G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 7, Number October 2006 Q10P17, doi: /2005gc ISSN: Neogloboquadrina pachyderma (dex. and sin.) Mg/Ca and d 18 O records from the Norwegian Sea Birgitte F. Nyland and Eystein Jansen Bjerknes Centre for Climate Research, University of Bergen, Allégaten 55, 5007 Bergen, Norway (birgitte.nyland@bjerknes.uib.no) Department of Earth Sciences, University of Bergen, Allégaten 41, 5007 Bergen, Norway Henry Elderfield Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Carin Andersson Bjerknes Centre for Climate Research, University of Bergen, Allégaten 55, 5007 Bergen, Norway [1] Magnesium/calcium (Mg/Ca) records based on N. pachyderma (dex.) and N. pachyderma (sin.) from the last 1200 years have been retrieved from high-resolution cores in the Norwegian Sea. Comparing temperatures inferred from Mg/Ca and the oxygen isotopic composition of calcite (d 18 O C ) with instrumental temperature records from Ocean Weather Ship Mike suggests that N. pachyderma calcifies during the summer season with an offset to oxygen isotope equilibrium of 0.6%. In this region, summer temperatures below the thermocline are related to the winter season ventilation and breakdown of the seasonal thermocline; hence the deeper dwelling N. pachyderma (sin.) provides a winter season signal. Down-core N. pachyderma (dex.) Mg/Ca temperatures display larger variance than observed in d 18 O C temperatures record derived from the same morphotype. The smaller variance in the d 18 O C N. pachyderma (dex.) temperatures is probably linked to salinity changes in the upper 50 m of the water column. The Mg/Ca temperature and d 18 O C records are used to reconstruct changes in the oxygen isotopic composition of water (d 18 O w ) during the last 1200 years. The reconstructed d 18 O W records show variation within a realistic range, allowing the influence of other water masses than those present at the site today, and suggest that the Mg/Ca signal in morphotypes of N. pachyderma in the Norwegian Sea responds to changes in climatic parameters. Components: 9424 words, 8 figures, 2 tables. Keywords: Mg/Ca; planktonic foraminifera; Norwegian Sea. Index Terms: 4954 Paleoceanography: Sea surface temperature. Received 26 June 2005; Revised 7 April 2006; Accepted 19 June 2006; Published 12 October Nyland, B. F., E. Jansen, H. Elderfield, and C. Andersson (2006), Neogloboquadrina pachyderma (dex. and sin.) Mg/Ca and d 18 O records from the Norwegian Sea, Geochem. Geophys. Geosyst., 7, Q10P17, doi: /2005gc Theme: Development of the Foraminiferal Mg/Ca Proxy for Paleoceanography Guest Editor: Pamela Martin Copyright 2006 by the American Geophysical Union 1 of 18

2 1. Introduction [2] Reconstructions of past temperature changes in the Norwegian Sea from before the instrumental record have been based on indirect indicators such as variations in diatom assemblages [Koç and Jansen, 1992; Birks and Koç, 2002], alkenones [Calvo et al., 2002], relative abundance of foraminifers [Andersson et al., 2003; Risebrobakken et al., 2003], and d 18 O in foraminifers [Dokken and Jansen, 1999; Berstad et al., 2003; Risebrobakken et al., 2003; Kristensen et al., 2004]. Recently, the correlation between temperature and Mg in biogenic calcite has been studied extensively, although mostly in other regions [e.g., Cronblad and Malmgren, 1981; Nürnberg et al., 1996; Mashiotta et al., 1999; Elderfield and Ganssen, 2000; Lea et al., 2002; Martin et al., 2002; Lea et al., 2003]. Larger amounts of magnesium are incorporated into foraminiferal calcite with increasing temperature [e.g., Chave, 1954; Nürnberg, 1995; Lea et al., 1999; Anand et al., 2003]. Salinity and ph are secondary factors that exert minor influences on shell Mg [Nürnberg et al., 1996; Lea et al., 1999]. Mg/Ca temperature sensitivity in N. pachyderma (dextral) and N. pachyderma (sinistral) has been verified in numerous studies [Nürnberg, 1995; Nürnberg et al., 1996; Elderfield and Ganssen, 2000; Pak et al., 2004; von Langen et al., 2005] and is a useful proxy for palaeotemperature reconstructions [Mashiotta et al., 1999; Kiefer and Kienast, 2005]. However, few works have been performed on Mg/Ca in N. pachyderma (dex. or sin.) in high latitude regions [Nürnberg, 1995; Meland et al., 2005, 2006], although the species, N. pachyderma (sin.) in particular, is widely used within the palaeoceanographic community. Studies from different areas have shown that the regional distribution of N. pachyderma is controlled by water temperature and that its vertical distribution is related to the depth of the pycnocline [Kohfeld et al., 1996; Simstich et al., 2003; Kuroyangi and Kawahata, 2004]. N. pachyderma (sin.) is a dominantly polar-dwelling species, which thrives in cold waters at high latitudes [Pflaumann et al., 1996] whereas the right-coiling morphotype is typically found in subpolar and transitional waters [Bé and Tolderlund, 1971]. [3] The aim of this study is to test the fidelity of Mg/Ca in planktonic foraminifera as a palaeoclimatic proxy in high latitude regions. We present down-core Mg/Ca records from two planktonic foraminifera, N. pachyderma (dex.) and N. pachyderma (sin.), and compare them with paired d 18 O C records of the same species generated by Andersson et al. [2003]. Two sediment cores from a highlatitude site in the Norwegian Sea covering the last 1200 years were analyzed. The site is located underneath the Norwegian Atlantic current, which brings warm, saline Atlantic water (T > 3 C, S > 35% [Swift, 1986]) northward toward the Arctic. Speculations about calcification habitat are constrained by comparing geochemical data from box core surface sediments with instrumental observations from Ocean Weather Station Mike (OWSM) [Østerhus et al., 1996] located in the Norwegian Sea (Figure 1). The advantage of Mg/Ca as an independent proxy for temperature is that it may be used together with oxygen isotopes in calcite (d 18 O C ) to reconstruct changes in the oxygen isotope composition of seawater (d 18 O W ), a parameter that correlates with salinity. This study shows overall consistency in the reconstructed d 18 O W curves, indicating that Mg/Ca changes in N. pachyderma (both sin. and dex.) from high latitudes represent a climate signal. 2. Material and Age Control [4] The two cores (MD and JM97-948/2A) were recovered from the same location on the Vøring Plateau in the Eastern Norwegian Sea (66 N, 7 E) at a water depth of 1048 m (Figure 1). Box core JM97-948/2A was taken to recover surface sediments and represents the last 564 years. MD is m long, however, only the upper 90 cm were examined in this study. Together with the box core this represents the last 1200 years. The age-depth models for both cores are adopted from Andersson et al. [2003]. The agedepth model for the box core was reconstructed on the basis of nine 210 Pb dates from the uppermost 10 cm and two Accelerator Mass Spectrometry (AMS) 14 C dates. The age-depth model for the upper 90 cm in MD rests on five AMS 14 C dates. The 14 C dates were calibrated using DR =0 and converted to calendar ages using Calib 4.3 [Stuvier et al., 1998]. [5] The site has a high sedimentation rate: 53 cm/kyr on average in the box core, whereas the piston core suggests a sedimentation rate of 147 cm/kyr on average. Piston core sedimentation rates are normally expected to be less than in the box cores due to increased compaction of overlaying sediment. However, this appeared to not be the case and the suggested age-depth model shows an increased and unrealistic sedimentation rate between cm (AD ) of 268 cm/kyr. This is caused by the 2of18

