Oxygen isotope records of Holocene climate variability in the Pacific Northwest

Oxygen isotope (δ¹⁸O) measurements of authigenic carbonate from Cleland Lake (southeastern British Columbia), Paradise Lake (central British Columbia), and Lime Lake (eastern Washington) provide a ∼9000 year Holocene record of precipitation-evaporation balance variations in the Pacific Northwest. Both Cleland Lake and Paradise Lake are small, surficially closed-basin systems with no active inflows or outflows. Lime Lake is surficially open with a seasonally active overflow. Water isotope values from Cleland and Paradise plot along the local evaporation line, indicating that precipitation-evaporation balance is a strong influence on lake hydrology. In contrast, Lime Lake water isotope values plot on the local meteoric water line, signifying minimal influence by evaporation. To infer past hydrologic balance variations at a high temporal resolution, we sampled the Cleland, Paradise, and Lime Lake sediment cores at 1–60 mm intervals (∼3–33 years per sample on average) and measured the isotopic composition of fine-grained (<63 μm) authigenic CaCO3 in each sample. Negative δ¹⁸O values, which indicate wetter conditions in closed-basin lakes, occur in Cleland Lake sediment from 7600 to 2200 years before present (yr BP), and are followed by more positive δ¹⁸O values, which suggest drier conditions, after 2200 yr BP. Highly negative δ¹⁸O values in the Cleland Lake record centered on ∼2400 yr BP suggest that lake levels were high (and that the lake may have been overflowing) at this time as a result of a substantially wetter climate. Similarly, Paradise Lake sediment δ¹⁸O values are relatively low from 7600 to 4000 yr BP and increase from ∼4000 to 3000 yr BP and from ∼2000 yr BP to present, indicating that climate became drier from the middle through the late Holocene. The δ¹⁸O record from Lime Lake, which principally reflects changes in the isotopic composition of precipitation, exhibits less variability than the closed-basin lake records and follows a generally increasing trend from the mid-Holocene to present. These results are consistent with several proximal reconstructions of changes in lake-level, precipitation amount, and precipitation isotopic composition and may also reflect the establishment of modern El Niño Southern Oscillation (ENSO) variability in the late Holocene, as inferred from proxy evidence of synoptic ocean-atmosphere changes in the Pacific basin. Results from mid-Holocene (6000 yr BP) climate model simulations conducted as part of the Paleoclimate Modeling Intercomparison Project Phase 3 (PMIP3) indicate that in much of western North America, the cold season (October–March) was wetter and the warm season (April–September) was considerably drier relative to the late Holocene, leading to an overall drier climate in western North America with enhanced hydroclimatic seasonality. This is consistent with inferences from the Cleland and Paradise δ¹⁸O records, which lake modeling experiments indicate are strongly influenced by cold season precipitation-evaporation balance. This also explains apparent inconsistencies between the lake δ¹⁸O records and other proxies of hydroclimatic change from the greater Pacific Northwest region that are less sensitive to cold season climate and thus indicate relatively drier conditions during the mid-Holocene. The abrupt negative excursion at ∼2400 yr BP in the Cleland Lake δ¹⁸O data, as well as the marked shift to more positive values after this time, demonstrate that gradual changes in ocean-atmosphere dynamics can produce abrupt, non-linear hydroclimate responses in the interior regions of western North America.