Evolution of subpolar North Atlantic surface circulation since the early Holocene inferred from planktic foraminifera faunal and stable isotope records

Quaternary Science Reviews 76 (2013) 66e81

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Evolution of subpolar North Atlantic surface circulation since the early Holocene inferred from planktic foraminifera faunal and stable isotope records Francisca Staines-Urías a, *, Antoon Kuijpers a, Christoph Korte b a b

Geological Survey of Denmark and Greenland, Marine Geology and Glaciology Department, Øster Voldgade 10, 1350 K Copenhagen, Denmark University of Copenhagen, Department of Geosciences and Natural Resource Management, Øster Voldgade 10, 1350 K Copenhagen, Denmark

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 December 2012 Received in revised form 17 June 2013 Accepted 21 June 2013 Available online 18 July 2013

Past changes in the surface flow regime of two main eastern North Atlantic warm water pathways toward the Nordic seas were reconstructed based on faunal analyses in combination with carbon and oxygen stable isotope measurements in planktic foraminifera. The investigated sites, in the surroundings of the Faroe Islands, are located in the transitional area where surface waters of subpolar and subtropical origin mix before entering the Arctic Mediterranean. In these areas, large-amplitude millennial variability in the characteristics of the upper-water column appears modulated by changes in the intensity of the Subpolar Gyre circulation. From 7.8 to 6 ka BP, faunal records indicate a deep mixed-layer which, in conjunction with lighter d18O values, suggest that the inflowing Atlantic waters were dominated by a relatively cooler and fresher water mass, reflecting a strengthening of the Subpolar Gyre under conditions of enhanced positive NAO-like forcing and reduced meltwater input. A shift in the hydrographic conditions occurred during the Mid-Holocene (centered at 5 ka BP). At this time, increasing upper water column stratification and the incipient differentiation of the stable isotopic signal of the IcelandeFaroe and FaroeeShetland surface water masses, suggest increasing influx of warmer, more saline surface waters from the Subtropical Gyre, as Subpolar Gyre circulation weakened. The mid-Holocene decline in Subpolar Gyre strength is presumably related to a shift toward a low state of the NAO-like forcing associated with decreased solar irradiance. Later in the Holocene, from 4 ka BP to present, the increased frequency and reduced amplitude of the surface hydrographic changes reflect corresponding fluctuations in Subpolar Gyre circulation. These high frequency oscillations in Subpolar Gyre strength suggest increased surface circulation sensitivity to moderate freshwater fluxes to the LabradoreIrminger Sea basin, highlighting the importance of the salinity balance in modulating Subpolar Gyre dynamics, particularly under conditions of low NAO atmospheric forcing. Ó 2013 Elsevier Ltd. All rights reserved.

Keywords: Eastern North Atlantic Planktic foraminifera Paleoceanography Subpolar Gyre North Atlantic Oscillation

1. Introduction Surface waters in the eastern North Atlantic are much warmer than in every other ocean region at similar latitudes. The main reason for this is the large amount of ocean heat that is transported northward from tropical latitudes, across the GreenlandeScotland Ridge and into the Nordic seas and Arctic Ocean (e.g. Hansen and Østerhus, 2000). This transport of warm and saline water masses derived from the subtropics regulates climate over Europe and controls ocean convection and associated deep water formation (e.g. Hansen et al., 2004; Hátún et al., 2005; Holliday et al., 2008). It

* Corresponding author. E-mail addresses: [email protected], [email protected] (F. Staines-Urías). 0277-3791/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.quascirev.2013.06.016

is here, north of the GreenlandeScotland Ridge, in the so-called Arctic Mediterranean (AM) where through airesea exchange the warm Atlantic Inflow waters loose much heat but not salt, and are transformed into the denser water masses that return southwards as overflows through the deep Ridge passages (Hansen and Østerhus, 2000). This makes the AM region to play a crucial role for the Atlantic thermohaline circulation (Hansen et al., 2004). As density changes at these latitudes are largely controlled by salinity, changes in upper ocean salinity in this region will have a significant impact on the development of the thermohaline circulation (Rasmussen et al., 1996; Kuijpers et al., 1998; Holliday et al., 2008; Sarafanov et al., 2008). Accordingly, a predicted freshening of this region related to an enhanced hydrological cycle due to global warming (Cubasch et al., 2001) might have significant consequences for the strength of the Atlantic meridional overturning

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circulation, thus affecting the distribution of heat toward higher latitudes, and impacting the entire high latitude climate system. This assumption is further substantiated by modeling data (e.g. Bersch et al., 2007; Zhang, 2008), recent observations (Hansen and Østerhus, 2000; Häkkinen and Rhines, 2004) and numerous paleoceanographic studies (e.g. Boyle and Keigwin, 1987; Hall et al., 2004; Thornalley et al., 2009, 2010, 2011) which indicate that, on orbital and suborbital time scales, changes in the production of North Atlantic Deep Water are linked to climate variability and fluctuations in sea surface temperature (SST) and sea surface salinity (SSS). Nonetheless, reconstructions of Holocene climate variability in the North Atlantic show inconsistent results. Some paleoceanographic studies indicate basin-wide North Atlantic SST decrease during the Holocenedcommonly attributed to insolation changes (e.g. Marchal et al., 2002)dwhile others have shown that North Atlantic oceanographic changes were not uniform in amplitude or spatial extent (e.g. Moros et al., 2004; Solignac et al., 2006; Renssen et al., 2009; Andersson et al., 2010). Thus, the need for a better understanding of the various components of the regional oceanic circulation as well as the modes of internal variability of the climate system, remains. Here, we investigate Holocene oceanographic changes in two important pathway areas of warm surface flow to the high-latitude deep-water formation regions of the AM. Our paleoceanographic reconstructions are based on three sedimentary records from the Faroe Islands region. So far, relatively little detail is known about Holocene oceanographic changes in this area, as previous studies (Rasmussen et al., 1996; Kuijpers et al., 1998; Bäckström et al., 2001; Lassen et al., 2002) have mainly dealt with late Pleistocene ocean variability in this region. We have focused on the surface flow regime of the two main warm water pathways where today most of the volume flux into the Nordic seas is concentrated (Hansen and Østerhus, 2000), i.e. the warm water route crossing the Icelande Faroe Ridge west of the Faroe Islands and the second important branch found in the FaroeeShetland Channel gateway. There are indications that the water masses in these areas change over time in accordance with regional climate variability (Pollard et al., 2004; Hátún et al., 2005; Holliday et al., 2008). Past changes in the surface hydrography of each site were reconstructed based on planktic foraminiferal faunal analyses, the isotopic composition of planktic foraminifera carbonate, and textural/compositional analyses of sedimentary samples. The position of the selected locations (Fig. 1), in the transitional area where surface waters of subpolar and subtropical origin mix before entering the AM (Hátún et al., 2005, 2009; Sarafanov et al., 2008), provides the opportunity to evaluate past variability in the properties of the surface water masses feeding the deep convection areas, and therefore, to evaluate the influence of the North Atlantic Subpolar Gyre (SPG) relative to the Subtropical Gyre (STG) as a possible mechanism controlling the hydrographic characteristics of the Inflow waters. 1.1. Regional setting North Atlantic surface waters flow into the AM by three paths: via the Irminger Current passing west of Iceland, through the Iceland basin over the IcelandeFaroe Ridge, and through the Rockall Trough into the FaroeeShetland Channel gateway (Fig. 1). The flow pattern of warm and saline waters of subtropical origin across the North Atlantic shows a splitting of the Gulf Stream into a southern and a northern branch around 55
W (Holliday et al., 2008). Eventually, as the northern branch splits again, one branch turns north to form the Irminger Current, constituting one of the northern limbs of the SPG (Pollard et al., 2004), whereas a second branch flows eastward along the 50
N parallel, crossing the North Atlantic Basin to constitute the North Atlantic Current (NAC, Read, 2001).

