The onset of the Messinian salinity crisis in the Eastern Mediterranean (Pissouri Basin, Cyprus)

Transcription

1 Earth and Planetary Science Letters 194 (2002) 299^310 The onset of the Messinian salinity crisis in the Eastern Mediterranean (Pissouri Basin, Cyprus) W. Krijgsman a; *, M.-M. Blanc-Valleron b, R. Flecker c;d, F.J. Hilgen e, T.J. Kouwenhoven e, D. Merle f, F. Orszag-Sperber g, J.-M. Rouchy b f a Paleomagnetic Laboratory `Fort Hoofddijk', Budapestlaan 17, 3584 CD Utrecht, The Netherlands b CNRS FRE 2400, Laboratoire de Gëologie du Musëum National d'histoire Naturelle, 43 rue Bu on, Paris, France c Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK d Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QF, UK e Department of Geology, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands UMR 8569 CNRS, Laboratoire de Palëontologie, Musëum National d'histoire Naturelle, 8, rue Bu on, Paris, France g Universitë Paris-Sud (Orsay), Dëpartement des Sciences de la Terre, Paris, France Received 26 June 2001; received in revised form 2 November 2001; accepted 2 November 2001 Abstract The Pissouri Basin in Cyprus contains one of the most suitable sedimentary successions with which to study the onset of the Messinian Salinity Crisis in the Eastern Mediterranean. Exposures along the new Paphos^Limassol motorway near Pissouri exhibit distinct cyclic bedding which permits the construction of a chronology based on orbital tuning. Biostratigraphic results reveal 10 planktonic foraminifera events that have been astronomically dated in other Mediterranean sections, and as such provide an excellent first-order age control. Magnetostratigraphic results are in good agreement with the biostratigraphic data and show that all magnetic chrons between C4n.1n and C3An.1n are present. The pattern of sedimentary cycles generally fits well with the insolation curve. Astronomical tuning of the succession shows that the first gypsum bed at Pissouri overlies a 40^60 kyr stromatolite-bearing transitional interval and correlates with the amplitude increase in insolation at 5.96 Ma, as in the western Mediterranean. This indicates that the onset of evaporite precipitation was synchronous right across the entire Mediterranean Basin. ß 2002 Elsevier Science B.V. All rights reserved. Keywords: stratigraphy; evaporites; Miocene; Mediterranean region; Cyprus 1. Introduction Successions recording the late Miocene evolution of the Mediterranean Sea have been the focus * Corresponding author. Tel.: ; Fax: address: krijgsma@geo.uu.nl (W. Krijgsman). of numerous recent investigations searching for the processes which triggered the Messinian Salinity Crisis (MSC) [1^6]. Restriction of the Mediterranean^Atlantic connection ultimately resulted in the deposition of massive evaporites (gypsum and halite) followed by brackish water sedimentation (Lago Mare facies). Marine conditions were re-established at the beginning of the Pliocene as a consequence of ooding from the Atlantic [7,8] X / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved. PII: S X(01)00574-X

2 300 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^310 In most parts of the Mediterranean basin, the Messinian evaporites overlie cyclically bedded marine sediments which commonly comprise diatomites, marls and carbonates (e.g. Tripoli Formation on Sicily). This sedimentary cyclicity developed in response to an astronomically induced climate signal, ampli ed by the marginal setting of the Mediterranean. As such these sediments represent ideal successions for the construction of an astronomically calibrated time scale for the late Miocene [2,4,9]. Messinian astrochronology and integrated stratigraphic results from Sicily, the northern Apennines and SE Spain have demonstrated that in the western Mediterranean the onset of evaporite formation during the MSC was a remarkably synchronous event at 5.96 Ma [1^3]. The total mass of evaporites deposited in the Eastern Mediterranean is much greater than in the western basin, and many Messinian gypsumbearing sequences are now exposed on land (e.g. Ionian Islands [10]; Crete [11]; and Cyprus [12]). However, a high-resolution age model for the Eastern Mediterranean gypsum sequences is lacking since they have yet to be calibrated to the astronomical time scale. The astronomically dated Metochia section on Gavdos (Greece) does not contain gypsum, but shows an abrupt transition from diatomites to carbonates at the same time as evaporite deposition was initiated in the Western Mediterranean [2]. Accurate age control of the onset of Eastern Mediterranean evaporite precipitation is a critical component in understanding of the processes and mechanisms that underlie the MSC. This is particularly important because the Eastern Mediterranean is separated from the Western Mediterranean by a structural high in the Sicily Strait, which could have played a major role for evaporite precipitation in the Eastern Mediterranean during