Phase relationship and surface water mass change in the Northeast Atlantic during Marine Isotope Stage 11 (MIS 11)

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Quaternary Research 68 (2007) 445 – 455 www.elsevier.com/locate/yqres

Phase relationship and surface water mass change in the Northeast Atlantic during Marine Isotope Stage 11 (MIS 11) Evgeniya S. Kandiano a,⁎, Henning A. Bauch b a

b

IFM-GEOMAR, Kiel, Germany Mainz Academy of Sciences, Humanities, and Literature, c/o IFM-GEOMAR, Kiel, Germany Received 15 December 2006 Available online 1 October 2007

Abstract Planktic foraminiferal census data, faunal sea surface temperatures (SSTs) and oxygen isotopic and lithic records from a site in the northeast Atlantic were analyzed to study the interglacial dynamics of Marine Isotope Stage (MIS) 11, a period thought to closely resemble the Holocene on the basis of orbital forcing. Interglacial conditions during MIS 11 persisted for approximately 26 ka. After the main deglacial meltwater processes ceased, a 10- to 12-ka-long transitional period marked by significant water mass circulation changes occurred before surface waters finally reached their thermal maximum. This SST peak occurred between 400 and 397 ka, inferred from the abundance of the most thermophilic foraminiferal species and was coincident with lowest sea level according to benthic isotope values. The ensuing stepwise SST decrease characterizes the overall climate deterioration preceding the increase in global ice volume by ∼3 ka. This cooling trend was followed by a more pronounced cold event that began at 388 ka, and that terminated in the recurrence of icebergs at the site around 382 ka. Because the water mass configuration of early MIS 11 evolved quite differently from that of the early Holocene, there is little evidence that MIS 11 can serve as an appropriate analogue for a future Holocene climate, despite the similarity in some orbital parameters. © 2007 University of Washington. All rights reserved. Keywords: Marine Isotope Stage (MIS) 11; Northeast Atlantic; Interglacial climate variability; Planktic foraminifera; Faunal SSTs

Introduction The necessity to predict future climate changes encourages the search among previous interglacial periods for a suitable climate analogue of present-day and future climate conditions. Due to similarities in major climatic variables (insolation and atmospheric composition), the interglacial Marine Isotope Stage (MIS) 11 (∼ 400 ka) has been considered the best analogue of the Holocene and suggested as a target for detailed paleoclimate investigations (Berger and Loutre, 1991; Petit et al., 1999; Loutre and Berger, 2003; EPICA Community Members, 2004; Spahni et al., 2005). However, a good analogy between these two time intervals is only warranted if the climatic forcing and feedback mechanisms involved also correspond to comparable climate conditions on a larger spatial scale (Broecker and Stocker, 2006; Ruddiman, 2006). To date, ⁎ Corresponding author. E-mail address: [email protected] (E.S. Kandiano).

marine archives covering MIS 11 of a sufficient time resolution to examine interglacial climate changes in detail are sparse. Consequently, there are a number of outstanding issues concerning regional climatic conditions, and they carry global implications. A common paradigm to explain the particular climatic conditions of interglacials invokes a strong stable Meridional Overturning Circulation (MOC), which maintains relative climate stability in the northeast Atlantic Ocean (McManus et al., 2003). In general, marine sediment records from this region show MIS 11 as a prolonged warm period with sea-surface properties similar to the Holocene (Ruddiman et al., 1986; McManus et al., 1999; Kandiano and Bauch, 2003). The assumption of such a climate analogy, however, may be questioned by the observation of multiple abrupt changes found in bottom waters at low northern latitudes during MIS 11 (Healey and Thunell, 2004). Moreover, considerable dissimilarities in climate evolution between the Holocene and MIS 11 are recognized in marine archives at high northern

