Geochemical study of the origin of debris bands at Storglaciaren, Sweden

Geochemical data from glacial ice and meltwater at Storglaciaren, Sweden were used to evaluate the origin and age of a debris band located near the terminus of the glacier. Three ice facies identified and evaluated in this study were debris-rich ice, clear ice, and bubble ice. Stable hydrogen (6 H) and oxygen (5 O) isotope ratios were used to test for freezing. 3H (tritium) and dissolved gas measurements were used to evaluate the 3H-3He age-dating method in ice; the diffusion coefficient (Di) and solubility (S) of He and Ne in ice were also determined in a laboratory experiment. Stable isotope data suggest that debris-rich ice formed by freezing of subglacial water. The presence of anthropogenic tritium (3.7 TU) in one of two debris-rich ice samples indicates that the water is "post-bomb" in age. In contrast, clear ice directly above and below the debris band has nil tritium. These data argue against regelation infiltration and instead suggest net freeze-on processes at the glacier bed or within englacial conduits as the debris entrainment mechanism. Dissolved gas data from glacier and lab suggest a first order, size-dependent exclusion of heavy noble gases due to freezing. The data also show that He and Ne are more soluble in ice than water by a factor of ~2 to 3. In lab ice, measured Sue and SNe are 9.3xl0"8 cc (STP)/g and 1.3xl0~7 cc (STP)/g, respectively. DH e and D N e in lab ice are both approximately 1x10-10m2 sec"1. Based on the DHc , ice samples for 3H-3He age-dating should be collected from at least 1.25 m back from the ice-air interface or in areas of high ablation (~>0.2 m yr"1). A preliminary H- He age-date for one debris-rich ice sample (DB1) is 31+ 5 years BP. Excess (radiogenic) 4He age-dating of the same sample yields age dates ranging from 17 + 5 years BP to 65 + 18 years BP, depending on the method used to calculate excess 4He. Both age-dating methods require development of a model to account for excess air in ice and a more accurate value of SHZ before reliable age-dates can be determined.