Oxygen isotopes of diatoms (δ¹⁸O_diatom) record the isotopic composition of lake water (δW) in lakes of maritime south Alaska and provide insights into past changes in atmospheric circulation. Modern δW and climate data show a strong gradient from maritime to interior sites (Fig. 1). Lake water collected along an elevational transect shows that δW from lakes in maritime south Alaska reflect the isotope composition of local precipitation (δP) and define a local meteoric water line (LMWL) that is parallel to the global meteoric water line (Fig. 1 inset graph). δW from lakes of interior south Alaska, however, are influenced by evaporation and exhibit a lower slope (~6 versus 8). The modern δW data provides a better understanding of δP in the North Pacific, a region with few International Atomic Energy Agency-Global Network Isotopes of Precipitation stations (Fig. 1 map inset). Mica Lake (60.96° N, 148.15° W; 100 meters above sea level) is an evaporative-insensitive lake in the Prince William Sound (PWS) (Fig. 1). Sediment cores were taken from the lake in June 2006 to recover a Holocene record of δ¹⁸O_diatom. Thirteen calibrated ages on terrestrial and macrofossil samples were used to construct an age-depth model for core MC-2, which spans the last 9910 yr. Diatoms from 46, 0.5-cm-thick samples were isolated and analyzed for their oxygen isotope ratios. The analyses employed a new, stepwise fluorination technique, which uses a CO₂ laser-ablation system, coupled to a mass spectrometer, and has a reproducibility of ±0.2‰.

δ¹⁸O_diatom values from Mica Lake, core MC-2, range between 25.2 and 29.8‰ (Fig. 2b). δ¹⁸O_diatom values are relatively uniform between 9.6 and 2.5 ka, but exhibit a four-fold increase in variability since 2.5 ka. During the 20th century, δ¹⁸O_diatom increased by 4.0‰. Late Holocene excursions to lower d18Odiatom values suggest a more western or southwestern moisture source delivered by zonal flow, whereas, higher δ¹⁸O_diatom values reflect more southern moisture, delivered by meridional flow (Fig. 2a). Zonal flow likely corresponds to a weakened Aleutian Low because the south-to-north storm track is less prominent, allowing moisture from the west to reach south Alaska. Comparisons with regional δP records support the moisture-source hypothesis and further document pronounced shifts in late Holocene atmospheric circulation.