Holocene hydroclimatic change in south-central Alaska inferred from δ18Odiatom at Sunken Island Lake, Kenai Peninsula lowlands

Reconstructing the Holocene hydroclimate of southern Alaska is important for understanding the evolution of North Pacific ocean-atmosphere circulation. Previous research suggests a shift in the climate of the Northeast Pacific continental margin occurred at 4 ka. This shift is characterized in part by a strengthened Aleutian Low (AL) associated with mean conditions that resemble a positive phase of the Pacific Decadal Oscillation (PDO). Oxygen isotope data from lake sediments can be used to infer the isotope composition of lake water (δ¹⁸O_lake), which is sensitive to hydroclimatic variables such as the δ¹⁸O of meteoric water and precipitation-evaporation balance (P-E). While the δ¹⁸O of calcites is most often analyzed, in southern Alaska carbonate-bearing lakes are relatively rare; in such cases, the δ¹⁸O of diatoms can be studied. We present a new Holocene δ¹⁸O_diatom record from Sunken Island Lake, a hydrologically-closed kettle lake in the Kenai Peninsula lowlands, where the local evaporation line indicates δ¹⁸O_lake is influenced by changes in P-E.

The δ¹⁸O_diatom data show a Holocene range of 5.7‰ (+26.5 to +32.2‰ VSMOW, n = 98). Mean δ¹⁸O_diatom prior to 4.5 ka was +29.3‰ (n = 45), which increased to a mean of +30.7‰ (n = 25) between 4.5 and 1 ka, then decreased to a mean of +28.3‰ (n = 28) over the past 1 ka. Although higher values from 4.5 to 1 ka might indicate decreased P-E, a regional study of lake-level fluctuations suggests water levels were at their lowest in the Kenai lowlands during the early Holocene and then rose progressively. In the context of the evidence for rising regional lake levels since the early Holocene, our observed shift to higher δ¹⁸O_diatom at 4.5 ka suggests a change in meteoric source water, which dominated the δ¹⁸O_lake signal rather than decreased P-E. An increase in δ¹⁸O_{precipitation} in this region implies positive PDO-like conditions and a strengthened AL, which promote moisture transport from the relatively enriched North Pacific Ocean through the Gulf of Alaska to the Kenai Peninsula. This increase in δ¹⁸O_diatom at 4.5 ka adds to evidence for an ocean-basin-scale shift involving the PDO and AL. The origin of the decrease in δ¹⁸O_diatom at 1 ka is uncertain. In absence of regional evidence for a change in moisture source, we ascribe it to local factors that increased P-E at this site.