HR: 1340h
AN: PP33B-0930    [Abstracts]
TI: A 13,000-yr high-resolution record of Rocky Mountain Holocene climate change
AU: * Stone, J R
EM: jstone@unlserve.unl.edu
AF: University of Nebraska-Lincoln, 214 Bessey Hall Department of Geosciences, Lincoln, NE 68588 United States
AU: Fritz, S C
EM: sfritz2@unl.edu
AF: University of Nebraska-Lincoln, 214 Bessey Hall Department of Geosciences, Lincoln, NE 68588 United States
AB: A high-resolution study of 13,000 years of climatic changes recovered from Foy Lake, Montana, sampled continuously at 5 - 10 yr intervals throughout the record, demonstrates dramatic changes in fossil diatom assemblages throughout the lake's history. The power of examining lake records at this resolution is detailed by many sharp transitions in fossil diatom assemblages and multiple scales of climate variability imprinted on the sediment record. At 11,550 ybp, near the termination of the Younger Dryas, low-nutrient species give way to higher-nutrient, planktonic diatom species. At roughly 10,000 ybp, alkalinity increases dramatically, resulting in a major assemblage shift and $\sim$1,200 years where the diatom record is repeatedly dissolved, likely because of enhanced alkalinity during extremely dry intervals. From ca 8,800 - 7,100 ybp, the fossil diatom assemblage shows cycles between benthic and tychoplankton species, indicative of cycles of lake-level change in a relatively shallow lake. At $\sim$8,200 ybp, a 200-yr period of dominance by planktonic diatoms suggests a pronounced increase in lake level in the midst of several thousand years of otherwise very dry conditions. Decadal sampling throughout the mid-Holocene indicates that composition of the diatom assemblage remained constant, but high-frequency changes and sharp transitions in dominant species were common. For about 1,000 years, ending at approximately 6,100 ybp, a sudden increase in the percentage of planktonic diatoms suggests a moderate increase in effective moisture, followed by a $\sim$2000 yr period of tychoplankton and benthic species indicating increased alkalinity and reduced moisture. Planktonic diatoms again dominate the record from ca 4,400 - 2,800 ybp. These patterns of change indicate that mid-Holocene climate was neither exceedingly stable nor consistently dry in the northeastern Rocky Mountain region. From 2,800 - 1,400 ybp, the diatom record, dominated by benthic flora, is characterized by long periods of heavy dissolution suggesting higher alkalinity during an intensely dry period. A dramatic transition occurs at 1,400 ybp, when the diatom record becomes overwhelmed by a single planktonic diatom species, which dominates throughout much of the remainder of the late Holocene and from which we infer a modern maximum in effective moisture.
DE: 3344 Paleoclimatology
DE: 4215 Climate and interannual variability (3309)
DE: 4239 Limnology
DE: 3030 Micropaleontology
DE: 1812 Drought
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting