Browsing by Subject "Paleoclimate"

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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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    Evidence of Late Pleistocene Heinrich Stadials in Northeastern Brazil
    (2018-11) Berry, Akemi
    Quaternary paleoclimatology is an ever-expanding field. The last glacial period has been characterized by temporally small-scale periods of warming and cooling. Long-term climate oscillations have been attributed to solar forcing, while the cause of these more frequent oscillations are of greater debate. One such cycle is characterized by periods of extreme cooling that triggered freshwater and iceberg excursions into the North Atlantic. The massive release of freshwater and ice into the North Atlantic is thought to have caused an abrupt global reorganization of the ocean-atmosphere circulation. These extreme cooling events are named Heinrich Stadials. Paleoclimate studies have sought to better understand the climatic response to North Atlantic cooling events across the world. In the case of Heinrich Stadials, understanding the response of the hydroclimate throughout the world, and the relationship between local responses, is essential to further our understanding of Heinrich Stadials and their possible causes. In the ocean-atmospheric circulation changes related to Heinrich Stadials, the position of the Intertropical Convergence Zone (ITCZ) plays a pivotal role. Paleo-records suggest that the cooling in the North Atlantic associated with Heinrich Stadials triggered a southward migration in the ITCZ, a response consistent with altering ocean-atmospheric circulation due to influx hemispheric and ocean cooling. Likewise, a shift in the ITCZ could result in a shift in the location and amount of tropical precipitation. Here, we reconstruct one such paleo-record to continue the global reconstruction of climatic response to Heinrich Stadials. Semi-arid Northeastern Brazil is located south of the modern-day ITCZ position. However, large numbers of speleothems in Toca da Boa Vista (TBV) and Toca da Barriguda (TBR) caves in Northeastern Brazil suggest that there were periods of intense rainfall. One possible source of this past rainfall is a southern migration of the ITCZ. Previous studies have linked short-lived speleothem growth in both caves to Heinrich Stadials (X. Wang et al., 2004; Wendt, 2015). In the following chapters, we present chronologies from 5 distinct periods: 134.8 kyr-130.2 kyr, 72.5 kyr- 71.6 kyr, 66.5 kyr-59.7 kyr,54.9 kyr-54.8 kyr and 48.9 kyr-48.2 kyr. This high-resolution, absolute dated, multi-stalagmite oxygen isotope record considers the hydroclimate of Northeastern Brazil. The main record herein extends through the latter portion of Marine Isotope Stage 4, including HS-6, spanning from 66.4 to 59.7 kyr. We have dated three stalagmites using the U/Th method, with uncertainties in age of average ±0.2 kyr. Growth is continuous in TBV-63 from 62.7 to 59.7 kyr, in TBV-67 from 63.6 to 59.7 kyr and in TBV-34 from 66.4 to 63.0 kyr, with a growth hiatus between 64.8 and 64.0 kyr, within error of Greenland Interstadial (GIS) 18. Immediately following this hiatus, TBV-34 shows an abrupt drop in δ18O values, implying a rapid increase in rainfall likely related to southward migration of the ITCZ associated with stadial conditions in the North Atlantic. The δ18O record of these NE Brazil stalagmites is largely anti-phased with the East Asian Monsoon (EAM). Due to the steady decrease in δ18O values until 59.7 kyr and the correlation with the weak monsoon interval, we infer that rainfall over the cave site steadily increased during the prolonged stadial. An abrupt increase in δ18O values marks the end of growth in these samples and is synchronous within uncertainties with the strengthening EAM at 59.8 kyr, which marks the beginning of GIS-17 and Chinese Interstadial (CIS) 17. The anti-phase relationship between these distant, low-latitude caves supports the hypothesis of a southward migrating ITCZ sensitive to abrupt North Atlantic climate anomalies, and suggests a rapid teleconnection of atmospheric signals during HS-6.
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    Forced oscillators with Dynamic Hopf bifurcations and applications to paleoclimate
    (2014-05) Oestreicher, Samantha Megan
    Mathematical modeling is an important tool for understanding historic and future climate. The 100,000 year problem, or the mid-Pleistocene transition, has generated a variety of models to understand Earth's climate. In this work a collection of dynamic Hopf bifurcation models are analyzed to isolate the problems and challenges of this type of model. A classic Maasch and Saltzman model is shown to be insufficient. Hopf bifurcation is generalized to the McGehee and Peckham model to conclude that, in certain idealized situations, the phase of a numerical solution can be predicted. However, when small stochastic noise is added to the system, all structure is lost. Dynamic Hopf bifurcation models do not reproduce the phase correlations which δ18O has with obliquity and eccentricity. Some directions for future mathematical research are described and several oddities about quasi-periodic forcing of a Hopf bifurcation model are presented. Finally a discussion of discontinuing the use of dynamic Hopf bifurcations in mid-Pleistocene transition research is presented. Dynamic Hopf bifurcations provide a rich field for mathematical inquiry. However, as the understanding of the δ18O data and of dynamic Hopf bifurcations increases, the Hopf bifurcation models become less viable choices for modeling the mid-Pleistocene transition.
