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Limiting the Limits of Bioturbation, or At Least Focusing on the Positive

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Marginal-marine and sublittoral depositional environments capture some of the most valuable short-term ecological signals yielding (presumably) high-resolution sea-level, storm, and pollution records. Study of taphonomy and bioturbation inherently leads to skepticism about the potential resolution of these microfossil-derived signals. As a coastal geologist working the salt marshes along the southeastern Atlantic coast, I've slowly found my research opportunities hindered or even diminished by my awareness of this stratigraphic destruction. After quantifying mixing rates from fiddler crabs via artificial tracer studies in the marshes near Charleston, South Carolina, colleagues and I nearly abandoned projects designed to measure late-Holocene hurricane frequency or determine local rates of sea-level rise.

These tracer studies demonstrated the intensity and, perhaps as disturbing, the variability of mixing in salt-marsh subenvironments. The time resolution possible in high-marsh sediments far surpasses that of the low marsh because the primary bioturbators prefer muddier, softer, low-marsh sediments (Sharma et al., 1987; Hippensteel and Martin, 1999). This relationship between elevation and bioturbation has been documented in paleo-marsh deposits as well: preservation of buried washover fans and storm deposits in the high marsh exceeds that in the low marsh (Hippensteel and Martin, 2000). Shifts in marsh subenvironments through time equate to changes in bioturbation intensity and differential preservation of event layers. This temporal-spatial differential mixing could provide misleading interpretations of sea-level change or hurricane frequency and even suggest incorrect causal mechanisms. As Martin (1999, p. 385) points out: "Differential preservation of foraminiferal assemblages can, by itself, mimic the magnitude and frequency of sea-level change." This same phenomenon . . . [Full Text of this Article]