Despite several decades of paleo-studies, the Holocene temperature variation in Alaska remains largely unknown. Apart from limited isotope-based temperature estimates (e.g., Hu et al. 2001, Anderson et al. 2001) and recent tree ring records from interior Alaska, centennial to millennial-scale patterns of Holocene temperature change are primarily based on qualitative inferences from pollen records. The sequential expansions of the dominant tree and shrub taxa of the Alaskan boreal forests have been interpreted as evidence of warm early-Holocene conditions (Populus deciduous woodland), followed by progressive cooling (the arrivals of Picea glauca and then Picea mariana). However, it has been speculated that severe moisture deficits during the early Holocene contributed to restricting P. glauca’s expansion until the middle Holocene. Similarly, changes in the moisture regime have been implicated as important controls on the subsequent P. mariana and Alnus expansions. Pollen-independent proxies are needed to help resolve Holocene climatic changes in the region and to elucidate how climatic and ecological factors have affected vegetational dynamics.

Here we present new chironomid-assemblage records from six Alaskan sites: Malamute Lake (67°07’N, 153°09’W; southern Brooks Range), Low Lake (62°33’N, 153°37’W; southern Interior), Ongoke Lake (59°15’N 159°25’W, long; Southwest), Rainbow Lake (60°23’N,149°42’W; Kenai Peninsula), and Moose Lake and Hudson Lake (61°22’N, 143°35’W and 61°54’N, 145°41’W, respectively; Copper River Basin). Temperature estimates are based on a new inference model of mean July air temperature from Alaska and the Yukon Territory (Barley et al. 2006). All of our records span the entire Holocene, except Moose Lake, which covers the last 6,000 years. Four of the six records are retained for inter-site temperature comparisons. The chironomid assemblages of Ongoke Lake appear to be temperature-insensitive, likely due to a large influx of riparian taxa and the dominance of Corynocera ambigua, a taxon with an unusual modern geographic distribution. The chironomid assemblages of Low Lake appear to have been predominately affected by lake-level changes, especially between 9 – 6 ka when the lake might have nearly dried up.

Comparison of first-order fluctuations in the four records reveals pronounced differences in the temperature histories across the sites. Maximal temperatures occurred during the earliest Holocene between 12 and 10 thousand calibrated years before present (ka) at Malamute Lake, coinciding with the Populus period. Temperatures were similar to lower than modern starting at 10 ka. This contrasts with the records from southern Alaska, where cool early-Holocene conditions were followed by mid-Holocene temperature maxima centered at 5-6 ka and 4-5 ka for the Kenai and Copper River Basin sites, respectively. All records exhibit marked temperature fluctuations during the late Holocene with periods that were up to 1°C cooler than present. The most prominent of these fluctuations centered at 0.4 ka, coincident with the Little Ice Age.

Similarities of the temperature records from Moose and Hudson lakes (both located within the Copper River Basin) and between these records and tree ring-based temperature estimates indicate that chironomid assemblages hold great potential for inferring centennial-scale temperature changes with amplitudes of ~0.5-0.75°C, despite the large error envelopes on absolute temperature inferences. However, some of our records also illustrate that sites where chironomid assemblages appear to have been temperature-sensitive over the last 400 years may have intermittently lost their temperature sensitivity when local basin-conditions changed. Thus multiple records are needed from each area in order to obtain reliable estimates of Holocene temperature change from chironomid assemblage.