the MSC (e.g. [13]). The objective of this study is to obtain astronomical age control for the interval transitional to evaporite precipitation of the MSC in the easternmost part of the Mediterranean basin. We have selected the Pissouri Motorway Section in SW Cyprus because it comprises one of the most continuous upper Miocene successions from late Tortonian up to the onset of Messinian gypsum deposition. Moreover, the section exhibits the distinctive cyclic bedding typical of late Neogene Mediterranean sequences. An integrated stratigraphic approach, resulting in astronomical tuning of the sedimentary cycles, permits correlation and comparison of environmental changes in the Eastern and Western Mediterranean during the Messinian at an unprecedented level of detail. 2. Geological setting In the southern part of Cyprus, a Tertiary sedimentary cover fringes the Troodos Massif. Troodos is a fragment of ancient oceanic crust that originated during intra-continental rifting in late Triassic times [14]. Neogene sediments are distributed in small sub-basins, like the Polemi, Pissouri and Psematismenos basins (Fig. 1), which formed during a middle Miocene phase of palaeogeographic reorganisation [12,15^17]. This change in basin con guration re ects tectonic instability related to both the uplift of the Troodos Massif [18] and the position of Cyprus in a fore-arc setting within the complex zone of convergence between Africa and Eurasia [16,19]. The Messinian evaporites were deposited in these sub-basins at a time when the African plate was undergoing northward subduction beneath Eurasia [17]. The Neogene sedimentary sequence on Cyprus shows a gradual shallowing from Palaeocene^Oligocene deep-water pelagic carbonates (Lefkara Formation [20]), through mixed detrital and carbonate sediments (Pakhna Formation [21]) and Fig. 1. Location of the Pissouri Motorway Section and the Messinian gypsum deposits on Cyprus.

3 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^ reefal limestones (Koronia Member [22]), to the Messinian evaporites [12,17]. Deeper water conditions were re-established at the beginning of the Pliocene before renewed shallowing in middle Pliocene times [17,23]. The Messinian sedimentary succession of Cyprus is characterised by pre-evaporitic deposits, overlain by two gypsum units which are separated by a breccia [12,17]. In turn, the upper gypsum unit is overlain by lagoonal^lacustrine deposits (Lago Mare facies), followed by marine marls that re ect the basal Pliocene ooding [23]. 3. The Pissouri Motorway Section One of the best exposed upper Miocene sections on Cyprus is located along the new Limassol^Paphos motorway (Fig. 1), approximately 1 km west of Pissouri village. The section comprises two separate exposures (Fig. 2a) across which the characteristic cyclic sediments dip gently to the southwest (S 0 = 230/5). As a result of the road building programme, these exposures were completely fresh when they were sampled. Taking advantage of a small platform, built to reduce the risk of rock fall, we were able to log and sample a complete succession from upper Tortonian marls to the Messinian evaporites (Fig. 2a). The Pissouri Motorway Section shows a distinct sedimentary cyclicity which is generally characterised by an alternation of indurated calcareous beds and less indurated marls. The section was sampled and measured from the base of the evaporites downward (Fig. 3). Consequently, the indurated beds have been numbered successively with increasing stratigraphic depth (PC 1^48). Several weakly developed carbonate beds may indicate additional cycles. These have been labelled `a' following the number of the preceding cycle. In the lowermost part of the section, the sedimentary cyclicity is not very distinct and only expressed by colour alternations of blue marls and darker beds (Fig. 3, I^VII). Upward, the cyclicity becomes more prominent and is identi ed by distinct di erences in induration of the sediments (Fig. 2b). The average thickness of these cycles is 40^50 cm. This part of the section consists dominantly of blue-greyish marls which commonly contain bivalve shells, pteropods, and abundant burrows. The bulk carbonate content does not reveal signi cant variations (Fig. 3). Detrital levels, characterised by intercalations of centimetre- to decimetre-thick layers of reddish gypsiferous sands, are abundant in the interval between PC 28 and PC 42. A gradual transition in lithology is observed in the middle part of the section (PC 33^23). The carbonate content of the more recessive-weathering beds decreases resulting nally in a cyclic alternation of whitish indurated homogeneous carbonates and laminated marls. Dark laminated organic-rich clay layers, commonly referred to as sapropels, are present in the sedimentary cycles Fig. 2. Photographs showing the sample trajectory of the Pissouri Motorway Section (a) and the sedimentary cyclicity in the lower part of the section (b).

