The Deep-Sea Record of the Eastern Mediterranean in the Last 150,000 Years

During this time span the Eastern Mediterranean became stagnant five times and was subject to sixteen discrete ash falls, some of them with destructive effects.

INTRODUCTION

Sediments accumulating on the floor of deep-sea basins essentially consist of pelagic oozes, that is to say of numberless microscopic shells of planktonic foraminifers and skeletons of calcareous phytoplankton.

Both groups of organisms live at or near the surface of the ocean, and the mineral parts of their bodies fall to the bottom after death, or reproduction (pelagic fall out).

Because the Eastern Mediterranean is an enclosed basin surrounded by land, a fine terrigenous input (partially wind borne) is almost ubiquitous, being mixed in variable amounts with biogenic components. The resulting "normal" sediment, called hemipelagic, is fine-grained, white to pale brown in color, and extremely rich in micro and nannofossils.

In the last 150,000 years, or approximately 1/12 the total duration of the Quaternary Era, the Eastern Mediterranean underwent several significant changes and sometimes experienced sudden events, many of which are faithfully recorded in the sediment record, and which can be deciphered with appropriate techniques.

The events include:

(a).   A number of regional and global climatic fluctuations, including the coldest and warmest episodes ever recorded in the entire Quaternary era

(b).   Five episodes of abyssal stagnation, resulting in the deposition of organic-rich muds following the annihilation of bottom-dwelling and deep-swimming organisms, and

(c).  Sixteen ash falls, some of them widespread, of large volume and with destructive effects

MATERIAL

Deep-sea sediments are recovered by oceanographic ships either by means of tethered piston-corers, or by wire-line coring through drill-stems.

More than a hundred piston cores of 2 to 17 meter lengths have been raised from the Eastern Mediterranean by different institutions; only a limited number of these cores, however, has been investigated in detail.

Moreover, 12 drill-sites of the Deep Sea Drilling Project have penetrated 50 to 700 meter thick intervals of Quaternary sediments: seven from the Glomar Challenger drilling campaign of 1970 (Ryan, Hsü et al. 1973), and five from the 1975 campaign (Hsü, Montadert et al. 1977).

TECHNIQUES

The techniques used to investigate the deep-sea record are multiple and refer to different disciplines. The lithology of the "normal" sediment is studied with a reference to

(a).   the biogenic components, often dominant, and

(b).   the terrigenous components.

Routine analyses include textural and mineralogical observations, in particular:

1. Quantitative evaluation of the various grain-size fractions, that is clay (< 2 microns), silt (< 2 μ > 63 μ), and sand (> 63 μ);
2. Carbonate content
3. X-ray mineralogy and X-ray fluorescence with identification of the major and minor elements.

Besides the "normal" sediments and occasional turbidities two lithologies are peculiar to the Eastern Mediterranean deep-sea record. They are tephra and sapropel, and are very important because they represent strictly isochronous events. Their study requires techniques different from those used to investigate normal hemipelagic sediments.

• Tephra (Volcanic ash) is studied with analytical methods suitable for identifying its composition and volcanic province. Silica, alkali to calcium ratio and sodium to potassium ratio are measured on shards of volcanic glass. The refractive index is also useful in identifying tephra layers: it is measured on glass shards and is related to silica content (Keller et al. 1978).
• Sapropels (Organic-rich muds) contain organic carbon in unusually high abundance, up to 16% in weight (Kidd, Cita & Ryan, 1977). Authigenic minerals, especially pyrite, are well represented. The clay mineral composition of sapropels is usually different from that of the intercalated normal sediments: chlorite is unusually abundant, illite is better crystallized and smectite is less abundant (Chamley in Cita et al. 1977).

