Changes in planktic foraminiferal faunas, temperature and salinity in the Gulf Stream during the last 30,000 years: influence of meltwater via the Mississippi River

Quaternary Science Reviews 33 (2012) 42e54

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Changes in planktic foraminiferal faunas, temperature and salinity in the Gulf Stream during the last 30,000 years: influence of meltwater via the Mississippi River T.L. Rasmussen a, *, E. Thomsen b a b

Department of Geology, University of Tromsø, Dramsveien 201, N-9037 Tromsø, Norway Department of Geoscience, University of Aarhus, DK-8000 Aarhus C, Denmark

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 February 2011 Received in revised form 22 November 2011 Accepted 24 November 2011 Available online 21 December 2011

Variations in the distribution of planktic foraminiferal faunas, temperature and salinity in the surface water of the Gulf Stream during the last 30,000 years have been reconstructed based on a sediment core from the Blake Ridge, subtropical western Atlantic Ocean. Temperatures and salinities were calculated using planktic foraminiferal transfer functions and previously published oxygen isotope values. Productivity was estimated using the >106 mm size fraction of planktic foraminifera. The reconstructed values show that temperature variations have been small during the investigated period with a total range of only 3
C. The highest temperatures, about 1.5
C above the present temperature in the area, occurred during the late Glacial, Heinrich event 1 and the mid Holocene time periods. The lowest temperatures, about 1.5
C below the present temperatures, occurred during the Bølling interstadial. The early Holocene period was relatively cool. The low temperatures during the Bølling interstadial and during the early Holocene are in contrast to the northeastern Atlantic, where the Bølling interstadial was the warmest period of the deglaciation, and the early Holocene the warmest during the Holocene. We attribute the lower temperatures during the Bølling and early Holocene periods to colder meltwater from the Laurentide Ice Sheet flowing into the Gulf of Mexico through the Mississippi River system and carried to the Blake Ridge via the Gulf Stream. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Planktic foraminifera Sea surface temperature Salinity Gulf stream Meltwater Mississippi river

1. Introduction In the Atlantic Ocean, the Gulf Stream and North Atlantic Drift transport warm water north into colder areas, where it cools and sinks. The convected water then returns as North Atlantic Deep Water (NADW) to low latitudes. The convection process is a major driving force in the Atlantic Meridional Overturning Circulation (AMOC). Today, the main convection areas are located in the Nordic seas and in the Labrador Sea (Fig. 1). The AMOC system has been relatively stable during the Holocene. However, this was not the case during the last glaciation and deglaciation, when icebergs and meltwater, from the ice sheets surrounding the North Atlantic and Arctic oceans, by decreasing the surface water density, repeatedly stopped or reduced the deep convection (e.g., Bond et al., 1993). The strongest of these events during the deglaciation was Heinrich event H1, 18,000e15,500 cal years BP, when icebergs from the Hudson Strait spread a meltwater plume north of 45
N, completely halting convection in the North Atlantic and Nordic seas

* Corresponding author. Tel.: þ47 7764 4408; fax: þ47 7764 5600. E-mail address: [email protected] (T.L. Rasmussen). 0277-3791/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.quascirev.2011.11.019

(Bond et al., 1993). During the later part of the deglaciation cold freshwater from the melting Laurentide Ice Sheet was routed into the Gulf of Mexico, the North Atlantic and the Arctic Oceans resulting in several strong cooling events including the Younger Dryas, the Pre-Boreal Oscillation and the so-called 8.2 ka event (Broecker et al., 1989; Andrews et al., 1993; Aharon, 2003; Carlson et al., 2007; Lewis and Teller, 2007; Murton et al., 2010). Meltwater outflow continued until c. 7 ka BP, when most of the ice sheet had finally disappeared (Dyke and Prest, 1987). Variations in the North Atlantic Drift in relation to past climate changes and the rate of convection have been studied intensely in cores from the central and eastern part of the North Atlantic Ocean (e.g., Duplessy et al., 1986; Sarnthein et al., 1994; Kroon et al., 1997) and from the Nordic seas (e.g., Koç and Jansen, 1992). However, only a few studies have been concerned with the Gulf Stream itself and then rarely in detail or over longer time periods (see e.g., Ruddiman, 1968; Balsam, 1981; Lynch-Stieglitz et al., 1999; Keigwin, 2004; Vautravers et al., 2004; LeGrande and Lynch-Stieglitz, 2007; Carlson et al., 2008; Schmidt and Lynch-Stieglitz, 2011). In terms of planktic foraminifera the western subtropical North Atlantic Ocean constitutes a “white spot” with only a few studies since the CLIMAP Project Members (1981) reconstruction of sea surface temperatures

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Fig. 1. Map of the western North Atlantic Ocean showing position of KNR140-2/51GGC core site and major current systems.

(SST) for the Last glacial Maximum (LGM) (see e.g., Pflaumann et al., 2003; Kim et al., 2004; Kucera et al., 2005). In the present study we investigate the distribution pattern of planktic foraminiferal species off northwestern Florida over the last 30,000 years. The purpose of the study is to reconstruct changes in sea surface temperature (SST), salinity and productivity in the Gulf Stream in the western subtropical North Atlantic. The study is based on core KNR140-2/51GGC taken on the Blake Ridge below the northeastward flowing stream (Fig. 1). We investigate the distribution patterns of planktic foraminifera in both the >106 mm and >150 mm size fractions. The >150 mm size fraction allows us to calculate SST by transfer functions, while the >106 mm fraction captures a high proportion of the small, productivity indicator species (see e.g., Kipp,1976; Pflaumann et al.,1996; Rasmussen and Thomsen, 2008). We reconstruct d18Owater and salinity in order to analyse changes in water mass properties. The core has been studied previously and the lithology, the distribution of selected benthic foraminiferal species, and benthic and planktic oxygen isotope records are described in Keigwin (2001, 2004), together with several AMS-14C dates. SST estimates for the deglaciation based on planktic Mg/Ca ratios have been published in Carlson et al. (2008), while the distribution of neodymium isotopes is treated in Gutjahr et al. (2008). 2. Physical settings Core KNR140-2/51GGC was retrieved from Blake Outer Ridge (32
47’N, 76
17’W) at a water depth of 1790 m. The core site is a sediment drift deposited from the bottom currents of the NADW (Keigwin, 2004) in an area with low current activity and a high sedimentation rate (Gutjahr et al., 2008). The core site is located below the Gulf Stream. The Gulf Stream constitutes the continuation of the Florida Current and the Loop Current of the Gulf of Mexico and Caribbean Sea.

