210Po-210Pb Disequilibrium in the Western North Pacific Ocean: Particle Cycling and POC Export

Estimating the particulate organic carbon (POC) export flux from the upper ocean is fundamental for understanding the efficiency of the biological carbon pump driven by sinking particles in the oceans. The downward POC flux from the surface ocean based on 210Po-210Pb disequilibria in seawater samples from the western North Pacific Ocean (w-NPO) was measured in the early summer (May-June) of 2018. All the profiles showed a large 210Po deficiency relative to 210Pb in the euphotic zone (0–150 m), while this 210Po deficiency vanished below ∼500 m (with 210Po/210Pb ∼1 or > 1). A one-dimensional steady-state irreversible scavenging model was used to quantify the scavenging and removal fluxes of 210Po and 210Pb in the euphotic zone of the w-NPO. In the upper ocean (0–150 m), dissolved 210Po (D-Po) was scavenged into particles with a residence time of 0.6–5.5 year, and the 210Po export flux out of the euphotic zone was estimated as (0.33–3.49) × 104 dpm/m2/year, resulting in a wide range of particulate 210Po (P-Po) residence times (83–921 days). However, in the deep ocean (150–1,000 m), 210Po was transferred from the particulate phase to the dissolved phase. Using an integrated POC inventory and the P-Po residence times (Eppley model) in the w-NPO euphotic zone, the POC export fluxes (mmol C/m2/d) varied from 0.6 ± 0.2 to 8.8 ± 0.4. In comparison, applying the POC/210Po ratio of all (>0.45 μm) particles to 210Po export flux (Buesseler model), the obtained POC export fluxes (mmol C/m2/d) ranged from 0.7 ± 0.1 to 8.6 ± 0.8. Both Buesseler and Eppley methods showed enhanced POC export fluxes at stations near the continental shelf (i.e., Luzon Strait and the Oyashio-Kuroshio mixing region). The Eppley model-based 210Po-derived POC fluxes agreed well with the Buesseler model-based fluxes, indicating that both models are suitable for assessing POC fluxes in the w-NPO. The POC export efficiency was < 15%, suggesting a moderate biological carbon pump efficiency in the w-NPO. These low export efficiencies may be associated with the dominance of smaller particles and the processes of degradation and subsequent remineralization of these small particles in the euphotic zone of oligotrophic regions in the w-NPO.


INTRODUCTION
As the Earth's largest carbon reservoir (IPCC in Climate Change, 2013), the marine ecosystem plays a fundamental role in regulating the atmospheric CO 2 concentration and buffering the effects of global climate change (Sabine, 2004) by assimilating carbon from the atmosphere via dissolution and photosynthesis in the upper ocean. Even small changes in the magnitude of the downward transport of carbon in the upper ocean can have serious impacts on oceanic carbon sequestration and atmospheric CO 2 concentration (Kwon et al., 2009) and thus on global climate change (including global warming). It has been predicted that without the marine biological carbon pump (BCP), the atmospheric CO 2 concentration could be approximately 50% higher than its current value (Sanders et al., 2014).
To fully understand how the oceans regulate the atmospheric CO 2 concentration, detailed knowledge of the marine carbon cycle is needed. The well-known BCP is one of three major mechanisms by which the ocean takes up atmospheric CO 2 (Liu et al., 2018). The BCP is described by these major processes: Phytoplankton convert CO 2 into fixed carbon, e.g., carbohydrates or calcium carbonate through photosynthesis in the euphotic zone; Part of the CO 2 fixed is transferred to the ocean interior of the ocean, mainly by gravitational sinking of particulate organic carbon (POC); Diffusion, advection and vertical mixing of dissolved organic carbon (DOC) and active bio-transport of organic and inorganic carbon by diel vertical migrating zooplankton are also important carbon removal pathways; Finally very few percentage of fixed carbon will be sequestered in the deep ocean (Buesseler et al., 2007). All these biologically mediated processes constitute the BCP. Thus, sinking particles play an important role in driving the BCP (Falkowski et al., 1998;Ducklow et al., 2001;Wei et al., 2011;Liu et al., 2018).
The ratio of export production (or the POC export flux) to total primary production, known as the "f-ratio" is defined as the export efficiency of the BCP and is used to quantity the strength of the pump (Buesseler and Boyd, 2009). Hence, an investigation of the sinking POC export flux from the upper ocean would provide fundamental parameters for predicting future changes in the marine carbon cycle. To constrain the POC export flux from the upper ocean, two traditional approaches are frequently applied: sediment traps (Honjo et al., 2008;Hayes et al., 2018) and the disequilibrium of the natural radionuclide pair, 234 Th-238 U (Charette et al., 1999;Benitez-Nelson et al., 2001;Buesseler et al., , 2020Zhou et al., 2020).
