Milankovitch Cyclicity in the Ohio and Sunbury shales: Astronomical Calibration of the Late Devonian-Early Carboniferous Timescale
Thomas J. Algeo, Linda A. Hinnov, and D. Jeffrey Over
The Late Devonian (Famennian) Ohio Shale and Early Carboniferous (Tournaisian) Sunbury Shale of the Appalachian Basin exhibit subtle yet ubiquitous compositional cyclicity at several length scales. We have recorded this cyclicity using an ultra-slow (0.1 m hr-1) spectral-gamma core logging procedure that yielded reproducible records from a series of four two-inch-diam. drillcores between northern Kentucky and northern Ohio. Evolutionary harmonic analysis was carried out to allow for changes in the length scales of dominant cycles arising from variable sedimentation rates and to assist in identification of gaps/condensed intervals within the sections. The resulting spectra reveal composite cycles with dominant wavelengths at ~0.5 m and 2.5 m in the lower part of the Ohio Shale, shifting to shorter wavelengths upsection but maintaining a ~5:1 length ratio. These cycles likely represent the Earth’s orbital ~20-kyr precession and ~100-kyr short eccentricity cycles, yielding an average (compacted) sedimentation rate of ~25 mm kyr-1 for the lower Ohio Shale. All four drillcores exhibit similar spectral patterns, although with higher sedimentation rates at the environmentally more proximal northern locales. The results of the present study may allow development of "floating" astronomical time scales for portions of the Devonian timescale. Although some problems with the time scale have been resolved by recent high-precision radiometric dates for the Frasnian-Famennian (376.1±1.6 Ma) and Devonian-Carboniferous (360.5±0.8 Ma) boundaries (Tucker et al., 1998; Richards et al., 2002; Kaufmann et al., 2004; Trapp et al., 2004), there is still little agreement about the absolute or relative durations of individual Devonian stages and biozones. Our proposed astronomical calibration can provide minimum durations for each Upper Devonian stage and biozones and, in combination with graphic correlation analysis, may assist in determining the stratigraphic location and duration of intraformational hiati.
Elemental Proxies for Hydrographic Conditions in Restricted Marine Systems
Thomas J. Algeo, Harry Rowe and Barry Maynard
Patterns of trace metal-TOC covariation have considerable potential to reveal changes in hydrographic conditions in marine paleoenvironments, especially those with some degree of watermass restriction. In modern anoxic silled basins, sedimentary Mo and TOC exhibit strong positive covariation, yielding correlation-line slopes m ranging from ~4.5 to 45 (x 10-4)*. Smaller values of m are associated with greater restriction of the subpycnoclinal watermass, as reflected in deepwater renewal times (τdw). This relationship exists because restricted watermasses develop reduced aqueous Mo concentrations ([Mo]aq) owing to removal of Mo to the sediment at a rate faster than its resupply through deepwater renewal events. Sedimentary Mo concentrations exhibit significant positive covariation with TOC because they are controlled by the availability of organic host phases as well as [Mo]aq. Organic-rich facies of Middle Devonian to Early Carboniferous age from the North American craton also exhibit significant Mo-TOC covariation, with m ranging from ~2 to 65 (x 10-4)#. Systematic patterns of geographic and stratigraphic variation in m suggest control by slowly evolving watermass characteristics of the Devono-Carboniferous North American Seaway. Based on modern analogs, watermasses in this seaway had aqueous Mo concentrations ranging from <20% to >70% that of modern seawater, and deepwater renewal times ranging from <100 y to >1000 y. Trace metal-TOC relationships in anoxic black shales of the Late Pennsylvanian Midcontinent Sea (LPMS) exhibit a completely different pattern of covariation, one suggesting strongly dynamic fluctuations in eustasy and redox conditions†. The dynamic character of the LPMS is counterintuitive in view of its nearly landlocked setting and geochemical evidence of sulfidic deepwater conditions. The resolution of this conundrum is that LPMS deepwaters were “renewed” by lateral advection of oxygen-deficient waters from below the elevated pycnocline on the eastern margin of tropical Panthalassa.
