Geological Society of America

T218 Inverting Source from Sink: Decoding Sedimentary Records of Climate, Erosion, and Vegetation

Tuesday, 24 September 2019
1:30 PM - 5:30 PM
Room 132ABC, North Building

PALEOCLIMATE RECONSTRUCTIONS USING TRIPLE OXYGEN ISOTOPE ANALYSES OF LACUSTRINE CHERT: AN EXAMPLE FROM THE GREEN RIVER FORMATION

Paper number: T218-7
Program: Technical Sessions
Time: 2:50 PM – 3:10 PM

Authors

Daniel E. Ibarra Presenting Author
University of California, Berkeley, Berkeley, CA
Daniel E. Ibarra Presenting Author
Stanford University, Stanford, CA
Max Lloyd
University of California, Berkeley, Berkeley, CA
Tyler Kukla
Stanford UniversityGeological Sciences, Palo Alto, CA
Zachary Sharp
University of New Mexico, Albuquerque, NM
C. Page Chamberlain
Stanford University, Stanford, CA

Abstract

Triple oxygen isotope measurements are emerging as a new tool in terrestrial paleoclimate and paleoaltimetry reconstructions. In this contribution we develop the application of triple oxygen isotope measurements to lacustrine sediments, focusing here on a dataset from Eocene Lake Flagstaff (Green River Formation, UT) using chert samples previously measured for δ¹⁸O and δD. This basin is suggested to be indicative of a terminal balanced-filled lake system.

We present new Δ¹⁷O measurements made by laser fluorination of chert. Across an evaporation trend spanning 12.2‰ in δ¹⁸O we observe a strong negative correlation with Δ¹⁷O ranging from –0.081‰ to –0.203‰ (λ_RL= 0.528), with an empirical slope (λ_chert, δ’¹⁷O vs. δ’¹⁸O) of 0.523.

Given temperature-dependent fractionation factors for the SiO₂-H₂O system and compilations of trends in meteoric water, we derive mixing relationships analogous to modern lake trends in δ¹⁸O-δD. In doing so, we demonstrate calculation of the δ¹⁸O and Δ¹⁷O composition of the unevaporated lake source water. For the Flagstaff Basin chert samples we derive a source water of δ’¹⁸O = –17.9±1.4‰ and Δ¹⁷O = 0.030±0.021‰, a comparison point for climate model simulations and paleoaltimetry reconstructions. Our λ_chert is between equilibrium processes related to meteoric water and SiO₂precipitation (~0.528), and kinetic fractionation of evaporating water (~0.519), indicating that freshwater lacustrine cherts may be suitable for quantitative relative humidity reconstructions similar to those made on gypsum hydration waters. These analyses of δ¹⁸O-Δ¹⁷O allow us to test the fidelity of paleoclimate reconstructions using just δD or paired δ¹⁸O and δD measurements. Further, development of robust proxy system models is needed to establish the use of triple oxygen isotope measurements of chert to quantify patterns of regional moisture balance in the past.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-337554

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