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

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 (kd). kd 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.


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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