*Algeo, T.J., Lyons, T.W., 2006, Paleoceanography, 21, PA1016, doi:10.1029/2004PA001112.
#Algeo, T.J., Lyons, T.W., Blakey, R.C., Over, D.J., in press, Palaeo-3.
†Algeo, T.J., Rowe, H., Schwark, L., Hower, J.C., Heckel, P.H., in prep., Palaeo-3.
The Permian-Triassic Boundary at Nhi Tao, Vietnam: Evidence for Recurrent Influx of Sulfidic Watermasses to a Shallow-Marine Carbonate Platform
Thomas J. Algeo, Yanan Shen, Timothy W. Lyons, Brooks B. Ellwood, Harry Rowe, and Tonggang Zhang
The Permian-Triassic boundary (PTB) at Nhi Tao, Vietnam* is within a nearly pure carbonate succession, facilitating recognition of changes in the hydrogenous (seawater-derived) fraction that may represent contemporaneous changes in paleocean chemistry. The extinction horizon (EH) at which most Late Permian faunal elements disappear coincides with a 12-cm-thick oolitic-pisolitic grainstone layer that also marks the appearance of several geochemical anomalies that continue into the Lower Triassic part of the section. Eight pyrite [S] peaks within a 5-m-thick interval above the EH exhibit a strong negative relationship to both carbonate 13C and pyrite 34S (i.e., higher S concentrations associated with isotopically more depleted values). The most likely mechanism to account for these relationships is episodic upwelling of sulfidic, 12C- and 32S-enriched deep-ocean waters onto the shallow Jinxi Platform, located on the southern margin of the South China Craton. Such quasi-cyclic episodes of upwelling may represent multiple attempts at renewed global-ocean overturn modulated by Earth’s 20-kyr orbital precession cycle following prolonged Late Permian stagnation of the deep ocean. Large-scale upwelling of deepwaters implies that the end-Permian marine mass extinction was caused by anoxia and/or sulfide toxicity, and that recurrent episodes during the Early Triassic may have been a factor in delaying significant recovery of marine ecosystems until the start of the Middle Triassic. The rate and magnitude (-3‰) of the carbonate 13C shift across the PTB are consistent with a “Strangelove ocean” model with a sustained ~50% decline in primary productivity. A major increase in magnetic susceptibility values 12 cm above the EH may record the influx of fine soil-derived particles following destruction of terrestrial ecosystems. The lag is consistent with the interval required for transport of fine particles across the Nanpanjiang Basin to the relatively distal site of the Jinxi Platform. Within existing limits of uncertainty, the end-Permian marine and terrestrial crises recorded at Nhi Tao appear to have occurred synchronously. * Algeo, T.J., Ellwood, B.B., Nguyen, T.K.T., Rowe, H., Maynard, J.B., in press, The Permian-Triassic boundary at Nhi Tao, Vietnam: Evidence for recurrent influx of sulfidic watermasses to a shallow-marine carbonate platform: Palaeogeography Palaeoclimatology Palaeoecology.
