Source and Seasonality Effects on Precipitation Isotope Values in the Gulf of Alaska

Abstract:

A number of paleoisotope records from the Gulf of Alaska region attribute long-term shifts in δ¹⁸O and δ²H to changes in moisture source and air mass trajectory. However, the inability to fully quantify the modern relationship between proxy and climate has been problematic due to the paucity of modern precipitation isotopedata in this region. To provide these data, event-based sampling was used between 2005−2014 at Tideview sampling station in Anchorage, Alaska. Precipitation samples were analyzed for stable isotope ratios (δ¹⁸O_, δ²H, d-excess) and examined in context of climate variables including temperature, air mass source and trajectory, precipitation amount, and relative humidity. Local surface air temperature explains a relatively small (< 30%) proportion of isotope variability, suggesting that paleoisotope archives related to precipitation might not be useful for quantitative temperature reconstructions in this area. Instead, results from a Hybrid Single-Particle Lagrangian Integrated Trajectory Model demonstrate that air mass source and trajectory has the greatest influence on the isotopic composition of precipitation at our site. Specifically, lower (higher) δ¹⁸O (d-excess) values are associated with Arctic sourced moisture from the north, while Pacific marine sourced moisture from the south is characterized by the reverse (higher δ¹⁸O_, lower d-excess).These findings have important implications for local and regional paleoisotope reconstructions which are often interpreted in terms of moisture source and trajectory variations, and which currently rely on a sparse network of isotope measurement sites from across NW North America. Furthermore, these new data highlight the relative importance of regional atmospheric circulation patterns and climate indices (e.g. the North Pacific Index and Pacific Decadal Oscillation) for determining the source, transport, and rain-out history of precipitating moisture in the Gulf of Alaska region.