A 10,000-year lake-sediment based reconstruction of precipitation isotope values in the Canadian Rocky Mountains

Abstract

We present a 10,000-year carbonate oxygen isotope (δ¹⁸O) record from Shark Lake, Alberta, (50.8411°N, 115.3983°W; 1,857 m above sea level) that provides insight on past changes in precipitation seasonality, regional hydroclimate dynamics and atmospheric circulation. Shark Lake has a hydrologically open configuration, with water isotope values that mirror annual mean precipitation, and contains a continuous sediment sequence consisting largely of authigenic carbonate minerals precipitated from the water column. The sediment δ¹⁸O record exhibits a shift from lower to higher-than-average δ¹⁸O values from the middle to late Holocene circa 4,100 calendar years before present (yr BP). This transition generally aligns with hypothesized transformations in atmospheric circulation that ostensibly led to reduced winter precipitation in northwestern North America and heightened aridity in the southwest during the late Holocene. Monte Carlo cross-correlation analysis demonstrates that other precipitation δ¹⁸O records from the Pacific Northwest have a significant positive correlation with the Shark Lake record, in contrast with records from the southern Rocky Mountains and central/eastern North America, which have a negative correlation with the Shark Lake record and others from the Northwest. We apply the previously established strong relationship between atmospheric circulation associated with the Pacific North American Pattern (PNA) and precipitation isotope values across North America to explain the marked coherency in precipitation δ¹⁸O reconstructions. Collectively, the paleo-records suggest a middle to late Holocene transition in the mean state of atmospheric circulation over North America from one similar to the negative phase of the PNA to one comparable to the positive PNA phase, in response to non-linear changes in ocean-atmosphere circulation driven by ice sheet decay and insolation forcing. This shift had substantial impacts on the source and isotopic composition of air masses traversing the continent and their interplay with circulation from the Gulf of Mexico as well as the seasonal amount and distributions of precipitation. Our results underscore the need for additional isotope-enabled climate model simulations and additional δ¹⁸O records, especially from understudied regions across North America, for a comprehensive view of seasonal-scale hydroclimate variations and synoptic-scale atmospheric circulation pattern changes over the Holocene.