THE LUDLOW LAU EVENT - ANOTHER GLACIATION IN THE SILURIAN GREENHOUSE?

Different paleoclimatic and paleooceanographic models have been discussed for the Silurian in order to explain the instability of the environment as indicated by oceanic and extinction events, sea-level changes and the fast switch between warm and cold climates. Changes in the Silurian δ¹⁸O record of brachiopod calcite coincide with positive excursions in δ¹³C, suggesting that changes in the global carbon cycle may have resulted in prominent climatic changes. Indeed, well-dated Llandovery through Wenlock diamictites from Western Gondwana give evidence for Lower Silurian glaciations, but up to today, there is no geological record of Upper Silurian glacial sediments.

Oxygen isotope values measured on conodonts from Gotland (Sweden) and from several sections of the Prague Basin (Czech Republic) show an increase by more than 2 ‰ during the Ludfordian Lau Event. This increase in δ¹⁸O translates into cooling of surface waters of about 8° C assuming no major changes in salinity. However, cooling of low to mid latitude surface water temperatures by 8° C seems unrealistic. Consequently, we interpret the +2 ‰ increase in δ¹⁸O as the combined signal of the build-up of polar ice caps and climatic cooling. The +2 ‰ change in conodont apatite compares relatively well to Pleistocene interglacial-glacial changes in δ¹⁸O measured on surface dwelling foraminifera suggesting that the amplitude of Pleistocene (~120 m) and inferred Late Silurian glacio-eustatic sea-level changes might have been of comparable magnitude. Independent sedimentary evidence from Gotland and southernmost Sweden supports a Late Silurian glaciation. A substantial and stepped downward shift in coastal onlap and development of karst suggest that the forced sea-level fall associated with the Lau Event was of greater magnitude than any other Silurian short-term sea-level change in the Baltic Basin. Even the most conservative estimation of the magnitude of the sea-level fall amounts to several tens of meters.