Summary: For the heavily glaciated mountains of southern Alaska, few high-
resolution, millennial-scale proxy temperature reconstructions are
available for comparison with modern temperatures or with the history
of glacier fluctuations. Recent catastrophic drainage of glacier-
dammed Iceberg Lake, on the northern margin of the Bagley Icefield,
exposed subaerial outcrops of varved lacustrine sediments that span
the period 442¿1998 AD. Here, an updated chronology of varve
thickness measurements is used to quantitatively reconstruct
melt-season temperature anomalies. From 1958 to 1998, varve thickness
has a positive and marginally significant correlation with May¿June
temperatures at the nearest coastal measurement stations.
Varve sensitivity to temperature has changed over time, however,
in response to lake level changes in 1957 and earlier. I compensate
for this by log-transforming the varve thickness chronology,
and also by using a 400-year-long tree-ring-based temperature proxy
to reconstruct melt-season temperatures at Iceberg Lake.
Regression against this longer proxy record is statistically weak,
but spans the full range of occupied lake levels and varve sensitivities.
Reconstructed temperature anomalies have broad confidence intervals,
but nominally span 1.1°C over the last 1500+ years. Maximum temperatures
occurred in the late twentieth century, with a minimum in the late sixth
century. The Little Ice Age is present as three cool periods between
1350 and 1850 AD with maximum cooling around 1650 AD. A Medieval Warm
Period is evident from 1000 to 1100 AD, but the temperature
reconstruction suggests it was less warm than recent decades -
an observation supported by independent geological evidence of recent
glacier retreat that is unprecedented over the period of record.
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