Introduction

Active biogeochemical processes associated with sediment resuspension have occurred in the global ocean1. The interactions between sediment and seawater play an important role in the burial of materials and their resupplementation into the overlying water column, which greatly affect the carbon cycle, nutrients, trace metals, and other pollutants. The bottom nepheloid layer occurs at the boundary between sediment and seawater and was widely studied in the GEOSECS-JGOFS-GEOTRACES era2. Although the bottom nepheloid layer can be identified by physical3, biological4, chemical5, and geological parameters6, the quantitative particle dynamic processes have been investigated by 234Th/238U disequilibrium method with particulate 234Th, dissolved 234Th, activity ratio of 234Th to 238U, activity ratio of particulate 234Th to dissolved 234Th, and residence time of 234Th6,7,8,9,10.

The 234Th/238U disequilibrium method is on the basis of the distinct behaviours of 234Th and 238U in seawater (the particle-reactive 234Th and conservative 238U). This method is used to quantify the particle process of sediment resuspension7,9. However, photosynthesis generally occurs alongside sediment resuspension on the shallow continental shelf due to the penetration of sun-light into the full seawater column. The 234Th/238U disequilibrium method reflects the integrated results of all particle processes in seawater and cannot distinguish sediment resuspension from photosynthesis. Additionally, sediment resuspension enhances the scavenging of 234Th and probably leads to the overestimation of export flux of 234Th on the shallow continental shelf. Owing to the shortcomings of 234Th/238U disequilibrium method, Residual β activity of particulate 234Th (RAP234) is proposed for the first time to track sediment resuspension.

RAP234 derived from the second counting rate of particulate 234Th could be a powerful complement to 234Th/238U disequilibrium method in tracking sediment resuspension from low- to high-latitude oceans, including the western Arctic Ocean, the South China Sea, and the Southern Ocean. Seawater turbidity, residence time of total 234Th, and the ratio of RAP234 to particulate organic carbon (POC) were also measured to support the occurrence of sediment resuspension. The mechanism and conceptual model of RAP234 are represented and illustrated. This new definition of RAP234 is analogous to that of gross β in drinking water11. RAP234 is a sensitive proxy for distinguishing sediment resuspension from photosynthesis and for indicating the intensity of sediment resuspension without any additional sampling and measurement. Although further work needs to be conducted, RAP234 behaves in a similar manner to lithophile elements and can be a novel approach to investigate quantitative particle dynamics in the estuary-shelf-slope-open ocean continuum.

Results and Discussion

Abnormally high activity of RAP234

The second counting rate of particulate 234Th is generally overlooked due to its constant value of 0.3–0.4 counts per minute (cpm)12,13,14. In this study, RAP234 was calculated (see Appendix A1–A3), and data at selected stations are depicted in Fig. 1. High activity of RAP234 was commonly observed from low- to high-latitude oceans.

Figure 1
figure 1

Vertical profiles of RAP234 in the western Arctic Ocean (a), South China Sea (b), and Southern Ocean (c). High Activity of RAP234 was highlighted with brown circle.

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In the western Arctic Ocean (Fig. 1a), RAP234 did not vary with depth at deep stations (SR12 and SR15) with a mean activity of 1.95 ± 0.28 Bq/m3. However, RAP234 increased with depth on the continental shelf (e.g., SR3). The layers (SR3) can be divided into upper layers (2.08 ± 0.24 Bq/m3) and deep layers (4.92 ± 0.30 Bq/m3), which were separated by the halocline at a depth of 10m according to the salinity profile. RAP234 of the upper layers at SR3 was comparable with that of stations SR12 and SR15, while RAP234 of the deep layers at SR3 was significantly higher than that of SR12 and SR15.

This high RAP234 value could be qualitatively related to sediment resuspension and is used to determine the impact of sediment resuspension on export flux of 234Th without any additional sampling and measurement. The layer with a high RAP234 value must be screened out during the integration of 234Th and POC export fluxes, because sediment resuspension can bias 234Th and POC fluxes related to photosynthesis. The finer grain size of the surface sediment as well as intensive hydrodynamics were observed at station SR3 in shallow waters, which favoured sediment resuspension15,16. Additionally, the high particulate 210Pb and deficit of 234Th to 238U had also been attributed to sediment resuspension on the continental shelf of the Chukchi Sea10,17.

