Application of Organic Geochemical Proxies in the Environment

• Other Authors: Das, Oindrila (authoraut)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Earth sciences; Oceanography; Atmospheric sciences; Geophysics
• Online Access: http://purl.flvc.org/fsu/fd/FSU_migr_etd-6905

This dissertation research has used stable and radioactive isotope techniques to address several important issues in paleoclimate and paleoecoloical research which includes the reliability of using stable carbon isotope composition of black carbon or charcoal to reconstruct paleovegetation, the utility of organic geochemical proxies as paleo-storm indicators, and the reliability of radiocarbon dating of both organic and inorganic carbon in coastal dune lake sediments. The results are below: (i) C4 and C3 grasses were subjected to burning in the laboratory to determine whether there was any significant fractionation of carbon (C) isotopes between plant material and corresponding ash and smoke produced from burning. The results show that smoke produced from C4 grasses is generally depleted in 13C relative to the original plant, but the magnitude of the 13C depletion varies with species from less than 0.5 / to a maximum of 7.2 /. Ash derived from C4 grasses, on the other hand, is either depleted (by 0.1 to 3.5 /) or slightly enriched (<1 >/) in 13C relative to the original grass depending on species. In contrast, both smoke and ash produced from C3 plants do not show any significant deviation in 13C from that of the original plant material. Our data also show that the C isotope fractionation between ash and smoke and the original plant material depends not only on plant species and plant type but also on burning temperature. The weight percentage of C in ash and smoke decreases with increasing burning time in the temperature range 400-7000C. Multi-elemental thermo analysis of ash, smoke and original plant material reveal distinctly different chemical characteristics for these materials. Ash is preferentially enriched in compounds with higher thermal stability whereas smoke contains a wide spectrum of compounds with different stability in comparison with the original plant material. C4 grass appears to be more thermally stable than C3 grass. These results have important implications for paleoecological or ecological studies based on 13C signatures of black C or charcoal. (ii) Late Holocene paleoclimate records from coastal regions are important for understanding long-term variability of hurricane activity. Here we present a nearly 4000-year record of severe storms and environmental changes based on organic geochemical proxies (OGPs) preserved in sediment cores from two coastal dune lakes in northwest Florida. The OGPs data show that there are significant variations in δ13C, δ15N, C%, N% and C/N with depth, reflecting changes in lake environment that affected the processes delivering water/sediment to the lake as well as biological productivity within the lake. Analysis of modern organic materials in the lakes and their surrounding areas shows that the major sources of sedimentary organic matter in the lake are aquatic and terrestrial C3 vegetation. C4 grasses, although can be found in the mostly forest watershed, do not contribute significantly to the sedimentary organic matter in the lake. Thus, the positive C and N isotopic shifts, concurrent with negative shifts in C/N ratios, most likely indicate shifts to a marine-like environment in coastal lakes following the influx of marine water/nutrients and marine biota associated with major storm events. Some of these isotopic shifts observed in the sediment cores correspond to visible sand layers presumably representing overwash deposits associated with severe storm events. Radiocarbon dating of bulk sediment organic matter, wood fragments and shells indicates that the sediment in these cores was deposited over the last 3-4 thousand years. Assuming the radiocarbon dates are reliable, Eastern Lake data suggest that the recurrence interval of severe storms (i.e., large enough to cause seawater flooding of the lakes) is approximately 83 years over the last 2900 years, whereas Western lake data suggest an average recurrence interval of 87 years in the past 3900 years. iii) Our preliminary radiocarbon data from two coastal lakes in NW Florida show that DOM in these lakes has modern 14C signatures while POM is depleted in 14C compared to the DOM. The positive 14C values of DOM are due to the influence of the "bomb" radiocarbon and indicate that DOM in these lakes during the time of our sampling were originated primarily from organic matter photosynthesized in recent decades. The variable radiocarbon ages of POM and DOM in the lakes at different times reflect the heterogeneity in the sources of organic carbon. Our data also reveal inconsistencies in radiocarbon ages derived from bulk sediment organic matter. Radiocarbon ages of sediment organic matter are relatively older than contemporaneous shell or wood samples found in the sediment cores. Because bulk sediment organic matter is a mixture of organic matter from both aquatic and terrestrial sources, influx of old or dead organic carbon from different old organic matter deposits or soils due to erosion by runoff is most likely the cause of radiocarbon deficiencies in the lake sediments, resulting in radiocarbon ages of bulk sediment organic matter being older and erroneous. On the other hand, our limited 14C dates on shells seem to be more or less consistent with the dates of wood fragments in the same core, suggesting that these carbonate shells may have been formed in 14C equilibrium with the atmosphere and therefore may serve a reliable and useful substrate for radiocarbon dating when wood fragments cannot be found. Analysis of additional samples is required to investigate and quantify the reservoir effect on organic and inorganic carbon in coastal lakes. === A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester, 2012. === October 17, 2012. === Includes bibliographical references. === Yang Wang, Professor Directing Dissertation; Jeff Chanton, University Representative; A. Leroy Odom, Committee Member; Stephen A. Kish, Committee Member; Yuch Ping Hsieh, Committee Member.