Zhang et al.
Progress in Earth and Planetary Science(2023) 10:65
https://doi.org/10.1186/s40645-023-00600-z

Progress in Earth andPlanetary Science

Open Access

METHODOLOGY

Improvements in the determinationof attogram‑sized 231Pa in dissolved
and particulate fractions of seawater
via multi‑collector inductively coupled plasma
mass spectrometry
Pu Zhang1,2,3*†, Yanbin Lu2†, Zhe Zhang4, Richard Lawrence Edwards2, Robert Anderson5,6 and Phoebe Lam7

Abstract
A technique is developed to quantify the ultra-trace 231Pa (35–3904 ag) concentration in seawater using multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS). The method is a modification of the processdeveloped by Shen et al. (Anal Chem 75(5):1075–1079, 2003. https://​doi.​org/​10.​1021/​ac026​247r) and extends itto the application of very low levels of actinides, and the 35 ag 231Pa can be measured with a precision of 15%. The
total process blank for the water column was 0.02 ag/g, while the values of the large and small particles were ~ 30
ag/g. The ionization efficiency (ions generated/atom loaded) varies from 0.7 to 2.4%. The measurement time is 2–5
min. The amount of 231Pa needed to produce 231Pa data with an uncertainty of ± 0.8–15% is 35–3904 ag (~ 0.9 × ­105
to 10 × ­106 atoms). Replicate measurements of known standards and seawater samples demonstrate that the analytical precision approximates that expected from counting statistics, and that based on detection limits of 52 ag, 55 ag,
and 28 ag, protactinium can be detected in a minimum seawater sample size of ~ 2.6 L for small suspended particulate matter (> 0.8 μm and < 51 μm), ~ 3.0 L for large suspended particulate matter (> 51 μm), and ~ 56 mL for filtered(< 0.45 μm) seawater. The concentration of 231Pa (several attograms per liter) can be determined with an uncertainty
of ± 2–8% (2σ) for suspended particulate matter filtered from ~ 60 L of seawater. For the dissolved fraction, ~ 1 Lof seawater yields 231Pa measurements with a precision of 0.8–10%. The sample size requirements are several ordersof magnitude less than traditional decay-counting techniques, and the precision is better than that previously
reported for ICP-MS techniques. Our technique can also be applied to other environmental samples, including river,lake, and cave water samples.
Keywords 231Pa, MC-ICP-MS, Suspended particulate matter, Precision, Reproducibility†

Pu Zhang and Yanbin Lu have contributed equally to this work and share firstauthorship.
*Correspondence:
Pu Zhangzhangpu035@cdut.edu.cn
Full list of author information is available at the end of the article© The Author(s) 2023. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which
permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to theoriginal author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images orother third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line
to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory
regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of thislicence, visit http://creativecommons.org/licenses/by/4.0/.

Zhang et al. Progress in Earth and Planetary Science(2023) 10:65

1 Introduction
231
Pa is the only long-lived intermediate daughter product of 235U (t1/2 = 32,760 years) (Robert et al. 1969) andhas been broadly applied as a geochemical tracer in fields
as diverse as oceanography, geochronology, and paleoclimatology (Anderson et al. 1983; Asmerom et al. 2000;
Bradtmiller et al. 2014; Edmonds et al. 1998; Edwards
et al. 1997; Hayes et al. 2014, 2015; Hoffmann et al. 2013;
Moran et al. 1997). It is noteworthy that the concentrations of 231Pa and 230Th in seawater and sediments, aswell as their isotopic ratios, can elucidate the representative processes responsible for the removal of particle-reactive elements from the ocean (Anderson et al.
1983; Chase et al. 2003; Hayes et al. 2015; Kretschmer
et al. 2011; Lao et al. 1993; Luo and Ku 1999, 2004; Siddall et al. 2005; Walter et al. 1997, 2001). The 231Pa and230
Th in seawater not only contribute to the clarificationof the mechanisms of global ocean circulation processes
but also deepen our understanding of the paleoenvironmental record, including processes such as boundary
scavenging (Anderson et al. 1983; Hayes et al. 2014, 2015;
Kretschmer et al. 2011; Moran et al. 2001, 2002) and ventilation of the modern (Deng et al. 2014; Edmonds et al.
1998; Thomas et al. 2006) and last-glacial (Bradtmiller
et al. 2014; Thomas et al. 2007) periods in the Atlantic,
Pacific, Indian, and Arctic oceans.
231
Pa has an extremely low natural abundance in themarine environment, leading to uncertainty in its measurement. In surface seawater, the dissolved fraction of231
Pa is ~ 500 ag/L, while the particulate fraction is only
10 ag/L (Deng et al. 2014; Edmonds et al. 1998; Hayes
et al. 2014; Moran et al. 2001, 2002; Thomas et al. 2006).
There are differences in the precisions and the required
sample sizes of the different techniques. For example, thetraditional a and ß counting techniques typically yield aprecision of ± 9–28% (2σ) for the determination of 231Pa(100–4000 fg) (Anderson and Fleer 1982). In addition,these methods require larger sample sizes (~ 75 large
samples per measurement batch) and longer counting
times (Anderson and Fleer 1982; Anderson et al. 1983;
Nozaki and Nakanishi 1985). Thermal ionization mass
spectrometry (TIMS) was first applied by Pickett et al.
(1994) to perform Pa measurements with a precision of
better than ± 1% for sample sizes of hundreds of femtograms of 231Pa. Subsequently, Shen et al. (2003) used
TIMS with a precision of ± 4–12% (2σ) for samples containing 100–1000 ag 231Pa (~ 3 × ­105 to 3 × ­106 atoms).
TIMS has been widely used for the determination of231
Pa in different natural materials, such as carbonates,
silicates, and seawater (Edmonds et al. 1998; Edwards
et al. 1997; Pickett and Murrell 1997). In addition, inductively coupled plasma mass spectrometry (ICP-MS) has
also been used for the measurement of 231Pa in bothPage 2 of 11

