The Distal and Local Volcanic Ash in the Late Pleistocene Sediments of the Termination I Interval at the Reykjanes Ridge, North Atlantic, Based on the Study of the Core AMK-340

: We made the geochemical analysis of the volcanic material from the sediment core AMK-340 (the Russian research vessel “Akademik Mstislav Keldysh” station 340), the central zone of the Reykjanes Ridge. Two ash-bearing sediment units within the interval of the Termination I can be detected. They correlate with the Ash Zone I in the North Atlantic Late Quaternary sediments having an age of 12,170–12,840 years within the Younger Dryas cold chronozone and 13,600–14,540 years within the Bølling–Allerød warm chronozone. The ash of the Younger Dryas unit is presented mostly by the maﬁc and persilicic material originated from the Icelandic volcanoes. One sediment sample from this unit contained Vedde Ash material. The ash of the Bølling–Allerød unit is presented mostly by the maﬁc shards which are related to the basalts of the rift zone on the Reykjanes Ridge, having presumably local origin. Possible detection of Vedde Ash could help to specify the timing of the previously reconstructed paleoceanographic changes for the Termination I in the point of the study: signiﬁcant warming in the area might have occurred as early as 300 years before the end of the conventional Younger Dryas cold chronozone.


Introduction
Tephrochronology is a widely used tool for dating and correlating the marine and terrestrial sediment sequences, especially within the Quaternary [1]. Recent detailed mineralogical and geochemical studies of volcanic material revealed a high-resolution Late Pleistocene and Holocene chronostratigraphy for the North Atlantic [2][3][4]. Icelandic volcanoes are the major source of the ash in the marine sediments of the Nordic Seas and North Atlantic [5]. Extensive studies of the Icelandic soil, lake, and shelf sediments documented >150 tephra layers within the late glacial and Holocene time [6]. Thornalley et al. [7] detected numerous ash-bearing marine sediment layers south of Iceland within the last deglacial and Holocene time. Such data help to refine the regional and local sediment stratigraphy and synchronize the paleoclimatic archives between the distal oceanic and land regions. Numerous studies of the Late Pleistocene tephra layers in the North Atlantic domain exhibited a complicated mineral/geochemical composition of the ash material accumulated in the marine sediments during the Termination I between approximately 18 and 11 ka (e.g., [8][9][10]). Different processes influence the deposition of the specific ash layers: single eruption or closely-timed chain of eruptions in one locality, eruptions in one locality, dispersal by the atmospheric and oceanic flows, iceberg and sea-ice rafting, and reworking by the bottom currents and bioturbation [11].
Our study's aim is to get additional information on the occurrence and composition of the tephra in the North Atlantic sediments at the transition from the last glacial period to the Holocene. The position of the sediment core AMK-340 (the Russian research vessel "Akademik Mstislav Keldysh" station 340) is in the central zone of the Reykjanes Ridge with a local source of the eruptive material. Therefore, we tried to recognize the different sources of the volcanic ash, local or distal ones. Geochemical analysis of the volcanic shards using scanning electron microscopy will help to reveal the specific, well-known tephra layers like Vedde Ash which can be a good marker for a refinement of the core stratigraphy and chronology of the local paleoceanographic changes.

Materials and Methods
The sediment core AMK-340 was obtained during the 4th cruise of the Russian research vessel "Akademik Mstislav Keldysh" [12] in the central part of the Reykjanes Ridge, North Atlantic south of Iceland ( Figure 1): 58°30.6´ N, 31°31.2´ W, water depth of 1689 m, core length 387 cm. Figure 2 presents some general litho-and biostratigraphic and chronological data on the core AMK-340. More data are presented in [13][14][15]. The core unit of 0-307 cm is composed of the white pelitic calcareous to weakly calcareous (foraminiferal-coccolith) oozes with a CaCO3 content from 10-25% to 40-50%. In the lower core unit of 307-387 cm, the sediment color becomes mainly grey with thin alternations of greenish-grey, yellowish-grey, and dark, almost black, bands. This unit is enriched in the diatoms (sometimes up to 10-30%). The CaCO3 content varies there between 5.5% and 20% [12]. A visual lithological description of the core exhibited no signs of the volcanic material.
