On the north slope of the Brooks Range, Alaska, at least 70 cirque glaciers of <2 km2 occur within a 1500 km2 area centered about the Atigun River drainage basin (Fig. 1). All are now below the regional snow-line and wasting away rapidly with mass balances on the order of −1 m per year (Reference Ellis and CalkinEllis and Calkin, 1979). The Neoglacial deposits associated with 40 of these glaciers have been lichenometrically mapped and all but those of “Golden Eagle” glacier (unofficial name) display bouldery surfaces nearly barren of vegetation at the receding margins. At the gently sloping toe of “Golden Eagle” glacier (Fig. 2), retreat has exposed an area of about 800 m2 bearing undisturbed, non-sorted patterned ground, and in-situ patches of unidentified dead mosses partially enveloping hundreds of lichen-covered cobbles and boulders ranging to 1.5 m across. The lichen population is dominated by the long-living, green crustose Rhizocarpon geographicum s.l. In addition, some R. inarense/eupetraeoides, Lecidea, and unidentified gray crustose lichens were recorded. However, the black fruticose and foliose taxa such as Alectoria and Umbilicaria, common in the normal regional lichen cover, are conspicuously absent. The arrangement of the crustose lichens suggests that the hosting stones are largely undisturbed by the glacier that overrode them; however, there is a scattering (c. 20%) of boulders or cobbles set down from supraglacial or englacial positions. These latter are distinguished by their more angular and predominantly lichen-free character (Fig. 3).
Fig. 1. (a) and (b): location maps of “Golden Eagle” glacier (unofficial name); and (c): the site of relict patterned ground, mosses, and lichen-covered boulders in the east-central Brooks Range, Alaska. TAPS indicates the Trans-Alaska Pipeline System.
Fig. 2. Oblique aerial view southward of “Golden Eagle” glacier. The glacier is 1200 m long. Neoglacial limit is marked by the pronounced lateral moraine on the west side (right) of glacier and by dashed line. Arrow indicates location of emergent relict zone, radiocarbon-dated moss, and position of camera for Figure 3.
Fig. 3. View northward of the preserved patterned ground and boulders from the toe of “Golden Eagle” glacier. Scale is shown by P. E. Calkin at right center. Light-colored, angular boulders were set down on preserved terrain from englacial and supraglacial positions during recent recessional phase of “Golden Eagle” glacier.
Near the ice margin, the relict lichens are brightly colored, morphologically undamaged, and visually indistinguishable from those of the same species found elsewhere in the region. However, with increasing distance from the ice margin, the thalli deteriorate and colors become progressively bleached. Beyond 40 m of the glacier toe the lichens disappear. The emergence of patterned ground (Reference SmithSmith, 1961; Reference SwinzowSwinzow, 1962; Reference FalconerFalconer, 1966; Reference Black, Fahey and ThompsonBlack, [c1973]) and of vegetation (Reference ArnoldArnold, 1965; Reference Lowden and BlakeLowden and Blake, 1970; Reference CollinsCollins, 1976) from beneath glaciers is well documented in polar areas. Furthermore, on Baffin Island, emergent mosses have been dated at 350±75 years B.P. (1–1204) (Reference FalconerFalconer, 1966). Emergent sites with lichens indistinguishable from living forms are less common (see Reference KoernerKoerner, 1980), but include those reported from the Canadian Arctic islands (Reference Beschel and MüllerBeschel, 1961; Reference FalconerFalconer, 1966; Reference HarrisonHarrison, 1966). Lichens have also been exposed in a subglacial tunnel in Greenland (Reference HiltyHilty, 1959; Reference GoldthwaitGoldthwait, 1960, Reference Goldthwait1961).
At the “Golden Eagle” glacier site, we have obtained a radiocarbon age of 1120 ± 180 years B.P. (BGS-614) for the dead mosses surrounding the undisturbed lichen-covered boulders. The sample was obtained within 1 m of the July 1979 ice margin (Fig. 1c). This dates a Neoglacial advance across the presently deglaciated site. The length of the preceding ice-free episode is inferred from lichenometric measurements on the emergent boulders. Crustose lichen cover reached 60% on some boulders, and included a wide range of thallus diameters. The largest Rhizocarpon geographicum s.l. had well-defined thalli with maximum diameters ranging from 62 to 72 mm. A growth curve developed for the central Brooks Range (Reference Calkin and EllisCalkin and Ellis, 1980) allows this diameter range to be converted to a lichenometric age of 2000±500 years, suggesting a minimum duration for an ice-free interval prior to the 1120 years B.P. advance. This advance overran and preserved the patterned ground and vegetation under apparently cold-based conditions.
More recent cirque-glacier expansions have been recorded elsewhere in the Atigun drainage basin, but at “Golden Eagle” glacier only the last major advance dated at ~320 years B.P. (Reference Calkin and EllisCalkin and Ellis, 1980) is evident, as it formed the terminal Neoglacial deposit (Figs 1c and 2). Based on glacier reconstruction from this terminal deposit, the glacier was c. 60 m thick over the site, and had an area of 0.6 km2, 33% larger than at present. The equilibrium-line altitude (ELA) was depressed to 1860 m and the snout extended 250 m farther than present during this A.D. 1500–1600 ± 100 year advance. Retreat from the maximum was not uniform over this interval, as the lichen pattern on the lateral moraine suggests most rapid recession commenced after A.D. 1750. The present ELA is estimated to be between 2000–2 100 m at “Golden Eagle” glacier.
The relict patterned ground does not appear to be in equilibrium with the present periglacial regime. Stone-fronted terraces about 0.4 m high and 1.5 m broad are being exposed along the receding snout at this site. Although they resemble terraces located beyond the Neoglacial terminal moraine, these deposits are undergoing more active frost heaving.
We are unable to explain the reason(s) why this relict site with patterned ground, mosses, and lichen-bearing boulders was preserved during glacial expansion. However, the glacier may have initially advanced through the mechanism of accumulation rather than by active down-hill movement (which would normally remove evidence of former vegetation or patterned ground). The evidence is available only because the overrun surface on the east side of “Golden Eagle” glacier was not buried under a continuous sheet of drift during the most recent recession (Fig. 2). “Golden Eagle” glacier is the only known glacier within the region where this situation exists.
The explanation for the disappearance of emergent lichens in the down-valley direction across this relict zone is also unknown but may be related to periglacial re-sorting and substrate instability. It is rarely possible to see at a glance whether a crustose lichen is alive or dead (Reference SmithSmith, 1962); however, lichens are known to have a resistance to freezing (Reference BecquerelBecquerel, 1951; Reference LangeLange, 1972; Reference LarsonLarson, 1978) as well as other environmental extremes found in polar areas ( Reference GannutzGannutz, 1970; Reference Kappen and LangeKappen and Lange, 1970, Reference Kappen and Lange1972; Reference Lange, Kappen and LlanoLange and Kappen, 1972), and to endure long periods of inactivity (semi-dormant) in a frozen state. Therefore, we presume that the preserved lichens may have been killed due to unfavorable conditions for respiration and photosynthesis upon (1) initial submergence, (2) prolonged burial, or (3) emergence (Reference Beschel and MüllerBeschel, 1961; Reference Kappen, Ahmadjian and HaleKappen, 1973). They are now wasting away as duration of exposure increases.
Acknowledgements
This work was supported by U.S. National Science Foundation Grant DPP7819982. J. R. Graham of Placid Oil Company provided partial logistical support.
