We present a varve thickness chronology from glacier-dammed Iceberg Lake in the southern Alaska icefields. Radiogenic evidence confirms that laminations are annual and record continuous sediment deposition from A.D. 442 to A.D. 1998. Varve thickness is positively correlated with Northern Hemisphere temperature trends, and more strongly with a local, ∼600 yr long tree ring width chronology. Varve thickness increases in warm summers because of higher melt, runoff, and sediment transport (as expected), but also because shrinkage of the glacier dam allows shoreline regression that concentrates sediment in the smaller lake. Varve thickness provides a sensitive record of relative changes in warm season temperatures. Relative to the entire record, temperatures implied by this chronology were lowest around A.D. 600, warm between A.D. 1000 and A.D. 1300, cooler between A.D. 1500 and A.D. 1850, and have increased dramatically since then. Combined with stratigraphic evidence that contemporary jökulhlaups (which began in 1999) are unprecedented since at least A.D. 442, this record suggests that 20th century warming is more intense, and accompanied by more extensive glacier retreat, than the Medieval Warm Period or any other time in the last 1500 yr.
Bibliographical noteFunding Information:
We are grateful for field assistance from Kelly and Natalie Bay, Mike Booth, Dan Doak, Rebekah Helms, Bob Jacobs, Jennifer Jolliffe, Ed LaChappelle, Kevin Smith, Greg Stock, Diane Thompson, Don Welty, and Rusty, who spent his last summer perpetually coated in Iceberg Lake's tenacious sediments. Thanks to at Wrangell-St. Elias National Park; LRC; UNAVCO; the Wrangell Mountains Center; and especially to Dan Engstrom, Bruce Finney, Kena Fox-Dobbs, Brian Fulfrost, Christie Rowe, Doug Schnurrenberger, Bruce Tanner, and Kellie Townsend. Bill Bull, Nicole Davi, Hamish Fulton, Dave Hollis, Feng Sheng Hu, Gordon Jacoby, Justin Revenaugh, and Greg Wiles shared data, pictures, and ideas for this project, which was supported by an NSF SGER grant (EAR-00113400), an NSF Graduate Research Fellowship and a UCSC Chancellor's Fellowship to MGL, by the University of California CSIDE and the University Collaborative Research Program of CAMS/LLNL, and by a GSA Graduate Student Research Grant.
Copyright 2011 Elsevier B.V., All rights reserved.
- Arctic climate
- Climatic history
- Glacial lakes
- Glacial sediments
- Glaciation fluctuations
- Little Ice Age
- Medieval Warm Period