Geoscience Reference
In-Depth Information
1 Introduction
The state of the North Atlantic Oscillation (NAO) in combination with subpolar gyre
dynamics determines regional sea-surface heat loss and winter convection over the
subpolar basin by modulating North Atlantic westerly wind stress and fresh water
budgets (Hatun et al.
2005
). During the winter, NAO modulated wind-stress favors
the subduction of Subpolar Mode Water, which in turn comprises a large fraction of
Eastern North Atlantic Central Water (ENACW) (Poole and Tomczak
1999
).
Flowing south along the eastern margin of the North Atlantic basin at densities
between
σ
ʸ
= 27.3 and 27.6 kg/m
3
, ENACW circulation thus contributes to the
ventilation of the thermocline and provides an
, transmitting subpolar
ocean-atmospheric climate signals to lower latitudes between 300 and 900 m water
depth throughout the Holocene (Bamberg et al.
2010
; Morley et al.
2011
,
2014
).
Little is known concerning the Holocene sensitivity, spatial and temporal
response of central water circulation to abrupt or gradual climate forcing on mul-
tidecadal to millennial timescales. Major limitations on our ability to reconstruct
central water properties arise from the scarcity of high resolution marine records
from central water depth, a lack of central water proxies and limited investigation of
central water circulation in numerical paleoclimate models. Using a combination of
paleoclimate
'
oceanic tunnel
'
fingerprinting and proxy development we have developed a new
method for reconstructing past central water properties on decadal to millennial
time scales. This uni
ed numerical and experimental approach has enabled us to
evaluate the spatial and temporal response of Holocene central water circulation to
various changes in climate forcing.
2 Material and Methods
For trace metal quanti
cation, we analyzed up to 25 tests of the benthic forami-
nifera Hyalinea balthica from the 250 to 350
µ
m size fraction (mean: 20 individ-
uals), using a modi
ed reductive, oxidative cleaning protocol (Barker et al.
2003
)
and a Sector Field Inductively Coupled Plasma Mass Spectrometer (Thermo Ele-
ment XR) at Rutgers Inorganic Analytical Laboratory. The long-term analytical
precision of Mg/Ca ratios based on repeated analysis of three consistency standards
was 1.6, 1.2 and 1.2 % RSD (relative standard deviation), respectively. The isotopic
oxygen and carbon compositions (
ʴ
13
C) of the foraminiferal shells were
measured at the Stable Isotope Laboratory of the University of Bremen using a
Finnigan MAT 251 mass spectrometer equipped with an automatic carbonate
preparation device. Internal precision, based on replicates of a limestone standard,
was better than
18
O and
ʴ
0.07 % (VPDB).
Palaeoclimatic modeling was performed with the University of Victoria Earth
System Climate Model (UVic ESCM version 2.8, Weaver et al.
2001
) consisting of a
two-dimensional atmospheric energy-moisture balance model, a dynamic-
thermodynamic sea ice model and a three-dimensional ocean model (MOM2).
±
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