A 2000 year record of climatic change at Ongoke Lake, southwest Alaska
We analyzed sediments of the past 2000 years from Ongoke Lake, southwest Alaska, for organic carbon, organic nitrogen, biogenic silica, and diatom assemblages at decadal to centennial resolution to infer limnological changes that may be related to climatic variation in southwestern Alaska. The chronology is based on a 210Pb profile from bulk sediments and nine AMS 14C ages from terrestrial plant macrofossils. Four of the 14C ages span a core depth interval of 60.5 cm but are statistically indistinguishable from one another with a mean of ~1300 AD, which compromises the determination of temporal trends at Ongoke Lake and comparison with other paleoclimate records. The diatom record suggests changes in the duration of ice cover and strength of thermal stratification that are probably related to temperature variation. This variation includes a cold interval around the First Millennium Cooling and a warm interval spanning the Medieval Climate Anomaly. However, the lake-sediment record shows no clear signals of temperature variation for the period of the Little Ice Age (LIA) or the 20th century. Climatic changes during these periods may have been manifested through effective-moisture (precipitation minus evapotranspiration) variation in the Ongoke Lake area. We estimated water depths and infer effective-moisture fluctuations by applying a regional transfer function to our diatom record. Together with inferences from diatom autecologies, this water-depth reconstruction suggests that effective moisture increased steadily from 50 BC to 350 AD, which was followed by relatively dry conditions between 550 and 750 AD and relatively wet conditions between 750 and 1450 AD. Effective moisture was low from ~1450 to 1850 AD, coinciding with the LIA; an alternative age model places this interval between ~1315 and 1850 AD. During the past 150 years, effective moisture increased, with estimated water depths reaching peak values in the second half of the 20th century. This study offers the first paleolimnological record for inferring centennial-scale climatic variation over the past two millennia from southwestern Alaska.