Radium and Lead Radioisotopes Composition of Sediment and Its Biogeochemical Implication in Polymetallic Nodule Area of Clario-Clipperton Zone

Radioactivity levels of 210Pb and 226Ra were detected in a sediment core obtained using a multi-corer from the polymetallic nodule area inside the Clarion-Clipperton Zone (CCZ), a contract area of the China Ocean Mineral Resources Association (COMR) in the eastern Pacific Ocean. The profile of excess 210Pb (210Pbex) shows that the specific activity of 210Pbex has three parts with different distributions at depths of 0–16 cm (I), 17–36 cm (II), and 37–48 cm (III). When the I section of nonlocal mixing was excluded, using a steady-state diffusion mode, the bioturbation coefficients of the core were estimated to be 24.2 cm2/a at 17–36 cm deep and 5.9 cm2/a at 37–48 cm deep, which were greater compared to previously published results. This is most likely owing to bioturbations caused by various organism species in the two sections.


Introduction
There are approximately 54 × 10 6 km 2 of polymetallic nodules in the global oceans, of which the largest area is in the Pacific Ocean, with approximately 23 × 10 6 km 2 , especially in the Clarion-Clipperton Zone (CCZ) of the eastern Pacific Ocean, which is the most abundant and potentially most economically valuable [1]. The estimated reserves of polymetallic nodules in the CCZ are approximately 21 × 10 9 t, including approximately 6 × 10 9 t of Mn, which is larger than all known terrestrial Mn reserves. Meanwhile, the Ni content (270 Mt) and Co content (44 Mt) of polymetallic nodules in the CCZ are three and five times higher than the terrestrial reserves, respectively [2]. With the expansion of the global economy, all nations face a significant resource dilemma, and deep-sea mineral resources dispersed in international public seas are gaining attention from governments and organizations throughout the world.
Traditionally, the primary diagenetic processes of deep-sea polymetallic nodules have been diagenesis and hydrogenesis, but the relevance of biomineral production of deep-sea polymetallic nodules has garnered widespread attention in recent years [3][4][5]). Furthermore, benthic fauna plays a significant role in the production of both polymetallic nodules and Corich crusts. It is acknowledged that mining and future exploitation would erode the hard substrate and bottom deposition, and may generate sediment plumes, potentially impacting benthic faunal ecosystems across broad regions. Although seabed habitats are particularly vulnerable, data on density, richness, and community structure are missing in many sections of the CCZ [6]. 210 Pb is a naturally occurring radioisotope in the uranium decay series that has been regularly used to assess sedimentation rates and biological mixing records during the last 100 years. The gaseous decay product of its parent 226 Ra, 222 Rn, escapes from land and decays through a sequence of short-lived daughter bodies to produce 210 Pb. 210 Pb remains in the atmosphere for a brief period of time (about 10 days) before migrating out of the 2 of 8 sky via rain, snow, dust, etc. [7,8], and settling on the ocean surface before settling into the sediment, resulting in 210 Pb having higher specific activity than its parent 226 Ra [9]. Sinking particles convey 210 Pb to the bottom, which is produced by the disintegration of 226 Ra and 222 Rn in the atmosphere and water column. The 210 Pb incorporated into the sediment as a result of this process is known as "excess 210 Pb". The 210 Pb ex signal decays with time, eventually approaching zero after around 100 years. If sediment accumulation alone affects 210 Pb ex distribution, the extra 210 Pb signal will be unnoticeable after 100 years, the excess 210 Pb relevant to 226 Ra indicates a distinct mixing process in the sediments, which was caused by benthic activity [10].
The purpose of this study was to estimate the bioturbation coefficients at the Polymetallic Nodule Area of CCZ based on high-resolution radionuclide activity distributions in the sediment core.