3 Geosystems G 3 nyland et al.: norwegian sea mg/ca records /2005GC Figure 1. Map of study area showing principal currents: thin arrows denote surface currents; thick arrow denote deep-water flow. The large dot denotes the core site, and the small dot denotes Ocean Weather Ship Mike. coring technique which stretched the sediment in the piston core and is a persistent problem with older Marion Dufresne cores [Széréméta et al., 2004]. This is unlikely to affect the reliability of the results assuming that the sediment was not mixed. The box and piston cores were sampled every 0.5 and 1.0 cm, respectively. According to the agedepth model developed for the box core, the most recent N. pachyderma (sin.) Mg/Ca measurement is about 1975 AD. Since several data points from the uppermost 26 cm in the Mg/Ca N. pachyderma (dex.) record, including the core top, were rejected, the most recent Mg/Ca sample in this record is found at 6.5 cm and is dated to about 1937 AD. Almost all individuals in the samples were required for single analyses of both Mg/Ca and oxygen isotopes; thus our ability to analyze replicate samples was limited. 3. Methods 3.1. Mg/Ca Analyses [6] The samples were cleaned before trace element analysis to eliminate contamination by magnesium from surrounding sediments. The sample preparation for Mg/Ca analysis follows the cleaning protocol of Barker et al. [2003], which is a modified version of the cleaning protocol for cadmium analysis in biogenic calcite of Boyle [1981] and Boyle and Keigwin [1985]. Around individuals were used for the Mg/Ca analyses. Tests in samples from the box core and the upper 26 cm in MD were picked from the mm fraction. Individuals in samples from cm ( AD) in MD were picked from a limited fraction range ( mm) since a large range in test size results in larger Mg/Ca variability [Anand et al., 2003]. The analyzed foraminifera show no visible sign of dissolution, which is in agreement with observation of well-preserved carbonate tests found along the pathway of inflowing Atlantic water in a study by Huber et al. [2000]. [7] All cleaning preparation and Mg/Ca analyses were performed at the Department of Earth Sciences, University of Cambridge, UK. The samples were analyzed using a Varian Vista ICP-AES axial viewed instrument [de Villiers et al., 2002]. Measurements of a standard solution of Mg/Ca (5.13 mmol/mol) run in parallel had an analytical error of <0.011 mmol/mol (1s). At low Ca concentrations the content of magnesium may 3of18

