Friday May 4 CSL Meeting

Practice Master's Thesis Defense: Glacial-Interglacial Climate Variability in the North American Southern Great Plains from Stable Isotopes in a Texas Stalagmite

Presenter: Ellen Bartow-Gillies
Time: Friday, 4 May 2018, 2:00 p.m.
Location: 805 O&M

Abstract: In the US Southern Great Plains (SGP), projections of changes in rainfall under future warming scenarios differ greatly in their sign and intensity. The lack of continuous, well- dated paleoclimate records before the Last Glacial Maximum (LGM) hinders a comprehensive understanding of past variability in regional hydroclimate patterns. We present three absolutely U/Th-dated oxygen and carbon isotope records from a calcite stalagmite collected near Georgetown, Texas (Cobbs Cavern at 30°N, 98°W), spanning 350 to 3800 years before present (BP), 98 to 130 kyr BP, and 279 to 208 kyr BP. Based on our two-year dataset of central Texas precipitation and Cobbs Cavern dripwaters, we interpret the oxygen isotopic composition of the stalagmite to reflect variability in regional rainwater δ¹⁸O composition through time, which we determine is largely driven by amount of precipitation and storm structure. There is no evidence for kinetic isotope effects in the stalagmite, so we conclude that more negative δ¹⁸O values reflect wetter conditions with larger and more organized storms, whereas more positive δ¹⁸O values reflect drier conditions with unorganized, sporadic storms. Stalagmite δ¹³C variations may be driven by shifts in overlying vegetation type, soil bioproductivity, karst flow rate variability, and prior calcite precipitation such that more negative (positive) δ¹³C values reflect increased (decreased) moisture availability in the region. The stalagmite records include Marine Isotope Stage (MIS) 5e, a period where global temperatures may have been as much as 2°C warmer and sea level 4-6 m higher than present. Thus, our δ¹⁸O record provides context for how SGP hydroclimate may respond to future warming. Prominent features in the δ¹⁸O record, including a wet MIS 5e appear to be paced by precession, with the timing of δ¹⁸O minima (maxima) broadly consistent with that of maxima (minima) in summer insolation at 30°N. Our SGP stalagmite records shed light on the fundamental character of SGP hydroclimate response to glacial-interglacial forcings, and provide evidence for increased precipitation and persistent convective storm activity under past warming conditions.

Texas stalagmite oxygen record (blue) with U/Th ages (pink) and northern hemisphere summer insolation at 30°N (black). Glacial terminations are noted. More negative δ¹⁸O values are interpreted as indicative of wetter conditions, and more positive values drier.