67

Further east, the NAC splits again. One branch flows toward the IcelandeFaroe Gap to become the relatively fresher and colder Modified North Atlantic Water (MNAW, Read, 2001; Brambilla and Talley, 2006; Holliday et al., 2008), while a second branch flows northeastward into the southern Rockall Trough. The second branch, commonly acknowledged as the RockalleHatton branch (RHB), also carries MNAW (Häkinen and Rhines, 2009). However, the RHB is significantly influenced by waters from the intergyre region (Holliday, 2003; Pollard et al., 2004). The intergyre waters are represented by the Eastern North Atlantic Water (ENAW), which compare to waters derived from the NAC is warmer, more saline and nutrient enriched. The ENAW moves northwards, following the path of the Slope Current (SC), which originates in the Bay of Biscay and constitutes one of the main ENAW carriers. The SC moves from the northern limb of the STG (Häkkinen and Rhines, 2009) through the Rockall Trough, into the FaroeeShetland area (Fig. 1). Thus, in the FaroeeShetland area the upper water mass results from the mixing of MNAW and ENAW (Holliday, 2003; Sherwin et al., 2011). Modern inter-annual variability in the characteristics of the Atlantic Inflow is controlled by SPG dynamics (Hátún et al., 2005; Sherwin et al., 2011). Reduced freshwater input to the Labrador Sea causes the SPG to expand eastward, decreasing the influence of the warm saline STG waters (Fig. 1a), which allows the MNAW to dominate the Inflow. On the contrary, enhanced freshening of the Labrador Sea results in the westward retraction of the SPG, allowing the ENAW to dominate (Fig. 1b). Fluctuations in regional atmospheric patterns over the North Atlantic further affect the dynamics of the SPG. South of Iceland ocean conditions are episodically influenced by sea-ice and subsurface subpolar waters advected by the East Icelandic Current (EIC) exiting the AM, which typically occurs when the gradient between the Azores High and Icelandic Low centers of atmospheric pressure is reduced (Fig. 1b, Blindheim and Osterhus, 2005; Hátún et al., 2009; Thornalley et al., 2011). Consistently, a substantial part of the SPG dynamics and North Atlantic climate variability is associated with the North Atlantic Oscillation (NAO), which is a natural mode of atmospheric variability that refers to a meridional oscillation of the atmospheric pressure between the Azores High and the Icelandic Low (van Loon and Rogers, 1978). Because the signature of the NAO is strongly regional, to quantify its intensity, an index was defined as the difference between sea-level pressure anomalies in Portugal and in Iceland (Hurrell, 1996). High (positive) NAO values denote larger sea-level pressure gradients, while low (negative) values indicate reduced gradients. Climatically, when the NAO index is positive, enhanced sea-level pressure gradients translate into enhanced westerly winds over the North Atlantic moving relatively warm and moist air over part of Europe toward Asia. Over Greenland and northeastern Canada, these enhanced gradients produce stronger northwesterlies that carry cold air southward decreasing land temperatures (Zhang et al., 2008). In the northern North Atlantic, stronger westerly winds increase the depth of the mixed-layer and strengthen the SPG (Sarafanov, 2009; Sarafanov et al., 2010). Eventually, the strengthening of the SPG produces the eastward expansion of the NAC (Fig. 1a). In the eastern North Atlantic, an expanded NAC produces the southward shift of the Subpolar Front (SF) and the retreat of the warm saline waters from the STG (Hátún et al., 2005; Holliday et al., 2008), resulting in lower surface temperatures and salinities within the northeast North Atlantic region (Bersch et al., 2007; Holliday et al., 2008; Sarafanov, 2009). Because the surface waters of this region get entrapped in the IcelandeScotland surface overflow and contribute to the Atlantic Inflow that feeds the AM (Hátún et al., 2005; Sarafanov et al., 2009, 2010), the intensification of the SPG also produces the cooling and freshening of the entire

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Fig. 1. Modern North Atlantic Ocean surface circulation and cores locations (stars). Solid arrows indicate warm currents; dashed arrows, cold currents. Bottom inserts show a simplified schematic representation of the upper-ocean circulation in the North Atlantic under positive (left) and negative (right) NAO conditions. AM, Arctic Mediterranean; SPG, Subpolar gyre; STG, Subtropical gyre; NAC, North Atlantic Current; SC, Slope Current; EGC, East Greenland Current; IC, Irminger Current; RHB, Rockall Trough Branch; IFB, Icelande Faroe Branch.

eastern North Atlantic (Dickson et al., 1988; Holliday et al., 2008; Sarafanov, 2009). Thus, the intensification of the SPG reduces the salinity and temperature gradients between the IcelandeFaroe and the FaroeeShetland areas (Hátún et al., 2005). Considering the significant influence of the NAO on North Atlantic surface circulation, it is pertinent to ask whether this atmospheric mode may have played a significant role in Holocene climate variability. The detection of the NAO signature in North Atlantic centennial SST variability suggests a possible role of NAO in generating millennial time-scale changes (Keigwin and Pickart, 1999). Additionally, basin-wide paleoreconstructions show that North Atlantic millennial-scale temperature and salinity fluctuations display spatial patterns that resemble those of the NAO phases (Rimbu et al., 2003; Mann et al., 2009; Thornalley et al., 2009; Jessen et al., 2011). A mechanism for such NAO-like variations has been proposed based of the effect that the surface density changes associated with meltwater input to the western North Atlantic have on SPG circulation. As freshwater input to the

SPG region increases, the reduction in the density gradient at the edge of the gyre weakens SPG circulation. Climate model simulations show that, on millennial time scales, this process eventually creates surface hydrographic changes reproducing the spatial pattern of the NAO signal (Schmidt et al., 2004; Brewer et al., 2007; Mann et al., 2009). 1.2. Foraminiferal habitat and environmental preferences Many studies have documented a distinct relationship between the distribution of planktic foraminifera and ocean hydrography (e.g. Kucera et al., 2005). Modern planktic foraminifers show a vertical distribution and pattern of abundance in the water column that is related to their species-specific environmental adaptations and demands (e.g., Bé and Hutson, 1977; Deuser et al., 1981; Pflaumann et al., 2003). Table 3 summarizes modern geographical distributions, depth habitats and environmental preferences of the planktic foraminifera species most significant for this investigation.

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2. Material and methods Three sedimentary cores were recovered off the Faroe Islands shelf (Fig. 1). Two cores, LINK14 and HV03, where recovered west of the Faroe Islands within the FaroeeShetland region. This region comprises the eastern Faroese shelf, the FaroeeShetland channel and sections of the western ScottisheNorwegian continental shelf. A third core, ENAM33BC, was recovered east of the Faroe Islands near the IcelandeFaroe gap. Cores characteristics, retrieval location, water depth, and core length are presented in Table 1. For core ENAM33BC, Ca concentrations were measured by XRF scanning at the Netherlands Institute for Sea Research following the protocol described by Richter et al. (2006). All cores were sampled every 5 mm. Sediment samples were air-dried, weighted and washed over a 63 mm mesh sieve. The obtained residues were weighted to determine the mass of the sand fraction (>63 mm). The textural and mineralogical characteristics of the sand fraction were studied using a modified version (Trumbull, 1972) of the coarsefraction analysis originally developed by Shepard and Moore (1954). It has been observed that subpolar and polar foraminifera are generally small, and when using larger size fractions, e.g., >125 mm or >150 mm, much faunal information is lost (Husum and Hald, 2012). Thus, to include small species of ecological importance (e.g. Globigerinita uvula and Turborotalia quinqueloba), the >100 mm fraction was selected for faunal analyses (Bauch, 1994; Carstens et al., 1997; Kandiano and Bauch, 2002; Rasmussen and Thomsen, 2010; Husum and Hald, 2012). The total volume of the >100 mm fraction was reduced by splitting, ensuring that at least 300 specimens remain in the final split. All the planktic specimens contained in each of the preselected splits were picked, mounted and counted at species level. The fluxes of total planktonic foraminifera were computed in by using the following relationship: FLUX ¼ DBD  LSR  C, where DBD is the dry bulk density of the sediment (g cm3), LSR is the linear sedimentation rate (cm y1), and C is the number of specimens per gram of dry sediment in the >100 mm fraction. The flux is given as the number of specimens per square centimeter, per year (forams/cm2/y). Several biological processes cause deviations from equilibrium in the stable isotopic composition of foraminiferal calcite, both in planktic (e.g. Bouvier-Soumagnac and Duplessy, 1985; Bemis et al., 1998) and in benthic foraminifera (e.g. Woodruff et al., 1980; Wefer and Berger, 1991). To prevent d18O and d13C deviations due to changes in irradiance leveldthe so-called symbiont effect (Spero and Lea, 1993)dwe selected the non-symbiont bearing species Globigerina bulloides. Additionally, to minimize inconsistent isotopic values due to ontogenic effects (Spero and Lea, 1996; Bemis et al., 1998) the specimens used for isotopic measurements were selected within a narrower size range (250e300 mm) than the range used for the faunal analysis. The oxygen and carbon stable

