Limnologic Comparison of Two Small, Varved Lakes Based on Seismic-Reflection Data
Abstract
Seismic-reflection data are not commonly collected in the course of paleolimnological studies of small lakes, yet these data potentially contain considerable limnological information. In this study we compare high-resolution (Chirp) seismic-reflection data from two small glacial lakes in the northwestern United States, lakes that contain varved (annually laminated) sediments. Foy Lake (1.1 km2, 42 m deep), near Kalispell, Montana, is an oligotrophic lake in calcareous till and bedrock. It contains late Holocene varves whose summer layers are formed by precipitation of calcium carbonate, underlain by non-varved sediments. The seismic data indicate that biogenic methane occurs sporadically in the sediments, commonly accumulating at the level of the Mazama tephra, and in some cases forming mud lumps, or gas escape structures. Despite the occurrence of gas, the seismic profiles commonly image the varved upper sediments, as well as the Mazama tephra and underlying non-varved units. The data are especially useful for defining the depositional patterns of these units. Gillette Lake (0.19 km2, 26 m deep) in Stevens County, Washington, is a mesotrophic lake in non-calcareous till and bedrock. It contains non-calcareous varves whose summer layer consists mostly of diatom remains. The sediments are highly organic and charged with biogenic methane, to the point where sections of cored sediment float. Acoustic signals were reflected from the lake floor, but were completely scattered from below the sediment surface in all but shallow (<5 m) water. Despite this, the seismic profiles were still useful for defining the lake floor morphology in the course of selecting coring sites. The high-resolution geophysical data reveal properties of the two lacustrine sediment sequences that are entirely consistent with limnological and sedimentological data. The main limnological differences between the two are related to their productivity and trophic status, and to the composition of the underlying sediments and bedrock.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFMPP32B0290C
- Keywords:
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- 1699 General or miscellaneous;
- 1845 Limnology;
- 4239 Limnology