Paleomagnetic investigations on the Pleistocene lacustrine sequence of Piànico-Sèllere (northern Italy)

The Piànico-Sèllere is a lacustrine succession from northern Italy that records a sequence of climatic transitions across two Pleistocene glacial stages. The intervening interglacial stage is represented by well-preserved varves with calcitic (summer) and clastic (winter) laminae. There is a tight coupling between climate-driven lithologic changes and magnetic susceptibility variations, and stable paleomagnetic components were retrieved from all investigated lithologies including the largely diamagnetic calcite varves. These components were used to delineate a sequence of magnetic polarity reversals that was interpreted as a record of excursions of the Earth’s magnetic field. Comparison of the magnetostratigraphic results with previously published data allows discussion of two possible models which have generated controversy regarding the age of the Piànico Formation. The data indicates that the Piànico Formation magnetostratigraphy correlates to geomagnetic field excursions across the Brunhes/Matuyama transition, and consequently the Piànico interglacial correlates to marine isotope stage 19. This correlation option is substantially consistent with K-Ar radiometric age estimates recently obtained from a tepha layer interbedded in the Piànico Formation. The alternative option, considering the Piànico interglacial correlative to marine isotope stage 11 within the Brunhes Chron as supported by tephrochronological dating reported in the literature, is not supported by the magnetostratigraphic results

Paleomagnetic investigations on the Pleistocene lacustrine sequence of Piànico-Sèllere (northern Italy)

The Piànico-Sèllere is a lacustrine succession from northern Italy that records a sequence of climatic transitions across two Pleistocene glacial stages. The intervening interglacial stage is represented by well-preserved varves with calcitic (summer) and clastic (winter) laminae. There is a tight coupling between climate-driven lithologic changes and magnetic susceptibility variations, and stable paleomagnetic components were retrieved from all investigated lithologies including the largely diamagnetic calcite varves. These components were used to delineate a sequence of magnetic polarity reversals that was interpreted as a record of excursions of the Earth’s magnetic field. Comparison of the magnetostratigraphic results with previously published data allows discussion of two possible models which have generated controversy regarding the age of the Piànico Formation. The data indicates that the Piànico Formation magnetostratigraphy correlates to geomagnetic field excursions across the Brunhes/Matuyama transition, and consequently the Piànico interglacial correlates to marine isotope stage 19. This correlation option is substantially consistent with K-Ar radiometric age estimates recently obtained from a tepha layer interbedded in the Piànico Formation. The alternative option, considering the Piànico interglacial correlative to marine isotope stage 11 within the Brunhes Chron as supported by tephrochronological dating reported in the literature, is not supported by the magnetostratigraphic results

Il contributo dei pozzi perforati dalla Regione Lombardia alla conoscenza del Pleistocene lombardo

Facies analysis applied to several up to 220-m-deep cores, taken by Regione Lombardia in the central-northern Po Plain, allowed to recognize an overall regressive sequence consisting of cyclotemic shallow marine and fluvial-deltaic deposits overlain by distal to proximal braidplain sediments. Magnetostratigraphy, coupled with calcareous nannoplankton biostratigraphy, was used to date marine and fluvial-deltaic sediments to the early Pleistocene and continental sediments to the middle–late Pleistocene. Sediment accumulation rates were of ~0.3-0.4 mm/yr in the early Pleistocene, whereas an overall reduction in sediment accumulation rates to ~0.06-0.08 mm/yr, associated to relevant unconformities, characterized the middle-late Pleistocene. Stratigraphic evidences from petrographic, sedimentologic and palynologic analyses highlight in the Regione Lombardia cores a drastic reorganization of vegetational, fluvial, and Alpine drainage patterns, associated to a sequence boundary termed the “R surface”. The “R surface”, seismically traceable across the Po Plain subsurface, was constrained magnetostratigraphically to the first prominent Pleistocene glacio-eustatic lowstand of marine isotope stage (MIS) 22 at 0.87 Ma at the end of the Mid-Pleistocene Revolution, when climate worsened globally and locally caused the onset of the first major Pleistocene glaciation in the Alps. Most marine deposits in the cores lie above sea level highstands of corresponding age, suggesting that they have been uplifted. In order to estimate the observed rock uplift, sediments were back-stripped to elevations at times of deposition (expressed in meters above current sea level) by applying a simple Airy compensation model. The correlation of the isostatically corrected sedimentary facies to a glacio-eustatic reference curve obtained from classic oxygen isotope studies highlights a positive elevation mismatch (rock uplift) in the range of 70-120 m, which occurred after the onset of the major Pleistocene glacial-interglacial cycles at rates of at least 0.15-0.09 mm/yr. Although the driving forces of the observed rock uplift cannot be unambiguously identified, but its timing of onset after the beginning of the major Pleistocene glacial-interglacial cycles and the low seismicity observed in the most of the Regione Lombardia area seem to point to an isostatic readjustment of the chain probably due to the long-term erosional removal of sediments during major Pleistocene glacial advances