In recent years, another pair of natural radionuclide tracers, 210 Po (T 1/2 = 138.4 days) and its progenitor 210 Pb (T 1/2 = 22.4 year), has been shown to be effective at predicting the POC export flux (Murray et al., 2005;Stewart et al., 2007Stewart et al., , 2010Verdeny et al., 2009;Wei et al., 2011;Le Moigne et al., 2013;Roca-Martí et al., 2016;Hayes et al., 2018;Subha Anand et al., 2018;Horowitz et al., 2020); however, this tracer pair has not been applied frequently. The 210 Po/ 210 Pb ratio exhibits different behavior from the 234 Th/ 238 U ratio in terms of particlebinding properties in the ocean. 234 Th is particle reactive and is scavenged from surface waters when particles sink into the deep sea, while naturally occurring 238 U is conserved and remains dissolved in well-oxygenated seawater. In contrast, both 210 Po and 210 Pb are particle reactive. In addition, 210 Pb and 234 Th are only adsorbed onto the particle surface, whereas 210 Po is both adsorbed onto surfaces and biologically assimilated into cells by some species of phytoplankton and bacteria (Fisher et al., 1983;Cherrier et al., 1995;LaRock et al., 1996;Stewart and Fisher, 2003a,b). As a result, phytoplankton cells accumulating 210 Po are further ingested by zooplankton, while sinking particles excreted as fecal pellets and biogenic detritus are depleted in 210 Po (Heyraud et al., 1976). This biogeochemical behavior leads to the distribution coefficient of 210 Po being higher than that of 210 Pb (Tang et al., 2017;Zhong et al., 2019). Thus, when particles sink from the upper ocean, the large difference in particle affinity leads to secular disequilibrium between 210 Po and 210 Pb, which can be used to estimate particulate material export in a manner similar to the use of 234 Th-238 U disequilibrium (Friedrich and Rutgers van der Loeff, 2002;Cochran and Masqué, 2003;Verdeny et al., 2009;Wei et al., 2011;Hayes et al., 2018;Subha Anand et al., 2018;Horowitz et al., 2020). However, relatively few studies have used the 210 Po- 210 Pb disequilibrium method to quantify the POC export flux at a basin wide scale (Ceballos-Romero et al., 2016;Roca-Martí et al., 2016;. Oligotrophic open waters compose a major proportion (∼75%) of the surface ocean and account for over 30% of global marine carbon fixation (Shih et al., 2015). The NPO is an important atmospheric carbon sink, it can contribute to ∼25% of the total ocean CO 2 uptake per year (Takahashi et al., 2009). By assessing the carbon export in the NPO, the goal of peak carbon dioxide emissions and carbon neutrality for humanity can be achieved more scientifically, and the pace of global climate change can be gradually showed down. As a part of the "Marine Environment Monitoring and Early Warning System Construction in the West Pacific Ocean (MEMEWSC)" project, we took advantage of a unique opportunity to obtain measurements of 210 Po and 210 Pb activity over a large tract of the w-NPO (from 117 • E to 146 • E and from 20 • N to 40 • N). In this study, we describe the activity profiles (1,000 m) of 210 Po and 210 Pb at 7 stations spanning different oceanographic regions with different aeolian inputs, distances from the nearest coast and rates of primary production. The goals of this study are listed as follows: (1) To investigate the spatial variability of particle scavenging and removal processes in the water column of each oceanographic region; and (2) to estimate the magnitude and efficiency of the POC export flux derived from the 210 Po-210 Pb disequilibrium in the euphotic layer of the w-NPO.

Sampling and Preparation
The study area located in the western part of the NPO, covering an area of 20-40 • N and 118-153 • E. The main surface ocean currents in this region are shown in Figure 1. The Kuroshio Current (KC) is the most important western boundary current in the w-NPO, it originates from the North Equatorial Current and subsequently intrudes from the western Philippine Sea into the northern South China Sea (SCS) in the Luzon Strait. The  210 Pb activities were measured. The arrows represent the surface circulation systems of the w-NPO (Yasuda, 2003;Gallagher et al., 2015;Hu et al., 2015). TWC, Taiwan Warm Current; KC, Kuroshio Current; TC, Tsushima Current; OC, Oyashio Current. main KC enters the East China Sea (ECS) through a channel in east of Taiwan Island and then passes through Tokara Strait at around 30 • N. After the current has passed the Shikoku Basin, the northeastward KC merges with the southwestward subarctic Oyashio Current (OC) before leaving the coast of Japan to form the KC Extension in the NPO, resulting in a system known as the Oyashio-Kuroshio boundary region (Figure 1).
The MEMEWSC cruise was carried out in May-June 2018 on board RV Xiangyanghong III from the SCS to the Oyashio-Kuroshio mixing region of the western subarctic NPO (Figure 1). Dissolved and particulate samples for 210 Po and 210 Pb activity analysis were collected from the water column at 7 stations during the ∼1 month duration of the cruise. Discrete seawater samples were collected using a stainless steel sampling rosette equipped with 24 × 12 L Niskin bottles and a CTD Seabird sensor package. Seawater samples (∼20 L each) were taken from 9 depths from 0 to 1,000 m, with higher resolution in the upper 200 m of the water column. Salinity and temperature were obtained from these sensors (Seabird SBE 9/11plus CTD deployed with two Sea-Bird SBE 3P temperature sensors and two Sea-Bird SBE 4C conductivity sensors). To evaluate the POC export flux, 150 m is chosen as the integration depth according to estimated depth for the base of the euphotic zone on the ship from the Chl-a profiles.

Po and Pb Determination
Analysis for 210 Pb and 210 Po was described in Zhong et al. (2019). In brief, seawater samples (18-24 L each) were rapidly filtered through 142-mm diameter Nucleopore filters (0.45 µm pore size) to separate the particulate and dissolved 210 Po and 210 Pb. The filtered (dissolved) seawater was then acidified to pH 1-2 with concentrated HCl immediately after collection and spiked with a known amount of 209 Po (No. 7299, Eckert and Ziegler Isotope Products) and stable Pb 2+ to quantify any subsequent losses of Po and Pb. After equilibration for 6-12 h, 100 mg of Fe 3+ was added to the solution, the pH was adjusted to approximately 8-9, using concentrated NH 4 OH to co-precipitate Po and Pb with Fe(OH) 3 precipitate. After settling for 8-12 h, the precipitate was transferred into a 1.5-L polyethylene bottle, and stored on board for processing upon arrival on land. In the laboratory, the precipitate was centrifuged and dissolved in 6 M HCl solution in a clean Teflon beaker, and the pH was neutralized to 1-2. Both 210 Po and 209 Po were auto-plated onto a nickel disc  after adding 0.3 g of ascorbic acid, 1 mL of 25% sodium citrate, and 1 mL of 20% hydroxylamine hydrochloride to the solution. The particulate filters were spiked with known quantities of 209 Po and stable Pb 2+ and totally dissolved in a mixture of HF, HNO 3 and HClO 4 . After evaporating to nearly dryness several times to ensure that all the radionuclides were in dissolved form, the residue was then picked up in dilute HCl (0.1 M) for plating of Po onto a nickel disc. The plating procedure was similar to that utilized for dissolved samples. 210 Po and 209 Po activities were determined by alpha spectrometry (Canberra series 7200-08).
For 210 Pb determination, any remaining Po isotopes in the plating solution was removed by resuspending another nickel disc, and the purified sample solution was re-spiked with additional 209 Po and stored for ∼12 months to allow ingrowth of 210 Po from 210 Pb, and then polonium isotopes were again autodeposited, and counted. The recoveries of 210 Pb were determined through the added stable Pb and the measured Pb, using atomic absorption spectrometry (AAS). The in-situ activities of 210 Po and 210 Pb at the sampling date were determined by correcting for decay, ingrowth, chemical recoveries, detector backgrounds, and reagent blanks ( 210 Pb and 210 Po present in the stable lead) (Church et al., 2012;Baskaran et al., 2013;Rigaud et al., 2013).

