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Hill et al., 2006; Williams et al., submitted manuscript, 2011;
this study]. Age control for these cores is based on acceler-
ator mass spectrometry
14
C dates. All raw
14
C dates were
recalibrated to calendar years before present using the MA-
RINE09 calibration and the Calib 6.0 program (http://intcal.
qub.ac.uk/calib/) that includes an assumed constant 405 year
reservoir correction [Stuiver et al., 1998; Reimer et al.,
2009]. The lack of age reversals in the
14
C data set lends
some support to the assumed constant reservoir age, but we
later relax this assumption in examining the timing of
changes near the onset of the Younger Dryas.
4. CESSATION OF MELTWATER INPUT
AND THE YOUNGER DRYAS EVENT
18
O values were reached during
the last deglaciation, there was a sharp increase that is widely
interpreted as a reduction in southerly LIS meltwater
Shortly after minimum
δ
ow
(Figure 3). Because sea level was still rising [Fairbanks,
1989; Bard et al., 2010], a redirection of LIS meltwater has
been inferred. Kennett and Shackleton [1975] suggested that
the sharp
δ
18
O increase
“
represents a major change in the
direction of meltwater
flow from southward to eastward.
”
Broecker et al. [1989] linked this
to the
onset of the Younger Dryas and postulated that eastward flow
could have reduced AMOC and caused regional cooling in
the North Atlantic.
Critical to testing the routing hypothesis is the age of the
cessation event in the Gulf of Mexico. The most detailed
records of this event come fromOrca Basin cores EN32-PC4,
EN32-PC6, and MD02-2550. Each core exhibits a
δ
“
cessation event
”
18
O records EN32-PC4
[Flower and Kennett, 1990], EN32-PC6 [Leventer et al., 1982;
Flower et al., 2004], and MD02-2550 (Williams et al., submitted
manuscript, 2011) showing cessation event and subsequent YD
interval. Following minimum
δ
Figure 5.
Orca Basin G. ruber (white)
δ
18
O
18
O values (
3
‰
to
4
‰
), note
cessation event marked by a 2
‰-
3
‰
increase. Triangles indicate
reservoir-corrected accelerator mass spectrometry
14
C dates.
increase of 2
10 cm (Figure 5).
Our radiocarbon ages place this event from 11,200 to 10,900
14
C years ago, assuming a constant 400 year reservoir age,
which is synchronous with the onset of the Younger Dryas
determined in European lake sequences [Ammann and Lotter,
1989]. Conversion of a central date of 11,050
14
C years ago
to calendar years yields 13.0 ka based on the new MA-
RINE09 calibration (Calib 6.0) [Reimer et al., 2009].
The duration of the cessation event may also provide
insight into the mechanisms of reduction of southern melt-
water and its potential routing elsewhere. Because the
‰-
3
‰
over a thickness of 5
-
are consistent with the fact that the first two cores were piston
cores, while the last was a giant gravity core, and hence, the
sediments were probably differentially expanded/com-
pressed during core retrieval.
Radiocarbon age control can be used to estimate the dura-
tion of the cessation event. In cores EN32-PC4 and EN32-
PC6,
14
C dates converted to calendar years constrain the
accumulation rate at about 40 cm kyr
1
yielding a total
duration of about 500 years and the rapid portion at about
250 years. In MD02-2550, the dating is sufficiently detailed
to bracket the temporary increase and the rapid increase
separately with three
14
C dates. This is because of a
14
C date
at the inflection point. Accepting the calibrated
14
C dates at
face value, the calculated total duration is 362 years, and the
rapid
δ
18
O
increase involves a combination of SST cooling and salinity
increase, a more precise de
δ
nition of the cessation event is
18
Osw data (Williams et al., submitted manuscript,
2011). In
δ
based on
δ
18
O, it consists of a temporary increase after local
18
O values, a recovery, and then a large, rapid
minimum
δ
18
O increase of 1.4
δ
(Figure 5). The total thickness is
20, 20, and 11 cm in cores EN32-PC4, EN32-PC6, and
MD02-2550, respectively. The rapid
‰-
3
‰
18
O increase is 10,
10, and 5.5 cm thick, respectively. These relative thicknesses
δ
18
O increase of 3.1
‰
took about 184 years. Interest-
ingly, this interval lies within a section of continuously
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