Deep-Water Dynamics in the Subpolar North Atlantic at the End of the Quaternary

Abstract

In the subpolar North Atlantic, four sediment cores were taken. All of them were suitable for reconstructing the dynamics of the meridional overturning circulation in the late Quaternary. Stratigraphy of the cores was performed by carbonate analyses, study of planktonic foraminifera, and oxygen isotopic composition in Neogloboquadrina pachyderma sin. Study of benthonic foraminifera assemblages has shown significant differences in the deep-water dynamics in the late Quaternary related to water exchange between the North Atlantic and Arctic seas. It has been established that at the end of the middle and most of the late Pleistocene, deep circulation in the subpolar North Atlantic was poor. Its strengthening took place in the time of deglaciations (Termination III and Termination II). During the optimum of the last interglacial (MIS 5e), water flow from the Norwegian Sea into the North Atlantic happened mainly in its western part. The modern deep water in the eastern part of the North Atlantic began to form at the end of the last glaciation.

INTRODUCTION

An important component of Global Thermohaline Circulation (GTC) is the formation of deep water in the subpolar North Atlantic—one of the key regions of the World Ocean. It is here that North Atlantic meridional transfer (NAMT) of water masses takes place, which significantly affects the Earth’s climate. One of the two main components in this system is surface water of the North Atlantic Current, which penetrates the Norwegian–Greenland Basin, where it cools and sinks to great depths. Then, past the Iceland sills, it returns to the North Atlantic and, mixing with other water masses, forms the North Atlantic Deep Water (NADW), the second component of the NAMT.

Today, in the eastern part of the North Atlantic, more specifically, in the Iceland Basin, owing to Norwegian water flowing past the Ireland Trough and via the Faroe–Iceland sill, the Northeastern Deep Water forms. Rounding the Reykjanes Ridge, it enters the Northwest Atlantic. Here, southeast of Greenland, it mixes with water of the Irminger Current, water of the East Greenland Current, and Labrador water, forming the NADW as a result [7, 60].

The NAMT weakened during glaciations. The Arctic seas were covered by ice, which hindered the northward movement of the surface North Atlantic Current, due to which the Irminger Current deviated to the west and not to the northwest, like now. As a result, the sinking of warm surface water and the formation of deep water (overturning) occurred more to the south, in the subpolar North Atlantic [5, 56, 61]. The stable isotopic composition shows that during glacial maxima, the volume of the NADW was strongly reduced compared to now [23, 5254, 60], as a result of which, the GHC became less intense than now.

Despite the fact that the climatic optimum of the last interglaciation was similar to the present-day one, the circulation of deep and surface water masses in the North Atlantic was different [22, 29, 31]. It is assumed that a branch of the surface North Atlantic Current, the Irminger Current, was stronger than now and warm surface water penetrated into the Norwegian–Greenland Basin primarily over the Greenland–Iceland sill [10, 12].

Deep-water benthic foraminifera complexes contain information on deep-water oceanic environments, and their replacement in bottom sediments demonstrates significant changes in water mass dynamics during glacial–interglacial cycles, especially in the North Atlantic. The aim of this study is to reveal, based on a study of benthic foraminifera from the subpolar North Atlantic, differences in late Quaternary deep circulation from present-day circulation, as well as the reason thereof.

MATERIALS AND METHODS

On cruise 48 of the Akademik Mstislav Keldysh in the North Atlantic, four cores of bottom sediments were taken. The sampling points of the cores were simultaneously under the influence of both surface and deep elements of NAMT and therefore were ideal for detailed reconstruction of the two structural zones. In the northern part of the Iceland Basin in the NEDW propagation zone, two cores were raised: AMK 4438 and AMK 4442. The western branch of the surface North Atlantic Current passes over the first core, while Norwegian water from the Iceland Trough enters the deep zone and skirts the Rockall Plateau. The second core is in the propagation path of Norwegian water flowing over the Faroe–Iceland sill to the south (Fig. 1).

Fig. 1.
figure1

Scheme of North Atlantic meridional transport of water masses and location of studied cores [7, 60]. Solid lines, surface current; dotted lines, deep currents. NAC, North Atlantic Current; IC, Irgminger Current; EGC, East Greenland Current; WGC, West Greenland Current; LC, Labrador Current; NADW, North Atlantic Deep Water; NEDW, Northeast Deep Water.

Cores AMK 4453 and AMK 4493 were taken from the western part of the North Atlantic. The former is located at the foot of the western slope of the Reykjanes Ridge, along which the NEDW passes from the Iceland Basin to the north. Core AMK 4493 was taken in the Labrador Sea, in the propagation zone of NADW with an admixture of Labrador bottom water (Table 1).

