Climate archives and the Late Ordovician Boda Event

Abstract: Climate change has become an increasingly important subject in geological research as past climates can teach us about the global climate systems. This thesis is a literature study on natural climate archives with examples from the Late Ordovician Boda Event. The Ordovician Period represented a greenhouse world with atmospheric CO2 levels almost twenty times higher then today. The climate became cooler in the Late Ordovician and the period was terminated by a global ice-age in the Hirnantian. Much of the studies of Ordovician climate have focused on this glaciation. Recent studies have, however, revealed another drastic climatic change preceding the terminal Ordovician glaciation. In the pre-Hirnantian (latest Katian) there was an episode of increased carbonate deposition worldwide. Reefs and carbonate mud-mound developed in high latitudes in several areas that were previously associated with siliciclastic sedimentation. There was a contemporaneous pole-ward migration of marine faunas, resulting in a breakdown of climatically controlled endemism. These drastic changes have been termed as the Boda Event by Fortey and Cocks (2005) who interpreted this as an episode of global warming. They argue that higher ocean temperatures would allow carbonate producers to spread into higher latitudes. Cherns and Wheeley (2007) on the other hand, suggested that the Boda Event represented a cooling event, resulting in lowering of global sea-level and increased oceanic circulation and overturn. This would oxygenise bottom waters and allow carbonate production and faunal migration. In this thesis the contradicting theories are evaluated and exemplified by the Swedish Boda Limestone that formed during this time interval.