69

isotopes (d18O and d13C) analyses were performed on samples weighing between 50 and 400 mg (w35e75 G. bulloides specimens). To improve the repeatability and accuracy of the measurements, the shells were cleaned and prepared by following the procedure developed by Boyle (1995) and modified to work with samples <2 mg (Staines-Urías and Douglas, 2009). For cores ENAM33BC and HV03, the isotope analyses were carried out at the Stable Isotope Laboratory of the Department of Geology at the University of California Davis. Based on replicated measurements of a reference standard, the long-term standard reproducibility for d18O is 0.03& and for d13C is 0.02&. For core LINK14, isotope analyses were carried out at the Department of Geosciences and Natural Resource Management, University of Copenhagen. The accuracy of the produced data was controlled by multiple analyses of the Copenhagen Stable Isotope laboratory standard (Carrara Marble ¼ LEO) and international standards. No disagreements of mean values >0.06& were found for the range of isotope ratios accepted or adopted values. The measured d18O values were corrected for whole-ocean ice volume changes based on the relation proposed by Fairbanks (1989). Sea level changes throughout the studied period were estimated using the minimum sea level curve created by Lighty et al. (1982) and data provided by Waelbroeck et al. (2002). 2.1. Geochronology All age models were constructed based on AMS radiocarbon dates. Conventional radiocarbon ages were corrected for isotope fractionation by using the 14C/13C ratios relative to the PDB standard (Stuiver and Reimer, 1993). Subsequently, age determinations (AMS 14 C dates) were calibrated using the CALIB 6.0 software (Stuiver and Reimer, 1993) and applying a correction for the regional variation from the marine reservoir age (DR) of 23  99 (Olsson, 1980). Values are reported on a 95% confidence interval (Table 2). For Core ENAM33BC, the age model was calculated assuming linear sedimentation rates between three AMS dates from G. bulloides monospecific samples (Fig. 2, left). Likewise, core HV03 age model was constructed based on three AMS dates from G. bulloides (Fig. 2, center). The age model for core LINK14, previously published by Rasmussen and Thomsen (2010), is based on five 14C dates from bivalves and benthic foraminifera (Fig. 2, right). Throughout this manuscript, all reported dates refer to Calibrated dates BP. 3. Results 3.1. Sediment analyses Textural and mineralogical analyses show that in all the cores the biogenic component represents the bulk of the sediment sand

Table 1 Core location, water depth and characteristics, including analyses performed on each core. Core ID

Latitude

Longitude

Type

Retrieval location

Water depth (m)

Core length (cm)

ENAM33BC

61
15.8840 N

11
09.6540 W

Gravity core

IcelandeFaroe area (Faroe Bank Channel outlet)

1217

32

LINK14

61
43.030 N

05
49.360 W

Piston core

FaroeeShetland area (Eastern Faroese Shelf)

346

HV03

61
44.2460 N

05
47.5480 W

Gravity core

FaroeeShetland area (Shallow section of the Sandoy Trough)

314

Sampling depth (cm)

Sample size (cm3)

Analyses performed

32

12.5

565

101

18.9

157

40

18.9

-

Planktic foraminifera census Stable isotope Textural and mineralogical XRF core scanning Planktic foraminifera census Stable isotope Textural and mineralogical Stable isotope Textural and mineralogical

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3.2. Foraminifera faunal analysis

Table 2 Radiocarbon AMS age determinations and calibrated ages for cores ENAM33BC, LINK14 and HV03. Calibrations computed with Calib 6.0 software (Marine09 dataset) with a DR ¼ 23  99 (Olsson, 1980). (*) AMS data for core LINK14 from Rasmussen and Thomsen (2010). Core name/ depth (cm) ENAM33BC 10 25 32 LINK14* 10 35 75 140 195 HV03 20 65 132.5

14 C age  standard deviation

Calibrated age (cal yr. BP)

Material dated

Lab no.

Globigerina bulloides Globigerina bulloides Globigerina bulloides

AAR13325 AAR13326 AAR3812

3293  32 6196  44 7085  55

Bivalve Hyalina balthica Cassidulina laevigata Cassidulina laevigata Bivalve

AAR6841 AAR8143 AAR8144 AAR8145 AAR-6842

525 3885 6295 8640 9700

Globigerina bulloides Globigerina bulloides Globigerina bulloides

AAR14639 AAR14640 AAR15199

    

LINK14 and ENAM33BC faunal analyses reveal that in both areas T. quinqueloba, G. bulloides (d’Orbigny, 1826), Globorotalia inflata (d’Orbigny, 1839), Neogloboquadrina incompta (Cifelli, 1961; [syn. N. pachyderma (Ehrenberg 1861), dextral]), G. uvula (Ehrenberg, 1861), and Neogloboquadrina pachyderma (sensu Darling et al., 2006; [syn. N. pachyderma (Ehrenberg, 1861), sinistral]) are the most common species of the planktic foraminiferal assemblage (Fig. 4). Other species of subtropical origin occurred in a small number of samples (Fig. 3), i.e. Orbulina universa, Globigerinoides sacculifer, Globigerinella siphonifera, Globigerinoides ruber, Globigerinita glutinata and Globigerinita iota, although each of these species represents less than 1% of the assemblage. No significant trends were detected in the relative abundance of N. pachyderma, neither in core LINK14 nor in ENAM33BC. From 7.8 ka BP to present, percentages of this species were consistently low (4%, Fig. 4g) indicating that summer SSTs in both areas were 9
C (Tolderlund and Bé, 1971; Thornalley et al., 2011). From w7.8 to 6 ka BP, G. bulloides and T. quinqueloba dominate the planktic assemblage in both the FaroeeShetland and the IcelandeFaroe areas (Fig. 4b and c). The dominance of these two species, as well as the significantly higher planktic foraminiferal fluxes observed through this period (Fig. 4a), suggest that at this time upper-water column stratification was reduced and the surface mixed-layer was deeper (Thunell and Reynolds, 1984; Thunell and Sautter, 1992; Rohling and Cooke, 2003; Bergami, 2006, Table 3). It is also during this period, that in both records G. inflata exhibits the highest relative abundances (Fig. 4d) suggesting increased influence of the NAC (Pflaumann et al., 2003; Farmer et al., 2011; Thornalley et al., 2011, Table 3), hence increased influx of MNAW to both areas (Holliday, 2003). Moreover, in good agreement with their geographical location (Fig. 1), G. inflata percentages are higher (w2%) in core ENAM33BC than in core LINK14 (Table 4), suggesting greater contributions from the NAC west of the Faroe Islands, in the IcelandeFaroe area. In both areas, the period between 6 and 5 ka BP is characterized by a sharp decline in planktic foraminiferal fluxes, a moderate increase in T. quinqueloba, N. incompta and G. uvula relative abundance, a moderate decrease in the G. bulloides relative abundance and a substantial decrease in G. inflata percentages (Fig. 4, Table 4).

