Entrainment and emplacement of englacial debris bands near the margin of Storglaciären, Sweden

Abstract

P. 2013 (January): Entrainment and emplacement of englacial debris bands near the margin of Storglaciären, Sweden.Internal structure, stable isotope composition and tritium concentration were measured in and around debrisbearing 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 d 18 O and dD 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.

Isotopic investigation of runoff generation in a glacierized catchment in northern Sweden

“…For example, Moore et al . () estimated the mean isotopic composition of clean glacier ice at −13.5‰ on the basis of near‐surface ice samples collected from the margin of Storglaciären in 2006–2008. The switch of streamwater isotope values from more depleted to more enriched values (indicated by horizontal lines in Figure ) coincided in both years with the occurrence of the first major rainfall event on predominantly snow‐free ground.…”

“…Despite the increased variability in streamwater isotope values in 2011, both years showed distinct seasonal trends. Isotope values were more depleted in the heavy isotopes during the spring snowmelt season (May until beginning of July), which is consistent with long‐term observations of the isotopic composition of snow and glacier ice in Tarfala (Jansson et al ., 2007; Moore et al ., ), and became abruptly more enriched in heavy isotopes from the middle of July until the fall. The generally lower isotope values observed during spring and early summer reflect the increased input of snowmelt water, which varied between −13.3‰ and −16.0‰ with an arithmetic mean of −14.3‰ based on depth‐integrated averages for 14 snow cores and 20 snowmelt water samples collected during spring 2011 (Dahlke and Lyon, ).…”

Structures and fabrics in glacial ice: A review

“…There is a serious problem in interpreting the structures described as faults or thrusts as the result of shear fracturing, in that the stresses required to produce shear fractures in ice are much larger than the stresses usually inferred throughout glaciers (Moore et al, 2013). Estimates of shear stresses near the base of glaciers are typically in the order of 50-200 kPa (Cuffey and Paterson, 2010, table 8.3).…”

Thermal structure of Svalbard glaciers and implications for thermal switch models of glacier surging

“…Longyearbreen has been less studied, but investigation of the newly collected radargrams reveals a large number of elongated internal reflectors extending from the bed toward the surface in the lower half of the glacier (see arrows in Figure ). Similar englacial bands have been found close to the margins of Storglaciären, Sweden [ Moore et al ., ]. The features midway up the glacier dip upglacier at about 28°, while they become less steep downglacier and reach a dip of 12° close to the terminus.…”