Taphonomy of Marsh Foraminiferal Assemblages: Implications for Rates and Mechanisms of Holocene Sea-Level Rise

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9614155 Martin/Pizzuto Over the last decade, the applied Earth Sciences have moved from an emphasis on resource exploitation to one of resource conservation and management. In order for paleontology to fully participate in this "paradigm shift", the science of taphonomy, which has largely stressed information loss and stratigraphic disorder, must instead accentuate what can be regained from the fossil record. Data must be used to reconstruct stratigraphic signals in order to evaluate natural versus anthropogenic disturbances. Precise understanding of taphonomic filters bears strongly on the future application of paleontology and other traditional fields of the Earth sciences to environmental problems. Marshes provide a fertile proving ground for applied taphonomy. Marsh environments are extremely susceptible to natural and anthropogenic disturbance, so they are ideal for distinguishing between natural and anthropogenic change using historical (pre-anthropogenic) records. Marshes also display exceptional temporal resolution (low signal attenuation) that approaches or surpasses that of deep-sea cores, and which makes them ideal for taphonomic modeling. Nevertheless, differential preservation of marsh foraminiferal assemblages, which have been used to analyze natural and anthropogenic causes of sea-level rise, has been largely ignored. By itself, though, differential preservation can mimic sea-level change when none has actually occurred. We propose to construct taphonomic-age-environment models that describe shell input, preservation, and temporal resolution of foraminiferal assemblages in easily accessible intertidal Holocene marsh facies of the Delaware coast. We will then test our forward models (taphonomic filters) by using them to reconstruct the same inputs used to produce the filters. We will further test our models by using them to reconstruct paleoenvironments of the last ~2 ka preserved in vibracores taken from nearby sites, and assess the implications for distinguishin g natural versus anthropogenic sea-level rise. We will use multiple techniques to constrain sedimentary parameters and the reconstructions based on them. Inputs and decay rates of tests ("half-lives"), which affect the abundance, preservation, probability of reworking, and temporal resolution of foraminiferal assemblages, will be determined by seasonal sampling of selected marsh sites. Mixed layer thickness (m), sedimentation rate (v), and the bioturbation coefficient (D) will be estimated using x-rays and radiotracers (7Be, 137Cs, 210Pb, 14C). Field experiments involving artificial conservative (non-decaying) "impulse" layers of glass beads (analogous to microtektite and volcanic ash layers of similar deep-sea studies) will assess sedimentary parameters independently of radiotracers. Temporal resolution will be examined by applying several quantitative approaches, including graphic correlation, to dispersion (attenuation or smearing) of bead layers and other datums. Pollens profiles will control for possible anthropogenic changes (eutrophication due to deforestation and runoff) in test production and sedimentation. Besides the implications for the role of paleontology and stratigraphy in the environmental sciences, our investigation also bears on a number of critical (paleo) environmental issues, such as: 1) Calculation of the magnitude and rates of Holocene sea-level rise, their correlation with rates and mechanisms of proposed causal climatic phenomena, and distinguishing between natural and anthropogenic sea-level change; 2) Paleoenvironmental resolution in the Holocene below that of available climate proxies such as the marine oxygen isotope curve; 3) The natural evolution of wetlands and their management in light of the Holocene transgression; 4) Precise understanding of taphonomic filters an the integrity, time-averaging, and temporal resolution of stratigraphic sections, all of which ultimately bear on such profound paleobiological questions as patterns of extinction and the record of biodiversity dynamics in the fossil record. Answers to these questions are crucial to understanding, for example, past (and therefore current) biodiversity crises, but they are dependent upon very high temporal resolution. Other modeling efforts have concentrated on deep-sea assemblages and stable isotopes, but because of the intrinsically high temporal resolution of march assemblages, our study serves as an excellent starting point for applying quantitative taphonomic models to more stratigraphically-complex shelf and slope settings where much of the fossil record has formed.