2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 11
Presentation Time: 10:45 AM


EKDAHL, Erik, Department of Geosciences, Univ of Nebraska-Lincoln, 214 Bessey Hall, Lincoln, NE 68588-0340, TERANES, Jane, Scripps Institution of Oceanography, La Jolla, CA 92093 and STOERMER, Eugene, School of Natural Resources, Univ of Michigan, Ann Arbor, MI 48109, eekdahl@umich.edu

Two cores from meromictic Crawford Lake, Ontario, Canada record periods of human settlement and disturbance in annually laminated sediments. Geochemical, palynological, and biological records were used to describe changing settlement and land-use patterns within the watershed. The first settlement period, marked by the sudden appearance of Zea (maize) pollen in core sediments, was related to Iroquois agricultural communities dated to the mid-13th century with a chronology constructed using 25 AMS 14C dates and varve counts.

Total organic carbon (TOC) and CaCO3 mass accumulation rates (MARs), with C/N, δ13C from bulk carbonate, and diatom stratigraphies were used to describe the impact of human settlement upon the Crawford Lake ecosystem. TOC and CaCO3 MARs increased immediately following Iroquois settlement. C/N ratios decreased, indicating elevated input of algal organic material to lake sediments. Carbon isotopes increased from –6‰ to –1‰ during maximum Iroquois construction. Pristine diatom communities were altered within only a few years of human occupation, and increased towards assemblages indicative of higher water nutrient concentrations as Iroquois population increased. These results are interpreted as the consequence of increased nutrient input to Crawford Lake resulting from slash-and-burn agricultural practices and the construction of Iroquois shelters only a few hundred meters from the lake. Increased nutrient availability resulted in eutrophication of the lake and permanent bottom water anoxia.

Following Iroquois abandonment, geochemical parameters returned topre-disturbance levels. However, diatom assemblages remained altered, and C/N values remained lower than pristine conditions. Euro-Canadians began farming in the vicinity of Crawford Lake in the mid-1800s. Land use changes resulted in a second period of increased nutrient input to the lake, and a second period of eutrophication. However, carbon isotopes did not respond to Canadian eutrophication, and diatom communities were not altered from post-Iroquoian assemblages. These results suggest that initial perturbations are most important in altering biologic communities from pristine background conditions. Consequently, pristine ecological communities have a low resilience relative to post-disturbance communities.