Particulate Organic Carbon Analysis
POC samples were obtained by filtering 4-6 L seawater through pre-combusted and pre-weighted 25 mm QMA filters (with a pore size of 0.45 µm, Whatman) immediately after sampling, and the filters were then washed 3 times with Milli-Q water to remove salt before frozen storage prior to laboratory analysis. Before the POC content was measured, the sample was ovendried at 55 • C to a constant weight. Then, the inorganic carbon was removed from the filter by using acid vapor (concentrated HCl) in desiccators for 48 h. The POC was then determined using a Vario ELIII CHNOS Elemental Analyzer. The analytical precision of the method was estimated to be around ± 4%, using triple measurement of the same sample.

Hydrographic Characteristics and Particulate Organic Carbon Concentrations During the MEMEWSC Cruise
Vertical profiles of the temperature and salinity in the w-NPO are shown in Figure 2. Among the 7 stations, the sea surface temperature (SST) ranged from 15.5 to 28.9 • C. The SST was high (27.6-28.9 • C) at the low-latitude (21 • N) stations (XTW1-3, XTW1-11 and XTJ1-1) but decreased gradually with increasing latitude from station XTJ1-1 (28.4 • C) to station XTJ1-22 (15.5 • C). Influenced by the OC, station XTJ1-22 showed the lowest SST. Furthermore, the water temperature decreased with increasing depth and did not differ appreciably between stations at 1,000 m (Figure 2A).
The salinity profiles spatially differed among the 7 stations (Figure 2A). At station XTJ1-22, the salinity decreased rapidly with increasing depth and exhibited a subsurface minimum (33.82) at 150 m, displaying the features of low-salinity subarctic waters (OC), and then the salinity increased gradually to 1,000 m. In contrast to station XTJ1-22, the other six stations exhibited similar salinity distributions, with the maximum values occurring at depths of approximately 150-200 m (Figure 2A), indicating the existence of stratification in the water column at each station. However, the salinity in the top 150 m of the water column at station XTW1-3 (located in Luzon Strait) was lower (from 33.80 to 34.66) than that at the station in the NPO basin (from 34.55 to 35.05) (Figure 2A). From the temperaturesalinity-depth diagram (Figure 2B), the water masses can be clearly discriminated for the study area. Station XTJ1-22 showed a signature of OC, and station XTW1-3 might represent the South China Sea water. From the Figure 2B, the remaining five stations (XTJ1-1, XTW1-1, XTJ1-8, XTJ2-7, and XTJ1-13) could be divided into two groups with different hydrographic features for the surficial water mass (upper 200 m). One group includes stations XTJ1-1 and XTW1-11, and the other group contains stations XTJ1-8, XTJ1-13, and XTJ2-7, showing a much narrower ranges of salinity and temperature ( Figure 2B). In 1,000 m, all the stations displayed a similar temperature-salinity feature, with low temperatures of < 5 • C and a salinity range of 34.3-34.5. Based on the salinity and temperature profiles, the thermocline was observed between 100 and 200 m. To determine the euphotic zone depth, we use information for the base of the euphotic zone according to the subsurface chlorophyll-a maximums (SCMs) features. The profiles of chlorophyll-a concentrations showed that SCMs could occur between 120 and 150 m for most of the stations (33 out of 43 stations) during the cruise (unpublished data, personal communication with Dr. Jianhua Kang). Hence the euphotic zone could be delimited from 0 to 150 m. Overall, the water column could be divided into two boxes: 0-150 m box (the euphotic zone) and 150-1,000 m box (the mesopelagic zone).
All POC concentration data can be found in Supplementary Table 1. In total, the POC concentrations ranged from 1.85 to 14.35 µmol C/L, with an average of 4.15 ± 2.15 µmol C/L in the w-NPO (Figure 3). Station XTJ1-22 at the Kuroshio Current-Oyashio Current boundary had the highest POC concentration (mean: 6.23 ± 4.12 µmol C/L) in the upper 150 m, followed by station XTW1-3 (mean: 5.44 ± 1.28 µmol C/L) in the Luzon Strait, while the other five stations had similar POC concentrations with a mean value of 3.12-3.81 µmol C/L ( Table 1). Almost all POC profiles showed a decrease with increasing water column depth from 0 to 500 m, and remained relatively constant from 500 to 1,000 m except at two stations (XTJ1-8 and XTJ1-13) (Figure 3).  (Figure 4), which are comparable to the reported values in the upper Sargasso Sea (>500 m, 0.5-8.2 dpm/100 L) (Kim and Church, 2001), and slightly higher than those in the Aleutian Basin (negligible to 6.0 dpm/100 L) (Hu et al., 2014) and in the North Atlantic Ocean (0.1-7.0 dpm/100 L) (Horowitz et al., 2020). All stations showed a systematic decrease in P-Po activity with depth in the upper 1,000 m, except for station XTW1-3 in the Luzon Strait, which exhibited a subsurface maximum between 200 and 500 m (Figure 4).

Po and Pb Profiles
The activities of dissolved 210 Po (D-Po) varied from 2.87 to 14.57 dpm/100 L (Figure 4), which are comparable to other results previously measured in the w-NPO (5.28-14.02 dpm/100 L) (Nozaki and Tsunogai, 1976), the North Atlantic Ocean (0.6-13.1 dpm/100 L) (Horowitz et al., 2020) and the western equatorial Pacific Ocean (4.26-14.82 dpm/100 L) (Peck and Smith, 2000), although these regions might correspond to very different biogeochemical conditions. The D-Po concentration was lowest in surface seawater and gradually increased with depth; the subsurface D-Po peak was observed within approximately 50-150 m at all stations (Figure 4). This increase in the D-Po concentration with depth is consistent with the rapid scavenging of 210 Po near the surface and the remineralization of particulate 210 Po at depth. The profiles of total 210 Po (T-Po) showed vertical variations similar to those of P-Po, with a range of 4.61-20.69 dpm/100 L.
