Centennial- to millennial-scale ice-ocean interactions in the subpolar northeast Atlantic 18-41 kyr ago

Centennial‐to millennial‐scale ice‐ocean interactions in the subpolar northeast Atlantic 18–41 kyr ago I. R. Hall, 1 E. Colmenero‐Hidalgo, 1,2 R. Zahn, 3,4 V. L. Peck, 5 and S. R. Hemming 6 Received 16 November 2010; revised 14 February 2011; accepted 28 March 2011; published 30 June 2011. [1]In order to monitor the evolution of the British‐Irish Ice Sheet (BIIS) and its influence in surface ocean structure during marine isotopic stages (MIS) 2 and 3, we have analyzed the sediments recovered in core MD04‐2829CQ (Rosemary Bank, north Rockall Trough, northeast Atlantic) dated between ∼41 and ∼18 ka B.P. Ice‐rafted debris flux and composition, 40 Ar/ 39 Ar ages of individual hornblende grains, multispecies planktonic stable isotope records, planktonic foraminifera assemblage data and faunal‐based sea surface temperatures (SSTs) demonstrate a close interaction between BIIS dynamics and surface ocean structure and water properties in this region. The core location lies beneath the North Atlantic Current (NAC) and is ideal for monitoring the shifts in the position of its associated oceanic fronts, as recorded by faunal changes. These data reveal a succession of BIIS‐sourced iceberg calving events related to low SST, usually synchronous with dramatic changes in the composition of the planktonic foraminifera assemblage and with variations in the stable isotope records of the taxa Neogloboquadrina pachyderma (sinistral coiling) and Globigerina bulloides. The pacing of the calving events, from typically Dansgaard‐Oeschger millennial timescales during late MIS 3 to multicentennial cyclicity from ∼28 ka B.P., represents the build‐up of the BIIS and its growing instability toward Heinrich Event (HE) 2 and the Last Glacial Maximum. Our data confirm the strong coupling between BIIS instabilities and the temperature and salinity of surface waters in the adjacent northeast Atlantic and demonstrate the BIIS’s ability to modify the NAC on its flow toward the Nordic Seas. In contrast, subsurface water masses were less affected except during the Greenland stadials that contain HEs, when most intense water column reorganizations occurred simultaneously with the deposition of cream‐colored carbonate sourced from the Laurentide Ice Sheet. Citation: Hall, I. R., E. Colmenero‐Hidalgo, R. Zahn, V. L. Peck, and S. R. Hemming (2011), Centennial‐to millennial‐scale ice‐ocean interactions in the subpolar northeast Atlantic 18–41 kyr ago, Paleoceanography,26, PA2224, doi:10.1029/2010PA002084. 1. Introduction [2] Since the discovery of millennial and faster climate variability in Greenland ice cores in the early 1990s [Dansgaard et al., 1993; Greenland Ice Core Project Members, 1993; Grootes et al., 1993], understanding of these climatic oscillations has increased rapidly. The Dansgaard‐ Oeschger (DO) cycles are the most prominent manifestation of climate instability in the recent geological past as they represent substantial decadal air temperature fluctuations between Greenland stadials (GSs) and interstadials (GIs) of the order of 5°–10°C in the North Atlantic region [Dansgaard et al., 1993]. They coincide with variations in the properties of oceanic water masses recorded in sediment cores in the North Atlantic and beyond. These records reveal the close links between atmospheric changes and ocean dynamics [e.g., Bond et al., 1992, 1993; Cortijo et al., 1997; Keigwin and Lehman, 1994; Vidal et al., 1997]. An increasing number of sediment records recovered offshore the British Isles [Dickson et al., 2008; Haapaniemi et al., 2010; Hibbert et al.,2010;Knutz et al., 2001, 2002, 2007; Peck et al., 2006, 2007a, 2007b, 2008; Peters et al.,2008, 2010; Scourse et al., 2000, 2009] and in the Nordic Seas [Fronval et al., 1995; Kuijpers et al., 1998; Lekens et al., 2006, 2009; Meland et al., 2008; Rasmussen et al., 1996a, 1996b; Rasmussen and Thomsen, 2004] show that the Northwestern European Ice Sheet (NWEIS) was an important element of the climate of the last glacial, in that it played a major 1 School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK. 2 Now at Facultad de Ciencias Biológicas y Ambientales, Universidad de León, Campus de Vegazana, León, Spain. 