to longitudinally monitor both the neutrophil recruitment dynamics and in vivo bacterial burden at the site of infection in anesthetized mice from onset of infection to resolution or death. Mice are more resistant to a number of virulence factors produced by S. aureus that facilitate effective colonization and infection in humans. Immunodeficient mice provide a more sensitive animal model to examine persistent S. aureus infections and the ability of therapeutics to boost innate immune responses. Herein, we characterize responses in LysM-EGFP mice that have been bred to MyD88-deficient mice (LysM-EGFP×MyD88-/- mice) along with wild-type LysM-EGFP mice to investigate S. aureus skin wound infection. Multispectral simultaneous detection enabled study of neutrophil recruitment dynamics by using in vivo FLI, bacterial burden by using in vivo BLI, and wound healing longitudinally and noninvasively over time.

The increasingly common disappearance of glaciers is striking, and it is often used to highlight the effects of anthropogenic climate change to the public. This approach is more effective when modern glacier and climate change is placed within a frame of reference. Here we present a new, efficient method to assess the timing of glacier birth and death. We model equilibrium line altitudes (ELAs) of 22 Icelandic glaciers through time, and in doing so produce the first regional estimates of glacier lifespans. We force the model with three Holocene temperature reconstructions and five future-climate projections, while also exploring the effects of glacial-isostatic adjustment and variable precipitation. Mass balance parameters are tuned for each ice cap using modern data. Geologically-constrained inception ages are used to validate individual simulations. The timing of Icelandic glacier inception likely ranged from before the Holocene to as late as the Little Ice Age. Anthropogenic warming is expected to cut short the lifespans of glaciers that have existed for thousands of years. By quantifying the controlling factors we show that topography and the rate of summer temperature change are the primary controls of glacier lifespans in Iceland. Topography, represented by the difference between the modern ELA and the highest local topography, is ten-times as important as ELA sensitivity to temperature. This easy-to-measure topographic metric exerts a first-order control on the timing of inception and the future loss of accumulation area and it can be used to contextualize ongoing glacier disappearance.

Non-linear climate change is often linked to rapid changes in ocean circulation, especially around the North Atlantic. As the Polar Front fluctuated its latitudinal position during the Holocene, Iceland's climate was influenced by both the warm Atlantic currents and cool, sea ice-bearing Arctic currents. Drangajokull is Iceland's fifth largest ice cap. Climate proxies in lake sediment cores, dead vegetation emerging from beneath the ice cap, and moraine segments identified in a new DEM constrain the episodic expansion of the ice cap over the past 3 ka. Collectively, our data show that Drangajokull was advancing at ∼320 BCE, 180 CE, 560 CE, 950 CE and 1400 CE and in a state of recession at ∼450 CE, 1250 CE and after 1850 CE. The Late Holocene maximum extent of Drangajokull occurred during the Little Ice Age (LIA), occupying 262 km2, almost twice its area in 2011 CE and ∼20% larger than recent estimates of its LIA dimensions. Biological proxies from the sediment fill in a high- and low-elevation lake suggest limited vegetation and soil cover at high elevations proximal to the ice cap, whereas thick soil cover persisted until ∼750 CE at lower elevations near the coast. As Drangajokull expanded into the catchment of the high-elevation lake beginning at ∼950 CE, aquatic productivity diminished, following a trend of regional cooling supported by proxy records elsewhere in Iceland. Correlations between episodes of Drangajokull's advance and the documented occurrence of drift ice on the North Icelandic Shelf suggest export and local production of sea ice influenced the evolution of NW Iceland's Late Holocene climate.

