INSIGHTS INTO THE RELATIONSHIP BETWEEN CONTINENTAL WEATHERING AND GLACIO-EUSTASY FROM CYCLOSTRATIGRAPHY AND TRENDS IN εNd

Nd-isotope trends (reported as ε_Nd) from modern and ancient marine deposits have been utilized to document patterns and changes in marine circulation and continental weathering. Here we present whole-rock ε_Nd values and cyclostratigraphy from limestones formed under icehouse conditions to evaluate potential relationships between high-frequency (10⁴-10⁵ yr) glacio-eustasy and continental weathering rates.

Samples were collected from three consecutive high-frequency cycles (2-4 m thick) in the Middle Pennsylvanian Gray Mesa Formation of central New Mexico. Upward-shallowing facies trends within cycles are characterized by thin-bedded skeletal mudstone/wackestone (deep subtidal facies) overlain by massive skeletal wackestone/packstone (shallow subtidal facies), and are capped by calcretes or regolith breccias indicative of subaerial exposure during sea-level fall/lowstand. These asymmetric facies patterns suggest abrupt transgression above the underlying cycle cap followed by gradual regression through most of cycle development.

Preliminary data from bulk carbonate samples show two contrasting trends. One trend illustrates decreasing ε_Nd (2.5 ε units) up-cycle within early regressive facies, followed by an increase in ε_Nd in late regressive facies. This decreasing trend is expected in regressive facies as a result of an increase in continental area available for weathering during sea-level fall/lowstand. In this model, ε_Nd increases as the continent is flooded during deglaciation and transgression. The second trend, less pronounced, is characterized by initially increasing, then decreasing ε_Nd values up-cycle (1.8 ε units), which may record the initial transgression (decreasing area available for continental weathering) followed by regression. These initial results suggest that combined ε_Nd and cyclostratigraphy can be used to evaluate relationships between eustasy and continental weathering and to refine the nature of sea-level curves without relying solely on facies analysis. More high-resolution sampling through additional cycles is being conducted along with comparison of ε_Nd values from coeval biogenic apatite to determine if either trend is dominant for cycles formed by glacio-eustasy and to check for integrity of ε_Nd data.