3 Institut de Ciencia i Tecnología Ambientals, Departament de Física, Universitat Autónoma de Barcelona, Bellaterra, Spain. 4 Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain. 5 British Antarctic Survey, Cambridge, UK. 6 Lamont‐Doherty Earth Observatory and Department of Earth and Environmental Sciences, Columbia University, Palisades, New York, USA. Copyright 2011 by the American Geophysical Union. 0883‐8305/11/2010PA002084 PALEOCEANOGRAPHY, VOL. 26, PA2224, doi:10.1029/2010PA002084, 2011 PA2224 1of18 role in the freshwater release that led to disruptions in deep‐ water convection and influenced the Atlantic Meridional Overturning Circulation (AMOC). Of all NWEIS sectors, the British‐Irish Ice Sheet (BIIS) was directly situated in, and highly dependent on, the track of moisture and heat transport derived from the North Atlantic Current (NAC) [Hansen and Østerhus, 2000]. As a result, the dynamics of the small‐sized, fast responding BIIS were tightly coupled to variations in such transports, making nearby oceanic sediment cores highly sensitive records in which to monitor ocean‐ice interactions. Evidence of BIIS growth and instability has been documented in high‐resolution sedimentary sequences along its margins [e.g., Dickson et al., 2008; Knutz et al., 2002, 2007; Peck et al., 2006, 2007a, 2007b, 2008; Scourse et al., 2009] and recent modeling studies confirm that the BIIS was the dominant source of ‘background’ice‐ rafted debris (IRD) supplied to the North Atlantic during the last glacial [Bigg et al., 2010], but additional data are needed to achieve a comprehensive regional view of ocean‐ BIIS interactions. [3] Here we present new high‐resolution paleoceanographic records from a sediment core recovered close to the northwestern margin of the BIIS, aimed at extending results of previous records [Knutz et al., 2002, 2007] well into marine isotopic stage (MIS) 3 in order to depict in fine detail the sequence of BIIS growth and decay between 41 and 18 ka B.P. We present extensive 40 Ar/ 39 Ar dating results on hornblende grains that, together with IRD composition, enable IRD to be evaluated. We also compare IRD flux with multispecies planktonic foraminifera stable isotope records and assemblage data, and with faunal‐based sea surface temperature estimates (SST) as a means of closely assessing the interactions between ice sheet dynamics and surface water structure. Bringing these two approaches together enables a comprehensive and high‐resolution view of the role of the BIIS in the forcing of past short‐term climate variability. 2. Core Location and Oceanographic Setting [4] Sediment core MD04‐2829CQ (58°56.93′N, 9°34.30′W; 1007 cm) was retrieved at 1743 m water depth on a contourite drift located southeast of Rosemary Bank in the northern Rockall Trough during the R/V Marion Dufresne cruise MD141, within the SEQUOIA (Sequencing Ocean‐ Ice‐Climate Interaction in the NE Atlantic during the Last Glacial) program [Hall and Scourse, 2005] (Figure 1). The core reoccupies the site of core DAPC2 that retrieved the interval 10–27 ka B.P. [Hall et al., 2006; Knutz et al., 2002, 2007] and was primarily targeted to extend the DAPC2 paleoceanographic records into MIS 3. Rosemary Bank is an ideal location for the recovery of continuous, high‐resolution sediment records containing evidence of the variability of the BIIS, yet sufficiently far, so as not to be directly influenced by downslope processes triggered by shelf ice activity [Sejrup et al., 2005], such as those shown in other cores from the nearby Barra‐Donegal Fan area [e.g., Knutz et al., 2001; O’Reilly et al., 2007]. [5] MD04‐2829CQ is also sensitively located to monitor changes of surface ocean conditions related to variability in circulation of the North Atlantic. At present, surface water circulation in the area is dominated by the northeastward flow of the warm and saline (>7°C, >35 psu) NAC, responsible for a large amount of heat and moisture transport to high latitudes, reaching as far north as the Arctic Ocean [Hansen and Østerhus, 2000]. The