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latitudes (Bauch, 1997; Bauch et al., 2000; Bauch and Erlenkeuser, 2003) as well as in the northeast Atlantic (Helmke and Bauch, 2003; de Abreu et al., 2005). This study presents further details about the specific climate conditions in the mid-latitude northeast Atlantic during the peak interglacial period of MIS 11 through analyzing planktic foraminiferal assemblages in combination with other proxy data. The high correlation between planktic foraminiferal assemblages and SSTs makes this faunal group a serviceable tool for reconstructing changes in sea-surface conditions. This foraminferal SST method proved its usability when reconstructing glacial–interglacial climate fluctuations as well as high amplitude short-term climate variability during glacial periods and transitional phases (e.g., Ruddiman and McIntyre, 1976; Chapman et al., 2000). However, because of some methodological limitations (e.g., SST error, species dominance and biogeographical range), it is quite difficult to investigate intra– interglacial climate dynamics on the basis of foraminiferal SST reconstructions alone (Barash, 1988; Pflaumann et al., 1996; Waelbroeck et al., 1998;). To overcome many of these problems, we employed a recently introduced analytical approach (Bauch and Kandiano, 2007) to make use of the relatively rare, but paleoceanographically important, tropical to subtropical–species found at northern latitudes, and relate their occurrences to

the temporal variability in warm water advection and surface ocean properties there. Core selection and stratigraphy We selected a core location sensitive to record changes in surface water mass configuration and intensity of warm water advection during full interglacial times. Marine sediment core M23414 (53°32′ N, 20°17′ W; 2196 m water depth) appears to be well suited to our purpose, as it is located at the intersection of various water masses (Fig. 1). These sediments underlie the western edge of the North Atlantic Drift (NAD) and therefore potentially capture flow deflections that might have occurred during past times. In the modern ocean, remnants of Mediterranean Overflow Water (MOW) have also been found in the close vicinity of site M23414 (Harvey, 1982). This water mass is formed in the Mediterranean Sea, enters the North Atlantic through Gibraltar Strait and propagates west and northward at intermediate depths as a highly saline water layer (Reid, 1979). The presence of this water mass might influence abundances of deepwater-living foraminifera in the investigated region. The stratigraphic subdivisions of M23414 are based on planktic and benthic oxygen isotope records, as well as on a centimeter-spaced sediment reflectance record. Correlating the

Figure 1. (A) General surface ocean circulation in the North Atlantic and geographical positions of the investigated core site (M23414) and the discussed reference cores (ODP980, 55°29′N, 14°42′W 2189 m water depth, Oppo et al., 1998; MD01-2443, 37°52.89′N, 10°10.57′W, 2941 m water depth, de Abreu et al., 2005). NAC—North Atlantic Current; NAD—North Atlantic Drift; NC—Norwegian Current; IC—Irminger Current; EGC—East Greenland Current; LC—Labrador Current; MOW— Mediterranean Overflow Water. The black line indicates the transect of salinity and temperature profiles (shown on panel B). (B) Sections across the North Atlantic Drift showing temperature and salinity distribution (Schlitzer, 2000, Ocean Data View, http://odv.awi-bremerhaven.de; http://woceatlas.tamu.edu/; Conkright et al., 2001).

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studied core section (the late deglacial phase) were sampled in 1-cm-thick slices (average time resolution 242 yr). The overall time resolution of the entire 1-m-thick section of MIS 11 is 385 yr (range 50–530 yr). The time span in each of 0.5-cm-thick slice averages 210 yr (range 125–215 yr). The time resolution of the Holocene section, which was sampled as 1-cm-thick slices, averages 333 yr. Materials and methods Foraminiferal census counts of dominant and rare species In core M23414, foraminiferal assemblages consist of seven dominant species, composing 88 to 100% of the total assemblages. Abundances of 15 other accompanying species usually do not exceed 2% (Kandiano et al., 2004). The latter group will hereinafter be referred to as “rare species”. Because faunal-based SST reconstructions are generated using only dominant assemblage members, species with abundances lower than 2% are commonly not incorporated in SST estimates (Klovan and Imbrie, 1971). Therefore, involving more species, in spite of their low abundances, could be regarded as an additional and independent tool for further paleoenvironmental interpretation. Foraminiferal census counts were made from the N 150-μm fraction and separately for both dominant and rare species. Dominant species were counted following the conventional procedure using a microsplitter (counting a minimum of 300 foraminiferal tests). For counting rare species, a much larger sediment split was needed to achieve a sufficient statistical robustness (Fig. 3). Figure 2. (A) Stratigraphic subdivision of core M23414 showing the downcore records of sediment lightness reflectance L∗ (Helmke et al., 2002), planktic isotopes and IRD content in size fraction N250 μm (Helmke et al., 2002; Helmke and Bauch, 2003). In addition, IRD content in size fraction N150 μm is shown for MIS 1 and MIS 11 (this study). (B) Blow-up of the studied core section of MIS 11 showing benthic isotope records and relative abundance of N. pachyderma (s) in cores M23414 and ODP 980 (Oppo et al., 1998). Marine isotope stages (MIS) are indicated at the top.