4 rise to unrealistic levels due to the blank effect. Samples with Ca concentration below 30 ppm were therefore excluded as were samples enriched in Fe/Ca (above 0.1 mmol/mol) and/or Al/Ca since these elements are likely to be found in contaminants that may also contain high amounts of magnesium [Barker et al., 2003]. [8] 28 duplicate Mg/Ca N. pachyderma (sin.) measurements were performed on surface and LGM samples from the Nordic Seas [Meland et al., 2006]. A pooled standard deviation of the residuals with 56 degrees of freedom indicates reproducibility within ±0.08 mmol/mol. This equals to an error of ±0.8 C for Mg/Ca in N. pachyderma (sin.) within the concentration range used in this study according to Elderfield and Ganssen [2000] and is adopted in this study as a general estimate of the reproducibility of each sample. The Mg/Ca temperature calibration of Elderfield and Ganssen [2000] was used to convert Mg/Ca concentrations in N. pachyderma (dex. and sin.) to temperatures (see section 5.2) Oxygen Isotope Measurements [9] Paired oxygen isotope measurements on the same samples were generated previously [Andersson et al., 2003]; therefore Mg/Ca data were not generated on the same homogenized samples used for d 18 O. The measurements were performed at the Geological Mass Spectrometer (GMS) laboratory at the University of Bergen, using Finnigan MAT 251 and MAT 252 mass spectrometers both equipped with automatic preparation lines. Long-term replicate measurements of a carbonate standard gave a reproducibility of ±0.07% for the oxygen isotope measurements. Duplicate d 18 O measurements of 39 core top samples of N. pachyderma (sin.) have been reported elsewhere and show a modified standard deviation of ±0.13% (0.5 C) [Meland et al., 2006]. The d 18 O results are reported in % versus VPDB, calibrated against NBS-19 and NBS-18. A vital effect of 0.6% was added to the isotopic measurements before the palaeotemperature equation of O Neil et al. [1969], as cited by Shackleton [1974], was applied to calculate temperatures from the oxygen isotopic measurements (see section 5.3). [10] Changes in the local oxygen isotopic composition in seawater (d 18 O W ) were derived from d 18 O C measurements (%) and Mg/Ca temperatures (T C) by rearranging the palaeotemperature equation of O Neil et al. [1969] and Shackleton [1974] (equation (1)). A factor of 0.27 was used to convert the calcite value from the PeeDee Belemnite (PDB) to the Standard Mean Ocean Water scale (SMOW). d 18 O w ¼ d 18 O c þ 0:27 2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 4:38 4:38 2 0:4 * 16:9 T Mg=Ca 4 5 ð1þ 0:2 4. Down-Core Results 4.1. Oxygen Isotopes [11] The d 18 O C values from right- and left-coiling N. pachyderma for the last 1200 years have averages of 1.34% and 2.02% (Figure 2, Table 1) and a similar variance of 0.23% and 0.28%, respectively. The records are in general visually strongly related. A cooling of 2 C (0.5%) from AD is synchronous in the d 18 O C records for both morphotypes. The heaviest isotopic values are seen between 1400 and 1900 AD, suggesting colder conditions in the period. The isotopic variability in N. pachyderma (sin.) is less in this period compared to an earlier period ( AD). A discrete cooling event at 1900 AD is observed in both records, which is more pronounced in the d 18 O C record of N. pachyderma (dex) Mg/Ca [12] The Mg/Ca records in N. pachyderma (dex.) and N. pachyderma (sin.) have average values of 1.17 mmol/mol (1s = 0.116) and 0.94 mmol/mol (1s = 0.087), respectively (Figure 3, Table 1). The N. pachyderma (dex.) Mg/Ca results show two periods at and with temperatures about C warmer than the 20th century (Figure 4a). These warm intervals are apparent in the oxygen isotope record of N. pachyderma (dex.) (Figure 4a). Between AD temperatures are generally warmer than the subsequent period, but are interrupted by colder spells. Cooling of 2 C in the Mg/Ca temperatures of N. pachyderma (dex.) at 1050 and 1400 AD is mirrored in the oxygen isotope-derived temperatures, but not at 1200 AD. The N. pachyderma (sin.) Mg/Ca record exhibits weaker warming and cooling trends between AD. At 1360 AD the Mg/Ca concentrations drop suddenly, the most prominent temperature minimum in the record, whereas the d 18 O C N. pachyderma (sin.) indicate a smaller temperature decrease. Both records show a following increase in temperature. The smoothed temperatures derived from Mg/Ca 4of18

5 Figure 2. Down-core oxygen isotope records (% versus VPDB) in N. pachyderma (dex.) and N. pachyderma (sin.) in JM97-948/2A and MD Colored thick lines show the 5-point moving average. Double-headed arrow marks the pooled standard deviation of the reproducibility of ±0.13% in d 18 O. Triangles along the bottom axis indicate the AMS 14 C dates of JM97-948/2A and MD of N. pachyderma (dex.) show variance twice as high as those deduced from N. pachyderma (sin.); 0.57 and 0.25, respectively. The visual correlation between the smoothed Mg/Ca curves of the two morphotypes changes after 1500 AD: there is a negative correlation from AD but a more positive correlation from 1500 and 1900 AD. A cooling event seen in both Mg/Ca records at 1900 AD and the observed subsequent warming are also synchronous with the oxygen isotope records. 5. Discussion 5.1. Constraints on Calcification Habitat [13] Bauch et al. [2003] suggests that the identification of N. pachyderma (sin.) based on morphologic distinction is not suitable when the percentage of N. pachyderma (dex.) is less than 5%. On the basis of abundance data from Andersson et al. [2003] (40 70% N. pachyderma (dex.) and 10 30% N. pachyderma (sin.) through the last 3000 years) the N. pachyderma morphotypes in our site are distinguishable and questions regarding calcification habitat are therefore discussed separately for each of the morphotypes. [14] Studies suggest that N. pachyderma (sin.) calcifies within the mixed layer and sinks below the pycnocline to reproduce and gains secondary calcite [Kohfeld et al., 1996; Simstich et al., 2003]. This modifies the shell s geochemistry such that the end product reflects colder temperatures, giving an apparent calcification depth of between m depending on the depth of the pycnocline. The depth of the pycnocline in the Norwegian Sea is within the upper 100 m [National Oceanographic Data Center, 1998]. It is generally assumed that N. pachyderma (dex.) also migrates through the water column during reproduction. [15] Temperatures for the last 50 years inferred from N. pachyderma (dex.) oxygen isotopes are 8.5 C (±0.5 C) on average after adjustment for the vital effect (section 5.3), while temperatures derived from oxygen isotopes and Mg/Ca in N. pachyderma (sin.), indicate temperatures around 6 C (±0.5 C) and 7 C (±0.8 C), respectively (Figures 4a and 4b). Calcification habitat can be further constrained when these temperatures are compared to instrumental data from OWSM (Figure 1). Water with temperatures within the range of reconstructed temperatures from N. pachyderma (dex.) is found at the surface in spring and at around 50 m during summer (Figure 5a). An apparent calcification at the sea surface is regarded as implausible since N. pachyderma (dex.) is believed to dwell in sub-surface water, not at the surface [Pak and Kennett, 2002; Berstad et al., 5of18

