The Rb isotope composition of modern seawater and outputs to deep-sea sediments

Abstract

The rubidium (Rb) isotope system has the potential for tracing water–silicate interactions and providing information on the global Rb cycling. However, the Rb isotope compositions of modern seawater and its major inputs and outputs remain poorly understood. Here we measured Rb isotope compositions of seawaters, pelagic clay sediments and porewaters from the western and central equatorial Pacific Ocean. Our results show that the δ⁸⁷Rb of modern seawater is homogeneous (0.13 ± 0.04‰; 2SD, n = 13) and higher than both the local sediments (-0.17‰ to 0.03‰) and the bulk lithosphere (Δ⁸⁷Rb_seawater-UCC = 0.27‰). The Rb in pelagic clay sediments is primarily associated with silicates (> 90%) and partially with exchangeable fractions (∼ 4%). The exchangeable fractions display relatively lower δ⁸⁷Rb (-0.07 ± 0.05‰; 2SD, n = 6). Meanwhile, the correlation between K/Rb and δ⁸⁷Rb of bulk sediments, along with investigations on the clay sized particles (δ⁸⁷Rb = -0.06‰), represents that lithogenic silicates have relatively low K/Rb and δ⁸⁷Rb close to the UCC while formation of authigenic phillipsite or clays can result in higher bulk K/Rb (up to 930) and δ⁸⁷Rb (up to 0.03‰). The δ⁸⁷Rb of both authigenic silicates and absorbed fractions in deep–sea sediments are lower than seawater, which can partially contribute to the removal of isotopically light Rb from seawater. The δ⁸⁷Rb of the measured marine porewaters are approximately homogeneous (0.08‰ to 0.14‰) and similar to seawater. The result consistent with previous K isotope investigation in this region with limited impact of authigenic silicates. Using a mass balance estimation in a steady state with isotope data, the flux of sediment removal for Rb in the ocean is about 2.2 – 12.0 × 10⁷ kg/year.