3099 6605 7494

40 150 65 90 80

230 3870 6750 9350 10,570

4138  46 5743  41 19,141  56

4025 6125 22,322

fraction. This component includes mainly tests and test fragments of foraminifera, both benthic and planktic. The downcore preservation of the foraminiferal shells is good and unaffected by fluctuations in grain size or calcium concentration, indicating that fluctuations in the relative abundance of all species, including those susceptible to dissolution (e.g. T. quinqueloba and G. uvula), is not due to preservation changes but to ecological fluctuations (Li et al., 2000). In the FaroeeShetland area (cores LINK14 and HV03), the textural analysis of the detrital component of the sand fraction (after removal of the biogenic component by acidification) shows rounded, well-sorted grains indicating mature sediment and a moderately low-energy environment (van Weering, 1981). In the IcelandeFaroe area (core ENAM33BC), the comparison of the sand fraction percentage and sediment calcium concentration shows that both variables are highly correlated (Fig. 3b), suggesting that biogenic carbonate production is the most likely control on the composition of the sand fraction (Andersson, 1998). Compared to the FaroeeShetland area, ENAM33BC sand fraction percentages are higher (Fig. 3) and the detrital component is better sorted, suggesting a more energetic bottom environment (van Weering, 1981).

Age (cal. ka BP) 0

2

4

6

8

0

0

5

10

15

20

25

0

0

0

3

20 4025 cal y BP

10

6

9

12

230 cal y BP

3834 cal y BP

40

40

3099 cal y BP

Depth (cm)

6736 cal y BP

80

60 20

6125 cal y BP

80

120

6605 cal y BP

100

30

9242 cal y BP

160 7494 cal y BP

120 22322 cal y BP

40

ENAM33BC 0

2

140 4

6

8

HV03 0

5

LINK14

200 10

15

20

25

0

12

10570 cal y BP

24

36

48

Sedimentation rates (cm/ka) Fig. 2. Age models for cores ENAM33BC, LINK14 and HV03, including sedimentation rate (cm ka1). All plotted ages refer to calibrated AMS dates.

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Fig. 3. Sand fraction content (>63 mm) and simplified core-stratigraphy indicating the occurrence of rare subtropical foraminifera species. (a) Core LINK14 and (b) core ENAM33BC (diamonds), including XRF-Ca concentration measurements (squares).

In both cores, G. inflata percentages depict a well-defined negative trend (Table 4). The faunal changes of this period suggest increase upper-water column stratification (Koeve, 2001; Schiebel et al., 2001; Bergami et al., 2009) and a gradual westward contraction of the NAC (Kipp, 1976; Ottens, 1992; Pflaumann et al., 2003; Farmer et al., 2011). This hydrographic shift is also reflected in the marked occurrence of subtropical foraminifera species observed at the end of this period indicating larger contributions of water masses from the STG region (Fig. 3). Between 5 and 4 ka BP, foraminiferal fluxes are lower than previously (Table 4). Early in this period, subtropical species occur in both cores but disappear from the sedimentary record past w4.5 ka BP (Fig. 3). From 5 to 4 ka BP, T. quinqueloba persist as the most abundant species. However, in core ENAM33BC, its relative abundance is comparable to that of N. incompta (Table 4). A short-lived increase in G. bulloides relative abundance is observed during this period. The timing of this event is slightly different in each core but of similar magnitude (Fig. 4a). For most of this period, G. inflata percentages are low (LINK14  4%, ENAM33BC  6%) showing only a moderate increase ca 4 ka BP. Based on the ecology of individual species and species groups (Table 3), this faunal configuration indicates a shoaling of the surface mixed-layer and increasing water column stratification. Later in the Holocene, from 4 to 0.8 ka BP, planktic foraminiferal fluxes are persistently low (1 forams/cm2/y). During this period,

N. incompta becomes the most abundant species (Table 4, Fig. 4c). Throughout this interval, N. incompta and G. uvula show increasing relative abundances, while T. quinqueloba and G. bulloides show decreasing percentages. This shift in faunal dominance, and the reduction in foraminiferal fluxes, suggests that upper-ocean conditions were characterized by a well-stratified upper-water column and a shallow surface mixed-layer (Reynolds and Thunell, 1985; Sautter and Thunell, 1991; Schiebel et al., 2001, Table 3). Stronger water column stratification decreases seasonal vertical mixing, isolating the surface layer, further ensuing nutrient depletion (Edwards et al., 2001; Holliday et al., 2006; Henson et al., 2009; Jang et al., 2011; Hickman et al., 2012). Such conditions explain the sharp decrease in foraminiferal fluxes observed in this period. From 4 to 0.8 ka BP, G. inflata also shows decreasing relative abundance. It is during this period that the lowest percentages of this species occur in both records (Fig. 4d), indicating a considerable westward narrowing of the NAC (Pflaumann et al., 2003, Table 3). During the last 800 years, the trends in the relative abundance of all species appear inverted, although the general composition of the foraminiferal assemblage shows no significant change (Fig. 4, Table 4). This reversal suggests the return to a less stratified upperwater column and deeper surface mixed-layer, as well as the eastward broadening of the area influenced by the NAC. This change coincides with the onset of the Little Ice Age (LIA, Mann et al., 2009).

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a

ENAM33BC LINK14

8 6 4 2

50

b

0

40 30

T. quinqueloba (%)

20

G. bulloides (%)

Foraminiferal Flux ( forams / cm 2 / year )

10

10 50

c 40 30 20 10 8 6 4 2

G. inflata (%)

d

10

N. incompta (%)

0 50

e

40 30 20 10 40

0

30

N. pachyderma(%)

20 10 6

g

G. uvula (%)

f

0

4 2 0

0

1

2

3

4 5 Age cal. ka BP

6

7

8

Fig. 4. Planktic foraminifera census data. Foraminiferal flux (a) and the relative abundance of most common planktic species: G. bulloides (b), T. quinqueloba (c), G. inflata (d), N. incompta (e), G. uvula (f) and N. pachyderma (g) for cores ENAM33 (dark lines) and LINK14 (light lines).

Intensification of the NAC during cold climate periods (Miettinen et al., 2011), and particularly during the LIA, has also been inferred from sedimentary records off Iceland and Ireland (Eiríksson et al., 2006) as well as Southeast Greenland (Andresen et al., 2012). 3.3. G. bulloides carbon and oxygen stable isotope composition The distinct periods revealed by the downcore variability in the composition of the planktic foraminiferal assemblage, are also apparent in the stable isotope signature of G. bulloides. Earlier in the

