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Debra A. Willard

Publications and source records attributed to Debra A. Willard.

102 records · Page 6Linked to original sources
USGS1995

Paleoecology of the Springfield Coal Member (Desmoinesian, Illinois Basin) near the Leslie Cemetery paleochannel, southwestern Indiana

The Springfield Coal Member (Carbondale Group, Petersburg Formation of Indiana) is split locally in Warrick and Gibson Counties, Indiana, by clastic rocks of the Folsomville Member (Carbondale Group, Petersburg Formation) that represent the Leslie Cemetery paleochannel, part of a large, interconnected paleochannel system in the Springfield coal bed. This study incorporated analysis of miospore and megaspore assemblages, coal petrography, plant compression fossils and conodonts from the coal and clastic split to document changes in the swamp and its vegetation in response to the activity of the Leslie Cemetery paleochannel. Palynological and petrographic data indicate that environmental conditions and vegetation in the lower bench of coal near the Leslie Cemetery paleochannel were similar to those found in profiles through the coal bed at sites near the larger, more extensive Galatia paleochannel. Miospore floras of the high-vitrinite, lower bench of coal were dominated by tree-fern miospores, with those of lycopsids ranking second in abundance; megaspore assemblages were dominated by lycopsid megaspores. Near the contacy between the lower bench of coal and clastic split, medullosan prepollen increases in abundance and fusain bands are more common, which may indicate the occurrence of fire, either within or outside the swamp. Vegetation in the Folsomville Member and upper bench of coal differ markedly from that of the lower bench of coal. Plant megafossils from the Folsomville Member indicate dominance by pteridosperms and/or lycopsids and Folsomville Member miospore assemblages are dominated by lycopsid miospores most typically found in mudstones, including Lycospora torquifer and higher than normal abundances of Granasporites medius . In the transition from clastic rocks of the Folsomville Member to the upper bench of coal, miospores of ‘ecotonal’ lycopsids ( Paralycopodites ) reach their peak abundance, pteridosperms dominate megafossil assemblages and inertinite levels are relatively high. At higher levels of the upper bench of coal, vitrinite levels are higher and miospore assemblages are dominated by lycopsid miospores typical of coal swamps ( Lycospora granulata, L. pusilla ). The presence of conodonts in coalballs in the upper bench of coal and spirorbid worms in the split indicate that the paleoswamp may have received some marine or brackish water influence. Higher than normal salinity levels may explain the vegetational changes observed in the upper bench of coal near the Leslie Cemetery paleochannel.

Indiana↗
USGS1995

Preliminary analysis of down-core biotic assemblages Bob Allen Keys, Everglades National Park, Florida Bay

Introduction A series of short piston cores (< 2m) were taken from eleven stations in Florida Bay in May, 1994 by researchers from the U.S. Geological Survey (St. Petersburg, FL., Woods Hole, MA., and Denver CO.) in cooperation with South Florida Water Management District, and the Everglades National Park, and the National Oceanic and Atmospheric Administration (NOAA). Core 6A from Bob Allen Keys (25° 1.391” N, 80°39.41” W) penetrated 172 cm of Holocene sediments in 0.6 m of water on a grass covered mud bank, approximately 1.75 miles (2.82 km) east of the water monitoring station on the southern end of the Bob Allen Keys. Core 6A was sampled for particle size, insoluble residue, water content, loss on ignition, Pb 210 , Rasup>222, and paleontologic analyses. Here we present the results of the preliminary paleontologic analyses of the biotic components from core #6A. The Everglades/Florida Bay ecosystem has formed over the last 5000 years at the southern tip of peninsular Florida. Here it has been influenced by Atlantic, Caribbean and Gulf of Mexico waters, and by tropical and subtropical climatic regimes. This location ensures that over time the ecosystem has undergone climatic changes on both a seasonal and long term basis, and that it has been subjected to many major storms. Additionally, in the last century, the hydrologic regime of the region has been altered profoundly through construction of a canal system to control flooding in southern Florida. This system regulates the timing and amount of freshwater flow into Florida Bay. Recently, algal blooms, seagrass, and sponge die-offs, and declining numbers of shellfish, have been reported in Florida Bay; although it has been assumed that these changes have resulted from human alteration of freshwater flow into the bay, this assumption has not been rigorously tested. The research described here is part of a project designed to examine the history of the Everglades/Florida Bay ecosystem over the last 150 years and to test assumptions of cause and effect. The purpose of the project is two-fold; first, to determine the characteristics of the ecosystem prior to significant human-induced alteration, including the natural range of variation in the ecosystem. This information will establish a baseline for restoration of the system. Second, the project aims to establish the extent, range, and timing of changes to the ecosystem over the last 150 years, and to determine whether these changes correlate with human alteration of the environment, or meteorological patterns, such as precipitation and major storms, or a combination of factors.

