Leonardo Sagnotti

Obliquity-paced Pliocene West Antarctic ice sheet oscillations

Thirty years after oxygen isotope records from microfossils deposited in ocean sediments confirmed the hypothesis that variations in the Earth’s orbital geometry control the ice ages, fundamental questions remain over the response of the Antarctic ice sheets to orbital cycles. Furthermore, an understanding of the behaviour of the marine-based West Antarctic ice sheet (WAIS) during the ‘warmer-than-present’ early-Pliocene epoch (∼5–3 Myr ago) is needed to better constrain the possible range of ice-sheet behaviour in the context of future global warming. Here we present a marine glacial record from the upper 600 m of the AND-1B sediment core recovered from beneath the northwest part of the Ross ice shelf by the ANDRILL programme and demonstrate well-dated, ∼40-kyr cyclic variations in ice-sheet extent linked to cycles in insolation influenced by changes in the Earth’s axial tilt (obliquity) during the Pliocene. Our data provide direct evidence for orbitally induced oscillations in the WAIS, which periodically collapsed, resulting in a switch from grounded ice, or ice shelves, to open waters in the Ross embayment when planetary temperatures were up to ∼3 °C warmer than today and atmospheric CO2 concentration was as high as ∼400 p.p.m.v. (refs 5, 6). The evidence is consistent with a new ice-sheet/ice-shelf model that simulates fluctuations in Antarctic ice volume of up to +7 m in equivalent sea level associated with the loss of the WAIS and up to +3 m in equivalent sea level from the East Antarctic ice sheet, in response to ocean-induced melting paced by obliquity. During interglacial times, diatomaceous sediments indicate high surface-water productivity, minimal summer sea ice and air temperatures above freezing, suggesting an additional influence of surface melt under conditions of elevated CO2.

Orbitally induced oscillations in the East Antarctic ice sheet at the Oligocene/Miocene boundary

Between 34 and 15 million years (Myr) ago, when planetary temperatures were 3–4 °C warmer than at present and atmospheric CO2 concentrations were twice as high as today, the Antarctic ice sheets may have been unstable. Oxygen isotope records from deep-sea sediment cores suggest that during this time fluctuations in global temperatures and high-latitude continental ice volumes were influenced by orbital cycles. But it has hitherto not been possible to calibrate the inferred changes in ice volume with direct evidence for oscillations of the Antarctic ice sheets. Here we present sediment data from shallow marine cores in the western Ross Sea that exhibit well dated cyclic variations, and which link the extent of the East Antarctic ice sheet directly to orbital cycles during the Oligocene/Miocene transition (24.1–23.7 Myr ago). Three rapidly deposited glacimarine sequences are constrained to a period of less than 450 kyr by our age model, suggesting that orbital influences at the frequencies of obliquity (40 kyr) and eccentricity (125 kyr) controlled the oscillations of the ice margin at that time. An erosional hiatus covering 250 kyr provides direct evidence for a major episode of global cooling and ice-sheet expansion about 23.7 Myr ago, which had previously been inferred from oxygen isotope data (Mi1 event).

Age of the Corsica–Sardinia rotation and Liguro–Provençal Basin spreading: new paleomagnetic and Ar/Ar evidence

Abstract The age of spreading of the Liguro–Provencal Basin is still poorly constrained due to the lack of boreholes penetrating the whole sedimentary sequence above the oceanic crust and the lack of a clear magnetic anomaly pattern. In the past, a consensus developed over a fast (20.5–19 Ma) spreading event, relying on old paleomagnetic data from Oligo–Miocene Sardinian volcanics showing a drift-related 30° counterclockwise (CCW) rotation. Here we report new paleomagnetic data from a 10-m-thick lower–middle Miocene marine sedimentary sequence from southwestern Sardinia. Ar/Ar dating of two volcanoclastic levels in the lower part of the sequence yields ages of 18.94±0.13 and 19.20±0.12 Ma (lower–mid Burdigalian). Sedimentary strata below the upper volcanic level document a 23.3±4.6° CCW rotation with respect to Europe, while younger strata rapidly evolve to null rotation values. A recent magnetic overprint can be excluded by several lines of evidence, particularly by the significant difference between the in situ paleomagnetic and geocentric axial dipole (GAD) field directions. In both the rotated and unrotated part of the section, only normal polarity directions were obtained. As the global magnetic polarity time scale (MPTS) documents several geomagnetic reversals in the Burdigalian, a continuous sedimentary record would imply that (unrealistically) the whole documented rotation occurred in few thousands years only. We conclude that the section contains one (or more) hiatus(es), and that the minimum age of the unrotated sediments above the volcanic levels is unconstrained. Typical back-arc basin spreading rates translate to a duration ≥3 Ma for the opening of the Liguro–Provencal Basin. Thus, spreading and rotation of Corsica–Sardinia ended no earlier than 16 Ma (early Langhian). A 16–19 Ma, spreading is corroborated by other evidences, such as the age of the breakup unconformity in Sardinia, the age of igneous rocks dredged west of Corsica, the heat flow in the Liguro–Provencal Basin, and recent paleomagnetic data from Sardinian sediments and volcanics. Since Corsica was still rotating/drifting eastward at 16 Ma, it presumably induced significant shortening to the east, in the Apennine belt. Therefore, the lower Miocene extensional basins in the northern Tyrrhenian Sea and margins can be interpreted as synorogenic “intra-wedge” basins due to the thickening and collapse of the northern Apennine wedge.