Sampling
On the Monitoring and Protection of Ecology and Environment Cruise in the northeast Pacific in 2017, a sediment core KW1-S05-MC13 was collected on the R/V XIANGYANGHONG03 (Third Institute of Oceanography, MNR, Xiamen, China) ( Figure 1, Table 1). Which was discovered at a bottom depth of 5167 m at the sampling site (10. decays through a sequence of short-lived daughter bodies to produce 210 Pb. 210 Pb remains in the atmosphere for a brief period of time (about 10 days) before migrating out of the sky via rain, snow, dust, etc. [7,8], and settling on the ocean surface before settling into the sediment, resulting in 210 Pb having higher specific activity than its parent 226 Ra [9]. Sinking particles convey 210 Pb to the bottom, which is produced by the disintegration of 226 Ra and 222 Rn in the atmosphere and water column. The 210 Pb incorporated into the sediment as a result of this process is known as "excess 210 Pb". The 210 Pbex signal decays with time, eventually approaching zero after around 100 years. If sediment accumulation alone affects 210 Pbex distribution, the extra 210 Pb signal will be unnoticeable after 100 years, the excess 210 Pb relevant to 226 Ra indicates a distinct mixing process in the sediments, which was caused by benthic activity [10]. The purpose of this study was to estimate the bioturbation coefficients at the Polymetallic Nodule Area of CCZ based on high-resolution radionuclide activity distributions in the sediment core.

Sampling
On the Monitoring and Protection of Ecology and Environment Cruise in the northeast Pacific in 2017, a sediment core KW1-S05-MC13 was collected on the R/V XIANG-YANGHONG03 (Third Institute of Oceanography, MNR, Xiamen, China) ( Figure 1, Table  1). Which was discovered at a bottom depth of 5167 m at the sampling site (10   The sediment sample was taken using a multi-core sampler with an inner diameter of 9.5 cm, which may gather many undisturbed sediment samples from the seafloor with a deploy, including the sediment-water interface and benthic life. Within 24 h of being collected, the sediment cores were extruded and sectioned at 1 cm intervals. Each  The sediment sample was taken using a multi-core sampler with an inner diameter of 9.5 cm, which may gather many undisturbed sediment samples from the seafloor with a deploy, including the sediment-water interface and benthic life. Within 24 h of being collected, the sediment cores were extruded and sectioned at 1 cm intervals. Each subsample was packed individually in a clean polyethylene bag and kept frozen at −18 • C. The samples were sent to a land-based laboratory for examination.

210 Pb and 226 Ra Analysis
The freeze-dried materials were crushed, pulverized, sieved, and homogenized before being powdered. Due to the detector shield's restricted size, about 2-3 g of the samples was moved to cylindrical plastic containers for gamma spectrometric counting. The cylindrical plastic canisters were sealed for roughly 20 days prior to gamma counting to allow radon and its short-lived progeny to attain secular radioactive equilibrium.
One lead-shielded high-purity germanium (HPGe) detector (Model 9030, Ortec Inc., Atlanta, GA, USA), with an efficiency of 20% and a full width at half maxima (at 1332 keV) less than 2.1, was used for the gamma spectrometry. The characteristic gamma ray used in 210 Pb counting is 46.5 keV. The specific radioactivity of 210 Pb was calculated according to Formula (1). A self-absorption correction for low-energy 210 Pb γ rays was performed following the method described by Cutshall et al. (1983) [11]: where A is the radioactivity of 210 Pb (Bq/kg); N i and N bi are respectively the counts of the peaks at 46.5 keV of the sample and the blank; t and t b are the counting times for the sample and blank, respectively; ε i is the detecting efficiency of 210 Pb calibrated with the marine sediment reference material produced by the International Atomic Energy Agency (IAEA-385); m is the mass of the sample (kg); and Y is branch ratio (4.25%). The radioactivity of 226 Ra was derived from the radioactivity levels of its daughter isotopes 214 Pb and 214 Bi, assuming that they reached equilibrium in the 3-week storage prior to counting. The energy transitions are 295.2 keV (18.4%) of 214 Pb and 609.3 keV (45.5%) of 214 Bi. The HPGe spectrometer was calibrated for the 226 Ra quantitative analysis with the sediment reference material (GBW08304a, produced by the National Center of Reference Material, Beijing, China) containing a known amount of 226 Ra.
The acquisition time for the spectrum and the blank were 72 h and 120 h, respectively. All the reported activity values were decay corrected to the date of sample collection.
The excess 210 Pb ( 210 Pb ex ), which is the fraction of 210 Pb activity exceeding the activity of the parent isotope 226 Ra, was determined by subtracting the activity of 226 Ra from the activity of 210 Pb.

