ee . - . a 4 “ag ami wads! F POLLEN ZONES Maximum on land (/8, 19}, dates and isotopic temperatures of deep-sea sediments (20), and estimates of ice vol- 0 19,000 years ago; before this, the levels 2?3 umes (21). These sources suggest that sea level was as low as —{23 m about Fig. 4. A generalized standard pollen zonation for northeastern coastal United States for postglacial time [after Leopold (40)]. Table 3. The §Cfor peat samples. Analyses were made in duplicate by J. M. Hunt, Woods Hole Oceanographic Institution, relative to Chicago standard Cretaceous belemnite (PDB). Sample No. Depth (m) G 2198 Ei —§2 —64 to —66 —59 —26,9 —27.6 Wood Matrix Debris — 23.5 —26.2 —25.8 Matrix —33 Matrix —28 Matrix —20to Matrix —27 Barnstable sait marsh —26.8 — 26.0 26.1 —26.0 BT 9A Vv AL RL 3 sc (Se) Wood Matrix RL 1 RL2 Type of material —43 —40 Six samples —1.5 to Wood Matrix —27.1 —11.6to +1.5 15,5 Spartina patens , Blades —11.7 Rhizomes —12.7 were higher. Data on low sea levels of postglacial time are provided on the Atlantic continental shelf of the United States and Canada by radiocarbon dates on shells of the commercial oyster, Crassostrea virginica (Gmelin) that generally lives in a restricted zone between high tide and a depth of several meters in estuaries and lagoons. The depth-date measurements for the oysters (22, 23) and for a salt-marsh component of the peat in sample RL 1 are somewhat erratic, perhaps because the oysters were not completely restricted to shaflow water or because currents moved someshells seaward. A rather wide variation was also present in the data used by Shepard (17), whose best-fit curve is shallower than our oyster-shell and peat data. We believe that our data indicate a postglacial subsidence of the continental shelf off northeastern United States relative to most other shelves of the world. In Fig. 6 the points plotted for the freshwater peats are shallower than the sea level of the same general age, as one would expect. blades of ten specimens of Spartina patens from Barnstable salt marsh and for another composite sample of their rhizomes (rootlike parts). Spartina patens is a high-marsh plant, submerged only at high tide. The values of §C™ for blades and rhizomes are similar and within the range obtained for the salt-marsh peat. Thus §C! appears to be a useful tool for distinguishing between salt-marsh and freshwater is useful; this version is based upon During the past decade about 400 radiocarbon dates of calcareous and carbonaceous materials from known depths below sea level have permitted the construction of generalized curves dates, based mostly upon borehole samples from salt marshes, show that sea to the right on the diagram shows glacial ice occupying part of the shelf off of sea level versus time. Radiocarbon level rose slowly to its present position from about —3 m approximately 4000 years ago (15, /6). Prior to that date the rise was much faster. Dates of shells, coral, and other materials ex- tend this faster rate back to about 15,000 years ago (17). Earlier dates from the sea floor are rare, but they are supplemented by dates of the glacial Yr YZ 2 > 5 o Li. 6 w" 7 o - Lo Yy — BARN c LA as 3 ELLA _ a 15 7 _ 4 — Yy o 44 = 10 " —v¥ =RL4 SRL2 —oP2 —SeQ —TB —ROG : 7 + -~ -_ Fig. 5. Positions of the sea-floor and coastal peats of Table 1 with respect to the pollen zones of Fig. 4. Age limits for the pollen zones are those of Davis (42). Most peat samples are plotted according to both radiocarbon ages and pollen percentages, but V and AZ are plotted only by their pollen percentages because both samples were too small for accurate radiocarbon measurements. In order to avoid the complexities of Fig. 6, a simplified version (Fig. 7) the past sea levels shown in Fig. 6 for the shelf off northeastern United States. The zone a few meters below or above this line (illustrated by vertical marks on the sea-level line of Fig. 7) serves to limit the age and present depth of salt-marsh peats and of most remains of oyster shells. This zone defines the shore or lagoonal environment, The region below this zone and the one extending beyond the lower depth limit of the figure represents the ocean environment. The region above the zone and that extending beyond the upper limit of the figure is the subaerial or the peats. SAMPLE NUMBER ZONE S _ \ + NS . —_ w sennaNeth dotnim vie LM. Lealaetetas Lae eee eo freshwater pond environment. The area New England; in other regions the pond environment extends to the right through this area of the graph until limited by high sea levels 25,000 to 30,000 years ago (24). In other words, Fig. 7 is a boundary diagram which shows that the dates and depths for fu- ture recoveries of marine shells should lie within the area marked OCEAN and that the dates and depths for future recoveries of freshwater peats should lie within the area marked PONDS. The presence of freshwater peats on the continental shelf reinforces the belief that during glacial stages of low sea levels the shelves became seaward extensions of the previous (and present) land areas, Coastal plains were enormously widened. The belt of new land soon became covered by vegetation similar to that of the adjacent old land, so that the former shore zones were almost obliterated. Land animals, in- cluding elephants, moose, musk Ox (25), and probably man (26), soon followed. Their new habitation was temporary, however, for when the sea advanced several thousand years later, it flooded the land vegetation, leaving only some peat deposits as evidence of former forests and grasslands. Similarly, the advancing sea either drove ahead of it the animal inhabitants or submerged their remains, as, even now, it continues to displace them with its present advance of a few millimeters per year. 1305