4 302 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^310 PC 25^22. This middle part of the succession commonly contains bivalve shells, pteropods and burrows and benthic diversity is at a maximum in the PC 27^22 interval [24]. Unfortunately, this middle interval also contains bedding-parallel shear planes that strongly reduce the stratigraphic thickness of the sapropelitic layers. In contrast, high-angle faults are not observed. The upper pre-evaporitic part of the Pissouri Motorway Section (PC 22^1) shows cyclic alternation of whitish indurated homogeneous limestones, marls and marly limestones. The thickness

5 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^ Fig. 3. Carbonate content, lithology, biostratigraphy and magnetostratigraphy of the Pissouri Motorway Section. In the lithology column, white levels correspond to more indurated calcareous beds (numbered in descending order), black levels are dark green organic-rich layers, termed sapropels. Grey levels correspond to softer blue-greyish and laminated marls, dotted levels represent the transitional interval to the evaporites composed of stromatolitic limestones, and `v'-shaped levels symbolise gypsum. Fishes represent levels with fossil sh remains. The lithological alternations of soft laminated marls and more indurated limestones are also determined by variations in the bulk carbonate content which ranges between 18 and 100%. The carbonate content of the lower part of the section does not vary signi cantly explaining the di culties in distinguishing sedimentary cycles. From PC 18a upward, the carbonate content is exclusively composed of calcite and occasionally very small quantities (up to 13%) of aragonite, while biosiliceous remains become more abundant. The planktonic foraminiferal events correspond to: (1) LO of G. menardii 4; (2) LO of G. falconarae; (3) FO of G. menardii 5; (4) FRO of the G. miotumida group; (5) LCO of dominantly sinistral G. scitula; (6) FO of G. nicolae; (7) LO of G. nicolae; (8) LO of the G. miotumida group; (9) sinistral/dextral coiling change of Neogloboquadrina acostaensis; (10) rst in ux ( s 80%) of sinistral neogloboquadrinids. In the magnetic polarity column black denotes normal polarity, white is reversed and grey indicates intervals of uncertain polarity. Closed symbols represent reliable directions, open symbols are unreliable and crosses correspond to samples for which no meaningful direction was determined. 6 of the individual cycles varies from to V1.5 m. The softer marls are commonly laminated and exhibit a diatomitic appearance. However, no pure diatomites such as those found in other Mediterranean areas (e.g. northern Morocco (Melilla, Boudinar), Algeria (Chelif), SE Spain (Sorbas, Nijar), Sicily and Gavdos [25]) were identi ed in the section. The biosiliceous remains of the marls comprise abundant sponge spicules, while the abundance of diatoms, silico agellates and radiolarians varies. This interval commonly contains bivalve shells, Discospirina, sh remains, some pteropods, and gastropods. In the uppermost part of the section, a 3.5-m-thick slumped interval, characterised by large contorted fragments of laminated marls and carbonates, is intercalated between PC 3 and 4 (Fig. 3), above a more organic-rich interval. This slump does not signi cantly truncate the underlying deposits of the Pissouri Motorway Section. However, its thickness rapidly increases southwards, reaching up to 8 m, where it forms a chaotic accumulation of reef limestone blocks (up to several metres in diameter) mixed with fragments of marls, diatomites and homogeneous carbonates. The transitional interval from cyclic carbonates to evaporites, informally known as `barre jaune' in the Polemi Basin [26], is about 1.5 m thick here. It is composed of indurated and nely laminated limestones which display features characteristic of microbial deposits (stromatolites). We have interpreted the `barre jaune' to represent two or three sedimentary cycles. This interval marks an important environmental change towards the top of the section, but the sediments were not suitable for magnetostratigraphy. 