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Article

Towards an Integrated and Accurate Planktonic-Foraminiferal-Deduced Bio-Chrono-Stratigraphic Framework of Late Quaternary Mediterranean Marine Cores

by
George Kontakiotis
*,
Assimina Antonarakou
,
Evangelia Besiou
,
Elisavet Skampa
and
Maria V. Triantaphyllou
Department of Historical Geology-Paleontology, Faculty of Geology and Geoenvironment, School of Earth Sciences, National and Kapodistrian University of Athens, Panepistimiopolis, Zografou, 15784 Athens, Greece
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2023, 11(12), 2345; https://doi.org/10.3390/jmse11122345
Submission received: 17 November 2023 / Revised: 7 December 2023 / Accepted: 10 December 2023 / Published: 12 December 2023
(This article belongs to the Special Issue Advances in Marine Micropaleontology)
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Abstract

:
The late Quaternary is a key stratigraphic interval as it encompasses the Late Glacial to Holocene transition, which is characterized by a series of pronounced centennial climate oscillations and subsequent short-term events of paleoceanographic variability. Across this stratigraphic interval, significant turnovers and faunal changes in the composition and abundance of planktonic foraminiferal assemblages are well-documented through their high-resolution quantitative analysis performed in the south Aegean core NS-18. The identification of 10 synchronous bioevents among the Mediterranean sub-basins allows accurate inter-basinal correlations over the study time interval, thus contributing to the development of a robust chronostratigraphic framework for this setting. Moreover, the identification and timing of additional 20 diachronous bioevents, in conjunction with the already established bio-ecozonation scheme of the Aegean Sea, provide a continuous record of faunal changes (in terms of species-specific distributional abundances) which can be used as an additional locally expressed biochronological tool for the eastern Mediterranean deep-sea cores. The present study certainly indicates that the cause-and-effect relationships between the paleoceanographic/paleoclimatic perturbations and biological response require a highly resolved regional chronostratigraphy.

1. Introduction

The Mediterranean Sea is of particular importance to paleoceanography and paleoclimatology, as the marginal character of the basin promotes rapid responses to climate change, in such a way that any hydroclimate signal can be registered in an amplified fashion in its hydrological properties compared to the global oceans. Particularly, the Aegean Sea in its eastern sector is among the most studied basins due to its latitudinal position at the interface between the higher- and lower-latitude climate systems [1], and its heightened climate sensitivity because of the combination of its small size and semi-enclosed character due to the close atmospheric and partial isolated oceanic connections [2]. The peculiarities of this setting, mostly the large topographic contrasts, the oligotrophic nature, and the relatively small volume of the Aegean Sea, cause perturbations that can be recorded instantaneously in paleoceanographic data, such as microfossil abundances [3,4,5,6,7,8] and/or geochemical proxies [9,10,11,12,13,14,15]. Signals reflected by the changes in size, abundance, and/or distribution of the modern planktonic foraminiferal population have also been documented for that basin [16,17,18], providing high-quality information, mostly related to their calcification, ecology, and related controlling factors [19,20,21,22,23,24,25], as well as comparative constraints at both the regional (entire Mediterranean basin; [26,27,28,29]) and global [30,31,32] scales. Moreover, the exceptionally high sedimentation rates reported for this basin (~20 cm/ky; [4,33,34,35]), compared to the open eastern Mediterranean (~4 cm/ky; [36]), provide a powerful tool for monitoring the paleoenvironmental conditions over the late Quaternary.
Geochronology is an essential tool in paleoceanographic studies aimed at providing an accurate chronostratigraphic framework of marine sediment cores through the combination of radiometric dating techniques (e.g., 14C, 210Pb, and 137Cs), geochemical methods (e.g., stable oxygen isotopes δ18O; [37,38]), tephrostratigraphy [39,40,41,42,43], and micropaleontological datings (e.g., planktonic foraminiferal eco-biostratigraphic events; [44,45,46,47,48,49,50]). The application of high-resolution planktonic-foraminiferal-deduced biostratigraphic schemes, especially, during the late Quaternary has an exceedingly high potential for correlating (hemi)pelagic successions in several sub-basins (e.g., Siculo-Tunisian Strait [46,51], Tyrrhenian Sea [52,53,54,55,56], Adriatic Sea: [57,58,59,60,61], Mediterranean Ridge [62], Ionian Sea [63], Aegean Sea [3,6], and Levantine Sea [46]) across the entire Mediterranean Sea, owing to the continuously increasing and multidisciplinary nature of the planktonic foraminiferal contribution in biochronology (including biostratigraphy and bio-ecozonation). The concept of event stratigraphy enables the assembling of datings in considerable detail from high-sedimentation-rate cores, and, hence, provides guidance to the interpretation of the paleoclimatic data through stratigraphic correlations. The evidence of such high-resolution data facilitates inter-correlations between Mediterranean sub-basins and further offers a better comprehension of the paleoceanographic/paleoclimatic evolution of the entire Mediterranean region.
Within this context, this study carried out in a high-sedimentation-rate core retrieved from the southeastern Aegean emphasizes that the planktonic foraminiferal assemblages over the last glacial cycle provide useful insights to define biochronological events that can be used for the construction of an accurate chronostratigraphic framework both locally for the Aegean Sea and regionally for the eastern Mediterranean. The deglacial-to-Holocene period is an ideal time interval target for such an investigation because the sea level history is well-determined, the centennial scale chronostratigraphic resolution is highly achievable through planktonic foraminiferal eco-biozonation, and the paleoclimate history during that period is accurately constrained through previous studies [3,6,8,9,64]. The sapropel boundaries and the well-constrained bio-ecozonation scheme of Triantaphyllou et al. [3] for the study area further increase the accuracy of the planktonic foraminiferal stratigraphic ranges, thus improving the final biochronological scheme. Our micropaleontological dataset is strengthened by stratigraphically targeted accelerator mass spectroscopy (AMS) radiocarbon datings performed on planktonic foraminifera. Overall, the approach presented here is relatively simple and fast, and does not require highly professional skills or complicated instruments, and is mostly an inexpensive and non-time-consuming process. Such an integrated proxy could be considered a significant advance in the biostratigraphic structure of marginal settings, while it could also act as a template for future work on the late Quaternary by refining or extending previously established biostratigraphic schemes.

