Geotimes - July 2003 - Highlights

Geotimes

Highlights

Limnogeology
Elizabeth H. Gierlowski-Kordesch

Limnogeology is the study of lake systems and their deposits. "Limno" is based on the Greek word, "limne," meaning marsh, lake or pool. "Limnology "is the biologic, chemical and physical study of modern lakes and rivers while "paleolimnology" refers to the study of lake sediments to unravel past environmental conditions. Lakes are defined as inland bodies of water occupying a depression in Earth's crust, with salinities ranging from dilute to over 200 parts per thousand. Lakes and their sedimentary sequences are data archives containing high-resolution records of past conditions. Lakes and their deposits are more widespread through space and time than formerly believed and have great potential as a new frontier in environmental and tectonic studies as well as in resource exploration for hydrocarbons, zeolites, salts and ores.

Through the efforts of the late Kerry Kelts, limnogeologists around the world have established the International Association of Limnogeology (IAL), which held its third congress in Tucson (ILIC3), Ariz., this past spring (2003). The congress was convened by Andrew Cohen (University of Arizona) with abstracts submitted from 34 countries. In addition, a new limnogeology division within the Geological Society of America was established in 2002, with its first sessions held at the GSA annual meeting last October. This discipline is, indeed, blossoming as it attempts to answer some key questions about the past.

At the GSA annual meeting, the Limnogeology Division sponsored two oral sessions, "Geochemical and Mineralogical Records from Ancient Lake Sediments" and "The Green River Formation Revisited," and associated poster sessions. New frontiers in geochemical research in the first session included the use of strontium isotopes to reconstruct the hydrologic history of a lake basin and the use of biomarkers, clay chemistry and organic matter to interpret past climatic signatures. The session on the Green River Formation included innovative sedimentologic and geochemical work as well as summaries on paleontologic research.
Topics discussed at ILIC3 included varve records, flood and drought signals for climate reconstruction, arid lakes and their sedimentary records, lakes on the edge (extreme environments), Quaternary lake signals from seven continents, and climate change and human activity in Meso-America. Other subjects included the use of isotopes in unraveling paleoclimate, paleoweathering and the paleodrainage of lakes, recognition of seismic events in lake sequences, and ancient lake deposits from the Phanerozoic.

Research on the Green River Formation, as presented at GSA and ILIC3, has finally entered the modern era with the use of detailed basin analysis to map facies relationships, strontium isotopes to interpret drainage changes through time, more precise dating of tuffs, and the counting/correlation of laminations across the basin. New discoveries include the confirmation of a tectonic origin (drainage diversion) for a change in sedimentation from the lower to upper LaClede Bed in the Green River Basin (GRB); tracking the source and the input of sediment over time within the Wilkins Peak, Tipton Shale, and Laney members of the GRB; and identifying provenance in the Paleozoic carbonates flanking the uplifts around the basin. Most illuminating was the measurement of sediment cycles in the Wilkins Peak member using argon-40/argon-39 dating of the included tuffs to derive an estimate of 10,000-year cycles not related to Milankovitch cycles. The fact that the number of rhythmites across the GRB varied from the depocenter to the margin was intriguing as well. The assumption that all millennial-scale cycles in ancient lacustrine deposits are astronomically driven clearly needs to be re-assessed.

Of great interest at ILIC3 was the research on Quaternary lakes, focusing on high temporal resolution of sediments to tease out short-term climatic change and predict short-term events on a human scale. Paleomagnetics was suggested as a tool to extend and verify chronology in lakes and to correlate among a suite of lakes. Research also continues in the correlation of continental records to marine events, such as the Heinrich and Dansgaard-Oeschger events. The goal of the lake drilling activities of the International Continental Drilling Program (ICDP) is to collect climatic records from a series of lakes for assessing the effects of orbital forcing vs. ice sheet dynamics in controlling climate at various latitudes. The Great Salt Lake and Lake Titicaca data are now being processed, while Lake Malawi and Lake Bosumtwi (Ghana) are due to be drilled in 2004. Work is also underway in Asia and South America to collect and analyze lake signal archives to tie together data in regional syntheses.

A newer topic in the geosciences is the impact of microbial life on sedimentation, especially in relation to extreme conditions. The "Lakes on the Edge" session at ILIC3 included highly acidic, Antarctic, highly saline, and hot spring lakes with their associated records of sedimentation and life. In addition, organic matter archives from hydrocarbons and bacteriochlorophylls to alkanes were probed in various types of lakes using biomarkers and sulfur, carbon, oxygen, and hydrogen isotopes.

Of prime importance to understanding the evolution of lake basins and their resources, as well as the relative effects of climate and tectonics on the sedimentary record, is the establishment of a universal lake model. The integrated sequence-stratigraphic-geochemical lake model proposed by Kevin Bohacs, Alan Carroll and colleagues (balanced fill, overfilled, and underfilled basins) explains lake sequence lithofacies packaging and predicts hydrocarbon resource potential for any lake basin. This model will continue to be tested for its robustness and predictive qualities.

On a personal note, the IAL awarded its W.H. Bradley Medal to Thomas C. Johnson of the Large Lakes Observatory (University of Minnesota-Duluth) for his service to the global lake community and his impressive research accomplishments. In addition to establishing and managing the IDEAL program (International Decade for the East African Lakes), he coordinated research in the African lakes and promoted large lake research, particularly the North American Great Lakes, and its funding needs in the United States. Johnson has published 71papers on limnogeology in peer-reviewed journals and books. Major review papers include the l984 article on sedimentation in large lakes and his 1996 paper on sedimentary processes and signals of past climatic change in the large lakes of the East African Rift Valley. He has made noteworthy contributions to understanding the history, paleoclimatic records and sedimentary processes of Lakes Superior, Turkana, Victoria and Malawi.

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Gierlowski-Kordesch is an associate professor in the Department of Geological Sciences at Ohio University. E-mail: gierlows@ohio.edu.

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