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  • 1
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    Uncertainty in Peat Volume and Soil Carbon Estimated Using Ground-Penetrating Radar and Probing (2012)
    Wiley
    Details
    Publication Date: 2012-09-01
    Print ISSN: 0361-5995
    Electronic ISSN: 1435-0661
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Published by Wiley
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  • 2
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    The effect of peat structure on the spatial distribution of biogenic gases within bogs (2013)
    Wiley
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    Publication Date: 2013-09-07
    Description: Northern peatlands are a large source of atmospheric methane (CH 4 ) and both a source and sink of atmospheric carbon dioxide (CO 2 ). The rate and temporal variability in gas exchanges with peat soils is directly related to the spatial distribution of these free-phase gases within the peat column. In this paper we present results from surface and borehole ground penetrating radar (GPR) surveys – constrained with direct soil and gas sampling – that compare the spatial distribution of gas accumulations in two raised bogs: one in Wales (UK), the other in Maine (USA). Although the two peatlands have similar average thickness, physical properties of the peat matrix differ, particularly in terms of peat type and degree of humification. We hypothesize that these variations in physical properties are responsible for the differences in gas distribution between the two peatlands characterized by: 1) gas content up to 10.8 % associated with woody peat and presence of wood layers in Caribou Bog (Maine), and 2) a more homogenous distribution with gas content up to 5.7 % at the surface (i.e. 〈 0.5 m deep) in Cors Fochno (Wales). Our results highlight the variability in biogenic gas accumulation and distribution across peatlands and suggest that the nature of the peat matrix has a key role in defining how biogenic gas accumulates within, and is released to the atmosphere from, peat soils. This article is protected by copyright. All rights reserved.
    Print ISSN: 0885-6087
    Electronic ISSN: 1099-1085
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley
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  • 3
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    Threshold Sensitivity of Shallow Arctic Lakes and Sub-lake Permafrost to Changing Winter Climate (2016)
    Wiley
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    Publication Date: 2016-06-17
    Description: Interactions and feedbacks between abundant surface waters and permafrost fundamentally shape lowland Arctic landscapes. Sub-lake permafrost is maintained when the maximum ice thickness (MIT) exceeds lake depth and mean annual bed temperatures (MABT) remain below freezing. However, declining MIT since the 1970s is likely causing talik development below shallow lakes. Here we show high temperature sensitivity to winter ice growth at the water-sediment interface of shallow lakes based on year-round lake sensor data. Empirical model experiments suggest that shallow (1-m depth) lakes have warmed substantially over the last 30 years (2.4 °C), with MABT above freezing five of the last seven years. This is in comparison to slower rates of warming in deeper (3-m) lakes (0.9 °C), with already well-developed taliks. Our findings indicate that permafrost below shallow lakes has already begun crossing a critical thawing threshold approximately 70 years prior to predicted terrestrial permafrost thaw in northern Alaska.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Detecting unfrozen sediments below thermokarst lakes with Surface Nuclear Magnetic Resonance (2013)
    Wiley
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    Publication Date: 2013-01-17
    Description: [1]  A talik is a layer or body of unfrozen ground that occurs in permafrost due to an anomaly in thermal, hydrological, or hydrochemical conditions. Information about talik geometry is important for understanding regional surface water and ground water interactions as well as sub-lacustrine methane production in thermokarst lakes. Due to the direct measure of unfrozen water content, surface nuclear magnetic resonance (NMR) is a promising geophysical method for noninvasively estimating talik dimensions. We made surface NMR measurements on thermokarst lakes and terrestrial permafrost near Fairbanks, Alaska and confirmed our results using limited direct measurements. At an 8 m deep lake, we observed thaw bulb at least 22 m below the surface; at a 1.4 m deep lake we detected a talik extending between 5 and 6 m below the surface. Our study demonstrates the value surface NMR may have in the cryosphere for studies of thermokarst lake hydrology and their related role in the carbon cycle.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    Application of ground-penetrating radar to measure near-saturation soil water content in peat soils (2012)
    Wiley
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    Publication Date: 2012-02-28
    Description: The presence and flux of biogenic methane-rich free phase gas that is eventually released to the atmosphere influence water content (θ) of peat soils below the water table. Small variations in gas content in peat soils at near-saturation θ could be inferred by changes in dielectric permittivity, but detailed measurements in that range of θ needed to develop calibration functions are lacking. Our experiment uses a new method for varying θ in the sample using elevated pressure to reduce the naturally occurring volumetric gas content in a manner similar to what occurs in situ under atmospheric pressure change, which relevant to understanding carbon gas cycling in peatlands. We recorded dielectric permittivity using a 1.6 MHz ground-penetrating radar antenna at multiple water contents between 0.87 and 0.95 m3 m−3 on four peat monoliths with varying levels of humification and with
    Print ISSN: 0043-1397
    Electronic ISSN: 1944-7973
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    Geophysical evidence for the lateral distribution of free phase gas at the peat basin scale in a large northern peatland (2011)
    Wiley
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    Publication Date: 2011-07-27
    Description: Northern peatlands contribute to the global carbon cycle by way of gas emissions in the form of biogenic carbon dioxide and methane originating in the subsurface. Determining how much gas is present and where it is trapped is essential to understanding the role of northern peatlands on the global carbon cycle, particularly in regard to methane cycling. In this study, spatial variability in free phase gas content was estimated using ground penetrating radar (GPR) along a 1.4 km transect crossing distinct peat landforms including a bog crest, midslope lawn, and fen. Variations in gas content and distribution were observed in landforms crossed by this transect. Estimated gas content up to 25% by volume was observed in landforms dominated by woody surface vegetation. In areas of the midslope lawn, the estimated gas content was greater than 15%, while estimated gas contents between 0% and 7% were found in the fen. Changes in estimated gas content of up to 20% were observed over distances of 100 m across a transition between a stand of 10 m tall trees and an adjacent fen. The mean of the estimated errors in gas content is 1.3%, suggesting that these large changes in estimated gas content between landforms can be clearly determined using geophysical methods. A geophysical image of the distribution of free phase across this transect reveals evidence for gas storage in deep peat in intervals as thin as 0.35 m. These data support conceptual models based on accumulation and storage of free phase gas in deep peat.
