OROMBELLI G. & PORTER S.C.
Late Holocene fluctuations of Brenva Glacier
Pages 14-37
Abstract
Fluctuations of the terminus of Brenva Glacier during the last several centuries have been reconstructed using documentary evidence in the form of maps, painting, lithographs, drawings, written accounts, photographs, and instrumental surveys, supplemented by geologic mapping and botanical dating. The glacier terminus lay close to its present position during the late l8th century and advanced to its Holocene maximum in 1818. Following this culmi-nation the glacier retreated several hundred meters before readvancing in the 1840′s to reach a new maximum about 1850 only slightly short of the earlier one. During the next three decades the terminus receded about 1 km upvalley but then readvanced sharply during the 1880′s to a secondary culmination about 1890-1895. Slow persistent retreat until about 1914 was then followed by renewed advance. Massive rockfalls from M. Bianco in 1920 thickly mantled the ablation zone with granitic rubble. This debris cover inhibited ablation of ice and caused the terminus to continue its advance until a new maximum was reached in 1940-1941 only about 50 m behind the 1818 limit. From 1940-41 until the mid-1960′ the glacier front receded some 400 m, but renewed advance was detected between 1965 and 1967 which has continued to the present. The terminal fluctuations of Brenva Glacier show a consistent relationship to a meteorological record from Great St. Bernard Pass (1818 – present) and to a long temperature record from Milano (1763 – present). Glacier advances followed intervals when winter accumulation was above average and air temperature, especially during the ablation season, was below average. Buried logs found embedded in the upper right-lateral moraine of Brenva Glacier record earlier ice advances when supramorainal forests were killed and buried by morainal debris. One log is 1170±55 14C yr old (=760 – 980 AD.), whereas a younger log has an age of 285±60 yr, probably equivalent to a calendar age of 1660 A.D. or older. The dated samples lie near the crest of the 150 m – high moraine which may contain in its core a succession of still-older morainal accretions providing a record of multiple ice advances extending far back into the Holocene.