sediment reflectance record to SPECMAP chronology shows that the core extends back to MIS 13 (Helmke and Bauch, 2001; Helmke et al., 2002), thus covering the last five glacial– interglacial transitions (Fig. 2). The chronology of the Holocene section of M23414 is also supported by AMS 14C dates (Didié et al., 2002). Our independent age model for MIS 11 is relatively well matched with that of the nearby core ODP 980 (Fig. 1; Oppo et al., 1998), which allows further comparison of data and interpretations. To achieve a better time resolution in comparison to all previous studies from core M23414, the main interval of MIS 11 (i.e., substage 11c) was resampled in 1-cm steps and as fairly large slices (∼30–40 cm2; 1-cm-thick for the termination section and 0.5 cm thick for the rest of the studied core interval). Through the relatively large area sampled, this method minimizes bioturbational effects. Only the lower 16 cm of the

Environmental preferences of selected foraminiferal species From the dominant assemblage, two species, Neogloboquadrina pachyderma sinistral (s) and Globigerina bulloides, were selected for the further analysis down to species level. N. pachyderma (s) belongs to polar environments (Fig. 4; Bé and Tolderlund, 1971; Kohfeld et al., 1996) and is useful to define the time when interglacial conditions prevailed at site M23414 (Kandiano and Bauch, 2003). According to its modern distribution in the North Atlantic, changes in abundance of G. bulloides (this species thrives westward from the NAD) at our site provide important information about advances and retreats of cooler waters (Fig. 4; Vincent and Berger, 1981; Hemleben et al., 1989; Pflaumann et al., 2003).

Figure 3. Total number of specimens counted for the rare species analysis.

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Figure 4. Geographical distribution of the selected dominant species from coretop sediments (data set: Pflaumann et al., 2003).

Ten rare species were considered for additional counts (Table 1). From these three rare species, whose ecological preferences are quite well known, were selected for further interpretations: tropical surface ocean species Globigerinoides ruber white (w) and the two subtropical deep ocean species Globorotalia truncatulinoides sinistral (s) and dextral (d). These species have relatively high abundances in their optimal environments (Fig. 5, Pflaumann et al., 2003) as well as in our record (Table 1). The modern distribution of G. ruber (w) implies that it thrives in water with temperatures between 16° and 30 °C (Bé and Tolderlund, 1971; Barash, 1988) and its abundance shows a high correlation with SST. Therefore, the occurrence of this species represents an unequivocal marker for temperature maxima. Globorotalia truncatulinoides is a species that consists of several morpho- and genotypes with different ecological preferences (Healy-Williams et al., 1985; de Vargas et al., 2001). In the North Atlantic, right- and left-coiling varieties of this species show a different geographical distribution (Thiede, 1971; Herman, 1972). Genetical investigations indicate that the right-coiling variety, G. truncatulinoides (d), is mainly related to high SSTs and thrives in waters of the subtropical gyre (de Vargas et al., 2001; Renaud and Schmidt, 2003). Moreover, G. truncatulinoides (d) is regarded as the deepest subtropical dweller among all modern foraminiferal species (Hemleben et al., 1989; Lohmann and Schweitzer, 1990; Ravelo and Fairbanks, 1992; Mulitza et al., 1997) and therefore has a potential to record changes in the intensity of warm water advection to our site. In contrast, the left-coiling variety of G. truncatulinoides, G. truncatulinoides (s), tends to tolerate colder SSTs (de Vargas, 2001; Renaud and Schmidt, 2003). SST reconstructions, planktic and benthic oxygen-isotope measurements, IRD content SSTs were calculated on the basis of foraminiferal census data using three mathematical approaches: Transfer Function Technique (TFT; Imbrie and Kipp, 1971), Modern Analogue Technique (MAT; Prell, 1985) and Revised Analogue Method (RAM; Waelbroeck et al., 1998). The degree of analogy between the core-top data base (Pflaumann et al., 2003) and the fossil assemblages was estimated by a mean of square chord distance (Overpeck et al., 1985). Results of foraminiferal census counts

