DID NEOGLACIATION BEGIN AS EARLY AS 6400 CAL YEARS AGO?

Published evidence from western North America, Europe, the Himalaya, New Zealand, and the Andes suggests that the first substantial Holocene glacier advance began by 6400 cal yr BP. We define “substantial advance” as one that achieves a downvalley extent comparable to that of the last 3000 years. In Washington state, South Cascade Glacier sheared trees dated 4700+/-130 (W-1030) and 4960+/-90 14C yr BP (UW-99) (Miller, 1969, Arct. Alp. Res.) and Easton Glacier on Mount Baker deposited till on a deformed mat of trunks and peat dated 5260+/-70 (Beta-207260) and 5240+/-70 14C yr BP (Beta-207279). In situ stumps in forefields of glaciers in the mountains of southwest British Columbia date 4770+/-120 (I/GSC-182), 5260+/-200 (Y-140bis), and 5300+/-70 14C yr BP (GSC-2027), and are the basis for a middle Holocene glacial event termed the “Garibaldi phase” (Ryder and Thomson, 1986, Can. J. Earth Sci.). The calibrated age range of these dated samples is 6410-4570 cal yr BP. Evidence from other mountain ranges shows that the Garibaldi advance is a global event. Fourteen radiocarbon ages on wood and soils beneath till in the forefields of Austre Okstindbreen in Norway, Rutor Glacier in the Italian Alps, and Findelengletscher and Glacier de Brenay in the Swiss Alps, as well as ages on Ötzi the Iceman in the Austrian Alps, range from 7335 to 3910 cal yr BP. Six radiocarbon ages from soils buried in moraines in the Nepalese and Indian Himalaya, 11 radiocarbon ages from glacier forefields in the Southern Alps of New Zealand, and 13 radiocarbon ages from forefields in the Andes are also within this interval. Porter and Denton (1967, Am. J. Sci.) defined Neoglaciation as a time-transgressive event, during which alpine glaciers reformed or readvanced following a period of early Holocene warmth. There is broad agreement that Neoglaciation culminated with the climactic advances of the Little Ice Age in the last half of the past millennium, but the time of the beginning of Neoglaciation has long been debated. A growing body of data suggests that Neoglaciation began much earlier than 3000 cal yr BP, which is commonly taken as its starting point. We suggest that the maximum Holocene rate of change (dW/dt) in summer insolation (W cm-2/kyr-1) for 60oN at 6000-5000 cal yr BP triggered Neoglaciation.