2. Regional Setting

The Aegean Sea is situated in the northeastern Mediterranean, between the Turkish and Greek coastlines (Figure 1). It is separated into two distinct (north and south Aegean) sub-basins with different hydrographic characteristics, which are controlled by the exchange of the Levantine and Black Sea water masses and subsequent climate contrasts between more humid conditions in the north and semi-arid conditions in the south [65]. It is one of the most oligotrophic areas in the world (especially its south basin; [66,67,68,69]) and is generally characterized by a cyclonic surface water circulation, supplemented by several permanent or transient mesoscale cyclonic and anticyclonic eddies [65,70,71]. The complex topography is another dynamic feature of the Aegean basin due to the great variability of its shores and bottom relief [72,73].
Apart from the Adriatic Sea, the region is the second deep-water formation zone within the Mediterranean basin, with its main source areas reported to be the North Aegean continental shelf and the eastern Cretan margin [74,75,76]. The Black Sea freshwater discharges during cold atmospheric fronts in the winter determine the magnitude of dense deep-water supply [77], while the Cretan Gyre (including its peripheries to the east up to Rhodes Island) in the south Aegean further functions as a source of both intermediate and deep waters [76,78], and, therefore, can be considered to be the main contributor in triggering changes in the eastern Mediterranean thermohaline circulation (Eastern Mediterranean Transient; [75]).

3. Material and Methods

3.1. Study Core

The 2.8 m length gravity core NS-18 used in this study was retrieved during the 1998 R/V Aegaeo cruise from 496 m water depth at the easternmost edge of the recent Aegean volcanic arc (36°31.32′ N, 27°16.15′ E) in the south Aegean Sea. NS-18 core contains distinct organic-rich dark interval representing the regional expression of sapropel S1 [79], and comprises the last ~20 ka. The lower part of the core, below 230 cm, consists of olive-gray coarse sands of turbiditic origin, and, therefore, was excluded from further analyses. For the detailed description of the core lithology, we referred to Triantaphyllou [80].

3.2. Radiocarbon Analyses

Four accelerator mass spectroscopy (AMS) 14C measurements were performed on hand-picked mixed Globigerinoides ruber and Globigerinoides trilobus specimens in the size fraction >125 μm. It is worthy to note that the selected samples were related to abundance peaks to minimize the effect of bioturbation. The datings at 57 and 255 cm were performed at Beta Analytics laboratories at Miami, Florida (USA) in 2009, and those at 159 and 201 cm at the University of California, Irvine in 2022 (Table 1). Conventional radiocarbon datings were corrected using the regional marine reservoir correction of 58 ± 85 years [81], and then calibrated to calendar years using the INTCAL20 calibration curve in the Calib 8.2 software [82].

3.3. Micropaleontological Analyses

The planktonic foraminiferal assemblages identified in the 49 downcore sediments were picked from ~10 cm3 of wet sediment after washing through a 125 μm mesh sieve and cleaning using the HyPerCal protocol [83]. Following the findings of Capotondi et al. [84], particularly for the study area, the adopted size fraction provides a more realistic spectrum of the planktonic foraminiferal assemblage in the Mediterranean Sea. The dry residues (~3 g) were split using an Otto micro-splitter into aliquots of at least 300 planktonic foraminiferal specimens, thus providing a significant level of at least 95% for species of greater than 3% relative abundance [85]. After identification to species level based on Hemleben et al. [86] and Brummer and Kučera [87], they were sorted on Chapman slides and counted. Raw data were transformed into percentages of the total absolute abundance, and relative percentage abundance curves of the most representative species were plotted versus core depth in Grapher. For detailed species-specific morphological recognition and micropaleontological groupings among the studied species, we refer to the studies of Kontakiotis et al. [17], and, for the subsequent ecological interpretations, we refer to those of Zarkogiannis et al. [16], Kontakiotis et al. [28], Giamali et al. [19], and Kozanoglou et al. [88], particularly for the Mediterranean Sea.