It is fed by equatorial surface water and consists of warm, oligotrophic surface water (Fig. 1). The current system is partly wind-driven and partly driven by the thermohaline circulation (e.g., Kanzow et al., 2007). To the southeast the stream is bounded by the Sargasso Sea subtropical gyre and to the northwest by the colder and fresher water of the Labrador Current (Fig. 1). The Gulf Stream meanders and where it mixes with the colder more eutrophic surface water cold and productive eddies are generated. The flow strength of the Gulf Stream varies on a seasonal scale. During summer, the stream is warmer, stronger and flows further northward than during winter-spring, when it is wider, weaker and cooler (Tracey and Watts, 1986). 3. Methods The Holocene interval was sampled in 2 cm-thick slices for every 10 cm, while the glacial interval was sampled for every 2e4 cm. The samples were dried, weighed and washed over 63 mm sieves. The residues were dry sieved into >106 mm fractions and 63e106 mm fractions. Between 400 and 600 specimens of planktic foraminifera were picked from the >106 mm fraction on a picking tray, counted, and identified. The >106 mm fractions were subsequently dry sieved over 150 mm sieves and 300 to 400 specimens of foraminifera were counted and identified. The concentration of planktic foraminifera >106 mm and >150 mm were calculated per gram dry weight sediment. The methods for the stable isotope and CaCO3 analyses and the glacial AMS-14C dates are described in Keigwin (2001, 2004). Three new dates for the Holocene interval (D.W. Oppo, unpublished data) are presented in Table 1. All 14C dates were calibrated to calendar years using the the Fairbanks et al. (2005) program (Table 1; Fig. 2). The d18O record measured in Globigerinoides ruber (Keigwin, 2004) was corrected for isotopic disequilibrium by 0.4& (e.g., Fairbanks

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Table 1 Carbon-14 ages and calibrated ages of new dates of core KNR140-2/51GGC. Depth cm

14 C age (uncorr.)

Error (1s)

Calibrated age

Error (1s)

Lab code NOSAMS

Species

4 100 210

615 3930 7310

35 45 55

240 3815 7730

80 65 65

OS-50126 OS-50127 OS-50128

Mixed plank. G. sacculifer G. sacculifer

et al., 1980, 1982; Loncariç et al., 2006; Steph et al., 2009) and for ice volume changes using the sea level curve of Fairbanks (1989). Summer sea surface temperatures were reconstructed by transfer functions using the C2 program of Juggins (2007). The calculations are based on the distribution of planktic foraminifera in the >150 mm fraction, the summer sea surface temperatures of the World Ocean Atlas (WOA, 1998), and the MARGO database of the distribution of planktic foraminifera >150 mm in the North Atlantic Ocean (Kucera et al., 2005). Samples from stations with summer SST below 3
C and five apparent outlier samples were excluded from the calculations (see Juggins, 2007). We applied the WAPLS (Weighed Average Partial Least-Squares) technique using three components and following the recommendations of Birks (1998). Three components reduced the RMSEP (Root mean Square Error of Prediction) with 10% as compared to two components and with 21% as compared to one component. Four components gave a further reduction of 4.4%. However, to be useful a component should give a reduction of at least 5% (Birks, 1998, p. 312). The various options created essentially similar temperature curves, but with downward parallel displacements of about 1
C for one and two components and similar upward displacement for four and five components. The three component option produced the recent temperatures that are most similar to present temperatures in the area. The RMSEP using three components is 1.52
C of the calculated temperatures. The calculated summer SST were used to calculate the d18Owater applying the formula published by Shackleton (1974) as presented in Lubinski et al. (2001). The standard error for the d18Owater is 0.35&. Salinity was calculated using the GEOSECS data (Østlund et al., 1987). The quantitative faunistic data from the >106 mm size fraction were submitted to fuzzy c-means (FCM) clustering analysis (Bezdek, 1987; Gary et al., 2009). FCM allows samples to group with multiple clusters, in contrast to standard hierarchical clustering methods, where samples only can belong to one cluster. The method is advantageous in dynamic systems with small, but frequent faunistic changes. A cluster membership is represented by a value that ranges between 0 and 1, so that for each sample all of its cluster membership will sum up to 1 (Gary et al., 2009; Erbs-Hansen et al., 2011). The number of clusters to which a sample can belong is controlled by the fuzzy exponent. The fuzzy exponent is determined subjectively. If the exponent is set at 1 a sample can only belong to one cluster. In this study, the fuzzy exponent was set at a value of 1.5 as recommended by Gary et al. (2009) and Erbs-Hansen et al. (2011) for biostratigraphical and paleoecological data. For each cluster, a characteristic assemblage is determined algorithmically by locating the centre of gravity of samples that are grouped together in the variable space (Gary et al., 2009; Erbs-Hansen et al., 2011). The program TACSWorks was used for the FCM analysis (Gary et al., 2009). 4. Stratigraphy The core comprises the last c. 30,000 years and includes the LGM (25,000-c. 18,000 years BP) from 403 to 370 cm, the deglaciation (18,000e11,700 years BP) from 370 to 298 cm, and the Holocene (11,700 years BP-Recent) from 298 to 0 cm (Keigwin, 2004) (Figs. 2 and 3). The deglaciation comprises Heinrich event

Fig. 2. Age model for KNR140-2/51GGC with calibrated ages versus core depth (cm). Abbreviations: LGM, Last Glacial Maximum; H1, Heinrich event 1; B-A, Bølling-Allerød interstadials; YD, Younger Dryas.

H1 (18,000e15,500 years) from 370 to 352 cm, the Bølling-Allerød interstadials (15,500e13,000 years) from 352 to 317 cm, and the Younger Dryas stadial (13,000e11,700 years) from 317 to 298 cm. The sedimentation rate was highest in the Holocene with rates of 22e27 cm/ka and lowest in the glacial interval with rates of about 3 cm/ka (see also Gutjahr et al., 2008) (Figs. 2 and 3). 5. Results 5.1. Overall distribution of planktic foraminifera The concentration of planktic foraminifera is highest in the glacial part of the record and lowest in the Holocene (Fig. 3). A small but distinct peak at 190 cm (c. 7000 years BP) (Figs. 3 and 4) shows indication of size sorting, as small species here are either reduced in number or completely absent. An increased number of shallow water, phytal benthic foraminifera suggests that the sample may represent a turbidite. A total of 26 planktic foraminiferal taxa were identified. Large specimens of Neogloboquadrina dutertrei were counted separately from smaller juvenile specimens. These smaller forms are often referred to as ‘N. dutertrei-N. pachyderma d-intergrades’ (Bé and Tolderlund, 1971; Kipp, 1976; Hillbrecht, 1997) (Fig. 4). A few specimens of Globoturborotalita tenella were included in the counts of Globoturborotalita rubescens. The distribution patterns of the various species are fairly similar in the >106 mm and >150 mm size fractions (Fig. 4). However, species with large tests such as G. ruber, Globigerinita glutinata and N. dutertrei are relatively more abundant in the >150 mm size fraction (Fig. 4a,c,r), whereas species with smaller tests like G. rubescens and Turborotalita quinqueloba are more abundant in the >106 mm size fraction (Fig. 4b,p). The planktic foraminiferal faunas are in all samples dominated by G. ruber, G. rubescens and G. glutinata (Fig. 4a,b,c). Most species occur in all samples and changes in the species composition are generally gradual. 5.2. Sea surface temperature, oxygen isotope and salinity variations The average summer SST for the uppermost three samples is 27.6
C, as calculated by transfer functions. This value is very close