When Is a Species Not a Species? Quantifying Regional and Evolutionary Differences in Morphology for the Genus Cucullaea [Bivalvia]
BUICK, Devin P., Department of Geology
Variation in morphology within species arises primarily because of changes in the timing and rate of developmental events (ontogeny), and from adaptations to local environmental conditions (ecophenotypy). Over time, these processes may lead to speciation, as existing morphological features are modified, leading to new species diverging into separate regions of morphospace. The bivalve genus Cucullaea has a globally extensive fossil record dating back 200 million years, providing an opportunity to test i) how shape variation within species relates to ontogenetic and ecophenotypic effects, and ii) how morphological difference among species evolves through time in concert with changes in geographic distribution and diversity. Within a broader, ongoing investigation of the synergistic roles of geographic distribution, species richness and morphological variability within Cucullaea, this study focuses on quantifying intra- and inter-specific variation in shape. To assess morphological disparity, numerous well-preserved museum specimens from various geographic areas and stratigraphic intervals were digitized for morphometric analyses. Landmark techniques, in addition to traditional and outline methods, were then used to recognize i) regional, ecophenotypic variation within the present day, Indo-Pacific species, C. labiata, and ii) variation among geographically and temporally distinct cucullaeid species from the present-day and fossil record. Combining morphometric methods provides the opportunity to explore the significance of multiple morphological characteristics, ranging from those features that represent phylogenetic relationships and best discriminate species as taxonomic units, to those that evolve primarily in response to environmental conditions and life strategies. This multifaceted, quantitative approach provides insight into the dominant selective pressures operating on individual shell characteristics, separate populations or on entire species. Because cucullaeid species often exhibit wide morphological ranges, taxonomic designations must be made with consideration of intraspecific disparity throughout the genus, allowing for rigorous analyses of regional differences within species and evolutionary differences among species.
Three-Dimensional Modeling of the Degradation of Civil War Earthworks
BULLARD, Reuben G. Jr, Geology, and NASH, David B., Geology
Civil War-era earthworks provide a useful laboratory for scientists studying erosion. Unlike natural hills, earthworks can be dated – often to the precise day on which construction stopped. Reuben Ballard and David Nash found that three-dimensional modeling of the Civil War earthworks near Charleston, South Carolina, accurately predicts the observed erosion of the structure.
Evidence of Changes in Glacial Thermal Regime: A Test Case from Nevado Sajama, Bolivia
SMITH, Colby A., Department of Geology, and LOWELL, Thomas V., Department of Geology
Basal thermal regime is a primary control on glacial erosion and deposition. For example, glacial erosion rates can vary by up to two orders of magnitude depending rather the glacier bed is frozen or not. However, insights to the temporal evolution of basal regime is lacking for small glacier systems. Recent field investigation on Nevado Sajama in the Western Cordillera, Bolivia provides geomorphic evidence of wet based glacial activity in an area that currently contains only cold based ice. Thus, glaciers have changed thermal regime at sometime in the past.
A numerical model has been used to estimate past equilibrium line altitudes (ELA) and glacial extents in Aychuta Valley on the west side of Sajama. Model inputs include paleo-precipitation data from the Sajama ice-core, tropical paleo-temperature data derived from noble gas concentrations in ground water, and precipitation and temperature lapse rates derived from local meteorological observations.
Preliminary results indicate that environmental conditions during the past 25 ka (the length of the ice-core) have not been simultaneously warm and wet enough for our model to predict a glacial margin extending below the 0 oC isotherm. Thus, we hypothesize that wet based ice has not existed on the mountain during this time period implying that the outermost moraines which formed under wet based conditions are older than 25 ka. A field campaign has been undertaken to test the modeling results by dating glacial moraines on Nevado Sajama.
Erosion Evolution in Arid Environments Derived from the Study of Pre-Columbian Agricultural Terraces in Southern Peru
LONDONO, Ana Cristina, Geology
Ancient people of Peru, notably the Wari (600–1000 AD) and the Inca (1450–1532 AD), built agricultural terraces that offer a very good setting for testing of models of landscape evolution, and open the possibility of dating hillsides on a thousand-year timescale. Ana Londono of the University of Cincinnati closely inspected the remains of Peruvian terraces, and found patterns of erosion that may help understand the erosional history of geoforms.
