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Keywords:

  • Atlantic thermohaline circulation;
  • benthic carbon isotopes;
  • Heinrich events;
  • meltwater forcing

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[1] We present high-resolution benthic δ13C records from intermediate water depth core site MD01-2461 (1153 m water depth), from the Porcupine Seabight, NE Atlantic, spanning 43 to 8 kyr B.P. At an average proxy time step of 160 ± 56 years this core provides information on the linkage between records from the Portuguese Margin and high-latitude North Atlantic basin, allowing additional insights into North Atlantic thermohaline circulation (THC) variability during millennial-scale climatic events of the last glacial. Together, these records document both discrete and progressive reductions in Glacial North Atlantic Intermediate Water (GNAIW) formation preceding Heinrich (H) events 1, 2, and 4, recorded through the apparent interchange of glacial northern and southern-sourced intermediate water signatures along the European Margin. Close coupling of NW European ice sheet (NWEIS) instability and GNAIW formation is observed through transient advances of SCW along the European margin concurrent with pulses of ice-rafted debris and meltwater release into the NE Atlantic between 27 and 16 kyr B.P., when the NWEIS was at maximum extent and proximal to Last Glacial Maximum convection zones in the open North Atlantic. It is such NWEIS instability and meltwater forcing that may have triggered reduced North Atlantic THC prior to collapse of the Laurentide ice sheet at H1 and H2. Precursory reduction in GNAIW formation prior to H4 may also be inferred. However, limited NWEIS ice volume prior to H4 and convection occurring in the Norwegian-Greenland Sea require that if a meltwater trigger is invoked, as appears to be the case at H1 and H2, the source of meltwater prior to H4 is elsewhere, likely higher-latitude ice sheets. Clarification of the sequencing and likely mechanisms of precursory decrease of the North Atlantic THC support theories of H event initiation relating to ice shelf growth during cold periods associated with reduced North Atlantic THC and subsequent ablation through subsurface warming and sea level rise associated with further reductions in meridional overturning.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[2] An increasing number of theories concerning the initiation of periodic collapse of the Laurentide ice sheet (LIS), so-called Heinrich (H) events call for improved understanding of phase relationships within the North Atlantic ice-ocean-climate system. A key unknown and critical component to several H event initiation theories [e.g., Moros et al., 2002; Flückiger et al., 2006] is the phasing of Atlantic Meridional Overturning Circulation (AMOC) perturbation with respect to LIS collapse.

[3] Advance of Southern Component Water (SCW) into the North Atlantic during the last glacial is apparent from extensive mapping of benthic foraminiferal carbon isotopes from North Atlantic core sites, with high nutrient, low δ13C Antarctic Bottom Water (AABW) bathing the North Atlantic basin at depths >2000 m as far north as the Rockall Trough [e.g., Oppo and Lehman, 1993; Sarnthein et al., 1994; Curry and Oppo, 2005]. The glacial mode of North Atlantic thermohaline circulation (THC) was highly sensitive to density contrasts between Glacial North Atlantic Intermediate Water (GNAIW) and SCW, most likely Antarctic Intermediate Water (AAIW) [e.g., Broecker et al., 1990; Weaver et al., 2003; Rickaby and Elderfield, 2005]. Significantly reduced ventilation of waters bathing intermediate depth North Atlantic sites during episodes of northern hemisphere ice sheet instability, particularly the H events, suggests reduced GNAIW formation, likely due to increased freshwater flux obstructing North Atlantic deep/intermediate water formation [e.g., Cortijo et al., 1997; Vidal et al., 1997; Ganopolski and Rahmstorf, 2001], and invasion of SCW to high northern latitudes (at depths <2000 m) [Sarnthein et al., 1994; Zahn et al., 1997; Willamowski and Zahn, 2000; Elliot et al., 2002; Rickaby and Elderfield, 2005]. However, this apparently straightforward cause-effect relationship between H event meltwater and intermediate water formation is questioned, with North Atlantic benthic δ13C records suggesting that weakening of the North Atlantic THC may have begun up to 2.5 kyr prior to LIS instability [Zahn et al., 1997; Willamowski and Zahn, 2000]. Furthermore, recent investigation into AMOC variability using 231Paxs/230Thxs suggest a substantial decrease in the rate of overturning prior to deposition of LIS-sourced ice-rafted debris (IRD) [McManus et al., 2004; Hall et al., 2006], possibly driven by NW European ice sheet (NWEIS) sourced meltwater [Hall et al., 2006]. Flückiger et al. [2006] suggest that subsurface warming and relative sea level rise likely resulted from reduced AMOC prior to the H events, ablating and destabilizing northern hemisphere ice sheet margins and initiating ice sheet collapse. Conversely, Moros et al. [2002] suggest that enhanced subsurface heat flux driven by a strengthened AMOC may have melted ice shelves and forced the retreat of grounding lines. These divergent theories require further investigation into the records of AMOC variability and phasing with ice sheet-surface ocean conditions.