dissolved and particulate forms throughout ocean water
profiles. This method improves the detection limits andsample size requirements and has a precision of ~  ± 2%
for 100–200 fg seawater samples (Choi et al. 2001). In
contrast to the above techniques, a multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS)
technique with lower detection limits and a higher precision is proposed in this study. This technique can beapplied to various special environments, for example, thedetection of small (0.8–51 μm) and large (> 51 μm) particles and surface dissolved samples with low 231Pa concentrations in the Arctic with a better precision. The divisionof the particle size is of great oceanographic importance(Hayes et al. 2015). It has also been shown that scavenging intensity for the natural radionuclides 230Th and 231Pa
can vary with particle type in ocean. Changing proportions of particle types over geologic time may overprint
other influences on the sediment record. While previous
work has shown that there is still debate about what are
the major carrier phases. The research result of Hayes
et al. (2015) suggests that a negligible fraction of Pa or
Th exists in the 0.45–0.8 μm size class. Most of Pa andTh exist in particle of size 0.8–51 μm size. The particle
collected in ocean includes lithogenic material, particulate organic matter, calcium carbonate, biogenic opal
and authigenic Fe and Mn oxides. And absorbed 230Th ismainly originating from decay of 234U in dissolved seawater namely < 0.45 μm. Absorbed 230Th originating from
dissolution of U-bearing minerals usually exists small
particulate about 0.8–51 μm. Particulate 230Th originating from decay of 234U within intact mineral lattices
usually exists large particulate (> 51 μm). This detailed
classification of particle size is fully linked to the differences in the chemical behavior of the 231Pa and 230Th
being measured.
(1) Samples passing through 0.8 μm filters contain colloidal U and dissolved U, and a negligible fraction of Pa
or Th exists in the 0.45–0.8 μm size class. In the watercolumn, Th has a tetravalent oxidation state with a highparticle adsorption that is consistent with Pa, and the Pa
in the water column is readily absorbed onto suspended
particulate matter. (2) Samples between the 0.8 μm and51 μm filters contain small particles, which usually occur
in the convergence, adsorption, and reconvergence processes in the water column due to the slow settling
velocity. (3) Samples passing through 51 μm filters usually exist as large particle and usually have a fast settling
velocity, indicating that the detrital component in thewater column is less affected before sinking to sediments
and the sizes of the particles have with oceanographic
significance. Among them, the Arctic surface dissolved
fraction contains ~ 70 ag/L, and the ice hole contains ~ 30
ag/L. In addition, there are differences in the particulateZhang et al. Progress in Earth and Planetary Science(2023) 10:65

fraction of 231Pa at different depths in the same environment (Additional file 1: Figs. S1 and S2). The small particulate fraction of 231Pa in the Pacific surface seawater
is only 3 ag/L ­(10–21–10–20 molar or 1
­ 03–104 atoms/L),
while the large particulate fraction at a depth of ~ 1000 m
is 15 ag/L. The particulates analyzed are mainly inorganic
particles with a limited organic effect. The concentrations of the elements in the dissolved fraction cannot bemeasured via ICP-MS due to the high detection limitsof several ppb and ppt. All of the samples measured inthis study were at the ag and fg levels. The 230Th and 231Pa
concentrations of the particulates with sub-fg and subpg concentrations also cannot be measured via ICP-MS
or CHNS analysis because of the high detection limits ofseveral ppb and ppt. All measured samples here are allag and fg levels. 230Th and 231Pa of particulate with subfg and sub-pg also cannot be measured by ICP-MS or
CHNS analysis because of high detection limits getting
to ppb and ppt. In conclusion, the quantification of ultratrace 231Pa (35–3904 ag) concentrations in seawater via
MC-ICP-MS is available for the measurement of samplesof several orders of magnitude at the ag and fg levels, andthis extension of the determination of very low 231Pa concentrations is both novel and challenging.
The lowest value previously reported for the blanks
was 16 ± 15 ag (Shen et al. 2003), and the detection
limits were 38 ag for particulate matter and 49 ag forthe dissolved fraction. In this study, we mainly modified the method proposed by Shen et al. (2003), and themain contribution of this study is the characterizationand testing of this method for very small protactinium
loads in the dissolved fractions of seawater for small
and large particulates (Fig. 1). The signal/noise ratio
was improved due to the high ionization efficiency andPage 3 of 11

fast analysis of a large-quantity samples with a limited
analysis time in a case producing more 233U decayedfrom 233Pa (decay constant λ223Pa = 0.0256486 ­d−1) foranalysis with a slow time. However, the application of
MC-ICP-MS to the analysis of the seawater particulate
and dissolved fractions containing as little as tens of
attograms of Pa still remains challenges. (1) The procedural blank should be kept at as a low level as possible. (2) A high ionization efficiency must be maintained(e.g., ~ 0.7–2.4%). (3) Measurements with small ion
beams must be precise and accurate. Among the othercharacteristics, a low detector dark noise is required.
With the introduction of 233Pa (t1/2 = 27.0 days; Joneset al. 1986) as a spike in MC-ICP-MS analysis, the darknoise inevitably increases. Due to the decay of 233Pa to
233
U in a short time range, the dark noise of the detector increases from 70 to 143 cpm in 10 h. When thesamples have been measured, we increase the machinesensitivity (ionization efficiency get to from 0.7 to 2.4%)
to obtain a high signal/noise ratio when we measuresample in a limited time from 2 to 5 min/per sample.
TIMS analysis of 233Pa is slow, usually ~ 1 h/per sample, which is not good for a large quantity of samples
per batch, because of the short decay time from 233Pa
to 233U in the entire analysis period. 4) A high sample
throughput is required to obtain large-scale ocean 231Pa
data. The analysis of ultra-trace 231Pa in the dissolved
seawater and particulate fractions requires the methodwith small sample sizes, short measurement times, alow procedural blank, a high instrumental sensitivity,
and a high signal/noise ratio. In this study, we mainly
utilized MC-ICP-MS to determine the concentration ofultra-trace 231Pa (35–3904 ag) in seawater. This method
is feasible and has a better sample size, precision, andaccuracy than previously reported ICP-MS techniques.