Biomonitoring of traffic air pollution in Rome using magnetic properties of tree leaves

Abstract We report a biomonitoring study of air pollution in Rome based on the magnetic properties of tree leaves. In a first step, magnetic properties of leaves from different tree species from the same location were compared. It was observed that leaves of evergreen species, like Quercus ilex, present much higher magnetic intensities than those of deciduous species, like Platanus sp., suggesting that leaves accumulate magnetic pollutants during their whole lifespan. In a second step, leaves from Q. ilex and Platanus sp. trees, both very common in Rome, have been used to monitor traffic emission pollution in two different periods. A Platanus sp. sampling campaign was undertaken in October 2001, at the end of the seasonal vegetational cycle, and 5 Q. ilex monthly sampling campaigns from April to August 2002. The strong difference observed in the magnetic susceptibility from leaves collected in green areas and roads allowed the realization of detailed pollution distribution maps from the south of Rome. Magnetic properties indicate that high concentrations and relatively larger grain-sizes of magnetic particles are observed in trees located along roads with high vehicle traffic and in the vicinity of railways. The decrease in concentration and grain size of magnetic particles with distance from the roadside confirms that magnetic properties of leaves are related to air pollution from vehicle emissions. The results indicate that a magnetic survey of tree leaves, which is relatively rapid and inexpensive, may be used in addition to the classical air quality monitoring systems to identify and delineate high-polluted areas in urban environments.

Magnetic fabric of weakly deformed clay-rich sediments in the Italian peninsula: Relationship with compressional and extensional tectonics

Abstract We present the results of anisotropy of magnetic susceptibility (AMS) analyses carried out in weakly deformed Neogene and Quaternary clay-rich sediments from different compressional and extensional settings of the Italian peninsula, discussing the relationships between the magnetic fabrics and the tectonic settings. A well defined magnetic lineation of tectonic origin was found in several structures. The studied cases indicate that the AMS analysis of fine-grained sediments constitutes a powerful method to better constrain the tectonic evolution of sedimentary basins, where strain markers are not available. In the extensional basins the magnetic lineation coincides almost always with the stretching direction, obtained from mesostructral analysis of faults and joints, and it is generally aligned with the bedding dip. This geometric relationship is different in the compressional basins, where the magnetic lineation is almost parallel to the bedding strike. In both the extensional and compressional environments the magnetic lineation was acquired during the early stages of deformation, when the bedding was still sub-horizontal and it was not modified by the subsequent tectonic phases.

Magnetic fabric of clay sediments from the external northern Apennines (Italy)

