HR: 0800h
AN: PP41A-0177    [Abstracts]
TI: Late Holocene environmental change at three glacier-fed lakes, southern Alaska
AU: * Kaufman, D S
EM: Darrell.Kaufman@nau.edu
AF: Northern Arizona Univ, Dept of Geology, Flagstaff, AZ 86011, United States
AU: * Kaufman, D S
EM: Darrell.Kaufman@nau.edu
AF: Northern Arizona Univ, Environmental Sciences, Flagstaff, AZ 86011, United States
AU: Anderson, R S
EM: scott.anderson@nau.edu
AF: Northern Arizona Univ, Environmental Sciences, Flagstaff, AZ 86011, United States
AU: Daigle, T A
EM: tad58@NAU.EDU
AF: Northern Arizona Univ, Dept of Geology, Flagstaff, AZ 86011, United States
AU: Kathan, K M
EM: kmk238@nau.edu
AF: Northern Arizona Univ, Dept of Geology, Flagstaff, AZ 86011, United States
AU: McKay, N P
EM: npm4@nau.edu
AF: Northern Arizona Univ, Dept of Geology, Flagstaff, AZ 86011, United States
AU: Michelutti, N N
EM: michelut@biology.queensu.ca
AF: Queen's Univ, Dept of Biology, Kingston, ONT K7L 3N6, Canada
AU: Werner, A
EM: awerner@mtholyoke.edu
AF: Dept of Earth and Environment, Mount Holyoke College, South Hadley, MA 01075, United States
AB: Lake-sediment cores and glacial geomorphology were used to infer late Holocene paleoenvironmental changes at three glacier-fed lakes across southern Alaska. The lakes form a 730-km-long transect around 60N lat, and they span the transition zone between two centers of opposite surface air-temperature responses attributed to fluctuations in the strength of the Aleutian Low, the primary indicator of winter climate in the North Pacific. Sediment cores from Hallet Lake in the NE Chugach Range display varying concentrations of biogenic silica (BSi), a measure of overall lake production. A transfer function was developed to infer summer temperature from downcore BSi content. The reconstruction shows clear evidence of first millennium AD cooling, warmth from 1300-1500 AD, Little Ice Age (LIA) cooling between 1750 and 1900 AD, and recent warming beginning ca. 1900 AD. During the last 30 yr, summer temperatures were nearly 2C warmer than the reconstructed mean of the past 2 millennia. Goat Lake is near treeline in the Kenai Mountains, and about 1 km from an outlet glacier of the Harding Icefield. Pollen assemblages show increasing abundances of mountain hemlock from 700-1200 AD, which we interpret as an expansion of treeline. The expansion was terminated around 1230 AD when 10 cm of tephra was deposited in the lake. Treeline above the modern and prior to the LIA is further indicated by a 14C age of 1470 ± 85 AD on logs exposed below till at the present glacier terminus. By 1660 AD the outlet glacier thickened by 150 m where it overtopped its drainage divide and spilled meltwater into Goat Lake, which continued until around 1890 AD. Since then, hemlock pollen has increased to levels comparable to the 1200 AD peak, and the outlet glacier has retreated 1.4 km to the location of the 1470 AD logs. At Cascade Lake, sediment traps installed for 2 yr collected 77% less BSi when spring and summer temperatures were lower, suggesting that BSi flux in the lake is related to growing-season conditions. BSi was at its minimum early during the first millennium AD. It peaked around 700 AD, then decreased during the next 400 yr. BSi flux was relatively constant until the 19th century when it decreased to near-minima values, then attained its highest values of the last 2000 yr late during the 20th century. BSi and hemlock pollen are probably related more strongly to summer conditions than to winter, whereas glaciers respond to a combination of winter and summer climate variability. Late Holocene moraines in the forefields of cirque glaciers around all study lakes were mapped and dated roughly with lichenometry. The moraines delimit maximum glacier positions attained late in the 19th century, when glacier snouts generally descended less than 100 m in elevation relative to their 1950-1970 positions. This limited LIA expansion, together with tree-ring and other independent evidence for decades-long LIA summer cooling of at least 0.8çC in south-central Alaska, indicates a reduction in accumulation-season precipitation during the LIA. A simultaneous reduction in winter precipitation across southern Alaska is difficult to ascribe to a shift in the Aleutian Low pressure system because instrumental data show dipolar responses across this region. This implies a longer- term, more general climate forcing that supersedes inter-decadal variability in the Aleutian Low.
DE: 0746 Lakes (9345)
DE: 1616 Climate variability (1635, 3305, 3309, 4215, 4513)
DE: 1637 Regional climate change
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting