Variations in mid-latitude North Atlantic surface water properties during the mid-Brunhes : Does Marine Isotope Stage 11 stand out ?

Variations in mid-latitude North Atlantic surface water properties during the mid-Brunhes: Does Marine Isotope Stage 11 stand out? A. H. L. Voelker, T. Rodrigues, R. Stein, J. Hefter, K. Billups, D. Oppo, J. McManus, and J. O. Grimalt Dept. Geologia Marinha, Laboratorio Nacional de Energia e Geologia (LNEG; ex-INETI), Estrada da Portela, Zambujal, 2721-866 Alfragide, Portugal CIMAR Associate Laboratory, Rua dos Bragas 289, 4050–123 Porto, Portugal Alfred-Wegener-Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany College of Marine and Earth Studies, University of Delaware, 700 Pilottown Road, Lewes, DE 19958, USA


Introduction
The Brunhes polarity chron encompasses the last 780 ka (kiloannum = thousand year) and its middle section is often considered as a particularly warm period during the last 1000 ka, when warm surface waters penetrated polewards and sea levels were generally higher than at Present . MIS 11 and 9 are part of 5 this warm interval. Interglacial MIS 11c was the first interglacial period after the mid-Pleistocene transition with atmospheric greenhouse gas concentrations and temperatures over Antarctica (Petit et al., 1999;Siegenthaler et al., 2005;Spahni et al., 2005;Jouzel et al., 2007) at levels similar to those during subsequent interglacials including the current one, the Holocene. Based on temperature related proxy records from the 10 oceans (Hodell et al., 2000;Lea et al., 2003;McManus et al., 2003;de Abreu et al., 2005;Helmke et al., 2008) and from Antarctica (Petit et al., 1999;Jouzel et al., 2007) it was an unusually long lasting interglacial and northern heat piracy, i.e. the enhanced advection of warm waters from the South into the North Atlantic, was at its maximum (Berger and Wefer, 2003). The early temperature rise during the high amplitude transi- 15 tion from glacial MIS 12 to MIS 11c leads to two possible definitions for the duration of the interglacial period within MIS 11. Based on the interval of maximum warmth in marine records, the interglacial period lasted at minimum from to 420 to 396 ka (McManus et al., 2003;Helmke et al., 2008). The definition of an interglacial as the period of ice volume minimum/sea-level highstand (Shackleton, 1969), however, shortens this inter-interglacial interval within MIS 13, even if atmospheric carbon dioxide concentrations were at a similar level during both warm substages, MIS 13c and 13a (Siegenthaler et al., 2005), and nitrous dioxide peaked during MIS 13c (Spahni et al., 2005).
One of the reasons why MIS 13 is so different, might be that its preceding glacial, MIS 14, was so weak with sea level lowering only about as half as during MIS 16, 12 15 or 10. Consequently, also the amplitude of Termination VI was much lower than during Terminations V or IV. MIS 12, on the other hand, was one of the most extreme glacials during the last 1000 ka, when sea level was probably lower than during the last glacial maximum (MIS 2) (Lisiecki and Raymo, 2005). Sea level during MIS 10 was similar to MIS 2, even though MIS 10 lasted only half as long as MIS 12. In regard to dust flux 20 in Antarctica MIS 14 is also the weakest and MIS 12 the strongest glacial (Lambert et al., 2008). This pronounced difference between the mid-Brunhes glacials is, however, not evident in the EDC temperature and greenhouse gas records (Jouzel et al., 2007;Loulergue et al., 2008;Lüthi et al., 2008). Greenhouse gas concentrations during the MIS 10 glacial maximum were actually lower than during MIS 12 and 14. Introduction

Tables Figures
Back Close

Full Screen / Esc
Printer-friendly Version Interactive Discussion Manus et al., 1999;Hodell et al., 2008;Ji et al., 2009). McManus et al. (1999) showed that the onset of millennial-scale climate variability, including ice-rafting events, is linked to a threshold value of 3.5‰ in benthic δ 18 O. As soon as this ice volume threshold was passed the Atlantic meridional overturning circulation (AMOC) became less stable resulting in oscillations between weaker and stronger AMOC modes. Most of the existing 5 evidence for millennial-scale AMOC variability during the mid-Brunhes and its impacts on surface and deep waters is linked to the inception of MIS 10 (Poli et al., 2000;Billups et al., 2004;de Abreu et al., 2005;Hall and Becker, 2007;Martrat et al., 2007;Dickson et al., 2008;Stein et al., 2009). Only records from ODP Sites 980 and 1058 cover the older glacials with sufficient resolution (McManus et al., 1999;Flower et al., 10 2000; Billups et al., 2006;Weirauch et al., 2008) and here we present the previously unpublished planktonic δ 13 C records for these sites.
The new records fill the gap between ODP Sites 1056 and 980 and reveal, if hydrographic conditions in the mid-latitude North Atlantic, which encompasses the southern edge of the North Atlantic ice-rafted debris (IRD) belt (Ruddiman, 1977;Hemming, 15 2004) and experienced large SST gradients during glacials (Calvo et al., 2001;Pflaumann et al., 2003) and stadials (Chapman and Maslin, 1999;Oppo et al., 2001), were more similar to those in the subtropical or the subpolar gyre during interglacials, especially during MIS 11. The two new records off Portugal, MD01-2446 and MD03-2699, furthermore, allow for the first time to reconstruct and evaluate the full transition from 20 glacial MIS 12 to MIS 11 in this eastern boundary upwelling system. By combining the planktonic foraminifer stable isotope records from three new sites in the mid-latitude North Atlantic Ocean with those from ODP Sites 980 and 1056/1058 we aim (1) to map hydrographic conditions within the major surface currents (Fig. 1) and thus to identify potential latitudinal or longitudinal gradients in the North Atlantic during the interval 25 spanning from MIS 9c to 14 (300-540 ka); (2) to trace the potential sources of the subsurface waters and their changes on glacial and interglacial timescales; and (3) to address the question, how stable or variable hydrographic conditions were in the mid-latitude North Atlantic during MIS 11 and how they differed from those during its Introduction

Conclusions
References Tables  Figures   Back  Close Full Screen / Esc

Printer-friendly Version
Interactive Discussion neighboring interglacials, MIS 13 and 9. Our interpretation is supported by IRD records for the three new sites and for alkenone-based SST data for MIS 11 from two of those sites.

