Magnetic susceptibility of Permian-Triassic terrestrial mudstone of southern Britain: features and application for paleoenvironment reconstruction

The late Permian to Triassic (~ 260-201 Ma) is one of the hottest periods in the last 500 Ma, witnessing dramatic changes in the environment on the Pangea supercontinent, extensive low mid latitude dusty desert formation and the development of a strong monsoonal regime (Ruffell & Hounslow 2006). However, one aspect of Permo-Triassic climate that is relatively unexplored is the extent to which orbital forcing drove terrestrial environmental change in the monsoon and desert areas. The terrestrial mudstone deposits of the Aylesbeare and Mercia Mudstone groups formed under a dusty desert environment crop out along the southwest coast of England (Hounslow and Gallois, 2023) and potentially provide a means to address this gap.

Here we apply dual frequency magnetic susceptibility and analysis of its frequency dependence on mudstones of the Aylesbeare and Mercia Mudstone groups in order to constrain the environmental controls on these parameters and reconstruct Permo-Triassic environmental changes. We sample for these parameters at closely spaced regular intervals and use the new magnetostratigraphic timescale of Hounslow and Gallois (2023) to ascertain variations in these parameters with both age and depth. Both bulk magnetic susceptibility at low frequency (976 Hz) and frequency dependence show considerable variation, which correspond to stratigraphic alternations between green-grey silty mudstones and red mudstones, potentially reflecting wider scale changes in water table level and humidity. Evidence from temperature‐dependent magnetic susceptibility experiments suggested prevailing of magnetite in all types of sequences with hematite evidence in a red mudstone. We propose a model where changes in a dissolution of iron oxides due to climatically driven water table level changes explains these variations in the mudstones. Spectral and wavelet analysis reveals clear orbital periodicities in the proxy data, implying orbital control on water table in these desert environments, potentially via distal monsoon precipitation. Notably, there are substantial changes in the importance of key orbital parameters in the data through time, implying strong sensitivity of this hyper-arid climate to external boundary conditions.  

 

References

Ruffell, A. & Hounslow, M., 2006: In P. F. Rawson, & P. Brenchley (Eds.), The Geology of England & Wales. Geological Society of London; 295-325.

Hounslow, M.  & Gallois, R., 2023: Magnetostratigraphy of the Mercia Mudstone Group (Devon, UK): implications for regional relationships and chronostratigraphy in the Middle to Late Triassic of Western Europe. Journal of the Geological Society 180, jgs2022-173.