234Th/238U & 228Ra & 129I to study the ocean-currents driven transport of dissolved polycyclic aromatic hydrocarbons (PAHs) at the southern Okhotsk Sea

PAHs have high dissolved concentrations (Σ₁₃PAH_diss) at the northern area off the Sakhalin Island (33-78 ng L⁻¹) where oil and natural gas is extracted (Chizova et al.,2013). East Sakhalin Current (ESC) starting northern of the Sakhalin Island and reaching the southern Okhotsk Sea during autumn-winter, presents a latent risk of PAHs transboundary pollution. The southern Okhotsk Sea is also fed by Soya Warm Current (SWC, Summer) originating in the Sea of Japan. The seasonal interchange of SWC and ESC makes the area an excellent fishing ground, with 352,000 tons of scallops, crabs, salmon, and other major species of fish caught annually.

50+ seawater samples (~10 L) were collected from 2017 to 2022 with collaboration of the Japan Fisheries Research and Education Agency to evaluate the PAHs migration around southern Okhotsk Sea. Dissolved PAHs were concentrated, from pre-filtered (0.5 µm) samples, using C18 membranes and measured with a HPLC-fluorescence. Low-background γ-spectrometryof ²³⁴Th and ²²⁸Ra were measured at the Ogoya-Underground Laboratory, Kanazawa University using HPGe-detectors. ¹²⁹I was measured using accelerator mass spectrometry at the University of Tsukuba. ²³⁴Th is constantly produced from ²³⁸U in seawater, thus the disbalance of the particle-reactive ²³⁴Th (t_1/2=24.1 d) tells the scavenging due to new production. ²²⁸Ra (t_1/2=5.75 y) is produced by the decay of ²³²Th, in the shallow continental shelves. ¹²⁹I (t_1/2=15.7x 10⁶ y) is released from nuclear fuel reprocessing facilities and transported by the westerlies to the West-Pacific’s marginal seas.

The Σ₁₃PAH_diss averaged 2.59 (0.87−5.56) ng L⁻¹. Using isomers ratios and statistical analyses, PAHs’ sources were inferred as primarily pyrogenic for all samples. Oceanographic structure was characterized by a coastal area [high salinity (>33.0 PSU), high ²²⁸Ra (0.80−1.90 mBq L⁻¹) and low PAHs (<2 ng L⁻¹)], dominated by the SWC and an offshore area [low salinity (<33.0 PSU), low ²²⁸Ra (0.25−0.80 mBq L⁻¹) and high PAHs (>2 ng L⁻¹)], dominated by the ESC-influenced Okhotsk Sea Surface Water. Σ₁₃PAH_diss were not necessarily low (1.71−5.15 ng L⁻¹) at the endmember of SWC, and thus surface migration and isopycnal mixing does not fully explain PAHs distribution. ²³⁴Th/²³⁸U ratio was 0.65 at the endmember of SWC, 0.51 ± 0.03 in the offshore area, but only 0.36 ± 0.10 in the coastal area. Chlorophyll-a, PO₄^3-, and SiO₂ at the coastal (1.38 ± 0.76 mg m³, 0.07 ± 0.05 μM, 1.77 ± 0.65 μM, respectively) and offshore (0.61 ± 0.57 mg m³, 0.20 ± 0.08 μM, 3.64 ± 1.44 μM, respectively) areas also indicated higher primary productivity at the former, and thus scavenging of dissolved PAHs at the coastal area of southern Okhotsk Sea was nominated as a major geochemical process at the southern Okhotsk Sea. The high primary productivity and downward carbon export is key for the successful of scallop farming, but the co-scavenging of organic pollutants found by this study emphasizes the environmental risk that Sakhalin I, II present. ²²⁸Ra’s origins are multiple at the West-Pacific’s marginal seas, therefore a multiple-tracer assessment including the seaice’s role will be prepared when ¹²⁹I analysis are completed.