Post‐Younger Dryas deglaciation of the Greenland western margin as revealed by spatial analysis of lakes

Lake shapes and their spatial distribution are important geomorphological indicators in previously glaciated areas. Their shapes are influenced by the underlying geological structure and processes of glacial sediment deposition or erosion. Since these processes act on large areas, distribution of lakes can reflect the intensity of glacial erosional/depositional processes and their spatial extent. Landsat imagery was used to extract lake outlines from a selected pilot‐study area on the widest ice‐free coastal margin of the south‐western Greenland north of Kangerlussuaq. Analysis included image classification and spatial analysis of lakes with elevation data using geographic information system (GIS) tools. A morphometric index was applied to extract kettle lakes as indicators of a specific glacial process – ice stagnation. Analysis of their spatial distribution helped in the reconstruction of glacial dynamics in formerly glaciated terrain. Our results show that spatial lake distribution combined with elevation analysis can be used to identify zones of glacial erosion and deposition. The highest concentrations of lakes within the study area occupy the elevation range between 164 and 361 m above sea level (a.s.l.). This zone can be identified as an area where intensive glacial erosion took place in the past. The widespread distribution of modeled kettle lake features within the same elevation range and across the study area suggests that the last deglaciation process was accompanied by abandonment of blocks of stagnant ice. This conclusion is supported by surface exposure ages obtained in the same study area and published elsewhere. Copyright © 2009 John Wiley & Sons, Ltd.     

Multidimensional Investigation of Bedrock Heterogeneity/Unconformities at a DNAPL‐Impacted Site

Organic solvent (i.e., dense nonaqueous phase liquid, DNAPL) migration in the subsurface is known to be extremely sensitive to geologic heterogeneity. There is often a focus on heterogeneity that results from changing depositional conditions over short spatial scales. Similar or even more extreme spatial heterogeneity can result postdeposition due to erosional processes. This study applies a synergistic approach based on a combination of high‐resolution lithologic logs of continuous cores, borehole geophysical logs, surface electrical resistivity, and seismic refraction tomography models to assess spatial heterogeneity in a shallow bedrock sequence subject to multiple unconformities and contaminated with a mixture of organic chemicals. The persistence of DNAPL in the source zone and an associated dissolved‐phase plume led to variable impacts on formation resistivity across the study site. Seismic refraction in combination with electrical resistivity tomography improved interpretation of highly irregular erosional boundaries by delineating sharp lateral transitions in lithologic composition near the source zone and across the dissolved‐phase plume. Electrical resistivity was effective at differentiating between clean and mud‐rich sandstones and their unconformable contact with an underlying dolostone. Geophysical measurements revealed eroded dolostone mounds encased by a network of younger mud‐rich sandstones channelized by clean semi‐lithified sand, all of which was buried beneath variable glacial drift. Our synergistic multidimensional approach resulted in the development of a detailed three‐dimensional shallow bedrock geospatial model, which has led to an improved understanding of DNAPL migration and contaminant plume heterogeneity.     

Global distribution of coastal cliffs

Previous studies have estimated that coastal cliffs exist on about 80% of the global shoreline, but have not been validated on a global scale. This study uses two approaches to capture information on the worldwide existence and erosion of coastal cliffs: a detailed literature survey and imagery search, and a GIS-based global mapping analysis. The literature and imagery review show coastal cliffs exist in 93% of the combined recognized independent coastal states and non-independent coastal regions worldwide (total of 213 geographic units). Additionally, cliff retreat rates have been quantified in at least one location within 33% of independent coastal states and 15% of non-independent regions. The GIS-based mapping used the near-global Shuttle Radar Topography Mission 3 arc second digital elevation model and Arctic Coastal Dynamics Database to obtain near-global backshore coastal elevations at 1 km alongshore intervals comprising about 1,340,000 locations (81% of the world vector shoreline). Backshore coastal elevations were compared with the mapped distribution of European coastal cliffs to produce a model training set, and this relationship was extended globally to map the likelihood of coastal cliff locations. About 21% of the transects (17% of the world vector shoreline) were identified as mangroves and eliminated as potential cliff locations. The results were combined with estimates of cliff percentages for Greenland and Antarctica from the literature, extending the global coverage to estimate cliff occurrence across 89% of the world vector shoreline. The results suggest coastal cliffs likely exist on about 52% of the global shoreline. © 2018 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

