HOLOCENE PALEOCLIMATE RECONSTRUCTIONS FROM STABLE ISOTOPE GEOCHEMISTRY IN CARBONATE LAKES FROM THE SOUTHERN YUKON TERRITORY, CANADA

We seek to test hypotheses concerning the causes of shifts in the precipitation/evaporation (P/E) balance during the late Pleistocene and Holocene at all four closed basin lakes in the upper reaches of the Yukon River watershed. ƒÔ2H and ƒÔ18O measurements on modern surface waters across the region indicate the proposed sites are sensitive to changes in the moisture balance. Initial oxygen-18 analyses from surface sediment authigenic carbonate suggest that it records lake-water ƒÔ18O. Previous work based on pollen studies provides a broad paleoclimatic picture for this period. However, significant paleoclimatic questions remain unanswered, and high-resolution (< century-scale) climate reconstructions have not been attempted.

We propose to reconstruct the climate history of the Yukon River watershed by applying an integrated methodology that combines core transects, sedimentology and sediment magnetic characteristics, elemental and isotopic geochemistry, and pollen. We will use varve counts, 137Cs, 210Pb, and tephrochronology for this time period. We will use AMS 14C measurements on carefully screened organic constituents (macrofossils and pollen) and paleomagnetic studies for the whole length of the record.

These records are needed to address the following three broad hypotheses:

(i) During the late Pleistocene and glacial to interglacial transition the P-E balance increased across the Yukon River watershed resulting in higher lake-levels.

(ii) A prolonged low lake stand between 13.5 and 12.5 cal ka B.P., interrupted a long-term increase in lake levels and is a function of higher summer insulation and subsequent increased evaporation that outweighed any increase in precipitation. Two modern synoptic patterns (Edwards et al., in press) can be tested as analogues for a warm-dry or cool-dry scenario.

(iii) Significant sub-decadal to century-scale P-E variability occurred during the Holocene across the region as suggested by the results from our ongoing work in the central Brooks Range. These climatic fluctuations are superimposed on the more general millennial-scale increase in the moisture balance of the region. If changes relate to the magnitude of westerly moisture sources, then variations should be well represented in lakes in southwestern Yukon Territory.