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Debra A. Willard

Publications and source records attributed to Debra A. Willard.

At least 91 records · Page 5Linked to original sources
USGS2001

Bedrock cores from 89° North: Implications for the geologic framework and Neogene paleoceanography of Lomonosov Ridge and a tie to the Barents shelf

Two piston cores from the Eurasian flank of Lomonosov Ridge near lat 88.9°N, long 140°E provide the first samples of bedrock from this high-standing trans-Arctic ridge. Core 94-PC27 sampled nonmarine siltstone similar in facies and age to uppermost Triassic to lower Lower Jurassic and mid– Lower Cretaceous beds in the 4 to > 5 km Mesozoic section on Franz Josef Land, on the outer Barents shelf. A ca. 250 Ma peak in the cumulative frequency curve of detrital zircons from the siltstone, dated by U- Th-Pb analysis, suggests a source in the post-tectonic syenites of northern Taymyr and nearby islands in the Kara Sea. Textural trends reported in the literature indicate that the Lower Jurassic nonmarine strata of Franz Josef Land coarsen to the southeast; this suggests the existence of a sedimentary system in which detrital zircons could be transported from the northern Taymyr Peninsula to the outer Barents shelf near the position of core 94-PC27 prior to opening of the Eurasia Basin. Correlation of the coaly siltstone in core 94-PC27 with part of the Mesozoic section on Franz Josef Land is compatible with the strong evidence from seafloor magnetic anomalies and bathymetry that Lomonosov Ridge is a continental fragment rifted from the Barents shelf during the Cenozoic. It also suggests that Lomonosov Ridge near the North Pole is underlain by a substantial section of unmetamorphosed Mesozoic marine and nonmarine sedimentary strata. Core 94-PC29 sampled cyclical deposits containing ice-rafted debris (IRD) overlying weakly consolidated laminated olive-black anoxic Neogene siltstone and mudstone with an average total organic carbon (TOC) of 4.1 wt%. The high TOC content of the mudstone indicates that during the Neogene, prior to the introduction of IRD into the Arctic seas about 3.3 Ma (early late Pliocene), the shallow waters of the central Arctic Ocean supported significant primary photosynthetic organic production near the North Pole. These deposits also contain fine grains of siltstone that resemble the breccia-clast siltstone of core 94-PC27 and reworked Carboniferous, Cretaceous, and Tertiary palynomorphs that may have also originated in the bedrock of Lomonosov Ridge.

GSA Bulletin↗
USGS2000

Climatic variability in the eastern United States over the past millennium from Chesapeake Bay sediments

Salinity oscillations caused by multidecadal climatic variability had major impacts on the Chesapeake Bay estuarine ecosystem during the past 1000 yr. Microfossils from sediments dated by radiometry (14C, 137Cs, 210Pb) and pollen stratigraphy indicate that salinity in mesohaline regions oscillated 10-15 ppt during periods of extreme drought (low fresh-water discharge) and wet climate (high discharge). During the past 500 yr, 14 wet-dry cycles occurred, including sixteenth and early seventeenth century megadroughts that exceeded twentieth century droughts in their severity. These droughts correspond to extremely dry climate also recorded in North American tree-ring records and by early colonists. Wet periods occurred every ~60-70 yr, began abruptly, lasted <20 yr, and had mean annual rainfall ~25%-30% and fresh-water discharge ~40%-50% greater than during droughts. A shift toward wetter regional climate occurred in the early nineteenth century, lowering salinity and compounding the effects of agricultural land clearance on bay ecosystems.

Chesapeake Bay↗
USGS2000

Historical trends in Chesapeake Bay dissolved oxygen based on benthic foraminifera from sediment cores