were linked to surface water properties using oceanographical data of Levitus and Boyer (1994). SSTs were reconstructed for the uppermost 10-m water layer. We will discuss only the summer season, considering that summer temperature range has a larger impact on climate-driving mechanisms than winter SSTs (Paillard, 1998). Stable isotope measurements were performed on benthic epifaunal species Cibicidoides wuellerstorfi and the two planktic species: N. pachyderma dextral (d) and G. bulloides. In the modern North Atlantic both planktic species inhabit the upper 60-m water layer but show different seasonal preferences. G. bulloides has its bloom during April–May, whereas N. pachyderma (d) is more abundant during late summer (Ottens, 1992; Schiebel and Hemleben, 2000; Schiebel et al., 2001). Ice-rafted debris (IRD) content was estimated by counting mineral grains in the sediment fraction N 150 μm. All data are available via internet databank (www.pangea.de). Results The periods of full interglacial conditions of MIS 11 and the Holocene are clearly identified in our records by low values of benthic and planktic δ18O in combination with minimum IRD content and low abundance of N. pachyderma (s), as well as enhanced SSTs (Figs. 2 and 6). Calculated core-top SSTs of 15.1 °C (MAT), 15 °C (TFT) and 15.9 °C (RAM) agree, within error, with the modern value of 14.3 °C for site M23414 (Fig. 6; Levitus and Boyer, 1994). Dissimilarity coefficients calculated Table 1 Comparison of taxonomical diversity of rare species during the Holocene and MIS 11 Species Hastigerina pelagica Globorotalia conglobatus Globigerinella aequilateralis Globorotalia crassaformis Beella digitata Globorotalia hirsuta Globigerinoides ruber (w) Globorotalia truncatulinoides (s) Globorotalia truncatulinoides (d) Orbulina universa

MIS 1

MIS 11

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Figure 5. Geographical distribution of the selected rare species from coretop sediments (data set: Pflaumann et al., 2003).

with the MAT method range between 0.03 and 0.13. This does not exceed the admissible value of 0.14 determined for the temperate climate zone (Fig. 6; Kandiano and Bauch, 2003).

a thermal ocean maximum of 16 to 18 °C is recognized in all three SST records (Fig. 6). This time is roughly coincident with the highest sea-level stand as indicated by minimal

The Holocene The three Holocene SST records are highly consistent. They reach 14.5 °C at the end of deglaciation and exhibit a general warming trend over the entire interglacial subsequent period (Fig. 6). The same gradual warming trend is mirrored by the δ18O curve produced from G. bulloides, whereas the isotope record from N. pachyderma (d) exhibits a plateau-like character during the entire postglacial time. The largest offset between the two isotope records is between 11 and 7 ka (Fig. 6). The benthic δ18O curve gradually trends towards lighter values until 6 ka. After this time the benthic record shows slightly increasing values. In the Holocene, diversity and abundance of rare species are substantially depleted compared to MIS 11 (Fig. 7). All rare species started to appear in the record during the late deglaciation. The abundance of G. truncatulinoides (s) reached its maximum of 2‰ at 10 ka, immediately after deglaciation, whereas the abundance of G. ruber (w) was slightly enhanced between 10 and 6 ka but does not exceed 0.4‰. After 6 ka it decreased again and remained on a negligible level not exceeding 0.1‰. The abundance of G. truncatulinoides (d) ranged between 1‰ and 3‰ during the Holocene and increased to 5‰ only in the core top sample. The relative abundance of the dominant species G. bulloides showed only minor fluctuations during most of the Holocene but increased considerably from 16 to 35% at 3 ka (Fig. 7). MIS 11 According to our age model, the major local deglaciation/ meltwater processes (Termination V) lasted until about 411 ka (Fig. 6). Although the relative abundance of N. pachyderma (s) started to drop (from 90% to 35%) notably earlier (416 ka), there was still significant IRD deposition at site M23414. Towards the end of deglaciation the SST increased to 14.5 to 17 °C, depending on the estimation method used. However, SST records did not reach their highest value immediately after Termination V. Instead, they exhibit a gradual increase until about 402 ka. Between 402 and 393 ka,

Figure 6. Comparison of conventional paleoceanographical proxies in core M23414 for sections MIS 11 and MIS 1. (A) Relative abundance of N. pachyderma (s); (B) IRD content in size fraction N150 μm; (C) dissimilarity coefficients (DC) derived from MAT illustrating the degree of analogy between modern and fossil foraminiferal assemblages with the dotted line indicating the admissible value for the temperate climate zone (Kandiano and Bauch, 2003); (D) foraminiferal SST results derived from different mathematical approaches (MAT, TFT; RAM); (E) benthic oxygen isotopes measured on C. wuellerstorfi; (F) planktic oxygen isotopes measured on G. bulloides and N. pachyderma (d). Final phases of Terminations I and V are indicated by grey bars. Marine isotope stages (MIS) are indicated at the top.