The linear interpolation between four AMS 14 C-datings [13] (Figure 2) helped to develop an age model of the core AMK-340, taking into account litho-and biostratigraphy [14,15].  [13][14][15]. The core unit of 0-307 cm is composed of the white pelitic calcareous to weakly calcareous (foraminiferal-coccolith) oozes with a CaCO 3 content from 10-25% to 40-50%. In the lower core unit of 307-387 cm, the sediment color becomes mainly grey with thin alternations of greenish-grey, yellowish-grey, and dark, almost black, bands. This unit is enriched in the diatoms (sometimes up to 10-30%). The CaCO 3 content varies there between 5.5% and 20% [12]. A visual lithological description of the core exhibited no signs of the volcanic material. Lithostratigraphy of the AMK-340. Lithological description (a): number 1 is pelitic calcareous oozes with a CaCO3 content of >40-50%, number 2 is pelitic weakly calcareous muds with a CaCO3 content of 10-25%, and number 3 is silty-pelitic muds with a CaCO3 content between 5.5% and 20% and an admixture of biogenic SiO2 of up to 10-30% [12]. The green-colored graph (b) is a distribution of the radiolarian species Lithelius spiralis group as a marker of the temperate (non-glacial) conditions in the North Atlantic [13]. The blue-colored graph (c) is a distribution of the polar planktic foraminiferal species Neogloboquadrina pachyderma sinistral [13] as a glacial marker. The dark-blue line (d) is a distribution of the mineral grains in the sediment fraction of >100 μm [13] as a marker of the ice-rafted material. Pink bars are the samples where the ash material was analyzed. Black dotted arrows with captions show positions of the AMS (Accelerator Mass Spectrometry) 14 C-dates. Violet rectangle indicates the Younger Dryas unit.
Calibration of the 14C-datings in the calendar age data was made in the CALIB 7.1.0 computer software using the MARINE13 calibration scale with the standard reservoir correction of 405 years and its regional deviation of 85 ± 79 years [16]. The core spans the time interval of the last approximately 14,500 years or the Termination I and Holocene. We assign the core unit of 315-340 cm the Younger Dryas interval based, in addition to the radiocarbon dating, on the litho-and biostratigraphic information [13,15]. As in many other northern North Atlantic sediment cores, an abundance peak of the polar planktic foraminiferal species Neogloboquadrina pachyderma sinistral (left- Figure 2. Lithostratigraphy of the AMK-340. Lithological description (a): number 1 is pelitic calcareous oozes with a CaCO 3 content of >40-50%, number 2 is pelitic weakly calcareous muds with a CaCO 3 content of 10-25%, and number 3 is silty-pelitic muds with a CaCO 3 content between 5.5% and 20% and an admixture of biogenic SiO 2 of up to 10-30% [12]. The green-colored graph (b) is a distribution of the radiolarian species Lithelius spiralis group as a marker of the temperate (non-glacial) conditions in the North Atlantic [13]. The blue-colored graph (c) is a distribution of the polar planktic foraminiferal species Neogloboquadrina pachyderma sinistral [13] as a glacial marker. The dark-blue line (d) is a distribution of the mineral grains in the sediment fraction of >100 µm [13] as a marker of the ice-rafted material. Pink bars are the samples where the ash material was analyzed. Black dotted arrows with captions show positions of the AMS (Accelerator Mass Spectrometry) 14 C-dates. Violet rectangle indicates the Younger Dryas unit.
Calibration of the 14C-datings in the calendar age data was made in the CALIB 7.1.0 computer software using the MARINE13 calibration scale with the standard reservoir correction of 405 years and its regional deviation of 85 ± 79 years [16]. The core spans the time interval of the last approximately 14,500 years or the Termination I and Holocene. We assign the core unit of 315-340 cm the Younger Dryas interval based, in addition to the radiocarbon dating, on the litho-and biostratigraphic information [13,15]. As in many other northern North Atlantic sediment cores, an abundance peak of the polar planktic foraminiferal species Neogloboquadrina pachyderma sinistral (left-coiled shells) of up to >99% together with an occurrence of the Heinrich 0 layer (maximum of ice-rafted material accumulation-see a mineral grains distribution in Figure 2) marks the Younger Dryas cooling event.
In the same unit, the radiolarian species Amphimelissa setosa, which is now typical for the cold-water environments of the Nordic Seas and the Arctic, has a very noticeable increase in abundance; overall, the radiolarian assemblages were subarctic there. Rich benthic foraminiferal microfauna, including the specific bioproductivity-indicating species, appeared in the core just above this unit. A possible detection of Vedde Ash with the age of 12,170 years [3,17,18] in the sample 323-325 cm (see Discussion Section) allows us to make a more accurate age model of the sediment core AMK-340 for the time interval of the Termination I (sharp warming with the climatic fluctuations between the Last Glacial Maximum and Holocene). From new calculations regarding possible Vedde Ash detection, the lower time limit of the core AMK-340 could be 14,540 years B.P. The core units of 323-340 and 355-378 cm could have the presumable age of 12,170-12,840 and 13,600-14,540 years, respectively.
During the micropaleontological analysis of the core AMK-340 sediments under the stereomicroscope (foraminiferal study of the sediment fraction of >100 µm) and transmitted light microscope (radiolarian study of the sediment fraction of >50 µm) (Figure 3), we could recognize a remarkable admixture of the eruptive shards in the two core units (Figure 1), 323-340 cm (four samples: 323-325, 328-330, 334-336, and 338-340 cm) and 355-378 cm (four samples: 355-357, 360-368, 370-372, and 376-378 cm). In all these samples, the content of the eruptive material was >10-15% from the whole number of grains in the sediment fraction >100 µm. Volcanic shards in other samples of the core within the interval of 0-323 cm were absent or occasional. From these samples, eruptive shards in the fraction >100 µm were picked out for the subsequent studies of their chemical composition. All in all, data on 16 shards in the natural state, 24 shards from the core unit of 323-340 cm, and 16 shards from the core unit of 355-378 cm in the polished thin sections are presented (Table 1).
