The response of wetlands to sea level rise: Ecologic, paleoecologic, and taphonomic models
Hoge, Bradley Earle
Fisher, Frank M.
Doctor of Philosophy thesis
Texas coastal wetlands will be increasingly endangered over the coming century as sea level rises due to global warming. Since estimates of sea-level rise may be similar to trends interpreted from Holocene sediments, a paleoecologic analysis of wetland peats may provide a predictive model for wetland succession. Three areas of focus are necessary to achieve predictive power, however: ecologic, sedimentalogic, and taphonomic. No single microtaxonomic group is well preserved in all depositional environments. Foraminiferal assemblages delineate salt to brackish marsh environments, but are absent in fresh marsh sediments. Ostracodes and diatoms occur in all marsh types, but ostracodes are sporadically preserved and diatom thanatocoenoses from different environments become too similar to significantly indicate different biocoenoses. A combined thanatocoenosis composed of microtaxa with similar ecological requirements but different taphonomic signatures provides an effective method of identifying sea-level history from wetland sediment cores, however, since taphonomic effects on selected groups overlap. Calcium carbonate and silicate dissolve according to pH, while proteinaceous cements of many agglutinated foraminifera withstand dissolution and are removed only through oxidation-reduction reactions. An analysis of these trends in the Galveston Bay system supports the following taphonomic model: During accretionary still stands, in-situ preservation is high for each thanatocoenosis. This is due to the effects of time averaging over an otherwise harsh and patchy chemical environment. During rapid sea-level rise, fresh marsh sediments become inundated with brackish to salt water. This tends to raise the pH and Eh, enhancing preservation. The thanatocoenoses grade sharply from mixed to distinct. During flooding, or progradation, fresh water inundates brackish to salt water sediments. This tends to lower pH and Eh, decreasing preservation. In situ thanatocoenoses are reduced to only a few species or eliminated completely. This taphonomic model suggests that the traditional approach of using a single microfossil group to interpret sea-level trends may be inadequate. The combined thanatocoenosis approach can produce better controls on paleoecologic and paleoclimatic interpretations, and will allow for better predictions of future trends.