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.
=============================================================
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.