Two instruments examined the chemical composition of the shards. The first one is a scanning electron microscope CamScan MV2300 with the energy dispersive analysis system (EDS) INCA at the Geological Institute of the Russian Academy of Sciences, Moscow, Russia, (gold-sprayed shards in the natural state). The selected locality of measurements was 1 µm, and the voltage was 20 kV. The microanalyzer was equipped with a mylar input window of 0.5 µm in caliber which allowed to analyze light elements including oxygen and carbon. The reference samples for the quantitative analysis were obtained from the standard mineral collection provided by Oxford Instruments. From this collection, an analytical program tried to find a number of reference samples which were chemically close to the analyzed samples. The analytical accuracy was as high as 0.15%. Further, the scanning electron microscope VEGA3 TESCAN with EDS (Electron Dispersive Spectrometry) X-Max was used at the Institute of Oceanology of the Russian Academy of Sciences, Moscow, Russia (carbon-sprayed shards in the polished thin sections). The diameter of the electron beam was 1 µm, and the accelerating voltage was 20 kV. The concentrations of elements were detected using a correction on the absorption and dispersion of the X-rays with the help of the INCA standard software. The analytical accuracy was as high as 0.2%. Table S1 presents the normalized (along with some raw) data of the measurements. Two instruments examined the chemical composition of the shards. The first one is a scanning electron microscope CamScan MV2300 with the energy dispersive analysis system (EDS) INCA at the Geological Institute of the Russian Academy of Sciences, Moscow, Russia, (gold-sprayed shards in the natural state). The selected locality of measurements was 1 μm, and the voltage was 20 kV. The microanalyzer was equipped with a mylar input window of 0.5 μm in caliber which allowed to analyze light elements including oxygen and carbon. The reference samples for the quantitative analysis were obtained from the standard mineral collection provided by Oxford Instruments. From this collection, an analytical program tried to find a number of reference samples which were chemically close to the analyzed samples. The analytical accuracy was as high as 0.15%. Further, the scanning electron microscope VEGA3 TESCAN with EDS (Electron Dispersive Spectrometry) X-Max was used at the Institute of Oceanology of the Russian Academy of Sciences, Moscow, Russia (carbonsprayed shards in the polished thin sections). The diameter of the electron beam was 1 μm, and the accelerating voltage was 20 kV. The concentrations of elements were detected using a correction on the absorption and dispersion of the X-rays with the help of the INCA standard software. The analytical accuracy was as high as 0.2%. Table S1 presents the normalized (along with some raw) data of the measurements.

Morphological Types of the Eruptive Material in the Studied Sediment Samples
Angular fragments of the pumiceous basalts and basaltic andesites were black and sometimes greenish, aphanitic (microlithic) and porous, with cavities filled with the light volcanic ash and sometimes with the fragmented diatom frustules ( Figure 4).

Morphological Types of the Eruptive Material in the Studied Sediment Samples
Angular fragments of the pumiceous basalts and basaltic andesites were black and sometimes greenish, aphanitic (microlithic) and porous, with cavities filled with the light volcanic ash and sometimes with the fragmented diatom frustules ( Figure 4). Angular transparent/subtransparent fragments of the persilicic glass were olive and bottlegreen, elongate (columnar) with rough scratching on the surface parallel to the elongation or having an irregular shape, and aphanitic ( Figure 5). Angular transparent/subtransparent fragments of the persilicic glass were olive and bottle-green, elongate (columnar) with rough scratching on the surface parallel to the elongation or having an irregular shape, and aphanitic ( Figure 5).  Pisolites were found as semi-rounded/non-rounded grains, black, and composed of the andesitic and mafic ash with inclusions of the fragmented diatom frustules, quartz, and pyroxene ( Figures 6  and 7а-b,d).
Besides, mostly rounded, transparent, colorless or sometimes rose or yellow-brown (ferruginized) quartz was found.
What are the pisolites in our samples? Part of the ash material consists of the rounded and semi-rounded aggregates of the very thin, usually slightly cemented ash particles of a size of ≤50 μm having the various components with a sometimes substantial admixture of the fragmented diatom frustules, sponge spicules, and other microfossils. We refer to them as pisolites or ash shatters. They are aggregates of thin volcanic ash. They can form during the penetration of raindrops within ash clouds and during the vapor condensation on the ash particles in the eruptive clouds [19][20][21][22]. According to the classification of the Pisolites were found as semi-rounded/non-rounded grains, black, and composed of the andesitic and mafic ash with inclusions of the fragmented diatom frustules, quartz, and pyroxene ( Figures 6 and 7a,b,d).
Besides, mostly rounded, transparent, colorless or sometimes rose or yellow-brown (ferruginized) quartz was found.