Quality Controls
10% to 15% of the samples were selected for parallel determination. All the standard deviation of the parallel results was within the permissible range (15%), and then the average of the results was taken as the final result for the specific sample.
Besides, the laboratory has participated in the annual environmental radionuclides analysis proficiency tests organized by the International Atomic Energy Agency (IAEA) and achieved good results, demonstrating that the research team has good quality control in radionuclide measurements.

The Distribution of 226 Ra and 210 Pb
The profiles of 226 Ra and 210 Pb specific radioactivity are given in Figure 2. The specific radioactivity of 226 Ra varied from 298 Bq/kg at the surface to 122 Bq/kg at the bottom, with an average of 214 ± 47 Bq/kg. (The value after the "±" sign is the uncertainty of the measured result, which depends on the net counting rate obtained in the measuring process and is a characteristic of radioactivity measurement result). The profiles of 226 Ra-specific radioactivity in the core decreased gradually with the depth.
The specific radioactivity of 210 Pb varied from 525 Bq/kg to 1488 Bq/kg, with an average of 1074 ± 263 Bq/kg. It increased with increasing depth and then descended in an exponential manner with the depth after reaching a maximum at a depth of 18 cm.
The specific radioactivity of 210 Pb varied from 525 Bq/kg to 1488 Bq/kg, with an average of 1074 ± 263 Bq/kg. It increased with increasing depth and then descended in an exponential manner with the depth after reaching a maximum at a depth of 18 cm.
Specific Activity (Bq/kg) The activity of 210 Pb supported by 226 Ra is substantially regulated by the activity of 230 Th, 226 Ra's parent. Scavenging from the water column and ingrowth from 234 U, which is initially in excess due to the high U content of these aragonite-rich sediments, influence 230 Th activities in turn.
The main source of 210 Pb in deep-sea sediments is the decay of the parent 226 Ra. If there are no other processes, the activity distributions of 210 Pb and 226 Ra should be similar. However, the showed results indicate that the activity of 210 Pb did not change with 226 Ra. The difference between the two radionuclides indicates that 210 Pb is not entirely from the production of 226 Ra in the sediment. At the same time, there is a significant excess of 210 Pb in the whole column.
The specific radioactivity of 210 Pbex ranged from 391 Bq/kg to 1252 Bq/kg, with an average of 860 ± 234 Bq/kg. It gradually increases with depth, and reaches a maximum of 20 cm. Then it gradually decreases with depth. But there is an uptick at 36-40 cm.
The significant excess of 210 Pb further confirms other sources of 210 Pb in the surface sediments. Combined with the strong particle activity of 210 Pb, the excess 210 Pb should come from 210 Pb removed by particulate matter in the bottom water, which would result in a deficit of 210 Pb relative to 226 Ra in the bottom water. Elevated particulate 210 Pb in the

