Jessica L. Oster

Uranium isotopes in soils as a proxy for past infiltration and precipitation across the western United States

The intermittent presence of large Pleistocene lakes in the southwestern interior of North America, a region that is now a semi-arid desert, suggests repeated oscillations between profoundly different climatic conditions. The origin of these shifts is still unresolved due to inconsistencies in existing climate proxy data (for example, pollen, lake levels, and oxygen isotopes in speleothems). To resolve the inconsistencies in the water balance over the last 10 to 60 kyr, we use uranium isotopic variations in secondary soil minerals to quantify net infiltration and precipitation along a north-south transect in western North America. We show that winter infiltration increased by 30 to 100 percent, and precipitation by a lesser amount, in the valleys of the Great Basin and Mojave deserts between 60 and ∼26 ka. This increase in infiltration and precipitation preceded the Last Glacial Maximum (LGM) and the timing of most lake highstands in the region by 5 to 10 kyr, respectively, suggesting a possible Last Precipitation Maximum (LPM) that coincided with a minimum in winter insolation. Subsequent decreases in infiltration and precipitation after the LGM can be reconciled with the timing of lake highstands if colder summer temperatures due to a minimum in summer insolation reduced lake evaporation. The soil records, combined with a range of proxy data, suggest that seasonal insolation is the long-term driver for large shifts in both precipitation and surface water variability in the region.

Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India

Two types of El Nino events are distinguished by sea surface temperature (SST) anomalies centered in the central or eastern equatorial Pacific. The Central Pacific El Nino events (CP-El Nino) are more highly correlated with weakening of the central Indian Summer Monsoon and linked to decadal Pacific climate variability. We present a 50 year, subannually resolved speleothem δ18O record from northeast India that exhibits a significant correlation with northern Pacific decadal variability and central equatorial Pacific SSTs. Accordingly, we suggest that δ18O time series in similar northeast Indian speleothems are effective tools for investigating preinstrumental changes in Pacific climate, including changes in El Nino dynamics. In contrast to central India, rainfall amounts in northeast India are relatively unaffected by El Nino. However, back trajectory analysis indicates that during CP-El Nino events moisture transport distance to northeast India is reduced, suggesting that variations in moisture transport primarily control δ18O in the region.

Pedothem carbonates reveal anomalous North American atmospheric circulation 70,000–55,000 years ago

Significance We show for the first time, to our knowledge, that pedogenic (soil) carbonate mineral accumulations can preserve continuous paleoclimate records that rival the temporal resolution of widely used archives, such as speleothems or lake sediments. Using microanalysis of oxygen, carbon, and uranium isotopes coupled with uranium series dating, we find evidence for a distinct shift in atmospheric circulation in North America’s interior from 70,000 to 55,000 years ago, a finding that highlights the influence of large continental ice sheets on atmospheric circulation. Perhaps most significantly, this work shows that pedothems, which are common in arid and semiarid regions around the world, are a rich archive of paleoclimate information for continental landscapes. Our understanding of climatic conditions, and therefore forcing factors, in North America during the past two glacial cycles is limited in part by the scarcity of long, well-dated, continuous paleoclimate records. Here, we present the first, to our knowledge, continuous, millennial-resolution paleoclimate proxy record derived from millimeter-thick pedogenic carbonate clast coatings (pedothems), which are widely distributed in semiarid to arid regions worldwide. Our new multiisotope pedothem record from the Wind River Basin in Wyoming confirms a previously hypothesized period of increased transport of Gulf of Mexico moisture northward into the continental interior from 70,000 to 55,000 years ago based on oxygen and carbon isotopes determined by ion microprobe and uranium isotopes and U-Th dating by laser ablation inductively coupled plasma mass spectrometry. This pronounced meridional moisture transport, which contrasts with the dominant zonal transport of Pacific moisture into the North American interior by westerly winds before and after 70,000–55,000 years ago, may have resulted from a persistent anticyclone developed above the North American ice sheet during Marine Isotope Stage 4. We conclude that pedothems, when analyzed using microanalytical techniques, can provide high-resolution paleoclimate records that may open new avenues into understanding past terrestrial climates in regions where paleoclimate records are not otherwise available. When pedothem paleoclimate records are combined with existing records they will add complimentary soil-based perspectives on paleoclimate conditions.

Evolution of moisture transport to the western U.S. during the last deglaciation

We investigate climate dynamics and teleconnections governing moisture transport to the western U.S. during past warm and cool intervals of the last deglaciation using paleoclimate simulations of the Bølling warm (~14ka) and Younger Dryas cool (~12 ka) events. Results suggest that the waning continental ice sheet weakened atmospheric pressure centers in the region leading to a progression from a more sinuous to more zonal Pacific winter storm track throughout the deglaciation. Furthermore, variations in meltwater flux to the Atlantic influenced the meridional temperature gradient over the Pacific and thereby modulated storm track intensity. Changing sinuosity of the storm track may be reflected in broad increases in modeled δOprecip and observed δ Ospeleothem values from the western U.S. over the last deglaciation, whereas abrupt δOspeleothem shifts are dynamically consistent with the response of storm track intensity to variations in meltwater flux to the Atlantic.

Climate response to the 8.2 ka event in coastal California

A fast-growing stalagmite from the central California coast provides a high-resolution record of climatic changes synchronous with global perturbations resulting from the catastrophic drainage of proglacial Lake Agassiz at ca. 8.2 ka. High frequency, large amplitude variations in carbon isotopes during the 8.2 ka event, coupled with pulsed increases in phosphorus concentrations, indicate more frequent or intense winter storms on the California coast. Decreased magnesium-calcium ratios point toward a sustained increase in effective moisture during the event, however the magnitude of change in Mg/Ca suggests this event was not as pronounced on the western North American coast as anomalies seen in the high northern latitudes and monsoon-influenced areas. Nevertheless, shifts in the White Moon Cave record that are synchronous within age uncertainties with cooling of Greenland, and changes in global monsoon systems, suggest rapid changes in atmospheric circulation occurred in response to freshwater input and associated cooling in the North Atlantic region. Our record is consistent with intensification of the Pacific winter storm track in response to North Atlantic freshwater forcing, a mechanism suggested by simulations of the last deglaciation, and indicates this intensification led to increases in precipitation and infiltration along the California coast during the Holocene.

The hydrologic record of karst systems: linking soil moisture to the carbon isotope signatures of soils above the Blue Spring cave system

Speleothem carbonates are precipitated continuously from inorganic carbon dissolved in seepage water infiltrating from the land surface, that reflects a mixture of atmospheric CO2, respired soil carbon, and epikarst sources, each with distinct δ13C values. To aid in deconvolving these signatures, soil samples were collected above the Blue Spring cave system in Sparta, Tennessee, USA and subjected to a series of incubation experiments, in order to constrain the correlation between CO2 respiration rates and soil moisture. This relationship is used to parameterize a simple mixing model which predicts the relationship between δ13C and soil moisture in fluids infiltrating into the underlying cave system.