What are the pisolites in our samples? Part of the ash material consists of the rounded and semi-rounded aggregates of the very thin, usually slightly cemented ash particles of a size of ≤50 µm having the various components with a sometimes substantial admixture of the fragmented diatom frustules, sponge spicules, and other microfossils. We refer to them as pisolites or ash shatters. They are aggregates of thin volcanic ash. They can form during the penetration of raindrops within ash clouds and during the vapor condensation on the ash particles in the eruptive clouds [19][20][21][22]. According to the classification of the volcaniclastic material in [23], particles <0.1 mm are fine-grained ash dust. This material appears at andesite volcanic eruptions, and the area of its dispersal is unlimited. It can be contaminated with terrestrial particles (e.g., minerals, fresh-water diatoms) during the eolian transport and with marine particles (e.g., microfossils) during the sedimentation in the ocean.
We recognized two types of pisolites in our samples: (1) semi-rounded, black, dense, sometimes pumiceous aggregates of the mixed composition and (2) rounded, white, loose aggregates of the intermediate composition (Figures 6 and 7). volcaniclastic material in [23], particles <0.1 mm are fine-grained ash dust. This material appears at andesite volcanic eruptions, and the area of its dispersal is unlimited. It can be contaminated with terrestrial particles (e.g., minerals, fresh-water diatoms) during the eolian transport and with marine particles (e.g., microfossils) during the sedimentation in the ocean. We recognized two types of pisolites in our samples: 1) semi-rounded, black, dense, sometimes pumiceous aggregates of the mixed composition and 2) rounded, white, loose aggregates of the intermediate composition (Figures 6 and 7).    The content and size of the black eruptive fragments decrease sharply in the sample of 328-330 cm; they comprised 5-10% of the sediment fraction, and their size was typically <0.5-1 mm. In this sample, the light pisolites of 0.25-0.5 mm in size, composed of persilicic and andesitic ash with quartz, prevailed, reaching 50-70% of the sediment fraction.

Distribution of the Eruptive Material in the Studied Sediment Samples
The content of the volcanic particles in the lowermost part of the unit, sample 338-340 cm, decreased significantly to 8-10% of the sediment fraction. They are present mostly in the form of black volcanic glass with an admixture of the olive-green ash, white pisolites of the persilicic composition, quartz, and rare fragments of the pyroxene and plagioclase. The size of the particles rarely exceeded 1 mm, with a maximum of up to 2.5 mm. The eruptive material is present in the form of sharply angular, mainly pumiceous, black basalt fragments with white ash dust in the cavities, the semi-transparent bottle-green, sometimes yellowish, mafic glass particles, quartz, feldspar, and sporadic semi-rounded white pisolites composed of persilicic and andesitic ash dust (Figure 8). In the analyzed sediment fraction, fragments of the eruptive rocks and minerals are larger compared to the biogenic particles, being 0.5-1 mm, occasionally 2-4 mm, in size. The eruptive material within the unit is distributed irregularly with the highest amounts of up to 20-25% of the sediment fraction in the middle (sample 360-362 cm) and lower (sample 376-378 cm) parts; its content in other samples stands at 5-10%. and 334-336 cm, respectively, with a content of 30-40% of the whole sediment fraction (Figure 3). The largest fragments of >1-5 mm in size are typical for the upper part of the unit. The content and size of the black eruptive fragments decrease sharply in the sample of 328-330 cm; they comprised 5-10% of the sediment fraction, and their size was typically <0.5-1 mm. In this sample, the light pisolites of 0.25-0.5 mm in size, composed of persilicic and andesitic ash with quartz, prevailed, reaching 50-70% of the sediment fraction.
The content of the volcanic particles in the lowermost part of the unit, sample 338-340 cm, decreased significantly to 8-10% of the sediment fraction. They are present mostly in the form of black volcanic glass with an admixture of the olive-green ash, white pisolites of the persilicic composition, quartz, and rare fragments of the pyroxene and plagioclase. The size of the particles rarely exceeded 1 mm, with a maximum of up to 2.5 mm.

The Core Unit of 355-378 cm with an Age of 13,600-14,540 Years
The eruptive material is present in the form of sharply angular, mainly pumiceous, black basalt fragments with white ash dust in the cavities, the semi-transparent bottle-green, sometimes yellowish, mafic glass particles, quartz, feldspar, and sporadic semi-rounded white pisolites composed of persilicic and andesitic ash dust ( Figure 8). In the analyzed sediment fraction, fragments of the eruptive rocks and minerals are larger compared to the biogenic particles, being 0.5-1 mm, occasionally 2-4 mm, in size. The eruptive material within the unit is distributed irregularly with the highest amounts of up to 20-25% of the sediment fraction in the middle (sample 360-362 cm) and lower (sample 376-378 cm) parts; its content in other samples stands at 5-10%.