Holocene, from 7.8 to 6 ka, d18OGB values are the lowest on record (Fig. 5a, Table 4). During this period, ENAM33BC d18OGB values are similar to those of LINK14 and HV03, indicating comparable upperocean densities in both areas. Nonetheless, d18OGB values in the IcelandeFaroe area are persistently lower (0.2&, Fig. 5c), perhaps because of greater influence of the NAC resulting in increased influx of the 18O-depleted MNAW (Frew et al., 2000) to the aforementioned region compared to the FaroeeShetland area (Fig. 1). The stronger influence of the NAC on the IcelandeFaroe area can also be inferred from the higher relative abundances displayed by G. inflata in samples from ENAM33BC compare to LINK14 (Fig. 4d, Table 4). All through this early period, the d13C signature of G. bulloides shows that surface waters in the IcelandeFaroe region had a heavier carbon stable isotope composition than at the Faroee Shetland area (Table 4, Fig. 5b). The distinct d13C signals most likely reflect the different sources of the surface water masses moving through the IcelandeFaroe and the FaroeeShetland areas (Hátún et al., 2009; McGrath et al., 2012), rather than indicating substantial differences in water column structure between locations (Pollard et al., 2004; Häkkinen and Rhines, 2009). Accordingly, the marked 12C depletion observed in the upper-water column of the IcelandeFaroe area indicates higher contributions of a nutrient depleted water mass, such as the MNAW, to this area, and explains the lower planktic foraminiferal fluxes observed in ENAM33BC compared to LINK14 (Fig. 4a, Table 4). From 6 to 5 ka BP, the d18OGB gradient between locations persists (Fig. 5c), however all the d18OGB records show positive trends resulting in d18O enrichment of w0.2& (Fig. 5a). Attributing the increase in d18OGB solely to a water temperature change implies a temperature decrease of w0.8
C (Bemis et al., 1998). Such option, challenges earlier paleotemperature records that show warming of the eastern North Atlantic between 6 and 5 ka BP (Fig. 6g, Rimbu et al., 2004; Solignac et al., 2008; Thornalley et al., 2009). Thus, it is more likely that the observed change in G. bulloides d18O reflects a reduction in the influx of the 18O-depleted MNAW (Frew et al., 2000) and increasing SSS due to increasing contributions from the STG (Solignac et al., 2008; Thornalley et al., 2009). The synchronicity between the decrease in G. inflata percentages and the increase in d18OGB is consistent with such scenario (Fig. 6). Higher SST and SSS also contribute to strengthen the stratification of the upper water column (Holliday et al., 2006). Accordingly, HV03 and LINK14 d13CGB records show positive trends indicating a gradual 12C depletion of the upper-water column through this period. ENAM33BC d13CGB values, however, are not significantly different from the previous period (Fig. 5b). Higher d13CGB values are congruent with a well-stratified water column, reflecting the nutrient depletion of the surface mixed-layer as it becomes further isolated from the nutrient-rich subsurface waters. Furthermore, nutrient depleted surface waters are congruent with the drastic reduction in foraminiferal fluxes observed at this time (Fig. 4, Table 4). From 5 to 4 ka BP, the d18OGB gradient between locations increases. However, in both areas, d18OGB records show a short-lived decrease (Fig. 5a) that is synchronous to a similar signal in the d13CGB records (Fig. 5a and b). The isotopic changes observed at this time coincide with a temporary increase in planktic foraminiferal fluxes (Fig. 4a). Collectively, these changes indicate an event characterized by decrease upper-water column stratification and a deeper surface mixed-layer. The effect of such conditions is an increase in seasonal mixing, resulting in 12C (and nutrient) enrichment of the near-surface waters, and perhaps higher surface productivity rates (Edwards et al., 2001; Holliday et al., 2006; Henson et al., 2009; Jang et al., 2011; Hickman et al., 2012), which explain the larger planktic foraminiferal fluxes observed during this short-lived event.

Table 3 Distribution, depth habitat and other ecological attributes of selected planktic foraminifera species according to the literature. Depth habitat

Environmental preferences

Shell characteristics

References

G. bulloides

Temperate to subpolar regions. North Atlantic: more abundant where SSTs are between 4 and 14
C.

Surface dweller (0e50 m depth)

Prefers well-mixed upper water column environments where phytoplankton density and prey abundance are high. Distribution and abundance are primarily controlled by food availability, rather than by temperature.

Shell size is small to medium (max. 800 mm). Moderately resistant to dissolution.

T. quinqueloba

Temperate to subpolar regions. North Atlantic: more abundant where SSTs are between 4 and 18
C. In the Polar North Atlantic, this species is more abundant in areas influenced by waters from the NAC.

Mid-surface dweller (10e60 m depth)

Abundance increases as the depth of the surface mixed-layer and chlorophyll concentrations increase. This species also responds rapidly to changes in nutrient supply. High relative abundance in surface sediments indicates enhanced surface productivity

Shell size is small (max. 250 mm). Susceptible to dissolution. Changes in carbonate preservation might affect its relative abundance in sediments.

G. inflata

Subtropical to subpolar regions. North Atlantic: common in areas where SSTs are between 4 and 11
C. In Arctic regions, this species occurs only in areas influenced by warm waters from the NAC. Temperate to subpolar regions. Abundance peaks in waters between 10 and 14
C. North Atlantic: more abundant where summer SSTs are >8
C.

Deep dweller (100e400 m depth)

In the eastern North Atlantic, the presence of this species in surface sediments indicates direct influence of the NAC and intensified influx of MNAW. Juveniles favor shallower habitats. Thrives in environments characterized by low intra-annual temperature gradients (i.e. reduced seasonal variability). It migrates to deeper habitats as surface temperatures increase. In the eastern North Atlantic it is well adapted to live below the pycnocline. Prefers environments with a well-developed thermocline. Inhabits below the deep chlorophyll maximum. Migrates down the water column in response to increased interspecific competition and temperature changes. Shows large variability in depth habitat preferences among seasons. Geographic distribution is mainly controlled by seawater temperature. Vertical distribution is related to the location of the pycnocline. Juveniles calcify within the mixed-layer and then sink below the pycnocline to reproduce.

Shell size is small to medium (max. 700 mm). Moderately resistant to dissolution.

Deuser et al., 1981; Durazzi, 1981; Ganssen and Kroon, 2000; Gifford et al., 1995; Hilbrecht, 1996; Marchant et al., 2004; Mohtadi et al., 2005; Rohling and Cooke, 2003; Sautter and Thunell, 1991; Spero and Lea, 1996; StainesUrías et al., 2009; Steens et al., 1992; Thunell and Reynolds 1984; Thunell and Sautter, 1992 Bergami et al., 2009; Bergami, 2006; Carstens et al., 1997; Frydas and Bellas, 1994; Hald et al., 2007; Husum and Hald, 2012; Johannesen et al., 1994; Malmgren, 1983; Marino et al., 2011; Mortyn and Charles, 2003; Reynolds and Thunell, 1985; Rohling and Cooke, 2003; Schulz and Mulitza, 2006; Smart, 2002; Thompson, 1981 Farmer et al., 2011; Ganssen and Kroon, 2000; Hilbrecht, 1996; Kipp, 1976; Ottens, 1992; Pflaumann et al., 2003; Tolderlund and Bé, 1971

N. incompta

Mid-deep dweller (60e150 m depth)

G. uvula

Colder/subpolar waters. North Atlantic: associated with oceanic fronts. In the Polar North Atlantic, this species is more abundant in areas dominated by the relatively warmer and fresher Norwegian Coastal Water.

Mid dweller (40e100 m depth)

N. pachyderma

Subpolar to polar regions. North Atlantic: more abundant when summer SSTs are <9
C. In surface sediments, relative abundance <50% indicates the presence of warm Atlantic waters.

Mid-deep dweller (50e300 m depth)

Shell size is small to medium (max. 500 mm). Resistant to dissolution.

Small-sized (max. 120 mm), thin-shelled species. Susceptible to dissolution. Changes in carbonate preservation may have an effect on its relative abundance in surface sediments. Shell size is small to medium (max. 500 mm). Resistant to dissolution.

Fairbanks et al., 1982; Husum and Hald, 2012; Kozdon et al., 2009; Kuroyanagi et al., 2006; Kuroyanagi and Kawahata, 2004; Reynolds and Thunell, 1985; Schiebel et al., 2001; Simstich et al., 2003; Tolderlund and Bé, 1971; Wu and Hillaire-Marcel, 1994 Bé and Hutson, 1977; Bergami et al., 2009; Berger 1969; Boltovskoy et al., 2000; Boltovskoy, 1969; Husum and Hald, 2012; Malmgren, 1983; Smart, 2002; Tolderlund and Bé, 1971

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Distribution

Bäckström et al., 2001; Bé and Hamlin, 1967; Bé, 1969; Hemleben et al., 1989; Husum and Hald, 2012; Kozdon et al., 2009; Negri et al., 2003; Simstich et al., 2003; Tolderlund and Bé, 1971

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Table 4 Overview of proxy changes through time. Presented values correspond to the proxy average and standard deviation per period. The shaded numbers indicate dominant species in the corresponding period.

