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• 1. [Article] Past climate variability in southwestern Oregon and relationships with regional and hemispheric climate

  This dissertation presents the results of three studies that assessed climate variability on short and long timescales in western United States. The growth of carbonate formations in caves (speleothems) ...

  Citation × Citation Citation Abstract Copy Title: Past climate variability in southwestern Oregon and relationships with regional and hemispheric climate Author: Ersek, Vasile This dissertation presents the results of three studies that assessed climate variability on short and long timescales in western United States. The growth of carbonate formations in caves (speleothems) is used to infer the timing and amplitude of past climate variability. We first assess the controls on speleothem growth for the past 380 000 years by combining high-density U-Th dating with a regional climate model and an energy balance model. The majority of speleothem growth occurred overwhelmingly during interglacial periods and glacial periods were characterized by little or no growth. We assess the mechanisms responsible for growth cessation during the peak of the last glaciation (LGM), 21 000 years ago, with a regional climate model and determine that a combination of drier LGM conditions with a change in the seasonal cycle of the surface water balance provides a feedback mechanism that limited recharge to the deeper soil and reduced or eliminated drip water in the cave. The energy balance model supports this mechanism by indicating that cave temperatures did not drop below the freezing point of water at any time during the last glaciation. We then evaluate the climatic significance of stable isotopes of oxygen in rainwater collected in southwestern Oregon in order to accurately interpret the isotopic record of speleothems. We establish that temperature plays an important role in controlling the distribution of oxygen isotopes in rainwater and therefore speleothems in southwestern Oregon can preserve a record of temperature changes through time. Finally, we present a high-resolution paleoclimate record using speleothems from a cave in Oregon that grew during the last 9000 years and during the middle of the last interglaciation. We find the winter temperatures were sensitive to winter solar insolation and varied in a quasi-cyclical pattern on millennial timescales. This pattern was characteristic of both regional and hemispheric variability as indicated by similarities of our record with other regional and hemispheric climate reconstructions. It appears that higher climate variability in the past was associated with warming trends, with the implication that global warming may make the climate more unstable in the future. Climate variability during the last interglacial period was characterized by different scales of millennial and centennial climate variability compared with the Holocene. Title: Past climate variability in southwestern Oregon and relationships with regional and hemispheric climate Author: Ersek, Vasile This dissertation presents the results of three studies that assessed climate variability on short and long timescales in western United States. The growth of carbonate formations in caves (speleothems) is used to infer the timing and amplitude of past climate variability. We first assess the controls on speleothem growth for the past 380 000 years by combining high-density U-Th dating with a regional climate model and an energy balance model. The majority of speleothem growth occurred overwhelmingly during interglacial periods and glacial periods were characterized by little or no growth. We assess the mechanisms responsible for growth cessation during the peak of the last glaciation (LGM), 21 000 years ago, with a regional climate model and determine that a combination of drier LGM conditions with a change in the seasonal cycle of the surface water balance provides a feedback mechanism that limited recharge to the deeper soil and reduced or eliminated drip water in the cave. The energy balance model supports this mechanism by indicating that cave temperatures did not drop below the freezing point of water at any time during the last glaciation. We then evaluate the climatic significance of stable isotopes of oxygen in rainwater collected in southwestern Oregon in order to accurately interpret the isotopic record of speleothems. We establish that temperature plays an important role in controlling the distribution of oxygen isotopes in rainwater and therefore speleothems in southwestern Oregon can preserve a record of temperature changes through time. Finally, we present a high-resolution paleoclimate record using speleothems from a cave in Oregon that grew during the last 9000 years and during the middle of the last interglaciation. We find the winter temperatures were sensitive to winter solar insolation and varied in a quasi-cyclical pattern on millennial timescales. This pattern was characteristic of both regional and hemispheric variability as indicated by similarities of our record with other regional and hemispheric climate reconstructions. It appears that higher climate variability in the past was associated with warming trends, with the implication that global warming may make the climate more unstable in the future. Climate variability during the last interglacial period was characterized by different scales of millennial and centennial climate variability compared with the Holocene. Close

• 2. [Article] Deep-sea sediment paleomagnetism : a case study from the North Atlantic

  Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding ...