In the South China Sea (Fig. 1b), high activity of RAP234 was evident on the continental shelf (A6 and A7) outside the Pearl River Estuary during autumn. The average RAP234 activities for coastal stations (A6 and A7) and open ocean stations (A1 and A2) were 2.16 ± 0.37 Bq/m3 and 0.68 ± 0.19 Bq/m3, respectively. This region was significantly affected by monsoon winds, especially during autumn. Sediment resuspension was stimulated under the influence of strong winds and shallow depth on the continental shelf, which had previously been demonstrated via the activity ratio of particulate 234Th to dissolved 234Th8.

As for the Southern Ocean (Fig. 1c), sediment resuspension also occurred at coastal station (D2-4B) due to active hydrodynamics and shallow depths near Elephant Island. The ‘Island Effect’ had been demonstrated to be a significant process for providing iron from the sediment to stimulate primary production in this ‘High Nutrient Low Chlorophyll’ region18. The average RAP234 value at station D2-4B (2.27 ± 0.42 Bq/m3) was higher than that of D2-2 and D3-4 (1.46 ± 0.20 Bq/m3) in the open ocean, which could be used as a novel approach to identify sediment resuspension.

Mechanism of RAP234

The potential radionuclides associated with high RAP234 activity and suspended particles in the seawater can be classified into external and internal radionuclides. The external radionuclides associated with suspended particles refer to the surface-bound radionuclides with high particle reactivity. The radionuclides and their activities in natural seawater have previously been compiled19 and can be classified into low and high particle reactivity (Table 1) according to their particle-seawater distribution coefficient (Kd)20. A high Kd indicates high particle reactivity. Other artificial radionuclides with short half-lives are not considered due to a lack of nuclear facilities in our sampling region. Otherwise, radionuclides, such as 91Y, 152Eu, etc., should be taken into account when nuclear fuel reprocessing facilities are in operation near this sea area21.

Table 1 Typical activities of radionuclides in natural seawater.
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Although the components of suspended particles, including POC, lithogenic materials, biogenic inorganic materials, and hydrogenous materials, display distinct affinity for radionuclides22, the requirements for major external radionuclides are high activity in seawater and high Kd. The high activity of 234Th with high particle reactivity results in the direct measurement of surface-bound particulate 234Th without additional radiochemical separation. The β counts contributed by 210Pb/210Bi and other radionuclides with low activity and low energy β particles were shielded by a layer of Mylar film and two layers of aluminium foil (16 mg/cm2)23,24, to prevent external contributions to the β counting21. Therefore, the dominant external radionuclide associated with the suspended particles is 234Th after sample collection.

Surface-bound 234Th with a short half-life (24.1 days) on the suspended particles was unsupported by its parent radionuclides 238U, which remains in the seawater due to low Kd. RAP234 was measured over 120 days after sampling. This surface-bound 234Th on external particles will decay away. Consequently, the unsupported 234Th adsorbed on external particles should not contribute to RAP234.

In our study, only RAP234 derived from the second counting rate of particulate 234Th was investigated. Low activity and low Kd of radium in seawater lead to extremely low activity of radium for adsorption onto external suspended particles. Radium and its progenies, such as 224Ra25, should not contribute to RAP234 via surface adsorption. Additionally, to our knowledge, there is no tectonically active region on the continental shelf of the Chukchi Sea to provide high radium activity26. Therefore, the radionuclides with low Kd in seawater, such as radium, should not significantly contribute to RAP234.

The internal radionuclides of suspended particles were more complicate than the external radionuclides. The suspended particles can be classified into terrigenous and biogenic particles in order to analyse the internal radionuclides.

In the bottom nepheloid layer, terrigenous particles resuspended from marine sediment could reach 70%27. The concentration of suspended particles in the bottom layer of seawater reached values of up to 9.5 mg/L on the continental shelf of the western Arctic Ocean during the 5th Chinese National Arctic Research Expedition (CHINARE-5), which was significantly higher than that of the upper seawater and indicated the occurrence of sediment resuspension. High concentrations of suspended particle material were also observed for bottom seawater in the western Arctic Ocean10. The activity, emitting particle type with energy, and the yield for radionuclides in the marine sediment are presented in Table 2. The activities of some radionuclides are consistent with the limiting direct measurement in the Chukchi Sea28. Some α-particle-emitting radionuclides are also presented, because the daughter radionuclides supported by these α-particle-emitting radionuclides could contribute to the β count, such as 226Ra and its daughter radionuclides. Therefore, an exhaustive overview of the radionuclides in biogenic and terrigenous particles will benefit comprehensive understanding.

