Lake oxygen isotopes as recorders of North American Rocky Mountain hydroclimate: Holocene patterns and variability at multi-decadal to millennial time scales

Lake sediment oxygen isotope records (calcium carbonate-δ¹⁸O) in the western North American Cordillera developed during the past decade provide substantial evidence of Pacific ocean-atmosphere forcing of hydroclimatic variability during the Holocene. Here we present an overview of 18 lake sediment δ¹⁸O records along with a new compilation of lake water δ¹⁸O and δ²H that are used to characterize lake sediment sensitivity to precipitation-δ¹⁸O in contrast to fractionation by evaporation. Of the 18 records, 14 have substantial sensitivity to evaporation. Two records reflect precipitation-δ¹⁸O since the middle Holocene, Jellybean and Bison Lakes, and are geographically positioned in the northern and southern regions of the study area. Their comparative analysis indicates a sequence of time-varying north-south precipitation-δ¹⁸O patterns that is evidence for a highly non-stationary influence by Pacific ocean-atmosphere processes on the hydroclimate of western North America. These observations are discussed within the context of previous research on North Pacific precipitation-δ¹⁸O based on empirical and modeling methods. The Jellybean and Bison Lake records indicate that a prominent precipitation-δ¹⁸O dipole (enriched-north and depleted-south) was sustained between ~3.5 and 1.5ka, which contrasts with earlier Holocene patterns, and appears to indicate the onset of a dominant tropical control on North Pacific ocean-atmosphere dynamics. This remains the state of the system today. Higher frequency reversals of the north-south precipitation-δ¹⁸O dipole between ~2.5 and 1.5ka, and during the Medieval Climate Anomaly and the Little Ice Age, also suggest more varieties of Pacific ocean-atmosphere modes than a single Pacific Decadal Oscillation (PDO) type analogue. Results indicate that further investigation of precipitation-δ¹⁸O patterns on short (observational) and long (Holocene) time scales is needed to improve our understanding of the processes that drive regional precipitation-δ¹⁸O responses to Pacific ocean-atmosphere variability, which in turn, will lead to a better understanding of internal Pacific ocean-atmosphere variability and its response to external climate forcing mechanisms.