Stefano Claudio Vaiani

Palaeogeographic and palaeoclimatic evolution of the Po Plain from 150-ky core records

Integrated sedimentological and micropalaeontological (foraminifers, ostracods, pollen) analyses of eight continuously cored boreholes, up to 140 m deep, reveal the depositional history and the palaeoclimatic evolution of southeastern Po Plain (northern Italy) over the last 150 ky. Age assignments are supported by the chronostratigraphic sequence framework calibrated by radiocarbon and pollen data, allowing detailed correlation with the oxygen-isotope record. Facies analysis (including identification of 12 distinct microfossils associations) and detailed stratigraphic correlations across several tens of km document a cyclic sedimentation pattern (including continental, coastal and shallow-marine deposits), which defines two transgressive–regressive sequences, deposited over the last two interglacial–glacial cycles. Two prominent stratigraphic markers, corresponding to wedge-shaped coastal sand bodies, are recorded between 0–30 and 100–125 m core depths. These sedimentary bodies were deposited during the two major transgressive pulsations and subsequent sea-level highstands of the last 150 ky, assigned to the Holocene and the Tyrrhenian (oxygen-isotope Substage 5e), respectively. The stratigraphic architecture of post-Substage 5e deposits shows consistent patterns of coastal evolution with changing sea-level position. Lowering of sea level between 125 and 70 ky (onset of Substages 5d and 5b, and Stage 4) resulted in extensive and repeated basinward shifts of facies, which can be observed across closely spaced unconformity surfaces associated to alluvial plain sedimentation (falling-stage systems tract). The general phase of sea-level fall was punctuated by short transgressive phases (Substages 5c, 5a and Stage 3 interstadials), which led to widespread deposition of organic-rich (lagoonal and swamp) deposits. Upper Stage 3 and Stage 2 deposits (lowstand systems tract) are replaced across a significant part of the study area by a hiatal surface. Holocene interglacial deposits are characterized by a retrogradational stacking pattern of coastal plain and littoral facies (transgressive systems tract), reflecting invariably the landward migration of a barrier–lagoon–estuary system. Subsequent highstand deposition (highstand systems tract) was characterized by extensive progradation of wave-influenced deltas and strandplains. The two major transgressive surfaces, of Tyrrhenian and Holocene age, are considerably easier to identify than the other key surfaces for sequence stratigraphic interpretation. The lowermost transgressive deposits display a pollen signature diagnostic of warm climate (interglacial) periods, showing wide forest development and relatively high pollen concentrations. By contrast, the deposits overlying the unconformable surfaces related to successive phases of sea-level fall are fingerprinted by pollen spectra dominated by Pinus and non-arboreal pollen types, with very low pollen concentrations, reflecting the onset of a stable cold climate vegetation characteristic of stadial to fully glacial conditions. The good match between facies architecture, pollen distribution and global sea-level evolution strongly suggests that Late Quaternary sedimentation in the Po Basin developed under a predominantly glacio-eustatic control. Stratigraphic architecture of the Po Basin thus can provide a useful analog for interpretation and correlation in the stratigraphic record of very rapid glacio-eustatic (fourth-order) cycles, with frequencies of about 100 ky.

Facies Architecture and Latest Pleistocene-Holocene Depositional History of the Po Delta (Comacchio Area), Italy

Integrated sedimentological and micropaleontological study of 16 cores and 137 piezocone penetration tests, approximately 40 m deep, in the Comacchio area enables the documentation of the depositional history of southeastern Po Plain in the last 30 ka, in response to fluctuating sea level. Sedimentation within an alluvial plain was the dominant feature across the entire study area during the pronounced sea level fall that culminated in the Last Glacial Maximum. Thin lowstand fluvial sediments form the lower part of a shallow incised valley above the Last Glacial Maximum unconformity, whereas a characteristic paleosol separates the last glacial alluvial plain deposits from the overlying postglacial deposits in the interfluves. Transgressive and highstand deposits show a well‐developed stacking pattern of retrogradational (coastal plain and estuarine) and progradational (deltaic) facies. Detailed reconstruction of transgressive paleogeography shows evolutionary features that can be useful for refined interpretation of coeval and ancient analogs. At relatively early stages of transgression (10.5–9 ka B.P.), sedimentation in a coastal plain was restricted to the incised valley, whereas nondeposition and pedogenesis took place on the interfluves. With rising sea level (9–6 ka B.P.), a wave‐dominated, barred estuary developed in the former topographic low. At peak transgression, after filling up of the estuarine systems with coastal, back‐barrier sediments, wide areas outside the valleys were flooded, aggradation extended onto the interfluve unconformity, and a shallow marine depositional environment developed across most of the study area. The depositional history during the subsequent highstand phase was dominated by progradation of the early Po Delta and reflects the complex interplay between high‐frequency sea level fluctuations, climate, subsidence, and autocyclic processes.

