The late Eocene-Miocene is a key interval in the evolution of the global climate. At the Eocene-Oligocene boundary, the Earth faced the transition from a greenhouse period, to the present icehouse climate. This transition culminated with the onset of the Antarctica polar ice cap, testified by a positive oxygen isotope shift recorded in the deep-sea carbonate successions, known in literature as Oi-1 Event (~33.5 Ma). At the Oligocene-Miocene boundary (~23 Ma) another positive oxygen isotope excursion, the Mi-1 Event, testifies for the increase of the Antarctica ice-volume. Conversely, a long-term negative ẟ18O excursion documents the Mid Miocene Climatic Optimum (17-13.5 Ma). These climatic events were associated to two sharp positive ẟ13C shifts recorded contemporary to the Oi-1 and Mi-1 events, and a long-term positive carbon isotope excursion recorded in the middle Miocene, known in literature as Monterey Event. On a regional scale, the Eocene-Miocene interval is extremely dynamic too. In fact, the Mediterranean evolved from a wide open to the modern closed basin, and the Apennine orogenesis, coupled with the associated volcanism, affected the Central Mediterranean area. This work aims to identify how the Central Mediterranean shallow- and deep-sea carbonate successions responded to the major carbon-cycle perturbations, and recorded the main carbon isotope events between late Eocene and middle Miocene. Furthermore, this study intends to discriminate the global and the regional factors controlling carbonate production and changes, to understand which one prevailed in affecting the Central Mediterranean water chemistry and its isotope signature during the interval of interest. Both shallow-water and basinal carbonate successions have been investigated. The shallow-water record has been investigated in the Central Apennines platforms, the Latium-Abruzzi Platform and the Apula Platform, while the basinal isotope signature has been investigated in the Umbria-Marche hemipelagic succession, in the Northern Apennines. A robust chronostratigraphic framework of the platform successions has been assessed through calcareous nannofossil biostratigraphy coupled with Strontium Isotope Stratigraphy. The reconstruction of the stratigraphic architecture of the platforms and the high-resolution biostratigraphy allowed to correlate the shallow-water isotope signals with the Mediterranean hemipelagic record and the global deep-sea isotope curves. The carbon-cycle perturbation related to the Eocene-Oligocene transition and the Oi-1 Event has been analysed in the northern extension of the Apula Platform, cropping out in the Majella Mountain (Santo Spirito Formation), and in the contemporary Umbria-Marche basinal succession (Massignano section, Conero area, Northern Apennines). The shallow-water δ13C has been measured on whole-rock samples and correlated with the δ13CTOC of the hemipelagic succession. The results show that the upper Eocene carbon isotope record of the Central Mediterranean follows the global trend, but that local factors related to the oceanography of the Mediterranean affected the hemipelagic carbon isotope record. Furthermore, no major carbon isotope shifts are recorded contemporary to the Oi-1 Event in the Santo Spirito Formation since the normal bedding is interrupted by extensive slumps, which are interpreted as the main consequence of the sea-level drop occurred due to the establishment of the Antarctica ice-sheet. Lastly, no significant changes in the carbonate factory of the Santo Spirito Formation are recorded at the Eocene-Oligocene transition since the sampled sections represent an outer ramp setting, thus colonised only by photo-independent organisms. However, the observation of skeletal assemblages within the resedimented material across the Eocene-Oligocene transition document the demise of the Larger Benthic Foraminifera, the disappearance of the ortophragminids and the reduction of the nummulitids, and a contemporary increase of the organisms related to the seagrass and the spread of the corals. The Monterey Event has been investigated in both the Central Apennine carbonate platforms: the Latium-Abruzzi Platform (Lithothamnion and Bryozoan Limestone Formation) and the Apula Platform (Bolognano Formation), and correlated with the contemporary Umbria-Marche pelagic signal. Differently from the carbon isotope record of the Eocene-Oligocene transition, the middle Miocene ẟ13C curves show a wider positive excursion in the shallow-water records in comparison to the hemipelagic one. This amplification of the signal has been interpreted as due to the photosynthetic