CORRELATIONS BETWEEN SEA-LEVEL CHANGE, NUTRIENT AVAILABILITY, AND PENNSYLVANIAN BRACHIOPOD MORPHOLOGY

Brachiopod faunas may represent an important source of information about the biotic response to paleogeographic and climatic change during the Carboniferous Period. However, little is known about how those changes modified brachiopod assemblages in the field. To address this question, studies on Pennsylvanian brachiopods from the eastern Great Basin, Nevada, have been conducted. These brachiopods were selected because of their stratigraphic context within cyclic depositional sequences, the origin of which may be related to eustatic sea-level change linked to fluctuations in ice volume on Gondwana at Milankovitch time scales.

Preliminary observations have shown a general trend in the distribution of brachiopods within the cycles. Spire-bearing brachiopods represent the major fraction of biomass at the top of each cycle (in the shallowest part) and productids are dominant in deeper facies. This general distribution suggests that water depth, type of substrate, and temperature might be viewed as primary paleoecological factors since they vary with eustatic and climatic changes related to the glaciation on Gondwana. Some brachiopod species, however, were found at different stratigraphic levels within the cycles and with changes in size and volume of the specimens. These findings suggest that additional factors other than fluctuating sea level are significant as paleoecological controls. Fluctuations in nutrients along the cycles seem to be important because different types of brachiopods correlate with changes in paleoproductivity indeces (e.g.P2O5). Spire-bearing brachiopods increase in size and volume with high levels of P2O5 and Fe while larger productid brachiopods are dominant with low levels of nutrients. Such a distinction is likely related to different ways of filtering water currents through the lophophore.