Current Research in the Pleistocene, vol. 12, pp. 92-95 (1995).
Timothy H. Heaton
Department of Earth Sciences
University of South Dakota
Vermillion, SD 57069
The extensive karstlands of Prince of Wales Island in the southern Alexander Archipelago have yielded early post-glacial vertebrate deposits in several different habitats (Heaton and Grady 1992, 1993; Heaton and Love 1995). The dominant mammalian species are grizzly bear (Ursus arctos) and black bear (Ursus americanus) dating to at least 12,300 yr B.P. Several of the caves are located in glacial valleys or cirques that were disrupted by glaciation. Radiocarbon dates on these remains provide useful age limits for glacial retreat and post-glacial colonization but offer little hope of elucidating biogeographic patterns during or before the last glaciation. But two caves have recently been found to contain vertebrate remains that predate the late Wisconsin glacial maximum (21,000-16,000 yr B.P.; Blaise et al. 1990), and these provide the first glimpses of the Archipelago's more distant biotic past.
During the 1994 field season the proximal end of a femur (including half the shaft) of a large bear was collected from On Your Knees Cave and was radiocarbon dated to 35,365 ñ 800 yr B.P. (AA-15227). This femur appears to be from a grizzly bear and is nearly as large as the femur of the grizzly giant recovered from El Capitan Cave (Heaton and Grady 1992, 1993). On Your Knees Cave consists of two small passages, each extending about 40 m into the hillside. The femur was found eroding out of the shallow silt floor 10 m inside the entrance. Found nearby in an organic surface layer were a canine fragment and broken calcaneum of a smaller bear (probably black bear), the nearly complete skeleton of a river otter (Lutra canadensis), a bird femur, and an abundance of ground fish bone. Since these bones were not buried in silt, they may be Holocene.
Deeper in the same passageway, 25 m from the cave entrance, other bear bones were found eroding from silt deposits on the walls of the tight crawlway. The distal end of a tibia and a first, second, and third phalanx were collected, all of which appear to be from a small black bear. The tibia was radiocarbon dated to 41,600 ñ 1,500 yr B.P. (AA-16831). This age is very close to the limit of radiocarbon dating and may not be finite, so the lab issued an alternate date of >39,100 yr B.P.
The association of grizzly and black bear, river otter, and fish is identical to the El Capitan Cave fauna (Heaton and Grady 1992, 1993), although at On Your Knees Cave they may not all be contemporaneous. The apparent coexistence of grizzly and black bears both before and after the peak of late Wisconsin glaciation is highly significant. Prior to 1990 grizzly bears were thought never to have inhabited Prince of Wales Island since they do not occur there today (Klein 1965). It now appears that both species either colonized the island twice or survived on it through the peak of glaciation. The high ë13C value of the femur (AA-15227) suggests that this bear was more of a fish eater than its post-glacial counterparts (Heaton 1995). If nunataks or coastal refugia were present and fish were available as food, then there is no reason to discount the possibility that bears survived the glacial maximum in the Alexander Archipelago. Evidence for refugia during the height of Wisconsin glaciation has been found on the Queen Charlotte Islands, 250 km to the south (Clague 1989; Mathewes 1989; Warner et al. 1982).
On Your Knees Cave is located in the extreme northwest corner of Prince of Wales Island on a peninsula called Protection Head. This site is adjacent to Sumner Strait, a major glacier-cut channel through the archipelago. Protection head is not in the path of any former valley glaciers but may have been overridden during the glacial maximum. The cave was likely a shallow den site 42,000 to 35,000 years ago and has not been disturbed by extensive erosion or deposition since then. Cave sediments hold the only clue to the glacial history of the peninsula's karstland, and they have not yet been studied.
A vertebrate microfauna of even older age has been found in Devil's Canopy Cave, located 35 km southeast of On Your Knees Cave in a narrow neck of land between El Capitan Passage and Whale Passage. This cave contains a stream and is part of an extensive hydrologic cavern complex. Rodent remains have been recovered from a 1 m thick silt deposit located in a dry passage 5 m above the present stream level. In 1992 a marmot (Marmota) incisor was found which is beyond the age of radiocarbon dating, or >44,500 yr B.P. (AA-8871A). Marmots do not currently live on Prince of Wales Island and have not been found in post-glacial deposits. In 1994, 100 kg of slumping silt were wet screened in the cave and searched for bone. Recovered elements include a marmot molar, a lower jaw of deer mouse (Peromyscus), a number of skeletal elements of small rodents, and some insect and plant fragments. The marmot molar is smaller than the hoary marmot (M. caligata) that lives on the Alaska mainland but is similar in size to the yellow-bellied marmot (M. flaviventris) of the western United States. Further investigation may reveal fossiliferous units within the undisturbed portion of the deposit.
The silt deposit in Devil's Canopy Cave is of glacial origin (containing many mineral types besides calcite) and apparently filled the cave passage prior to subsequent dissolution and downcutting by the cave stream. This part of the island was likely overridden by glaciers, but, like Protection Head, was not in a position to experience deep glacial scouring that would obliterate shallow cave deposits. These two sites, as well as refugia on the Queen Charlotte Islands (Clague 1989; Mathewes 1989; Warner et al. 1982), suggest that low elevation ridges are the best places to search for fossil deposits predating the late Wisconsin glacial maximum.
Kevin Allred took me to On Your Knees Cave and assisted with sample collection. James Baichtal introduced me to Devil's Canopy Cave. David Love assisted with screen washing and sediment sorting. The radiocarbon dates were funded by Tongass National Forest.
Clague, J. J. 1989 Quaternary geology of the Queen Charlotte Islands. In The Outer Shores, edited by Geoffrey G. E. Scudder and Nicholas Gessler, pp. 65-74.
Heaton, T. H. 1995 Interpretation of ë13C values from vertebrate remains of the Alexander Archipelago, southeast Alaska. Current Research in the Pleistocene 12 (this volume).
Heaton, T. H., and F. Grady. 1992 Preliminary report on the fossil bears of El Capitan Cave, Prince of Wales Island, Alaska. Current Research in the Pleistocene 9:97-99.
Heaton, T. H., and F. Grady. 1993 Fossil grizzly bears (Ursus arctos) from Prince of Wales Island, Alaska, offer new insights into animal dispersal, interspecific competition, and age of deglaciation. Current Research in the Pleistocene 10:98-100.
Heaton, T. H., and D. Love. 1995 The 1994 excavation of a Quaternary vertebrate fossil deposit from Bumper Cave, Prince of Wales Island, Alaska. Geological Society of America Abstracts with Programs 27(3):57.
Klein, D. R. 1965 Postglacial distribution patterns of mammals in the southern coastal regions of Alaska. Arctic 18:7-20.
Mathewes, R. W. 1989 The Queen Charlotte Islands refugium: a paleoecological perspective. In Quaternary Geology of Canada and Greenland, edited by Robert J. Fulton, pp. 486-491.
Warner, B. G., R. W. Mathewes, and J. J. Clague 1982 Ice-free conditions on the Queen Charlotte Islands, British Columbia, at the height of Late Wisconsin Glaciation. Science 218:675-677.