The Radioactive Disequilibrium between 226 Ra and 210 Pb
210 Pb created in the atmosphere and water column by the decay of 226 Ra and 222 Rn, was transported on particles to the seafloor by sinking. The 210 Pb integrated into the sediment as a result of this process is referred to as "excess 210 Pb", since it exists in excess of the 210 Pb created in situ by the decay of 226 Ra and 222 Rn within the sediment column ("supported 210 Pb").
The activity of 210 Pb supported by 226 Ra is substantially regulated by the activity of 230 Th, 226 Ra's parent. Scavenging from the water column and ingrowth from 234 U, which is initially in excess due to the high U content of these aragonite-rich sediments, influence 230 Th activities in turn.
The main source of 210 Pb in deep-sea sediments is the decay of the parent 226 Ra. If there are no other processes, the activity distributions of 210 Pb and 226 Ra should be similar. However, the showed results indicate that the activity of 210 Pb did not change with 226 Ra. The difference between the two radionuclides indicates that 210 Pb is not entirely from the production of 226 Ra in the sediment. At the same time, there is a significant excess of 210 Pb in the whole column.
The specific radioactivity of 210 Pb ex ranged from 391 Bq/kg to 1252 Bq/kg, with an average of 860 ± 234 Bq/kg. It gradually increases with depth, and reaches a maximum of 20 cm. Then it gradually decreases with depth. But there is an uptick at 36-40 cm.
The significant excess of 210 Pb further confirms other sources of 210 Pb in the surface sediments. Combined with the strong particle activity of 210 Pb, the excess 210 Pb should come from 210 Pb removed by particulate matter in the bottom water, which would result in a deficit of 210 Pb relative to 226 Ra in the bottom water. Elevated particulate 210 Pb in the Pacific Ocean bottom water supports a stronger particle-scavenging 210 Pb process in the bottom water [12,13].
Although the 210 Pb removed from the bottom water settled on the sediment surface, a significant excess of 210 Pb was still observed in the entire column below the surface to a depth of 48 cm. The sedimentation rate in the vicinity of the study sea area was less than 1 cm/ka. If there is no mixing process, with a half-life of only 22.3 a for 210 Pb, it should reach equilibrium with 226 Ra within 1 mm of the surface layer. The actual observations were the opposite, revealing a distinct mixing process in the sediments, which was caused by benthic activity.
Smith and Schafer (1984) used a "conveyor belt" hybrid model to explain the existing 210 Pb subsurface sediment maximum value distribution in the North-East Atlantic 4000-5000 m depth [14]; specifically, the benthos feeding is rich in organic matter and radioactive nuclide surface sediment after its discharge to the surface sediment, and the recent single suddenly mixed event model results were consistent with the measured radionuclide section. Smith et al. (1997) discovered 210 Pb ex in deep ocean sediments of the equatorial Pacific Ocean, which displayed a "shoulder phenomenon" at a depth of 23 cm, which researchers suspected was caused by the burrowing operations of huge sea urchins [13]. The equatorial Pacific Ocean's bottom sediments are densely packed with sea urchins. Sea urchins frequently excavate trenches a few decimeters wide, and they disrupt the surface sediments in these locations every 5 to 7 years [15].
The second largest value of 210 Pb ex in sediments from various Peruvian Sea basin sites emerged at depths of 5-20 cm (the first maximum value was at the surface), which was thought to be generated by surface eating and subsurface excretion of animals such as Echinaceae [16]. The Yi insects are responsible for the tropical Pacific Ocean sediments in northeast 210 Pb ex at 22 to 28 cm and 6-10 cm in the subsurface peak, and they rotated the trail of the in situ hybrid [17].

