Evolution of Cretaceous phytogeoprovinces, continents and climates

Abstract

A phytogeoprovince is defined as an isolated landmass with its characteristic flora. Modern pollen depositional patterns indicate that pollen produced on a landmass are transported mostly by water and deposited on landmass edges under various ecological conditions. Preserved pollen assemblages can be recovered and used to recognize past phytogeoprovinces, continents, and their climates. Such a model is used here to interpret the evolution of Cretaceous phytogeoprovinces and their climates. Jurassic continents remained in close proximity, devoid of major elevations and inland seas, until the Neocomian. Jurassic flora, fauna, and climates were cosmopolitan globally. Fretaceous phytogeoprovinces evolved from the two Jurassic ones, namely a boreal Cerebropollenites and an austral Microcachryidites phytogeoprovince separated by the Tethys. The creation of a third phytogeoprovince during the Neocomian, by the appearance of Dicheiropollis in the equatorial region, coincided with the initial rift of South America and Africa and the opening of the Labrador Sea. An extensive Neocomian marine regression exposed large landmasses. Except for a warm temperate climate in the boreal-most province, a subtropical to tropical climate prevailed globally during the Neocomian. The opening og the South Atlantic separated South America and Africa completely by the end of the Albian. Except in northern Siberia, the Aptian-Albian interval was a transgressive marine phase which inundated landmasses and created inland seas. A progressive cooling trend had set in the Aptian climate. After reaching a minimum in the late Aptian, the climate started warming up in the Albian. An elater-bearing flora replaced the equatorial Dicheiropollis flora in the Aptian-Albian. The Cenomanian was a cool phase in the Cretaceous, the Coniacian-Santonian a little warmer, and the Maastrichtian a little cooler again. During the Late Cretaceous, active continental movement displaced landmasses whereas marine transgressions created new inland seas separating the land areas. Climates were differentiated latitudinally. The early Cenomanian appearance of Normapolles pollen in the Urals area initiated the development of one of the boreal provinces which became discrete in the Senonian. Similarly, the other boreal Aquilapollenites phytogeoprovince was established during Turonian-early Maastrichtian time. These two boreal provinces were separated longitudinally by N-S epeiric seas in North America and the Urals area. A palmate Constantinisporis flora replaced the elater-bearing flora of the equatorial phytogeoprovince in the Senonian. An austral province was established by the appearance of Nothofagus flora in the Coniancian. The integrity of the Late Cretaceous boreal and equatorial phytogeoprovinces was destroyed by the late Maastrichtian marine regression by exposing land connections across provinces and allowing land-plant migration interprovincially. The austral phytogeoprovince remained isolated and its proteaceous and Nothofagus floras of the Late Cretaceous have survived up to the present.