Messinian event in the black sea

Palaeogeography, Palaeoclimatology, Palaeoecology, 29(1979/1980): 75-93 o Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

75

MESSINIAN EVENT IN THE BLACK SEA’ KENNETH J. HSU and FEDERICO GIOVANOLI Geological

Institute,

Swiss Federal Institute

of Technology,

Ziirich

(Switzerland)

(Received July 20, 1979)

ABSTRACT Hsu, K. J. and Giovanoli, F,, 1979. Messinian event in the Black Sea. Palaeogeogr., Palaeoclimatol., Palaeoecol., 29: 75-93. Three holes were drilled during the 1975 DSDP Leg 42B drilling the Black Sea. A section from Hole 380, at 2107 m water depth on the western edge of the abyssal plain, is 1074 m thick, and provides the most complete stratigraphic section. Dating of the sediments is based upon (1) fossil evidence from pollen, crustaceans, benthic foraminifera, and diatoms, (2) correlation with climatio changes and with unusual isochronous events that have been dated elsewhere, (3) paleomagnetic data, and (4) estimates of sedimentation rate. The history of Black Sea sedimentation recorded by the DSDP cores includes black shale sedimentation during the Late Miocene, followed by periodic chemical sedimentation from Late Miocene to Early Quaternary, and a change to dominantly terrigenous sedimentation from the Middle Quaternary. These hemipelagic and turbiditic sediments were deposited in lacustrine and brackish marine environments. The Messinian sediments, however, consist of stromatolitic dolomite, oolitic sands, and coarse gravels, deposited in supratidal and intertidal environments. The intercalation of the shallow-water sediments in a deep-water sequence suggests a drastic lowering of the water-level within the Black Sea basin during the Messinian so that the edge of the present abyssal plain was then the edge of a shallow lake. The Messinian draw-down phase of the Black Sea was in existence for about 100,000 years during the Lago-Mare stage of the salinity crisis. The evaporated waters formed an alkaline lake before it was drowned by a brackish marine transgression correlative to the Trubi transgression of the Mediterranean.

INTRODUCTION

The Black Sea and the Red Sea might be considered as two satellite basins of the Mediterranean. Continuous seismic reflection discovered a very strong S-reflector at a few hundred meters depth subbottom in the Red Sea (Coleman, 1974). The S-reflector bears considerable resemblance to the M-reflector of the Mediterranean. Like the latter, the reflector is identified as the top of an evaporite formation in the Red Sea dated as Messinian by Leg ’ ContributionNo. 132 of the Laboratory of Experimental Geology, E.T.H., Zurich (Switzerland).

76 SW E1 ,, NE

*

PROFILE 21

0(

7

I?

aiioyE~~s Continuous seismic reflection profile showing the stratigraphy of the Black Sea sediments near Sites 380 (designated B-2 on the profile) and 381 (projected location). The first prominent subbottom reflector at Site 380 probably marks the Middle Quaternary transition from carbonate to terrigenous mud sedimentation there (Letouzey et al., 1978). The first prominent subbottom reflector in the area near 381 is the top of the Messinian, which plunges down to about 900 m subbottom depth in the area near Site 380. Note that the Neogene sequence at Site 381 is thinner than that at Site 380, because much of the Quaternary there has been removed by down-slope slumping.

ng.1.

23 deepsea drilling (Stoffers and Ross, 1974). Geophysical surveys of the Black Sea revealed the presence of several subbottom reflectors (Fig.1 ), but none of them seemed to be correlative with the M-reflector (Letouzey et al., 1978). It has been thought that brackish sedimentation continued during the Late Miocene, and the Messinian event did not affect this Paratethys basin. The Leg 42B drilling was scheduled primarily to sample the thick Quaternary sequence of the Black Sea, to establish a paleoclimatic record, and to study diagenetic changes of organic matter in sediments (Ross et al., 1978). It was recognized that the geological history of the Black Sea should have been influenced by its relation to the Mediterranean, but we could not predict the expression of the Messinian event in the Black Sea. Three sites were drilled in 1975 and more than 2000 m of the Neogene . sequence were continuously cored (Fig. 2). The most complete stratigraphical sequence was encountered at Site 380, where Hole 380k reached 1074 m subbottom. Unfortunately, except for some Quaternary nannofossils found near the top of the section, no chronostratigraphically diagnostic marine planktonic fossils have been found. Different lines of approach have been used and there have been considerable differences in chronostratigraphical interpretations (Ross, 1978).