[4] We present benthic δ13C records from sediment core MD01-2461 from the Porcupine Seabight, an intermediary for pre-existing NE Atlantic records, concentrated on the Iberian margin [Zahn et al., 1997] and Rockall Trough-Irminger Sea [van Kreveld et al., 2000; Elliot et al., 2002; Knutz et al., 2002; Hall et al., 2006; Rickaby and Elderfield, 2005]. Combined with previously published records of ice sheet instability and surface ocean conditions from core MD01-2461 [Peck et al., 2006], we investigate at multicentennial to millennial time steps the distribution of intermediate waters along the European Margin to determine sequencing of North Atlantic ice sheet vs. THC events.

2. Materials and Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[5] Sediment core MD01-2461 was recovered from a seismically laminated sequence on the NW flank of the Porcupine Seabight (51°45′N, 12°55′W) at a water depth of 1153 m (Figure 1b). The Porcupine Seabight represents a N-S trending embayment in the continental shelf SW of Ireland, gradually deepening to the south, to ∼3000 m water depth and opening onto the Porcupine Abyssal Plain. Thermohaline convection in the Labrador Sea and Norwegian-Greenland Sea drives present-day circulation, through the formation of intermediate level Labrador Seawater (LSW) and deeper North Atlantic Deep water (NADW). The latter water mass comprises three components that enter the North Atlantic Basin through the Wyville-Thomson, Iceland-Faeroe and Greenland-Iceland ridges [McCartney and Talley, 1984] to mix with the overlying LSW and re-circulate over the Porcupine Abyssal Plain [van Aken, 2000]. This southward penetration of deepwater masses is compensated for by the northward flow of warm, saline surface waters to the Norwegian Sea via the North Atlantic Drift (NAD) and Eastern North Atlantic Waters (ENAW). Today, a pole-ward flow occurs at all depth levels along the eastern slope of the Porcupine Seabight [Rüggeberg et al., 2005], however, the morphology of the Porcupine Seabight prevents direct throughflow of waters below ∼500 m water depth where currents are topographically steered in a cyclonic direction [Frank et al., 2004]. ENAW overlies and mixes with Mediterranean Outflow Water (MOW), which currently bathes the site of MD01-2461 and is characterized by a salinity maximum and oxygen minimum at a depth of 1000–1200 m [Frank et al., 2004].

image

Figure 1. Positioning of core sites discussed in this study and modern circulation scheme. (a) Locations of cores discussed in this study and the modern North Atlantic circulation scheme. Blue arrows illustrate the pathways of the NADW across the Wyville-Thomson Ridge (W-T) and recirculation in the eastern North Atlantic basin at ∼2000–3000 m water depth. Yellow arrows represent LSW at 1500–2000 m. The dashed orange shows the path of MOW at ∼1000 m depth, and red arrows show surface North Atlantic Current and ENAW (to depths of ∼750 m). GEOSECS (http://ingrid.ldeo.columbia.edu/SOURCES/.GEOSECS/) Station 23 (black, open diamond); V28-73 (black, filled circle) and VM29-198 (black, filled square) both from Oppo and Lehman [1993]. Figure adapted from Frank et al. [2004]. (b) eWOCE (http://www.ewoce.org/) meridional section A16 showing modern-day nitrate concentrations, with nutrient-rich AAIW clearly expressed at intermediate depth reaching as far north as ∼20°N. Nutrient-depleted NADW fills the basin and flows south at depth. Cores discussed in this study are plotted on this meridional section to show their relation to modern-day hydrography.

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[6] The 20.2 m core recovered olive-gray silty-clay sediments with frequent drop stones. Visual and x-radiograph evidence present no evidence of core disturbance or turbidite sequences within the interval studied. Geochronology of MD01-2461 is based on 25 monospecific foraminifera (Globigerina bulloides or Neogloboquadrina pachyderma sinistral) AMS 14C dates and fine-tuning of the relative abundance of Neogloboquadrina pachyderma sinistral within the planktonic assemblage (N. pachyderma sin.%) with the GISPII δ18O record [Peck et al., 2007]. The GISPII-tuned age model (r2 = 0.83, 27–8 kyr B.P.) suggests significantly enhanced 14C-marine reservoir ages, exceeding 2 kyr during the deglacial [Peck et al., 2006] (Figure 2a). Sedimentation rates range between 12 and 60 cm kyr−1.

