Summer temperatures during the Medieval Warm Period and Little Ice Age inferred from varved proglacial lake sediments in southern Alaska

Loso, M.G.

Abstract

Study Location

Iceberg Lake is a proglacial, ice-dammed lake located on the northern margin of the Bagley Icefield in southern Alaska’s Wrangell St. Elias National Park and Preserve. The lake was 60 m deep and 4.4 km² prior to 1999, when the first of several jokulhlaups catastrophically drained the previously stable lake. Lacustrine sediments exposed by these drainage events were examined in the summers of 2001-2003.

Climate Proxy

Varve thickness:

• Varves are rhythmically laminated lacustrine sediments composed typically of a thin, fine-grained winter deposit alternating with a thicker, coarser-grained summer deposit. Varves are common where warm temperatures in summer enhance glacier melt, increase water and sediment discharge in proglacial streams, and hence deposit more lacustrine sediment. Thicker varves are therefore correlated with warmer summer temperatures.

Results

Varve thickness at Iceberg Lake is positively correlated with short (<50 yr) instrumental temperature records from nearby stations, and also with a longer (>400 yr) temperature reconstruction based on maximum latewood density of white spruce. The correlations are weak, but trends in the varve-based temperature reconstruction are corroborated by other geological evidence, and suggest that maximum temperatures over the last 1500+ years have occurred in the late 20th century, with a minimum in the late 6th century. Click here to see the temperature reconstruction from Iceberg Lake.

References

• Davi NK, Jacoby GC, Wiles GC (2003) Boreal temperature variability inferred from maximum latewood density and tree-ring width data, Wrangell Mountain region, Alaska. Quaternary Research 60: 252-262

Related Websites

• Michael Loso's website