77

t Fig.2. Correlation of the lithologic units of the Black Sea sequences at Sites 379, 380, and 381.

The Black Sea sediments obtained by DSDP are mainly deep-water hemipelagic, lacustrine, or brackish marine muds and marls. The only unusual sediments are an association of oolitic sands, gravels and stromatolitic carbonates encountered at the 864-884 m interval in. Hole 380A. Paleontological and paleomagnetic evidence, as well as considerations of sedimentation rates, suggested that these unusual sediments are Pliocene or Late Miocene in age (Ross, 1978). The only unusual event that might be responsible for the genesis of those unusual sediments was the Messinian salinity crisis in latest Miocene (Hsii, 1978a). This opinion was, however, not shared by all other shipboard scientists of Leg 42B (see Ross, 1978; Stoffers et al., 1978). The purpose of this paper is to recapitulate the criteria adopted for the recognition of the Messinian event in the Black Sea, and to present additional new evidence to support the hypothesis linking an episode of

78

unusual sedimentation in the Black Sea basin to the desiccation of the Mediterranean. CHRONOSTRATIGRAPHY

AT SITE 380

‘The history of the Black Sea sedimentation recorded by the DSDP cores is shown diagrammatically by Fig.3. The earliest record was represented by brackish marine sedimentation of black shales when the climate was subthe

LithOlO~k

UllltS Ewch

Pig. 3. History of changes in sedimentation, .in climate, and in water salinity of the Black Sea. Shown on the farthest left of the diagram is an interpretation of the paleomagnetic stratigraphy. The measurements shown however, indicate that the sediments in some intervals (stippled) have acquired positive polarity because of diagenesis (see text for explanation). Note also that the boundary between the Pliocene and the Alpha Glacial sediments at 650 m is not the Pliocene-Pleistocene boundary, which should be located at 615 m because the Alpha Glaciation began during the Late Pliocene.

79

tropical. This was followed. by periodic chemical sedimentation, when laminated carbonates, aragonite, chalks and siderite units were laid down, in brackish or fresh-water lakes; the climate was then becoming cooler and continental glaciation started in northeastern Europe as the first lacustrine chalks were precipitated. The dominant lithology of the sediments deposited during the last period of oscillating climatic changes was terrigenous mud. Since chronostratigraphically meaningful planktic fossils are practically absent, the dating of the more than 1000 m of the Black Sea sequence is at best a working hypothesis. One could try to match the epochs of glaciation as suggested by the pollen data with the glacial stages in the Black Sea region or those in northwestern Europe (see Ross, 1978). This approach makes improbable assumptions of a completeness of records everywhere, and an exact correspondence of climatic stages, and has led to a conclusion that the bottom of the sequence was still as young as Pliocene (Stoffers et al., 1978; Koroneva and Kartashova, 1978). This interpretation has been proven wrong by paleontological evidence to be discussed in the following sections. The philosophy adopted by us is to accept first the paleontological evidence. A second step is to refine the dating mainly on the basis of climatical history. The chronostratigraphy thus constructed is then checked by paleomagnetic results, and with consideration of sedimentation rates.

Fossil evidence Brackish marine benthic foraminifera, belonging to Ammonia and EZphidium, including endemic forms typically found in Paratethyan basins of eastern Europe, have been found near the bottom of Hole 380A in the 978-1054 m interval (Gheorghian, 1978). Associated with those are mysid statoliths of Purumysis mihaii; they have been used as index fossils of the Sarmatian for the sediments of central and eastern Paratethys (Gheorghian, 1978). The age of the Sarmatian has been determined by radiometric dating to range from 10.5 to 13 m.y. (R&g1 et al., 1978), and is equivalent to the latest Serravallian and Early Tortonian of the Mediterranean stages (Fig.4). This Late Miocene age of Hole 380 sediments at about 1000 m subbottom depth has been confirmed by palynological evidence (Traverse, 1978). The diatom evidence further confirms this chronology. Schrader (1978, p. 856) identified a Late Miocene diatom flora in the oldest sediments at Site 380. Jouse and Mukhina (1978, p. 909) found in the cores of the 845-865 m interval (at Site 380) just above the presumably Messinian gravel unit, marine diatoms typical of the Pontic and Kimmerian floras of the Taman Peninsula; they added that this age corresponds to the close of the Late Miocene (Messinian), or the earliest Pliocene (see Fig.4), because nannofossils defining zone NN 11 have been found in Lower and Middle Pontian, and zone NN 12 in uppermost Pontian and Kimmerian sediments (Semenenko, 1978). The brackish marine sediments above the gravel are thus an expression

