Magnetic anisotropy reveals the depositional and postdepositional history of a loess-paleosol sequence at Nussloch (Germany)

Samuel N. Taylor (1) , France Lagroix (1)

1 IPGP - Institut de Physique du Globe de Paris

Anisotropy of magnetic susceptibility (AMS) is employed as a tool to unravel the depositional history of the 17 m thick Nussloch P8 Weichselian loess sequence located 10 km south of Heidelberg, Germany. Through an AMS study, the primary aeolian depositional origin of the magnetic fabrics is evaluated, and overprinting due to postdepositional reworking and/or deformation is identified. Primary fabrics along the P8 sequence are defined by near-vertical K MIN axes and horizontal foliations. Eight intervals display secondary fabrics, characterized by either prolate orientation distributions or oblate orientation distributions with dipping foliation planes. These postdepositional fabrics are associated with laminated loess and tundra gley horizons. It is proposed that increased moisture (due to higher precipitation or enhanced snowmelt) and repeated cryogenic processes were able to reorganize and rework the accumulated loess. Primary aeolian fabrics are archived within 6.45 m of cumulated depth or 38% of the profile and dominantly within the Upper Pleniglacial units. Even though maximum susceptibility axes of primary fabrics are statistically well resolved at the specimen (ε 12 = 10.1° ± 8.6) and population (ε 12 = 6°) level, any inferred paleowind directions from the magnetic lineation remains speculative given the low concentration of ferrimagnetic minerals (<0.03 wt %). Tundra gley horizons upprofile display primary magnetic fabrics and no major changes in the degree of anisotropy or AMS orientation distributions. This suggests a weakening in gley-induced diagenesis and therefore favorable environmental conditions needed (moisture and presence of permafrost active layer) to initiate their formation in loess deposits. Finally, pedogenesis has not played an important role in modifying the magnetic fabric since paleosols display the same magnetic fabrics observed in primary loess.

Domaines: Planète et Univers [physics]

Samuel N. Taylor, France Lagroix. Magnetic anisotropy reveals the depositional and postdepositional history of a loess-paleosol sequence at Nussloch (Germany). Journal of Geophysical Research : Solid Earth, 2015, ⟨10.1002/2014JB011803⟩. ⟨insu-01351755⟩