227-2 Legacy Amino Acid Ratios from Terrestrial Snails in Alluvial Deposits in Colorado Show >8-10° Average Quaternary Temperature Depression but Inconsistent River-terrace Correlations

Session: Recent Advances in Glacial Geology, Geomorphology, and Chronology (Posters)

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Abstract:

Amino-acid ratios (D-alloisoleucine/L-isoleucine of the total hydrolysate fraction; aIle/Ile) that we measured (using high-performance liquid chromatography) on terrestrial gastropod shells from alluvial terrace deposits in Colorado (1) confirm independent estimates of at least 8-10°C lowering of temperature in the Rocky Mountain region during the middle and late Quaternary, but (2) do not provide consistent relative ages or allow correlations among terraces of differing heights along the Front Range piedmont, or along the Yampa and White rivers in northwestern Colorado. Data for this and related studies of terrestrial gastropod aIle/Ile ratios during 1977-1984 can be found at: https://doi.org/10.7910/DVN/JLDPAD.

Ratios on shells of the genus Vallonia from a  Lava Creek B ash bed (~631 ka) near Carbondale, Colorado, on Succinea-Catinella from just above the same ash near Meeker, Colorado, and on Succinea-Catinella from just beneath the same ash near Baggs, Wyoming, when used with a kinetic equation for isoleucine epimerization in Vallonia that we determined through laboratory heating analyses and ¹⁴C-dated shells, yields an EDT (effective diagenetic temperature) of 7-11°C lower than the present MAT (mean annual temperature) in these areas. Because our EDT is derived with an equation in which the reaction rate is exponentially dependent on temperature, the arithmetic mean temperature for the period 0-631 ka  must  have  been lower than our  EDT.

Our attempt to use aIle/Ile ratios on shells of Vallonia, Succinea-Catinella, and Pupilla from alluvial terrace deposits among the three study areas for relative dating and correlation was largely unsuccessful. Even along the two rivers in northwestern Colorado where MATs are ~6°C, ratios are too variable or analyses too few to show the <10% precision among ratios in multiple genera needed to support correlations of terraces between the two river valleys. A more significant issue is that all ratios for shells from terrace deposits along the Front Range are dramatically lower than expected, considering the present Front Range MAT (10°C). This suggests that EDTs along the Front Range were lower than suggested by the present differences between Front Range MAT and the MATs of other areas where aIle/Ile ratios have been measured on these genera of terrestrial gastropods.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7031

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