• Radiometric dating of deep-sea sediments is difficult to apply for the interval under discussion. In fact, the C¹⁴ method, which is very popular, easy and inexpensive, can only be used for the topmost part of the fossil record, younger than approximately 50,000 years. The abundance of both carbonaceous and carbonatic components in the Mediterranean deep-sea sediments renders this method highly suitable. However, the extrapolation downcore of sedimentation rates calculated on the basis of radiocarbon ages obtained from the topmost part of the cores has proven fallacious (Ryan 1972; Cita et al. 1977). Notwithstanding the occurrence of several K-rich tephra layers, the K/Ar method is not applicable, its reliability being too low for ages younger than 1 X 10⁶ years. Fission tracks dating has been used successfully on land exposures (Bonadonna & Bigazzi 1970), but not for the deep-sea record so far.
• Micropaleontology is carried out routinely on planktonic foraminifers and calcareous nannofossils in "normal" sediments, whereas palynology is specially suited for the study of sapropels, where pollens and spores are preserved in unusual abundance. Due to the briefness of the time span considered, classical micropaleontology based on the study of evolutionary changes recorded in the fossils as a function of time can not be applied. No detectable and univocal evolutionary trend can be recorded in the late Pleistocene in any of the over 20 species of planktonic foraminifers, all of them still living today. In terms of calcareous nannofossils, the first record of Emiliania huxleyi, with an interpolated age of 0.27 m.y., falls slightly below the interval under discussion, whereas the first abundant record of that taxon (base of Emiliana huzleyi Acme-zone of Gartner, 1977) is only 7,000 years old. Quantitative micropaleontology based on planktonic foraminifers (Parker 1958; Luz & Bernstein 1976; Cita et al. 1977 inter alios) proved competent to decipher the climatic record of the Eastern Mediterranean: the observations concern the relative abundance of warm-water indicators versus cold-water indicators in the fossil assemblages. This technique also showed the interaction of a factor other than climate (see below) in affecting the populations recorded at or near some of the sapropel layers. Though spores and pollens of terrestrial plants are extraneous to the pelagic realm, they are recorded in abundance in the sapropel layers. Quantitative palynological studies (Rossignol - Strick 1973) demonstate that the mechanisms of transport of the pollen grains to the sea-floor are multiple.
• Isotopic geochemistry applied to the composition in stable isotopes of Oxygen and Carbon, as measures on calcitic tests of monospecific assemblages of planktonic foraminifers, permits one to evaluate the climatic evolution independently from the faunal evidence discussed above. This technique was first used by its founder Cesare Emiliani in the early days of isotopic geochemistry (Emiliani 1955) and has been used quite recently to investigate the long piston cores from the Eastern Mediterranean (Vergnaud - Grazzini in Cita et al. 1977; Ryan & Cita 1977; and Thunell et al. 1977). The isotopic signal as recorded in the Eastern Mediterranean is similar in shape and timing, but much greater in amplitude than that recorded in the open ocean (Emiliani 1971; Shackleton & Opdyke 1973).

THE CLIMATIC RECORDS

The last 150,000 years of the history of our planet includes the latter part of the ice ages and the Post-glacial or Holocene. This is a time of drastic and rapid climatic change. That climatic changes are induced by astronomic factors in the earth's orbit, including obliquity and precession, has recently been substantiated by CLIMAP investigators after several years of integrated efforts (McIntyre et al. 1976; Hays et al. 1976).

Global effect

Climatic fluctuations in an oceanic realm result (Figure 1) in changes in surface water temperature induced by changes in ocean circulation and convection, by the temperature of the air, and by the opaqueness of the atmosphere. Ocean surface temperatures in turn result in:

(a).   changes in qualitative and quantitative composition of planktonic foraminiferal assemblages (Faunal Signal), and

(b).   changes in isotopic composition of shells of planktonic foraminifers (Isotopic Signal)

Extreme climatic fluctuations on land result in glaciation or deglaciation. Besides the possibility of freezing inland seas, continental glaciation and/or deglaciation also affects the oceanic realm in two respects (Figure 1).

Glaciation results in:

- eustatically induced lowering of the sea-level up to -120 meters (regression), which in turn results in:

- enhanced circulation

- higher oxygenation of bottom waters

- higher organic productivity, as well as

- enhanced erosion due to lowering of base-level of erosion and thus

- higher sedimentation rates

Deglaciation results instead in:

- eustatically induced rise of sea-level (transgression), which in turn results in:

- reduced circulation

- reduced oxygenation

- reduced organic productivity, as well as

- reduced erosion, and

- reduced sedimentation rates

Glaciations and/or deglaciations also result in changes in isotopic composition of sea-water, which is recorded by the Isotopic Signal. Melt-water is isotopically lighter than sea-water. That changes in isotopic composition as recorded in foraminiferal shells reflect changes in ice volume more than changes in paleotemperature at the surface of the ocean was argued by Broecker and Van Donk (1970), and recently proved by Shackleton and Opdyke (1973) by means of investigations carried out on bottom-living foraminifers.

