Distribution of ²¹⁰Pb and ²¹⁰Po and Particulate Organic Carbon (POC) Fluxes in the Northwestern Pacific Ocean in Summer 2024

Abstract

During the 71st cruise of the R/V Akademik Oparin in the summer of 2024, we assessed the distributions of dissolved and particulate forms of ²¹⁰Pb and ²¹⁰Po in the Sea of Japan, the Sea of Okhotsk, and the northwestern Pacific Ocean. Quantitative estimates of vertical fluxes were derived based on measured concentrations of suspended particulate matter (SPM) and particulate organic carbon (POC). This study provides the first in situ measurements of these radionuclides and the first estimates of derived fluxes for the Sea of Okhotsk. The study confirmed the existence of two contrasting biogeochemical regimes: a sedimentation regime in the productive waters of the Sea of Okhotsk and a recycling regime in the oligotrophic waters of the open ocean, separated by the dynamic transition zone of the Kuril Islands. The calculated POC fluxes confirmed the high efficiency of the biological pump in the coastal seas. The identified anomalies in the distribution of radionuclides indicate a significant role of lateral transport and the sorption of organic carbon onto mineral particles in shaping vertical fluxes matter.

1. Introduction

Due to their high radiotoxicity, ²¹⁰Pb and ²¹⁰Po are considered priority radionuclides. Regulatory requirements stipulate that they must be quantified in drinking water when total alpha/beta activity exceeds established levels [1]. For the same reason, monitoring of ²¹⁰Pb and ²¹⁰Po in drinking water is mandatory in several European countries. Analyzing these radionuclides at low concentrations is challenging due to their nuclear-physical characteristics: ²¹⁰Po is a pure alpha emitter, while ²¹⁰Pb is a low-energy beta emitter with a low-energy gamma line (47 keV) that is highly absorbed by the sample matrix, resulting in low detection efficiency. This challenge is compounded by their low maximum permissible activity concentrations (0.11 Bq/L for ²¹⁰Po and 0.2 Bq/L for ²¹⁰Pb) [2].

In addition to the radiotoxicological aspect, the ²¹⁰Po/²¹⁰Pb pair serves as a powerful tool for studying biogeochemical processes in the ocean. Specifically, this pair is used to quantify SPM sedimentation and POC export fluxes [3,4]. The application of the ²¹⁰Po/²¹⁰Pb imbalance is based on the premise that the sinking of organic particles creates a deficit of ²¹⁰Po relative to ²¹⁰Pb in surface waters, due to the preferential biological uptake of ²¹⁰Po [5,6]. Unlike ²¹⁰Pb, ²¹⁰Po is more actively incorporated into the biological cycle, taken up by organism cells, and recycled during the regeneration of organic matter [7].

Combining the ²¹⁰Po/²¹⁰Pb ratio with other approaches, such as sediment traps or the use of the ²³⁴Th/²³⁸U pair, allows for a more complete assessment of downward particle fluxes by integrating data over timescales ranging from days to months [8]. Specifically, the ²³⁴Th/²³⁸U pair provides information on POC export over a timescale of 2–3 weeks, whereas the ²¹⁰Po/²¹⁰Pb pair yields estimates of seasonal fluxes integrated over 2–3 months [4,9].

The study of these fluxes is critically important for understanding the role of the biological pump—a key mechanism of carbon dioxide absorption by the ocean, which converts CO₂ into biomass and transports it to deep layers [10,11]. Although most of the carbon is recycled near the surface, a variable portion of it leaves the euphotic zone through gravitational settling, with less than 1% reaching the seafloor [12,13]. The efficiency of this pump, which influences the planet’s thermal balance, varies significantly in space and time, making its global assessment challenging [9].

In deep oligotrophic waters with low suspended particle concentrations, radioactive equilibrium is typically established between ²¹⁰Pb and ²¹⁰Po. However, a significant deficit of ²¹⁰Po has been observed in the meso- and bathypelagic zones of various regions of the World Ocean. This disequilibrium is explained either by the scavenging of ²¹⁰Po by particles originating from productive surface waters or from the shelf [14], or by its effective adsorption onto particles with minimal desorption at depth [15]. An alternative hypothesis suggests the absence of active uptake of ²¹⁰Po by bacteria in deep oligotrophic waters.

Studies of POC fluxes using the ²¹⁰Po/²¹⁰Pb pair have been conducted in various regions of the World Ocean: the northern [16], southern [17], and equatorial [18] Atlantic, the Sargasso Sea [19], the northern [4,20] and equatorial [20,21] Pacific, the western Arctic Ocean [22], the East China Sea [23] and the South China Sea [24], and Prydz Bay (Antarctica) [25,26]. However, for the waters of the Russian Federation, particularly the northwestern part of the Pacific Ocean, such studies are currently lacking. The unique differences in the half-lives and biogeochemical behavior of these radionuclides provide an opportunity to gain new insights into processes in the water column of this region [4], which determines the relevance of the present work.

