Abstract
The mixed siliciclastic/carbonate sediments of Baffin Bay, Texas, provide a record of the evolution of the Bay for the last 10 ka. Flooding surfaces at 8 ka and 5.5 ka provided an a priori separation of sediments in a core into three groups. Discriminant analysis and interpretation of species composition of the foraminifera from these groups indicated a progression from deltaic to open-bay to hypersaline environments. This traditional paleoecological analysis, however, does not utilize the information available in the relative abundance distribution (RAD) within each community. An approach capable of assessing within community change is provided by S (species richness), H (Shannon information function) and E (evenness) analysis. Using this approach not only can communities be designated without a priori assumptions and environments identified easily, but also the RAD within each community can be evaluated, providing a record of community growth, decline or stasis with time. Stasis, or the ecological balance of nature, is mathematically defined as an equilibrium between S and E such that diversity (H) remains constant with time. This stasis requires that, as the number of individuals (N) gets larger with time, the value of H remains constant. Thus, at stasis a regression between H versus lnN will have a 0 value for the regression coefficient (β1H), here termed the Relative Abundance Distribution Index, RADI. A positive value of RADI indicates community growth, a negative value indicates community decline. In the Baffin Bay core 6, communities were identified from 46 samples using SHE analysis. At ∼9 ka the RADI was positive, indicating the growth of a normal marine community with a high S. A second community, still largely normal marine but with a slightly negative RADI, formed at ∼8 ka. Beginning at ∼6.4 ka, the 3rd and 4th marginal marine communities with highly negative RADIs formed, indicating a sharp decline for ∼1 ka during the formation of Padre Island, which may have taken ∼1000 years. At ∼5 ka the 5th and 6th marginal marine communities were established with RADIs indicating a prolonged period of stasis. The 5th community was dominated by Elphidium with a high percentage of miliolids. The 6th community, established at ∼2 ka, was dominated by Ammonia and a moderate percentage of miliolids. These last two communities, both at stasis, were apparently responding to changes in salinity brought on by changes in rainfall.
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