Magnetostratigraphy of the Milan subsurface

Four cores have been taken from the surroundings of the city of Milan, in the framework of the Milan CARG project. The northernmost drillings (Milano Triulza RL10, Milano Parco Nord RL11) were drilled to 100-m depth; the southernmost drillings (Peschiera Borromeo RL8, Gaggiano RL9) reached a depth of 180 m and 150 m, respectively. A total of 530 m of sediments was recovered. The overall core lithostratigraphy is composed by three superimposed lithologic sequences, consisting, from the bottom, of alternated silt and medium- to fine-grained sand, arranged in fining-upward cycles, interpreted as meandering alluvial plain; the central sequence develops with coarse-grained sand, pebbly sand and subordinated gravel, interpreted as distal braidplain. Medium- to coarse-grained, poorly sorted, massive sand and pebbly sand, and clast-supported gravels with sandy matrix, interpreted as proximal braidplain, characterize the upper sequence. As a whole, the central and the upper sequence can be regarded as a prograding braidplain, composed by severall small-scale fining-upward cycles. Paleomagnetic properties were studied on a total of 79 samples collected from cohesive fine-grained sediments with a common average sampling frequency in the order of one sample every 3/4 core-meters. The intensity of the NRM (measured at the Alpine Laboratory of Paleomagnetism) was in the order of 10 -3 - 10-4 A/m and orthogonal projections of demagnetization data typically indicated the existence of a lower unblocking temperature component, superimposed to a higher unblocking temperature component. The higher temperature component was removed to the origin of the demagnetization axes mainly in the magnetite and hematite temperature ranges between ~350 and ~680 °C and it is interpreted as the characteristic component. This characteristic component bears either positive (down-pointing) or negative (up-pointing) inclinations with overall mean values of 60° ± 15 and -54° ± 16, respectively, and is regarded as acquired at or shortly after sediment deposition (DRM or pDRM). At least a magnetic polarity reversal has been recognized in each core, in the depth range of 60-80 m, and it has been interpreted, by means of the available pollen biostratigraphy and the regional framework reported in Carcano & Piccin (2002), Muttoni et al. (2003), Scardia et al. (2006), as the Brunhes/Matuyama boundary; in cores RL 8 and RL9 also the Jaramillo Subchron was recognized. The major lithologic change observed in each core, produced by a depositional switch from distal meandering alluvial plain to a prograding braidplain, occurs during a reverse polarity period, interpreted as Subchron Late Matuyama, and it is well constrained between the Subchron Jaramillo and the Brunhes/Matuyama boundary in cores RL8 and RL9; the same age constrain can be inferred in cores RL10 and RL11. This episode, already observed by Carcano & Piccin (2002), has been correlated by Muttoni et al. (2003) to an important Pleistocene climatic event, related to the onset of the major glaciations at the southern foothills of the Alps occurred at ~0.87 ka, during the Subchron Late Matuyama

Modulation of Late Cretaceous and Cenozoic climate by variable drawdown of atmospheric pCO2 from weathering of basaltic provinces on continents drifting through the equatorial humid belt

