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Environmental Magnetism


The “Environmental magnetism” involves the new and significant applications of the paleomagnetism for the study of the paleoenvironmental and paleoclimatic variations, via (obtained by a) detailed analysis of the magnetic mineralogy (mineral magnetic assemblages) of sedimentary sequences (e.g. marine, lake or loess deposits). In all these cases the magnetic properties variations can be also used to carry out stratigraphic correlations, obtain reliable age models, identify sediment sources and mechanisms of transport, characterize the depositional and post-depositional processes (es. formation of authigenetic/diagenetic phases), and to identify the presence of a ciclicity in the magnetic property of sediments and it relation with environmental and astronomical processes.

The fundamental assumption of the environmental magnetism is that the variations of the magnetic property of sediments are closely tied to the quantitative, the nature and the granulometry of the terrigenous fraction, and to the different diagenetic processes operating in the sedimentation environment. Through the analysis of the stratigraphic trend of magnetic parameters it is therefore possible to provide a semi-quantitative estimation of the concentration, composition and granulometry variations of the magnetic minerals present in a successions, commonly correlated to paleoclimatic, paleoenvironmental and paleoceanographic changes occurred in a sedimentary basin.

In the last decades, with the improving in the understanding of the magnetization processes of sediments and the development of the dating techniques, the paleomagnetic database has become remarkably rich, concurring, in many cases, to progress the information on the climatic changes at a local and global scale, and to produce new ties for the modelling. Of particular interest, with this regard, it is the use of magnetic measurement in the study of the stratigraphic sequences sampled in different areas of the Antarctic continental margins.

Study of marine sedimentary series from Antarctic margins

In the frame of the SEDANO (SEdiment Drifts of ANtarctic Offshore)-PNRA project, we studied sedimentary series, covering the last glacial-interglacial cycle, coming from sediment cores of the western continental margin of Antarctic Peninsula (fig 1).

Figure 1 Location of the studied SEDANO cores collected on the continental rise of the Pacific margin of the Antarctic Peninsula.

The paleomagnetic analysis, carry out in continuous every centimetre on u-channel samples, allowed to obtain stratigraphic variations of different magnetic parameters: between them, the coercitivity dependent parameter MDFNRM (median destructive field of the NRM) show pronounced minima in correspondence of characteristic decimetric dark levels present in last glacial period (fig. 2). According to the interpretation of magnetic mineralogy such dark levels represent a clear climatic signal, characterized from diagenetic variations during the sulphides formation processes, due to changes of amount of organic matter contribution in the sedimentary basin, that is forced from the extension of the Ice sheet of the Antarctic Peninsula.

Figure 2 Downcore variation of the coercivity-dependent MDFNRM parameter showing pronounced sharp minima in glacial unit B, corresponding to marker dark grey beds (shaded areas).

Beside the Magnetostratigraphy techniques, that employ the reversals and the excursions of the geomagnetic field eventually recorded in the sediments for dating sedimentary series, in the last years another method of dating has been developed allowing  to obtain high resolution age models by correlation the curves of relative paleointensity obtained for each sedimentary sequence and various synthetic reference curves available on a global scale. Fig. 3 show the comparison between a synthetic curve (Stack) obtained for the SEDANO cores and the reference paleointensity curve SINT-800 of Guyodo and Valet, 1999.

Figure 3click to enlarge In order to obtain a reliable age model, the individual relative paleointensity (RPI) records obtained for each of the SEDANO cores, and the ChRM inclination curves, were “staked” and compared to the SINT-800 reference curve of Guyodo and Valet, 1999.

The high resolution age model, obtained for the SEDANO core through the Paleointensity methods, has evidenced that there is a temporal correlation between the dark decimetric levels with distinguished magnetic mineralogy present in the last glacial period, and some millennian scale climatic episodes, recognized in the North Atlantic sediments and in others areas around the globe, already known as "Heinrich events" (fig. 4). In this study, we obtained the first evidences of the presence of climatic marker, coevals to the events recognized in the northern hemisphere, also in high latitudes sediments of the southern hemisphere or else in the sedimentary successions of the peri-antarctic margins.

