THE AGE OF SALT MARSH PEAT AND ITS RELATION TO RECENT CHANGES IN SEA LEVEL AT BARNSTABLE

Several features of the salt marshes of New England are most readily explained on the assumption that sea level has risen during their development.l, In particular, salt marsh peat occurs at depths many feet below the levels at which it is presently being formed, a fact which is cited among the evidence for subsidence of the coast.3-5 The measurements to be reported of the radiocarbon age of peat recovered from various depths in salt marshes on Cape Cod, at Barnstable, Massachusetts, were made to determine the rate of vertical accretion of the high marsh peat and thus provide a chronology of the development of these marshes. The results are interpreted to indicate the change of the relative elevation of the sea and land which has occurred in the Cape Cod region during the past 3,700 years. The accumulation of peat in the salt marshes of New England is initiated by the colonization of previously barren areas of the intertidal zone by Spartina alterniflora. The entrapment of sediment by the grass permits the peat to thicken until the surface is raised to approximately the level of mean high water. Near this level the marsh surface becomes flat, is populated by a more varied assemblage of plants including a dwarf variety of S. alterniflora, S. patens, Distichlis spicata and others, and is referred to as high marsh.5, The vertical accretion of salt marsh peat may thus be separated into an intertidal stage in which the rate of accretion is governed largely by the supply of sediment and a high marsh stage controlled by the rate of rise in sea level. In the marsh at West Barnstable, Massachusetts, Spartina alterniflora grows in the intertidal zone from the high marsh level down to a little below mean sea level, a vertical range of approximately six feet. We have reason to believe that intertidal peat of this thickness may develop comparatively rapidly--in a matter of two or three hundred years. The subsequent accretion of peat at the high marsh level is much slower and falls within a time scale to which radiocarbon dating is appropriate. Because the high marsh peat is naturally formed in a limited tidal range, the age of samples collected at depth may be expected to provide an indication of the rate of rise in sea level with less uncertainty than is frequently the case with other submerged organic materials. Material and Methods.-In collecting samples for radiocarbon analysis, it is desirable to avoid errors due to contamination, to the possible settling and compaction of the peat, and to other changes in the elevation of the layer sampled which might have occurred since its deposition. It is also necessary to avoid taking samples which may have formed in the intertidal zone. Samples were taken at five sites within the Town of Barnstable. At sites 1 and 2 a small piston corer, which was closed duiing descent, was employed. Contamination was avoided by rejecting the lower portion of the core. At the other sites, samples were taken with a corer made from a 2" I. D. pipe which secured cores 2 feet in length. At site 3, the peat was unusually soft and contamination with material from higher levels was indicated in the core of one sample (W-678). Later collections at this site were made with an arrangement which cased the hole as the corer descended. At sites 4 and 5, the corer was fitted with a piston arrangement, similar to that de1728