Latitudinal Diversity Gradients and the Cenozoic Fossil Record of the Western Atlantic: Consideration of Spatial Scale and Sampling Issues
HENDY, Austin J.W., BUICK, Devin P., BULINSKI, Katherine V., FERGUSON, Chad A., and MILLER, Arnold I., Department of Geology
There is widespread agreement that the biodiversity of modern oceans is highest in tropical regions of the world, and decreases in richness towards the poles. Austin Hendy of the University of Cincinnati reviewed the fossil record for the past 50 million years and found evidence that this pattern held true throughout that time period. Hendy based his analysis largely on museum specimens, and discovered ways to minimize biases introduced by collectors assembling specimens for various reasons.
Sample, Locality and Regional-Level Biodiversity: Implications of Aggregating Paleontological Data at Multiple Spatial Scales
BULINSKI, Katherine V., Department of Geology
Biodiversity can be measured on multiple scales, ranging from the sample-level (e.g. a bulk sample), to the local and regional (e.g. the Cincinnati Arch), provincial (e.g. Baltoscandia) and global-level. At each of these spatial scales, components of biodiversity, such as richness, evenness and the number of fossil individuals influence one another and affect the composite biodiversity signal. As a precursor to the present study, I investigated the relationship among these properties at the sample-level with respect to lithology and position along an environmental gradient. Within assemblages, richness was influenced by sample size and evenness, while evenness and the number of individuals in a sample were significantly correlated only when considering samples from disparate communities. While the interactions of these variables are understood at the sample-level, it is not clear how the components of biodiversity influence one another when aggregating sample-by-sample and locality-by-locality. The purpose of this study is to determine how variables such as richness, evenness and sample size as well as paleoenvironmental variation and lithology can influence assessments of local and regional biodiversity.
To accomplish this goal, a total of 140 bulk samples yielding over 5,000 individuals were collected from the highly fossiliferous Fulton submember of the Kope Formation (type Cincinnatian, Upper Ordovician) of Ohio and Kentucky. These data were aggregated sample-by-sample to generate a series of local and regional-level biodiversity assessments. At each spatial scale, I generated rarefaction curves, dominance diversity assessments, collector's curves and assessed evenness. Additionally within the Fulton samples, the rare taxa (defined here as taxa with less than 1% of total taxonomic abundance) comprise only 3.2% of total fossil abundance, yet yield over 60% of the richness. This “rare” component of fossil communities, often overlooked, may also influence the way that biodiversity assembles. Since paleontologists seek to build biodiversity assessments and trends from composite data sets, this investigation into the quantitative relationships among sample-level and locality-level properties is an important step towards understanding how biodiversity assembles on multiple scales.
The Role of Low Pressure Fractionation in the Differentiation of Calc Alkaline Ankara Volcanics
USTUNISIK, Gokce K., Geology
In Central Ankara, Turkey, Middle Eocene (38.8-42.0±1.6 Ma) volcanism has produced calc alkaline dacites and rhyolites. Previous work on these volcanics has failed to reach a consensus on the nature of the differentiation process. Our hypothesis is that the calc alkaline dacites and rhyolites of the Ankara Volcanics are the products of fractional crystallization of calc alkaline parental dacitic composition of D-16.
Linear trends in major oxide variation diagrams suggest that fractional crystallization of a dacitic parental magma may be responsible for the chemical spectrum shown by the Ankara Volcanics. In order to test this hypothesis, we chose a dacite composition (D-16 with Mg number of 48.64) as the parental magma and using the MELTs algorithm we allowed it to differentiate under fractional crystallization conditions at fO2≈NNO.
Results of the MELTs calculations show that the best fit between the bulk composition of the Ankara Volcanics and calculated melt compositions is in the pressure range of 1.5-2.0 kbars with water content of 3.0-4.0 %.
These results suggest that low pressure fractionation of a dacitic magma located at about 6-8 km depth in the continental crust may be the principal process for the calc alkaline series of Ankara Volcanics.