The primary objective of this work was therefore to use the distributions of ²¹⁰Pb and ²¹⁰Po as tracers to investigate contrasting particle export regimes and to provide the first quantitative estimates of SPM and POC fluxes for the Sea of Okhotsk and the adjacent northwestern Pacific Ocean.

2. Materials and Methods

2.1. Sampling and Filtration

Field investigations were conducted during the 71st cruise of the R/V Akademik Oparin (4 June–15 July 2024) in the Sea of Japan, the Sea of Okhotsk, and the northwestern Pacific Ocean. The study focused on investigating the distributions of dissolved and particulate ²¹⁰Pb and ²¹⁰Po, as well as determining SPM and POC concentrations.

During the cruise, samples were collected at 14 stations. Vertical profiles were obtained at 11 stations, while only surface samples were collected at the remaining three stations (21, 46, and 87), resulting in a total of 29 discrete water samples. Sampling depths were chosen to include the surface, the chlorophyll maximum, and/or the base of the mixed layer. A map of the sampling stations is presented in Figure 1. Station numbers correspond to the cruise’s general station list. The stations were selected to represent the key biogeochemical provinces of the region: the coastal seas (Japan Sea, Sea of Okhotsk), the dynamic Kuril Islands transition zone, and the oligotrophic open Pacific, to enable a comparative analysis of particle export regimes.

To determine the activity of particulate forms, water samples (1100–5100 L) were filtered through pre-weighed polypropylene cartridge prefilters (Geyer PP05-10SL, LLC “Akvatoriya”, St. Petersburg, Russia) with a pore size of 0.5 μm. The flow rate was maintained at 15 L/min. A high flow rate was used to minimize the sorption of radionuclides (particularly ²¹⁰Pb and ²¹⁰Po) onto the prefilter matrix itself. Such sorption can occur as the filter becomes saturated with suspended particles and would distort the original phase distribution.

Subsequently, the prefilters were dried in a drying oven at 70–80 °C and weighed to determine the SPM concentration in the seawater using the gravimetric method. The filters were then ashed at 600 °C in a muffle furnace (SAFTherm, Luan, China) and the resulting ash was used to determine the activity of particulate forms of ²¹⁰Pb and ²¹⁰Po.

To determine the activity of dissolved forms of ²¹⁰Pb and ²¹⁰Po, seawater samples from various depths were collected into 20 L plastic containers using a UNIPUMP BCP 3.5-0.5-65 submersible pump (LLC “Sabline Service”, Moscow, Russia) installed on board the vessel.

Samples for POC analysis were collected by filtering seawater (up to 10 L) through pre-combusted (4–6 h at 450–500 °C) GF/F glass fiber filters, 47 mm in diameter, with a pore size of 0.7 μm (GVS Life Sciences, Albino, Italy).

2.2. Hydrological Survey

The distribution of temperature, salinity, and chlorophyll fluorescence was measured during the cruise of the R/V Akademik Oparin using a multiparameter CTD probe from the CTD-63 system (Sea & Sun Technology GmbH, Trappenkamp, Germany).

2.3. Concentration of Dissolved Forms of ²¹⁰Pb and ²¹⁰Po

A scheme for the preconcentration of ²¹⁰Pb and ²¹⁰Po by precipitation using iron(III) chloride from natural waters is presented in detail in Figure 2.

For the concentration of dissolved radionuclides, 20 L water samples were collected after filtration. Subsequently, nitric acid was added to the sample container at a rate of at least 1 mL of concentrated nitric acid per 1 L of sample (until reaching pH ≤ 2), and the mixture was left to stand for 24 h.

After this holding period, 5 mg of stable lead isotope in the form of lead nitrate and 200 mg of iron in the form of iron(III) chloride were added to the collected sample. The mixture was thoroughly stirred and left to stand for 1–2 h.

An ammonia solution was then added to adjust the pH to 8 for the coprecipitation of ²¹⁰Pb and ²¹⁰Po with iron(III) hydroxide. After settling for 24–48 h, the precipitate was decanted and transferred to 1 L containers for further processing in the laboratory after the expedition.

2.4. Determination of ²¹⁰Pb and ²¹⁰Po Activity by Alpha-Spectrometry and Radiometric Method After Radiochemical Preparation

The obtained iron(III) hydroxide precipitate was separated by filtration through a “White Ribbon” filter, followed by dissolution in 20 mL of dilute hydrochloric acid.

