Magnetostratigraphy of Lower Pleistocene Banyoles palaeolake carbonate sediments from Catalonia, NE Spain: Evidence for relocation of the Cobb Mountain sub-chron

Abstract

The Banyoles palaeolake carbonate sediments cored at Bòbila Ordis (hole BO IV, 52.15 m long) carry a weak natural remanent magnetization (mean NRM intensity: 0.59 ± 0.54 mAm −1) of reversed magnetic polarity except for a short zone of normal polarity close to the top (16.7−14.05 m). The NRM is probably carried by magnetite as well as by greigite (Fe 3S 4) in some zones. A chronostratigraphic constraint based on the presence of rodent teeth belonging to the Lower Pleistocene, coinciding climatic events, duration of the deposit estimated by cyclostratigraphy as well as sedimentation rates, suggests that the short normal polarity zone can be correlated to the Cobb Mountain sub-chron (1.19 Ma). This event is recorded at the Oxygen Isotope Stage 36/35 transition in marine sediment cores. In core BO IV, the normal polarity zone probably belongs to the end of an interglacial period as indicated by a cool and humid climate inferred from conifer pollen grains. There is thus an apparent time difference between the oxygen isotope and pollen signal, which is tentatively attributed to a significantly larger post-depositional lag in the acquisition of the remanent magnetization in marine sediments relative to the lacustrine sediment in question. The Cobb Mountain sub-chron might therefore have actually occurred at the end of the interglacial isotope stage 35.