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Figure 7. Down core distribution of relative abundances of rare species in comparison with other paleoceanographical data during MIS 11 and MIS 1 from core M23414. (A) IRD content in size fraction N150 μm; (B) averaged results of three different SST calculations (MAT, TFT, RAM), which are shown separately in Figure 6; (C) benthic oxygen isotope records produced on C. wuellerstorfi; (D) planktic oxygen isotope records produced on N. pachyderma (d) and G. bulloides; (E) relative abundance of G. bulloides; (F) mid-June summer insolation at 65 °N (Berger, 1978) and relative abundance of G. ruber (w); (G) relative abundance of G. truncatulinoides (s); (H) relative abundance of G. truncatulinoides (d). Note that relative abundances of rare species are presented in per mil of the total assemblage. The SST optimum during MIS 11, inferred from relative abundance of the most thermophilic species G. ruber (w), is indicated by grey bars. Marine isotope stages (MIS) are indicated at the top.

benthic δ 18 O values. All SST data reveal continuous temperature variability throughout the entire MIS 11. During peak interglacial conditions, SSTs fluctuate by 2 to 3 °C depending on the method employed. Due to singularities in the mathematics, TFT calculations result in considerably higher and more variable SST estimates in the temperate climate zone than MAT and RAM (Kandiano et al., 2004). In addition, MIS 11 is characterized by less consistent SST estimates than the Holocene, which makes an assessment of absolute SST changes in MIS 11 more difficult. Offsets between the SST records reach up to 2.5 and 5 °C during the peak interglacial conditions and the ensuing glacial inception, respectively (Fig. 6).

In agreement with the SST estimates, the planktic δ18O would indicate a late climate optimum after 400 ka (Fig. 6). After 393 ka there was a significant increase in benthic δ18O associated with slightly cooler and more variable SSTs. A particular event centered at about 390 ka, which appears to be an intermittent warming, is observed in the δ18 O record of N. pachyderma (d) (Fig. 6). Until 388 ka both the planktic δ18O records co-vary within error, even though G. bulloides exhibits slightly higher amplitude changes than N. pachyderma (d). A rather pronounced event, when δ18O increased by 0.4‰, is recorded by G. bulloides at 397 ka. After 388 ka, all planktic δ18O and SST records indicate pronounced surface cooling. Because this general cooling trend

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at the surface was coeval with increased bottom water δ18O, it reflects the onset of glacial inception (Fig. 6). It is noteworthy that the δ18O record of G. bulloides deviates from that of N. pachyderma (d) after 388 ka by up to 0.6‰. By comparison to the Holocene, both planktic δ18O records show consistently heavier values (on average by 0.5‰) during peak interglacial conditions of MIS 11. The benthic δ18O record, however, reveals approximately the same values during the two interglacials. This implies a similar interglacial sea level highstand, provided bottom water had about the same temperature. The distribution of rare species during MIS 11 reveals a distinct phase relationship in their peak abundances (Fig. 7). Globorotalia truncatulinoides (s) was the first to appear in the record, near the end of the main deglaciation, at 416 ka. The abundance of this species reached a maximum of 6.4‰ between 412 and 410 ka, at a time when G. ruber (w) and G. truncatulinoides (d) appeared in the record. The abundance of G. truncatulinoides (d) remained mainly on a level between 3 and 6‰ before it started to vanish from the record at 397 ka. Slightly earlier, at 400 ka, the abundance of G. ruber (w) sharply increased to reach a maximum around 399 ka. All three rare species had completely disappeared from the area around 388 ka, roughly coincident in time with the divergence of the two planktic δ18O records and with a steep increase in the abundance of G. bulloides. This increase in abundance of the latter, to about 45%, emphasizes that G. bulloides is not strictly a peak interglacial indicator but rather thrives in our study area during times when cooler interglacial conditions prevail. Discussion Despite the resemblance of insolation forcing during the present time and MIS 11, the earlier interglaciation was preceded by a considerably longer glacial–interglacial transition, which was recognized as a prominent feature of the late Quaternary climate dnamics (Rohling et al., 1998; Healey and Thunell, 2004). Indeed, marine records from middle (Oppo et al., 1998) to high latitudes (Bauch et al., 2000; Bauch and Erlenkeuser, 2003) of the North Atlantic indicate that Termination V extended well into MIS 11. The absence of IRD clearly indicates that, after circum-North Atlantic ice sheets had melted off in the course of Termination V, a 26-ka-long period of full interglacial conditions (from 414 to 388 ka) prevailed in the Northeast Atlantic. At nearby site ODP 980, sea-surface conditions of this period have been described as rather uniform, with a diminished temperature variability (≤1 °C) similar to the Holocene (Oppo et al., 1998; McManus et al., 1999, 2003). In contrast, our SST reconstructions show persistent intra–interglacial changes ranging from 1 to 3 °C, depending on the method, and a pronounced temperature maximum that occurred in the middle of MIS 11 (from 400 to 390 ka). Reconstructing sea-surface dynamics during periods of full interglacial conditions is difficult using faunal SSTs alone because temperature changes might fall within the errors of the modern calibration (Pflaumann et al., 1996; Waelbroeck et al., 1998; Kandiano et al., 2004). In addition, greater faunal dis-