Table 2 Activity, emitting particle (with energy), and radionuclide yield in marine sediments59.
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The requirements of the major contributors to internal radionuclides include high activity, high energy of β particles, and high yield. 234mPa, the daughter radionuclide of 234Th, emits β particles with a maximum energy of 2.28 MeV (Table 2). The small-volume technique via β counting of 234Th is on the basis of 234mPa measurement. The lower energy β particles from other radionuclides were significantly shielded during source preparation with a layer of Mylar film and two layers of aluminium foil23.

234Th, the parent radionuclide of 234mPa, is supported by the primordial radionuclide 238U in the minerals derived from the crust. 238U in marine sediments was reported to have a mean activity of 50 Bq/kg in the Chukchi Sea28. Terrigenous particles from marine sediment via sediment resuspension can reach 70% in the bottom nepheloid layer27. It has also been reported that particulate 238U can even reach 95% of total 238U due to sediment resuspension29. Therefore, 234Th supported by 238U is probably the dominant contribution to internal radionuclides, especially on the shallow continental shelf with active hydrodynamics.

Although the β energy of 90Y was the same as 234mPa (Table 2), the activity of particulate 90Y was very low in seawater. The anthropogenic radionuclide of 90Sr, the parent radionuclide of 90Y, was mainly from global fallout in the Chukchi Sea. Although direct measurement for 90Sr had not, to our knowledge, been reported, a fingerprint activity ratio of 90Sr to 137Cs had a value of 0.63 for global fallout30. Combining the reported activity of 137Cs with an average value of 2.0 Bq/kg for surface sediment in the Chukchi Sea28, the activity of 90Sr was about 1.3 Bq/kg, which was less than 5% of that of 234mPa supported by 234Th/238U. Therefore, the β counting rate contributed by 90Y should be minor.

212Bi, the progeny of 228Th-232Th, has the β energy of 2.25 MeV, with a yield of 48.4%. The β energy of 212Bi is also close to that of 234mPa with similar detector efficiency. The activity of 232Th was generally comparable with that of 238U in the marine sediment derived from the crust31,32. High activity of 228Th in bottom layer seawater had previously been directly measured as a result of sediment resuspension in the Baltic Sea31. Therefore, 212Bi supported by 228Th-232Th should be considered when sediment resuspension occurs due to its high activity, β energy, and yield.

Although the existence of 40K, 226Ra, and 210Pb had been confirmed by γ spectrometry for bottom seawater in the northeast Atlantic Ocean33, these radionuclides and their progenies have lower β particle energies. Their contribution to β counting rate should be minimal due to the shielding effect of aluminium foil24.

Biogenic particles make a major contribution to suspended particles in the upper ocean when photosynthesis occurs. The biotas make preferential use of low atomic number elements, such as C, N, P, S and others. Many radionuclides with high atomic numbers are not essential elements for these biotas. Typical radionuclides found in marine biotas are shown in Table 3. The dominant radionuclide amongst the marine biotas is 40K, the activity of which is two orders of magnitude greater than that of the other radionuclides. The shield effect of aluminium foil during source preparation of particulate 234Th limits the contribution by 40K to RAP234 due to low β energy. Therefore, the activity of RAP234 was low in the euphotic layer due to a major fraction of suspended particles from photosynthesis.

Table 3 Typical activities of radionuclides in marine biotas (in Bq/kg-f.w.)59,60.
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The dominant radionuclides contributing to the high activity of RAP234 in bottom seawater are likely to be 234Th-238U and 212Bi-228Th in the particles resuspended from marine sediments. RAP234 was re-measured three times 120 days after the sampling date to check its stability. It indicated that RAP234 is constant due to the long half-lives of 238U and 228Th-232Th and the relatively enclosed environment of the crystal lattice in the mineral derived from marine sediments that constrain any deficit or ingrowth process of the daughter-parent radionuclides.

