Interpretation of del-13C Values from Vertebrate Remains of the Alexander Archipelago, Southeast Alaska

Current Research in the Pleistocene, vol. 12, pp. 95-97 (1995).

Timothy H. Heaton
Department of Earth Sciences
University of South Dakota
Vermillion, SD 57069

Late Pleistocene vertebrate remains have been found and radiocarbon dated from seven caves on Prince of Wales Island and several smaller islands nearby. Twenty one dated samples have been provided with 13C corrections, and these stable isotope values offer insights into the ecology and diet of the island's prehistoric vertebrates. At El Capitan Cave a large volume of ground fish bone (flatfish, sculpin, and at least 20 other marine species) was found associated with remains of grizzly bears (Ursus arctos), black bears (Ursus americanus), river otters (Lutra canadensis), shrews, bats, and several species of rodents (Heaton and Grady 1992, 1993). The association of bears, otters, and fish was also found at On Your Knees Cave, though possibly of different ages (Heaton, 1995). Both appear to be den sites. A higher elevation grizzly bear den in Bumper Cave lacks fish, otter, and black bear (Heaton and Love 1995).

It has been established that different primary producers select carbon isotopes from their environment in different ratios, and that these isotopic signatures are passed on, with predictable modifications, to higher levels in the food web. Different body tissues concentrate 13C in different proportions, but the concentration remains the same or increases only slightly in higher trophic levels (Tieszen and Boutton 1988). The Alexander Archipelago closely approximates the simple two food source model outlined by Fry and Sherr (1984). Terrestrial plants of northern latitudes are virtually all of the C3 photosynthetic type and yield del-13C values averaging -27% (Tieszen and Boutton 1988), with a del-13C enrichment correction to about -21% for the bone collagen of animals getting their food from terrestrial plant sources. A charcoal sample from El Capitan Cave provided a del-13C value of -23.0%, which is at the high end of the normal range for C3 wood. By contrast, three early Holocene fish bone samples from El Capitan Cave have yielded del-13C values of -16.1%, -13.2%, and -11.1% (mean of -13.5%). These high del-13C values are typical of marine fishes, though the food web by which they obtain these values is complex (Fry and Sherr 1984). Since these values are based on bone collagen samples, similar or slightly higher values would be expected in the bone collagen of any predator that uses them as its primary food source.

The purpose of this analysis is to determine the diet of prehistoric island mammals and establish which species is responsible for the extensive ground fish remains in coastal caves. Based on the analysis above, expected del-13C values in the bone collagen of mammals would be approximately as follows: -21% for exclusive terrestrial plant feeders, -13% for exclusive marine fish feeders, and an intermediate value for mixed feeders.

Remains of four post-glacial black bears have been radiocarbon dated, all from El Capitan Cave (Heaton and Grady 1992, 1993). The del-13C values range from -18.7% to -22.1% with a mean of -20.5%. A middle Wisconsin (or older) black bear from On Your Knees Cave (Heaton 1995) has a similar del-13C value of -20.7%. In spite of the coastal location of these caves, such low values suggest that the black bears obtained their food almost exclusively from terrestrial sources. These values match closely with those that Bocherens et al. (1994) found in modern Alaska black bears and several species of fossil bears interpreted as being C3 plant eaters, thus strengthening this conclusion.

The consistent terrestrial signature of fossil black bear del-13C values found in this study is curious because modern black bears on Prince of Wales Island have frequently been seen feeding on marine fishes. This is an unusual habit for black bears, and it may not have begun until after grizzly bears disappeared from the island. A stable isotope study of modern black bears on Prince of Wales island is needed to confirm this proposed dietary shift.

