Potential and limitation of 230Th-excess as a chronostratigraphic tool for late Quaternary Arctic Ocean sediment studies: An example from the Southern Lomonosov Ridge

Recently, the use of extinction ages of excesses in U-series isotopes (Th-230(xs), Pa-231(xs)) has been proposed for the setting of benchmark ages of up to ~350 and ~150 ka, respectively, in late Quaternary marine records from the Arctic Ocean. However, the use of such U-series-based chronostratigraphic approaches has some limitations. These limitations are illustrated by U-series measurements in a cored sequence from the southern Lomonosov Ridge (PS2757). In this core, the final measurable excess in Th-230 (Th-230(xs)), strictly linked to the sedimentary flux of this isotope from the overlying water column (Th-230(xs)-marine), is observed at a depth of ~590 cm downcore. An extinction age of ~230 ka can be estimated for the residual (230)Th(xs )at this depth. It approximately matches the Marine Isotope Stage 7/8 transition. Below this transition, strong redox gradients constrained by a layer enriched in organic carbon resulted in a late-diagenetic relocation of uranium leached from detrital minerals in the over-and underlying oxidized layers. This uranium relocation resulted in large amplitude radioactive disequilibria within a core section otherwise characterized by near secular equilibria between inventories of U-238-series isotopes, implying an age greater than the Th-230(xs)-marine extinction age for the whole section. In the overlying part of the core, the (230)Th(xs )distribution correlates with other Th-230(xs)-documented sequences from the Central Arctic Ocean. (230)Th(xs )can be thus used for stratigraphic correlations between the relatively low-sedimentation rate marine sequences of this basin, over the last two or three glacial cycles, but special attention to potential diagenetic effects is recommended. Moreover, as for a given Th-230(xs)-marine flux at the seafloor, initial Th-230(xs)-values are broadly inversely-proportional to the sedimentation rate, the resulting estimates of Th-230(xs ) extinction age vary accordingly. This variability restricts the chronostratigraphic use of (230)Th(xs )to sequences with relatively low sedimentation rates, such as those where the initial Th-230(xs)-marine significantly exceeds the Th-230-fraction carried by detrital minerals.

Automated summary

The study proposes using extinction ages of U-series isotopes to determine benchmark ages in late Quaternary marine records from the Arctic Ocean. However, this method has limitations including diagenetic effects and sedimentation rate.