4. Results

4.1. Planktonic Foraminiferal Assemblages

A qualitative analysis of planktonic foraminifera allows us to identify 20 species lumped into 14 groups: Globigerinoides ruber alba, Globigerinoides ruber rosea, Globigerinoides trilobus gr., Globigerinoides bulloides gr., Globigerinella siphonifera gr., Orbulina universa, Globoturborotalita rubescens gr., Globoturborotalita glutinata, Globorotalia scitula, Globorotalia inflata, Neogloboquadrina pachyderma, Neogloboquadrina dutertrei, Globorotalia truncatulinoides, and Turborotalita quinqueloba. Their relative abundances (%) versus core depth are displayed in Figure 2. Along the study interval, assemblages are mainly represented by G. ruber (alba and rosea) and G. bulloides. The former is the most abundant species, especially in the south Aegean with percentages of up to 50%, and, together with the latter, represent about 80% of the entire population. The remaining species are composed mostly of the surface- and subsurface-dwelling species (G. trilobus, G. rubescens, O. universa, and G. siphonifera) corresponding to the SPRUDTS group [89], with average percentages of ~10%, while deeper-dwellers and cold-water species (e.g., G. scitula, G. inflata, and T. quinqueloba) appear with lower percentages. Both representatives of Neogloboquadriniids (N. pachyderma and N. dutertrei) present a sporadic distribution since they are significant contributors only in the lower part of the core with an average contribution of ~30–40%.

4.2. Bioevent Stratigraphy

The observed planktonic foraminiferal abundance fluctuations are consistent with previously documented changes in the study area, the comparison of which allows us to define a series of biochronological events. Each event is defined by sharp frequency shifts or by local occurrence/disappearance alternations of the identified species. The advantage of the combination of these bioevents with the most complete (combining planktonic foraminifera, coccolithophores, and pollen) eco-biozonation scheme of Triantaphyllou et al. [3] for the Aegean Sea, established for the well-dated core NS-14 (near to NS-18), offers a stratigraphically controlled and more high-resolution subdivision of the Late Pleistocene–Holocene. Overall, we identified, described, and dated up to 30 bioevents (Figure 2), of which 10 are Mediterranean-wide synchronous (symbolized with “s”) and 20 (those of Pérez-Folgado et al. [48] enriched with the newly defined) are diachronous (symbolized with “d”). In the case of the Pérez-Folgado et al. [48] bioevents, we kept the nomenclature introduced by these authors by also determining their timing ranges, in which certain species reach values well above or below their average percentages. The earlier or delayed event among different Mediterranean sub-basins could be mostly attributed to species-specific ecological preferences in conjunction with relevant hydrographic parameters and oceanic circulation. In addition, the identification of the described bioevents in other more or less distant cores permits precise intra- and inter-basinal correlations and facilitates regional palaeoceanographic interpretations for the studied time interval.
Jmse 11 02345 g002
Figure 2. Frequency curves and recognized bioevents (synchronous in red and diachronous in blue) of the most representative planktonic foraminiferal species in core NS-18. To the right, the established Aegean bio-ecozones of Triantaphyllou et al. [3] are presented in green. Dark grey bands correlate to sapropel S1 depositional intervals, while the light grey one to the equivalent to the Sapropel Mid Holocene (SMH) [90] layer.
Figure 2. Frequency curves and recognized bioevents (synchronous in red and diachronous in blue) of the most representative planktonic foraminiferal species in core NS-18. To the right, the established Aegean bio-ecozones of Triantaphyllou et al. [3] are presented in green. Dark grey bands correlate to sapropel S1 depositional intervals, while the light grey one to the equivalent to the Sapropel Mid Holocene (SMH) [90] layer.
Jmse 11 02345 g002

5. Discussion

5.1. High-Resolution Mediterranean Bio-Ecochronological Events Deduced from Planktonic Foraminiferal Assemblages

In biochronological studies, bioevents are often identified on entries, exits, and significant shifts in the faunal record [44], usually displayed by species-specific relative abundances. Moreover, frequency shifts illustrated in relative abundance plots reflect population shifts rather than evolutionary changes among species. Especially in marginal basins like the Aegean Sea, the successive planktonic foraminiferal zones can be indicated as bio-ecozones, because most of the species are sensitive indicators of past environmental changes with amplified signals [45]. Therefore, the integrated concept of “assemblage bio-ecozones” supplemented with the “species-specific planktonic foraminiferal bioevents” adopted here refers to their response to environmental changes in specific time intervals. The synchronous or diachronous nature of these bioevents could be further considered as Mediterranean-basin-wide or only local (or even an asynchronous expression), depending on their occurrence.