Environmentally sensitive benthic foraminifera (protists) from Chesapeake Bay were used as bioindicators to estimate the timing and degree of changes in dissolved oxygen (DO) over the past five centuries. Living foraminifers from 19 surface samples and fossil assemblages from 11 sediment cores dated by 210 Pb, 137 Cs, 14 C, and pollen stratigraphy were analyzed from the tidal portions of the Patuxent, Potomac, and Choptank Rivers and the main channel of the Chesapeake Bay. Ammonia parkinsoniana , a facultative anaerobe tolerant of periodic anoxic conditions, comprises an average of 74% of modern Chesapeake foraminiferal assemblages (DO-0.47 and 1.72 ml l −1 ) compared to 0% to 15% of assemblages collected in the 1960s. Paleoecological analyses show that A. parkinsoniana was absent prior to the late 17th century, increased to 10–25% relative frequency between approximately 1670–1720 and 1810–1900, and became the dominant (60–90%) benthic formaniferal species in channel environments beginning in the early 1970s. Since the 1970s, deformed tests of A. parkinsoniana occur in all cores (10–20% of Ammonia ), suggesting unprecedented stressful benthic conditions. These cores indicate that prior to the late 17th century, there was limited oxygen depletion. During the past 200 years, decadal scale variability in oxygen depletion has occurred, as dysoxic (DO=0.1–1.0 ml l −1 ), perhaps short-term anoxic (DO<0.1 ml l −1 ) conditions developed. The most extensive (spatially and temporally) anoxic conditions were reached during the 1970s. Over decadal timescales, DO variability seems to be linked closely to climatological factors influencing river discharge; the unprecedented anoxia since the early 1970s is attributed mainly to high freshwater flow and to an increase in nutrient concentrations from the watershed.

Maryland, Virginia↗
USGS2000

Effects of climate variability and human activities on Chesapeake Bay and the implications for ecosystem restoration

Chesapeake Bay, the Nation’s largest and most productive estuary (fig. 1), faces complex environmental issues related to nutrients and oxygen, turbidity and sedimentation, toxic dinoflagellates, sea-level rise, and coastal erosion. The Chesapeake Bay Program (CBP) is a partnership among the Chesapeake Bay Commission, the Federal Government, the District of Columbia, and the States of Maryland, Virginia, and Pennsylvania. The CBP is working to preserve, restore, and protect the bay’s living resources, vital habitats, and water quality, to protect human health, and to promote sound land-use policies in the watershed. The CBP began to set restoration goals for the ecosystem in the mid-1980’s and is now refining current goals and setting new ones as part of a new bay agreement— Chesapeake 2000. As the CBP sets restoration goals for the next 10–20 years, it will be critical to understand the long-term changes of the bay ecosystem due to climate variability and the influence of past and future human activities. For the past 4 years, the U.S. Geological Survey (USGS) has been engaged in research designed to provide objective scientific answers to questions about long-term changes in the bay ecosystem: What paleoecological and geochemical methods are best for documenting trends in the bay ecosystem? How does climate variability, including drought, affect the bay? What are historical trends in dissolved oxygen? What is the relationship between sedimentation and water clarity, and what is the effect of turbidity on living resources? How have past land-use changes affected bay habitats and living resources?

Chesapeake Bay Watershed↗
USGS1998

Phanerozoic stratigraphy of Northwind Ridge, magnetic anomalies in the Canada Basin, and the geometry and timing of rifting in the Amerasia Basin, Arctic Ocean

Cores from Northwind Ridge, a high-standing continental fragment in the Chukchi borderland of the oceanic Amerasia basin, Arctic Ocean, contain representatives of every Phanerozoic system except the Silurian and Devonian systems. Cambrian and Ordovician shallow-water marine carbonates in Northwind Ridge are similar to basement rocks beneath the Sverdrup basin of the Canadian Arctic Archipelago. Upper Mississippian(?) to Permian shelf carbonate and spicularite and Triassic turbidite and shelf lutite resemble coeval strata in the Sverdrup basin and the western Arctic Alaska basin (Hanna trough). These resemblances indicate that Triassic and older strata in southern Northwind Ridge were attached to both Arctic Canada and Arctic Alaska prior to the rifting that created the Amerasia basin. Late Jurassic marine lutite in Northwind Ridge was structurally isolated from coeval strata in the Sverdrup and Arctic Alaska basins by rift shoulders and grabens, and is interpreted to be a riftogenic deposit. This lutite may be the oldest deposit in the Canada basin. A cap of late Cenomanian or Turonian rhyodacite air-fall ash that lacks terrigenous material shows that Northwind Ridge was structurally isolated from the adjacent continental margins by earliest Late Cretaceous time. Closing Amerasia basin by conjoining sea-floor magnetic anomalies beneath the Canada basin or by uniting the pre-Jurassic strata of Northwind Ridge with kindred sections in the Sverdrup basin and Hanna trough yield similar tectonic reconstructions. Together with the orientation and age of rift-margin structures, these data suggest that (1) prior to opening of the Amerasia basin, both northern Alaska and the continental ridges of the Chukchi borderland were part of North America, (2) the extension that created the Amerasia basin formed rift-margin grabens beginning in Early Jurassic time and new oceanic crust probably beginning in Late Jurassic or early Neocomian time.

