Biological Indicators of Sea Level Change

Biological indicators, like shells or tree stumps, for reconstructing former sea levels can be very persuasive evidence of significant sea level changes. This is the case especially where fossils of upland tree species are recovered from considerable depth of water in continental shelves or where fossils of deepwa-

ter fauna crop out in sediments occurring substantially above modern sea level. One need only recall the example of Charles Darwin, clambering among rocks high in the Andes during the voyage of the Beagle, and the inescapable conclusions he reached regarding crustal uplift upon encountering fossils of rugose corals. Similarly, fossils of mammoths and mastodons found in surficial sediments of the inner shelf of the U.S. east coast (Emery, 1967) testify to the depth of the regression during the last glacial maximum.

Unfortunately, as telling as this evidence can be, there are numerous problems that confront any researcher in attempting to infer more than relative changes in sea level position from such material. The first question that must be answered is, was the fossil organism buried in place (i.e., in situ) after death? Rooted tree stumps are an obvious instance where it is generally safe to assume that no post-mortem transport of the organism occurred (Fig. 2.3). However, with logs, for example, this can be a risky assumption—the author once found a large remnant of a telephone pole lodged in the beach sands of Parramore Island, Virginia, with a stamp indicating it had originated in New Jersey some 300 miles north! Shells of smaller marine organisms like oysters and clams can also be transported considerable distances upon death, depending on currents and the incidence and intensity of coastal storms. Ultimately, any researcher must determine whether the organism remains are in life position or, in the case of shell material, a life assemblage. There is a considerable paleontological literature on this (cf. Easton, 1960). But if the material lacks obvious signs of transport like abrasion or breakage—not uncommon for benthic species unexposed to wave action and significant transport—it may be very difficult to judge whether it is in situ.

A second, and perhaps more meaningful, question to ask is, what is the relation of the species' habitat to mean sea level—how accurately can you reconstruct former sea level position upon finding fossils of coastal marine or mainland species in buried littoral sediments? Kidson and Heyworth (1979) and van der Plassche (1977), among others, have reviewed extensively the errors in determining past sea levels from material as diverse as tree stumps to the shells of shallow water bivalves. As any inveterate beachcomber knows, many pelecypod species can occur over a wide range of depths, often from the surf zone down to more than 10 m depth below mean sea level (Fig. 2.4). The example of Mytilus edulis is illustrative. In German Bay in the North Sea, this species has been reported to occur from the shelf bottom (depth of water around 20 m) to relatively shallow channel areas (<5 m depth) (Reineck and Singh, 1975). Hence, the best that can be said for fossils of Mytilus is that they have only "a crude relationship to sea level" (Kidson, 1982), and incorporating these data in sea level records requires a realistic appraisal of possible errors. As Kidson (1982) noted, the case for past sea level fluctuations is hardly unimpeachable with such sea level indicators.

Submerged tree stumps of riparian species have figured prominently in many sea level records. The danger here is similar to injudicious use of pelecy-

Figure 2.3 Use of rooted tree stumps occurring along Deal Island, Maryland, for reconstruction of sea level changes during the last thousand years in the region. The tree stumps, dating from around 800 BP, were exhumed from the overlying nearshore sands by a storm. In dating such material, there are several problems, not the least of which is contamination of the old wood by boring marine organisms. A more subtle problem is the actual age of the tree upon death, which relates to the timing of sea level change. It is highly possible that these trees were dead snags long before burial by the sand layer, probably being killed by inundation of their roots by an increase in groundwater level as sea level first rose. Thus, the age of the trees may predate actual submergence of the site. (Reproduced from Kearney, M. S., Sea level change during the last thousand years in Chesapeake Bay. Journal of Coastal Research, 12, 977-983, 1996.)

Figure 2.3 Use of rooted tree stumps occurring along Deal Island, Maryland, for reconstruction of sea level changes during the last thousand years in the region. The tree stumps, dating from around 800 BP, were exhumed from the overlying nearshore sands by a storm. In dating such material, there are several problems, not the least of which is contamination of the old wood by boring marine organisms. A more subtle problem is the actual age of the tree upon death, which relates to the timing of sea level change. It is highly possible that these trees were dead snags long before burial by the sand layer, probably being killed by inundation of their roots by an increase in groundwater level as sea level first rose. Thus, the age of the trees may predate actual submergence of the site. (Reproduced from Kearney, M. S., Sea level change during the last thousand years in Chesapeake Bay. Journal of Coastal Research, 12, 977-983, 1996.)

pod shells, since many riparian tree species are only nominally so, and can survive quite easily in upland environments. An example is loblolly pine (Pinus taeda), a common tree in shoreline areas where soils are poor along the

Figure 2.4 Diagram illustrating the potential depth variation in the occurrence of a common pelecypod species, such as Mytilus edulis.

U.S. Atlantic Coastal Plain—even in high levees of Chesapeake Bay coastal marshes—but ranges into the Piedmont (Silberhorn, 1982). Tree species like bald cypress (Taxodium distichum) are at least restricted to aquatic, if not necessarily shoreline, habitats, but do not occur in marine environments, tolerating little salt. Thus, finding in situ fossil wood of a species like bald cypress buried beneath littoral sediments certainly is powerfully evocative of a dramatic rise in sea level; however, not knowing where the tree was growing in relation to past shorelines before submergence and death obviates any real reconstruction of the degree of shoreline change.

Ideally, the most useful biological indicators of sea level change are those whose occurrence can be demonstrated to be restricted to within a very narrow elevation/depth range with respect to mean sea level—Kidson (1982) suggests about ±1 m. Even that restricted a range is still too coarse for determining sea level changes within the last 2 millennia, where the total change in mean sea level in most areas has been only on the order of a few meters. Of course, like all biological indicators of past environmental conditions, the fossil must be common in older sediments and resistant to problems of postburial contamination or changes in chemical composition which introduce errors in the typical radiometric dating techniques (e.g., 14C dating) for determining age. Regrettably, biological sea level indicators used in many sea level records often fall short in meeting one or more of these criteria.

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