부산대학교 도서관

In 2017 part of an ancient stone causeway was uncovered at Bjæverskov on the island of Sjælland, Denmark. Previous approaches to dating such ancient stone structures have almost always been relative, or by association, with methodologies relying upon interpretation of temporally diagnostic artifacts, radiometric dating of organic remains (14C AMS), or surrounding sediments (optically stimulated luminescence dating). Such techniques can fail to conclusively address whether these materials are actually associated with construction of the ancient structure itself. With no artifacts found at the site, optically stimulated luminescence dating of sedimentary coarse grains, grains derived from crushed rock cores, and surficial rock chips obtained directly from granitic road cobbles were used to determine the time of construction. Some granitic road cobbles were visibly disaggregating at the time of excavation, and lab measurements revealed surprising fast-component dominated quartz sensitivity from these samples. It was concluded that at least some of the rocks used in the causeway had been heated, presumably prior to/or during incorporation in the structure. Dose recovery plateau experiments using quartz grains recovered by crushing apparently heated rocks suggested the use of a 220/180°C preheat/cut-heat combination (DR ratio 0.99±0.01; n=40); the low pretreatment temperatures were preferred to reduce the risk of thermal transfer in these young samples. IRSL signals were used with 4 rocks that could not be disaggregated. All yielded L/T burial profiles that indicated prior light exposure, and of these 4, two (1 granite, 1 felsic gneiss) had apparently been exposed for sufficient time for us to be confident of obtaining accurate IRSL ages. IR50 fading corrections were undertaken by determining L/T at field saturation (from deep within the rock), giving the same aliquots a 2kGy dose in a 60Co gamma cell, and remeasuring the laboratory saturated L/T. The post-IR IRSL180 signals were also measured in these two cobbles; the bleaching front was shallow and the signal was only sufficiently reset to allow accurate determination of De on one rock. In total 8 ages were accepted; 4 quartz ages from coarse grained sediment, 2 fading corrected IR50 ages from surface slices from non- disaggregated cobbles, and 2 quartz ages from disaggregated (apparently heated) cobbles. IRSL signals from the sedimentary and heated samples were used primarily to assess the degree of resetting of the quartz blue-stimulated OSL. In all the sedimentary samples, both the post-IR IRSL and IR50 signals significantly over-estimated the quartz age. However, the quartz ages, the heated cobbles and the fading corrected IR50 ages from the unheated road cobbles, are consistent and likely reflect the construction age. Quartz from the overlying sediments appears to be mainly well bleached, but is younger than the construction age. We conclude that the stone causeway was constructed ~2ka ago. [ABSTRACT FROM AUTHOR]