Current state of Alaska's glaciers and evolution of Black Rapids Glacier constrained by observations and modeling

Thesis (Ph.D.) University of Alaska Fairbanks, 2016Glaciological studies rely on a wide range of input data, the most basic of which, accurate glacier extents, were not available on an Alaska wide scale prior to this work. We thus compiled a glacier database for Alaska and neighboring Canada using multi-sensor satellite data from 2000 to 2011. The inventory yielded a glacierized area of 86,720 km², which corresponds to ~12% of the global glacierized area outside the ice sheets. For each of the ~27,100 glaciers, we derived outlines and 51 variables, including centerline lengths, outline types, and debris cover, which provide key input for observational and modeling studies across Alaska. Expanding on this large-scale observational snapshot, we conducted two case studies on Black Rapids Glacier, Eastern Alaska Range, to assess its evolution during the late 20th and 21st centuries. Black Rapids Glacier, 250 km² in area, was chosen given its surge-type dynamics and proximity to critical infrastructure. Remotely sensed and in-situ elevation observations over the 1980--2001--2010 period indicated strong mass loss of Black Rapids Glacier (~0.5 m w.e. a⁻¹), with higher thinning rates over the 2001--2010 (~0.65 m w.e. a⁻¹) than the 1980--2001 period (~0.4 m w.e. a⁻¹). A coupled surface mass balance-glacier dynamics model, driven by reanalysis climate data, reproduced the glacier shrinkage. It identified the increasingly negative summer balances, a consequence of the warming atmosphere, as the main driver for the negative mass balance trend. Elevation observations in Black Rapids' surge reservoir suggested a surge was not imminent at the time of the analysis due to the lack of ice thickening. Re-initiation of sufficient elevation growth in the surge reservoir would require more favorable surface mass balances, as observed in the early 1980s. Compared to nearby Gulkana Glacier (a USGS benchmark glacier), the observed specific mass losses at Black Rapids Glacier were less pronounced, ~0.4 vs. 0.5 m w.e. a⁻¹ (1980--2001) and ~0.65 vs. 0.95 m w.e. a⁻¹ (2001--2010). The larger difference between the two glaciers' mass balances over the 2001--2010 period was partly caused by rockslide debris deposited on Black Rapids Glacier in 2002. This ~4.5 m thick debris layer, spread across 11.7 km² of Black Rapids lower ablation area, was modeled to suppress Black Rapids' glacier wide mass loss by ~20%. Modeling Black Rapids' evolution until 2100 suggested sustained glacier retreat, even under a repeated constant climate scenario, with ~225 km² of area remaining in 2100. Using a warming scenario (RCP 8.5), the modeled retreat was strongly accelerated with only ~50 km² of glacier area left in 2100. Given its thick, low-slope valley portion, Black Rapids Glacier is very susceptible to climate change. Its neighboring glaciers in the Eastern Alaska Range have similar properties, suggesting region wide glacier retreat in the future. To constrain this further, the Black Rapids case studies should be extended to the regional scale, a step now facilitated by the new Alaska wide glacier database.Chapter 1 General Introduction -- Chapter 2 Derivation and analysis of a complete modern-date glacier inventory for Alaska and northwest Canada -- Chapter 3 Geodetic mass balance of surge-type Black Rapids Glacier, Alaska, 1980--2001--2010, including role of rockslide deposition and earthquake displacement -- Chapter 4 Mass balance evolution of Black Rapids Glacier, Alaska, 1980--2015--2100, and its implications for surge recurrence -- Chapter 5 General Conclusions

