USING OXYGEN AND NEODYMIUM ISOTOPES TO UNDERSTAND THE ORIGINS OF SILURIAN AND DEVONIAN GREENHOUSE CYCLES AND RELATIONSHIPS BETWEEN SEA-LEVEL CHANGE AND CONTINENTAL WEATHERING FLUX

High-frequency (10⁴-10⁵ yr) cycles are the fundamental building blocks of carbonate platforms in both greenhouse and icehouse time intervals. While icehouse cycles are commonly attributed to orbitally-controlled glacio-eustasy, the origin of greenhouse cycles is unclear. In this study, we utilize δ¹⁸O from conodonts and ε_Nd values from whole-rock carbonates to investigate the origin of cycles developed in two different greenhouse time intervals and the relationships between high-frequency sea-level change and continental weathering flux.

Samples were collected from Upper Silurian subtidal cycles in central Oklahoma and Upper Devonian subtidal cycles in central Nevada. Upper Silurian cycles (2-3 m thick) are composed of thin bedded, skeletal mudstone-wackestone, coarsening upward into medium to thick bedded skeletal wackestone-packstone. Upper Devonian (Frasnian) cycles (2-5.5 m thick) are composed of thin bedded skeletal mudstones overlain by medium bedded skeletal wackestone-packstone. Asymmetric facies trends in both greenhouse examples suggest abrupt transgression followed by gradual regression.

Preliminary δ¹⁸O results from Devonian cycles range from 17-18‰ and indicate initially decreasing, followed by increasing values, with the lowest δ¹⁸O values occurring mid-cycle. The total isotopic shift over cycle development is <0.6‰, and supports the interpretation of minor eustatic changes related to glacio- and/or thermo-eustasy. These trends suggest the lowest glacial ice volumes and/or warmest seawater temperatures occurred during sea level highstands. Preliminary ε_Nd trends from Silurian cycles range from -7.3 to -6.7 and are characterized by initially decreasing, then increasing values, with the lowest ε_Nd values occurring mid-cycle. These trends suggest that continental weathering flux was highest during sea-level highstands. If these cycles formed by glacio-eustasy, ε_Nd trends may be explained by regionally wetter climate (increasing weathering flux) during interglacial intervals and decreasing flux during drier glacial intervals. These relationships between regional wet/dry and glacial/interglacial cycles agree with δ¹⁸O and ε_Nd trends from cycles formed in Pleistocene and Pennsylvanian icehouse climates.