1. Introduction
Tephra (volcanic ash) layers can potentially enable the precise alignment of sediment records to resolve the relative timings of palaeoenvironmental events between a wide range of terrestrial and marine archives [
1,
2,
3]. If they can be dated directly, these markers provide independent checks on age models based on alternative dating methods [
4,
5,
6]. Records preserving several superimposed tephra layers together constitute a tephrostratigraphy which can contribute to a more detailed understanding of volcanic histories [
7,
8,
9,
10,
11,
12].
The development of an Eastern Mediterranean marine tephrostratigraphy, which includes not only Italian but also Aegean Arc source volcanoes (e.g., Santorini, Kos, Nisyros, Yali), is still in its infancy. Data available from the Sea of Marmara and the northern Aegean Sea only extend back through the last ~80 kyrs and rarely include cryptotephra data [
13,
14,
15,
16]. Longer sediment sequences from the southern Aegean Sea and the Levantine Basin have only occasionally been studied in terms of their(crypto) tephra content despite the favourable downwind position of these sites with respect to Quaternary active volcanoes and their importance for palaeoceanographic research [
17]. For example, marine cores in this area have provided detailed evidence of orbital-scale African monsoon variability over at least the last 3 Ma, forced by changes in Nile run-off into the basin [
18,
19,
20,
21,
22], as well as centennial- to millennial-scale variations in sea surface temperature and circulation modes throughout the eastern Mediterranean [
23,
24,
25,
26,
27,
28,
29]. Hence, independent dating and linking of these palaeo-records via (crypto)tephras is crucial.
The application of tephrochronological studies has been spurred to some extent by improved methods for recovering and analysing non-visible ash (cryptotephra) layers from sediment sequences worldwide. This has greatly extended the traceable “footprints” of some tephra isochrons and helped to extend or refine regional tephrostratigraphic frameworks (or “lattices”), particularly for the Late Quaternary in the Mediterranean [
5,
17,
30,
31,
32,
33].
The development of a tephra framework for the Mediterranean Sea started with the first recovery of deep-sea cores from the Ionian and Levantine Seas in the late 1940s and has been updated ever since [
13,
14,
34,
35,
36,
37,
38,
39,
40]. Detailed marine tephrostratigraphies for Central Mediterranean (Italian) volcanoes were compiled for the last 200 kyrs from Tyrrhenian, Ionian and Adriatic Sea cores and the first cryptotephra studies were also undertaken in this region [
5,
8,
30,
41,
42,
43,
44,
45,
46,
47].
The Aegean and Levantine Seas are situated downwind from volcanoes in the Aegean Arc (Methana, Milos, Santorini, Kos, Yali, Nisyros), and, further to the west, on the Italian peninsula (Campanian and Roman Provinces) and islands (Etna, Aeolian Islands, Pantelleria) (
Figure 1)—all of which were frequently active during the Late Quaternary (see [
48] and references therein). The Levantine Sea in particular can also receive tephra from the North from highly active Western and Central Anatolian volcanoes (Acigöl, Hasan Dag, Erciyes Dag) (
Figure 1). However, so far, very few tephra layers have been characterised in the eastern sector of the Mediterranean Basin and tephra studies have, therefore, been underexploited in the construction of volcanic histories. In the Levantine Sea, the known tephrostratigraphical record consists only of an Early Holocene tephra thought to originate from the Ericyes Dag stratovolcano in Central Anatolia [
49], the ~22 ka Cape Riva tephra from Santorini [
50,
51], and the ~161 ka Kos Plateau Tuff (KPT; [
50,
52,
53]). All these tephras form visible layers in the marine sediment records, and this sector of the Mediterranean has not previously been investigated for the presence of cryptotephra layers. The only published marine cryptotephra record in the Eastern Mediterranean Sea is derived from core LC21 in the southern Aegean Sea [
17]. This core encompasses the last ~160 kyrs and has been used for high-resolution palaeoceanographic reconstructions [
18,
21,
22,
23,
26,
27,
54]. A detailed study identified nine visible tephras and eight cryptotephra layers in core LC21 with provenances from Italian and Aegean Arc volcanoes [
17], demonstrating the potential for tephra to correlate the region’s marine sediment cores.
Here, we present updated (crypto)tephra results from core LC21 and two other deep-sea cores; M40/4-67 (GeoTÜ-KL51) from the southeastern Aegean Sea and Ocean Drilling Project (ODP) Site 967B from the central Levantine Basin (
Supplementary Materials 1 and 2). These cores hold sapropel sediments deposited during the Eemian (last) interglacial period (~130–115 ka) (sapropels S3–S5).