The Core Unit of 323-340 cm with an Age of 12,170-12,840 Years
Sample 323-325 cm Basalt fragments, which prevail here, have a typical content of SiO 2 of 49.05-51.67%, a degraded concentration of K 2 O of 0-0.91%, and a high TiO 2 amount of 1.49-5.43%. The TiO 2 content drops down to 0.68% in some grains with the highest SiO 2 concentration. The glass composition on the surface of one shard can vary in some cases from basaltic, with a SiO 2 content of 49.10%, to andesitic, with a SiO 2 content of 57.15% (Figure 4a). Cavities of the black pumiceous basalt fragments are filled in many cases with volcanic ash dust of the different compositions, from the andesitic one, with a SiO 2 content of 55.79%, to the persilicic one, with a SiO 2 content of 63.89-65.97% (Figure 4c).
Rhyolites (volcanic persilicic shards) were less numerous than the basaltic fragments. We analyzed the chemical composition of one grain (Figure 5a) where the SiO 2 and K 2 O contents were high, up to 76.37% and 3.92%, respectively. TiO 2 was absent.
Pisolites, which were present here mostly in the form of a white, rounded, loose intermediate variety (Figures 6 and 7), were composed of the ash dust with a SiO 2 content from 53.30-53.66% to 58.18% with occasional inclusions of titanomagnetite. Black pisolites had fragments of the mixed mafic (SiO 2 content of 42.08-50.76%) to persilicic (SiO 2 content of 65.49% on average) composition. The persilicic shards are notable here for their elongated shape, presence of the lengthwise scratching, and dense texture. Basalt shards had an irregular shape and were pumiceous (Figures 4 and 5). Shards of the mixed composition were close to the persilicic ones in shape; on their transverse shear surfaces, we could see many small round cavities (bubbles) filled with the fragmented diatom frustules (Figure 5c,d).
Some pisolites consisted of thin andesitic ash dust with a SiO 2 content of 59.94%, and abundant diatom fragments were found.

Discussion
The diagram of SiO 2 versus K 2 O in Figure 9 shows that most analyzed volcanic grains from the core unit of 323-340 cm match the ash material from the Icelandic volcano eruptions.
53.46% and sharply angular transparent persilicic glass shards with a SiO2 content of 71.45-71.84% were less numerous. The latter had a relatively high К2О content of 3.31-3.48%.
Sporadic black, semi-rounded grains of the isometric or oval form of pisolites consisted of andesitic ash dust with a SiO2 content of 52.56-56.30% and occasionally more massive basalt shards (Figure 7d)

Discussion
The diagram of SiO2 versus К2О in Figure 9 shows that most analyzed volcanic grains from the core unit of 323-340 cm match the ash material from the Icelandic volcano eruptions. Figure 9. Position of the eruptive shards from the ash-bearing core unit of 323-340 cm in the binary SiO2/К2О system according to [24]. GIN (Geological Institute) samples present data on the shards in the natural state. SIO (Shirshov Institute of Oceanology) samples show data on the shards in the polished thin sections. Areas of the volcanic material from different sources are according to [25,26]. Figure 9. Position of the eruptive shards from the ash-bearing core unit of 323-340 cm in the binary SiO 2 /K 2 O system according to [24]. GIN (Geological Institute) samples present data on the shards in the natural state. SIO (Shirshov Institute of Oceanology) samples show data on the shards in the polished thin sections. Areas of the volcanic material from different sources are according to [25,26].
The sample of 323-325 cm contains basalt shards and pisolites enriched in TiO 2 and occasional rhyolites with an elevated K 2 O and high FeO concentrations. The bi-plots in Figure 10 present the composition of the presumable Vedde Ash material in the sample of 323-325 cm in comparison with well-known published data. Such ash composition is typical for Vedde Ash whose age in the Greenland ice-core chronology appears to be 12,170 ± 57 years [3,17,18]. A source of the eruptive material for Vedde Ash could be the Katla volcano in Southern Iceland [8,27]. Besides, we found some intermediate ash shards, also as dust in pisolites, with a low K 2 O content, but their origin is unclear.
Geosciences 2019, 9,379 13 of 18 The sample of 323-325 cm contains basalt shards and pisolites enriched in TiO2 and occasional rhyolites with an elevated К2О and high FeO concentrations. The bi-plots in Figure 10 present the composition of the presumable Vedde Ash material in the sample of 323-325 cm in comparison with well-known published data. Such ash composition is typical for Vedde Ash whose age in the Greenland ice-core chronology appears to be 12,170 ± 57 years [3,17,18]. A source of the eruptive material for Vedde Ash could be the Katla volcano in Southern Iceland [8,27]. Besides, we found some intermediate ash shards, also as dust in pisolites, with a low К2О content, but their origin is unclear. Figure 10. Bi-plots of major oxides in the ash material from the core AMK-340 sample of 323-325 cm in comparison to the published data. Black-outlined red circles are the geochemical measurements of the solid shards in the sample of 323-325 cm. Pink circles are the geochemical measurements of the pisolites in the sample of 323-325 cm. Blue circles are data of the Vedde Ash from the lake Kråkenes in western Norway, green circles are data from the lake Loch Ashik in Scotland, and black-outlined yellow squares are data from the Greenland NGRIP ice core [9].
