Oscillating Sea Level Curves The South Carolina Coast

No sea level curve for the U.S. Atlantic Coast departs more from the present model for attempting to reconstruct a record of late Holocene sea level change than that of Colquhoun and his colleagues (Fig. 2.9). This curve, the first version of which was published almost 20 years ago (Colquhoun et al. 1980),



Fairbridge Sea Level Curve
Figure 2.9 Holocene sea level curve for the South Carolina coast by Colquhoun and Brooks (1986). Notice, as in the Fairbridge curve for Brazil, that the archaeological evidence figures prominently in the reconstruction of sea level fluctuations, particularly former high stands.

incorporates data from historical sources, archaeological evidence, and basal and other marsh peats. Eight major transgressive phases, the earliest beginning about 6000 BP, are indicated in the curve and depict temporary high stands in mean sea level of almost 2 m. Significantly, however, none of these high stands were above modern sea level, though the most recent, dated around 1000 BP, appears to have been within a few decimeters of present limits.

Colquhoun et al. (1995) readily admit that some of the short transgressive phases may not have occurred (especially where there is little regional corroboration elsewhere) because of bad data. Perhaps less tractable than simply poor data is the acknowledged influence of hydrological variations in interriverine and upper reaches of the tributary estuaries where much of the supporting archaeological data (i.e., middens) appear to have been gathered. Though Colquhoun et al. (1981) presented persuasive arguments for the eustatic interpretation of spatial changes in the distribution of these sites (both midden and local resource exploitation sites), an alternate explanation could be that increased flooding during wetter climatic periods encouraged the adoption of sites somewhat higher above mean water level than usual. Exceptionally high water levels from tropical storms in the riverine and oligohaline reaches of major Coastal Plain estuaries can remain elevated for days (sometimes weeks; e.g., Tropical Storm Agnes in 1972 and Hurricane Floyd in North Carolina, September, 1999).

The incorporation of archaeological data, then, is the strength and weakness of the Colquhoun curve. Without these data (i.e., relying solely on the basal peats), the curve becomes another example of a smooth, nonoscillating curve (Fig. 2.9). It highlights the dilemma for attempting to go beyond basal peat curves: the evidence can be intriguing, but not necessarily defensible.

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