Core sites and modern hydrographic setting
The three new core sites are IODP Site U1313, MD01-2446 and MD03-2699 (Table 1;  (Table 1; Fig. 1a). Surface waters at all sites are derived in one form or another from the Gulf Stream and the North Atlantic Current (NAC). ODP Sites 1056 and 1058 are located directly 15 below the Gulf Stream, while ODP Site 980 is influenced by the Rockall Trough branch of the NAC (Fratantoni, 2001;Brambilla and Talley, 2008). Even though IODP Site U1313 is located south of the core NAC pathway, drifter data shows that surface waters in this area are derived from the NAC, partly through recirculation off the Grand Banks (Fratantoni, 2001;Reverdin et al., 2003). 20 Gulf Stream/NAC derived surface waters off the western Iberian Peninsula are transported by two currents, the Portugal Current and the Azores Current. The Portugal Current (PC) is the NAC recirculation branch within the northeastern North Atlantic (Fig. 2a) and is centered west of 10 • W off Portugal (Peliz et al., 2005). Thus it is the current influencing site MD01-2446 (Fig. 1b) and Talley, 1982), including along the NAC's Rockall Trough branch (Brambilla and Talley, 2008). The Azores Current (AzC) diverges from the Gulf Stream and moves in large meanders between 35 and 37 • N across the North Atlantic. Its northern boundary forms the subtropical Azores front.
In the eastern basin the AzC splits into several branches, one of which is the Canary 5 Current, its major recirculation, and another, the eastern branch, enters into the Gulf of Cadiz (Fig. 1). During winter, waters from this eastern branch recirculate northward as the Iberian Poleward Current (IPC) (Peliz et al., 2005), thereby bending the subtropical front northward along the western Iberian margin (Fig. 1b). Similar to the PC, the IPC includes a subsurface component: ENACW of subtropical origin. Subtropi-10 cal ENACW is formed by strong evaporation and winter cooling along the Azores front (Rios et al., 1992) and is less ventilated, warmer and saltier than its subpolar counterpart ( van Aken, 2001 Outside of the western Iberian upwelling zone plankton blooms drive surface water productivity (Table 1). Site MD01-2446 falls within the mid-latitude regime of (Levy et al., 2005) that is associated with a bloom that starts in fall and peaks in spring. The northern boundary of this regime is at 40±2 in filter paper at 40 • C and weighted. Sample intervals are 1-3 cm for core MD03-2699 and 2-3 cm for core MD01-2446. IODP Site U1313 was sampled continuously with 2 cm-wide scoops. Site U1313 stable isotope samples were taken from the secondary splice ) and biomarker samples from the primary splice (Stein et al., 2009).
For planktonic stable isotope measurements in cores MD03-2699 and MD01-2446 and IODP Site U1313, 8-10 clean specimens of Globorotalia inflata were picked from the fraction >315 µm. G. inflata is one of the dominant species in the planktonic foraminifer fauna associated with the NAC (Ottens, 1991). Its stable isotope values reflect hydrographic conditions at the base of the seasonal thermocline (Cléroux et al., 15 2007); conditions that are close to those in the winter mixed layer. Details on stable isotope measurements for ODP Site 980 are given by Oppo et al. (1998) and McManus et al. (1999), for ODP Site 1056 by Chaisson et al. (2002) and Billups et al. (2004) and for ODP Site 1058 by Billups et al. (2006) Germany). The mass spectrometer is coupled to an automated Kiel carbonate preparation system and the long-term precision is ±0.07‰ for δ 18 O and ±0.05‰ for δ 13 C based on repeated analyses of internal and external (NBS-19) carbonate standards. The number of lithic fragments was determined in the fraction >315 µm and is presented as "#/g" (normalized by the respective sample's dry weight). Lithics are primarily 5 interpreted as IRD. The coarser size fraction was chosen 1) to minimize modification of the IRD signal by wind deposition and lateral advection at slope site MD03-2699 and 2) to avoid scientific overlap for Site U1313 within the science party of IODP Exp. 306. Using a coarser size fraction allows to identify all the ice-rafting events (e.g. Voelker, 1999) and only this is relevant for the current study, but might underestimate the absolute intensity of an ice-rafting event.
Biomarker samples of IODP Site U1313 and core MD03-2699 were prepared following established procedures (Villanueva et al., 1997;Calvo et al., 2003). Core MD03-2699 samples were analysed in a Varian gas chromatograph either at the Dept. of Environmental Chemistry of CSIC (Barcelona) or at the Dept. de Geologia Marinha 15 of LNEG (Rodrigues et al., 2009). Site U1313 samples were measured in a gas chromatograph/time-of-flight mass spectrometer at the Alfred-Wegner Institute, Bremerhaven (Hefter, 2008;Stein et al., 2009). Alkenone-based sea surface temperatures (SST) for both sites were calculated using the unsaturation index U 37 k of Müller et al. (1998)  at the depth of 1895 cm which has an age of 453.6 ka with the current age model, in agreement with (Raffi et al., 2006). Due to stronger winnowing by the MOW as evidenced by foraminifer sands, sedimentation rates in core MD03-2699 subsided during glacial maxima, especially during MIS 12 (Fig. 2c). Overall, sedimentation rates of core MD03-2699 and IODP Site 20 U1313 are similar, while they are lower in core MD01-2446 (Fig. 2c). Temporal resolution of the planktonic stable isotope records are 100-600 years for IODP Site U1313, 90-1210 years for core MD03-2699 and 280-1820 years for core MD01-2446.
ODP Site 980 records are shown on the LR04 age model of Lisiecki and Raymo (2005) in this paper and the N. pachyderma (r) stable isotope records have 25 a temporal resolution of 40 to 3230 years. The age scale of ODP Site 1056 was transferred to LR04 time using the Billups et al. (2004)  V is the one of the largest glacial to interglacial transition of the middle Pleistocene while Termination VI is probably one of the smallest (Lisiecki and Raymo, 2005  Although MIS 11 may have been the warmest interval of the past 1000 ka, the G. inflata δ 13 C record contains a maximum in the later stage of MIS 13 (MIS 13a; Fig. 3).
High δ 13 C values are a typical signal for mid-Brunhes planktonic and benthic δ 13 C records (e.g. Hodell et al., 2003). During MIS 13c and 11c, δ 13 C values were at a similar level, but on average 0.5‰ lower than during MIS 13a. Terminations VI and V 25 were associated with pronounced δ 13 C minima. Introduction

