ISOTOPIC TEMPERATURES FROM THE EARLY AND MID-PLIOCENE OF THE US MIDDLE ATLANTIC COASTAL PLAIN, AND THEIR IMPLICATIONS FOR THE CAUSE OF REGIONAL MARINE CLIMATE CHANGE

Mean seasonal extreme temperatures on the seafloor calculated from the shell
d
18
Oofthescallop
Placopecten clintonius
from the basal part of the early Pliocene Sunken Meadow Member (Yorktown Formation) in
Virginia are very similar to those from the same horizon at the latitude of Cape Hatteras in North Carolina (
~
210 km
to the south). The lowest and highest temperatures calculated from each shell (using
d
18
O
seawater
¼þ
0.7
%
)givemean
values for winter and summer of 8.4
6
1.1
8
C(
6
1
r
) and 18.2
6
0.6
8
CinVirginia,and8.6
6
0.4
8
Cand16.5
6
1.1
8
C in North Carolina (respective median temperatures: 13.3
8
C and 12.6
8
C). Patterns of ontogenetic variation in
d
18
O,
d
13
C and microgrowth increment size indicate summer water-column stratification in both areas, with summer
surface temperatures perhaps 6
8
C higher than on the seafloor. The low winter paleotemperatures in both areas are
most simply explained by the greater southward penetration of cool northern waters in the absence of a feature
equivalent to Cape Hatteras. The same current configuration but a warmer general climate can account for the high
benthic seasonal range (over 15.0
8
C in some cases) but warmer median temperatures (15.7–21.3
8
C) derived from
existing
d
18
O data from scallops of the higher Yorktown Formation (using
d
18
O
seawater
¼þ
0.7
%
for the upper Sunken
Meadow Member and
d
18
O
seawater
¼þ
1.1
%
for the mid-Pliocene Rushmere, Morgarts Beach, and Moore House
members). Existing
d
18
O data from the infaunal bivalve
Mercenaria
of the Rushmere Member yields a similarly high
median temperature (21.6
8
C) but a low seasonal range (9.2
8
C), pointing to the periodic influence of warm currents,
possibly at times when the Gulf Stream was exceptionally vigorous.