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

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 (similar to 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 degrees is observed in winter beds, along with an 11.3 degrees shallowing in summer beds. Magnetic declinations follow, on average, the expected field direction, but differences of up to 20 degrees 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.

Automated summary

The high sedimentation rate sediments' natural remanent magnetization (NRM) provides valuable information about the paleomagnetic secular variation of the Earth's magnetic field and can be used for stratigraphy. However, the acquisition of NRM is dependent on the specific conditions of the depositional environment. Varved sediments, in addition to providing an accurate annual chronology, also reflect significant annual sedimentary changes.