The Question Of Higher Sea Levels Than Present During The Late Holocene

Perhaps no aspect of the famous "Fairbridge curve" (Fairbridge, 1961,1976) so captured the attention of sea level researchers worldwide than the portrayal of sea levels higher than present throughout most of the late Holocene (see Fig. 2.1). This curve not only stood in sharp contradiction of a smooth gradual rise in sea level depicted by curves like that for the Delaware coast discussed above, but went further: modern sea level had been attained by an overall decline in sea levels over the last 5000 years. In fact, sea levels during the middle Holocene warm period were as much as 2 m higher than present.

Theoretical work on the influences of hydro-isostasy and glacio-isostasy (e.g., Clark et al, 1978; see also Chapter 4) has indicated that the general trajectory of sea level changes shown in the Fairbridge curve is at least possible, if generally restricted to emerged beach ridges at higher latitudes around the Atlantic Ocean basin. On the other hand, for middle to lower latitudes above

30° N, theoretical considerations support a rising sea level that approached within a meter of modern limits by 5000 BP, and therefore slowly (if perhaps unevenly) rose to present mean sea level.

The work summarized above has important implications for any discussion of Holocene sea level highstands. First, it points to the primacy of geophysical causes for an apparent sea level high during the middle Holocene in many areas (especially around the North Atlantic). Second, by showing that sea levels were within "striking distance" of modern sea level relatively early, it clearly leaves open the possibility that sea levels could have exceeded present limits at some time during the late Holocene. After all, the rise would only have been on the order of less than a meter to reach present sea level and exceed it.

Reports of late Holocene sea level highstands recur in the sea level literature. Undoubtedly, early descriptions of such phenomena were marred by poor dating and inconclusive proof that the features in question (often beach ridges) were created by higher sea levels and not other factors like storms (see earlier discussion). But more recent reports are less likely to have these problems—the wide appreciation of the methodological traps among potential reviewers almost ensures that this would not be the case. Thus, such phenomena, like the ca. 2000 BP highstand discussed by Walker et al. (1995), have to be taken as real, at least in the sense that they exist. However, what they mean is another matter, particularly in terms of a eustatic response to climatic changes. Here there are considerable problems if one compares the level of climatic forcing that is necessary to achieve such rises by steric expansion of the oceans to the energy that was actually available.

Consider, as a reasonable approximation, that a 20-cm rise in global mean sea level requires at least 1°C increase in world mean temperature. This approximation is a whole ocean estimate and does not decouple surface waters of the euphotic zone from deeper ocean waters—which clearly is desirable, but brings into play such unknowns as rates of thermal diffusion with depth and other complexities. Nor does it consider shallow seas like the Caribbean, where light can reach the sea floor.

Reconstructions of climatic changes prior to the advent of instrumental records rely on proxy information that can have a high degree of imprecision. Nonetheless, there is sufficient agreement that temperatures during the warmest phases of the last several millennia were no more than 1°C higher than now. In fact, temperatures in east-central North America during the Hypsi-thermal ca. 8-5 ka BP, the warmest period of the Holocene, were just 2°C above present (Davis, 1984). Because this estimate is largely based on paleobo-tanical data (which generally reflect summer temperatures) and oceans warm up less than interior continental areas in summer, the degree of potential thermal expansion of the oceans even at this time was limited. Even recent computer models for a global sea level rise from a 2 to 2.5°C anthropogenic greenhouse warming fall considerably short of 1 m (Houghton et al., 1996).

What, then, to make of reported late Holocene sea level highstands in excess of 1 m? First, the strict steric change argument is probably not supportable-the extensive work on late Holocene climate has yet to reveal a temperature increase during warm periods anywhere commensurate with this magnitude of sea level rise. In particular, those highstands within the last millennia purported to be on the order of 2-3 m above the mean sea level trend of any period certainly need reexamination; the possibility that the data are unreliable is simply too great. Second, alternative explanations for moderate highstands should be considered. Among these are geoidal variations, as first proposed by Morner (1976); local neotectonics (i.e., faults); and possibly tsunamis, from unsuspected offshore tectonism. Finally, the expectation that such highstands be widely correlated is perhaps unwarranted as it would be surprising if they were given the confounding effects of crustal dynamics and geoidal variations.

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