Strong similarities in Holocene climate reconstructions derived from multiple proxies (BSi, TOC, δ13C, C/N, MS, δ15N) preserved in sediments from both glacial and non-glacial lakes across Iceland indicate a relatively warm early-to-mid Holocene from 10 to 6 ka, overprinted with cold excursions presumably related to meltwater impact on North Atlantic circulation until 7.9 ka. Sediment in lakes from glacial catchments indicates their catchments were ice-free during this interval. Statistical treatment of the high-resolution multiproxy paleoclimate lake records shows that despite great variability in catchment characteristics, the records document more or less synchronous abrupt, cold departures as opposed to the smoothly decreasing trend in Northern Hemisphere summer insolation. Although all lake records document a decline in summer temperature through the Holocene consistent with the regular decline in summer insolation, the onset of significant summer cooling, occurs ~5 ka in high-elevation interior sites, but is variably later in sites closer to the coast, suggesting some combination of changing ocean currents and sea ice modulate the impact from decreasing summer insolation. The timing of glacier inception during the mid-Holocene is determined by the decent of the Equilibrium Line Altitude (ELA), which is dominated by the evolution of summer temperature as summer insolation declined as well as changes in sea surface temperature for glacial systems particularly in coastal settings. The glacial response to the ELA decline is also highly dependent on the local topography. The initial nucleation of Langjökull in the highlands of Iceland starting by ca 5 ka, was followed by a stepwise expansion of both Langjökull and northeast Vatnajökull between 4.5 and 4.0 ka, with a second abrupt expansion ca. 3 ka. However, the initial appearance of Drangajökull in the NW of Iceland was delayed until after 2.5 ka. All lake records reflect abrupt summer temperature and catchment disturbance at about 4.5 ka, statistically indistinguishable from the ~4.2 ka event with a second widespread abrupt disturbance centered on 3.0 ka. Both are intervals of large explosive volcanism on Iceland. The most widespread increase in glacier advance, landscape instability, and soil erosion occurred shortly after 2 ka, likely due to a complex combination of increased impact from volcanic activity, cooling climate, and increased sea ice off the coast of Iceland. All lake records indicate a strong decline in temperature ~1.5 ka, culminating during the Little Ice Age between 1300 and 1900 CE when most glaciers reached their maximum dimensions.

Debris-covered glaciers are common in rapidly eroding alpine landscapes. When thicker than a few centimeters, surface debris suppresses melt rates. If continuous debris cover is present, ablation rates can be significantly reduced leading to increases in glacier length. In order to quantify feedbacks in the debris–glacier–climate system, we developed a 2-D long-valley numerical glacier model that includes englacial and supraglacial debris advection. We ran 120 simulations on a linear bed profile in which a hypothetical steady state debris-free glacier responds to a step increase of surface debris deposition. Simulated glaciers advance to steady states in which ice accumulation equals ice ablation, and debris input equals debris loss from the glacier terminus. Our model and parameter selections can produce 2-fold increases in glacier length. Debris flux onto the glacier and the relationship between debris thickness and melt rate strongly control glacier length. Debris deposited near the equilibrium-line altitude, where ice discharge is high, results in the greatest glacier extension when other debris-related variables are held constant. Debris deposited near the equilibrium-line altitude re-emerges high in the ablation zone and therefore impacts melt rate over a greater fraction of the glacier surface. Continuous debris cover reduces ice discharge gradients, ice thickness gradients, and velocity gradients relative to initial debris-free glaciers. Debris-forced glacier extension decreases the ratio of accumulation zone to total glacier area (AAR). Our simulations reproduce the "general trends" between debris cover, AARs, and glacier surface velocity patterns from modern debris-covered glaciers. We provide a quantitative, theoretical foundation to interpret the effect of debris cover on the moraine record, and to assess the effects of climate change on debris-covered glaciers.