Tables Figures
Back Close

Full Screen / Esc
Printer-friendly Version

Interactive Discussion
Melting icebergs reached IODP Site U1313, located within the area of high IRD sedimentation during Heinrich events (Hemming, 2004), during all the mid-Brunhes glacials, but IRD deposition was more pronounced and frequent during MIS 12 (Fig. 3). The first IRD event recorded at site U1313 during the transition from MIS 13 to 12 occurred at 490 ka followed by a 23 ka long period with continuous, but not intense 5 IRD sedimentation. After 467 ka, the record reveals four intervals of increased IRD deposition with the last interval exhibiting three short-term maxima. All of the IRD maxima as well as the IRD peak during MIS 10c (isotopic event 10.4) contain dolomite grains (Stein et al., 2009) and are thus interpreted as Heinrich-type ice-rafting events. During MIS 13a IRD deposition ceased for 12 ka (506.4-494.3 ka). During Termination 10 V continuous IRD deposition ended already at 415.9 ka and during MIS 11c melting icebergs did not reach Site U1313 between 410.7 and 399.6 ka (Fig. 6).

Core MD01-2446
The G. inflata δ 18 O record of core MD01-2446, the offshore site off Portugal, shows the same features as the Site U1313 record with relative stable conditions during the inter- 15 glacials and millennial-scale variability during glacial inceptions and glacials (Fig. 4) (Fig. 4). δ 13 C minima during glacials were lower than at Site U1313. In contrast to Site U1313, G. inflata recorded a pronounced δ 13 C minimum between 452 and 443 ka offshore Portugal. During MIS 10, lower δ 13 C values occurred especially between 351 and 342 ka.
Although Site U1313 received continuous but in comparison to the other glacials 5 small amounts of IRD during MIS 14 ( Fig. 3), (coarse) IRD deposition offshore Portugal was nearly negligible and no IRD was deposited during MIS 13 after 523 ka (Fig. 4). During MIS 12, melting icebergs started to reach the Portuguese margin after 472 ka, i.e. significantly later than at IODP Site U1313. The last two IRD peaks in core MD01-2446 during MIS 12 coincided with the last interval of increased IRD deposition at Site 10 U1313 and its Heinrich-type IRD events. At site MD01-2446, however, IRD deposition greatly diminished between the Heinrich-type events. During MIS 10, phases of intensive ice rafting coincided with stadial MIS 10c and with Termination IV. Minor amounts of lithic grains, generally clear quartz grains, were deposited throughout MIS 9e (Fig. 4), especially until 322 ka. However, as the tropical planktonic foraminifera species G. 15 menardii is found in the same samples as the quartz grains and mean annual SST further south on the margin exceeded 19 • C (Martrat et al., 2007) these grains were more likely deposited by strong westerly winds during the upwelling season than by melting ice. During MIS 11c, coarse lithics were not deposited between 410.8 and 393.6 ka with the exception of one quartz grain found at 404.2 ka that was probably also wind-20 transported. After 393.6 ka minor amounts of lithics were encountered throughout the glacial inception.

Core MD03-2699
At nearshore site MD03-2699 the glacial to interglacial pattern that is so clearly evident at the other two sites is more difficult to detect (Fig. 5 Overall, the shape of the δ 13 C record mimics the pattern described for IODP Site U1313 and site MD01-2446 with heaviest δ 13 C values recorded during MIS 13 and 11. 15 Contrary to those records heavy δ 13 C values persisted throughout MIS 12b (Fig. 6).
The same is seen during the inception of MIS 10 and in particular during MIS 10c and 10b. Like at site MD01-2446 interglacial MIS 9e is associated with increasing δ 13 C values that reached higher levels only during MIS 9d and 9c. During MIS 11c the δ 13 C record shows a stepwise recovery from the minimum during Termination V. 20 The first "plateau" with values generally between 0.75 and 0.9‰ lasted from 416.5 to 401.4 ka, followed by a second maximum with values mainly between 1 and 1.25‰ from 401.1 to 394.4 ka. Also the MIS 13 record of MD03-2699 shows more structure than in the previous records. Although highly variable, early MIS 13c is associated with a maximum in δ 13 C values, followed by a broad minimum lasting from late MIS 13c to 25 MIS 13b  MIS 13b (Fig. 5). During MIS 12 the first, but minor IRD peak occurred at 470 ka. Continuous IRD deposition started after 442 ka and lasted until 422.8 ka. IRD peaks during this interval coincided with the Heinrich-type events recorded at IODP Site U1313. After this interval with intensive IRD deposition, minor amounts of lithics (mainly quartz grains) were detected until 410 ka. In three levels during MIS 11c ( Fig. 6e) 1 or 5 2 quartz grains were observed, but these grains are most likely wind-transported from the Portuguese coast. A significant IRD peak occurred around 388 ka within MIS 11b (isotopic event 11.24; Figs. 5, 6e). After this first MIS 11 stadial, minor amounts of lithics were deposited on and off throughout MIS 11a (Fig. 6e). As all those periods of lithic grain deposition coincided with the presence of tetra-unsaturated alkenones 10 (Rodrigues et al., 2009), which are linked to fresher surface waters (Bard et al., 2000), the lithics are interpreted as IRD. MIS 10 is associated with another extended period (362.2-333.4 ka) of ice rafting. Maximum IRD concentrations, however, occurred during MIS 10b. A small IRD peak is also associated with stadial MIS 9d. 15 Annual mean SST at IODP Site U1313 varied between 7.7 and 20.2 • C (Fig. 6b). The coldest SST was recorded during the Heinrich-type ice-rafting events during MIS 12 and 10c (isotopic event 10.4). SST rose quickly after the onset of Termination V increasing by nearly 8.5 • C between 427 and 423 ka. During MIS 11c two SST plateaus with values around 18 • C are observed with the second plateau, which also experi-20 enced minimally warmer SST, coinciding with the interglacial sea level highstand (408-396 ka). During the subsequent glacial inception, the more pronounced cooling occurred during the MIS 11b stadial (isotopic event 11.24; ∼390 ka). At core site MD03-2699 mean annual SST were relatively warm during MIS 12 with 11.7 to 15.8 • C (Fig. 6e). SST then dropped to values below 8 • C during the Heinrich- 25 type event at the beginning of Termination V. Similar to the G. inflata δ 13 C records the SST data also shows two plateaus for MIS 11c. The first plateau with SST around 17.6

