Acquisition of detrital magnetization in four turbidites

Abstract

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.

Magnetic fabric of Bengal fan sediments: Holocene record of sedimentary processes and turbidite activity from the Ganges-Brahmaputra river system

“…The turbidite sequences have higher Km values which indicates an increase in the concentration of magnetite and the Anhysteretic Remanent Magnetization (ARM) to magnetic susceptibility ratio (ARM/ k) as well as the hysteresis parameters reveal an increase in the magnetite grain-size within the youngest coarse-grained turbidite located between 120 and 155 cm (Tanty et al, 2016). Similar results were found in other turbidite sequences from core MD17&18 (Moreno et al, 2014;Fig.…”

“…3. The turbidite level between 120 and 155 cm was already studied by AMS in a previous study (Tanty et al, 2016). Additional measurements were performed on new samples.…”

“…Such high Pj and/or F values in turbidite sequences in marine sediments have been observed elsewhere in rapid sediments deposits, for example, sediments from Bermuda Rise, northern California (Joseph et al, 1998) and mass transport deposits (MTD) from Ursa Basin, northern Gulf of Mexico (Meissl et al, 2011). Besides, Tanty et al (2016) investigated de AMS properties of four turbidites layers with different environmental conditions and different thickness. The results showed that the highest values in the degree of anisotropy were observed in the thickest turbidite (174 cm) in core MD98-2194 located in the Okinawa trough.…”

Impact of turbulence on magnetic alignment in sediments

“…Cores from Tanty et al (2016) The study of Tanty et al (2016) focused on the detrital remanent magnetization of four turbidites found in cores MD12-3418, MD01-2477 and MD98-2194. Two turbidites were sampled and studied in core MD12-3418 from the Bay of Bengal (16 °30.27 N, 87 °47.92 E, water depth 1).…”

“…A magnetic study of four distinct turbidites from the Bay of Bengal, Gulf of Corinth and Eastern China Sea was recently published by Tanty et al (2016) (Figure 1). The sedimentary and magnetic grain sizes revealed a significant coarsening of both sediment particles and magnetic grains within the bottom layers.…”