New Mexico Geological Society Annual Spring Meeting — Abstracts

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. When combined with δ¹⁸O trends from marine deposits, we hypothesize that ε_Nd will provide new insights into local climate fluctuations during high-frequency (104 -105 yr) glacial-interglacial cycles. Here we present whole-rock εNd, conodont apatite δ¹⁸O, and cyclostratigraphy from the Middle Pennsylvanian Gray Mesa Formation of central New Mexico to evaluate potential relationships between glacio-eustasy and continental weathering rates during an icehouse climate.

Samples were collected from seven consecutive high-frequency cycles (2-4 m thick). Upwardshallowing facies trends within the cycles are characterized by thin-bedded skeletal mudstone/wackestone (deep subtidal facies), overlain by massive skeletal wackestone/packstone (shallow subtidal facies), and are commonly capped by calcretes, regolith breccias, and/or δ¹³C values indicative of prolonged 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.

δ¹⁸O trends for 3 of the 4 sampled cycles record 0.8‰ to 1.2‰ up-cycle increases, which is consistent with a glacio-euastic origin for cycle formation. ε_Nd trends from 6 of the 7 sampled cycles are characterized by mid-cycle minimum ε_Nd values (before maximum regression). We interpret that maximum influx of local fluvial sediment (lowest ε_Nd values) occurred during interglacial intervals when the local paleotropical climate was wettest, and minimum fluvial influx (maximum ε_Nd values) occurred during glacial intervals when the climate was the driest. This interpretation is supported by Pleistocene climate records, which show that during glacial intervals, the tropics were drier, whereas during interglacial intervals, the tropics were wetter. These preliminary results suggest that local paleotropical climate changes and associated continental weathering were influenced by the same 104 -105 yr mechanisms that drove Middle Pennsylvanian glacio-eustasy.