80

BIOSTRATIGRAPHIC CORRELATION OF CENTRAL PARATETHYS, EASTERN PARATRTHYS AND MEDITERRANEAN NEOGENE STAGES

7

Biostratigr. Zonations I Mediterranean

E I B ;" .j

5

s

z

.i

2

B w"

3 3

j

PLIO1 CENE NN13 s- _

;:;y

N19‘1. ZANCLIAN

NNl2 _=

Central Paratethys sages

Eastern hratetilys Stages

ROMANIAN DACIAN

KIMMERIAN

PONTIAN

PONTIAN

NlS=. MESSINIAN N17

NNll

_ TORTONIAN

1 lo-

!_

N16

lNNlO

PANNONIAN

--i

I--

1 MEOTIAN N AN

SARMATIAN

SERRAVALLIAN

BADENIAN

1

KONKIAN KARAGANIA TSHOKRAKI AN TARKSANIAN

N

LANGHIAN RARPATIAN BURDIGALIAN OTTNANGIAN

KOZACHURIAN

EGGENBURGIAN

SAKARAULIAN

EGERIAN

CAUCASIAN

2

AQOITANIAN

25-

NNl

N4

NP25

/P22

Pig.4. Correlation of Black Sea DSDP section at Site 380 to Mediterranean and Paratethys stages. The correlation of the central Paratethys and the Mediterranean is based upon Rogl et al. (1978). The correlation of the eastern Paratethys sections to the Paratethys is based upon Papp (1969), and Semenenko (1978). The correlation of DSDP cores to the Paratethys is the working hypothesis of this paper. Gheorghian (1978) found in DSDP Unit V black shales, brackish marine benthic faunas that are similar to those in the Bessarabian and Volhnian of Paratethys basins; this suggests an age of 12 or 13 m.y. for the bottom of the oldest sediments.

of the latest Pontian and Early Kimmerian transgression, which is well known from land sections on the Caucasus (Tchelidze, 1974). Comparison of diatom assemblages of the Black Sea cores with those on land by Joust5 and Mukhina (1978) placed the Plio-Pleistocene boundary at 615 m at Hole 380A. This boundary has been dated to about 2 m.y. on the basis of paleomagnetical evidence on land (just above the Matuyama-Gauss contact; Kochegura and Zubakov, 1977). Diatom assemblages defined the top of the Ponto-Caspian Stage Chauda (Giinz, or Mindel Glacial) at 360 m in Hole 380A, which has been dated paleomagnetically as being 0.6 m-y. of age (Kochegura and Zubakov, 1977). Jouse and Mukhina recognized other Ponto-Caspian Quatemary stages, including the Karangat (Riss-Wiirm interglacial) at about the 50-100 m interval.

8 3.

In summary, we feel that the paleontological data have constraints on the chronostratigraphical boundaries at Site we are still in need of more exact datum horizons. The best age of about 10.5-13 m.y. for the sediment at the bottom (H&u, 1978a).

given us broad 380, even though estimate gives an of Hole 380A

Climatic history

The climatic changes of the region surrounding the Black Sea can be determined by pollen investigations. A correlation of the regional paleoclimate with the global climate permits a tentative identification of some datum horizons. The methodology of expressing climatic variations quantitatively has been resolved by Traverse (1978) through his introduction of the concept of steppe index. The index is defined by the relation

S.I. =

% steppe pollen steppe pollen + forest pollen

x 100

The steppe index can also be expressed by a curve of sliding averages. The individual values of the index at Site 380 and their five-point sliding averages are shown by Fig.3. The latter give an insight into overall climatic changes, but such averages tend to eliminate some short interludes of real changes. Two “milestones” in the climatic changes are located at 780 m and 650 m, respectively (Fig.3). The first represents the first significant climate cooling, which resulted eventually in a replacement of forests by steppes, so that the steppe index of sediments became as high as 50. The second represents the beginning of an even more intense cooling, when continental glaciation to an extent comparable to that of Late Quatemary began to take place. Eventually the steppe index reached almost 100, as forests were-completely replaced by grass, shrubs, and other steppe vegetations. Studies of marine climates as recorded by the sediments of piston cores suggest that the two datum horizons might be dated as 3.2 m.y. (end Gilbert Magnetic Epoch) and 2.5 m.y. (end Gauss Magnetic Epoch) respectively (Berggren, 1972; McDougall and Wensink, 1966; Shackleton and Kennett, 1975; Shackleton and Opdyke, 1977). Shackleton and Opdyke, for example, found isotopic evidence that glacial and interglacial fluctuations have characterized the Earth’s climate for the past 3.2 m.y., and that the intensity of glaciation increased to that comparable to the Late Quaternary some 2.5 m.y. ago. A further similarity of the climatical histories as recorded by the Black Sea cores and by the marine sediments is afforded by a comparison of the sliding averages of steppe index values at Site 380 with the sliding averages of winter-temperature variations at Equatorial Atlantic established by Briskin and Berggren (1976) on the basis of their microfaunal analyses: the Glacial