Eastern Mediterranean interactions on the global climatic record

The global effects discussed above interact in the Eastern Mediterranean with other factors which are peculiar to the area. For example, the Mediterranean is an enclosed basin, communicating with the Atlantic Ocean by means of the Gibraltar straits, whose mean depth is only 350 m. below sea-level. This east-west trending oceanic seaway which played a fundamental role in controlling oceanic circulation and the distribution of marine life in the Mesozoic, ceased to exist as an unrestricted passageway after the alpine orogeny. Counter-clockwise rotation of the African plate interrupted the connections with the Indian Ocean by middle Miocene times, over 15 m.y. ago. Since that closure, the Mediterranean Behaves as an Atlantic gulf.

The Eastern Mediterranean in particular, being separated from the Western by one more intervening threshold (the Sicily straits, some 450 m deep) is subject to excess evaporation resulting in:

(a).   enrichment of isotopically heavier Oxygen (Isotopic Signal higer than in the open ocean), and

(b).   higher salinities (Figure 1).

During period of deglaciations, which are thought to coincide with times of rising sea-level, the Eastern Mediterranean received melt-water deriving from the front of the ice-cap covering large parts of central and northern continental Europe (Figure 2). Discharge from this fed the Black Sea via the Dnieper and Don drainage systems. Moreover, melt-water from the Alpine glaciers was largely conveyed to the Eastern Mediterranean by means of rivers originating from the southern slopes of the Alps via the Adriatic, and from the Danube drainage system via the Black and Aegean Seas.

Many parts of the world experienced a great increase in precipitation during the transition form glacial maximum to the late glacial and post-glacial warm-up. The increase in humidity in Europe is in part exemplified by the transition from cold steppe to open grasslands by 12.000 years and to shrub land and eventually forests by 11.000 years. In Africa and on the Levant coast of the Middle East the increase in regional humidity at 9.000 years is revealed by the very high levels of enclosed lakes (Street and Grove 1976). The strength of the previous arid phase prior to 14.500 years is attested by the almost complete drying up of the White Nile (Williams and Adamson 1974) when the level of Lake Victoria was so depressed as to prevent its outflow across Murchison Falls.

Excess meltwater and enhanced pluvial conditions both result in dilution of the superficial water layers, affecting the foraminiferal faunal assemblages (Salinity Faunal Index) and the isotopic composition of their test (Isotopic Signal), see Figure 3.

Density stratification induced by a global effect (decreased circulation, see above) summed with three local effects (reduced evaporation, enhanced precipitation and excess melt-water) resulted in cyclically repeated episodes of stagnation (Ryan and Cita 1977). In absence of active circulation at depth, bottom waters are oxygen depleted or entirely devoid of oxygen (oxygen crises) and consequently barren. Organic matter falling to the sea-floor from the surface can be neither decomposed by bacteria, nor consumed and recycled, and accumulates in form of organic-rich muds. Sapropels are the sedimentary expression of such stagnations, each one lasting a few to several thousand years.

The deep-sea record from the Eastern Mediterranean is subject to another interaction related to volcanic activity. Indeed, several areas located in the Eastern Mediterranean were subject to intense fallout of volcanic dust put into the atmosphere by violent eruptions in the volcanic arcs of Italy and Greece.

The Hellenic arc, in the Aegean, has a volcanic belt active today and in the recent past. Besides the products of this local volcanism, the Eastern Mediterranean deep-sea record also contains the expression of volcanic activity which occurred in areas located to the west, as the Campanian and Romana provinces of the Tyrrhenian coast of Italy, or the Eolian province, whose relationships with an island arc are postulated by Barberi et al. (1973, 1977) and by Keller et al. (1977).

Tephra layers as detectable in piston cores are centimetric to decimetric beds consisting of sand - and silt-sized shards of volcanic glass either pure, or mixed with biogenic components.

