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

•Modern seawater has a homogeneous δ87Rb with mean value at 0.13 ± 0.04‰ (2SD, n = 13);.•Authigenic phillipsite and clays can be isotopically light sinks of oceanic Rb cycling with δ87Rb at around −0.06‰ to 0.03‰.•The exchangeable fraction in sediments has δ87Rb of −0.07 ± 0.05‰ (2SD, n = 6), reflecting preferential absorption of 85Rb by deep-sea sediment.•The δ87Rb of the porewaters are homogeneous (0.08‰ to 0.14‰) and similar to seawater, suggesting a limited modification by authigenic silicates in the study region. 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 (Δ87Rbseawater-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.