Anthropogenic litter is omnipresent in terrestrial and freshwater systems, and can have major economic and ecological impacts. Monitoring and modeling of anthropogenic litter comes with large uncertainties due to the wide variety of litter characteristics, including size, mass, and item type. It is unclear as to what the effect of sample set size is on the reliability and representativeness of litter item statistics. Reliable item statistics are needed to (1) improve monitoring strategies, (2) parameterize litter in transport models, and (3) convert litter counts to mass for stock and flux calculations. In this paper, we quantify sample set size requirement for riverbank litter characterization, using a database of more than 14,000 macrolitter items (>0.5 cm), sampled for 1 year at eight riverbank locations along the Dutch Rhine, IJssel, and Meuse rivers. We use this database to perform a Monte Carlo based bootstrap analysis on the item statistics, to determine the relation between sample size and variability in the mean and median values. Based on this, we present sample set size requirements, corresponding to selected uncertainty and confidence levels. Optima between sampling effort and information gain is suggested (depending on the acceptable uncertainty level), which is a function of litter type heterogeneity. We found that the heterogeneity of the characteristics of litter items varies between different litter categories, and demonstrate that the minimum required sample set size depends on the heterogeneity of the litter category. This implies that more items of heterogeneous litter categories need to be sampled than of heterogeneous item categories to reach the same uncertainty level in item statistics. For example, to describe the mean mass the heterogeneous category soft fragments (>2.5 cm) with 90% confidence, 990 items were needed, while only 39 items were needed for the uniform category metal bottle caps. Finally, we use the heterogeneity within litter categories to assess the sample size requirements for each river system. All data collected for this study are freely available, and may form the basis of an open access global database which can be used by scientists, practitioners, and policymakers to improve future monitoring strategies and modeling efforts.

Quantifying components of the hydrological cycle in glacierised catchments is important for the assessment of the temporal distribution, quantity and quality of water resources available to downstream regions, especially under a changing climate. However, this assessment requires long time series of observations, which are typically unavailable for remote catchments, such as those in mountainous areas. In this study, we leverage a unique similar to 40 year time series of hydrological data recorded in the subarctic glacierised Tarfala catchment (Sweden) to explore temporal trends in the components of the catchment water balance (precipitation, runoff, change in storage, and evaporation), and to assess if water balance residuals are associated with specific hydro-climatic conditions. No significant temporal trends were found in precipitation and storage changes of the glacierised area, but significant increases were found in evaporation and summer discharge (in part attributed to glacier volume losses). The annual water balance could not be perfectly closed, and water losses were on average 112 mm y(-1) larger than the water inputs over the study period. Among the water balance components, discharge contributed most to the total water balance uncertainty, and storage surplus due to antecedent meteorological conditions could explain why water losses in specific years exceeded the uncertainty bounds. It is therefore essential to consider legacy effects from previous years when applying water balance calculations in mountainous and/or glacierised catchments.

Geophysical surveys provide an efficient and non-invasive means of studying subsurface conditions in numerous sedimentary settings. In this study, we explore the application of three geophysical methods to a proglacial environment, namely ground penetrating radar (GPR), seismic refraction and multi-channel analysis of surface waves (MASW). We apply these geophysical methods to three glacial landforms with contrasting morphologies and sedimentary characteristics, and we use the various responses to assess the applicability and limitations of each method for these proglacial targets. Our analysis shows that GPR and seismic (refraction and MASW) techniques can provide spatially extensive information on the internal architecture and composition of moraines, but careful survey designs are required to optimise data quality in these geologically complex environments. Based on our findings, we define a number of recommendations and a potential workflow to guide future geophysical investigations in analogous settings. We recommend the initial use of GPR in future studies of proglacial environments to inform (a) seismic survey design and (b) the selection of seismic interpretation techniques. We show the benefits of using multiple GPR antenna frequencies (e.g., 25 and 100 MHz) to provide decimetre scale imaging in the near surface (e.g., < 15 m) while also enabling signal penetration to targets at up to ∼40 m depth (e.g., bedrock). This strategy helps to circumvent changes in radar signal penetration resulting from variations in substrate conductivity or abundant scatterers. Our study also demonstrates the importance of combining multiple geophysical methods together with ground-truthing through sedimentological observations to reduce ambiguity in interpretations. Implementing our recommendations will improve geophysical survey practice in the field of glacial geology and allow geophysical methods to play an increasing role in the interpretation of glacial landforms and sediments.

