Evidence for Early-Holocene Warmth in Southwestern Alaska: Biogenic Silica, Diatom Assemblage, and Diatom-Isotope Records From Ongoke Lake.

Evidence of a Holocene Thermal Maximum remains equivocal in Alaska where few non-pollen proxy records exist for Holocene climatic reconstruction. We analyzed sediment cores from Ongoke Lake (59° 15`N, 159° 25`W, 400 m a.s.l., 7m. max. depth), southwest Alaska, for biogenic silica content (BSi) (at contiguous < 35 yr. resolution), sedimentary diatom assemblages (< 250 yr. resolution), and diatom oxygen isotopes (< 250 resolution). Fluctuations in BSi over the past 12,000 cal. yr BP appear to reflect lake primary productivity, which is probably related to climatic changes and associated variations in nutrient availability. Spectral analysis of the BSi record reveals pronounced productivity cycles (1200, 750, and 200 years), possibly caused by centennial to millennial variations in solar irradiance as evident in the BSi record of nearby Arolik lake (59° 28`N, 161° 07`W). Four periods of high BSi (> 353 mg/g) each lasting c. 1,000- 1,500 yrs, delineated by negative excursions of short duration (< 100 yrs.), occurred between 10,000 and 4,500 cal. yr BP. These periods of high BSi coincide with diatom assemblages characteristic of higher nutrient concentration and a longer ice-free season. Reduced ice cover duration, strengthening of the lake's stratification, and alteration to the timing of lake mixing probably increased the internal nutrient loading of the lake leading to increased BSi and a rise in planktonic, mesotrophic diatom species. Between 4,000 and 500 cal. yr BP, BSi fluctuates around the mean Holocene value, and rises in the relative abundance of littoral, oligotrophic diatom species and those flourishing in more turbulent conditions suggest cooler conditions with increased storminess during the late Holocene. The millennial-scale climatic inferences from BSi and diatom assemblages are supported by the diatom oxygen isotope (δ18Odiat) data from the same lake, which suggests the early Holocene was warmer and drier in comparison to the late Holocene.