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similarities between the modern reference and past data sets increase the degree of non-analogy between modern and fossil foraminiferal assemblages (Mix et al., 1999). Applying our rare species analysis in combination with the other paleoceanographical proxies clearly reveals a phase relationship of water mass changes in the middle latitudes during the full interglacial conditions of MIS 11. The rapid invasion of a fauna, including subtropical species G. truncatulinoides (s), immediately after the end of Termination V indicates early warming (Fig. 7), something also observed at lower latitudes (Poli et al., 2000; Healey and Thunell, 2004; de Abreu et al., 2005). In contrast to the early to mid-Holocene post-deglacial climate maximum, which is unambiguously registered at low and high northern latitudes during times of highest northern insolation (Eglington et al., 1992; Koç et al., 1993; Bauch et al., 2001), surface warming during MIS 11 peaked some 10–12 ka after the main deglaciation. This warm peak was more or less coincident with the lowest insolation during MIS 11 (Fig. 7). During the development of the warm peak of MIS 11, enhanced water advection occurred first at greater, intermediate water depth and later nearer to the ocean surface. This suggestion is deduced from the stepwise and earlier appearance of the deepliving species G. truncatulinoides versus the shallow-water dweller G. ruber (w). It is interesting to note that, despite some uncertainties in age models, planktic δ18O in core MD01-2443 from the Iberian margin show pronounced changes towards lighter values between 408 and 406 ka (de Abreu et al., 2005). In the Northeast Atlantic, MOW has a relatively high density. Thus, the water mass changes that started to occur at 410 ka first at greater depth, as inferred by the occurrence of G. truncatulinoides (s), could be connected to an early influence of MOW at site M23414. However, the short SST optimum, represented by the highest abundance of G. ruber(w), occurred somewhat later (400–397 ka). Farther north in the Nordic seas, the interval between 408 and 398 ka was the only time during MIS 11 that was associated with a westward expansion of relatively warm, Atlantic-derived surface waters (Bauch et al., 2000; Helmke and Bauch, 2003). Thus, the peak expansion of warm surface water advection into the polar North Atlantic most likely correlates to the SST optimum revealed at site M23414 (represented by G. ruber (w)). This situation would have eventually resulted in the most diminished SST gradient between the two regions during entire MIS 11. The abrupt decline in the abundance of G. ruber (w) at 397 ka, together with simultaneous increases in the abundance of G. bulloides, provides evidence for an early climate deterioration. This change in surface waters preceded regrowth in global ice volume by several thousand years. This regrowth of global ice volume upon a progressive, but initially slow, cooling seems to be a characteristic feature of early interglacial–glacial transitions in general (Bauch and Kandiano, 2007). Moreover, like at the MIS 5e–5d climate transition, the steady temperature reduction after 397 ka terminated in a more pronounced, cooler episode; this phase is well expressed in the two planktic δ18O records of G. bulloides and N. pachyderma (d), SST records and the disappearance of rare species after 388 ka (Fig. 7). The increase in G. bulloides, coincident with this time, is evidence