Seven early post-glacial grizzly bears have been dated from Prince of Wales Island (Heaton and Grady 1993, Heaton and Love 1995): two from El Capitan Cave (coastal), four from Bumper Cave (subalpine), and one from Blowing in the Wind Cave (alpine). The del-13C values range from -16.8% to -19.5% with a mean of -18.1%. The -19.5% value came from the only sample based on tooth rather than bone. Bocherens et al. (1994) found tooth collagen samples in bears to have del-13C values up to 1.1% lower than bone collagen samples. The lowest of the bone del-13C values is -18.5%, which is from a juvenile, and Bocherens et al. (1994) found juvenile del-13C values to be lower than those of adults. del-13C values of the five adult bone samples ranges from -16.8% to -18.3% with a mean of -17.8%. These values are all higher than the highest value for black bears (-18.7%), and they suggest that the grizzly bears had a marine component to their diet. Curiously, grizzly remains recovered from caves higher in elevation and farther from the ocean do not have lower del-13C values than their coastal counterparts as might be expected.

A ninth grizzly bear, recovered from On Your Knees Cave (Heaton 1995), dates to the middle of the Wisconsin glacial and has a del-13C value of -15.9%, the highest of any bear in this study. Plant food may have been scarce during the glacial epoch, and a predominantly fish diet would account for this high value.

A river otter bone from El Capitan Cave has been radiocarbon dated, and its del-13C value is -10.0%, the highest of the 21 samples. This suggests a predominantly marine diet with a del-13C trophic level enrichment in addition. This data, combined with the unique association of otter and fish bone, suggests that the otters are responsible for the extensive fish deposits in El Capitan and On Your Knees Caves. This conclusion is consistent in detail with studies of river otter diets and den sites in the Alexander Archipelago (Larsen 1984).

A marmot incisor from Devil's Canopy Cave, reported by Heaton (1995), has a del-13C value of -23.7%. This indicates a terrestrial plant diet, as would be expected for this species. The unusually low del-13C value might be due to the tooth sample, though continuously-growing teeth like rodent incisors are generally not depleted in 13C as are rooted teeth (Bocherens et al. 1994). Since the age of this tooth is beyond the range of radiocarbon dating, no specific environmental interpretation can be made.

A deer humerus from Nautilus Cave (Heceta Island), radiocarbon dated at 8,180 +/- 70 yr B.P. (AA-10574), has a del-13C value of -25.2%, by far the lowest of the 21 samples. This value is so low that the deer likely obtained its plant food from a closed-canopy forest. In such forests much of the CO2 taken in by vegetation low in the canopy is recycled from the forest itself, and therefore the carbon is repeatedly depleted in 13C by the C3 plants (Tieszen and Boutton 1988). This level of depletion suggests that the dense rain forest conditions of the Alexander Archipelago were established prior to 8,000 years ago.

This paper benefitted greatly from discussions with Larry L. Tieszen. Kevin Allred and James Baichtal discovered most of the vertebrate remains reported herein. Becky Wigen identified the fish remains. Isotope analyses were conducted by the University of Arizona and Beta Analytic Inc. Funding for the analyses was provided by Tongass National Forest and the National Geographic Society.

References Cited

Bocherens, H., M. Fizet, and A. Mariotti. 1994 Diet, physiology and ecology of fossil mammals as inferred from stable carbon and nitrogen isotope biogeochemistry: implications for Pleistocene bears. Paleogeography, Paleoclimatology, Paleoecology 107:213-225.

Fry, B., and E. B. Sherr. 1984 del-13C measurements as indicators of carbon flow in marine and freshwater ecosystems. Contributions in Marine Science 27:13-47. Reprinted (1988) in Stable Isotopes in Ecological Research, edited by P. W. Rundel, J. R. Ehleringer, and K. A. Nagy, pp. 196-229.

Heaton, T. H. 1995 Middle Wisconsin bear and rodent remains discovered on Prince of Wales Island, 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.

Larsen, D. N. 1984 Feeding habits of river otters in coastal southeastern Alaska. Journal of Wildlife Management 48:1446-1452.

Tieszen, L. L., and T. W. Boutton. 1988 Stable carbon isotopes terrestrial ecosystem research. In Stable Isotopes in Ecological Research, edited by P. W. Rundel, J. R. Ehleringer, and K. A. Nagy, pp. 167-195.

Timothy H. Heaton: E-mail, Home page, Phone (605) 677-6122, FAX (605) 677-6121