The salinity history of Florida Bay from the mid-1800's to present has been estimated from the metal/calcium ratios of fossil ostracode shells from sediment cores collected from central Florida Bay. The ostracode metal/Ca ratio methodology relies on two main tenets which appear to be valid: (1) metal/Ca ratios of Florida Bay waters co-vary with salinity; and (2) the metal/Ca ratios of calcite shells of ostracodes (micro-crustaceans) are controlled by the metal/Ca ratios Florida Bay waters.

Down-core results reveal a quasi-decadal scale oscillatory pattern in ostracode metal/Ca ratios that is interpreted to reflect past fluctuations in salinity. Salinity maxima occurred in the early 1920's, early 1930's, late 1940's, late 1950's to early 1960's, mid 1970's, and late 1980's, and generally correlate with multi-year intervals of low winter rainfall. Similarly, intervening intervals of estimated low salinity roughly correlate with periods of higher than normal winter rainfall. As reported by others, winter rainfall in south Florida is closely linked to ocean-atmosphere oscillations in the equatorial Pacific, and our metal/Ca -based salinity record appears to show good correspondence with the El Nino Southern Oscillation Index that describes the state of this large-scale climatic phenomenon.

These results support the hypothesis that seasonal to decadal scale salinity fluctuations in Florida Bay are a natural part of the South Florida Ecosystem, and that these fluctuations are largely a function of natural variability of regional climate (rainfall). While a clear long-term trend in salinity change is not evident in our records, beginning around 1940 there was a shift in the bay's salinity that has persisted up to the present. Our data suggest that this 60-year period was marked by large salinity oscillations and, moreover, that these oscillations generally are shifted toward higher salinity.

Thus far, the ostracode metal/Ca ratio we have focused on to reconstruct salinity is the ratio of magnesium to calcium (Mg/Ca) and the above results are based on Mg/Ca data. However, assigning absolute salinity values based on shell Mg/Ca ratios is complicated, because, in addition to the Mg/Ca ratio of the water, ostracode shell Mg/Ca ratios are also dependent on ambient water temperature. We initially operated under the assumption that water temperature effects would be minimal when considered as annual averages. However, based on the analysis of seasonally collected modern specimens, it appears that the down-core Mg/Ca ratio signal may include some temperature effects as well.

To better assess the potential influence of temperature on Mg/Ca ratios we are beginning work on shell Ba/Ca ratios. Recent (preliminary) results suggest that ostracode Ba/Ca ratios may be better suited as a salinity proxy because Ba/Ca ratios exhibit a much larger change with salinity than Mg/Ca in Florida Bay waters and there appears to be no thermodependence on ostracode shell Ba/Ca ratios. Ultimately, by coupling Ba/Ca and Mg/Ca ratio analysis on the same shells, we may be able to derive both salinity and temperature records of Florida Bay from ostracodes, possibly at seasonal time-scales.

(This abstract was taken from "Programs and Abstracts - 1999 Florida Bay and Adjacent Marine Systems Science Conference". (PDF, 1 MB))

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