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The inclusion of freshwater forcing in the model runs to drive the AMOC state
through a collection of partial hysteresis cycles, so-called
first-order reversal curves
(FORC), demonstrated that large areas of the Atlantic exhibit an asymmetrical
temperature response to a declining and then recovering AMOC. At central water
depths the temperature of the eastern subtropical gyre shows an approximately
reversible relationship with AMOC evolution through a given FORC (Heslop and
Paul
2012
). This region is therefore suitable to derive high-
delity records of past
AMOC activity on the basis of proxy seawater temperature reconstructions. In
contrast, other areas of the Atlantic exhibit strongly nonreversible responses, which
imply that similar temperatures during the cooling and subsequent warming of a
given episode cannot be assumed to correspond to the same AMOC strength.
We calibrated Mg/Ca ratios and
18
O in tests of H. balthica to bottom water
temperature (BWT) analyzing core tops from Indonesian and northeast Atlantic
depth transects. The resulting calibration, Mg/Ca (mmol mol
−
1
) = (0.488
ʴ
±
0.03)
BWT (
C), demonstrates H. balthicatests exhibit a temperature sensitivity four
times higher than other studied deep-sea benthic foraminifera. Small calibration
uncertainties [
°
±
0.7
°
C,
±
0.32 % VSMOW and
±
0.69 psu (2
σ
)], allow estimation of
seawaterdensity to <0.3
cient to reconstruct changes in
Holocene water mass properties and provenance (Rosenthal et al.
2011
).
To estimate the contribution of central water circulation to the AMOC and its
role in natural climate variability we applied the H. balthica calibration equation to
high resolution multi- (GeoB6007-1) and gravity-cores (GeoB6007-2/OC437-7 24
GGC) from the eastern boundary of the subtropical gyre. We show that strong sea-
surface heat loss (Westerlies) and enhanced Arctic freshwater exports resulted in
cooler [(
σ
ʸ
units, which is suf
0.2) central waters during positive
NAO phase shifts (past 165 years) and pronounced solar activity minima over the
past 2.7 thousand years (ka) before present (BP) (Fig.
2
) (Morley et al.
2011
,
2014
).
Over the transition from the Holocene Thermal Maximum to the cooler Late
Holocene (3.3
0.8
±
0.7)
°
C] and lighter (
σ
ʸ
=
0.3
±
−
−
2.6 ka BP), core OC437-7 24 GGC shows ENACW cooling of
approximately (1
-
0.2 (Fig.
2
). This
appears to be a dynamic response of ENACW circulation to changing ocean-
atmosphere circulation patterns forced by a gradual strengthening of latitudinal
temperature gradients. Further, we identi
±
0.7)
°
C and decreased densities of
σ
ʸ
= 0.4
±
ed ENACW circulation as an amplifying
feedback, which will help to constrain how regional climate change affects hemi-
spheric wide climate linkages (Morley et al.
2014
).
Given the current acceleration in Greenland Ice Sheet melting, we evaluated the
impact of early Holocene freshwater forcing on ENACW circulation. During times of
enhanced Laurentide Ice Sheet melting (9.0
8.5 ka BP) central water production
weakened. Additionally, two 150-year cooling events [(8.54
-
±
0.2) ka BP and
±
(8.24
0.1) ka BP] provide evidence for an abrupt central water response to the
drainage of Lake Agassiz (8.54 ka BP) in addition to the maximumAMOC slowdown
at 8.2 ka BP (Fig.
2
) (Bamberg et al.
2010
). These observations provide a possible
analogue for future effects on meridional heat transfer at central water depth from
enhanced Greenland Ice Sheet melting and increases in Arctic freshwater export.
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