Bioturbation Coefficient
The quantification of bioturbation is required for the numerical modeling of sediment diagenesis. Using a steady-state diffusion mode, the bioturbation coefficient (D B ) was calculated from the 210 Pb ex activity profile in the sediment. Because of the simple and fast sample measurement, the simultaneous measuring of 226 Ra and 210 Pb in the sample is capable. Thus the steady-state diffusion mode has been widely applied in studies of sediment bioturbation. In this model, bioturbation and sedimentation are considered to be similar to diffusion and advection. The biological mixing process of tracer material in sediments must fulfill two characteristics when a biological diffusion model is used: (1) The frequency of biological mixing must be significantly larger than the tracer's disappearance rate, and (2) the particle exchange size must be less than the size of the tracer profile and the thickness of the mixing layer.
Since the half-life of 210 Pb is 22.3 a, 210 Pb will reach a radioactivity equilibrium with 226 Ra in five half-lives (~100 years). Therefore, the 210 Pb ex signal will appear in the sediment that accumulated in the recent period of approximately 100 years. Considering that the sedimentation rates in the study area are estimated to be lower than 1 mm per thousand years, 210 Pb ex should be detectable only within the top 1 mm of the sediment core, if there is no bioturbation or other disturbance. However, in the core samples collected in this study, 210 Pb ex signal was found down to a depth of dozens of centimeters, which was interpreted to be a result of bioturbation. To represent the process of bioturbation, Guinasso and Schink (1975) and Nozaki (1977) devised and modified a steady-state diffusion model [18,19]: where z is the depth of the sediment sample (cm), A is the 210 Pb ex radioactivity (Bq/kg) at the depth of z, ρ is the density (g/cm 3 ), D B is the bioturbation coefficient (cm 2 /a), S is the sedimentation rate (cm/a), λ is the decay constant of 210 Pb (0.031 a −1 ) and t is the time (a).
Assuming that in the mixing layer, D B , S and ρ are in a steady state, then ∂ ∂t (ρA) = 0. With the boundary conditions: A = A 0 at z = 0; and (2) A→0 at z→∞: Since S for oceanic sediments is on the order of mm/ka and the valid time scale of applying 210 Pb in tracing bioturbation is approximately 100 a, the sediment deposition process can be ignored, so Equation (3) is simplified as: Therefore, the exponential fit of the measured profile of 210 Pb ex quantifies the process of bioturbation.
The depth resolution of radiotracers in this study is very high compared to other research [20][21][22]. The profile of 210 Pb ex shows that the specific activity of 210 Pb ex has three different distributions at depths of 0-16 cm (I), 17-36 cm (II), and 37-48 cm (III), indicating that the bioturbation coefficient has changed (Figure 3). This suggested that the particle mixing process should be complicated. The complicated vertical profiles were also observed in the Pacific [22,23]. The maximum value in the subsurface layer indicates heterogeneous mixing of particles by benthic organisms [14,24,25]. The 210 Pb ex data affected by nonlocal mixing are excluded when estimating D B .

Conclusions
The radioactivity profiles of 210 Pb and 226 Ra in sediment core from CCZ were measured. A significant 210 Pbex signal was found throughout the core and has three parts with different distributions at depths of 0-16 cm (I), 17-36 cm (II), and 37-48 cm (III). Based on the radioactivity profiles of 210 Pbex in sediment cores from CCZ and the one-dimensional steady-state diffusion model, bioturbation coefficients at one station of different sections were estimated to be 24.2 cm 2 /a at 17-36 cm deep and 5.9 cm 2 /a at 37-48 cm deep, which are larger than the reported values, indicating that this region is biologically abundant and has evident bioturbation processes. This is most likely owing to bioturbations caused by various organism species in the two sections. This will necessitate further investigation in the future.   [20]. On the other hand, it is clear that the D B of section II is higher than that of section III. The D B of section III is the same as Hyeong et al. (2018) [20]. This is most likely owing to bioturbations caused by various organism species in the two sections. This will necessitate further investigation in the future.
In the meantime, it should be reminded that it is not reasonable to portray the entire area with the bioturbation coefficients estimated with a single sediment core sample. Although obtaining sediment core from the deep ocean is very cost-consuming, further investigation is needed in the future to better understand the characteristics of this important region.

Conclusions
The radioactivity profiles of 210 Pb and 226 Ra in sediment core from CCZ were measured. A significant 210 Pb ex signal was found throughout the core and has three parts with Molecules 2022, 27, 5061 7 of 8 different distributions at depths of 0-16 cm (I), 17-36 cm (II), and 37-48 cm (III). Based on the radioactivity profiles of 210 Pb ex in sediment cores from CCZ and the one-dimensional steady-state diffusion model, bioturbation coefficients at one station of different sections were estimated to be 24.2 cm 2 /a at 17-36 cm deep and 5.9 cm 2 /a at 37-48 cm deep, which are larger than the reported values, indicating that this region is biologically abundant and has evident bioturbation processes. This is most likely owing to bioturbations caused by various organism species in the two sections. This will necessitate further investigation in the future.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.