5.2. Planktonic Foraminifera Fluctuations along the Mediterranean Basin: Calibrating the Synchronous Bioevents

The comparison of NS-18 planktonic foraminiferal distribution patterns with those reported from deep-sea cores from eastern (e.g., NS-14, C69, C40, SK-1, SL-11, SL-21, SL-31, SLA-9, LC-21, ST5, and MD84-641; [3,6,44,46,88]) and western (e.g., MD95-2043; ODP 977, REC13-53, KET80-19, Hole 963D, BS79-22, BS79-38, and GNS84-C106; [48,51,52,53,54]) basins displayed some Mediterranean-wide assemblage fluctuations over the glacial–interglacial timescale. At the regional scale, these trends could be considered quite synchronous, considering the 2σ uncertainty, even though minor dating differences (possibly related to the available radiocarbon datasets and uncertainties in the different age models) exist.
Based on the existing, well-documented records across the Mediterranean Sea, a mean age of each recognized bioevent was determined, and then intercalibrated through the comparison with the planktonic foraminiferal bioevents and related bio-ecozonation derived from the nearby well-dated core NS-14 [3]. The sequence of the resulting ages (Table 2) in the core NS-18 stratigraphy revealed an excellent match with AMS datings and, therefore, manifests that these events are well-constrained temporally in the eastern Mediterranean, also reflecting the accuracy of the adopted chronostratigraphic approach.
Over the last ~20 kyr, we identified ten bioevents that can be further used as control-points to define or strengthen the chronology of Mediterranean marine sediment cores. Most of them correspond to the termination T1, while one is reported for the latest Glacial and two for the Holocene (Table 2).
During the latest Glacial period, the last increase of T. quinqueloba (bioevent s1) dated at 18.3 ka is marked. This T. quinqueloba spike close to the end of the Last Glacial Maximum (LGM) has also been observed in several records across the Mediterranean Sea from the Alboran (bioevent Q2 at 18.3 ka [48]) to the Tyrrhenian basin (18.2 ka [52]; ~18 ka [54]), and through the Siculo-Tunisian Strait (18.3 ka [91,92]) up to the Levantine Sea (18.3 ka [46]).
Seven distinct and quite synchronous bioevents characterize the deglaciation period. The first post-glacial occurrences of G. inflata and G. truncatulinoides (bioevents s2 and s3) are recorded at 15.3 and 14.6 ka, respectively [46]. The post-glacial reappearance of G. inflata during the Bølling–Allerød (B/A) has been dated at 15.3 ka in Hole 963D [51] and core Z1 [63], at 15.4 ka in MD90-917 [61], at 15.1–15.2 ka in cores BS79-22 and BS79-38 [52,53], and at 15.6 ka in core MD84-641 [46] in the Levantine Sea. The first post-glacial occurrence of G. truncatulinoides was dated at 14.5 ka in the Levantine Sea (core MD84-641 [46]) and the south Adriatic Sea (core MD90-917 [61]), in accordance with other records from the central Adriatic (core CM92-43 [59,60]) and Tyrrhenian Sea (core KET80-19 [46]). The time equivalent with the reoccurrence of G. truncatulinoides is the last common postglacial disappearance of G. scitula (bioevent s4), which has also been dated at around 14.5 ka within the Mediterranean basin [7,46,63]. The existing dating discrepancies in the literature regarding this bioevent cannot be considered as a significant offset, since, in some cases, it is described as an abundance decline (at 14.8 ka in core REC13-53 and at 14.3 ka in core KET80-19; [46]), while, in others, as the last occurrence of this subpolar species (at 13.9 ka core MD90-917; [61] or ~14.0 ka in cores MD04-2797 and MD 04-2797 CQ; [91,92]; core BS79-38; [53,54]; core KIM-2A; [5]; and core SK-1; [9,93]). In all locations, this bioevent reflects the base of Marine Isotope Stage 1 [92,94], and, consequently, the climate amelioration during the B/A warm period in the entire Mediterranean basin [4,8,9,61,63]. The next bioevent corresponds to an increase in the abundance of G. inflata during that period (bioevent s5), characterized by two consecutive peaks, the highest of which has been dated at 13.5 ka [59]. Compared to other Mediterranean sites, the relatively lower percentages of this deep-dwelling species recorded here could be attributed to the lower bathymetry of the NS-18 core site (496 m water depth). This shallow depth site probably prevents the occurrence of deep-dwelling species such as G. scitula, G. truncatulinoides, and G. inflata, favoring, in parallel, the increased abundance of the surface-dwelling species (e.g., G. ruber).
At 12.8 ka, the assemblage is punctuated by the simultaneous peaks of G. bulloides and T. quinqueloba (bioevents s6 and s7). Similarly with other planktonic records across the Mediterranean Sea [3,46,51,54], these bioevents were accompanied by a decline in G. ruber (w). The Aegean records (e.g., NS-18 and NS-14) show a remarkable resemblance with those from the central Mediterranean (e.g., REC13-53, KET80-19, and GNS84-C106), with only some slightly discrepancies to be observed on the related taxa abundances, which could be attributed to the different mesh sieves used in these studies that essentially led to the loss of the small-sized species such as T. quinqueloba and, consequently, increased percentages of G. ruber (w). However, the high occurrence of the G. ruber taxon living in warm surface waters reflects the SST increase in the studied area. The end of the B/A and the onset of the Younger Dryas (YD) period was marked by two coeval peaks of G. ruber (w) (53%) and G. glutinata (9%; bioevent s8) at 12.3 ka. Similar bioevents have been also recognized in several basins along the Mediterranean basin (e.g., Sicily Channel—ODP Leg 160, Site 963; [51], Tyrrhenian Sea—GNS84-C106, KET80-19, BS79-22, BS79-38; [52,53], Aegean cores of [44], MNB3; [8]; MD84-641; [46]). The age of 12.26 ka derived from an AMS analysis in NS-18 (159 cm depth) fully supports the synchroneity of this bioevent into the Mediterranean basin. According to Casford et al. [95], the peak of G. glutinata (up to 15% in NS-18) within the YD reflects the Termination T1b onset.
During the Holocene, the maximum abundances of G. inflata during the interruption (3% at 7.7 ka; bioevent s9) and after the end (20% at 5.8 ka; bioevent s10) of sapropel S1 are synchronous along the central-eastern Mediterranean Basin. The former (at 7.7 ka bioevent I; [48]) has also been recognized in the western Mediterranean, while the latter, particularly, has been interpreted as the re-establishment of deep-water ventilation in the eastern Mediterranean [3,6,9]. Both the bioevents have been documented in numerous central and eastern Mediterranean cores [3,4,5,6,7,8,19,44,46,52,61,63,95,96,97], and interpreted as the results of the intensification of vertical mixing during a cold event [4].