Table 1.   Location of studied cores and number of studied samples

Stratification of the columns was performed by studying the carbonate content of sediments, complexes of planktonic foraminifera, and the isotopic composition of oxygen for the plankton species Neogloboquadrina pachyderma sin. (for all columns except AMK 4442) (Fig. 2). For several horizons of AMK 4438, the absolute age was determined by 230Th and 14C dating (Table 2).

Fig. 2.
figure2

Correlation of studied cores in terms of oxygen isotopic composition in Neogloboquadrina pachyderma. TI, TII, TIII—terminations.

Table 2.   230Th and 14С datings for core AMK-4438

The carbonate content of sediments and planktonic foraminifera were studied at the Geoecology Laboratory of the Atlantic Branch, Shirshov Institute of Oceanology, Russian Academy of Sciences [9, 42]; the isotopic composition of oxygen was determined at the laboratory of Kiel University, and absolute age, at the laboratory of St. Petersburg State University. Attention was focused on the content of ice-rafted debris (IRD) in sediments, the accumulation of which is related to Heinrich events [28].

Benthic foraminifera are the main component of benthic communities. Their distribution in deep-water sediments of the World Ocean is affected by several factors, the most important of which is the influx of organic material to the sea bottom and the oxygen concentration in the near-bottom water layer or in pore water [40, 66, 71]. These factors determine not only the population density of benthic foraminifera and their species diversity, but also species composition. All bottom-dwelling benthic organisms are divided into epifauna and infauna. As research shows, all epifaunal species of benthic foraminifera are sensitive to oxygen content and prefer an oligotrophic environment, whereas infaunal species, conversely, prefer an environment rich in organic matter with a low oxygen content [24, 26]. The ratio of epifauna and infauna among benthic foraminifera is an indicator of the degree of ventilation of near-bottom water masses [26, 33, 39, 40, 43].

Changes in the deep structure of the subpolar North Atlantic zone are well reflected in the alteration of benthic foraminifera complexes in the glacial–interglacial bottom sediments. As a result of studying benthic foraminifera, we determined more than 70 secreting and around 10 agglutinating species. The species composition in all cores was identical to a significant degree. Dominant components greater than 20% in at least one of the profile intervals were Hoeglundina elegans, Planulina wuellerstorfi, Epistominella exigua, Oridorsalis umbonatus, Melonis pompilioides, Melonis barleeanum, Pullenia bulloides, Bulimina marginata, Uvigerina peregrinа, Osangularia umbonifera, Eggerella bradyi,and complexes of Cibecides, miliolids, and agglutinating species.

Hoeglundina elegans and Planulina wuellerstorfi are epifaunal species [34, 45, 63]. Hoeglundina elegans in the North Atlantic dominates at depths from 2000 to 3500 m on the southern slopes of the Rockall Rise, on the Mid-Atlantic Ridge, and on the continental slope of North America, which are washed by the NADW [30, 47, 68]. The structure of its aragonite plain with small pores confirms the preference of Hoeglundina elegans for a high oxygen concentration [18]. Planulina wuellerstorfi in the North Atlantic dominates in the same regions as Hoeglundina elegans, as well as on the slopes of the Norwegian–Greenland Basin, where its abundance reaches 30–60% [5, 70]. Planulina wuellerstorfi is widespread in places where the influx of organic matter to the bottom fluctuates between 1 to 3 g m–2 yr–1 [8] and the oxygen concentration in bottom water is greater than 4 mL/L [43]. On the slope of northwest Africa, the complex in which Hoeglundina elegans and Planulina wuellerstorfi dominate is characteristic of oligotrophic conditions with a high oxygen concentration in near-bottom water [48].

Epistominella exigua is one of the most widespread species in the North Atlantic. It dominates in the Irminger, Iceland, Labrador, and West European basins, where its abundance frequently exceeds 50% [5]. It is assumed that Epistominella exigua lives in regions where the mean annual influx of organic matter to the bottom is not very high. However, periodically, when the seasonal phytoplankton bloom occurs and influx of fresh organic matter increases, the population of this species rapidly grows [35, 67, 69, 71].

Oridorsalis umbonatus is considered an epifaunal–shallow-water infaunal species widespread in areas with cold oxygen-rich bottom water, where the influx of organic matter to the bottom is quite low [20, 44, 49]. Together with Epistominella exigua in the present-day North Atlantic, Oridorsalis umbonatus dominates in sediments of the Labrador Basin [5], where seasonal eutrophication of surface waters occurs [69]. The distribution of Oridorsalis umbonatus, together with Pyrgo murrhyna, correlates in the Indian Ocean with poor food availability [49]. Meanwhile, in the eastern equatorial part of the Pacific Ocean, it prevails in areas with weak productivity [49]. Clearly, this species may be tolerant to low-oxygen conditions [58].