The activity concentrations of dissolved 210 Pb (D-Pb) and particulate 210 Pb (P-Pb) ranged from 1.86 to 24.45 dpm/100 L and from 1.88 to 31.33 dpm/100 L, respectively ( Figure 5). The P-Pb activity was highest in the near-surface seawaters at all stations; thus, 210 Pb activity was predominantly in the particulate phase (P-Pb/T-Pb > 50%) in the upper 50 m (Figure 6). Below 50 m, P-Pb showed a systematic downward trend with depth.    However, the activity of D-Pb was lowest in the surface seawater; then, D-Pb increased with depth from 50 to 200 m and finally decreased below 200 m. All stations had the highest total 210 Pb (T-Pb) concentration at the surface, and T-Pb decreased with depth from the surface to 1,000 m, suggesting a surface source of 210 Pb due to atmospheric input, as indicated by the 210 Pb activity (7.22-38.61 dpm/100 L) being greater than the 226 Ra activity (6.0-14.4 dpm/100 L, Kawakami and Kusakabe, 2008) in the upper water column. At all stations, the percentages of P-Po in the w-NPO surface seawater (0-50 m) were higher than 50%, showing that P-Po often exceeded D-Po and dominated T-Po. The high fraction of particulate 210 Pb indicated a signature of atmospheric input. Moreover, the P-Po/T-Po percentages decreased slowly to constant values of 20-30% as the water depth increased up to 500 m ( Figure 6). P-Pb was generally 20-50% of the T-Pb at depths below the mixed layer, demonstrating that T-Pb was dominated by D-Pb in most layers of the water column at all stations in the w-NPO (Figure 6). Overall, these findings reveal that both 210 Po and 210 Pb were predominantly present in the dissolved phase below 50 m at all stations (Figure 6), as is commonly the case in other oceans.

Po/ Pb Activity Ratio
Vertical profiles of the 210 Po/ 210 Pb activity ratios in the particulate, dissolved and total fractions are plotted in Figure 7. Particulate matter was enriched in 210 Po (particulate 210 Po/ 210 Pb activity ratios > 1) at the subsurface (50-100 m) at all stations, especially in the particulate sample from a depth of 75 m at station XTJ1-1, where the P-Po/P-Pb ratio reached 3.04 ± 0.73 (Figure 7). These data indicates that there was an excess of P-Po relative to P-Pb. The dissolved 210 Po/ 210 Pb activity ratios (D-Po/D-Pb) increased with depth, with values < 1 at the surface and increasing toward 1 or even > 1 below the euphotic zone, which complemented the elevated P-Po/P-Pb ratio. All stations showed similar vertical distributions of T-Po/T-Pb with a significant deficiency of 210 Po relative to 210 Pb from the surface to 200 m. The deficit of 210 Po relative to 210 Pb declined with depth and then approached equilibrium or shifted to an excess ( 210 Po/ 210 Pb≈1 or > 1) at different depths for different stations, except for stations XTJ1-1 and XTJ1-8, which displayed an excess of T-Po relative to T-Pb (T-Po/T-Pb ratio > 1) at 50-75 m and at 500-1,000 m. Interestingly, 210 Po excess (T-Po/T-Pb ratios > 1.2) was observed in seawater at 500 and 1,000 m of station XTJ1-1 and in seawater at 1,000 m of station XTJ1-8 (Figure 7). From Figure 7, we can find that this 210 Po excess is caused by the high dissolved 210 Po activity concentration in the deep layers (500-1,000 m), because D-Po/D-Pb ratios at deep layers (500-1,000 m) were much higher than unity (Figure 7). And this high dissolved 210 Po activity concentration implied the occurrence of a strong 210 Po dissolution in the deep ocean.

DISCUSSION
Enhanced Particle Scavenging of 210 Po and 210 Pb and Deficiency of 210 Po in the Western North Pacific Ocean Both Pb and Po are particle-reactive elements that can be scavenged effectively from the water column by lithogenic and biogenic particles. Figure 4 reveals that both the P-Po and the  210 Pb is the atmospheric deposition, and the 210 Pb is mainly in dissolved phase in the atmospheric deposition samples. 210 Pb is particle-reactive, therefore, in the upper layer with higher primary productivity, some 210 Pb sourced from the atmosphere would be absorbed and enriched by the biogenic particles. In addition, our study area is under the cover of dust deposition from the East Asia (Hayes et al., 2013); during every spring (especially in April-May), the prevailing dust storms from the East Asia would possibly increase the particulate 210 Pb activity.
Furthermore, the P-Po/P-Pb ratios were nearly 1 or higher than 1 within 30-75 m at all stations (Figure 7), supporting the previously findings that 210 Po was preferentially scavenged over 210 Pb (Bacon et al., 1976;Murray et al., 2005;. In addition, on all profiles, both P-Po and P-Pb decreased with depth in the upper 100 m, which is consistent with the increases in D-Po and D-Pb (Figures 4, 5). The highest P-Po activity concentrations were presented at the surface for all stations (Figure 4), which could be related to particle concentration, since particle concentration is usually higher than in the twilight zone or deeper layers. D-Po increased from the surface to 1,000 m (Figure 4), especially at stations XTW1-11 and XTJ1-8, which showed subsurface D-Po maxima at 150 and 50 m, respectively. This phenomenon implies that sinking particulate matter is re-mineralized at depth in the w-NPO, which is supported by an excess of D-Po relative to 210 Pb, with the D-Po/D-Pb activity ratios often > 1 below 200 m (Figure 7). Specifically, at station XTW1-11, the feature of D-Po/D-Pb > 1 appeared between 150 and 1,000 m, while at station XTJ1-1, D-Po/D-Pb > 1 only appeared between 500 and 1,000 m. Except for station XTJ2-7, the feature of D-Po/D-Pb > 1 appeared at 1,000 m for all stations (Figure 7). Based on the above discussion, it can be concluded that enhanced particle scavenging of both 210 Po and 210 Pb occurred in the upper ocean and that 210 Po adsorbed onto particles from the upper ocean was released in the deeper ocean due to the remineralization of particulate organic matter below the mixed layer.
