Reconstructing Past Elevations From Triple Oxygen Isotopes Of Lacustrine Chert: Application To The Eocene Nevadaplano, Elko Basin, Nevada, United States

Triple oxygen isotope measurements are an emerging tool in paleoclimate reconstructions. In this contribution we develop the application of triple oxygen isotope measurements to lacustrine sediments to reconstruct past elevations. We focus on a well-constrained sample set from the Eocene North American Cordillera (Cherty Limestone Formation, Elko Basin, NV, United States, 42-43.5 Ma) on the east side of the elevated Nevadaplano. We present triple oxygen isotope measurements on freshwater lacustrine chert samples from the Cherty Limestone Formation. Across an evaporation trend spanning 6.5 parts per thousand in delta O-18 values we observe a negative correlation with Delta ' O-17 ranging from -0.066 to -0.111 parts per thousand (lambda(RL) = 0.528), with an empirical slope (lambda(chert), delta ' O-17 vs. delta ' O-18) of 0.5236. Additionally, we present new carbonate clumped isotope (Delta(47)) temperature results on the overlying fluvial-lacustrine Elko Formation, which indicate an error-weighted mean temperature of 32.5 +/- 3.8 degrees C (1 sigma), and evaporatively enriched lake water spanning delta O-18 values of -3.7 to +3.5 parts per thousand (VSMOW). Paired chert and carbonate delta O-18 values demonstrate that co-equilbrium among the carbonate and chert phases is unlikely. Thus, as also previously suggested, it is most likely that Elko Basin chert formed during early diagenesis in equilbirium with pore waters that reflect evaporatively O-18-enriched lake water. Using this scenario we apply a model for back-calculating unevaporated water composition to derive a source water of delta ' O-18 = -16.1 parts per thousand (VSMOW), similar to modern local meteoric waters but lower than previous work on paired delta O-18- delta D measurements from the same chert samples. Further, this back-calculated unevaporated source water is higher than those derived using delta D measurements of Late Eocene hydrated volcanic glass from the Elko Basin (average delta ' O-18 equivalent of approximately -18.4 parts per thousand, VSMOW). This suggests, assuming Eocene meteoric water Delta ' O-17 values similar to today (similar to 0.032 parts per thousand), either that: (1) the hypsometric mean elevation recorded by the lacustrine Cherty Limestone was lower than that derived from the average of the volcanic glass delta D measurements alone; or (2) there was hydrogen exchange in volcanic glass with later low delta D meteoric fluids. Nonetheless, our new findings support a relatively high (similar to 2.5-3 km) plateau recorded in the Elko Basin during the mid-Eocene.