Miriam (Mimi) Fearn, Ph.D.
Assistant Professor

Department of Geology and Geography
mfearn@usouthal.edu
(334) 460-6381  office

(334) 461-1487  fax 
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Paleoecology of southwestern Louisiana:
... My Ph.D. dissertation at Louisiana State University was a paleoecological study of southwestern Louisiana. Travelers on Interstate 10 between Lafayette and Lake Charles pass through a region of rice fields which was originally a natural tall grass prairie. I like grasslands, and the idea of looking for changes in the southern prairie-forest ecotone appealed to me. Under Dr. Kam-biu Liu's direction, I started my study using pollen, went to the University of Minnesota to learn phytoliths in order to refine the pollen data, and eventually included diatoms because they were in my phytolith samples. My dissertation research was supported primarily by the National Science Foundation, the Association of American Geographers, and the Robert West Fund of the LSU Geography and Anthropology Department. 
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The Southwestern Louisiana Prairie

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Publications on southwestern Louisiana include:
Fearn, M.L. 1995. Louisiana's Cajun Prairie: Holocene History of a Southern Grassland. Ph.D. dissertation. Louisiana State University, Baton Rouge, Louisiana.
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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.

 

Fearn, M.L. 1998. Phytoliths in sediment as indicators of grass pollen source. Review of Palaeobotany and Palynology 103: 75-81.
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ABSTRACT 
Phytolith and pollen analyses of soil samples and of sediment cores in southwestern Louisiana show that phytoliths add valuable information especially when Gramineae pollen is a significant component of the pollen assemblage. The environments sampled include dry prairie, poorly drained prairie, intermediate marsh, and salt marsh. Dumbbell shaped phytoliths dominate samples from the dry prairie. Rondel shaped phytoliths increase along the waterlogging/salinity gradient and dominate salt marsh samples. The ratio of dumbbell shaped phytoliths to rondel shaped phytoliths changes significantly along this environmental gradient and provides a means of refining pollen based interpretations of an estuarine sediment core. The phytolith ratio indicates whether grass pollen in the core comes from marsh taxa or from southwestern Louisiana's natural tallgrass prairie. This simple phytolith ratio shows promise for resolving the interpretive dilemma resulting from Gramineae's prominence in both very dry and very wet environments. 

Keywords: Phytoliths, pollen, grasslands, marshes, Louisiana

 

Fearn, M.L. In review. Holocene history of Louisiana’s tallgrass prairie outlier.
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ABSTRACT
Pollen, phytolith, charcoal, and diatom analysis 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 a natural tallgrass prairie outlier. Microfossil analysis indicates that the grassland has neither expanded nor contracted over the last 6000 years and that fire has contributed to its maintenance. Phytoliths reveal that high percentages of Gramineae pollen in Lake Arthur cores between 6000 and 3000 B.P. come from expanding marsh vegetation associated with drowning of the low gradient river by slowly rising sea level. Throughout the period of record, Pinus, Quercus, and Taxodium dominate southwestern Louisiana's forest; and a distinct Ambrosia rise marks the settlement horizon. In Lake Arthur, low pollen influx and high percentages of broken phytoliths indicate high rates of erosion and sedimentation after settlement.

 

See also: Presentations with Published Abstracts
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Mimi Fearn Home Page
University of South Alabama
Last modified: 5/26/99