6 Table 1. Summary of d 18 O c and Mg/Ca in N. pachyderma (dex.) and N. pachyderma (sin.) From JM97-948/2A and MD a N. pachyderma (dex.) N. pachyderma (sin.) Core Core Depth, cm d 18 O % Versus VPDB Mg/Ca, mmol/mol d 18 O % Versus VPDB Mg/Ca, mmol/mol JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A JM97-948/2A MD MD MD MD MD MD of18

7 Table 1. (continued) N. pachyderma (dex.) N. pachyderma (sin.) Core Core Depth, cm d 18 O % Versus VPDB Mg/Ca, mmol/mol d 18 O % Versus VPDB Mg/Ca, mmol/mol MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD of18

8 Table 1. (continued) N. pachyderma (dex.) N. pachyderma (sin.) Core Core Depth, cm d 18 O % Versus VPDB Mg/Ca, mmol/mol d 18 O % Versus VPDB Mg/Ca, mmol/mol MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD MD a Oxygen isotope records are adopted from Andersson et al. [2003]. 2003; Kuroyangi and Kawahata, 2004, von Langen et al., 2005]. Therefore we suggest that the reconstructed d 18 O C temperatures indicate calcification depths at 50 m for N. pachyderma (dex.). Proxy temperatures derived from N. pachyderma (sin.) indicate an apparent calcification depth at 100 m or deeper. These findings are in agreement with previous studies of N. pachyderma (dex.) and N. pachyderma (sin.) in the Norwegian Sea [Berstad et al., 2003; Simstich et al., 2003] and of N. pachyderma (sin.) in the polar and northern arctic water masses [Kohfeld et al., 1996; Bauch et al., 1997]. The uncertainties in the methods result in a large range in calcification depth especially of Figure 3. Mg/Ca concentration (mmol/mol) records in cores JM97-948/2A and MD Thick red and blue lines show the 5-point moving average of the Mg/Ca concentrations in N. pachyderma (dex.) and N. pachyderma (sin.), respectively. Double-headed arrow shows the pooled standard deviation of the reproducibility of ±0.08 mmol/ mol in Mg/Ca. Black triangles denote the radiocarbon dates. 8of18

9 Figure 4. (a) Mg/Ca and d 18 O N. pachyderma (dex.) inferred temperature records from JM97-948/2A and MD The upper, half stippled, light green curve shows oxygen isotope temperatures inferred from the palaeotemperature equation of O Neil et al. [1969] and Shackleton [1974]. The stippled, dark green curve shows oxygen isotopic values with a vital effect of 0.6% added. The solid curve denotes Mg/Ca temperatures calculated according to Elderfield and Ganssen [2000] (equation (2b) in text). Each record is smoothed with a 5-point moving average. The double-headed arrows show the pooled standard deviations of the reproducibility (in temperature) for the smoothed records in each of the proxies (black for Mg/Ca and green for d 18 O). (b) As Figure 4a but inferred N. pachyderma (sin.) proxy temperatures. N. pachyderma (sin.) because the temperature gradient is relatively small below the seasonal thermocline within the Atlantic water mass. Also, the season in which N. pachyderma (sin.) calcifies is difficult to determine with our data since the temperature differences between the seasons at the supposed calcification depth are smaller than the error of the methods. However, sediment trap studies reveal only one major production bloom in summer (July August-September) in the Nordic 9of18

10 Figure 5. (a) Seasonal and annual water temperature averages for the last 50 years from the Ocean Weather Ship Mike in the upper 100 m of the water column. The averages are based on the temperature records presented in Figure 5b. Green curve represents spring (May June), red represents summer (July September), blue represents winter (October April), and gray represents seasonal average. (b) Instrumental temperature records for the last 50 years at 0, 10, 25, 50, and 100 m water depth at Ocean Weather Ship Mike during winter (October April), spring (May June), and summer (July September). Seas [Simstich et al., 2003, and references therein], which is in agreement with the findings of maximum flux for N. pachyderma (sin.) between August and September in Arctic water [Kohfeld et al., 1996]. We therefore assume that the calcification of N. pachyderma (sin.) occurs in summer Reconstruction of Past Temperatures: Mg/Ca Calibration [16] Several studies suggest an exponential correlation between N. pachyderma Mg/Ca concentration ratio and temperature [e.g., Nürnberg, 1995; Elderfield and Ganssen, 2000, von Langen et al., 2005]. The appropriate choice of which calibration to use is crucial when reconstructing past temperatures. The selection of calibration is based on the most recent N. pachyderma (sin.) Mg/Ca samples since these can be linked to the instrumental temperature record, whereas no N. pachyderma (dex.) Mg/Ca results are within in this 50-year time window. To decide which temperature calibration to apply, the reconstructed temperatures calculated according to the multispecies calibration approach of Elderfield and Ganssen [2000] (equation (2a)) and the species-specific calibration based on N. pachyderma (sin.) of Nürnberg [1995] (equation (2b)) were compared to instrumental data from OWSM. Mg=Ca ¼ 0:52 expð0:10 * TÞ ð2aþ Mg=Ca ¼ 0:55 expð0:099 * TÞ ð2bþ 10 of 18

