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

Paleomagnetism of latest Anisian (Middle Triassic) sections of the Prezzo Limestone and the Buchenstein Formation, Southern Alps, Italy

A paleomagnetic study was carried out at six stratigraphic sections (309 specimens) in the latest Anisian (Middle Triassic) Prezzo Limestone and the overlying Buchenstein Formation. These units outcrop over a wide area in the western Southern Alps, although most of the sampled sections are in the vicinity of the Late Eocene-Early Oligocene Adamello batholith. Three sites suffered a complete remagnetization induced by the Adamello, whereas a characteristic component with a positive fold test has been isolated at the three other sites. The mean pole of the characteristic component (Lat. 63.2°N, Long. 229.3°E,N = 3,A95 = 8°,k = 236) is in agreement with the Triassic portion of the West Gondwana apparent polar wander path (APWP), supporting the use of paleopoles from well-dated rocks in the Southern Alps as useful proxies for the African APWP. The characteristic component, where isolated in the sampled sections, is of normal polarity only, corresponding to the latest Anisian on the basis of well-defined ammonoid and conodont biostratigraphy, but the present results suggest that there are good opportunities for extending Middle Triassic magnetostratigraphy in these Southern Alps rock units. The mean pole of the Adamello-induced component (Lat. 74.5°N, Long. 172.1°E,N = 4,A95 = 7.6°,K = 145) lies close to the Early Tertiary portion of the APWP for stable Europe. The post-folding, Adamello-induced directions confirm that the 30–42 Ma Adamello batholith intruded after Alpine deformation and that no further deformation apparently occurred in post-emplacement times

Mobility of Pangea: Implications for Late Paleozoic and Early Mesozoic paleoclimate

Several recent analyses of paleomagnetic data support the concept of Pangea, an assemblage of most of the world‘s continents that was mobile in terms of large-scale internal deformation and with respect to paleolatitude. The main feature of internal deformation involved the transformation from a Pangea B—type configuration in the late Paleozoic, with northwestern South America adjacent to eastern North America, to a more traditional Pangea A—type configuration in the early Mesozoic, with northwestern Africa adjacent to eastern North America. Pangea B thus seems to coincide in time with extensive low-latitude coal deposition and high southern-latitude Gondwana glaciations, whereas Pangea A coincides with generally drier conditions over the continents and no polar ice sheets. Although the configuration of Pangea may have been more stable as an A-type configuration in the Early and Middle Jurassic prior to breakup, the paleomagnetic evidence suggests that there was appreciable latitudinal change of the assembly. Such changing tectonic boundary conditions emphasize the practical importance of age registry of paleoclimate data in making valid comparisons with model results. A simple zonal climate model coupled with the geocentric axial dipole hypothesis for establishing paleolatitudes in precisely controlled paleogeographic reconstructions can explain many of the climate patterns in both the late Paleozoic and the early Mesozoic, but it cannot explain the presence or absence of continental ice sheets

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

ADRIA AS PROMONTORY OF AFRICA AND ITS CONCEPTUAL ROLE IN THE TETHYS TWIST AND PANGEA B TO PANGEA A TRANSFORMATION IN THE PERMIAN