Neutrophils are central players in inflammation and the innate immune response to invading microbial pathogens. At any given time, the majority of neutrophils are sequestered in the microvasculature of the lung, a location that is well suited for them to respond immediately to inflammatory stimuli originating from the external environment. They sequester in the lung capillaries through unique hemodynamic and geometric properties of the pulmonary microvasculature, as well as changes in the rheological properties of neutrophils during activation. In contrast to the systemic circulation, the branching pulmonary microvasculature has the capacity to shunt blood flow away from inflamed areas through vasoconstriction that acts to limit the extent of neutrophil emigration into the airspaces. Due to these unique features of the pulmonary circulation, neutrophil migration in acute pulmonary inflammation is not always dependent on the typical multistep process of selectin-mediated rolling that transitions to chemokine-activated integrin-dependent arrest on immunoglobulin-like cell-adhesion molecules (Ig-CAMs). Neutrophils can cause extensive damage to the lung in a variety of infectious and toxicological inflammatory disorders. This is accomplished by their production of reactive oxygen species (ROS), release of granule components such as proteolytic enzymes and cationic proteins and excretion of neutrophil extracellular traps (NETs) upon activation. Experimental models suggest that repetitive pulmonary inflammation and accompanying epithelial injury exacerbate the inflammatory cascade and enhance the extent of pulmonary inflammation and epithelial injury in subsequent cycles. However, there is also evidence that neutrophils play an important role in removing injured pulmonary cells to enhance repair of the lung. The mechanisms utilized by neutrophils to target injured cells and selectively kill them or cause their junctional detachment in the absence of parenchymal derangement are just beginning to be understood. Clearly this beneficial action of the neutrophils occurs only in mild toxicological injury to the lung since it appears to be a dose-dependent response with mild injury resulting in low-to-moderate neutrophil emigration, while severe injury is associated with robust neutrophil recruitment, degranulation, and reactive oxygen elaboration. Under these conditions of widespread and severe injury, the main function of neutrophils appears to be removal of dead cells and debris and stimulation of the remaining viable cells to spread and proliferate in order to promote reepithelialization of the lung, the first step to organ restoration.

Long-term in␣vitro maintenance of primary hepatocytes has been a challenge in hepatic tissue engineering, since these cells dedifferentiate and lose their phenotype in standard culture conditions. Some of the most successful culture systems employ collagen or Matrigel matrices. As an alternative matrix, our laboratory has been exploring the use of heparin-containing hydrogels as scaffolds for cultivation of functional hepatocytes. The present study characterized a heparin gel sandwich culture system for maintenance of primary rat hepatocytes. The heparin gel layers were fabricated via UV light induced thiol-ene coupling reaction of thiolated heparin (Hep-SH) and diacrylated poly(ethylene glycol) (PEG-DA) precursors. Analysis of hepatic function revealed that cells sustained albumin secretion for at least three weeks at similarly high levels for collagen and heparin gel sandwich culture systems. Furthermore, cytochrome P450 activity of hepatocytes was ∼1.6 times higher in heparin gels compared to collagen gels. Another distinct advantage of heparin is its ability to bind and release growth factors in a controlled manner. We observed that after 24 days hepatocytes cultured in heparin gel sandwich containing hepatocyte growth factor (HGF) produced ∼1.8 times higher amounts of albumin compared to cells in collagen sandwiches. Overall, we demonstrate heparin gel sandwich to be an excellent system for maintaining differentiated, polarized primary hepatocytes for over three weeks. The biomaterial used in this study, heparin–PEG hydrogel, offers significant flexibility in terms of tuning mechanical properties, adjusting heparin content or introducing photolabile chemistries to promote gel degradation. We see multiple applications for this novel system in hepatocyte maintenance and stem cell differentiation studies.

SummaryX-linked Severe Combined Immunodeficiency (SCID-X1) is a genetic disease that leaves newborns at high risk of serious infection and a predicted life span of less than 1 year in the absence of a matched bone marrow donor. The disease pathogenesis is due to mutations in the gene encoding the Interleukin-2 receptor gamma chain (IL-2Rγ), leading to a lack of functional lymphocytes. With the leukemogenic concerns of viral gene therapy there is a need to explore alternative therapeutic options. We have utilized induced pluripotent stem cell (iPSC) technology and genome editing mediated by TALENs to generate isogenic subject-specific mutant and gene-corrected iPSC lines. While the subject-derived mutant iPSCs have the capacity to generate hematopoietic precursors and myeloid cells, only wild-type and gene-corrected iPSCs can additionally generate mature NK cells and T cell precursors expressing the correctly spliced IL-2Rγ. This study highlights the potential for the development of autologous cell therapy for SCID-X1 subjects.

