THE TAGHANIC EVENT: A LATE MID DEVONIAN ARIDITY CRISIS

The latest Givetian (late Mid Devonian) Taghanic Event is the first of a series of extinctions events that characterize the Late Devonian to Early Carboniferous interval. Continuing work on terrestrial sediments from the Old Red Sandstone continent enables these events to be contrasted and reveals much about the underlying causes. For example, it will be shown that the D-C boundary Hangenberg Event and the earliest Carboniferous Alum Shale Event result from high latitude deglaciations with, at low latitude, a near coincident monsoon driven humidity event. The marine Taghanic Event in Scotland has its expression as a limestone found in a few offshore oil exploration wells and represents an extreme sea-level highstand. In terrestrial sections the equivalent level is the Eday Marl well known from Orkney. Offshore its correlative is found in many wells drilled north of the Highland Boundary Fault. These have generally been mis-identified as a Permian siltstone. Onshore sections show that subtle marine transgressions occur at the top and base of the Eday Marl. The Eday Marl was formed within a sandy sabkha environment with the sediments showing a clear internal climatic cyclicity. The interval represents an aridity episode within a long sequence of fluvial sandstone. The contrast with the D-C boundary events is the association of high sea-levels with both humidity (Hangenberg) and aridity (Taghanic)episodes. Both are associated with Milankovitch driven climatic maxima but in the case of the Taghanic the high sea levels are believed to result largely from thermal expansion of sea water. The aridity driven reduction in the terrestrial hydrological cycle and linked changes in the shelf sea circulation and water temperature provide a machanism for the very significant Taghanic extinctions that occur in both the marine and terrestrial realms.