The polypropylene cartridges (mechanical filters) were ashed in a muffle furnace at 600 °C for 4 h. The ash was dissolved in 50 mL of concentrated nitric acid and 50 mL of 30% hydrogen peroxide [27]. Although elemental polonium is volatile, the ²¹⁰Po in the particulate samples is immobilized within the refractory mineral matrix of the ash, preventing its loss during this step.

Subsequent preparation of the counting source was performed according to the methodology described in [28,29]. The scheme of radiochemical preparation is presented in Figure 3.

The resulting sample solution was evaporated to a minimal volume (approximately 5 mL), ensuring the residue did not dry completely. It was then transferred to a water bath and evaporated to dryness, taking care to prevent the residue from burning. The dry residue was moistened with 2 mL of concentrated hydrochloric acid and evaporated to dryness again. Subsequently, the process proceeded to the electrodeposition of ²¹⁰Po and ²¹⁰Bi onto the counting source.

2 mL of concentrated hydrochloric acid was added to the beaker containing the dry residue (concentrate) of the sample. Distilled water was then added to reach a total volume of 50 mL. The beaker was covered with a watch glass and heated on a laboratory hotplate until the residue dissolved. Ascorbic acid (approximately 100–200 mg) was added to the solution until it became colorless (reducing Fe³⁺ to Fe²⁺), followed by 0.5 g of oxalic acid to prevent the precipitation of U.

Prior to electrodeposition, 0.25 mL of a stable ²⁰⁹Bi carrier solution (Certified Reference Material CRM 0541:2003, 1 g/L Bi) was added to yield a final Bi concentration of approximately 5 mg/L. The solution was stirred, and electrochemical deposition of ²¹⁰Po and ²¹⁰Bi isotopes onto a clean steel disk (34 mm in diameter) was carried out. Prior to deposition, one side of the disk was treated with sandpaper and wiped with cotton wool soaked in ethyl alcohol. The spontaneous electrochemical deposition of ²¹⁰Po and ²¹⁰Bi was carried out onto a steel disk from the vigorously boiling solution for 2 h. As the solution evaporated, hot distilled water was added to maintain the original volume.

After 2 h, the beaker with the sample was removed from the hotplate. After the precipitate settled, the solution was decanted. The holder with the disk was carefully removed using tweezers and rinsed with distilled water. The disk was then carefully removed from the holder to avoid damaging the thin active deposit, rinsed again with water, and air-dried to constant weight.

Stable carriers were used to monitor the efficiency (yield) at all stages. The chemical yield of lead at the coprecipitation and dissolution stages was monitored using the stable isotope ²⁰⁷Pb added at the beginning of the analysis. Its content in aliquots of the solution was determined by atomic absorption spectroscopy (AAS). The efficiency of the electrochemical deposition of polonium and bismuth was determined using the stable ²⁰⁹Bi carrier added immediately before electrolysis. Its concentration in the electrolyte before and after the procedure was also measured by AAS. The obtained radionuclide activity values were corrected for the calculated chemical and electrochemical yields, and the uncertainties associated with these determinations were included in the overall measurement uncertainty.

The activity of ²¹⁰Pb (via its daughter ²¹⁰Bi) in the prepared counting sources (disks), prepared as described above, was measured no earlier than 10 h afterward using an UMF-2000 alpha–beta radiometer (LLC NPP “Doza,” Zelenograd, Russia) for a duration of at least 8 h several times. Multiple measurements were performed over time. The initial ²¹⁰Bi activity was derived through standard decay-curve analysis. Following this, the activity of ²¹⁰Po was measured by alpha-spectrometry using the “UMF-Spectr” spectrometric complex (LLC NPP “Doza,” Zelenograd, Russia) for a duration of at least 8 h.

The combined standard uncertainty for each activity concentration was calculated by propagating the individual uncertainties. The main uncertainty components included: Poisson counting statistics for the sample and background, the uncertainty in detector efficiency calibration (for β-counting) or spectrometric energy calibration (for α-spectrometry), and the uncertainty in chemical yield determination. The reported relative combined uncertainties for ²¹⁰Pb and ²¹⁰Po activities reflect these contributions and typically ranged between 9% and 15%.

The overall sequence for the concentration, separation, and determination of ²¹⁰Pb and ²¹⁰Po in environmental samples is presented in Figure 4.