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Figure 2. Benthic and planktonic stable isotope and IRD stacks from MD01-2461, SO75-26KL, and NEAP4K. Unless otherwise stated, records from MD01-2461 are black, closed circles; SO75-26KL are green, open circles; and NEAP4K are purple, open circles. (a) Inferred 14C marine reservoir ages for MD01-2461 [Peck et al., 2006] and SO75-26KL (see text). (b) δ18O C. wuellerstorfi. Vertical arrows indicate tie-points synchronizing the record of SO75-26KL to MD01-2461 (SO75-26KL age model as Zahn et al. [1997] prior to 24 kyr B.P.). (c) δ18O planktonic. Surface-dwelling G. bulloides (MD01-2461, red; SO75-26KL, green) and subsurface-dwelling N. pachyderma sin. from MD01-2461 (blue). (d) δ13C C. wuellerstorfi. (e) Flux of total IRD at MD01-2461 (>150 μm cm−2 kyr−1). (f) Flux of dolomtic carbonate at MD01-2461 (orange); concentration of total IRD at SO75-26KL (>355 μm g−1); weight% >1 mm at NEAP4K. (g) Radiocarbon (black) and tuned (red) sedimentation rates at SO75-26KL. Blue vertical bars highlight episodes of surface ocean stratification. Orange vertical bars highlight H layers 4, 2, and 1, with H3 located by the purple bar.

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[7] Lithic grains >150 μm embedded within the silty clay sediment are considered to be ice rafted, and lithological and geochemical examination has been used to determine the phasing of circum-North Atlantic ice sheet instability and iceberg rafting to the core site [Peck et al., 2007]. Vertical structure of the upper water column is inferred from δ18O records of surface- and subsurface-dwelling planktonic foraminifera Globigerina bulloides and Neogloboquadrina pachyderma sinistral, calcifying at an average depth of 30 m and 150 m respectively [see Peck et al., 2006]. Benthic, epifaunal species Cidicidoides wuellerstorfi has been found to accurately record the δ13C of dissolved inorganic carbon in ambient bottom water, making down-core benthic δ13C records a useful proxy for the reconstruction of past bottom water mass changes [Duplessy et al., 1984]. Youngest benthic δ13C values from MD01-2461 of +1.1‰ VPDB compare well with core top values derived from nearby core sites at similar water depths (1.07‰ VPDB at V29–198, 1139 m water depth; 1.17‰ VPDB at V28–73, 2063 m water depth) [Oppo and Lehman, 1993] and closely correlates with δ13C of ambient water total CO2 of +1.0 to +1.1‰ VPDB (between 1000–2000 m water depth, GEOSECS station 23; Figure 1a). Between 1 and 4 specimens of C. wuellerstorfi were selected from the >250 μm size fraction wherever present, providing an average time step of 160 ± 56 kyr for benthic isotope records. Stable isotope analyses were made using a ThermoFinnigan MAT 252 with an external reproducibility of ≤0.08‰ for δ18O and 0.03‰ for δ13C. Following Shackleton and Opdyke [1973] a disequilibrium correction of +0.64‰ is applied to δ18OC. wuellerstorfi to allow direct comparison with other benthic records.

[8] Whole ocean δ13C changes [e.g., Curry et al., 1988; Duplessy and Shackleton, 1985; Duplessy et al., 1988] will have affected the epibenthic δ13C records from the different core sites used in this study equally and therefore are not considered an obstacle in our documentation of regional δ13C gradients. Correction of our record for changes of the global ocean carbon reservoir demand robust constraints on mean ocean δ13C variations that occurred during the last 40 kyr at high temporal resolution, which are currently unavailable.