Sea surface temperature reconstructions for MIS 10 to 12
• C lasted from 425 to 413.7 ka, followed until 410 ka by an interval with more vari- able SST and some values as low as 16.8 • C. The second SST plateau with values exceeding 18 • C lasted from 410 to 402.5 ka, but SST dropped permanently below 17.5 • C only after 396.6 ka with the transition into stadial MIS 11b. During the glacial inception of MIS 10, the SST record reveals four cold/warm cycles whose amplitude weakened towards MIS 10d (Fig. 6e). MIS 10b was associated with warmer SST that were as 5 warm as the warm oscillations within MIS 11a.

Comparison to published records
North Atlantic ODP Site 980 s δ 18 O data was discussed in previous publications (Oppo et al., 1998;McManus et al., 1999) and is shown in Fig. 7. This site's δ 13 C record of N. pachyderma (r) reveals the lowest δ 13 C values during the earliest phase of MIS 12 and 10 during the colder intervals of MIS 10 ( Fig. 7). δ 13 C levels recorded during interglacials MIS 13a and 11c were similar and about 0.5‰ heavier than those during interglacial MIS 9e. With the onset of the inception of glacial MIS 12 (480 ka Fig. 7. N. dutertrei records conditions towards the bottom of the seasonal thermocline (Billups et al., 2004) with the highest flux in winter (Deuser 20 and Ross, 1989). Thus its living conditions are comparable to those of G. inflata (Fairbanks et al., 1980;Deuser and Ross, 1989;Cléroux et al., 2007). Contrary to the other sites in this study, heaviest δ 13 C values were not concurrent with the sea level highstands of MIS 13a and 11c, both of which exhibit relatively low values (Fig. 7). Times with highest δ 13 C values coincided with late MIS 14 to 13c and with stadial MIS 11b. 25 For most of the record covered by Site 1056 (MIS 12-10; darker green line in Fig. 7 As this cooling of about 4 • C occurred during the MIS 11c sea-level highstand, upwelling of the deeper subpolar ENACW induced by strong winds -thus making the lithic grain a dust grain -is more likely the cause for the cooling. The short cooling at 326.3 ka during MIS 9e and the high variability in both δ 18 O and δ 13 C during MIS 13c and b are probably also related to the influence of upwelled waters. Increased productivity 5 during MIS 13c, most probably related to upwelling, is indicated by higher organic carbon concentrations (Voelker et al., unpubl. data). While the interglacial δ 18 O levels are similar at the two sites and for MIS 11c also in agreement with those recorded for core MD01-2443 ( Fig. 6f; de Abreu et al., 2005), G. inflata δ 13 C values at site MD01-2446 are generally higher than at site MD03-2699 indicating that more nutrients 10 were available in the offshore waters either because of lower nutrient consumption (open ocean vs. upwelling regime) or because the waters offshore had already higher preformed nutrient concentrations. The alkenone-derived SST (Fig. 6e) indicates extremely stable mean annual surface water temperatures during MIS 11c in the nearshore waters off Portugal. The new 15 record from core MD03-2699 agrees well with the one of core MD01-2443 ( Fig. 6g; Martrat et al., 2007). Both records show two plateaus within MIS 11c and a short minimum prior to the second, warmer plateau coinciding with the MIS 11c sea-level highstand (note that the minimum in MD01-2443 is shifted towards older ages with the Tzedakis et al. (2009) age model (Fig. 6g), while with the de Abreu et al. (2005) age 20 model (not shown) the minima are aligned). Such a strong SST stability over thousands of years is not seen in the NAC waters at IODP Site U1313 (Fig. 6b) where temperatures, however, reached values similar to those off Portugal. While not as clearly marked as at the Portuguese sites, Site U1313 also recorded two intervals with warmer SST during MIS 11c separated by a minimum, which at 41 • N was more pro- 25 nounced. More variable conditions in the NAC waters are probably linked to admixing of subpolar surface waters, especially during the first MIS11c temperature plateau when hydrographic conditions in the Nordic Seas (Helmke and Bauch, 2003) and the Arctic Ocean (Knies et al., 2007), thus in the subpolar and polar regions, were still unstable CPD Introduction

