PROXY CLIMATE TRENDS IN THE BLUE RIDGE OF VIRGINIA, 0-45 KA, AND COMPARISON TO THE GREENLAND ICE CORE RECORD

Pollen and ¹⁴C evidence has enabled us to create a composite climate history for the Blue Ridge of Virginia that extends to 45,000 years BP. Our samples: 1) were collected from a small geographic area- the upper part of the Rapidan River drainage, 2) were ordered chronologically by independent ¹⁴C AMS dates, and 3) were calibrated for prehistoric ¹⁴C atmospheric flux (ref. 1). Pollen samples, recovered from outcrops and shallow core evidence(Shenandoah NP), were used to estimate the probable series of forests that occupied this part of the Blue Ridge from 0-45 ka. Each sample yielded a time-specific 'snapshot' of vegetation that was then time-linked into a composite serial record. Using modern forest analogues, spanning from 30⁰ N to 52⁰ N latitude along the U.S. Atlantic Coast, modern tree distributions were compared to presence-absence and relative abundance pollen data of the same arboreal taxa, to estimate prehistoric forest zones by pollen analysis. Modern forest zones, in turn, appear to be limited by thresholds of Mean Annual Temperature (MAT), which makes these fossil samples useful indicators of proxy climate. Our model suggests that Blue Ridge vegetation appears to have responded largely in phase with large-scale climate shifts in the Northern Hemisphere. This climate model for the Blue Ridge exhibits strong similarity to estimated paleotemperature sequences in the Greenland GRIP ice core record (ref. 2), in trends but not in absolute values. Our current estimates suggest that the minimum range of MATs that were experienced in this study area over the past 45 ky spanned from 1.5⁰ C to 13⁰ C. We think this Blue Ridge record, although still fragmentary, extends back to approximately Interstadial 12 (IS12) of the GRIP chronology (ref. 3). This record includes evidence of ‘Younger Dryas’-aged cooling, and an overall thermal minimum trend in MATs between 21 ky BP and 25 ky BP, suggesting a potential temporal placement for the Northern Hemisphere last glacial maximum (LGM).

References

1. Stuiver, M., and Reimer, P.J., 1993. Extended ¹⁴C database and revised CALIB radiocarbon calibration program. Radiocarbon, 35:215-230.

2. Jouzel, J., et al., 1997. Validity of the temperature reconstruction from water isotopes in ice cores. Journal of Geophysical Research, 102(C12):26,471—26,487.

3. Dansgaard, W., et al., 1993. Evidence for general instability of past climate from a 250-kyr ice-core record. Nature, 364:218-220.