Hurricane signatures and landforms-toward improved interpretations and global storm climate chronology

The correct understanding of the geological and geomorphic evolution of paralic basins and coastal barriers is a prerequisite for establishing Holocene hurricane archives worldwide. Detection and documentation of hurricane signatures in coastal deposits are becoming instrumental in attempts to reconstruct mid and late Holocene storm climate history. Subsurface exploration and field studies on the NE Gulf coast and data gleaned from the literature suggest the need for a complex approach that provides well-documented details of coastal landscape evolution. Until recently, prehistoric hurricanes and their intensity levels have been automatically interpreted from the frequency and thickness of sand layers in muddy paralic deposits. However, in addition to wind velocity, several hydro-, aerodynamic, and other factors a wide range of stratigraphic conditions, topographic setting, and sand sources also determine variations in the extent and intensity of storm flooding, erosion, transfer, and deposition of sand layers in small paralic basins. Inherent limitations in these sedimentary records and their interpretation must be overcome in order to improve local hurricane chronologies and establish interregional correlations. Before sedimentary and chemical signatures are accepted as diagnostic indicators of overwash especially by catastrophic hurricanes, alternative causes must also be considered. A parallel scale of storm flood-level stages should supplement the Saffir-Simpson hurricane categories. The relationship between storm intensities and flooding elevations is highly complex. Changes in sea-level, shore positions, formation and extinction of preexisting incised valley networks, the impact of transgression and regression processes on estuaries and their deposits play determining roles in setting the stage for overwash processes. Strandplain progradation followed by erosive shore retreat played a key role in framing certain paralic lakes. Without overwash, sand layers and marine microfossils may be also emplaced by storm-driven currents that enter unobstructed estuaries, inlets and storm breaches. Bioturbation also distorts the storm record. Significant lateral variations in the number and thickness of intercalated sand layers at closely adjacent sites suggest the importance of local differences in sources, transport and preservation processes: in the dimensions, relative positions and accessibility of sand resources inside and adjacent to coastal basins. Variations in the number, thickness, and ages of sand laminae, may not be related to the number of storms and their intensities. This may impair correlations between regionally interpreted calm and intensive phases. In partial overlap with an assumed quiescent storm phase, stable isotope ratios in lacustrine muds impacted by salt water influx reveal frequent hurricane activity during the last 1.5 ka as well. By mimicking effects of reduced storminess, the fewer and thinner sand laminae deposited between similar to 7.0 and 3.4 ka B.P. may be attributed to greater distances of estuarine landward transport by storm tidal currents at times of lower sea-levels. (C) 2011 Elsevier B.V. All rights reserved.