NAC meets colder and fresher (<5°C, <34.9 psu) polar waters north of the Faeroe Islands, resulting in the development of the Iceland‐Faeroe Front not far from the core site, generally close to the Iceland‐Scotland Ridge [Hansen and Østerhus, 2000; Perkins et al., 1998]. Past migrations of this oceanic front due to changes in the strength of the surface currents are likely registered in our record. [6] Two main deep water masses affect the core site: (1) a branch of the cold and saline (<3°C, <35 psu) Norwegian Sea Overflow Water (NSOW), a precursor to North Atlantic Deep Water (NADW) that enters the North Atlantic basin through the Faeroe‐Shetland Channel and over the Wyville‐Thomson Ridge, just northeast of the site, and (2) a boundary current of the main Northeastern Atlantic Deep Water (NEADW, recirculated upper North Atlantic Deep Water) that flows northward along the eastern European margin at 2–3 km water depth and reaches the core location regularly [Hansen and Østerhus, 2000; McCartney, 1992; New and Smythe‐Wright, 2001; van Aken, 2000]. 3. Materials and Methods 3.1. Core Sampling [7] Sediment core MD04‐2829CQ was sampled every centimeter across the interval 300–1008 cm, yielding a total of 708 sample slices of 1 cm thickness. Each sample was wet sieved with distilled water over a 63 mm mesh and both coarse and fine fractions recovered, dried at 40°C and weighed. Coarse (>63 mm) fractions were split into two aliquots for stable isotope, and petrological and micropaleontological analyses, which were carried out every 2 to 4 cm. 3.2. Faunal and Lithic Counts [8] The subsamples for foraminifera faunal assemblage and lithic counts were dry‐sieved over a 150 mm mesh, and the portion >150 mm was split again to yield an aliquot fraction of at least 400 planktonic foraminifera and 300 mineral grains, which were identified and counted. Planktonic foraminifera were identified following Kennett and Srinivasan [1983]. Relative proportions of selected taxa are presented in this work. [9] All mineral grains >150 mm, with the exception of authigenic glauconite nodules, were considered to be IRD, and were divided into transparent quartz, haematite‐coated grains, finely crystalline, cream‐colored carbonate, volcanics (grey pumice, rhyolitic glass), volcanic basalt, dark grey and brown carbonates, black limestones, metamorphic and igneous rock fragments, mica flakes and hornblende grains, among the most common. The provenance of the IRD assemblage was established following the criteria outlined by Peck et al. [2007a]. Results are presented as fluxes in units of grains per square centimeter per kiloyear. 3.3. Sea Surface Temperature [10] The resulting planktonic foraminifera assemblage counts were employed to estimate SST at 10 m water depth by running a transfer function based on a back propagation HALL ET AL.: BRITISH‐IRISH ICE SHEET INSTABILITY PA2224PA2224 2of18 artificial neural network (ANN) [Malmgren et al., 2001] trained on the North Atlantic MARGO Project data set [Kucera et al., 2005]. The same data set was used to apply a Modern Analogue Technique (MAT) [Prell, 1985] to the faunal counts. For the ANN, a set of ten neural networks was considered, providing 10 different mean annual SST reconstructions, which were averaged to estimate the SSTs and standard deviations from the calibration data set. The MAT was applied using a set of ten modern analogs, allowing the calculation of a similarity index for MAT SSTs. 3.4. Dating of 40 Ar/ 39 Ar on Hornblende Grains [11] A total of 421 individual hornblende fragments from the >150 mm fraction were picked when present and analyzed to provide for provenance data of IRD. Grains from the same horizon were grouped in the same sample, yielding a total of 184 samples covering the interval from ∼18 to ∼40 ka B.P., that were sent to the Cd‐lined core facility (CLICIT) at the Oregon State University reactor (USA) for their coirradiation with the hornblende monitor standard Mmhb (525 Ma [Samson and Alexander, 1987]). Argon activity analyses were performed at the Ar Geochronology Laboratory, Lamont‐Doherty Earth Observatory (LDEO, USA), where individual grains were fused with a CO 2 laser and ages were calculated from the resulting Ar isotope ratios, corrected for mass discrimination, interfering nuclear reactions, procedural blanks and atmospheric Ar contamination following the standard procedure of McDougall and Harrison [1999]. 