Mangerud et al. [28] and Kvamme et al. [8] summarized findings of Vedde Ash, starting as early as the 1940s, within the Younger Dryas chronozone sediments in the North Atlantic, Nordic Seas, and surrounding European continental areas. Bond et al. [29] and Thornalley et al. [7], as Kvamme et al. [8] before, demonstrated a complicated geochemical composition of the ash beds in the Ash Zone I. Vedde Ash cannot be a simple synonym of the Ash Zone I. Moreover, Lane et al. [9] and Tomlinson et al. [10] concluded that the Icelandic eruptions could have produced ash material comparable to the Vedde Ash before, within and after the Younger Dryas chronozone. Therefore, they claimed an appropriate stratigraphic/chronological control to prove a Vedde-like ash occurrence within the late Younger Dryas time interval. As we presented in the Materials and Methods Section and in Figure 2, the core AMK-340 sample of 323-325 cm, where the possible Vedde Ash is detected, is from the Younger Dryas unit. However, we are aware that this sample does not contain pure Vedde Ash shards but mixed eruptive material originated from possible distal airfall, local sources within the Reykjanes Ridge rift zone, iceberg and sea-ice rafting, reworking during the sedimentation and bioturbation. Unfortunately, we are not able to quantify an exact proportion of the airfall material. An admixture of the local eruptive shards must take place. Ice-rafting influenced the ash accumulation in the area. Ruddiman and Glover [30] described a rhyolitic ash bed in the pre-Holocene sediments in the North Atlantic which can be referred to as the regional Ash Zone I. As the authors suggested, this ash bed must be mostly ice-rafted. Kvamme et al. [20] and Gibbs et al. supposed rather fast Late Pleistocene sea-ice rafting in the North Atlantic ranging from a few years to a few decades, and this is not critical for the time assignment of the core AMK-340 sample of 323-325 cm. According to studies of Ruddiman and Glover [30], the bioturbation can mix the ash through the sediment thickness of dozens of centimeters at pelagic sedimentation rates of 2.4-6.7 cm×10 −3 years. In the core AMK-340, the thickness of the ash-bearing sediments from two core units is 40 cm within the time interval of approximately 12,100-14,500 years B.P. The average sedimentation rate Mangerud et al. [28] and Kvamme et al. [8] summarized findings of Vedde Ash, starting as early as the 1940s, within the Younger Dryas chronozone sediments in the North Atlantic, Nordic Seas, and surrounding European continental areas. Bond et al. [29] and Thornalley et al. [7], as Kvamme et al. [8] before, demonstrated a complicated geochemical composition of the ash beds in the Ash Zone I. Vedde Ash cannot be a simple synonym of the Ash Zone I. Moreover, Lane et al. [9] and Tomlinson et al. [10] concluded that the Icelandic eruptions could have produced ash material comparable to the Vedde Ash before, within and after the Younger Dryas chronozone. Therefore, they claimed an appropriate stratigraphic/chronological control to prove a Vedde-like ash occurrence within the late Younger Dryas time interval. As we presented in the Materials and Methods Section and in Figure 2, the core AMK-340 sample of 323-325 cm, where the possible Vedde Ash is detected, is from the Younger Dryas unit. However, we are aware that this sample does not contain pure Vedde Ash shards but mixed eruptive material originated from possible distal airfall, local sources within the Reykjanes Ridge rift zone, iceberg and sea-ice rafting, reworking during the sedimentation and bioturbation. Unfortunately, we are not able to quantify an exact proportion of the airfall material. An admixture of the local eruptive shards must take place. Ice-rafting influenced the ash accumulation in the area. Ruddiman and Glover [30] described a rhyolitic ash bed in the pre-Holocene sediments in the North Atlantic which can be referred to as the regional Ash Zone I. As the authors suggested, this ash bed must be mostly ice-rafted. Kvamme et al. [20] and Gibbs et al. supposed rather fast Late Pleistocene sea-ice rafting in the North Atlantic ranging from a few years to a few decades, and this is not critical for the time assignment of the core AMK-340 sample of 323-325 cm. According to studies of Ruddiman and Glover [30], the bioturbation can mix the ash through the sediment thickness of dozens of centimeters at pelagic sedimentation rates of 2.4-6.7 cm × 10 −3 years. In the core AMK-340, the thickness of the ash-bearing sediments from two core units is 40 cm within the time interval of approximately 12,100-14,500 years B.P. The average sedimentation rate there is approximately 17 cm × 10 −3 years or much higher compared to that mentioned by Ruddiman and Glover [30]. We can expect a lower than dozens of centimeters sediment mixing depth at the bioturbation within the Younger Dryas, especially if a high activity of the benthic organisms during the cold paleoclimatic environments is not assumed, because the faunal activity (benthic foraminiferas) in the core point started to increase after the Younger Dryas [13]. Studies of the modern sediment mixed depth at the bioturbation in the deep-sea Temperate North Atlantic and Subarctic Nordic Seas exhibited values as high as 3.9 and 6-7 cm [31]. Therefore, we accept possible bottom reworking of the ash-bearing sediments. However, based on the litho-and biostratigraphic information together with the radiocarbon datings, we suppose that the ice-rafting and sediment reworking cannot mask/dispose of the occurrence of the Younger Dryas unit in the core AMK-340. If so, eruptive material from the sample of 323-325 cm can partially have a Vedde Ash origin.
