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Abstract(s)
Speleothems are secondary mineral deposits formed in caves and are considered as very good archives of the Earth’s magnetic field and climate. In this thesis, I provide new highresolution magnetic data of a speleothem (SPA) collected in Algarve, in order to evaluate the effect of speleothem surface slope on the remanent magnetic directions, a still poorly studied and undemonstrated aspect of speleothem magnetism, and to provide new paleomagnetic data for the calibration of paleosecular variation (PSV) models. Paleomagnetic directions obtained from samples collected along subhorizontal to gradually subvertical calcite layers of a transversal cross-section of the SPA speleothem show very stable and high intensity magnetic directions, but with magnetic inclinations varying according to the slope of the calcite layers. Increased misalignment of the ferromagnetic particles due to their rolling along the surface, determined by Anisotropy of Anhysteretic Remanent Magnetization (AARM) techniques, results in a net distribution of shallower inclinations compared to PSV models. A correction factor is calculated based on the extrapolation of the magnetic inclinations to hypothetical horizontal layers, allowing a better comparison with PSV models, particularly consistent with the SHA.DIF.14K model. In addition, relative paleointensity data estimated using two different methods display consistent and comparable results. Finally, I provide detailed concentration- and coercivity-dependent magnetic proxy obtained through the analysis of isothermal remanent magnetization (IRM) curves and compared them with carbon and oxygen isotope composition measured in selected samples of the SPA stalagmite. Results show a low statistical correlation between magnetic and isotopic composition. However, higher content of pedogenic magnetite, often correlated with lower 13δC and 18δO compositions, suggests a casual-to-effect link between climate and magnetic mineralogy. These findings open new perspectives for reconstructing high-resolution PSV and climate records from speleothems and provide new insights into their NRM acquisition mechanisms.
Description
Keywords
Speleothems paleomagnetism environmental magnetism paleosecular variation