11 [17] The past 50 years of the record show temperatures of 6 7 C according to Elderfield and Ganssen [2000]. This is closer to the observed water temperatures at depths around 100 m at OWSM (Figure 5a), which is believed to be the calcification depth of N. pachyderma (sin.) [Kohfeld et al., 1996; Bauch et al., 1997; Simstich et al., 2003], than to temperatures according to Nürnberg [1995] (5 6 C). The Mg/Ca cleaning protocol and analysis used in this study is similar to the method Elderfield and Ganssen [2000] used to generate data for their calibration (although cleaning protocol has improved since that study), whereas the data used in the calibration of Nürnberg [1995] was obtained by electron microprobe analysis, which suggests that the first calibration is more appropriate. Mg/Ca temperatures calculated using von Langen et al. [2005] calibration, based on cultured N. pachyderma (dex.), have similar constants to Elderfield and Ganssen [2000] (with speciesspecific pre-exponential constant) and thus gives similar temperatures. Therefore the Mg/Ca temperature reconstructions (both left- and right-coiling morphotypes) were calculated according to Elderfield and Ganssen [2000] (equation (2a)) Oxygen Isotope Temperatures and Vital Effect [18] Different equations have been proposed to convert d 18 O in calcite into temperatures. In this study we have used the O Neil et al. [1969] and Shackleton [1974] palaeotemperature equation as it is based on benthic foraminifers dwelling in colder water rather than Bemis et al. [1998], which are based on planktonic foraminifera dwelling in warmer water (15 25 C). The equation used is widely applied in the determination of palaeotemperatures from high-latitude planktonic species. [19] Modern d 18 O W data collected in the upper 300 m of the Norwegian Sea (5 W 7 E, N, S >35%, T>3 C) show an average of 0.3%, which is used when calculating isotopic temperatures [Schmidt et al., 1999]. The d 18 O C temperatures of N. pachyderma (dex.) and N. pachyderma (sin.) ranges between 8 13 C (mean = 11 C) and 6 11 C (mean = 8 C) respectively. The mean temperature derived from N. pachyderma (dex.) would suggest a calcification depth at the sea surface and is in the same range as temperatures derived from diatoms and alkenones representing sea surface conditions [Calvo et al., 2002; Jansen and Koç, 2000]. We find these temperature estimates to be highly unlikely, as explained in section 5.1. The d 18 O C temperatures deviate by 2 3 C from the Mg/Ca temperature records of the same morphotypes (Figures 4a and 4b). Deviation between Mg/ Ca and d 18 O C N. pachyderma (dex.) derived temperatures has also been found in a study on sediment trap samples from the Santa Barbara Basin by Pak et al. [2004]. The weak correlation in their study was linked to the d 18 O C temperature calibration equation, seasonal or interannual changes in d 18 O W, inaccuracies in the Mg temperature equation, or secondary effects on either Mg/ Ca or d 18 O C. [20] Assuming Mg/Ca derived temperatures represent true values, the reason for deviation of 2 3 C between the two proxies can lie in the reconstructed d 18 O C temperatures as the morphotypes of N. pachyderma build their calcite shells in disequilibrium with predicted inorganic d 18 O (d 18 O Eq ). It has been hypothesized that the disequilibrium is linked to postgametogenetic processes as planktonic foraminiferas continue to calcify when they sink through the water column [Bemis et al., 1998, and references therein]. The d 18 O C may be further modified by selective dissolution since enriched gametogenetic calcite is more resistant than primary calcite [Bemis et al., 1998, and references therein]. [21] Predicted inorganic d 18 O Eq are estimated from Mg/Ca temperatures inferred from six N. pachyderma (sin.) samples coincident with the instrumental record so that modern d 18 O W can be used in the palaeotemperature equation of O Neil et al. [1969] and Shackleton [1974] (Table 2). The median offset between predicted and measured d 18 O C is 0.59% (n = 6). Two of the values are unrealistically low because Mg/Ca is high, despite all concentrations of potential contaminating elements being within threshold values. When these samples are excluded the average disequilibrium is 0.6%. The d 18 O C N. pachyderma (sin.) temperatures found in this study were thus estimated with an adjustment for the vital effect of 0.6% (Figure 4b). The same vital effect of 0.6% was adopted for calculating d 18 O C temperatures from N. pachyderma (dex.), because it was not possible to calculate a specific d 18 O Eq as its Mg/Ca record does not contain samples comparable to the instrumental record. The N. pachyderma (dex.) d 18 O C temperatures show an offset to the d 18 O temperatures from N. pachyderma (sin.) of 2 3 C asis the difference between the reconstructed Mg/Ca temperature curves. The resulting oxygen isotope temperatures lie in the same range as the Mg/Ca 11 of 18