Fig. 1 a Natural processes of 231Pa in seawater and b procedures for the determination of 231Pa concentration via the modified MC-ICP-MS
technique developed in this study

Zhang et al. Progress in Earth and Planetary Science(2023) 10:65

2 Experimental methods
2.1 Reagents and materials

This study was performed in the clean room at the Minnesota Isotope Laboratory (MIL) for chemical procedure analysis. A Super-Q purification system was used
to purify the deionized/distilled water, and the bottles
and beakers were cleaned in an acid vat (Edwards 1988).
The ultrapure reagents were obtained from Aristar Ultra
Chemicals, including HCl, HF, ­HClO4, and ­NH4OH. The
redistilled ­HNO3 was obtained from GFS Chemicals Inc.
First, 200 g of anion-exchange resin (Spectrum SpectraGel, AG 1-X8, 100–200 mesh) was filled into a quartz
column and was sequentially cleaned using the following cleaning steps: (1) 250 mL of ­H2O, 250 mL of 14 M
­HNO3; (2) 500 mL of ­H2O; (3) 250 mL of ­H2O, 250 mL of
14 M HCl; (4) 500 mL of ­H2O; (5) 490 mL of H
­ 2O, 10 mL
of HCl; and (6) 500 mL of ­H2O. The columns used forthe ion separation were constructed of heat-shrink Teflon tubing (4:1, ½ in.) from Texloc Ltd. and were cleaned
sequentially via boiling in aqua regia, AR ­HNO3 (7 M),
AR HCl (5 M), distilled H­ NO3 (0.1 M), and Aristar Ultra
HCl (0.1 M).
223
2.2 
Pa spike solution preparation

Our modification of the 237Np milking method describedby Shen et al. (2003) was to obtain the internal standard(233Pa), which is described below. The measurement conditions and improved separation methods for MC-ICPMS are presented in Additional file 1 (Tables S1 and S2).
(1) The solution in the stored Np vial was dried, andfive drops of ­HClO4 were added to the 237Np solution.
(2) The solution was dried, redissolved using 1 column
volume (cv) of 9 M HCl, and passed through an 850 μL
ion-exchange column, and the eluant was collected in
the Np vial. (3) The Pa fraction was eluted twice via theaddition of 2 CVs of 9 M HCl + 0.05 M HF to the Pa vial.
The collected Pa solution was dried using three drops of­HClO4. (4) The Np fraction was eluted into the Np vial byadding 8 CVs of 1 M HCl + 0.5 M HF. (5) The collected
Pa fraction was dissolved using 1 cv of 9 M HCl + 0.05 M
HF and was loaded into column 2. (6) Steps (3), (4), and(5) were repeated 10 times in columns 2 and 3 to purify
the Pa from the Np. (7) The collected Pa fraction was dissolved using 1 cv of 9 M HCl and was loaded into column
4. The Th fraction was eluted twice via the addition of 2
CVs of 9 M HCl to the Np vial. (8) The Pa fraction was
eluted twice via the addition of 2 CVs of 9 M HCl + 0.05
M HF to the Pa vial. The collected Pa solution was driedusing three drops of ­HClO4. (9) Steps (7) and (8) wereconducted one time in column 5 to further purify the Pa
from Th interference. After collecting the last Pa fraction,
the solution was dried and redissolved using five drops

Page 4 of 11

of ­HNO3. In a clean room, the Pa solution was diluted
to ~ 100 mL using 1 M ­HNO3 + 0.05 M HF and at heated
at 70 °C overnight to equilibrate the spike solution.
2.3 Standards and samples

The 231Pa solutions (Pa-I, 3.04 × ­109 atoms/g; Pa-II,
4.09 × ­109 atoms/g) were separated from the uranium ore
solutions. The 231Pa was calibrated using a 233Pa tracer
solution, which was calibrated with a 233Pa tracer solution, which was calibrated with a solution of a zircon
from the Piper Gulch Granodiorite that records concordant U-Pa ages (Asmerom et al. 1990) and may bein secular equilibrium with 234U and 230Th (Cheng et al.
2000). Interlaboratory calibration between the Minnesota Isotope Lab (based on the same mixed 231Pa–230Th–
232Th) and Lamont Laboratory demonstrated that theindependent calibrations were reproducible to better that
0.2% (Edwards et al. 1997). A more dilute standard (PaIII, 1.85 × ­109 atoms/g or 710 fg/g) was prepared via dilution of an aliquot of Pa-I.
The particulate fraction of seawater samples (~ 500 L)
collected from three depths (25–5000 m) in the Pacific
was used for the replicate analyses. An additional 25 seawater samples (including dissolved samples collected in
the Arctic during Geotrace 2015 and small and large particulate fractions collected in the Pacific during Geotrace2013) were filtered through an acid-cleaned 51 μm poresize Sefar polyester mesh prefilter and a 0.8 μm pore-size
Pall Supor 800 polyethersulfone filter, and approximately
500 L of seawater was filtered to collect the particulate
fractions. The filtered seawater samples were stored andacidified (20 mL of 6 M HCl) in acid-cleaned polyethylene bottles.
2.4 Chemical analysis

The chemical procedures were optimized according to
the methods used in previous studies, and Th samples
were also collected. The measurement time has been
2–5 min according to the sample content based on thesignal/noise ratio, usually the signal of 233Pa is 600 cps.
When we measured a 1 L sample, the 231Pa content of thesample varied from 35 to 4000 ag. The dissolved fractionsof the seawater samples were acidified (pH of ~ 2) using 6
M HCl, 30–80 fg of 233Pa tracer was added, and 450 mg of
Fe was added to 1 L of dissolved sample. To ensure sample-spike equilibrium, the solution was shaken by hands
for 4 days, and then, ­NH4OH was added to the solution
to co-precipitate Pa with Fe (OH)3 (pH of 8–9 for thissolution). The concentration of 231Pa (35–3904 ag) in seawater is an ultra-trace level, and the background value of231
Pa is very low. A large number of samples (~ 56) wereanalyzed in the first batch and the other 28 samples wereanalyzed in the second batch to avoid contamination,