5.3. Integrated Age Model

The integrated chronostratigraphy of the core NS-18 is based on four AMS 14C dates, and on the additional control points which correspond to well-dated lithostratigraphic levels (e.g., sapropel S1 and SMH boundaries) within the eastern Mediterranean basin (Table 2, Figure 3). Although the SMH is not lithologically evident in the study core, the similar faunal signals derived from NS-18 (Figure 2) and the reference NS-14 core [3] indicate that it can be used as an additional stratigraphic point, particularly for that sub-basin.
Furthermore, the calibrated planktonic foraminiferal bioevents described above as Mediterranean-wide synchronous (Table 2) have been integrated in the NS-18 age model as additional dating points allowing for a high-resolution chronostratigraphic framework covering the last ~20 kyr. The resulting age model was constructed by linear interpolation between all the above dating points as shown in Figure 3. We note that the AMS dating at 255 cm, although it seems reliable (21.26 ka cal BP; Table 1), was not used in the age model construction because the entire interval below 230 cm consists of sands, interpreted as being of turbiditic origin linked to the rapid sea level rise during the early stages of the last deglaciation [3,33,90] or associated with seismically induced landslide events [98]. Nevertheless, we consider the analyzed sedimentary succession (0–230 cm) above this event as continuous, which is further supported by the fact that equivalent bio-ecozones and the relevant bioevents were recognized in several basins within the entire Mediterranean Sea.

