LATE ORDOVICIAN CLIMATE CHANGE: IMPACT ON GRAPTOLITE BIODIVERSITY

Late Ordovician biodiversity changes (notably extinctions but also certain radiations) may be linked to greenhouse -icehouse (glaciation over South Pole)-greenhouse climate changes. Planktic graptolites, most of which lived within or on the margins of hypoxic waters, nearly became extinct during the climate changes. A few planktic graptolites , based on their occurrences in bioturbated strata of shallow marine origin, lived in oxic waters. Glaciation during the icehouse interval drained or at least markedly lowered sea level over large shelf sea areas which led to loss or marked restriction of hypoxic waters. As well, sea level changes led to loss of oceanic upwelling and related oxygen minimum zones in which most graptolites lived. The losses of hypoxic waters, which varied somewhat in time from place to place during the Late Ordovician, resulted in extinction of many graptolite taxa. Hypoxic waters did persist, although they became somewhat reduced in area as marine conditions changed throughout the Late Ordovician, in the Yangtze region shelf seas in modern south China. Those hypoxic waters served as a refugium for planktic graptolites. Graptolite species living in the refugium exhibit gradual extinctions as environmental conditions changed. Some new species appeared in the refugium in the icehouse interval. Graptolites that lived in relatively oxic environments persisted and even radiated modestly during icehouse glaciation. Late Ordovician climate changes thus led to two primary patterns in graptolite biodiversity: 1) extinctions that took place, commonly relatively rapidly, as hypoxic habitats were reduced or lost, and 2) speciation and development of new lineages among graptolites living in oxic waters as well as among graptolites living within a hypoxic water refugium.