Impact of turbulence on magnetic alignment in sediments

Abstract

Rapidly deposited layers (RDL) such as turbidites or hyperpycnites are mostly studied for their sedimentological properties, but are carefully avoided in paleomagnetic studies due to the disturbances caused by such sudden and rapid sediment accumulation. Therefore, these layers can also be seen as potential indicators of sediment parameters susceptible of affecting the alignment of magnetic grains and ultimately the acquisition of the natural remanent magnetization (NRM). We have compiled 13 Holocene rapidly deposited layers from core MD99-2222 in the Saguenay Fjord, eastern Canada (St-Onge and al., 2004) with varying thicknesses (from 7.1 cm to 1,510 cm) and 4 Quaternary turbidites of different origins, to document the influence of sedimentary and magnetic parameters on natural remanent magnetization acquisition. We found a logarithmic relationship between rapidly deposited layers thickness on the one hand, and the amplitude of inclination changes and magnetic grain sizes on the other. Inclination and magnetic grain sizes are themselves correlated to each other by a logarithmic law. As there is no relationship between inclination deviation and stratigraphic depth, compaction alone cannot account for such large effects on inclination. Flocculation is grain size sensitive, but it is expected to affect mainly the natural remanent magnetization intensity, rather than its direction. Turbulence that prevails during the rapid deposition of sediments during such events is most likely the dominant factor.

Influence of Seasonal Post‐Depositional Processes on the Remanent Magnetization in Varved Sediments From Glacial Lake Ojibway (Canada)

“…Varve formation involves reduced turbidity caused by surface ice‐cover that allows clay flocculation in winter beds, and increased silt influx in summer (e.g., Smith & Ashley, 1985). Rapidly deposited sediments such as turbidites can record shallow inclinations, with the event size, measured by thickness, determining the extent of the shallowing effect (Philippe et al., 2022; Tanty et al., 2016). Unusually large lock‐in depths of up to 30–70 cm, have been reported for varved lake sediments (Mellström et al., 2015; Nilsson et al., 2018), despite the absence of obvious post‐depositional sediment mixing.…”

“…The aim of this study was to constrain NRM acquisition mechanisms. Several parameters have an impact on magnetic particle alignment by the geomagnetic field (Anson & Kodama, 1987; Arason & Levi, 1990; Katari & Bloxham, 2001; Philippe et al., 2022; Roberts et al., 2013; Shcherbakov & Sycheva, 2010; Tanty et al., 2016; Tauxe, 1993; Tauxe et al., 2006). First, magnetic particle alignment depends on various factors related to depositional conditions and sediment lithology.…”

“…The sediment response function depends on several parameters related to the depositional environment, sediment matrix, and nature of magnetic grains. Numerous experiments and models aimed at simulating depositional processes and sediment magnetization have been proposed over the last 60 years (Nagata, 1961; Philippe et al., 2022; Spassov & Valet, 2012; Tanty et al., 2016; Tauxe, 1993; Tauxe et al., 2006; Yoshida & Katsura, 1985). However, a major limitation of laboratory deposition experiments is the impossibility to simulate natural sedimentation conditions in the absence of adequate scaling that accounts for the much longer duration of the magnetization process in nature (Katari et al., 2000; Spassov & Valet, 2012; Tauxe, 1993; Tauxe et al., 2006; Valet et al., 2017; Yoshida & Katsura, 1985).…”

“…It is also possible to compare the results of laboratory experiments with natural analogs of rapidly deposited sediments. The role of lithology and grain size in the magnetization process can be studied in turbidite sequences that provide useful indications about the influence of hydrodynamic conditions and the role played by magnetic and sediment grain sizes (Philippe et al., 2022; Tanty et al., 2016). Another interesting approach focuses on varved sediments (Mellström et al., 2015; Nilsson et al., 2018), which can potentially provide highly detailed magnetic records with annual temporal resolution.…”

“…grain sizes (Philippe et al, 2022;Tanty et al, 2016). Another interesting approach focuses on varved sediments (Mellström et al, 2015;Nilsson et al, 2018), which can potentially provide highly detailed magnetic records with annual temporal resolution.…”