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  • 1. Böttner, Christoph
    et al.
    Berndt, Christian
    Reinardy, Benedict T.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Geersen, Jacob
    Karstens, Jens
    Bull, Jonathan M.
    Callow, Ben J.
    Lichtschlag, Anna
    Schmidt, Mark
    Elger, Judith
    Schramm, Bettina
    Haeckel, Matthias
    Pockmarks in the Witch Ground Basin, Central North Sea2019In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 20, no 4, p. 1698-1719Article in journal (Refereed)
    Abstract [en]

    Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. Quantitative estimates for methane release from marine sediments are scarce, and a poorly constrained temporal variability leads to large uncertainties in methane emission scenarios. Here, we use 2-D and 3-D seismic reflection, multibeam bathymetric, geochemical, and sedimentological data to (I) map and describe pockmarks in the Witch Ground Basin (central North Sea), (II) characterize associated sedimentological and fluid migration structures, and (III) analyze the related methane release. More than 1,500 pockmarks of two distinct morphological classes spread over an area of 225 km(2). The two classes form independently from another and are corresponding to at least two different sources of fluids. Class 1 pockmarks are large in size (> 6 m deep, > 250 m long, and > 75 m wide), show active venting, and are located above vertical fluid conduits that hydraulically connect the seafloor with deep methane sources. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller (0.9-3.1 m deep, 26-140 m long, and 14-57 m wide) and are limited to the soft, fine-grained sediments of the Witch Ground Formation and possibly sourced by compaction-related dewatering. Buried pockmarks within the Witch Ground Formation document distinct phases of pockmark formation, likely triggered by external forces related to environmental changes after deglaciation. Thus, greenhouse gas emissions from pockmark fields cannot be based on pockmark numbers and present-day fluxes but require an analysis of the pockmark forming processes through geological time. Plain Language Summary Marine sediments host large amounts of methane (CH4), which is a potent greenhouse gas. The amount of methane released into the atmosphere is, however, largely unknown making it difficult to implement this methane source in climate models. Here we use geophysical, geochemical, and sedimentological data to map the distribution of fluid escape structures in the central North Sea. More than 1,500 pockmarks, which are circular to semicircular depressions of the seafloor, indicate fluid flow from the subsurface. There are two distinct morphological classes of pockmarks corresponding to at least two different fluid sources. Class 1 pockmarks are large, show active venting, and are located above vertical fluid conduits in the subsurface, which feed fluids from deeper strata. Class 2 pockmarks, which comprise 99.5% of all pockmarks, are smaller and limited to the soft sediments directly below the seafloor. Older pockmarks in the subsurface document distinct phases of pockmark formation, likely triggered by external forces after the retreat of ice in the North Sea. The amount of methane released from natural geological sources based on pockmark numbers may be wrong as these do not take into account the origin and composition of released fluids.

  • 2. Killingbeck, Siobhan F.
    et al.
    Booth, Adam D.
    Livermore, Philip W.
    West, Landis J.
    Reinardy, Benedict T.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Nesje, Atle
    Subglacial sediment distribution from constrained seismic inversion, using MuLTI software: examples from Midtdalsbreen, Norway2019In: Annals of Glaciology, ISSN 0260-3055, E-ISSN 1727-5644, Vol. 60, no 79, p. 206-219Article in journal (Refereed)
    Abstract [en]

    Fast ice flow is associated with the deformation of subglacial sediment. Seismic shear velocities, Vs, increase with the rigidity of material and hence can be used to distinguish soft sediment from hard bedrock substrates. Depth profiles of Vs can be obtained from inversions of Rayleigh wave dispersion curves, from passive or active-sources, but these can be highly ambiguous and lack depth sensitivity. Our novel Bayesian transdimensional algorithm, MuLTI, circumvents these issues by adding independent depth constraints to the inversion, also allowing comprehensive uncertainty analysis. We apply MuLTI to the inversion of a Rayleigh wave dataset, acquired using active-source (Multichannel Analysis of Surface Waves) techniques, to characterise sediment distribution beneath the frontal margin of Midtdalsbreen, an outlet of Norway's Hardangerjokulen ice cap. Ice thickness (0-20 m) is constrained using co-located GPR data. Outputs from MuLTI suggest that partly-frozen sediment (Vs 500-1000 m s(-1)), overlying bedrock (Vs 2000-2500 m s(-1)), is present in patches with a thickness of similar to 4 m, although this approaches the resolvable limit of our Rayleigh wave frequencies (14-100 Hz). Uncertainties immediately beneath the glacier bed are <280 m s(-1), implying that MuLTI cannot only distinguish bedrock and sediment substrates but does so with an accuracy sufficient for resolving variations in sediment properties.

