Evaluating Zr/Rb Ratio From XRF Scanning as an Indicator of Grain-Size Variations of Glaciomarine Sediments in the Southern Ocean

• Published in: Geochemistry, Geophysics, Geosystems
• Main Authors: Wu, Li, Wilson, David J., Wang, Rujian, Yin, Xuebo, Chen, Zhihua, Xiao, Wenshen, Huang, Mengxue
• Format: Report
• Language: English
• Published: AMER GEOPHYSICAL UNION 2020
• Subjects: bottom currents; glaciomarine sediment; grain‐ size; iceberg rafted debris; Southern Ocean; XRF core scanning Zr; Rb; Geochemistry & Geophysics; ANTARCTIC ICE-SHEET; SORTABLE SILT; PALEOCURRENT SPEED; CONTINENTAL-SLOPE; LABRADOR CURRENT; EAST ANTARCTICA; ATMOSPHERIC CO2; ROSS SEA; CHINA; CORE; Antarctic; East Antarctica; Ross Sea; Antarc*; Antarctica; Ice Sheet; Iceberg*
• Online Access: http://ir.qdio.ac.cn/handle/337002/169305; https://doi.org/10.1029/2020GC009350

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 (SS over bar ) grain-size. Universal gradients exist in plots of SS% versus ln(Zr/Rb), and SS over bar versus ln(Zr/Rb), such that the ln(Zr/Rb) ratio provides a convenient way to estimate the magnitude of changes in SS% and SS over bar . 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.