5.4. Diachronous Bioevents for Improving Local Aegean Biostratigraphic Schemes and Their Paleoceanographic Implications

In a regional context, the Aegean Sea represents an ideal basin to develop an integrated biochronological framework owing to exceptionally high accumulation rates and enhanced paleoceanographic information derived from the high-resolution planktonic foraminiferal records throughout the basin since the last glacial–interglacial transition [3,4,5,6,7,8,9,19,88]. Hence, supplementary to the known bioevents of Pérez-Folgado et al. [48], we identified 20 additional bioevents expressed locally in the Aegean Sea (Table 3).
Due to the turbidites’ presence at the base of the core NS-18, only the uppermost part of the Late Glacial period, including the LGM, is present and is ascribed to the bio-ecozone APFE-11 [3]. The most pronounced faunal fluctuation recovered during this timespan is the peak in the abundance of Neogloboquadriniids (bioevent P3; [48]). The maximum in the abundance of both the deep-dwelling representatives N. pachyderma (dex) and N. dutertrei (dex) of this group is indicative of cold and productive waters associated with a well-developed Deep Chlorophyll Maximum (DCM) at the base of the euphotic layer [96,99,100]. The particularity of the LGM in the Aegean Sea is the mixture between cold (G. scitula, T. quinqueloba, N. pachyderma, N. dutertrei, and G. glutinata)- and warm (G. ruber (w) and G. rubescens)-water species. Despite the dominance of cold-water species during that time, noticeable percentages (up to ~40%) of G. ruber (w) were observed in the Aegean in accordance with other sub-basins (e.g., Adriatic, Tyrrhenian, and Alboran; [3,61,101,102,103]), probably reflecting a slight SST rise. A sharp decrease (in the half ~20%; bioevent d1) of G. ruber at 16.2 ka marks the end of the LGM, which is also coeval with the upper boundary of this biozone. Consistent with previous records from the central Mediterranean, the simultaneous increase of G. scitula (bioevent d2) characterizes the Oldest Dryas climatic event and possibly reflects a decrease in salinity [61,91,104]. In the north Aegean, this enhanced faunal proportion of G. scitula has been recorded with two distinct peaks at 17.0 and 15.8 ka [8], while, in the Adriatic, it has been traced at 16.9 ka [61] respectively.
The interval covered by the bio-ecozone APFE-10 corresponds to the first climatic phase of the last deglaciation in terms of the interstadial Bølling–Allerød dated between ~15.0 and 13 ka in the Aegean Sea. A slight increase in the planktonic foraminiferal diversity characterizes this interval dominated by warm-to-temperate species (e.g., G. ruber (w), G. inflata, G. bulloides, N. pachyderma, and G. glutinata). The onset of the Bølling in the NS-18 core is dated at 14.9 ka, comparable to the Greenland ice core records (14.69 ka), as well as other records within the entire Mediterranean [5,8,55,61], and is characterized mostly by the sudden increase of G. ruber (w) (bioevent d3 at the base of bioevent Ra2 at 14.8 ka; [48]) reflecting the climatic amelioration. The increased contribution of this species in the fauna, which almost reached Holocene values, was accompanied by the first occurrence of G. ruber rosea at 14.0 ka (bioevent d4), the re-entry of the warm indicators G. siphonifera and O. universa, and a significant decrease of G. scitula and T. quinqueloba. The G. glutinata shift displayed by maximum percentages of 14% is recorded contemporaneously with the major peak of G. inflata (biooevent I2; [48]) at 13.5 ka. These bioevents, together with the G. truncatulinoides peak at 12.8 ka (bioevent d5), suggest strong vertical mixing within a relatively cool, mesotrophic water column in the south Aegean Sea. The productivity increase during that time is further supported by the significant rise in G. bulloides (bioevent B1; [48]), culminating in a peak (35%) at 12.8 ka.
The bio-ecozone APFE-10 between 12.3–11.6 ka corresponds to the Younger Dryas cooling event revealed by the increase in cool-water and decrease in warm-water species, respectively. The significant contribution of G. bulloides, and N. dutertrei suggest a strengthening of winter convection and strong marine eutrophication, which is further supported by the noticeable occurrence of G. glutinata, an opportunistic species to nutrient availability. Moreover, the progressive reduction in G. inflata and G. truncatulinoides and the increase in Neogloboquadriniids (also evident in the Sicily Channel; [51]) further imply that seasonal water column homogenization favored during the B/A became inhibited during that period. The minimum concentrations of G. bulloides (bioevent d6) and G. glutinata (bioevent d7) at 11.97 ka may indicate mild climate conditions in the middle of the YD, reinforcing the findings about distinct climate phases (two cold phases at the top and the base and a third warm in between, based on terrestrial proxies, in contrast to a more weakened signal with two phases in the marine microfauna records) identified both locally within the Mediterranean basin [8,9,105,106,107] and globally [108,109,110].
This stadial environment appears to alter at 11.6 ka, representing a transitory phase at the beginning of the Holocene until the onset of the sapropel deposition. Τhe abrupt switch to warmer conditions is marked by the near-disappearance of all cold water indicators and the abrupt increase in warm-water species belonging to the SPRUDTS group. This regime in the Aegean Sea is also characterized by the re-occurrence of O. universa at 10.73 ka (bioevent d8), a species which progressively increases its abundance from its lowest values recorded at 10.48 ka [88] and reaches a maximum abundance at 10.36 ka (bioevent d9), as well as the turnover between G. inflata and G. glutinata which has been interpreted in terms of seasonality [9,95]. The increased percentages of T. quinqueloba at 10.73 ka (bioevent Q1; [48]) could be associated with the brief cooling event centered at around 10.5 ka [4,8] and the drop in the warm/cold plot below the sapropel at 10.48 ka [44,88].
By 10.0 ka, the onset of the sapropel deposition represents the Holocene Climatic Optimum testified mostly by the acme interval of both chromotypes of G. ruber (bioevents Ra and R1a; [48]) in the planktonic fauna. However, the detailed planktonic foraminiferal record throughout both sapropel units (S1a and S1b), including the interruption (S1i), allows the identification of several bioevents indicative of the changing environmental conditions, particularly for that marginal basin. Generally, the fauna shows an abrupt increase in warm mixed layer species and the near absence of cold-water species, reflecting a pronounced warming of surface waters that is also portrayed in the Aegean SST records [90,111]. A remarkable feature of the planktonic foraminiferal assemblages is also the distinctive decrease in Neogloboquadriniids within the sapropel, previously recognized as the bioevent Pm [48]. At the base of the bio-ecozone APFE-7 corresponding to the lower part of S1a, a rise in G. glutinata is recorded at 9.2 ka (bioevent d10). Considering the cosmopolitan character of this species together with similar reports from the south Adriatic basin at around 9.0 ka [96], the G. glutinata spike could possibly reflect enhanced primary productivity with the sapropel onset in the southeastern Mediterranean. Such an interpretation is further supported by elevated Ba/Al values measured in many cores retrieved from the Aegean and Adriatic sub-basins [4,44,79,90,112]. The top of S1a (eco-biozone APFE-6; [3]) is characterized by the increase in warm oligotrophic species, as evidenced by the relevant spikes (in order from the oldest to the newest) of G. ruber rosea, G. siphonifera, O. universa, and G. rubescens. Among the above representative species, G. ruber rosea (bioevent d11) and O. universa (bioevent d12) present a peak at the onset of the bio-ecozone APFE-6 (8.87 ka), followed by the G. siphonifera maximum abundance dated at 8.55 ka (bioevent d13), and an increasing trend of G. rubescens centered at 8.23 ka (bioevent d14). The progressive increase in abundance observed for all these members of the SPRUDTS group is indicative of an increase in the depth and extent of the thermocline during the deposition of S1a.
Within the brief interval of the bio-ecozone APFE-5, a significant turnover was documented in the planktonic fauna reflected by the decline in all warm water species and the increase in cold-water species (e.g., G. glutinata and G. inflata), respectively. The reoccurrence of G. inflata during the sapropel interruption S1i was the result of temporary stronger water column ventilation, probably triggered by cold and dry conditions (8.2 kyr event; [113]), as evidenced all over the Mediterranean basin [1,4,8,51,114,115,116]. Vigorous vertical mixing of a relatively cool water column enriched in nutrients is testified by the G. inflata peak (bioevent I; [48]). The mixing was intense to prevent water mass stratification during S1i in the south Aegean Sea. However, the NS-18 record shows that these conditions did not prevail for a long time. The multi-centennial climate deterioration was accompanied by warm climate conditions reflected by the increase in warm indicators during the deposition of S1b (eco-biozone APFE4; [3]). The considerable increase in the SPRUDTS group reflected mostly by peaks of G. trilobus (bioevent d15) and G. siphonifera (bioevent d16) at 6.7 ka and, subordinately, of O. universa (bioevent d17) indicate a shallow pycnocline. The near absence of G. inflata within S1b (particularly at the top; bioevent d18), compared to its previous peak during the S1i, hints that reduced oxygenation in the deep-water column was detrimental to this species, suggesting the persistent lack of winter mixing in a quite stratified (poorly ventilated) water column (supported also by the absence of oxic benthic species). In accordance with similar records along the Aegean Sea, its last entrance in the top of S1b was dated at 6.4 ka [44] or 6.87 ka [88].
After the termination of the sapropel S1 deposition (bio-ecozone APFE-3), G. inflata registers another peak (bioevent s10 at 5.8 ka) before its local disappearance at 5.51 ka [88], which has been ascribed to an overturning reinforcement and reventilating of the Aegean deep waters [117]. Despite the predominance of G. ruber (bioevent R0; [48]), the rise in the abundance of G. bulloides (bioevent B0; [48]) might be explained by phytoplankton blooms induced by enhanced nutrient availability in shallow waters. During the eco-biozone APFE-2, the local deposition of SMH in the south Aegean of between 5.4 and 4.3 ka is the most pronounced environmental perturbation, pinpointed by the sapropel-like fauna. G. ruber alba (bioevent R0; [48]) and G. ruber rosea (bioevent R1c; [48]) are the dominant species, while a significant rise in G. rubescens is also observed at 4.91 ka (bioevent d19). Interestingly, the G. bulloides decline (bioevent d20) is marked at the same time, culminating in the bioevent B0. The NS-18 core recorded significant percentages of G. bulloides and G. trilobus, indicating fresher (sub)surface waters. Overall, the regime of that warm and humid phase is characterized by the strengthening of the summer stratification [80]. Finally, the bio-ecozone APFE-1 is dominated by the G. ruber alba, G. rubescens, G. siphonifera, and G. glutinata assemblage which is consistent with the salinity increase and oligotrophic and dry nature of the water column of the modern Aegean Sea [9,16].