Glacier forelands contain valuable information on past glacier dynamics and associated climatic conditions, particularly at small mountain glaciers where responses to climate change are rapid. To maximize the potential of glacial landforms as palaeoclimate indicators, a thorough understanding of the controls on landform genesis and subsequent evolution is required. Traditionally, such landforms have been studied using glacial geological techniques such as sedimentary logging. While these provide valuable in situ information they have numerous limitations, namely poor availability and spatial extent of exposures. Near-surface geophysics provides an efficient and non-invasive means of studying subsurface conditions in numerous sedimentary settings, offering spatially extensive information on substrate material properties and architecture. However, the logistically challenging terrain, remote location and complex structure of proglacial environments has limited the development of geophysical techniques for studying the internal architecture of glacial landforms. Here, we explore the application of three geophysical methods to investigate proglacial substrates: ground penetrating radar (GPR), seismic refraction and multi-channel analysis of surface waves (MASW). Three sites with contrasting sediment properties were surveyed at the foreland of Midtdalsbreen glacier in southern Norway; (a) a 100 m2 area of glaciotectonised sandy sediments, (b) a ~2 m high lateral moraine ridge containing stratified silts, sands, and gravel and (c) a terminal moraine ridge with a peak crest height of ~5 m and an open blockwork of cobbles and boulders at its surface. At all sites, we deployed 25 MHz and 100 MHz GPR antennas and undertook seismic surveys with 50−75 m long geophone spreads and a sledge-hammer source to sample to target depths of around 10−15 m. Through comparing the results from sites (a) to (c), we assess the capabilities and limitations of each of the aforementioned techniques for proglacial substrate imaging and characterisation, we analyse how their performances vary across these settings and outline factors that contribute to a successful geophysical investigation. The ease of analysis and achievable investigation depths of the geophysical data and the applicability of seismic interpretation methods varied considerably depending on the surface terrain and structural complexity of the site. Our results show how the combination of GPR and seismic data can assist with the internal characterisation of glacial moraines when a relatively simple subsurface structure is present. However, basic seismic inversions likely lack the sophistication to resolve seismic structure in all but the simplest of layered models. We offer suggestions on how to optimise field time in more complex settings, where more sophisticated seismic inversion algorithms (e.g. tomography) or 3-D GPR surveys could be better-suited. Our experience should help advance the use of geophysics in proglacial studies. It should serve as a guide for future survey planning, and help avoid typical pitfalls such that field time can be optimised. It is hoped that geophysical survey methods will play an increasing role in the understanding of proglacial sedimentary landforms and their associated palaeoenvironments.

Estimations of peatland carbon stocks often use generalised values for peat thickness and carbon content. Ground penetrating radar (GPR), a rapid technique for field data collection, has been increasingly demonstrated as an appropriate method of mapping peat thickness. Light Detection and Ranging (LiDAR) data as a method for understanding peatland surface elevation are also becoming more widely available. Reliable mapping and quantification of site-specific carbon stocks (e.g. upland raised bogs) is therefore, becoming increasingly feasible, providing a valuable contribution to regional, national and potentially global carbon stock assessments. This is particularly important because raised bogs, such as those found in South Wales are considerable carbon stores. They are, however, susceptible to climate warming owing to their southerly location within the UK. Accurate estimates of peatland carbon stocks has broader importance because world-wide peatland carbon stores are significant and threatened by climate change, posing a substantial challenge not only due to climate feedbacks if this stored carbon is released into the atmosphere, but also the impact on the other ecosystem services that they provide. Here, we assess the value of an integrated GPR, LiDAR and Geographic Information System (GIS) approach to improve estimation of regional carbon stocks. We apply the approach to three ombrotrophic raised bogs in South Wales, UK, selected for their conservation value and their topographically-confined raised bog form. GPR and LiDAR are found to be well suited, respectively, to mapping peat thickness at bog scale and surface elevation, thus allowing surface and basal topographies to be evaluated using GIS. In turn, this allows peat volumes to be estimated. For the first time, we record values between 55,200 m(3) and 163,000 m(3) for the sites considered here. The greater confidence in these peat volume estimates results from the ability to calibrate the GPR velocity using a depth-to-target calibration with peat cores extracted at locations encompassing the deepest bog area. Peat thickness is mapped at the bog scale with near centimetre precision, improving the robustness of subsequent volume calculations and our understanding of the contribution of these small but numerous sites to regional carbon stocks. Our evaluation shows that GPR corresponds well with conventional manual probing but is minimally invasive and therefore less disturbing of sensitive peatland sites, while also offering improved coverage and spatial resolution with less time and cost. In combination with measured bulk density and organic carbon contents, these peat volumes allow carbon stocks to be estimated with greater confidence compared to conventional aproaches, having values between 2181 +/- 122 tonnes carbon and 6305 +/- 351 tonnes carbon at our three sites.