Zhang et al. Progress in Earth and Planetary Science(2023) 10:65

and more magnetic stirrers were used at the same time.
We preferred the hand-shaking method to the magnetic
stirrer method. Hand shaking was conducted for 2 days
during the experimental phase. Each sample was shaken
hourly and eight times per day to ensure adequate mixingof the solution, diluent, and ­FeCl2. The sample was thenleft to stand for 2 days in order to achieve effective flocculation and precipitation of Fe(OH)3 using N­ H3·H2O.
The overall chemical procedure yield was determined bymeasuring the 233Pa content of the diluent, and the yieldconsistently exceeded 90%. The replicate sample resultsof every batch were accurate within the error, and theresults of the surface water (SW) standards STD 2010-1
and STD 2015-1 for every batch with the same column
chemistry were also consistent within error. This precipitate was allowed to settle for 1–2 days before theoverlaying seawater was siphoned off. Then, the Fe precipitate was transferred to a centrifuge tube for centrifugation and as rinsed with deionized H­ 2O (> 18 MΩ) toremove the major ions from the seawater. The precipitate
was then dissolved in 14 M H­ NO3 and transferred to a
Teflon beaker. It was then dried and subjected to anionexchange chromatography in 7 M H­ NO3 using AG1-X8,
100–200 mesh-size resin, and a polyethylene frit. The initial separation was performed in Teflon columns with a
0.75 mL column volume. The sample was loaded in one
CV of 7 M ­HNO3, followed by the addition of 1.5 CVsof 7 M H­ NO3, 3 CVs of 8 M HCl (to collect the Th fraction), and 3 CVs of 8 M HCl + 0.015 M HF (collect Pa
fraction). The Pa and Th fractions were then dried downin the presence of two drops of ­HClO4 and dissolved in
7MH­ NO3. They were each passed through the second
and third columns (each with a 0.5 mL column volume)using similar elution schemes. The final Pa and Th fractions were then dried down in the presence of two dropsof ­HClO4 and were dissolved in 2% ­HNO3 + 0.01% HF for
analysis on the mass spectrometer.
The filtered particulate fraction was analyzed as follows: 1/8 of a filter was placed in a 30 mL Teflon beaker,
and the aliquots of the artificial isotope yield monitors of 229Th and 233Pa were weighed. The filters werecompletely submerged in 7 M ­HNO3 combined with 10
drops of HF, were tightly capped with a Teflon threaded
cap, and were heated for 10 h at ~ 93.3 °C (200°F) todissolve/leach the particulate samples under pressure.
The leached solution was then transferred to a second
acid-cleaned Teflon beaker and was separated from theremaining filter, and then, five drops of ­HClO4 wereadded to the leached solution in the second beaker. The
original beaker walls and caps were washed with small
amounts of diluted ­HNO3 for low concentration ~ 10%
and the resulting solution was added to the second
beaker. The solution was then dried down, dissolved inPage 5 of 11

2 M HCl, and transferred to a 15 mL centrifuge tube,
along with the 2 M HCl rinse solution. One drop of dissolved Fe and six to nine drops of ­NH4OH were added
to raise the pH to 8–8.5, at which time the iron (oxy)hydroxide precipitated. This precipitate was then centrifuged, decanted, washed with deionized ­H2O (> 18MΩ), centrifuged, dissolved in 14 M ­HNO3, and transferred to a Teflon beaker. It was then dried down anddissolved in 7 M H­ NO3 for anion-exchange chromatography using AG1-X8, 100–200 mesh resin, and apolyethylene frit. The initial separation was performed
using Teflon columns (internal diameter of ~ 0.35 cm)
with a ~ 0.55 mL column volume. The sample was
loaded in one CV of 7 M ­HNO3, followed by 1.5 CVsof 7 M ­HNO3, 3 CVs of 8 M HCl (collection of Th fraction), and 3 CVs of 8 M HCl + 0.015 M HF (collectionof Pa fraction). The Pa and Th fractions were then dried
down in the presence of two drops of ­HClO4 and weredissolved in 7 M ­HNO3. They were each passed throughthe second and third columns (each with a column volume of ~ 0.55 mL) using similar elution schemes. The
final Pa and Th fractions were then dried down in thepresence of two drops of H
­ ClO4 and were dissolved in
2% ­HNO3 + 0.01% HF for mass spectrometer analysis.
The chemical recoveries were typically 90–95% for the
dissolved and particulate fractions.
The concentrations of 232Th, 230Th, and 231Pa were
calculated via isotope dilution using nuclide ratios
determined on a Thermo-Finnigan Neptune mass spectrometer. All of the measurements were conducted using
a peak jumping routine in ion counting mode on thediscrete dynode multiplier after the retarding potential
quadrupole. A total of 50–250 sets of data were measured for each sample. Since the centering of peaks could
not be easily verified with small ion beams for 231Pa and233
Pa, an additional mixed 231Pa–233Pa standard wasused for this purpose. A solution of 233U–236U tracer was
analyzed to determine the mass deviation correction(assuming that the mass fractionation for Th and Pa was
the same as that for U). Each sample measurement was
bracketed by measurements of an aliquot of the run solution in order to correct for the influence of the instrument background count rates on the masses measured.
The analyses (MC-ICP-MS) were conducted within 24 hof the chemical processing, and the study revealed thatthe small amount of 233U produced via the decay of 233Pa
during this interval evaporated prior to the start of thedata acquisition to diminish this isobaric interference,i.e., to minimize the decay of 233Pa to 233U in the Pa cut(Thomas et al. 2006). Throughout the analysis, the multiplier dark noise gradually increased from an initial valueof < 1.0 counts per second (cps) to 1.2–1.4 cps due to theintroduction of the short-lived 233Pa radionuclide. This

