Protactinium-231 budget of the Atlantic sector of Southern Ocean

20 October 2023

Levier and colleagues (2023, see reference below) analysed dissolved and particulate Protactinium-231 (²³¹Pa) in samples collected as part of the BONUS GoodHope GEOTRACES GIPY4 cruise in the SE Atlantic sector of the Southern Ocean. They established a first direct estimate of the ²³¹Pa partition coefficient between suspended opal and seawater (~ 1.42 ± 0.55 × 106 g/g). They used this data to simulate the particulate ²³¹Pa concentration based on the dissolved Pa fraction and the opal concentration. The authors demonstrate that adding a new parameter (²³¹Pa) to the isopycnal advection-diffusion-scavenging model built for thorium isotopes along the same section (Levier, et al, 2021) leads to reduction by half of the estimated uncertainties among the different transport parameters of the model…

**Figure 1:** Conceptual diagram of the advection-diffusion-scavenging model. This model consider the transport of protactinium through the water masses circulation (advection), the molecular transport (diffusion) and the vertical transport by adsorption on particles (scavenging), which are occurring along the upwell of deep Atlantic Ocean waters into surface Southern Ocean.

**Figure 2:** Map showing the study area in the Atlantic sector of the Southern Ocean, with the position of the hydrological fronts identified based on the physical characteristics of the water masses. The positions of the analyzed seawater sampling stations (S1-S5) are plotted, as well as the position of station PS-131, which is used as an intercalibration reference carried out by another research unit.

**Figure 3:** Representation of protactinium mixing between the deep Atlantic Ocean waters and the Southern Ocean surface along two isopycnal pathways, (left) γ =27.865 kg/m3 and (right) γ =28.094 kg/m3. The green dots are the extrapolated protactinium concentration on the isopycnal surface based on a linear trend on the density gradient between the nearest samples in the water column, while the red lines represent the results from the advection-diffusion-scavenging model obtained with a simulation by randomization of transport parameters (Monte Carlo).

References:

Levier, M., Roy-Barman, M., Foliot, L., Dapoigny, A., & Lacan, F. (2023). Distribution of Pa in the Atlantic sector of the Southern Ocean: Tracking scavenging during water mass mixing along neutral density surfaces. Deep Sea Research Part I: Oceanographic Research Papers, 194, 103951. Access the paper:10.1016/j.dsr.2022.103951

Levier, M., Roy-Barman, M., Colin, C., Dapoigny, A. (2021). Determination of low level of actinium 227 in seawater and freshwater by isotope dilution and mass spectrometry. Mar. Chem. 233, 103986. Access the paper: 10.1016/j.marchem.2021.103986

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