Abstract We report on the anisotropy of magnetic susceptibility (AMS) analyses of fine-grained sediments deposited during the Messinian in foredeep basins at the front of the northern Apenninic chain. The data refer to 32 sampling sites, mostly distributed in the fine-grained intervals of the Laga and Colombacci formations, extending along the belt for a total length of about 300 km. Rock magnetism analyses indicate that the magnetic susceptibility and its anisotropy are in most cases dominated by the paramagnetic minerals of the clay matrix. In order to delineate the contribution of the ferrimagnetic fraction to the overall susceptibility fabric, the anisotropy of the anhysteretic remanent magnetisation was investigated at some representative sites. The magnetic fabric of the studied sediments mostly reflects the effects of compaction, showing a predominant magnetic foliation parallel to the bedding plane. At all the sites a well distinct magnetic lineation was also found, which is parallel to the fold axes and thrust fronts, both at local and regional scales. This feature is maintained in sequences that differ for sedimentological character and age, implying that the magnetic lineation was produced by a mild tectonic overprint of the primary sedimentary-compactional fabric. The relationship between the magnetic lineation trends and the vertical axis rotations detected by Speranza et al. [Speranza, F., Sagnotti, L., Mattei, M., 1997. J. Geophys. Res. 102, 3153–3166] indicates that the magnetic lineation formed during the compressive phases of the Messinian-early Pliocene, when the Apenninic front was almost rectilinear and oriented N320°.

Rock magnetism and palaeomagnetism of greigite-bearing mudstones in the Italian peninsula

Magnetic measurements were carried out on a collection of samples from different geological settings throughout the Italian peninsula. The samples display magnetic properties that indicate the presence of ferrimagnetic iron sulphide minerals. Paired samples were also investigated to compare the palaeomagnetic behaviour with respect to different demagnetization treatments (thermal, static and tumbling alternating field (AF) demagnetization). Greigite (Fe3S4) was positively identified in most samples. In agreement with previous studies of greigite, the sediments display: thermal decomposition of the magnetic carriers at temperatures above ca. 230°C, resulting in decreased magnetic susceptibility values and maximum unblocking temperatures (Tub) mostly in the range 320–350°C; high saturation isothermal remanent magnetization (SIRM) to low-field magnetic susceptibility (k) ratios (SIRM/k > 10 kA/m); hysteresis ratios that are typical for single domain (SD) grains (Mrs/Ms>0.5 and (B0)cr/(B0)c<1.5, where Mrs is the saturation remanence, Ms is the saturation magnetization, (B0)c is the coercive force and (B0)cr is the coercivity of remanence); moderate coercivity, with (B0)cr values that range between 52 and 81 mT and a tendency to acquire a significant rotational remanent magnetization (RRM). We also found that greigite-bearing sediments are particularly sensitive to field impressed anisotropy. We propose a new magnetic parameter that can be used as a rapid mean to screen a rock sample for the presence of greigite, which relies on its SD behaviour in field impressed anisotropy experiments. In greigite-bearing sediments, the magnetic susceptibility measured in a given direction is increased by the application of a relatively high magnetic field (0.9 T in our experiments) at right angles to this direction. The susceptibility increase is directly correlated to the concentration of greigite in the sediment. The comparative palaeomagnetic analysis of standard demagnetization techniques indicates that thermal demagnetization, in close steps up to 350–380°C, is clearly the most effective treatment for the isolation of the remanent magnetization carried by greigite, whereas AF demagnetization treatments display significant acquisition of unwanted magnetic remanences (gyroremanent remanent magnetization (GRM) and RRM) at fields above ca. 40 mT.

Tectonics of the Umbria-Marche-Romagna Arc (central northern Apennines, Italy): New paleomagnetic constraints

We present the results of a paleomagnetic study carried out on 32 sites from mainly Messinian clayey sediments distributed throughout the external Umbria-Marche-Romagna Arc (UMRA). These data, together with published results from coeval sediments, demonstrate that this arc is an orocline in its central northern sector. Bending, not well constrained in time, was due to about 15° clockwise rotations of the central part of this arc and to counterclockwise rotations farther north. In this latter area, post-Messinian counterclockwise rotations are of the same amplitude as those calculated for some classic Mesozoic paleomagnetic sections in northern Umbria, suggesting a Plio-Pleistocene age for the rotations reported from the older sequences.

Magnetobiostratigraphic chronology of the Eocene—Oligocene transition in the CIROS-1 core, Victoria Land margin, Antarctica: Implications for Antarctic glacial history

