EVALUATING THE EFFECTIVENESS OF MICROFOSSIL-BASED PALEOTEMPESTOLOGY PROXIES IN BACKBARRIER MARSHES

Sedimentological and micropaleontological paleotempestology proxies are only effective if they consistently record hurricane strikes in sedimentary archives and the resulting storm signatures are not destroyed by processes such as bioturbation. Two studies from North and South Carolina and Delaware suggest that one such approach, storm overwash enriched with offshore-indicative calcareous foraminifers, fails both criteria.

In the first study, analysis of sediment and microfossils from fifteen 3-m cores revealed a lack of spatio-lateral continuity for storm deposits from the marshes of Folly Island, SC. The foraminifers in some deposits were taphonomically altered and in many cases cores taken 10 m apart provided significantly different records. In several low-marsh cores, where bioturbation is more intense, storm deposits were unrecognizable using foraminiferal or sedimentological criteria.

In the second study, sediments and microfossils from four marshes along Onslow Bay, NC, were analyzed immediately after Hurricane Irene made landfall in 2011. Irene did not produce sandy overwash deposits and the storm signature did not contain significant numbers of displaced marine foraminifers. Only one subenvironment, the tidal creek at Alligator Bay, received shallow-marine foraminifers and this was the sole marsh with meteorological conditions that were ideal for such transport: a relatively high maximum onshore wind [18.5 m/s] and a substantial storm surge [0.94 m]. This spatio-laterally limited storm signature will probably not be detectable in the marsh strata in the future and is discouragingly similar to the downcore record of storms preserved in these marshes: only five archived storm deposits from the past 1,500 years.

Both studies indicate that preservation of storm deposits is highly variable and the Irene study demonstrates that microfossil-based proxies may only work under ideal meteorological conditions. This research suggests that only the most robust storms produce a durable sedimentological or micropaleontological record of hurricane landfall and even these records are often destroyed via bioturbation before it is preserved in the marginal-marine strata.