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  • paleolimnology  (2)
  • 1
    ISSN: 1573-0417
    Keywords: Pliocene ; Oklahoma ; Pyrgophorus hibbardi ; paleolimnology ; stable isotopes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences
    Notes: Abstract The basal portion of the Ogallala Formation (=‘Laverne Formation’) (Lower Pliocene) Beaver County, Oklahoma, contains an interesting assemblage of non-marine fossil molluscs that include both spinose and non-spinose forms of the aquatic gastropod species Pyrgophorus hibbardi. The origin and paleolimnological significance of the spinose morph has been a source of much conjecture that has influenced environmental reconstructions of this assemblage. In one hypothesis the spinose forms of P. hibbardi are assumed to be associated with brackish water conditions by analogy with some populations of a related hydrobiid Potamopyrgus jenkinsi. To test the hypothesis that the spinose forms lived under different water conditions than the non-spinose morphs, we analyzed 10 specimens each of the two varieties for stable oxygen and carbon isotope ratios in the shell aragonite. The mean isotope ratios for the smooth and spinose morphs show no significant difference (oxygen: t = 0.28, df = 18, P (T ≤ t) 0.78 n.s.; carbon: t = 0.96, df = 18, P (T ≤ t) 0.35 n.s). We conclude that the lack of a statistically significant difference between the means of the oxygen and carbon isotope values for the smooth and spinose morphs suggests that the two forms lived in waters having similar isotope signatures. The considerable range in oxygen isotope values recorded by both morphs of P. hibbardi, including values as high as 5–6‰, suggest that both morphs were associated with waters which were periodically evaporatively enriched in 18O.
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  • 2
    Electronic Resource
    Electronic Resource
    Springer
    Journal of paleolimnology 23 (2000), S. 175-183 
    ISSN: 1573-0417
    Keywords: Late Holocene ; paleolimnology ; oxygen isotopes ; molluscs ; ostracodes
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Geosciences
    Notes: Abstract Europe Lake occupies a small, closed, basin that would have been an embayment in Lake Michigan during the high water level events in the larger lake. Cores recovered from the lake reveal late Holocene water level fluctuations in the basin that are inferred from changes in taxa and abundance of molluscs, ostracodes, magnetic susceptibility, organic carbon, and oxygen isotopes. Non-glacial, Holocene lacustrine/paludal sedimentation in this portion of the Europe Lake basin started after 6600 RCYBP and was probably initiated by a rise in the water table of the deep bedrock aquifer, during the Nipissing transgression in Lake Michigan. Isotopically light ground water from this source was probably a major contributor during this phase to the negative δ18O spikes in Valvata tricarinata and Amnicola limosa. The start of stable lacustrine conditions is marked by maximum diversity of ostracode and mollusc taxa and a shift toward much more positive δ18O values. The Europe Lake basin at this time became an embayment of Lake Michigan. This event was probably coeval with the peak of the Nipissing transgression, when the water plane reached an altitude of about 183 m. The isolation of Europe Lake from Lake Michigan started at about 2390 RCYBP and is probably due to a drop in water level in Lake Michigan and/or to isostatic uplift of the Door Peninsula. Since isolation from Lake Michigan, water levels in Europe lake have been controlled primarily by fluctuations in local precipitation, evaporation and ground water discharge.
    Type of Medium: Electronic Resource
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