Date

2017

Document Type

Thesis

Degree

Master of Science

Department

Earth and Environmental Sciences

First Adviser

Peters, Stephen C.

Other advisers/committee members

Kodama, Kenneth P.; Pazzaglia, Frank J.

Abstract

The goethite/hematite (G/H) ratio in modern soils around the world has a strong, positive linear relationship with mean annual precipitation (MAP) (Hyland et al., 2015). If this relationship holds true in paleosols, then the G/H could be a reliable proxy of paleoclimate conditions recorded in soils. The Holocene, brown soils in the mid-Atlantic region are observably different than the older, red paleosols buried meters below, suggesting they formed in response to different environmental conditions, and are ideally suited to explore the G/H paleoclimate proxy. Soil development occurs in the critical zone, where parent material is altered by physical, chemical, and biological processes. These soils can be preserved on the landscape, and record paleoprecipitation conditions. MAP at the time of soil development can be encoded in elemental composition of pedogenic minerals, such as iron oxides. MAP controls the relative formation of goethite and hematite at the time of soil formation, resulting in the creation of the G/H ratio. Iron oxide minerals form in soils and give soils a characteristic color based on the relative abundance of different iron oxide minerals. Goethite and hematite are the two most commonly occurring of the iron (III) oxides and are formed under competing environmental conditions, with goethite (FeOOH) preferring high soil moisture, whereas hematite (Fe2O3) prefers low soil moisture. This work applies the relationship between G/H and MAP established by Hyland et al. (2015) to reconstruct MAP for 11 soils in the mid-Atlantic region of the US using novel magnetic methods to characterize G/H abundance. The particle size distribution, iron oxide crystallinity, and bulk elemental analysis are characterized for each soil profile. Interpreted MAP values indicate relative wetter or drier conditions through geologic time. Compared to present day conditions, the Middle to Late Miocene experienced a wetter climate than present, whereas the Middle and Late Pleistocene had lower MAP or experienced drier conditions than present. In comparison to the Middle Pleistocene, the Late Pleistocene shows relatively wetter conditions. MAP values obtained from the G/H proxy were compared to the Chemical Index of Weathering (CIW) proxy from Sheldon et al. (2002). The CIW proxy relates MAP with the weathering of feldspar minerals and accumulation of clays. Disagreement between the G/H and CIW proxies indicate possible over or under prediction of MAP in some soils. The influence of inheritance from parent material, organic matter leaching of secondary iron oxides, water level fluctuations and gleying, total dithionite-extractable (DCB) iron, and time of pedogenesis are discussed as they control the encoding of G/H in the soil profile.

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