Derivation of paleoceanographic proxy values
• Reduced sea surface salinity and meltwater dilution of local
surface and subsurface waters are inferred from strongly reduced
planktic δ18O values of Neogloboquadrina
pachyd erma sin. (Nps) that in part also serve as tracer of
increased sea surface temperature (SST) both below an ongoing sea ice
cover in the Icelandic Sea and in the eastern Nordic Seas (Voelker,
1999; van Kreveld et al., 2000; Sarnthein et al., 2001; Simstich et al.,
2003; Sadatzki et al., 2020).
• Straight estimates of SST are derived from census counts of
planktic foraminiferal species (Pflaumann et al., 2003; Sarnthein et
al., 2001; Voelker, 1999).
• The particular minimum of bottom water temperature (BWT) at
Site PS2644 has been deduced from maximum epibenthic
δ18O values measured on single specimens of epibenthicCibicides lobatulus and C. wuellerstorfi (suppl. by 0.64 ‰
for each value; data of Voelker, 1999). Actually, interpretation of this
record is complex, since each sediment sample is providing a 1–2 ‰
broad array of isotope values as result of seasonal and/or interannual
temperature oscillations, if we assume a widely constant bottom water
salinity. For the interval 22–18.4 cal. ka, however, the maximum value
of each δ18O array is strictly confined to 5.6 ‰ per
mil, in contrast to the time span 18.4–15.1 cal. ka, where maximum
δ18O values are confined to 4.8 ‰ (Fig. 4). The
outlined difference by 0.8-‰ may serve to constrain a short-term major
shift in minimum bottom water temperature corresponding to
~3.4°C (following the conversion of
δ18O values by Shackleton, 1974; Ganssen, 1983).
• Short-term oscillations of BWT were also
derived from Mg/Ca ratios of C. neoteretis at neighbor Site
GS15-198-36CC for stadials GS4 to GS9 and interstadials GI5-GI8 during
MIS3 (Sessford et al., 2018). The latter temperature record forms a
valuable analog to identify potential differential bottom water sources
assumed for the interstadial scenario of late LGM and the onset and
culmination of stadial HS1.
• Different levels of bottom water ventilation and their
seasonal and/or interannual variability at PS2644 are traced by means of
epibenthic δ13C values of single specimens of
epibenthic C. lobatulus (Fig. 4). Analytical details of measuring
stable C and C isotope data of single specimens are given in Voelker
(1999).
• Spatial and temporal MRA variations of subsurface waters(Table 1) are deduced by means of 14C plateau tuning
outlined in the main text and presented in 14C yr
(details in Sarnthein et al., 2020). Low MRA serve as tracer of open,
high MRA as tracer of impeded CO2 exchange surface
waters with the atmosphere. At Site PS2644 the exchange was blocked by
long-term predominant sea ice cover (per analogy to MIS3; Sadatzki et
al., 2020) and by Arctic sourced waters of the EGC (Fig. 2a). Likewise,
MRA at Site GIK23074 are temporarily very high (~2000 yr
and more), hence also suggesting a lid of Arctic sourced surface waters
(Fig. 2b).
• (Raw, i.e., uncorrected) estimates of ventilation
ages of bottom waters (Table 1) record their last contact with the
atmosphere and form a robust tracer of deep-water masses since their
last contact with the atmosphere (Matsumoto, 2007). The short-term age
variations are simply deduced from the age difference between paired
epibenthic and planktic 14C ages in addition to the
paired planktic MRA (following rules defined by Cook and Keigwin, 2015,
and Sarnthein et al., 2020). Temporal and spatial differences in benthic
ventilation age serve as tracer of a differential origin of bottom
waters either in the Nordic Seas and/or the North Atlantic. Brine
water-derived bottom waters are earmarked by the close affinity of their
ventilation age to paired high planktic MRA and by ’aberrant’ light
benthic δ18O values that closely reflect the
δ18O level of nearby of surface waters freezing during
late summer (Bauch and Bauch, 2001).
• The geometry of past bottom water circulation is further
constrained by authigenic and detrital Nd and Pb isotopes at Site
PS2644. We use both isotope systems together in order to constrain the
provenance of sediment and deep-water currents. However, it is well
established that the authigenic sediment fraction in regions of volcanic
input can be overprinted in situ by detrital contributions of radiogenic
Nd; accordingly, the authigenic Nd isotope data must be interpreted with
caution (Elmore et al., 2011, Blaser et al., 2016). We thus largely
constrain out interpretations on the radiogenic isotope data of the
detrital sediment fraction (see also Struve et al., 2019).
Nd and Pb isotope data were analysed from the same solutions. Sediments
were leached with a weak acid-reductive solution as described by Blaser
et al. (2020) and Blaser et al. (2016). Afterwards the remaining
sediment was digested with an automated micro wave system employing a
mixture of HNO3 and HBF4 under high
pressure and temperature. The sample solutions were purified with
established column chromatography methods (Blaser et al. (2016), Gutjahr
et al. (2007)) and their isotopic ratios measured with a Neptune Plus
MC-ICP-MS at the University of Lausanne.
For Nd isotopes, instrument-induced mass fractionation was corrected to
a 146Nd/144Nd value of 0.7219. The
corrected 143Nd/144Nd ratios were
then normalised to the accepted value of 0.512115 based on repeatedly
measured JNdi-1 standard solutions (Tanaka et al., 2000). Nd isotope
signatures are reported as εNd =
([(143Nd/144Nd)sample/ (143Nd/144Nd)CHUR] − 1) ∗ 10,000, where
(143Nd/144 Nd)CHUR =
0.512638 (Jacobsen and Wasserburg, 1980). The reproducibility was
determined via in-house standard solutions to be 0.3 εNd units (2
standard deviations).
Pb isotopes were measured with a Tl-doping technique for exponential
mass bias correction and all three isotopic ratios were normalised to
SRM NBS 981 (Gutjahr et al., 2007, Galer and Abouchami, 1998).
Reproducibilities of 206Pb/204Pb
varied between 0.3 and 4.3 * 10-3, which is far
smaller than the variation observed in sediments of Site PS2644 or the
regional end members.
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REFERENCES
Andrews, J.T., Voelker, A.H.L. 2018: ”Heinrich events” (& sediments): A
history of terminology and recommendations for future usage.Quaternary Science Reviews 187, 31-40.