66 Paleoclimate Records and Future Climate Trends in the American Southwest (Posters)

Thursday, 17 May 2018
8:30 AM - 4:30 PM
Outdoor Tents

CARBONATE CLUMPED ISOTOPES AND IN SITU TEMPERATURE MONITORING FOR HOLOCENE SOILS IN THE SAN LUIS VALLEY, USA INDICATE SPRINGTIME CARBONATE FORMATION

Paper number: 66-6
Program: Theme Sessions
Time: 8:30 AM – 4:30 PM

Authors

Abstract

Pedogenic carbonate horizons are abundant in semi-arid and arid regions worldwide and within the geologic record. They present a widely distributed archive of past environmental conditions, driven by global climate or tectonically-controlled elevation changes. Oxygen and carbon isotopes in calcite-rich nodules and clast rinds are widely-applied indicators of past soil water and CO2 composition linked to changing precipitation and plant communities. The temperature of carbonate formation, however, provides key constraint on past water and CO2 isotope values and can elucidate why they may have changed in the past. Clumped isotope thermometry can provide temperature constraints and additional climate information, if the carbonate forming system is well understood. We present preliminary clumped isotope (∆47) temperatures for Holocene soil carbonates dated by 14C and U-Th disequilibrium, and compare those results to two years of in situ soil temperature data to better understand the mechanism and seasonality of carbonate formation in the San Luis Valley region of the southern Rocky Mountains. Five temperature-monitoring sites ranging in elevation (1940-2450 m) and latitude (36.2-37.9°N) were installed in a variety of settings (range front, valley center, and canyon). All sites have similar temperature variations at depths >60 cm in terms of both seasonal amplitude (~18°C) and absolute value on the same day (within <4°C). This suggests ∆47 temperatures determined from soil carbonate that formed under similar conditions should be comparable at sites across the region. Temperatures based on ∆47 measurements of Holocene (>1.8 to 11.0 ka BP) carbonate from multiple sites yield consistent temperatures of 10±4°C, which is similar to modern springtime soil temperatures at >60 cm depth. This season of formation matches previous results of isotopic modeling at sites further south along the Rio Grande corridor. Temperatures measured between March and May show multiple, abrupt warming and cooling cycles on weekly timescales caused by wetting and drying of the soil during spring precipitation events. This may drive carbonate precipitation under low pCO2 conditions before increased plant respiration increases soil pCO2 later in the season.

Geological Society of America Abstracts with Programs. Vol. 50, No. 5

doi: 10.1130/abs/2018RM-313683

© Copyright 2018 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.