Palaeoclimate analysis using spectral gamma-ray data from the Aptian (Cretaceous) of southern England and southern France
Abstract The abundance and half-lives of potassium (K), uranium (U) and thorium (Th) make these three elements the dominant sources of gamma-rays detected in rocks both at outcrop and in the subsurface. With the advent of the spectral gamma-ray tool (SGR), the variation shown by these three elements in rocks has been considered to reflect sorting at flooding surfaces and changes in the weathering regime of sediment source-lands. K and U are considered to be more soluble than Th, thus elevated Th to K or U ratios in soils and eroded clays can reflect hot, humid climates and palaeoclimates. This study makes an assessment of the usefulness of SGR data to palaeoclimate studies by comparing changing Th/K and Th/U ratios from a known period of palaeoclimate change to extensive published and new data from other analytical methods. New SGR data from the Cretaceous of southern England and southern France compare favourably to outcrop sedimentological facies interpretations and laboratory geochemical data. Palaeoclimate data from two other sections (northern Germany, northern Italy) are compared as controls to the study. Together these analyses suggest that there was a gradual change from semi-arid climate in the Barremian and early Aptian, to a semi-humid climate in the mid-Aptian. Superimposed on this long-term rise in humidity were a number of short-term (ammonite zone-length) changes in Th/K and Th/U ratios and the abundance of climate-sensitive materials. This study shows that SGR data are more useful than originally thought. When problems of overprinting from heavy minerals and diagenesis are overcome, outcrop and borehole spectral gamma-ray logs from mudstones or the clay-rich beds of mixed successions may be used to interpret past patterns of hinterland weathering.