82

Stage Alpha is correlative with the first two and the Glacial Stages Beta and Gamma with the last two glacial stages recorded by the Atlantic cores (see Hsii, 1978a, for details). Tentative correlation of individual oscillations at Site 380 with the climatical epochs established by Emiliani (1966) and by Shackleton and Opdyke (1977) has been made for the upper parts of the section (Fig.3). The one epoch that can be correlated with certainty is a brief, yet intensely warm episode recorded by the floras in the sediments 60-100 m sub-bottom (Fig.3). This is correlative to the well-known Climatic Epoch 5 of Emiliani (1966), 80-l 20 thousand years before present. This interglacial epoch is also known as the Karangat Stage in the terminology of Black Sea shoreterraces, as the Riss-Wiirm Interglacial in the Quatemary terminology of Central Europe, and as the Eemian in northwestern Europe. At that time, the Dardanelles Strait connecting the Black Sea and the Mediterranean had been eroded down to about the present level, so that the Mediterranean water could then, as now, enter and bring in brackish marine organisms including the Gephyrocupsa carribbeanica nannoflora (Percival, 1978), and the Thalassiosira diatom flora (Jous& and Mukhina, 1978). Counting peaks and valleys of the steppe index, we estimate that the Climatic Epoch 19 of Shackleton and Opdyke (1977) should fall somewhere between 300 and 400 m sub-bottom at Site 380 (Fig.3). This Epoch 19 should be about 0.7 m.y. old, and should mark the Brunhes-Matuyama transition. Farther down the sedimentation rate is low and the sampling interval of Black Sea cores (1 per 9 m) has been too widely spaced to permit a definite comparison of the data from older sediments with the marine data. Correlation with unusual events in nearby regions Two major events in the history of the Black Sea sedimentation are (1) the deposition of the gravel unit, and (2) the transition from chemical to detrital sedimentation. The gravel unit is assumed to have been deposited during the Messinian (Hsi_i, 1978a). This would place the top of the Miocene, or the transition from Magnetic Epoch 5 to Gilbert Magnetic Epoch at 864 m subbottom. Such an age assignment is in perfect accord with the diatomchronostratigraphy of Jouse and Mukhina (1978). The transition from dominantly carbonate to dominantly terrigenous sedimentation, marked by the bottom of the terrigenous mud unit at 325 m subbottom, was caused by a sudden influx of detritus from the Danube after it took its present course sometime during the Middle Quaternary. This event cannot be accurately dated, but available evidence from the Danube Delta region suggests that the 325 m horizon should fall within the Chauda Stage of the Ponto-Caspian region (I-Is& 1978a), which ranges from 0.6 to 1.0 m.y. of age (Kochegura and Zubakov, 1977). Diatom stratigraphy also supports this conclusion (Jouse and Mukhina, 1978).