However, the exact percentage of 234Th-238U and 212Bi-228Th for RAP234 was not obtained in our study due to limitations on the volume of seawater available. Only 4–8 L of seawater was sampled on the continental shelf for 234Th analysis. The phenomenon, abnormally high activity of RAP234, was found during data analysis. Although the radionuclides of 234Th-238U and 212Bi-228Th could be checked by α-spectrometry and γ-spectrometry, a large amount of seawater (>100 L) should be sampled due to the lower detector efficiency of γ-spectrometry (<1%) and α-spectrometry (10%~20%) relative to that of β-counter in this study (47 ± 2%). Chemical recovery should also be considered via α-spectrometry. RAP234, which combines the dominant β signal from 234Th-238U and 212Bi-228Th, is sensitive enough to indicate sediment resuspension with small volume of seawater via β-counter with high detector efficiency.

The relative contributions of 234Th-238U and 212Bi-228Th should vary with distinct sea areas. The specific character of sediment and intensity of sediment resuspension will determine their relative contributions as well as activity of RAP234.

Seawater turbidity to indicate sediment resuspension

Seawater turbidity was measured at six stations ranging in location from the southern Chukchi Sea to the open Arctic Ocean during the CHINARE-6. Extremely high turbidity was observed within the bottom 10 m layer at stations SR1, SR3, SR5, and SR7 (Fig. 2a–d), which were located in the southern and central Chukchi Sea, suggesting that intensive sediment resuspension occurred near the bottom on the shallow continental shelf. This is expected because strong bottom currents in the Chukchi Sea are often observed in summer15,34. In contrast, seawater turbidity was almost invariable with depth at stations SR9 and R10 (Figs 2e and 4f), which were located on the northern shelf or in the open ocean. Neither weak currents nor great depth should favor sediment resuspension15,34.

Figure 2
figure 2

Seawater turbidity at stations SR1 (a), SR3 (b), SR5 (c), SR7 (d), SR9 (e), R10 (f).

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Figure 3: RAP234 versus POC in the western Arctic Ocean.
figure 3

The photosynthesis indicated by green arrow can elevate POC but lower RAP234. Resuspension is represented by brown arrow and characterized by high RAP234.

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Figure 4: Conceptual model of RAP234.
figure 4

The green and grey particles represent biogenic and resuspended particles, respectively. The yellow particles refer to the surface-bound 234Th on the particles, which decays away after 120 days. The half-yellow/half-red particles represent internal radionuclides with long half-life parent radionuclides, such as 234Th supported by 238U and 212Bi supported by 228Th.

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Long term time series of measurements for ocean currents and other parameters had been collected in the Chukchi Sea15,34. These observations indicate that there was an active interaction between the sediment and the seawater on the shallow continental shelf, especially during the ice-free summer season. This provides an environmental benefit for generating and sustaining sediment resuspension. An exceptionally strong summer cyclone was reported in early August, 2012 in the Chukchi Sea35. The strong mixing and upwelling caused by the cyclone resulted in a relatively well-mixed, vertically homogeneous water column on the continental shelf. Thus, the summer cyclone is another factor that is favourable to the generation of sediment resuspension. Therefore, it is reasonable to infer that sediment resuspension probably took place and redistributed RAP234 on the continental shelf.

Residence time of total 234Th to indicate sediment resuspension

The residence time of total 234Th was calculated and represented using an irreversible steady-state model (Appendix Table A1)36. Our results were consistent with other studies of this region37,38,39. The residence time of total 234Th on the continental shelf was significantly shorter than that in the open Arctic Ocean. The high nutrients waters supplied from the North Pacific Ocean can support high photosynthesis and scavenge 234Th on the continental shelf relating to nutrient depletion and low photosynthesis in the open ocean40,41.

The residence time of total 234Th for bottom seawater was shorter than that for upper-layer seawater, which had been attributed to sediment resuspension to enhance the scavenging of 234Th from seawater42. Therefore, the short residence time of total 234Th for the bottom layer also provided another clue to sediment resuspension on this shallow, but hydrodynamically active, continental shelf.

