Browsing by Subject "Holocene"

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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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    Changes in seasonal precipitation of East Central North America with connections to global climate.
    (2010-10) Hardt, Benjamin Fulper
    Research on oxygen isotopes in stalagmites collected in West Virginia caves has yielded several new insights into regional climate. Oxygen isotopes most likely represent changes in the mean annual isotopic composition of precipitation, a parameter determined locally by the seasonal distribution of precipitation (Hardt et al., 2010). Holocene samples indicate that summer precipitation represented a greater proportion of annual totals, consistent with lake level results (Shuman and Donnelly, 2006) and climate models (Braconnot et al., 2007; Diffenbaugh et al., 2006). During the Pleistocene, seasonal precipitation varies on precessional timescales, although the phasing appears unusual in that it is in-phase with September insolation rather than June. This offset could be due to several processes, most likely in conjunction with each other. These processes include: changes in Gulf of Mexico sea surface temperatures, partially modulated by glacial meltwater routed through the Mississippi (Oglesby et al., 1989; H Wang et al., 2010); changes in the mean state of the tropical Pacific ocean between to El Nino- and La Nina-like conditions (Timmermann et al., 2007); and a seasonally-lagged sea surface temperature response in the subtropical North Atlantic, which would enhance the anticyclonic circulation of the Bermuda High. These same processes also appear to influence isotopic behavior over the last glacial Termination. During the last glacial maximum, oxygen isotopic composition is enriched, consistent with model results indicating wetter summers near the southern margin of the ice sheet (Bromwich et al., 2005), but inconsistent with a temperature control. Antiphasing in summer precipitation between West Virginia and Florida support a control on seasonal precipitation by changes in the position or size of the Bermuda High (Donders and de Boer et al., 2009). Comparison with speleothems from Northeast Brazil (Cruz et al, 2009) indicates a role for the El Nino Southern Oscillation due to its effect on the position of the Bermuda High (Seager et al., 2005).
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    The Holocene History of Lake Kivu (East Africa): New perspectives from new cores
    (2014-07) Votava, Jillian Emilia
    Lake Kivu lies at the heart of East Africa&lsquos rift lakes in a volcanically active region. Hydrothermal seeps impose a complex stratification regime with heated, high-salinity waters entering below 280 m water depth. Previous detailed studies of fossil diatoms and mineralogy of the sediment record suggest this hydrothermal activity began 5,000 years BP. Unfortunately, dating bulk organic matter of these original cores was problematic due to dissolved volcanogenic CO2. This study offers a new chronology and a detailed perspective on the limnologic history of Lake Kivu through investigation of carbonates and bulk organic matter from sediment cores recovered in 2012 and 2013. A Holocene history was compiled by 14C dating of recovered terrestrial macro fossils from a deep, central basin core, and by 210Pb geochronology of recent sediments from a near shore core. Water levels in Lake Kivu rose during the African Humid Period (AHP) pluvial from 12 ka to 5 ka. Authigenic CaCO3 deposition began around 4.2 ka in the deep, main basin with all subsequent carbonate intervals composed of endogenic aragonite. A solute budget reveals that most of the Ca2+ ion is supplied at depth via the hydrothermal seeps and suggests that this sub-lacustrine input was initiated just prior to onset of carbonate deposition. Stable isotopic analyses of δ13Caragonite and δ13COM both indicate slight enrichment beyond the expected kinetic fractionation and above other East African lakes suggesting volcanogenic influence on water column DIC began around 4.2 ka. Some of the alternating intervals of carbonate deposition and cessation in the late-Holocene, and associated δ18Oaragonite enrichment, coincide with records of drought from nearby Lake Edward, such as at the AHP termination, at 2 ka, and during the Little Ice Age. This suggests a climate overprint on the predominantly volcanogenic record of carbonate sedimentation in Lake Kivu.