Sedimentary Processes Associated with Deglaciation near Sioux Pond Lookout, Northwest Ontario
MILAWSKI, James E., Department of Geology; DENDRAMIS, Nancy C., Department of Civil and Environmental Engineering; MCFADDIN, Jared, Department of Civil and Environmental Engineering; MICHAELS, Joshua A., Department of Geology; REDDER, Myles, Department of Geology; LOWELL, Thomas V. Department of Geology and HUFF, Warren D., Department of Geology
Lacustrine sediments at Sioux Pond Lookout, northwest Ontario record the depositional events during the abrupt transition from the end of the Younger Dryas to the early Holocene. Published radiocarbon ages at Sioux Pond average 9740 ± 100 BP (Svante Bjorck, 1984). A core thrust taken from 650-750 cm depth at Sioux Pond includes 25 cm of gray clay-silt rhythmites overlain by a 20 cm gray massive silt unit overlain, in turn, by 50 cm of gyttya deposits. Grain size analysis in the gray silt region and rhythmite layers indicates a strong overprint of eolian processes. SEM images of sub-angular quartz grains support the partial influence of a wind blown sediment source. Total Gamma (API) spectral intensities are high in the rhythmite section, reflecting a high potassium content in the clay minerals. Gamma intensities are low in the gray massive silt due to a higher quartz and lower illite content, and at moderate levels in the gyttya. Total carbon is high in the gyttya, as expected, and moderate in the massive gray silt and rythmite sections. X-ray powder diffraction analysis of both bulk and <2?m size fractions show clay minerals, quartz, feldspar and carbonate in varying proportions throughout the section. Variable levels of illite and chlorite found throughout the core probably reflect sediment sources in the Canadian Shield. The massive gray silt region and the rythmite section both show significant levels of illite/smectite (I/S). We hypothesize that the I/S also reflects an eolian source, most likely from westerly winds transporting soil clays from smectite-rich Cretaceous shales in the Western Interior. A general decrease in detrital illite and chlorite was detected from the bottom of the core up through the massive gray silt. This trend is consistent with grain size data showing that the size of non-clay particles decreases upwards as eolian supply became more dominant. The influx of organics which increase upwards into the gyttya deposits suggests that the sediment source changed from eolian-dominated to fluvial-dominated. Relatively articulated pennate Diatoms (250-300um) were discovered in the middle gray silt layer below the gyttya deposits, reflecting the onset of an episode of climatic warming.
Shell Bed Genesis in Mudrocks: Sedimentary Dynamics and Taphonomic Feedback in a Mixed Siliciclastic-Carbonate Foreland Basin System (Upper Ordovician, Southwest Ohio and Northern Kentucky)
BRETT, Carlton E., Department of Geology, MCLAUGHLIN, Patrick, Geology Department, Bucknell University (UC Department of Geology alumnus)
The sedimentary rocks found throughout the Cincinnati area have been described as a layer cake. The conventional view is that the soft grey shale represents a deep-water environment, while the harder limestones were laid down, mostly by storms, in shallower water. Carl Brett of the University of Cincinnati and Patrick McLaughlin of Bucknell University have raised questions about this accepted interpretation. Brett and McLaughlin found evidence that a variety of factors including fluctuating sea levels, gradual accumulations of skeletal materials, and hurricane-generated deposition of muds as well as wave erosion may combine to create the patterns we see today.