RAP234 and POC to indicate sediment resuspension

The relationship between RAP234 and POC was investigated in the western Arctic Ocean (Fig. 3). The slope of linear regression line between RAP234 and POC was about 0.160 Bq/mmol C for suspended particles. As for the end-member of sediment in the Chukchi Sea, the activity of 238U was about 50 Bq/kg28, while the activity of 232Th was generally comparable with that of 238U in the marine sediment32. The average concentration of POC was about 1% with a range of 0.5% to 2% in the marine sediment43. Thus, the sediment fingerprint is characterized by its ratio of 238U-234Th and 232Th-228Th to POC of 0.09 Bq/mmolC with a range of 0.045 to 0.18, which was consistent with the ratio of 0.16 for RAP234 to POC (Fig. 3). The linear regression between RAP234 and POC also gave a clue to sediment resuspension.

Conceptual model of RAP234

The conceptual model of RAP234 is illustrated in Fig. 4. Biogenic and terrigenous particles make the dominant contributions to suspended particles in the upper ocean and bottom nepheloid layer, respectively. Both kinds of particles can adsorb high particle-reactive radionuclides onto particle surfaces. In seawater, the dominant surface-bound radionuclide is 234Th. The external and unsupported 234Th adsorbed on biogenic and terrigenous particles decays away after 120 days and should not contribute to RAP234. Radionuclides with high atomic number are seldom taken up by biotas as essential elements. Thus, biogenic particles play a minor role in RAP234. The internal radionuclides of terrigenous particles, dominated by 234Th supported by 238U and 212Bi supported by 228Th, still exist and contribute to the second β counting rate of particulate 234Th after 120 days due to the long half-lives of 238U (4.47 × 109y) and 228Th-232Th (1.91y and 1.4 × 1010y) in the minerals. Both 228Th and 238U have been categorised as lithophile elements according to Goldschmidt’s classification44, which is analogous to aluminium, titanium and other lithophile elements to trace the terrigenous fraction45. Although 228Th and 238U were not measured directly in our study due to the limitation of seawater volume, both 228Th and 238U in resuspended particles had been directly measured and attributed to sediment resuspension in other studies29,31.

On the continental shelf, low activity of RAP234 in the upper layer and high value in the deep layer at SR3 can be interpreted as dominant photosynthesis and sediment resuspension, respectively. In comparison, low activity of RAP234 remained stable considering of its activity uncertainty at SR15 in the open ocean (Fig. 1a), while a peak value of POC was observed in the subsurface layer at a depth of 47 m (Appendix Table A1, 3.57 mmolC/m3 at SR15). Subsurface chlorophyll maximum had been widely observed in the Arctic Ocean due to the supplement of nutrients in the subsurface layer46. Although POC was variable due to heterogeneous photosynthesis, RAP234 was vertically uniform as a result of small contributions to RAP234 from biogenic particles.

Consequently, RAP234 refers to the terrigenous particles resuspended from marine sediment, which is probable to trace sediment resuspension with sufficient sensitivity via β counter. RAP234 could be a nice addition to the 234Th/238U disequilibrium and seawater turbidity methods to distinguish particle processes related to photosynthesis and sediment resuspension.

Advantages of RAP234

The relationship between RAP234 and POC was utilized to distinguish particle processes, including photosynthesis and sediment resuspension, in the western Arctic Ocean (Fig. 3). Biogenic particle were characterized by low RAP234 in addition to variable concentrations of POC, which depended on intensity of photosynthesis. In comparison, sediment resuspension can elevate RAP234. Therefore, sediment resuspension and photosynthesis could be distinguished with distinct RAP234, while seawater turbidity and 234Th/238U disequilibrium method could not differentiate sediment resuspension from photosynthesis. Additionally, the slope of linear regression between RAP234 and POC, ‘slope assumption’, has the potential to indicate the intensity of sediment resuspension (Fig. 3).

