New Mexico Geological Society Annual Spring Meeting — Abstracts

Pedogenic carbonates in arid and semi-arid soils could be used as a proxy indicator of Quaternary paleo-environmental conditions. Previous studies have utilized C and O isotopic compositions in pedogenic carbonates of the desert southwest USA to reconstruct paleo-vegetation and paleo-precipitation. Climatic changes affect hydrological budgets thus prediction of water balances in future climates within the arid Southwest. However, due to inconsistency of paleo-climate information revealed from various proxies, constraining the timing of Quaternary climatic oscillations in the Southwest remains a challenge. Uranium and Sr isotopes are co-precipitated with Ca in carbonates. Here, we use Sr and U-series isotopes in stage V pedogenic carbonates at the La Mesa geomorphic surface within the Jornada Basin, New Mexico to: (1) identify Ca sources, (2) determine carbonate ages, and (3) infer paleo-hydrologic soil conditions during their formation in soils. We analyzed calcareous soil (H1, at 40 cm depth from surface), pedogenic carbonate/ caliche (H2 and H3, at 60 and 200 cm depths respectively) obtained along a soil profile, and modern dust collected within the basin. ⁸⁷Sr/⁸⁶Sr of the labile fraction of H1, H2, and modern dust showed ratios between 0.7086 and 0.709. This suggests significant contribution of dust-sourced Sr to the formation of young carbonates in shallow soils, assuming a constant dust composition since H2 carbonate formation with atmospheric circulations similar to modern conditions.H3 labile fraction in the deep, old carbonates showed slightly radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.711), suggesting a source of highly radiogenic dust (derived from areas different from modern dust source regions). Uranium concentrations in bulk caliche and soils range from 1.00 to 3.67 ppm and the (²³⁴U/²³⁸U) activity ratios (UAR) range from 0.91 to 1.34. The depth profiles of [U] and UAR suggests surface addition of uranium probably from dust. H3 and H2 carbonates yielded U-series ages ranging from 145.7 ± 3.6 to 5.94 ± 0.1 ka (2σ) and initial (²³⁴U/²³⁸U) ₀ between 0.95 and 1.35 ( ±7‰, 2σ). The calcareous soils (H1) did not yield any reliable age, but had a calculated (²³⁴U/²³⁸U) ₀ of 0.92. These ages represent a mixture of old and younger carbonate at respective depths and show a general decreasing trend towards surface consistent with the carbonate growth history inferred from the stratigraphic relationship. The (²³⁴U/²³⁸U) ₀ could be used as a proxy for soil paleo-waters during carbonate formation and shows considerable variations with depth in the soil profile. If the (²³⁴U/²³⁸U) ₀ ratios vary based on soil moisture availability, these observations suggest that H2 carbonates were formed during a relatively wet period while H1 and H3 carbonates formed under dry periods. This study demonstrates the relevance of radiogenic isotopes in reconstructing Quaternary paleo-hydrologic events as well as shedding light on formation of pedogenic carbonates and paleoclimates of New Mexico.