New algorithms for the compilation of glacier inventories

Thesis (M.S.) University of Alaska Fairbanks, 2013Glacier inventories are used for many applications in glaciology, however, their manual compilation is time-consuming. Here, we present two new algorithms for the automatic compilation of glacier inventories. The first approach is based on hydrological modeling tools and separates glacier complexes into individual glaciers, requiring a digital elevation model (DEM) and glacier complex outlines as input. Its application to > 60,000 km² of ice in Alaska (~98% success rate) and southern Arctic Canada (~97% success rate) indicates the method is robust if DEMs and glacier complex outlines of good quality are available. The second algorithm relies on glacier outlines and a DEM and derives centerlines in a three-step 'cost grid -- least cost route' procedure. First, termini and heads are determined for every glacier. Second, centerlines are derived by determining the least cost route on a previously determined cost grid. Third, the centerlines are split into branches, followed by the attribution of a branch order. Application to > 21,000 Alaska glaciers shows that ~5.5% of the glacier heads and ~3.5% of the termini require manual correction. With corrected heads and termini, ~1.5% of the actual derived centerlines need edits. Comparison with alternative approaches reveals that the centerlines vary significantly depending on the algorithm used.Chapter 1. General introduction -- Chapter 2. A new semi-automatic approach for dividing glacier complexes into individual glaciers -- Chapter 3. A new method for deriving glacier centerlines applied to glaciers in Alaska and northwest Canada -- Chapter 4. General conclusions

Tracking icebergs with time-lapse photography and sparse optical flow, LeConte Bay, Alaska, 2016–2017

We present a workflow to track icebergs in proglacial fjords using oblique time-lapse photos and the Lucas-Kanade optical flow algorithm. We employ the workflow at LeConte Bay, Alaska, where we ran five time-lapse cameras between April 2016 and September 2017, capturing more than 400 000 photos at frame rates of 0.5–4.0 min−1. Hourly to daily average velocity fields in map coordinates illustrate dynamic currents in the bay, with dominant downfjord velocities (exceeding 0.5 m s−1 intermittently) and several eddies. Comparisons with simultaneous Acoustic Doppler Current Profiler (ADCP) measurements yield best agreement for the uppermost ADCP levels (∼ 12 m and above), in line with prevalent small icebergs that trace near-surface currents. Tracking results from multiple cameras compare favorably, although cameras with lower frame rates (0.5 min−1) tend to underestimate high flow speeds. Tests to determine requisite temporal and spatial image resolution confirm the importance of high image frame rates, while spatial resolution is of secondary importance. Application of our procedure to other fjords will be successful if iceberg concentrations are high enough and if the camera frame rates are sufficiently rapid (at least 1 min−1 for conditions similar to LeConte Bay).This work was funded by the U.S. National Science Foundation (OPP-1503910, OPP-1504288, OPP-1504521 and OPP-1504191).Ye

Towards a better understanding of debris flow sediment sources: Monitoring of an active rock slope at Spitze Stei, Switzerland

Rapid process cascades can lead to destructive debris flows. Identifying and characterizing the processes conditioning debris-flow occurrence will strongly contribute to the mitigation of debris-flow hazards. In recent years, the rock slope near “Spitze Stei”, in the Kandersteg region, Switzerland, has exhibited elevated displacement rates exceeding 10 cm per day, suggesting a growing instability up to 20 million m3. The accumulated sediments at the bottom of the Spitze Stei slope are mobilized as debris flows by melting snow and heavy summer precipitations. Here, we use seismology combined with an intelligent algorithm to automatically detect rockfall and landslides at the Spitze Stei rock slope. These mass movements act as primary sediment sources delivering sediments to a debris-prone channel. Our initial results quantify mass movement activity before two debris flow events that occurred in 2022 and identify their triggers. Such analysis can contribute towards mitigating debris flow hazards and extending warning time, especially for debris flows triggered by factors other than precipitation

Morainal Bank Evolution and Impact on Terminus Dynamics During a Tidewater Glacier Stillstand

Sedimentary processes are known to help facilitate tidewater glacier advance, but their role in modulating retreat is uncertain and poorly quantified. In this study we use repeated seafloor bathymetric surveys and satellite‐derived terminus positions from LeConte Glacier, Alaska, to evaluate the evolution of a morainal bank and related changes in terminus dynamics over a 17‐year period. The glacier experienced a rapid retreat between 1994 and 1999, before stabilizing at a constriction in the fjord. Since then, the glacier terminus has remained stabilized while constructing a morainal bank up to 140 m high in water depths of 240–260 m, with rates of sediment delivery of 3.3 Å~ 105 to 3.8 Å~ 105 m3 a−1. Based on repeated interannual surveys between 2016 and 2018, the moraine is a dynamic feature characterized by push ridges, evidence of active gravity flows, and bulldozing by the glacier at rates of up to meters per day. Beginning in 2016, the summertime terminus has become increasingly retracted, revealing a newly emerging basin potentially signaling the onset of renewed retreat. Between 2000 and 2016, the growing moraine reduced the exposed submarine area of the terminus by up to 22%, altered the geometry of the terminus during seasonal advances, and altered the terminus stress balance. These feedbacks for calving, melting, and ice flow likely represent mechanisms whereby moraine growth may delay glacier retreat, in a system where readvance is unlikely.This work was supported by NSF Arctic Natural Sciences Grants OPP—1503910, 1504191, 1504288, and 1504521. National Geographic CP4‐171R‐17 to E. Pettit and J. Nash helped support 2018 cruise logistics.Ye