12 Table 2. Estimated d 18 O Vital Effect Calculated by Subtracting Predicted Inorganic d 18 O Eq From Measured in Shell d 18 O C in Six Samples of N. pachyderma (sin.) a Depth, cm Year AD b Predicted d 18 O Eq, % Measured d 18 O C, % Mg/Ca, C Vital Effect, % a The predicted d 18 O Eq is calculated from the palaeotemperature equation of O Neil et al. [1969] and Shackleton [1974] with Mg/Ca temperatures and modern d 18 O w of 0.3%. The samples used are all within the instrumental time period. b Age model is adopted from Andersson et al. [2003]. temperature record from N. pachyderma (dex.) and is therefore deemed acceptable (Figure 4). Simstich et al. [2003] suggests an offset between d 18 O C in N. pachyderma (sin.) and d 18 O Eq of 0.6% in surface samples from the Nordic Seas, after including a second correction for post depositional process related to bioturbational admixture of cold sub-fossiliferous foraminifers. This is in agreement with our estimate, though we do not believe that bioturbation is the cause. Also, a plankton tow study in polar regions found a similar disequilibrium correction (between %) for N. pachyderma (sin.) [Stangeew, 2001]. However, other works from the Okhotsk Sea and within polar and northern arctic water masses indicate a higher disequilibrium of d 18 O C in N. pachyderma (sin.) of about 1.0% [Bauch et al., 1997, 2002; Kohfeld et al., 1996, Volkmann and Mensch, 2001], while work on surface samples in Icelandic water found a lower vital effect of 0.25% [Smith et al., 2005] and studies on surface and LGM samples in Arctic water found that use of vital effect of 0.6% (from this study) when calculating temperature from d 18 O C N. pachyderma (sin.) gave an unrealistically deep calcification depth [Meland et al., 2006]. Contradicting studies indicate that this issue is not well constrained with a need for further study Difference in Variance in Down-Core Temperature Reconstructions [22] The two proxy records have similar variance in N. pachyderma (sin.) but dissimilar variance in N. pachyderma (dex.). In an attempt to resolve this, other Mg/Ca temperature calibrations were applied [Nürnberg, 1995; Anand et al., 2003], but they did not change the pattern of higher variability in the Mg/Ca temperatures of N. pachyderma (dex.). The variances in the Mg/Ca records are controlled by the temperature sensitivity that is expressed by the exponential constant of the equation (equation (2a)). By varying the exponential constant in the Mg/Ca equation we find that a value of 0.14 is needed to achieve a variance similar to that observed in the oxygen isotope temperature records (Figure 6). However, culture work on N. pachyderma (dex.) from the Santa Barbara basin shows a Figure 6. Temperature records inferred from Mg/Ca in N. pachyderma (dex.) using different temperature equations. Stippled black curves in both panels are the d 18 O C temperatures with a vital effect of 0.6%. (top) Mg/Ca N. pachyderma (dex.) temperatures calculated according to Elderfield and Ganssen [2000] (equation (2b) in text) (red curve). (bottom) Mg/Ca temperatures inferred using a modified equation with a temperature sensitivity of 0.14 (purple curve) which gives a similar variance to that observed in the d 18 O C smoothed temperature curve. 12 of 18

13 Mg/Ca temperature sensitivity of 0.1 [von Langen et al., 2005] and virtually all planktonic calibrations seem to converge on an exponential value of [Elderfield and Ganssen, 2000; Anand et al., 2003]; thus the value of 0.14 is rather high. Also, calculations according to Skinner and Elderfield [2005] suggest that the two morphotypes require a similar Mg/Ca calibration. The explanation may more likely lie in the lower variance of the d 18 O C temperatures of N. pachyderma (dex.). [23] The oxygen isotopic temperatures are based on biogenic d 18 O C which is affected by d 18 O W. Modern data show that d 18 O W in the study area (60 70 N, 5 E 2 W, S > 35%, upper 300 mwd) range from % [Schmidt et al., 1999]. This is equivalent to a DT value of 0.6 C when a fixed d 18 O C value of 1.4% is used in the palaeotemperature equation of O Neil et al. [1969] and Shackleton [1974]. This temperature uncertainty plays a critical role when considering high-resolution data over a narrow temperature range. Salinity and d 18 O W are correlated in the Nordic Seas, since salinity changes are caused by precipitation, river runoff or glacial meltwater [e.g., Bauch et al., 1995], whereby most sources of low salinity originate from generally cold water masses with reduced d 18 O. Thus changes in d 18 O C temperatures associated with movement of a thermal front may result in reduced calculated temperature levels relative to Mg/Ca temperatures if a change in d 18 O W is not accounted for within the palaeotemperature equation. This indicates that the variability between the Mg/Ca and oxygen isotope temperature reconstructions of N. pachyderma (dex.) may, in part, be explained by changes in d 18 O W N. pachyderma (sin.): A Winter Signal [24] Differences in patterns between species, as observed in the d 18 O C and Mg/Ca records in this study, have been used by others as an argument that the two species calcify in different seasons [Berstad et al., 2003]. Figure 5b show that the summer temperatures at 0 and 100 m at Ocean Weather Ship Mike during the last 50 years display different patterns. This difference is caused by stratification of the water column during summer, so that the lower boundary of the mixed layer is less than 100 m. When atmospheric warming ceases during winter and turbulence is enhanced due to increased wind stress, the stratified structure is lost and the base of the mixed layer deepens to m [Nilsen and Falck, 2006] causing the whole layer to have a similar temperature. At 100 m the temperature stays approximately constant throughout the year (Figure 5b). As N. pachyderma (sin.) is thought to calcify around 100 m, where the mean temperature is a remnant of the wintertime mixing, their shells would reflect the winter situation even though the calcification occurs during the summer season. Multiproxy results from the same cores (JM98-947/2A and MD ) support this theory. d 18 O C in N. pachyderma (dex. and sin.) show a different pattern than the SST reconstructions and have a trend that resembles the winter insolation at 65 N through the Holocene, showing a late Holocene temperature maximum [Risebrobakken et al., 2003], while surface proxies such as alkenones and diatoms are correlated with summer insolation and show an early Holocene temperature optimum [Birks and Koç, 2002; Calvo et al., 2002]. This suggests that the climate signal derived from the deeper dweller N. pachyderm (sin.) may be an indicator for wintertime conditions Climatic Implications [25] The observed change in correlation in the Mg/ Ca curves at 1500 AD (Figure 3) is synchronous with the transition from the Medieval Warm Period (MWP), characterized by generally warmer conditions ( AD), into the Little Ice Age (LIA) ( AD) [Lamb, 1979; Grove and Switsur, 1994; Hughes and Diaz, 1994], suggesting a decrease in stratification of the water column. The extent and timing of the MWP and the subsequent colder LIA is debated [Hughes and Diaz, 1994; Mann et al., 1999; Nesje and Dahl, 2003; von Storch et al., 2004; Moberg et al., 2005]. N. pachyderma (dex.) Mg/Ca temperatures in this study suggest that from 800 to 1500 AD temperatures were 0.5 C warmer than modern (i.e., 20th century) though interrupted by colder spells (D C = 1.2). These spells are colder than those of the LIA. Further, the Mg/Ca temperature records indicate that water temperature during the LIA was not continuously cold. Several studies in the Nordic Seas [Dahl-Jensen et al., 1998; Koç and Jansen, 2002; Andersson et al., 2003; Berstad et al., 2003; Kristensen et al., 2004; Knudsen et al., 2004] and elsewhere [Keigwin, 1996; demenocal et al., 2000; Cronin et al., 2003] suggest that the LIA consists of at least two cold stages. The contemporaneous warming observed in all proxy records from the Vøring Plateau at around 1920 AD is also observed in other marine palaeo records [e.g., Kristensen et al., 2004] and continental records such as tree rings [e.g., Briffa, 2000]. This is related to the early 20th century warming. 13 of 18

