USING CARBON AND OXYGEN ISOTOPES TO UNDERSTAND RELATIONSHIPS BETWEEN ORBITAL-SCALE CLIMATE AND CARBON BUDGET CHANGES IN THE EARLY MISSISSIPPIAN

The relationships between orbital-scale glacial-interglacial cycles and δ¹³C variations (as a proxy for the global carbon budget) are not well understood. Evidence from some Plio-Pleistocene ODP records indicate that interglacial times correspond to higher δ¹³C values and glacial times to lower δ¹³C values; however, other Pleistocene records show the opposite trends. We are investigating the relationships between orbitally driven glacio-eustasy and δ¹³C trends in deep time (Early Mississippian) glacial-interglacial cycles to better understand the interactions between climate and sea-level change and the carbon budget.

The Lower Mississippian (Tournaisian) Lodgepole Formation of southwestern Montana is characterized by subtidal carbonate cycles or parasequences (2-8 m thick) composed of substorm wave base facies, overlain by distal storm deposits, and capped by coarse proximal deposits. Cycles were sampled for oxygen isotopes from conodont apatite and for carbon isotopes from whole-rock limestone.

Preliminary stable isotope data from Lower Mississippian cycles suggests that 1) they developed in response to relatively high amplitude (several 10’s m) glacio-eustatic sea-level change which implies that the Late Paleozoic Ice Age began ~25 My earlier than previously proposed, and 2) δ¹³C values increase as sea-level falls (ice buildup), which we interpret to represent enhanced organic carbon burial related to increased upwelling and/or increased sedimentation rates leading to increased production and preservation of organic carbon.