Sapropel S5 is represented in numerous sediment cores throughout the eastern Mediterranean and dated to between 128.3 and 121.5 ka [
21]. It is the best developed (often annually laminated) sapropel of the last glacial/interglacial cycle (see [
18] and references therein). Sapropel boundaries are frequently used as isochrons to synchronise eastern Mediterranean sediment records, but these can be blurred by post-depositional oxidation/reduction at the top/base of each layer. Furthermore, the exact onset and cessation of sapropel formation at each site depends on the development of benthic anoxia, which in turn depends on water depth, vertical mixing, and carbon export. These factors may result in differences of a few centuries between sites, for the start and end of sapropel formation [
55]. Uncertainties (±2 ka) on current dates for the start and end of sapropels S3–S5 [
21] are large enough to accommodate these centennial geographic differences in the timing of sapropel formation. Nonetheless, the tephra layers reported here may prove particularly valuable for clarifying centennial- to millennial-scale changes during the Eemian in this climatically sensitive region.
Where the geochemistry of a tephra layer can be matched to a proximal volcanic deposit, the date derived for the tephra layer can also be used to apply age constraints to the development and activity of a volcanic system [
56,
57]. This is sometimes the only way to define dates for volcanoes, particularly those with magmatic compositions which are not conducive to radiometric dating. For example, in the Aegean, dates for eruptions of Yali [
17], Nisyros [
58] and Santorini [
59] are only known from the correlation of proximal volcanic stratigraphies to tephra deposits in marine sediment cores. The tephra deposits described here, therefore, provide a new opportunity to define eruption dates for the volcanic systems of the Eastern Mediterranean.
5. Discussions
In this section, we assess the possible correlations of the individual tephra deposits reported in
Section 4, and their potential to serve as regional isochrons. We do not attempt to definitively assign a detailed source to the tephra deposits, as a major constraint hampering this endeavour is the current paucity of relevant geochemical and geochronological information for the region’s proximal volcanic strata of Eemian age, especially for nearby Aegean Arc and Anatolian volcanic provinces. For example, Santorini’s proximal stratigraphy is well documented [
82,
83], but lacks glass chemical datasets for comparisons and precise radioisotopic dating of tephras >100 ka. The volcanic stratigraphy of Kos, Yali and Nisyros is not well documented [
84] or dated [
85,
86,
87] for this time frame, and only few glass chemical data are available [
13,
14,
58,
88]. The eruptive history of the Central and Eastern Anatolian volcanoes is reasonably well established for the last 35 kyrs [
48,
78,
89] and for >160 ka [
79], but is lacking data for the Eemian time interval. Other potential but more distal sources include Italian volcanic provinces. A few less widespread tephra markers of Italian provenance and Eemian age have been reported from terrestrial sites in central and southern Italy [
4,
90], but those layers are all K-alkaline (trachytic-phonolitic) in composition and, therefore, unrelated to the silica saturated tephra layers presented here (see
Figure 5). In the Adriatic Sea, [
5] found shards in the middle of sapropel S5 preserved in core PRAD 1–2 (sample PRAD-3065), but unfortunately these were not able to be geochemically characterised, precluding a comparison to the data presented here. Therefore, in the absence of a robust regional proximal database, attempts to adduce the provenance of the tephra layers reported here will necessarily be preliminary in nature.
5.1. Correlations Between Cores and Possible Sources of the Tephra
Attempts to correlate the tephra deposits between the three cores described above used a two-stage approach: first, bi-plots of key elements were used to identify overlapping chemical ranges, then DFA (
Figure 6) was employed to test for possible correlations. The first and second discriminant functions were plotted and any similarity visually inspected (
Figure 6) in tandem with ad hoc utilisation of the stratigraphic positions of samples to identify any clear correlations between the tephra layers, and hence between the three sediment cores examined here (
Figure 7,
Table 1).
The oldest tephra deposits reported here are those lying close to the lower boundary of the S5 sediment units in ODP967B and LC21 (
Figure 2). Layers ODP967B-2H1 129.5 and LC21 993.5 cannot be differentiated either through their major element concentrations (
Figure 5) or the DFA plot (
Figure 6). Given their indistinguishable high-silica rhyolitic chemical compositions and similar stratigraphic position, it is reasonable to interpret that they represent the same eruption event, and hence provide a proxy-independent isochron for linking Eemian and S5 records between the Aegean and Levantine seas (
Figure 1,
Figure 7 and
Figure 8). The high SiO
2 content (~77 wt%) and very low CaO and FeO concentrations of these rhyolitic shards indicate a likely origin from either Central Anatolia volcanoes (e.g., Acigöl) or from the Kos-Nisyros-Yali volcanic system (SE Aegean Arc) (
Figure 1 and
Figure 5).