3.5. Isotope Measurements [12] Aliquots for stable isotope analyses were dry‐sieved over 250 and 355 mm meshes. Around 10–15 specimens of the planktonic foraminifera Neogloboquadrina pachyderma (sinistral coiling), mostly the encrusted type, were obtained every 2 cm, while up to 15–20 individuals of Globigerina bulloides were picked when possible. Around 25 specimens of Turborotalita quinqueloba (>150 to <250 mm) were also Figure 1. Bathymetric map (in meters) showing the location of cores MD04‐2829CQ and MD01‐2461 and the approximate path of present‐day surface and deep‐water masses. NAD, North Atlantic Drift (grey arrows); NSOW, Nordic Seas Overflow Water (dashed black arrows); NEADW, Northeastern Atlantic Deep Water (black arrows); FSC, Faroe‐Shetland Channel; WTR, Wyville‐Thomson Ridge; RB, Rosemary Bank; BDF, Barra and Donegal Fans; and HS, Hebrides Shelf. Double dashed black lines indicate the estimated maximum extent of the northeastern European and Faroe Islands ice sheets during the Last Glacial Maximum, and grey arrows indicate approximate location of the associated ice streams (references in text). HALL ET AL.: BRITISH‐IRISH ICE SHEET INSTABILITY PA2224PA2224 3of18 picked every 2 cm in the interval 780–830 cm. Prior to analyses samples were submerged in hydrogen peroxide (3%) for 30 min and briefly ultrasonicated in ethanol. Excess liquid and residues were quickly removed and the tests dried at low temperatures (40°C). T. quinqueloba samples were not cleaned or crushed prior to analysis due to their fragility and small size. All samples were analyzed on a ThermoFinnigan MAT 252 mass spectrometer coupled to a fully automated CARBO Kiel carbonate preparation device at the Stable Isotope Laboratory of Cardiff University (UK). Isotope results are reported in standard delta notation relative to Vienna Pee Dee Belemnite (VPDB), using NBS‐19 for calibration. The external reproducibility of the d 18 O determination is ±0.06 ‰. [13] Possible contamination of isotopic results by clay‐ size dolomitic carbonate fragments attached to shell walls [Hodell and Curtis, 2008] was reduced to a minimum since all N. pachyderma sin. samples from cream‐colored carbonate bearing IRD horizons (mainly HE 2 and HE 4) were crushed before cleaning, as well as G. bulloides samples corresponding to HE 4. [14] Extra analyses on specimens from intermediate horizons within intervals with anomalous isotopic values (e.g., spikes in the G. bulloides d 18 O record) were performed when possible, yielding a final 1 cm resolution on those specific intervals. All paleodata will be publicly available through the World Data Centre for Marine Environmental Sciences (http://www.wdc‐mare.org; http://www.pangaea.de). 4. Age Model [15] The chronostratigraphic framework of core MD04‐ 2829CQ is based on the calibration to calendar years (cal years B.P.) of 24 AMS 14 C dates determined from monospecific samples containing more >1000 specimens (equivalent >10 mg of carbonate) of N. pachyderma sin. or G. bulloides (Table 1). Samples for dating were selected from sections where abundance of either taxon maximized in order to minimize the influence of bioturbation [Bard et al.,1987].AMS 14 C analyses were run at the NERC Radiocarbon Laboratory facility (East Kilbride, UK). Carbon‐ 14dates were converted to calendar years using the online calibration program of Fairbanks et al. [2005] (www. radiocarbon.ldeo.columbia.edu) with its January 2007 database, applying a 400 year correction for global marine reservoir age [Stuiver et al., 1998] that coincides with modern local estimates for the area between 40°–70°N in the North Atlantic [Bard, 1988; Butzin et al., 2005] (Figure 2). The resulting age model was then fine tuned and extended beyond current AMS 14 C coverage by correlating relative abundance of N. pachyderma sin. to the GISP 2 Greenland ice core d 18 O ice record [Grootes and Stuiver, 1997] (Figures 2 