The linear regression between activity ratio of 234Th to 238U and POC (Fig. 5) was compared with that of RAP234 and POC (Fig. 3). The correlation coefficient of RAP234 and POC (0.815) is greater than that of 234Th/238U and POC (0.44). Both sediment resuspension and photosynthesis can enhance the scavenging of 234Th. It is difficult to distinguish these two processes via 234Th/238U method. However, RAP234 is directly related to the terrigenous fraction from sediment resuspension based on the conceptual model (Fig. 4). Additionally, the 234Th/238U disequilibrium method has a memory effect that records the integrated particle dynamics during the past several months47. Both RAP234 and POC are instantaneous parameters relative to the parameters of 234Th/238U disequilibrium method with memory effects. Therefore, a better regression result for RAP234 and POC was obtained compared with the 234Th/238U disequilibrium method.

Figure 5
figure 5

Activity ratio of 234Th to 238U versus POC in the western Arctic Ocean.

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RAP234 and its implications for export flux of 234Th

The 234Th/238U disequilibrium method reflects the integrated particle dynamics, including sediment resuspension and photosynthesis, on the shallow continental shelf. Sediment resuspension can enhance the scavenging of 234Th and deficit of 234Th to 238U, overestimating export flux of 234Th. From the conceptual model of RAP234, high activity of RAP234 was directly related to sediment resuspension. Sediment resuspension can be qualitatively identified on the basis of RAP234 to screen out the layer in which sediment resuspension occurred when export flux of 234Th was integrated into the shallow water column. However, export flux of 234Th may be underestimated following screening when photosynthesis occurs in conjunction with sediment resuspension.

Two endmembers, biogenic particles and resuspended particles, are assumed to exist in bottom seawater. The surface-bound concentrations of 234Th on biogenic and resuspended particles were assumed to be f1 and f2, respectively, in order to estimate the export fluxes of 234Th from these two kinds of particles. The exact values of f1 and f2 were determined by two factors: particle concentration and the adsorbing ability of the particles. Most of the time, the particle concentration could be quantified by chemical proxies with distinct values for biogenic and resuspended particles, such as δ13C, Al, Ti and others. Biogenic and resuspended particles have low and high activity of RAP234, respectively. Therefore, there is a potential to quantify the concentrations of biogenic and resuspended particles by mean of RAP234.

The adsorbing capacity of distinct particle compositions can be quantified by different values of Kd for thorium48,49,50. The particle compositions include lithogenic particles, opal, carbonate carbon, organic carbon, etc. If the Kd for thorium can be obtained for biogenic and resuspended particles, f1 and f2 can be calculated (Eqs 1 and 2).

where a and b represent the fraction of biogenic and resuspended particles derived from chemical proxies. TSP is the total suspended particles in the seawater (mg/L). Kd−bio. and Kd−res. are the particle-seawater distribution coefficients for biogenic and resuspended particles (L/kg), and AD234 is the dissolved activity of 234Th in seawater (Bq/m3). If f1 and f2 (Bq/m3) can be calculated, the export flux of 234Th (F234−bio.) derived from the biogenic process can be obtained:

Substituting for f1 and f2 gives

Therefore, the export flux of 234Th (F234−bio.) derived from biogenic process can be determined from the fraction of biogenic particles and Kd. In natural seawater, the fraction of biogenic particles, along with its uncertainty, can be quantified by chemical proxy. Large uncertainties in the fraction of particles occur, because the chemical proxies for endmember are generally difficult to identify. Although the Kd of thorium for distinct particle compositions had been derived under the laboratory conditions48,49,50, Kd−Bio. and Kd−Res. are difficult to obtain in natural seawater, especially when complex particle compositions co-occur in biogenic and resuspended particles. The accuracy of the particle fraction and Kd will constrain the exact estimation for export fluxes of 234Th derived from biogenic particles.

RAP234: a linkage of the atmosphere-ocean-sediment system

To validate the ‘slope assumption’, the A transect was revisited to investigate RAP234 and POC in the South China Sea during spring and autumn. The slope of linear regression between RAP234 and POC in Fig. 6 was also greater in autumn (0.30) than in spring (0.19), which may be attributed to sediment resuspension. Sediment resuspension could increase the terrigenous fraction and elevate RAP234. The intensity of sediment resuspension had been indicated to be high in the same sea region in autumn relative to spring via the ratio of particulate 234Th to dissolved 234Th under the influence of the East Asian monsoon system8. Therefore, the assumption of the slope of linear regression between RAP234 and POC is confirmed in the South China Sea. RAP234 will shed new light on 234Th-based particle dynamics to investigate the linkage of the atmosphere-ocean-sediment system, such as the typhoons and their impacts on sediment.