Sample LC21 975.5 represents a cryptotephra peak 18 cm above the basal LC21 993.5 layer and only 5 cm below the visible tephra layer LC21 970.9 (
Figure 2). As noted in
Section 4, this sample yields shards with similar compositions to samples LC21 993.5 (below) and LC21 970.9 (above) (
Figure 6), possibly indicating reworking of these tephra shards through bioturbation or redeposition. It does, however, also include rhyolitic shards (
n = 6) which are chemically distinct from those of all other layers analysed within this study (tephra layer 3? in the DFA, where the question mark indicates uncertainty, see
Figure 6). These could signal an additional distinctive eruption event associated with sapropel S5. Alternatively, the analyses obtained from this sample, considered together, could simply represent geochemical variation within one eruption. Here, we tentatively propose a tephra layer on the basis of a sharp peak in shard concentrations, but caution that these results would need to be replicated in other sediment sequences from the region for this interpretation to be confirmed. The rhyolitic shards show some affinities to Central and Eastern Anatolian tephras (
Figure 5).
Just above the samples described above (see
Figure 2) is the ~2 mm thick visible layer LC21 970.9, first reported in [
17] with a glass composition that does not match any of the other samples presented here (
Figure 5 and
Figure 6). It, therefore, represents a distinctive eruption event. Being dacitic to trachydacitic in glass composition (
Figure 5), it conforms to the geochemical ranges of proximal deposits on Santorini [
17] and is outside the chemical range of other nearby Aegean Volcanic Arc sources (Nisyros, Yali, Kos), as well as of Anatolian or Italian volcanoes [
17]. Glass chemical data available for Santorini proximal tephras is limited to only a few major eruptions from the time intervals <100 ka [
48] and >200 ka [
33], hence still hampering detailed correlations. Several of the Plinian events evident in the proximal stratigraphy of Santorini are undated. However, the similarly dacitic-trachydacitic Vourvoulos eruption is currently constrained to between ~152 and ~47 ka by the dates of the underlying (Middle Pumice) and overlying (Upper Scoria 2) deposits and is thus a viable candidate eruption for this Eemian tephra layer.
Figure 5 shows that the EPMA of LC21 970.9 (visible) matches the composition of the silicic end-member of the Vourvoulos proximal deposits [
57] and we propose an attribution to this eruption.
The youngest discrete tephra layer reported here is the KL51 346.8 tephra. It is a visible layer occurring close to the upper boundary of the S5 sediment unit (
Figure 3). Both the major element bi-plots (
Figure 5) and DFA plot (
Figure 6), where these shards together form cluster 1, indicate that the glass chemistries of these samples are indistinguishable from (and, therefore, correlate to) the uppermost three tephra samples from core LC21, namely LC21 899.5, 941.5, and 959.5. A sample at 957.5 cm depth (from [
17]) also matches this composition (
Figure 5). The three LC21 samples represent tephra peaks selected for analysis from a series of tephra samples that extend continuously over a >70 cm sediment interval (
Figure 2) with LC21 959.5 being the basal sample. As previously discussed, the thick sequence of tephra peaks in the upper part of S5 in core LC21 (
Figure 2) results most probably from prolonged redeposition of tephra from the slopes surrounding the sedimentary basin, rather than in-situ reworking (for example by bioturbation). This interpretation is supported by the observation of undisturbed laminations in S5 in LC21 [
54]. A range of possible correlations between the KL51 346.8 tephra and the LC21 shard count peaks presented in this paper are discussed in the
supplementary information (Supplementary Material 4) with respect to the known biostratigraphy of the core. However, as no precise correlation can be made between the cores at present, these results highlight the need for trace element analyses of the tephra layers [
17,
58].
If differences in concentrations of fractionation-sensitive trace elements such as Ba or Sr can be detected between otherwise geochemically identical samples within the same core (such as LC21 899.5, 941.5 and 959.5 cm), this would support an interpretation that each peak in shard concentrations represents a separate eruption event. In addition, trace element analyses could also assist in the attribution of volcanic source regions to the tephra layers. For example, Rb, Nb, Ta, Y and Th are proposed to good discriminators between active and post-subduction tectonic settings [
48] and so could discriminate between the Aegean Arc (active) and Anatolian (post-subduction) sources proposed here for tephra samples LC21 993.5 cm and ODP967B 2H1 129.5 cm.
Notwithstanding the difficulty in determining the precise correlation between LC21 and KL51, the andesitic to dacitic composition of these tephras strongly suggests an origin from an eruption of the Santorini volcano (
Figure 5), which is known to have produced such compositions at least within the last 100 kyrs [
48].
5.2. Age Estimates of the Tephra Deposits
Ideally, to maximise their utility, the ages of tephra layers should be derived by direct or indirect radiometric measurements from sequences with well-understood sedimentary contexts. However, the increasing numbers of detected tephra layers that are only registered at distal locations means this is not always possible, as these layers usually do not have sufficient material to obtain absolute age estimates in this manner. Hence, the inferred ages of such tephra layers are usually reliant on the robustness of age models based on independent evidence [
4].