3. Results and Discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

3.1. Records From MD01-2461

[9] Elevated benthic δ13C values of 1.65‰ VPDB during the last glacial reflect the influence of a well-ventilated mid-depth water mass, GNAIW [Oppo and Lehman, 1993] or Upper North Atlantic Deep Water (UNADW) [Boyle and Keigwin, 1987]. H events recorded in MD01-2461, identified through the occurrence of IRD deposition with contribution of distinctive Hudson Strait-sourced dolomitic carbonate (Figure 2f) [Peck et al., 2006], coincide with benthic δ13C depletions of 0.6‰ to 1.25‰. A smaller negative δ13C anomaly, of ∼0.3‰, is associated with an enhanced flux of IRD at ∼30 kyr B.P. likely recording the occurrence of H3. This suggestion is supported by the apparent lack of LIS-sourced IRD in this interval, which has been considered a type-characteristic of H3 along with a strong volcanic component to the IRD assemblage [e.g., Grousset et al., 1993; Gwiazda et al., 1996], likely of East Greenland/Icelandic origin [Peck et al., 2007]. Decreased benthic δ13C can potentially be ascribed to high vertical fluxes of organic carbon [Mackensen et al., 1993]. However, the negative benthic δ13C anomalies associated with H events at MD01-2461 are considered to reflect, principally, bottom water mass changes [cf. Willamowski and Zahn, 2000], most feasibly advances of SCW, likely a glacial version of AAIW, to our core site in response to decreased or halted GNAIW formation, confirming restriction of deep-intermediate convection in the northern North Atlantic in the course of the events [e.g., Cortijo et al., 1997; Vidal et al., 1997; Zahn et al., 1997; Rickaby and Elderfield, 2005].

[10] An additional control on the ventilation of deep-intermediate waters in the North Atlantic during meltwater events might involve a change in the mode of North Atlantic deep water formation, from open-ocean convection under glacial conditions to buoyancy loss due to brine formation in the course of sea-ice formation in the Nordic Seas following meltwater surges [Dokken and Jansen, 1999]. Recent studies have suggested that brine formation proximal to ice margins persisted throughout the last glacial in the northern North Atlantic concurrent with open-ocean convection in the North Atlantic [Labeyrie et al., 2005]. The most likely preformed poorly ventilated signal [Vidal et al., 1997; Elliot et al., 2002] and low δ18O signature of these intermediate brine waters being masked during ambient glacial conditions by the well-ventilated GNAIW and only becoming apparent during episodes of reduced MOC, when simultaneous negative anomalies in both planktonic and benthic δ18O and benthic δ13C are observed in North Atlantic records [Dokken and Jansen, 1999; Labeyrie et al., 2005; Waelbroeck et al., 2006]. Such an interpretation challenges the interpretation of North Atlantic benthic δ13C anomalies as documenting the northward penetration of southern sourced waters in response to reduced MOC as the rapid transmission of the North Atlantic-derived brine signal to both the Indian and Pacific Ocean necessitates active intermediate water formation [Labeyrie et al., 2005; Waelbroeck et al., 2006].

[11] At site MD01-2461 we also see some evidence for the injection of low δ18O intermediate waters, characteristic of formation via brine rejection, notably at meltwater event “b” (18–17.7 kyr B.P.) with a well defined decrease of benthic δ18O by 0.7‰ associated with a transient negative benthic δ13C anomaly. Further benthic δ18O (brine) anomalies are hinted at during H4, H3 and meltwater events at ∼26 kyr B.P. and “a” (21.8–20.9 kyr) although these anomalies are much less distinct and appear absent at H2. This lack of a clear brine signal together with the recently reported covariance of 231Paxs/230Thxs and benthic δ13C records from core DAPC2 [Hall et al., 2006] provides convincing evidence that changes in bottom water ventilation along the northern European margin (inferred from benthic δ13C) relate to changes in the rate of MOC and the interchange of poorly ventilated southern and well ventilated northern sourced water masses (at least up to ∼1700 m water depth) rather than the dominant injection of poorly ventilated brine waters from the Nordic Seas [Labeyrie et al., 2005]. This interpretation of benthic δ13C data provides the basis for the following discussion.