The possible Vedde Ash occurrence in the sediment sample of 323-325 cm allows to re-estimate the timing of the paleoenvironmental changes during the Younger Dryas chronozone (Glacial Stadial 1 in Rasmussen et al. [18]) for the core AMK-340 [13]. Figure 11 presented the age-depth plot for the core AMK-340 based on the radiocarbon datings and corrected the possible Vedde Ash detection with age of 12,170 years [32]. Matul et al. [13] assigned the pre-Holocene warming at the Reykjanes Ridge in the North Atlantic within the Younger Dryas cold chronozone to the time interval of 12,500-12,200 years B.P. or significantly earlier than the beginning of the Holocene (11,700 years B.P.). Now, we can assume that this change occurred 12,200-12,000 years B.P., which was at least 300 years before the Holocene start. there is approximately 17 cm × 10 −3 years or much higher compared to that mentioned by Ruddiman and Glover [30]. We can expect a lower than dozens of centimeters sediment mixing depth at the bioturbation within the Younger Dryas, especially if a high activity of the benthic organisms during the cold paleoclimatic environments is not assumed, because the faunal activity (benthic foraminiferas) in the core point started to increase after the Younger Dryas [13]. Studies of the modern sediment mixed depth at the bioturbation in the deep-sea Temperate North Atlantic and Subarctic Nordic Seas exhibited values as high as 3.9 and 6-7 cm [31]. Therefore, we accept possible bottom reworking of the ash-bearing sediments. However, based on the litho-and biostratigraphic information together with the radiocarbon datings, we suppose that the ice-rafting and sediment reworking cannot mask/dispose of the occurrence of the Younger Dryas unit in the core AMK-340. If so, eruptive material from the sample of 323-325 cm can partially have a Vedde Ash origin. The possible Vedde Ash occurrence in the sediment sample of 323-325 cm allows to re-estimate the timing of the paleoenvironmental changes during the Younger Dryas chronozone (Glacial Stadial 1 in Rasmussen et al. [18]) for the core АМК-340 [13]. Figure 11 presented the age-depth plot for the core AMK-340 based on the radiocarbon datings and corrected the possible Vedde Ash detection with age of 12,170 years [32]. Matul et al. [13] assigned the pre-Holocene warming at the Reykjanes Ridge in the North Atlantic within the Younger Dryas cold chronozone to the time interval of 12,500-12,200 years B.P. or significantly earlier than the beginning of the Holocene (11,700 years B.P.). Now, we can assume that this change occurred 12,200-12,000 years B.P., which was at least 300 years before the Holocene start. Figure 11. Corrected age-depth plot for the Termination I interval of the core AMK-340 (red line) according to the occurrence of Vedde Ash in the sample of 323-325 cm (black-outlined red square). Black squares and black captions on the plot show the radiocarbon datings converted to calendar dates (see also Figure 2).
In the core unit of 328-335 cm with the corrected age estimate of approximately 12,700-12,900 years, the ash is present mainly in the form of andesitic pisolites with the admixture of basalts and rhyolites. Most of them suit the material from the Icelandic volcanoes. However, some basalt fragments with an elevated К2О in the andesitic pisolites can be related to the Jan Mayen volcanoes (Figure 9). A common occurrence of the marine microfossil (diatom) fragments inside pisolites and ash shard cavities may suggest that the marine transportation and sedimentation could have influenced an accumulation of the volcanic material to a large degree in the area of study. However, we cannot be sure of the Jan Mayen origin of the above-mentioned andesitic pisolites because Abbot and Davies [3] noted that it is rare to find the Jan Mayen volcanic deposits in the distal areas.
The ash in the core unit of 355-378 cm with an age of 13,600-14,540 years differs from that in the core unit of 323-340 cm with an age of 12,170-12,840 years. Black squares and black captions on the plot show the radiocarbon datings converted to calendar dates (see also Figure 2).
In the core unit of 328-335 cm with the corrected age estimate of approximately 12,700-12,900 years, the ash is present mainly in the form of andesitic pisolites with the admixture of basalts and rhyolites. Most of them suit the material from the Icelandic volcanoes. However, some basalt fragments with an elevated K 2 O in the andesitic pisolites can be related to the Jan Mayen volcanoes (Figure 9). A common occurrence of the marine microfossil (diatom) fragments inside pisolites and ash shard cavities may suggest that the marine transportation and sedimentation could have influenced an accumulation of the volcanic material to a large degree in the area of study. However, we cannot be sure of the Jan Mayen origin of the above-mentioned andesitic pisolites because Abbot and Davies [3] noted that it is rare to find the Jan Mayen volcanic deposits in the distal areas.
The ash in the core unit of 355-378 cm with an age of 13,600-14,540 years differs from that in the core unit of 323-340 cm with an age of 12,170-12,840 years.