activity within the oligophotic zone. Furthermore, in both the carbonate platforms, the Monterey Event coincides with the spread of bryozoans within the lower portion of the middle ramp and in the outer ramp settings. Filter-feeding biota must have been favoured by the increased nutrient availability related to the Mid Miocene Climatic Optimum and enhanced by regional factors, e.g. the volcanism of the Sardinia-Corsica Block and the closure of the Indo-Pacific gateway between the late Burdigalian and the Langhian. The high productivity of the bryozoan-dominated carbonate factory, in turn, led to the development of low-angle ramps. The complex interplay between global and regional factors in controlling the Central Mediterranean seawater chemistry has been investigated through the integrated study of the Sr and the Nd isotope signatures of the Miocene hemipelagic Umbria-Marche and Apula successions. The results document that the geodynamic evolution of the Mediterranean had an unequivocal control on the seawater chemistry of this basin. The Sr isotope record shows three critical intervals: the late Aquitanian, the middle to late Burdigalian and the Messinian. During late Aquitanian, the increased runoff in a moment of sea-level low-stand related to a glaciation affected the Sr isotope record of the Central Mediterranean. During Burdigalian, the highly explosive subduction-related volcanism of the Western Mediterranean led the Sr isotope ratios to fall below the global ocean signal. Lastly, during late Miocene, the Sr isotope record testifies for the onset of restricted water exchanges between several marginal basins, e.g the proto-Adriatic basins, and the main Central Mediterranean water body. The onset of restricted water conditions has been documented not only in the hemipelagic record of the Majella Mountain succession (Turborotalita multiloba marls, Messinian), but also in the shallow-water record of the Lithothamnion Limestone (late Tortonian-early Messinian). In fact, the Lithothamnion Limestone Sr isotope signature falls significantly below the global ocean record during lower Messinian due to the eastward migration of the Apennine accretionary wedge, which led to an increased freshwater input and sediment runoff into the basin. The Miocene Nd isotope record testifies for the evolution of the Mediterranean from a wide open to the modern closed basin. The volcanic influence on the Nd isotopes is recorded during Aquitanian, when the Cenozoic volcanism affected the northern Indian Ocean, and the subduction-related volcanism of the Western Mediterranean was active. During middle Miocene, the Nd isotope ratios document a Central Mediterranean exchanging with the Paratethys, especially during the sea-level high-stand contemporary to the Middle Miocene Climatic Optimum. During the Messinian, the Nd isotope datum is coherent with the Sr isotope record, showing a good affinity with the Atlantic Ocean signal, but also contamination by the local freshwater input. Lastly, the Nd isotope record of the Mediterranean reflects the physiography of this basin, where the proximity of the land is a major controlling factor on its water chemistry due to runoff and freshwater input. Thus, the Nd isotope signature of the Mediterranean could never be the same as the surrounding Oceans, but it will always be a mix of two opposite controlling factors: the main oceanic masses entering the basin and its oceanography, and the local geodynamic evolution coupled with volcanism. This work states that the Central Mediterranean carbonate successions recorded the main carbon isotope trends and the global carbon-cycle perturbations occurred between late Eocene and middle Miocene, but that the local factors related to the geodynamic and oceanographic evolution of the Mediterranean area affected the local water chemistry, complicating the isotope signature of the carbonate successions. For these reasons, this study stresses out how caution must be taken prior to drive any conclusion on the global climatic and the trophic events analysing the signal of a closed basin. Secondly, this work proves that Strontium Isotope Stratigraphy must be applied very carefully on carbonate successions of marginal basins, even when fully marine conditions are established, and diagenetic overprint is ruled out. Therefore, this work reaffirms the necessity to asses a good biostratigraphic framework prior to apply chemostratigraphy, especially on carbonate successions belonging a closed basin, because the local factors superimposed to the global signal can lead to mistakes and misinterpretations of the isotope curves.

