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

Abstract

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 (²¹⁰Pb_ex) shows that the specific activity of ²¹⁰Pb_ex 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.

1. Introduction

There are approximately 54 × 10⁶ km² of polymetallic nodules in the global oceans, of which the largest area is in the Pacific Ocean, with approximately 23 × 10⁶ km², 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 × 10⁹ t, including approximately 6 × 10⁹ 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].

²¹⁰Pb 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 ²²⁶Ra, ²²²Rn, escapes from land and decays through a sequence of short-lived daughter bodies to produce ²¹⁰Pb. ²¹⁰Pb 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 ²¹⁰Pb having higher specific activity than its parent ²²⁶Ra [9]. Sinking particles convey ²¹⁰Pb to the bottom, which is produced by the disintegration of ²²⁶Ra and ²²²Rn in the atmosphere and water column. The ²¹⁰Pb incorporated into the sediment as a result of this process is known as “excess ²¹⁰Pb”. The ²¹⁰Pb_ex signal decays with time, eventually approaching zero after around 100 years. If sediment accumulation alone affects ²¹⁰Pb_ex distribution, the extra ²¹⁰Pb signal will be unnoticeable after 100 years, the excess ²¹⁰Pb relevant to ²²⁶Ra 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.

2. Materials and Methods

2.1. 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. Coordinates of sampling stations and bioturbation coefficients.

Station	Latitude (°N)	Longitude (°E)	Bottom Depth (m)	Db (cm2/a)
KW1-S05-MC13	10.0833	−154.3334	5167	24.2 (II)
				5.9 (III)

). Which was discovered at a bottom depth of 5167 m at the sampling site (10.0833° N, −154.3334° E) in the China Ocean Mineral Resources Association’s (COMR) northern polymetallic nodule exploration contract region.

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.

2.2. ²¹⁰Pb and ²²⁶Ra 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 ²¹⁰Pb counting is 46.5 keV. The specific radioactivity of ²¹⁰Pb was calculated according to Formula (1). A self-absorption correction for low-energy ²¹⁰Pb γ rays was performed following the method described by Cutshall et al. (1983) [11]:

$$\mathrm{A}={\displaystyle \sum }_{i=1}^n\left(\frac{N_i}{t}-\frac{N_{bi}}{t_b}\right)\times \frac{1}{{\epsilon }_i{Y}_{i}m}$$

(1)

where A is the radioactivity of ²¹⁰Pb (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; ${\epsilon }_i$ is the detecting efficiency of ²¹⁰Pb 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 ²²⁶Ra was derived from the radioactivity levels of its daughter isotopes ²¹⁴Pb and ²¹⁴Bi, assuming that they reached equilibrium in the 3-week storage prior to counting. The energy transitions are 295.2 keV (18.4%) of ²¹⁴Pb and 609.3 keV (45.5%) of ²¹⁴Bi. The HPGe spectrometer was calibrated for the ²²⁶Ra quantitative analysis with the sediment reference material (GBW08304a, produced by the National Center of Reference Material, Beijing, China) containing a known amount of ²²⁶Ra.

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 ²¹⁰Pb (²¹⁰Pb_ex), which is the fraction of ²¹⁰Pb activity exceeding the activity of the parent isotope ²²⁶Ra, was determined by subtracting the activity of ²²⁶Ra from the activity of ²¹⁰Pb.

2.3. 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.

3. Results

The Distribution of ²²⁶Ra and ²¹⁰Pb

The profiles of ²²⁶Ra and²¹⁰Pb specific radioactivity are given in Figure 2. The specific radioactivity of ²²⁶Ra 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 ²²⁶Ra-specific radioactivity in the core decreased gradually with the depth.

The specific radioactivity of ²¹⁰Pb 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.

4. Discussion

4.1. The Radioactive Disequilibrium between ²²⁶Ra and ²¹⁰Pb

²¹⁰Pb created in the atmosphere and water column by the decay of ²²⁶Ra and ²²²Rn, was transported on particles to the seafloor by sinking. The ²¹⁰Pb integrated into the sediment as a result of this process is referred to as “excess ²¹⁰Pb”, since it exists in excess of the ²¹⁰Pb created in situ by the decay of ²²⁶Ra and ²²²Rn within the sediment column (“supported ²¹⁰Pb”).

The activity of ²¹⁰Pb supported by ²²⁶Ra is substantially regulated by the activity of ²³⁰Th, ²²⁶Ra’s parent. Scavenging from the water column and ingrowth from ²³⁴U, which is initially in excess due to the high U content of these aragonite-rich sediments, influence ²³⁰Th activities in turn.

