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Great Lakes Sensitivity to Climatic Forcing
Project Overview
Previous reconstructions of late glacial and post-glacial lake phases in the Great Lakes have attributed major changes in lake levels to non-climatic processes (e.g. isostatic rebound and shifts in outlet elevation). New findings of early-middle Holocene lake closure events that could only have been forced by abrupt periods of severe dry climate contrast with relatively small changes in lake levels recorded within the last two centuries. Knowledge of past occurrences of high-amplitude rapid hydrological change is relevant as some scenarios of future climate driven by global warming, suggest lake-level reductions below presently-known variability may be possible in the important watershed of the Laurentian Great Lakes.
We hypothesize that the lakes were driven below their outlets after 9000 cal BP
(8 ka) by the combined effects of enhanced southward incursions of dry Arctic air during late deglaciation in Hudson Bay and the intensification of eastward flows of warm-dry Pacific air. The latter effect was associated with shifting atmospheric circulation about 8200 cal BP that is manifested for example in the eastward expansion of prairie vegetation in the latitudes of southern Lake Michigan and Lake Erie. This effect is thought to have delayed recovery of lakes Michigan and Huron from closed status until about 7800 cal BP (7 ka). These closed-lake events afford a useful example of past high-amplitude climate-hydrology variation because the hydrology of the Great Lakes basin had already entered its present non-glacial state.
Research Components
The project has three major components:
(1) Corroborate and test Great-Lake closed lowstands. Available sediment cores and seismic profiles (Fig. 1) from previous studies will be used to corroborate past altitudes of water surfaces (large lake response to climate change), and to ensure that lake water sources in the modeled periods were mainly a result of local hydrological processes, as at present. Limited new seismic surveys and new coring will be undertaken in southern Lake Huron and Lake Erie.
Figure 1: Existing selected seismic profiles and cores. Black dots in Lakes Michigan and Huron (excluding 9 and 37) comprise a network of cores for analysis to constrain source of δ18O-depleted water at 7.9-7.5 ka.
(2) Evaluate paleoclimate change using multi-proxy data from small lakes in the Great Lakes watershed. Fieldwork is proposed to survey and sample sediments in selected small lakes distributed around the periphery of the Great Lakes drainage basin to determine past atmospheric conditions and their gradients across the Great Lakes basin. Laboratory studies will identify the targeted time interval in cores selected for study, both from large and small lakes using paleomagnetic and AMS 14C methods. Paleoecological transfer functions and isotopic geochemical methods will be used to derive proxy records of changes in atmospheric and hydrological conditions. These records will be supplemented by dendroclimatological studies of fossil wood from submerged in situ tree stumps.
(3) Reconstruct paleogeography and model the paleoclimate-hydrologic relationship of the Great Lakes. Data generated in 1) and 2) above will constrain modeling of the climate-lake hydrology relationship using both an operational hydrological process model (AHPS) and an isotopic hydrological model for the Great Lakes. Both will be modified for paleogeographic conditions in the targeted time interval. These approaches will determine hydrological sensitivity of the lake system to abrupt high-amplitude climate change.
This portion of the project will be headed by Dr. Thomas Croley at the Great Lakes Environmental Research Laboratory (GLERL). More information about GLERL's role in the project can be found in the Hydrological Models section of this website
Objectives
The knowledge and understanding of the Great Lakes we plan to generate from the proposed research consists of:
(1) Documentation of a widespread episode of closed lakes, which will add a new chapter to the history of the Great Lakes.
(2) Multi-proxy quantification of climate changes in the Great Lakes basin and comparison with known paleoclimatic events, (e.g. the final disintegration of the Laurentide ice sheet, the 8200 cal BP cool event, and vegetation changes (e.g. expansion of the prairie peninsula).
(3) An understanding of the climate-hydrology relationship, and derivation of the sensitivity of lake levels to high-amplitude climate change through numerical modeling.
(4) An explanation for depleted isotopic composition of Huron basin waters during the target interval.
Last updated: 2006-08-08 ks