Using longitudinal survival probabilities to test field vigour estimates in sugar maple (Acer saccharum Marsh.)

Tree mortality is a major force driving forest dynamics. To foresters, however, tree mortality is often considered a loss in productivity. To reduce tree mortality, silvicultural systems, such as selection cuts, aim at removing trees that are more likely to die. In order to identify trees with higher risks of mortality, field classifications are employed that assess vigour based on external characteristics of trees. We used a novel longitudinal approach for estimating survival probabilities based on ring-width measurements, initially developed by Bigler and Bugmann [Bigler, C., Bugmann, H., 2004. Predicting the time of tree death using dendrochronological data. Ecol. Appl. 14 (3), 902-914], to parameterize a survival probability model for sugar maple (Acer saccharum Marsh.) and to test whether field-assessed tree vigour classes are corroborated by survival probabilities determined from radial growth history. Data from 56 dead and 321 live sugar maples were collected in stands in western Quebec (Canada) that had undergone a selection cut ≈10 years prior to sampling. Our results showed that tree vigour established from external defects and pathological symptoms, using the classification of Boulet [Boulet, B., 2005. D

Subglacial Discharge Reflux and Buoyancy Forcing Drive Seasonality in a Silled Glacial Fjord

Fjords are conduits for heat and mass exchange between tidewater glaciers and the coastal ocean, and thus regulate near-glacier water properties and submarine melting of glaciers. Entrainment into subglacial discharge plumes is a primary driver of seasonal glacial fjord circulation; however, outflowing plumes may continue to influence circulation after reaching neutral buoyancy through the sill-driven mixing and recycling, or reflux, of glacial freshwater. Despite its importance in non-glacial fjords, no framework exists for how freshwater reflux may affect circulation in glacial fjords, where strong buoyancy forcing is also present. Here, we pair a suite of hydrographic observations measured throughout 2016–2017 in LeConte Bay, Alaska, with a three-dimensional numerical model of the fjord to quantify sill-driven reflux of glacial freshwater, and determine its influence on glacial fjord circulation. When paired with subglacial discharge plume-driven buoyancy forcing, sill-generated mixing drives distinct seasonal circulation regimes that differ greatly in their ability to transport heat to the glacier terminus. During the summer, 53%–72% of the surface outflow is refluxed at the fjord's shallow entrance sill and is subsequently re-entrained into the subglacial discharge plume at the fjord head. As a result, near-terminus water properties are heavily influenced by mixing at the entrance sill, and circulation is altered to draw warm, modified external surface water to the glacier grounding line at 200 m depth. This circulatory cell does not exist in the winter when freshwater reflux is minimal. Similar seasonal behavior may exist at other glacial fjords throughout Southeast Alaska, Patagonia, Greenland, and elsewhere.This work was supported by NSF Arctic Natural Sciences grants OPP-1503910, 1504191, 1504288, and 1504521. The authors thank Pat Dryer, Dylan Winters, Erin Pettit, and the crews of the R/V Pelican and M/V Stellar for their contributions to the fieldwork. The authors thank Petersburg High School and the U.S. Forest Service for accommodating this project, and our two anonymous reviewers for their feedback in improving the manuscript. The authors also acknowledge the Shtax'héen Kwáan Tlingits, whose ancestral lands lie in this region.Ye

Sugar maple (Acer saccharum March.) growth is influenced by close conspecifics and skid trait proximity following selection harvest