L’intervallo compreso tra l’Eocene superiore e il Miocene è estremamente interessante e dinanico a livello globale come alla scala regionale del Mediterraneo. Al passaggio Eocene-Oligocene si registra la transizione da una fase di greenhouse, in cui la Terra era priva di calotte polari e la concentrazione di CO2 in atmosfera raggiungeva le 1000 ppm, al periodo di icehouse attuale, caratterizzato dalla presenza di due calotte polari stabili e concentrazioni di gas serra dell’ordine delle centinaia di ppm. È, infatti, in questo momento che ha inizio la glaciazione della calotta polare Antartica, così come testimoniato da un picco positivo nel record globale del ẟ18O, registrato nell’Oligocene basale (33.5 Ma) e noto in letteratura come Oi-1 Event. Un secondo picco positivo di ẟ18O, denominato Mi-1 Event, viene registrato al passaggio Oligocene-Miocene (~23 Ma), mentre il Miocene medio è caratterizzato da un’escursione negativa dei rapporti isotopici dell’ossigeno che attestano un aumento delle temperature delle acque a livello globale. Questa fase di riscaldamento culmina nel Middle Miocene Climatic Optmimum (17-13.5 Ma), momento in cui si stima che le temperature delle acque superifciali alle latitudini temperate abbiano raggiunto valori fino a 6°C superiori a quelli odierni. Questi cambiamenti climatici hanno influenzato il ciclo del carbonio innescando perturbazioni di diversa entità. La curva globale del ẟ13C mostra, infatti, due picchi positivi, rispettivamente in corrispondenza dell’Oi-1 Event e del Mi-1 Event, e una lunga escursione positiva durante il Miocene medio, nota in letterature come Monterey Event. A scala regionale, l’intervallo stratigrafico in esame è ancora più dinamico a causa della complessa evoluzione geodinamica e oceanografica del Mediterraneo, ed in particolare del Mediterraneo Centro-Occidentale, dove si sviluppa l’orogenesi appenninica ed un forte vulcanismo ad essa associata. Lo scopo di questo studio è quindi quello di identificare la risposta dei sistemi carbonatici del Mediterraneo Centrale ai principali eventi di perturbazione del ciclo del carbonio registrati a livello globale tra l’Eocene superiore e il Miocene medio. Inoltre, analizzando il record isotopico di Sr e Nd delle successioni carbonatiche del Mediterraneo Centrale, si intende discriminare i fattori di controllo globali da quelli locali, legati all’evoluzione geodinamica ed oceanografica dell’area Mediterranea, che possono aver influenzato il chimismo delle acque di questo bacino, concorrendo quindi ad innescare delle crisi o dei cambi della produzione carbonatica. A tal fine sono stati analizzati i record isotopici del carbonio dei due domini di piattaforma dell’Appennino Centrale (la Piattaforma Laziale-Abruzzese e la Piattaforma Apula), ed il record isotopico del carbonio della successione emipelagica umbro-marchigiana (Appennino Settentrionale). Al fine di indentificare l’influenza dei fattori di controllo regionali sul chimismo delle acque del Mediterraneo durante il Miocene, inoltre, si propone lo studio integrato del record isotopico dello Sr e del Nd, misurato nella successione Umbro-Marchigiana (Aquitaniano-Tortoniano inferiore), e in quella Apula (Tortoniano-Messiniano). Il segnale isotopico del carbonio durante la transizione Eocene-Oligocene è stato investigato nella porzione settentrionale della Piattaforma Apula (Montagna della Majella), dove l’intervallo in esame corrisponde alla Formazione di Santo Spirito (Daniano-Rupeliano). Il ẟ13C della Formazione di Santo Spirito è stato correlato al ẟ13CTOC della contemporanea successione bacinale Umbro-Marchigiana, misurata nella sezione di Massignano, GSSP del limite Eocene-Oligocene. Entrambe le curve del carbonio analizzate per l’Eocene superiore seguono il trend globale, che attesta una ridotta produttività primaria delle