An abrupt drowning of the Black Sea Shelf

William B. F. Ryan, Walter C. Pitman III. and Candace O. Major, Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA

Kazimieras Shimkus and Vladamir Moskalenko, Southern Branch of the P.P. Shirshov Oceanology Institute, Gelendzhik-7, 353470, Russia

Glenn A. Jones, Texas A&M University at Galveston, Galveston, TX, USA

Petko Dimitrov, Bulgarian Academy of Sciences, Institute of Oceanology, 9000 Varna, PO Box 152, Bulgaria

Naci Gorür and Mehmet Sakinç, Î.T.Ü. Maden Fakültesi Jeoloji Bölümü, 80626 Ayazaga, Istanbul, Turkey

Hüseyin Yüce, Seyir, Hidrografi ve Osinografi Dairesi, 81647 Çubuklu, Istanbul, Turkey

Abstract

During latest Quaternary glaciation, the Black Sea became a giant freshwater lake. The surface of this lake drew down to levels more than 100 m below its outlet. When the Mediterranean rose to the Bosporus sill at 7,150 yr bp¹, saltwater poured through this spillway to refill the lake and submerge, catastrophically, more than 100,000 km2 of its exposed continental shelf. The permanent drowning of a vast terrestrial landscape may possibly have accelerated the dispersal of early neolithic foragers and farmers into the interior of Europe at that time.

Introduction
The most widely-accepted hypothesis, based on data from the deep basin floor sediments (Degens and Ross, 1972; Deuser, 1972; Ross and Degens, 1974; Ross, Degens et al., 1970), holds that post-glacial inflow of Mediterranean water into the Black Sea's isolated freshwater lake began as a trickle 9,000 yrbp. As saline spills intermittently reached the basin floor, they supplied a thickening puddle of stratified brackish bottom water, which eventually became anoxic two thousand years later (Degens and Ross, 1972; Deuser, 1974). Physical models (Deuser, 1974; Glenn and Arthur, 1985) based on this scenario have predicted a shoaling oxic/anoxic interface leading to sapropel accumulation first on the basin plain and then later on the margins. However, new data from the shallower continental margin and from the inlet itself offer a contradictory picture of influx commencing later at 7,150 yrbp and initiated by a cascade of saltwater pouring into a partly-emptied Black Sea lake following the breaching of an intervening barrier. Catastrophic flooding produced a simultaneous submergence of broad regions of the continental shelf, an invasion of immigrant marine organisms (Shimkus, Mukhina et al., 1973; Wall and Dale, 1974), a four-fold decrease in sediment accumulation rate (Degens and Ross, 1972; Shimkus, Emelyanov et al., 1975), and a simultaneous onset of organic-rich sediments at all depths (Jones and Gagnon, 1994).

New Data
We report the findings of a joint Russian/US expedition in 1993 which surveyed two regions of the outer continental shelf where, at times of a lowered lake surface, ancient rivers may have once approached the shelf edge (Fig. 1). These surveys revealed a buried erosion surface strewn with shelly gravel extending across the broad continental margin of the northern Black Sea to beyond its shelf break (Evsylekov and Shimkus, 1995; Major, 1994). This unconformity (Fig. 2) truncates strata belonging to a terrestrial flood plain with meandering river beds, coastal deltas and wave-cut beaches developed in the last glacial period when the Black Sea had become a giant freshwater lake (Federov, 1971; Nevesskaya, 1965). The gravel is composed entirely of bleached fragments of the freshwater "Caspian" mollusk (Dreissena rostriformis) eroded from underlying dry (water content <10%) coquina bearing sand and clay layers (Fig. 3O) where the intact shells are coated with fossilized algae and have an 18 O/ 16 O isotopic composition ranging from -3.8 to -7.8 o/oo (Fig. 3P). The stiff clay contains abundant leafy plant material, fluvial gastropods (Viviparus viviparus ), desiccation cracks and roots indicative of former alluvial to coastal marsh environments presently submerged in water depths down to -140 m. Our AMS 14 C ages on individual intact valves of D. rostriformis in the Neoeuxine coquina below the gravel extend from 14,700 ± 65 to 10,400 ± 55 yrbp. Ages of reworked mollusks in the gravel are as young as 8,250± 35 yrbp.