4. Biostratigraphy Planktonic foraminiferal biostratigraphy of the Pissouri highway section is based on the stratigraphic distribution of several late Miocene marker species plus the coiling ratios of Globorotalia scitula and Neogloboquadrina acostaensis. Ten biostratigraphic events, which have previously been dated astronomically in other Mediterranean sections, are also recognised in the Pissouri Motorway Section (Fig. 3). Keeled globorotaliids, which play a key role in biostratigraphic studies of the late Tortonian^early Messinian time interval, are well represented. In the lowermost part of the section, a brief in ux of sinistrally coiled Globorotalia menardii (form 4) is observed with its last common occurrence (LCO) slightly above dark layer PC II. Higher in the section, keeled globorotaliids are absent up to the rst occurrence (FO) level of dextrally coiled Globorotalia menardii (form 5) between PC 47 and PC 46. The rst regular occurrence (FRO) of the Globorotalia miotumida group, which closely corresponds to the recently de ned Tortonian^Messinian boundary [27], is located between PC 44 and PC 43, while the LO of the Globorotalia miotumida group is determined between PC 21 and PC 20. Other important biostratigraphic events in stratigraphic order are: the LO of Globorotaloides falconarae at approximately 46 m, the

6 304 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^310 LCO of dominantly sinistral Globorotalia scitula in PC 38, the FO and LO of Globorotalia nicolae in PC 29 and PC 26, respectively, the sinistral/ dextral coiling change of Neogloboquadrina acostaensis between PC 14 and PC 13, and the rst in ux ( s 80%) of sinistral neogloboquadrinids between PC 5a and PC Magnetostratigraphy Palaeomagnetic samples were taken from 199 levels with a water-cooled drill and oriented to modern north with a magnetic compass. At least one specimen per sampling level was thermally demagnetised with small temperature increments of 30^50³C up to a maximum temperature of 600³C in a magnetically shielded, laboratory-built furnace. The natural remanent magnetisation (NRM) was measured on a 2G Enterprises DC SQUID cryogenic magnetometer. Least-squares analysis was applied to determine the component directions of the NRM, chosen by inspection of vector end-point demagnetisation diagrams. Some rock magnetic tests were performed to identify the dominant carriers of the magnetism, including acquisition of an isothermal remanent magnetisation (IRM) and subsequent demagnetisation of three orthogonal IRM components. The IRM was induced in a pulse magnetiser and measured on a digitised spinner magnetometer based on a Jelinek JR5 driver unit. In the lower part of the section, NRM intensities range between 0.1 and 10 ma/m, and thermal demagnetisation diagrams are generally of good quality. In most cases, a normal polarity component is removed at temperatures between 100 and 180³C (Fig. 4a). This relatively low-temperature component has a present-day eld direction before bedding plane correction and can thus be regarded as a secondary overprint, probably caused by sub-recent weathering or viscous mag- Fig. 4. IRM acquisition, three-component IRM demagnetisation curves and thermal demagnetisation diagrams of selected samples from the lower part (a) and upper part (b) of the Pissouri Motorway Section. In the IRM diagrams, the saturation at elds mt and the total unblocking at temperatures of 570³C indicate that magnetite is the dominant carrier of the magnetisation.