acque. Tuttavia, la curva del ẟ13CTOC della sezione di Massignano mostra delle anomalie negative di breve durata, innescate da picchi momentanei della produttività delle acque tra i 36.5 e i 36.0 Ma. Queste perturbazioni transienti sono legate all’attività della Subtropical Eocene Neo-Tethys current (STENT), la quale, entrando nel Mediterraneo attraverso l’Indian Gateway, portava nel Mediterraneo Centrale acque ricche in ferro, innescando così un aumento della produttività primaria nelle acque superficiali. Il segnale isotopico di piattaforma non registra queste perturbazioni di breve durata, così come non registra l’anomalia del carbonio contemporanea all’Oi-1 Event. La mancata registrazione di questo evento è legata alla presenza di slump che interrompono la normale sedimentazione della Formazione di Santo Spirito nell’Oligocene basale. Questi slump, identificati sull’intera rampa carbonatica nello stesso intervallo stratigrafico, vengono qui interpretati come conseguenti alla caduta del livello del mare dovuta alla formazione della calotta polare Antartica. Una caduta del livello marino, infatti, comporta un abbassamento del limite inferiore di risentimento dell’onda di tempesta, andando a far aumentare l’instabilità della rampa. In ultimo, le sezioni stratigrafiche analizzate per la Formazione di Santo Spirito non permettono di vedere qualora ci siano stati cambi importanti della carbonate factory tra l’Eocene e l’Oligocene, poiché i depositi studiati identificano un ambiente di rampa esterna, colonizzato dai soli organismi foto-indipendenti. Tuttavia, l’analisi dei componenti scheletrici ritrovati nei conglomerati e nei depositi risedimentati mostra una diminuzione dei macroforaminiferi bentonici nell’Oligocene, ed un contemporaneo aumento degli organismi legati al seagrass e dei coralli. Il segnale isotopico del Miocene inferiore e medio è stato analizzato nella successione della Montagna della Majella (Formazione di Bolognano), rappresentativa del dominio della Piattaforma Apula, e in quella della Piattaforma Laziale-Abruzzese (Calcari a Briozoi e Litotamni). Il record di piattaforma è stato poi correlato con il contemporaneo segnale isotopico del bacino umbro-marchigiano. A differenza di quanto visto per la transizione Eocene-Oligocene, il Monterey Event mostra un’anomalia positiva del ẟ13C ben quattro volte più ampia nel record di piattaforma che in quello di bacino. In entrambi i domini di piattaforma il Monterey Event coincide con un momento di massima diffusione dei briozoi tra la porzione inferiore della rampa intermedia e la rampa esterna. A sua volta, una produttività così elevata nella zona afotica è andata a determinare la geometria complessiva di queste piattaforme, che infatti evolvono come rampe carbonatiche a basso angolo. Lo studio integrato di Sr e Nd nelle successioni emipelagiche mioceniche del Mediterraneo Centrale mostra come sia assolutamente evidente l’influenza dell’evoluzione geodinamica dell’area Mediterranea sul chimismo delle acque di questo bacino. Il segnale isotopico dello Sr indica tre intervalli critici, durante i quali il segnale del Mediterraneo devia rispetto a quello globale: l’Aquitaniano superiore, il Burdigaliano medio e superiore e il Messiniano. Nell’Aquitaniano superiore, durante una fase di caduta del livello del mare legata ad una glaciazione, l’aumento del runoff nel Mediterraneo viene registrato dagli isotopi dello Sr, che mostrano un 87Sr/86Sr molto più alto di quello globale. Al contrario, durante il Burdigaliano, lo 87Sr/86Sr del Mediterraneo Centrale cade al di sotto della curva globale a causa del vulcanismo altamente esplosivo che interessava il Mediterraneo Occidentale. Infine, nel Messiniano il record isotopico dello Sr delle Marne a T. multiloba (Montagna della Majella, Appennino Centrale) indica l’onset di una circolazione ristretta