Lichenometry: Its Application to Holocene Moraine Studies in Southern Alaska and Swedish Lapland

Abstract

In White River Valley and Skolai Pass in the Wrangell and St. Elias mountains of southern Alaska, several well-dated drift surfaces and abandoned alluvial channels covered with numerous lichens served as control points of a growth curve for Rhizocarpon geographicum. This curve shows initial rapid increase in thallus diameter, followed after a few centuries by a long interval of nearly linear increase of about 3.4 mm per century. The largest living Rhizocarpon geographicum thallus in the region is 155 mm in diameter and presumably is about 3,700 years old. Unfortunately, use of the lichen-growth curve for absolute dating and for correlation of moraines among the nine glaciers studied was restricted by the unsuitable character of many older moraine surfaces for lichen measurements, due to melting of internal ice cores or to encroachment of vegetation over rock surfaces on moraines. With some exceptions, only young moraine surfaces proved reliable for lichenometric dating. Therefore, in this region lichenometry served as a valuable reconnaissance tool, but in most cases could not be used for detailed dating.

Lichenometry proved more useful in the Kebnekaise and Sarek mountains of Swedish Lapland. Measurement of maximum thallus diameters of Rhizocarpon geographicum and Rhizocarpon alpicola on mapped Holocene drift units fronting 40 glaciers afforded consistent results that permitted regional correlation and placement of Holocene moraines into four groups, each representing a broad interval of glacier expansion. Lichen measurements on numerous surfaces of known historical age dispersed through the mountains, first, suggested that growth rates were uniform throughout the area and, second, provided control points for a detailed growth curve through the last several centuries. An attempted extension of this growth curve through the last several thousand years proved to be tentative, for it was based only on three ¹⁴C dates of soils buried beneath moraines. The initial portion of the extended curve represents both Rhizocarpon geographicum and Rhizocarpon alpicola, which have similar growth rates while contemporaneous individuals occur together; however, Rhizocarpon geographicum is relatively short-lived in this area and hence the older portion of the curve pertains only to Rhizocarpon alpicola. The extended curve, which shows initial rapid increase in thallus diameter followed by a long interval of slower growth that averaged about 3.6 mm per century, places provisional age estimates on older Holocene moraines. Moreover, it suggests that the largest living Rhizocarpon alpicola thallus in the region, which is 480 mm in diameter, may be as much as 9,000 years old.