2.5. Determination of POC Concentration

The determination of organic carbon concentration was performed using a TOC-L CSN carbon analyzer equipped with an SSM-5000A solid sample module (Shimadzu, Kyoto, Japan). The analysis was carried out using the difference method, calculating the concentration of total carbon versus inorganic carbon. The difference method (POC = TC − IC) is based on first determining total carbon (TC). Subsequently, inorganic carbon is removed by combustion of the solid sample at a high temperature (680 °C) and subsequent determination of inorganic carbon (IC) using a non-dispersive infrared (NDIR) detector.

2.6. Calculation of SPM and POC Fluxes

A one-dimensional steady-state model [30,31,32,33] was used to estimate the fluxes. The particulate ²¹⁰Po flux was calculated using formulas that employ the trapezoidal method for integrating the radionuclide inventory in the water column:

$${\mathrm{F}}_{{\mathrm{210}}_{\mathrm{Po}}}={\displaystyle \underset{0}{\overset{\mathrm{z}}{\int }}{\lambda }_{\mathrm{Po}}\cdot \left({\mathrm{A}}_{{\mathrm{210}}_{\mathrm{Pb}}}-{\mathrm{A}}_{{\mathrm{210}}_{\mathrm{Po}}}\right)}\cdot \mathrm{dz} \mathrm{Bq}/({\mathrm{m}}^2\cdot \mathrm{day}),$$

(1)

where λ_Po is the decay constant of ²¹⁰Po, day^–1 (0.00502 day^–1); ${\mathrm{A}}_{{\mathrm{210}}_{\mathrm{Pb}}}$ is the total activity of ²¹⁰Pb in dissolved and particulate forms, Bq/m³; and ${\mathrm{A}}_{{\mathrm{210}}_{\mathrm{Po}}}$ is the total activity of ²¹⁰Po in dissolved and particulate forms, Bq/m³.

The specific activity of particulate ²¹⁰Po was calculated using equation:

$${\mathrm{A}}_{{\mathrm{210}}_{\mathrm{Po}(\mathrm{Part})}}^{\mathrm{sp}}={\displaystyle \frac{{\mathrm{A}}_{{\mathrm{210}}_{\mathrm{Po}(\mathrm{Part})}}}{{\mathrm{C}}_{\mathrm{SPM}}}} \mathrm{Bq}/\mathrm{g},$$

(2)

where C_SPM is the SPM concentration, g/m³.

Based on these quantities, the SPM flux was calculated using Equation:

$${\mathrm{F}}_{\mathrm{SPM}}={\displaystyle \frac{{\mathrm{F}}_{{\mathrm{210}}_{\mathrm{Po}}}}{{\mathrm{A}}_{{\mathrm{210}}_{\mathrm{Po}(\mathrm{Part})}}^{\mathrm{sp}.\mathrm{avg}}}} \mathrm{g}/({\mathrm{m}}^2\cdot \mathrm{day}),$$

(3)

The POC flux was calculated using equation:

$${\mathrm{F}}_{\mathrm{POC}}={\mathrm{F}}_{\mathrm{SPM}}\cdot {\left({\displaystyle \frac{{\mathrm{C}}_{\mathrm{POC}}}{{\mathrm{C}}_{\mathrm{SPM}}}}\right)}_{\mathrm{avg}} \mathrm{g} \mathrm{C}/({\mathrm{m}}^2\cdot \mathrm{day}),$$

(4)

where C_POC is the POC concentration, g/m³.

3. Results and Discussion

3.1. Distribution of ²¹⁰Pb and ²¹⁰Po

The spatial and vertical distributions of dissolved and particulate ²¹⁰Pb and ²¹⁰Po in the study area during summer 2024 are presented in Figure 5 and Figure 6, respectively, with the complete dataset provided in

Table 1. Hydrological parameters and measured activities obtained during the 71st cruise of the R/V Akademik Oparin (4 June–15 July 2024).