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Fig. 3. Percent calcium carbonate, grain size distribution in percent, concentration of planktic foraminifera >106 mm and >150 mm. Concentration values are in numbers per g dry weight sediment. Column to the right shows sedimentation rates. Abbreviations as in Fig. 2.

to modern summer SST in the area of 27.8
C (WOA, 1998) (Fig. 5a). The mid Holocene, Heinrich event H1, and the late glacial were the warmest periods with summer SST 1e1.5
C higher than today. The coldest periods were the Bølling interstadial and the Younger Dryas stadial with SST about 1.5
C below the modern values. In general, temperature variations have been small during the last 30,000 years with a total range in summer SST of about 3
C (Fig. 5a). The calculated SST closely follows the distribution patterns of the planktic foraminiferal faunas (Fig. 4; see also discussion below). The relative abundance of the individual planktic species varies with less than 20% in good agreement with the calculated small temperature fluctuations in the surface water. The d18O values show relatively high variability (Fig. 5b). This is typical for G. ruber and probably reflects seasonality as the species calcify over a wide range of water temperatures (Deuser et al., 1981; Anand et al., 2003; Lund and Curry, 2004). The average d18Owater value for the last 2000 years is 1.22& corresponding to a salinity value of 36.4 (Fig. 5c). The modern d18Owater and salinity values in the area are c. 1.2& and 36.5e37, respectively (LeGrande and Schmidt, 2006). The average d18Owater value for the whole of the Holocene is 1.29& corresponding to a salinity of c. 36.7 (Fig. 5c). The average d18Owater value is 1.75& for Heinrich event H1 and 2.25& for the late glacial samples. These values, which correspond to salinities between 37.7 and 38.5 are significantly higher than Holocene and modern values. The few data points in the glacial interval are because most of the samples analysed for oxygen isotopes and the samples analysed for SST were taken at different levels (Fig. 5). 6. Analysis and interpretation of planktic foraminiferal assemblages 6.1. Fuzzy c-means cluster analysis Despite of the overall small variability, the FCM cluster analysis reveals that the faunas have undergone distinctive changes (Fig. 6). Using four clusters as the best solution for the data set, the

section can be subdivided into four assemblage zones each with a characteristic combination of species. Zone A comprises the late glacial, LGM and Heinrich event H1, 30,000e15,500 years BP (Fig. 6a). The zone is clearly defined by cluster FC 1. Zone B comprises the Bølling-Allerød interstadials, the Younger Dryas stadial, and the early Holocene 15,000e9300 years BP. The lower part of the zone is defined almost exclusively by cluster FC 2, but from the beginning of the Younger Dryas the assemblages show greater variability as indicated by an increasing influence of cluster FC 4 (Fig. 6a). Zone C comprises the mid Holocene interval 9300-2300 years BP, while zone D comprises the late Holocene 2300 years BP to Recent. The mid Holocene interval is mainly linked to cluster FC 4, whereas the late Holocene interval is mainly linked to cluster FC 3 (Fig. 6a). 6.1.1. Zone A (late glacial and Heinrich event H1, 30,000e15,500 years BP) The zone is defined by fuzzy cluster FC 1. The temperatures were as today or slightly higher (Fig. 5a and Fig. 7a). The salinity was relatively high and probably 1e2 & higher than at present (Fig. 5c). The characteristic assemblage of FC 1 (Assemblage group 1) is dominated by G. ruber, but G. rubescens is also abundant (Fig. 4a,b and Fig. 6b). They are characteristic ‘tropical and subtropical’ species (Bé and Tolderlund, 1971). G. ruber is a surface dweller in the warm well-mixed water above the thermocline (Fairbanks et al., 1980). Together, G. ruber and G. rubescens are typical of oligotrophic, high salinity surface water (Fairbanks et al., 1982; Conan and Brummer, 2000; Schmuker and Schiebel, 2002). Other characteristic species are Globigerinella calida and Globigerinella siphonifera (¼G. aequilateralis; e.g., Anand et al., 2003) (Fig. 4e and Fig. 6b). G. siphonifera and G. calida are often treated as one species as they have very similar ecology and can be difficult to distinguish (Hillbrecht, 1996). They are both surface dwellers thriving in the transitional zone between temperate and tropical water masses (Parker, 1958; Hillbrecht, 1996; Cayre et al., 1999; Anand et al., 2003). They appear to proliferate in the oligotrophic waters of the Sargasso Sea and Gulf Stream (Deuser et al., 1981; Schiebel et al.,

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Fig. 4. Relative abundance of selected planktic foraminifera species plotted versus core depth (cm) for both size fractions >106 mm and >150 mm. Abbreviations as in Fig. 2.