In 1986, cores were obtained to a depth of 702 m (with 98% recovery) from the CIROS-1 drill hole beneath the Ross Sea on the Victoria Land margin. Glaciogene sediments identified near the base of the hole mark the earliest known record of Antarctic glaciation. Initial biostratigraphic analysis indicated that the lower 336 m of the core is early Oligocene in age, and that the upper 366 m is of late Oligocene‐early Miocene age. Recently, the chronology of the CIROS-1 core has been questioned. We developed a magnetostratigraphy for the lower 400 m of the CIROS-1 core to clarify the chronology. Our magnetobiostratigraphic results indicate that the base of the CIROS-1 core is early-late Eocene in age (corresponding to Chron C16r; ca. 36.5 Ma). We identify the Eocene-Oligocene boundary at about 410‐420 m, within a 20-m-thick, poorly stratified, bioturbated sandy mudstone. This makes the CIROS-1 core the highest latitude site (77.1°S) from which this datum event has been recognized. At 366 m, a 4 m.y. hiatus, which lies immediately beneath fluvial sediments, accounts for most of Chrons C11 and C12. We recognize three major climatic episodes in the CIROS-1 core: (1) the late Eocene (34.5‐36.5 Ma, 430‐702 m), when relatively warm conditions dominated and there were high sedimentation rates and some glacial activity; (2) the late Eocene‐early Oligocene boundary interval (28.5‐34.5 Ma, 340‐430 m), which was a transition from relatively warm to cooler conditions that coincided with glacial intensification, sea-level fall, and subaerial erosion of the shelf; and; (3) the late Oligocene‐ early Miocene (22‐28.5 Ma, 50‐340 m), when large-scale glaciation dominated the region and glaciers grounded across the continental shelf. From correlation with global oxygen isotope and sea-level records, we infer that the Antarctic climate and surrounding oceans cooled after separation of Australia and Antarctica and development of deep-water circulation between them. This marked the onset of the Eocene‐Oligocene transition at ca. 34.5 Ma. A major East Antarctic ice sheet did not develop until the early-late Oligocene boundary, toward the end of the Eocene‐Oligocene transition (ca. 28.5 Ma). Outlet glaciers did not breach the Transantarctic Mountains and ground across the Ross Sea Shelf until 0.5 m.y. later (ca. 28 Ma).

Inter-laboratory calibration of low-field magnetic and anhysteretic susceptibility measurements

Inter-laboratory and absolute calibrations of rock magnetic parameters are fundamental for grounding a rock magnetic database and for semi-quantitative estimates about the magnetic mineral assemblage of a natural sample. Even a dimensionless ratio, such as anhysteretic susceptibility normalized by magnetic susceptibility (Ka/K) may be biased by improper calibration of one or both of the two instruments used to measure Ka and K. In addition, the intensity of the anhysteretic remanent magnetization (ARM) of a given sample depends on the experimental process by which the remanence is imparted. We report an inter-laboratory calibration of these two key parameters, using two sets of artificial reference samples: a paramagnetic rare earth salt, Gd2O3 and a commercial “pozzolanico” cement containing oxidized magnetite with grain size of less than 0.1 μm according to hysteresis properties. Using Gd2O3 the 10 Kappabridges magnetic susceptibility meters (AGICO KLY-2 or KLY-3 models) tested prove to be cross-calibrated to within 1%. On the other hand, Kappabridges provide a low-field susceptibility value that is ca. 6% lower than the tabulated value for Gd2O3, while average high-field susceptibility values measured on a range of instruments are indistinguishable from the tabulated value. Therefore, we suggest that Kappabridge values should be multiplied by 1.06 to achieve absolute calibration. Bartington Instruments magnetic susceptibility meters with MS2B sensors produce values that are 2–13% lower than Kappabridge values, with a strong dependence on sample centering within the sensor. The Ka/K ratio of ca. 11, originally obtained on discrete cement samples with a 2G Enterprises superconducting rock magnetometer and a KLY-2, is consistent with reference parameters for magnetites of grain size <0.1 μm. On the other hand, Ka values from a 2G Enterprises magnetometer and K values from a Bartington Instruments MS2C loop sensor for u-channel and discrete cement samples, will produce average Ka/K values that are unrealistically high if not properly corrected for the nominal volume detected by the sensors for these instruments. Inter-laboratory measurements of K and Ka for standard paleomagnetic plastic cubes filled with cement indicate remarkable differences in the intensity of the newly produced ARMs (with a standard deviation of ca. 21%), that are significantly larger than the differences observed from the calibration of the different magnetometers employed in each laboratory. Differences in the alternating field decay rate are likely the major source of these variations, but cannot account for all the observed variability. With such large variations in experimental conditions, classical interpretation of a “King plot” of Ka versus K would imply significant differences in the determination of grain size of magnetite particles on the same material.