These findings of a former emerged land surface complement prior investigations which report loess soils (Kuprin, Scherbakov et al., 1974), alluvium (Popov, 1973; Skiba, Scherbakov et al., 1975), littoral deposits (Dimitrov, 1982; Kuprin, Scherbakov et al., 1974; Scherbakov, 1983; Scherbakov, Kuprin et al., 1978; Shimkus, Emelyanov et al., 1975), and beach terraces (Shimkus, Evsyukov et al., 1980) in formations on the continental shelf spanning 17,780±200 to 9,660±70 yrbp at depths between -93 and -122 m, as well as deeply entrenched river valleys (Ostrovskiy, Izmaylov et al., 1977; Panin, 1972; Skiba, Scherbakov et al., 1975) indicative of a past lake surface in Neoeuxine time at or below -110 m.

The sediment overlying the ubiquitous erosion surface is a wet (water content >50%), dark, jelly-like sapropel with organic carbon contents ranging from 1.5 to 8% (Fig. 3N). Over much of the shelf this deposit comprises a structureless layer of uniform thickness which drapes all of the undulations of the unconformity surface (Fig. 2, insert). The drape shows no evidence of a transgressive systems tract bedding geometry or coastal onlap as is seen on Mediterranean and Atlantic shelves.


Discussion
A sapropel has been previously described from the deeper basin where its base records the first post-glacial appearance of euryhaline marine dinoflagellates (Wall and Dale, 1974) (Fig. 3C) and diatoms (Mayard, 1974; Shimkus, Mukhina et al., 1973) (Fig. 3D&E). Cosmopolitan and diverse salinity-tolerant dinoflagellates abruptly replace a novel low-diversity stenohaline flora that lived in an Neoeuxine "lake-sea" of extremely reduced salinities and whose disappearance is indicative that they were unable to survive the influx of saline water which carried in the euryhaline-marine organisms (Wall and Dale, 1974). On the shelf the Neoeuxine mollusk fauna (Dreissina polymorpha, D. rostriformis, Monodacna caspia, Clathrocaspia gmelini and Theodoxus pallas) have "Caspian" affinities. Several species continue to live today but are confined to river mouths and the fresher parts of the Azov, Caspian and Aral Seas.
The faunal/floral successions on the shelf parallel those from the deep basin cores by also recording at the base of the shelf sapropel the first post-glacial arrival of Mediterranean immigrantsthe euryhaline marine mollusks (Cardium edule, Mytilaster lineatus, Mytilus galloprovincialis, Hydrobia ventrosa, and Abra ovata) indicative of the Old Black Sea stage of the Holocene (Federov, 1971; Nevesskaya, 1965). This horizon has been dated by the 14 C Accelerator Mass Spectrometer (AMS) method using single intact specimens of C. edule and M. Caspia (bivalves which dwell in the uppermost substrate no deeper than the diameter of their shell) and M. galloprovincialis (an epibenthic mussel which dwells on the substrate). In cores from water depths of -49 to -123 m our dates from five cores (Fig. 2) cluster at 7,150±100 yrbp and fall within the error limits of ages obtained at the base of the sapropel in another nine cores spanning the depths of -200 to -2200 m (Jones and Gagnon, 1994). Apparently organic carbon-rich sedimentation began synchronously throughout the Black Sea once its barrier to the Mediterranean was breached.

The Neoeuxine sediments on the shelf and basin floor have roughly comparable light 18 O/ 16 O isotopic compositions (-8.0 to -4.0 o/oo) considered to be typical of freshwater environments (Fig. 3H &P). Though there is insufficient carbonate in the unit 2 sapropel to measure its isotopic variability, the heavier ratios of the mollusks throughout the Old and New Black Sea stages on the shelf (0.0 ± 0.8 o/oo) suggest that the Black Seas water composition may have changed substantially with the invasion of the euryhaline-marine mollusks, for they show little subsequent isotopic variation in the last seven thousand years.

The hypothesis of a Mediterranean inflow beginning at 9,000 years bp was strongly influenced by consideration of global eustatic sea level rise (Milliman and Emery, 1968) to a presumed -50 m sill in inlet at that time (Degens and Ross, 1972). However, reflection profiling in the Strait of Istanbul (SOI-Bosporus) and Strait of Çanakkale (SOÇ-Dardanelles) by the Turkish Navy reveals the straits as exhumed and now partly sediment-filled channels whose bedrock sill depth is no shallower than -85 m (Fig. 4) . The sediment body within the SOI-Bosporus has been sampled by drilling (Derman, 1990). The vertical distribution of its mollusk (Taner, 1990) (Corba gibba, Cerithidea insulaemaris) and benthic foraminiferal (Meriç and Sakinç, 1990) (Ammonia beccarii, Eliphidium crispum) assemblages indicate that the first environmental conditions were euryhaline.