It is assumed that the most widespread representative of miliolids, Pyrgo murrhyna, in the North Atlantic adapted to variable conditions of organic matter influx [38] and prefers well-ventilated NADW with a high oxygen content [27]. Miliolids, as well as various cibicidids, are quite widespread in the present-day Atlantic Ocean, but their abundance rarely exceeds 10%.

Bulimina aculeata is a typical shallow-water infaunal species [65] and an indicator of high surface productivity [50]. In dominates between 70 and 250 m near the coast of West Africa [48] and between 200 and 600 m in the Gulf of Mexico [21].

Another infaunal species, Uvigerina peregrina, is widespread in highly productive zones of the World Ocean [16, 33, 43]. In the North Atlantic, its habitat is primarily on the continental slope of North America, when its complex correlates not only with increased organic carbon and reduction in oxygen, but also with the increased silt component in sediments [47].

Melonis barleeanum is frequently encountered in upwelling zones with a high organic matter input to sediments and a low oxygen content [62]. In particular, the complex of benthic foraminifera on the northwest coast of Africa, represented by the species Uvigerina peregrina and Melonis barleeanum, correlates with a Corg content from 3 to 6 g m–2 yr–1 and an oxygen content in the near bottom water of around 3 mL/L [48]. However, it is assumed that Melonis barleeanum is less sensitive to organic matter in sediments compared to Uvigerina peregrina, while the bulimina complex reflects an even lower oxygen content compared to the uvigerina complex [15, 16].

Melonis pompilioides is also an abundant species in areas with high surface productivity [33, 45]. However, in the present-day North Atlantic, this species extremely rarely reaches 10% in the complexes. Shells of Melonis pompilioides, just like shells of Melonis barleeanum, have large pores, which speaks to the low oxygen content in the environment of its habitat [18].

Pullenia bulloides is also an infaunal species, which is widespread in the Atlantic Ocean in areas with a high continuous influx of organic matter and low oxygen content in the water, frequently in areas of upwelling [16, 25, 45]. However, in the Indian Ocean, it is associated with areas with low productivity and a high oxygen concentration [20]. Today, the abundance of Pullenia bulloides in the North Atlantic does not exceed 10%.

Osangularia umbonifera is an abyssal species that has adapted to poor food supply, low temperature, and salinity. It is associated with Atlantic Bottom Water, which is undersaturated in calcium carbonate [14, 17, 39, 46]. Sigmoilopsis schlumbergeri lives in the first few centimeters of bottom sediments and feeds on detritus deposited in them; it is able to develop in conditions with a lack of dissolved oxygen [19]. Cassidulina teretis inhabits the substrate. This species is not encountered in the North Atlantic today. It dominated in the Norwegian Sea during the last glaciation at depths from 800 to 1600 m, where it was associated with Norwegian intermediate glacier water [5].

The distribution of agglutinating benthic foraminifera also significantly depends on the organic matter content at the bottom. They cannot tolerate an oxygen-rich near-bottom environment [37]. They frequently inhabit continental slopes covered in weakly carbonate sediments to which a large amount or organic carbon is delivered by geostrophic or vertical currents [32, 51]. The most abundant of the agglutinating species in the profiles studied by us are rhabdammins and hyperammins with elongated cylindrical tubular shells. The tubule diameter of tubular species is an indicator of the trophicity of the environment [36]. In the present-day North Atlantic, both genera prevail on the slopes of the North American Basin, on the continental slope of the Bay of Biscay, and on the slopes of the Rockall Plateau [32, 47, 51, 55].

BASIN DISTRIBUTIONS OF BENTHIC FORAMINIFERA

Core AMK 4438. In core AMK 4438, nine upper marine isotopic stages (MIS) were distinguished [42] (Fig. 3).

Fig. 3.
figure3

Distribution of dominant benthic foraminifera species in core AMK 4438 (%). CaCO3 column shows presence of terrigenous fragments in sediments.

In the sediments of MIS 9 with a high concentration of benthic foraminifera (up to 600 ind./g, the dominants were Melonis pompilioides (up to 35%), Pullenia bulloides, and Planulina wuellerstorfi (more than 20%).

In the glacial sediments of MIS 8 with a high concentration of IRD, the concentration of benthic foraminifera decreases more than twofold; here, three species dominate: Pullenia bulloides, Melonis pompilioides (up to 40%), and Oridorsalis umbonatus (up to 30%). At the MIS 8/MIS 7 boundary (Termination III), the abundance of these species is reduced to a minimum due to an increase in the abundance of Planulina wuellerstorfi and Hoeglundina elegans (up to 30 and 15%, respectively).