It has been almost 60 years since the first results from the Early Permian Bolzano Quartz Porphyries from the Trento Plateau of northern Italy (Southern Alps) showed paleomagnetic inclinations steeper than inclinations from broadly coeval units from central Europe. This experimental discrepancy, confirmed ever since at varying levels of magnitude and certitude, implied that northern Italy had paleolatitudes too northerly relative to Europe to be considered part of the European continent. On the other hand, it became progressively more apparent that paleomagnetic data from northern Italy were more compatible with data from Africa than with data from Europe, and this observation revived and complemented Argand’s original concept of Adria as a promontory of Africa. But if Adria was part of Africa, then the paleolatitude anomaly of Adria relative to Europe translated into a huge crustal misfit of Gondwana relative to Laurasia when these landmasses were forced into a classic Wegenerian Pangea as typified by the Bullard fit of the circum-Atlantic continents. This crustal misfit between Gondwana and Laurasia was shown to persist in the ever-growing paleomagnetic database even when data from Adria were provisionally excluded as non-cratonic in nature. Various solutions were offered that ultimately involved placing Gondwana to the east (allowing it to be more northerly) relative to Laurasia and envisaging a dextral shear occurring in the Tethys (Mediterranean) realm between these supercontinental landmasses. This shear or transformation was initially thought to occur as a continuum over the course of the Mesozoic–Cenozoic (the so-called ‘Tethys Twist’) but soon afterwards when plate tectonics came into play and limited the younger extent, as a discrete event during the post-Triassic, Triassic or most probably – as in the latest and preferred reconstructions – the Permian between a configuration of Pangea termed B – with the northwestern margin of Africa against southern Europe – to a configuration termed Pangea A-2, with the northwestern margin of Africa against eastern North America, that is more proximal in shape to the classic Pangea A-1 that started fragmenting in the Jurassic with the opening of the Atlantic Ocean. The Permian timing and presumed locus of the ~2300 km dextral shear is supported by rotated tectonic domains in Sardinia and elsewhere along the interface between Lauarasia and Gondwana. The concept of Pangea B and its transformation into Pangea A developed therefore in close conjunction with the concept and paleomagnetic support of Adria as a promontory of Africa, and has ramifications to many aspects of tectonics, climate change and biogeography yet to be explored

Widespread formation of cherts during the early Eocene climate optimum

Radiolarian cherts in the Tethyan realm of Jurassic age were recently interpreted as resulting from high biosiliceous productivity along upwelling zones in subequatorial paleolatitudes the locations of which were confirmed by revised paleomagnetic estimates. However, the widespread occurrence of cherts in the Eocene suggests that cherts may not always be reliable proxies of latitude and upwelling zones. In a new survey of the global spatio-temporal distribution of Cenozoic cherts in Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores, we found that cherts occur most frequently in the Paleocene and early Eocene, with a peak in occurrences at ~50 Ma that is coincident with the time of highest bottom water temperatures of the early Eocene climatic optimum (EECO) when the global ocean was presumably characterized by reduced upwelling efficiency and biosiliceous productivity. Cherts occur less commonly during the subsequent Eocene global cooling trend. Primary paleoclimatic factors rather than secondary diagenetic processes seem therefore to control chert formation. This timing of peak Eocene chert occurrence, which is supported by detailed stratigraphic correlations, contradicts currently accepted models that involve an initial loading of large amounts of dissolved silica from enhanced weathering and/or volcanism in a supposedly sluggish ocean of the EECO, followed during the subsequent middle Eocene global cooling by more vigorous oceanic circulation and consequent upwelling that made this silica reservoir available for enhanced biosilicification, with the formation of chert as a result of biosilica transformation during diagenesis. Instead, we suggest that basin-basin fractionation by deep-sea circulation could have raised the concentration of EECO dissolved silica especially in the North Atlantic, where an alternative mode of silica burial involving widespread direct precipitation and/or absorption of silica by clay minerals could have been operative in order to maintain balance between silica input and output during the upwelling-deficient conditions of the EECO. Cherts may therefore not always be proxies of biosiliceous productivity associated with latitudinally focused upwelling zones

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

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

A Novel Plate Tectonic Scenario for the Genesis and Sealing of Some Major Mesozoic Oil Fields

Recent research reveals evidence of a major event of global plate motion during the Jurassic, with a magnitude and tempo hitherto not fully appreciated. Yet, its legacy persists today as the potent Jurassic source-reservoir-seal oil systems in the Persian Gulf region. We suggest that these formed as a consequence of a sweeping tectonic movement whereby Arabia drifted rapidly from the oil- forming Intertropical Convergence Zone along the equator to the arid tropics of the southern hemisphere with rapid deposition of relatively uncemented carbonate reservoirs and anhydrite seals, all during as little as 15 m.y. in the Late Jurassic. The Ghawar super- giant oil field of Saudi Arabia was one of the results. Rapid latitu- dinal change may have influenced the development of some source-reservoir-seal oil systems elsewhere as well