Contourites

Abstract

The ln(Zr/Rb) count ratio derived from X-ray fluorescence (XRF) core scanning holds potential as a high-resolution tracer for grain-size variations of glaciomarine sediments, and hence current strength. To evaluate this approach, we conducted high-resolution grain-size measurements, together with Rb and Zr measurements by XRF core scanning and inductively coupled plasma-mass spectrometry (ICP-MS), on a series of sediment cores from different regions of the Southern Ocean. Downcore changes of the ln(Zr/Rb) count ratio from XRF core scanning are consistent with Zr/Rb concentration ratios derived from ICP-MS analyses, even though Rb and Zr counts deviate significantly from concentrations due to specimen and matrix effects. The ln(Zr/Rb) count ratio displays discrepancies with the bulk mean grain-size, but correlates well with the mean grain-size of the sediment fractions that do not include unsorted sand delivered by ice-rafting. These observations are supported by evidence from a grain-size separation experiment, which indicates that Zr and Rb are concentrated in different grain-size fractions. Consistent with its lack of sensitivity to coarse grain-size fractions derived from ice-rafting, the ln(Zr/Rb) ratio records similar trends to the sortable silt percent (SS%) and sortable silt mean ((Formula presented.)) grain-size. Universal gradients exist in plots of SS% versus ln(Zr/Rb), and (Formula presented.) versus ln(Zr/Rb), such that the ln(Zr/Rb) ratio provides a convenient way to estimate the magnitude of changes in SS% and (Formula presented.). Overall, our results support the use of the ln(Zr/Rb) ratio as an indicator of bottom current strength in cases where the sediment is current-sorted.