83

Paleomagnetic stratigraphy Data presented so far permitted us to recognize several datum horizons and to construct a theoretical paleomagnetic stratigraphy for the sediments at Site 380, as shown by the farthest left column in Fig.3. The adjacent column is the polarity of samples as indicated by direct measurements. Good NRM (natural remnant magnetism) is present in practically all the cores studied. The results show considerable discrepancies between the prediction and measurements. We have found evidence that diagenetic formation of magnetic minerals may have greatly complicated the problem of magnetic stratigraphy. This problem is particularly difficult at Site 380, where we identified a magnetic sulphide’ mineral greigite (Fe,&) as the source of NRM. Greigite is magnetically stable, its NRM is not eliminated by demagnetization in fields up to 500 Oe. Greigite is common as concretions in chemical sediments. Consequently, some of those sediments must have acquired their NRM during diagenesis, and may indicate a different polarity than that extant at the time of ~deposition (Giovanoli, 1979). This fact must be considered in the interpretation of paleomagnetic results! The datum horizons determined by paleontological and other criteria are 330 m subbottom for the Brunhes-Matuyama boundary at 0.7 m.y., 640 m for the Matuyama-Gauss boundary at 2.4 m.y., 780 m for the Gauss-Gilbert boundary at 3.2 m.y., and 865 m for the Gilbert Epoch 5 boundary at 5.0 m.y. The 330 m datum is identified on the basis of (1) the recognition of Climatic Epoch 19, which should straddle the Brunhes-Matuyama boundary, and (2) the recognition of significant terrigenous influx in Chauda time. This designation is in general accord with the diatom biostratigraphy of Jouse and Mukhina (1978). Paleomagnetic data at Site 380 do not show a long interval of reverse polarity that could be designated as the Matuyama. Fortunately, the magnetic signature at Site 379 is less obscured by diagenesis, because the non-magnetic pyrite (Fe&) is the common sulphide. We are able to recognize the transition to Matuyama there, and determined the transition at 420 m in Hole 379 (Giovanoli, 1979). This horizon is correlative to the 330 m horizon in Hole 380 (Fig.2, see also H&i, 197813). We believe, therefore, that the paleomagnetic results are in agreement with other lines of evidence. A 650 m datum has been tentatively identified as the beginning of intense glaciation at 2.5 m.y., just below the Plio-Pleistocene contact of diatom stratigraphy. Paleomagnetic data showed a significant change of polarity at about 640 m which could be assigned as that of the Gauss-Matuyama boundary at 2.43 m.y . Here the prediction is in accordance with measured results. The 780 m interval has been tentatively identified as the beginning of cooling at 3.2 m.y. We recognize a polarity change at about this horizon, which could be assigned as that of the Gauss-Gilbert boundary.

84

The 864 m datum has been identified on the basis that the gravel unit is Messinian, an interpretation which has been confirmed by diatom evidence. Unfortunately we do not have sufficient data to determine this boundary paleomagnetically . In summary we might state that the paleomagnetic results have not given a clearcut chronostratigraphy, but the results could be best interpreted within the framework of the chronostratigraphy that has been constructed on the basis of other lines of evidence.

Sedinaen ta tion rate A final check on the credibility of our chronostratigraphical interpretations is provided by a calculation of sedimentation rates for different intervals. The dissolved and suspended loads of rivers draining into the Black Sea are known (Shimkus and Trimonis, 1974), and their relation to rate of sedimentation during glacial and interglacial times has been clarified through a study of well-dated piston cores (Ross and Degens, 1974). The sedimentation of Late Quatemary Black Sea sediments is about 10 cm/t.y. (t.y. = thousand years) for the Holocene and about 90 cm/t.y. for the Wiirm Glacial stage. The average Late Quatemary rate is thus 50 cm/t.y. Using this rate, we could predict a Brunhes-Matuyama boundary at about 350 m in Hole 380 which lies remarkably close to the 330 m datum determined by us. The sedimentation rate determined on the basis of our chronology is 47 cm/t.y. (Table I). It has been pointed out that the terrigenous detritus of the Danube had been largely trapped in a Precarpathian basin prior to the Middle Pleistocene (Hsti, 1973a); detritus of the Don may have been similarly trapped in a TABLE I Bate of sedimentation in the Black Sea basin Age of datum (m.y. 1

0.7 1.7 2.4 3.2 5.0 10.0

Sediment lithology between successive datum horizons

Thickness of sediment (ml

Sedimentation rate (cm/thousand year)

terrigenous mud glacial + interglacial Upper Siderite Upper Chalk Upper Chalk Lower Siderite Lower Chalk Lower Chalk pre-glacial laminated carbonates + black shales

