- Email: [email protected]
Siple Dome Ice Cores: Implications for West Antarctic Climate and ENSO Events
Abstracts / Quaternary International 279-280 (2012) 121–232
(234U/230Th) constrain the general period of growth to MIS 5d to MIS 5b. Preliminary results suggest that the stable oxygen isotope profile obtained from the axial zone largely reflects the unaltered isotopic composition of the cave drip water. The observed shifts in the isotope records display long-term changing climate conditions from temperate warm and humid conditions to relatively colder and drier conditions. High-amplitude oscillations superimposed on this trend are interpreted as short warm and dry intervals during which calcite precipitation is primarily affected by non-equilibrium conditions. In light of modern climate in the region, these findings suggest that the proxy record primarily reflects a shift in precipitation regimes that in turn may be linked to the long-term seasonal displacement of the subtropical high-pressure ridge and its seasonal duration. PEDOGENIC CLIMATE SIGNALS IN THE GREAT PLAINS (USA) DURING THE PLEISTOCENE-HOLOCENE TRANSITION (BØLLING/ALLERØD – BOREAL) William C. Johnson. University of Kansas, United States E-mail address: [email protected]
Despite major environmental shift at the Pleistocene-Holocene transition, landscapes of the Great Plains remained remarkably stable. The Brady Soil of the Central Great Plains typifies this stability, with soil formation beginning between 15 and 14 cal ka and ending 10.5 to 9.2 cal ka. During the late Pleistocene prior to Brady Soil formation, loess accumulated at high rates and dunes activated. As dust load subsided and dunes stabilized, Brady Soil formation began. Subsequently, early-Holocene loess accumulation extinguished the Brady Soil. Pedogenesis spanned the Bølling/ Allerød, Younger Dryas (YD), and early Boreal episodes, i.e., the Brady Soil represents a complex sequence of paleoenvironmental changes. Several metrics were used to capture the paleoenvironmental record within the Brady Soil. Stable C isotope data indicate dominance of C3 plants during early Brady Soil development, with C4 plants progressively increasing and reaching a peak about 10 cal ka, just prior to soil termination. The rate of increase in C4 plants was retarded somewhat during the YD, but only minor landscape instability has been recorded, in contrast to the welldocumented activity during the early Holocene and subsequent dry periods, such as the Medieval Climatic Anomaly. Biosilicate data exhibit an overall shift from Pooids (C3 grasses) and arboreal components to Chloridoids (xeric C4 grasses) and provide detail such as decreased Panicoids (mesic C4 grasses) during the YD, and increased Stipa type (dry-adapted C3) biosilicates during the early Boreal. As pedogenesis waned, total biosilicate concentration, extraneous (loess-borne) components (e.g., diatoms, sponge spicules), and charcoal and charred biosilicates all increased, suggesting increasing landscape instability. Ichnological data show a transition from cicada-like insects (boreal affinity) to burrowing rodents (grassland affinity). Other metrics include whole soil and carbonate geochemistry, and climo-function development. SIPLE DOME ICE CORES: IMPLICATIONS FOR WEST ANTARCTIC CLIMATE AND ENSO EVENTS Tyler Jones. Institute of Arctic and Alpine Research, Universit, United States E-mail address: [email protected]
Ice cores at Siple Dome, West Antarctic receive the majority of their precipitation from Pacific Ocean moisture sources. Pacific climate patterns, particularly the El Niño-Southern Oscillation, affect the local temperature, atmospheric circulation, and snow accumulation at Siple Dome, as well as isotopic signals (dD and d18O). We examined isotopes, accumulation and borehole temperatures from a number of shallow ice cores distributed 60km across the Dome. The data reveal a strong microclimate heavily influenced by South Pacific climate and the location of the Amundsen Sea Low Pressure Area. The Dome Summit and Pacific Flank respond to La Niña conditions by warming, increasing isotope ratios and increased snowfall. The Inland Flank responds to El Niño conditions and cold interior air masses by cooling, decreasing isotope ratios and decreased snowfall. Spectral analysis of the dD record over the last few centuries shows a distinct shift in ocean-atmosphere climate dynamics in the late 19th
225
century, where scattered bi-decadal to decadal periodicities change to include more intensely grouped and decreasing periodicities at the end of the 20th century. This record is compared with other proxy records of ENSO over the last few centuries. DATA BASED MODELS OF LAKE WATER OXYGEN ISOTOPE VARIABILITY: IMPLICATIONS FOR PALAEOCLIMATE RECONSTRUCTION Matthew Jones. School of Geography, University of Nottingham, United Kingdom E-mail address: [email protected]
