Holocene glacial and climate history of the Jostedalsbreen region, Western Norway; evidence from lake sediments and terrestrial deposits

Abstract

The valleys surrounding the Jostedalsbreen ice cap were deglaciated during the latter half of the Preboreal Chronozone. At the end of the Preboreal Chronozone, however, a glacier readvance occurred. Terminal moraines were deposited by outlet valley glaciers from the Jostedalsbreen Plateau up to 1 km beyond Little Ice Age moraines. Inferred from the altitude of lateral moraines formed during this readvance and calculations of the equilibrium-line altitude (ELA) depression based on an accumulation area ratio (AAR) of 0.6, the average depression of the ELA was 325 + 75 −115 m below the present. By assuming a similar precipitation pattern as at present, this suggests a mean temperature decline of about 2°C. Palynological investigations from Sygneskardet, Sunndalen, indicate that climate like the present was achieved just after 9000 BP. The Holocene climatic optimum occurred during the Atlantic Chronozone, with elm ( Ulmus) stands growing at the present birch ( Betula) forest limit in Sunndalen and pine ( Pinus) growing at Styggevatnet to an altitude of at least 1160 m. During this period the mean summer temperature is estimated to have been at least 2.7 and 1.8°C warmer than at present, with and without the local climatic effect of Jostedalsbreen, respectively. An inferred rise of the ELA of about 400 m from the present altitude suggests that possibly no glaciers existed on the Jostedalsbreen Plateau during the Holocene climatic optimum. Vegetational changes as deduced from palynological studies, lowered tree limits and increased resedimentation in peat bogs indicate general climatic deterioration since the Late Atlantic Chronozone. A significant Alnus decline 6300 BP and a Betula expansion 5300 BP, as recorded from palynological studies of peat bogs in Sprongdalen, is interpreted to represent the initial Neoglaciation on the Jostedalsbreen Plateau. Lithostratigraphic evidence at a section in the upper part of Glomsdalen may indicate a minor Neoglacial phase close to 6000 BP. The first pronounced Neoglacial event in the Jostedalsbreen region, however, was bracketed between 3700 and 3100 BP. Spørteggbreen, the upper part lying ca. 30 m above the present regional glaciation threshold, was formed around 500 BP at the initiation of the Little Ice Age. Gelifluction at 1000 m altitude began after 3200-2800 BP. During the Medieval Period, glaciers were smaller than now. At the northwestern part of Jostedalsbreen an initial Little Ice Age glacier expansion is dated to have occurred after 890 ± 60 BP (A.D. 1030–1220). From the early 14th century, and especially from the mid-17th century, a severe climatic deterioration during the Little Ice Age is historically documented. Around Jostedalsbreen this advance culminated during the mid-18th century. Documents suggest that the western outlet glaciers from Jostedalsbreen reached their maximum Little Ice Age position some years before the longer eastern outlet valley glaciers. A typical depression of the ELA during the Little Ice Age of 100–150 m indicates a mean temperature decline from the present of ca. 0.5–1°C.