12. Abrupt Climate Change: Causes Due to Glacial Lake Discharge in the Northeastern United States

The postdoctoral fellow will conduct research on the relationship between pro-glacial lake history in the northeastern United States and abrupt climate change during the Younger Dryas climatic cooling interval ~ 12.8 to 11.4 yr BP. Research emphasis will be on surficial and deglacial geology in the Champlain and St. Lawrence Valleys carried out by field studies; analyses and correlation of varved and marine sediments; measurement of paleo-lake, marine shorelines and glacio-isostatic uplift; quantification of water-volume in large glacial lakes; and modeling the potential impact of their discharge on North Atlantic surface ocean salinity and thermohaline circulation. Results from the surficial geological record will be integrated with proxy-based paleoclimate reconstructions based on sedimentary records in the region being carried out under the auspices of a new USGS project on Abrupt Climate Change. The fellow will relate Champlain Valley deglacial history to that known from correlative deposits in the Hudson and St. Lawrence Valleys and parts of New England in order to establish the relationships between glacial lake Vermont, Lake St. Lawrence (Lake Candona), Lake Hitchcock, and Glacial Lake Agassiz in the midcontinent, and to deep-sea and ice-core records of the Younger Dryas climate event. This research dovetails with surficial geological investigations in New England and the Great Lakes regions.

The Younger Dryas (YD) is the prototypical abrupt climate event - atmospheric temperatures in the North Atlantic region dropped by as much as 10°C in as little as a few decades ~ 12.8 ky BP– during the rapid warming following the last glacial maximum ~ 24 ky BP. An equally dramatic rise in temperatures occurred at the termination of the Younger Dryas when deglacial warming resumed. Moreover, other abrupt “millennial” climate events occurring at sub-orbital frequencies are known to punctuate much of the last glacial period and, though lower in amplitude, the Holocene interglacial as well. The YD event prompted the controversial movie “Day After Tomorrow” and has led to widespread concern among policymakers that anthropogenic greenhouse gases might cause a large-scale abrupt shift in global climate. These climate “surprises” are perhaps of greatest concern to the global scientific community, as well as politicians and the public, because they do not represent a slow, steady warming that many climate models suggest might result from greenhouse gasses or changes in solar insolation, but rather an abrupt and potentially catastrophic climate shift occurring over decades.

The most widely accepted hypothesis to explain the Younger Dryas cooling event is massive freshwater discharge of glacial-lake and ice-sheet melt water from North America out the St. Lawrence River Estuary, which may have diluted North Atlantic surface water salinity and slowed down the global ocean thermohaline circulation. Although there are numerous studies of Younger Dryas-age sediments from around the world, surprisingly little detailed chronological and paleoclimatic data are available from the Champlain–St Lawrence Valley region, the provenance of this purported fresh-water pulse to the North Atlantic. More generally, there is at present a confusing literature regarding the timing and scale of glacial lake discharge from the Hudson, Mississippi, St. Lawrence, and McKenzie River systems and thus locating the freshwater source and determining when it discharged is a critical factor in understanding its influence on ocean circulation and climate.

The Champlain–St. Lawrence region may be only place where one can directly link deglacial and voluminous melt-water-discharge history to deep-sea and ice-core records of the Younger Dryas climate reversal and thus establish causality between deglaciation history and this global climate event. This is because the region’s stratigraphic record of paleo-lake sedimentation prior to and at the onset of the Younger Dryas provides an opportunity to reconstruct at a high temporal resolution the sequence of events during the northward retreat of the Laurentide Ice Sheet margin in this critical area. These events include the formation and drainage of large glacial lakes and the marine inflow into the isostatically depressed St. Lawrence and Champlain Basins known as the Champlain Sea. It is an opportunity to test the recent hypothesis for the age of the switch from southward drainage through the Hudson River, to northward through the St. Lawrence Valley. The Lake Vermont–Champlain Sea sedimentary sequence in northeastern New York and northwestern Vermont is more than 10 m thick in some onshore sections now under investigation and as much as 80 to 200 m in the modern Lake Champlain Basin itself, where long sediment coring is planned.

The postdoctoral fellow, in collaboration with the two USGS supervisors, will test the hypothesis that glacial lake discharge through the St. Lawrence caused the Younger Dryas interval. Methods will include field investigation of post-glacial surficial processes, sediment coring, stratigraphic and chronologic analysis of varves, modeling isostatic adjustment of shorelines, quantification of lake volumes, acquiring and interpreting new radiocarbon dates and other chronological data, and integrating the post-glacial surficial geology with paleoclimate proxy records from sediments in the Champlain region of northeastern New York and Vermont, and south of the St. Lawrence Seaway in New York. Specifically, there is a need for detailed field investigation and improved chronology of Lake Vermont and the transition to Champlain Sea through AMS ¹⁴C dating of organic material and marine shells, varve counting and analysis, and acquiring magnetic intensity data. It is hoped that a standard timescale can be developed for the post-glacial deposits of the northeastern United States. Collaboration with Dr. Dave Franzi and Peter Kneupler (State University of New York), Guillaume St. Onge and Claude Hillaire-Marcel (GEOTOP, University of Quebec), and other geologists is planned. Multi-proxy paleoclimate records to be developed by USGS investigators also are needed to interpret the late Quaternary regional climate history and to identify the sources and histories of lake and marine waters entering the Champlain–St. Lawrence Basin. The chronologic and paleoclimate data also will provide a means to correlate climate records with those from other parts of the eastern U.S. and Canada. Specific sites for proposed field studies and methodology for paleo-proxy studies are available on request.

Proposed Duty Station: Reston, VA

Areas of Ph.D.: Geology, oceanography

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, Research Oceanographer

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Personnel specialist.)

Research Advisor(s): Thomas Cronin, (703) 648-6363, tcronin@usgs.gov; Byron Stone, (703) 648-6881, bdstone@usgs.gov

Personnel Office contact: Rosetta Alexander, (703) 648-7468, ralexand@usgs.gov

Summary of Opportunities