Age period (cal. ka BP) Location

Core

Proxies 18

LINK14

OGB

13

5.0-4.0

4.0-0.8

0.8-0

1.44±0.06

1.63±0.09

1.73±0.12

1.87±0.13

1.83±0.26

-0.41±0.03

-0.29±0.06

-0.21±0.09

-0.22±0.09

-0.29±0.09

‰ (VPDB)

CGB % dry weight

35.18±5.89

41.91±3.80

43.55±3.38

40.76±4.48

44.63±2.18

forams/cm2/y

4.5±1.6

2.4±0.9

2.0±1.4

0.5±0.2

0.9±0.5

G. bulloides

36.9±4.1

28.9±3.2

24.5±4.7

17.9±3.6

19.9±1.7

T. quinqueloba

38.4±4.8

40.6±3.7

36.2±3.6

25.9±6.0

17.6±1.9

5.2±1.3

4.5±1.2

2.9±0.9

1.5±1.2

2.0±1.5

G. inflata

10.4±2.9

13.6±2.5

22.5±2.9

26.4±5.3

27.5±1.2

G. uvula

7.7±2.2

10.6±2.5

12.1±2.8

25.9±3.6

31.1±2.6

1.46±0.10

1.56±0.14

1.75±0.10

1.83±0.10

1.96±0.15

-0.43±0.13

-0.25±0.15

-0.28+0.10

-0.31+0.11

-0.41+0.10

47.1±9.0

49.0±3.9

50.5±11.5

51.3±5.1

47.6±6.9

OGB

‰ (VPDB)

CGB

sand fraction content foraminiferal flux Relative abundance

%

N. incompta

13

Iceland-Faroe area

6.0-5.0

foraminiferal flux

18

ENAM33BC

7.8-6.0

sand fraction content

Relative abundance

Faroe-Shetland area

HV03

Units

% dry weight 2

3.0±0.9

2.2±1.0

1.6±0.7

0.4±0.2

1.0±0.3

G. bulloides

29.6±2.3

28.5±3.1

25.7±5.3

18.9±4.6

24.1±3.2

T. quinqueloba

32.5±3.4

33.7±3.5

33.6±2.2

23.0±3.5

22.8±4.8

7.7±1.2

4.9±1.0

4.1±0.8

2.3±1.0

3.7±2.5

N. incompta

24.1±3.6

25.5±2.8

29.4±3.7

40.2±6.4

34.6±3.0

G. uvula

4.4±2.6

4.9±1.8

5.5±3.1

13.2±2.7

13.0±2.6

1.28±0.06

1.40±0.05

1.34±0.14

1.34±0.09

1.46±0.08

0.06±0.11

0.07±0.10

-0.03±0.14

-0.05±0.10

-0.02±0.03

G. inflata

18

OGB

13

CGB

forams/cm /y

%

‰ (VPDB)

In the period between 4 and 0.8 ka BP, a d18OGB gradient >4& persists between the IcelandeFaroe and the FaroeeShetland areas (Fig. 5c). In both areas, this period is characterized by increasing d18OGB values, as the upper-water column becomes progressively 18 O-enriched. Simultaneously, the d13CGB gradient between areas increases (Fig. 5d). Because of these changes, by the end of this period, the surface water mass in the FaroeeShetland area resembles waters from the STG, which are warmer, saltier and nutrient enriched (d18O-enriched, d13C-depleted), while in the IcelandeFaroe area, surface waters resemble those from the NAC, colder, fresher and nutrient depleted (d18O-depleted, d13Cenriched). Within the last 800 years, no significant changes are observed in the carbon stable isotope composition of G. bulloides, neither in the FaroeeShetland nor in the IcelandeFaroe area. However, from the beginning of this period, d18OGB records shows a marked multicentury increase coinciding with the onset of the LIA (Fig. 5a). 4. Discussion In the early Holocene, from 7.8 to 6 ka BP, the low d18OGB gradient between the FaroeeShetland and the IcelandeFaroe sites (Fig. 5a) indicates that the properties of the upper-water column (i.e. temperature, salinity) were similar in both areas (Fig. 7). Modern controls on the stable oxygen isotope composition of North Atlantic surface water masses include (1) influx of d18O lighter, lowsalinity water from the Arctic advected by the East Greenland

Current, (2) evaporationeprecipitation processes leading to 18O enrichment and 18O depletion, respectively. The regionally different influences of each of these controls results in a d18O surface gradient across the North Atlantic (Frew et al., 2000). The isotopically lightest surface waters occur in the western region under the influence of low-salinity waters from the Arctic (Khatiwala et al., 1999). Toward the east, surface isotopic values become progressively heavier by the influx of 18O-enriched surface waters advected from the STG region, where evaporation rates are higher (e.g. Schmitt et al., 1989). Thus, the reduced d18O gradient between the study sites (Fig. 5c) suggests that early in the Holocene the surface water regime, both in the FaroeeShetland and IcelandeFaroe, was dominated by a single water mass. The light d18O composition of this water mass, further indicates a source area to the west (Frew et al., 2000) and, consequently, eastward transport by the NAC (Holliday, 2003). During this early period, the composition of the planktic foraminiferal assemblage suggests a deep mixed-layer, both east and west of the Faroe Islands. A well-mixed upper-water column, characterized by lighter d18O values, is consistent with an ocean state under positive NAO-like conditions (Henson et al., 2009), when enhanced sea-level pressure gradients associated with the positive NAO phase produce strong westerly winds that break up upper-water column stratification and deepen the mixedlayer (Hátún et al., 2009). Thus, from 7.8 to 6 ka BP, faunal and isotopic records suggest an ocean state of intensified SPG circulation that resulted in the eastward expansion of the NAC and reduced contributions of the warm, saline waters from the STG

F. Staines-Urías et al. / Quaternary Science Reviews 76 (2013) 66e81

2.4

a

ENAM33BC LINK14 HV03

2.2 OGB (‰)

2.0

18

75

1.8 1.6

1.2

32 0.2

b

0.0

1 3 5 7 9 11

36 ENAM33BC

b

c

36

c

-0.6

35 Iceland-Faroe Gap (Thornalley et al., 2009)

0.1

-0.7

0

1

2

3

4

5

6

7

34

1.6 OGB (‰)

-0.5

CL14-E33 (‰)

-0.3

13

-0.1

d

1.4

d

18

OL14-E33 (‰)

18

34

Labrador Sea (de Vernal & Hillaire-Marcel, 2006)

1.2

8

ENAM33BC

Age cal. ka BP

Sea Surface Salinity (‰)

0 -2

Subtropical E Atlantic (deMenocal et al., 2000)

-4 -6

35 34

Faroe-Shetland Channel (Solignac et al., 2008)

f

33

16

g

14 12

Faroe-Shetland Channel (Solignac et al., 2008)

8 6

1.7

LINK14

18

OGB (‰)

2.2

10

Summer SST (°C)

(Hátún et al., 2005; Holliday et al., 2008). Eventually, the decreased influx of ENAW to the FaroeeShetland area resulted in the homogenization of the upper-ocean conditions between areas (Bersch et al., 2007; Holliday et al., 2008, Fig. 8a). The intensification of the SPG circulation during the early Holocene has been recognized in dinoflagellate cyst records from the FaroeeShetland Channel (site HM03-133-25), indicating lower SSSs and SSTs from 8 to 6 ka BP (Fig. 6f and g, Solignac et al., 2008). Likewise, coupled Mg/Ca-d18O measurements in planktic foraminifera south of the IcelandeFaroe gap (site RAPID-12-1k) indicate reduced water column stratification from 8.5 to 6 ka BP (Thornalley et al., 2009).

2

e

Temperature anomaly (˚C)

Fig. 5. G. bulloides oxygen (a) and carbon (b) stable isotope composition for cores LINK14 (circles), HV03 (squares) and ENAM33BC (diamonds). d18OGB values have been corrected for whole-ocean ice-volume changes. Three-point moving averages are shown in bold. The G. bulloides d18O (c) and d13C (d) gradient between areas (Dd ¼ dLINK14  dENAM33BC) was calculated from points of equal age produced by the linear interpolation resampling of the original series. Three-point moving averages shown in bold.

Salinity (p.s.u.)

0.8 0.6 0.4 0.2 0.0 -0.2

CGB (‰)

-0.4

13

-0.2

NAC influx G. inflata(%)

1.0

Summer SSS (p.s.u.)