Tables Figures
Back Close

Full Screen / Esc
Printer-friendly Version Interactive Discussion due to freshwater release. The stability in annual mean temperatures off Portugal, on the other hand, must be related to a dominant influence of the subtropical AzC and IPC waters and thus confirm that the hydrographic (winter-time) situation off Portugal during MIS 11c was similar to the Present (Fig. 1b). Since the temperature trends were similar along the different North Atlantic surface currents it appears that the signal originated 5 in the tropical/subtropical regions of the Atlantic Ocean and was advected northwards. This agrees with the observation of Kandiano and Bauch (2007) that northward advection of subtropical waters is causing the SST optimum at Rockall Plateau site M23414. Tropical planktonic foraminiferal species also contributed significantly to the MIS 11c fauna of core MD01-2443 (de Abreu et al., 2005). In cores MD03-2699 and MD01-10 2446, the deeper dwelling tropical species Globorotalia menardii and Sphaeroidinella dehiscens, both of which do not occur in the modern fauna off western Iberia (Salgueiro et al., 2008), were found in MIS 11c (Fig. 6d) and 9e samples. Thus it appears that within the North Atlantic's eastern boundary system heat was transported northward not only by the IPC, but also at subsurface level through an enhanced contribution of 15 the eastern boundary undercurrent to the IPC's subsurface component. This enhanced flux of tropical subsurface waters, which are nutrient poor, could also contribute to the lower G. inflata δ 13 C values at site MD03-2699. It, furthermore, confirms that heat flux into the North Atlantic was at its maximum during the mid-Brunhes (Berger and Wefer, 2003). 20 MIS 11c interglacial conditions off Portugal ended around 395 ka with the onset of the 11b stadial (isotopic event 11.24). Cooling during this stadial was gradual and coldest conditions were reached only towards the end of the stadial coincident with IRD maxima around 388 ka (Figs. 4, 6). Even though site MD03-2699 received more IRD, δ 18 O-inferrred surface water-cooling seems to have been similar between the two 25 sites (Fig. 8d) , 2005) confirms maximum cooling towards the end of the stadial also for northern Iberia, but more importantly it shows that cooling along with the IRD peaks recorded at site MD03-2699 and MD01-2446 was restricted to the winters. Since melting icebergs and colder surface waters reached the western Iberian margin much later than IODP Site U1313 (Figs. 3, 6a, b) or ODP Site 980 (Fig. 6c), where IRD deposition peaked 5 1000 years prior but persisted until the end of the stadial (Oppo et al., 1998), a latitudinal front must have existed north of the Iberian Peninsula. In addition, it appears that it took ≥1000 years of meltwater flux into the subpolar and northern transitional regions to weaken AMOC enough to push this front as far south as 39 • N. MIS 11a and thus the glacial inception of MIS 10 is marked by four stadial/interstadial 10 cycles, the first interstadial of which is associated with isotopic event 11.23 (Figs. 3-6).
The cycles are best depicted in the alkenone SST records (Fig. 6b, e). Cooling during the second stadial is much stronger on the northern (Desprat et al., 2005) and middle Iberian margin (MD03-2699) than in the NAC waters (IODP Site U1313). Therefore a European or Scandinavian source for the cooling is more likely than advection with the 15 NAC from the western subpolar gyre, the typical source region for ice-rafting events during MIS 3. Such an eastern source region is supported by the stronger IRD signal at ODP Site 980 ( Fig. 6c; Oppo et al., 1998) than at IODP Site U1313, even given the differences in the IRD size fraction. The subsequent stadials had only small impacts on the SST at site MD03-2699, but were associated with short, but strong coolings in the 20 G. inflata δ 18 O record indicating the presence of colder surface waters during some of the winters (a more detailed discussion of the δ 18 O data follows below). The cold temperatures recorded by G. inflata might be linked to the ones recorded in the MD01-2443 alkenone record ( Fig. 6g; assuming an offset due to age models for stadial III). However, since alkenones represent annual mean temperatures and thus smooth a more 25 seasonal signal like the G. inflata δ 18 O data and since MD01-2443's cold alkenone SST are not reflected in the faunal based SST (de Abreu et al., 2005), these for this latitude unusually cold SST outside of a Heinrich event or glacial maximum need to be confirmed by other records from the northern Iberian margin or the Bay of Bis-