In the samples of 355-368 cm and 376-378 cm with an age of 13,600-14,100 and 14,540 years, respectively, we can see a dominance of the basalt and andesitic basalt shards with a SiO 2 content of 52.32-53.13%, FeO content of 9.71-11.13%, MgO content of 6.31-8.69%, and K 2 O content of <1%. In terms of the geochemical composition, they are close to the tholeiitic basalts and basalt glass in the rift zone of the Reykjanes Ridge [12] (Figure 12) but differ from them in terms of a higher SiO 2 concentration. The SiO 2 content is 49.76% and 50.56% on average in the tholeiitic basalts and the basalt shards of the Reykjanes Ridge, respectively. The ash in the core unit of 355-378 cm may be mainly the local eruptive material originated from the volcanism and tectonics in the Reykjanes Ridge rift zone [12] and may have transformed under the influence of the acidic hydrothermal fluids during sedimentation.
Geosciences 2019, 9,379 15 of 18 In the samples of 355-368 cm and 376-378 cm with an age of 13,600-14,100 and 14,540 years, respectively, we can see a dominance of the basalt and andesitic basalt shards with a SiO2 content of 52.32-53.13%, FeO content of 9.71-11.13%, MgO content of 6.31-8.69%, and К2О content of <1%. In terms of the geochemical composition, they are close to the tholeiitic basalts and basalt glass in the rift zone of the Reykjanes Ridge [12] (Figure 12) but differ from them in terms of a higher SiO2 concentration. The SiO2 content is 49.76% and 50.56% on average in the tholeiitic basalts and the basalt shards of the Reykjanes Ridge, respectively. The ash in the core unit of 355-378 cm may be mainly the local eruptive material originated from the volcanism and tectonics in the Reykjanes Ridge rift zone [12] and may have transformed under the influence of the acidic hydrothermal fluids during sedimentation. Figure 12. Position of the eruptive shards from the ash-bearing core unit of 355-378 cm in the binary SiO2/К2О system according to [24]. All data on our samples are from the polished thin sections. Areas of the volcanic material from different sources are according to [25,26].
Other ash materials in the core unit of 355-378 cm are (1) mafic shards with a decreased SiO2 content of 45.88% in the lowermost sample of 376-378 cm and (2) persilicic glass with a SiO2 content of 67.65% and a high К2О concentration in the sample of 370-372 cm; the sources of both are unclear. Persilicic pisolites in the sample of 370-372 cm and mafic pisolites with a SiO2 content of 51.12% in the sample of 376-378 cm can have a possible origin from the Icelandic volcanoes, but the source of the andesitic pisolites with a SiO2 content of 55.17% in the sample of 376-378 cm is not defined and could be more distal. Figure 12. Position of the eruptive shards from the ash-bearing core unit of 355-378 cm in the binary SiO 2 /K 2 O system according to [24]. All data on our samples are from the polished thin sections. Areas of the volcanic material from different sources are according to [25,26].
Other ash materials in the core unit of 355-378 cm are (1) mafic shards with a decreased SiO 2 content of 45.88% in the lowermost sample of 376-378 cm and (2) persilicic glass with a SiO 2 content of 67.65% and a high K 2 O concentration in the sample of 370-372 cm; the sources of both are unclear. Persilicic pisolites in the sample of 370-372 cm and mafic pisolites with a SiO 2 content of 51.12% in the sample of 376-378 cm can have a possible origin from the Icelandic volcanoes, but the source of the andesitic pisolites with a SiO 2 content of 55.17% in the sample of 376-378 cm is not defined and could be more distal.

Conclusions
Two sediment units of the core AMK-340, Reykjanes Ridge, North Atlantic, contain a significant amount of volcanic ash. They can be related to the Ash Zone I in the North Atlantic Late Quaternary sediments. Accumulation of the ash-bearing sediments in the studied core samples occurred within the time intervals of 12,170-12,840 and 13,600-14,540 years B.P. or the Younger Dryas cold chronozone and Bølling-Allerød warm chronozone, respectively.
The ash in the core AMK-340 within the Younger Dryas unit is present mostly in the form mafic and persilicic material originated from the Icelandic volcanoes, and ice-rafted to the point of the study. In one sediment sample, we could detect shards of possible bimodal Vedde Ash. If it was accepted that the core sample of 323-325 cm contains Vedde Ash, the age model of the core AMK-340 and timing of the previously reconstructed paleoceanographic changes for the Termination I time interval can be specified. However, in any case, significant warming in the area could have occurred as early as 300 years before the end of the conventional Younger Dryas cold chronozone.
The ash in the core AMK-340 within the Bølling-Allerød unit is present mostly in the form of mafic shards which are close to the basalts and basalt glass of the rift zone in the Reykjanes Ridge, i.e., have presumably a local origin.
In some samples of both units, we found aggregated pisolites of andesitic ash dust with inclusions of persilicic/mafic particles and fragments of marine diatom frustules. The origin of these pisolites is unclear.