Lake oxygen isotope records are often used to reconstruct past climate and environment. However, there are still questions as to what extent these changes can be quantitatively understood. The speed and magnitude of lake responses to climate change can only be established by longterm monitoring although this is often difficult with constraints of time and money. If isotope system models of lake variability can be shown to correctly reproduce observed changes then these models can be used to explain hydrological variability in the present and through geological time. Here we present models developed from monitoring data collected during a 6 year sampling project in the UK, over which time we measured monthly isotope variability in 2 hydrologically open and 2 hydrologically closed lakes and compared their response to common climate forcing. Local rainfall has also been sampled weekly throughout the sampling period and for 3 years atmospheric moisture was also collected and analysed daily. For the first 2 years of the sampling period we also regularly sampled 6 further lakes, across a broad hydrological gradient. The resulting models explain up to 80% of the isotope variability in the closed lakes, with the open systems recording a smoothed precipitation isotope record. Sensitivity analysis of the models suggests that most of the isotope variability can be explained if values of precipitation (P) and temperature (T) are known. This suggests that past P or T could be reconstructed given lake isotope values and an independent proxy for T or P respectively. WHAT DO WE MEAN BY WET? RECONSTRUCTING WATER AVAILABILITY IN THE NEAR EAST Matthew Jones. School of Geography, University of Nottingham, United Kingdom E-mail address: [email protected]
Water is a key societal resource, especially in areas marginal to drylands where changing climate is more likely to alter accessibility to water. Here we draw on ongoing research in two hydrologically marginal environments, eastern Jordan and southwestern Iran, where geoarchaeological investigations into past water availability are running alongside archaeological excavations focussing on the Late Glacial and Holocene respectively. Although separated in space and time the two projects raise similar questions for the methods used to reconstruct the amount of water available to past societies. We investigate what we mean by wet, both from the proxies available to us and in our interpretations of these proxies, and consider how spatially coherent these wetness reconstructions can be. Changes in hydroclimate recorded by proxies may be describing changes in precipitation, over a full year or only in a given season, effective precipitation (E:P), or humidity for example. How these different changes in “wetness” lead to changes in available water is not always clear and water availability is also dependent on changing geomorphology, either naturally driven or by people. Our examples from both Jordan and Iran show changes in sedimentation regime that could be controlled by either climatic or geomorphological processes, or a combination of the two; processes that are difficult to separate given the relative lack of palaeoenvironmental data from both regions. This leads to our second strand of discussion. We argue that local geoarchaeological investigation is key to all studies looking to draw environmentally deterministic conclusions to past societal change. Both our examples here show differences, either in the timing and/or expression of events, from the nearest palaeoclimatic reconstructions which therefore need to be used with care.
(234U/230Th) constrain the general period of growth to MIS 5d to MIS 5b. Preliminary results suggest that the stable oxygen isotope profile obtained from the axial zone largely reflects the unaltered isotopic composition of the cave drip water. The observed shifts in the isotope records display long-term changing climate conditions from temperate warm and humid conditions to relatively colder and drier conditions. High-amplitude oscillations superimposed on this trend are interpreted as short warm and dry intervals during which calcite precipitation is primarily affected by non-equilibrium conditions. In light of modern climate in the region, these findings suggest that the proxy record primarily reflects a shift in precipitation regimes that in turn may be linked to the long-term seasonal displacement of the subtropical high-pressure ridge and its seasonal duration. PEDOGENIC CLIMATE SIGNALS IN THE GREAT PLAINS (USA) DURING THE PLEISTOCENE-HOLOCENE TRANSITION (BØLLING/ALLERØD – BOREAL) William C. Johnson. University of Kansas, United States E-mail address: [email protected]
Despite major environmental shift at the Pleistocene-Holocene transition, landscapes of the Great Plains remained remarkably stable. The Brady Soil of the Central Great Plains typifies this stability, with soil formation beginning between 15 and 14 cal ka and ending 10.5 to 9.2 cal ka. During the late Pleistocene prior to Brady Soil formation, loess accumulated at high rates and dunes activated. As dust load subsided and dunes stabilized, Brady Soil formation began. Subsequently, early-Holocene loess accumulation extinguished the Brady Soil. Pedogenesis spanned the Bølling/ Allerød, Younger Dryas (YD), and early Boreal episodes, i.e., the Brady Soil represents a complex sequence of paleoenvironmental changes. Several metrics were used to capture the paleoenvironmental record within the Brady Soil. Stable C isotope data indicate dominance of C3 plants during early Brady Soil development, with C4 plants progressively increasing and reaching a peak about 10 cal ka, just prior to soil termination. The rate of increase in C4 plants was retarded somewhat during the YD, but only minor landscape instability has been recorded, in contrast to the welldocumented activity during the early Holocene and subsequent dry periods, such as the Medieval Climatic Anomaly. Biosilicate data exhibit an overall shift from Pooids (C3 grasses) and arboreal components to Chloridoids (xeric C4 grasses) and provide detail such as decreased Panicoids (mesic C4 grasses) during the YD, and increased Stipa type (dry-adapted C3) biosilicates during the early Boreal. As pedogenesis waned, total biosilicate concentration, extraneous (loess-borne) components (e.g., diatoms, sponge spicules), and charcoal and charred biosilicates all increased, suggesting increasing landscape instability. Ichnological data show a transition from cicada-like insects (boreal affinity) to burrowing rodents (grassland affinity). Other metrics include whole soil and carbonate geochemistry, and climo-function development. SIPLE DOME ICE CORES: IMPLICATIONS FOR WEST ANTARCTIC CLIMATE AND ENSO EVENTS Tyler Jones. Institute of Arctic and Alpine Research, Universit, United States E-mail address: [email protected]