Station No. *	Coordinates		Depth (m)	T (°C)	S (‰)	Chlorophyll Fluorescence (mg/m3)	A 210Pb (Bq/m3)		A 210Po (Bq/m3)		CSPM (mg/L)
	Lat. (N)	Long. (E)					Diss.	Part.	Diss.	Part.
1	136.74049	42.88050	5	10.8067	33.9373	24.28	4.32 ± 0.43	0.263 ± 0.026	0.30 ± 0.07	0.119 ± 0.012	10.8264
			27	8.0633	33.9610	84.42	5.26 ± 0.53	0.464 ± 0.046	0.49 ± 0.11	0.498 ± 0.050	18.9233
			40	6.0781	34.0110	21.60	5.37 ± 0.54	0.437 ± 0.044	0.32 ± 0.08	0.482 ± 0.048	18.2654
6	143.76805	51.36921	5	1.1533	31.6295	9.14	4.93 ± 0.49	0.702 ± 0.070	1.04 ± 0.14	0.230 ± 0.026	7.4075
			20	−0.1179	32.0683	30.49	4.28 ± 0.43	0.045 ± 0.009	0.30 ± 0.08	0.047 ± 0.010	12.8219
21	145.51447	46.84548	5	6.9583	32.5678	104.7	4.43 ± 0.44	1.904 ± 0.190	1.05 ± 0.15	0.737 ± 0.074	19.9269
22	145.98249	45.98332	5	8.4107	32.3414	7.40	4.51 ± 0.45	0.122 ± 0.013	0.50 ± 0.11	0.120 ± 0.013	4.5184
			20	3.4998	32.3457	60.26	4.64 ± 0.46	0.727 ± 0.073	0.43 ± 0.12	0.250 ± 0.030	11.9668
			40	−0.7164	32.7263	4.39	4.20 ± 0.42	0.571 ± 0.057	0.64 ± 0.11	0.054 ± 0.020	11.1527
23	146.48012	45.06959	5	9.0408	32.4497	11.95	4.80 ± 0.48	0.521 ± 0.052	0.68 ± 0.12	0.362 ± 0.042	10.1514
			30	4.1246	32.8411	47.16	4.53 ± 0.45	0.181 ± 0.021	0.77 ± 0.14	0.158 ± 0.020	13.6434
25	147.90582	44.06788	5	7.9192	32.9331	129.2	5.05 ± 0.51	0.042 ± 0.010	0.77 ± 0.12	0.262 ± 0.038	9.4139
			15	5.1283	32.9665	152.3	4.90 ± 0.49	0.060 ± 0.011	0.87 ± 0.11	0.193 ± 0.034	16.5764
			40	2.5627	33.0838	21.08	1.82 ± 0.18	0.065 ± 0.010	1.08 ± 0.11	0.062 ± 0.012	16.1874
31	148.00939	40.00293	5	20.2482	34.4823	8.66	4.70 ± 0.47	0.262 ± 0.026	1.88 ± 0.23	0.144 ± 0.027	4.4625
			20	16.6783	34.2747	20.23	5.21 ± 0.52	0.383 ± 0.038	1.17 ± 0.18	0.702 ± 0.070	17.7603
36	148.01845	37.10271	5	19.5928	33.8659	2.66	2.20 ± 0.22	0.056 ± 0.010	1.73 ± 0.17	0.108 ± 0.016	3.7826
			33	17.3377	34.2599	19.44	2.71 ± 0.27	0.048 ± 0.011	2.32 ± 0.23	0.093 ± 0.010	13.2455
			40	14.9937	34.0616	33.48	2.60 ± 0.26	0.064 ± 0.012	2.34 ± 0.23	0.081 ± 0.010	13.0464
46	145.89192	34.05742	5	23.5155	34.4775	6.44	4.31 ± 0.43	0.947 ± 0.095	1.17 ± 0.13	0.575 ± 0.058	12.2051
61	149.77839	36.66245	5	24.6333	34.4700	1.33	5.19 ± 0.52	1.234 ± 0.123	1.02 ± 0.14	0.417 ± 0.052	5.8806
			20	24.1555	34.4602	2.14	4.94 ± 0.49	0.562 ± 0.056	2.28 ± 0.23	0.334 ± 0.038	12.7721
68	149.53298	38.82417	5	21.7025	34.6008	2.03	5.55 ± 0.56	1.204 ± 0.120	2.13 ± 0.21	0.448 ± 0.048	10.4037
			20	21.3631	34.5912	2.57	5.49 ± 0.55	0.757 ± 0.076	2.05 ± 0.21	0.482 ± 0.050	14.7758
81	148.79276	43.08188	5	16.5603	34.0160	23.22	4.76 ± 0.48	0.154 ± 0.020	0.94 ± 0.13	0.340 ± 0.039	11.0367
			20	13.0126	34.0546	77.54	4.82 ± 0.48	0.087 ± 0.012	1.29 ± 0.14	0.125 ± 0.018	15.0450
87	149.89904	43.87579	5	4.7375	33.0093	161.57	3.08 ± 0.31	1.112 ± 0.111	0.90 ± 0.16	0.487 ± 0.050	16.5432
88	149.87744	44.96065	5	3.5710	33.0190	21.31	2.02 ± 0.20	0.318 ± 0.032	1.08 ± 0.16	0.239 ± 0.034	10.7690
			15	3.5203	33.0196	68.20	3.92 ± 0.39	0.066 ± 0.010	0.88 ± 0.12	0.144 ± 0.021	14.0514

* Station numbers correspond to those in the official station log of the 71st cruise of the R/V Akademik Oparin.

.

