Radiocarbon Dates

Radiocarbon dating has provided an age framework for the fossils collected in the caves of Southeast Alaska. Fossils from most sites studied date to the early postglacial interval (12,000-8,000 years BP). The intervals of the Last Glacial Maximum and preceding period are covered by fossils from On Your Knees Cave. Radiocarbon dates on bone beyond 40,000-45,000 years BP are considered limitless dates (no measurable Carbon-14), so it is difficult to work out a chronology beyond 40,000 years ago. Much of the sediment from On Your Knees Cave and Devil's Canopy Cave dates to beyond this radiocarbon limit.

The chart below provides a timeline for the best-documented species recovered from Prince of Wales Island. Actual dates are indicated by stars, while lines indicate when species are assumed to be present based on associations. Dashed lines indicate species that may have survived the glacial maximum in the region, but there is no conclusive evidence. The only rodent to be radiocarbon dated is marmot. Long-tailed vole is the only microtine living on Prince of Wales Island today and the only one found in postglacial deposits. Hoary marmot, heather vole, and brown lemming are found in the older sediments at On Your Knees Cave, but these species were apparently killed off by the cold conditions of the Last Glacial Maximum. Lemmings became more abundant relative to other rodents as the glacial maximum approached, suggesting the expansion of tundra, but they still eventually died off.

The dates on the chart below are those reported from laboratories at the University of Arizona, University of Colorado, and Stafford Labs, and they have not been corrected in any way. Animals with a marine diet are about 600 years younger than indicated (one position to the right on the chart) because the ocean reservoir contains some old carbon. No conversion has been made from radiocarbon years to calendar years (which would make most of the dates about 10% older than indicated).

Timeline for selected species from Prince of Wales Island.

                   --------Middle Wisconsin-------||---Glacial Maximum---||-------Postglacial------|

14C Age (KY BP)   40        35        30        25        20        15        10         5         0
                   |         |         |         |         |         |         |         |         |
Species
Brown Bear         ---------*------*----------*------- - - - - - - - - - -****-**----*
Black Bear         *****-*-------------*--*--------- - - - - - - - - - - - -*******---**---*--------

Arctic Fox         --------*-*-----*----------------------*--*------------*-*
Red Fox                                          ? --*------*------------------*-- ?
Wolf                                                                                            ? --

River Otter        -------------------------------- ?                        ? ---*--*-*------*-*---
Ringed Seal                                        ****-*-********----*-*
Harbor Seal        -----*--------------------*------------*--*--------------------------------------
Steller's Sea Lion -*------------------------------------*-*---------------------------*------------

Saiga (or goat)    ------------*---*--------------- ?
Caribou	        ----*-*--------------------*------- - - - - - - - - - ---*-*--
Black-tailed Deer                                                                --**---**-----*--*-

Hoary Marmot       *------------**---------**-------*
Heather Vole       ----------------------------------
Brown Lemming      -----------------------------------
Long-tailed Vole   --------------------------------                         ------------------------

Man & Artifacts                                                               *-*------*-----*------

Birds              *-------------*-------------*----------*----------------------*------------------
Fishes             ---------------------------------------------------------------*--*-*------------

All 14C Dates      ***** * *** *** *   *  ******   **** **********    * * **************** * **** * 

                   |         |         |         |         |         |         |         |         |
14C Age (KY BP)   40        35        30        25        20        15        10         5         0

                   --------Middle Wisconsin-------||---Glacial Maximum---||-------Postglacial------|

At the bottom of the chart above is a row of stars indicating all of the radiocarbon dates obtained from all sites on Prince of Wales Island. Most of the date gaps (i.e. 31,000 and 25,000 years BP) appear to be a random result of small numbers of samples rather than actual gaps in the record. The one possible exception is the gap between 17,100 and 14,500 years BP, which is late in the glacial maximum. Ringed seals are a climatic indicator of sea ice, and their remains are quite abundant in On Your Knees Cave. Twenty five samples of ringed seal have been dated so far (more than any other species from a single site) in order to determine 1) the time range of temperatures cold enough to create sea ice, and 2) whether On Your Knees Cave was available as a carnivore den during the entire glacial maximum. This gap of 2,600 appears to be significant, suggesting that the cave may have been covered by a snowfield or glacier during this interval.

All radiocarbon dates obtained thus far from parts of Southeast Alaska other than Prince of Wales Island are postglacial in age. Some of these dates are provided in the individual site descriptions. The ultimate goal of this research is to establish detailed timelines for many of the islands, as well as the mainland, so that colonization histories can be compared and glacial refugia can be identified.

   
© 2002 by Timothy H. Heaton