330

47

185

18.5

125 140

18 18

90

5

210

5.5

85

satellite basin. The Danube delivers about l/2, and both rivers deliver about 2/3 of the terrigenous detritus to the Black Sea today. When the Danube and Don contributed little or no terrigenous detritus, chemical sedimentation took place in the Black Sea basin (Hsii, 1978a). The sedimentation rate should then have been about i/3 or l/2 of the average Late Quaternary rate, ranging from 16 to 25 cm/t.y. Extending down from the 330 m datum, this would place the top of the Olduvai Event at a depth between 490 and 580 m subbottom. Our stratigraphy places this datum at 515 m, well within the predicted interval Our chronostratigraphy indicates a 18 cm/t.y. rate of chemical sedimentation during these alternating epochs of glacial and interglacial climates. The climate started to oscillate when the Alpha Glaciation began at 2.4 m.y. In fact, similar climatic conditions may have prevailed since the first cooling at 3.2 m.y. If we use the 18 cm/t.y. rate of extrapolation down from the 515 m datum, the top of the Gauss Magnetic Epoch should be about 640 m and the top of the Gilbert 785 m. These are almost exactly as the 640 and 780 m datum horizons determined by our chronostratigraphic considerations. Prior to the Late Pliocene cooling, the climate was warm and temperate, and conditions were favourable for slow erosion, deep weathering and siderite sedimentation. The preglacial rate of sedimentation should be similar to the Quatemary interglacial rate which was only about i/3 of the average Quatemary rate. We estimated, therefore, that the preglacial rate was only about 4 cm/t.y. at the time when the Danube detritus did not reach the Black Sea basin. Using the rate for extrapolation down from the 780 m datum, the predicted top of the Miocene would then be 850 m. The datum determined by us on the basis of assuming the top of the gravel unit as the top of the Messinian is 865 m. Again the agreement is good, as our chronostratigraphical correlation gives 5 cm/t.y. for the average pre-glacial rate of sedimentation. Using the 5 cm/t.y. rate and extrapolating downward from the 865 m datum, we obtain an age of 9.5 m.y. for the bottom of the hole, compared to the lo-13 m.y. estimate given by paleontological evidence. The rate is a little too small, because we did not take into consideration the rate of Messinian sedimentation, which must have been unusually high. To summarize this discussion, we feel that a proper evaluation of the sedimentation rate, with considerations of changes of climates and drainages, has given support to the chronostratigraphy we presented, and has further confirmed our interpretation that the gravel unit is the Black Sea expression of the Messinian Event. ORIGIN OF THE BLACK SEA BASIN

The presence of shallow-water flora and of shallow-water sediments in the Black Sea basin suggested the possibility that the basin floor was only

66

slightly below the world-wide sea level until it was lowered by a 2000 m subsidence during the Quaternary ..~(Stoffers and Miiller, 1978; Degens and Paluska, 1978). However, Ross (1978) presented convincing evidence that the basin has not undergone such a catastrophic subsidence: not only is there a general absence of seismic activities, but also the numerous seismic profiles available do not show any evidence of large-scale down-faulting during the Quaternary. Geothermal studies indicate that the Black Sea has a normal heat flow, not an unusually high heat flow typical of Late Neogene marginal basins such as the Tyrrhenian and the Aegean (Erickson and Von Herzen, 1978). The geothermic state suggests that the basin is either underlain by oceanic crust, formed 40-80 m.y. ago, or by a continental crust which has not been tectonically active within the last 100 m.y. Geological data suggest that the Black Sea was a marginal basin formed in Late Cretaceous behind the volcanic island-arc of Anatolia (Hsii et al., 1977b; Letouzey et al., 1978). The basin has remained a deep-water basin throughout the Cenozoic, despite the accumulation of some 10 km sediments. This tectonic interpretation is supported by the sedimentological evidence from DSDP cores: Except for the gravel unit, the sediments in the sections penetrated, are pelagic, hemipelagic, or turbiditic and ah of them could have been laid down in a deep lake or a deep brackish marine environment. THE MESSINIAN EVENT OF THE BLACK SEA

The gravel unit includes a most unusual lithology: mud-pebble conglomerates, cobble conglomerates, breccias, and dolomite. The most remarkable sediment is the horizontally laminated stromatolitic dolomite (Fig.5). The rock consists of pellets, ooids, with partially filled vugs,

FEg.5. Stromatolitic dolomite found on a supratidal flat during the Messinian Stage when the water level within the Black Sea was drawn down to the level of the abyssal plain.