The activity of dissolved ²¹⁰Pb in the surface layer ranged from 2.02 to 5.55 Bq/m³, with an average of 4.28 ± 0.43 Bq/m³. The activity of particulate ²¹⁰Pb ranged from 0.042 to 1.904 Bq/m³, averaging 0.632 ± 0.063 Bq/m³. The activity of dissolved ²¹⁰Po in the surface layer ranged from 0.30 to 2.13 Bq/m³, with an average of 1.09 ± 0.11 Bq/m³, while particulate ²¹⁰Po activity ranged from 0.108 to 0.737 Bq/m³, averaging 0.328 ± 0.033 Bq/m³.

It should be noted that the activity of ²¹⁰Pb and ²¹⁰Po in dissolved and particulate forms had not been previously determined in the Sea of Okhotsk. At the same time, the established values are consistent with those previously obtained for the northwestern Pacific Ocean [34] and the Sea of Japan and the East China Sea [35].

The distribution maps (Figure 5 and Figure 7) show that particulate ²¹⁰Pb and ²¹⁰Po concentrations are correlated with SPM distribution. Specifically, particulate radionuclide activities decrease with decreasing SPM concentration. The obtained relationship between the activity of particulate ²¹⁰Pb and ²¹⁰Po in surface samples and the concentration of SPM is presented in Figure 8.

The correlation coefficient between particulate ²¹⁰Po concentration and SPM concentration (R = 0.79) was higher than that for ²¹⁰Pb (R = 0.64). This difference serves as direct evidence of their distinct interaction mechanisms with marine biogenic and lithogenic particles. While ²¹⁰Pb is primarily sorbed onto the surface of inorganic particles (an abiotic process), ²¹⁰Po is actively incorporated into the biological cycle, showing a strong affinity for biogenic particles [4], which demonstrates its closer association with the biogenic fraction of SPM. This confirms that ²¹⁰Po is a more specific tracer for studying the biological pump.

Analysis of the vertical profiles (Table 1, Figure 7) revealed a clear gradient in the behavior of the ²¹⁰Po/²¹⁰Pb pair from the coastal seas (the Sea of Japan and the Sea of Okhotsk) towards the open waters of the Pacific Ocean, with a transition zone in the region of the Kuril Islands. This observation is consistent with current understanding of the biogeochemistry of these radionuclides [4].

The productive waters of the Sea of Japan (Station 1) and the Sea of Okhotsk (Stations 22, 23) exhibit the characteristic pattern of an efficient biological pump. At Station 1, the chlorophyll maximum at a depth of 27 m (84.42 mg/m³) coincides precisely with the maxima of particulate ²¹⁰Pb and ²¹⁰Po (0.464 Bq/m³ and 0.498 Bq/m³, respectively). This provides direct evidence that the thermocline acts not only as a physical barrier but also as an active biogeochemical zone where radionuclides are scavenged by sinking particles and exported from the surface water column. At Station 22, within the layer of maximum fluorescence (60.26 mg/m³ at 20 m), a similar but more pronounced process of active removal of dissolved ²¹⁰Po and its conversion to the particulate form is observed.

In the transition zone of the Kuril Islands (Stations 25, 88), extremely high fluorescence values were recorded (up to 152.3 mg/m³ at Station 25), which is consistent with known data on the high productivity of strait zones [34]. However, an anomaly was identified at Station 25: despite high chlorophyll concentrations (a proxy for phytoplankton biomass), particulate radionuclide concentrations remained low. This, combined with an anomalously low content of dissolved ²¹⁰Pb at depth (1.82 Bq/m³), indicates a complex interplay of advection and biological uptake processes in this dynamic region.

In the oligotrophic waters of the open ocean (Stations 36, 61, 68), a pronounced decoupling between fluorescence and radionuclide distribution is observed. Low fluorescence values (1.33–2.66 mg/m³) indicate a low biomass of photosynthetic organisms. At the same time, the dissolved ²¹⁰Po/²¹⁰Pb ratio is close to equilibrium (~0.9 at Station 36), which is characteristic of a “recycling regime” [35], where remineralization processes dominate over sedimentation. At Stations 61 and 68, in the subsurface layer (20 m), an increase in particulate ²¹⁰Po is observed with a minimal increase in fluorescence, which may indicate sorption onto lithogenic particle surfaces or association with non-phytoplankton particulate matter.