Although the indicator used most assertively as a proxy for salinification 9,000 yrbp is the 18 O/ 16 O ratio of the carbonate component of unit 3 sediment, its excursion beyond the typical range of unit 3 variability (-5.5 ± 0.8 o/oo) only occurs in the topmost sample from this unit (-3.9 o/oo) with an interpolated age of 7,900 yrbp (Fig. 3H). The tiny grain size of this carbonate material is but a few hundred angstrom. Such grains, no larger than 15 unit cells dimensions in any direction, present the possibility of post-depositional oxygen exchange (Degens and Ross, 1972; Deuser, 1972), caused by dense saline bottom water percolating into the seabed for a few decimeters after inflow had commenced.

Conclusions
Our reluctance to accept an early salinification at 9,000 years ago (Fig. 5) is identical to the concerns of Wall and Dale ( 1974) who has asked "why was the major reentry of marine organism delayed for nearly 2,000 years?" Furthermore, we have looked for and find no evidence for a pre-sapropel lithostratigraphic layer on the outer shelf at a time when it would have been overlain by more than 60 m of water (Fairbanks, 1989) if, indeed, an early connection had been made with the Mediterranean.

We interpret the instantaneous submergence of a broad former land surface of 100,000 km 2 as the consequence of Mediterranean waters invading an isolated inland lake whose surface had been drawn down beyond its shelf break by evaporation and reduced river input (Fig. 5). Such a cascade, once underway, would posses the power to further enlarge its orifice through positive-feedback erosion. The bedrock cross-sections of the SOI-Bosporus and SOÇ-Dardanelles observed at dozens of points along their lengths present a flume capable delivering a flux in excess of 50 km 3 per day, initially filling the lake at a rate approaching 10's of cm/day.

At this time (7,550 calendar years BP), farming, which had already been established in Greece, Bulgaria, Romania and along the coast of the Marmara Sea (Özdagan, 1983; van Andel and Runnels, 1995), spread rapidly inland along the major river valleys of southeastern Europe (Greg, 1988; Hodder, 1990). The light plow and simple irrigation appear abruptly in the Transcaucasus (Glumac and Anthony, 1991). Such "wave-of-advance" population movements (Sokal, Oden et al., 1991) could have been induced by the permanent expulsion of inhabitants which had adapted to the natural resources of the formerly-emerged Black Sea peripherynamely, its arable loess, alluvial soil, and the moist loam of the freshly exposed bed of its shrinking shoreline.


Figure Captions
Fig. 1. The arid landscape of the Neoeuxine stage in the Black Sea deduced from the lithologies of more than 250 sediment cores (Kuprin, Scherbakov et al., 1974; Scherbakov, Kuprin et al., 1978). The soils of the emerged shelf are dominated by wind-blown loess (wavy pattern) and the alluvial deposits (stippled pattern) of meandering rivers that flowed hundreds of km beyond their present mouths to shelf-edge deltas. The ancient littoral zone (brick pattern) was explored during two surveys of a joint Russia/US expedition in 1993.The line with small circles extending northwest from the survey west of the Crimea indicates a transect of new cores.


Fig. 2. Interpretation of one of our high-resolution seismic profiles across the outer continental shelf in the Black Sea south of the Ukraine illustrating an erosional unconformity (bold line) which has truncated an underlying glacial-age alluvial and delta deposit. This scar is ubiquitous everywhere above the -150 m isobath. Sediment cores obtained in 1993 are indicated by vertical lines which mark the extent of sub-bottom penetration into and through the unconformity. Circled numbers are for cores which had to be projected onto the profile. The asterisks point to locations where mollusks were extracted from the base of the uniform drape resting on the unconformity. The identical radiocarbon ages (7,150±100 yrbp) of the shells document a flooding surface which reached inland from the shelf edge to at least the -49 m isobath. The rapid transgression left the erosion surface intact as shown by the uniform burial of a former river channel.