Tables Figures
Back Close

Full Screen / Esc
Printer-friendly Version Interactive Discussion caye, i.e. regions closer to potential sources for such cold waters. Such confirmation is especially important because stadials II and III did not coincide with Heinrich-type events (Hodell et al., 2008;Stein et al., 2009) and such strong coolings are (so far) not detected in other records either along the NAC path -IODP Site U1313 (Fig. 6a, b) and core M23414 (Kandiano and Bauch, 2003) -or more locally in the planktonic 5 δ 18 O records of cores MD01-2446 (Fig. 4) and MD01-2447(Desprat et al., 2005. In addition, at ODP Site 980 ice-rafting and cooling as indicated by the presence of N. pachyderma (s) was much lower during stadial III than stadial II (Fig. 6c). The pollen based terrestrial temperature records for core MD01-2447 also reveal only minor cooling during the younger stadials (Desprat et al., 2005), conform with the MD03-2699 10 SST record. Thus for the moment the SSTs recorded in core MD03-2699 appear to be more realistic for the western Iberian margin during MIS 11a than the MD01-2443 signal.
With the onset of MIS 11a the G. inflata δ 18 O and δ 13 C records of core MD03-2699 start to diverge from the offshore signal at site MD01-2446 (Fig. 8d). For most 15 of the glacial inception, δ 18 O values in core MD03-2699 stayed low, while values in core MD01-2446 increased as is to be expected with gradual cooling and increasing ice volume. The difference between these two relative closely located core sites can only be caused by a strong hydrographic front. Because G. inflata is reflecting winter mixed-layer conditions this front must have been the northward trending subtropical 20 front, in a manner similar to the present when the front allows warmer subtropical water to flow across the location of core MD03-2699 (Fig. 1b) (Fig. 8d), like during stadials II and III, did the front not exist and colder waters also penetrated into the nearshore regions. During early MIS 10 the subsurface component of the IPC was strengthened again with deep dwelling tropical foraminifer species (Fig. 6d) being advected to the mid-latitude nearshore Portuguese 5 margin. The increased (temporary) subsurface northward heat flux could explain why mean annual SST and the winter mixed layer at site MD03-2699 were hardly impacted by the Heinrich-type event during stadial MIS 10c (isotopic event 10.4), even though melting icebergs reached this site (Figs. 5, 6e). Since the signals for warmer surface to subsurface waters and IRD deposition occur in the same levels, seasonality at site 10 MD03-2699 must have been very high during MIS 10.
The pattern with a strong subtropical front separating sites MD03-2699 and MD01-2446 and with subtropical IPC waters dominating the nearshore waters of the Portuguese margin is not only seen for the glacial inception of MIS 10, but also during MIS 12 and with lesser intensity during MIS 14b (isotopic event 14.3) and the glacial 15 inception starting with stadial MIS 9d (Fig. 8d). Therefore the presence of a northward extending subtropical front off Portugal is a typical feature for the glacial inceptions and glacials of the mid-Brunhes period. For the transition from MIS 13a to 12a four short-term colder episodes are detected in the G. inflata δ 18 O record of core MD03-2699. Values during those times did not, however, reach the colder MD01-2446 levels 20 (Fig. 8d), except for the short IRD event at 470 ka (Fig. 5) that like stadial MIS 11b also had more likely an eastern source because a pronounced IRD peak is recorded at site M23414 (Kandiano and Bauch, 2003) and ODP Site 980 (Oppo et al., 1998) but not at IODP Site U1313 (Fig. 3). Overall, wintertime hydrographic conditions in the nearshore waters off Portugal appear to have been warmer and more stable dur- 25 ing MIS 12 than during the subsequent glacial inception. Such temperature "stability" points to a strong influence of the subtropical Azores Current in this region and is conform to evidence from the western Mediterranean Sea. There planktonic δ 18 O records of ODP Sites 976, 977 and 975 (Pierre et al., 1999;von Grafenstein et al., 1999) (Figs. 3, 4). During late MIS 12, minimum alkenone SST of core MD03-2699 are similar to minimum temperatures recorded during early MIS 10 (Fig. 6e). SST in nearshore waters off Portugal, however, rose continuously towards the deglaciation. During the MIS 12 glacial maximum at 430 ka a SST gradient of ≥6 • C existed between IODP Site U1313 10 and MD03-2699 ( Fig. 6b and e, respectively) indicating that a front -either just the subtropical front off Portugal or even the polar front -separated the surface waters in this mid-latitudinal band. The trend of rising SST off Portugal was abruptly interrupted by the significant cooling associated with the Heinrich-type ice-rafting event around 427 ka at the onset of the deglaciation (Hodell et al., 2008;Stein et al., 2009). Without this 15 event, SST off Portugal would probably have increased gradually towards MIS 11. This Heinrich-type event had a major impact on the hydrography in the North Atlantic, even leaving a fresher water signal in the G. inflata records of core MD01-2446 (low δ 18 O values contemporary with light δ 13 C values; Fig. 4), and led to a temporary AMOC shut down , just like its younger counterparts. A Heinrich-type event is 20 also associated with Termination IV as evident by core MD01-2446's records (Fig. 5), where the presence of melting icebergs led to strong oscillations in the planktonic δ 18 O values. 25 Both the PC and the IPC also have a subsurface component of subpolar or subtropical waters, respectively, whose properties contribute to those in the deep winter mixed layer in which deeper dwelling foraminifer like G. inflata calcify their tests. By tapping 1577 Introduction

