Relative paleointensity of the geomagnetic field over the last 4,500 years BP from sediment cores of Laguna Chaltel (Patagonia, Argentina)

GOGORZA, CLAUDIA SUSANA; M.A. IRURZUN; SINITO, ANA MARÍA; ALDANA, MILAGROSA; FEY, MICHAEL; OHLENDORF, CHRISTIAN; B. ZOLITSCHKA

Cancún

Congreso; American Geophysical Union (AGU) - Meeting of the Americas; 2013

American Geophysical Union (AGU)

One motivation to investigate the magnetism of rocks is to study the behaviour of Earth’s magnetic field of the past. The magnetic field is a vector field, having both direction and intensity. A complete understanding of it requires the study of all vector properties. However, paleointensity determinations are much more difficult than directions alone. This is one reason why the majority of paleomagnetic studies is concerned only with the directional variability of the magnetic field.  Four short gravity cores from Laguna Chaltel in Patagonia, Argentina (49° 57’S, 71° 06’W) have been used to estimate the regional geomagnetic paleointensity. Measurements of intensity and directions of Natural Remanent Magnetization (NRM), magnetic susceptibility (k), isothermal remanent magnetization (IRM), saturation isothermal remanent magnetization (SIRM), as well as back field and anhysteretic remanent magnetization at 100 mT (ARM100mT) were performed and associated parameters calculated (ARM100mT/k, SIRM/ ARM100mT). In order to identify the magnetic mineralogy of the samples, IRM curves of a group of pilot samples were decomposed applying a Direct Signal Analysis (DSA) (Aldana et al., 2011). Results indicate the presence of magnetite, with logB1/2=1.8 and a relative proportion of 80%. Two other magnetic phases are observed at lower and higher logB1/2 values, probably greigite and goethite, respectively. DSA also indicates that the relative proportion of these minerals is the same in all samples analyzed. Studies also show that the magnetic grain size varies between 1 and 8 µm and that their concentration is between 0.01 and 0.08%. This range of the studied parameters indicates that our samples are suitable for paleointensity studies. The remanent magnetization at 15 mT (NRM15mT) has been normalized using the anhysteretic remanent magnetization at 15 mT (ARM15 mT), the saturation of isothermal remanent magnetization at 15 mT (SIRM15mT) and low field magnetic susceptibility (k). Our paleointensity (NRM15mT/ARM15mT) versus age curve shows good agreement with the published record from Laguna Potrok Aike, Argentina (Gogorza et al., 2012) suggesting that, in suitable sediments, paleointensity of the geomagnetic field can give a globally coherent and dominantly bipolar signal.   References Aldana, M., Costanzo-Álvarez, V., Gómez, L., González, C., Díaz, M., Silva, P., Rada, M., 2011. Identification of magnetic minerals related to hydrocarbon authigenesis in Venezuelan oil fields using an alternative decomposition of isothermal remanence curves. Studia Geophysica et Geodaetica. 55, 343-358.  Gogorza, C.S.G., Irurzun, M.A., Sinito, A.M., Lisé-Pronovost, A., St-Onge, G., Haberzettl, T., Ohlendorf, C., Kastner, S., Zolitschka, B., 2012.High-resolution paleomagnetic records from Laguna Potrok Aike (Patagonia, Argentina) for the last 16,000 years. Geochemistry Geophysics Geosystems. 13, Q12Z37.