Description: Reconstructions of Quaternary climate are often based on the isotopic content of paleo-precipitation preserved in proxy records. While many paleo-precipitation isotope records are available, few studies have synthesized these dispersed records to explore spatial patterns of late-glacial precipitation δ18O. Here we present a synthesis of 86 globally distributed groundwater (n = 59), 
cave calcite (n = 15) and ice core (n = 12)
 isotope records spanning the late-glacial (defined as
 ~ 50000 to ∼ 20000 years ago) to the late-Holocene (within the past ∼5000 years). We show that precipitation δ18O changes from the late-glacial to the late-Holocene range from −7.1 ‰ (δ18Olate-Holocene 〉 δ18Olate-glacial) to +1.7 ‰ (δ18Olate-glacial 〉 δ18Olate-Holocene), with the majority (77 %) of records having lower late-glacial δ18O than late-Holocene δ18O values. High-magnitude, negative precipitation δ18O shifts are common at high latitudes, high altitudes and continental interiors (δ18Olate-Holocene 〉 δ18Olate-glacial by more than 3‰). Conversely, low-magnitude, positive precipitation δ18O shifts are concentrated along tropical and subtropical coasts (δ18Olate-glacial 〉 δ18Olate-Holocene by less than 2 ‰). Broad, global patterns of late-glacial to late-Holocene precipitation δ18O shifts suggest that stronger-than-modern isotopic distillation of air masses prevailed during the late-glacial, likely impacted by larger global temperature differences between the tropics and the poles. Further, to test how well general circulation models reproduce global precipitation δ18O shifts, we compiled simulated precipitation δ18O shifts from five isotope-enabled general circulation models simulated under recent and last glacial maximum climate states. Climate simulations generally show better inter-model and model-measurement agreement in temperate regions than in the tropics, highlighting a need for further research to better understand how inter-model spread in convective rainout, seawater δ18O and glacial topography parameterizations impact simulated precipitation δ18O. Future research on paleo-precipitation δ18O records can use the global maps of measured and simulated late-glacial precipitation isotope compositions to target and prioritize field sites.

Description: Stable isotope paleoaltimetry studies often implicitly assume that atmospheric flow interactions with topography can be simply modeled as a Rayleigh distillation process in which air parcels consistently ascend topographic barriers. We present a modern (A.D. 1979–2010) air parcel trajectory analysis using the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model that shows that this fundamental assumption is often violated in the Sierra Nevada region of the western United States. Observed trajectory patterns and scaling calculations indicate that windward orographic blocking frequently occurs when trajectories encounter high elevations (〉2.5 km) in the central and southern Sierra Nevada. As a result, trajectories reaching the Sierran lee commonly travel around, rather than over, the highest range elevations. Redirection effects are particularly pronounced at leeward sites distal (〉150 km) to the Sierran crest, but are also evident in trajectory patterns in the northern Sierra Nevada. This trajectory analysis improves the interpretability of regional meteoric water and proxy isotopic records and has particular relevance to stable isotope–based reconstructions of Sierran paleoelevations. Specifically, stable isotope methods alone provide only limited insight into the elevation history of the Sierra Nevada and are likely insufficient to resolve proposed late Cenozoic elevation gains on the order of 1–2 km that may have raised the Sierra to its modern elevations.