  2001; Schmuker and Schiebel, 2002; Zari c et al., 2005) and are associated with areas of very high salinities (Machain-Castillo et al., 2008). In sum, the planktic foraminiferal species forming assemblage group 1 indicate surface conditions almost as today, but more oligotrophic and probably much more saline as also indicated by the oxygen isotope values. 6.1.2. Zone B (Bølling-Allerød, Younger Dryas and early Holocene, 15,500e9300 years BP) The transition from Zone A to Zone B is revealed by a shift in dominance from FC 1 to FC 2, but with some influence of FC 4 especially in the upper part (Fig. 6a). The characteristic assemblage of FC 2 (Assemblage group 2) is dominated by G. glutinata and T. quinqueloba, which increase in abundance mainly on the expense of G. ruber, which decreases (Fig. 4a,b,c, p and Fig. 6b,c). G. glutinata is a ubiquitous species present from tropical to polar regions. It is described as a gyre-margin species in the North Atlantic Ocean (Bé and Hamlin, 1967; Bé and Tolderlund, 1971). It is generally attracted

to areas with increased productivity in upwelling areas and with productive eddies (Fairbanks et al., 1980, 1982; Thunell and Reynolds, 1984; Ottens, 1991; Cayre et al., 1999; Schmuker and Schiebel, 2002; Machain-Castillo et al., 2008) feeding, preferably, on phytoplankton, particularly diatoms, close to the thermocline (e.g., Hemleben et al., 1989). The latter species (Fig. 4p) is a subpolar form found mostly in the cooler, more fertile areas of the North Atlantic Ocean (Bé and Tolderlund, 1971; Bé et al., 1971) including upwelling regions and ocean fronts (e.g., Ortiz et al., 1995; Boltovskoy et al., 1996; Ufkes et al., 1998; Cayre et al., 1999; Oda and Yamasaki, 2005; see also; Smart, 2002). Altogether, the planktic foraminiferal species of Assemblage group 2 can be characterised as ‘food’ species related to cooler surface water. They are therefore indicative of a shift towards colder, more productive conditions. This interpretation is in agreement with the transfer functions, which indicate that the summer SST decreased with more than 2.5
C at the beginning of the Bølling interstadial (Fig. 5a and Fig. 7a). The SST increased slightly during the Allerød period, but

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nutrient rich water. The two first events are associated with decreased surface water salinity as indicated by the lower d18Owater values (Fig. 5c). Apart from the short, abrupt increases, N. dutertrei is a relatively rare species and it has little influence on the fuzzy cmeans clustering analysis (Fig. 6bee). This applies also for P. obliquiloculata, which increases during the Bølling interstadial (Fig. 4g and Fig. 6bee). It is a predominantly tropical species, which in the Gulf of Mexico lives in the deep current of the Loop Current (Brunner, 1979). Both N. dutertrei and P. obliquiloculata obtain their highest relative abundance in Assemblage group 4.

Fig. 5. Oxygen isotope records and temperature and salinity proxy data for KNR140-2/ 51GGC plotted versus depth. (a) Summer SST-10 m calculated from transfer functions. (b) Oxygen isotope values measured on G. ruber. (c) d18Owater and salinities based on the SST calculated from transfer functions. Abbreviations as in Fig. 2.

the temperatures of H1 were not approached until the mid Holocene. However, the most distinct events in the zone are three abrupt increases in the relative abundance of N. dutertrei (Fig. 4r and Fig. 7e). In KNR140-2/51GGC, N. dutertrei normally makes up between 3 and 7% of the faunas. The species lives in productive areas at the deep chlorophyll maximum in stratified surface water in tropical and subtropical waters (Fairbanks et al., 1982; Curry et al., 1983; Hemleben et al., 1989; Ortiz et al., 1995; Watkins et al., 1996; Hillbrecht, 1997; Ufkes et al., 1998; Lin and Hsieh, 2007; Machain-Castillo et al., 2008; Mohtadi et al., 2009). N. dutertrei is also found connected to nutrient rich freshwater runoff from rivers (Brunner, 1979; Ufkes et al., 1998; Maslin and Burns, 2000; Schmuker and Schiebel, 2002). It reaches up to >40% of the total planktic fauna together with abundant G. ruber outside the Congo River mouth (Ufkes et al., 1998). N. dutertrei is here linked to the highly productive, warm and low-saline surface water. In the Caribbean and Gulf of Mexico it is most abundant off the two largest rivers, the Mississippi and the Rio Grijalva (Brunner, 1979). In the Amazon River outlet it proliferates in association with lenses of river water from the river discharge (Maslin and Burns, 2000). The first and strongest event started during H1 at c. 17,000 BP and culminated around 14,800 years ago in the early Bølling interstadial, when the species makes up c. 21% of the planktic foraminiferal fauna (Fig. 4r and Fig. 7e). The second, and smaller event, occurred around 13,700 years BP during the Allerød interstadial. The third event started at the Younger DryaseHolocene transition and continued into the early Holocene with elevated values until c. 7200 years BP. The shifts in the relative abundance of N. dutertrei indicate that during the deglaciation the surface water at KNR140-2/51GGC at least three times was affected by fresher,

6.1.3. Zone C (mid Holocene, 9300-2300 years BP) Zone C is defined mainly by fuzzy cluster FC 3 (Fig. 6a). The characteristic assemblage is dominated by G. ruber, but G. rubescens and G. glutinata are also common (Fig. 4a,b,c, 6d). In addition to these omnipresent species the zone is characterised by Globigerinoides sacculifer, Globorotalia scitula, and G. menardii (Fig. 4d,l,h and Fig. 6d). These species are all tropical-subtropical. G. sacculifer lives in the upper mixed layer of the water column, while the latter two species live at the thermocline or deeper (e.g., Bé et al., 1971; Fairbanks et al., 1982; Thunell and Reynolds, 1984; Faul et al., 2000; Schmuker and Schiebel, 2002; Anand et al., 2003). They are all abundant in the Gulf Stream proper (Bé and Hamlin, 1967; Ruddiman, 1968; Kipp, 1976) and indicate a well-developed and deep seasonal thermocline (Faul et al., 2000). In the Caribbean and Gulf of Mexico, G. sacculifer is most common in the Loop Current (Brunner, 1979). The decrease in the relative abundance of G. glutinata and N. dutertrei and increase in G. calida/G. siphonifera (Fig. 4c,r,e) indicate more oligotrophic conditions and higher salinity. This time interval is the warmest in the Holocene record with summer SST up to 28.7
C from 7500 to 5500 years BP (Fig. 5a and Fig. 7). These temperatures are about 1
C above present temperatures in the area. Species Assemblage group 3 is thus characteristic of warm subtropical, oligotroph and salty conditions. 6.1.4. Zone D (late Holocene, 2300 years BP-recent) During the latest Holocene, we see a return of fuzzy cluster FC 4 and the fauna type that characterised the early Holocene (Fig. 6a,e). The three dominant species G. ruber, G. rubescens, and G. glutinata are almost equally abundant (Fig. 4a,b,c and Fig. 6d). However, there are also some differences between the early and late Holocene periods, in particular the decrease in G. inflata in the uppermost samples (Fig. 4q). This species is rare or absent in the coring area today. It is strictly confined to a narrow belt at the northern edge of the Gulf Stream at the transition from subtropical to subpolar conditions (Bé and Tolderlund, 1971). The calculated summer SST shows a slight decrease as compared to the mid Holocene. Altogether, the changes indicate a return to the cooler and more productive conditions that characterized the deglaciation and early Holocene. 7. Gulf Stream variability The overall pattern in summer SST in core KNR140-2/51GGC reconstructed by transfer functions is in good accordance with the pattern calculated from Mg/Ca ratios measured on G. ruber (Carlson et al., 2008) (Fig. 7a,b). However, the results are not always in complete agreement, as the transfer function shows a tendency to produce slightly higher temperatures than the Mg/Ca ratio. The reason for this difference is not clear, but part of the explanation may be that the transfer function calculates an average summer temperature maximum, whereas the Mg/Ca ratio measures the temperature only for the time period, when the species calcifies (see Carlson et al., 2008). Under tropical to subtropical conditions

