Intertidal Deposits Coastal Marshes

The recognition of coastal marshes as repositories of information on past sea levels goes back at least to Johnson's (1925) insight that only global sea level rise, not just the vagaries of local land subsidence, could explain the worldwide occurrence of coastal marshes (cf. Newman et al., 1980). This, together with earlier observations of Mudge (1858)—who asked the question, "whence all this mud? . . ."—and Shaler (1885) that marshes grow upward and laterally (vertical and lateral accretion) by trapping mineral sediment carried in with the tides, has led to the increasing investigation of marsh sediments as the preferred archives of regional sea level history (Stevenson et al, 1986). At present, coastal marshes are being investigated for evidence of regional changes in sea level by dating the sediments themselves and by examining faunal remains and geochemical indicators of former sea level position.

Modern study of marsh deposits for reconstructing past regional variations in sea level began after World War II. The first sea level curves based on marsh records that are still cited today began to appear in the 1960s (e.g., Bloom and Stuiver, 1963). After Kaye and Barghoorn (1964) published their seminal paper on the autocompaction of older marsh peats, it was quickly recognized that a single marsh sediment core would no longer suffice for an accurate depiction of changes in sea level elevation over time. Essentially, because older peats collapse under their own weight, as well as lose volume from decay and dewatering, they tend to sink lower than the depth at which they formed in relation to a former sea level. The amount of collapse and subsidence of old marsh peats increases with age (i.e., depth), and although this could conceivably be accommodated by developing a functional model of age versus autocompaction, it would have to be assumed that the lithology of the peat (especially bulk density) remain unchanged over time. Unfortunately, this is decidedly not the case, as the development of the marsh produces changes in peat composition in response to temporal shifts in plants species across the marsh and the differences in sediment trapping they cause (cf. Ward et al., 1998). Moreover, these considerable obstacles to deriving any relationship of autocompaction to age can be further complicated by the occasional large quantities of mineral sediment that overwash the marsh during storms (Stumpf, 1983).

It was realized some time ago (cf. Redfield, 1972) that a reliable record of vertical growth of the marsh as a result of rising sea levels could be obtained by analyzing the horizontal spread of the marsh (lateral accretion) across mainland surfaces as they are submerged by sea level rise. In this approach, only the basal peat in contact with the old upland surface is dated. Since only thin wedges of peat at the highly compacted, old surface are dated, the problem of autocompaction is effectively solved, and a true picture of former sea level elevations is possible (Fig. 2.6).

Of course, there are always additional problems. The most important is being aware of just what part of the marsh formed on the upland surface as it was being submerged. Surface elevations change across marshes, generally rising from the shoreline to the upland boundary. In true Atlantic Coast salt marshes, this gradual elevation has been divided into principal zones: the low marsh dominated by a single species, Spartina alterniflora, and the high marsh, again dominated by single species, Spartina patens. At the upland margin, the marsh surface may be flooded only during the highest mean water conditions (i.e., spring tides), or what is referred to as the mean high high water (MHHW) mark. The connection of this upper landward boundary with the tidal exchange that influences most of the marsh is thus less direct and, by extension, also with mean sea level. This also theoretically comprises the leading edge of the marsh as it first colonizes the newly drowned upland surface. To further

Figure 2.6 Sampling strategy for reconstruction of Holocene sea level rise using the encroachment of the marsh upon the former upland surface (lateral accretion) as sea levels rose.

confuse matters, the plants that characterize this part of the marsh, like Juncus sp. or Phragmites sp., can also be found in essentially freshwater environments, with little connection to mean sea level (Fig. 2.7). In salt marshes of the open coast, where the sea was presumably somewhere nearby throughout the history of the marsh, at least it can be assumed that the basal peat (even if consisting of upper marsh boundary species) does indicate in some way the first intrusion of a rising sea upon the land. In estuaries, however, nontidal, riverine marshes probably came slowly under the influence of an advancing saltwater front driven by coastal submergence. Here, basal peats may only record the establishment of the marsh, but not sea level. The actual transition to a truly brackish, estuarine marsh may have occurred long afterwards and this, of

Age Phragmites

Age Phragmites

Figure 2.7 Relationship of various common salt marsh plants to former mean sea level. As indicated, the actual elevation of the plants to MSL can vary significantly, perhaps as much as 1 m in some marshes. While the potential error is probably negligible to the determination of former sea level position in the early (ca. 10-8 BP) and middle (8-5 BP) Holocene, when relative sea levels were tens of meters below modern limits, it becomes critical in the reconstruction of the sea level record of the past several millennia where the total range of sea level change is often only a few meters.

Figure 2.7 Relationship of various common salt marsh plants to former mean sea level. As indicated, the actual elevation of the plants to MSL can vary significantly, perhaps as much as 1 m in some marshes. While the potential error is probably negligible to the determination of former sea level position in the early (ca. 10-8 BP) and middle (8-5 BP) Holocene, when relative sea levels were tens of meters below modern limits, it becomes critical in the reconstruction of the sea level record of the past several millennia where the total range of sea level change is often only a few meters.

course, would have been farther up the sediment column in levels affected by autocompaction.

Scott and Medioli (1978) pioneered the use of marsh foraminifera to delineate former zones in marshes as a means of obtaining better insights into how sea level variations are mediated through changes in marsh environments. Together with closer attention to marsh peat composition (Allen, 1977), marsh sediments can be repositories for the fine-scale records of sea level change requisite for the past several thousand years. Moreover, new approaches, like those of Varekamp et al. (1992), demonstrate that the possibilities for extraction of sea level information from salt marsh sediments are still not fully exploited. Geochemical and other parameters tied to changing water levels in marshes can illuminate a richer record of past sea level history than is possible in the general road map of former sea level positions demarcated by basal peat curves.

This last point highlights the intrinsic limitation of basal peat curves. Regressions, even still stands, are not well served by a sole reliance on basal peats because fundamentally such curves are predisposed to portray rising sea trends as the marsh upland boundary extends landwards with further submergence. By comparison, a fall in sea level, especially if prolonged, lowers water levels in the marsh and can lead to oxidation and degradation of surface sediments— hence loss of existing record. Thus, consider the old cliché that the child is father to the man, and whether the (alleged) smooth, nonoscillating sea level curve has been truly independent of the type of data that created it.

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