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for a southeast shift of colder waters to site M23414; this water mass change may have also caused the abrupt changes found in SSTs at the Iberian margin during this time (de Abreu et al., 2005). All of our findings contradict previous suggestions about the potential of the MOC to maintain a relatively unaltered interglacial warmth over the North Atlantic region during times when the major climate driving mechanisms would be forcing the system towards glacial inception (McManus et al., 2003). The fact that a first more notable increase in abundance of N. pachyderma (s) and IRD deposition occurred only after 384 ka, and only when δ18O values in N. pachyderma (d) were already significantly increased, shows the inherent insensitivity of these two polar-water proxies to record any early interglacial climate deterioration in this region (Bauch and Kandiano, 2007). To a certain degree, an early interglacial climate cooling may be linked to enhanced seasonal contrasts, which would then find expression in a distinct time lag in the δ18O values of N. pachyderma (d) and G. bulloides (Bauch and Kandiano, 2007). Our refined faunal analyses reveal a number of differences in the climate evolution of both the Holocene and MIS 11, and it appears evident that the reason that underlies these discrepancies was associated with some characteristic features of the preceding glacial–deglacial transition. The glacial period just prior to MIS 11, called Elsterian (MIS 12) in Europe, is thought to have been the most extreme Quaternary glaciation in terms of expansion of the European ice sheets (e.g., Donner, 1995). The huge ice sheet extent is confirmed by marine evidence that shows by far the highest input of IRD into the subpolar and polar North Atlantic during MIS 12 and Termination V (Bauch and Erlenkeuser, 2003; Helmke and Bauch, 2003). It can also be expected that the time required to melt such large ice sheets was much longer during Termination V than at the end of less extensive glaciations. In addition, the dispersion of meltwater across the North Atlantic region, which resulted from this melting, is regarded to be responsible for a distinctly different water mass development during the full interglacial phase of MIS 11 as compared to younger interglacials (Bauch et al., 2000). Some researchers have suggested a higher sea level and, therefore, less global ice volume during MIS 11 versus the present interglacial (e.g., Hearty et al., 1999; Poore and Dowsett, 2001). But our records from site M23414, as well as the data from nearby site 980 (McManus et al., 2003), the Nordic Seas (Bauch and Erlenkeuser, 2003) and elsewhere (Hodell et al., 2000; de Abreu et al., 2005), all support a sea level for MIS 11 of similar magnitude to the Holocene (Fig. 2). Moreover, Helmke and Bauch (2003) already noted a major Holocene versus MIS 11 discrepancy between sites 980 and M23414 in the planktic δ18O values: at site M23414, consistently heavier values (0.3‰) were recorded in G. bulloides in MIS 11 compared to the Holocene, whereas no difference between the two warm periods was found in N. pachyderma (d) δ18O at site 980 (McManus et al., 1999, 2003). The original finding by Helmke and Bauch (2003) is now reconfirmed on the basis of our new planktic δ18O records, for N. pachyderma (d) as well as for G. bulloides (Fig. 7). The