The small reservoir of carbon dioxide in the atmosphere (pCO2) that modulates climate through the greenhouse effect reflects a delicate balance between large fluxes of sources and sinks. The major long-term source of CO2 is global outgassing from sea-floor spreading, subduction, hotspot activity, and metamorphism; the ultimate sink is through weathering of continental silicates and deposition of carbonates. Most carbon cycle models are driven by changes in the source flux scaled to variable rates of ocean floor production, but ocean floor production may not be distinguishable from being steady since 180 Ma. We evaluate potential changes in sources and sinks of CO2 for the past 120 Ma in a paleogeographic context. Our new calculations show that decarbonation of pelagic sediments by Tethyan subduction contributed only modestly to generally high pCO2 levels from the Late Cretaceous until the early Eocene, and thus shutdown of this CO2 source with the collision of India and Asia at the early Eocene climate optimum at around 50 Ma was inadequate to account for the large and prolonged decrease in pCO2 that eventually allowed the growth of significant Antarctic ice sheets by around 34 Ma. Instead, variation in area of continental basalt terranes in the equatorial humid belt (5° S–5° N) seems to be a dominant factor controlling how much CO2 is retained in the atmosphere via the silicate weathering feedback. The arrival of the highly weatherable Deccan Traps in the equatorial humid belt at around 50 Ma was decisive in initiating the long-term slide to lower atmospheric pCO2, which was pushed further down by the emplacement of the 30 Ma Ethiopian Traps near the equator and the southerly tectonic extrusion of SE Asia, an arc terrane that presently is estimated to account for 1/4 of CO2 consumption from all basaltic provinces that account for ~1/3 of the total CO2 consumption by continental silicate weathering (Dessert et al., 2003). A negative climate-feedback mechanism that (usually) inhibits the complete collapse of atmospheric pCO2 is the accelerating formation of thick cation-deficient soils that retard chemical weathering of the underlying bedrock. Nevertheless, equatorial climate seems to be relatively insensitive to pCO2 greenhouse forcing and thus with availability of some rejuvenating relief as in arc terranes or thick basaltic provinces, silicate weathering in this venue is not subject to a strong negative feedback, providing an avenue for ice ages. The safety valve that prevents excessive atmospheric pCO2 levels is the triggering of silicate weathering of continental areas and basaltic provinces in the temperate humid belt. Excess organic carbon burial seems to have played a negligible role in atmospheric pCO2 over the Late Cretaceous and Cenozoic

Magnetostratigraphic confirmation of a much faster tempo for sea-level change for the Middle Triassic Latemar platform carbonates

New magnetostratigraphic data for the 3c470-m-thick Latemar carbonate platform, which includes 3c600 shallowing-upward bedding cycles, are consistent with litho- and biostratigraphic correlations of the section to a 3c10-m-thick interval in the basinal Buchenstein Beds that most likely represents only 3c1 m.y. of deposition according to published U-Pb single-crystal zircon dates. A reappraisal of reported cycle stratigraphic analyses of the Latemar suggests that the visibly obvious meter-scale bedding is not due to Milankovitch precessional forcing but rather reflects tempos an order of magnitude faster that may involve millennial-scale tidal amplitude variations

Magnetostratigraphy of a Lower/Middle Triassic boundary section from Chios (Greece)

The Marmarotrapeza Formation at Chios Island (northern Aegean Sea, Greece) is renowned for its Lower-Middle Triassic boundary sections in a marine Tethyan setting. Two sections have been sampled bed by bed to develop a magnetostratigraphic framework for the ammonoid and conodont biostratigraphy. The boundary sections occur within a lower normal (A+)-reverse (B-)-upper normal (C+) polarity sequence. The Lower-Middle Triassic boundary, placed at the first occurrence of the ammonoid genera Aegeiceras ugra Diener, Paracrochordiceras spp., Paradanubites depressus Fantini Sestini and Japonites sp., and close to the first appearance of the conodont species Gondolella timorensis Nogami, occurs in normal polarity zone Chios C+. The overall mean direction of the reversal-bearing characteristic component, whose early acquisition is suggested by a tilt test, is D = 271.2°, I = 33.2° (α95 = 11.7°, k = 112.5, N = 3). The inferred paleolatitude of the sampling sites is about 18°N, consistent with either an African or stable European affinity, although the declinations suggest large-scale counter-clockwise rotations with respect to Africa or stable Europe since the Early-Middle Triassic