The minimum activity concentration of 210 Po appeared at the 100 m layer at XTW1-3, XTJ1-8 and XTJ2-7, while the minimum 210 Po activity concentration appeared at the 200 m layer at station XTW1-11, XTJ1-1, XTJ1-13, and XTJ1-22 (Figure 4). As shown in Figure 7, T-Po was largely deficient relative to T-Pb in the upper 200 m of the w-NPO, with the T-Po/T-Pb ratios as low as 0.4. This phenomenon is similar to the general pattern in which T-Po is usually deficient with respect to T-Pb (T-Po/T-Pb ratio = 0.5) in the euphotic zone due to the rapid removal of Po caused by the sinking of biogenic particles (Bacon et al., 1976;Nozaki et al., 1998;Stewart et al., 2010;Roca-Martí et al., 2016;Subha Anand et al., 2018). In contrast, 210 Po gradually reached secular equilibrium with 210 Pb or even an excess relative to 210 Pb (Figure 7) at the bottom of the euphotic zone (∼150 m) or in the mesopelagic waters (500 or 1,000 m) due to the regeneration of 210 Po from sinking particles during particle remineralization. For example, at stations XTJ1-1 and XTJ1-8, T-Po/T-Pb ratios were high up to 1.61-2.02 at 500-1,000 m layers. From Figures 4, 5, we can clearly find that the D-Po activity concentration increased significantly, but at the same layers, both D-Pb and P-Pb activity concentration decreased significantly. The explanation for this increased D-Po activity at the deep layer may be related to strong particle export events. We believe that a strong particle export event occurred in the upper ocean before our sampling, and subsequently sinking particles dissolved at the deep layers. This ultimately increased the activity concentration of dissolved 210 Po in the mesopelagic zone. Similar research cases have also been reported in a review paper published by Verdeny et al. (2009). Such disequilibrium between 210 Po and 210 Pb in the water column could help depict the scavenging rates of 210  The 210 Po and 210 Pb activities in the ocean are the result of a balance among atmospheric inputs, continuous production from the decay of mother nuclides ( 210 Pb or 226 Ra) in seawater, the radioactive decay of 210 Po and 210 Pb, removal onto sinking particles, and transport into or out of the system by advection and diffusion. Generally, the general form of the mass balance equation in the upper ocean for 210 Po and 210 Pb between sources and sinks can be designed as follows: where ∂Pb/∂t and ∂Pb/∂t are the changes in the 210  The advection-diffusion term is relatively important only in the case of algal blooms, mesoscale eddies, and upwelling regions Horowitz et al., 2020). Generally, a steady-state model can be adopted by ignoring advection and diffusion (V≈0). Figure 8 displays the one-dimensional irreversible scavenging conceptual model for 210 Po and 210 Pb in surface and deep waters. Thus, Eqs. (1) and (2) can be rearranged as follows. For the upper ocean (0-150 m), we obtain: where Eqs. (3) and (4) are mass-balance equations for D-Pb and P-Pb, and Eqs (5) and (6) are mass-balance equations for D-Po and P-Po. The residence times of 210 Po and 210 Pb in the upper ocean can therefore be written as: τ Po−P(0−150m) = I Po−P P Po(0−150m) , τ Pb−P(0−150m) = I Pb−P P Pb(0−150m) .
In the above equations, I Pb and I Po are the inventories of 210 Pb and 210 Po, respectively, in the different boxes; P Pb and P Po are the removal fluxes of P-Pb and P-Po, respectively, by particles  Bacon et al., 1976;Shimmield et al., 1995).
sinking out of the box; J Pb and J Po are the rates of scavenging of 210 Pb and 210 Po, respectively (if a radionuclide is transferred from solution to the particulate phase by particle scavenging, then J is positive, while J is negative for opposite); and τ P and τ D are the residence times of the radionuclides in the particulate and dissolved phases, respectively.
The average annual atmospheric 210 Pb deposition flux (F Pb ) in East Asian is 18540 dpm/m 2 /yr (n = 25, Zhong, 2020), while the atmospheric flux of 210 Po (F Po ) is usually assumed to be only 10% of F Pb (Turekian et al., 1977;Masqué et al., 2002;Baskaran, 2011). Table 1 shows the scavenging rates, removal fluxes and residence times of 210 Po and 210 Pb in the near-surface and deeper ocean. Inventories of 226 Ra were calculated based on published data from station KH-71-3-S (which is very close to station XTJ1-8) in the w-NPO (Nozaki and Tsunogai, 1976), the 226 Ra inventories in the 0-150 m and 150-1,000 m boxes were calculated to be 14,625 dpm/m 2 and 172,530 dpm/m 2 , respectively (see Table 2).
Within the 0-150 m surface layer, J Po ranged from 1,693 to 17,644 dpm/m 2 /yr; these positive values confirm that 210 Po was transferred from the dissolved phase to the particulate phase due to the scavenging of sinking particles in the upper ocean. Correspondingly, the removal flux of 210 Po ranged from 3,327 to 34,941 dpm/m 2 /year, showing a large spatial variation in the w-NPO. The residence times of D-Po (τ Po−D ) and P-Po (τ Po−P ) in the upper 0-150 m layer ranged from 0.6 to 5.5 year and from 83 to 921 days, respectively. Interestingly, the values of τ Po−D (0.6-1.2 year) and τ Po−P (83-93 days) at the three more northerly stations (XTJ1-13, XTJ1-22, and XTJ2-7) were much lower than the values of τ Po−D (1.6-5.5 year) and τ Po−P (130-921 days) of the more southerly stations (XTW1-3, XTW1-11, XTJ1-1, and XTJ1-8). Generally, the residence time of 0.6 years was common for dissolved 210 Po under more biologically productive conditions in the ocean (Shimmield et al., 1995). The τ Po−D at stations XTJ1-13, XTJ1-22, and XTJ2-7 were much shorter than the other four stations, and correspondently, the average POC concentration at 0-150 m box were higher than the other four stations (Table 1), which supports that the residence time of dissolved 210 Po was shorter when the biological activity was higher. In addition, longer particulate residence times are related to inefficient vertical removal processes (low export fluxes), which can be confirmed from the removal flux of 210 Po (P Po ). As seen from Table 1, the P Po values of the three more northerly stations (3,327-17,242 dpm/m 2 /year) were much lower than those of the four more southerly stations (14,785-34,941 dpm/m 2 /year). The above phenomena suggest that the upper waters of the southern part of the w-NPO are characterized by inefficient vertical removal processes and high turnover rates, while strong particle export occurred in the northern part of the w-NPO.
At greater depths (150-1,000 m), the J Po values were negative at all stations except stations XTJ1-13 and XTJ2-7 (Table 1), further demonstrating the occurrence of 210 Po remineralization in the deeper ocean. However, due to the release of 210 Po, τ Po−D could not be calculated at almost all stations in the deeper layer (150-1,000 m) except at station XTJ1-13. The P Po values were negative at stations XTJ1-1 and XTJ1-8, implying that 210 Po could not be removed from the 150-1,000 m layer to the much deeper (>1,000 m) ocean at these two sampling sites. In addition, the τ Po−P in the 150-1,000 m layer ranged from 85 to 331 d at the remaining four stations (XTW1-3, XTJ1-13, XTJ 1-22, and XTJ2-7).