[12] While initial benthic δ13C decreases in MD01-2461 directly coincide with the onset of H layer deposition, peak minima in benthic δ13C coincide with maximum divergence in δ18O from surface- and subsurface-dwelling planktonic foraminifers (Figure 2b) that we interpret as indications of meltwater stratification of the upper water column [Peck et al., 2006]. Benthic δ13C values fall to their lowest values within the entire record, +0.4‰ VPDB, at 16.2 kyr B.P., some 0.6–1.1 kyr after the incursion of the detrital carbonate peak indicative of H1 deposition at this site, coincident with offset between δ18O G. bulloides and N. pachyderma sin. of >2.5‰ indicating prominent meltwater stratification. Deglacial meltwater forcing is evident from persistent upper ocean stratification through 16.4–14.0 kyr B.P., associated with significant sea level rise at this time [Bard et al., 1990, 1996]. Benthic δ13C values do not return to their elevated glacial levels but vary between 0.7–1.2‰ VPDB throughout the deglacial, plausibly reflecting a combination of changes involving the terrestrial biosphere and its influence on marine carbon reservoir δ13C and establishment of the modern mode of North Atlantic THC. A decrease in benthic δ13C of 0.5‰ at 13 kyr B.P. is captured by two data points only, but potentially reflects a convection slow-down in the North Atlantic during the Younger Dryas cold period [Boyle and Keigwin, 1987; Sarnthein et al., 1994; Rickaby and Elderfield, 2005]. In addition to the H events, short-lived negative δ13C anomalies are observed at 26–25.7 kyr, 21.8–20.9 kyr (“a”) and 18–17.7 kyr (“b”) B.P., and directly coincide with intermittent meltwater stratification of the upper water column, and enhanced deposition of NWEIS-sourced IRD. Our timings of the two most recent events, “a” and “b” support dates of British ice sheet (BIS) ice sheet retreat determined from cosmogenic nuclide (36Cl) surface-exposure dating of glacial erratic boulders and glacially smoothed bedrock sampled around the former ice margins in Ireland [Bowen et al., 2002]. IRD associated with meltwater event “b” and an IRD peak with a similar NWEIS-signature prior to H2 (at 25.0 kyr B.P.) likely correspond to the well-documented “European precursor” events [e.g., Grousset et al., 2000; Scourse et al., 2000]. Freshwater surging associated with these episodes of NWEIS instability conceivably reached the area of GNAIW formation and caused transient, 200–500 year, reduction in northern sourced intermediate water flux, allowing brief northward penetration of SCW to the MD01-2461 site [Peck et al., 2006].

3.2. Intermediate Water Ventilation Changes From NE Atlantic Records

[13] We compare the benthic δ13C record of MD01-2461 with similar records of other North Atlantic sites (Table 1) to determine regional variability of intermediate water masses. In particular, we use the high-resolution record from core SO75-26KL from the Portuguese Margin at a water depth of 1099 m, similar to that of MD01-2461 [Zahn et al., 1997]. Data records from SO75-26KL have been published on a radiocarbon timescale applying a constant 14C-marine reservoir age correction of 400 years [Zahn et al., 1997; Willamowski and Zahn, 2000]. Assessment of multiple 14C data [e.g., Voelker et al., 1998; Waelbroeck et al., 2001], in agreement with age modeling of the MD01-2461 records [Peck et al., 2006], suggest that NE Atlantic marine 14C reservoir ages were highly variable during the glacial period such that the radiocarbon timescale of SO75-26KL does not allow for a detailed comparison with MD01-2461. To attempt a synchronized timescale for SO75-26KL we graphically correlate the benthic δ18O records of both cores by “tuning” the record of benthic δ18O record from SO75-26KL to that of MD01-2461 across the deglaciation (24–8 kyr B.P.) (Figure 2b). This procedure receives independent support from the observation that an IRD peak at SO75-26KL, on the synchronized timescale, is concurrent with the lithologically and geochemically distinct H layer 1 at MD01-2461, consistent with synchronous deposition of H layers 4 and 2 at the two sites (Figure 2f). Additionally, the onset of deglacial warming/surface ocean freshening in the planktonic δ18O records is simultaneous at the two sites (Figure 2c). The synchronized timescale of SO75-26KL suggests marine 14C-reservoir ages between 0.4 kyr to 0.9–1.0 kyr in the period 17.3–16.4 kyr B.P. at this site (Figure 2a). A latitudinal gradient of reservoir ages during the deglaciation from ∼1 kyr at 37°N (SO75-26KL) up to ∼2 kyr at 52°N (MD01-2461) [Peck et al., 2006] is comparable with the findings of Waelbroeck et al. [2001].

Table 1. Site Locations Discussed in This Study
CoreLocationWater Depth, mReference
NEAP 4KBjörn Drift 61°29′N, 24°10′W1627Rickaby and Elderfield [2005]
DAPC2Rockall Trough 58°58′N, 09°37′W1709Knutz et al. [2002]
SO75-26KLIberian Margin 37°49′N, 20°30′W1099Zahn et al. [1997]
SU81-18Iberian Margin 37°46′N, 10°11′W3135Gherardi et al. [2005]

[14] The lower resolution benthic δ13C record from core NEAP 4K at Björn Drift (1627 m water depth; Figure 1) [Rickaby and Elderfield, 2005] is used as reference for comparison with mid-depth ventilation changes in the high-latitude North Atlantic. The age model of NEAP 4K (>13 kyr B.P.) is based on stratigraphical correlation (benthic and planktonic δ18O) with core BOFS 5K [Barker et al., 2004], which has a radiocarbon-based age model incorporating the elevated marine reservoir ages of Waelbroeck et al. [2001] (1.9 kyr) for this time period. Temporal resolution at this site averages 500 years and does not allow for a detailed correlation of the benthic δ18O record with that of MD01-2461. A broad peak in weight% of the >1 mm size fraction, spanning ∼20–9 kyr B.P. is of little use for correlation to the IRD events at the European Margin sites. Therefore we have no firm control on the timing of isotope patterns along the benthic isotope records of NEAP 4K in relation to MD01-2461 and SO75-26KL and will use the record for a qualitative assessment of regional gradients only.