reason for the isotopic difference in surface waters between the two sites might be due to the circulation of water masses surrounding the Rockall Plateau. Site 980, located at the eastern flank of the Rockall Plateau, is today under the direct influence of NAD, whereas site M23414 is more prone to an influence of cooler waters from the west and northwest (Fig. 1). Considering this, the isotopic gradient found between the two sites during MIS 11 was probably caused by an eastward shift of the NAD axis. This shift led to an enhanced influence of colder surface waters at site M23414. But a colder MIS 11 compared to the Holocene at site M23414, as inferred from planktic δ18O values, is in contradiction to our reconstructed SSTs and the higher abundance of rare species such as G. ruber (w). The increased abundance of G. truncatulinoides (d) during MIS 11 may further imply that a thicker body of upper warm water was advected to site M23414 during MIS 11. Although G. ruber (w) clearly indicates the interval when the warmest surface conditions prevailed, this rare species probably still reflected relatively rare conditions, namely when high summer temperature extremes occurred. In contrast, our planktic δ18O records were measured on typical temperate species, well adapted to the mid-latitude seasonal temperature range. Because deep-sea sediments usually cannot provide resolution on annual or even seasonal frequency, it is conceivable that the temperature signals inferred from the δ18O records and foraminiferal species assemblages also respond to a seasonal change. In spite of the complexity, changes in different seasonal behavior of a given foraminiferal species should still be regarded as also reflecting the larger scale climate situation. To what extent cooler water masses from farther northwest can influence site M23414 may also be deduced from the first rapid increase in G. bulloides abundance after 395 ka, during the early phase of climate deterioration. Such an influence of cooler water masses should have caused a deflection of the NAD that led to enhanced zonality in ocean circulation pattern. According to the modern pattern of ocean–atmospheric circulation, enhanced zonality over the North Atlantic is associated with an eastward expansion of polar waters as well as intensified seasonal ice drift and coverage in the north polar regions (e.g., Dickson et al., 2000). Although the abovementioned limitations of SST methods complicate a comparison of absolute intra-interglacial temperature variability, our planktic δ18O record from G. bulloides exhibits considerably higher oscillations during MIS 11 than during the Holocene. Independent from that, the timing in the appearances of rare species is evidence that the NAD went through several stepwise changes until the warmest phase of MIS 11 was reached. These stepwise changes reflect oceanic processes that have no comparable analogue in the Holocene. In the light of discussing interglacial climate analogies between the Holocene and MIS 11 (their duration and atmosphere changes in CO2 concentration and temperature), the recent proposal by Broecker and Stocker (2006)–of a continuing warm Holocene for many more millennia–gains little support from our data. It has previously been shown that, at north polar latitudes, the warmest conditions in MIS 11 occurred during the later part of the full interglacial phase (Bauch et al.,

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2000). This rather late warm phase is now reconstructed for the Northeast Atlantic (this study), and it is also a feature clearly recognizable in the new ice core from Antarctica (EPICA Community Members, 2004). We suggest that a delay in interglacial development was caused by the long-lasting deglacial meltwater influence during Termination V. This prolonged period with meltwater release into the open ocean affected surface ocean properties and thus deepwater formation processes at the high northern latitudes. Therefore, overlaying the entire Holocene and last deglacial CO2 record onto what appears to be largely Termination V and the ensuing transition towards peak full-interglacial conditions in MIS 11, as done by Broecker and Stocker (2006), seems indeed erroneous (Ruddiman, 2006). Summary and conclusions Occurrences of certain planktic foraminiferal species, in combination with other paleoceanographic proxies, reveal a distinct phase relationship in the northeast Atlantic Ocean during full interglacial times of MIS 11, which lasted approximately 26 ka, according to our age model. During the early interglacial phase of MIS 11, climate conditions evolved quite differently in comparison to the Holocene. In the Holocene, a rapid postglacial SST increase immediately culminated in an interglacial climate optimum. During MIS 11, the coincidence of lowest benthic δ18O values with proxy data indicating highest SSTs 10–12 ka after iceberg activity had ceased suggests a late oceanic recovery from the deglacial processes of Termination V. During the posttermination transition, warm water advection into the midlatitude North Atlantic experienced a prolonged stepwise intensification. Moreover, changes in the relative abundances of tropical deep- and shallow-water foraminifers indicate that the establishment of peak warmth during MIS 11 was characterized by complex interactions in the upper water column that have no analogue in the Holocene. In particular, intrusions of water masses at intermediate depth, which may correspond to the modern Mediterranean Outflow Water, preceded changes at the surface. The dynamics in abundance of the most thermophilic species, G. ruber (w), imply that the surface interglacial temperature optimum occurred between 400 and 397 ka, when insolation was at its lowest. After this climatic optimum, a progressive SST decrease indicates the onset of climate deterioration, which started 3 ka before a renewed growth of global ice volume is observed. At 388 ka, this general climate deterioration abruptly culminated in full stadial conditions, as inferred from planktic δ18O and faunal records. The difference seen between the Holocene and MIS 11 in the early post-termination climate behavior was probably caused by effects linked to the melting of the huge northern ice sheets of the MIS 12 glaciation. Because the meltwater from these large ice sheets determined surface water conditions in the polar latitudes for a considerable time, it also influenced the interglacial water mass development in the subpolar North Atlantic. As inferred from a significant difference in planktic δ18O for MIS 11, it seems as if the NAD axis in the North Atlantic was likely shifted eastward in comparison to its present position.

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