and 3a and Table 2.). The GISP 2 record has estimated age uncertainties of about ±2% for the age interval studied here [Meese et al., 1997], and shows a good long‐term agreement with the more recent NGRIP ice core [Svensson et al., 2008]. Therefore it was selected for correlation instead of the NGRIP in order to facilitate comparison of our results to those from cores MD01‐2461 and DAPC2, also tuned to GISP 2 [Knutz et al., 2007; Peck et al., 2006, 2007a, 2007b, 2008]. The name convention of the INTIMATE group for the GSs and GIs is used [Lowe et al., 2007]. [16] Tuning N. pachyderma sin. abundance maxima to Greenland stadials is a common procedure in North Atlantic records [e.g., Bond et al., 1993; Bond and Lotti, 1995; Knutz et al., 2007; Peck et al., 2006] and is based on the obserTable 1. AMS Radiocarbon Ages for MD04‐2829CQ and Their Calibrated Correspondence as Well as Their Calendar Age According to the Tuned‐to‐GISP 2 Age Model and the Difference in Years Between Both Age Models as Plotted in Figure 3b Laboratory Code Depth (cm) Material 14 C Age (years) Error Age (±1syears) Calendar Age (cal years B.P.) a Error Age (± years) Age GISP 2 (cal years B.P.) Difference Age Models (years) SUERC‐8793 312.5 N. pachyderma l.c. 16,732 65 19,450 86 18,498 952 SUERC‐8794 376.5 N. pachyderma l.c. 17,254 69 20,024 107 19,380 644 SUERC‐8795 391.5 N. pachyderma l.c. 17,382 70 20,174 91 19,595 579 SUERC‐8797 422.5 N. pachyderma l.c. 17,706 73 20,470 94 20,024 446 SUERC‐8798 438.5 N. pachyderma l.c. 17,992 76 20,806 128 20,242 564 SUERC‐8799 457.5 N. pachyderma l.c. 18,231 78 21,127 141 20,373 754 SUERC‐8802 495.5 N. pachyderma l.c. 18,312 80 21,239 146 20,649 590 SUERC‐8803 510.5 N. pachyderma l.c. 18,569 83 21,626 165 20,758 868 SUERC‐8804 534.5 N. pachyderma l.c. 18,670 82 21,802 162 21,159 643 SUERC‐8805 544.5 N. pachyderma l.c. 18,804 84 22,012 132 21,436 576 SUERC‐8807 560.5 N. pachyderma l.c. 19,597 92 22,793 130 21,879 914 SUERC‐8808 592.5 N. pachyderma l.c. 20,328 101 23,813 143 22,764 1049 SUERC‐8809 618.5 N. pachyderma l.c. 20,512 104 24,021 132 23,252 769 SUERC‐8812 636.5 N. pachyderma l.c. 20,696 106 24,203 121 23,539 664 SUERC‐8813 648.5 N. pachyderma l.c. 21,383 117 25,107 178 23,911 1196 SUERC‐8814 664.5 N. pachyderma l.c. 21,447 116 25,190 171 24,408 782 SUERC‐8815 720.5 N. pachyderma l.c. 23,505 151 27,726 220 26,441 1285 SUERC‐8816 762.5 N. pachyderma l.c. 24,779 174 29,160 234 28,112 1048 SUERC‐8817 784.5 G. bulloides 25,711 197 30,465 269 28,792 1673 SUERC‐10904 807.5 N. pachyderma l.c. 26,480 146 31,318 210 30,017 1301 SUERC‐10899 816.5 N. pachyderma l.c. 26,963 154 31,839 224 30,679 1160 SUERC‐10900 880.5 N. pachyderma l.c. 30,073 223 35,092 254 34,146 946 SUERC‐10901 916.5 N. pachyderma l.c. 32,909 306 37,905 344 36,755 1150 SUERC‐10902 950.5 N. pachyderma l.c. 35,136 399 40,092 421 39,020 1072 a Converted using the online calibration program of Fairbanks et al. [2005], January 2007 version, and assuming a constant 400 year marine reservoir correction. HALL ET AL.: BRITISH‐IRISH ICE SHEET INSTABILITY PA2224PA2224 4of18 vation that this taxon is dominant in present‐day polar and subpolar assemblages [Bé, 1977; Ruddiman and McIntyre, 1981]. Hence it is considered a proxy for both arctic water masses with seasonal ice cover [Johannessen et al., 1994] and polar water masses with perennial sea ice cover [Elliot et al., 1998; Kuijpers et al., 1998; Pflaumann et al., 1996]. [17] The resulting chronostratigraphic framework (Figure 2) shows that our records span an interval of about 23 kyr, extending between mid MIS 3 (∼41.1 ka B.P.) and mid MIS 2(∼18.3 ka B.P.). The age model has independently been constrained by the identification of the Laschamp excursion (∼41 ka B.P.) [Lund et al., 2005] in the magnetic record (E. Moreno, personal communication, 2010). Mean sedimentation rates for the studied interval are approximately 31 cm kyr −1 , yielding a mean time step of ∼65 years for each 2 cm interval, with maximum rates of up to 137 cm kyr −1 recorded during MIS 2. Sedimentation rates increase from ∼26 ka B.P. toward the Last Glacial Maximum (LGM), in a similar fashion to those recorded in nearby cores MD95‐ 