LATE MISSISSIPPIAN (CHESTERIAN) PALEOCLIMATE INFERRED FROM PALEOSOL NIOBIUM CONCENTRATIONS, MAUCH CHUNK FORMATION, EAST-CENTRAL PENNSYLVANIA

An approximately 630 m thick conformable section containing the Mississippian - Pennsylvanian boundary is exposed in a roadcut south of Pottsville, PA. The lowermost 270 m of this section is occupied by the late Mississippian Mauch Chunk Formation, which consists of alternating conglomeratic sandstones and mudstones. The mudstones are primarily paleosols, with features comparable to modern Vertisols and Aridisols. Paleosols are calcareous lower in the section and become noncalcareous up-section. Overlying the Mauch Chunk Formation are the coal-bearing early to middle Pennsylvanian Pottsville and Llewellyn Formations, indicating a shift from a predominantly arid to alternating arid-humid climate during the latest Mississippian into the earliest Pennsylvanian. To assess changes in paleoclimate during the latest Mississippian, decompacted thickness of paleosol carbonate horizons is used as a proxy for paleoprecipitation. Niobium (Nb) concentrations from these horizons will also be tested as a secondary paleoclimate proxy. Concentrations of Nb in paleosol B horizons have been shown to covary with the geochemical climate proxy chemical index of alteration without potash (CIA-K). However, this relationship was established in paleosols subject to much shallower burial than the Mauch Chunk Formation. The objective of this study is to assess the reliability of Nb as a paleosol climate proxy under different burial and diagenetic conditions and to evaluate the role of climate change as a possible forcing mechanism for changes in depositional environments across the Mississippian-Pennsylvanian boundary.