Certain stations exhibit unique features that highlight the complexity of processes in the region. At Station 81, a distinct chlorophyll maximum (77.54 mg/m³) is accompanied by an increase in particulate ²¹⁰Po, aligning it with the coastal sedimentation regime. The profile of Station 6 indicates the dominance of lithogenic SPM: at the 20 m depth horizon, an extremely low specific activity of particulate radionuclides is observed alongside a high SPM concentration. The anomalously high content of particulate ²¹⁰Po at Station 31, coupled with moderate chlorophyll values, may be attributed to the predominance of fine organic SPM (detritus, bacterioplankton) with high sorption capacity [36].

A comparison of the highly productive Stations 21 and 87 revealed an important pattern. At Station 87, despite the record chlorophyll concentration (161.6 mg/m³), the content of particulate radionuclides remains relatively moderate. In contrast, Station 21, with a somewhat lower biomass (104.7 mg/m³), exhibits the maximum concentrations of particulate ²¹⁰Pb and ²¹⁰Po. This suggests that these stations represent different phases of bloom development. Station 87 corresponds to an active growth phase, whereas Station 21 represents a mature phase, when the processes of radionuclide sorption and bioassimilation reach their maximum.

3.2. Fluxes of SPM and POC

Using the distributions presented in Table 1 and the flux model described in Section 2.6, we performed quantitative assessments of SPM and POC export parameters.

The obtained results, presented in

Table 2. Results of sedimentation parameter calculations based on the obtained distributions of different forms of ²¹⁰Pb and ²¹⁰Po during the 71^st cruise of the R/V Akademik Oparin (4 June–15 July 2024).

Station No.	Coordinates		Depth (m)	CSPM (mg/L)	${\mathrm{F}}_{{\mathrm{210}}_{\mathrm{Po}}}$ (Bq/(m2·day))	FSPM (g/(m2·day))	CPOC (mg/L)	FPOC
	Lat. (N)	Long. (E)						(mg C/(m2·Day))	(mmol C/(m2·Day))
1	136.74049	42.88050	5	10.8264	0.9138	39.92	0.061	191.25	15.92
			27	18.9233			0.075
			40	18.2654			0.095
6	143.76805	51.36921	5	7.4075	0.4235	30.93	0.183	428.07	35.60
			20	12.8219			0.097
21 *	145.51447	46.84548	5	19.9269	–	–	0.160	–	–
22	145.98249	45.98332	5	4.5184	0.8682	56.59	0.085	513.01	42.71
			20	11.9668			0.146
			40	11.1527			0.020
23	146.48012	45.06959	5	10.1514	0.6133	28.06	0.080	166.20	13.84
			30	13.6434			0.061
25	147.90582	44.06788	5	9.4139	0.5928	48.36	0.103	536.60	44.68
			15	16.5764			0.198
			40	16.1874			0.167
31	148.00939	40.00293	5	4.4625	0.3245	8.52	0.062	49.09	4.09
			20	17.7603			0.066
36	148.01845	37.10271	5	3.7826	0.0744	7.94	0.064	53.33	4.44
			33	13.2455			0.075
			40	13.0464			0.063
46 *	145.89192	34.05742	5	12.2051	–	–	0.088	–	–
61	149.77839	36.66245	5	5.8806	0.4217	10.47	0.048	65.13	5.42
			20	12.7721			0.068
68	149.53298	38.82417	5	10.4037	0.4019	10.88	0.028	46.24	3.85
			20	14.7758			0.079
81	148.79276	43.08188	5	11.0367	0.3595	20.16	0.028	82.73	6.89
			20	15.0450			0.079
87 *	149.89904	43.87579	5	16.5432	–	–	0.119	–	–
88	149.87744	44.96065	5	10.7690	0.1255	8.13	0.117	74.38	6.19
			15	14.0514			0.110

* Flux calculations for Stations 21, 46, and 87 were not possible because only surface samples were collected at these stations.

and Figure 9, demonstrate a clear gradient in flux magnitudes, reflecting the transition from highly productive shelf waters to oligotrophic areas of the open ocean.

The maximum values of both SPM fluxes (up to 56.6 g/(m²·day)) and POC fluxes (up to 44.68 mmol C/(m²·day)) were obtained for the coastal areas of the Sea of Okhotsk near Sakhalin Island and the Kuril Islands (Stations 6, 22, 25). These values show good agreement with data from other productive marginal seas [9,34] and correlate with the previously identified zones of high biological productivity, confirming the efficient operation of the biological pump in these regions.

In the open waters of the Pacific Ocean (Stations 36, 61, 68), the fluxes decrease by an order of magnitude (F_POC: 3.85–6.89 mmol C/(m²·day)), which is characteristic of oligotrophic waters [35] and corresponds to the “recycling regime” identified earlier, where remineralization processes dominate over sedimentation.