  • 3.
    Reinardy, Benedict T.
    et al.
    Stockholm University, Faculty of Science, Department of Physical Geography.
    Booth, Adam D.
    Hughes, Anna L. C.
    Boston, Clare M.
    Åkesson, Henning
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bakke, Jostein
    Nesje, Atle
    Giesen, Rianne H.
    Pearce, Danni M.
    Pervasive cold ice within a temperate glacier - implications for glacier thermal regimes, sediment transport and foreland geomorphology2019In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 13, no 3, p. 827-843Article in journal (Refereed)
    Abstract [en]

    This study suggests that cold-ice processes may be more widespread than previously assumed, even within temperate glacial systems. We present the first systematic mapping of cold ice at the snout of the temperate glacier Midtdalsbreen, an outlet of the Hardangerjokulen icefield (Norway), from 43 line kilometres of ground-penetrating radar data. Results show a 40 m wide cold-ice zone within the majority of the glacier snout, where ice thickness is < 10 m. We interpret ice to be cold-based across this zone, consistent with basal freeze-on processes involved in the deposition of moraines. We also find at least two zones of cold ice up to 15 m thick within the ablation area, occasionally extending to the glacier bed. There are two further zones of cold ice up to 30 m thick in the accumulation area, also extending to the glacier bed. Cold-ice zones in the ablation area tend to correspond to areas of the glacier that are covered by late-lying seasonal snow patches that reoccur over multiple years. Subglacial topography and the location of the freezing isotherm within the glacier and underlying subglacial strata likely influence the transport and supply of supraglacial debris and formation of controlled moraines. The wider implication of this study is the possibility that, with continued climate warming, temperate environments with primarily temperate glaciers could become polythermal in forthcoming decades with (i) persisting thinning and (ii) retreat to higher altitudes where subglacial permafrost could be and/or become more widespread. Adversely, the number and size of late-lying snow patches in ablation areas may decrease and thereby reduce the extent of cold ice, reinforcing the postulated change in the thermal regime.

  • 4.
    Reinardy, Benedict T. I.
    et al.
    University of Bergen, Norway.
    Sejrup, Hans Petter
    Hjelstuen, Berit O.
    King, Edward
    Augedal, Hans
    A Quaternary aminostratigraphy constraining chronology of depositional environments in the North Sea Basin2018In: Marine Geology, ISSN 0025-3227, E-ISSN 1872-6151, Vol. 402, p. 139-152Article in journal (Refereed)
    Abstract [en]

    The aminostratigraphy of up to 1000 m of glacial and interglacial sediments in the North Sea Basin (NSB) is compiled from multiple boreholes sites and dated using strontium isotope (Sr) analysis to provide a chronological framework extending throughout the Quaternary. The Quaternary aminostratigraphy within the NSB can be divided into five amino zones (AZ), AZ1 (youngest) to AZ5 based on alle/Ile ratios which are used to correlate depositional events recorded in sedimentary facies from multiple borehole sites. The new relative and absolute chronology also ties to seismostratigraphy and multi-generational tunnel valley fill. Early Pleistocene sediments produce AZ5 ratios with Sr ages around 1.4–1.5 Ma deposited in marine to glacimarine conditions. Sediments with AZ4 ratios are constrained by Sr ages and magnetic polarity to 1.3–0.9 Ma, spanning the eccentricity to axial tilt insolation periodicity change of the Mid Pleistocene Transition (MPT). Some coarse-grained sedimentary units with AZ4 ratios may be linked to ice proximal deposition during the MPT. Sediments with AZ3 ratios are poorly chronologically constrained to between 1.1 and 0.26 Ma. At least one episode of diminished coarse clasts and increased foraminiferal diversity during this Mid Pleistocene time suggests a progression from ice proximal to warmer, possibly interglacial conditions. Furthermore, the stratigraphically deepest generation of tunnel valleys cut strata with AZ3 ratios indicating subglacial conditions at least once during the Mid Pleistocene. Sediments with AZ2 ratios corresponding to MIS7-5 are tied to seismic units including a thick (up to 40 m) deformation till, suggesting glacial incursion into the NSB during MIS6 followed by tunnel valley cutting of the this till, thought to be an MIS6 event also. The geometry and cross-cutting complexities of such tunnel valleys precludes further correlation of this ice extent. The youngest aminostratigraphic zone, AZ1, is recognised in all boreholes and well constrained to MIS4-2 and the youngest set of tunnel valleys, attributed to a single or multiple glacial advances at this time.

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