RECONSTRUCTING HIGH RESOLUTION RECORDS OF PALEOCLIMATE FROM MAGNETIC MINERAL ASSEMBLAGES IN MODERN SOILS AND PALEOSOLS SEQUENCES: PROGRESS AND CHALLENGES (Invited Presentation)

Magnetic mineral assemblages in modern and fossilized soils (paleosols) are typically indicative of the ambient climatic conditions present during soil formation, including mean annual precipitation and temperature. Yet, understanding the paleoclimate history recorded by pedogenic magnetic minerals requires sensitive differentiation of the grain size, composition, and concentration of various magnetic minerals within a soil or paleosol. Critically, paleoenvironmental interpretations hinge on our ability to separate and isolate the magnetic signals due to pedogenic magnetic minerals from those associated with a soil’s parent material and non-pedogenic processes.

Here, we summarize two case studies that highlight recent advances in using pedogenic magnetic mineral assemblages as paleoclimate proxies. First, we explore the influence of vegetation on the magnetic properties of modern soils. A unique site in Minnesota allows us to compare forest and prairie soils that developed over identical time periods, parent materials, and climate conditions. Although the structure of the forest and prairie soils are different, when the magnetic properties of the pedogenic magnetic minerals are isolated, both yield the same reliable estimate of modern annual rainfall, which argues that environmental information gathered from different contemporaneous soil types may be broadly comparable. A second case study on Paleocene-Eocene paleosols preserved in the Bighorn Basin, Wyoming, highlights the impacts of diagenesis and surficial weathering on pedogenic magnetic mineral preservation. While pedogenic magnetic minerals in this study have clearly experienced alteration, general trends in the magnetic data compare favorably to geochemical paleoprecipitation proxies on the same materials, suggesting that primary environmental information can be preserved in paleosols’ magnetic minerals in deep time.

Future research should aim to calibrate magnetic paleoproxies more precisely and constrain the role of microbial communities on the production of pedogenic magnetic minerals. Finally, we argue for improving paleosol paleoenvironmental proxies through integrating geochemical, mineralogical, and magnetic data into a single user-friendly, multivariate, community-supported paleoenvironmental tool.