The immune response to Staphylococcus aureus infection in skin involves the recruitment of polymorphonuclear neutrophils (PMNs) from the bone marrow via the circulation and local granulopoiesis from hematopoietic stem and progenitor cells (HSPCs) that also traffic to infected skin wounds. We focus on regulation of PMN number and function and the role of pore-forming α-toxin (AT), a virulence factor that causes host cell lysis and elicits inflammasome-mediated IL-1β secretion in wounds. Infection with wild-type S. aureus enriched in AT reduced PMN recruitment and resulted in sustained bacterial burden and delayed wound healing. In contrast, PMN recruitment to wounds infected with an isogenic AT-deficient S. aureus strain was unimpeded, exhibiting efficient bacterial clearance and hastened wound resolution. HSPCs recruited to infected wounds were unaffected by AT production and were activated to expand PMN numbers in proportion to S. aureus abundance in a manner regulated by TLR2 and IL-1R signaling. Immunodeficient MyD88-knockout mice infected with S. aureus experienced lethal sepsis that was reversed by PMN expansion mediated by injection of wild-type HSPCs directly into wounds. We conclude that AT-induced IL-1β promotes local granulopoiesis and effective resolution of S. aureus–infected wounds, revealing a potential antibiotic-free strategy for tuning the innate immune response to treat methicillin-resistant S. aureus infection in immunodeficient patients.

The immune response to Staphylococcus aureus infection in skin involves the recruitment of neutrophils (PMN) from the bone marrow via the circulation and local granulopoiesis from hematopoietic stem and progenitor cells (HSPC) that also traffic to infected skin wounds. We focus on regulation of PMN number and function and the role of pore-forming alpha-toxin (AT), a virulence factor that causes host cell lysis and elicits inflammasome-mediated IL-1β secretion in wounds. Infection with wild type S. aureus enriched in AT reduced PMN recruitment and resulted in sustained bacterial burden and delayed wound healing. In contrast, PMN recruitment to wounds infected with an isogenic AT mutant strain (ΔAT) was unimpeded, exhibiting efficient bacterial clearance and hastened wound resolution. HSPC recruited to infected wounds was unaffected by AT production and were activated to expand PMN numbers in proportion to S. aureus abundance in a manner regulated by TLR2 and IL-1 receptor signaling. Immunodeficient MyD88 knockout mice infected with S. aureus experienced lethal sepsis that was reversed by PMN expansion mediated by injection of wild type HSPC directly into wounds. We conclude that AT induced IL-1β promotes local granulopoiesis and effective resolution of S. aureus-infected wounds, revealing a potential antibiotic free strategy for tuning the innate immune response to treat MRSA infection in immunodeficient patients.

Glaciers respond to climate variations and leave geomorphic evidence that represents an important terrestrial paleoclimate record. However, the accuracy of paleoclimate reconstructions from glacial geology is limited by the challenge of representing mountain meteorology in numerical models. Precipitation is usually treated in a simple manner and yet represents difficult-to-characterize variables such as amount, distribution, and phase. Furthermore, precipitation distributions during a glacial probably differed from present-day interglacial patterns. We applied two models to investigate glacier sensitivity to temperature and precipitation in the eastern Southern Alps of New Zealand. A 2-D model was used to quantify variations in the length of the reconstructed glaciers resulting from plausible precipitation distributions compared to variations in length resulting from change in mean annual air temperature and precipitation amount. A 1-D model was used to quantify variations in length resulting from interannual climate variability. Assuming that present-day interglacial values represent precipitation distributions during the last glacial, a range of plausible present-day precipitation distributions resulted in uncertainty in the Last Glacial Maximum length of the Pukaki Glacier of 17.1 km (24%) and the Rakaia Glacier of 9.3 km (25%), corresponding to a 0.5°C difference in temperature. Smaller changes in glacier length resulted from a 50% decrease in precipitation amount from present-day values (−14% and −18%) and from a 50% increase in precipitation amount (5% and 9%). Our results demonstrate that precipitation distribution can produce considerable variation in simulated glacier extents and that reconstructions of paleoglaciers should include this uncertainty.

