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GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 218-6
Presentation Time: 2:50 PM

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


IBARRA, Daniel E.1, LLOYD, Max2, KUKLA, Tyler3, SHARP, Zachary4 and CHAMBERLAIN, C. Page3, (1)Earth and Planetary Sciences, University of California, Berkeley, Berkeley, CA 94709; Geological Sciences, Stanford University, Stanford, CA 94305, (2)Earth and Planetary Sciences, University of California, Berkeley, Berkeley, CA 94709, (3)Geological Sciences, Stanford University, Stanford, CA 94305, (4)Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131

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‰ (λRL = 0.528), with an empirical slope (λchert, 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 λchert is between equilibrium processes related to meteoric water and SiO2 precipitation (~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.

Session No. 218

T86. 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 (Phoenix Convention Center)
Geological Society of America Abstracts with Programs. Vol. 51, No. 5
doi: 10.1130/abs/2019AM-337554
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