2006, DAPC2 and ENAM32 [Knutz et al., 2001, 2007; Lassen et al., 1999] and sites in the southern Norwegian Sea [Nielsen et al., 2007; Rasmussen et al., 1996b], probably reflecting a combination of changing bottom current intensity and terrigenous sediment supply to the core site. [18] Significant temporal discrepancies between the initial 14 C‐calibrated and the calendar year timescale derived from tuning to the GISP 2 age model (Figure 2) suggest that local marine reservoir ages (DR) have shifted within a range of ∼430 to ∼1700 years, far larger than the modern global average 400 year reservoir correction (Figure 3b). Mean implied DR values during MIS 2 are typically ∼700 years, while in MIS 3 the divergence increases to ∼1000 years. Maximum DR values are found close to H events. High reservoir ages during colder periods are systematically registered in records recovered off the British Isles [Knutz et al., 2007; Lekens et al.,2006;Peck et al., 2006] and in other regions of the glacial North Atlantic, due to a combination of sea‐ice and meltwater induced reduction of air‐sea gas exchange, ventilation and isotope equilibration [Austin et al., 1995; Bard et al.,1994;Voelker et al., 1998; Sarnthein et al., 2007; Waelbroeck et al., 2001]. Increased contribution of 14 C‐depleted Antarctic waters reaching the high latitude North Atlantic [Cao et al., 2007] possibly have added to the elevated reservoir ages. The effect of these differences apparently decreases toward lower latitudes, deglacial warm periods and the Holocene [Cao et al., 2007; Waelbroeck et al., 2001]. 5. Results 5.1. Abundance Distribution of Planktonic Foraminifera [19] Changes in the relative abundances of planktonic foraminifera closely reproduce the variability seen in the GISP 2 d 18 O ice record (Figure 4). Large fluctuations in the assemblage composition characterize the period between 41 and 27 ka B.P., in accordance with the high‐amplitude GS‐GI cycles 4 to 11. N. pachyderma sin. dominates during HE and GSs, while transient increases of temperate/ subtropical taxa (e.g., Globorotalia scitula) mark the warmer GIs, dominated by G. bulloides and T. quinqueloba. Abundance of N. pachyderma (dextral coiling; abundances not shown here) also increases during GIs but never reaches values above 5%. Table 2. Depths and Corresponding Calendar Ages of the Tie Points Used for the Fine Tuning of the Radiocarbon‐Based Age Model to the GISP 2 Oxygen Isotope Record a MD04‐2829CQ Depth of Pointers (cm) Age GISP 2 (cal years B.P.) Stratigraphic Position Sedimentation Rates (cm kyr −1 ) 300 18,334 GS 1, top of record – 369 19,280 GS 1 72.90 434 20,215 GS 1 69.87 522 20,851 GS 1 137.62 600 23,017 base GI 2 36.13 633 23,475 base GI 2 71.69 687 25,151 GS 3 32.20 759 28,053 base GI 3 24.87 794 29,132 base GI 4 32.34 815 30,681 base “DO 4.1”13.59 826 32,312 base GI 5 6.75 864 33,672 GI 6 27.88 879 34,146 base GI 6′31.73 897 35,281 base GI 7 15.82 936 38,467 base GI 8 12.25 960 39,492 HE4 23.53 1007 41,153 top GI 10, base of record 28.24 a Explanation and references in text and in Figure 3. The approximate stratigraphic position of the selected intervals (Figure 3a) and the partial sedimentation rates (Figure 2) are also included. Figure 2. Age models of core MD04‐2829CQ. (a) Solid triangles, raw radiocarbon AMS dates; (b) thin black line, AMS 14 C dates calibration to calendar ages; and (c) thick black line, tuned‐to‐GISP2 age model. The grey area evidences the divergence between the AMS 14 C calibrated and the tuned‐to‐GISP2 age models. The sedimentation rates according to the tuned‐to‐GISP2 age model are also plotted. Positions of HE 2–4 are included for reference. HALL ET AL.: BRITISH‐IRISH ICE SHEET INSTABILITY PA2224PA2224 5of18 [20]N. pachyderma sin. is the dominant taxon between 27 and 18 ka B.P., showing small abundance shifts during the LGM interval coincident with centennial‐scale climate variations observed in Greenland. Major exceptions are two brief abrupt increases of the cold/temperate species Turborotalita quinqueloba and G. bulloides coincident with GI 2. 