Browsing by Subject "stable isotope"
Now showing 1 - 5 of 5
Results Per Page
Sort Options
Item Circulation of North American epicontinental seas during the Carboniferous using stable isotope and trace element analyses of brachiopod shells(2012-07-16) Flake, Ryan ChristopherPrevious studies have identified ???C events in the Carboniferous that imply major shifts in the carbon cycle. However, inherent in this interpretation is the assumption that epicontinental seas are chemically representative of the global ocean. Our study uses stable isotope and trace element analyses of brachiopod shells to examine changes in climate and circulation of the North American epeiric sea. Formations were selected for study to provide shallow marine environments with geographic coverage of North America. These units include the Grove Church and Mattoon Formations (Illinois Basin), Glenshaw Formation (Appalachian Basin), Bird Spring Formation (Bird Spring Basin), and Oread Formation (US midcontinent). In all, 98 brachiopod shells were found to be well preserved based on screening with plane light and cathodoluminescence microscopy of thin-sections, and trace element analyses. Upper Chesterian Grove Church (Illinois Basin) samples have ???C and ???O averages of 1.1% and -3.1% respectively. These low values are interpreted as a local or regional effect caused by terrestrial runoff. Terrestrial influences are also suggested by the depositional environment: nearshore marine. Chesterian samples from the Bird Spring Formation at Arrow Canyon, Nevada average 3.7% and -1.4% for ???C and ???O respectively. The higher ???C and ???O values, compared with samples from the time equivalent Grove Church, likely reflect the freer exchange with the Panthalassa Ocean at this most western edge of North America, and best represent open-ocean conditions. Samples from the Virgilian Ames-Shumway-Plattsmouth cyclothem show a progression of ???C and ???O enrichment moving west from near the Appalachians (1.9% and -3.8%) to the Illinois Basin (3.2% and -2.4%) and finally to the US midcontinent (4.2% and -1.5%). This is interpreted as the transition from nearshore, terrestrial influence with enhanced organic matter oxidation and lower salinity to well-mixed conditions with normal salinities and potential for seafloor ventilation and upwelling. This is supported by published sediment ?Nd(t) values from the Appalachian Basin (?Nd(t) = -9) that increase further westward (?Nd(t) = -6) due to higher influence from the eastern Panthalassa Ocean. Mass balance calculations based on the ???O of the brachiopod shells suggest salinities of 25 and 31 psu for the Appalachian and Illinois Basins, respectively, assuming salinities of 34.5 psu for the US midcontinent. Trace element analyses do not show a systematic east-west trend similar to stable isotopes. In both time slices, spiriferids from the intermediately-located Illinois Basin are enriched in Mg/Ca and Sr/Ca relative to those in other basins. This Mg and Sr enrichment in Illinois Basin brachiopods suggests delivery of Sr-rich fresh waters and restricted circulation in that basin.Item Ecohydrological Controls and Effects of Rhizome Integration on the Performance of Arundo donax in a Rio Grande Riparian Zone(2012-10-19) Kui, LiThis study focused on an invasive riparian reed grass, Arundo donax L., a clonal plant of the family Poaceae that is widely distributed in North America. Water availability, including water taken up from the roots locally or transported from the neighboring ramets, may affect the performance of A. donax in riparian zones. The first objective was to find out how moisture gradients affected the performance of A. donax in riparian zones. I measured leaf photosynthetic rate, leaf ?13C ratio, and plant growth-related parameters across two summer growing seasons at four transects perpendicular to the water course on the Rio Grande in South Texas. The second objective was to find out whether physiological integration existed in A. donax and how resource sharing, if any, affected plant growth. A rhizome severing experiment was conducted on five paired plots to compare growth-related parameters between plots with rhizomes severed and intact at 3, 7, and 11 weeks after treatment. Heavy water (? 