Energy, commons and the rest

Thoughts on energy as a commons, science and other stuffs by Cecile Blanchet

Papua New Guinea

Relative Geomagnetic Intensity during Marine Isotope Stage 3

This work has been pursued during my Master´s thesis in CEREGE. The aim was to reconstruct the paleomagnetic field using a sediment core that was collected on the southern margin of Papua New Guinea.

C. L. Blanchet, N. Thouveny, T. de Garidel-Thoron (2006). Evidence for multiple paleomagnetic intensity lows between 30 and 50 ka BP from a western Equatorial Pacific sedimentary sequence. Quaternary Science Reviews 25.

A paleomagnetic study was carried out on the radiocarbon dated MD97-2134 core located in the western Equatorial Pacific (southern Papua New Guinea margin). Rock magnetic investigations revealed changes of the magnetic mineralogy along the hemi-pelagic sedimentary sequence but the reconstruction of past direction and relative paleointensity variations of the geomagnetic field remained feasible. Four successive paleointensity drops are recorded between 30 and 50 ka BP. The largest one is associated with an abrupt swing of declination and inclination interpreted as a smoothed signature of the Laschamp excursion (ca. 41 ka BP). The succession of four events of weak intensity between 30 and 50 ka BP introduces a complex behaviour of the geomagnetic field in the time interval spanning over the Laschamp and the Mono Lake excursions.

This project was followed by another Master´s thesis by Guillaume Leduc, during which he analyzed the content in Beryllium 10 in order to assess the results obtained using standard paleomgnetic methods.

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Figure 10 of Blanchet et al. 2006, which shows the recontrscted relative paleointensity (for core MD97-2134), as compared with global southern (SAPIS) and northern (NAPIS) paelointensity stacks (see text for references).

G. Leduc, N. Thouveny, D.L. Bourlès, C.L. Blanchet, J.T. Carcaillet (2006). Authigenic 10Be/9Be signature of the Laschamp excursion: A tool for global synchronisation of paleoclimatic archives. Earth and Planetary Science Letters 245.

Authigenic 10Be and 9Be isotope concentrations have been measured in marine sediments deposited between 10 and 60 ka BP in the western equatorial Pacific and their ratio compared to the geomagnetic relative paleointensity (RPI) record obtained from the same core [C.L. Blanchet, N. Thouveny, T. de Garidel-Thoron, New evidences of geomagnetic moment lows between 30 and 45 ka BP from a sedimentary sequence of the west equatorial Pacific, Quat. Sci Rev. (in press)]. Over the studied time interval, dated by radiocarbon and oxygen isotope stratigraphy, three periods of reduced RPI occurring at ∼32, 37 and 45 ka, according to the constructed age model, are concomitant with three significant increases in atmospheric 10Be production. Since the most prominent 10Be/9Be peak necessarily results from the weakest geomagnetic moment event, i.e. that associated with the Laschamp excursion, we assign to this cosmogenic nuclide event the age of the Laschamp excursion (i.e. 40.4±2.0ka BP) [H. Guillou, B.S. Singer, C. Laj, C. Kissel, S. Scaillet, B.R. Jicha, On the age of the Laschamp geomagnetic excursion, Earth Planet. Sci. Lett. 227 (2004) 331–343]. Cosmogenic nuclide production peaks provide critical complementary evidence for the identification of geomagnetic dipole lows linked to excursions and constitute accurate markers for global intercorrelation of paleoclimatic archives.

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Figure 5 of Leduc et al., 2006, which shows the 10/9Be record for core MD97-2134 (b) at time of the Laschamp excursion, as compared with cosmogenic isotope records for similar time intervals in core MD95-2042 (Portuguese Margin, a) and in the Greenland Ice core record GRIP (b) (see text for references).
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