The main source of ²¹⁰Pb in deep-sea sediments is the decay of the parent ²²⁶Ra. If there are no other processes, the activity distributions of ²¹⁰Pb and ²²⁶Ra should be similar. However, the showed results indicate that the activity of ²¹⁰Pb did not change with ²²⁶Ra. The difference between the two radionuclides indicates that ²¹⁰Pb is not entirely from the production of ²²⁶Ra in the sediment. At the same time, there is a significant excess of ²¹⁰Pb in the whole column.

The specific radioactivity of ²¹⁰Pb_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 ²¹⁰Pb further confirms other sources of ²¹⁰Pb in the surface sediments. Combined with the strong particle activity of ²¹⁰Pb, the excess ²¹⁰Pb should come from ²¹⁰Pb removed by particulate matter in the bottom water, which would result in a deficit of ²¹⁰Pb relative to ²²⁶Ra in the bottom water. Elevated particulate ²¹⁰Pb in the Pacific Ocean bottom water supports a stronger particle-scavenging ²¹⁰Pb process in the bottom water [12,13].

Although the ²¹⁰Pb removed from the bottom water settled on the sediment surface, a significant excess of ²¹⁰Pb 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 ²¹⁰Pb, it should reach equilibrium with ²²⁶Ra 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 ²¹⁰Pb 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 ²¹⁰Pb_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 ²¹⁰Pb_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 ²¹⁰Pb_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].

4.2. 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 ²¹⁰Pb_ex activity profile in the sediment. Because of the simple and fast sample measurement, the simultaneous measuring of ²²⁶Ra and ²¹⁰Pb 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 ²¹⁰Pb is 22.3 a, ²¹⁰Pb will reach a radioactivity equilibrium with ²²⁶Ra in five half-lives (~100 years). Therefore, the ²¹⁰Pb_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, ²¹⁰Pb_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, ²¹⁰Pb_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]:

$$\frac{\partial }{\partial t}\left(\rho \mathrm{A}\right)=\frac{\partial }{\partial z}\left(\rho D_B\frac{\partial \mathrm{A}}{\partial z}\right)-\frac{\partial }{\partial z}\left(\rho S\mathrm{A}\right)-\mathsf{\lambda }\rho \mathrm{A}$$

(2)

where z is the depth of the sediment sample (cm), A is the ²¹⁰Pb_ex radioactivity (Bq/kg) at the depth of z, ρ is the density (g/cm³), D_B is the bioturbation coefficient (cm²/a), S is the sedimentation rate (cm/a), $\mathsf{\lambda }$ is the decay constant of ²¹⁰Pb (0.031 a⁻¹) and t is the time (a).

Assuming that in the mixing layer, D_B, S and $\rho$ are in a steady state, then $\frac{\partial }{\partial t}\left(\rho \mathrm{A}\right)=0$.

With the boundary conditions: A = A₀ at z = 0; and (2) A→0 at z→∞:

$$\mathrm{A}={\mathrm{A}}_{0}exp\left[\frac{S-\sqrt{S^2+4\mathsf{\lambda }{D}_B}}{2D_B}z\right]$$

(3)

Since S for oceanic sediments is on the order of mm/ka and the valid time scale of applying ²¹⁰Pb in tracing bioturbation is approximately 100 a, the sediment deposition process can be ignored, so Equation (3) is simplified as:

$$\mathrm{A}={\mathrm{A}}_{0}exp\left[-z\sqrt{\mathsf{\lambda }/D_B}\right]$$

(4)

Therefore, the exponential fit of the measured profile of ²¹⁰Pb_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 ²¹⁰Pb_ex shows that the specific activity of ²¹⁰Pb_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 ²¹⁰Pb_ex data affected by nonlocal mixing are excluded when estimating D_B.

The calculated D_B are 24.2 cm²/a and 5.9 cm²/a in sections II and III, respectively. The D_B values of our research are significantly higher than Yang and Zhou (2004) (0.26–2.75 cm²/a) [15] and Hyeong et al. (2018) (1.1–9.0 cm²/a) [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.

5. Conclusions

The radioactivity profiles of ²¹⁰Pb and ²²⁶Ra in sediment core from CCZ were measured. A significant ²¹⁰Pb_ex 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 ²¹⁰Pb_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 cm²/a at 17–36 cm deep and 5.9 cm²/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.