Fig. 3. Indicators of abrupt environmental change in sediment cores from deep basin (left ) and shallow shelf (right ) regions of the Black Sea. The horizontal bold gray line at 7,150 kybp marks a time of simultaneous change in many properties of the sediments
Columns:
A) lithostratigraphic units (Degens and Ross, 1972).
B) predominant lithology (Ross, Degens et al., 1970).
C) dinoflagellate abundance (Wall and Dale, 1974) Lingulodinium machaerophorum (1), Peridinium claudicans (2),Spiniferites bulloides (3), Tectatodinium psilatum (4) and Spiniferites cruciformis (5).
D) diatom abundance in a core at -920 m depth (Shimkus, Mukhina et al., 1973).
E) location of fresh (F), brackish (B) and marine (M) diatom specimens in cores below -2000 m depth (Mayard, 1974).
F) percent organic carbon (Ross, Degens et al., 1970).
G) 13 C/ 12 C ratio (o/oo) in the carbonate fraction (Deuser, 1972).
H) 18 O/ 16 O ratio (o/oo) in the carbonate fraction (Deuser, 1972).
I) Sedimentation rate (cm/kyr) (Degens and Ross, 1972).
J) calculated percent fraction of reworked detritus (Deuser, 1972).
K) stratigraphic nomenclature (Shimkus, Emelyanov et al., 1975).
L) predominant lithology (Kuprin, Scherbakov et al., 1974; Nevesskaya, 1965; Scherbakov, 1983; Shimkus, Emelyanov et al., 1975).
M) stratigraphic range of selected mollusk species in our core AK93-1 at -68 m depth (Major, 1994).
N) percent organic carbon in our core AK93-11 at -91 m depth.
O) percent water content in our core AK93-11.
P) 18 O/ 16 O ratio (o/oo) measured on Cardium edule from core AK93-1 for the Old and New Black Sea sediments and on Dreissina rostriformis in core AK93-13 at -165 m depth for the Neoeuxine sediments (Major, 1994).
Q) synopsis of geometry of clinoforms seen in the high-resolution seismic reflection profiles across the outer shelf.

Fig. 4. Interpreted seismic reflection profiles across the SOÇ-Dardanelles showing the channel eroded into the bedrock. The vertical scale is in milliseconds, two-way travel time, which places the bedrock sill depth at -85 m. The sediments, which have subsequently buried the erosion surface, accumulated after the establishment of inflow to the Black Sea from the Mediterranean.

Fig. 5. Reconstructed levels in the Mediterranean (M) and Black Seas (B), and their connections/isolations via the SOÇ-Dardanelles and SOI-Bosporus sill (S). On the left is a portrayal of the gradual inflow model (Degens and Ross, 1972; Deuser, 1972; Deuser, 1974) in which the first connection was made at 9,000 yrbp. The depth of the strait deepened slowly as global eustatic sea level continued to rise, leading to an acceleration in the strengthening of the Black Sea's salinity. On the right is our catastrophic flood model in which the Mediterranean broke through a barrier in the inlet at 7,150 yrbp and rapidly flooded a partly evaporated freshwater lake. Here increase in the Black Sea's salinity is shown to have been initially abrupt and then to have later decelerated.



Acknowledgements
The 1993 expedition materialized though the generous assistance of the officers and crew of the R/V Aquanaut, the Datasonics Corporation, Falmouth, MA, R. Kosyan and E. Kontar of the P.P. Shirshov Oceanology Institute and V. Artemenko of Yuzhmorgeologiya Gelendzhik. M. Bruckner, R. Fairbanks, T. Guilderson, B. Hatau, W. Haxby, B. Il'ina, S. Klimley, W. Menke, L. Nevesskaya, S. O'Connell, Ç. Sengor and J. Weissel provided crucial technical and analytical help. Discussions with D. Bailey, D. Harris, I. Hodder, J. Mellaart, A. Moore, J. Oates, C. Renfrew, J. Renfrew and A. Sherratt have substantially contributed to our understanding of possible archaeological implications of the Black Sea connections with the Mediterranean. Lamont-Doherty Earth Observatory Contribution Number ####.



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Correspondence and requests for materials should be addressed to W.B.F.R. ( e-mail: billr@ldeo.columbia.edu). Additional supporting materials are available at http://imager.ldeo.columbia.edu/BlackSeaShelf.html