The dynamics of mountain erosion: Cirque growth slows as landscapes age

Glacial cirques are widely used palaeoenvironmental indicators, and are key to understanding the role of glaciers in shaping mountain topography. However, notable uncertainty persists regarding the rate and timing of cirque erosion. In order to address this uncertainty, we analyse the dimensions of 2208 cirques in Britain and Ireland and model ice accumulation to investigate the degree of coupling between glacier occupation times and cirque growth. Results indicate that during the last ~120 ka, cirques were glacier-free for an average of 52.0 ± 21.2 ka (43 ± 18%); occupied by small (largely cirque-confined) glaciers for 16.2 ± 9.9 ka (14 ± 8%); and occupied by large glaciers, including ice sheets, for 51.8 ± 18.6 ka (43 ± 16%). Over the entire Quaternary (i.e. 2.6 Ma), we estimate that cirques were glacier-free for 1.1 ± 0.5 Ma; occupied by small glaciers for 0.3 ± 0.2 Ma; and occupied by large glaciers for 1.1 ± 0.4 Ma. Comparing occupation times to cirque depths, and calculating required erosion rates, reveals that continuous cirque growth during glacier occupation is unlikely. Instead, we propose that cirques attained much of their size during the first occupation of a non-glacially sculpted landscape (perhaps during the timeframe of a single glacial cycle). During subsequent glacier occupations, cirque growth may have slowed considerably, with the highest rates of subglacial erosion focused during periods of marginal (small glacier) glaciation. We propose comparatively slow rates of growth following initial cirque development because a ‘least resistance’ shape is formed, and as cirques deepen, sediment becomes trapped subglacially, partly protecting the bedrock from subsequent erosion. In support of the idea of rapid cirque growth, we present evidence from northern British Columbia, where cirques of comparable size to those in Britain and Ireland developed in less than 140 ka. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Constraining sediment provenance for tsunami deposits using distributions of grain size and foraminifera from the Kujukuri coastline and shelf,Japan

Tsunami deposits preserved in the geological record provide a more comprehensive understanding of their patterns of frequency and intensity over longer timescales; but recognizing tsunami deposits can prove challenging due to post-depositional changes, lack of contrast between the deposits and surrounding sedimentary layers, and differentiating between tsunami and storm deposition. Modern baseline studies address these challenges by providing insight into modern spatial distributions that can be compared with palaeotsunami deposits. This study documents the spatial fingerprint of grain size and foraminifera from Hasunuma Beach and the Kujukuri shelf to provide a basis from which tsunami deposits can be interpreted. At Hasunuma Beach, approximately 50 km east of Tokyo, the spatial distribution of three common proxies (foraminiferal taxonomy, foraminiferal taphonomy and sediment grain size) for tsunami identification were mapped and clustered using Partitioning Around Medoids cluster analysis. Partitioning Around Medoids cluster analysis objectively discriminated two coastal zones corresponding to onshore and offshore sample locations. Results show that onshore samples are characterized by coarser grain sizes (medium to coarse sand) and higher abundances of Pararotalia nipponica (27 to 63%) than offshore samples, which are characterized by finer grain sizes (fine to medium sand), lower abundances of Pararotalia nipponica (2 to 19%) and Ammonia parkinsoniana (0 to 10%), higher abundances of planktonics (15 to 58%) and species with fragile tests including Uvigerinella glabra. When compared to grain-size and foraminiferal taxonomy, foraminiferal taphonomy; i.e. surface condition of foraminifera, a proxy not commonly used to identify tsunami deposits, was most effective in discriminating modern coastal zones (identified supratidal, intertidal and offshore environments) and determining sediment provenance for tsunami deposits at Kujukuri. This modern baseline study assists the interpretation of tsunami deposits in the geological record because it provides a basis for sediment provenance to be determined.     

Recent and historical tsunami deposits from Lake Tokotan,eastern Hokkaido,Japan, inferred from nondestructive,grain size,and radioactive cesium analyses

Geological evidence of recent tsunamis from sediment samples collected from Lake Tokotan, a coastal lagoon in eastern Hokkaido, northern Japan, was detected using computed tomography (CT) and soft X-ray images, grain size, and radionuclide profiles. Initial field observations revealed that sediments had no discernable sedimentary structures at the top of the core. However, results of CT imaging, soft X-ray, and grain size analyses show evidence for three invisible sand layers that are intercalated with mud layers. These sand layers exhibit trends of landward fining and thinning. Furthermore, the distribution of sand layers was limited to the center and seaward parts of the lake. Vertical profiles of cesium and lead concentrations in combination with recent eyewitness accounts indicated that these sand layers are correlated with the 1973 Nemuro-oki, 1960 Chilean, and 1952 Tokachi-oki tsunami events. The deeper part of the sediment cores includes three volcanic ash layers and three prehistoric coarse sand layers. The prehistoric layers are correlated with unusually large tsunamis that were geologically identified in previous studies from eastern Hokkaido. These findings suggest that nondestructive techniques, in combination with radionuclide analysis, allow for detection of frequent but faint tsunami deposits. This technique allows for an improved understanding of the history of tsunami inundation in Lake Tokotan and of other locations for which stratigraphic evidence for faint tsunamis layers is not readily apparent from field assessments.

