2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 1
Presentation Time: 9:00 AM-6:00 PM


SIDLE, W.C., National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 26 West MLK Dr, MS 679, Cincinnati, OH 45268 and CVETIC, V., Groundwater Research Co, PO Box 132A, Port Angeles, WA 98363, sidle.william@epa.gov

Noble gas temperature proxies are examined from 52 springs in the Olympic Mountains, Washington. Groundwater flows from seeps to pooled springs at <0.1 L s-1 - 2.5 L s-1 in the Elwha watershed (≈692 km2). About 85% of sampled springs issue from confined fracture reservoirs preserving noble gas temperatures (NGT) from recharge areas. Closed system equilibration modeling exceeds goodness of fit testing (χ 2 = 37.8; p >0.010) among other treatment methods for trapped excess air during groundwater infiltration. Calculated mean recharge air temperatures (MRAT) range from 3.9 ± 1.0 C to 12.4 ± 0.6 C. About 17% of the NGT’s indicate some past air temperatures to be warmer during the Holocene than present climate trends in the region. Apparent ages of recharge events that equilibrated with these surface air temperatures were calculated from cosmogenic 39Ar, 14C, and fissiogenic 85Kr, 3H, and its progeny 3H-3He. The multi-tracer ages of these springs record recharge events from 5 ± 3 yr BP (85Kr) to 8510 ± 420 yr BP (14C). Fourier transform time-series modeling supports at least two pronounced regional warming trends with significant centennial-scale variability during the Holocene. Where decadal variability can be discerned among the NGT proxies, its past millennium variability exceeds that observed from the most recent half-century of weather instrument records within subalpine areas of the Olympic Peninsula.