[15] During the last glacial, an eastern branch of GNAIW was advected along the European Margin toward the Portuguese Margin, while northward flowing SCW (likely AAIW) penetrated as far north as the Moroccan Margin as is indicated in paired benthic δ13C-Cd/Ca profiles (Figure 1a) [Willamowski and Zahn, 2000]. GNAIW therefore maintained the well-ventilated ambient bottom water conditions (δ13C > 1.4‰ VPDB) recorded at both MD01-2461 and SO75-26KL during mean glacial conditions. Conversely, the offset, of up to 0.5‰, between the MD02-2461 and SO75-26KL benthic δ18O records over the glacial interval suggests the additional influence of a warmer and/or low-δ18O glacial mid-depth water mass at the upper Portuguese margin [Zahn et al., 1997].

[16] Similar absolute values of benthic δ13C are recorded at both SO75-26KL and MD01-2461 associated with and following IRD deposition at H4, 2 and 1. However, decline into these benthic δ13C anomalies starts considerably earlier at the Portuguese margin and appears more gradual, notably in the periods prior to H4 and H1. That is, mid-depth ventilation appears to deteriorate at the Portuguese margin up to 5 kyr prior to these two H events, and importantly, before the collapse of mid-depth ventilation at the site of MD01-2461. This contrast in mid-depth ventilation plausibly reflects the proximity of the northerly MD01-2461 to the site of mid-depth convection that provided the site with well-ventilated mid-depth waters even though the production of these waters was in decline. If so, the early decrease in benthic δ13C at the Portuguese margin suggests that the production of mid-depth waters started to deteriorate well before H1 and H4 presumably because of a gradual built-up of meltwater surging before the LIS destabilized and large-scale iceberg calving occurred. The benthic δ13C proxy close to the centers of convection would not resolve such an early decline in mid-depth convection.

3.3. Precursory Meltwater Forcing From the NWEIS

[17] GNAIW convection in the open North Atlantic at the LGM [Sarnthein et al., 1994; Vidal et al., 1997], proximal to the fully advanced NWEIS, facilitated close coupling of NWEIS meltwater and overturning circulation. Within the H2 to H1 interval, benthic δ13C at MD01-2461 suggests that bottom waters at this site remained well ventilated by GNAIW until ∼16.6 kyr B.P., the exception being the apparent transient advance of the GNAIW/SCW hydrographic front north of ∼52°N at meltwater events “a” and “b.” At SO75-26KL, benthic δ13C fell steadily over a ∼5 kyr period approaching H1, following an abrupt shift by −0.5‰ at 21.4 kyr B.P. Initiation of this trend of decreasing ventilation at SO75-26KL is coincident with event “a” at site MD01-2461 when the apparent advance of SCW as displayed in a likewise brief episode of benthic δ13C depletion centered on 21.2 kyr B.P. On the basis of an IRD assemblage at MD01-2461 dominated by BIS-derived lithologies, this event has been suggested to constitute a reduction in GNAIW formation during an episode of NWEIS instability and associated meltwater surging into the NE Atlantic [Peck et al., 2006]. The end of this freshwater surge allowed intermediate water production to resume, reverting benthic δ13C values at MD01-2461 back to elevated/GNAIW values at 20.9 kyr B.P. However, continuing decrease of benthic δ13C values recorded at SO75-26KL suggest GNAIW production did not fully recover and that North Atlantic THC was progressively weakening prior to the incursion of H1 icebergs and meltwater to the NE Atlantic. As no further meltwater forcing is evident in the planktonic δ18O records of MD01-2461 until event “b,” at 17.8 kyr B.P., convection was plausibly reduced by freshwater forcing at higher latitudes [e.g., Elliot et al., 2002] or further to the west. 231Paxs/230Thxs ratios at DAPC2 (1709 m water depth) in the Rockall Trough, increase toward production values at ∼18.0 kyr B.P., suggest substantially reduced rates of overturning [Hall et al., 2006] concurrent with event “b” which may represent either a second transitory advance of SCW or an episode of brine injection to MD01-2461 preceding H1 by ∼0.9 kyr. However, Gherardi et al. [2005] use 231Paxs/230Thxs records from the Iberian Margin to suggest that “shallow” overturning in the NE Atlantic basin was vigorous until 16.5 kyr B.P. Core SU81-18 used in their study is located at 3135 m water depth, some 2000 m deeper than both MD01-2461 and SO75-26KL. It may be possible that 231Pa export at depths below cores SO75-26KL, MD01-2461 and DAPC2 is recorded at SU81-18 accounting for these divergent signals, supporting a reduction in shallow overturning (≤1700 m), while deeper convection was perhaps maintained, accounting for the lower 231Paxs/230Thxs values recorded at SU81-18 at this time.