Ecce Homo in Milan

Come ogni essere vivente anche l\u2019uomo ha colonizzato gli habitat solo nel momento in cui questi presentavano le condizioni favorevoli alla sua sopravvivenza. La colonizzazione del continente europeo \ue8 avvenuta molto probabilmente da Est verso Ovest e l\u2019ingresso in Italia pu\uf2 essere avvenuto solo dal punto pi\uf9 comodo in cui si poteva superare la catena Alpina, ossia passando dai Balcani (Figura 2). Dal record geologico si deduce il susseguirsi di periodi caldi a periodi freddi durante tutto il Pleistocene, tuttavia non ci sono evidenze di un passaggio ad un clima glaciale nel nostro paese prima del Pleistocene Superiore. Questo cambiamento ha come prima conseguenza l\u2019espansione delle calotte glaciali legata all\u2019intrappolamento dell\u2019acqua degli oceani e il conseguente abbassamento del livello del mare che provoca il passaggio di molte zone precedentemente sommerse prima ad un ambiente litorale e inne emerso. Questo \ue8 quanto si \ue8 vericato in Pianura Padana. Il cambiamento di ambiente ha provocato una variazione nel materiale che si stava depositando: prima si avevano limi e sabbie marine, poi ciottoli legati al trasporto uviale e materiale ne legato alle esondazioni dei umi. La dierenza nel materiale deposto \ue8 stata riconosciuta lungo tutta la Pianura Padana ed \ue8 stata nominata Discontinuit\ue0 R. La Discontinuit\ue0 R \ue8 quindi legata a un cambiamento nel clima, e all\u2019istaurarsi delle grandi glaciazioni nel continente europeo. Mediante lo studio di diverse carote prelevate in Pianura Padana \ue8 stato possibile creare un modello di et\ue0 che ha permesso di associare la Discontinuit\ue0 R al MIS22, datandola a circa 870.000 anni fa (Figura 1). Il MIS22 corrisponde inoltre alla ne della cosiddetta \u201cRivoluzione del Pleistocene Medio\u201d, un momento in cui si \ue8 assistito ad un cambiamento nella ora con la comparsa delle prime piante legate all\u2019ambiente glaciale, ma anche ad un cambiamento nella fauna legato all\u2019arrivo di specie come il mammuth (Mammuthus meridionalis), l\u2019elefante antico (Elephas antiquus), e il rinoceronte lanoso (Coelodonta antiquitatis). La nostra ipotesi \ue8 che l\u2019arrivo dell\u2019uomo in Europa e in Italia sia stato legato alla sua tendenza a seguire le onde migratorie degli animali che costituivano le sue principali prede (Figura 2), e che sia avvenuto in concomitanza con l\u2019instaurarsi delle grandi glaciazioni Pleistocenica, ossia attorno all\u2019et\ue0 del MIS22

Organic Carbon Burial following the Middle Eocene Climatic Optimum (MECO) in the central - western Tethys

We present trace metal geochemistry and stable isotope records for the middle Eocene Alano di Piave section, NE Italy, deposited during magnetochron C18n in the marginal Tethys Ocean. We identify a $\sim$ 500 kyr long carbon isotope perturbation event we infer to be the middle Eocene Climatic Optimum (MECO) confirming the northern hemisphere expression and global occurrence of MECO. Interpreted peak climatic conditions are followed by the rapid deposition of two organic rich intervals ($\le$3\% TOC) and contemporaneous positive $\delta^{13}$C excursions. These two intervals are associated with increases in the concentration of sulphur and redox-sensitive trace metals, and low concentrations of Mn, as well as coupled with the occurrence of pyrite. Together these changes imply low, possibly dysoxic, bottom water O$_{2}$ conditions promoting increased organic carbon burial. We hypothesize that this rapid burial of organic carbon lowered global {\it p}CO$_{2}$ following the peak warming and returned the climate system to the general Eocene cooling trend