In the upper 0-150 m, the scavenging rates of 210 Pb (J Pb ) varied between 18,373 and 18,720 dpm/m 2 /yr and the removal fluxes of 210 Pb (P Pb ) ranged from 17,835 to 18,454 dpm/m 2 /year ( Table 1). As atmospheric input is the dominant source term for 210 Pb in the upper ocean, the removal flux of 210 Pb was close to the constant atmospheric 210 Pb deposition flux of 18,540 dpm/m 2 /year. The values of τ Pb−D and τ Pb−P in the upper ocean (0-150 m) were nearly on the same level, ranging from 0.47 to 1.09 year and from 0.33 to 0.98 year, respectively. However, in the deeper ocean (150-1,000 m), the scavenging rate of 210 Pb decreased significantly to a low level of 3,103-3,800 dpm/m 2 /year, whereas the removal flux of 210 Pb remained at a high level (15,575-17,649 dpm/m 2 /year). Moreover, the residence times of D-Pb and P-Pb below 150 m were calculated to be 13.1-22.9 and 1.1-5.5 year, respectively.

Particulate Organic Carbon Export Flux Estimated From 210 Po Deficiency
The scavenging of particle-reactive nuclides produces a deficiency of the daughter nuclides with respect to their parents in the water column. From these daughter deficiencies, Buesseler et al. (1992) developed a method to estimate the POC export flux (F POC ) from the upper ocean by using 234 Th-238 U disequilibrium, and this method has been applied extensively. F POC can be derived by multiplying the daughter nuclide removal flux from the euphotic zone by the ratio of the POC concentration to the daughter nuclide activity of the total particulate material. Similarly, based on the 210 Po deficiency, the POC export flux is expressed as follows: where POC/A P Po denotes the measured POC/ 210 Po ratio in particulate matter, and P Po denotes the removal flux of 210 Po at the output interface of 150 m.
Similarly, Coale and Bruland (1987) showed that the profile of 234 Th was closely linked to the profiles of nutrients and chlorophyll, implying that the cycling of 234 Th was linked to the cycling of organic matter. This led Eppley (1989) to assume that, if the residence times of POC and 234 Th in surface waters are similar, the 234 Th flux can be used to calculate F POC . Because Dissolved 226 Ra data is cited from Nozaki and Tsunogai (1976).
Frontiers in Marine Science | www.frontiersin.org 210 Po has stronger particle affinity in organic matter than 234 Th, 210 Po could be superior to 234 Th as a tracer for POC; hence, based on Eppley's assumption, the 210 Po-based POC flux can also be derived as follows: where I POC stands for the integral inventory of POC in the upper ocean and τ Po−P is the residence time of 210 Po in the particulate phase. The POC fluxes estimated by the Eppley and Buesseler models in the upper layer of the w-NPO (0-150 m) are displayed in Table 3 and Figure 9. From Figure 9A, we can divide these seven stations into two groups. One group (Group I) includes the station XTW1-3 in the Luzon Strait, station XTJ2-7 in the northern Shikoku Basin, and station XTJ1-22 located in the Oyashio Current -Kuroshio Current boundary near the main island of Japan, which showed a higher estimated F POC level of 6-9 mmol C/m 2 /d (see Table 3 and Figure 9A). The common characteristic of Group I was that all of them were located near the continental shelf of East Asia. However, station XTJ1-22 was also affected by the cold and nutrient-rich Oyashio Current, which appears to promote high primary production and subsequently high POC export flux. Therefore, the FPOC of station XTJ1-22 was the highest among all stations ( Figure 9A).
By contrast, the other four stations (XTJ1-1, XTW1-11, XTJ1-8, and XTJ1-13 station) can form another group (Group II), which denoted a lower POC export flux level in the w-NPO ( Figure 9A). Concretely, the lowest POC export (<1 mmol C/m 2 /day) was found at station XTJ1-1 in the Marianas Trench, at which the negligible sinking flux of 210 Po (3,327 dpm/m 2 /year, Table 1) was the most obvious feature. The relatively lower POC export fluxes (1-2 mmol C/m 2 /day) at stations XTW1-11 and XTJ1-8 were derived from the relatively low sinking fluxes of 210 Po (Table 1) and relatively low particulate POC/ 210 Po ratios (Table 3). Geographically, these four stations of Group II (XTW1-11, XTJ1-1, XTJ1-8, and XTJ1-13) were located in the central gyre of the w-NPO, which had the lower particulate POC/ 210 Po ratios in total suspended matters comparing with values of the stations (XTW1-3, XTJ1-22, and XTJ2-7) in Group I close to the East Asian continent (Table 3). F POC of stations in Group II were much lower than those of Group I, which may be related to the dominated phytoplankton. The central gyre of the w-NPO (stations of Group II) is an oligotrophic area where the phytoplankton community is dominated by picoplankton (such as smaller diatoms or coccolithophores), and these small size phytoplankton produce a low POC flux due to the oligotrophic living conditions in this region. Interestingly, in these areas, stations in Group II had much higher particulate POC/ 210 Po ratios than those of stations in the central gyre of the w-NPO (Group I). For example, the stations XTW1-3, XTJ1-22, and XTJ2-7, located at the margins of the Chinese mainland and the main island of Japan, have higher particulate POC/ 210 Po ratios of 181 ± 16, 184 ± 16, and 108 ± 10 µmol/dpm, respectively. And the other four stations XTW1-11, XTJ1-1, XTJ1-8, and XTJ1-13, located in the central gyre of the w-NPO, have much lower particulate POC/ 210 Po ratios of 73 ± 5, 76 ± 6, 84 ± 7, and 69 ± 5 µmol/dpm, respectively ( Table 2). Generally, smaller phytoplankton cells can scavenge more 210 Po (smaller particles dominate higher particulate 210 Po activity relative to larger particles in the non-oligotrophic area) due to their larger surface area per unit of volume, lowering their particulate POC/ 210 Po ratios .