Did the A.D. 365 Crete earthquake/tsunami trigger synchronous giant turbidity currents in the Mediterranean Sea?

In the Ionian Sea, one of the most seismically active regions in the Mediterranean, subduction is commonly associated with uplift of coastal mountains, enhanced erosion, and seismic activity along the Calabrian Arc and Hellenic Arc, thus potentially resulting in repetitive mass failures. Some of the turbidites observed in the deep basins are thick and prominent on seismic records because of the acoustic transparency of their upper structureless mud layer. Our high-resolution study of the most recent of these megabeds, the homogenite Augias turbidite (HAT), provides key proxies to identify pelagic sediments deposited following the catastrophic causative event. Radiometric dating in an area >150,000 km2 indicates that the different Mediterranean so-called homogenite deposits are in fact synchronous and were deposited during a single basin-wide event within the time window A.D. 364–415. Unlike interpretations that relate this turbidite to different triggering events, including the Santorini caldera collapse, the turbidite can be traced back to a large tsunami sourced from the A.D. 365 Crete megathrust earthquake. Correlation of the single-event HAT over a wide area of the Mediterranean, from the northern Ionian Sea to the Mediterranean Ridge and the anoxic Tyro Basin south of Crete, suggests that the A.D. 365 Crete earthquake and tsunami must have produced devastating effects, including widespread massive sediment remobilization in the eastern Mediterranean Sea.

Metaxytherium subapenninum (Bruno, 1839) (Mammalia, Dugongidae), the latest sirenian of the Mediterranean Basin

ABSTRACT Metaxytherium subapenninum was a halitheriine dugongid distributed along the northwestern coasts of the Mediterranean Basin during the early and late Pliocene. It represents the latest sirenian species of the Mediterranean Basin, the latest Metaxytherium species in the world, and also the latest species belonging to the paraphyletic subfamily Halitheriinae. We review M. subapenninum in the light of new discoveries, including its stratigraphic and geographic distribution, osteology, paleoecology, and relationships. M. subapenninum represents a more derived stage of evolution in comparison with the earlier Metaxytherium species. It is characterized in particular by an increase in body size, an increase in tusk size, and a dorsal broadening of the nasal process of the premaxilla. Its variation in tusk size does not appear to represent sexual dimorphism as in the modern Dugong, but instead progressive intraspecific evolution of larger tusks as a feeding adaptation convergent on that of derived dugongines. It was a relict species limited to the Mediterranean Basin, and responded to long-term climatic cooling by an increase in body size and by an increase in tusk size and rostral reinforcement in order to obtain a more nutritious diet richer in rhizomes.

Testing the Applicability of Strontium Isotope Stratigraphy in Marine to Deltaic Pleistocene Deposits: An Example from the Lamone River Valley (Northern Italy)

The Quaternary deposits of the Lamone River Valley in the Northern Apennines are mainly composed of marine clays and sands. Micropaleontological investigations allow the reconstruction of the paleoenvironmental evolution of that succession by recognition of 13 foraminifer and ostracod biofacies diagnostic of upper slope to deltaic environments. 87Sr/86Sr measurements were performed in well‐preserved fossils throughout most of the recognized biofacies. These are compared with the strontium (Sr) isotopic ratios of Pleistocene seawater. Samples collected within biofacies characteristic of slope to shallow marine environments, without significant riverine influence, provide Sr isotope ratios consistent with Pleistocene seawater. In contrast, fossils from shallow marine deposits of a river‐influenced zone may exhibit Sr isotope ratios lower than Pleistocene seawater. These anomalous ratios may be due to dilution by low‐87Sr/86Sr fluvial waters from the Lamone River that largely drain Tertiary sedimentary rocks. 87Sr/86Sr ratios measured in brackish and freshwater ostracods from backshore sediments are substantially lower than that in Pleistocene seawater and support this interpretation. Sr isotope stratigraphy is an important tool for dating marginal marine sequences; however, caution should be used in applying this technique in shallow marine river‐influenced deposits.

NEW SIRENIAN RECORD FROM LOWER PLIOCENE SEDIMENTS OF TUSCANY (ITALY)

A left humerus of a sirenian found in the upper part of a marine succession from Camigliano (Siena, Italy) is described. Foraminiferal assemblages reveal that this humerus belongs to a specimen living during the early Zanclean ( Globorotalia margaritae Zone) in a shallow marine environment and that was probably subjected to post-mortem transport to a shelf environment under storm wave action. This new Tuscan record can be ascribed, for size and stratigraphic position, to Metaxytherium cf. subapenninum. The dimensional comparison of this humerus with those of its supposed ancestor M. serresii confirms the remarkable increase in body size of M. subapenninum compared with the former M. serresii .