nel proto-Adriatico. La deviazione del segnale isotopico dello Sr nel bacino proto-Adriatico viene registrata nel Dominio della Piattaforma Apula già a partire dai Lithothamnion Limestone (Tortoniano-Messiniano inferiore). I rapporti isotopici dello Sr, misurati su gusci di bivalvi e brachiopodi, mostrano una deviazione dalla curva globale nel Messiniano inferiore, significativamente prima della crisi di salinità messiniana. L’ abbassamento del rapporto 87Sr/86Sr nel bacino del proto-Adriatico è legato al verificarsi di condizioni di circolazione ristretta dovute alla fisiografia del bacino stesso. L’avanzamento del fronte appenninico ha, infatti, provocato un progressivo restringimento del bacino ed un concomitante aumento del runoff, ed una maggiore influenza delle acque dolci, fluviali nel bacino. Il record isotopico del Nd durante il Miocene testimonia perfettamente l’evoluzione del Mediterraneo da bacino aperto ed alimentato principalmente dall’Oceano Indiano, all’attuale bacino chiuso, in connessione con il solo Oceano Atlantico. Nell’Aquitaniano il segnale isotopico del Nd attesta l’influenza del vulcanismo, attivo sia nell’Oceano Indiano settentrionale che nel Mediterraneo Occidentale, sul chimismo delle acque del Mediterraneo. Durante il Miocene medio, invece, il rapporto 144Nd/143Nd della successione Umbro-Marchigiana testimonia degli scambi di masse d’acqua tra il Mediterraneo Centrale e il bacino della Paratetide durante la fase di alto stazionamento del livello del mare connessa al Mid Miocene Climatic Optimum. Infine, il dato isotopico del Nd del Miocene superiore è coerente con quanto visto nel record dello Sr, testimoniando un bacino Mediterraneo in comunicazione con l’Oceano Atlantico, e un bacino proto-Adriatico contaminato dall’apporto di acque dolci e dal runoff continentale. Un dato estremamente interessante che si evince dall’analisi del record del Nd è che questo riflette la fisiografia del Mediterraneo stesso. In un mare interno, infatti, la vicinanza dei continenti non può non influire sul chimismo delle sue acque, il quale segnale isotopico sarà il risultato di due fattori distinti: l’assetto oceanografico e le principali masse oceaniche che entrano in questo mare interno da un lato, e l’evoluzione geodinamica e del vulcanismo dall’altro. In conclusione, questa tesi dimostra che il record isotopico del carbonio delle successioni del Mediterraneo Centrale segue i trend globali registrati tra l’Eocene superiore e il Miocene medio. Allo stesso modo però, risulta evidente che diversi fattori locali, legati all’evoluzione geodinamica del Mediterraneo, ne hanno influenzato il chimismo delle acque, controllando il record isotopico delle successioni carbonatiche. Secondariamente, questo lavoro intende porre l’attenzione su quanto sia rischioso applicare la stratigrafia isotopica dello Sr su successioni carbonatiche di mari interni o bacini chiusi, anche dopo aver stabilito, per mezzo di un’accurata analisi di facies, condizioni marine franche, escluso variazioni significative di salinità, o qualsiasi sovraimpronta diagenetica sui campioni analizzati. Infine, sebbene forse non necessario, questa tesi afferma l’assoluta necessità di costruire un robusto framework stratigrafico, basato prima di tutto su dati biostratigrafici, prima di analizzare il record isotopico di successioni carbonatiche rappresentative di mari interni. Questo lavoro dimostra, infatti, come siano molteplici i fattori locali che, sovrapposti al segnale globale, possono complicare la risposta delle successioni locali al forcing climatico globale, e come questo possa portare a facili errori o interpretazioni sbagliate delle curve isotopiche locali.

The impact of isotopic events on the Central Mediterranean carbonate successions between late Eocene and Miocene / Cornacchia, Irene. - (2018 Feb 21).