In this study, we quantified the effects of local neighbourhood competition, light availability, and proximity to skid trails on the growth of sugar maple (Acer saccharum Marsh.) trees following selection harvest. We hypothesized that growth would increase with decreasing competition and increasing light availability, but that proximity to skid trails would negatively affect growth. A total of 300 sugar maples were sampled 10 years after selection harvesting in 18 stands in Témiscamingue (Québec, Canada). Detailed tree and skid trail maps were obtained in one 0.4 ha plot per stand. Square-root transformed radial growth data were fitted to a linear mixed model that included tree diameter, crown position, a neighbourhood competition index, light availability (estimated using the SORTIE light model), and distance to the nearest skid trail as explanatory variables. We considered various distance-dependent or -independent indices based on neighbourhood radii ranging from 6 to 12 m. The competition index that provided the best fit to the data was a distance-dependent index computed in a 6 m search radius, but a\ud distance-independent version of the competition index provided an almost equivalent fit to data. Models corresponding to all combinations of main effects were fit to data using maximum likelihood, and weighted averages of parameter estimates were obtained usingmultimodel inference. All predictors had\ud an influence on growth, with the exception of light. Radial growth decreased with increasing tree diameter, level of competition and proximity to skid trails, and varied among crown positions with trees in suppressed and intermediate positions having lower growth rates than codominants and dominants. Our results indicate that in selection managed stands, the radial growth of sugarmaple trees depends on\ud competition from close (6 m) conspecific neighbours, and is still affected by proximity to skid trails 10 years after harvesting. Such results underscore the importance of minimizing the extent of skid trail networks by careful pre-harvest planning of trail layout. We also conclude that the impact of heterogeneity among individual-tree neighbourhoods, such as those resulting from alternative spatial patterns of harvest, can usefully be integrated into models of post-harvest tree growth

Formation, flow and break-up of ephemeral ice mélange at LeConte Glacier and Bay, Alaska.

© The Author(s), 2020. Published by Cambridge University Press. This is an Open Access article, distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike licence (http://creativecommons.org/licenses/by-nc-sa/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the same Creative Commons licence is included and the original work is properly cited. The written permission of Cambridge University Press must be obtained for commercial re-use. Supplementary material. The supplementary material for this article can be found at https://doi.org/10.1017/jog.2020.29Ice mélange has been postulated to impact glacier and fjord dynamics through a variety of mechanical and thermodynamic couplings. However, observations of these interactions are very limited. Here, we report on glaciological and oceanographic data that were collected from 2016 to 2017 at LeConte Glacier and Bay, Alaska, and serendipitously captured the formation, flow and break-up of ephemeral ice mélange. Sea ice formed overnight in mid-February. Over the subsequent week, the sea ice and icebergs were compacted by the advancing glacier terminus, after which the ice mélange flowed quasi-statically. The presence of ice mélange coincided with the lowest glacier velocities and frontal ablation rates in our record. In early April, increasing glacier runoff and the formation of a sub-ice-mélange plume began to melt and pull apart the ice mélange. The plume, outgoing tides and large calving events contributed to its break-up, which took place over a week and occurred in pulses. Unlike observations from elsewhere, the loss of ice mélange integrity did not coincide with the onset of seasonal glacier retreat. Our observations provide a challenge to ice mélange models aimed at quantifying the mechanical and thermodynamic couplings between ice mélange, glaciers and fjords.This work was supported by the US NSF awards OPP-1503910, OPP-1504191, OPP-1504288, OPP-1504521 and DMR-1506307. The WorldView imagery and DEM were provided by the Polar Geospatial Center under US NSF awards OPP-1043681, OPP-1559691 and OPP-1542736. The IfSAR DEM is distributed through the USGS Earth Resources Observation Center. Field logistics was provided by CH2MHill Polar Field Services and would not have been possible without the help of the crew of the MV Steller and MV Pelican, Temsco Helicopters, Petersburg High School and the US Forest Service. We also thank J.B. Mickett, D.S. Winters, W.P. Dryer, A. Stewart, M. Michels, C. Carr, T. Moon, A. Simpson and E.C. Pettit for assistance with field work and data processing and M. Truffer for loaning the GPRI radar interferometer.Ye