Valley glacier moraines are commonly used to infer past mean annual precipitation and mean melt-season temperature. However, recent research has demonstrated that, even in steady climates, multi-decadal, kilometer-scale fluctuations in glacier length occur in response to stochastic, year-to-year variability in mass balance. When interpreting moraine sequences it is important to include the effect of interannual weather variability on glacier length; moraines record advances that are forced either by interannual variability or by a combination of climate change and interannual variability. We address this issue for the Last Glacial Maximum (LGM) glaciers of the Colorado Front Range, United States. Using a linear glacier model that allows thorough exploration of parameter uncertainties, supplemented by a shallow-ice flowline model, our analyses suggest that (1) glacial standstills longer than 50 years were unlikely; (2) mean glacier lengths are ∼10%–15% up-valley from maximum glacier lengths; and (3) individual LGM terminal moraines were formed by a combination of a climate change and interannual variability–forced advances.

Sub-meter resolution, stereoscopic satellite images allow for the generation of accurate and high-resolution digital elevation models (DEMs) over glaciers and ice caps. Here, repeated stereo images of Drangajökull ice cap (NW Iceland) from Pléiades and WorldView2 (WV2) are combined with in situ estimates of snow density and densification of firn and fresh snow to provide the first estimates of the glacier-wide geodetic winter mass balance obtained from satellite imagery. Statistics in snow- and ice-free areas reveal similar vertical relative accuracy (, This study was funded by the University of Iceland (UI) Research Fund. Pleiades images were acquired at research price thanks to the CNES ISIS program (http://www.isis-cnes.fr). The WV2 DEM was obtained through the ArcticDEM project. This work is a contribution to the Rannis grant of excellence project, ANATILS. Collaboration and travels between IES and LEGOS were funded by the Jules Verne research fund and the TOSCA program from the French Space Agency, CNES. This study used the recent lidar mapping of the glaciers in Iceland that was funded by the Icelandic Research Fund, the Landsvirkjun research fund, the Icelandic Road Administration, the Reykjavik Energy Environmental and Energy Research Fund, the Klima-og Luftgruppen (KoL) research fund of the Nordic Council of Ministers, the Vatnajokull National Park, the organization Friends of Vatnajokull, the National Land Survey of Iceland, the Icelandic Meteorological Office and the UI research fund. The ground-based mass balance measurements on Drangajokull have been jointly funded by Orkubu Vestfjarda (Westfjord Power Company), the National Energy Authority (2004-2009) and the Icelandic Meteorological Office (2009-2015).

Debris-covered glaciers are common in rapidly-eroding alpine landscapes. When thicker than a few centimeters, surface debris suppresses melt rates. If continuous debris cover is present, mass balance gradients can be reduced leading to increases in glacier length. In order to quantify feedbacks in the debris-glacier-climate system, we developed a 2-D long-valley numerical glacier model that includes englacial and supraglacial advection. We ran 120 simulations in which a steady state debris-free glacier responds to a step increase of surface debris deposition. Simulated glaciers advance to steady states in which ice accumulation equals ice ablation, and debris input equals debris loss from the glacier. Our model and parameter selections produce two-fold increases in glacier length. Debris flux onto the glacier and the relationship between debris thickness and melt rate strongly control glacier length. Debris deposited near the equilibrium-line altitude, where ice discharge is high, results in the greatest glacier extension when other debris related variables are held constant. Continuous debris cover reduces ice discharge gradients, ice thickness gradients, and velocity gradients relative to initial debris-free glaciers. Debris-forced glacier extension decreases the ratio of accumulation zone to total glacier area (AAR). The model reproduces first-order relationships between debris cover, AARs, and glacier surface velocities from glaciers in High Asia. We provide a quantitative, theoretical foundation to interpret the effect of debris cover on the moraine record, and to assess the effects of climate change on debris-covered glaciers.