14 Figure 7. The offset between Mg/Ca in N. pachyderma (dex.) and N. pachyderma (sin.) (solid curve) and the oxygen isotopic offset for the same morphotypes (stippled curve). [26] A temperature difference of 2 C between the two proxy records of N. pachyderma (dex.) from AD may be explained by changes in d 18 O W within the upper 50 m of the water column (Figure 4a, section 5.7). This is also evident in the difference between the two morphotypes (offset curve) within each proxy (DMg/Ca and Dd 18 O, Figure 7). The offset curves show in general a coherent picture, but larger amplitude in DMg/Ca than Dd 18 O. The reduced temperature magnitude in Dd 18 O suggests influence of water mass with different d 18 O W signature. However, the overall similarity of the Dd 18 O with the DMg/Ca ( C) highlights the utility of the Mg/Ca records as a climate indicator (Figure 7) Down-Core d 18 O W Changes [27] Reconstructions of d 18 O W can be used to validate the Mg/Ca method, if the combination of Mg/Ca temperatures and d 18 O C values from the same levels give realistic d 18 O W values. d 18 O W correlates with salinity in seawater [Craig and Gordon, 1965]. Processes that change local salinity without affecting temperature (other than global ice volume, which is not important over the period studied) are evaporation and precipitation. Modern surface d 18 O W values in Atlantic water near the core site vary between % [Schmidt et al., 1999]. In a geographically wider context, the range of modern d 18 O W values increases due to the influence of low-salinity Polar water, containing variable amounts of d 18 O-depleted water originating from rivers and glacier meltwater. In the upper 300 m of the water column in the area between 7 W and 7 E and 60 and 75 N, d 18 O W varies from 1.13% to 0.63% (note that these data are not normally distributed) [Schmidt et al., 1999]. [28] The reconstructed d 18 O W values show an overall increase from 1250 until 1700 AD of 0.7% for N. pachyderma (dex.) and 0.8% for N. pachyderma (sin.) following a decrease of 0.9% and 0.7% from 900 to 1250 AD (Figure 8). The two d 18 O W record display minima at 1250 AD for N. pachyderma (dex.) and 1350 AD for N. pachyderma (sin.). During the LIA d 18 O W values are generally higher than earlier. The high reconstructed d 18 O W values during the LIA are surprising. Colder termperatures are normally associated with lower salinity and lower d 18 O W. Therefore the expected results is that colder temperatures during the LIA would be accompanied by lower d 18 O W. The higher d 18 O W would indicate that the surface layer was less stratified in the LIA interval than before and was less influenced by low-salinity waters. The reconstructed d 18 O W curves differ in detail, but the overall consistency is encouraging for Mg/Ca in N. pachyderma (dex.) and N. pachyderma (sin.) as climate proxies. [29] The reconstructed d 18 O W ranges between 0.2% and 0.7% (s = 0.37%) (Figure 8). This range is toward the upper limit of what is reasonable and would require the influence of other water masses than those present at the site today. Thus, 14 of 18