[18] The low-resolution record of NEAP-4K closely follows the structure of the benthic δ13C record of MD01-2461 and documents elevated δ13C values around 1.4‰ throughout the interval between H2 and H1, suggesting persistent bathing of this site with GNAIW. This pattern confirms our contention of continued production of GNAIW, albeit at lower rates in response to NWEIS instabilities, at “a” and “b.”

[19] Benthic δ13C values of <0.7‰ VPDB at all three intermediate water depth sites reflect the prominence SCW between 16.2–15.4 kyr B.P., implying large-scale collapse of GNAIW production following H1. Unlike the coupled H layer-benthic δ13C collapse records of SO75-26KL, NEAP-4K and DAPC2 [Knutz et al., 2002; Hall et al., 2006], significantly reduced ventilation at MD01-2461 is not recorded until several hundred years after H1. Geochemical (40Ar/39Ar dates of individual hornblende grains, Sr-Nd isotopic composition of carbonate-free IRD), magnetic susceptibility and lithological classification (dolomitic carbonate) each suggest that this is the only horizon within the deglacial interval (20–10 kyr B.P.) of MD01-2461 that contains a notable contribution of LIS-derived debris [Peck et al., 2006]. The H1 layer in MD01-2461 spans a few hundred years, whereas deposition at both DAPC2 and SO75-26KL is in the order of 1.0 kyr. While it appears plausible that MD01-12461 may have witnessed the earlier stages of H1 deposition in the NE Atlantic only, perhaps reflecting changing surface current patterns, it remains an issue to explain why benthic δ13C at MD01-2461 does not decrease until after the H1 layer in this core. One possible explanation is that the elevated δ13C value recorded immediately following H1 layer deposition and measured from a single specimen of C. wuellerstorfi (asterisk on Figure 2d) is not representative of ambient bottom water ventilation at this time and ventilation reduction at MD01-2461 was effectively simultaneous with H1. Alternatively, taking the benthic δ13C record at MD01-2461 at face value across H1, the elevated benthic δ13C levels may reflect a time-transgressive shoaling of reduced ventilation that reached the deeper sites in the north (NEAP-4K, DAPC2) first, before affecting MD01-2461.

[20] At H2, a “precursory” reduction in benthic δ13C is observed at SO75-26KL at 25 kyr B.P., ∼1 kyr prior to H layer deposition, accompanied by a brief anomaly at MD01-2461 at 24.6 kyr B.P. The abrupt decrease in benthic δ13C at SO75-26KL is synchronous with the equally abrupt increase in IRD from the NWEIS at MD01-2461, representing instability of the BIS immediately preceding H2 [Peck et al., 2007]. Surface and subsurface planktonic δ18O display an only minor divergence at this time (up to 0.5‰) indicating freshwater forcing at a smaller scale than during the “a” and “b” events, but its influence on GNAIW formation appears significant enough to produce the reduction in ventilation observed at SO75-26KL. Mid-depth ventilation recovers abruptly after this event before large-scale ventilation collapse occurs in the course of H2. Minimum ventilation of intermediate waters along the European Margin is seen in the benthic δ13C record of both SO75-26KL and MD01-2461 that immediately follows the H2 IRD deposition in MD01-2461. Significant destabilization of the BIS is suggested by a substantial increase in the flux of BIS-derived debris immediately following deposition of H2 at MD01-2461, perhaps triggered by sea level rise of up to 15 m associated with LIS-collapse [Yokoyama et al., 2001; Chappell, 2002]. Consistently the maximum (>2.5‰) offset between δ18O of the G. bulloides and N. pachyderma sin. which lags H layer deposition by ∼300 years is thought to be derived principally from NWEIS meltwater.