In the lower part of the interglacial sediments of MIS 7, as before, there is a high abundance of Planulina wuellerstorfi, but in the upper intervals of the horizon, Pullenia bulloides again dominates (up to 25%). The number of foraminifera fluctuates from 200 to 400 ind./g.

In the penultimate glaciation sediments of MIS 6, which are also characterized by a large amount of IRD, the concentration of benthic foraminifera decreases on average down to 100 ind./g of sediment. Here, Pullenia bulloides dominates (25–45%), while in the middle part of the horizon, a complex of agglutinating species appears (mainly from the genus Rhizammina), which in total makes up 20–30%. At the MIS 6/MIS 5 boundary (Termination II), the abundance of Pullenia bulloides reduces to 10%, while the share of Planulina wuellerstorfi increases up to 30%.

In the interglacial sediments of MIS 5, the concentration of benthic foraminifera is maximum in the profile and reaches 800–1200 ind./g; the maximum falls on cold substages 5b and 5d, and the minimum, on warm substages 5е, 5c, and 5а. At substage 5e, Melonis pompilioides dominates (up to 20%); higher, Pullenia bulloides dominates (up to 30–40%), decreasing quantitatively to 15% at the MIS 5/MIS 4 boundary. Higher in the profile, this species never again reaches the prior values.

The concentration of benthic foraminifera in the glacial sediments of MIS 4 decreases more than twofold. In the lower part of the horizon, Cassidulina teretis dominates (up to 30%), and in the upper part, Hoeglundina elegans (up to 20%).

In the glacial sediments of MIS 3, the abundance of benthic foraminifera increases on average up to 300 ind./g of sediment, but at a level of 103–95 cm, a peak is detected where their concentration reaches approximately 600 ind./g of sediment. The horizon is distinguished by the dominance of Bulimina aculeatа, the abundance of which is maximum in the middle part (on average, 30%). The remaining dominant species—Cassidulina teretis (up to 40%), Hoeglundina elegans (up to 40%), and Melonis pompilioides (up to 30%)—have short-lived maxima.

The glacial sediments of MIS 2 are characterized by a large amount of IRD, the accumulation of which probably corresponds to H-1. The horizon is distinguished by an abundance of agglutinating foraminifers, which are represented by groups of Astrorhizida (genus Rhabdammina), Ammodiscida (genus Hyperammina), Lituolida (genera Haplophragmoides and Ciclammina), Textulariida (genus Siphotextularia), and Ataxophragmiida (genera Trochammina and Martinotiella). Their total abundance reaches 30%, and the concentration of shells in sediment does not exceed 100 ind./g. At the MIS 2/MIS 1 boundary (Termination I), agglutinating foraminifera almost completely vanish.

In Holocene sediments, among the diverse and quite numerous (up to 800 ind./g of sediment) benthic foraminifera, there is no starkly pronounced dominance. Planulina wuellerstorfi, Hoeglundina elegans, and miliolids are present in approximately equal numbers (10–20%), and melonises, pullenia, gyroidins, cibicids, etc., are also encountered.

Core AMK 4442. In core AMK 4442, seven MIS were distinguished (Fig. 4). The concentration of benthic foraminifera shells in the entire profile was quite low, not exceeding 400 ind./g sediment, with the exception of MIS 5d, where it reaches 8000 ind./g.

Fig. 4.
figure4

Distribution of dominant benthic foraminifera species in core AMK 4442 (%).

In the sediments of MIS 7, the dominants are Pullenia bulloides (up to 35%) and Melonis pompilioides (20% on average). In the lower part of MIS 6, Uvigerina peregrinа is distinguished (30%), and in the remainder, the Cibicides sp. sp./Miliolida complex (30% on average for each genus); periodically Pullenia bulloides dominates (20% on average). It also prevails in the sediments of MIS 5. The high value for Melonis pompilioides (up to 40%) is characteristic of MIS 5d.

In the sediments of MIS 4, Planulina wuellerstorfi dominates (up to 30%). In MIS 3, there is no starkly expressed benthic foraminifera complex. However, in sediments at the MIS 3/MIS 2 boundary with a large amount of IRD (H-1 event) and a low concentration of benthic foraminifera in sediment (less that 200 ind./g), a numerous and diverse complex of agglutinating foraminifera is distinguished, in which tubular rhabdammins and hyperammins dominate (40–60%). Their dominance also continued in the lower part of MIS 2. In the upper part of MIS 2, they were replaced by the Hoeglundina elegans/Planulina wuellerstorfi complex (40/25%).

The upper sediment layer corresponding to MIS 1 contains a benthic foraminifera complex in which Melonis barleeanum and Melonis pompilioides dominate (approximately 20% each).

