We document here the threat of large scale destruction (collapse) of barrier islands based on the study of many cores taken along the Outer Banks and in Pamlico Sound, North Carolina.Around 1,100 cal yr BP, probably as the result of hurricane activity, portions of the southern Outer Banks must have collapsed to allow normal salinity waters to bathe southern Pamlico Sound for several hundred years. Such collapse could occur again during our current regime of global warming, rising sea level and increased tropical cyclone activity. The economic effect of barrier island break collapse on Outer Banks communities would be devastating.
We present an objective and quantitative technique to reconstruct former sea level from assemblages of salt-marsh foraminifera using partitioning around medoids (PAM) cluster analysis and linear discriminant functions. Three salt marshes, representing different physiographic environments in southern New Jersey, were selected for describing the modern distribution of foraminifera from 56 surface samples. PAM estimated the number and composition of assemblages present at each site and showed that foraminifera adhered to the concept of elevation-dependent ecological zones, making them appropriate sea-level indicators.This approach has several advantages in comparison to hierarchical clustering. Application of PAM to a regional dataset generated by combining all samples identified the presence of five distinctive biozones occupying defined elevation ranges. The characteristics of these biozones were similar to those identified elsewhere on the mid-Atlantic coast of the USA. Classification of each of the 56 samples as belonging to one of the five biozones enabled us to develop linear discriminant functions, which confirmed their distinctiveness. These functions can estimate the probability that assemblages of foraminifera preserved in samples of salt-marsh sediment represent one of the five modern biozones. Recognition of these biozones in sequences of salt-marsh sediment provides a means to reconstruct sea level. We collected a 4.0 m core from Leeds Point, New Jersey to investigate the practical application of this approach to reconstructing former sea level. The linear discriminant functions were used to indicate the faunal origin of 32 core samples and in cross validation tests were accurate in 54 of 56 cases.The approach described can be used as an independent means to reconstruct sea level or to check the ecological plausibility of results from other techniques including transfer functions.
[1] Stratigraphic evidence is found for two coseismic subsidence events that underlie a floodplain 20 km south of Padang, West Sumatra along the Mentawai segment (0.5°S-0.3°S) of the Sunda subduction zone. Each earthquake is marked by a sharp soil-mud contact that represents a sudden change from mangrove to tidal flat. The earthquakes occurred about 4000 and 3000 cal years B.P. based on radiocarbon ages of detrital plant fragments and seeds. The absence of younger paleoseismic evidence suggests that late Holocene relative sea level fall left the floodplain too high for an earthquake to lower it into the intertidal zone. Our results point to a brief, few thousand year window of preservation of subsidence events in tidal-wetland stratigraphic sequences, a result that is generally applicable to other emergent coastlines of West Sumatra.Citation: Dura, T
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