Late Holocene Climate Fluctuations at Cascade Lake, Ahklun Mountains, Southwestern Alaska
Abstract
Climate change, including changes in growing-season length and precipitation patterns, directly impacts lake productivity. Sediment cores from Cascade Lake, Ahklun Mountains, SW Alaska, indicate that biogenic silica production shifted with climate fluctuations during the late Holocene. Inter-decadal variations in North Pacific oceanographic and atmospheric processes influence the climate of this region, and this study addresses their influence on lake productivity for the last 2000 years. Cascade Lake (3.1 km2; 60.1°N; 159.4°W) is glacier-fed with 12% of its drainage basin occupied by modern (1970) glaciers. The primary sediment contributor is Cascade Glacier located 5.4 km up-valley. Three sediment cores were recovered from the lake and the lake was instrumented with water-temperature loggers and sediment traps from 2004-2006. 137-Cs, 210-Pb profiles, and the Aniakchak tephra (3500 cal yr BP) provide the age control and indicate an average late Holocene sedimentation rate of ~0.07 cm/yr. The cores are composed of rhythmically laminated mud with an average lamination thickness of 0.4 cm, and were not deposited annually. Sediment accumulation in the traps (and likely lamination thickness) is directly related to spring/summer temperatures with 70% less sediment (by mass) deposited between Aug 2005-2006, when summer temperature averaged 1.5°C cooler, than during 2004-2005. Biogenic silica (BSi) content was analyzed at a nearly annual resolution (average=3 yr) for the last 100 yr, and at sub-decadal resolution for the last 2200 yr. Generally, BSi values are low (average 2% by mass), but show three major fluctuations: values are relatively low from 2.2 to 1.6 ka (1.5%), rise from 1.6 to 1.4 ka (2.5%), then decrease again from 1.4 ka to 1830 AD (1.9%). BSi subsequently rises and reaches its highest values in 1995 (3%). These fluctuations generally correspond to the widely recognized climate events of the Medieval Warm Anomaly, Little Ice Age, and 20th century warming. The timing of the low productivity associated with the Little Ice Age and the high productivity of the 20th century corresponds to the advance and retreat of local cirque glaciers. Lichenometric ages on glacial deposits indicate that glaciers reached their maximum Holocene extent ~1870 AD. The subsequent retreat rate was slow, then increased significantly in the late 20th century (~30 vs. ~10 m/yr), corresponding to the rise in BSi content. The BSi values of the sediment deposited post-1919 are linearly correlated with records from Dillingham, Alaska (140 km SE of Cascade Lake) and with published indices of North Pacific climate. BSi content shows the strongest correlation with a 5-year running average of the Aleutian Low Pacific Index (ALPI), and annual precipitation at Dillingham (r= 0.46, 0.39, respectively). However, correlations between the ALPI and Dillingham meteorological data indicate a stronger relation between temperature and this index than with precipitation. The ALPI appears to influence productivity within the lake (e.g., extended growing season) whereas the relation between productivity and precipitation may indicate a local effect related to nutrient input to the lake, which is entirely terrestrial (i.e., the lake is not used by salmon). Applying the correlation between BSi and the ALPI to this record indicates a strengthening of the Aleutian low-pressure system during the Medieval Warm Anomaly, but not to the degree that it strengthened during the mid-1990s.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMPP54A..04K
- Keywords:
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- 1637 Regional climate change