230Th-excess inventory and distribution in a southern Mendeleev Ridge core (Arctic Ocean): linkage with late Quaternary sedimentological and paleogeographical changes

In addition to ¹⁴C-data, sedimentary excesses in ²³⁰Th  (²³⁰Th_xs) in central Arctic Ocean cored sequences yielded critical time constrains and sedimentation rates estimates, at least, at sites characterized by very low sedimentation rates (<< 1cm/ka). Closer to the Russian margin, where higher accumulation rates are recorded based on ¹⁴C-ages, the setting of a reliable stratigraphy based on ²³⁰Th_xs reveals more challenging, as illustrated here, based on the analysis of  a gravity core raised from the southern Mendeleev Ridge (core ARC7-E25; -179.4°E, 79.0°N; 1200 m water depth; 320 cm long). Subsamples were collected at a 4 to 8 cm interval. Measurements included: AMS ¹⁴C in foraminifera, grain size, bulk Xray mineralogy, clay mineralogy, geochemistry (C_org, C_inorg,¹³C_org, ²³⁸U, ²³⁴U, ²³⁰Th, ²²⁶Ra, ²¹⁰Pb). Data indicate that some sediment were lost at core top. Nevertheless, ¹⁴C and ²³⁰Th_xs  data allow estimating a mean sedimentation rate of about 6 to 7 mm/ka during the last two climatic cycles. A comparison of the ²³⁰Th_xs inventory and distribution pattern with those from other cores allows identifying important parameters involved in the cycling of the water column-produced ²³⁰Th in this basin and its sporadic sedimentary accumulation, in particular linkages with sea-ice production over shelves, thus sea-levels, sea-ice rafting routes, grain-size and mineralogy, potential winnowing of fine fractions, role of brines and relative duration of intervals with reduced or nil sedimentation preceding ²³⁰Th_xs-accumulation intervals.