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Fig. 6. (a) Membership values for each sample from the fuzzy-c means (FCM) clustering analysis based on the distribution of planktic foraminifera species in the>106 mm size fraction. Four distinct clusters, FC 1 to FC 4, are separated. Based on the clustering, the section is subdivided into four assemblage zone (Zones A-D). (b) Characteristic planktic foraminiferal assemblage (Assemblage group 1) for fuzzy cluster FC 1 obtained by fuzzy c-means clustering analysis. (c) Characteristic planktonic foraminiferal assemblage (Assemblage group 2) for fuzzy cluster FC 2. (d) Characteristic planktonic foraminiferal assemblage (Assemblage group 3) for fuzzy cluster FC 3. (e) Characteristic planktonic foraminiferal assemblage (Assemblage group 4) for fuzzy cluster FC 4. Abbreviations as in Fig. 2.

G. ruber calcifies over 2-week periods, but all year round (e.g., Bijma et al., 1990). The record of G. ruber may therefore be influenced by other seasons than summer. The changes in SST and d18Owater values in KNR140-2/51GGC are generally small. Similar low variability is seen in a recently published study from the Florida Straits comprising the time period 8000e21,000 years BP (Schmidt and Lynch-Stieglitz, 2011, see below) and it appears that the environmental conditions in the Gulf

Stream around Florida have undergone only small changes over the last 30,000 years, apart from some distinct coolings and freshenings during the deglaciation (Figs. 4e7). Low variability have also been observed in the Caribbean Sea in studies comprising the LGM-Holocene transition (Rühlemann et al., 1999; Schmidt et al., 2004) and the Holocene (Poore et al., 2003), and Martinez et al. (2007) estimated that over the last 500,000 years SST in the Caribbean Sea have varied with only 2
C. Furthermore, from the

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Fig. 7. Sea surface temperature and salinity proxy data for KNR140-2/51GGC plotted versus age (calendar years BP). (a) Summer SST-10 m calculated from transfer functions. (b) SST calculated from Mg/Ca data measured in G. ruber (data from Carlson et al., 2008). (c) Calculated d18Owater from d18O measured in G. ruber and summer SST calculated from transfer functions (see Fig. 5). (d) Calculated d18Owater based on SST measured by Mg/Ca ratios in G. ruber from Carlson et al. (2008). (e) Relative abundance of N. dutertrei. (f,g) Sea surface temperature and salinity proxy data for core KNR166-2-26JPC in the Florida Straits plotted versus age (from Schmidt and Lynch-Stieglitz, 2011). (f) SST measured by Mg/Ca ratios in G. ruber. (g) Calculated d18Owater from d18O values measured in G. ruber. Abbreviations as in Fig. 2.

Blake Ridge, Chaisson et al. (2002) demonstrated comparable low variability during isotope stages MIS12 and MIS11. 7.1. Late glacial, LGM and H1 (30,000e15,500 years BP) The SST and d18Owater records of KNR140-2/51GGC show many similarities to SST and d18Owater records calculated from Mg/Ca ratios and d18O values measured on G. ruber from core KNR166-2-26JPC in the Florida Straits (Schmidt and Lynch-Stieglitz, 2011). At both sites, we see a cooling centred at 17,500 years BP followed by a warming correlating with H1 and a strong cooling at the beginning of the Bølling interstadial c. 15,300-c. 14,800 years BP (Fig. 7a,b,f). Furthermore, in both studies the cooling in the early part of the Bølling interstadial is coupled to decreasing d18Owater values (Fig. 7c,d,g). The summer SST records of KNR140-2/51GGC (Fig. 7a,b) are also in fairly good agreement with the results presented by CLIMAP (CLIMAP Project Members, 1981), Mix et al. (1999), Pflaumann et al. (2003) and Kucera et al. (2005). Their reconstructions of the subtropical gyres during the LGM show SST almost as at present. The high salinity values in the subtropical western North Atlantic during the LGM (Fig. 7c,d) have been attributed to a more southern position of the Inter-Tropical Conversion Zone (ITCZ) (Prell and Hays, 1976; Flower et al., 2004; Schmidt et al., 2004, 2006; Lund et al., 2006). This should lead to drier conditions and hence a lower input of freshwater (Lynch-Stieglitz et al., 1999; Lund et al., 2006; Nürnberg et al., 2008). The reconstructed high salinity (Fig. 7) corresponds well with the foraminiferal faunas, which are dominated by Assemblage group 1 (Fig. 6a,b). Species of this group are indicative of high SST, high salinity and oligotrophic conditions. The total absence of cold water foraminiferal species in KNR140-2/ 51GGC indicates that the site was not reached by meltwater from

the North Atlantic region of the so-called ‘Heinrich meltwater plume’ (e.g., Broecker, 1991). The high temperatures continued during Heinrich event H1 (Fig. 7a,b), which according to the Mg/Ca ratios may have been even warmer than the LGM. These high estimates are in good agreement with records from the tropical and subtropical Atlantic, which indicate increasing SST during H1 (Rühlemann et al., 1999; Hüls and Zahn, 2000; Flower et al., 2004; Schmidt et al., 2004; Weldeab et al., 2006). An increase in the percentage of N. dutertrei from c. 17,000 years BP indicates a change towards more productive conditions (see below) (Fig. 7e). The relative high SST of the Gulf Stream during the late glacial, LGM and H1 has been attributed to a reduction in northward flow of the current allowing for the accumulation of warm water in the subtropical and tropical Atlantic (Rühlemann et al., 1999; Carlson et al., 2008). Terrestrial records from Florida also showed warming of the neighbouring land during this period (Grimm et al., 2006). During the LGM and H1, deep-water formation in the North Atlantic had completely stopped or was severely reduced (e.g., Broecker, 1991; Bond et al., 1993) and the AMOC was weak (Hillaire-Marcel et al., 2001; Renssen et al., 2005). According to Lynch-Stieglitz et al. (1999) water flow in the Loop Current and the Florida Current was reduced. Studies from the northeastern Atlantic and the Nordic seas indicate that the inflow to the Nordic seas during the LGM was low and unstable (Rasmussen and Thomsen, 2008). Summer SST in the northeastern Atlantic was only 3e6
C as compared to 9e12
C today (Knutz et al., 2007; Rasmussen and Thomsen, 2008). H1 was here even colder with meltwater from the ice sheets and numerous melting icebergs producing a low-density surface water layer. The North Atlantic Drift clearly cooled at a much faster rate during the glacial than it does today, probably because of a much steeper temperature gradient between the subtropical and the polar areas (CLIMAP

