Figure 1

INTRODUCTION

The last interglacial period has been cited as a possible analog for a future climate under an increased-CO₂ greenhouse warming. Previous studies have shown that during the last interglacial CO₂ concentrations in the atmosphere were relatively high (see Fig. 1) temperatures may have been higher than the present, and sea level may have been ~6 m higher (see Fig. 2). The ultimate goals of the LITE project are to (1) develop an accurate estimate of the duration of the last interglacial period, with improved understanding of its primary cause or causes, and (2) using the geologic record, reconstruct the climate of the last interglacial period in the U.S. Both of these goals are intended to provide a basis for improvement of atmospheric general circulation models (AGCMs) that are critical for modeling of future climate.

Figure 2

PROJECT OBJECTIVES AND STRATEGY

The overall project objectives are to: (1) determine the precise timing of the last interglacial period, (2) reconstruct the last interglacial climate of the U.S., including Alaska, and (3) compare the paleoclimate, derived from the geologic record, of the last interglacial with several AGCM reconstructions of this paleoclimate.

The timing of the last interglacial period is determined from precise TIMS U-series dating of emergent marine terrace and reef corals. Less precise timing of the last interglacial period for continental interior localities is determined through a combination of minimum-limiting radiocarbon ages, thermoluminescence dating, and ¹⁰Be dating. Paleotemperature reconstruction of coastal waters around the U.S. during the last interglacial is accomplished through oxygen isotope analyses of last interglacial mollusks and/or corals collected from emergent terraces and reefs. Climate of the continental interior of the U.S. and Alaska is reconstructed from transects of last interglacial soils found in thick loess sections that occur across modern temperature and precipitation gradients. Because soil properties are in part a function of climate and vegetation, modern soil geography is a function of modern climatology and biogeography (see Fig. 3: map of modern North American vegetation). By analogy, last-interglacial soil geography is a function of last interglacial paleoclimate and paleobiogeography. Soil properties most closely linked to climate and vegetation are morphology, clay content, mineralogy, chemistry, magnetic properties, and carbon isotopic composition of organic matter.

Figure 3

POTENTIAL IMPACTS

At present, precise climate forecasting is not possible, but may be accomplished in a broad way with AGCMs. These models are continually being refined, in part by means of testing simulated climates of the past with geologic reconstructions. The last interglacial period is the most recent warm interval that may have been significantly warmer than today's climate; therefore, it is a critical time period for testing of AGCMs. Several AGCMs have been used to reconstruct this period, but there are few geologic records that synthesize the last interglacial period. This project will have its greatest impact in the climate modeling community, where the AGCM reconstructions of the last interglacial can be compared to the geologic record of this period.

COLLABORATORS