W.B.F. Ryan et al. - An abruptdrowning of the BlackSea she/faf 7.5 Kyr B.P. AN ABRUPT DROWNING OF THE BLACK SEA SHELF AT 7.5 KYR BP WilliamB.F. Rvnru1,Walter C. Prrunrulll1, Candace O. MR.loR1, KazimierasSHrurus', Vladimir MoscRtrruro', Glenn A. JoruEsr, PetkoDrMrrRova, Naci GonURs, Mehmet SRrcruqs & Hiiseyin Ytice SEvlR6 1 Lamont-DoheityEarth Observatory of ColumbiaUniversity, Palisades, NY 10964,USA 'Southern. Branchof the P.P. ShirshovOceanology Institute, Gelendzhik-7, 353470, Russia 3 TexasA & M Universityat Galveston,Galveston, TX, USA a Bulgarian-Academyof Sciences, lnstitute of Oceanology,9000 Vama, PO Box 152,Bulgaria " l.T.U.Maden FakUltesi Jeoloji B6lUmU, 80626 Ayazga, lstanbul, Turkey 6 SeyirHidrografi ve OsinografiDairesi, 81647 gLiUuXtu, lstanbul, Turkey Abstract: During latest Quaternaryglaciation, the BlackSea becamea giant freshwaterlake, The surface of this lake drew down to levels morethan 100mbelow its outlet.When the Mediterraneanrose to the Bosporussill at 7.5 kyr BPl, saltwaterpoured through this spiltway to refillthe and submerge,in lessthan a year,more than 100,000Km'of its exposedcontinental shelf. Key words: continentalshelf, erosion surface, climatic condition, faunal succession, radiocarbon data, Neoeuxine Lake, Black Sea the ecology of the fossil molluscs2s' 32' INTRODUCTION ^^ tt, dinoflagellatqs33and diatoms3a' from strongly The earliestwritten accountson the Bosporus negative O'o''o isotopic ratios^^inthe carbonate and DardanellesStraits, date to the Roman era'-" component of the sediments'o, and from the and chroniclea catastrophicbreakthrough of the chemistry of pore waters drawn out of the sea. The winding course of both straits, their Neoeuxine-agestrata3T-3e. terraced cross-sections, and their dendritic Near-shorelittoral deposits2627' 40' 41, with tributarieswere interpretedby nineteenthcentury 6 radiocarbonages of the msllusc shells spanning geomorphologistss as indicative of a stream 19 to I kyr BP, have been found at depths origin.Today the channelis submerged 9eeply between -93 and -122 m. Linear steps that and pafilyfilled with sediment'-'"to a sill depthof- resemblethe profilesof beachterraces have been 33 m". lt hosts a two-way exchange of water mapped for tens of km along the outer shelf betweenthe partly marineBlack Sea and the fully between-90 to -110 m. A substantiallowering of marine MediterraneanSea11-16. In fact, the the pastlake levelto at least-110 m has also been northward-flowingundercurrentlT had long ago inferredfrom the discoveryof formerlyentrenched been known to mariners who lowered baskets river valleysa2-45which are now partlyfilled in with laden with rocks into its core to tow their vessels estuarinedeposits under the bedsof modernrivers upstreamtoward the BlackSea18. as they approach the coast. Boreholes in the '' Dated fossil coral reefs in the Caribbeanle-21Kerch Strait'o have recovered coarse gravel showthat, at the time of the maximumexpansion withfluvial fauna at -62 m in the streambed of the of continentalice sheets around 20 kry BP, the ancient Don River > 200 km seaward of the surfaceof the globalocean was at -120 m. Thus presentriver mouth. Mud, silt and sand on the prior to the glaciers,the Black Sea was without basin plain dated between I to 7.5 kyr BPas (Fig.1). connectionto an externalocean ln the contain .A significant cgmponent of reworked absence of a saltwaterinlet it had transformed plan 23, molluscsae,microfossilsso and detritus33 itself into a giant freshwater body22 dwarfing washed downslope from coastal swamps. The anymodern lake in areaand volume. contaminationof former proximityof wetlandsto NEOEUXINELAKE the headsof the submarinecanyons which indent the shelf edge and act as conduitsfor sediment glacial postglacial Lacustrinedeposits of and transportto the basinplain. The inferredrelocation age, assigned to the local Neoeuxine"-'" of the coastline in the vicinity of the current stratigraphicstage of the late Quaternary,have shelfbreak is shown in Fig.2, along with the beencored at a large numberof sites on the floor ubiquitousdistribution of loessand alluvialsoils on of the Black Sea extendinofrom the continental the broadand emergedcontinental shelf south of plain26'2e-31 shelf26-28to the centerof itibasin at a Ukraineand throughoutthe Sea of Azov27. depth of 2.2 km. The fresheningis deducedfrom GEO-ECO-MARINA,41997 Nationallnstitute of Maine Geologyand Geo-ecologyof Romania Prcc. lntern. Workshopon "Fluvial-Mainelnteractions" in Malnas,Romania, OcLl-7, 1996 W.B.F. Ryanet al. - An abruptdrowning af the Black Sea she/f af 7.5 Kyr B.P. Fig.1 The BlackSea lake with its reconstructed9 kyr BP shoreline(blue/green boundary) at -120 m belowthe moderncoastline (bold line).The regressionportrayed here has exposedthe entirecontinental shelf and the Seaof Azov,thereby reducing the regioncovered by waterto two-thirdsof its presentextent. Fig. 2 The arid landscapeof the Neoeuxinestage in the BlackSea deduced from the lithologiesof morethan 250 sedimentcores'u' ". The soils of the emergedshelf are dominatedby wind-blownloess (wavypattern) and the alluvialdeposits (stippled pattern) of meanderingrivers that flowedhundreds of km beyondtheir present mouths to shelf-edgedeltas. The ancientlittoral zone (brickpaftern) was exploredduring two surveysof a jointRussiai US expeditionin 1993.The linewith smallcircles extending north-west from the survey westof the Crimeaindicates a transectof newcores whose lithr.rlogies and ages are portrayed in Fig.6. 