Abstract

Stratigraphic and geochemical studies from tropical through polar latitude records now recognize that the late Paleozoic ice age (LPIA) began earlier and ended later than traditionally thought and was characterized by discrete, My-scale intervals of increased glacial ice volumes that were separated by My-scale intervals of diminished ice volumes. Whereas the onset of LPIA glaciation now is recognized to have started as early as the Late Devonian, the continuity and patterns of glaciation in the > 15 My interval between this onset date and the well-documented mid-Carboniferous portion of the LPIA are less well understood. This study utilizes detailed cyclostratigraphic and δ¹⁸O records from Lower Mississippian (mid Tournaisian) tropical marine deposits and conodont apatite to argue that in addition to evidence for a discrete, My-scale early LPIA glaciation, there is compelling evidence for superimposed high-frequency (orbital-scale) glacial–interglacial climate oscillations driving glacio-eustasy and changes in sea-surface temperature (SST).

Cyclostratigraphic evidence includes the development of asymmetric (progressive shallowing) and symmetric (deepening followed by shallowing) subtidal cycles in the Tournaisian Woodhurst Member of Lodgepole Formation which record repeated high-frequency water-depth changes from below storm wave base to above fairweather wave base. δ¹⁸O values from samples originating from six cycles range from 17.8‰ to 21.6‰, and intracycle trends record progressive increases in isotopic values from the deepest-water facies (at cycle base for asymmetric cycles or mid cycle for symmetric cycles) into the shallower-water cycle caps. Four of the cycles record average intracycle isotopic shifts of 1.4‰, and two cycles record shifts of ≥ 2.5‰. These co-varying trends between interpreted water-depth and δ¹⁸O changes are consistent with the hypothesis that the water-depth changes were controlled by orbital-scale glacio-eustasy. Assuming that the observed intracycle δ¹⁸O shifts represent the combined effects of ice-volume and SST changes, calculated glacio-eustatic magnitudes were < 50 m with tropical SST changes of < 4°C. Cycles recording ≥ 2.5‰ shifts may reflect additional SST cooling related to intensification of coastal upwelling during the onset of glacial stages.

Orbital-scale glacio-eustatic interpretations imply that in addition to overall long-term Early Mississippian cooling and glaciation, the mid Tournaisian was characterized by superimposed high-frequency orbital-scale climate changes which controlled marked changes in glacial ice volumes, eustatic sea levels, coastal-upwelling intensities, and tropical SSTs. These long-term and superimposed short-term climate changes highlight the dynamic nature of the early LPIA and are reminiscent of climate patterns observed for Cenozoic and Late Ordovician greenhouse-to-icehouse climatic transitions.