30

a

1.4

h

1.2

0 2 4 6

i LINK14

0

1

2

3

4 5 6 Age (cal. ka BP)

7

8

NAC influx G. inflata (%)

Fig. 6. Proxy records indicating changes in the strength of the SPG circulation. In all records, SPG strength increases downwards. (a) Near-surface salinity estimates based on dinocyst assemblages from the Labrador Sea (site MD2227, de Vernal and HillaireMarcel, 2006) on reversed axis. (b) G. inflata relative abundance (core ENAM33BC) on reverse axis, three-point moving average shown in bold. (c) Subsurface salinity estimates derived from paired Mg/Ca-d18O measurements in G. inflata near the Icelande Faroe Gap (site RAPID-12-1k, Thornalley et al., 2009), three-point moving average shown in bold. (d) G. bulloides d18O (core ENAM33BC), three-point moving average shown in bold. (e) Faunal-based linear detrended temperature anomalies record from the subtropical eastern Atlantic (ODP Hole 658C, deMenocal et al., 2000). (f) Nearsurface salinity estimates based on dinocyst assemblages from the FaroeeShetland Channel (site HM03-133-25, Solignac et al., 2008), three-point moving average shown in bold. (g) Near-surface temperature estimates based on dinocyst assemblages from the FaroeeShetland Channel (site HM03-133-25, Solignac et al., 2008), three-point moving average shown in bold. (h) G. bulloides d18O (core LINK14), three-point moving average shown in bold. (i) G. inflata relative abundance (core ELINK14) on reverse axis, three-point moving average shown in bold.

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HV03

LINK14 2.20

7.8 to 6 ka BP

ENAM33

6 to 4 ka BP

ENAW

2.20 2.00 1.80

1.60

1.60

1.40

1.40

1.20

MNAW

OGB (‰)

1.80

18

OGB (‰)

2.00

18

4 to 0 ka BP

1.20 1.00

1.00 -0.60 -0.40 -0.20 0.00 0.20

-0.60 -0.40 -0.20 0.00 0.20 13

-0.60 -0.40 -0.20 0.00 0.20

CGB (‰)

Fig. 7. Crossplot of G. bulloides stable carbon and ice-volume-corrected oxygen isotopes between 7.8 and 6 ka BP (left), 6 and 4 ka BP (center), and 4 ka BP to present (right). The time progression shows the gradual segregation of the water masses. Shaded regions show modern d18O and d13C estimates of biogenic calcite precipitated in equilibrium with MNAW and ENAW. For modern estimations, the original d18Owater were derived from salinity values using the relation presented by Frew et al. (2000). Subsequently, G. bulloides calcite d18O values were calculated using Bemis et al. (1998) empirical relation (Equation 5) for water-calcite fractionation. Modern temperature and salinity ranges were obtained from Holliday et al. (2008) for MNAW and from McGrath et al. (2012) for ENAW. The d13C values were obtained from Weidman and Millner (2000).

In present times, positive NAO conditions have been correlated with intensified SPG circulation (Sarafanov, 2009; Sarafanov et al., 2010). Model simulations show that during positive NAO phases, strong cold winds over the northern North Atlantic lead, through the large loss of heat from the ocean, to surface cooling in the SPG region (Hurrell et al., 2003; Lohmann et al., 2009). However, apart from wind forcing, the strength of the SPG circulation also depends on the baroclinic circulation regime driven by buoyancy forcing associated with deep convection (Häkkinen and Rhines, 2004; Born and Levermann, 2011). Accordingly, through cooling and subsequent formation of intermediate and deep water masses, positive NAO-like buoyancy forcing leads to the thickening of the intermediate layers, creating a doming structure of the isopycnals of the SPG, increasing baroclinic forcing, and ultimately strengthening SPG circulation (Lohmann et al., 2009). Likewise, the change in density structure associated with decreased meltwater input to the Labrador Sea, i.e., enhanced density gradient at the edge of the SPG, increases baroclinic circulation and favors deep convection (Hillaire-Marcel et al., 2001; Thornalley et al., 2009; Born and Levermann, 2011), further intensifying SPG circulation. Thus, the reduction of deglacial freshwater input occurred after 8.2 ka BP (Mayewski et al., 2004) in conjunction with predominantly positive NAO-like conditions early in the Holocene (Dima et al., 2002; Rimbu et al., 2003) are perhaps the forcing mechanisms behind the long-term SPG strengthening observed from 7.8 to 6 ka BP. Fig. 6 shows proxy evidence of such SPG strengthening, as higher SSS in the Labrador Sea (Fig. 6a) coincides with fresher surface conditions in the IcelandeFaroe (Fig. 6c and d) and FaroeeShetland areas (Fig. 6f and h). In the subtropical Atlantic, where cooler temperatures are caused by increased advection of subpolar waters during intensified SPG circulation, a faunal record of SST variations (ODP Hole 658C) shows negative SST anomalies from 9 to 6 ka BP (Fig. 6e, deMenocal et al., 2000). Collectively, these proxies indicate increased MNAW influx to the northeastern North Atlantic and an overall increase in SPG influence in the eastern subtropical North Atlantic. Modern oceanographic data show that temperature and salinity variations in the eastern North Atlantic are significantly influenced by changes in regional circulation, manifested as a greater or lesser contribution of the cooler and fresher MNAW, and the warm and saline ENAW (Holliday, 2003). Consistent with modern controls on regional surface oceanographic conditions, between 6 and 5 ka BP, proxy records show increasing ENAW contributions associated

with SPG circulation weakening (Fig. 8b). At this time, paleorecords show an uncharacteristic occurrence of subtropical species (Fig. 3a), rapid upper-ocean warming (Fig. 6g), increasing SSS (Fig. 6c and f), and weakening of the NAC (Fig. 6b and i). Globally, the period between 6 and 5 ka BP has been recognized as a period of rapid climate change (Mayewski et al., 2004), featuring increasing water column stratification in the Iceland Sea (Simstich et al., 2012), North Atlantic ice rafting events (Bond et al., 1997), northern Europe alpine glacier advances (Denton and Karlén, 1973), and Scandinavian cooling (Karlén and Kuylenstierna, 1996). Notably, these climate changes in conjunction with weaker SPG circulation resemble the pattern of climate variability associated with the low state (negative phase) of the NAO (Hurrell et al., 2003). The comparison of foraminiferal stable isotope data to insolation induced thermal stratification changes in the northern North Atlantic, as obtained by solar-forced general circulation model simulations, suggests a linkage between oceanic stratification and insolation during the Holocene, not observed in previous glacial times (Simstich et al., 2012). Moreover, climate models and empirical reconstructions have shown that, compared to the global average, late Holocene temperature changes in the North Atlantic associated with solar irradiance changes are quite large (Bond et al., 2001; Shindell et al., 2001). This regional amplification appears to occur primarily through a forced shift toward the low (negative) state of the NAO as solar irradiance decreases (Shindell et al., 2001). Hence, solar variability appears as the more plausible forcing mechanism for the surface circulation changes, and the shift toward a negative NAO-like state, observed between 6 and 5 ka BP. Particularly, as this period coincides with maxima in D14C and 10Be records (residual proxies for solar variability) suggesting a decline in solar output (Beer, 2000). In the period between 5 and 4 ka BP, proxy records indicate a secondary phase of SPG circulation strengthening, although less pronounced and shorter than the one observed prior to 6 ka BP. At this time, the intensification of the SPG circulation is expressed as decreasing SSS in the IcelandeFaroe gap (Fig. 6c), 18O depletion of the surface water masses of the IcelandeFaroe area (Fig. 6d), negative temperature anomalies off the west coast of Africa (Fig. 6e), and decreasing SSS and SST along the FaroeeShetland Cannel (Fig. 6f and g). In the Iceland Sea, this period is also characterized by a decrease in upper-water column stratification (Simstich et al., 2012). However, in the IcelandeFaroe and Faroee Shetland areas, the increase in the relative abundance of the NAC

F. Staines-Urías et al. / Quaternary Science Reviews 76 (2013) 66e81

Fig. 8. Surface circulation and convection intensity in the subpolar North Atlantic under high (a) and low (c) Subpolar Gyre (SPG) activity corresponding to positive and negative North Atlantic Oscillation-like conditions. Panel (b) represents a transitional stage. The position of deep convection sites is denoted by the solid (intense) and dashed (incipient) circles (Sarafanov, 2009). Larger, shaded ovals represent areas where the Atlantic Inflow waters are entrained into the Arctic Mediterranean (Sarafanov et al., 2010). STG, Subtropical Gyre.

