CLIMATIC CHANGES IN THE MEDITERRANEAN DURING THE NEOGENE

CLIMATIC CHANGES IN THE MEDITERRANEAN DURING THE NEOGENE (10 -4 Ma) DEDUCED FROM TEMPERATE AND TROPICAL CARBONATES JOSÉ M. MARTÍN 1, JUAN C. BRAGA 1, ISABEL M. SÁNCHEZ-ALMAZO 2 and JULIO AGUIRRE 1 (1) Departamento de Estratigrafía y Paleontología Universidad de Granada (2) Centro de Instrumentación Científica (CIC), Universidad de Granada

MEDITERRANEAN-NEOGENE, SHELFCARBONATE RECORD - Alternating temperate and tropical carbonate episodes are common feature in the Upper Neogene stratigraphic record of the western Mediterranean. - Temperate carbonates appear in the early Tortonian, latest Tortonian-earliest Messinian and in the Zanclean. By constrat, tropical carbonate formation took place in the earliestmost Tortonian, late Tortonian and the Messinian.

TEMPERATE AND TROPICAL CARBONATES CONSTRASTED: THE TEMPERATE (FORAMOL) AND TROPICAL (CHLOROZOAN) ASSOCIATIONS

TEMPERATE AND TROPICAL CARBONATES CONSTRASTED: OTHER MAJOR DIFFERENCES - Micrite is neither chemically or biochemically precipitated on temperate-carbonate environments. If present, it forms as result of bioclast grinding. - Early sea-floor lithification is a significant feature in tropical carbonates but is almost absence in temperate carbonates. As a result temperate-carbonate particles are loose on the sea-floor and can be easily remobilized. In this respect temperate carbonates can be compared with and behave very much like siliciclastic sediments. - Sedimentation rates are one order of magnitude (ten times) higher in tropical than in temperate carbonates.

A CASE STUDY: THE NEOGENE BASINS OF THE BETIC CORDILLERA

THE MEDITERANEAN-LINKED, NEOGENE BETIC BASINS - They differentiated as small, interconnected seaway passages, linked to the open Mediterranean sea to the S and to the E, as the major Betic reliefs were being uplifted and emerged as islands. - Sediments deposited at the margins of the Mediterranean-linked, Neogene Betic Basins are either carbonates or coarse-grained (conglomerates and sands) siliciclastics, the latter in connection with “rambla” and river discharge areas. In deeper, basinal settings silts, clays and marls accumulated. Evaporites also formed locally. - Carbonates deposited on the siliclastic-free, shelf areas are of two types (temperate and tropical). Both types contain abundant bioclast (fossil) remains.

THE TEMPERATE CARBONATES A first type are the non-reefal, cold-water temperate carbonates. They formed in similar conditions to those occurring on the Present-day Mediterranean sea, with mean winter, sea-surface, water temperatures between 14° C and 17° C.

DEPOSITIONAL MODELS OF TEMPERATE CARBONATES IN THE BETIC CORDILLERA

THE FACTORY ZONE

“THE FACTORY FACIES”

MOBILIZATION PROCESSES - The lack of early lithification favours mobilization of skeletal particles from the “factory”. - Waves and currents accumulate bioclastic particles in shoals, spits and beaches. - Skeletal grains are also mobilized downslope as sediment gravity flows.

REDEPOSITED TEMPERATE CARBONATES - Temperate-carbonate sediments are also mobilized via submarine channels and canyons.

THE TROPICAL EPISODES - A second type of Mediterranean Neogene carbonate deposits are tropical carbonates, exemplified by coral reefs with abundant hermatypic coral remains. These reefs are strong evidence of the existence of a warm Mediterranean sea at certain times during the Neogene (reefs formed by hermatypic corals only develop in shallow marine environments, with mean winter sea-surface water temperatures of 20° C or higher).