A comparative analysis of the anomalous stations highlights the complexity of the processes controlling vertical fluxes. At Station 31, the high content of particulate ²¹⁰Po is not accompanied by a significant POC flux, indicating specific sorption processes in this area. In the case of Station 6, a unique combination is observed: a high POC flux (35.6 mmol C/(m²·day)) alongside signs of lithogenic SPM dominance. This suggests that in the coastal areas of the Sea of Okhotsk, the mechanisms of vertical organic carbon transport may not only involve the classical biological pump but also effective sorption of POC onto mineral particles, which enhances the total flux of organic matter to deeper layers.

The calculated SPM and POC fluxes (Table 2) provide a quantitative expression of the contrasting biogeochemical regimes identified earlier. Station representative of the efficient sedimentation regime in productive coastal waters (e.g., Station 22, Sea of Okhotsk) yielded substantial POC fluxes (42.71 mmol C/(m²·day)). This confirms the direct link between high chlorophyll levels, active ²¹⁰Po assimilation, and effective vertical export of organic matter.

The transition zone of the Kuril Islands (e.g., Station 25), characterized by extreme surface productivity, also resulted in very high POC flux (44.68 mmol C/(m²·day)). However, as noted earlier, the decoupling between high biomass and relatively low particulate radionuclide concentrations at this station suggests that lateral advection and particle dynamics modulate the export efficiency, highlighting the complex interplay of biology and physics in this dynamic region.

In stark contrast, stations within the recycling regime of the oligotrophic open ocean (e.g., Stations 36, 68), which exhibited near-equilibrium ²¹⁰Po/²¹⁰Pb ratios and low fluorescence, produced an order-of-magnitude lower POC fluxes (3.85–4.44 mmol C/(m²·day)). This quantifies the dominance of remineralization over particle export in these waters. This direct correlation between the biogeochemical patterns and the calculated fluxes.

Our POC export estimates are based on ²¹⁰Po/²¹⁰Pb disequilibrium measured using the Fe(OH)₃ co-precipitation method, with chemical yields rigorously monitored using stable ²⁰⁷Pb and ²⁰⁹Bi carriers. We note that a method intercomparison study suggested this approach may yield lower total ²¹⁰Po activities compared to the Co-APDC technique in certain marine settings [33]. While our protocol of direct yield control ensures the internal accuracy of our measurements, we acknowledge this methodological consideration. Therefore, the absolute values of our derived POC fluxes may represent upper-bound estimates. Importantly, any potential systematic bias does not affect the relative patterns central to our study. The clear quantitative contrast between the efficient export regime of the Sea of Okhotsk and the recycling-dominated regime of the oligotrophic open ocean remains robust.

Thus, the calculated fluxes quantitatively confirm the previously identified spatial heterogeneity in the ecosystem functioning of the region: from active vertical export of organic carbon in the coastal seas to its efficient recycling in the oligotrophic waters of the open ocean.

4. Conclusions

We have presented a comprehensive dataset on the distributions of dissolved and particulate ²¹⁰Pb and ²¹⁰Po, SPM, and POC for the waters of the Sea of Okhotsk and the northwestern Pacific Ocean. These data represent the first such measurements for the Sea of Okhotsk.

A clear distinction between biogeochemical regimes has been established. The waters of the Sea of Okhotsk are characterized by a “sedimentation regime” with an efficient biological pump, where the maxima of particulate ²¹⁰Pb and ²¹⁰Po in the thermocline layer correlate with the chlorophyll maximum. The open oligotrophic waters of the Pacific Ocean exhibit a “recycling regime” with a near-equilibrium dissolved ²¹⁰Po/²¹⁰Pb ratio and the dominance of remineralization processes.

Our results demonstrate the key role of the Kuril Islands zone as a dynamic transition region. Extremely high values of primary productivity were recorded; however, the distribution of radionuclides in this area is complicated by intense processes of lateral water mass transport.

We provide quantitative assessments of SPM and POC export fluxes. POC fluxes in the productive coastal waters of the Sea of Okhotsk (tens of mmol C/(m²·day)) were an order of magnitude higher than in the oligotrophic open ocean waters ((single digits of mmol C/(m²·day)), which quantitatively confirms the high efficiency of the biological pump in the marginal seas.

We identified and interpreted several anomalies that indicate a complex interplay of physicochemical and biological processes. The high POC flux at a station showing signs of lithogenic SPM (Station 6) highlights the importance of organic carbon sorption onto mineral particle surfaces. The anomalously high concentration of particulate ²¹⁰Po under moderate productivity (Station 31) underscores the role of specific sorption mechanisms potentially mediated by non-phytoplankton particles or dissolved organic matter.

The obtained data demonstrate the potential of using radioactive tracers for the quantitative assessment of matter fluxes.