Core AMK 4453. In core AMK 4453, six MIS are distinguished (Fig. 5). In the lower part of MIS 6, the benthic foraminifera complex is represented almost exclusively by the species Bulimina aculeata (up to 80%), owing to which the shell concentration in sediment is maximum in the entire profile and amounts to almost 30 000 ind./g. The sediments of MIS 6 contain a certain amount of terrigenous material, which most likely has a turbidite character. In the middle and upper parts of MIS 6, miliolids (more that 20%, mainly Triloqulina tricarinata) and Pullenia bulloides are abundantly represented. The abundance of foraminifera decreases to approximately 1000 ind./g of sediment.

Fig. 5.
figure5

Distribution of dominant benthic foraminifera species in core AMK 4453 (%).

In the lower part of the interglacial sediments of MIS 5e, Planulina wuellerstorfi dominates (up to 30%), owing to which the abundance of shells in sediment increases up to 1500–2000 ind./g. In MIS 5d, where the species Melonis pompilioides and Pullenia bulloides dominate (around 30%), it amounts to less than 1000 ind./g. Higher (MIS 5с–5а), the abundance of foraminifera decreases down to 500 ind./g of sediment. Here, terrigenous fragments are encountered. At the MIS 5/MIS 4 boundary, the abundance of Pullenia bulloides decreases approximately down to 10%.

In the lower part of the glacial MIS 4, a peak for Epistominella exigua appears (almost 40%); here, shells of Hoglundina elegans have also been found, which have been poorly preserved, likely due to their oversedimentation.

In interstadial MIS 3, Bulimina marginata again appears (up to 20%), which dominated in MIS 6. The glacial sediments of MIS 2 with a small amount of terrigenous fragments, just like in the northern part of the Iceland Basin, are also distinguished by a complex of agglutinating foraminifera (up to 30%), among which, like before, tubular species of the genus Rhabdammina dominate. Here, Planulina wuellerstorfi, Melonis pompilioides, and Epistominella exigua periodically dominate.

In MIS 1, the agglutinating species almost completely disappear, Epistominella exigua dominates (up to 30%), and Hoglundina elegans appears again (up to 16%), the shells of which this time are very well preserved.

Core AMK 4493. In Core AMK 4493, six MIS are distinguished (Fig. 6); terrigenous fragments are characteristic of the entire profile, which may have been transported from the Gloria Drift. The concentration of benthic foraminifera shells in sediment is very low (on average from 20 to 100 ind./g), with the exception of MIS 5e, where it increases approximately to 200 ind./g. Sometimes, shells are represented literally by single individuals, owing to which, when recalculating to a percentage concentration, invalid peaks occur, which should not enter into calculations when analyzing complexes.

Fig. 6.
figure6

Distribution of dominant benthic foraminifera species in core AMK 4493 (%).

In the sediments of MIS 6, Pullenia bulloides consistently dominates (20–30%); periodically Melonis pompilioides, Uvigerina peregrina, Planulina wuellerstorf, Eggerella bradyi, Osangularia umbonifera, Epistominella exigua, and various cibicids reach 20–30%. At the MIS 6/MIS 5е boundary (Termination II), Uvigerina peregrina accounts for more than half of all species in the complex.

In the interglacial sediments of MIS 5е, Pullenia bulloides, Melonis pompilioides, and Uvigerina peregrina almost completely disappear, whereas the share of Planulina wuellerstorfi increases up to 40%. The middle part of MIS 5 diverse agglutinating species and milionids dominate, which reach up to 50% and show an inverse correlation. In MIS 5c, Oridorsalis umbonatus dominates (more than 30%), and in the upper part of MIS 5a, Melonis barleeanum dominates (up to 30%).

In MIS 4, agglutinating foraminifera are absent; in the lower part, Uvigerina peregrinа reaches a peak (up to 40%), while in the upper part, Osangularia umbonifera demonstrates a peak. The sediments of MIS 3 are distinguished by periodic dominance of Uvigerina peregrinа (30–40%), Melonis pompilioides (up to 20%), and Oridorsalis umbonatus (up to 20%).

In the glacial sediments of MIS 2, the complex consists mainly of Pullenia bulloides, Melonis pompilioides, and agglutinating species whose abundance fluctuates in a sawtooth pattern from 5 to 30%. The sediments of MIS 1 are well distinguished by a sharp decrease in the earlier abundant Pullenia bulloides,Uvigerina peregrinа, and agglutinating species, as well as by an increase in Hoeglundina elegans up to 30%.

PALEORECONSTRUCTION BASED ON THE STUDIED CORES

Our experiments show that at the end of the middle Pleistocene (MIS 8–6) and the beginning of the late Pleistocene (MIS 5), the spilling of Norwegian water in the North Atlantic over the Faroe–Iceland sill hardly occurred. In these periods, in the northern part of the Iceland Basin, near-bottom water was depleted in oxygen and rich in nutrients, which is evidenced by the benthic foraminifera complexes with the dominant infaunal species Pullenia bulloides, sometimes together with the infaunal species Melonis pompilioides.

