Time constraints from 230Th and 231Pa data in late Quaternary, low sedimentation rate sequences from the Arctic Ocean: an example from the northern Mendeleev Ridge

Abstract

Due to the lack of ‘conventional’ oxygen isotope stratigraphies, the setting of unquestionable time frames in Late Quaternary records from low sedimentation rate settings of the Arctic Ocean often encounters difficulties. Here we use HOTRAX cores raised from the Mendeleev Ridge (multicore HLY0503-12, ∼1600 m water depth; multicore and trigger weight core HLY0503-11, ∼2500 m water depth) to demonstrate that U-series data may supply reliable time constraints and sedimentation rate estimates. Grain size, mineralogical, geochemical (C org, C inorg) and isotopic ( 210Pb, 238U, 234U, 232Th, 230Th, 231Pa) measurements were made on ground bulk sediment, whereas isotopic ( 13C, 14C, 18O) measurements were performed on planktonic foraminifers when present. The two multicores present remarkably similar profiles despite their >1 km-bathymetric difference. Both show three majors units. The top (0–8 cm) and bottom (26–38 cm) units have high clay and carbonate contents, whereas the intermediate unit (8–26 cm) shows high sand and low clay and inorganic carbon contents. An apparently short event with higher clay and inorganic content is recorded at ∼17 cm. 230Th-excess and 231Pa-excess are observed down to 35 and 27 cm, respectively, thus locking minimum ages of ∼340 ka and ∼140 ka at these depths. Age-model estimates allowed us to assign the top unit to the Marine Isotopic Stages (MIS) 1–3 interval, and the bottom unit to the Termination III-MIS 7 interval. The intermediate unit would represent the last two glacial cycles, interrupted by a short Termination II-MIS 5e “excursion”. Data from the TWC allow this stratigraphy to be extended to MIS 13. Glacial units correspond to very low sedimentation rates essentially linked to ice rafting deposition, as indicated by their high sand content. This study suggests a mean sedimentation rate of about 1.5 mm ka −1 for this sector of the Mendeleev Ridge, which supports the very low sedimentation rate suggested by 10Be data in a nearby site from other studies.