Hydroclimate variability in the United States continental interior during the early Eocene Climatic Optimum

Abstract

The early Eocene (56.0 to 47.8 million years ago) was punctuated by a series of transient episodes of rapid global warming superimposed on the long-term early Cenozoic warming trend, culminating in the early Eocene Climatic Optimum (EECO; 53.3 to 49.1 million years ago). Details of the hydroclimate regime operating during the EECO are poorly constrained, especially for continental interior sites. The Green River Formation (GRF) of Utah and Colorado was deposited in a suite of large, unusually productive lakes that offer an ideal opportunity to study the hydrological response to warming. Here we report the hydrogen isotopic composition (δ2H) of leaf wax (long-chain n-alkanes) and algal (phytane) lipids preserved in the organic-rich Mahogany Zone (49.3 to 48.7 Ma) and use these data to reconstruct precipitation and lake water δ2H records, respectively. We observe large inter-site variations in algal and leaf wax δ2H values (~50 to 75‰), suggesting that additional local controls influence precipitation and/or lake water δ2H (e.g., salinity). Intriguingly, leaf wax and algal lipid δ2H values show little variation through the Mahogany Zone, implying a relatively stable hydrological regime during the latter phase of the EECO. This contrasts with the more variable hydrological regime that prevailed during early Eocene hyperthermals. Unlike the EECO, the early Eocene hyperthermals in the Uinta region do not coincide with the deposition of organic-rich sediments. This suggests that a stable hydrological regime during the EECO may enable the preservation of organic matter within continental-interior lake systems, potentially leading to an important negative climate feedback during the early Eocene and other greenhouse climates.

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Identifiers

ORCID for John Marshall: orcid.org/0000-0002-9242-3646

ORCID for Gordon Inglis: orcid.org/0000-0002-0032-4668

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