Conclusions

The controversy of an oscillating versus smooth rise in late Holocene sea level emerged almost immediately following publication of the first modern sea level curves in the 1950s. For most of the decades following, advances in both data collection and understanding of the climatological and geophysical frameworks for late Holocene sea level change did nothing to diminish the arguments. If anything, the increasing detail on late Holocene climates almost enjoined sea level researchers to find comparable variations in past sea level. And, in fact, sea level records at very fine temporal scales, generally supporting a fluctuating sea level, have appeared for several areas over the past 2 decades. For the past millennium, such records make clear the widespread flattening of the global secular sea level trend as the Little Ice Age reached its maximum in the 17th and 18th centuries (cf. Varekamp et al., 1992).

Nevertheless, obtaining that equally detailed pictures of earlier sea level variations older than the last several millennia is uncertain. In many areas, coastal marshes, the best (i.e., least equivocal) archives of former sea levels, are seldom older than a few thousand years. It is rare to find a continuous marsh sediment record extending back 4000 to 5000 years, and then generally in the upper reaches of estuaries, where freshwater conditions (i.e., not clearly marine and tied to sea level) may have dominated the early part of the record. True salt marshes of the open coast (especially those in barrier lagoons) can be short-lived, tending to fall victim to some of the same forces produced by the transgression that created them.

Ultimately, length of record in intertidal deposits may be less of an issue than whether present relationships between accretion, sea level, and biological indicators have always been the same. This is a point raised by Pirazolli (1991), and it strikes at the heart of any expectations of increasing the resolution of sea level changes during the late Holocene prior to the past 2 millennia. Basal peat curves, though well tied to former sea level positions, are, as noted, essentially limited to portraying transgressions, not regressions. Geochemical and other indicators, as employed by Varekamp etal. (1992), can accommodate both rises and falls in sea level—the latter, where the sediments have not been destroyed by supertidal erosion or oxidation—but can give only the direction and relative magnitude of sea level change. Thus, improvements in fine scale temporal resolution of the late Holocene sea level history may rest in our understanding of how variations in sedimentary environment influenced sea level rise affects the ability of organisms to record those changes. As Scott and Medioli (1978) demonstrated over 20 years ago in the use of marsh foraminifera, the potential for detailed information on past sea level changes from biological indicators has yet to be fully exploited.

Was this article helpful?

0 0

Post a comment