Optical dating of quartz sediments and accelerator mass spectrometry 14C dating of bone gelatin and moa eggshell: a comparison of age estimates for non-archaeological deposits in New Zealand

摘要

A consensus has not been reached on the validity of "old" (pre-Polynesian settlement) C-14 ages for Pacific rat bones from New Zealand. As an independent test of their validity, we have applied optical dating techniques to fossiliferous sediments at three non-archaeological sites in the North and South Islands. In this paper, we report the optical ages obtained from quartz grains and compare them with a suite of accelerator mass spectrometry (AMS) C-14 ages obtained from the bone gelatin of Pacific rats (Rattus exulans) and five species of bird (four herbivores and one omnivore). An AMS C-14 age is also reported for one sample of moa (Aves: Dinornithiformes)eggshell. All dated fossil remains were collected from known stratigraphic positions. Additional chronological control is provided by two known-age volcanic tephras at the Hukanui sites in the North Island. At the South Island site (Earthquakes #1), an infilled burrow provides independent age control, in that fossils inside the burrow should yield younger ages than the sediments and fossils outside the burrow. Bone preservation is uniformly good at all sites, as shown by surface detail, nitrogen content, and C:N ratios. In addition, amino acid profiles are consistent with those of collagen from modem Pacific rats and the laboratory rat collagen standard. Single-aliquot optical dating protocols were employed to avoid age overestimation due to incomplete bleaching of sediments before burial and to permit the identification of any post-depositional disturbance. At the Hukanui sites, C-14 ages on bird bone and eggshell agree with the optical ages for the enclosing sediments, and both chronologies are consistent with the accepted ages for the overlying Taupo Ignimbrite (c. 1850 yr BP) and the underlying Waimihia Tephra (c. 3300 yr BP). Two C-14 ages from Finsch's duck (Chenonetta finschi) bones excavated from within the Taupo Ignimbrite agree with the accepted age of the ignimbrite. In addition, a gelatin sample from a Pacific rat bone reportedly excavated from beneath the Taupo Ignimbrite has a similar amino acid profile to that of modem collagen. This measure of good bone preservation provides confidence in the C-14 age for this sample (1775+/-93 yr BP), which accords with its position beneath the ignimbrite. At Earthquakes #1, five C-14 determinations were made: three on Pacific rat, one on New Zealand pigeon.