Paleoecology of ostracoda from quanternary sediments of the Great Salt Lake Basin, Utah

Abstract

Stratigraphic collections were made from two cores more than 200 m long taken in Quaternary sediments of the Great Salt Lake Basin, Utah, in order to trace temporal changes in the complexity of the lacustrine ostracode community. It is intended to test the hypothesis that the fauna changed in a regular and predictable manner with time and in response to changing physical conditions. According to theory, during maturation of a community, diversity tends to increase in an undisturbed environment.

Pleistocene sediments of the Great Salt Lake Basin were deposited during a number of successive lacustrine episodes which were separated by shrinkage and drying of the lake. Each episode can be treated as a separate period of ecosystem development. Samples from the cores were taken at intervals of 10 to 40 cm. Data on faunal composition and relative abundance of species were collected for each sample. These data were compared to each other and to data on the physical stratigraphy of the cores in several ways. Information theory diversities, equitabilities, and number of species were calculated; moving-average curves of each of these parameters were generated for each core. These data were correlated with data on Pleistocene climates.

Changes in diversity with stratigraphic level in the cores delineate four episodes of ostracode community development. These episodes correlate well with inferred climatic changes as indicated by other studies. Within each episode, diversity gradually increased. The number of species fluctuated because of climatic change during lacustrine episodes. Equitability increased during the early part of each episode but thereafter levelled off and did not fluctuate as severely as the number of species did. The end of each faunal episode was marked by a drop in diversity more pronounced than those due to minor climatic change.

Results can be explained by a random-migration random-extinct ion model. If, on the average, a certain number of species migrated into the lake per unit time and a certain proportion of the species were exterminated from the lake per unit time, diversity curves similar to those found would have been generated. Curves of equitability suggest that community structure remained stable within lacustrine episodes, whereas number of species fluctuated due to introductions and exterminations of the less common ostracode species. This model is similar to an island ecosystem model, and in many respects lakes may be regarded as aquatic analogs of true islands.