6. Conclusions

We performed here a detailed micropaleontological study to generate accurate biochronological events, considered to be the key requirements for a precise age model construction. The high-resolution micropaleontological study performed on the core NS-18 from the south Aegean Sea provides a continuous record of faunal changes over the last 20 ka, reflected mostly by turnovers and abundance variations in the planktonic fauna during the Late-Glacial-to-Holocene transition. Major changes in the planktonic foraminiferal record, reflected mostly by deglacial appearance/disappearance alterations, display synchronous patterns along the different Mediterranean sub-basins, and, therefore, such bioevents can be used for the establishment of the core chronology. Moreover, downcore variations in the abundance distributional patterns of the most representative species represent additional diachronous bioevents that can be used to refine or extend previous local eastern Mediterranean biostratigraphic schemes. These results, integrated with the existing Aegean bioecozonation, are also consistent with the most important Late-Glacial-to-Holocene paleoclimatic (e.g., Bølling–Allerød, Younger Dryas) and paleoceanographic (e.g., S1 and SMH deposition) events in the eastern Mediterranean, facilitating regional climatic and/or microfaunal correlations. The correlation between the planktonic stratigraphic distribution and paleoclimatic/paleoceanographic conditions expressed by the species-specific ecological preferences further reveal the high potential of this multi-disciplinary approach in stratigraphic, paleoclimatic, and paleoecological studies. However, we emphasize that additional records covering all intra-Mediterranean sub-basins are required to ensure the applicability of this multi-disciplinary proxy.

Author Contributions

Conceptualization, G.K. and A.A.; methodology, G.K., A.A. and M.V.T.; software, G.K.; validation, G.K., A.A., E.B., E.S. and M.V.T.; formal analysis, G.K.; investigation, G.K., A.A., E.B., E.S. and M.V.T.; resources, M.V.T.; data curation, G.K., A.A. and M.V.T.; writing—original draft preparation, G.K.; writing—review and editing, G.K., A.A., E.B., E.S. and M.V.T.; visualization, G.K., A.A. and M.V.T.; supervision, A.A. and M.V.T.; project administration, G.K., A.A. and M.V.T.; funding acquisition, G.K. and M.V.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data used in this work are available upon request from the corresponding author.

Acknowledgments

Τhe APC of this article has been funded by the National and Kapodistrian University of Athens (KE 19432); Internationalization of Higher Education actions MIS 5164455.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Label in Figure 1Core NameReference
1MD84-641[46]
2LC-31[44]
3NS-18This study
4NS-14[3,90]
5LC-21[44]
6KIM-2A[5]
7C69[6]
8C40[7,44]
9SL-9[44]
10SL-11[44]
11SL-21[44]
12SL-31[44]
13SK-1[9,44]
14MNB-3[111]
15AEX-15[4,19]
16AEX-23[4]
17M4-G[118]
18SL-152[119,120]
19Z1[63]
20MD90-917[61]
21IN68-9[44,57]
22CM92-43[59,60]
23GNS84-C106[54]
24KET80-19[46]
25BS79-22[52]
26BS79-38[53]
27ODP SITE-963[51]
28REC13-53[46]
29Hole 963 D[51]
30MD95-2043[48]
31ODP977[48]
32ST5[88]