The impact of isotopic events on the Central Mediterranean carbonate successions between late Eocene and Miocene

CORNACCHIA, IRENE
21/02/2018

Abstract

The late Eocene-Miocene is a key interval in the evolution of the global climate. At the Eocene-Oligocene boundary, the Earth faced the transition from a greenhouse period, to the present icehouse climate. This transition culminated with the onset of the Antarctica polar ice cap, testified by a positive oxygen isotope shift recorded in the deep-sea carbonate successions, known in literature as Oi-1 Event (~33.5 Ma). At the Oligocene-Miocene boundary (~23 Ma) another positive oxygen isotope excursion, the Mi-1 Event, testifies for the increase of the Antarctica ice-volume. Conversely, a long-term negative ẟ18O excursion documents the Mid Miocene Climatic Optimum (17-13.5 Ma). These climatic events were associated to two sharp positive ẟ13C shifts recorded contemporary to the Oi-1 and Mi-1 events, and a long-term positive carbon isotope excursion recorded in the middle Miocene, known in literature as Monterey Event. On a regional scale, the Eocene-Miocene interval is extremely dynamic too. In fact, the Mediterranean evolved from a wide open to the modern closed basin, and the Apennine orogenesis, coupled with the associated volcanism, affected the Central Mediterranean area. This work aims to identify how the Central Mediterranean shallow- and deep-sea carbonate successions responded to the major carbon-cycle perturbations, and recorded the main carbon isotope events between late Eocene and middle Miocene. Furthermore, this study intends to discriminate the global and the regional factors controlling carbonate production and changes, to understand which one prevailed in affecting the Central Mediterranean water chemistry and its isotope signature during the interval of interest. Both shallow-water and basinal carbonate successions have been investigated. The shallow-water record has been investigated in the Central Apennines platforms, the Latium-Abruzzi Platform and the Apula Platform, while the basinal isotope signature has been investigated in the Umbria-Marche hemipelagic succession, in the Northern Apennines. A robust chronostratigraphic framework of the platform successions has been assessed through calcareous nannofossil biostratigraphy coupled with Strontium Isotope Stratigraphy. The reconstruction of the stratigraphic architecture of the platforms and the high-resolution biostratigraphy allowed to correlate the shallow-water isotope signals with the Mediterranean hemipelagic record and the global deep-sea isotope curves. The carbon-cycle perturbation related to the Eocene-Oligocene transition and the Oi-1 Event has been analysed in the northern extension of the Apula Platform, cropping out in the Majella Mountain (Santo Spirito Formation), and in the contemporary Umbria-Marche basinal succession (Massignano section, Conero area, Northern Apennines). The shallow-water δ13C has been measured on whole-rock samples and correlated with the δ13CTOC of the hemipelagic succession. The results show that the upper Eocene carbon isotope record of the Central Mediterranean follows the global trend, but that local factors related to the oceanography of the Mediterranean affected the hemipelagic carbon isotope record. Furthermore, no major carbon isotope shifts are recorded contemporary to the Oi-1 Event in the Santo Spirito Formation since the normal bedding is interrupted by extensive slumps, which are interpreted as the main consequence of the sea-level drop occurred due to the establishment of the Antarctica ice-sheet. Lastly, no significant changes in the carbonate factory of the Santo Spirito Formation are recorded at the Eocene-Oligocene transition since the sampled sections represent an outer ramp setting, thus colonised only by photo-independent organisms. However, the observation of skeletal assemblages within the resedimented material across the Eocene-Oligocene transition document the demise of the Larger Benthic Foraminifera, the disappearance of the ortophragminids and the reduction of the nummulitids, and a contemporary increase of the organisms related to the seagrass and the spread of the corals. The Monterey Event has been investigated in both the Central Apennine carbonate platforms: the Latium-Abruzzi Platform (Lithothamnion and Bryozoan Limestone Formation) and the Apula Platform (Bolognano Formation), and correlated with the contemporary Umbria-Marche pelagic signal. Differently from the carbon isotope record of the Eocene-Oligocene transition, the middle Miocene ẟ13C curves show a wider positive excursion in the shallow-water records in comparison to the hemipelagic one. This amplification of the signal has