The tephra layers reported in this study have not been dated directly, so the ages presented here (
Table 1) are derived from linear interpolation between age estimates with 2σ uncertainty previously established for the upper (121.5 ± 2 ka) and lower (128.3 ± 2 ka) boundaries of sapropel S5 in Eastern Mediterranean sediments [
21]. These sapropel ages themselves are derived from an age model developed for the LC21 core sequence by [
54]. Briefly, this is based on radiocarbon dates and two well-dated tephra layers (the Minoan and Campanian Ignimbrite eruption events) for the 0–40 ka interval, and constrained by tuning the LC21 planktonic foraminiferal δ
18O record to the U/Th-dated Soreq Cave (Israel) speleothem δ
18O record [
91] for the 40–150 ka interval. Robust tie points link the top and base of sapropel S5 in LC21 to Soreq δ
18O record, but crucially no intermediate tie points were proposed [
54]. Hence the tephra age estimates provided here were necessarily derived by linear interpolation between the top and base of S5. They should, therefore, be regarded as provisional, because their derivation rests on the assumed correspondence of stratigraphic markers (the top and bottom of sapropel S5) between deep sea-floor pelagic sediments of cores ODP967B-2H1, LC21 and KL51, and between changes in the δ
18O of eastern Mediterranean surface waters and Soreq cave speleothems. The latter relationship derives from a shared physical mechanism whereby evaporation from the eastern Mediterranean Sea provides the dominant source of precipitation over Soreq Cave (hence, the δ
18O “source” signal is preserved in Soreq speleothems) [
54,
91]. Although there is thought to be a lag time of up to 400–600 years between deposition of S5 in the west and east of the Eastern Mediterranean basin [
55], this is much smaller than the 2σ uncertainty of ages defined for the top (121.5 ± 2 ka) and base (128.3 ± 2 ka) of sapropel S5. Any asynchronicity between cores is, therefore, accommodated by the uncertainties of these dates. Defining age estimates for the tephra layers here does not preclude their valuable use as proxy-independent isochrones. It also allows them to provide a unique chronological contribution to the eruptive history of the region.
The age of 121.8 ± 2 ka (2σ uncertainty;
Table 1) for the upper tephra marker KL51 346.8 (corresponding to samples LC21 959.5, 941.5 and 899.5) is derived from linear interpolation in core KL51 between the ages of the start and end of sapropel S5, as outlined above. The age estimate for the oldest tephra marker ODP967B-2H1 129.5 (corresponding to LC21 993.5) at 126.4 ± 2 ka is derived by linear interpolation in core ODP967B-2H1 between the ages of the start and the end of sapropel S5 (
Table 1).
Bracketed between these two tephra deposits, but only represented in core LC21, are two further tephra layers, LC21 970.9 (visible) and LC21 975.5 (clusters/tephra layers 2 and 3? in
Figure 7). Due to the uncertainty in the correlation of the tephra in the uppermost part of S5 in LC21 with the KL51 core (layer 1,
Figure 7), their ages can only be constrained stratigraphically by being older than the KL51 346.8 tephra (121.8 ± 2 ka) and younger than the ODP967B-2H1 129.5 tephra at 126.4 ± 2 ka.
Table 1 summarises these stratigraphical and chronological interpretations.
5.3. Synthesis with the Regional Tephrostratigraphy
The four tephra layers described here are the first to be characterised within the Eemian, Eastern Mediterranean sapropel S5 and significantly add to the known tephrostratigraphy in this region (
Figure 8). These Eemian sediments preserve a valuable record of the climate of the last interglacial period. Furthermore, volcanic stratigraphies in the region are often poorly dated [
82,
87] and so this updated regional tephrostratigraphy (
Figure 8) has both palaeoenvironmental and volcanological applications. Prior to this publication, Eemian tephra layers in the Mediterranean have only been characterised in terrestrial sites in central and southern Italy, namely in Fucino [
90] and Lago Grande di Monticchio [
4]. Unfortunately, those are layers of Italian provenance and do not allow correlation to the marine sequences described here (
Figure 8). However, other older and younger tephras such as the Pantelleria P-11 tephra (133 ka) and possibly the Campanian POP2/TM-24a tephra (102 ka) [
92] are valuable markers which may prove invaluable in the precise synchronisation of terrestrial and marine records throughout the Mediterranean (
Figure 8). In many cases, these tephra layers also provide the only chronological constraints for volcanic stratigraphies. For example, here, we suggest a date range for the Vourvoulos eruption of Santorini (
Figure 8,
Table 1).