Zhang et al. Progress in Earth and Planetary Science(2023) 10:65

background dark noise was tracked between samples,and a correction was applied during the data processing.
An aliquot of two intercalibrated working standard
solutions of 232Th, 230Th, and 231Pa, SW STD 2010-1 andSTD 2015-1, were added to a separate acid-cleaned Teflon beaker along with weighed aliquots of the 229Th spikeand 233Pa spike. The spike and standard were equilibrated
for 3 days. They were then dried down and dissolved in
7 M ­HNO3 for anion-exchange chromatography using
AG1-X8, 100–200 mesh-size resin, and a polyethylene
frit, and they were also processed using the sample procedure used for the samples. The procedural blanks forthe chemical and mass spectrometric analyses conductedin the Minnesota lab were approximately 28 ag for 230Th
and 18 ag for 231Pa.
An improved and more consistent ionization efficiencyof 0.7–2.4% was used for the sample measurement,
which was better than our previous work (ionization
efficiency of 3–5‰) (Shen et al. 2003). Additional advantages include a reduced measurement time, from ~ 1 h to
2–5 min, and an increased ion beam intensity. The typical beam intensities for 231Pa and 233Pa were 8–470 cps
and 800–5000 cps, respectively, and thus, the signal/darknoise ratio was increased by a factor of 3–6.
2.5 Data processing

We completed the data processing offline, and the measurements were calibrated sequentially, including massfractionation correction, instrumental tail correction,
diluent factor adjustment, blank subtraction, and internal growth correction for 231Pa and 230Th. The uncertainties were calculated to be at the 2σ level, representing
the maximum of the counting statistics. The corrections
were fully propagated via mass fraction correction, darknoise, abundant sensitivity, blanks, and spikes. The abundance sensitivity was 2.6 × ­10–7 at a one atomic mass unit(amu) difference and 5.2 × ­10–8 at a 2 amu difference after
the RPQ filter. Radioactive decay corrections were performed for the 233Pa in the spike solution between calibration and analysis time.

3 Results and discussion
3.1 Blank and limit of detection

After 11 separation and purification step repetitions of
the 233Pa spike from the 237Np solution, the 231Pa was nolonger measurable (231Pa/233Pa < 7 ppm) and the amountof 231Pa in each sample was negligible. The total 231Pa
blank from the chemicals (including the spikes, resins,
iron, and acids) in the dissolved sample was 0.02 ± 0.006
ag/g (Table 1). The total procedural blanks, including filtration, were 34 ± 6 ag/g (1/8 filter) (0.8 μm Pall Supor800
polyethersulfone filters) and 29 ± 8 ag/g (1/8 filter) (51 μm

Page 6 of 11

Sefar polyester mesh prefilter), all of which are used forthe seawater particulate sampling (Table 1).
The dark noise in our system was ~ 1.0 cps, and thenoise/signal ratio was typically between 5 and 10% for
samples with 231Pa < 0.3 ag/g, and less than 3% for the
samples had 231Pa > 0.4 ag/g (Fig. 2). On the basis of thegeneral definition of the detection limit, the blank + 3δB,
where δB is the standard deviation of the blank, the minimum detectable amount of the dissolved sample 231Pa
was 38 ag, that for using one-eighth of a 0.8 μm Pall
Supor800 polyethersulfone filter was 52 ag, and that for
using one-eighth of a 51 μm Sefar polyester mesh prefilter was 53 ag.
3.2 Precision and accuracy

The precision is eventually constrained by the counting statistics: 2σ = 2 (N−1/2), where N is the total numberof ions counted. Due to the high ionization efficiency(0.7–2.4%), the high 231Pa ion beam to dark noise ratio(9–457), and the low procedural blank (0.02 ag/g), the231
Pa measurements had a precision of 1–3.9 ag/g (0.8–2%), including the counting error for 233Pa. Figure 2
shows the relationship between the sources of error(including the sample size, chemical blank, and massfractionation correction) and the total analytical uncertainty for the Pacific seawater particulate samples andArctic seawater dissolved samples. For samples containing 231Pa > 0.3 ag/g, the Pa counting error accounted forthe majority of the total uncertainty, and the proceduralblank accounted for less than 7%. For samples containing
231
Pa < 0.3 ag/g, the intensity of the 231Pa ion beam wasalways less than 8 cps. When the amount of 231Pa was lessthan 0.1 ag/g, the portion of the total uncertainty causedby the counting error was less than 10%, and the portion
caused by the blanks was greater than 20%.
The results from replicate analyses of the 231Pa-2010
and 231Pa-2015 standards are presented in Table 2. Theinternal precision (intra-run) was 0.5% for a sample sizeof 18 fg. For multiple measurements of the sample sizes,
the external uncertainties (between-runs) were comparable to the internal uncertainties for small particulates.
These results indicate that the internal uncertainty was an
accurate measure of the true uncertainty. The key sourcesof this uncertainty were the counting statistics (primarily for samples greater than 300 ag) and the blank correction uncertainties (primarily for samples less than 100ag). The measured concentrations for all of the replicateswere consistent with the gravimetric measurements andwith measurements obtained in the Lamont–Doherty
Earth Observatory (LDEO) Laboratory, Columbia University (Table 2), indicating that the MC-ICP-MS technique is accurate overall.

Zhang et al. Progress in Earth and Planetary Science(2023) 10:65

Page 7 of 11

Table 1 The total amount of 231Pa in the blank derived from the chemicals, including the spike, resin, Fe, and acids, and the total
procedural blank, including filtering for an acid-cleaned 0.8 μm Pall Supor800 polyethersulfone filter (for small particulates) and 51 μm
Sefar polyester mesh prefilter (for large particulates)
231

Pa

Error

(ag/g)/(2σ)

Chem-blank-1

0.015

0.008

Dissolved

Chem-blank-2

0.013

0.006

Dissolved

Chem-blank-3

0.025

0.008

Dissolved

Chem-blank-4

0.020

0.003

Dissolved

Chem-blank-5

0.025

0.004

Dissolved

Average

0.020

0.006

Sample no.

