Oxygen and hydrogen isotope ratios in freshwater chert as indicators of ancient climate and hydrologic regime

Abstract

The oxygen and hydrogen isotopic composition of Eocene and Miocene freshwater cherts in the western United States records regional climatic variation in the Cenozoic. Here, we present isotopic measurements of 47 freshwater cherts of Eocene and Miocene age from the Great Basin of the western United States at two different sites and interpret them in light of regional climatic and tectonic history. The large range of ␦ 18 O of terrestrial cherts measured in this study, from 11.2‰ to 31.2‰ (SMOW: Standard Mean Ocean), is shown to be primarily the result of variations in ␦ 18 O of surface water. The following trends and patterns are recognized within this range of ␦ 18 O values. First, in Cenozoic rocks of northern Nevada, chert ␦ 18 O records the same shift observed in authigenic calcite between the Eocene and Miocene that has been attributed to regional surface uplift. The consistent covariation of proxies suggests that chert reliably records and retains a signal of ancient meteoric water isotopic composition, even though our analyses show that chert formed from warmer waters (40°C) than coexisting calcite (20°C). Second, there is a strong positive correlation between ␦ 18 O and ␦D in Eocene age chert from Elko, Nevada and Salina, Utah that suggests large changes in lake water isotopic composition due to evaporation. Evaporative effects on lake water isotopic composition, rather than surface temperature, exert the primary control on the isotopic composition of chert, accounting for 10‰ of the 16‰ range in ␦ 18 O measured in Eocene cherts. From authigenic mineral data, we calculate a range in isotopic composition of Eocene precipitation in the north-central Great Basin of Ϫ10 to Ϫ14‰ for ␦ 18 O and Ϫ70 to Ϫ100‰ for ␦D, which is in agreement with previous estimates for Eocene basins of the western United States. Due to its resistance to alteration and record of variations in both ␦ 18 O and ␦D of water, chert has the potential to corroborate and constrain the cause of variations in isotope stratigraphies.

Paleogene landscape evolution of the central North American Cordillera: Developing topography and hydrology in the Laramide foreland

“…4), which we interpret to be the result of formation from evaporatively Dand 18 O-enriched lake water. This interpretation is supported by the study of Abruzzese et al (2005), who showed that cherts throughout the Green River Formation in the Flagstaff Basin have δD chert and δ…”

“…O measured in a single proxy that instead describes a line of lower slope than the GMWL, such as that of ray e in Figure 4, indicates that lake waters were evaporatively enriched in 18 O and D (Abruzzese et al, 2005). We use the approach of Abruzzese et al (2005) to determine the role of evaporation in chert-bearing facies.…”

“…We use the approach of Abruzzese et al (2005) to determine the role of evaporation in chert-bearing facies. O trend representative of a mixing line between unaltered and diagenetic carbonate (Garzione et al, 2004).…”

Stable isotope constraints on the elevation history of the Sierra Nevada Mountains, California

“…Fossil teeth have been used to reconstruct ancient isotopic rain shadows in other areas Fricke, 2003). For the Sierras, there have been studies using authigenic minerals on either the leeward (Chamberlain et al, 1999;Chamberlain and Poage, 2000;Horton et al, 2004;Abruzzese et al, 2005;Takeuchi and Larson, 2005) or windward side (Mulch et al, 2006) of the range. Yet because these studies do not reconstruct the composition of meteoric water on both sides of the range, it is diffi cult to tease apart the orographic signal from other factors that infl uence isotopic value, especially climate change.…”

Continental paleoclimate estimates from the late Mississippian Redwall karst event: northern and north-central Arizona (USA)

“…Microorganisms embedded and permineralized in Precambrian chert attest to the chemical integrity and physical stability of silica (Schopf et al 2007;Knauth and Lowe 2003). Remarkable preservation of microorganisms and previous oxygen and deuterium isotopic studies on chert (Knauth and Epstein 1976;Kenny and Knauth 1992;Sharp et al 2002;Abruzzese et al 2005) suggest excellent preservation of original isotopic values in chert dating from the late Proterozoic. Nevertheless, one process that could produce the observed isotopic depletions is metamorphism.…”