Overall, the POC fluxes calculated by the Eppley method and Buesseler method ranged from 0.6 to 8.8 mmol C/m 2 /d and from 0.7 to 8.6 mmol C/m 2 /d, respectively (Figure 9A). These values are in good agreement with the estimated POC fluxes derived via the steady-state 210 Po-210 Pb method in other open oceans with similar latitudes (negligible to 8.5 mmol C/m 2 /day) (Kim and Church, 2001;Stewart et al., 2007;Verdeny et al., 2009;Roca-Martí et al., 2016;Horowitz et al., 2020). From Figure 9A, although there are some differences, Eppley model and Buesseler model-derived F POC , are very similar within uncertainties. Buesseler model estimated F POC were 24 and 43% higher than that of Eppley model for stations XTW1-3 and XTJ2-7, respectively. However, Eppley modelderived F POC was 17% higher than that of Buesseler model The uncertainties in the 210 Po-derived POC flux were estimated based on the propagation of error. The export interface is defined at 150 m. *The primary production for calculating the e-ratio values was 62 ± 19 mmol C/m 2 /d (cited from Palevsky et al., 2016). for station XTJ1-22. The difference of F POC results given by different models may be related to the assumptions of the models. For example, the Eppley model's assumption that the residence times of POC and particulate daughter radionuclides ( 234 Th or 210 Po) should be the same within the upper ocean is possibly too demanding in the real ocean and therefore is often difficult to achieve. Murray et al. (1989) compared the residence times of 234 Th and POC in the euphotic zone of the eastern equatorial Pacific and found that the residence time of 234 Th was approximately three times less than that of POC. In contrast, the Buesseler model needs to assume only that the same particulate matter serves as the carrier of POC and 234 Th (or 210 Po), which is relatively easy to achieve in the actual sea. For some specific stations, the difference between two models may be significant, hence, more care should be taken when selecting the model. But on a larger spatial scale in our study area, Eppley model can also provide reasonable estimation results. For example, Figure 9B shows that Eppley model-derived F POC was well correlated with the Buesseler model-derived F POC in the w-NPO (n = 7, slope = 1.06, R 2 = 0.92, p = 0.003); additionally, the 210 Po-POC export fluxes based on the Eppley and Buesseler models had very similar spatial distribution features ( Figure 9A). These evidences suggest that the hypothesis of the Eppley model is also valid in the w-NPO and this agreement of Eppley model-derived F POC and Buesseler model-derived F POC in the w-NPO probably suggest the residence times of 210 Po and POC were similar in the upper ocean.

Comparison of Particulate Organic Carbon Export Fluxes Among Different Regions of the Western North Pacific Ocean and Its Surrounding Marginal Seas
The 210 Po-derived POC export fluxes from the upper ocean (5.9-8.8 mmol C/m 2 /day) at the stations along the edges of the w-NPO (close to the East Asian land masses) were significantly higher (by a factor of 2∼14) than the fluxes determined from the interior (oligotrophic) seawater in the w-NPO (i.e., XTW1-11, XTJ1-1, and XTJ1-8: 0.6-2.5 mmol C/m 2 /d) ( Table 3 and Figure 9). Comparing our POC fluxes with other fluxes in different regions of the Pacific Ocean and its surrounding marginal seas ( Table 4, in which the POC fluxes were estimated either by sediment traps or by the 234 Th-238 U and 210 Po-210 Pb disequilibrium methods), reveals systematic higher POC fluxes occurring in the marginal seas surrounding the w-NPO (Figure 10). For example, the POC export fluxes in the Pacific Ocean were much smaller than those reported for the SCS (2.0-16.7 mmol C/m 2 /day, 234 Th-238 U disequilibrium, Zhou et al., 2020), inner shelf of the ECS (4.8-65.4 mmol C/m 2 /day, trap, Hung et al., 2013), and the Sea of Japan (3.2-26.3 mmol C/m 2 /day, 234 Th-238 U disequilibrium, Kawakami et al., 2015). The NPO is a region of particular interest for quantifying the rate and efficiency of the BCP, as this region is a major sink for atmospheric CO 2 , absorbing 0.5 Pg C/year, ∼25% of the total ocean CO 2 uptake (Takahashi et al., 2009). As an important part of the NPO, the subtropical oligotrophic w-NPO is a typical nitrate-deficient water region and is characterized by low chlorophyll-a (Chl a) concentrations , and this region occurs a band of strong CO 2 uptake between 30 and 40 • N, overlapping with the transition zone between the subtropical and subarctic gyres (Shih et al., 2015). Researchers stated the importance of the contributions of biological carbon export to the NPO sink (Ayers and Lozier, 2012). However, the characteristics of the POC export flux in the w-NPO region are not well understood (Kawakami et al., 2015;Shih et al., 2015). Using the information available, based on the summary of the published POC export fluxes in the North Pacific Ocean (Table 4 and Figure 10), we can obtain a preliminary general view for the distribution feature of POC export flux. Although there were spatial differences, the overall mean level of POC export flux was below 10 mmol C/m 2/ day. Specifically, the POC fluxes in our study area of the w-NPO (0.6-8.8 mmol C/m 2 /day) are comparable to the results of other published studies obtained with the 234 Th-238 U method in other Pacific Ocean regions under general production condition (Figure 10A), such as the subarctic northeastern Pacific Ocean (2.8-7.6 mmol C/m 2 /d, from February 1996 to February 1997, Charette et al., 19992.01 ± 0.56 (n = 61) Table 4 and this study.
Export efficiency can be defined as the ratio of POC export to primary production (the e-ratio, Buesseler, 1998). Although primary productivity levels were not available for the studied sampling stations, Palevsky et al. (2016) summarized the annual and seasonal net primary productivity (NPP) estimates for the western North Pacific. Since our sampling activities took place from late May to late June 2018, we preliminarily evaluated the e-ratios of the BCP in the w-NPO by using the summer NPP estimates for the regions of the North Pacific (62 ± 19 mmol C/m 2 /day), hence, the e-ratio values were calculated to be in the range of 1.0-14.2% (Table 3), which are similar to the previously reported values in the North Pacific (Table 4 and Figure 10B; Charette et al., 1999;Benitez-Nelson et al., 2001;Buesseler et al., , 2020. As can be seen from Figure 10B, the e-ratios of biological carbon pump were more variable in the marginal seas than that in the Pacific Ocean. For example, the e-ratios were very consistent (in a level of < 20%) in the NE Pacific, North Pacific, and w-NPO. However, in the marginal seas, the POC fluxes and e-ratios changed over a very wide range ( Figure 10B). One extreme case reported in the inner shelf of ECS showed that the e-ratios ranged from 10 to 90% . The eratios in our study were below 15%, suggesting a moderate BCP efficiency in the w-NPO. By comparison, it can be found that the BCP strengths in some marginal seas of the Pacific Ocean [for example, e-ratio = 9-91% in the inner shelf of ECS , 13-28% in the Southern ECS , and 7-56% in the Sea of Japan (Kim et al., 2011)] is stronger than that in the interior of the Pacific Ocean (for example, e-ratio = ∼10% at station XTJ2-7 in the Shikoku Basin, 3-4% at station XTW1-11 in the Philippine Basin, and ∼1% at station XTJ1-1 in the Mariana Trench).