    Print ISSN: 0148-0227
    Topics: Biology , Geosciences
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Expansion rate and geometry of floating vegetation mats on the margins of thermokarst lakes, northern Seward Peninsula, Alaska, USA (2011)
    Wiley
    Details
    Publication Date: 2011-07-16
    Description: ABSTRACT Investigations on the northern Seward Peninsula in Alaska identified zones of recent (〈50 years) permafrost collapse that led to the formation of floating vegetation mats along thermokarst lake margins. The occurrence of floating vegetation mat features indicates rapid degradation of near-surface permafrost and lake expansion. We report on the recent expansion of these collapse features and determine their geometry using geophysical and remote sensing measurements. The vegetation mats were observed to have an average thickness of 0.57 m and petrophysical modeling indicated that gas content of 1.5 - 5% enabled floatation above the lake surface. Furthermore, the geophysical investigation provides evidence that the mats form by thaw and subsidence of the underlying permafrost rather than terrestrialization. The temperature of the water below a vegetation mat was observed to remain above freezing late in the winter. Analysis of satellite and aerial imagery indicates that these features have expanded at maximum rates of 1 – 2 m yr -1 over a 56-year period. Including the spatial coverage of floating “thermokarst mats” increases estimates of lake area by as much as 4% in some lakes. Copyright © 2011 John Wiley & Sons, Ltd.
    Print ISSN: 0197-9337
    Electronic ISSN: 1096-9837
    Topics: Geography , Geosciences
    Published by Wiley
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  • 8
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    Uniform and lateral preferential flows under flood irrigation at field scale (2019)
    Wiley
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    Publication Date: 2019
    Description: Abstract Flood irrigation is globally one of the most used irrigation methods. Typically, not all water that is applied during flood irrigation is consumed by plants or lost to evaporation. Return flow, the portion of applied water from flood irrigation that returns back to streams either via surface or subsurface flow, can constitute a large part of the water balance. Few studies have addressed the connection between vertical and lateral subsurface flows and its potential role in determining return flow pathways due to the difficulty in observing and quantifying these processes at plot or field scale. We employed a novel approach, combining induced polarization, time‐lapse electrical resistivity tomography, and time‐lapse borehole nuclear magnetic resonance, to identify flow paths and quantify changes in soil hydrological conditions under nonuniform application of flood irrigation water. We developed and tested a new method to track the wetting front in the subsurface using the full range of inverted resistivity values. Antecedent soil moisture conditions did not play an important role in preferential flow path activation. More importantly, boundaries between lithological zones in the soil profile were observed to control preferential flow pathways with subsurface run‐off occurring at these boundaries when saturation occurred. Using the new method to analyse time‐lapse resistivity measurements, we were able to track the wetting front and identify subsurface flow paths. Both uniform infiltration and preferential lateral flows were observed. Combining three geophysical methods, we documented the influence of lithology on subsurface flow processes. This study highlights the importance of characterizing the subsurface when the objective is to identify and quantify subsurface return flow pathways under flood irrigation.
    Print ISSN: 0885-6087
    Electronic ISSN: 1099-1085
    Topics: Architecture, Civil Engineering, Surveying , Geography
    Published by Wiley
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  • 9
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    Small Root Biomass Effect on the Dielectric Properties of Soil (2012)
    Soil Science Society of America (SSSA)
    Details
    Publication Date: 2012-02-01
    Description: The biomass attributed to fine and small roots contained within soils is challenging to quantify in situ. We examined the effect of fine roots on the bulk dielectric properties of soil to explore the potential for assessing volumetric root content using ground-penetrating radar (GPR). We found a positive relationship between fine root content and bulk dielectric permittivity in relatively dry sand. We fit the data with a mixing model that accounted for the mineral, air, and root components of the sample. Our results indicate that changes in volumetric root content can be detected by GPR under such simple, controlled conditions; however, the presence of pore water, along with its spatial variability and the inherent structural heterogeneity of soils, will probably render detection of fine root biomass from a GPR measurement very challenging in the field.
    Electronic ISSN: 1539-1663
    Topics: Geosciences , Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Published by Soil Science Society of America (SSSA)
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  • 10
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    Near-surface geophysics for informed water-management decisions in the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands of South Australia (2014)
    Society of Exploration Geophysicists (SEG)
    Details
    Publication Date: 2014-12-03
    Description: The Aboriginal population of the Aṉangu Pitjantjatjara Yankunytjatjara (APY) lands in South Australia is dependent on groundwater for nearly all water needs. In that region, placement of wells in productive aquifers of appropriate water quality is challenging because of lack of hydrologic data and variable aquifer properties. It is desirable to have an improved ability to identify and evaluate groundwater resources in this remote region with cost-effective methods that make minimal impact on the environment. A project supported by the Society of Exploration Geophysicists program Geoscientists Without Borders tested a combined geophysical approach with airborne and ground-based data sets to locate a potential aquifer, confirm water content, and estimate the subsurface extent of the water-bearing zone. This hydrogeophysical approach was an effective means for exploration and evaluation of groundwater resources in APY lands generally, and it characterized a specific aquifer as a case study.
    Print ISSN: 1070-485X
    Electronic ISSN: 1938-3789
    Topics: Geosciences
    Published by Society of Exploration Geophysicists (SEG)
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