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Jmse 11 02345 g001
Figure 1. Bathymetric map of the Mediterranean Sea including surface oceanography and the location of the study core (numbered in red), as well as all the additional cores (numbered in white; Appendix A) used for comparison in different sub-basins.
Figure 1. Bathymetric map of the Mediterranean Sea including surface oceanography and the location of the study core (numbered in red), as well as all the additional cores (numbered in white; Appendix A) used for comparison in different sub-basins.
Jmse 11 02345 g001
Jmse 11 02345 g003
Figure 3. Integrated age model (black line), using the calibrated ages of the Mediterranean-wide synchronous planktonic bioevents (s1–s10, grey circles; Table 2), the available AMS datings (green triangles; Table 1), and sapropel layer control points (red diamonds; Table 2), along with the derived sedimentation rates (blue line) of the core NS-18. Dark grey bands correlate to sapropel S1 depositional intervals, while the light grey one corresponds to the equivalent to the SMH interval [90].
Figure 3. Integrated age model (black line), using the calibrated ages of the Mediterranean-wide synchronous planktonic bioevents (s1–s10, grey circles; Table 2), the available AMS datings (green triangles; Table 1), and sapropel layer control points (red diamonds; Table 2), along with the derived sedimentation rates (blue line) of the core NS-18. Dark grey bands correlate to sapropel S1 depositional intervals, while the light grey one corresponds to the equivalent to the SMH interval [90].
Jmse 11 02345 g003
Table 1. Accelerator mass spectroscopy (AMS) 14C datings for the core NS-18.
Table 1. Accelerator mass spectroscopy (AMS) 14C datings for the core NS-18.
Core Depth (cm)AMS Lab CodeConventional 14C Age (yr)Calibrated Age (yr cal BP)2σ Age Range
56–58Beta-2537094690 ± 4047104570–4850
158–160UCIAMS-25686411,020 ± 2511,02011,956–12,572
200–202UCIAMS-25686514,350 ± 3516,47216,144–16,800
254–256Beta-30365118,300 ± 8021,26021,110–21,410
Table 2. Mediterranean-wide synchronous age model pointers, derived from the study core NS-18 and calibrated with other available Mediterranean reference records. Sapropel S1 and SMH layers control points are also added.
Table 2. Mediterranean-wide synchronous age model pointers, derived from the study core NS-18 and calibrated with other available Mediterranean reference records. Sapropel S1 and SMH layers control points are also added.
Synchronous Planktonic Foraminiferal Bioevents within the Mediterranean SeaControl PointsAMS 14CDepth (cm)Age (ka cal BP)
(s1) Last glacial peak of T. quinqueloba 21518.3
AMS20116.47
(s2) First post-glacial occurrence of G. inflata 19515.3
(s3-s4) Last post-glacial occurrence of G. truncatulinoides and disappearance of G. scitula 18514.5
(s5) G. inflata maximum abundance during B/A 17513.5
(s6-s7) T. quinqueloba and G. bulloides peaks during YD 17012.8
(s8) G. glutinata peak at the B/A-YD boundary 16012.3
AMS15912.26
Base S1a 12810
Top S1a 1027.9
(s9) Frequency increase of G. inflata during the sapropel interruption S1i 987.7
Base S1b 947.3
Top S1b 826.4
(s10) G. inflata peak after the end of sapropel S1 745.8
Base SMH 645.4
AMS574.71
Top SMH 494.3
Table 3. Newly defined diachronous planktonic bioevents for the Aegean Sea, derived from the integrated ΝS-18 age model.
Table 3. Newly defined diachronous planktonic bioevents for the Aegean Sea, derived from the integrated ΝS-18 age model.
Newly Defined Diachronous Planktonic Bioevents in Aegean SeaDepth (cm)Age (ka cal BP)
(d1) Decrease of G. ruber (w) at the end of the Latest Glacial20016.2
(d2) Increase of G. scitula during the Oldest Dryas20016.2
(d3) Increase of G. ruber (w) at the start of Bølling–Allerød (base of bioevent Ra2)19014.9
(d4) First occurrence of G. ruber rosea during the deglaciation18014.0
(d5) G. truncatulinoides maximum during the Bølling–Allerød 17012.8
(d6) Minimum abundance of G. bulloides during the Younger Dryas15511.97
(d7) Minimum abundance of G. glutinata during the Younger Dryas15511.97
(d8) Re-occurrence of O. universa with the start of Holocene13810.73
(d9) Maximum abundance of O. universa below the sapropel S113310.36
(d10) Peak in abundance of G. glutinata within S1a1189.2
(d11) Peak in abundance of G. ruber rosea within S1a1148.87
(d12) Peak in abundance of O. universa within S1a1148.87
(d13) Maximum abundance of G. siphonifera within S1a1108.55
(d14) G. rubescens peak within S1a1068.23
(d15) G. trilobus peak within S1b866.7
(d16) G. siphonifera peak within s1b866.7
(d17) O. universa peak within s1b907.0
(d18) Near absence of G. inflata at the top of S1b826.40
(d19) G. rubescens peak within SMH594.91
(d20) G. bulloides decline during SMH (top of bioevent B0)594.91
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Kontakiotis, G.; Antonarakou, A.; Besiou, E.; Skampa, E.; Triantaphyllou, M.V. Towards an Integrated and Accurate Planktonic-Foraminiferal-Deduced Bio-Chrono-Stratigraphic Framework of Late Quaternary Mediterranean Marine Cores. J. Mar. Sci. Eng. 2023, 11, 2345. https://doi.org/10.3390/jmse11122345

AMA Style

Kontakiotis G, Antonarakou A, Besiou E, Skampa E, Triantaphyllou MV. Towards an Integrated and Accurate Planktonic-Foraminiferal-Deduced Bio-Chrono-Stratigraphic Framework of Late Quaternary Mediterranean Marine Cores. Journal of Marine Science and Engineering. 2023; 11(12):2345. https://doi.org/10.3390/jmse11122345

Chicago/Turabian Style

Kontakiotis, George, Assimina Antonarakou, Evangelia Besiou, Elisavet Skampa, and Maria V. Triantaphyllou. 2023. "Towards an Integrated and Accurate Planktonic-Foraminiferal-Deduced Bio-Chrono-Stratigraphic Framework of Late Quaternary Mediterranean Marine Cores" Journal of Marine Science and Engineering 11, no. 12: 2345. https://doi.org/10.3390/jmse11122345

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