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    Heinrich Stadials recorded in Northeast Brazilian Stalagmites
    (2015-05) Wendt, Kathleen
    During the last glaciation, periods of extreme cooling triggered massive freshwater and iceberg discharges into the North Atlantic. These periods of cooling, referred to as Heinrich Stadials, are believed to have caused an abrupt reorganization of atmospheric and oceanic circulation. Studying the behavior of tropical precipitation systems during Heinrich Stadials is important to understanding the response of tropical hydroclimate change to North Atlantic climate anomalies. In addition, tropical precipitation records may provide insight into the detailed timing and structure of Heinrich Stadials. Doing so is critical to understanding the underlying causes of Heinrich Stadials and their associated discharge events. The position of the Inter-Tropical Convergence Zone (ITCZ) plays a key role in the locality and amount of tropical precipitation worldwide. Paleo-records suggest that Heinrich Stadials triggered a southward migration of the ITCZ. The semiarid region of Northeast (NE) Brazil is located immediately south of the modern-day Atlantic ITCZ position. Short-lived speleothems that decorate the dry caves of NE Brazil suggest past periods of intense rainfall due to southerly migrations of the ITCZ. Previous studies have linked NE Brazil speleothem growth phases to Heinrich Stadials (Wang et al. 2004). Thus, reconstructing NE Brazilian pluvial periods will provide important insight into the chronology and structure of Atlantic ITCZ migrations associated with Heinrich Stadials. Here, we present a high-resolution, absolute dated, multi-stalagmite record of Heinrich Stadial (HS) 1, 4, 5 and 6 as recorded in NE Brazilian stalagmites. This thesis will focus on HS4 and HS1 time periods. Results show a detailed anti-correlation between NE Brazil and Northern Hemisphere subtropical records during HS4 and HS1, such as the Hulu Cave record from China (Wang et al. 2001). The synchronicity of these two distant records suggests a rapid transmission of atmospheric signals, likely through the global migration of the ITCZ. In addition, the 2-phased precipitation structure of HS1 and HS4 recorded in NE Brazil may reflect 2-stepped cooling observed in North and mid-Atlantic sea surface temperatures (Bond et al. 1992; Martrat et al. 2007). This supports the hypothesis that climate-ocean forcings were the underlying cause of Heinrich Events and highlights the relationship between mid-Atlantic sea surface temperatures and the position of the Atlantic ITCZ during the last glaciation. Arid NE Brazil is situated between the Amazon and the Atlantic rainforests. Evidence suggests that the pluvial periods associated with Heinrich Stadials promoted an ecological "bridge"� between both rainforests. This bridge may have permitted the transfer of species between rainforests. In this thesis, we compare NE Brazil speleothem precipitation records to NE Brazil palynological marine records to suggest the precise timing of rainforest expansion during HS1. Characterizing these NE Brazil pluvial anomalies are critical in understanding the link between climate change and the response of environmental systems.
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    Mountain hemlock (Tsuga mertensiana [Bong.] Carr.) growth and cool-season precipitation in Crater Lake National Park, Oregon
    (2015-04) Appleton, Sarah Nicole
    This study describes the development of a new network of mountain hemlock (Tsuga mertensiana [Bong.] Carr.) records at high-elevation sites in Crater Lake National Park (CRLA), Oregon, and uses these data to make inferences about past climate in the park during the last five and a half centuries. The seven hemlock chronologies, which are constructed from 53-80 tree-ring samples at each location, are highly synchronous over time, which suggests that tree growth across the Park is controlled by similar sets of environmental factors. The climate signal preserved in tree-ring-width series was estimated by comparing them against local monthly climate data, and that analysis showed an inverse relationship between tree growth and cool-season precipitation (previous November, previous December, January, March and April) and a more modest positive relationship with growing season temperature (April-August). The inverse association between cool-season precipitation and hemlock growth is opposite to the relationship displayed by most tree-ring records in western North America. Based on the observed association between hemlock growth and climate, I identify several periods during the past five centuries where snowpack in CRLA was persistently high or low. Tree growth was combined with observed anatomical anomalies, including locally absent rings, traumatic lenses, and light latewood, to estimate past hydroclimate conditions within the park. The sequence from A.D. 1809, 1810, and 1811 stands out over the last five centuries because of its low growth and high concentration of anatomical anomalies, which are interpreted as evidence for the early onset of winter in A.D. 1809 and an exceptionally short growing season in A.D. 1810.
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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.