An evaluation of paired δ18O and (234U/238U)0 in opal as a tool for paleoclimate reconstruction in semi-arid environments

Authigenic opal deposits are widespread in semi-arid and arid environments and often contain high uranium concentrations that facilitate high-precision dating by U-series and U-Pb geochronology. However, despite their ubiquity and utility for placing temporal constraints on surficial processes, the potential for authigenic opal deposits to be used as archives of past climate variability remains largely unexplored compared with more frequently used authigenic carbonate deposits. Here we investigate the potential for authigenic opal deposits to provide records of paleoclimate and paleohydrologic variability through the development of a combined record of uranium ((234U/238U)0) and oxygen (δ18O) isotope variability using in situ ion microprobe methods on opal deposits from Pinnacles National Park, California that precipitated from approximately 16 to 3 ka. Pinnacles opal (234U/238U)0 suggests a moderate decrease in net infiltration from the early to middle Holocene, coincident with the timing of increasing aridity inferred from Sierra Nevada lake records, while Pinnacles opal δ18O indicates a relative increase in subtropical storms at the same time. A shift to more North Pacific sourced storms commenced after ~ 5 ka coincident with the easing of aridity in Northern California and increased storminess in the Sierra Nevada as indicated by lake sediment records. These results suggest that useful insights about past hydrologic change can be gleaned from authigenic opal deposits, and that these deposits have the potential to enhance the spatial and temporal coverage of paleoclimate reconstructions in regions where other archives might be scarce. However, further study of the influence of water in opal on δ18O variability and additional in situ analysis of δ18O variability on longer-lived opal deposits with uniform growth histories are necessary to verify the controls on this potential proxy. Similarly, further investigation into short-term and spatial variability in (234U/238U)sw will improve qualitative and quantitative applications of (234U/238U)0 in authigenic opal deposits, enhancing our ability to use this promising new paleoclimate archive.