5.2. Faunal‐Based Sea Surface Temperatures [21] SST from ANN and MAT techniques generated very similar records and values, with the occasional exception, for example peak warmth during GI 3 and 4, where ANN estimates suggest higher temperatures up to 3°C higher than MAT (Figure 4). A few levels from both reconstructions, however, are affected by no‐analog situations, as indicated by MAT similarity values below 0.7 and ANN standard deviations larger than 1s[Kucera et al., 2005] (Figure 4e). Most of these intervals also show the highest degree of disagreement between the two techniques and so should be considered with some caution. Nevertheless, it is also the case that these no‐analog situations typically occur within warm intervals, when assemblages are dominated by G. bulloides and T. quinqueloba with less than 5% of N. pachyderma dex., an unusual assemblage with no present‐ day analog in the MARGO Atlantic database (M. Kucera, personal communication, 2008). However, low N. pachyderma dex. abundance, even during GIs, is reported in other cores recovered from the Rockall Trough and surrounding area [e.g., Knutz et al., 2007; Kuijpers et al., 1998; van Kreveld et al., 2000] suggesting a regional ecological feature in response to oceanographic conditions. Therefore, we consider that these SST estimates provide a generally faithful trend reconstruction, supported by the similarity of the curves to that of the GISP 2 d 18 O ice record. [22] MIS 3 GS‐GI abrupt SST warmings average 5°C and generally vary between ∼4 and ∼10°C, which is consistent with the values obtained in nearby core MD01‐2006 for the period between ∼55.5 and ∼40 ka B.P. [Dickson et al., 2008] andinMIS3‐2 records further to the northwest [van Kreveld et al., 2000]. The colder extreme (∼4°C) is lower than coldest temperatures (∼6°C) reconstructed for MIS 3 for core MD01‐2461 [Peck et al., 2008], likely due to latitudinal differences. Mean SSTs of ∼9°C during GIs are within the range of values displayed by modern NAC in the area, GS mean values of ∼5°C represent the influence of more Arctic waters [Hansen and Østerhus, 2000]. Remarkably, SSTs increase during MIS 2 toward the LGM (18 to 21 ka B.P.) from an initial value of ∼4°C at HE 2 to ∼7°–8°C at 20 ka B.P. This ∼4°C to ∼8°C SST oscillation displayed in the LGM fits within the SST ranges obtained in several Figure 3. (a) Correlation of the relative abundance of N. pachyderma sin. recorded in core MD04‐ 2829CQ with the GISP 2 oxygen isotope record [Grootes and Stuiver, 1997]. Dotted arrows connect tie points used for the correlation, while black triangles indicate the horizons dated with AMS 14 C. (b) Difference in years between the calibrated AMS 14 C ages and the calendar ages for the same horizons as given by the tuned‐to‐GISP2 age model. The horizontal dashed line marks the average modern mean reservoir age. The position of HE 2 to HE 4 is plotted for reference. HALL ET AL.: BRITISH‐IRISH ICE SHEET INSTABILITY PA2224PA2224 6of18 cores south of the Iceland/Faroe Ridge for the same time interval [Weinelt et al., 2003]. 5.3. Lithic Composition and Abundance [23] The flux of total IRD to the core site is continuous throughout the 41–18 ka B.P. interval (Figure 5); however warmer GIs are characterized by very low abundances of mineral grains while increased fluxes occur during HE, GSs and MIS 2. The sequence of centennial‐scale IRD events superimposed on a background of increasing IRD fluxes from ∼28 ka B.P. toward HE 2 is a major aspect of the record. This feature is also observed in the IRD record of cores SU90‐09 and MD95‐2002 [Auffret et al., 2002; Grousset et al., 2000, 2001] in the southeastern margin of the main IRD belt, and is similar to the increases shownincoresDAPC2[Knutz et al., 2002, 2007] and MD01‐2461 [Peck et al., 2006] before HE 1. Also, spectral analysis of the record (multitaper method [Pardo‐Igúzquiza Figure 4. Planktonic foraminifera and SST records from core MD04‐2829CQ. (a) GISP 2 ice core record, (b) relative abundances of N. pachyderma sin., (c) percent of other planktonic foraminifera taxa, (d) summer SST estimates at 10 m depth using the ANN and MAT techniques, and (e) estimated mean similarity for MAT and 1serror for ANN. Intervals of dubious SST reconstructions are marked