231

Error

(ag/g(1/8filter))/(2σ)

GT8735db

65.5590.011

Small particulate

GT8702db

8.212

0.003

Small particulate

GT9004db

25.652

0.007

Small particulate

GT9062db

29.761

0.006

Small particulate

GT9587db

50.1320.006

Small particulate

GT9594db

34.9500.005

Small particulate

GT9966db

85.8450.012

Small particulate

GT9971db

38.7200.006

Small particulate

GT8292db

11.1150.001

Small particulate

GT8348db

62.3860.004

Small particulate

GT3160db

16.2890.008

Small particulate

GT3193db

9.858

0.004

Small particulate

GT2441db

16.1650.004

Small particulate

GT2326db

9.326

0.006

Small particulate

GT2276db

41.4270.009

Small particulate

Average

33.6930.006

GT8824dbQP

21.4500.009

Large particulate

GT8735dbQP

60.6540.011

Large particulate

GT8702dbQP

26.3720.008

Large particulate

GT9253dbQP

22.6280.006

Large particulate

GT9971dbQP

27.3830.009

Large particulate

GT3160dbQP

17.8000.006

Large particulate

Average29.381

0.008

Sample no.

3.3 Seawater sample analysis

Pa

231

The ionization efficiency was 1.0% when Pa was analyzed via MC-ICP-MS, and the precision was ± 0.8–6%
when the sample size was 100–3900 ag 231Pa. As a result,
we would need 5 L of surface seawater (231Pa of ~ 20 ag/L)or 2 L of deep seawater (231Pa of ~ 50 ag/L) in order toobtain suspended particulate matter samples of this precision via filtration (Moran et al. 2001, 2002). In contrast,
0.2 L of seawater (231Pa of 500–4000 ag/L) was required
for the dissolved fraction samples (Moran et al. 2001,
2002). With respect to the detection limits, the minimum
surface seawater sample size was 2.5 L for the particulate
fraction and 0.2 L for the dissolved fraction. Therefore,

Sample type

Sample typefor routine seawater 231Pa analyses, we recommend sample sizes of 0.5–1 L for the dissolved fraction and 5–10 L
for the suspended particulate fraction.
Figure 3 presents the results of the replicate analysesof seawater (Arctic and Pacific) and small/large particulate samples (Pacific). The dissolved and small particulate samples are consistent with each sampling depth
within the margin of error. The similarity of the magnitudes of the external and internal uncertainties for eachsample shows that the internal error can represent thetrue precision. Nevertheless, the external uncertainties
for the Pacific large particle samples are larger than theindividual internal uncertainties (Fig. 3c), indicating that

Zhang et al. Progress in Earth and Planetary Science(2023) 10:65

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Table 2 Replicate analyses of the 231Pa-2010 and 231Pa-2015
standards
231

Error

(× ­103 ag/g)(2σ)

LDEO 2015-1

39.00

LDEO 2010-1

37.50

STD 2010-1

Sample no.

Pa

Notes

Measured in Lab

1.36

STD 2015-1

LDEO

1.87

STD 2010-1

LDEO

37.78

0.29

STD 2010-1

UMN

STD 2010-2

37.63

0.20

STD 2010-1

UMN

STD 2010-3

37.63

0.18

STD 2010-1

UMN

STD 2010-4

37.92

0.18

STD 2010-1

UMN

STD 2010-1-ave

37.74

0.21

STD 2015-1

39.58

0.16

STD 2015-1

UMN

STD 2015-2

39.20

0.15

STD 2015-1

UMN

STD 2015-3

38.79

0.32

STD 2015-1

UMN

STD 2015-4

39.42

0.22

STD 2015-1

UMN

STD 2015-5

38.73

0.16

STD 2015-1

UMN

STD 2015-6

39.23

0.17

STD 2015-1

UMN

STD 2015-7

39.25

0.17

STD 2015-1

UMN

STD 2015-1-ave

39.17

0.19

LDEO Lamont-Doherty Earth Observatory, Columbia University, UMN Universityof Minnesota

Fig. 2 Relationship between the analytical uncertainty
for the sample (triangle), chemical blank (diamond), and secondary
electron multiplier (SEM) dark noise (squares) and the amount of 231Pa
measured in a the dissolved fraction of 25 seawater samples, b
the small particulate fraction of 14 seawater samples, and c the large
particulate fraction of six seawater samples. The seawater dissolved
samples were collected from the Pacific Ocean and the particulate
samples were collected from the Arctic Ocean

the internal errors may not be representative of the true
precision of the large particulate samples, but the external uncertainty was predominantly less than 12%. The
suspended particulate matter samples obtained filtering ~ 500 L seawater at different depths through 1/8 of afilter contained 200–2000 ag of 231Pa and had a measurement accuracy of ± 1.0–5.6%. The precision of the two

replicates of one particle sample was ± 1.5–5.9% from two
replicates of six small particulate samples obtained via
1/8 filtration of ~ 500 L of seawater containing 200, 280,
380, 1830, and 5780 ag. All of the samples were from theGeotrace program. The particulate matter is not only for
Pa and Th but also for the analysis of other trace metalelements (Schlitzer et al. 2018), and all of the samples
were filtered in the field when sampling seawater. The
concentration detection limit was also useful for particles. Due to the heterogeneity of the particulate matter,in this study, 1/8 of a filter was used uniformly for thechemical analysis, and realistic sample sizes were used
to obtain reproducible evaluation results. This method
is useful for sub-femtogram size applications, with anerror of ~ 1%, and it is useful for attogram size applications, with an error of ~ 15% (Fig. 3). From two replicatesof five large particulate samples, two replicates of one
large particulate sample were analyzed with an accuracyof ± 1–20% for 1/8 filtration of ~ 500 L of seawater from
depths of 900–3900 m, with a 231Pa content of 500–3800
ag. The results demonstrate that the 231Pa chemically separated from the seawater had essentially the same analytical power as the 231Pa in the standard solution.