Because climate change can affect the carbon balance and hydrology in permafrost peatlands, a better understanding of their sensitivity to changes in temperature and precipitation is needed. In Tavvavuoma, northernmost Sweden, meteorological parameters and ground thermal properties have been monitored in a peat plateau from 2006 to 2013. During this time period, the air temperature record shows no warming trend, and the late-season thaw depth has been relatively stable at around 55-60cm. Meanwhile, the mean annual ground temperature at 1m depth has increased by 0.06 degrees C/yr and at 2-5m depth the permafrost is currently warmer than -0.3 degrees C. Statistical analyses suggest that interannual changes in thaw depth and ground temperatures are affected by different meteorological factors. Summer air temperatures and annual thawing degree-days control thaw depth (p0.05), whereas winter precipitation/snow depth affects ground temperatures (p0.1). The permafrost in this peat plateau is likely relict and not in equilibrium with current climatic conditions. Since the early 20(th) century, there has been a regional increase in air temperature and snow depth. If the ongoing permafrost warming in Tavvavuoma is a result of these long-term trends, short-term variability in meteorological parameters can still have an impact on the rate of permafrost degradation, but unless pronounced climate cooling occurs, thawing of the peat plateau is inevitable. Copyright (c) 2015 John Wiley & Sons, Ltd.

Abstract In this study, snow particle size variability was investigated along a transect in Dronning Maud Land from the coast to the polar plateau. The aim of the study was to better understand the spatial and temporal variations in surface snow properties. Samples were collected twice daily during a traverse in 2007–08 to capture regional variability. Local variability was assessed by sampling in 10×10 m grids (5 m spacing) at selected locations. The particle size and shape distributions for each site were analysed through digital image analysis. Snow particle size variability is complex at different scales, and shows an internal variability of 0.18–3.31 mm depending on the sample type (surface, grid or pit). Relationships were verified between particle size and both elevation and distance to the coast (moisture source). Regional seasonal changes were also identified, particularly on the lower elevations of the polar plateau. This dataset may be used to quantitatively analyse the optical properties of surface snow for remote sensing. The details of the spatial and temporal variations observed in our data provide a basis for further studies of the complex and coupled processes affecting snow particle size and the interpretation of remote sensing of snow covered areas.

The study of glacier surface mass balance provides key data and insights into how the cryosphere interacts with climate and hydrology. The glacier monitoring programme of the Tarfala Research Stati ...

Steady-state water pressures in subglacial conduits : corrections to a model and recommendations for its use

Over 11 000 L of kerosene was deposited on the surface of Rabots glaciär on the Kebnekaise Massif, northern Sweden, following the crash of a Royal Norwegian Air Force aircraft in March 2012. An environmental monitoring programme was subsequently commissioned, including a series of dye tracing experiments during the 2013 melt season, conducted to investigate the transport of pollutants through the glacier hydrological system. This experimental set-up provided a basis from which we could gain new insight into the internal hydrological system of Rabots glaciär. Results of dye tracing experiments reveal a degree of homogeneity in the topology of the drainage system throughout July and August, with an increase in efficiency as the season progresses, as reflected by decreasing temporary storage and dispersivity. Early onset of melting likely led to formation of an efficient, discrete drainage system early in the melt season, subject to decreasing sinuosity and braiding as the season progressed. Four distinct meltwater flow regimes are identified to summarize the temporal and spatial evolution of the system. Analysis of turbidity-discharge hysteresis further supports the formation of discrete, efficient drainage, with clockwise diurnal hysteresis suggesting easy mobilization of readily available sediments in channels. Dye injection immediately downstream of the pollution source zone reveals prolonged storage of dye followed by fast, efficient release. Twinned with a low dye recovery, and supported by sporadic detection of hydrocarbons in the proglacial river, we suggest that meltwater, and thus pollutants in solution, may be released periodically through an efficient, and likely pressurized, hydrological system within the upper reaches of the glacier.