Florida↗
USGS1994

Palynological record from the North Atlantic region at 3 Ma: Vegetational distribution during a period of global warmth

Pollen assemblages from five mid-Pliocene, deep-marine and shallow-marine sites in the North Atlantic Ocean region were analyzed quantitatively to estimate vegetational composition and terrestrial paleoclimates during a time interval centered at about 3 Ma. Three subarctic sites (58–67°N latitude) from the Norwegian Sea, northern Iceland, and the Labrador Sea yielded pollen assemblages characteristic of mixed conifer-hardwood vegetation; these assemblages have no close modern analogs, but the most similar modern vegetation is that of the Tsuga (hemlock)-northern hardwood forests of Newfoundland and Nova Scotia (45–50°N). The presence of such vegetation at these sites indicates mid-Pliocene January temperatures from 4 to 10°C warmer than today and a northward shift of the deciduous forest zone into areas presently occupied by boreal forests. Pollen assemblages from the mid-latitude (36°N) Yorktown Formation site in southeastern Virginia, USA represent Quercus-Carya-Pinus (oak-hickory-pine) forests, similar to those presently occupying much of the Atlantic coastal plain of the United States. Modern analogs for this site are located off the coasts of North Carolina and South Carolina, indicating mid-Pliocene January temperatures 3–5°C warmer than today. In southwestern Florida (27°N), pollen assemblages from the mid-Pliocene Pincrest Beds are representative of pine-dominated communities, similar to Pinus palustris/P. ellittii forests (longleaf/slash pine) growing in Florida today. Close modern analogs for the Pinecrest Beds are located at numerous sites around the Florida peninsula, indicating mid-Pliocene temperatures about the same as today for this region. These sites show a gradient from much higher than present temperatures at high latitudes to little change from present temperatures at low latitudes and low elevations; a similar pattern is shown by mid-Pliocene sea-surface and isotopic data. Such a gradient also is consistent with the northward migration of the boreal forest zone, covering sites as far north as Meighen and Banks Islands, a northward shift of the deciduous-boreal forest boundary in both North America and Europe, and little change in vegetational distribution in low elevation sites in subtropical latitudes during the mid-Pliocene warm interval. Atmospheric temperaturs estimated from vegetational distributions indicate a Pliocene to modern temperature anomaly of a similar pattern and magnitude to that indicated by sea-surface temperatures. These data are most consistent with current general circulation model simulations of climate changes in response to increased meridional heat transport.

Florida, Newfoundland and Labrador, Virginia↗
USGS1994

Joint investigations of the Middle Pliocene climate I: PRISM paleoenvironmental reconstructions

The Pliocene epoch represents an important transition from a climate regime with high-frequency, low-amplitude oscillations when the Northern Hemisphere lacked substantial ice sheets, to the typical high-frequency, high-amplitude Middle to Late Pleistocene regime characterized by glacial—interglacial cycles that involve waxing and waning of major Northern Hemisphere ice sheets. Analysis of middle Pliocene (∼3 Ma) marine and terrestrial records throughout the Northern Hemisphere forms the basis of an integrated synoptic Pliocene paleoclimate reconstruction of the last significantly warmer than present interval in Earth history. This reconstruction, developed primarily from paleontological data, includes middle Pliocene sea level, vegetation, land—ice distribution, sea—ice distribution, and sea-surface temperature (SST), all of which contribute to our conceptual understanding of this climate system. These data indicate middle Pliocene sea level was at least 25 m higher than present, presumably due in large part to a reduction in the size of the East Antarctic Ice Sheet. Sea surface temperatures were essentially equivalent to modern temperatures in tropical regions but were significantly warmer at higher latitudes. Due to increased heat flux to high latitudes, both the Arctic and Antarctic appear to have been seasonally ice free during the middle Pliocene with greatly reduced sea ice extent relative to today during winter. Vegetation changes, while more complex, are generally consistent with marine SST changes and show increased warmth and moisture at higher latitudes during the middle Pliocene.

Global and Planetary Change↗
USGS1994

PRISM 8 degrees X 10 degrees North Hemisphere paleoclimate reconstruction; digital data

The PRISM 8?x10? data set represents several years of investigation by PRISM (Pliocene Research, Interpretation, and Synoptic Mapping) Project members. One of the goals of PRISM is to produce time-slice reconstructions of intervals of warmer than modern climate within the Pliocene Epoch. The first of these was chosen to be at 3.0 Ma (time scale of Berggren et al., 1985) and is published in Global and Planetary Change (Dowsett et al., 1994). This document contains the actual data sets and a brief explanation of how they were constructed. For paleoenvironmental interpretations and discussion of each data set, see Dowsett et al., in press. The data sets includes sea level, land ice distribution, vegetation or land cover, sea surface temperature and sea-ice cover matrices. This reconstruction of Middle Pliocene climate is organized as a series of datasets representing different environmental attributes. The data sets are designed for use with the GISS Model II atmospheric general circulation model (GCM) using an 8?x10? resolution (Hansen et al., 1983). The first step in documenting the Pliocene climate involves assigning an appropriate fraction of land versus ocean to each grid box. Following grid cell by grid cell, land versus ocean allocations, winter and summer sea ice coverage of ocean areas are assigned and then winter and summer sea surface temperatures are assigned to open ocean areas. Average land ice cover is recorded for land areas and then land areas not covered by ice are assigned proportions of six vegetation or land cover categories modified from Hansen et al. (1983).

Open-File Report↗
USGS1993

Terrestrial and marine records of climatic and environmental changes during the Pliocene in subtropical Florida

Pollen, ostracode, and benthic foraminifer assemblages deposited during sea-level high-stands in subtropical Florida record a climate change during the period 4.5-1.0 Ma. Before 3.5 Ma, open-shelf marine faunas and pollen assemblages with abundant Pinus , Quercus , Fagus , Carya , and nonarboreal pollen were present, indicating cooler conditions than today. From ∼3.5 to 1.0 Ma, marine and terrestrial records indicate warmer conditions, similar to those existing in south Florida today. Combined with evidence for much warmer than modern conditions at high latitudes, these data suggest that increased poleward oceanic heat transport, possibly related to the emergence of the Central American isthmus between ∼3.5 and 2.5 Ma, was a major influence on mid-Pliocene warmth.

Geology↗