Figure 6
figure 6

Regression analysis of RAP234 and POC in the South China Sea during spring and autumn.

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A novel approach of RAP234 is proposed for the first time to trace sediment resuspension from low- to high-latitude oceans. High activity of RAP234 was widely observed on the continental shelf in relation to sediment resuspension (Fig. 1). Sediment resuspension was also corroborated by seawater turbidity, residence time of total 234Th, Goldschmidt’s classification, and fingerprint ratio of RAP234 to POC from the sediment endmember in the western Arctic Ocean. The mechanism and conceptual model of RAP234 was investigated and illustrated (Fig. 4.). RAP234 is sufficiently sensitive to identify sediment resuspension via β counter with high detector efficiency. The advantage of RAP234 is that it is a supplementary parameter to the 234Th/238U disequilibrium method and does not require any additional sampling and measurement to distinguish sediment resuspension from photosynthesis, while both the 234Th/238U disequilibrium and seawater turbidity methods cannot differentiate biogenic particles from terrigenous particles. RAP234 is a potential proxy to trace sediment resuspension without a memory effect. RAP234 could also be used to screen out the layer to bias integration of 234Th and POC fluxes. The slope of the linear regression between RAP234 and POC was used to indicate the higher intensity of sediment resuspension in the South China Sea during autumn. Similar to the definition of gross β, RAP234 may stimulate some debate but is also meaningful to identify and indicate the intensity of sediment resuspension. From the mechanism proposed, RAP234 refers to the terrigenous fraction and has potentially broad implications for investigating the dynamics of suspended particles in the estuary-shelf-slope-ocean continuum and the linkage of the atmosphere-ocean-sediment system.

Methods

Sampling

Seawater samples were collected for 234Th analysis from low- to high-latitude ocean in the western Arctic Ocean, the South China Sea, and the Southern Ocean (Fig. 7). Seven stations (SR1, SR3, SR5, SR7, SR9, SR12, SR15) were sampled in the western Arctic Ocean during the 5th Chinese National Arctic Research Expedition (CHINARE-5) in September, 2012 (Fig. 7b.). The sea ice extent during the sampling period was the lowest since the first satellite measurement taken in 197951. Seawater turbidity was measured and was indicated by red stars on the continental shelf (SR1, SR3, SR5, SR7, SR9) and the open ocean (R10) (Fig. 7b).

Figure 7: Station map for the western Arctic Ocean (1), the South China Sea (2), and the Southern Ocean (3).
figure 7

These maps were drawn by ODV 4.7.4 (https://odv.awi.de/)58.

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A transect (six stations) was taken from the continental shelf to the open ocean outside the mouth of the Pearl River in the northern South China Sea during 2–8 November, 2010 (autumn) and 16–18 May, 2011 (spring)8 (Fig. 7c). Stations A7, A6, and A5 were on the continental shelf (depth < 100 m). Three stations were analysed around Elephant Island, off the north-eastern Antarctic Peninsula on 22–25 January, 2012 during the 28th CHINARE-Antarctic (Fig. 7d). Only station D2-4B was near coast of Elephant Island with the depth of 53 m. Two stations (D2-2 and D3-4) were in the open ocean with the depth over 3000 m.

Analysis of 234Th

The 234Th/238U disequilibrium method has been widely applied in the global ocean with a huge database to quantify the marine biological carbon pump52, which modulates glacial/interglacial atmospheric carbon dioxide and climate change53. The international calibration of 234Th was conducted under the framework of GEOTRACES54. The small-volume technique via β counting of 234Th has been extensively studied due to its high sampling resolution13,23. The radiochemical analysis of 234Th had been described8,37.

Following filtration of seawater with 25-mm diameter Quartz Microfiber (QMA, nominal pore size 1.0 μm), the direct measurement of particulate 234Th without radiochemical separation was obtained from the difference in values between the first β counting after sampling and the second β counting after 120 days as a result of high activity of 234Th in seawater19. After treating with MnO2 co-precipitation, the activity of total 234Th was also calculated from the difference between the first and second β counting rates of total 234Th. The activity of 234Th and its associated uncertainty were calculated according to Eqs 4, 5, 6, 7, 8, 9.

