Browsing by Subject "Carbon isotopes"

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    Anthropogenic climate change has driven Lake Superior productivity beyond the range of holocene variability: an organic and stable isotopic study of Human impacts on a pristine biogeochemical system
    (2013-07) O'Beirne, Molly D.
    Recent studies have noted that changes in Lake Superior's physical, chemical and biological processes are apparent - including a warming of the surface waters at a rate twice as great as the surrounding airshed in the last 20 years. These changes are often difficult to perceive as cause for concern when not placed within a historical context. In this study, bulk C and N elemental abundance and stable isotope analysis of sediments from three piston and corresponding gravity cores, representing a record of lake-wide paleoproductivity trends spanning the Holocene, allows for the historical comparison with recent (1800 A.D. to present) productivity trends. Overall, Lake Superior experiences a slow, steady increase in productivity consistent with the concept of `natural' eutrophication, which is characterized by gradual increases in TOC and TON, as well as the steady 13C-enrichment of bulk sedimentary organic carbon and 15N-enrichment of bulk sedimentary organic nitrogen compositions. Over the last 200 years bulk concentrations and stable isotope compositions of carbon and nitrogen from eight multicores sampled at high resolution indicate that the Lake Superior basin has undergone productivity changes in the last two centuries (1800 to present) which are unique in the context of the Holocene. Overall, lake-wide sedimentary bulk organic proxy data show increasing primary production between 1900 and present, as indicated by an ~2 ‰ increase in δ13Corg. The most recent increases in productivity are likely a response to increasing water temperatures and longer stratified periods reported in Lake Superior. Down-core variations in the δ13C composition of algal-derived short-chain n-alkanes do not exhibit the same trend as that observed for bulk sedimentary organic matter (δ13Corg). The δ13C of bulk sedimentary organic matter shows systematic 13C-enrichment over the last ~9000 years, while the δ13C values of aquatic derived n-alkanes exhibit a systematic 13C-depletion to present-day. Down-core variation in δ13C values of n-alkanes likely reflect multiple isotope effects associated with carbon partitioning and fractionation associated with the biosynthesis of n-alkanes.
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    Stable isotopic investigation of late Neogene terrestrial paleoecology and paleoclimate of the circum-Mediterranean region
    (2010-08) Matson, Samuel Dean
    The late Neogene was an interval of important global change, in which gradual cooling and aridification resulted in terrestrial ecosystems over much of the world that became essentially modern. The geologic record of the circum-Mediterranean region presents an exceptional opportunity to examine the interplay of tectonics, biology, and climate during this important transition, because the paleogeography of this region was influenced heavily by a unique tectonic situation governed by both large-scale convergence between Europe and Africa and smaller-scale extension within the Mediterranean Basin. Effects of this distinct tectonic regime include the establishment of land bridges that allowed migration of animals between Europe, Africa, and Asia, large-scale desiccation of inland seas due to tectonic closure of seaways connecting the Mediterranean Sea and the Atlantic Ocean, and development of intramontane basins that preserve an extensive sedimentary record of past continental environments. In combination with this unique tectonic situation, regional climatic and ecological effects in the circum-Mediterranean region resulted in a late Neogene transition to modern terrestrial ecosystems that was in many ways different than general global patterns. In order to better undstand the late Neogene transition both regionally and globally, the research presented here focuses on reconstruction of terrestrial paleoclimate and paleoecology in Spain and Italy through the development of a stable isotopic record from biogenic and and authigenic minerals preserved in fossil mammals and continental sediments. vii A reconstruction of Late Miocene to Pleistocene paleoclimate and paleoecology in Spain was developed through analysis of the oxygen isotopic composition (δ18O) of biogenic phosphate in tooth enamel and dentine from fossil mammals. Comparisons of δ18O between clades are consistent with morphological interpretations of habitat and physiology, and suggest a semi-aquatic habitat for anthracotheres, hippopotamids, and castorids, and open or mixed habitats for most gracile taxa such as equids and cervids. Comparisons of enamel and dentine δ18O indicate slight diagenetic alteration of dentine, but also suggest that such comparisons can be used to reconstruct reasonable values of diagenetic water δ18O. Since the δ18O of modern horses has been demonstrated to be a reasonable proxy for the δ18O of local meteoric water, which is in turn strongly dependent on mean annual temperature (MAT) for modern mid- to high-latitudes, the δ18O of fossil horses from Spain was used to reconstruct terrestrial paleotemperature. These reconstructions are consistent with global cooling during the late Cenozoic, with MAT for the late Miocene that is warmer than today by ~1–2 ºC in NE Spain and by ~4–5 ºC in SE Spain. The difference of ~8–9 ºC between NE and SE Spain for the Late Miocene is ~60% greater than the MAT difference between these same areas today. To examine the ways in which a desiccated Mediterranean Basin affected surrounding terrestrial environments during the Messinian Salinity Crisis (MSC), a paleoclimatic record of this event was developed through integrated analyses of sedimentology, δ18O, and the stable carbon isotopic composition (δ13C) of latest Miocene authigenic carbonates from the Baza Basin in southern Spain. A transition from dolomite- and calcite-rich palustrine and distal alluvial fan sediments to lacustrine diatomites and calcite-rich limestones is accompanied by a decrease in both δ13C and δ18O, reflecting increased lake level under a wetter climate. The mean δ18O of latest Miocene lacustrine calcite is significantly lower than that of modern closed-basin lakes in the Iberian Peninsula, and likely represents overflow or through-flow conditions with inflow waters derived from the surrounding Betic mountains. This result is consistent with some aspects of climate model reconstructions of the MSC, which suggest strengthened storm tracks from the Atlantic Ocean over southern Europe. Orographic uplift of these air masses along the Betic Cordillera may have resulted in enhanced precipitation and runoff in southern Spain. To examine the interplay between tectonics, environmental change, and biological evolution, a paleoecological record was developed from the δ13C of Late Miocene paleosols from the Baccinello Basin in northern Italy. These paleosols span the extinction of Oreopithecus bambolii, which was the only European hominoid to survive an important extinction event ca. 9.6 Ma. Oreopithecus is important for understanding the evolutionary history of Late Miocene hominoids, since its peculiar morphology precludes a simple interpretation of its phylogenetic position. The paleosol δ13C values show very low temporal and spatial variability (indicating plant ecosystem stability through time) and provide no evidence for ecologically significant changes in floral composition spanning the Oreopithecus extinction event. These results validate assumptions about the importance of tectonics and species interaction as an underlying cause for the extinction of Oreopithecus and its associated fauna. The paleosol δ13C values fall entirely within the range of isotopic variability for modern plants following the C3 photosynthetic pathway, indicating that C4 vegetation was not an important component of biomass. The research presented in this dissertation underscores the importance of the Mediterranean region for consideration of the interplay of climate, tectonics, and ecology during important global transitions occurring in the Late Miocene. The results of this research validate the utility of stable isotopic approaches to paleoenvironmental reconstruction, and provide a powerful complement to independent means of reconstructing terrestrial systems that are complex and often poorly understood, but nevertheless an extremely important component of the Earth System.