First direct measurement of actinium (but also radium and thorium) distribution coefficient values in marine sediments

16 October 2023

Actinium-227 (²²⁷Ac), radium-226 (²²⁶Ra) and radium-228 (²²⁸Ra) profiles in sediments were measured and modeled at 5 stations in the Northeast Pacific. Data allowed the distribution coefficient (Kd) between the solid and liquid phases for Ac and Ra.

A strong positive correlation is observed between Ra and ²²⁷Ac kd values, with ²²⁷Ac being almost 6.6 times higher than Ra kd values, and both co-varying with the MnO2 in solid phases.

The source function of ²²⁷Ac in the Northeast Pacific was determined by two independent methods: core incubation and reaction transport models. The authors also established the Ra fluxes.

The largest ²²⁷Ac and Ra isotope fluxes are near the center of the Northeast Pacific (~37ºN). Smaller ²²⁷Ac, ²²⁸Ra and ²²⁶Ra fluxes occur north of 40ºN, primarily due to dilution of their protactinium and thorium ancestors by higher sediment accumulation rates.

**Figure 1:** Study Area of the Northeast Pacific Basin (NEPB). Red dots and black triangles refer to stations where ²²⁷Ac was measured in the water column for GEOTRACES PMT (GP15) and C-Disk-IV cruises. Numbers next to symbols refer to station numbers. All sediment isotope profiles (Ra, Th, Ac, Pb, and Po) were made on C-Disk-IV cruise samples.

**Figure 2:** ²²⁵Ac vs. ²²⁸Ra distribution coefficients (k_d). k_d is the ratio of solid phase to dissolved phase concentration (k_d=dpm/g÷dpm/mL=mL/g).
Deep-sea sediments were added to Ra-free seawater and spiked with a known amount of ²²⁵Ac and ²²⁸Ra. The slurry was then rotated for one-hour, after which sediments and seawater were separated by centrifugation. Sediments and seawater were then measured separately by HPGe (High-Purity Germanium Detector). The regression line on the figure is forced through zero.

**Figure 3:** ²²⁶Ra, ²²⁸Ra, and ²²⁷Ac Flux vs. Station # for C-Disk-IV sediments. The ²²⁸Ra and ²²⁷Ac fluxes were multiplied x2 and x10 in order to produce comparable scales, since ²²⁶Ra flux was so much larger.

Reference:

Kemnitz, N., Hammond, D. E., Henderson, P., Le Roy, E., Charette, M., Moore, W., Anderson, R. F., Fleisher, M. Q., Leal, A., Black, E., Hayes, C. T., Adkins, J., Berelson, W., & Bian, X. (2023). Actinium and radium fluxes from the seabed in the northeast Pacific Basin. Marine Chemistry, 250, 104180. Access the paper: 10.1016/j.marchem.2022.104180

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