A PALEOCLIMATE SIMULATION OF THE WENLOCKIAN (LATE EARLY SILURIAN) WORLD USING A GENERAL-CIRCULATION MODEL WITH IMPLICATIONS FOR EARLY LAND PLANT PALEOECOLOGY

The Silurian Period (439-409 Ma) is known for its extensive organic-ri ch, graptolitic. black shales and graptolitic, shelf carbonates. Physi cal conditions drive paleoclimate and control the zonal deposition of lithotopes. Moreover, the paleoclimate created a paleoceanic environme nt favorable for widespread generation, deposition, and preservation o f plankton. The relationship between the paleogeographic framework, in cluding paleotopography, and the resultant bio- and lithostratigraphy are suitable for study with a general circulation model (GCM). For thi s study we chose the Wenlockian Stage (430-424 Ma), the late Early Sil urian. The Wenlockian northern hemispheric surface was dominated by a large ocean, the southern hemisphere by the giant Gondwanan continent. Much of Gondwana's extensive margin was in the mid-latitudes. Laurent ia and Baltica occupied a tropical position, and Siberia and Kazakh la id in warm temperate latitudes to the north. Silurian stratigraphy fit s a paleoatmosphere with elevated greenhouse conditions. Estimated Sil urian atmospheric CO2 values vary between wide limits. We used 1120 pp m CO2, 4 x that of the pre-industrial level and at the lower end of th e range of estimates. The paleoclimate was forced by the paleogeograph y of each hemisphere's circulation. The northern hemisphere Silurian s imulation is dominated by strong zonal circulation in all seasons. In contrast, the continental southern hemisphere reacts to the summer hea ting and winter cooling of Gondwana. This simulation furnishes paleocl imatic conditions that help explain the distribution of early land pla nts. This paleoclimate simulation supports a humid coastal paleoenviro nment for early Silurian land plants. Furthermore, these results imply that relative humidity was more important than precipitation rates, a nd that intercontinental spore dispersal between Laurentia/Europe and Gondwana was not wind aided. The GCM provides a good match with Siluri an lithological and paleobiological data.