The sediment record of Crawford Lake, Ontario and Derby Lake, Michigan: Anthropogenic modification of diatom communities and paleoclimate reconstructions.

摘要

Annually laminated lake sediments provide detailed records of climatic variability and human activity in a watershed. Laminated lake sediments from Derby Lake, Michigan have been used to reconstruct Holocene climate variability for the last 8,600 years. Using a detailed grayscale record, spectral analysis shows distinct periods of variability centered at ∼250 and 44 years. These periodicities are similar to observed and inferred fluctuations in solar variability, and indicate that solar-climate linkages may be important factors governing regional climate.;A sediment record from Crawford Lake, Ontario, Canada, has been used to reconstruct human activity in the watershed for the past ∼1000 years. The Crawford Lake watershed has long been of interest to archaeologists and anthropologists due to settlement by native Iroquois people in the mid 13 th century. Detailed fossil diatom assemblages reconstructed from Crawford Lake sediment show that diatom assemblages were extremely susceptible to human activity in the watershed. The diatom flora changed simultaneously with the arrival of humans in the watershed, despite very low numbers of people. The Crawford Lake record is unique in that nearly 400 years of lake recovery separate Iroquoian abandonment of the watershed and resettlement by Canadians in 1867 AD. The diatom flora, combined with a detailed geochemical record, show that despite decreased nutrient availability during this recovery phase, diatoms in Crawford Lake did not return to a pre-disturbance assemblage. Rather, diatoms maintained a meso-eutrophic assemblage throughout the recovery phase. Diatom community structure may be related to hysteresis, or non-linear behavior. The diatom community may have shifted to an alternative stable state following Iroquois village construction, thereby making recovery to pristine assemblages difficult or not possible.