Geoscience Reference
In-Depth Information
MD95-2040 and MD01-2443 (Figure 5f) between 271 and
262 ka clearly indicate that the AMOC was reduced or shut
off. The more depleted signal recorded in core MD95-2040
is most likely related to marine snow [Mackensen et al.,
1993] during a period of high productivity [Thomson et al.,
2000]. In comparison to the northern areas, cooling in the
surface waters off Sines was reduced, similar to the previous
glacials, but this time, the peak cooling in the south was
signi
comparison because they have been discussed in detail by
Voelker et al. [2009]. Data from north of the front existing
between 38°N and 40°N, i.e., core MD95-2040, are com-
pared to records from south of the front, i.e., core MD95-
2041 for the last glacial cycle and core MD01-2443 for MIS 6.
One peculiarity associated with the deep dwelling foramini-
fers used is the dominant coiling direction of G. truncatuli-
noides. During MIS 2 and 6, the right-coiling variety, which
is known from just one genotype [de Vargas et al., 2001],
dominated, while during MIS 4, the left-coiling variety that
can be attributed to all four known genotypes is more abun-
dant. Since the genotype often found in the subtropical
waters of the Sargasso and Mediterranean Sea [de Vargas
et al., 2001] is the only one with both coiling directions, we
assume that our species belong to the same genotype.
The hydrography during the glacial maxima of MIS 2 and
4 at site MD95-2040 was similar (Plate 1), with the IPC, as
indicated by the G. ruber white values, being absent during
the latest part and during the deglaciations, i.e., Heinrich
stadials 1 and 6, respectively. The interval when G. ruber
white was absent during late MIS 4 is also the one when N.
pachyderma (s), and thus subpolar waters, were continuously
present. Along with the rise in percent N. pachyderma (s),
just prior to Greenland interstadial 18, seasonality (
cantly shorter in the planktic foraminifer records (Fig-
ures 5c and 5d). Its duration was, however, comparable in the
alkenone SST record (Figure 5b), indicating decoupling in
the response of the two plankton groups. On the other hand,
the second abrupt cold event within MIS 8 (242.5
246.5 ka)
is only evident in the foraminifer records and not in the
alkenone SST. This event, associated with termination III,
had a lesser impact on the AMOC because the benthic
-
13
C
values were less depleted and thus indicate a GNAIW/
AABW boundary deeper in the water column than during
the previous event.
During glacial MIS 10, only the Heinrich-type event asso-
ciated with termination IV [Hodell et al., 2008; Stein et al.,
2009] had a pronounced impact on the hydrography off
Iberia. The percent N. pachyderma (s) levels at site MD01-
2443 were again lower than during the last glacial cycle and
during the MIS 8 Heinrich-type event (Figure 5d), but they
were comparable to Heinrich event 11. The benthic
δ
δ
18
O,
Plate 1d) increased, but the highest seasonal contrast was
associated with Heinrich stadial 6. Then, seasonality was in
the same range as the values observed during the MIS 2
Heinrich stadials. Greenland interstadial 18 was associated
with warming (lower
δ
Δδ
13
C
levels at both sites were, however, similar to the MIS 8 event
(Figure 5f ) and again indicate a much reduced AMOC and a
modified signal at site MD95-2040 due to high productivity
[Thomson et al., 2000]. Including evidence from other cold
stages such as MIS 7d, it is clear that a boundary, sometimes
the Arctic, sometimes the Subarctic Front, always separated
the two core sites during abrupt cooling events. The longer
records show, moreover, that the strong coolings associated
with the Heinrich stadials of the last glacial cycle were close
to unique and had only two counterparts during the last
420 kyr.
18
O values) in the surface to subsur-
face waters shown in particular by G. bulloides, N. pachy-
derma (r) and G. in
ata (Plates 1a and 1b). The earlier
Greenland interstadials 19 and 20 are poorly resolved, but
the presence of G. ruber white and reduced seasonality
indicates relative warm and stable conditions. This constancy
also referred to the subsurface waters as indicated by the
relative stable
18
O records of G. in
ata and G. truncatuli-
noides (Plate 1b). The MIS 4 deep-dweller records are clearly
different from MIS 2 when extremely light values were
measured [Voelker et al., 2009]. During this early part of
MIS 4, the subsurface waters were well ventilated, especially
the ENACW
st
recorded in the G. truncatulinoides
δ
δ
6. IMPACTS ON THE GLACIAL UPPER
WATER COLUMN
13
C
values (Plate 1e). We relate the G. truncatulinoides data of
core MD95-2040 to the ENACW
st
, and thus a signal trans-
ported northward because of the similar isotopic levels ob-
served in both MD95-2041 (Plate 2e) and MD95-2040. The
good ventilation in the subsurface waters is also common to
both glacial periods. Another difference to MIS 2 or more
speci
6.1. The Last Glacial Cycle: MIS 4 and MIS 2
Abrupt climate events not only affected the uppermost
waters but also left their imprints in the subsurface waters
[Rashid and Boyle, 2007; Voelker et al., 2009], information
on which is often sparse. The structure of the water column
from 0 to about 400 m can be investigated by combining the
isotope data of various planktic foraminifer species (for de-
tails see Voelker et al. [2009, Table 4]). Here we focus on the
last three glacial periods but using the MIS 2 data only for
cally to the younger Heinrich stadials at site MD95-
2040 is, however, that G. in
ata was present during some
intervals of Heinrich event 6 with the light
δ
18
Ovalues
pointing to lower salinities in the subsurface waters.
Search WWH ::
Custom Search