indicator species G. inflata is small and occurs later, near the end of the period w4.1 ka BP (Figs. 1 and 6b). The analysis of modern meteorological and oceanographic data shows that as the SPG retreats during extreme and/or prolonged negative NAO conditions, the warmer, saltier waters from the STG region invade the central part of the SPG, decreasing the density gradient across the gyre, further reducing SPG circulation strength (Häkkinen and Rhines, 2004; Reverdin, 2010). This process leads to a more saline Atlantic Inflow to the AM, which is eventually carry into the Labrador Sea, via the East Greenland Current, increasing once again the density gradient at the edge of the gyre and eventually reinforcing SPG circulation. This feedback mechanism has also been associated with the restart of the Atlantic meridional overturning circulation at 5 and 2.8 ka BP (Sarafanov, 2009; Thornalley et al., 2009, 2011). It appears, then, that the salinity

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increase occurred in the entire eastern North Atlantic during the previous negative NAO-like state eventually resulted in a long-term strengthening of the SPG circulation between 4 and 5 ka BP. In the late Holocene, between 4 and 0.8 ka BP, upper-ocean conditions in both study areas are characterized by higher frequency, lower amplitude variability and the further differentiation of the stable isotopic composition of the surface water masses flowing through each area (Fig. 7). Thus, rapid oceanographic fluctuations in the eastern North Atlantic appear associated with significant changes in the properties of the surface waters flowing into the AM (Fig. 8c). At this time, lake records from southwestern Greenland indicate a conspicuous, long-term change toward the low state of the NAO, including extended periods of strong negative NAO conditions (Olsen et al., 2012). In addition, Holocene reconstructions of NAO variability indicate that the oceaneatmosphere circulation changes associated with this mode of atmospheric variability coincide with the large-scale, orbitally induced shifts in Northern Hemisphere climate observed at the end of the mid-Holocene (Andresen et al., 2012; Olsen et al., 2012). Notably, these ocean-atmosphere circulation changes also involve regional fluctuations in North Atlantic SST, which are observed in both North Atlantic terrestrial climate records and ice rafting episodes (Moros et al., 2004). Consistent with paleoclimate reconstructions, a general decline of the NAO state through the late Holocene is observed in atmospheric general circulation model simulations solely forced by insolation and CO2 (Dima et al., 2002; Rimbu et al., 2003). Hence, the late Holocene weakening of the NAO has been attributed to winter tropical warming due to increasing low-latitude solar insolation associated with the Earth’s precession cycle (Rimbu et al., 2003; Risebrobakken et al., 2011). In relation to ocean surface circulation changes, the gradual decline in NAO-like forcing (Rimbu et al., 2003; Jessen et al., 2011; Olsen et al., 2012) presumably contributes to enhance the sensitivity of the system to other forcing mechanisms involved in ocean dynamics, such as baroclinic circulation and buoyancy (Schulz et al., 2007; Thornalley et al., 2010; Risebrobakken et al., 2011). Congruently, the episodic freshening of Labrador Sea observed during the late Holocenednot primarily enhanced by meltwater fluxes but associated with changes in sea-ice formation and drift, and influx of fresher polar ocean waters (Mayewski et al., 2004; Thornalley et al., 2009)dis less pervasive and of shorter duration (Fig. 6a, de Vernal and Hillaire-Marcel, 2006). Thus, without meltwater freshening and under weak NAO forcing, surface circulation appears to oscillate between two modes involving stronger and weaker SPG circulation (Thornalley et al., 2011). This interpretation highlights the impact of moderate freshwater fluxes to the Labrador Sea in forcing oceanographic variability, whether the freshening is produced by ice melting or increased precipitation, corroborating the paramount importance of North Atlantic salinity balance, i.e. the transport of salt from low to high latitudes, in modulating SPG dynamics and its relation to climate changes. In this context, the eastward broadening of the NAC coinciding with the onset of the LIA ca 0.8 ka BP, as illustrated by the increased in G. inflata abundance (Fig. 6b and h), is to be noted. As the SPG system oscillates, a stronger SPG results in intensified NAC salinity (and heat) transport, affecting both the central and eastern SPG, and the Irminger Seawater pathways (Lassen et al., 2004; Seidenkrantz et al., 2007; Andresen et al., 2012). Thus, the intensification of the NAC acts as a feedback mechanism by affecting the density gradients at the edge of the gyre, and hence baroclinic circulation. 5. Conclusions Faunal and stable isotope foraminiferal data reveal that sine the early Holocene, large-amplitude millennial changes in the

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characteristics of the upper-water column of two major Northeast Atlantic warm water pathways into the AM were modulated by changes in the intensity of SPG circulation. The interplay between the Subpolar and Subtropical Gyres modulates the properties of the inflowing waters to the AM by controlling the transfer pattern of subtropical salinity to high latitudes via the NAC and the SC. The strengthening of the SPG circulation observed earlier in the Holocene (7.8e6 ka BP) was presumably related to enhanced positive NAO-like atmospheric forcing and decrease meltwater input to the Labrador Sea, and resulted in relatively fresher and cooler Inflow waters. The mid-Holocene (6e4 ka BP) SPG circulation weakening/ strengthening cycle resulted from a millennial decline in NAO-like atmospheric forcing, linked to decreased solar irradiance, followed by a long-term intensification in baroclinic circulation associated with the higher density of the Atlantic Inflow waters fed to the Labrador Sea later in the period. The high frequency/low amplitude changes in SPG circulation observed later in the Holocene (4e0 ka BP), suggest a progressive SPG weakening and increasing influx of STG waters, both resulting in relatively warmer and saltier Inflow waters. Such changes are presumably linked to a declining NAO-like atmospheric forcing as winter low latitude solar insolation increased. The low (negative) NAO-like forcing likely contributed to increase the sensitivity of the SPG circulation system to small changes in SSS, hence enhancing the role of other forcing mechanisms such as baroclinic circulation and buoyancy. Acknowledgments We highly acknowledge financial support from the Danish Agency for Science and Technology provided to the Faroese Fishery Laboratory (Grant 2144-09-0003), which funded this study. We thank Bogi Hansen and Hjalmar Hátún for inspiring collaboration and discussions and the Faroe Marine Research Institute for making shiptime available for collecting additional core material. Master and crew of RV Magnus Heinason are thanked for their collaboration during the work at sea. We very much appreciate collaboration with Tine L. Rasmussen, University of Tromsø, who made core material (LINK14) and radiocarbon dating information of this core available. The manuscript was improved by helpful comments of two anonymous reviewers. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.quascirev.2013.06.016. References Andresen, C.S., Hansen, M.J., Seidenkrantz, M.-S., Jennings, A.E., Knudsen, M.F., Nørghaard-Pedersen, N., Larsden, N.K., Kuijpers, A., Pearce, C., 2012. Mid- to late-Holocene oceanographic variability on the Southeast Greenland shelf. The Holocene. http://dx.doi.org/10.1177/0959683612460789. Andersson, C., 1998. Pliocene calcium carbonate sedimentation patterns of the Ontong Java Plateau: ODP Sites 804 and 806. Marine Geology 150. http:// dx.doi.org/10.1016/S0025-3227(98)00053-X. Andersson, C., Pausata, F.S.R., Jansen, E., Risebrobakken, B., Telford, R.J., 2010. Holocene trends in the foraminifer record from the Norwegian Sea and the North Atlantic Ocean. Climate of the Past 6. http://dx.doi.org/10.5194/cp-6-179-2010. Bäckström, D.L., Kuijpers, A., Heinemeier, J., 2001. Late Quaternary North Atlantic paleoceanographic records and stable isotopic variability in four planktonic foraminiferal species. Journal of Foraminiferal Research 31. http://dx.doi.org/ 10.2113/0310025. Bauch, H.A., 1994. Significance of variability in Turborotalita quinqueloba (Natland) test size and abundance for paleoceanographic interpretations in the NorwegianeGreenland Sea. Marine Geology 121. http://dx.doi.org/10.1016/00253227(94)90162-7.

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