The Clay Mineral Alteration Index (CMAI) as an improved indicator of climate change

Several indices for measuring the degree of chemical weathering in soils, sediments and sedimentary rocks based on selective clay minerals have been proposed but quantitative links to climatic parameters, such as mean annual temperature or rainfall are generally lacking. This contribution provides some first constraints from published case studies on a more comprehensive Clay Mineral Alteration Index (CMAI) to establish relationships between silicate weathering, clay mineral formation and climate by using the complete clay mineral assemblage. On a global scale, lithological controls on clay mineral assemblages introduce a large degree of heterogeneity to the system, which makes direct interpretations based on numerical indices difficult to implement. Some general correlations of the CMAI and climate occur for selective soil types (i.e. alfisols), but correlations improve further when the host rocks are compositionally similar and only climatic factors vary. Soils located on Silurian and Oligoclase shale bedrock from different climatic zones also show good correlations when mean annual temperatures and rainfall are plotted separately or combined and appropriately weighted. Based on these relationships, the CMAI values for young clastic sediments of the Qionghai Lake (China) were successfully used to evaluate mean annual temperature and rainfall patterns from the sediment record over the last 21,000 cal BP. Although further refinement and validation of the methodology is required, a similar pattern in palaeotemperature variation was produced as established indicators for the last 10,000 years as well as detecting a strong monsoonal activity between 15,500 and 8300 cal BP. The CMAI was also applied to the Ediacaran mudstones and felsic palaeosols of the East European Craton that were deposited on the Baltic palaeocontinent. Together with consideration of the degrees of chemical alteration, a pattern of Late Neoproterozoic- Early Palaeozoic climatic change was revealed that is consistent with available palaeomagnetic studies. This supports the concept of the northerly migration of Baltica between 615 and 565 Ma followed by a southerly migration between 550 and 480 Ma. The CMAI is therefore suggested to represent a useful additional tool for tracking climate change through geological time. •The Clay Mineral Alteration Index (CMAI) is an improved proxy for studying climate change.•The CMAI is correlated against the climate parameters of soils developed on Siluri