Date of Award

1995

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geography and Anthropology

First Advisor

Kam-biu Liu

Abstract

Pollen, phytolith, charcoal, and diatom analysis, in conjunction with sediment stratigraphy and radiocarbon dating, of sediment cores from Lake Arthur and Prien Lake, estuaries in southwestern Louisiana, provide 6000 years of data used to reconstruct the late-Holocene vegetation history of Louisiana's natural prairie and to define coastal processes which affected the sites. The Cajun Prairie is a grassland outlier in a climate which normally supports forest. Poorly drained, impermeable soils with poor water yielding capability induce summer drought severe enough to limit the success of trees, resulting in a natural tallgrass prairie. Pollen, phytoliths, and charcoal suggest that the grassland island has neither expanded nor contracted over the last 6000 years and that fire has contributed to its maintenance. Pinus, Quercus, and Taxodium have been components of southwestern Louisiana's vegetation for the entire period of record with a minor increase in pine from 2000 to 1000 B.P. High pollen concentrations followed by an Ambrosia rise and a drop in Taxodium and Pinus mark the settlement horizon. Low pollen influx after settlement indicates high rates of erosion and clastic input to the lakes. A crash in the diatom flora of Lake Arthur probably relates to maximum pollution of the estuary by agricultural chemicals. The use of phytoliths in sediment to distinguish the source of Gramineae pollen is a promising new technique. Dramatic increases in Gramineae pollen accompanied by high percentages of rondel shaped phytoliths document expanding marsh vegetation around Lake Arthur and Prien Lake as rising sea level initiated drowning of the low gradient rivers by 5000 B.P. Radiocarbon dates on peat samples indicate relative sea level of $-$5.5 m by 6000 B.P. followed by slow steady rise at a rate of 9 cm/100 years up to present times. Freshwater diatom assemblages between 4500 and 3000 B.P. coincide with westward progradation of the Mississippi River's Teche Delta Complex, and an abrupt shift to brackish/marine diatom flora at 3000 B.P. signals abandonment of the Teche system.

Pages

283

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