Abstract
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‰ in δ18O values we observe a negative correlation with Δ′17O ranging from −0.066 to −0.111‰ (λRL= 0.528), with an empirical slope (λchert, δ′17O vs. δ′18O) of 0.5236. Additionally, we present new carbonate clumped isotope (Δ47) temperature results on the overlying fluvial-lacustrine Elko Formation, which indicate an error-weighted mean temperature of 32.5 ± 3.8°C (1σ), and evaporatively enriched lake water spanning δ18O values of −3.7 to +3.5‰ (VSMOW). Paired chert and carbonate δ18O 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 evaporatively18O-enriched lake water. Using this scenario we apply a model for back-calculating unevaporated water composition to derive a source water of δ′18O = −16.1‰ (VSMOW), similar to modern local meteoric waters but lower than previous work on paired δ18O- δD measurements from the same chert samples. Further, this back-calculated unevaporated source water is higher than those derived using δD measurements of Late Eocene hydrated volcanic glass from the Elko Basin (average δ′18O equivalent of approximately −18.4‰, VSMOW). This suggests, assuming Eocene meteoric water Δ′17O values similar to today (∼0.032‰), 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 δD measurements alone; or (2) there was hydrogen exchange in volcanic glass with later low δD meteoric fluids. Nonetheless, our new findings support a relatively high (∼2.5–3 km) plateau recorded in the Elko Basin during the mid-Eocene.
Highlights
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
In this study we present the first lacustrine chert triple oxygen isotope dataset from a Cenozoic basin in western North America and use this data set, with carbonate clumped isotope measurements from overlying strata, to derive an elevation estimate for the Eocene Nevadaplano
Our triple oxygen isotope external reproducibility (1σ) during the session of analyses based on the primary standard used to correct δ18O and 17O data is ±0.073 for δ18O and 0.016 for 17O ( ± 1σ; n = 13)
Summary
AND GEOLOGIC SETTINGReconstructing the topographic history of mountain belts relies heavily upon the oxygen (δ18O) and hydrogen (δD) isotopes of authigenic minerals in paleosols and paleolake sediments (e.g., Chamberlain et al, 1999; Poage and Chamberlain, 2002; Takeuchi and Larson, 2005; Ghosh et al, 2006a; Garzione et al, 2006; Davis et al, 2009; Takeuchi et al, 2010; Mulch et al, 2010, 2015; Gébelin et al, 2013; Schwartz et al, 2019; Pingel et al, 2020; Ingalls et al, 2020a; Quade et al, 2020; San Jose et al, 2020; Kukla et al, 2021). The interpretation of these isotopic data as signals of past elevation is often complicated by evaporation in soils and lakes that will enrich waters in 18O and D (e.g., Abruzzese et al, 2005; Davis et al, 2009; Mulch et al, 2015; Mulch, 2016; Ingalls et al, 2020a) These evaporitic effects are problematic in semi-arid to arid settings that often form in the rain shadow of uplifting mountains. Oxygen isotope ratios of the hydroxyl ions of chert are likely a robust indicator of waters from which they form (Knauth, 1973) since hydroxyl ions appear to resist post-depositional exchange (Micheelsen, 1966) It is strongly material dependent how resilient hydrogen isotopes are to later diagenesis and exchange given that other minerals (clays and micas) can exhibit some degree of later exchange of hydrogen (O’Neil and Kharaka, 1976; Chamberlain et al, 2020). Complementary to triple oxygen isotopes, carbonate clumped isotope analyses are needed to constrain the effect of evaporation on the δ18O values of the minerals by assessing the carbonate formation temperatures and the δ18O values of (evaporatively enriched) lake water from which the carbonate mineral formed
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