In this study, summer rainfall contributions to streamflow were quantified in the sub-arctic, 30% glacierized Tarfala (21.7km(2)) catchment in northern Sweden for two non-consecutive summer sampling seasons (2004 and 2011). We used two-component hydrograph separation along with isotope ratios (O-18 and D) of rainwater and daily streamwater samplings to estimate relative fraction and uncertainties (because of laboratory instrumentation, temporal variability and spatial gradients) of source water contributions. We hypothesized that the glacier influence on how rainfall becomes runoff is temporally variable and largely dependent on a combination of the timing of decreasing snow cover on glaciers and the relative moisture storage condition within the catchment. The results indicate that the majority of storm runoff was dominated by pre-event water. However, the average event water contribution during storm events differed slightly between both years with 11% reached in 2004 and 22% in 2011. Event water contributions to runoff generally increased over 2011 the sampling season in both the main stream of Tarfala catchment and in the two pro-glacial streams that drain Storglaciaren (the largest glacier in Tarfala catchment covering 2.9km(2)). We credit both the inter-annual and intra-annual differences in event water contributions to large rainfall events late in the summer melt season, low glacier snow cover and elevated soil moisture due to large antecedent precipitation. Together amplification of these two mechanisms under a warming climate might influence the timing and magnitude of floods, the sediment budget and nutrient cycling in glacierized catchments. Copyright (c) 2012 John Wiley & Sons, Ltd.

We present ice thickness and bed topography maps with a high spatial resolution (250–500 m) of a land-terminating section of the Greenland Ice Sheet derived from ground-based and airborne radar surveys. The data have a total area of ~12 000 km2 and cover the whole ablation area of the outlet glaciers of Isunnguata Sermia, Russell, Leverett, Ørkendalen and Isorlersuup up to the long-term mass balance equilibrium line altitude at ~1600 m above sea level. The bed topography shows highly variable subglacial trough systems, and the trough of Isunnguata Sermia Glacier is overdeepened and reaches an elevation of ~500 m below sea level. The ice surface is smooth and only reflects the bedrock topography in a subtle way, resulting in a highly variable ice thickness. The southern part of our study area consists of higher bed elevations compared to the northern part. The compiled data sets of ground-based and airborne radar surveys cover one of the most studied regions of the Greenland Ice Sheet and can be valuable for detailed studies of ice sheet dynamics and hydrology. The combined data set is freely available at doi:10.1594/pangaea.830314.

Ground-penetrating radar has been widely used to map the thermal structure of polythermal glaciers. Hitherto, the cold temperate transition surface (CTS) in radargrams has been identified by a labour-intensive and subjective manual picking method. We introduce a new automatic approach for picking the CTS that uses the difference in signal power exhibited by the cold and temperate ice layers. We compare our automatically computed CTS depths with manual picks. Our results show very good agreement between the two methods in most areas (r(2) > 0.7). RMSEs computed at each trace in two-way travel-time from three test sites range from 14 to 19 ns (2.4-3.2 m). The proposed automated method mostly fails in areas showing a rather gradual transition in signal power at the CTS. In some areas, high power originating from non-water sources is misinterpreted by the automatic picking method as 'temperate ice'.