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Project Members, 1981). Reductions in the AMOC are also considered to be responsible for the warmings of the climate and oceans in the tropics and on the Southern Hemisphere, the so-called ‘bipolar seesaw’ effect (Broecker, 1998; Ziegler et al., 2008). 7.2. Bølling-Allerød interstadials (15,500e13,000 years BP) The start of the Bølling is marked by the onset of deep convection in the Nordic seas, and renewal of the AMOC (e.g., Rasmussen et al., 1996; McManus et al., 2004; Barker et al., 2010). The increase is accompanied by an abrupt rise in SST and salinity in the North Atlantic Ocean and Nordic seas, and the beginning of the Bølling interstadial is generally seen as a warming (e.g., Koç and Jansen, 1992; Lehman and Keigwin, 1992; Austin and Kroon, 1996; Kroon et al., 1997; Rasmussen et al., 2002). In KNR140-2/51GGC, we see the opposite trend. Here, the start of the Bølling is characterised by distinct decreases in both SST and salinity (Fig. 7aed; Carlson et al., 2008). A similar decline in SST and salinity is seen in the Florida Straits (Schmidt and Lynch-Stieglitz, 2011) (Fig. 7f,g). The species dominating Assemblage group 2 (the ‘food’ and cooler water species) indicate higher productivity thus supporting the SST calculations (Fig. 4b,p, and 6a,c). The assemblage also indicates that the upper water masses may have been stratified with lower salinity at the surface (Fairbanks et al., 1982; Curry et al., 1983; Hemleben et al., 1989; Ortiz et al., 1995; Ufkes et al., 1998; Lin and Hsieh, 2007; Machain-Castillo et al., 2008). Carlson et al. (2008) speculates that the decrease in SST and salinity could be due to the rejuvenation of the AMOC and an increase in the northward transport of water and heat. However, the fact that the sea surface changes coincide with an abrupt increase in the relative abundance of the planktic foraminifera N. dutertrei indicates that there also is an influence of freshwater, most likely meltwater from the Mississippi River (Fig. 7). In the Caribbean and in other areas, N. dutertrei thrives in the nutrient-rich water outside of river mouths (Brunner, 1979; Ufkes et al., 1998; Maslin and Burns, 2000; Schmuker and Schiebel, 2002). The Bølling-Allerød period was characterised by the highest rates of meltwater runoff during the deglaciation (Meltwater Peak 1a, Fairbanks, 1989). The main runoff route from the Laurentide Ice Sheet was through the Mississippi River system and the timing of the peak abundances of N. dutertrei in KNR140-2/51GGC correlates well with the strongest discharges of meltwater. These are dated to the beginning of the Bølling interstadial, w15,300 years BP, to the Allerød interstadial, 13,700 years BP, and to the early Holocene 11,700 to c. 10,500 years BP, and they left distinct imprints on the SST and d18Owater records from the Gulf of Mexico (Kennett and Shackleton, 1975; Emiliani et al., 1978; Leventer et al., 1982; Kennett et al., 1985; Broecker et al., 1989; Flower and Kennett, 1990; Aharon, 2003; Flower et al., 2004; Montero-Serrano et al., 2009). Kennett et al. (1985) in a study of the deglaciation from the Orca Basin in the northwestern Gulf of Mexico also noted an increase in the abundance of N. dutertrei in association with increased outflow of meltwater from the Mississippi River valley. They observed a delay in the increase in the species, which they attributed to the initial cold conditions associated with the meltwater pulse. They speculated that the species would thrive better further downstream from the river, where conditions would be warmer and more optimal for the species. Other occurrences of N. dutertrei in connection with the Mississippi meltwater spike from the Gulf of Mexico are reported in Kennett and Shackleton (1975), Thunell (1976), Leventer et al. (1982), and Flower and Kennett (1990). During the strongest meltwater pulses, the input of freshwater to the Gulf of Mexico exceeded the modern discharge by a factor of eight (Aharon, 2003). According to Aharon (2003), parts of the meltwater were undoubtedly advected into the Atlantic Ocean. This

is corroborated by several modern observations. Drifters released at the mouth of the Mississippi can reach the Florida Keys within a month and Cape Hatteras a month later (Ortner et al., 1995) and studies have shown that Mississippi water may influence the salinity of the Gulf Stream as far as Bermuda (Atkinson and Grosch, 1999). In 1993, during a period with unusual high discharge from the Mississippi River, a large body of low salinity water was observed in the Florida Strait and east of Florida, entrained by the Loop Current in the western Gulf of Mexico (Ortner et al., 1995; Gilbert et al., 1996). The water had a profound effect on animal life in the Florida Keys (Yeung et al., 2000). In 2004, a somewhat similar plume, 10e20 km wide in the Florida Strait and 50 km wide passing the Blake Ridge off Georgia, caused detectable anomalies in the Gulf Stream in both salinities and temperatures and also slightly increased the nutrient content (Hu et al., 2005). Demise in shallow reefs on the south Florida platform margin has also been attributed to intermittent low salinity conditions due to freshwater influence from the Mississippi River (Mallinson et al., 2003). Increased freshwater influence during the late LGM and the early deglaciation is also indicated in core KNR166-2-26JPC from the Florida Strait, where the Ba/Ca ratio in tests of G. ruber increases from 17,200 years BP (Schmidt and Lynch-Stieglitz, 2011), synchronous with the first increase in the relative abundance of N. dutertrei on the Blake Ridge. The higher Ba/Ca ratio is assumed to reflect an increase in the input of riverine water from the melting Laurentide ice sheet. The elevated values continue until 13,600 years BP with a short, poorly understood, drop from 15,600e14,900 years BP (Schmidt and Lynch-Stieglitz, 2011). It is therefore surprising that Nürnberg et al. (2008) did not find any indications of freshwater influence in the deglaciation interval of a core from the De Soto Canyon in the northeastern Gulf of Mexico. On the other hand, Emiliani et al. (1978), in a core from the same general area, observed a strong negative excursion in the d18O values of planktic foraminifera dated to the Bølling interstadial. Altogether, we conclude that the massive discharges of glacial meltwater into the Gulf of Mexico during the deglaciation could easily have been advected into the Atlantic Ocean reaching to the Blake Ridge. We suggest that meltwater from the Mississippi River was the main factor in the reduced SST and salinity of the Gulf Steam west of Florida during the early deglaciation, although, possibly, in conjunction with the rejuvenation of the AMOC and the increased northward heat transport, such as implied by Carlson et al. (2008). It is unlikely that the meltwater came from the north as no true cold-water planktic foraminiferal species are present such as would be expected if this was the case. Instead, we see a rise in P. obliquiloculata, G. sacculifer and N. dutertrei, true tropical-subtropical species (Fig. 4g,d,r; see also above). 7.3. Younger Dryas stadial (13,000e11,700 years BP The two temperature proxies recorded from KNR140-2/51GGC give somewhat different results for the Younger Dryas stadial, as the transfer function indicates a slight cooling trend relative to the foregoing Allerød interstadial, whereas the Mg/Ca ratios indicate a warming (Fig. 7a,b). The reason for this discrepancy is not certain. However, both reconstructions indicate that the Younger Dryas was warmer than the Bølling interstadial. This is very different from the situation in the northeastern Atlantic, where the Younger Dryas is always much colder than the Bølling interstadial. A temperature decrease during the Younger Dryas has also been observed at several sites in the Caribbean Sea, Gulf of Mexico, and Florida Straits (Lea et al., 2003; Flower et al., 2004; Schmidt et al., 2004; Came et al., 2008; Schmidt and Lynch-Stieglitz, 2011), even though the evidence here is somewhat conflicting as others see a warming or no change (e.g., Rühlemann et al., 1999; Herbert and Schuffert, 2000; Weldeab

