BIOGENIC CARBONATE δ18O AS A PROXY FOR 10-100KY SCALE PALEOENVIRONMENTAL VARIATION DURING THE MIDDLE DEVONIAN: A PILOT STUDY

The causes and timing of the Middle-Late Devonian greenhouse to icehouse transition are ambiguous and debated. Lithologic cycles from the Middle Devonian are interpreted to represent fluctuating sea level, possibly glacioeustatic in origin. Paleoclimatic models have typically represented the Middle Devonian as a greenhouse world, conflicting with a glacioeustatic interpretation of cyclicity. In order to gain insight into the drivers of lithostratigraphic cyclicity in the Middle Devonian, I explore variation in the stable isotope values of biogenic carbonate from the Hamilton Group of the Northeastern US. The expectation is that, if the source of cyclic lithologic variation is glacioeustatic, δ¹⁸O values should vary upsection coeval with lithology, driven by a combination of climate and ice volume. Specimens of the brachiopod Mucrospirifer, chosen for their abundance and ubiquity, were collected from an outcrop in the Ludlowville Formation exposing two complete fifth order (inferred 100ky) cycles. Scanning electron microscopy reveals consistently preserved primary shell microtexture. 42 shell fragments from 12 stratigraphic horizons spanning the two cycles were analyzed for their δ¹⁸O and δ¹³C values. Stable oxygen isotope values of shell calcite range from -10.42 to 6.14‰. Variability is high, averaging 1.54‰ within a single horizon. Such low and heterogeneous values suggest potential diagenetic alteration, although preserved microtextures, typical marine δ¹³C values of +1 to +3‰, and the lack of significant covariation between δ¹⁸O and δ¹³C (R²=0.097) counter that interpretation. Oxygen isotope values in the more densely sampled cycle suggest positive covariation with inferred sea level, but given the high variability within horizons, clear and consistent trends cannot be supported without additional data. This pilot study underscores the need for more research, including further assessment of alteration using SEM and elemental composition, greater sampling density to account for within-horizon variability, and repetition of this approach in additional inferred cycles.