87

resembling the dolomite crust formed by diagenesis in supratidal environments of the Bahama Islands (Stoffers and Miiller, 1978). The stromatolitic structure suggests the trapping of carbonate muds by an algal mat in intertidal environments. Also present are well-sorted oolitic calcarenites, typical of deposition in a shallow subtidal environment. The intercalated coarse elastics may have been deposited by debris flows, originating from the newly exposed steep slopes. This clear-cut evidence permitted a concensus by the Leg 42B shipboard scientists that the gravel unit was deposited in shallow-water and subaerial environments. The staff disagreed on the question of the floor depth of the Black Sea basin at the time of shallow-water sedimentation. Assuming that the shallow-water sediments could only have been deposited in a shallow basin, Degens and Paluska (1978), Stoffers and Muller (1978), and Schrader (1978), among others, postulated catastrophic subsidence during the Late Quaternary. However, as we discussed previously, the geological evidence indicates that the Black Sea is a relic back-arc basin of Late Mesozoic age: there was no evidence of such Quatemary “collapse”. Isostatic consideration permits a calculation of the paleobathymetry of the Neogene Black Sea. The total thickness of Black Sea sediments as shown by seismic studies is about 10 km in the region of Site 380 where the sequence lies above an oceanic crust (Neprochnov and Ross, 1978). Assuming a sediment density of 2.4 g/cm3, a crustal density of 2.84 g/cm3, and a mantle density of 3.27 g/cm3, the depth of the basin floor should be about 2 km when it is isostatically adjusted (see Hsu, 1958, for details in a computational method); this is indeed the present depth of the abyssal plain of the Black Sea. Tracing back to the Messinian time when the basin had some 900 m less sediments, the basin-floor under water should have been about 2.3 km. This line of evidence leads to the conclusion that such a deep Black Sea basin must have held very little water when the shallow-water Messinian sediments were deposited. At the time when there was little water in the deep basin, isostatic uplift in response to the negative load should have raised the basin floor to a level of about 1.6 km below the world-wide sea level. It was, nevertheless, still a deep desiccated basin like the Messinian Mediterranean although smaller and less depressed. Hydrographic budget of the Black Sea The hydrographic budget of the Black Sea at present is shown by Table II. The balance of inflow and outflow gives an estimate of evaporation rate, which is 250 km3, or some 55 cm3/cm2 per year. When the Black Sea was not connected to the Mediterranean by the Bosporus, the hydrographic budget should have a hydrographic surplus of 150 km3 which was to find its way via a river to the Mediterranean. Geological evidence indicates that the desiccated Mediterranean received an extraordinary inflow of freshwater during the latest Lago-Mare phase of the salinity crisis (Cita et al., 1978). The inflow has been attributed to a

88 TABLE II Hydrographic budget of the Black Sea today Volume of Basin Annual inflow of rivers Annual inflow from Mediterranean Annual outflow to Mediterranean Estimated evaporation (After Zenkovitch,

5.37 x 105kms 400 km3 200 km3 350 km3 250 km’

1966)

change of the central European drainage system, when the Danube was pirated and the headward erosion of Mediterranean drainage and the Danubian water was diverted to the Mediterranean (Fig.6, see also Hsii et al., 1977a; 1978). Losing half of its river inflow, the Black Sea basin was no

b

Fig. 6. Paleogeographic maps depicting the change from (a) the Paratethys (14-l 3 m. y.) to (b) the Messinian Lago-Mare stage (5 m.y.). A = Atlantic; M = Mediterranean; P = Paratethys.

89

longer blessed with a huge surplus but should have ended up with a deficit of some 50 km3 per year. At this rate, the Black Sea basin with a 5 X 10’ km3 volume could have been evaporated dry within 10,000 years. It should be recalled that the Messinian event lasted about 1 or 0.5 m.y., and that the final Lago-Mare phase may have spanned some 100,000 years (Hsii et al., 1978). Our estimates show that the time available was more than sufficient to evaporate dry the Black Sea basin. A question should be asked concerning the residues of evaporation: the absence of a salt deposit seems to contradict the postulate of desiccation. However, this paradox may be explained by the fact that rivers carry very little dissolved chloride and sulphates, commonly less than 100 ppm (Shimkus and Trimonis, 1974). According to our estimate the total river inflows to the Black Sea in 100,000 years should have been 2 X 10’ km3 during the Lago-Mare phase. If this body of water was evaporated down to 2 X lo4 km3 or 1/25th of the volume of the Black Sea basin, which would be barely enough to cover its abyssal plain, the salinity of the residual brine would have increased thousand-fold. Even so, the concentration should still have been less than 100,000 ppm or lo%, and not sufficiently saline for salt deposition. The salinity of interstitial waters of Late Miocene sediments reached 9% in Hole 380A and more than 6% in Hole 381 (Manheim and Schlug, 1978). This high salinity has remained despite of postdepositional ionic migration. We consider it sufficient evidence that a supersaline stage once existed in the Black Sea during the Late Miocene, although the supersaline condition did not reach the point of depositing significant amounts of solid evaporates, The lake did have alkaline affinities, as extremely high values (up to 95 meg/ kg) of alkalinity were suddenly obtained near the 870 m horizon in Hole 380 A (Ross et al., 1978, p. 159). Implications of the Black Sea desiccation If the Black Sea has been desiccated during the Late Messinian the stratigraphic sections on land should record such an event. Indeed, Gagic and Sokac (1970), and Jiricek (1975) both referred to a widespread regression at the end of the Pontian. This Messinian event in the Paratethys was recently discussed in some detail by Steininger and Papp (1979; also Steininger, personal communication). Whereas the Early Pontian was characterized by wet and temperate floras, the Late Pontian floras were typical of more arid climate. The pronounced climatic change has been correlated by Steininger and Papp to the Messinian event. The paleoclimatic and paleogeographic changes also resulted in a sudden invasion of steppe faunas such as antelopes, hyaenids, etc., as well as typically African faunas (giraffids, hyraxes, aardvarks, etc.) into the Paratethys realm. The final desiccation of the Vienna and western Pannonian basins took place during the Late Pontian (Bachmayer and Zapfe, 1969; Sauerzopf, 1952; Steininger and Papp, 1979).

