Collaborative Research: Tropical Taphonomy and Porewater Geochemistry: Comparative Analysis of Molluscan Preservation in Siliciclastic and Carbonate Sediments, Western Carribean

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9628369 Walter The post-mortem fate of molluscan carbonate is influenced by processes of biological and geochemical recycling, but little is known about the rates and selectivities of these processes in the tropics, nor what their net effect on the surviving fossil record might be. Understanding these phenomena is pre-requisite both in using taphonomic damage to reconstruct paleoenvironmental conditions (e.g. taphofacies criteria) and in controlling for bias in paleoecologic, biogeographic, and evolutionary analyses. These issues also have geochemical significance, because Sr isotope compositions of shell material may be affected by compositional changes driven by organic matter respiration and by carbonate and silicate mineral dissolution in pore waters. This proposed collaborative field study of tropical siliciiclastic and carbon sediments of the western Caribbean will focus primarily on the San Blas Archipelago of Panama, and secondarily on the Los Cochinos shelf of Honduras; it builds on a successful taphonomic pilot study in the Bocas del Toro area of Panama, and extensive pore-water work in carbonate sediments of Florida Bay. This research will produce the first information on taphonomic patterns and processes in tropical siliciclastics, which constitute about half of modern shelf areas in low latitudes, and will permit the first test of carbonate reactivies and pore-water evolution in both Fe-rich (siliciclastic) and Fe-poor sediments (carbonates) within a single region. The research will quantify: 1. post-mortem damage on dead bivalve shells as a function of physical environment (e.g., water depth, grain size, sedimentation rate as determined by Pb-210), including frequency of boring, encrustation, rounding, surface degradation, and fragmentation; 2. post-mortem damage as a function of pore-water chemistry, particularly carbonic acid, major and minor element, and sulfur system parameters that govern rates of carbonate dissolution and recrystallization; samples will also permit determination of stable isotope (C & S) and 87Sr/86Sr compositions in pore-waters to assess how pore-water chemistry influences the isotopic signatures of shell material with and without microstructural alteration; 3. selectivity in taphonomic destruction as a function of taxonomy, life habit, mineralogy and microstructure of bivalves, and the importance of these factors relative to environmental forces in determining geographic variation in death assemblage condition and bias; 4. environmental variation in scales of time-averaging, stratigraphic disordering, and relationships between shell damage and shell age-since-death (i.e., test for a taphonomic clock), especially as a function of sedimentation rate, dissolution rate, composition of local shell input, and other environmentally arrayed variables; and 5. rates and pathways of shell degradation under specific conditions, using experimental arrays of shells deployed over periods up to 3 years; this a corroborative means of testing for selecitivities in and controls on shell destruction, with implications for compositional fidelity of fossil assemblages.