'Little Ice Age' glacier variations in Jotunhcimcn, southern Norway: a study in regionally controlled lichenometric dating of recessional moraines with implications for climate and lichen growth rates

摘要

A new approach to regional lichenometric dating is developed and applied to 'Little Ice Age' moraine-ridge sequences on 16 glacier forelands in Jotunheimen, southern Norway. Lichenometric-dating curves, based on the Rhizocarpon subgenus, are constructed independently for west, central and east Jotunheimen. Although there are differences between the subregions, a composite regional moraine chronology for Jotunheimen identifies 12 episodes of moraine formation in AD 1743-1750 (the regional 'Little Ice Age' glacier maximum), 1762-1771, 1782-1790, 1796-1802, 1811-1818, 1833-1838, 1845-1854, 1860-1868, 1871-1879, 1886-1898, 1915-1922 and 1927-1934. Spatial and temporal patterns in glacier behaviour between the subregions and between Jotunheimen and the neighbouring Jostedalsbreen are explained in terms of the interaction of annual to decadal variations in summer temperature and winter precipitation: glacier advances and moraine-formation events driven primarily by winter-precipitation variations exhibit subregional patterns while summer-temperature forcing affects more synchronous glacier behaviour across the region. Regionally controlled lichenometric dating improves the accuracy of dating by up to about ±20 years on relatively old moraines and is dependent on regional patterns in the rate of lichen growth. On relatively young surfaces, mean cumulative growth rate declines from about 0.75 mm yr~(-1) in maritime west Jotunheimen to about 0.55 mm yr~(-1) in continental east Jotunheimen (though the differential in growth rate is less on older surfaces).