THE TROPICAL EPISODES: THE NEOGENE CORAL REEFS IN THE MEDITERRANEAN-LINKED BASINS

THE TORTONIAN REEFS AND CORALS

THE MESSINIAN REEFS Mound-like bioherms Fringing reefs

THE MESSINIAN CORALS

THE MESSINIAN BIOHERMS: THE CORALS AND HALIMEDA MOUNDS

THE FRINGING REEFS

TEMPERATE AND TROPICAL CARBONATE EPISODES IN THE UPPERMOST MIOCENE - In the upper Miocene, temperate carbonates accumulated on ramps in the cold stages of third-order eustatic sea-level cycles, during sea-level lowstands, while tropical carbonates formed on shelves in warm periods, during rising and high sea levels. - These relative sea-level variations can be correlated with the Late Neogene global sea-level curve. All this suggests that change in the type of carbonate formation was driven by temperature variations of seawater related to global temperature fluctuations.

THE CLIMATE CHANGES Temperate and tropical episodes follow each other and alternate in time. They reflect cyclical climate changes, of presumably global significance, affecting all the western Mediterranean area.

STABLE-ISOTOPE DATA - Oxygen-isotope data confirm the climate alternations. Lateral facies changes from upper Tortonian/Messinian marginal carbonates into pelagic marls can be physically traced in wellexposed sections in the Sorbas basin (SE Spain). Average d 18 O values of O. universa (0. 85‰) indicate surface-water temperatures during the deposition of the uppermost Tortonian - lowermost Messinian temperate carbonates close to the ones in the present-day western Mediterranean (Sánchez-Almazo et al. , 2001). d 18 O values, together with planktic-foraminiferal assemblages, suggest a rise in sea-surface temperatures in the early Messinian as the change from temperate to tropical, reef carbonate deposition took place (Sánchez. Almazo et al. , 2001).

reef units cool-water carbonate unit sea-surface temperature is the major factor controlling carbonate type

ADDITIONAL DATA - A cooling of Mediterranean waters at the Tortonian-Messinian boundary has also been detected with the d 18 O record of planktic and benthic foraminifers in the Monte Casino section in Italy (Kouwenhoven et al. , 1999), and in the Salé Briquetiere section in the Rifian Corridor in Morocco (Hodell et al. , 1994). - Increase in global ice volume has been invoked as a possible causal mechanism (Hodell et al. , 1994). Ice cap expansion has been reported from the southern hemisphere (Warnke et al. , 1992), and from the northern hemisphere (Thiede et al. , 1998), at about 7 Ma, near the Tortonian. Messinian boundary.

THE CLIMATIC-BELT FLUCTUATIONS In the cold stages (as in the Present-day situation) the western Mediterranean area was within the temperate climatic belt. In contrast, during warm periods the Mediterranean area fell within the (sub)tropical belt.

THE LAST MESSINIAN TEMPERATE EPISODE During the last Messinian, climatic temperateepisode evaporites formed in the Mediterranean floor instead of carbonates.

THE MEDITERRANEAN SEA DESSICATION In Messinian times, at ~ 5’ 9 Ma, the Mediteranean sea dried out completely as a consequence of the closing of the Atlantic-Mediterranean connections, leaving behind huge salt (halite) deposits in its deepest areas.

THE PLIOCENE ANOMALY Temperate carbonates formed as well in the western Mediterranean during the Lower Pliocene even though the global climate was warmer than during the latemost Miocene.

OXYGEN-ISOTOPE DATA FROM THE WESTERN MEDITERRANEAN Isotopic studies (oxygen stable isotope values), carried out on planktic foraminifers from a core (Hole 976 B) in the western Mediterranean (ODP Leg 161, Site 976), confirm a temperature decrease of Mediterranean surface waters during the Early Pliocene.

MAJOR CHANGES IN THE ATLANTICMEDITERRANEAN COMMUNICATIONS DURING THE LATE NEOGENE

CONSEQUENCES DERIVED FROM THE OPENING OF THE GIBRALTAR STRAITS A significant change in the Atlantic. Mediterranean connections took place with the closure of the Rifian Straits and the opening of the Gibraltar Straits. A new current pattern for the western Mediterranean was implemented, with temperate surface waters flowing into the Mediterranean Sea from a more northern, cooler source area. This new situation probably caused the disappearance of coral reefs in the Mediterranean Pliocene and favoured the widespread development of temperate carbonates on shelves. This regional cooling is in contrast with the subtle global warming recorded in the open oceans during the Early Pliocene (Zachos et al. , 2001).