Volcanically mediated plankton blooms in the Central Belt of the Southern Uplands, Scotland, during the Llandovery

At Thirlestane Score and at four other localities in the Southern Uplands, graptolites of the Llandovery gemmatus Zone occur in couplets of lithologie s immediately above thin 'high-U' bentonites. Above the bentonites, abundan t graptolites, especially siculae, and a straight-line survivorship trend i mplies high productivity coupled with environmentally mediated mortality: t he population structure expected in the early part of a plankton bloom. In the overlying facies, fewer, larger individuals and a convex survivorship c urve suggest reduced productivity and internally mediated mortality. This i s consistent with the later stages of a bloom where resources were waning b ut the ecological structure of the system was better developed. It is likel y that the introduction of trace-metals, Fe or Al, to the water column via volcanic ash increased primary productivity, suggesting that macronutrients were available in the Southern Uplands system, allowing a bloom to be stim ulated by the addition of volcanic products. This process is observed in mo dem open oceanic systems and implies a temporal continuity of control on th e plankton despite complete faunal turn-over since the Silurian. These inte rpretations are most consistent with an open ocean geotectonic setting for the region.