The Early Carboniferous (Asbian-Brigantian) seasonal tropical climate of northern Britain

Northern Britain lay close to the equator during the Early Carboniferous and evidence, primarily from paleosols and predictions from climate models, indicates that it experienced a seasonal climate, perhaps linked to monsoonal circulation. This study further investigates the Early Carboniferous climate of northern Britain using anatomically preserved plant fossils (gymnosperm woods and leaves) from seven Asbian-Brigantian (333-339 Ma) localities. Many of the fossil woods exhibit growth rings that possess subtle, discontinuous ring boundaries and ring increments of variable width. These growth rings are qualitatively and quantitatively analyzed and compared with growth rings in modern Araucariacean conifers, which are believed to have a very similar phenology to the Carboniferous gymnosperms in question. The Carboniferous growth rings bear a remarkable similarity to those of Araucariaceans growing in the seasonal tropics today. It is concluded that these Carboniferous growth rings record a tropical seasonality, characterized by dry seasons of irregular duration and non-annual frequency, comparable to the climate of present-day northern Australia. Fossil leaves also exhibit an array of xeromorphic features such as hairy or rolled surfaces and greatly thickened cell walls, which although not diagnostic, are certainly consistent with the notion of seasonal water stress. It is suggested that the Asbian-Brigantian flora of northern Britain (which is gymnosperm-dominated in contrast to the more characteristic lycopod-dominated flora elsewhere) represents a tropical savanna-like biome.