Ice cores at Siple Dome, West Antarctic receive the majority of their precipitation from Pacific Ocean moisture sources. Pacific climate patterns, particularly the El Niño-Southern Oscillation, affect the local temperature, atmospheric circulation, and snow accumulation at Siple Dome, as well as isotopic signals (dD and d18O). We examined isotopes, accumulation and borehole temperatures from a number of shallow ice cores distributed 60km across the Dome. The data reveal a strong microclimate heavily influenced by South Pacific climate and the location of the Amundsen Sea Low Pressure Area. The Dome Summit and Pacific Flank respond to La Niña conditions by warming, increasing isotope ratios and increased snowfall. The Inland Flank responds to El Niño conditions and cold interior air masses by cooling, decreasing isotope ratios and decreased snowfall. Spectral analysis of the dD record over the last few centuries shows a distinct shift in ocean-atmosphere climate dynamics in the late 19th
225
century, where scattered bi-decadal to decadal periodicities change to include more intensely grouped and decreasing periodicities at the end of the 20th century. This record is compared with other proxy records of ENSO over the last few centuries. DATA BASED MODELS OF LAKE WATER OXYGEN ISOTOPE VARIABILITY: IMPLICATIONS FOR PALAEOCLIMATE RECONSTRUCTION Matthew Jones. School of Geography, University of Nottingham, United Kingdom E-mail address: [email protected]
Lake oxygen isotope records are often used to reconstruct past climate and environment. However, there are still questions as to what extent these changes can be quantitatively understood. The speed and magnitude of lake responses to climate change can only be established by longterm monitoring although this is often difficult with constraints of time and money. If isotope system models of lake variability can be shown to correctly reproduce observed changes then these models can be used to explain hydrological variability in the present and through geological time. Here we present models developed from monitoring data collected during a 6 year sampling project in the UK, over which time we measured monthly isotope variability in 2 hydrologically open and 2 hydrologically closed lakes and compared their response to common climate forcing. Local rainfall has also been sampled weekly throughout the sampling period and for 3 years atmospheric moisture was also collected and analysed daily. For the first 2 years of the sampling period we also regularly sampled 6 further lakes, across a broad hydrological gradient. The resulting models explain up to 80% of the isotope variability in the closed lakes, with the open systems recording a smoothed precipitation isotope record. Sensitivity analysis of the models suggests that most of the isotope variability can be explained if values of precipitation (P) and temperature (T) are known. This suggests that past P or T could be reconstructed given lake isotope values and an independent proxy for T or P respectively. WHAT DO WE MEAN BY WET? RECONSTRUCTING WATER AVAILABILITY IN THE NEAR EAST Matthew Jones. School of Geography, University of Nottingham, United Kingdom E-mail address: [email protected]
Water is a key societal resource, especially in areas marginal to drylands where changing climate is more likely to alter accessibility to water. Here we draw on ongoing research in two hydrologically marginal environments, eastern Jordan and southwestern Iran, where geoarchaeological investigations into past water availability are running alongside archaeological excavations focussing on the Late Glacial and Holocene respectively. Although separated in space and time the two projects raise similar questions for the methods used to reconstruct the amount of water available to past societies. We investigate what we mean by wet, both from the proxies available to us and in our interpretations of these proxies, and consider how spatially coherent these wetness reconstructions can be. Changes in hydroclimate recorded by proxies may be describing changes in precipitation, over a full year or only in a given season, effective precipitation (E:P), or humidity for example. How these different changes in “wetness” lead to changes in available water is not always clear and water availability is also dependent on changing geomorphology, either naturally driven or by people. Our examples from both Jordan and Iran show changes in sedimentation regime that could be controlled by either climatic or geomorphological processes, or a combination of the two; processes that are difficult to separate given the relative lack of palaeoenvironmental data from both regions. This leads to our second strand of discussion. We argue that local geoarchaeological investigation is key to all studies looking to draw environmentally deterministic conclusions to past societal change. Both our examples here show differences, either in the timing and/or expression of events, from the nearest palaeoclimatic reconstructions which therefore need to be used with care.