7 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^ netisation. Demagnetisation at higher temperatures shows that a relatively high temperature component is gradually removed between temperatures of 180 and 580³C. This component has both reversed and normal polarities and is interpreted as the characteristic remanent magnetisation (ChRM). In the upper part of the section, NRM intensities are much lower (0.05^0.3 ma/ m) and demagnetisation diagrams are usually of lesser quality. After removal of the low-temperature component, the remaining intensity is not always su ciently high (especially in the indurated limestones) to reliably determine the characteristic directions, although in many samples the polarity can still be resolved (Fig. 4b). Normalised IRM acquisition and demagnetisation curves show no signi cant di erences between samples from the lower and upper part of the section (Fig. 4). The IRM acquisition curves all show an initial steep rise and saturation at elds of 300^500 mt. Blocking temperatures of 580³C indicate that magnetite is the dominant carrier of the magnetic signal. Moreover, a higher-coercivity mineral like maghaemite may also be present. Declinations and inclinations were calculated for each characteristic component after correction for bedding tilt. These ChRM directions and the resulting polarity zones show that 11 polarity reversals are recorded (Fig. 3). The overall mean direction in tilt-corrected co-ordinates is: Dec = 350³, Inc = 57³ for the normal components and Dec = 161³, Inc = 353 for the reversed components (Fig. 5). The reversal test is negative. Possibly, a later present-day eld or late diagenetic overprint has not been entirely removed because of partial blocking temperature spectrum overlap at temperatures higher than 200³C. Averaging all directions results in a mean counterclockwise rotation of 16³ at an age younger than late Messinian. 6. Palaeobathymetry Fig. 5. Equal-area projection of the ChRM directions after bedding correction. Grey circles give K 95 for the di erent means. N = number of samples; Dec. = declination; Inc. = inclination; k = Fisher's precision parameter; K 95 = 95% cone of con dence. Palaeodepth reconstructions in the Pissouri Motorway Section are based on foraminifera, bivalves and gastropods. Planktonic/benthic foraminifer (P/B) ratios and assemblage characteristics of benthic foraminifers have been evaluated for the non-indurated intervals. P/B ratios as applied in Van der Zwaan et al. [28] are not always reliable in the Pissouri Motorway Section, as sedimentary and faunal features frequently indicate downslope transport and, during the Messinian, periodic unstable benthic environments. Gypsum and pseudomorphs after gypsum occur occasionally in samples above PC 22, the frequency and amount increasing towards the top of the section. Tortonian and early Messinian depositional depth is estimated to be outer shelf/upper slope. Shallowing is apparent and mainly occurs in the approximately 10 m interval between PC 17 and PC9. Above PC 9, the sediments were probably deposited at depths less than 100 m. The bivalves and gastropods, present throughout the section, indicate a stepwise decrease in depositional depth from circalittoral^epibathyal conditions in the

8 306 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^310 lower part of the section to shallower conditions in the middle part [29]. All palaeoecological data thus indicate a signi cant shallowing from depths around 500 m in the upper Tortonian/lower Messinian interval to very shallow water conditions just before the onset of evaporitic conditions. 7. Correlation to the astronomical curves The advent of the astronomical time scale has provided an excellent chronostratigraphic framework for the late Miocene, permitting marine sections to be correlated on a bed-to-bed basis over long distances. High-resolution biostratigraphic studies on astronomically dated sections in western [6], central [4] and eastern [9] parts of the Mediterranean demonstrate that a large number of Tortonian^Messinian planktonic foraminiferal events are synchronous all over the basin [2], and can serve as very accurate time horizons for stratigraphic studies. In the Pissouri Motorway Section, 10 planktonic foraminiferal events are recorded that have been astronomically dated [4,9] in other Mediterranean sections (Fig. 6). These planktonic foraminiferal events provide an excellent rst-order