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Modeling the Paleocirculation of the Mediterranean

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Modeling the Paleocirculation of the Mediterranean PALEOCEANOGRAPHY, VOL. 13, NO. 6, PAGES 586-606, DECEMBER 1998 Modeling the palcocirculation of the Mediterranean- The last glacial maximum and the Holocene with emphasis on the formation of sapropel $1 Paul G. Myers and Keith Haines Departmentof Meteorology,University of Edinburgh,Edinburgh, Scotland, United Kingdom Eelco J. Rohling Departmentof Oceanography,Southampton University, Southampton, England, United Kingdom Abstract. An ocean generalcirculation model is used to simulatethe thermohalinecirculation in the Mediter- ranean sea during the last glacial maximum and the Holocene,when the sapropel$• was deposited.The model is forced by prescribedsurface temperatures and salinities,where present-dayvalues lead to very realistic sur- face buoyancyfluxes. Differentpaleoreconstructions for the surfacesalinity and temperaturedistributions during these periods are tested. In both periods, under all reconstructions,antiestuarine flow is maintained at Gibraltar and Sicily. The Holocenecirculation has fresh intermediatewater producedin the Adriatic and an upward salt flux from the old waters below help maintain its outflow at Sicily. The depth of ventilation around the basin is broadly consistentwith the shallowestsapropel layers observed. Shoaling of the easternpycnocline occurs in all experimentsin both periods,possibly indicating enhancedproductivity, although the reasonsfor this are different in each case. 1. Introduction 1.1. The Last Glacial Maximum At the southern boundary of Europe the Mediter- The most recent Pleistoceneglaciation reached its ranean region is today generally warm, with a semi maximumat •020,000years B.P. Duringthis last glacial arid climate. The present-dayMediterranean (Figure maximum (LGM) extensiveice sheets,up to several 1) circulationis lagoonalor antiestuarine(both as a kilometersthick, coveredthe northernparts of North wholeand for eachof its main subbasins),with surface Americaand Eurasia[CrowIcy, 1988]. Mediterranean inflow of relatively fresh, light waters. Strong evapo- temperatures were seriouslydepressed, by 5ø-10øC in ration allows the Mediterranean to act as a concentra- winterand 1ø-3øCin summer[Prentice et al., 1992], tion basin, converting the surface waters into a salty with regionsof permafrostextending into southern intermediatewater mass (LevantineIntermediate Wa- France[Goudie, 1992]. Aroundthe northernshores ter (LIW)). This water exits the basinas a deeperout- of the Mediterraneanand in the Levantthe presence flow of dense water, which may play a significantrole of steppe-likeand salt-tolerantplants suggesta low in the thermohaline circulation of the North Atlantic level of precipitationprevailed [Guoit, 1987; Goudie, [e.g., Reid, 1979]. The LIW alsoreaches several sites 1992;Cheddadi and Rossignol-Strick, 1995a]. Along the in the northern parts of the basin (Adriatic, Gulf of southernboundary of the Mediterraneanthere is more Lions, and the Aegean) where it can preconditionthe uncertainty regarding the water cycle. Fossil dunes water column for the formation of dense deep water [Goudie,1992], groundwater [Sonntag et al., 1980],and masses(Eastern Mediterranean Deep Water (EMDW) pollensamples [Rossignol-Strick and Duzer, 1980] sug- and Western MediterraneanDeep Water (WMDW)). gest an expansion of aridity in the southern Saharan However, paleø-øceanøgraphicrecords suggest that the region;however, radiocarbon dating of high lake lev- Mediterranean circulation has not always been as it is els may suggestwetter conditionsalong the southern at present. Mediterraneanshore [Streetand Grove,1979; Street- Perrott and Roberts, 1983], especiallyin the west [Goudie,1992]. There may alsohave been a shift in Copyright 1998 by the American GeophysicalUnion. the seasonalityof the precipitationto a more extreme Paper98PA02736. dry summerand cold wet winter regime [Prentice et al., 0883-8305/98/98PA-02736$12.00 1992]. Changesin the monsoonover East Africa led to a significantdecrease in precipitationthere [Goudie, 586 MYERS ET AL.: MODELING THE PALEO-MEDITERRANEAN 587 45 • I I I I Gulf of Lions 40 Aegean .... Tyrrhenian ß Sea ß', ea Strait IonianSea 35 o,s B Strait of Gibraltar C 3O -5 0 5 10 15 20 25 30 35 Longitude Figure 1. A map detailing some of the main Mediterranean locationsmentioned in this paper. The sectionsshown axe those for the schematicsin Figure 14. 