15 Figure 8. Seawater d 18 O W inferred from d 18 O C and Mg/Ca in N. pachyderma (dex.) (red curve) and in N. pachyderma (sin.) (blue curve) using a rearrangement of the palaeotemperature equation of Shackleton [1974]. The horizontal line represents the modern d 18 O W value of 0.3%. The records are smoothed with a 5-point moving average. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi The double-headed arrow indicates the uncertainty of the d 18 O W reconstructions (s w ), which is ±0.37%. s w = var d 18 pffiffiffi O w ; var (d 18 O w ) = var(d 18 O c )+ 0:4 0:2 var(k), where K = T from equation (1). 2 converted to salinity and given a plausible range of salinity/oxygen isotope relationships, the reconstructed d 18 O W values have an amplitude of variation that is quite high considering the location of the site within the Atlantic water mass, far from the coastal, low-salinity boundary currents on either side of the Nordic Seas. The possibility that some of the amplitude originates from imperfect knowledge of the Mg/Ca temperature calibration of these morphotypes remains an issue for further research. [30] However, studies have shown that neighboring Arctic water on the west side and Norwegian coastal water on the east side may at times stretch toward the site and influence the water hydrology bringing fresher water having a different d 18 O W signature. Influence of Arctic water at the site has been suggested to influence the subsurface layer of Atlantic water during the Holocene [Risebrobakken et al., 2003]. Instrumental records from OWSM indicate that precipitation can indirectly influence OWSM when increased northerly winds produce an increased Ekman transport of fresh water away from the Norwegian coast and into the Norwegian Atlantic water [Nilsen and Falck, 2006]. A comparison of the lateral advection of modern North Atlantic inflow with the North Atlantic Oscillation (NAO) index showed that reduced strength of the westerlies led to a more westward position of the Atlantic waters [Orvik et al., 2001]. An increased atmospheric pressure gradient increases the strength of the westerlies, which would bring d 18 O W -depleted Arctic water eastward over the site. This may explain the low d 18 O W observed before the onset of LIA. However, higher d 18 O W levels during LIA are in contradiction to findings by Nesje and Dahl [2003] suggesting that rapid glacier advance in the early eighteenth century in southern Norway was caused by increased winter precipitation due to an NAO+ situation. Consequently, it remains unclear if the NAO, or other types of atmospheric circulation changes that influence the water column at the Vøring Plateau, can explain our reconstructed d 18 O W. 6. Conclusions [31] Paired Mg/Ca and d 18 O C measurements from N. pachyderma (dex.) and N. pachyderma (sin.) have been used to constrain the oxygen isotopic vital effect and calcification habitat. A vital effect in the N. pachyderma d 18 O C of 0.6% is deduced from Mg/Ca N. pachyderma (sin.) reconstructed temperatures and the modern d 18 O W of 0.3%. The suggested vital effect is also a suitable estimate for calculating d 18 O C temperatures from N. pachy- 15 of 18

16 derma (dex.). The data indicate that N. pachyderma (dex.) and N. pachyderma (sin.) calcify in summer at around 50 and 100 m or deeper, respectively. Signals derived from N. pachyderma (sin.) primarily reflect remnant winter conditions. The N. pachyderma (dex.) record suggests temperatures were C warmer in the Medieval Warm Period than in the 20th century. A shift in the curve relation at 1500 AD is interpreted to represent the transition from the MWP with more stratified water mass then during LIA times. The variance in the Mg/Ca temperature record is greater for N. pachyderma (dex.) than N. pachyderma (sin.). In contrast, the d 18 O C records have similar variances. This discrepancy may be due to problems with the Mg/Ca temperature calibration or, more likely, d 18 O W changes. Reconstructed d 18 O W records indicate that water masses with different d 18 O W signature have influenced the site. Overall consistency in the morphotype offset curves derived from the Mg/Ca and d 18 O C and consistency in the reconstructed d 18 O W curves highlight the utility of Mg/Ca in N. pachyderma in the Norwegian Sea as a climate proxy. Acknowledgments [32] We are grateful to Mervyn Greaves for his patient training and assistance during the Mg/Ca cleaning and analyses at the Department of Earth Sciences at the University of Cambridge, UK. Trond M. Dokken and Marius Meland are thanked for useful discussions. We thank Ulysses Ninnemann for helpful comments on an earlier draft of the manuscript and Cathy Jenks, who improved the English. The manuscript was improved by suggestions and comments from Thomas M. Marchitto, Pamela Martin, and two anonymous reviewers. Svein Østerhus is thanked for providing the OWSM instrumental data, and Richard Telford and Einar Hegaard are thanked for introduction to statistics and calculation of standard deviation in d 18 O w reconstruction. B.F.N. was supported by the Norwegian Research Council and the University of Bergen. This paper is a contribution from the CESOP and PACLIVA projects funded by the FP5 of the EU and the NOClim project of the Norwegian Research Council and NERC awards NE/C509531/1. This is publication A 134 from the Bjerknes Centre for Climate Research. References Anand, P., H. Elderfield, and M. H. Conte (2003), Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series, Paleoceanography, 18(2), 1050, doi: /2002pa Andersson, C., B. Risebrobakken, E. Jansen, and S. O. Dahl (2003), Late Holocene surface-ocean conditions of the Norwegian Sea (Vøring Plateau), Paleoceanography, 18(2), 1044, doi: /2001pa Barker, S., M. Greaves, and H. Elderfield (2003), A study of cleaning procedures used for foraminiferal Mg/Ca paleothermometry, Geochem. Geophys. Geosyst., 4(9), 8407, doi: /2003gc Bauch, D., P. Schlosser, and R. G. 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