Late Early Silurian (Wenlockian) general circulation model-generated upwelling, graptolitic black shales, and organic-rich source rocks—An accident of plate tectonics?

Abstract

Through an evolutionary process of plate tectonics, the Wenlockian physical world was composed of an oceanic northern hemisphere and a mainly continental southern hemisphere dominated by the giant continent of Gondwana. The high-latitude position of Gondwana placed much of its extensive margin in the middle latitudes. Laurentia and Baltica occupied an equatorial position; Siberia and Kazakh were to the north. The global paleoclimate was created and driven by the diverse paleogeography of the two hemispheres. In the model, the northern hemisphere is dominated by strong zonality in all seasons. In contrast, the continental southern hemisphere reacts to wide temperature ranges of summer heating and winter cooling of Gondwana. The extensively flooded, ∼28000 km margin of Gondwana occupied the zone of the southern hemisphere westerlies. These westerlies produced intense surface upwelling and, by extension of present analogies, high primary biologic productivity. Gondwana's northern margin extended far enough toward the equator that significant winter sea ice did not form in the southern hemisphere: this allowed a vertically stratified sea to develop, as well as probable dysaerobic and anoxic conditions. A high organic flux of marine plankton accumulated on the sea floor, where it was subsequently buried and preserved. A strong positive correlation exists between upwelling generated by a general circulation model (GCM) on Gondwana's northern margin and the distribution of southern hemisphere basins with source rocks containing types I and II kerogens.