1992],turning the Nile River into a low discharge,sea- mate simulationfrom this modelalso completely lacked sonalriver [Adamsonet al., 1980]. deep water formation in both the western basin and es- With large amountsof freshwater lockedaway in the peciallythe easternbasin, where no Adriatic deepwater, continentalice sheets,global sea level was •,120 m be- was formed. lowthat of today [Fairbanks,1989]. With decreasedair 1.2. The Holocene and Sapropel S1 temperatures,Mediterranean sea surfacetemperatures were also lower by 6ø-10øC in winter, with generally Within the sedimentaryrecord obtained by deep-sea smallerdecreases in summer[Bigg, 1994]. In general, coring, numerouslayers of black, often laminated, sed- the temperature decreasesare thought to have been iments rich in organic matter (sapropels)have been larger in the westernbasin than in the east and Levan- found betweenthe normal pelagicsediments [Kullen- tine [Thiede,1978]. Changesin salinityalso occurred, berg, 1958]. Sapropelscontain abundantand well- although these are harder to quantify. It is generally preservedcalcareous microfossils of planktonic origin believed that the basin salinity increased,although the but are mostly devoidof benthic fossils[Castradori, sizeof the increaseis questioned.Some researchers have 1993;Rohling, 1994]. They havebeen found throughout postulated a large increaseof severalpractical salinity the easternMediterranean, with upperdepth limits (for units(psu) [(e.g., Bethoux, 1984; Thunell and Williams, the overlyingwater column)quoted at 700 m [Thunell 1989;Rohling and Bryden, 1994], while others have sug- et al., 1984],to 300m [Rohlingand Gieskes, 1989], to as gestedonly a smallincrease [Bigg, 1994, 1995]. shallowas 150 m in the North AegeanSea [Perissoratis One previousattempt to model the circulationof the and Piper, 1992]. Their presencesuggests anoxic con- Mediterranean at the LGM using an ocean general cir- ditions below the specifieddepths. Sapropelshave also culationmodel (OGCM) wasby Bigg [1994]. He used beenfound in the TyrrhenianSea [Thunellet al., 1984; a fairly coarseresolution (for the Mediterranean)ver- Castradori,1993] and possiblyother parts of the west- sionof the Bryan-Cox[Cox, 1984] OGCM, with a spa- ernMediterranean [Rohling, 1994]. Sapropels have been tial resolution of 0.5 ø and 18 vertical levels. The model formedthroughout the past 7 Myr [Rohlingand Hilgen, was forcedby the output from the atmosphericGCM of 1991; Nijenhuiset al., 1996],with at least 11 formed Kutzbachand Guetter[1986], spatially and temporally duringthe last 450 kyr [Cheddadiand Røssignøl-Strick' interpolated onto the ocean grid, using an interactive 1995b].Although the majoritywere formed during cli- heat and salt flux scheme.Bigg [1994] found that his matic warm periods, severalwere formed under glacial modeled sea surfacetemperatures were similar to the re- conditions,and astronomicalforcing is thought to con- constructions,but that his modelledsea surface salinity trol the timing of Sapropel formation, associatedwith (SSS)fields were at mostonly 0.1-0.2higher than today. minima of the precessioncycle which occur about every He also found a completeloss of stability in the western 21,000 years[Rohling and Hilgen,1991]. Theseastro- basin in winter, with WMDW overflowingthe Strait of nomical conditionscreate changesin the monsoon,and Sicily to fill the deep easternbasin. However,the model thus precipitation and runoff patterns in the Mediter- had a poor initial Mediterraneanclimatology which was ranean watershed. significantlytoo fresh and warm. The present-daycli- The most recent sapropel$1 was formed during the 588 MYERS ET AL.: MODELING THE PALEO-MEDITERRANEAN Holocene, a period of warm and wet climate in the arine circulation[Zahn et al., 1987; Vergnaud-Grazzini Mediterraneanregion [Mangini and Schlosser,1986; et al., 1989]. Rohlingand Hilgen, 1991]. It was depositedbetween Other work suggeststhat the productivity of the -•9000and 6000years B.P. [Jorissenet al., 1993;Troel- Mediterranean had substantially increased during stra et al., 1991; Perissoratis and Piper, 1992; Fontugne sapropelformation [e.g., de Langeand ten Haven,1983; et al., 1992],on the basisof acceleratormass spectrome- Calvert, 1983]. Drawing a parallel to conditionsin try (AMS) 14Cdating. Several studies suggest that the the present-dayBlack Sea, Pederson and Calvert[1990] processesculminating in the formation of $1 started suggestthat it is high productivity, not bottom water muchearlier [e.g., Howell and Thunell,1992; Rohling et anoxia, that controls the increaseof organic carbon in al., 1993],with the mostrecent minimum in the preces- sediments. The enhancedproductivity may have been sioncycle occurring at 11.5kyr B.P. [Bergerand Loutre, relatedto enhancedriverine nutrient fluxes [Rohling and 1994]. Hilgen,1991] compounded with basin-wideshoaling of Studiesof 5150 records[e.g., Vergnaud-Grazziniet the pycnoclineinto the euphoticzone [Rohling,1994]. al., 1977;Cita et al., 1977;Mangini and Schlosser, 1986] The consequentenhanced rain of organic debris from suggestedthat eastern Mediterranean surface salini- the euphoticzone may not have been fully decomposed ties were very low at
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