Geological Society of America Bulletin↗
USGS1998

Preliminary paleontologic report on core 37 from Pass Key, Everglades National Park, Florida Bay

Sediments from Pass Key core 37, in eastern Florida Bay (N 25.1478, W 80.5745) record a history of rapid sedimentation during this century. The lowest portion of the core contains benthic fauna indicative of relatively low salinities and sparse seagrass coverage. This period is followed by an increase in salinity and seagrass. In the middle portion of the core, a slight decrease in salinity and an increase in seagrass occur. These shifts in the benthic fauna correspond to a period when the terrestrial flora change, and an increase in dinocyst absolute abundance occurs, indicating changes in factors affecting the entire South Florida ecosystem. These changes may represent a period of increased terrestrial flushing, due to rainfall, water management practices or a combination of both. The benthic faunas in the upper portion of the core indicate an increase in salinity and seagrass density. This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Florida↗
USGS1997

Two new species of Scolecopteris (Marattiales), sources of Torispora securis Balme and Thymospora thiessenii (Kosanke) Wilson et Venkatachala

Two biostratigraphically significant miospore species, Thymospora thiessenii and Torispora securis , were isolated from marattialean synangia preserved in coal-ball permineralizations from Pennsylvanian strata of the Illinois Basin and Western Interior coal region. Synangia containing Thymospora thiessenii were recovered from the Missourian Parker Coal Member (Patoka Formation) of Indiana and are described here as Scolecopteris parkerensis Lesnikowska et Willard, sp. nov. The uniformly thin outer sporangial walls and broadly enclosing, hirsute pinnule margins place the species in the Latifolia group sensu Millay (1979). It is distinguished from the other species of the group by the size of the synangia, by the size and shape of the pinnules, by features of the pedicel and by the spores. Coal balls from the Desmoinesian Cliffland Coal Member (Kalo Formation) of Iowa yielded synangia producing two different spore types: a peripheral layer of spores with a distinct apical crassitude ( Torispora securis ) and interior spores assignable to Laevigatosporites globosus . The crassitude of Torispora is shown to be a structure distinct from the main spore body but is enclosed with it by the exine. The fructifications are described as Scolecopteris dispora Lesnikowska et Willard, sp. nov. , which belongs to the Oliveri group sensu Millay (1979) on the basis of the uniformly thick outer sporangial wall and downturned, hairless pinnule margins. It is distinguished from the other members of the group by the large size of the synangia, the uniform histology of the outer sporangial walls, features of the pedicel, and by the spores. This is the first reported association of either of these spores with permineralized fructifications from coal.

Review of Palaeobotany and Palynology↗
USGS1997

Pollen and geochronological data from South Florida; Taylor Creek Site 2

Many recent changes in plant and animal communities of the Everglades have been attributed to human alteration of the environment, such as changes in the hydrologic regime and increased agricultural activity, but cause-and-effect relationships between environmental and biotic changes have not been documented scientifically. This report on pollen and geochronological evidence from cores collected along Taylor Creek is the first of a series documenting the biotic history of a series of sites in southern Florida. Pollen and geochronology were analyzed from two cores collected at site 2 along Taylor Creek, one short core (35 cm long) to provide high-resolution data and one long core (98 cm long) to provide a record of the last few millenia. Analysis of pollen assemblages from these cores indicate that marsh and slough vegetation, primarily sawgrass with some incursions by cattails, dominated the area for most of the last two millenia, until about 1950-1960. At that point, sawgrass pollen declined to lower abundances than recorded elsewhere in the core, and tree pollen became much more abundant in the cores. This change reflects the vegetational response to alterations in the hydrologic system throughout much of the last century.

