Acquisition of detrital magnetization in four turbidites

Turbiditic events are mostly avoided in paleomagnetic studies and therefore their remanence and magnetic properties are poorly described. Turbidites are exempt of bioturbation and potentially provide pertinent information about depositional remanence. We studied four quaternary turbidites of different origins in marine sediment cores. Upward fining of both magnetic and sedimentary fractions indicates that coarser grains reached the bottom first. We observe a progressive shallowing of the magnetic inclinations between the upper and bottom layers of the turbidites that increases with the size of the events and obeys a simple linear scaling law. Measurements of magnetic anisotropy suggest that hydrodynamic conditions prevailing during deposition seem to be dominant for the alignment of the magnetic grains. We suggest that small spherical grains are randomly oriented with zero resultant magnetization in presence of strong turbulent conditions, while the alignment of elongated grains is constrained by the competition between gravity and magnetic forces. A possible scenario is that under turbulent conditions they tend to rest at the bottom with their long axes parallel to the sediment surface and therefore with shallow inclinations, whereas weakly turbulent conditions like during the smallest (26 cm thick) event do not disturb the magnetic alignment and therefore do not generate inclination shallowing.