Smooth Nonoscillating Sea Level Curves or Shepard Curves

In the 1960s through the 1980s, the smooth, nonoscillating sea level curve came to be viewed as the preferred portrayal of sea level change during the late Holocene. It is not surprising that this occurred, as evidence on the potential errors of many sea level indicators was amassed and the pitfalls of radiocarbon dating became more widely known. Recognition of the latter, of course, changed forever the notion of radiocarbon dating as a precise calendar of past events and led to increased efforts to control problems of contamination with foreign organic materials.

Thus, if any sea level indicator was probably suspect, with regard to both its accuracy in recording former sea level position and its age, the notion of "connecting-the dots" for constructing sea level curves stood largely discredited. The best that could done was to display the source of errors as explicitly as possible (using error bars, or similar devices, for depth and age) and to determine the relative trend in sea level over time. This guiding principle (if it can be called such) of sea level research along the U.S. Atlantic and Gulf Coasts for the past 30 years was not shared by European workers, who, despite admitting the errors in time and sea level position inherent in the construction of sea level curves, continued to investigate the possibility of sea level fluctuations (cf. Shennan, 1987).

The classic sea level curve of Kraft and his co-workers (Belknap and Kraft, 1977; Kraft et al., 1987) for the Delaware coast is one of the best and most widely cited smooth sea level curves for the U.S. Atlantic Coast (Fig. 2.8). Though the curve is mainly based on 16 dated basal peats, there are also over 70 additional dated materials (nonbasal marsh peats) incorporated in the overall sea level record of the area. Because the curve spans almost 12,000

AGE : 103 YRS. BEFORE PRESENT

AGE : 103 YRS. BEFORE PRESENT

Figure 2.8 The classic Holocene sea level curve for the Delaware coast of Kraft and his coworkers. This particular curve is an updated version based on I4C dates published in Kraft (1976), Belknap and Kraft (1997), and Chraztowski (1986). Figure supplied courtesy of Daria Nikitina and James Pizzuto, Department of Geology, University of Delaware.

BASAL PEAT

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TIDAL MARSH PEAT

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AGE RANGE

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DEPTH RANGE

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Figure 2.8 The classic Holocene sea level curve for the Delaware coast of Kraft and his coworkers. This particular curve is an updated version based on I4C dates published in Kraft (1976), Belknap and Kraft (1997), and Chraztowski (1986). Figure supplied courtesy of Daria Nikitina and James Pizzuto, Department of Geology, University of Delaware.

years of sea level history (and total changes in sea level on the order of 30 m), the oldest dated basal peats are of necessity not from existing coastal marshes, as the rise in sea level during the mid-Holocene appears to have been too rapid for long-term marsh survival (Rampino and Sanders, 1981).

The curve shows the steady diminution in the rate of sea level rise so typical of late Holocene sea level curves. Submergence of the Delaware coast appears to have been most rapid prior to 5000 BP, when sea levels rose at an average rate of 3 mm/yr (30 cm per century). Interestingly, this figure is considerably slower than the 10 mm/yr (1 m per century) that is conventionally cited as the rate of mid-Holocene submergence of most of the U.S. Atlantic Coast. During the past several millennia, sea levels along the Delaware coast rose at a rate of about 13 cm per century, although there are few dates in the original curve younger than 2000 BP. There is, however, a suggestion of a considerably slowing in the rate of sea level rise after 2000 BP.

More recently, Varekamp et al. (1992) discussed the climate-sea level linkages for the last 1500 years for the Connecticut coast, generating a quasi sea level curve based on benthic foraminiferal assemblages and changes in iron abundance. This curve, though not a classic basal peat curve, does suggest a distinct oscillating character to the sea level record of the last millennium and a half, with distinct regressions and transgressions, and obvious decelerations and accelerations in rate. With a vertical error of ±0.9 m, the relatively wide "bandwidth" of the curve for the Delaware coast is clearly too coarse to discriminate the small amplitude (<1 m) changes that probably have characterized the last several millennia. The classic oscillating sea level curve of Colquhoun stands in sharp contrast, belying the portrayal of a smooth (if gradually slowing) rise in late Holocene sea level, but, it is also illustrative of all the problems inherent in oscillating curves.

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