Origen de las variaciones magnéticas en sedimentos de la Ría de Pontevedra (NO de España)

Resumo

The magnetic properties of three cores from the Ria de Pontevedra (NW Spain) were determined to characterise their variability in terms of origin and diagenetic evolution of the magnetic minerals present. Three cores were taken from the inner; middle and external parts of the ria along its longitudinal axis. The low field magnetic susceptibility (c) of cores 2 (middle) and 3 (external) was higher at their upper part, rapidly decreasing with depth until a constant value was reached. Core 1 (internal) generally showed lower but more stable susceptibility values with depth. Frequency dependence of susceptibility (cfJ and IRM acquisition curves were obtained to study the origin of the observed variability in susceptibility. This data set was completed with total Fe and S, determined by XRF and sequential extractions of Fe. The results showed a strong correlation between c and Fe/S ratio with depth. Fe-rich oxides and oxihydroxides dominate the upper portion of the cores located in high energy areas (external and middle). The importance of sulfidic phases increase with depth in these areas, and completely dominates the sediment in low energy areas (internal). Hcr indicates a greater importance of the harder magnetic phases with depth in high energy areas. This is explained by the ongoing of dissolution and/or authigenic processes affecting the magnetic minerals with depth.