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