Carbon and oxygen isotope variations in shell beds from the Upper Ordovician (mid-Cincinnatian: Maysvillian to early Richmondian) of Ontario: Evaluation of the Warm Saline Deep Ocean hypothesis, paleoceanographic changes, and Milankovitch orbital cycles in the transition to the Hirnantian glaciation

Abstract

Carbon and oxygen isotopes of both well-preserved articulate brachiopod Paucicrura rogata and cement in mid-Cincinnatian samples (Maysvillian to early Richmondian) on the eastern North American passive margin shelf in SW Ontario show no simple relationships. The δ13C trends of the brachiopods, organic matter and cements are little altered and vary little up section. The brachiopod δ13Cbrach pattern is very close to the American mid-continent whole rock δ13CWRcarb pattern across the Fairview excursions. The δ18Obrach variations, if primary, indicate fluctuating but generally slightly cooling temperatures upwards, though the actual temperatures depend on inferred salinities of the seawater. The temperature/ salinity relationships variations are compatible with the warm deep saline water hypothesis. The paleoceanography shows a change from a warm open passive carbonate margin to a fluctuating warmer/cooler enclosed clastic marginal basin with freshwater input, heralding the cold conditions of the Hirnantian ice age. The Milankovitch cyclicity determined from δ13Ccarb patterns, is practically identical to that from determined from magnetic intensities on the contemporary Yangtse Shelf.