  Citation × Citation Citation Abstract Copy Title: Deep-sea sediment paleomagnetism : a case study from the North Atlantic Author: Strano, Sarah Elianna Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding of how the magnetization is acquired in the sediments is poorly understood. Primary magnetizations preserved in deep-sea sediments are known to be acquired through a depositional or possibly, a post-depositional remanent magnetization (DRM or pDRM). A pDRM process implies that the magnetization is locked-in at depth creating an offset between the age of the magnetization and the age of sediment. The process is not currently accounted for in paleomagnetic records despite the wide use of magnetic records to elucidate the timing and rate of change of many paleomagnetic and environmental processes. This dissertation uses seven Northern North Atlantic (NNA) deep-sea sediment cores that were studied by alternating field demagnetization of natural and laboratory imposed remanence on uchannel samples, providing for detailed paleomagnetic and environmental magnetic records. These high-quality Holocene and deglacial magnetic data are combined with independent radiocarbon chronologies to better understand the: (1) magnetic acquisition process, (2) the NNA paleo-geomagnetic signal and (3) the influence of rock magnetic parameters on the sedimentary paleomagnetic record. Under the traditional paradigm of magnetostratigraphy, sediment deposition and magnetization are assumed to occur synchronously and with little to no signal attenuation. In Chapter 2, we compare independently dated Holocene paleomagnetic records from the seven deep-sea sediments cores across the North Atlantic with regional paleo-geomagnetic reconstructions derived from ultra-high resolution sediment records. We find variable delays between the timing of these records, consistent with a magnetization “locked-in” at depth and over an interval that results in smoothing of the geomagnetic signal. Optimization modeling of the post-depositional remanent magnetization (pDRM) accounts for both offset and some of this smoothing. It also demonstrates that the preserved magnetization is acquired ~20 cm below the sediment-water interface. Consistent with previous observations, this potentially ubiquitous process results in age offsets of 350-2000 years even in deep-sea sediment accumulation rates in excess of 10 cm/kyr that is rarely if ever accounted for in magnetostratigraphy or paleomagnetic records. In Chapter 3, we assume that the new pDRM-corrected chronologies developed for Chapter 2 more accurately represent each paleomagnetic record and create a NNA stack of both direction and intensity from ~15,000 years ago to present (NAPstack15). Uncertainty analyses and comparison to data derived from global field models at the same locations suggest that both, our directional and intensity stacks robustly capture the evolution of the mean geomagnetic field variations of the NNA. Broader regional comparisons with data from North America and Europe begin to define the evolution of the geomagnetic field during this time interval. Geomagnetic morphology and spatial/temporal variability can be roughly broken into three time intervals consistent with the evolution of global intensity and implicate the dynamics of the high-latitude Holocene flux patches as a source of this variability. We evaluate the effect of rock magnetic properties on the fidelity of the NNA paleomagnetic record from the Holocene through the last deglaciation. NNA records have been argued to consistently record high-quality paleomagnetic records over millennial to orbital time scales with little concern for lithologic variability resulting from glacial-interglacial environmental changes. We find that rock magnetic variability has little effect on the fidelity of the NNA's directional record, but has a variable and sometimes large influence on normalized remanence records, which are commonly used as a relative paleomagnetic intensity proxy. We find that the eastern NNA records are most affected by the use of different normalizers during the Holocene. The western NNA cores are more affected by the use of different normalizers during the deglacial period but to a lesser extent. The Iceland Basin cores are an exception, providing consistent normalized remanence records regardless of the normalizer during both the Holocene and deglacial interval. This likely reflects their proximity to Icelandic basaltic sources and the consistent magnetite grain-size regardless of physical grain-size that these sources provide. Title: Deep-sea sediment paleomagnetism : a case study from the North Atlantic Author: Strano, Sarah Elianna Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding of how the magnetization is acquired in the sediments is poorly understood. Primary magnetizations preserved in deep-sea sediments are known to be acquired through a depositional or possibly, a post-depositional remanent magnetization (DRM or pDRM). A pDRM process implies that the magnetization is locked-in at depth creating an offset between the age of the magnetization and the age of sediment. The process is not currently accounted for in paleomagnetic records despite the wide use of magnetic records to elucidate the timing and rate of change of many paleomagnetic and environmental processes. This dissertation uses seven Northern North Atlantic (NNA) deep-sea sediment cores that were studied by alternating field demagnetization of natural and laboratory imposed remanence on uchannel samples, providing for detailed paleomagnetic and environmental magnetic records. These high-quality Holocene and deglacial magnetic data are combined with independent radiocarbon chronologies to better understand the: (1) magnetic acquisition process, (2) the NNA paleo-geomagnetic signal and (3) the influence of rock magnetic parameters on the sedimentary paleomagnetic record. Under the traditional paradigm of magnetostratigraphy, sediment deposition and magnetization are assumed to occur synchronously and with little to no signal attenuation. In Chapter 2, we compare independently dated Holocene paleomagnetic records from the seven deep-sea sediments cores across the North Atlantic with regional paleo-geomagnetic reconstructions derived from ultra-high resolution sediment records. We find variable delays between the timing of these records, consistent with a magnetization “locked-in” at depth and over an interval that results in smoothing of the geomagnetic signal. Optimization modeling of the post-depositional remanent magnetization (pDRM) accounts for both offset and some of this smoothing. It also demonstrates that the preserved magnetization is acquired ~20 cm below the sediment-water interface. Consistent with previous observations, this potentially ubiquitous process results in age offsets of 350-2000 years even in deep-sea sediment accumulation rates in excess of 10 cm/kyr that is rarely if ever accounted for in magnetostratigraphy or paleomagnetic records. In Chapter 3, we assume that the new pDRM-corrected chronologies developed for Chapter 2 more accurately represent each paleomagnetic record and create a NNA stack of both direction and intensity from ~15,000 years ago to present (NAPstack15). Uncertainty analyses and comparison to data derived from global field models at the same locations suggest that both, our directional and intensity stacks robustly capture the evolution of the mean geomagnetic field variations of the NNA. Broader regional comparisons with data from North America and Europe begin to define the evolution of the geomagnetic field during this time interval. Geomagnetic morphology and spatial/temporal variability can be roughly broken into three time intervals consistent with the evolution of global intensity and implicate the dynamics of the high-latitude Holocene flux patches as a source of this variability. We evaluate the effect of rock magnetic properties on the fidelity of the NNA paleomagnetic record from the Holocene through the last deglaciation. NNA records have been argued to consistently record high-quality paleomagnetic records over millennial to orbital time scales with little concern for lithologic variability resulting from glacial-interglacial environmental changes. We find that rock magnetic variability has little effect on the fidelity of the NNA's directional record, but has a variable and sometimes large influence on normalized remanence records, which are commonly used as a relative paleomagnetic intensity proxy. We find that the eastern NNA records are most affected by the use of different normalizers during the Holocene. The western NNA cores are more affected by the use of different normalizers during the deglacial period but to a lesser extent. The Iceland Basin cores are an exception, providing consistent normalized remanence records regardless of the normalizer during both the Holocene and deglacial interval. This likely reflects their proximity to Icelandic basaltic sources and the consistent magnetite grain-size regardless of physical grain-size that these sources provide. Close