by small bars (MAT) and lines (ANN). Grey vertical bars mark the position of HE (dark grey) and some of the Greenland stadial (light grey) horizons. HALL ET AL.: BRITISH‐IRISH ICE SHEET INSTABILITY PA2224PA2224 7of18 et al., 1994] with seven data tapers and spectral confidence levels located using the robust AR(1) modeling of median‐ smoothed spectra, not included here) shows significant centennial‐scale oscillations during MIS 2, which follow a rough 500 year cyclicity, while between 30 and 41 kyr the frequency of IRD events decreases to around 1100 years. [24] The dominant components of the mineral grain assemblage, transparent quartz and haematite‐stained grains, are likely sourced in a range of locations around the North Atlantic that are impossible to isolate. The remaining grains comprise a wide range of lithologies. HE 4 and 2 stand out as peaks of finely crystalline, cream‐colored carbonate (>750 grains cm −2 kyr −1 at HE 4; Figure 5d), classically diagnostic of HEs, identified as sourced from dolomitic limestones outcropping in the Hudson Bay Province of the LIS [Andrews and Tedesco, 1992; Bond et al., 1992]. Sedimentary lithics originating from the BIIS and igneous rock fragments typically increase during cold periods. Basalt grains, petrographically similar to those described as sourced in the Tertiary Volcanic Provinces outcropping in western Scotland [Knutz et al., 2007], show an abundance distribution that parallels that of the BIIS component (Figures 5b and 5c). [25] Other volcanic grains, usually associated with the Icelandic Ice Sheet (IIS), do not show a distinct increase toward HE 2 as the other groups (Figure 5b). Some peaks could be related to tephra layers identified in other cores of this area [Rasmussen and Thomsen, 2004]. 5.4. Hornblende Ages [26] Hornblende grains are present throughout most of the record, although their abundance is higher at certain intervals such as HE 4 and between 26 and 23 ka B.P. (including HE 2), and 22 and 20 ka B.P. (Figure 6). A total of 419 individual grains from 184 samples covering the interval Figure 5. Correlation of the major lithic records of core MD04‐2829CQ to GISP 2. (a) GISP 2 ice core record, (b and c) fluxes of the main IRD groups, and (d) total IRD and LIS‐sourced cream‐colored carbonate fluxes. Grey vertical bars mark HE (dark grey) and some of the Greenland stadial (light grey) horizons. HALL ET AL.: BRITISH‐IRISH ICE SHEET INSTABILITY PA2224PA2224 8of18 ∼18 to ∼40 ka B.P. produced good quality Ar 40 /Ar 39 ages, of which 10 were discarded as grains were suspected to be pyroxenes. [27] By dividing the sediment record into several stratigraphic intervals selected to separate HE from periods between HE (Figure 7a, <23.5 ka B.P.; Figure 7b, HE 2; Figure 7c, 24.3–29 ka B.P.; Figure 7d, cream‐colored carbonate peak time span within HE 2; Figure 7e, 31–38.6 ka B.P.; Figure 7f, HE 3; Figure 7g, >40 ka B.P.; Figure 7h, HE 4) and grouping the hornblende grains accordingly, we constructed histogram graphs in order to study the population composition in each period (Figure 7). We selected the histogram age bins based on known major geological provinces around the North Atlantic [Hemming et al., 1998, 2002; Hemming and Hajdas, 2003]. [28] The results highlight the importance of the Paleoproterozoic (1600–1900 Ma) and Paleozoic (200–600 Ma) hornblende grain populations, while the Mesoproterozoic (1400–1650 Ma) and the Early Paleoproterozoic (1900– 2400 Ma) are represented by far fewer grains. The Paleoproterozoic population is dominant during most of the record, although the quantity of grains within all groups tends to vary simultaneously and the Paleozoic population is also present in high numbers during most periods. These results contrast with sediment core V23‐14 from the western North Atlantic [Hemming and Hajdas, 2003], in where PaleoproFigure 6. Comparison of (b) fluxes of horblende grains and (c) the scatterplot of measured hornblende age (in Ma) to (d) the total IRD and cream‐colored carbonate grain fluxes in core MD04‐2829CQ. (a) The GISP 2 oxygen isotope record is also plotted for reference. 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