Tables Figures
Back Close

Full Screen / Esc
Printer-friendly Version Interactive Discussion into subsurface waters deep winter mixing replenishes the nutrients available in the upper water column. Since δ 13 C values measured in planktonic foraminifer tests are related to nutrient concentrations (Broecker and Peng, 1982;Ortiz et al., 1996) we are using them to trace the subsurface/mode waters in the North Atlantic. Such an approach is facilitated by the fact that the subpolar mode water is formed during winter 5 along the NAC branches around the Rockall Plateau (Brambilla and Talley, 2008) and thus in close vicinity to ODP Site 980. Because the mode water is directly advected southward with the Portugal current the transport way is relatively short minimizing the time during which the δ 13 C signal could be modified. Furthermore, the selected planktonic foraminifer species represent conditions in winter and thus prior to the spring 10 blooms and upwelling season during which the δ 13 C would be altered due to nutrient consumption. The comparison between sites focuses on trends and not on absolute values because the δ 13 C values of the different species were not corrected to dissolved inorganic carbon (DIC) levels (except for the ODP Site 980 N. pachyderma (r) values in Fig. 8a and b where the correction was added to minimize the plot's δ 13 C range). Thus In the direct comparison it becomes clear that the offshore core MD01-2446 is very similar to North Atlantic IODP Site U1313 (Figs. 7,8c) indicating that for most of the studied interval hydrographic conditions, i.e. temperature and salinity properties, were 20 not much different in the NAC and the PC. For most of the time the G. inflata δ 13 C values at both sites were comparable and generally heavier than at site MD03-2699. However, there were also intervals when the two open ocean records diverged. One such example is the interstadial associated with isotopic event 11.23 when warm conditions in the NAC at IODP Site U1313 lasted longer than in the PC record of core 25 MD01-2446 that is more comparable with the NAC record at ODP Site 980 (Figs. 7,. Thus it might be that the NAC's main flow path was shifted more southward than its current position (Fig. 1) and that the NAC waters reaching the Rockall Plateau and feeding the PC were already modified by entrainment of subpolar waters. A strong CPD 5,2009 Variations in mid-latitude North Atlantic surface water properties Interactive Discussion linkage in water mass conditions between the Rockall Plateau NAC branch and the PC is confirmed by the δ 13 C records (Figs. 7, 8b). This relationship is especially evident during MIS 12 when the MD01-2446 δ 13 C data follows the ODP Site 980 record, especially the pronounced δ 13 C minimum during early MIS 12 that is only recorded at these two sites (Fig. 7). Thus it appear that the poorly ventilated surface to subsurface waters 5 advected southward with the PC during MIS 12 were formed near or above the Rockall Plateau, probably in regions similar to those of modern subpolar mode water formation (Brambilla and Talley, 2008). Enhanced influence of eastern basin waters is also consistent with a stronger imprint of the 470 ka IRD event in these records. MIS 12 δ 13 C values at IODP Site U1313, on the other hand, stayed fairly constant despite the strong 10 temperature and salinity oscillations indicated by the δ 18 O data and the presence of melting icebergs (Fig. 3). Thermocline waters in this region were better ventilated than in the eastern basin indicating that Gulf Stream waters (ODP Site 1056 and 1058; Fig. 7) still reached this latitude, in agreement with the relative warm SST during most of MIS 12 (Stein et al., 2009) that also imply northward heat advection. The persistent 15 nutrient supply by the thermocline waters supported increased phytoplankton production near Site U1313 (Stein et al., 2009). Even though supported only by isotope data for early MIS 10, the same pattern with well ventilated thermocline waters supporting high productivity (Stein et al., 2009) is also seen then, while the MD01-2446 and ODP Site 980 records indicate that the Rockall Plateau was -similar to MIS 12 -the source 20 region for the PC waters (Fig. 8b). Thus while during mid-Brunhes glacial intervals Gulf Stream derived NAC waters penetrated at least until 41 • N in the western basin, their influence was diminished in the eastern basin. Especially during early MIS 12, the position of the polar front appears to have been tilted in the North Atlantic reaching further to the south in the eastern than western basin. The glacial differences are also well 25 seen in the scatter plots ( Fig. 9a) (Fig. 9b); thus making the NAC like today the dominant hydrographic feature in the mid-latitude North Atlantic. Although the NAC and PC records at IODP Site U1313, ODP Site 980 and site MD01-2446 agree well, there is one interval when the MD01-2446 record diverges from the others and this is the δ 13 C minimum associated with Termination V (Fig. 8a-5 c). During the onset of the deglaciation δ 13 C levels are similar at the three sites. However, while the δ 13 C minimum at Site U1313 lasted for 10 ka, which is similar to ODP Site 980 (Fig. 8a), at site MD01-2446 it ended after only 5 ka, which is also sooner than at site MD03-2699 (Fig. 8d). The late glacial and deglacial δ 13 C minima are generally linked to poorly ventilated and thus nutrient-rich water masses such as Subarctic Inter-10 mediate Water (Venz et al., 1999)  Interactive Discussion record of IODP Site U1313 (Stein et al., 2009). The prolonged δ 13 C minimum implies advection of poorly ventilated waters into the mid-latitude North Atlantic. These waters were most likely Subarctic Intermediate Water since the records of ODP Site 982 on the northern Reykjanes ridge (57.5 • N) reveal the same δ 13 C minimum accompanied by persistent, but low input of IRD (Venz et al., 1999). Thus ice-bearing arctic waters 5 must have penetrated from the Nordic Seas southward across the Iceland-Faeroe ridge into the subpolar gyre where they mixed with the NAC waters since this is one of the region were subpolar mode water is formed at Present (Brambilla and Talley, 2008). Besides the poor ventilation the advection of subpolar waters to the Iberian margin had no impact because mean annual and summer SST in the nearshore band stretching conditions during early MIS 9e, the lithic grains found at site MD01-2446 (Fig. 4) could be wind transported. As already mentioned above the δ 13 C values in records for NAC and PC waters were mostly heavier than at site MD03-2699 in agreement with subpolar source waters being better ventilated than subtropical ones. Given that the δ 18 O and SST data 20 implies a strong IPC influence on the latter record, the δ 13 C values of core MD03-2699 should show some resemblance to those values recorded in the subtropical Gulf Stream waters at ODP Sites 1056 and 1058 (Figs. 7, 8e). Based on the scatter plots (Fig. 9c, d) there is only minor overlap between the records from the western and eastern side of the North Atlantic basin and that seems mainly to be restricted to MIS 11 25 as implied by the MD01-2443 data (Fig. 9d). The N. dutertrei δ 13 C record of ODP Site 1056 shows a similar pattern to core MD03-2699 during MIS 11c with lighter values indicating relatively more nutrients during the early phase and heavier values during the later phase of the interglacial (Fig. 8e). Consequently, this two-step feature in nutri-

CPD Introduction
Conclusions References Tables  Figures   Back  Close Full Screen / Esc

Printer-friendly Version
Interactive Discussion ent concentrations is a subtropical gyre signal. Trends and even absolute δ 13 C values were also similar at ODP Sites 1056 and 1058 and site MD03-2699 between 495 and 440 ka and after 350 ka (Fig. 8e); thus during those glacial times, when SSTs and δ 18 O data of core MD03-2699 indicate a strong subtropical water influence. The good correspondence during MIS 12 might indicate that cross-Atlantic transport with the AzC 5 was strong with little admixing of transitional waters. This enhanced AzC (heat) flux towards the southern Iberian margin would also explain why MIS 12 is much warmer in western Mediterranean Sea records than MIS 10 (Pierre et al., 1999;von Grafenstein et al., 1999). During the inception of MIS 10, nutrient concentrations at site MD03-2699 were higher than at ODP Site 1056 indicating that the Gulf Stream waters were greatly modified before reaching the western Iberian margin, if they contributed to the subsurface waters there at all. The δ 13 C records between the two sites also diverged during MIS 13c and b, when upwelling from either subtropical or subpolar ENACW influenced site MD03-2699. The records were also decoupled during the glacial maximum of MIS 12, when the subtropical front did not separate waters at site MD03-2699 15 from those recorded in core MD01-2446 and subpolar waters influenced the western Iberian margin.