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et al., 2006; Nürnberg et al., 2008). The AMOC was reduced during the Younger Dryas (Keigwin and Lehman, 1994; Rasmussen et al., 1996; McManus et al., 2004) and according to Wan et al. (2009) change in local upwelling during times of weakened AMOC could be the cause of these highly variable responses in the region. Furthermore, the results from KNR140-2/51GGC indicate that differences between the various proxies used in the temperature reconstructions may contribute to the confusion. The surface water at KNR140-2/51GGC was probably more oligotrophic and saltier during the Younger Dryas than during the Bølling interstadial, as indicated by the almost total absence of N. dutertrei, an abrupt decrease in relative abundance of G. sacculifer, and a small increase in the abundance of Assemblage group 3 (Fig. 4r,d and Fig. 6a,d). N. dutertrei and G. sacculifer show also a decrease during the Younger Dryas in cores from the Orca Basin (Kennett et al., 1985; Flower and Kennett, 1990). G. sacculifer prefers intermediate salinities (Brunner, 1979) and the decrease in the abundance the two species in the Orca Basin probably indicates a higher salinity in the surface water of the Loop Current. This interpretation is in agreement with results from the Orca Basin (Williams et al., 2010) and the Florida Straits (Schmidt and LynchStieglitz, 2011). Both of these areas experienced pronounced salinity increases during the Younger Dryas. The changes in surface water conditions indicate that glacial runoff via the Gulf of Mexico during the Younger Dryas was strongly reduced. Apparently the meltwater was re-directed to more northern routes, e.g., the Hudson, St. Lawrence and Mackenzie rivers (Broecker et al., 1989; Andrews et al., 1993; Duplessy et al., 1996; Clark et al., 2001; Aharon, 2003; Fischer, 2003; Lewis and Teller, 2007; Murton et al., 2010). 7.4. Holocene 11,700-0 years BP-Recent In KNR140-2/51GGC, the Holocene period can be subdivided into a warm mid Holocene interval, 8000-4500 years BP, with SST up to 28.6
C separating slightly cooler early and late Holocene intervals (Fig. 7a). The planktic faunas of the mid Holocene interval belong mainly to Assemblage group 3, whereas the early and late Holocene faunas belong mainly to Assemblage group 4, although with significant contributions from Assemblage group 2 (Fig. 6a,d,e). The faunas indicate that the warm mid Holocene period also was the most oligotrophic. The SST proxy record of KNR140-2/51GGC differs from most other records in the western tropical Atlantic by the relatively cool early Holocene and a mid-Holocene temperature maximum (see e.g., Leduc et al., 2010). In the Cariaco Basin the highest temperatures are recorded from the early Holocene c. 11,500e8200 years BP, while the lowest occur from c. 8200-3000 years BP (Lea et al., 2003). In the northeastern Gulf of Mexico, Nürnberg et al. (2008) recorded an early Holocene maximum from c. 9500-6000 years BP followed by a cooling for the rest of the Holocene. In the Florida Straits, the early Holocene is relatively warm until c. 10,500 years BP, when a slight cooling sets in (Schmidt and Lynch-Stieglitz, 2011) (Fig. 7f). The development at the KNR140-2/51GGC site is more similar to the development in the western Gulf of Mexico with a similar distribution pattern of G. sacculifer (Poore et al., 2003). The last major meltwater discharge through the Mississippi River recorded in the Gulf of Mexico is dated to c. 10,000e9000 years BP (Broecker et al., 1989; Aharon, 2003; Poore et al., 2003; Flower et al., 2004; LoDico et al., 2006) and correlated with Meltwater peak 1b (Fairbanks, 1989). The relative high abundance of N. dutertrei and of species of Assemblage groups 4 and 2 (the ‘food’ group) in the early Holocene interval of KNR140-2/51GGC indicates conditions somewhat similar to those of the Bølling-Allerød interval with a slightly reduced salinity (Fig. 6a,c and Fig. 7) (see also Kennett et al., 1985).