Glaciolacustrine Sedimentation and Chronology of the Western Thunder Bay Lowland, Northwest Ontario
LOOPE, Henry; FISHER, Timothy; LOWELL, Thomas and HAJDAS, Irka
The deglacial chronology of the Laurentide Ice Sheet (LIS) within northwest Ontario is critical to understand the possible hydrologic linkage between glacial Lake Agassiz and the upper Great Lakes between 11.0 and 8.0 ka 14C yrs BP (13.0 to 8.9 ka cal yrs BP). Proposed outburst floods from Lake Agassiz through the Great Lakes to the North Atlantic Ocean have been implicated in the reduction of North Atlantic thermohaline circulation and pegged as a cause for the Younger Dryas cold event (11 – 10 ka 14C yrs BP; 13 – 11.4 ka cal yrs BP). Deglacial chronology and glacial stratigraphy are also important in determining the response of the LIS to the Younger Dryas cold period (i.e., regional readvance). The Marks Moraine west of Thunder Bay, Ontario has been previously correlated to the well-dated 10 ka 14C yrs BP ice margin positions on the southern end of the Superior Basin. Inside the Marks Moraine, glaciolacustrine sediments lie above and below brown-red Superior Lobe diamicton. Cutbank exposures along the Whitefish River and lacustrine cores from inside and outside the Marks Moraine were examined in order to discern the glacial chronology. Results from the Harstone Cut exposure (particle size analysis, loss on ignition, and magnetic susceptibility) show that at least 250 varves are present below Superior Lobe brown-red diamiction and 160 varves are present above diamiction from Mokomon Lake (~20 km to the north), although the basal diamicton was not reached in Mokomon Lake. Varves below diamicton indicate the Superior Lobe advanced into, and subsequently overrode, a glacial lake (glacial Lake O'Connor). These varves differ significantly from offshore deglacial Lake Superior varves - glacial Lake O'Connor summer beds are coarser (~15 µm) and rippled sand beds are present, indicating a proximal sediment source (i.e. the Superior Lobe). Varves above Superior Lobe diamicton are also distinct - they show more classic rhythmic sedimentation and are finer than those of Lake O'Connor. Glacial Lake O'Connor sediments indicate an advancing Superior Lobe within the lowland west of Thunder Bay and that the Superior Lobe was present in the lowland >250 years before the western end of the lowland was overridden. This indicates that the LIS was within the Superior Basin prior to the Superior Lobe advance, which is at odds with current interpretations.
Late Quaternary Landscape Evolution of Qaidam Basin, Northern Tibet
CHENG, Gongbi, Department of Geology
The Tibetan Plateau is one of the most important places influencing environment of surrounding areas and global climate change. Understanding the nature of climate change in this region is therefore important for modeling global climate change and the hydrological impact on the surrounding areas, which are home to one third of the world's populations. The Qaidam Basin floor has an average altitude of ~2700m asl and experiences hyper-arid conditions while the bordering mountains of the Kunlun, Altyn and Qilian Shan rise to over 5000m asl and are extensively glaciated. Alluvial fans, pediment surfaces, shorelines and a thick succession of sediments within the basin, coupled with moraines and associated landforms in the adjacent high mountain catchments of the Kunlun Mountains, record a complex history of Late Quaternary paleoenvironmental change and landscape evolution. The region provides an ideal natural laboratory to examine the interaction between tectonics and climate within a continent-continent collision zone and to quantify rates of landscape evolution as controlled by climate and the associated glacial and hydrological changes in hyper-arid and adjacent high altitude environments. The author focus on determining the evolutionary history and dynamic systems of the landscape during Late Quaternary in Qaidam Basin, associated with glacial and hydrological changes in this hyper-arid and adjacent high altitude environment. This will be compared with glaciations, shoreline changes and alluvial fan formation, combined with geomorphic mapping, analysis of sediments, and terrestrial cosmogenic radionuclide surface exposure and optically stimulated luminescence dating. The interpretation of landforms, their temporal and spatial relationships will allow assessing whether most of the landforms formed during times of climatic instability. Interpretation of the environment of deposition of the sediments and landforms provides proxies on the nature of earth surface processes, which when numerically dating will allow us to make paleoenvironmental reconstructions. Taken with the geomorphic evidence it will be able to provide a detailed history of paleoenvironmental change and landscape evolution for the Qaidam Basin for at least the last glacial cycle.
The Ordovician Radiation: A Macroevolutionary Crossroads
MILLER, Arnold I., BULINSKI, Katherine V., BUICK, Devin P., FERGUSON, Chad A., and HENDY, Austin J.W.