2H ~1800?) was applied on three 1-m2 area over 3 successive days and rhizome samples were collected beyond the watering zone after 5, 24, and 48 hours of last watering. At short-term scales, A. donax performance was adversely affected by both drought and inundated conditions; over longer time scales, plant performance decreased as water availability declined in general, but biomass and stem density were similar across moisture gradients. I also found evidence of physiological integration in A. donax. Water was transported through interconnected rhizomes at least 3.5 m; transport distances averaged 1.67 m. Rhizome severing stimulated higher ramet production initially but over longer periods produced shorter thinner stems with lower flood tolerance. However, after 11 weeks of re-growth, plot-level biomass was similar between plots with severed and intact rhizomes. These results suggest that performance of A. donax is affected by water availability in riparian zones; however, clonal plant plasticity, water use efficiency, and clonal integration ameliorate impacts of water stress on the performance of A. donax. Such traits enhance its resource use, which could potentially increase competitive ability rate of establishment, and extent of this invasive species in heterogeneous riparian environments.Item Influence of Insulin Resistance on Contractile Activity-Induced Anabolic Response of Skeletal Muscle(2011-02-22) Nilsson, Mats I.Although the long-term therapeutic benefits of exercise are indisputable, contractile activity may induce divergent adaptations in insulin-resistant vs. insulin-sensitive skeletal muscle. The purpose of this study was to elucidate if the anabolic response following resistance exercise (RE) is altered in myocellular sub-fractions in the face of insulin resistance. Lean (Fa/?) and obese (fa/fa) Zucker rats were assigned to sedentary and RE groups and engaged in either cage rest or four lower-body RE sessions over an 8-d period. Despite obese Zucker rats having significantly smaller hindlimb muscles when compared to age-matched lean rats, basal 24-h fractional synthesis rates (FSR) of mixed protein pools were near normal in distally located muscle groups (gastrocnemius, plantaris, and soleus) and even augmented in those located more proximally (P<0.05; quadriceps). Although 2 x 2 ANOVA indicated a significant main effect of phenotype on mixed FSR in gastrocnemius and soleus (P < 0.05), phenotypic differences were partially accounted for by an exercise effect in the lean phenotype. Interestingly, obese rats exhibited a significant suppression of myofibrillar FSR compared to their lean counterparts (P<0.05; gastrocnemius), while synthesis rates of mitochondrial and cytosolic proteins were normal (gastrocnemius and quadriceps), suggesting a mechanism whereby translation of specific mRNA pools encoding for metabolic enzymes may be favored over other transcripts (e.g., contractile proteins) to cope with nutrient excess in the insulin-resistant state. Immunoblotting of the cytosolic fraction in gastrocnemius muscle indicated an augmented phosporylation of eIF4EBP1 (+ 9%) and p70s6k (+85%) in obese vs. lean rats, but a more potent baseline inhibition of polypeptide-chain elongation as evidenced by an increased phospho/total ratio of eEF2 (+78%) in the obese phenotype. Resistance exercise did not improve synthesis rates of myofibrillar, cytosolic, or mitochondrial proteins to the same extent in obese vs. lean rats, suggesting a desensitization to contractile-induced anabolic stimuli in the insulin-resistant state. We conclude that insulin resistance has diverse effects on protein metabolism, which may vary between muscle groups depending on fiber type distribution, location along the proximodistal body axis, and myocellular sub-fraction, and may blunt the anabolic response to voluntary resistance exercise.Item Neogene Low-latitude Seasonal Environmental Variations: Stable Isotopic and Trace Elemental Records in Mollusks from the Florida Platform and the Central American Isthmus(2012-10-19) Tao, KaiThis Ph.D. dissertation integrates stable isotope and trace element geochemistry in modern and fossil gastropod shells to study low-latitude marine paleoenvironments. First, stable isotopes (delta18O and delta13C) and Sr/Ca ratios are used to examine low-latitude temperature and salinity variations recorded in Plio-Pleistocene (3.5-1.6 Ma) fossils from western Florida during periods of high-latitude warming and "global" cooling. The middle Pliocene Pinecrest Beds (Units 7 and 4) and the overlaying