Center for Geochronological Research

Center for Geochronological Research

The Geochronology Center promotes fundamental research in the development and application of geochronological methods and geochemical tracers that will lead to an improved understanding of processes controlling environmental change, and the rates at which those processes act. These techniques allow a quantitative reconstruction of past environmental changes, from which temporal linkages between components of the global climate system, particularly biosphere/atmosphere/ocean/ice-sheet interactions, can be identified. The active participation of graduate and undergraduate students is ensured through formal course offerings, access to specialized research facilities, and employment opportunities.

Research Highlights

2000 Years of Drought Variability in the Central United States

Droughts are one of the most devastating natural hazards faced by the United States today. Severe droughts of the 20th century have had significant impacts on economies, society, and the environment, especially in the central U.S. However, the instrumental record of drought for this region is only about 100 years long and contains only a limited subset of droughts. How representative is the 20th century record over time scales of hundreds to thousands of years? In a 1998 paper in the Bulletin of the American Meteorological Society, Connie Woodhouse and Jonathan Overpeck use paleoclimatic data to evaluate the characteristics of 20th century droughts in the context of longer time scales. In this study, a variety of proxy data, including historical documents, tree rings, archaeological remains, lake sediment and geomorphic data, are used to evaluate the representativeness of 20th century droughts compared to droughts which have occurred under the naturally-varying climate conditions of the past two thousand years.

The results of this review of paleoclimatic data suggest that 20th century droughts are not representative of the full range of drought variability that has occurred over the last several thousand years. Proxy data for the Great Plains region indicate that the severe droughts of the 20th century, although certainly major droughts, are by no means unprecedented and it is likely that droughts of a magnitude at least equal to those of the 1930s and 1950s have occurred with some regularity over the past 400 years. A look further back in time reveals evidence that multidecadal drought events occurred in the late 13th and 16th centuries that were of a greater duration and severity than 20th century droughts. Other proxy records, including the few annually resolved paleoclimatic records, provide some evidence for longer periods of drought or periods of more frequent drought prior to the 13th century, and support the idea of a drought regime shift roughly around the 13th -15th centuries.

This assessment of the paleoclimatic record suggests that droughts of the 20th century are not unusual in the context of the past 2,000 years, and that future droughts could be of a much greater severity and duration than what we have yet experienced. Assessments of future drought variability should consider the range of natural drought variability of the past several thousand years as documented by the paleoclimatic record, over which possible anthropogenically-induced climate changes will be superimposed.

A Long History of Human Caused Extinctions: Human Impact on Australian Megafauna 50,000 years Ago

Australia suffered a major loss of its large- and medium-sized land mammals in the Late Pleistocene. All marsupials larger than a human (19 species) and 75% of all marsupials weighing more than 10 kg became extinct, along with three large reptiles and the ostrich-sized Genyornis newtoni; two other large flightless birds, the emu and the cassowary, survived. Collectively, the lost species are often referred to as the Australian megafauna, although most are of modest body mass.

For more than a century the cause of this exceptional extinction has been debated without a clear consensus, largely because of the difficulty in dating faunal remains close to the limit of radiocarbon dating. The cause of megafauna extinction initially focused on climate change or human predation, but more recently has included indirect consequences of human activity, particularly ecosystem change resulting from burning practices. Resolving the debate requires secure dates on the extinction events, on the arrival of humans in Australia, and on major climate and environmental changes. In a 1999 paper in Science, Gifford Miller, Beverly Johnson and colleagues present new dates that constrain the ages of these key events based on amino acid racemization (AAR), AMS 14C and thermal ionization mass spectrometry (TIMS) U-series analyses on eggshells, and luminescence dates on associated sediment. These new dates allow us for the first time to rigorously evaluate the cause of this major extinction event.

At INSTAAR's Amino Acid Geochronology Laboratory the authors determined more than 700 AAR dates on Genyornis eggshells from three different climate regions that document the continuous presence of Genyornis from more than 100,000 years ago until their sudden disappearance about 50,000 years ago. This is about the same time that humans arrived in Australia. The extinction of Genyornis 50,000 ago is corroborated by more than 100 AMS 14C dates, 8 luminescence dates between 40 and 120 thousand years ago on eolian sand from which eggshell has been collected, and 5 TIMS U-series dates on Genyornis eggshell.

By evaluating paleoenvironmental reconstructions for the past 150,000 it was demonstrated that Genyornis was able to survive the range of natural environmental changes caused by Pleistocene climate oscillations. During the period of Genyornis extinction climate was moderate. Consequently, climate change as an explanation for Genyornis extinction is unlikely.

Humans colonized Australia about 55,000 years ago, shortly before the extinction event. The authors hypothesize that burning practices of the earliest human immigrants differed enough from that of the natural fire cycle to disrupt ecosystems across the semi-arid zone. The vegetation may have been particularly susceptible due to the continent's geological quiescence; soils in this low-relief landscape were depleted of most nutrients, resulting in a lack of ecosystem resiliency. The authors go on to postulate that human burning at times of the year and at frequencies to which the vegetation was not pre-adapted, resulted in a dramatic decrease in tree and shrub vegetation across the continental interior, which in turn placed unprecedented stress on the dependent fauna.

Synchronous Climate Changes in Antarctica and the North Atlantic

Central Greenland ice cores provide evidence of abrupt changes in climate over the past 100,000 years. These large temperature shifts of 5° to 10°C or more are very fast, occurring in few decades. Many of these changes have also been identified in sedimentary and geochemical signatures in deep-sea sediment cores from the North Atlantic, confirming the link between millennial scale climate variability and ocean thermohaline circulation. In a 1998 paper in Science, Eric Steig, James White and Scott Lehman, with a number of colleagues, showed that two of the most prominent North Atlantic events-the rapid warming that marks the end of the last glacial period and the Bølling/Allerød-Younger Dryas oscillation-are also recorded in an ice core from Taylor Dome, in the western Ross Sea sector of Antarctica. The study is based on stable isotope measurements on ice made in INSTAAR's Stable Isotope Laboratory. This result contrasts with evidence from ice cores in other regions of Antarctica, which show an asynchronous response between the Northern and Southern Hemispheres. For example, the Antarctic Cold Reversal (ACR), a period of cooling that appears in the Vostok and Byrd Station Antarctic stable isotope records, has in the past been compared with the Younger Dryas (YD), a prominent feature in Northern Hemisphere records (see figure).

Based on new measurements of atmospheric trace gas concentrations in trapped air bubbles by the French, we now know that the ACR occurred at least 1000 years before the YD. Just as we were mulling how to account for this asynchrony of polar climate, the Taylor Dome results show that for some parts of Antarctica, these major climate events are actually synchronous.

The study concludes that the differences between the isotope temperature history from Taylor Dome and those from other Antarctic sites are too large to be attributed to dating errors. Rather, the results indicate that the circum-Antarctic climate response to changes in major ocean circulation patterns, specifically the formation and export of deeper ocean water from the North Atlantic region. Given the current substantial difficulty of realistically simulating ocean atmosphere interactions in general and the dynamics of the Southern Ocean in particular, it may be some time before the role of North Atlantic Ocean circulation in shaping Antarctic climate can be rigorously evaluated. In the meantime, the authors are pushing forward with plans to test this observation by collection and analysis of additional Antarctic ice cores, especially from near coastal sites.

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