High-frequency (104-105 years) upward-shallowing carbonate cycles are common sedimentary features and are generally attributed to orbitally driven glacio-eustasy (changes in ice volume). But in warm, greenhouse conditions, glacial ice is thought not to have been significant, making this interpretation for high frequency carbonate cycles less compelling. We will evaluate the possibility that glacio-eustasy did occur in warm climates by analyzing oxygen isotope ratios of conodonts, a common fossil composed of diagenetically-resistant phosphate. Glacial ice strongly incorporates the light stable isotope of oxygen. Therefore, when large ice volumes are present (low sea level), the oxygen isotope ratio of the ocean becomes measurably heavier. The diagenetically resistant conodonts will allow us to search for this signal and address the long-debated origin of temporally persistent and widespread meter-scale carbonate cycles/parasequences, specifically whether the controversial proposal that transient ice sheets can form even in warm climatic periods is reasonable and supported by geochemical evidence. Our study will center on three greenhouse (Late Silurian, Early Devonian and Early Triassic) and one transitional (Early Mississippian) climatic mode. We plan to incorporate local middle and high school science teachers into our research project to help bring scientific inquiry into the classroom. This will be accomplished through collaboration with the Science Education Institute of the Southwest (SEIS), and by conducting a series of week-long summer workshops for professional teacher development in the sciences.
Ancient shallow marine limestones are characterized the occurrence of 1-9 meter-thick sedimentary packages (sedimentary 'cycles') characterized by deeper water deposits overlain by shallow water deposits; these repeated patterns of water-depth change suggest that global sea levels rose and fell on ~20,000 to 500,000 year time scales. Funding was requested to test the hypothesis that continental and/or alpine glaciers were present and waxing and waning on ~20,000 to 500,000 year time scales and drove global sea-level changes causing the formation of widespread and pervasive shallow-marine carbonate cycles in deep geologic time (between 200-500 million years ago; i.e., Paleozoic). To conduct this study, we analyzed oxgyen isotopes from apatitic microscopic teeth (conodonts) found in Paleozoic cyclic marine limestones. We targeted cycles forming in times of warmer-than-average climates (greenhouse climates), transistional, and cooler-than-average climates (icehouse climates) to better understand the range of climate changes. Results from studying cycles developed during six different geologic time periods spanning over 150 millions years (3 greenhouse, 1 icehouse, and 2 transitional climate intervals), indicate that during all six time intervals, cycle development was dominated by sea-level changes driven by the growth and melting of glacial ice (glacio-eustasy). In addition to glacio-eustasy, the ~20,000 to 500,000 year climate changes influenced sea surface water temperatures (driven by global climate change and local changes in coastal upwelling), and local changes in surface water evaporation rates. These results indicate Paleozoic greenhouse and transitional climates were not as uniformly warm as previously thought and that the temperature gradient between tropical oceans and polar oceans during these climates was significant steeper driving a more vigorous heat exchange between low and high latitudes.