Water exchange between the Arctic seas and the North Atlantic was established during deglaciations (Terminations III and II). Then, in the northern part of the Iceland Basin, oxygen-rich waters appeared, similar to today’s NEDW, which is indicated by the dominant epifaunal species Planulina wuellerstorfi and Hoeglundina elegans.

The most interesting moment is the change in deep circulation in the area of the MIS 6/MIS 5 boundary. First (at the end of MIS 6), the glacial NEDW appeared in the western part of the Iceland Basin (AMK 4442), clearly as a result of direct spilling over the Faroe–Iceland sill. In the eastern part of the basin (AMK 4438), it appeared later (at the MIS 6/MIS 5 boundary), taking a longer route from the Ireland Trough to the Iceland Basin.

Periodically, in the northern part of the Iceland Basin, other water masses appeared. In particular, at the onset of the glaciation (MIS 6), oxygen-poor so-called Uvigerina water arrived here from low latitudes.

At the end of the penultimate glaciation (MIS 6), a water mass favorable to the development of agglutinating foraminifera was in the Iceland Basin. All their dominance occurs on a background of a high content of fragmentary material in sediments, which is not related to IRD from shelves, since these foraminifera are of deep-water genera [51]. The tubular species identified by us have a tubule diameter of around 100 mm, which points to oligotrophic–mesotrophic near-bottom conditions [36].

During the temperature maximum of the last interglaciation (MIS 5е), when the climatic conditions were quite similar to today’s, in the northern part of the Iceland Basin, like before, there was water with a low oxygen content that had long been isolated from the surface, and the sediments were rich in organic carbon. Among the benthic foraminifera at this time, the infaunal species Pullenia bulloides and Melonis pompilioides dominated. The high food concentration in the bottom sediments of MIS 5e is also confirmed by the abundance of benthic foraminifera shells there.

At the end of the last interglaciation (MIS 5a), the abundance of infaunal specials began to decrease, which is probably related to a decrease in the volume of “old” water in the northern part of the Iceland Basin. In the West-European Basin, such water remained longer, up to the end of the last glaciation [5].

During the last glaciation (MIS 4–lower part of MIS 3), Norwegian intermediate glacial water flowed into the northern part of the Iceland Basic, which is related to the dominance of Cassidulina teretis on the western slope of the Rockall Rise. Intensified circulation in the intermediate structural zone at the interstadial of the last glaciation (MIS 3) caused active avalanche processes: coarser shallow-water sediments with Bulimina aculeata were transported from the western slope of the Rockall Plateau to the foot of the Rockall Rise.

At the end of MIS 3 and in MIS 2, in the southern part of the Iceland Basin, agglutinating foraminifera again began to dominate, which was also noted in [71] in the West European Basin at a depth of 3547 m. It is likely that at this time in the subpolar North Atlantic, approximately lower than 2000 m, there existed a water mass rich in organic matter with a low oxygen concentration, which is confirmed by the conclusions in [53].

At the end of the last glaciation, together with the Bølling–Allerød warming, the polar front began to deviate to the north, and warm North Atlantic surface water began to enter the NGB. The return flow of Norwegian water over the Faroe–Iceland sill facilitated the replacement of old deep water in the Iceland Basin with fresh oxygen-rich water, to which the sharp increase in the concentration of Hoeglundina elegans among the benthic foraminifera was related.

The upper horizon in core AMK 4442 contains a melonis complex uncharacteristic of the sampling point. This leads to the assumption that the upper sediment layer with a thickness of approximately 25 cm is allochthonous. It possibly slid down the eastern slope of the Gardar Drift, which closely approaches the core sampling area.

In the western North Atlantic, on the western coast of the Reykjanes Ridge (AMK 4453), during the penultimate glaciation (MIS 6), active avalanche processes took place, to which the dominance of the shallow-water species Bulimina aculeata is related. Later, these processes ceased and here, up to the end of the last interglaciation (MIS 5), including the interglacial optimum (MIS 5e), just like in the Iceland Basin, stagnant deep-water conditions existed, evidenced by the benthic foraminifera complex in which Pullenia bulloides dominated.

The peak of Planulina wuellerstorfi that occurred at the onset of the interglacial optimum (MIS 5e) is apparently related to the short-lived arrival of oxygen-rich deep water from the Iceland Basin. It is indicative that, here, this water appeared somewhat later than there, clearly losing time on the route around the Reykjanes Ridge.

