Eocene (50-55 Ma) greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA

Adrian P Broz; Devin Pritchard-Peterson; Diogo Spinola; Sarah Schneider; Gregory Retallack; Lucas C R Silva

doi:10.1038/s41598-024-53210-0

Eocene (50-55 Ma) greenhouse climate recorded in nonmarine rocks of San Diego, CA, USA

Sci Rep. 2024 Jan 31;14(1):2613. doi: 10.1038/s41598-024-53210-0.

Authors

Adrian P Broz^{1

2}, Devin Pritchard-Peterson³, Diogo Spinola⁴, Sarah Schneider⁵, Gregory Retallack⁵, Lucas C R Silva^{6

7}

Affiliations

¹ Department of Earth, Atmospheric and Planetary Sciences, Purdue University, Lafayette, IN, USA. abroz@uoregon.edu.
² Department of Earth Sciences, University of Oregon, Eugene, OR, USA. abroz@uoregon.edu.
³ Dudek Environmental Consulting, Encinitas, CA, USA.
⁴ Department of Ecosystem Science and Management, University of North British Columbia, Prince George, BC, Canada.
⁵ Department of Earth Sciences, University of Oregon, Eugene, OR, USA.
⁶ Environmental Studies Program, University of Oregon, Eugene, OR, USA.
⁷ Department of Biology, University of Oregon, Eugene, OR, USA.

Abstract

Nonmarine rocks in sea cliffs of southern California store a detailed record of weathering under tropical conditions millions of years ago, where today the climate is much drier and cooler. This work examines early Eocene (~ 50-55 million-year-old) deeply weathered paleosols (ancient, buried soils) exposed in marine terraces of northern San Diego County, California, and uses their geochemistry and mineralogy to reconstruct climate and weathering intensity during early Eocene greenhouse climates. These Eocene warm spikes have been modeled as prequels for ongoing anthropogenic global warming driven by a spike in atmospheric CO₂. Paleocene-Eocene thermal maximum (PETM, ~ 55 Ma) kaolinitic paleosols developed in volcaniclastic conglomerates are evidence of intense weathering (CIA > 98) under warm and wet conditions (mean annual temperature [MAT] of ~ 17 °C ± 4.4 °C and mean annual precipitation [MAP] of ~ 1500 ± 299 mm). Geologically younger Early Eocene climatic optimum (EECO, 50 Ma) high shrink-swell (Vertisol) paleosols developed in coarse sandstones are also intensely weathered (CIA > 80) with MAT estimates of ~ 20 °C ± 4.4 °C but have lower estimated MAP (~ 1100 ± 299 mm), suggesting a less humid climate for the EECO greenhouse spike than for the earlier PETM greenhouse spike.