Abstract. Over 11 000 L of hydrocarbon pollution was deposited on the surface of Rabots glaciär on the Kebnekaise Massif, northern Sweden, following the crash of a Royal Norwegian Air Force aircraft in March 2012. An environmental monitoring programme was subsequently commissioned, including water, snow and ice sampling. The scientific programme further included a series of dye tracing experiments during the 2013 melt season, conducted to investigate flow pathways for pollutants through the glacier hydrological system, and to gain new insight to the internal hydrological system of Rabots glaciär. Results of dye tracing reveal a degree of homogeneity in the topology of the drainage system throughout July and August, with an increase in efficiency as the season progresses, as reflected by decreasing temporary storage and dispersivity. Early onset of melting likely led to formation of an efficient, discrete drainage system early in the melt season, subject to decreasing sinuosity and braiding as the season progressed. Analysis of turbidity-discharge hysteresis further supports the formation of discrete, efficient drainage, with clockwise diurnal hysteresis suggesting easy mobilisation of readily-available sediments in channels. Dye injection immediately downstream of the pollution source zone revealed prolonged storage of dye followed by fast, efficient release. Twinned with a low dye recovery, and supported by sporadic detection of hydrocarbons in the proglacial river, we suggest that meltwater, and thus pollutants in solution, may be released periodically from this zone of the glacier hydrological system. The here identified dynamics of dye storage, dispersion and breakthrough indicate that the ultimate fate and permanence of pollutants in the glacier system is likely to be governed by storage of pollutants in the firn layer and ice mass, or within the internal hydrological system, where it may refreeze. This shows that future studies on the fate of hydrocarbons in pristine, glaciated mountain environments should address the extent to which pollutants in solution act like water molecules or whether they are more susceptible to, for example, refreezing into the surrounding ice, becoming stuck in micro-fractures and pore spaces, or sorption onto subglacial sediments.

AbstractWe show that geophysical methods offer an effective means of quantifying snow thickness and density. Opportunistic (efficient but non-optimized) seismic refraction and ground-penetrating radar (GPR) surveys were performed on Storglaciären, Sweden, co-located with a snow pit that shows the snowpack to be 1.73 m thick, with density increasing from ∼120 to ∼500 kg m–3 (with a +50 kg m–3 anomaly between 0.73 and 0.83 m depth). Depths estimated for two detectable GPR reflectors, 0.76 ±0.02 and 1.71 ± 0.03 m, correlate extremely well with ground-truth observations. Refraction seismic predicts an interface at 1.90 ± 0.31 m depth, with a refraction velocity (3730 ± 190 ms–1) indicative of underlying glacier ice. For density estimates, several standard velocity-density relationships are trialled. In the best case, GPR delivers an excellent density estimate for the upper snow layer (observed = 321 ± 74 kg m–3, estimated = 319 ± 10 kgm–3) but overestimates the density of the lower layer by 20%. Refraction seismic delivers a bulk density of 404 ±22 kgm–3 compared with a ground-truth average of 356 ± 22 kg m–3. We suggest that geophysical surveys are an effective complement to mass-balance measurements (particularly for controlling estimates of snow thickness between pits) but should always be validated against ground-truth observations.

Internal structure, stable isotope composition and tritium concentration were measured in and around debris‐bearing ice at the margin of Storglaciären, where englacial debris bands have previously been inferred to form by thrusting. Two types of debris bands were distinguished: (i) an unsorted diamicton band that is laterally continuous for more than 200 m, and (ii) well‐sorted sand and gravel bands that are lenticular and discontinuous. Above‐background tritium levels and enrichment of δ18 O and δD in ice from the diamicton band indicate entrainment by basal freeze‐on since 1952. Isotopic enrichment and tritium‐free ice in the sandy debris bands also indicate entrainment in freezing water, but prior to 1952. The lenticular cross‐section, sorting and stratification of the sandy bands suggest that they were deposited englacially. The basally accreted diamicton band has been elevated tens of metres above the bed and presently overlies the englacially deposited sandy bands, suggesting that the stratigraphy has been disrupted. Three interpretations could account for these observations: (i) thrusting of fast‐moving ice over slow, marginal ice uplifting recently accreted basal ice along the fault; (ii) folding near the margin, elevating young basal ice over older basal and englacial ice; and (iii) debris‐band formation by an unknown mechanism and subsequent contamination of ice geochemical properties by meltwater flow through debris bands. Although none of these interpretations is consistent with all measurements, folding is most compatible with observations and local ice‐flow kinematics.