Browsing by Subject "Oxygen isotopes"

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    A 10,000-year lake-sediment based reconstruction of precipitation isotope values in the Canadian Rocky Mountains and implications for past changes in North American hydroclimate
    (2019-01) Wagner, Zachary
    Hydroclimate change in the Canadian Rocky Mountains is an important area of research, as demand for water resources in the Great Plains has been continuously increasing in recent years. The population of Alberta has one of the fastest growth rates in Canada, and cities like Calgary and Edmonton are dependent on surface and groundwater resources that originate from precipitation in the mountains. Recent increases in petroleum exploration have also amplified demands for water, along with the growing water requirements of industrial agriculture. The application of sound water resource management policies is essential, and historical records span only ~200 years, a time frame too short to capture the full range of climate variability. The development of paleoclimate proxy records from the Rocky Mountains is therefore necessary to attain a thorough perspective on potential changes in climate. Such data can, for example, inform water resource managers of possible shifts in precipitation seasonality and drought/pluvial events on timescales of decades to millennia. To this end, we present a 10,000-year oxygen isotope record as a proxy for precipitation seasonality linked to the Pacific North American pattern (PNA) which adds to a growing body of research in a region of high spatial complexity of hydroclimate. Shark Lake in Alberta, CA (50.8412°N, 115.3990°W; 1857 m above sea level) is a hydrologically open basin lake in the Canadian Rocky Mountains with one large outlet and numerous small inlets and groundwater springs. Winter precipitation is more depleted in the heavier 18O isotope relative to summer precipitation due to equilibrium fractionation from rain-out and phase changes. δ18O and δD values of Shark Lake water samples (collected during the summer) range from -18.9 to -20.0 ‰ and -141 to -150 ‰, respectively. The annual weighted average precipitation isotope ratios are -16.6 and -126 ‰ for oxygen and hydrogen, respectively. This indicates that Shark Lake principally receives water inputs from runoff or shallow groundwater that originated as precipitation during the cold season. We collected 1 m and 1.5 m long sediment cores using a modified Livingstone corer and used loss-on-ignition (LOI), x-ray diffraction (XRD), x-ray fluorescence (XRF) analyses to destermine sediment texture and composition. The sediment was dated using 14C from terrestrial plant fossils, as well as 210Pb and 137Cs for the surface sediment. We analyzed the isotopic composition (δ18O) of authigenic carbonate sediment from Shark Lake using mass spectrometry and applied these results as a proxy for precipitation seasonality. Lake water oxygen isotope ratios are captured by authigenic carbonates, which form during the spring and summer in response to pH changes associated with primary production. These carbonate minerals (calcite) precipitate in isotopic equilibrium and are deposited on the lakebed where they are preserved. The Shark Lake δ18O record demonstrates a transition from lower to higher average δ18O values from the middle to the late-Holocene at around 4500 yr BP. This is consistent with previous findings of changes in PNA-like atmospheric patterns during the middle Holocene, when a gradual shift from a more negative to a more positive mean state phase of the PNA occurred. This produced enhanced zonal atmospheric circulation in the Pacific Basin that led to a reduction in winter precipitation in northwestern North America and drier conditions in the southwestern part of the continent in the late Holocene relative to the middle Holocene. The Shark Lake δ18O record has a positive, significant correlation with other similar records from the Pacific Northwest, specifically those from Lime Lake (WA) and the OCNM (OR) speleothem, and has a negative, significant correlation with records from the southern Rocky Mountains and eastern North America, namely those from Bison Lake (CO), Cheeseman Lake (NL), Grinnell Lake (NJ), and the Buckeye Creek Cave speleothems (WV). Decadally resolved records are useful for tracking changes in PNA state and its interaction with other related climate oscillations such as the El Niño Southern Oscillation (ENSO) and the associated Pacific Decadal Oscillation (PDO). Open-basin lake records can also be combined with hydrologically closed-basin lake records to reconstruct pluvial and drought periods over the Holocene. The Shark Lake record increases the spatial resolution of mid- to late Holocene hydroclimate climate data in the Rocky Mountains and provides a baseline for natural variability in precipitation seasonality in a hydrologically important region.
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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.