The dominance of Epistominella exigua at the beginning of the last glaciation (MIS 4) is possibly related to the seasonal spring–summer influx of organic matter to the bottom from an ice-free surface. It seems that the deep-water conditions of this period were quite similar to the Holocene (MIS 1).

At the end of the glaciation (MIS 2), at the western foot of the Reykjanes Ridge, just like in the Iceland Basin, oligotrophic–mesotrophic near-bottom conditions with a low oxygen concentration existed, which were favorable to agglutinating benthic foraminifera. The present-day deep conditions were established in the studied area at the end of the last stadial of the glaciation, to which the replacement of the agglutinating complex with today’s Epistominella exigua is related.

In the Labrador Basin (AMK 4493), in the late Pleistocene–Holocene (MIS 6–MIS 1), the influx of nutrients to the bottom was very weak, evidenced by the extremely low concentration of benthic foraminifera. However, it is possible that this is also related to the active dilution of carbonate matter with terrigenous matter.

At the end of the late Pleistocene (MIS 6), in the Labrador Basin, just like in other areas of the North Atlantic, there were stagnant deep conditions, judging from the dominance of Pullenia bulloides. The presence of Uvigerina peregrinа during the penultimate glaciation shows that water with a low oxygen concentration periodically entered here, which during the last glaciation was widespread at lower latitudes of the Atlantic Ocean [5]. At the beginning of MIS 6, Antarctic Bottom Water arriving from the south and also transporting Osangularia umbonifera also reached the Labrador Basin.

During the interglacial optimum (MIS 5e), the near-bottom conditions in the Labrador Basin, in contrast to the Iceland Basin and the eastern part of the Irminger Basin, were more similar to today’s conditions. Here, oxygen-rich deep water formed, evidenced by the dominance of Planulina wuellerstorfi. At later stages of the interglaciation (MIS 5с–5а), poor oxygen conditions favorable for the development of agglutinating benthic foraminifera were created.

Judging from the dominance of the same infaunal species—Uvigerina peregrinа, Melonis pompilioides, and Pullenia bulloides, the deep-water conditions in the Labrador Sea during the last glaciation (MIS 4–MIS 2) were very similar to those of the penultimate glaciation (MIS 6). At the onset of the last glaciation, Antarctic Bottom Water again appeared in the Labrador Sea. At the end of the glaciation, agglutinating foraminifera again began to dominate. Present-day North Atlantic water formed at the beginning of the Holocene, to which the appearance of Hoeglundina elegans is related.

DISCUSSION

Replacement of benthic foramina complex, as well as the composition of sediments themselves (the CaCO3 and terrigenous fragment contents) in the studied cores, shows that at the end of the Quaternary in the high latitudes of the North Atlantic, global deep-water hydrological restructuring occurred.

At the end of the middle–beginning of the late Pleistocene (MIS 8–MIS 5) in the subpolar Atlantic, the deep-water dynamics were stagnant, its basins were filled with low-oxygenated water, and sediments rich in organic matter. In particular, it was established that in the MIS 8 and 6, the isotherms in the North Atlantic had a predominantly sublatitudinal position [2], which facilitated the weak entry of warm, saline surface North Atlantic water into the NGB and a weak return flow. These conclusions confirm hypothesis [39] that for the majority of the Pleistocene, far less water from the NGB spilled into the North Atlantic than in the Holocene.

Periodically, short-lived fresh oxygen-rich and nutrient-depleted cold water from the NGB appeared in the basins. These events took place mainly during Terminations III and II. However, this deep water differed qualitatively from the present-day NEDW, since in the first case, it was distinguished by the Planulina wuellerstorfi complex, and in the second, mainly by the Hoeglundina elegans complex.

At the onset of the last interglaciation (MIS 5), deep-water circulation in the northern part of the Iceland Basin was also weaker than today, which is confirmed by the decrease in the abundance of Planulina wuellerstorfi in our sediment cores. The same situation was noted in the Iceland Basin in the area of the Gardar Drift [29], which is probably related to fresh surface water in the NGB due to active melting of Greenland ice.

During the temperature optimum of the last interglaciation (MIS 5e), the surface water temperature in the North Atlantic was 2–3.5°С higher than now [1, 11], evidenced by intensification of the NAMT. However, our research unambiguously indicates that in the northern part of the Iceland Basin, NEDW formation did not occur, which in the Holocene means that it did not propagate along the western slope of the Reykjanes Ridge, which our data show. The stagnant deep-water conditions in MIS 5e were characteristic not only of the Iceland Basin, but also for the West European Basin [3]. Thus, the deep-water situation in the eastern North Atlantic at this time differed fundamentally from today.

