Paleoclimate change in southern Indiana determined from speleothem climate proxies and analysis of modern precipitation oxygen isotope variations
Akers, Pete Douglas
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A narrative of southern Indiana climate and environmental changes for portions of the past 90,000 years has been created by integrating data from field-sampled stalagmites with a modern precipitation stable isotope database. Proper paleoenvironmental research in the American Midwest requires understanding the modern relationships between climate and precipitation stable oxygen isotope ratios (δ18O) at multiple temporal scales. While a novel weekly precipitation-day climate dataset did not produce results significantly different from previous studies, it did identify considerable isotope-climate relationship variability between individual months and seasons. Atmospheric flow patterns are the dominant control on multi-annual precipitation δ18O variability, with precipitation seasonality playing a lesser, yet related, role. This new understanding of Midwestern precipitation δ18O variability was applied to proxy climate data from fifteen stalagmites collected from four southern Indiana caves. A high-resolution analysis of two stalagmites from Upper Porter Cave provides a record of hydroclimate changes for much of the Holocene from multiple paleoenvironmental proxies. The timing of stalagmite growth and hiatus, with support from petrography, luminescence, and stable oxygen and carbon isotope values, suggests a drier-than-modern Indiana during the periods 2.3-2.7, 2.9-3.1, 3.6-4.2, 4.7-4.9, and 7.3-8.5 thousand years before present (ka BP). In contrast, the past 2300 years and the period of maximum Holocene warmth in the Midwest (5-7 ka BP) were relatively wet. Dry periods in southern Indiana during the Holocene may be driven partly by eastward shifts in the position of the Bermuda High in response to abrupt global climate changes. At millennial timescales, analysis of all sampled stalagmites suggests that high northern hemisphere summer insolation and relative global warmth favor stalagmite growth in the four sampled caves. The environment of southern Indiana during periods of stalagmite growth in the Pleistocene (57-59 and 80-85 ka BP) may have been roughly similar to early Holocene conditions, but the uncertainty involved with quantitative comparison between caves makes definitive conclusions difficult. The results from this research help fill a spatial void in the understanding of North American paleoenvironmental change and suggest that the transition to a future warmer climate may include significant dry episodes in southern Indiana.