Combining organic petrography and palynology to assess anthropogenic impacts on peatlands: Part 2. An example from a Carolina Bay wetland at the Savannah River Site in South Carolina

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Wetland peat deposits can be excellent archives of past changes in the depositional and ecological conditions under which they formed. Part of the story of these changes can be obtained from analysis of the palynomorphs preserved in these deposits; however, more meaningful reconstructions can be achieved by combining organic petrography with palynology. An example of this approach is presented from a study of a peat deposit at the Savannah River Site in South Carolina, a U.S. Department of Energy-run facility constructed during the 1950s to produce basic materials used in fabrication of nuclear weapons. The primary purpose of this study was to test the combined palyno-petrographic method for its utility in distinguishing between natural successions and recent anthropogenically-derived disruptions of the ecosystem. Petrographic analysis of core samples demonstrated the complexity of the layering and rapidity of past ecosystem changes at this site, with these rapid changes supporting the contention that the small size of this wetland made it especially vulnerable to minor changes in hydrologic and climatic conditions. Organic petrography, palynofacies analysis, and percentages of nonpollen palynomorphs were found to be most useful for assessing changes in local conditions at the core site and the pollen-spore counts were better for painting a broader paleoecological picture. At two of the sampling levels, significant changes in petrography and palynofacies characteristics (e.g., increases in fungal remains, concentration of siliceous lag layers, and decreases in palynomorph stain acceptance) as well as changes in palynomorph percentages (e.g., decreases in sedges, floating aquatics, and cypresses) were interpreted as being most likely related to recent anthropogenic activities. Combined 137 Cs/ 210 Pb dating indicated that the most significant of these changes occurred in the early 1950s, coinciding with the beginning of construction of the Savannah River Site. Additional changes at around the mid-1980s coincided with the damming of Steel Creek to produce L-Lake (a reactor cooling reservoir), very near to and up-gradient from the study area. Historical precipitation records could support an alternate interpretation that regional droughts caused or accentuated the ecological changes of the early 1950s; however, the mid-1980s event cannot be explained in this way because its occurrence corresponds to a period of greater than average precipitation. Contaminant-related changes were not investigated at this site; however, anomalous 137 Cs peaks at depth, detected during the dating procedure, indicated introduction of this contaminant from recent (post-1952) weapons research on-site, suggesting the need for further investigations of other possible contaminants at this study area. Furthermore, the petrographic results indicated that these anomalous 137 Cs peaks occurred at or directly above petrographic zone contacts, suggesting the additional use of organic petrography as a means for assessing modes of adsorption and directions of transportation of waterborne contaminants within organic-rich wetlands.