Iron-Oxide Geochronology to Constrain the Formation of Soils and Paleosols

In this thesis, I show how iron-oxide geochronology can be applied to soils and paleosols and I explore new applications of the (U-Th)/He and cosmogenic 3He dating methods. In Chapters II and III, I apply the (U-Th)/He method to goethite pisoliths in paleosols and fissure fillings of the Bohnerz deposits, which are a widespread erosional lag deposit in Central Europe. I show that this deposit formed between ~55 Ma and 2 Ma and not in the late Cretaceous-early Eocene, as was previously claimed. A map compiled from published sources shows the paleo-extent of the Bohnerz deposit, which were developed on every limestone plateau in Central Europe. Concentrations of cosmogenic 3He measured on pisoliths in paleosols, demonstrate that ancient cosmic-ray exposure occurred over at least 5 Ma, possibly 10-20 Ma. This shows that the Bohnerz deposits represent surfaces, which were stable for tens of millions of years before burial. Even today, these surfaces experience extremely low erosion rates of ~0.1 m/Ma, comparable to some of the Earth's most stable surfaces in arid environments. The hiatus represented by the Bohnerz unconformity lasts 125-150 Ma, yet only 30-40% of that duration is recorded in Bohnerz deposits. Pisoliths of the Bohnerz deposits are a continuous record of Central European continental climate for most of the Cenozoic. In Chapter IV, I develop a laboratory technique to enable (U-Th)/He dating of hematite samples by the single-aliquot method. Highly retentive hematite samples have to be heated to >1000 °C to fully degas helium, but U is lost from the sample at around 980 °C. Through infrared spectroscopy and trace element analysis of heated samples, I show that U-loss correlates with a phase change from hematite to magnetite. Delaying this phase change to higher temperatures extends the usable range of temperatures to which samples can be safely heated without U-loss. This is achieved by degassing samples in a partial oxygen pressure of around 100 mbar, which permits degassing of samples up to 1150 °C without loss of U. I demonstrate that precise and accurate (U-Th)/He ages can be obtained for hematite samples, which agree with established two-aliquot ages. I show how this method can be implemented and automated. In Chapter V, I extend the use of 3He cosmogenic dating to fine-grained iron-oxide particles, which are abundant in most types of modern soils. Diffusion modeling predicts that hematite particles down to ~10 nm should quantitatively retain helium for at least 1 Ma at Earth-surface conditions. In order to test whether pedogenic iron-oxides can be used for geochronology, I study a vertical profile of a relict soil developed on a fanglomerate terrace at Whitewater Hill, California. Profiles of 10Be and 26Al in detrital quartz agree well with an exponential decrease in cosmogenic nuclide production and they yield an exposure age of 52.4±2.2 ka assuming no erosion. The vertical profile of 3He concentrations in pedogenic iron-oxides shows a decrease with depth, but concentrations are higher than expected in the 40-100 cm depth range. This indicates vertical movement of iron-oxides in the soil, which is a well-known soil formation process. These observations are more consistent with a soil age of 208±44 ka. This approach yields information on both age constraints and the formation and migration of pedogenic iron-oxides in the soil column.