     

Snow disappearance timing is dominated by forest effects on snow accumulation in warm winter climates of the Pacific Northwest,United States

Forests modify snow processes and affect snow water storage as well as snow disappearance timing. However, forest influences on snow accumulation and ablation vary with climate and topography and are therefore subject to temporal and spatial variability. We utilize multiple years of snow observations from across the Pacific Northwest, United States, to assess forest–snow interactions in the relatively warm winter conditions characteristic of maritime and transitional maritime–continental climates. We (a) quantify the difference in snow magnitude and disappearance timing between forests and open areas and (b) assess how forest modifications of snow accumulation and ablation combine to determine whether snow disappears later in the forest or in the open. We find that snow disappearance timing at 12 (out of 14) sites ranges from synchronous in the forest and open to snow persisting up to 13 weeks longer in the open relative to a forested area. By analyzing accumulation and ablation rates up to the day when snow first disappears from the forest, we find that the difference between accumulation rates in the open and forest is larger than the difference between ablation rates. Thus, canopy snow interception and subsequent loss, rather than ablation, set up longer snow duration in the open. However, at two relatively windy sites (hourly average wind speeds up to 8 and 17 m/s), differential snow disappearance timing is reversed: Snow persists 2–5 weeks longer in the forest. At the windiest sites, accumulation rates in the forest and open are similar. Ablation rates are higher in the open, but the difference between ablation rates in the forest and open at these sites is approximately equivalent to the difference at less windy sites. Thus, longer snow retention in the forest at the windiest sites is controlled by depositional differences rather than by reduced ablation rates. These findings suggest that improved quantification of forest effects on snow accumulation processes is needed to accurately predict the effect of forest management or natural disturbance on snow water resources.     

Evolution of forest precipitation water storage measurement methods

Precipitation intercepted by forests plays a major role in more than one‐fourth of the global land area's hydrologic cycle. Direct in situ measurement of intercepted precipitation is challenging, and thus, it is typically indirectly estimated through comparing precipitation under forest cover and in the open. We discuss/compare measurement methods for forest precipitation interception beyond classical budgeting and then recommend future directions for improving water storage estimation. Comparison of techniques shows that methods submerging tree components produce the largest water storage capacity values. Whole‐tree lysimeters have been used with great success at quantifying water storage for the integrated system yet are unable to separate trunk versus canopy storage. Remote sensing, particularly signal attenuation, may permit this separation. Mechanical displacement methods show great promise and variety of techniques: pulley/spring system, branch strain sensors, trunk compression sensors and photography. Relating wind sway to water storage also shows great promise with negligible environmental disruption yet is currently at the proof‐of‐concept stage. Suggested future directions focus on development of common features regarding all discussed methods: (i) measurement uncertainties or processes beyond interception influencing the observed signal, (ii) scaling approaches to move from single tree components to the single‐tree and forest scales and (iii) temporal scaling to estimate the relevance of single‐interception components over longer timescales. Through addressing these research needs, we hope the scientific community can develop an ‘integrated’ monitoring plan incorporating multiple measurement techniques to characterize forest‐scale water storage dynamics while simultaneously investigating underlying (smaller‐scale) components driving those dynamics across the spectrum of precipitation and forest conditions. Copyright © 2014 John Wiley & Sons, Ltd.     

Heating experiments of the Tagish Lake meteorite: Investigation of the effects of short‐term heating on chondritic organics

We present in this study the effects of short‐term heating on organics in the Tagish Lake meteorite and how the difference in the heating conditions can modify the organic matter (OM) in a way that complicates the interpretation of a parent body's heating extent with common cosmothermometers. The kinetics of short‐term heating and its influence on the organic structure are not well understood, and any study of OM is further complicated by the complex alteration processes of the thermally metamorphosed carbonaceous chondrites—potential analogues of the target asteroid Ryugu of the Hayabusa2 mission—which had experienced posthydration, short‐duration local heating. In an attempt to understand the effects of short‐term heating on chondritic OM, we investigated the change in the OM contents of the experimentally heated Tagish Lake meteorite samples using Raman spectroscopy, scanning transmission X‐ray microscopy utilizing X‐ray absorption near edge structure spectroscopy, and ultraperformance liquid chromatography fluorescence detection and quadrupole time of flight hybrid mass spectrometry. Our experiment suggests that graphitization of OM did not take place despite the samples being heated to 900 °C for 96 h, as the OM maturity trend was influenced by the heating conditions, kinetics, and the nature of the OM precursor, such as the presence of abundant oxygenated moieties. Although both the intensity of the 1s?σ* exciton cannot be used to accurately interpret the peak metamorphic temperature of the experimentally heated Tagish Lake sample, the Raman graphite band widths of the heated products significantly differ from that of chondritic OM modified by long‐term internal heating.