90

However, continuous sedimentation seemed to have prevailed in the areas of the Black Sea throughout the Pontian, which argues against a postulate of Late Pontian desiccation of the Black Sea (Steininger and Papp, 1979). We believe, however, that a regression from peripheral basins may have been marked by an obscure disconformity: the controversy concerning the Vera Basin section (Cita and Vismara Schilling, 1978), where the salinity crisis seemed to have left no imprint in the section of that peripheral basin, clearly demonstrates the difficulty of recognizing regression by conventional stratigraphical methods. Another major implication of the Messinian desiccation is the expectation of a major erosional surface. The geomorphic evidence of the Mediterranean desiccation is abundantly clear (Ryan and Cita, 197 8). One should ask the whereabouts of the Messinian canyons in the Black Sea. Seismic reflection studies indicated the presence of numerous channels and a wide-spread erosional surface associated with a K-reflector in the Black Sea basin (Letouzey et al., 1978; L. Letouzey, pers. comm.). The reflector was tentatively identified as Late Miocene or older at Site 380 (Letouzey et al., 1978). However, the first bottom reflector at Site 381, which may be correlated to the K-reflector, is definitely the top of the Messinian cemented gravel (Fig. 1). Future seismic surveys may yield confirming evidence. Meanwhile, we offer a working hypothesis that the widespread erosional surface buried by sediments younger than the K-reflector is a manifestation of Messinian erosion in the Black Sea basin. A third problem concerns the correlation of the record of salinity changes as indicated by the Black Sea cores to marine transgression as shown by the land section. Semenenko (1978) recorded three major marine transgressions into the Ponto-Caspian regions during the Late Miocene and the Pliocene: Meotian (NN 10) during the Tortonian, latest Pontian and Kimmerian (NN ll), and Late Kimmerian (NN 12). Percival (1978) failed to find nannofloras diagnostic of those ages in the Black Sea cores. However, two occurrences of Bruarudosphaera flora (in sediments of Unit IV, and Unit IV,) are indicative of brackish marine transgressions which might be correlated’to the latest Pontian and Late Kimmerian transgressions on the basis of our stratigraphical interpretations, The evidence for the Meotian transgression, which seems to be widespread in the regions around the Black Sea, should be sought in the sediments of Unit IV,, underlying the Messinian gravel. Diatom specialists have failed to find marine diatoms in that unit, but Meotian marine diatoms have been found in the pebbly mudstones associated with the Messinian gravel at Site 381 (JousC and Mukhina, 1978), probably as allochthonous floras reworked from older sediment-s. If so, we might conclude that the Meotian transgression probably did bring marine waters into the Black Sea basin, but the identification of the event has been hampered because we do not have a continuous stratigraphical record.

91 SUMMARY

We have summarized in this paper our interpretation that the gravel unit found in the 864484 m interval in Hole 380A consists of shallow-water and subaerial sediments, deposited during the Messinian when the Black Sea water level was drawn down nearly to the level of the abyssal plain. The evaporative draw-down was related to drainage changes in central Europe, when the Danube water was diverted into the desiccated Mediterranean to form the Lago-Mare. The water in the Black Sea basin was then concentrated by evaporation in an enclosed basin for about 100,000 years until its salinity was increased to about 10%. An alkaline lake was then present on the site of the present abyssal plain. The Messinian sediments of the Black Sea were overlain by brackish marine aragonitic sediments of Early Pliocene age, deposited during a marine invasion correlative to the Trubi transgression of the Mediterranean. The connection between the Black Sea and the open sea, however, was soon severed. A desalinization of the water in the Black Sea basin led to a freshwater. lake environment until the Late Quaternary, when the Mediterranean water entered the Black Sea basin via the Bosporus Strait during an interglacial highstand of world-wide sea level.

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