[21] Similarly, to H1, a progressive decrease in benthic δ13C preceding H4 by ∼2 kyr is observed in SO75-26KL, while benthic δ13C depletion at MD01-2461 occurred abruptly associated with H layer deposition. The short-lived increase in IRD flux associated with H4 plausibly reflects the juvenile state of the BIS at this time [Peck et al., 2007], a contention that appears to be confirmed by the lack of a coeval freshwater signal in planktonic δ18O (Figure 2c). The limited extent of the NWEIS, coupled with North Atlantic deep-intermediate water convection likely occurring in a similar location to the contemporary ocean prior to the stage 3-2 boundary [Sarnthein et al., 1994; Vidal et al., 1997] suggest the NWEIS was an unlikely source of meltwater for triggering the reduction in mid-depth ventilation prior to H4. Rather, initial reduction in GNAIW production likely reflects other meltwater sources [van Kreveld et al., 2000; Elliot et al., 2002].

3.4. Ice Sheet Instability in Response to North Atlantic THC Changes

[22] Several hypotheses attempting to explain the occurrence of H events, incorporating a range of internal and external forcing factors, have been proposed [e.g., MacAyeal, 1993; Johnson and Lauritzen, 1995; Marshall and Clarke, 1997; Hunt and Maslin, 1998; Arbic et al., 2004]. Recent concepts have used the disintegration of ice shelves fringing the Antarctic Peninsula as a modern analogue for the sudden iceberg releases during H events [Hulbe, 1997; Hulbe et al., 2004] and consider ocean subsurface temperatures, coupled with North Atlantic THC variability, as a factor that may have destabilized the LIS through their effect on ice shelves and fringing ice margins [e.g., Moros et al., 2002; Shaffer et al., 2004; Flückiger et al., 2006].

[23] The concept of recurrent meltwater release from the NWEIS and surface ocean stratification, promoting transient weakening of the North Atlantic THC and regional cooling particularly appears to apply for the period preceding H1. Such cool conditions may also have promoted the growth of a LIS-fringing ice-shelf, perhaps priming the LIS for H event collapse [Hulbe, 1997; Hemming, 2004; Hulbe et al., 2004]. Subsequent subsurface warming and sea level rise (0.3–0.5 m), associated with the THC reduction, may then have played a role in undermining the ice shelf thus removing the buttressing support exerted on the feeder ice streams, leading to large-scale surging of the ice sheet [Flückiger et al., 2006].

4. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[24] Benthic δ13C records from NE Atlantic core MD01-2461 document interchange of well-ventilated northern-sourced and poorly ventilated southern-sourced intermediate waters during the last glacial. Enhanced ventilation at MD01-2461 documented by δ13C benthic values of >+1.6‰ VPDB, compared to early Holocene values of ≤1‰ VPDB suggests that well-ventilated GNAIW was bathing this core site, along with sites on the Björn Drift and Portuguese Margin at similar water depths during ambient LGM conditions. Frequent meltwater incursions, associated with instability of the NWEIS, appear coincident with weakened GNAIW formation leading to transient advances of SCW along the European margin. Such a scenario is likely to account for precursory reductions in North Atlantic THC prior to both H2 and H1 when NWEIS was at maximum extent and convection was more proximal. Progressive reduction in North Atlantic THC may also be inferred prior to H4; however, the limited extent of the NWEIS coupled with the likely location of convection centers at higher latitudes precludes meltwater release from the NWEIS as the trigger.

[25] LIS destabilization therefore appears to have consistently occurred following progressive (H1 and H4) or discrete (H2) reductions in the North Atlantic THC. The role that North Atlantic THC variability may have played in H event initiation is still debatable, yet a scenario where climatic cooling in response to surface ocean stratification and reduced North Atlantic THC promoted ice shelf growth, which were subsequently ablated through accompanying subsurface warming, triggering ice sheet collapse is consistent with the data reported here.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

[26] We thank Brian Long and Helen Medley for help with sample preparation and Giancarlo Bianchi for stable isotope analysis. Laurent Labeyrie, Jean Lynch-Stieglitz, and two anonymous reviewers provided constructive comments that helped to improve this manuscript. This work was supported by the Natural Environment Research Council (NERC) and NERC Radiocarbon Laboratory. We thank the Institut Polaire Francais Paul Emile Victor (IPEV) for technical support and for making the research vessel Marion Dufresne available for core retrieval. R.Z. acknowledges the support of the Ministerio de Educacion y Ciencias (MEC), Spain.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Materials and Methods
  5. 3. Results and Discussion
  6. 4. Conclusions
  7. Acknowledgments
  8. References
  9. Supporting Information
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