A combined U-series, radiocarbon and stable isotope approach for constructing a Pleistocene lake hydrograph: an example from Surprise Valley, California

Lake records and lake hydrographs provide an integrated record of the hydrologic conditions across a watershed. To provide useful constraints on past changes in climate, robust hydrographs require concordance among multiple geochronologic approaches as well as supporting geochemical and hydrologic evidence. Dating shoreline or near-shore lacustrine carbonates using U-series and radiocarbon methods is one approach for developing the age-elevation constraints to construct lake hydrographs. Geochemical analyses (e.g., stable isotopes, elemental ratios, U-series measurements) of modern waters and sediments, as well as the primary carbonate samples, can be used to assess the potential influence of open-system behavior, detrital Th corrections, or pedogenic overprinting on the calculated ages. Additionally, topographic analyses (e.g., basin pour point, shoreline elevations and sample locations) further constrain the spatial relevance and relationships between sample localities. To evaluate the timing and magnitude of the most recent late Pleistocene lake cycle in Surprise Valley, California, we analyzed 111 sub-samples from 22 laminated shoreline tufa samples using U-series disequilibrium geochronology, and pair these analyses with 15 radiocarbon ages. To further assess the radiocarbon and U-series ages, we measured the stable isotope (δ¹⁸O and δ¹³C) and elemental (Sr/Ca) signatures of the tufa samples, and characterized the range of (²³⁴U/²³⁸U) observed in the modern waters and playas within the watershed. Topographic analysis verified that Lake Surprise is a closed, inward draining basin, and demonstrated lateral correspondence between samples from the four targeted shoreline sets. Multiple lines of evidence revealed that samples from the highest shorelines are likely from older, higher lake cycles and were influenced by variable amounts of open-system exchange or pedogenic overprinting. The measured U concentrations of ~300 to 1200 ng/g, with (²³⁸U/²³²Th) from ~3 to 12, and (²³⁰Th/²³²Th) as low as 1.7, required correction procedures to produce accurate ages. Using the Total Sample Dissolution method on suites of >5 sub-samples, we calculated U-series isochron ages using paired 2-D Rosholt isochrons determined by error-weighted linear least squares regressions. We found concordance between most isochron ages and single sample detrital Th corrected ages. Additionally, Rosholt isochron ((²³⁸U/²³²Th) vs. (²³⁰Th/²³²Th)) intercepts and modern carbonate measurements indicate that the (²³⁰Th/²³²Th) of the detrital Th end-member has remained consistent, at a value of ~1.3. Most samples demonstrated concordance between the U-series ages and radiocarbon ages, but four samples had U-series ages that were on average ~1.35 kyr older than the paired radiocarbon ages. Spanning 10 to 30 ka, this new lake hydrograph places the highest lake level, ~176 m above present-day playa, at >15.2 ka. During the Last Glacial Maximum (19 to 26 ka) Lake Surprise stood at moderate levels, at ~80 m above modern playa. This coupled approach to hydrograph construction demonstrates how age-elevation data from multiple geochronologic methods can be corroborated and interpreted within the context of geochemical and topographic analyses.