been interpreted as due to the photosynthetic activity within the oligophotic zone. Furthermore, in both the carbonate platforms, the Monterey Event coincides with the spread of bryozoans within the lower portion of the middle ramp and in the outer ramp settings. Filter-feeding biota must have been favoured by the increased nutrient availability related to the Mid Miocene Climatic Optimum and enhanced by regional factors, e.g. the volcanism of the Sardinia-Corsica Block and the closure of the Indo-Pacific gateway between the late Burdigalian and the Langhian. The high productivity of the bryozoan-dominated carbonate factory, in turn, led to the development of low-angle ramps. The complex interplay between global and regional factors in controlling the Central Mediterranean seawater chemistry has been investigated through the integrated study of the Sr and the Nd isotope signatures of the Miocene hemipelagic Umbria-Marche and Apula successions. The results document that the geodynamic evolution of the Mediterranean had an unequivocal control on the seawater chemistry of this basin. The Sr isotope record shows three critical intervals: the late Aquitanian, the middle to late Burdigalian and the Messinian. During late Aquitanian, the increased runoff in a moment of sea-level low-stand related to a glaciation affected the Sr isotope record of the Central Mediterranean. During Burdigalian, the highly explosive subduction-related volcanism of the Western Mediterranean led the Sr isotope ratios to fall below the global ocean signal. Lastly, during late Miocene, the Sr isotope record testifies for the onset of restricted water exchanges between several marginal basins, e.g the proto-Adriatic basins, and the main Central Mediterranean water body. The onset of restricted water conditions has been documented not only in the hemipelagic record of the Majella Mountain succession (Turborotalita multiloba marls, Messinian), but also in the shallow-water record of the Lithothamnion Limestone (late Tortonian-early Messinian). In fact, the Lithothamnion Limestone Sr isotope signature falls significantly below the global ocean record during lower Messinian due to the eastward migration of the Apennine accretionary wedge, which led to an increased freshwater input and sediment runoff into the basin. The Miocene Nd isotope record testifies for the evolution of the Mediterranean from a wide open to the modern closed basin. The volcanic influence on the Nd isotopes is recorded during Aquitanian, when the Cenozoic volcanism affected the northern Indian Ocean, and the subduction-related volcanism of the Western Mediterranean was active. During middle Miocene, the Nd isotope ratios document a Central Mediterranean exchanging with the Paratethys, especially during the sea-level high-stand contemporary to the Middle Miocene Climatic Optimum. During the Messinian, the Nd isotope datum is coherent with the Sr isotope record, showing a good affinity with the Atlantic Ocean signal, but also contamination by the local freshwater input. Lastly, the Nd isotope record of the Mediterranean reflects the physiography of this basin, where the proximity of the land is a major controlling factor on its water chemistry due to runoff and freshwater input. Thus, the Nd isotope signature of the Mediterranean could never be the same as the surrounding Oceans, but it will always be a mix of two opposite controlling factors: the main oceanic masses entering the basin and its oceanography, and the local geodynamic evolution coupled with volcanism. This work states that the Central Mediterranean carbonate successions recorded the main carbon isotope trends and the global carbon-cycle perturbations occurred between late Eocene and middle Miocene, but that the local factors related to the geodynamic and oceanographic evolution of the Mediterranean area affected the local water chemistry, complicating the isotope signature of the carbonate successions. For these reasons, this study stresses out how caution must be taken prior to drive any conclusion on the global climatic and the trophic events analysing the signal of a closed basin. Secondly, this work proves that Strontium Isotope Stratigraphy must be applied very carefully on carbonate successions of marginal basins, even when fully marine conditions are established, and diagenetic overprint is ruled out. Therefore, this work reaffirms the necessity to asses a good biostratigraphic framework prior to apply chemostratigraphy, especially on carbonate successions belonging a closed basin, because the local factors superimposed to the global signal can lead to mistakes and misinterpretations of the isotope curves.
21-feb-2018
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