Introduction

In light of recent evidence validating the reality of anthropogenic warming, there is a concerted effort to understand Global Climate change and its effect on plants and animals. In the early Paleogene, approximately 55 million years ago, there was a rapid Global warming event called the Initial Eocene Thermal Maximum.

My project is part of a larger effort to understand floral response to rapid climate change in the past. The most effective way to study floral trends in the past is through dispersed fossil pollen, or palynoflora. Compared to most megafossils, pollen preserves very well in fine-grained sedimentary rocks. Stratigraphic sampling density is therefore much greater. However, when there are apparent changes in the palynoflora within a stratigraphic section, it is difficult to know whether they represent large regional changes in the plant community, or whether they merely represent local changes such as a windstorm, or a fire. In an attempt to clarify this problem, this study investigates the lateral palynofloral variation within an early Paleogene lignite.

Materials and Methods

The field area is near the town of Hebron in the Williston Basin of western North Dakota (Figure 1). This area was selected because of its considerable exposure of early Paleogene strata. In the area the Golden Valley formation lies conformably on the Sentinel Butte formation (Figure 2). In light of previous biostratigraphic investigations of the area, this lithologic contact probably represents the Paleocene – Eocene boundary (Bebout 1977, Hickey 1977, Robertson 1975). The Golden Valley formation is divided into a lower Bear Dem member and an upper Camels Butte member, which are separated by a laterally extensive lignite known as the Alamo Bluff (AB) lignite (Figure 3).

The lignite sampled in this research is laterally discontinuous, and it lies approximately three meters below the AB lignite in the Bear Den member (Figure 4). Samples were collected at six locations, in varying thickness of lignite. Palynological processing which included demineralization with hydrofluoric acid and sieving with a true ten-micron mesh, was performed at Global Geolab, Alberta.

Common elements of the palynoflora included Taxodiaceae, Betulaceae (at least two genera), Juglandaceae, Cycadaceae, Ulmaceae, and fern spores (Figure 5). Slides were viewed with a light microscope and approximately the first three hundred palynomorphs were identified and counted for relative abundance.

A hypothetical data set of time (represented as stratigraphic level) and some measure of floral composition is plotted in the figure above. It is difficult to know whether the perceived shift in floral composition at point X represents a significant change in the regional plant community. Large-scale regional community changes are most useful in biostratigraphy and paleoecological analyses. Small changes in local floral assemblages represent noise in our interpretation of larger trends. This study attempts to quantify the magnitude of local palynofloral change, so that it can be factored into larger studies of biostratigraphy.

Results

Preliminary analysis of the data indicates significant palynofloral variability between the samples. Figure 6 is a bar graph of three samples exhibiting some of this variability. The Betulaceous and Taxodiaceous taxa seem to be responsible for the majority of the variation between the samples. Where Betulaceae are abundant the Taxodiaceae are less abundant and vice versa. There also seems to be a relatively strong correlation between the thickness of the lignite and the abundant taxa in the sample. Samples within a thick section of the lignite tend to be dominated by Betulaceae, and thin sections tend to be dominated by Taxodiaceae. To understand this relationship much more data will have to be collected.

Future Work

More samples of the lignite will be analyzed to increase the precision of our analysis. Absolute abundance data will be collected in an attempt to understand how it relates to lignite thickness, or spatial distribution of samples.

Works Cited

Hickey, L.J. 1977. Stratigraphy and Paleobotany of the Golden Valley Formation (Early Tertiary) of WesternNorth Dakota. GSA Memoir 150, 181p.

Bebout, J.W. 1977. Palynology of the Paleocene-Eocene Golden Valley Formation of Western North Dakota. Unpublished Ph.D. thesis, Pennsylvania State University.

Robertson, E.B. 1975. Pollen and Spores Stratigraphic Indices to the North Dakota Paleocene. Unpublished Ph.D. thesis, University of Minnesota.

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