Holocene glacial chronologies of the Brooks Range, Alaska and their relationship to climate change

Regional Holocene glacial chronologies of the Brooks Range were developed from 119 moraines associated with 41 glaciers. When combined with previous studies, a composite chronology based on 101 glaciers indicates eleven periods of moraine formation centered around 7000, 4400, 3500, 2800, 2300, 1800, 1400, 1100, 770, 370, and 90 lichenometic (L) yr BP. Two moraines that formed about 7000 (L) yr BP mark the earliest known post-hypsitermal glacial advance in Alaska. After 4700 (L) yr BP, glacier advances became increasingly widespread; spectral analysis suggests an average frequency of every 340 years. Recent glacial episodes appear two-phased, with the first of more rapid and synchronous onset. The median age of all Holocene moraines increases towards the southwest, mimicking glaciation thresholds. Early-middle Holocene climatic deterioration may have been only strong enough to cause glacier expansion in areas with ready moisture sources. The Beaufort Sea may have become an important source for eastern areas only during late Holocene time.; Diameter increases measured in 92 Rhizocarpon geographicum s.l. lichens ranged from near 0 to 0.35 mm/yr and had an inverse and extremely variable relationship to thallus size. Comparison with the lichenometric dating curve suggests that thalli may sustain optimum growth for only short periods. Measurement of this species has not yet continued long enough to be used for dating, although a curve based on measurement of 57 Alectoria minuscula (Nyl.) thalli can be used for dating young deposits.; The Brooks Range glacial chronology suggests a similar climate history to that inferred from palynological, alluvial, and some dendroclimatologic evidence, but is for most of Holocene time more detailed. In response to twentieth century warming, eight glaciers in the eastern Brooks Range retreated 150 to 1330 m, although their response was generally not synchronous with temperature variation on a decade scale. However, there was a high correlation between seasonal temperature and equilibrium line altitude at one site in the central Brooks Range. When combined with topographic lapse rates, the data suggest that Holocene glacial episodes could have been associated with sustained summer temperatures 0.9 to 2.8{dollar}spcirc{dollar}C lower than today.