The Ordovician Period has long been a focal point for integrative research on the history of global biodiversity. Because it closely followed the Cambrian Explosion, researchers are seeking to understand whether the three-to-four fold increase in genus- and family-level diversity recognized during the Ordovician Radiation was an inevitable outcome of macroevolutionary transitions set in motion during the Cambrian, or whether it was contingent on physical or biological events unique to the Ordovician Period, or both.
Here, we consider three important themes related to the dissection of the Ordovician Radiation:
Given the remarkable research progress of the past two decades and the continued integration of data from new sources such as molecular biology, the Ordovician Radiation offers an unparalleled opportunity to understand the causes of a major global diversification, but a multidisciplinary effort will be required to achieve this goal.
Characterization Of Burgess Shale Type Biofacies From The Wheeler Shale, Utah
KRAMER, Anthony, Department of Geology
As probably the best-known example of Konservat Lagerstätten (fossil assemblages of unusually well-preserved remains), the Burgess Shale records a very rare image of both the typical hard part remains, and the almost exclusively absent soft parts remains of life in the aftermath of the Cambrian Explosion. The fauna of the Burgess Shale is well preserved in similar equivalent Burgess Shale Type (BST) deposits around the world, and has recently been characterized with a biofacies model (Gaines and Droser, 2005). One of the better known BST's is the Wheeler Shale. A multi-proxy analysis high resolution examination of the Upper Wheeler Shale of the Drum Mountains, Utah may provide new insight on the currently accepted model of Cambrian environments and the conditions that led to such wonderful preservation. The present model assumes the world has had relatively uniform environmental tolerances of benthic organisms since the early Cambrian. However, a potentially new biofacies below the predicted anoxic boundary could imply that life in the Cambrian did not comply with the assumptions of the model, and was indeed very different than previously acknowledged. Presently, there are three biofacies in the model based upon trace fossils seen similarly in younger and modern assemblages of fauna found in each: Oxic, Dysoxic, and Anoxic. The biofacies, which is characterized by an in situ diminutive fauna, is between the actual dysoxic/anoxic boundary and the last occurrence of benthic burrowers. The hypothesis is that the biofacies is a previously un-described biofacies that will either refute or refine the model for the progression of biofacies in equivalent faunas by application of independent proxies and detailed faunal study. The site includes all three previously recognized biofacies in succession with this new unit included in context. This interval could on a very practical scale serve as a good microcosm for BST's and depositional sequences around the world.
Coordinated Faunal Turnover in the Middle Devonian of Eastern North America
BARTHOLOMEW, Alexander J., Geology; BRETT, Carlton E., Department of Geology; DESANTIS, Michael K., Department of Geology
The hypothesis of Coordinated Stasis, developed from observations of extended periods of ecological stability in the Middle Devonian of Appalachian Basin, has been tested in various depositional basins throughout the rock record. However, no tests of the hypothesis have been conducted in coeval sediments in areas outside of the Appalachian Basin. With the advent of a well-resolved stratigraphic framework based on biostratigraphic and sequence stratigraphic data, it is now possible to test for patterns of Coordinated Stasis in sediments coeval with the “type strata” where the hypothesis was developed. The Middle Devonian (late Eifelian through mid-Givetian) strata of the Michigan and Illinois basins, identified as separate biogeographic subprovinces, preserve a diverse, benthic fauna. High resolution stratigraphic correlations between the Appalachian, Michigan, and Illinois basins through this interval provide a stratigraphic framework in which to test fine-scale patterns of faunal turnover across a large portion of Eastern North America (ENA). Identified within the Appalachian Basin are a series of Evolutionary Ecological Subunits (E.E. Subunits) defined as extended periods of relatively stable faunal composition bounded by short intervals with high rates of faunal turnover. Examination of the patterns of faunal composition through the Middle Devonian of the Michigan and Illinois basins has resulted in the delineation of E.E. Subunits in these areas for the first time. The lowest E.E. Subunit recognized contains the unique Stony Hollow Fauna (SH), dominated by warm-water taxa, and is identifiable across all areas. This fauna is associated with a major flooding event in ENA. A major faunal turnover occurs between the SH faunal interval and the overlying Hamilton/Traverse (HT) E.E. Subunit. The boundary between these units is correlative between the basins, suggesting an extra-basinal cause of the faunal turnover, i.e. sea level fall. The HT interval is marked by a long-lasting, diverse fauna in all basins. The upper boundary of this interval is preserved only in the Appalachian Basin, having been removed by erosion in other areas. However, faunas above the HT interval in the Michigan and Illinois basins are dissimilar from the HT Fauna indicating a period of turnover in roughly the same interval.
