Abrupt Climate Change Revisited - Abrupt Climate Change: Mechanisms, Patterns, and Impacts - page 3

that would indicate a shutdown or slowdown of the AMOC
during Heinrich events [Lynch-Stieglitz et al., 2007]. This
situation does not necessarily mean that changes in paleo-
bottom water composition did not occur but simply shows
that adequate supporting data are lacking. Even less certainty
can be applied to paleorecords older than the last glacial
cycle, when these abrupt climate changes were a recurrent
phenomenon. Whether such recurrent events occurred dur-
ing previous glacial cycles is not well documented because
of the scarcity of long paleoclimatic records with the requi-
site spatial and temporal resolution.
Several new areas of inquiry were discussed during the
meeting including (1) development of a new chronostrati-
graphy for Antarctic ice cores based on local insolation and
independent from bias-prone orbital tuning [Kawamura et
al., 2007; Laepple et al., 2011], (2) phasing between the deep
ocean and surface water warming during terminations as
derived from oxygen isotope records on benthic and plank-
tonic foraminifers [Rashid et al., 2009], (3) indication of
monsoon failure from atmospheric oxygen isotopes and deep
ocean temperature change from inert gases [Severinghaus et
al., 2009], (4) timing of elevated subantarctic opal

paleoceanographic records available from the North Atlantic,
the critical area for understanding changes in the MOC.
Voelker and de Abreu [this volume] provide an overview of
the last four glacial cycles from high-accumulation-rate sites
on the Iberian margin based mostly on data from Martarat
et al. [2007], Voelker et al. [2009], and Salgueiro et al.
[2010]. Latitudinal (43.20° to 35.89°N) and longitudinal
(10.39° to 7.53°W) gradients in sea surface temperature and
density are reconstructed in this chapter using a foraminiferal
assemblage - based transfer function (SIMMAX [P aumann

18 O from 11 sediment cores from the
northeastern Atlantic. In addition to sea surface conditions,
δ

et al., 1996]) and δ

13 C compositions in various planktonic foramini-
fers covering the depth range of 50 to 400 m indicate changes
in calci

18 O and

δ

cation depth during glacial intervals. Voelker and de
Abreu also evaluated changes in seasonality based on the
difference in δ

18 O between Globigerina bulloides and Glo-

borotalia in ata [Ganssen and Kroon, 2000]. As a result,

migration of subpolar and subtropical boundaries and hydro-
graphic fronts during abrupt climate events were identified.
Voelker and de Abreu suggest that nutrient levels and thus
ventilation of the upper 400 m of the water column were not
driven by millennial-scale events. In contrast, millennial-
scale oscillations in ventilation were recorded in the inter-
mediate to bottom waters, indicating the status of the
overturning circulation either in the North Atlantic or in the
Mediterranean Sea that admixed deep

uxes and
deep ocean carbon dioxide release to the atmosphere and
phasing between these features and the position of the west-
erlies [Anderson et al., 2009], (5) use of dynamic circulation
proxies (Pa/Th, Nd isotopes, etc.) and models of freshwater
forcing in assessing the strength of MOC, and (6) role of the
Antarctic Intermediate Water in distributing heat and trans-
porting old carbon around the ocean [Marchitto et al., 2007;
Basak et al., 2010].
Members of the paleoclimate modeling community
stressed the need to improve the temporal resolution and age
constraints on paleoclimatic records. In addition, it was sug-
gested to further explore

owing Mediterranean

Over ow Water to the Glacial North Atlantic Intermediate

Water. One of the important aspects of Voelker and de
Abreu is contribution is the detailed documentation of the

upper water structure during the penultimate glacial (marine
isotope stage (MIS) 6) that has been characterized by only
scarce data thus far.
Flower et al. [this volume] review planktonic Mg/Ca-

'

18 O
evidence from the Gulf of Mexico (GOM) to investigate the
role of meltwater input from the LIS in abrupt climate change
during MIS 3 and the last deglaciation. The chapter provides
an important summary of the current understanding of the
ACC in the GOM by synthesizing data mostly presented by
Flower et al. [2004], Hill et al. [2006], and Williams et al.
[2010]. These data show that the ice volume - corrected sea-
water

resolve
the leads and lags in important paleorecords, and provide
benchmark tests for climate sensitivity analysis. For exam-
ple, oxygen isotopes in speleothems could indicate the
amount of precipitation, changes in seasonality, or changes
in source region of moisture. More research is needed to
clarify the relationship of oxygen isotopes with these vari-
ables. Modelers also asked questions regarding the sources
of carbon dioxide increase during the Antarctic isotope max-
imum (AIM) warming events and factors that could be
attributed to the deep ocean warming during Heinrich events.

“ the meaning of proxies, ” δ

18 O in the GOM matches the East Antarctic ice core

δ

18 O[EPICA Community Members et al., 2006]. Flower et
al. [this volume] conclude that (1) LIS meltwater pulses
started during Heinrich stadials and lasted through the sub-
sequent D-O events; (2) LIS meltwater pulses appear to
coincide with the major AIM events; and (3) LIS meltwater
discharge is associated with distinct changes in deep ocean
circulation in the North Atlantic during H events. These
observations lead the authors to propose a direct link be-
tween GOM meltwater events and the weakening of the

δ

3. DISCUSSION OF MAJOR FINDINGS

3.1. Last Glacial-Interglacial Climate Cycle

After four decades of intense research, there are not
that many high-resolution (millennial to submillennial scale)