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Paleogene paleosols and changes in pedogenesis during the initial Eocene thermal maximum: Big Bend National Park, Texas, USA

Paul D. White^1, and Judith Schiebout²

¹ Physics Department, Community College of Rhode Island, 400 East Avenue, Warwick, Rhode Island, 02886, USA
² Museum of Natural Science and the Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA

Correspondence: E-mail: pdwhite1{at}ccri.edu

This paper tests the hypothesis that continental chemical weathering increased during the initial Eocene thermal maximum (IETM) by comparing paleosols that formed before and during the event in western Texas. The chemical index of alteration (CIA) was used to investigate the weathering of silicate minerals. Paleosols generated before the IETM have CIA values ranging from 62 to 72, and CIA values during the IETM range from 67 to 82. The CIA values for pre–initial Eocene thermal maximum paleosols indicate moderate weathering conditions, and CIA values during the event indicate moderate to extreme weathering conditions. The clay mineralogy of the paleosols is dominated by smectite, and it is only within paleosols that formed during the IETM that there is a change. There is a notable increase in the amount of kaolinite in one paleosol horizon that is associated with the carbon excursion. In addition, there is an increase in the translocation of clays and iron, and an increase in the leaching of calcite and plagioclase in initial Eocene thermal maximum paleosols. The differences between soils that formed before and during the initial Eocene thermal maximum indicate that chemical weathering did increase during this ancient global warming event. The mechanism responsible for increased weathering is interpreted to be an increase in hydrolysis reactions caused by an increase in humidity and an increase of carbonic acid in the soil due to elevated CO₂ levels during the initial Eocene thermal maximum. Documentation of an increase in chemical weathering during the initial Eocene thermal maximum is significant because it may have served as a negative feedback to reduce atmospheric CO₂.

Key Words: paleosols • chemical weathering • initial Eocene thermal maximum • kaolinite