116 GEO.ECO.MARINA,41997 NationalInstitute of Marine Geologyand Geo-ecologyof Romania Proe. lntern. Workshopon "Fluvial-Mainelnteractions" in Malnas,Romania, Oct"1-7, 1996 .L-_ W.B.F.Ryan et al.- An abruptdrowning of the Blac[,Seastlerf af 7.5Kyr B.P. 1220'E 45000'N 44050',N I formerriver channel (Fig. 5) I' Oll corelocadon shelf edgewest l+\ Ii .4 of the Crimea r,#tAune fietd(Fig.5) \4' scismicline (Fig.4) (bold dby Fig.SThe west Crimea survey, contoured in metersbelow present sea level. A beltof dunesliee between the -70 and -80 m isobath. Core14 at- 140 m containsan erosionsurface formed by emergence of thelake bottom. ttl A 9 710 2ltr DP craages 2 ,/ of rn) ey Fig.il Interpretationof selsmic profils A-A" lllustraungan erosionsurface (bold llne) which has truncated an underlyingglaolal-age deltaicdeposit and which ie ubiquitousin the west Crimeasurvey everywh€re above the- 156 m isobath.The cores obtainedin 1993 are proJectedontg the profile (llne6 showingthe depth of penetrationare those cores close to the profile,circled numbers are those further auny). In the bottomhalf of the figurethe reflectionprofile has been proJectedInto an age-distancespace to emphaeizethe major hiatus (outlinedin bold) formed by the regr€gsionsof the lake. The diamondsymbols locate AMS radiooarton-dat€dlevels in the cor€s. Two transgressionsare apparent,one starlingabout 15 kyr BP and the secondat 7.5 yr lcyrBP. The latterdrowns the shelf from -123 to49 m in the diagramin an interval,which appearsto be instantaneousgiven the resolutionof the AMS dates. GEO-ECO- MARINA, 2/1 997 Nationallnstitute of Maine Geologyand Geo-acologyof Romanla Proc. Intern. Workshapon "Fluvial-Mainglnteractions" in Malnas.Romania. Oct.1-7. 1996 W.B.F. Ryan ei al. - An abruptdrowning of tha BlackSea shelf al 7.5 Kyr B.P NEW DATA It truncates strata with a clinoform geometry The submerged shorelines on the northern diagnostic of an alluvial fan, flood plains, shelfof the BlackSea were revisited in June,1993 meandenngriver beds,and deltas.The surfaceis by a joint Russian/Us expedition.Two regions locallycovered with dunes(Fig.5) attributed to the were surveyedwith more than 1,000 km of high- wind drift of beach sands.The erosionsurface is resofutionsub-bottom reflection profiling and 24 widelystrewn with a shellygravel having a sharp new gravitycores (Fig.1). One surveylies southof top contact and gradational base. The gravel -45 the Kerch Strait in depthsbeyond the present component is comprised almost entirely of m isobathin a regionwhere the ancientDon River bleached and abraded fragments of a freshwater would have flowed to reach the freshwaterlake. molfusc (Dreissenarostiformis distincta). The most The other (Fig.3)is situatedwest of the Cfimea at common fragment is the robust hinge portionof -65 depthsbelow the m isobathwhere the ancient the valve.At depthsbelow -95 m the shellygravel Dneiperand DneisterRivers might have followed overliesa mud with sand-richlayers containing the routeslo the lake'sedge. same mollusc as found pulverisedin the shelly Widespreaderosion surface gravel.However, here the fragilemollusc has both valves attached shows An erosionsurface (Fig.4) reachesacross ihe still and no sing of abrasion or transport.The exteriorof the shell sholf to a depth of -'156 m. This unconformilyis is coatedwith a brown of what mappable without interruption throughout both hairy scum appears to be fossilisedalgae, which can be easily survey areas and along the track of the survey removedwith a fingernail In five cores (Fig.6) -95 vesselto and from the port of Gelendzhik. below the m isobath this shell-bearing er osion suf face sand restson a firm, dry clay in which one could identify river desiccationcracks filled with plant detritus and sur- chan:rel rounded by the preserved roots of plants and shrubs. The sand, rnud and clay layers contain gastropods (Viviparus viviparus) indi- cative of fluvial environ- ments. The fauna in the gravel, sand and mud are characteristicof the fresh- dunes waterNeoeuxine stage of the late Pleistocene Black sea25'5t.Mud