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Abstract The natural remanent magnetization (NRM) of high sedimentation rate sediments provides significant information about paleomagnetic secular variation of the Earth's magnetic field and can also potentially be used for stratigraphy. However, NRM acquisition depends on conditions inherent to the depositional environment. In addition to recording a precise annual chronology, varved sediments reflect marked annual sedimentary changes. The Earth's magnetic field does not vary significantly over such a short period, so magnetic changes recorded by varves are expected to reflect the influence of depositional parameters on the recording process. We focus here on a sequence of 27 ± 1 varves from the former proglacial Lake Ojibway (∼8.5 ka cal BP) from which individual cm‐thick summer and winter beds were sampled. Paleomagnetic, granulometric and geochemical analyses were conducted on each bed. A mean inclination shallowing of 24.3° is observed in winter beds, along with an 11.3° shallowing in summer beds. Magnetic declinations follow, on average, the expected field direction, but differences of up to 20° occur between successive beds. Summer beds are thicker than winter beds and have stronger magnetic susceptibility, higher Ca/Fe ratios and coarser sedimentary and magnetic grains. This grain size pattern reflects the input of coarser detrital particles during summer, while the finer fraction remained in suspension until it was deposited in winter. A combination of differential compaction between the winter and summer beds, seasonally varying physical and magnetic properties of sediments, and delayed NRM acquisition explains the variable and coercivity‐dependent inclination shallowing.

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.

Abstract Inclination is the angle of a magnetization vector from horizontal. Clastic sedimentary rocks often experience inclination shallowing whereby syn‐ to post‐depositional processes result in flattened detrital remanent magnetizations relative to local geomagnetic field inclinations. The deviation of recorded inclinations from true values presents challenges for reconstructing paleolatitudes. A widespread approach for estimating flattening factors (f) compares the shape of an assemblage of magnetization vectors to that derived from a paleosecular variation model (the elongation/inclination [E/I] method). Few studies exist that compare the results of this statistical approach with empirically determined flattening factors and none in the Proterozoic Eon. In this study, we evaluate inclination shallowing within 1.1 billion‐year‐old, hematite‐bearing red beds of the Cut Face Creek Sandstone that is bounded by lava flows of known inclination. Taking this inclination from the volcanics as the expected direction, we found that detrital hematite remanence is flattened with f=0.650.560.75 whereas the pigmentary hematite magnetization shares a common mean with the volcanics. Using the pigmentary hematite direction as the expected inclination results in f=0.610.550.67. These flattening factors are consistent with those estimated through the E/I method f=0.640.510.85 supporting its application in deep time. However, all methods have significant uncertainty associated with determining the flattening factor. This uncertainty can be incorporated into paleomagnetic poles with the resulting ellipse approximated with a Kent distribution. Rather than seeking to find “the flattening factor,” or assuming a single value, the inherent uncertainty in flattening factors should be recognized and incorporated into paleomagnetic syntheses.