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    A Multiple Proxy Lake Sediment Record Of Middle Through Late Holocene Climate Change In Southwestern British Columbia, Canada
    (2018-04) Shea, Christopher
    Holocene climate variability in the Pacific Northwest is the subject of ongoing debate due to the scarcity of continuous, high-resolution records and differences in proxy sensitivity. Analyses of the oxygen isotopic composition of lake sediment carbonates contribute to this debate by providing information on past changes in water δ18O values (δ18O) which respond to a variety of climate variables including changes in the balance between precipitation and evaporation, lake level, and/or the isotopic composition of precipitation. Here we present a decadally resolved carbonate oxygen isotope record spanning the last ~7500 years BP from Turquoise Lake (N50.83°, W121.69°, 807m), a small, alkaline, semi-closed basin system located in southwestern British Columbia. Turquoise Lake waters are dominated by cold season meltwater and fall along the local evaporation line but are not substantially enriched relative to local meteoric water, indicating that water losses from the lake occur principally through overflow and groundwater outseepage. We measured the δ18O values of carbonates in 3m of sediment recovered from the depocenter of Turquoise Lake, in order to make inferences on climate change during the Holocene. We dated the Turquoise Lake record using 210Pb, 137Cs, twelve 14C measurements, and one tephra layer and analyzed 375 samples of authigenic carbonate (<63µm fraction), collected at 2-10mm intervals. Multiple turbidite sequences possibly related to the Mazama ash deposition complicate the δ18O signal, but high-resolution X-ray fluorescence and magnetic susceptibility are used to identify and remove distinct intervals of detrital influence from the δ18O record. Detailed characterization of the sediment by scanning electron miscroscopy and powder x-ray analysis allowed for the distinction between useful authigenic sediment and clastic allochthonous input. With clastic turbidites removed, the Turquoise Lake δ18O record exhibits a decreasing trend of average δ18O values from -15 to -17‰ over the last 7500 years implying a dry middle Holocene and a transition to a wetter late Holocene. The past 4000 years are marked by overall steady conditions punctuated by decadal positive spikes in δ18O reflective of long-term drought events. More negative δ18O values occur from 1100 yr BP to 500 yr BP, implying wetter conditions during the Medieval Climate Anomaly (MCA). A steady increase in δ18O values from 400 yr BP indicates a drier climate early in the Little Ice Age (LIA), followed a steep decline at 200 yr BP, indicating a wetter late LIA. The results provide insight on middle to late Holocene climate and add to the growing network of lacustrine δ18O records and thus will improve our overall understanding of past hydrologic changes in North America.
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    A multiple proxy lake sediment record of Middle through Late Holocene climate change in southwestern British Columbia, Canada
    (2018-02) Shea, Christopher
    Holocene climate variability in the Pacific Northwest is the subject of ongoing debate due to the scarcity of continuous, high-resolution records and differences in proxy sensitivity. Analyses of the oxygen isotopic composition of lake sediment carbonates contribute to this debate by providing information on past changes in water δ18O values (δ18O) which respond to a variety of climate variables including changes in the balance between precipitation and evaporation, lake level, and/or the isotopic composition of precipitation. Here we present a decadally resolved carbonate oxygen isotope record spanning the last ~7500 years BP from Turquoise Lake (N50.83°, W121.69°, 807m), a small, alkaline, semi-closed basin system located in southwestern British Columbia. Turquoise Lake waters are dominated by cold season meltwater and fall along the local evaporation line but are not substantially enriched relative to local meteoric water, indicating that water losses from the lake occur principally through overflow and groundwater outseepage. We measured the δ18O values of carbonates in 3m of sediment recovered from the depocenter of Turquoise Lake, in order to make inferences on climate change during the Holocene. We dated the Turquoise Lake record using 210Pb, 137Cs, twelve 14C measurements, and one tephra layer and analyzed 375 samples of authigenic carbonate (<63µm fraction), collected at 2-10mm intervals. Multiple turbidite sequences possibly related to the Mazama ash deposition complicate the δ18O signal, but high-resolution X-ray fluorescence and magnetic susceptibility are used to identify and remove distinct intervals of detrital influence from the δ18O record. Detailed characterization of the sediment by scanning electron miscroscopy and powder x-ray analysis allowed for the distinction between useful authigenic sediment and clastic allochthonous input. With clastic turbidites removed, the Turquoise Lake δ18O record exhibits a decreasing trend of average δ18O values from -15 to -17‰ over the last 7500 years implying a dry middle Holocene and a transition to a wetter late Holocene. The past 4000 years are marked by overall steady conditions punctuated by decadal positive spikes in δ18O reflective of long-term drought events. More negative δ18O values occur from 1100 yr BP to 500 yr BP, implying wetter conditions during the Medieval Climate Anomaly (MCA). A steady increase in δ18O values from 400 yr BP indicates a drier climate early in the Little Ice Age (LIA), followed a steep decline at 200 yr BP, indicating a wetter late LIA. The results provide insight on middle to late Holocene climate and add to the growing network of lacustrine δ18O records and thus will improve our overall understanding of past hydrologic changes in North America.
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    Publications from LLO Faculty and Staff, 2016
    (2016) University of Minnesota Duluth. Large Lakes Observatory