CONODONT APATITE OXYGEN ISOTOPES: CONSTRAINING THE FAR-FIELD PALEOENVIRONMENTAL EFFECTS OF GLACIOEUSTATIC SEA-LEVEL FLUCTUATIONS ASSOCIATED WITH THE LATE PALEOZOIC ICE AGE DURING THE MIDDLE - LATE PENNSYLVANIAN (DESMOINESIAN – MISSOURIAN)

Conodonts from marine shales and limestones within cyclic deposits of terrestrial and marine strata (i.e., cyclothems) in the Illinois basin were analyzed for oxygen isotopes in order to constrain the far-field paleoenvironmental effects of glacioeustatic sea level fluctuations in the Middle-Late Pennsylvanian (Desmoinesian-Missourian) during the Late Paleozoic Ice Age (LPIA). Studies of Pennsylvanian paratropical terrestrial fossil floras from North America have revealed major vegetational changes across the Desmoinesian-Missourian boundary which have been interpreted to reflect a shift to a warmer climate, increased seasonality, and shorter periods of wet conditions in the late Pennsylvanian; however, such a trend has yet to be documented in the marine record.

Oxygen isotope analyses of conodont apatite from fourteen lithologic units (twelve black shales and two limestones) representing ~ 8 cylcothems are presented here. Average δ¹⁸O values for the black shale units range from 18.2‰ to 20.0‰ VSMOW with the average value around 19.1 ± 0.4‰ and δ¹⁸O values for the marine limestones range from 18.3‰ to 20.0‰ with the average value around 20.0 ± 0.4‰. There appears to be no systematic difference between the shale and limestone δ¹⁸O values. Within the limits of analytical uncertainty, the stratigraphic distribution of conodont δ¹⁸O values is generally non-trending; significantly, there is no considerable shift in δ¹⁸O values across the Desmoinesian-Missourian boundary. This marine isotope signal is in contrast with pedogenic sphaerosiderite δ¹⁸O values from stratigraphically bounding paleosols which display a ~-1.5‰ shift across the Desmoinesian-Missourian boundary.

Assuming the dominant control on the formation of Pennsylvanian cyclothems was glacioeustatic fluctuations, the conodont oxygen isotope data presented here in conjunction with pedogenic sphaerosiderite δ¹⁸O values may suggest more pronounced paleoenvironmental change (e.g., change in temperature, oxygen isotopic composition of meteoric or marine waters) occurred at paratropical latitudes during times of relative sea level lows, while in contrast, during times of relative sea level highs, such changes may have been minimal.