age control, allowing correlation of the sedimentary cycles to the summer insolation target curve [30]. On the basis of the phase relations ascertained for Miocene and Pliocene marine sequences elsewhere in the Mediterranean [9,31], indurated limestones and marls are assumed to correspond to insolation minima while laminated diatomitic and sapropelitic intervals correlate with insolation maxima. The cycle patterns in the Tortonian^Messinian boundary interval of the Pissouri Motorway Section, including the faintly developed indurated beds of PC 45a, 44a, 42 and 40, correlate well with (relatively) low-amplitude peaks in the insolation curve which result from precession obliquity interference (Fig. 6). This correlation is in good agreement with the known APTS ages of the three bioevents (3, 4, and 5) in this interval. Upward tuning of the sedimentary cycles to the insolation curve seems straightforward and reliable up to PC 33. The interval between PC 33 and PC 18 is problematic, as fewer cycles seem to be present between the positions of the G. nicolae FO and LO (events 6 and 7) and the G. miotumida LO (event 8) at Pissouri than in other Mediterranean sections. This may be the result of local dynamics whose e ects are commonly observed in the Messinian. For example, in the nearby Alektora section, water escape structures and small slumps are observed at various levels. This suggests ongoing tectonic instability of the basin which may mask or decrease the number of cycles preserved. However, our chronological resolution in this interval is not su cient to identify which cycles are missing. We can therefore only present a tentative correlation to insolation. Overlying this interval, the number of cycles again ts well with that of the astronomical target curve [30] from cycle PC 18 up to the S/D coiling change of N. acostaensis (event 9) and the rst sinistral in ux (event 10) (Fig. 3). In addition, the weakly developed limestone beds (5b, 5a, 2a and 1a) once again correlate well with relatively low-amplitude peaks in the insolation curve. The astronomical age for the last sedimentary cycle below the transitional interval to the evaporites is V6.0 Ma, assuming that no cycle is missing and the period of deposition of the slump is very short if not instantaneous. The magnetic polarity pattern of the Pissouri section is in excellent agreement with the biostratigraphic age constraints and demonstrates that every magnetic (sub)chron between C4n.1n and C3An.1n has been recorded (Fig. 3). However, there are some discrepancies in the cyclostratigraphic positions of particular palaeomagnetic reversals between the Pissouri Motorway Section and previously studied sections in the Mediterranean. The two short normal intervals between the LO of G. menardii 4 and the FRO of the G. miotumida group both comprise only 1^1.5 sedimentary cycles ( = 20^30 kyr) in the Pissouri Motorway Section. The corresponding chrons, C3Br.2n and C3Br.1n, by contrast, have an astronomical duration of 37 and 45 kyr, respectively [9]. Moreover, the reversal boundary C3Bn(y) is recorded between PC 40 and 39 and has an astronomical age of 7.15 Ma. This is 50 kyr older than the age

9 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^ Fig. 6. Astronomical tuning of the Pissouri Motorway Section to the insolation curve of Laskar et al. [30] and cyclostratigraphic bed-to-bed correlations with the composite section of the Sorbas Basin of the Western Mediterranean [2,6]. The grey shaded interval corresponds to the tectonically deformed middle part of the section where several cycles are missing and correlations are uncertain. The numbers in circles show planktonic foraminifera events that have previously been proven synchronous throughout the Mediterranean: (1) LO of G. menardii 4 (7.512 Ma); (2) LO of G. falconarae (7.456 Ma); (3) FO of G. menardii 5 (7.355 Ma); (4) FRO of the G. miotumida group (7.240 Ma); (5) LCO of dominantly sinistral G. scitula (7.095 Ma); (6) FO of G. nicolae (6.829 Ma); (7) LO of G. nicolae (6.722 Ma); (8) LO of the G. miotumida group (6.506 Ma); (9) sinistral/dextral coiling change of Neogloboquadrina acostaensis (6.337 Ma); (10) rst in ux ( s 80%) of sinistral neogloboquadrinids (6.126 Ma). For lithology column see legend to Fig. 3. Black levels from the Sorbas Basin represent sapropels. Note that the transition to MSC evaporites occurred synchronously in the Western and Eastern Mediterranean basin.