Florida↗
USGS1997

Preliminary paleontologic report on cores 19A and 19B, from Russell Bank, Everglades National Park, Florida Bay

The fauna and flora preserved in two cores, 19A and 19B, from the south side of Russell Bank (N 25 03.831', W 80 37.486') in north-central Florida Bay, Everglades National Park, Florida, record a history of environmental change over the last century. The benthic foraminifera and molluscs indicate fluctuating salinities with increasing average salinity upcore in core 19B. Shifts from low salinity (12-15 ppt) to higher average salinity (30 ppt) occurred at 70-66 cm and 24-18 cm in core 19B (approximately 1937-1940 and 1975-1980). The inverse, shifts from periods of higher average salinity to periods of lower salinity, occurred at 140 cm, 90 cm, and 42 cm (approximately 1880, 1921, and 1960). Significant changes in the molluscan fauna indicative of specific substrate types occur at 88 cm, 68 cm, and 22 cm. The lower portion of the core is dominated by a mixture of sediment and grass dwellers, the middle portion by sediment dwellers, and the upper portion of the core by grass and finally grass and algae dwellers. Changes occur in the floral assemblages in core 19A, but the significance of these changes is unclear. Three subtle shifts occur in the pollen assemblages indicating the onshore vegetation was responding to some environmental factor. Two peaks in dinocyst abundance occur in core 19A, but the composition of dinocyst assemblages remains relatively stable throughout the core. Correspondence between changes in salinity and onshore vegetation changes is consistent with results from previous cores. The pattern of increased salinity upcore is consistent with patterns seen in core T24 from the mouth of Taylor Creek and in core 6A from Bob Allen mudbank.

Florida↗
USGS1996

Pliocene–Pleistocene pollen assemblages from the Yermak Plateau, Arctic Ocean: Sites 910 and 911

Palynological and paleobotanical research on upper Pliocene sediments from Meighen Island, Kap København, and deep-sea sites in the North Atlantic Ocean has indicated the presence of forest tundra far north of its present limits and warmer conditions than present. New pollen data from samples obtained on Ocean Drilling Program Leg 151 (Sites 910 and 911) on the Yermak Plateau of the Arctic Ocean provide a record of late Pliocene terrestrial floras characteristic of at least boreal to subarctic climatic conditions in the source areas. Pollen and spores are present in upper Pliocene sediments from both cores, with typical concentrations of 1,000 to 2,000 pollen grains per gram of dry sediment. Pinus is the dominant genus in upper Pliocene assemblages, and Picea and Betula are subdominant. Other taxa typically present include Alnus , Corylus , Larix , Tsuga , Pterocarya , Sciadopitys and members of the Poaceae, Cyperaceae, Asteraceae, and Ericaceae. These assemblages are suggestive of open boreal vegetation with relatively temperate deciduous elements and indicate warmer Pliocene conditions than today in the pollen source areas. Pleistocene pollen assemblages from Site 911 consist primarily of Pinus and Picea pollen with few other taxa present. Such assemblages indicate cold conditions, probably similar to those of today, and palynomorphs are less common in Pleistocene sediments, typically <1,000 pollen grains per gram of dry sediment. In both cores, most reworked palynomorphs, which are two to three times more abundant than nonreworked ones, are Cretaceous in age and include genera characteristic of both the Aquilapollenites and Normapolles provinces; this composition suggests that the source of reworked material was the northern Asian coast between the Barents Sea and Laptev Sea, the main region where Cretaceous rocks containing both these assemblages are preserved. These reworked palynomorphs probably were transported either by sea ice or ocean currents via the Transpolar Drift to the Yermak Plateau sites.

Conference Paper↗
USGS1996

Neogene and Quaternary geology of a stratigraphic test hole on Horn Island, Mississippi Sound

During April and May, 1991, the U.S. Geological Survey (USGS) drilled a 510-ft-deep, continuously cored, stratigraphic test hole on Horn Island, Mississippi Sound, as part of a field study of the Neogene and Quaternary geology of the Mississippi coastal area. The USGS drilled two new holes at the Horn Island site. The first hole was continuously cored to a depth of 510 ft; coring stopped at this depth due to mechanical problems. To facilitate geophysical logging, an unsampled second hole was drilled to a depth of 519 ft at the same location.

Open-File Report↗