CHANGING PLANKTON COMMUNITY STRUCTURE IN THE LATE KATIAN-EARLY HIRNANTIAN: EVIDENCE FROM GRAPTOLITES AND NITROGEN ISOTOPES

Planktonic graptolites nearly became extinct during the latest Ordovician. Although graptolite diversity declined gradually through the latest Katian-early Hirnantian, most regions, especially in the paleotropics, experienced an abrupt change in clade dominance at the beginning of the Hirnantian. The Diplograptina that dominated Katian graptolite faunas were largely or completely replaced by Neograptina (normalograptids and their descendants). In addition, recent paleogeographic work has shown that neograptines were totally absent from the paleotropics in most of late Katian time, but were important components of the faunas of higher paleolatitudes. Thus, the abrupt faunal change at the beginning of the Hirnantian in the paleotropics must have involved migration of taxa from higher paleolatitudes. In order to link these patterns of faunal change and migration to changes in nutrient cycling and phytoplankton community structure we have undertaken N isotope analyses of late Katian-Hirnantian successions in Arctic Canada (paleotropics) and the Prague region (high paleolatitude). Late Katian, diplograptine-dominated black shales in Arctic Canada show δ¹⁵N values ~ 0-1‰. We interpret these to represent conditions of complete denitrification below the surface mixed layer with little or no vertical flux of fixed N. In these settings phytoplankton communities relied upon cyanobacteria for N fixation. At precisely the level of the abrupt change in graptolite faunas to normalograptid dominance, there is a significant (~ 3‰) positive shift in δ¹⁵N values, which we interpret to indicate less intense deep-water denitrification and algal dominance of the of the phytoplankton community, changes that we suggest led to the decline of the diplograptines. Recent biomarker studies support this inference. In our higher-latitude sections, which contain neograptines throughout the Katian-Hirnantian, our N isotope data indicate there was a significant flux of fixed N to the surface waters in both the late Katian and early Hirnantian. Thus, the high-latitude neograptines of the late Katian were well adapted to less N-limited, more algal dominated phytoplankton communities, which permitted their migration into, and preferential diversification within the early Hirnantian paleotropics.