CHARACTERIZATION OF THE GOETHITE/HEMATITE RATIO IN MODERN AND ANCIENT SOILS IN THE MID-ATLANTIC REGION AS A PALEOCLIMATIC INDICATOR

The Holocene, brown soils in the mid-Atlantic region are observably different than the older, red paleosols found meters below, suggesting they formed in response to different environmental conditions. 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 climate conditions, including: paleoprecipitation, paleotemperature, soil moisture, and gas composition. These parameters can be encoded in elemental composition of pedogenic minerals, such as iron oxides. Iron oxide minerals form in soils through precipitation of dissolved iron, and can 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. Climate conditions control the relative formation of goethite and hematite at the time the soil is forming. Hematite is favored in tropical to subtropical climate regimes, resulting in rubification of soils. Conversely, goethite formation is favored in cool and moist climate regimes, resulting in a yellowish-brown soil color. An analysis of the goethite/hematite (G/H) abundance in paleosols provides a paleoclimatic record in soils. Differences in the G/H likely reflect changes in soil moisture or surficial processes, which would indicate a possible change in regional climate.

This work applies the relationship between G/H and climate to reconstruct paleoclimate for 6 sites in the mid-Atlantic region of the US. Soil samples were analyzed for particle size distribution, bulk elemental analysis with the ICP-MS, iron geochemistry, and goethite and hematite abundance using x-ray diffraction and paleomagnetic methods. The G/H in modern, Holocene soils and in older paleosols is characterized to examine changes in the G/H with geologic time and within the B horizon. Additionally, an oxygen diffusion model constructed in Stella was parameterized for a particular soil profile to investigate the relationship between oxygen availability, which is controlled by mean annual soil moisture content, and the G/H in soils.