Water balance in the U.S. Great Basin largely reflects the position and intensity of the westerly storm track, suggesting that records of past hydrologic changes offer important insights into the behavior of the storm track in different climate states. The limited dating precision and sample resolution of presently available records of past water balance, however, leave scientists unable to address questions about the precise timing and rates of past water balance changes and their relation to abrupt events recorded in other paleoclimate archives. Further, few well-dated records extend past the last glacial period, allowing little understanding of previous interglacials and glacial terminations. This project will use cave deposits to reconstruct past abrupt water balance changes in the Bonneville Basin, the site of the largest of the Great Basin's paleolakes during the last glacial period. The work draws upon both a) lacustrine cave deposits formed in a series of caves flooded by lake waters and b) speleothems from Lehman Cave on the southwestern edge of the basin. Using stable isotope measurements and a combination of U-Th and 14C dating, this group will be able to produce records with unprecedented dating precision and sample resolution covering the most recent glacial termination as well as the previous two interglacials and terminations. Data from lacustrine cave deposits, a novel paleohydrologic archive, indicate that they formed during periods of the last 30 kyr when the lake was both above a given cave?s elevation and hydrologically closed. The deposits thus record the passage of the lake through each cave's elevation and the onset and cessation of basin overflow; they also preserve high-resolution records of the lake's oxygen isotope composition and chemistry. Importantly, the deposits can be precisely dated by U-Th methods, providing the first high-precision, absolute-dated records of Lake Bonneville's water balance changes. Calibrated 14C ages match U-Th ages in lacustrine cave deposits, indicating that 14C reservoir effects were minimal in the lake. As a part of the project, a series of paired U-Th and 14C ages will be used to improve the 14C calibration dataset, focusing on the period between 19 and 25 calendar ka. Lehman Cave speleothems comprise an important complement to the lacustrine cave carbonates, extending back through substantial portions of the last 300 kyr. Stable isotope records from the cave's stalagmites, anchored by U-Th dates, will provide some of the first well-dated archives of regional hydrology during the previous two interglacials and glacial terminations. Together, Lehman Cave and lacustrine cave records promise key insights into the eastern Great Basin's response to both abrupt and orbital-scale climate changes during the last three glacial cycles.
Models consistently predict that the world's drylands will become drier on average in response to greenhouse warming, but it is unclear how this drying will affect specific regions. In the U.S., there is particular interest in understanding the potential for future drying in the Great Basin, a drought-prone region that has experienced substantial population growth over the last two decades. Records of past water availability provide a key starting point for forecasting future changes in the Great Basin by showing us the region's responses to a wide variety of climates. The balance of precipitation and evaporation in the Great Basin in the western interior of the U.S.A. has changed dramatically over the past 30,000 years, leaving dry lake beds and hypersaline lakes today where extensive lakes existed during glacial times. Water availability in the region largely depends upon precipitation brought by the winter storm track, suggesting that records of past hydrologic changes offer important insights into the behavior of the storm track in different climate states. The limited dating precision and sample resolution of present records, however, leave us with only a general sense of past hydrologic changes, and no well-dated records currently extend to past interglacial warm periods. This project will use cave deposits to reconstruct past water balance changes in northern Utah's Bonneville Basin, the site of the largest of the Great Basin's paleolakes. These deposits record changes in precipitation in the basin over much of the past 300,000 years and are able to be precisely dated, offering us the opportunity to construct a detailed and extended picture of past climate changes in the region. Importantly, stalagmites from Lehman Cave on the western edge of the basin record the last two interglacial warm periods, allowing us the opportunity to study the mean state and variability of the region's hydrology during times when the region was as warm as or slightly warmer than at present. This work represents an important contribution to our understanding of past climate changes and will help calibrate models seeking to reproduce past changes in order to better forecast the future. In addition, these records will assist us in interpreting past ecological changes in the basin and the history of early human occupation in the region.
The interior western U.S. is highly vulnerable to changes in precipitation, making efforts to improve forecasts of the region’s response to future climate change an important research goal. The aim of this project was to document precipitation changes in Utah’s Bonneville Basin over the past 150,000 years in order to improve our understanding of the response of the region’s water balance to past climate changes and offer opportunities to test climate models’ representations of these past changes. The Bonneville Basin covers much of the western half of northern Utah and today holds Utah Lake and the Great Salt Lake. Shorelines preserved on the sides of the basin’s hills provide evidence of a paleo-lake with a surface area of similar to that of modern day Lake Michigan, pointing to dramatic changes in the region’s hydrology in past climates. Though radiocarbon dating documents that the lake reached its maximum extent at the end of the last ice age approximately 18,000 years ago, it has been difficult to build high-resolution records of past precipitation changes based on shoreline deposits, as their dating is relatively imprecise and they record only snapshots in time. In addition, lake shoreline deposits only record times wetter than present, whereas our interest clearly extends to times drier than present as well. In this project we aimed to develop a new generation of paleo-precipitation records from the region that offer more continuous coverage through time, are more precisely dated, and provide multiple lines of evidence as to the nature and timing of past hydrological changes. In the first stage of the project, we sampled dense lacustrine carbonates precipitated in caves, crevices and other protected spaces flooded by Lake Bonneville that are capable of being dated with great precision by both uranium-thorium and radiocarbon methods. Beginning and end dates for the deposits precisely record the lake's rise and fall past each cave site, while oxygen, strontium and uranium isotope data and trace element data provide nearly continuous high-resolution records of the basin's water balance. These lake deposits provide records that have the sampling resolution and dating precision to be directly comparable to other high-resolution records from ice cores, marine sediments and stalagmites from around the world, allowing us to correlate past precipitation changes in the western U.S. with climatic conditions elsewhere in the world. One key finding is that the wettest conditions in the basin occurred during periods of pronounced cooling in the North Atlantic in the last ice age known as "Heinrich Events", pointing out the sensitivity of the western U.S. winter storm track to warming or cooling at high latitudes. In the second phase of the project, we extended our records beyond deep-lake periods by sampling stalagmites from Lehman Cave in Great Basin National Park on the western boundary of the Bonneville Basin. This cave has a large number of previously broken stalagmites that the park staff have allowed us to use for research purposes. We dated small chips from over 90 stalagmites to identify those deposited during the last 150,000 years, and then developed detailed age models, stable isotope (oxygen and carbon) records, and trace element ratio (Mg/Ca, Sr/Ca) records from a subset of samples. Our work has developed a detailed record of the last 13,000 years that documents the drying of the basin at the end of the last ice age. A key finding of this record is that the final drying of the basin corresponds precisely in time with the final collapse of the remains of the large ice sheet remaining from the last ice age at 8,200 years ago. This finding points to the importance of even small changes in ice sheets for regional precipitation patterns. Ongoing work is now building detailed records of the previous interglacial period approximately 125,000 years ago, a time when high latitudes were approximately 4-6?F warmer than today. These new records document pronounced drying of the basin that tracks the warming of the Northern Hemisphere during the previous interglacial, offering important corroboration of the sensitivity of western U.S. precipitation to warming in the high latitudes of the Northern Hemisphere. These new results also document wet conditions during the Heinrich Events of the previous glacial period. Sophisticated climate models can now use these data as targets to explore the dynamics of past precipitation changes in the western U.S. However, even now their simple message is clear: that over the last 150,000 years the wettest conditions in U.S. Great Basin corresponded with the coldest temperatures in the high northern latitudes, and that drying occurred during times of warmer high-latitude temperatures and reduced ice sheet extent.