4 Conclusions
In this study, we improved and characterized the MCICP-MS technique for measuring attogram-sized 231Pa
concentrations in filtered seawater and particulateZhang et al. Progress in Earth and Planetary Science(2023) 10:65

Page 9 of 11

Fig. 3 Replicate measurements of the dissolved seawater sample from the Arctic and the two particulate fractions filtered from the seawatercollected from the Pacific: a concentration of 231Pa per gram of dissolved seawater; b concentration of 231Pa in the small particles (0.8–51 μm); c
concentration of 231Pa in the large particulate matter (> 51 μm). The blue color in the three panels indicates the actual measured concentrationof 231Pa in the sample, and the red color indicates the corresponding measurement error or uncertainty

matter samples. Compared to previous ICP-MS methods, the chemical blanks had low levels in our study.
The total process blank for the water column was 0.02
ag/g, while the concentrations of the large and small
particles were ~ 30 ag/g. Considering the higher ionization efficiency of the samples (0.7–2.4%), we achieveda relatively high ion beam signal to dark noise ratio,
resulting in the results having reduced sample size
capabilities (< 100 ag 231Pa). In addition, a large volume
of water sample is often required for routine seawater231
Pa analyses, but in this study, we achieved a reduction in the sample volume. We recommend volumesof 0.5–1 L for the dissolved phase and 5–10 L for thesuspended particulate matter. In addition, for the fluctuation of the dark noise during the sample analysis,
we evaluated the stability of the dark noise via multiple
corrections of the dark noise, multiple mass fractionation of 236U/233U (U-F), and cross-assessment of Pa
standards STD 2010-1 and STD 2015-1. In summary,measuring ultra-trace 231Pa (35–3904 ag) concentrations in seawater using MC-ICP-MS is applicable tomeasurement of samples with ag to fg concentrations.
This method is not only applicable to surface water anddeep water but also has broad applications to otherearth science fields, including 235U–231Pa geochronology, igneous geochemistry, and paleoclimatology.

Supplementary Information
The online version contains supplementary material available at https://​doi.​
org/​10.​1186/​s40645-​023-​00600-z.
Additional file 1. Supplementary information include 231Pa measurements method has been used in natural sample analysis of Pacific andArctic; Measurement conditions of MC-ICP-MS; A flowchart of the new
and improved 233Pa separation and purification method.
Acknowledgements
We would like to thank Phoebe Lam, Martin Q. Fleisher, and the Geotracegroup for the in situ measurements and processing of the particles samples
on board during an Arctic cruise. We also thank Yanbin Lv, Hai Cheng, RobertAnderson, and R. Lawrence Edwards for their support at the UMN Lab and
LDEO Lab when conducting the standard cross-calibration. Portions of thisresearch were carried out at the UMN isotope and U-Th dating lab supervised
by R. Lawrence Edwards. We thank Prof. Liu Yun for his guidance during themanuscript writing process. We thank LetPub (www.​letpub.​com) for its linguistic assistance during the preparation of this manuscript.
Author contributions
The research was supervised by RLE and RA. The samples were collected by
PL and RA. The experimental method was instructed by PZ and YL. The dataprocessing was completed in offline mode by PZ and HC. The figures werecreated by ZZ and PZ. The Pa and Th standards were provided by RA. The
paper was written by PZ and ZZ. All of the authors have read and agreed tothe published version of the manuscript.
Funding
This study was supported by the National Natural Science Foundation ofChina (No.42173027; No. 41873013), the Everest Talent Plan Project (Grant
No.10912-KYQD2022-09482), the Scientific Research Start-up Funds ofXi’an Jiaotong University (No.xxj032019007), the 111 program of China (No.
D19002), the U.S. National Science Foundation (No.1702816), Sichuan Tianfu

Zhang et al. Progress in Earth and Planetary Science(2023) 10:65

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Emei Young Talent Project (A.0104497), and the Northwest University YouthAcademic Backbone Talent Program.

