PALEOENVIRONMENTAL INFORMATION FROM HIGH-RESOLUTION U-SERIES AND TRACE ELEMENT MEASUREMENTS OF PEDOGENIC OPAL

Pedogenic opal commonly occurs in soil carbonate horizons. To determine if opal could be a useful paleoenvironmental archive, we considered opal and calcite pebble coatings from soils developed on Quaternary shorelines of the Great Basin Desert, NV. We hypothesize a link between opal ages, initial (²³⁴U/²³⁸U) activity ratios and the regional climate, but to explore this link requires an analytical technique with very high spatial resolution. The sensitive high-resolution ion microprobe reverse-geometry (SHRIMP-RG) was used to date opal layers in-situ within soil carbonate (using the ²³⁰Th-U system), and to determine trace element variations. Opal occurs as 2-200 µm thick layers in calcite-rich pebble coatings. Opal with 50 to 1000 ppm uranium adjacent to calcite with <1-2 ppm U, suggests that analytical techniques with a spatial resolution less than approximately 30 µm may bias measurements towards the values of the highly concentrated zones of opal. Trace elements measured in the calcite and opal also showed substantial differences between the minerals, and concentrations in both minerals were highly variable over time.

The pebble coatings range in age from 10 to 250 ka, however even the oldest and deepest soils often contain younger (early Holocene) material, thus providing long-term records. Fluctuations in initial (²³⁴U/²³⁸U) values and trace element concentrations also correspond to glacial-interglacial transitions. For instance, elevated initial (²³⁴U/²³⁸U) values occur during interglacial periods, then shift gradually to lower values during glacial periods due to an increase in precipitation. These oscillations suggest that U isotopes in authigenic soil minerals may provide useful paleoenvironmental information for both modern and relict soils.