Orbital-scale glacio-eustasy during the transition between the Early Ordovician warm period (greenhouse) and the Late Ordovician cool period (icehouse)

Recent studies using oxygen isotopes from conodont apatite and carbonate clumped isotopes suggest that the transition from the Early Ordovician warm period (greenhouse) to the Latest Ordovician glacial period (icehouse) occurred in two stages- cooling during the Early and Middle Ordovician and another, more abrupt cooling in the Latest Ordovician (Hirnantian). These results come from samples collected at time intervals of >0.5 My; therefore, the data does not resolve orbital-scale climatic changes. This study utilizes oxygen isotopes from conodont apatite collected across multiple, orbital-scale carbonate cycles (or parasequences) in the early Late Ordovician to evaluate high-frequency paleoclimate changes. More specifically, we are addressing whether the observed carbonate cycles formed in response to orbital-scale, glacio-eustasy and to shed light on the dynamics of the Ordovician warm-to-cool climatic transition. The Upper Ordovician (early Katian) Lexington Limestone of Kentucky is characterized by cyclic carbonates and shales deposited in southern subtropical waters of the Laurentian craton. Orbital-scale (~15-130 ky) subtidal cycles (1-3 m) are composed of offshore limestone and shale, overlain by lower shoreface, then upper shoreface skeletal carbonates and show no evidence of subaerial exposure at cycle tops. δ18Oapatite values from multiple cycles range from ~18-19.5%. The majority of cycles record low isotopic values in the deepest water facies (during sea-level rise and highstand) and higher isotopic values in the shallowest water facies (during sea-level fall and lowstand). The magnitude of isotopic shift across individual cycles is up to ~1%. Estimated subtropical SSTs range from ~28°-34°C. If the intracycle isotopic shifts were due only to SST changes, then the temperatures fluctuated up to 4°C during individual cycle formation; estimated thermo-eustatic changes given these SST changes are not sufficient in magnitude to generate the observed offshore to upper shoreface facies changes. We suggest that similar to the Pleistocene, the measured isotopic shifts were due to a combination of SST and ice-volume effects. If we assume Pleistocene relationships for partitioning of SST versus ice-volume effects, this suggests orbital-scale glacio-eustatic sea-level changes of many tens of meters along with SST changes of <2°C. Such magnitudes of glacio-eustasy imply that high-latitude continental ice sheets were waxing and waning at orbital time scales at least 6 My before the peak Hirnantian glacial period and that orbital-scale, glacial-interglacial cycles were superimposed upon the long-term Ordovician cooling trend- a pattern similar to that observed during the more recent Cenozoic cooling trend.