The dimensions and definitions of the parameters are given in Table 4. Equations 4, 5, 6, 7, 8, 9 had been deduced in detail with the similar principle55.

Table 4 List of parameters with dimensions and definitions.
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Definition and calculation of RAP234

The second counting rate of particulate 234Th (nP2) was usually overlooked, because only the difference between the first and second counting rates (nP1-nP2) was used to calculate particulate 234Th using Eq. 4. In the open ocean, the second counting rate of particulate 234Th (nP2) was relatively stable with a value of 0.3~0.4 cpm, which also depends on the instrumental background with a normal value of 0.15~0.2 cpm via gas-flow proportional low-level RISØ β-counter (Model GM-25-5, RISØ National Laboratory, Denmark)12,13,14. In this study, the abnormally high second counting rate of particulate 234Th was observed for bottom seawater on the continental shelf in the western Arctic Ocean. This phenomenon was further confirmed in the South China Sea and the Southern Ocean. RAP234 derived from the second counting rate of particulate 234Th and instrumental background was proposed for the first time to investigate this abnormal value of particulate 234Th. Equations 10 and 11 were used to calculate RAP234 and its uncertainty:

All the parameters in Eqs 10 and 11 are defined in Table 4. The detector efficiency of RAP234 is equal to that of particulate 234Th because of the similar energy of β particles being emitting by radionuclide candidates. The second counting rate of particulate 234Th (0.54 ± 0.02 cpm) was very constant after 136 days, 304 days, and 495 days from the sampling date, which indicates that it was mainly radionuclides with long half-lives that contributed to RAP234. The stability of the second counting rate of particulate 234Th has been demonstrated24.

Notice that RAP234 was not a signal from a certain radionuclide. In fact, RAP234 was the residual β activity for particulate 234Th after more than 120 days, which was usually recognized to be the stable methodological background for particulate 234Th and was therefore neglected. This residual β activity may include several radionuclides with long half-lives. It should be treated as a supplementary parameter for total 234Th and particulate 234Th and has advantage of being able to trace sediment resuspension without any additional sampling and analysis based on the small-volume technique for 234Th.

The definition of RAP234 is similar to that of gross β in drinking water. Most of the time, the exact radionuclides and their contributions to gross β cannot be identified11. However, gross β is an essential parameter for screening the level of radiological pollution, especially during nuclear emergency. The detector efficiencies of 90Sr and 137Cs are artificially chosen to calculate that of gross β for drinking water, although a spread of energies of β particles from distinct radionuclides (40K) with different detector efficiencies is very common11. Analogously, the definition of RAP234 is proposed and is convenient for tracing sediment resuspension without any additional sampling and measurement.

Although the abnormally high second counting rate of total 234Th was also observed for the bottom seawater on the continental shelf as that of particulate 234Th, the second counting rate of total 234Th was not discussed in this study. The radionuclides contributing to the second counting rate of total 234Th are more complex than that of particulate 234Th due to the additional MnO2 co-precipitation. The radiochemical treatment of MnO2 co-precipitation for total 234Th can scavenge other radionuclides of low Kd with variable chemical recovery, such as radium and its progenies, onto the MnO2-particle surface13,56, especially for the tectonically active sea region with 224Ra diffusion into the overlying seawater25.

Particulate organic carbon

Following the second counting of particulate 234Th, the POC was measured with an Elemental Analyzer (Elementar vario EL III) after removing the carbonate fraction by fuming with concentrated hydrochloric acid57. The blank of the method was subtracted. The analytical precision was always better than 10%.

Seawater turbidity

The seawater turbidity was measured using a turbidity sensor (Rinko-profiler) during the 6th CHINARE from 27 July to 7 August 2014. The turbidity sensor works on the basis of backscattering principle and has a range of 0~1 FTU. The reference material was Formazin. A few abnormal values over 1 FTU arising from the present of bubbles were discarded. The precision of the turbidity sensor was 0.03 FTU.

Additional Information

How to cite this article: Lin, W. et al. Residual β activity of particulate 234Th as a novel proxy for tracking sediment resuspension in the ocean. Sci. Rep. 6, 27069; doi: 10.1038/srep27069 (2016).