Then how did water from the NGB enter the North Atlantic? In MIS 5e, the Northern Polar Front had a submeridional direction, the main flow of surface North Atlantic water deviated to the northwest, and the Irgminger Current was more intense than today [2, 10, 12, 13]. Thus, in the area of the Labrador Basin, surface waters were 4–5°С warmer and 0.5–1.0‰ more saline than today [6, 31]. This gives grounds to assume that warm surface North Atlantic water entered the NGB mainly west of Iceland, over the Iceland–Greenland sill and not over the Faroe–Iceland sill, like in the Holocene. The reverse flow of deep water probably occurred through the Denmark Strait. Indeed, this assumption is confirmed by the data of core AMK 4493, in which the Pullenia complex disappears at the beginning of MIS 5e. Therefore, in the Labrador Sea, replacement of old water by fresh oxygen-rich water occurred at the very beginning of the last interglaciation, which is confirmed by other researchers [22]. The composition of the benthic foraminifera complex, as well as data on near-bottom carbon isotopes [57], shows that the near-bottom conditions in the Labrador Basin in MIS 5e itself were quite similar to today. During the last substage 5e, the value for glacial NADW began to decrease [54], to which the reduction in the share of Planulina wuellerstorfi in the complexes distinguished by us was related.

During the last glaciation, when the NGB was covered by year-round ice, turnover of water masses occurred in the North Atlantic at intermediate depths and the NAMT was more short-lived and weaker than today [5, 23]. At that time, Pacific Ocean water predominated in the Southern Ocean with a small share of NADW. In the Atlantic Ocean, the boundary between the NADW and South Atlantic water was located more to the north than today, allowing Southern Ocean water to enter the eastern Atlantic through the Romanche Trench [52]. Thus, in the Canary Basin during the last glaciation, Antarctic Bottom Water occupied not only the near-bottom, but also the deep structural zone (approximate down to 2500 m) without being counteracted by NADW from the north [4].

The propagation of agglutinating foraminifera in a wide deep-water range both in the western and eastern North Atlantic at the end of the last glaciation (the end of MIS 3–MIS 2) probably had a global character. Despite the fact that their abundance correlates with the abundance of IRD in sediments, this was not related to Heinrich events, since the tubular species identified by us are not encountered in shallow-water environments [51]. Probably, conditions favorable for them appeared in the deepest-water environment. This and the Cd/Ca and Zn/Ca ratios in shells of Planulina wuellerstorfi from several North Atlantic sediment cores presuppose a strong increase in nutrient content during the last glaciation [72].

The modern NAMT was established at the end of the last glaciation, during Bølling–Allerød warming, when isotherms began to assume a sublatitudinal direction [2]. Warm surface waters of the North Atlantic Current began to enter the NSG, and a return flow of cold, oxygen-rich, organic-matter-depleted water was established, which facilitated deep ventilation in the Northeast Atlantic. The acute renewal of deep water in the Northeast Atlantic began during the last termination [65]. Since then, Hoeglundina elegans began to dominate in the benthic foraminifera complexes of the subpolar North Atlantic. Turnover of water masses was shifted for a short time to the North Atlantic in the Older Dryas [23].

CONCLUSIONS

(1) At the end of the middle–beginning of the late Pleistocene, the basins of the subpolar North Atlantic were mainly filled with weakly ventilated, oxygen-depleted, and nutrient-rich water. The NAMT was weakened, as a result of which, the GHC was stagnant.

(2) During Terminations III and II, the NAMT intensified: fresh deep water similar to today’s NEDW formed in the Iceland Basin.

(3) During the last interglaciation, including during its temperature optimum, deep water from the NGB hardly entered the eastern part of the subpolar North Atlantic; stagnant conditions continued to exist there.

(4) At the beginning of the last glaciation, the NAMT was established in the western part of the subpolar North Atlantic; it was most active during the temperature optimum of the interglaciation. Water from the NGM entered the North Atlantic through the Denmark Strait.

(5) At the beginning of the last glaciation, when the NAMT was weakened, conditions favorable for the development of agglutinating foraminifera were established in the basins of the North Atlantic.

(6) In the eastern part of the North Atlantic, refreshing of deep water began at the end of the last glaciation, after which, the present-day NAMT model was formed, which provides for the activity of the GHC.

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ACKNOWLEDGMENTS

The author thanks V.B. Sivkov for providing the research materials. Determination of the oxygen isotopic compositions and absolute age was supported by a grant from the Russian Foundation for Basic Research (project no. 12-05-00240-a).

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Translated by A. Carpenter

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Lukashina, N.P. Deep-Water Dynamics in the Subpolar North Atlantic at the End of the Quaternary. Oceanology 58, 606–620 (2018). https://doi.org/10.1134/S0001437018040124

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Keywords

  • Planulina Wuellerstorfi
  • Iceland Basin
  • Pullenia Bulloides
  • Melonis Pompilioides
  • Hoeglundina Elegans