Figure 4 Chronostratigraphic variation of the MDFNRM in SED-02 and SED-04 cores, compared to the age range for the Heinrich events (H1-H6) from the North Atlantic. The H1-H6 ages are taken both from the “geomagnetic” chronostratigraphy developed for cores from the Labrador Sea (A) and from the recent compilation of radiocarbon ages of north Atlantic H-layers (B).

 

Analysis of the magnetic properties of PM10

A recent application of the Environmental magnetism is the study of the magnetic property of atmospheric particulate matter (PM10) carry out through a systematic analysis of the sampling filters collected by the quality air monitoring stations, or through the analysis of the thin powders deposited on the trees leaves present in the urban environments and in their countrysides.

In 2004 a study, of our laboratory get-together with the Regione Lazio, allowed to obtain, during a year of measurement on six regional stations (fig. 5), a semiquantitative estimation of the contribution to the entire concentration of PM10 of  the particles derived from pollution regarding to the fraction coming from other sources (es. natural). That is possible because the magnetic particles of the PM10 originating from the combustion processes of the vehicles, as submicroscopic spherules of maghemite or magnetite, are characterized by intense and stable magnetic properties measurable in terms of magnetic susceptibility.

Figure 5 Empirical linear correlation of PM10 concentration versus low-field magnetic susceptibility k. At every station such correlation was estimated for selected days with c comprised within ±20% of the arithmetic
mean value.

More in detail, for each daily filter (24 h) it has been measured, through a Kappabridge AGICO KLY-2, the low-field magnetic susceptibility per unit volume (k, adimensional in the International Units System), the values of the concentration of the PM10 have been supplied directly from the ARPA Lazio, while the magnetic susceptibility per unit mass (expressed in m3/kg) has been calculated from the relationship between the two. From the variations of these parameters is possible to distinguish three different cases: a) PM10 concentration vary as k (c is almost constant). This indicates that the magnetic composition of PM10 is constant over time, b)  the PM10 concentration increases more than (or decreases less than) k. These are episodes of χ minima suggest an input of exogenous dust, c) PM10 concentration decreases more (or increases less) than k. These episodes of χ maxima indicate a relative increase of the magnetic fraction of PM10.

The systematic analysis of the magnetic susceptibility of the filters allowed to define an empirical linear correlation, suitable for every station, tying the concentration of the PM10 to the magnetic susceptibility (fig. 6). From the analysis of the linear relations it is possible to calculate, for every station and for the entire analyzed period, the thin powder concentration that is not correlated to the magnetic susceptibility. We define such powders like "residual" PM10. In fig. 3 you can see the concentration of the measured PM10 and the residual PM10 at a various stations.

Figure 6 Diagrams and the daily variations of the measured PM10 concentration and the estimated ‘‘residual not magnetic’’ PM10 concentration for the period October–December 2004. We point out the similarity of the estimates for residual not magnetic PM10 at all stations during the same events for which a natural contribution is known (MA, marine aerosol; NAD, North African dust; SP, secondary pollution).

Integration of the magnetic data with those already available has finally demonstrated that the origin of such "not magnetic" fraction of atmospheric thin powders is mainly from episodes of powder contributions from North-Africa and in minor measure due to the contribution of marine spray aerosol or other secondary pollution events.

These considerations were anticipated by a former study on the measurement of magnetic susceptibility from leaves of trees in Rome; comparison of magnetic parameters from different tree species pointed out that the leaves Quercus Ilex (evergreen tree) are particularly suitable for the biomonitoring of urban atmospheric air pollution using magnetic properties.
The magnetic parameters indicate that the magnetic fraction of the urban dust in Rome is dominated by magnetite. The decrease in magnetite concentration and grain-size from high–traffic roads to green areas and with the distance to the roadside indicate that the main source of pollution is derived from vehicles emissions (fig. 7).

Figure 7click to enlarge Location of the six air-monitoring stations considered in the study get-together with the Regione Lazio: Roma-Villa Ada, Roma-Magnagrecia, Fontechiari, Viterbo, Latina, Frosinone scalo. Measurements were systematically carried out for every daily filter.

The lack of correlation with the hourly and daily PM10 records as measured in the automatic monitoring stations, and the small effect of the rainfalls on the magnetic parameters suggest that magnetic properties of leaves indicates a long-term pollution (in terms of accumulation of fine, mostly sub-micrometric, magnetic particles) rather than a punctual instantaneous record of dust load.


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