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Lacustrine carbonate sedimentation

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  1. Introduction.
  2. Characteristics of open and closed basin lakes.
  3. Sources of carbonate in lake systems.
  4. Deposition and preservation of lake sediments.
  5. Interpretation.
  6. Case study: Moon Lake, North Dakota.
    1. Demonstrating the importance of using various proxies when reconstructing paleoenvironment.
    2. A shift from calcite deposition to aragonite.
    3. The necessity to combine several techniques.
  7. Conclusion.

Lakes act as outdoor laboratories for the observation and understanding of natural processes. Every process has its own signature. If science looks at processes occurring today and their respective characteristics, then the door opens for understanding past processes that have similar characteristics and signatures. Sediments in lakes are the primary diagnostic source for determining paleoclimate, paleoenvironment and for developing a general understanding of lake processes. In particular carbonate sediments are unique archives for looking at a lake's specific chemical and environmental history. (Kelts and Talbot, 1990.) However it is necessary to look at a combination of factors other than carbonates and use various means of gathering data to fully understand a lake's history. Familiarity with the different sources of carbonate, the preservation processes and the general isotopic interpretation will provide bounds of information about biotic and abiotic interactions in the lake and local environmental change.

[...] Darker bands usually consist of mostly organic matter and diatoms from the fall and winter seasons when carbonate production does not occur. Some lakes freeze during the winter seasons and thus fine-grained suspended clay and silt settle out into the profundal sediments and add to this darker layer. Preserved laminated couplets can be interpreted as annual varves. Varves can be counted to determine lake chronology, however it may be difficult to distinguish annual couplets from seasonal couplets (Eugster and Kelts, 1983). [...]

[...] Kelts, K., and Hsu, K.J Freshwater carbonate sedimentation. In: Lerman, A. (eds.), Lakes: Chemistry, Geology, Physics . Springer-Verlag, Berlin p. 295-320. Kelts, K., and Talbot, M Lacustrine carbonates as geochemical archives of environmental change and biotic/abiotic interactions. In: M.M. Tilzer and C. Serruya (eds.), Ecological Structure and Function in Large Lakes. Science Tech. Publications, Madison WI, USA, p.290-317. Pearson, F.J., and Coplen, T.B Stable isotope studies of lakes. In: A. Lerman (eds.), Lakes: Chemistry, Geology, Physics. Springer-Verlag, Berlin p. 325-336. [...]

[...] A greater increase in carbonate and organic matter further on in Unit 3 (6170-4810 14C yr. B.P.) represents lake level rise and a decrease in clastic input. The change from Unit 3 to Unit 4 (4810- 3950 14C yr. B.P.) is marked as a distinct and abrupt sedimentological change. The sediments vary from finely laminated sediments of Unit 3 to massive and banded sediments of Unit 4. The silt-sized massive sediments and the large amounts of plant fragments indicate low lake level and the absence of lamination represent a decrease in anoxia present in the lake. [...]

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