This new finding supports the idea that these
abrupt transitions were unstable tipping points
and points to a possible mechanism that could have
pushed the system toward a climatic bifurcation.
We developed a high-resolution planktonic
foraminiferal d18O record as a composite from
three sediment cores in the Gulf of Alaska (GOA)
spanning the last 18,000 years (Fig. 1 and fig. S4).
Exceptionally high sedimentation rates provide
sample resolutions similar to that of polar ice
cores; average sample spacing is ~80 years in the
Holocene [0 to 11 thousand years ago (ka)], ~10
years in the deglacial interval (11 to 14.6 ka), and
~35 years during late glacial time (14.7 to 18 ka).
Age controls include 39 planktonic foraminiferal
radiocarbon dates, and a marine reservoir age
constrained by a tephra deposit dated on land
(see supplementary materials).
The GOA d18O record integrates local tempera-
ture, salinity, and global ice volume signals, whereas
the North Greenland Ice Core Project (NGRIP)
d18O record (26) reflects local temperature, moisture
transport, and global ice volume. The temperature
component of these d18O records is inversely rel-
ated (in marine carbonates higher d18O is colder,
whereas in ice cores lower d18O is colder); there-
fore, a high negative correlation implies synchro-
nization. The cross correlation was evaluated in
moving time windows for the raw records, as well
as for ice-volume–corrected, and low-pass– and
Fig. 1. Map of Gulf of Alaska and NGRIP core locations. Base
map shows average sea surface temperature and sea-ice extent for
December, 1985 to 2005 ( neo.sci.gsfc.na
sa.gov). Pink arrows depict
surface currents, with overturning indicated in the Norwegian seas.
Dashed white line across North America depicts the approximate
extent of the Laurentide ice sheet (LIS) during the Last Glacial
Maximum (LGM) (45). Dashed white line across the North Atlantic
depicts the extension of winter sea ice during the LGM (46).
Changes in the height and extent of the LIS and sea-ice margins
could have affected the atmospheric pathways connecting the
GOA and Greenland. Blue arrows across North America represent
the schematic differences in the position of the jetstream between
the modern and LGM climate states.
Fig. 2. Comparison of GOA and NGRIP climate variability. (A) The 25-year linearly interpolated GOA d18O
record (rose line, inverted axis) is plotted with the
NGRIP d18O record (dark blue) (26). Radiocarbon age
controls plotted as blue diamonds. The purple Xs near
the top of the plot indicate times when the subtropical
planktonic species O. universa was present in the faunal
assemblages, whereas the black dots indicate samples
in which they were not present. (B) A 2000-year moving
windowed cross correlation between the records is shown
for the original GOA age model (black line, inverted axis)
and for an alternative age model that is tuned to
Greenland (gray line). The pink shading represents the
time period of synchronization between records (see
Fig. 3 for expanded view), whereas the gray shading
reflects times with either no significant correlation or
a seesaw-like mode between records. The coarse fraction percent [green shading in (B)] is dominated by IRD
from 18.5 to 14.7 ka and virtually disappears near the
onset of the Bølling. The peaks in coarse fraction from
14.5 to 13.5 ka are all dominated by volcanic ash in the
sediments (purple shading).