Plio-Pleistocene Caloosahatchee Formation generate significantly different delta18O-derived paleotemperatures but identical Sr/Ca ratios. High delta18O values, together with low delta13C values and brackish fauna, indicate that Unit 4 was deposited in a lagoonal environment similar to modern Florida Bay. In contrast, relatively low delta18O and high delta13C values in Unit 7 and Caloosahatchee Formation represent deposition in an open-marine environment. The observed Unit 7 and Caloosahatchee paleotemperatures are inconsistent with middle Pliocene warming event, but consistent with the Plio-Pleistocene cooling trend. To quantify modern upwelling and freshening signals and contrast these signals between the tropical eastern Pacific (TEP) and southwestern Caribbean (SWC), methodologies are developed for reconstructing seasonal upwelling and freshening patterns from modern tropical gastropod shells from Panama using: 1) paired oxygen and carbon isotopic profiles and delta18O-delta13C (delta-delta) correlations, and 2) deviation from baseline delta18O values that represent conditions free of seasonal upwelling or freshening influences. Shell delta18O values normalized to the baseline faithfully record modern conditions of little or no upwelling in SWC and Gulf of Chiriqui, and strong upwelling in the Gulf of Panama, as well as strong freshwater input in most areas. The baseline and delta-delta methods are applied to identify and quantify changes in upwelling and freshening in the Neogene TEP and SWC seawaters associated with the final closure of Central American Isthmus. The records reveal significant upwelling in late Miocene SWC and mid Pliocene TEP waters, strong freshening in SWC waters from 5.7-2.2 Ma, and minimal seasonal upwelling and/or freshening variations in Plio-Pleistocene SWC waters. The reconstructed paleotemperatures agree with the global cooling trend through the late Miocene, but lack evidence for middle Pliocene warming or late Neogene global cooling.Item The role of grain sorghum in conservation of predatory arthropods of Texas cotton(Texas A&M University, 2004-09-30) Prasifka, Jarrad ReedFour separate but complimentary studies investigated the role of grain sorghum as a predator source for Southern Rolling Plains cotton in 2001 and 2002. Objectives were to: (1) determine the timing and magnitude of predator movement between crops, (2) test putative causes of movement by manipulating prey levels at different stages of crop phenology, (3) explore the feeding and reproductive behavior of a common predator colonizing cotton, and (4) examine the effects of grain sorghum and uncultivated areas on cotton predator abundance at an area-wide scale. Rubidium mark-recapture experiments indicated grain sorghum fields produced a net predator gain for adjacent cotton. Analysis suggested two coccinellids, Hippodamia convergens Gu?rin-M?neville and Scymnus loewii Mulsant, were responsible for the overall pattern of predator movement. Predator movement into cotton did not appear to be concentrated at specific stages of sorghum phenology. Manipulations of aphid levels in field cages were used to determine if prey abundance or phenology influenced the movement of H. convergens into cotton. In both years, more lady beetle adults were collected on cotton during the latest stages of sorghum phenology. In the second year, relatively low aphid densities (15 per plant) appeared to influence the movement of beetles onto caged cotton. Carbon isotope ratios of H. convergens were used to assess adult feeding behavior after colonizing cotton and to determine if prey consumed in sorghum contributed to egg production in cotton. Though aphids were absent 2001, H. convergens adults stayed in cotton, did not produce eggs and apparently consumed few prey. Cotton aphids were present in 2002 and H. convergens isotope ratios changed from prey consumed in cotton. The isotope ratios of egg masses collected in 2002 indicated prey consumed in grain sorghum contributed very little to egg production in cotton. An area-wide pattern analysis suggested the abundance of grain sorghum and uncultivated areas both positively influenced cotton predator levels. While these landscape effects were less important overall than prey levels and cotton planting dates, in some sampling periods landscape composition appeared to be the most important factor in determining cotton predator levels.