A paleoclimate simulation of the Wenlockian (Late Early Silurian) world using a general circulation model with implications for early land plant paleoecology

Abstract

The Silurian Period (439–409 Ma) is known for its extensive organic-rich, graptolitic, black shales and graptolitic, shelf carbonates. Physical conditions drive paleoclimate and control the zonal deposition of lithotopes. Moreover, the paleoclimate created a paleoceanic environment favorable for widespread generation, deposition, and preservation of plankton. The relationship between the paleogeographic framework, including paleotopography, and the resultant bio- and lithostratigraphy are suitable for study with a general circulation model (GCM). For this study we chose the Wenlockian Stage (430–424 Ma), the late Early Silurian. 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. Laurentia and Baltica occupied a tropical position, and Siberia and Kazakh laid in warm temperate latitudes to the north. Silurian stratigraphy fits a paleoatmosphere with elevated greenhouse conditions. Estimated Silurian atmospheric CO 2 values vary between wide limits. We used 1120 ppm CO 2 4x that of the pre-industrial level and at the lower end of the range of estimates. The paleoclimate was forced by the paleogeography of each hemisphere's circulation. The northern hemisphere Silurian simulation is dominated by strong zonal circulation in all seasons. In contrast, the continental southern hemisphere reacts to the summer heating and winter cooling of Gondwana. This simulation furnishes paleoclimatic conditions that help explain the distribution of early land plants. This paleoclimate simulation supports a humid coastal paleoenvironment for early Silurian land plants. Furthermore, these results imply that relative humidity was more important than precipitation rates, and that intercontinental spore dispersal between Laurentia/Europe and Gondwana was not wind-aided. The GCM provides a good match with Silurian lithological and paleobiological data.