Comparison of the interglacials
The new records from the mid-latitude North Atlantic confirm MIS 11c to be a long and relative stable interglacial in comparison to either MIS 13 or MIS 9. Especially the 20 NAC and PC waters experienced only minor changes in the hydrographic conditions and nutrient levels. Thus conditions in the mid-latitude North Atlantic were comparable to those of ODP Site 980 (Oppo et al., 1998;McManus et al., 2003). Based on the δ 18 O records hydrographic conditions in those waters were also stable during MIS 9e, while MIS 13c and 13a experienced some small-scale oscillations and consequently 25 less stable conditions (Fig. 7). MIS 13c and 13a are also confirmed to have been colder in than the subsequent interglacials with only short intervals during MIS 13a different. Hydrographic conditions during MIS 13c were highly variable due to intense upwelling (see Sect. 6.1), but were more stable and comparable to ODP Site 1058 during MIS 13a (Fig. 8e).
Overall, MIS 11c and 9e appear to have experienced comparable temperature and salinity conditions in the mid-latitude North Atlantic. However, there is one major dif-10 ference and that is the ventilation of the surface to subsurface waters and their impact on the AMOC. While no impact of the poorly ventilated and potentially fresher waters during MIS 9e is seen in the planktonic δ 18 O records presented here, they affected the ventilation of the deeper North Atlantic Deep Water (NADW) as evidenced by the benthic isotope record of IODP Site U1308 (Hodell et al., 2008), where well ventilated 15 NADW at 3870 m water depth was only recorded after 323.5 ka, and of core MD01-2446 (Voelker, unpublished data), where until 325 ka NADW ventilation was highly variable. Because the upper NADW as recorded at ODP Site 980 (McManus et al., 1999) was well ventilated, only the deeper branch was affected -either because the Iceland-Scotland Overflow Waters (ISOW) exiting from the Nordic Seas were poorly 20 ventilated or AMOC was shallower during early MIS 9e. Anyhow, a shallower AMOC or poorly ventilated ISOW sets MIS 9e apart from MIS 11c, when AMOC was so strong already early within the deglaciation that after 426 ka well ventilated NADW penetrated as deep as 3870 m (Hodell et al., 2008;Voelker et al., 2009 Interactive Discussion the enhanced northward heat flux hydrographic conditions in the offshore waters off Portugal were such that well ventilated mode waters were formed during the glacial to interglacial transition. So far these mode waters have only been recorded at site MD01-2446 and it remains to be seen in the future if their impact was locally restricted. MIS 13 is confirmed as colder than the subsequent interglacials and except for site MD03-2699 conditions during MIS 13c and 13a were not much different along the major currents. Due to apparently strong upwelling during MIS 13c the planktonic isotope records of core MD03-2699 experienced pronounced variability, while conditions during MIS 13a were relative stable and comparable to those in the Gulf Stream. Overall, it appears that conditions in the mid-latitude North Atlantic were not much different during 10 MIS 11c than during MIS 9e and, if one neglects the generally lower temperatures, also during MIS 13a. Current pathways and associated fronts were similar to today during all the interglacials. The difference between the interglacials lies in the ventilation of the subsurface waters. Here MIS 9e stands apart because the continuous admixing of arctic waters into the transitional waters of the mid-latitude North Atlantic. 15 Based on the closely spaced core sites MD03-2699 and MD01-2446 it is evident that a strong hydrographic front existed off Portugal, especially during the glacial inceptions and glacials. This front appears to have been equal to the northward extending subtropical front that exists during modern winters (Fig. 1b). East of the front, i.e. in the nearshore waters off Portugal, a strong influence of subtropical waters is recorded at 20 sites MD03-2699 and MD01-2443. Given this overprint of subtropical waters caution should be taken to interpret surface water records from nearshore sites off Portugal as basin-wide signals, at least for the mid-Brunhes interval studied here. Accordingly, the pronounced SST stability recorded in these waters during MIS 11c might be more typical for the subtropical gyre than a Gulf Stream/NAC signal (Fig. 6). Future records 25 off NW Iberia and off NW Africa, i.e. up-and downstream, might help to shed some light onto this question. The hydrographic front disappeared sporadically during stadials of the glacial inception of MIS 10, but cooling episodes in the nearshore waters were much shorter than those recorded offshore (Figs. 7, 8d) Increased heat transport with the AzC across the Atlantic is also seen during MIS 12 when δ 13 C records of ODP Sites 1056 and 1058, representing Gulf Stream waters, and of core MD03-2699 were most similar (Fig. 8e). The alkenone SST record of core MD03-2699 confirms relative warm waters at this site and warming from the glacial maximum to the interglacial would probably have been continuously if it had not been 5 interrupted by a Heinrich-type ice-rafting event at the onset of the deglaciation that left a freshwater signal in the offshore waters at site MD01-2446. Cooling during this event is comparable to the cooling observed during younger Heinrich events (e.g. de Abreu et al., 2003). While the nearshore waters off Portugal had a subtropical source during MIS 12, the offshore waters were derived from the Rockall Plateau region as the close 10 correspondence between the records of ODP Site 980 and core MD01-2446 reveal (Fig. 8b). The surface waters in the eastern basin were poorer ventilated than those in the western basin (IODP Site U1313), even though the western basin experienced stronger salinity oscillations linked to more frequent ice-rafting events.   (de Abreu et al., 2005;Martrat et al., 2007).G. inflata δ 18 O records are shown in blue (a, d, f), alkenone-based mean annual sea surface temperature (SST) records in red (b, e, g). The abundance of Lithics >315 µm (black; only sections with <4 grains/g) are shown for IODP Site U1313 (b) and core MD03-2699 (e) and >150 µm for ODP Site 980 (c; Oppo et al., 1998). Panel (c) also includes the % N. pachyderma (s) record of ODP Site 980 (orange; Oppo et al. 1998). In panel (d) magenta bars indicate presence of Globorotalia menardii and dark blue ones of Sphaeroidinella dehiscens in the respective levels. MD01-2443 data is shown using the age model of (Tzedakis et al., 2009) that links the MD01-2443 benthic δ 18 O data to the EDC δD record on the EDC 3 timescale (Jouzel et al., 2007) following the approach of (Shackleton et al., 2000). Grey bars indicate stadials (numbered I to IV) within MIS 11a. The bar outlined in grey during the oldest stadial marks the interval when cold conditions already prevailed at (I)ODP Sites U1313 and 980. H denotes the Heinrich-type ice-rafting event associated with Termination V.    de Abreu et al., 2005; with extreme values between 375 and 381 ka excluded).