10 308 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^310 determined from the APTS, which is based on the magnetostratigraphic and cyclostratigraphic data from sections on Crete [9]. However, it is synchronous with the boundary in the Oued Akrech section located on the Atlantic coast of Morocco [32]. The ages of the youngest palaeomagnetic reversals C3An.2n(y) and C3An.1n(o) are in good agreement with the astronomically derived ages for the same reversals in the Sorbas Basin of Spain [2], although the magnetic signal of this interval in the Pissouri Motorway Section is relatively weak. 8. Discussion and conclusions The Pissouri Motorway Section contains a cyclic sedimentary succession comprising well and poorly indurated marine marls in the lower part and laminated sapropelitic and diatomaceous sediments in the upper part. The section, which is topped by evaporites, documents the chronology of the Messinian events in this part of the Eastern Mediterranean and re ects the typical Mediterranean stratigraphy of the period immediately preceding the salinity crisis [2]. Palaeobathymetric analyses show that the late Miocene depositional environment on Cyprus was characterised by a progressive decrease in bathymetry from approximately 500 m to very shallow-water conditions. In the marginal areas of the Psematismenos Basin, mud cracks in the stromatolite-bearing interval below the gypsum indicate episodes of desiccation. This suggests that the onset of the MSC on Cyprus occurred in a shallow hydrologic setting subject to large uctuations in physical and chemical parameters such as salinity and depth. The most striking result outlined in this paper is the integrated stratigraphic data from the Pissouri Motorway Section, which permits tuning of its constituent sedimentary cycles and bed-to-bed correlation with other Messinian sequences. With the resolution of a few thousand years, such correlation allows detailed study of the palaeoclimate and environmental changes that precede the deposition of evaporites at a time scale much closer to that of the forcing function of the Mediterranean's hydrologic system. The abrupt transitions from carbonates to diatomites and then evaporites are the most conspicuous of the lithological changes recorded in the basinal successions of the Mediterranean. In the Pissouri Motorway Section, also the transitions to sapropelitic (PC 25) and biosiliceous-rich sediments (PC 19) have important implications but unfortunately they take place in the problematic interval. Hence, no reliable astronomical age can be determined for these transitions. We can make a good estimate, however, because the rst distinct sapropel is found in cycle PC 25 and corresponds closely to the G. nicolae LO (one cycle higher) dated astronomically at Ma [9]. This is coeval with a major lithological transition observed in other sections in the Western and Eastern Mediterranean around 6.7 Ma. On Crete, a sharp decrease in terrigenous supply occurred at 6.77 Ma, while on Gavdos, deposition of diatomites began [9]. This event, referred to as the `early Messinian starvation event' (EMSE of [33]), is probably a local Cretan phenomenon since sedimentation rate and terrigenous supply on Cyprus show an increase at the same time. In the Western Mediterranean, the Sorbas and Nijar basins in Spain experienced a major lithological transition from mainly homogeneous marls (`Lower Abad') to sapropel and diatomite-bearing sediments (`Upper Abad'); this transition is dated astronomically at 6.77 Ma [6]. Moreover, the onset of prograding reefs in the Alboran Basin is taken to represent the same event [34], and was recently dated radiometrically at 6.73 þ 0.02 Ma [5]. The onset of diatomite formation in the classical Tripoli Formaton on Sicily, however, started signi cantly earlier, at Ma [4]. Summarising, evidence is accumulating that a major palaeoenvironment change a ected the entire Mediterranean at 6.8^6.7 Ma. Unfortunately, age control is not straightforward in this interval of the Pissouri Motorway Section and sections in the Psematismenos Basin may be more suitable for constraining the exact age of this lithological transition on Cyprus. The high-resolution astrochronology for the Pissouri Motorway Section reveals that evaporite deposition began at 5.96 þ 0.02 Ma. Two to three sedimentary cycles consisting of stromatolitic

