LITTLE ICE-AGE VARIATIONS OF OUTLET GLACIERS FROM THE JOSTEDALSBREEN ICE-CAP, SOUTHERN NORWAY - A REGIONAL LICHENOMETRIC-DATING STUDY OF ICE-MARGINAL MORAINE SEQUENCES AND THEIR CLIMATIC SIGNIFICANCE

Complex moraine-ridge sequences in front of seven outlet glaciers of the Jostedalsbreen ice-cap (Austerdalsbreen, Bergsetbreen, Fabergstolsbreen, Lodalsbreen, Stegaholbreen, Tuftebreen, Bodalsbreen) are dated using families of lichenometric dating curves established previously at an eighth outlet (Nigardsbreen). Applicability of the Nigardsbreen curves at the regional level is tested using independent historical evidence: moraines deposited during the present century are dated to an accuracy of +/- 9.4 yr (16.0%), and most of them are dated to an accuracy of +/- 5.5 yr (9.4%). Results from the moraine sequences are combined to form a composite 'Jostedalsbreen' record. Median predicted dates for moraine ridges cluster around AD 1939 +/- 2 yr, 1929 +/- 3, 1908 +/- 3, 1886 +/- 5, 1875 +/- 2, 1867 +/- 4, 1855 +/- 3, 1842 +/- 5, 1822 +/- 5, 1807 +/- 4 and 1 785 +/- 5. At least four glaciers reached their 'Little Ice Age' maxima prior to AD 1780, two (Nigardsbreen and Bodalsbreen) at ca. 1750, one (Fabergstolsbreen) at ca. 1705. Stegaholbreen attained its maximum ca. 1863. Since the 'Little Ice Age' maximum, and despite large differences in glacier size, frontal variations of the various outlets have exhibited a high degree of synchroneity, which suggests that the moraine sequences contain a sensitive record of high-frequency climatic variations over the last ca. 250 yr. During the early twentieth century, measured readvances of the order of 5-150 m over 1-10 yr led to moraine formation. Dendroclimatic evidence indicates that since the late eighteenth century, moraine ridges formed about 5 yr after summer temperature minima and correlate with runs of cool summers (temperature depression of 0.5-1.0-degrees-C below the AD 1700-1950 average). Almost simultaneous glacier advances appear to have been caused by reduced ablation. This near-immediate response to climatic variation, by glacier tongues that descend to relative low altitudes, is superimposed upon the longer-term dynamic response of the ice cap to climate.