51

The late Holocene cooling at KNR140-2/51GGC from about 4000 years BP (Fig. 7a) is accompanied by a small increase in the relative abundance of the small species G. uvula and T. quinqueloba as well as in N. dutertrei and in Assemblage groups 4 and 2 (Fig. 4o,p,r and Fig. 6a,c,e). The faunistic changes indicate that the cooling is accompanied by an increase in nutrient and food supply. Records from the Florida Current also show a late Holocene cooling (Lund and Curry, 2004) and the distribution of planktic foraminifera in the Gulf of Mexico indicates a general reduction in the strength of the Loop Current (Poore et al., 2003). Data from the Orca Basin shows several large flood events of the Mississippi River in the last 2000 years (Brown et al., 1999). Each event is accompanied by an increase in relative abundance of G. glutinata. It is possible that the increase in species groups 2 and 4 over the Blake Ridge during last 2000 years is a reflection of these flood events. 8. Conclusions The planktic foraminifera and stable isotope records of core KNR140-2/51GGC from the Blake Ridge, western subtropical North Atlantic Ocean, indicate that the Gulf Stream has been relatively stable over the last 30,000 years with relatively small changes in SST and salinity. This is in contrast to the large variations seen in the more distal part of the current system in the polar and subpolar Northeast Atlantic, and to the general climatic development of the investigated time period, which includes the Last Glacial Maximum, the deglaciation, and the Holocene interglacial. The timing of the shifts recorded over the Blake Ridge appears to be in phase with the corresponding shifts in the northeast Atlantic, although the trends of the changes often seems to be in opposite directions. During the late glacial and early deglaciation, including Heinrich event H1, sea surface temperatures and salinities at KNR140-2/51GGC were as today or higher. The surface productivity was low and the Gulf Stream water more oligotrophic than at present. Elsewhere in the North Atlantic the period is characterised by cold conditions and the AMOC was reduced. At the transition between Heinrich event H1 and the Bølling Interstadial, summer SST dropped up to 2.5
C, while salinity decreased with about 1.5&. In the northern North Atlantic, the transition is marked by a significant temperature increase and the onset of a strong AMOC. The shift to colder and fresher conditions at KNR140-2/51GGC coincides with an abrupt increase in the planktic foraminifera N. dutertrei indicating a strong increase in the nutrient supply and an enhanced influence of fresh river water. The shift coincides exactly in time with a huge outflow of cold meltwater from the Laurentide ice sheet into the Gulf of Mexico via the Mississippi River. We suggest that the cooling at KNR140-2/51GGC is mainly due to this cold meltwater, although the rejuvenation of the AMOC may have contributed. Summer SST increased slightly during the Allerød interstadial, and the Younger Dryas stadial was warmer than the Bølling interstadial. The very cold conditions that characterise the Younger Dryas elsewhere in the North Atlantic region are not felt here. A significant decrease in the relative abundance of N. dutertrei indicates increased salinity and more oligotrophic conditions. The period coincides with a pause in the meltwater floods from the Mississippi River to the Gulf of Mexico, but also with a reduction in the AMOC. The early and mid Holocene development at KNR140-2/51GGC seems to deviate from the pattern seen in the northeastern Atlantic Ocean, where the early Holocene generally is warmer than the mid Holocene. At KNR140-2/51GGC, the early Holocene was slightly cooler, fresher and more eutrophic than the mid Holocene. The fresher and nutrient richer conditions over the Blake Ridge

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correlate with renewed outflow of meltwater via the Mississippi River valley. At KNR140-2/51GGC, the warmest, saltiest and most oligotrophic conditions occurred during the mid Holocene, when the sea surface turned cooler in the northeastern Atlantic Ocean. The shift coincides with a stop in the outflow of cold meltwater to the Mexican Gulf and a diminishing AMOC, and both factors may have contributed to the change. Only the cooling during the last 4000 years seems to have affected the entire Gulf Stream-North Atlantic Drift system uniformly. Acknowledgements We warmly thank D.W. Oppo, R.E. Came and L.D. Keigwin, Woods Hole Oceanographic Institution for kindly providing the samples and sharing core data and unpublished 14C dates with us. We thank L.D. Keigwin for helpful suggestions and D. Reng Erbs-Hansen for introducing us to the fuzzy c-mains cluster analysis and S.B. Andersen for carefully reading a draft of the manuscript. References Aharon, P., 2003. Meltwater flooding events in the Gulf of Mexico revisited: implications for rapid climate changes during the last deglaciation. Paleoceanography 18, 1079. doi:10.1029/2002PA000840. Anand, P., Elderfield, H., Conte, M.H., 2003. Calibration of Mg/Ca thermometry in planktonic foraminifera from a sediment trap time series. Paleoceanography 18, 1050. doi:10.1029/2002PA000846. Andrews, J.T., Dyke, A.S., Tedesco, K., White, J.W., 1993. Meltwater along the Arctic margin of the Laurentide Ice Sheet (8-12 ka): stable isotopic evidence and implications for past salinity anomalies. Geology 21, 881e884. Atkinson, L.P., Grosch, C.E., 1999. Continental runoff and effects on the North Atlantic subtropical mode water. Geophysical Research Letters 26, 2977e2980. Austin, W.E.N., Kroon, D., 1996. Late glacial sedimentology, foraminifera and stable isotope stratigraphy of the Hebridean shelf, northwest Scotland. In: Andrews, J.T., Austin, W.E.N., Bergsten, H. (Eds.), Late Quaternary Paleoceanography of the North Atlantic Margins. Geological Society of London, pp. 187e213. Special Publications 111. Balsam, W.L., 1981. Late Quaternary sedimentation in the western North Atlantic: stratigraphy and paleoceanography. Palaeogeography, Palaeoclimatology, Palaeoecology 35, 215e240. Barker, S., Knorr, G., Vautravers, M.J., Diz, P., Skinner, L.C., 2010. Extreme deepening of the Atlantic overturning circulation during deglaciation. Nature Geoscience 3, 567e571. Bé, A.W.H., Hamlin, W.H., 1967. Ecology of recent planktonic foraminifera, Part 3. distribution in the North Atlantic during the summer of 1962. Micropaleontology 13, 87e106. Bé, A.W.H., Tolderlund, D.S., 1971. Distribution and ecology of living planktonic foraminifera in surface waters of the Atlantic and Indian Oceans. In: Funnell, B.M., Riedel, W.R. (Eds.), The Micropaleontology of the Oceans. Cambridge University Press, pp. 105e149. Bé, A.W.H., Vilks, G., Lott, L., 1971. Winter distribution of planktonic foraminifera between the Grand Banks and the Caribbean. Micropaleontology 17, 31e42. Bezdek, J.C., 1987. Pattern Recognition with Fuzzy Objective Function Algorithms. New York Plenum Press, New York, p. 256. Bijma, J., Faber, W.W., Hemleben, C., 1990. Temperature and salinity limits for growth and survival of some planktonic foraminifers in laboratory cultures. Journal of Foraminiferal Research 20, 95e116. Birks, H.J.B., 1998. Numerical tools in palaeolimnology e progress, potentialities, and problems. Journal of Paleolimnology 20, 307e332. Boltovskoy, E., Boltovskoy, D., Correa, N., Brandini, F., 1996. Planktic foraminifera from the southwestern Atlantic (30
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