11 W. Krijgsman et al. / Earth and Planetary Science Letters 194 (2002) 299^ limestones are found beneath the rst gypsum. This stromatolite-bearing interval directly below the evaporites is also observed in the adjacent basins of Psematismenos and Polemi where they are locally intercalated with thick megabreccia beds [12,15,17]. Evaporite deposition during the MSC on Cyprus is thus preceded by an approximately 60-kyr transitional interval of deteriorating environmental conditions. The rst gypsum bed coincides with the amplitude increase in insolation following the V400-kyr eccentricity minimum dated around 6.0^6.1 Myr, suggesting that long-term orbital cycle forcing plays an important role in the exact timing of this event. Cyclostratigraphic correlations of the Pissouri Motorway Section with other astronomically dated sections in the Mediterranean reveal that the last regular sedimentary cycle of the pre-evaporitic sequence has exactly the same age (6.0 Ma) in the Sorbas Basin of Spain, the Caltanissetta Basin in Sicily, the Vena del Gesso Basin in Italy, and the Gavdos Basin south of Crete. In both Spain and northern Italy a similar approximately 60-kyr transitional interval is also documented. The high-resolution correlation of the Pissouri Motorway Section with other Mediterranean sections therefore indicates that the onset of evaporite precipitation was a remarkably synchronous event dated at 5.96 Ma þ This implies that the response to the event that triggered evaporite formation during the MSC was Mediterraneanwide. Furthermore, it follows that the structural high between Sicily and Malta ^ sometimes invoked as a key player in the evaporitic evolution of the eastern and western basins [13] ^ cannot have been a barrier to either the water or the salt ux prior to and during initial evaporite precipitation. Acknowledgements The Geological Survey of Cyprus provided access to the section and support during eldwork. The help of Dr A. Constantinou, Director of the Survey, I. Panayides and C. Xenophontos is particularly warmly acknowledged. We also thank E. Snel and C.E. Duermeijer for their help in the eld, G. Ittman, H.J. Meijer, P.J. Verplak and G.J. van 't Veld for their assistance with the laboratory procedures. M. Garcës, C.G. Langereis, A.H.F. Robertson, F.J. Sierro, J.A. Van Couvering and W.J. Zachariasse are thanked for their critical and constructive reviews of the manuscript. W.K. acknowledges nancial support from the Dutch research centre for Integrated Solid Earth Sciences (ISES). The French team was in part supported by the Crisevole programme. R.F.'s participation was funded by NERC, the Royal Society, The Carnegie Trust for the Universities of Scotland, the Geological Society of London and CASP.[RV] References [1] F. Gautier, G. Clauzon, J.-P. Suc, J. Cravatte, D. Violanti, Age et durëe de la crise de salinitë messinienne, C.R. Acad. Sci. Paris 318 (1994) 1103^1109. [2] W. Krijgsman, F.J. Hilgen, I. Ra, F.J. Sierro, D.S. Wilson, Chronology, causes and progression of the Messinian salinity crisis, Nature 400 (1999) 652^655. [3] W. Krijgsman, F.J. Hilgen, S. Marabini, G.B. Vai, New paleomagnetic and cyclostratigraphic age constraints on the Messinian of the Northern Apennines (Vena del Gesso Basin, Italy), Mem. Soc. Geol. It. 54 (1999) 25^33. [4] F.J. Hilgen, W. Krijgsman, Cyclostratigraphy and astrochronology of the Tripoli diatomite Formation (pre-evaporite Messinian, Sicily, Italy), Terra Nova 11 (1999) 16^ 22. [5] S. Roger, P. Mu«nch, J.J. Cornëe, J.P.S. Martin, G. Fëraud, S. Pestrea, G. Conesa, A.B. Moussa, 40 Ar/ 39 Ar dating of the pre-evaporitic Messinian marine sequences of the Melilla basin (Morocco): a proposal for some biosedimentary events as isochrons around the Alboran Sea, Earth Planet. Sci. Lett. 179 (2000) 101^113. [6] F.J. Sierro, F.J. Hilgen, W. Krijgsman, J.A. Flores, The Abad composite (SE Spain): A Messinian reference section for the Mediterranean and the APTS, Palaeogeogr. Palaeoclimatol. Palaeoecol. 168 (2001) 141^169. [7] K.J. Hsu«, W.B.F. Ryan, M.B. Cita, Late Miocene desiccation of the Mediterranean, Nature 242 (1973) 240^244. [8] K.J. Hsu«, L. Montadert, D. Bernoulli, M.B. Cita, A. Erickson, R.E. Garrison, R.B. Kidd, F. Melieres, C. Muller, R. Wright, History of the Mediterranean salinity crisis, Nature 267 (1977) 399^403. [9] F.J. Hilgen, W. Krijgsman, C.G. Langereis, L.J. Lourens, A. Santarelli, W.J. Zachariasse, Extending the astronomical (polarity) time scale into the Miocene, Earth Planet. Sci. Lett. 136 (1995) 495^510. [10] K.O. Heimann, The evaporite-bearing late Miocene on the Ionian Islands (Greece), Mem. Soc. Geol. It. 16 (1976) 319^325.

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