Doctoral Dissertations

Date of Award

5-1998

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Paul A. Delcourt

Committee Members

Hazel R. Delcourt, Thomas W. Broadhead, Steven G. Driese, Patricia L. Walne

Abstract

Lacustrine systems are influenced by changes in climate and surrounding vegetation. Changes in vegetation are themselves the result of changes in climate. The objective of this study was to investigate changes in a lacustrine system using fossil diatom assemblages and to compare changes in the diatom assemblages to known changes in aquatic and local terrestrial vegetation in the context of late Quaternary climate change. The study site, Anderson Pond in Middle Tennessee, is a karst sinkhole which has received continuous sedimentation for the last 19,000 years.

The fossil diatom assemblages indicate the presence of five diatom zones during the late Quaternary. The late-Pleistocene interval between 19,000 and 12,750 years Before Present (BP) is dominated by small, benthic species of the genus Fragilaria, which are characteristic of alkaline (diatom inferred pH: 6.8 to 7.7), high clarity water, with depths up to 6 meters. Between 12,500 BP and 10,000 BP, a new diatom assemblage establishes that is pH-indifferent (diatom inferred pH: 6.1 to 6.3), shallow-water epiphytic and epipelic, and dominated by the genera Eunotia, Pinnularia, and Stauroneis. The early to middle Holocene diatom assemblages from 9750 BP through 2250 BP are dominated by the acidophilous, epiphytic genera Eunotia and Gomphonema, indicating shallow-water levels with acidic water (diatom inferred pH: 5.6 to 5.8). From 2000 BP through 250 BP the diatom assemblages are dominated by planktonic diatoms of the genus Aulacoseira, indicating at least seasonal expansion of shallow, open water pools with continued acidic pH levels (diatom inferred pH: 5.5 to 5.8) and a decrease in water transparency. Modem sediment samples are dominated by the epiphytic genera Gomphonema, Eunotia, and Navicula indicating even shallower water levels and an increase in water pH to neutral conditions (diatom inferred pH: 6.2).

The disappearance of the Fragilaria-dominated assemblage between 12,750 BP and 12,500 BP in Anderson Pond coincides with a major change in upland forests from boreal jack pine and spruce to temperate oaks. The following three thousand years represent a period of cumulative changes in aquatic plant communities within the karst basin, associated with the local expansion of a mosaic wet meadow grassland and swamp thicket. Eunotia reached dominance by 9750 BP and this diatom assemblage remained relatively stable until 2250 BP. The early Holocene rise in Eunotia is linked to the availability of an epiphytic substrate—this is tied to the early Holocene infilling of the basin and the establishment, then expansion of dense alder thickets forming swamps. The rapid increase in Aulacoseira between 2250 BP and 2000 BP corresponds with a modest rise in water table and the expansion of open pools within the swamp thicket. The disappearance of Aulacoseira and the increase in Gomphonema, Eunotia, and Navicula during the last 250 years resulted from human activity leading to increased sediment influx into the basin.

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