CAN ZOOPLANKTON DERIVED PROXY DATA HELP TO RECONSTRUCT THE CLIMATE OF THE ORDOVICIAN?

Emerging evidence suggests that cooling towards the Hirnantian glacial maximum, and thus the onset of the Early Palaeozoic Ice Age, started earlier than previously assumed, during the Early Ordovician. Here, we test this hypothesis by examining climate proxies derived from zooplankton that was colonising new environments during these times.

We show that Ordovician chitinozoans, like graptolites, were “mixed layer” marine zooplankton and that their global distribution was primarily controlled by variations in Sea Surface Temperature. Data on the spatial distribution of chitinozoan provinces during the end-Ordovician Hirnantian glaciation (440Ma) are compared to those from the pre-glacial Sandbian (460Ma). Strikingly, Sandbian provinces map a steep latitudinal temperature gradient from equator to pole, which questions the notion that this interval had an intense greenhouse climate, and suggests that cooling may have started before (or during) the Sandbian. We also demonstrate that further cooling towards the Hirnantian glaciation resulted in a steeper latitudinal temperature gradient and an equator-ward shift of the Hirnantian austral Polar Front. This is deduced from an expansion of the Polar fauna. Late Ordovician surface ocean temperature gradients, and fluctuations between glacial and interglacial states, may have been more similar to modern oceans than hypothesized before.

In addition, we assess the potential for stable oxygen isotopes from the calcitic lenses of epipelagic trilobite eyes as a seawater palaeotemperature proxy. Current fossil-derived proxies for seawater temperature in the Ordovician are oxygen isotopes from conodont apatite and brachiopod calcite. Diagenetic alteration and the analytical techniques used are respectively problematic or still under development. We have analysed Floian age specimens of the widespread Carolinites from Spitsbergen. Well-preserved eyes can be distinguished from diagenetically altered eyes using EBSD C-axis mapping, microstructure preservation and geochemistry. Well-preserved eyes have low d¹⁸O values of –8‰ to –7‰ VPDB. Data from Mid Ordovician Australian specimens (Carolinites and Opipeuterella) provide a second case study and give similar results. However, emerging in situ d¹⁸O SIMS data may contradict this interpretation.