A Test of Ambocoeliid (Brachiopoda) Spinosity as an Adaptation for Soft Substrates Using Abundance Patterns Through Transgressive-Regressive Cycles and Within Pyritic Shale Beds
ZAMBITO, James J. IV, Department of Geology
“My research focuses on relating the presence and abundance of a family of brachiopods (shelled invertebrates) to sediment firmness and general sea level fluctuations,” Zambito says. “The brachiopods I am studying occur both with and without spines, which appear to be adaptations for staying on top of soft sediments. Implications include the use of the presence of certain species of brachiopods as general indicators for sea level rise and fall in the Middle Devonian Appalachian Basin of New York.”
Ecosystem Dynamics and the Sedimentary Record: Assessment of Faunal Patterns and Environmental History from Cross Bank, Florida Bay
FERGUSON, Chad Allen, Department of Geology
The effects of humans on ecosystems is a persistent and growing problem in coastal marine settings. Chad Ferguson returned to the site of nutrient pollution that was initiated during a two-decade-old study along Cross Bank, Florida Bay, in Everglades National Park. He found evidence of a transition in dominance of seagrass species, changing the substrate for animals (such as clams and snails) that dwell on the ocean floor. The objective of this research is to assess: 1) effects of changing environments on groups of mollusks in the upper sedimentary veneer; and 2) potential for similar transitions to be detected within the deeper subfossil record.
Comparison of surficial cores from altered and unaltered environments on Cross Bank demonstrates that molluscan death assemblages responded compositionally to change in seagrass community dominance. In particular, one grazing gastropod (or snail), is highly abundant at unaltered sites but occur at greatly reduced abundances at altered sites. Further analyses have shown an association between size attained by one species of seagrass and differences in blade width of the two seagrass species, with snails attaining greater size, suggesting a size threshold.
“By assembling a subfossil-based environmental history of Cross Bank,” says Ferguson, “I can enhance our understanding of environmental perturbations, human-induced or natural, and their ecological effects for evaluation of future changes in coastal systems.”
Size Distributions of Plagioclase Crystals in Lava Samples from Kilauea
BROCKMAN, Tracy L., Department of Geology
Crystal size distribution (CSD) is a theory first envisioned by chemical engineers Randolph and Larson in 1971, and brought to igneous petrology by Bruce Marsh in 1988. It is a measure of growth rate and age and generally predicts a log-linear distribution with increasing size. Many researchers have performed CSD studies on a number of different minerals, including garnet, pyroxene and plagioclase. Tracy Brockman used crystal size distribution analysis of basaltic lava from the 1985 Kilauea eruption to determine the magma residence time. (Magma residence time is roughly the time spent in the magma chamber. It has implications for input and output.) Crystal sizes of plagioclase feldspars from the 1985 flow at the Royal Gardens area have been measured in the Image Analysis lab and data have been analyzed. A plot of the number of plagioclase crystals per unit size per volume against crystal size yields a linear relationship. Using the published growth rate for plagioclase, the slope equation gives 2.69 years as the magma residence time.
“Since the ongoing eruption of Kilauea started in 1983 and my samples are from the 1985 flow, 2.6 years’ residence time based on my CSD analysis agrees very well with the residence time of the Kilauea magma,” says Brockman.