RECORD OF THE LAST 150.000 YEARS

The paleoclimatic and paleooceanographic reconstruction of the Eastern Mediterranean, as recorded in the following pages, is based on detailed, multi-disciplinary investigations carried out by the present authors and their associates (Ryan 1969, 1972; Cita et al. 1973; Cita d'Onofrio & Zocchi 1974; Keller et al. 1977; Cita et al. 1977; Vergnaud - Grazzini, Ryan & Cita 1977) to which reference is made. It also takes into account the pertinent literature on the subject, though not fully referenced.

The time-scale adopted here is followed by an increasing number of scientists belonging to different schools; however the present authors are fully aware that it is not yet generally accepted (see Hieke 1976; Stanley & Maldonado 1977 inter alios). As in any historical reconstruction, a chronological framework is essential in deciphering the record and in understanding the cause-and-effect relationship. Therefore, we want to clarify first of all the data points, and the interpolations on which our chronological framework is founded. From top to bottom, they are as follows (see Figure 4):

• Sapropel S1, dated with C¹⁴ methods at 9,000 years B.P. (several papers, including Stanley and Maldonado 1977).
• Ischia Tephra, dated with the K/Ar method at 41,000 ± years B.P. (Keller et al. 1977).
• Sapropel S 5, which corresponds to isotopic Stage 5e and Faunal stage 5e, is correlatable with the second Strombus raised beach of the Tyrrhenian interglacial, which has been dated radiometrically by U/Th techniques at 125,000 years (Bonadonna & Bigazzi 1070).
• The lowest occurrence of Emiliana huxleyi, which is calibrated at 170,000 years in equatorial Pacific piston cores (Gartner 1977), has been recorded in between Sapropels S 7 and S 8 in the Eastern Mediterranean (Sigl & Müller 1975).

All these findings are consistent between themselves, and are consistent with the highest occurrence of Pseudoemiliania lacunosa, recorded beneath Sapropel S 11 in Core RC9 - 181 and calibrated at 370,000 years (Gartner 1977). Since both tephra layers and sapropels are isochronous lithologies, representing geologically instantaneous events, these age determinations can be correlated from core to core with accuracy, over distances in excess of 1000 km.

Having clarified the time-scale problem, we can now comment on the record itself. From bottom to top, the most significant steps, as reconstructed from continuous lithological observations (that is to say by visual description of the sediment cores, and recognition of characteristic, isochronous lithologies), and from discontinuous micropaleontological and geochemical observations (that is to say by quantitative studies or measurements carried out on closely spaced samples, in order to assure a data point every 3 - 4000 years) are as follows:

• Some 150,000 years ago, the Eastern Mediterranean was characterized by well ventilated, cold superficial waters. We were in the latest part of a major glacial period (Riss glaciation of the Alpine terminology, Stage 6 of Emiliani 1955; Zone W of Ericson and Wollin 1968) and sediments deposited on the floor of deep basins yielded rich populations of cold-water planktonic foraminifers. Superficial waters were several degrees centigrade cooler than at present, though not as cold as they were some 20,000 years. ago. An ash fall originated from the Roman province (W - 1 of Keller et al. 1977) and it is recorded in several piston cores investigated from the Ionian basin, but not from the Levantine basin.
• A sudden and extremely rapid warming trend initiated after 130,000 y. B.P.: in a few thousand years it brought the Mediterranean waters to the highest temperatures ever reached (Stage 5e, well detectable both in the isotopic signal and in the faunal signal, see Figure 3), distinctly warmer than today. Calculations by Vergnaud-Grazzini (in Cita et al. 1977) indicate that the temperature change from Stage 6 to Stage 5e, as recorded by isotopic change, and corrected for glacial effect and for evaporation effect, is 8 ^oC. In all the cores investigated isotopically from the Eastern Mediterranean and represented in Figure 4, the isotopic change at the transition from Stage 6 to Stage 5e is the highest of the entire Pleistocene. Isotopic values for 5e are much lighter than in the open ocean, and this is accounted for by excess melt-water entering te Eastern Mediterranean  (see Figure 2) and by enhanced kataglacial pluvial activity. The effect of excess freshwater is to enhance density stratification, which results in stagnant conditions. Indeed, Sapropel S 5, which coincides with Stage 5e, is one of the thickest sapropels of the 12 recorded in the Late Pleistocene of the Eastern Mediterranean, but the foraminiferal faunas are almost normal (see Table 3 in Cita et al. 1978).
• Stage 5 (= Tyrrhenian stage of the chronostratigraphic sequence of marine stages = Riss/Würm interglacial of the Alpine terminology = X Zone of Ericson and Wollin 1968) lasted from approximately 130,000 to approximately 85,000 years B.P. It is characterized by three sapropel layers (S 5, S 4 and S 3 of Cita et al. 1977) and by six tephra layers of Italian origin (Campanian X-6 and X5; Etna X-4; Aeolian X-3; Campanian X-2) and one of Greek origin (Hellenic X-1, see Figure 5).The last is the only one recorded in piston cores from the Levantine basin. Three warm peaks (substages 5e, 5c and 5a) are recognizable both in the isotopic and in the faunal curves (Figure 3).The salinity faunal index (the percentage abundance of Globigerina eggeri, illustrated in Figure 6, a foraminiferal species whose distribution indicates low salinities, see Ruddiman 1971; Ryan 1972; Vergnaud-Grazzini et al. 1977) displays a marked freshening in correspondence with Sapropel S 4, and other minor freshenings within Stage 5. A time-lag between isotopic signal and faunal signal, with the isotopic signal coming first, and coinciding with peaks of the salinity faunal index, suggests that the increase in local Mediterranean superficial water temperature was delayed by excess melt-water, cold, fresh and isotopically depleted, deriving from north-European ice caps (see Figure 2).
• After 85,000 y. B.P. a new cold period began. It lasted approximately 60,000 years, encompassing Isotopic Stages 4 through 2 of Emiliani (1955), (see also Emiliani & Shackleton 1974). It corresponds to the Würm glaciation of Alpine terminology and to the Y Zone of Ericson and Wollin (1968). Once again the Mediterranean waters became cool and well ventilated. Sapropel layers, the sedimentary expression of stagnant conditions, which represent up to 20% of the pelagic sequences in Stage 5 (Zone X), disappear. Only a thin sapropel (S 2) is recorded in this interval, in correspondence with a minor warm peak (isotopic stage 3) probably corresponding to an interstadial. The sapropel is so thin that it can be destroyed by the activity of burrowing organisms.

On the cooling trend of Stage 3, we record the thickest and most widespread tephra layer of the Eastern Mediterranean deep-sea record, thought to have originated from the Citara - Serrata eruption in the island of Ischia (see Keller et al. 1977). It is a marker bed in the latest Pleistocene of the Eastern Mediterranean (see Figure 4). Besides the Ischia Tephra, six additional tephra layers are recorded within this interval (see Figure 5). The explosive volcanic activity is increasing both in the circum-Tyrrhenian volcanic provinces and in the Aegean. The first and second Santorini tephra layers (Y-4 and Y-2 of Keller et al. 1977) are recorded within Stage 2, with interpolated ages of 30,000 and 20,000 years respectively.

The latest part of this time-interval, at approximately 20,000 - 18,000 years, is the coldest ever experienced by the Mediterranean.

Immediately after the coldest peak, a climatic reversal occurs, leading to deglaciation and to the interglacial period in which we are living. The increase in temperature is rapid and marked, though not as large as the transition from Stage 6 to 5e : indeed, according to the calculations of Vergnaud - Grazzini (in Cita et al. 1977) the isotopic change, corrected for glacial effect and for evaporation effect, indicates a temperature increase of only 4 ^oC at the transition from Stage 2 to Stage 1.

Once again the Eastern Mediterranean became stagnant (Sapropel S 1); the event occured on the warming trend of the climatic cycle, and is dated at 9,000 years B.P.

Two important ash falls, both with destructive effects, occurred after the deposition of Sapropel S 1:

1. the Minoan Santorini eruption (Z-2 of Keller et al. 1977), which annihilated the Minoan culture, and dates back to 3,5000 years B.P. (Ninkovich & Heezen 1965; Keller 1971), and
2. the Vesuvius eruption (Z-1) of Keller et al. (1977), which is stratigraphically higher than the Minoan Santorini tephra, and has an interpolated age of 3,000 years B.P.
   

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