We compiled a table for comparing the export efficiencies of BCP in other open seas, as shown in the Table 5. It is worth noting that the e-ratios are similar in such different areas. For example, the e-ratios < 15% observed in our study are similar to those found in the North Atlantic (Iberian Basin, 0.5-2.5%; Irminger Basin, 3 ± 3%; western European Basin, 5 ± 5%; Iceland Basin, 6 ± 6%; and Labrador Basin, 10 ± 3%, see . The similarity between our study and  is that both sampling activities were carried out in May-June, however, the difference is that our study area was in an oligotrophic subtropical region (medium-low latitudes) and the study area of  was in a temperate system (medium-high latitudes). In addition,  started sampling when the bloom was starting, hence, the POC fluxes and e-ratios were still low. Ceballos-Romero et al. (2016) pointed that export efficiency was significantly discrepant in different stages of the bloom and the export efficiency also showed a strong seasonal variability in the North Atlantic. By comparison with study case of Ceballos-Romero et al. (2016) in the North Atlantic Ocean, we can find that algal blooming has a significant influence on BCP efficiency. Ceballos-Romero et al. (2016) reported two sampling campaigns: one sampling was carried out in the pre-bloom stage (April-May 2010), and the export efficiency was high at that moment (∼50%); another sampling was carried out in the decline of the bloom and post-bloom stage (July-August 2010), and the export efficiency was very low (3%). Obviously, our study displayed results from medium-low latitudes with no bloom, therefore, the low e-ratios of 1.0-14.2% were reasonable.
In addition, the e-ratios in the w-NPO (1.0-14.2%, this study) are close to or slightly higher than that in the lowlatitude tropical seas and oceans, for example, the equatorial Pacific (1-10%, Buesseler, 1998) and the Arabian Sea (1-10%, Buesseler, 1998;0.2-11.7%, Subha Anand et al., 2017). This low e-ratios of < 10% might represent the values of BCP efficiency under general condition for tropical ocean, considering the weak seasonal variation of tropical oceans and the oligotrophic features. However, the e-ratios in the w-NPO are much lower than those reported at high-latitude sites (>25%), such as the Eurasian Basin of the Arctic Ocean (60 ± 40%, Roca-Martí et al., 2016), the Bellingshausen Sea (37% at station K, Shimmield et al., 1995) and the Weddell Sea and the Antarctic Polar Front (16-100%, Rutgers van der Loeff et al., 1997), since most of these study cases in the high-latitude seas were related to the bloom situation or the increase of chlorophyll-a, POC and plankton biomass concentrations (Shimmield et al., 1995;Rutgers van der Loeff et al., 1997). The lower export efficiencies observed in the w-NPO may be consistent with the predominance of smaller phytoplankton in the oligotrophic seawater. The oligotrophic feature might weaken the export strength of the BCP in the w-NPO. Indeed, small particles are usually slowsinking particles that are more likely to be degraded during their descent (Villa-Alfageme et al., 2016), leading to lower export efficiencies. In our study, we also observed negative scavenging rates (J) of 210 Po (negative J values mean that radionuclides are transferred from particles to the dissolved phase) below 150 m in the w-NPO (Table 1), which supports the occurrence of particle degradation, leading to a low export efficiency. For example, the lowest J Po values in the 150-1,000 m segment were observed at stations XTW1-11 (J Po = -86,511 dpm/m 2 /year), XTJ1-1 (J Po = -64,036 dpm/m 2 /year) and XTJ1-8 (J Po = -26,022 dpm/m 2 /year), and correspondingly, these three stations have the lowest e-ratios (based on Eppley model: 1.0-2.7%; based on Buesseler model: 1.1-4.0% Table 3).  , 1997;Buesseler, 1998 CONCLUSION In this study, we reported the vertical distributions of D-Po, P-Po, D-Pb, and P-Pb activities in the w-NPO during late spring and early summer to constrain the particle dynamics (scavenging rates and removal fluxes of 210 Po and 210 Pb), and to estimate the carbon export production (POC export flux). More than 50% of the radionuclides were found in the dissolved phase below a depth of 50 m, while a small proportion was associated with the particulate phase. However, the percentages of P-Po and P-Pb were higher than 50% (even up to 90%) in the surface seawater (0-50 m). 210 Po deficits relative to 210 Pb were observed in the upper 150 m at all the stations, however such 210 Po deficiencies can even extend to deeper ocean (such as 500 m) at some stations. In the interior of the w-NPO, the excess 210 Po (total 210 Po/ 210 Pb > 1) activities in the deeper ocean (500-1,000 m) at stations XTJ1-1 and XTJ1-8 were attributed to the release of 210 Po due to biogenic particle release during sinking. Based on a conceptual one-dimensional irreversible scavenging model, the residence times of 210 Po and 210 Pb in the w-NPO were obtained. In the upper ocean (0-150 m), the residence times of D-Po (τ Po−D , 0.6-5.5 year) were much higher than those of P-Po (τ Po−P , 83-921 days), while in the deep ocean (below 150 m), the residence times of D-Pb were calculated to be 13.1-22.9 year, 2-10 times longer than those of P-Pb (1.1-5.5 year). Based on 210 Po-210 Pb disequilibrium, we found that the Eppley model-derived POC export fluxes agreed well with those derived from the Buesseler model in the upper ocean of the w-NPO, suggesting that particulate 210 Po and POC may have similar residence time in the water column. Overall, the 210 Poderived POC fluxes varied spatially, ranging from 0.6 to < 9 mmol C/m 2 /day, with the highest export fluxes at stations close to the East Asian continental shelf and the lowest export fluxes at stations in the ocean basin. We concluded that POC export fluxes tended to increase with decreasing distance from the continental margins. The POC export efficiencies also showed regional differences even within the same basin, with e-ratio values ranging from 1.0 to 14.2%, suggesting a moderate BCP efficiency in the w-NPO. The low export efficiencies may be associated with the dominance of smaller particles and particle release below the euphotic zone. The negative scavenging rates of 210 Po (-J Po ) below 150 m at stations in the central basin of the w-NPO supports the occurrence of particle dissolution. Comparing with the reported e-ratios in other open seas, relatively higher carbon export efficiencies occurred at high-latitude sites.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author/s.

AUTHOR CONTRIBUTIONS
QZ was responsible for conceptualization, methodology, sample analysis, data processing, writing original draft, writing-review, and revising. DH, TY, and JD provided a financial support and helped in article reviewing. JL, JJ, and JN contributed to the review of the manuscript. All authors contributed to the article and approved the submitted version.