Quaternary climate change. Science 276(5313):782–786. https://​doi.​org/​
10.​1126/​scien​ce.​276.​5313.​782
Edwards RL (1988) High precision thorium-230 age of corals and the timingof sea level fluctuations in the late Quaternary. Dissertation, California
Institute of Technology, US
Hayes CT, Anderson RF, Fleisher MQ, Huang KF, Robinson LF, Lu Y et al (2014)
230
Th and 231Pa on GEOTRACES GA03, the U.S. GEOTRACES North Atlantictransect, and implications for modern and paleoceanographic chemical
fluxes. Deep-Sea Res Part II Top Stud Oceanogr 116:29–41. https://​doi.​
org/​10.​1016/j.​dsr2.​2014.​07.​007
Hayes CT, Anderson RF, Fleisher MQ, Vivancos SM, Lam PJ, Ohnemus DC,
Huang KF, Robinson LF, Lu YB, Cheng H, Edwards RL, Moran SB (2015)
Intensity of Th and Pa scavenging partitioned by particle chemistry in the
North Atlantic Ocean. Mar Chem 170:49–60. https://​doi.​org/​10.​1016/j.​
march​em.​2015.​01.​006
Hoffmann SS, McManus JF, Curry WB, Brown-Leger LS (2013) Persistent exportof 231Pa from the deep central Arctic Ocean over the past 35,000 years.
Nature 497(7451):603–606. https://​doi.​org/​10.​1038/​natur​e12145
Jones RT, Merritt JS, Okazaki A (1986) A measurement of the thermal neutron
capture cross section of 232Th. Nucl Sci Eng 93:171–180. https://​doi.​org/​
10.​13182/​NSE86-​A17666
Kretschmer S, Geibert W, Rutgers van der Loeff MM, Schnabel C, Xu S, Mollenhauer G (2011) Fractionation of 230Th, 231Pa, and 10Be induced by particle
size and composition within an opal-rich sediment of the Atlantic Southern Ocean. Geochim Cosmochim Acta 75(22):6971–6987. https://​doi.​org/​
10.​1016/j.​gca.​2011.​09.​012
Lao Y, Anderson RF, Broecker WS, Hofmann HJ, Wolfli W (1993) Particulate
fluxes of 230Th, 231Pa, and 10Be in the northeastern Pacific Ocean. Geochim
Cosmochim Acta 57(1):205–217. https://​doi.​org/​10.​1016/​0016-​7037(93)​90479-gLuo S, Ku TL (1999) Oceanic 231Pa/230Th ratio influenced by particle composition and remineralization. Earth Planet Sci Lett 167(3–4):183–195. https://​doi.​org/​10.​1016/​s0012-​821x(99)​00035-7
Luo S, Ku TL (2004) On the importance of opal, carbonate, and lithogenic clays
in scavenging and fractionating 230Th, 231Pa and 10Be in the ocean. Earth
Planet Sci Lett 220:201–211. https://​doi.​org/​10.​1016/​s0012-​821x(04)​00027-5
Moran SB, Charette MA, Hoff JA, Edwards RL, Landing WM (1997) Distributionof 230Th in the Labrador Sea and its relation to ventilation. Earth Planet Sci
Lett 150(1–2):151–160. https://​doi.​org/​10.​1016/​s0012-​821x(97)​00081-2
Moran SB, Shen CC, Weinstein SE, Hettinger LH, Hoff JA, Edmonds HN, Edwards
RL (2001) Constraints on deep water age and particle flux in the Equatorial and South Atlantic Ocean based on seawater 231Pa and 230Th data.
Geophys Res Lett 28(18):3437–3440. https://​doi.​org/​10.​1029/​2001g​l0133​
39
Moran SB, Shen CC, Edmonds HN, Weinstein SE, Smith JN, Edwards RL(2002) Dissolved and particulate 231Pa and 230Th in the Atlantic Ocean:constraints on intermediate/deep water age, boundary scavenging, and231
Pa/230Th fractionation. Earth Planet Sci Lett 203(3–4):999–1014. https://​doi.​org/​10.​1016/​s0012-​821x(02)​00928-7
Nozaki Y, Nakanishi T (1985) 231Pa and 230Th profiles in the open ocean watercolumn. Deep-Sea Res 32(10):1209–1220. https://​doi.​org/​10.​1016/​0198-​
0149(85)​90004-4
Pickett DA, Murrell MT (1997) Observations of 231Pa/235U disequilibrium involcanic rocks. Earth Planet Sci Lett 148(1–2):259–271. https://​doi.​org/​10.​1016/​s0012-​821x(97)​00037-x
Pickett DA, Murrell MT, Williams RW (1994) Determination of femtogram
quantities of protactinium in geologic samples by thermal ionization
mass spectrometry. Anal Chem 66(7):1044–1049. https://​doi.​org/​10.​1021/​
ac000​79a020
Robert J, Miranda CF, Muxart R (1969) Mesure de laperiode du protactionium
231 parmicrocalorimetrie. Radiochim Acta 11(2):104–108. https://​doi.​org/​
10.​1524/​ract.​1969.​11.2.​104
Schlitzer R, Anderson RF, Dodas EM, Lohan M et al (2018) The GEOTRACES
intermediate data product 2017. Chem Geol 493:210–223. https://​doi.​
org/​10.​1016/j.​chemg​eo.​2018.​05.​040
Shen CC, Cheng H, Edwards RL, Moran SB, Edmonds HN, Hoff JA, Thomas RB(2003) Measurement of attogram quantities of 231Pa in dissolved andparticulate fractions of seawater by isotope dilution thermal ionization
mass spectroscopy. Anal Chem 75(5):1075–1079. https://​doi.​org/​10.​1021/​
ac026​247r

Availability of data and materials
The data have not been published in another paper.

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
College of Earth Sciences, Chengdu University of Technology, Chengdu,
China. 2 Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN, USA. 3 College of Urban and Environmental Sciences,
Northwest University, Xi’an, China. 4 College of Environmental Scienceand Engineering, Nankai University, Tianjin, China. 5 Lamont-Doherty Earth
Observatory of Columbia University, Palisades, NY, USA. 6 Department of Earthand Environmental Sciences, Columbia University, New York, NY, USA. 7 Department of Ocean Sciences, University of California, Santa Cruz, CA 95064, USA.
Received: 31 July 2023 Accepted: 14 November 2023

References
Anderson RF, Fleer AP (1982) Determination of natural actinides and plutonium in marine particulate material. Anal Chem 54(7):1142–1147. https://​doi.​org/​10.​1021/​ac002​44a030
Anderson RF, Bacon MP, Brewer PG (1983) Removal of 230Th and 231Pa from theopen ocean. Earth Planet Sci Lett 62(1):7–23. https://​doi.​org/​10.​1016/​
0012-​821x(83)​90067-5
Asmerom Y, Zartman RE, Damon PE, Shafigullah M (1990) Zircon U-Th-Pb
and whole-rock Rb-Sr age patterns of lower Mesozoic igneous rocks in
the Santa Rita Mountains, southeast Arizona: implications for Mesozoic
magmatism and tectonics in the southern Cordillera. Geol Soc Am Bull
102:961–968. https://​doi.​org/​10.​1130/​0016-​7606(1990)​102%​3c0961:​zu
Asmerom Y, Cheng H, Thomas RB, Hirschmann M, Edwards RL (2000) Meltingof the Earth’s lithospheric mantle inferred from protactinium–thorium–
uranium isotopic data. Nature 406(6793):293–296. https://​doi.​org/​10.​1038/​35018​550
Bradtmiller LI, McManus JF, Robinson LF (2014) 231Pa/230Th evidence for aweakened but persistent Atlantic meridional overturning circulation during Heinrich Stadial 1. Nat Commun 5(1):5817. https://​doi.​org/​10.​1038/​
ncomm​s6817
Chase Z, Anderson RF, Fleisher MQ, Kubik PW (2003) Scavenging of 230Th, 231Pa
and 10Be in the Southern Ocean (SW Pacific sector): the importance ofparticle flux, particle composition and advection. Deep-Sea Res Part II Top
Stud Oceanogr 50(3–4):739–768. https://​doi.​org/​10.​1016/​s0967-​0645(02)​00593-3
Cheng H, Edwards RL, Hoff JA, Gallup CD, Richards DA, Asmerom Y (2000) The
half-lives of uranium-234 and thorium-230. Chem Geol 169(1–2):17–33.
https://​doi.​org/​10.​1016/​S0009-​2541(99)​00157-6
Choi MS, Francois R, Sims K, Bacon MP, Brown-Leger S, Fleer AP, Ball L, Schneider D, Pichat S (2001) Rapid determination of 230Th and 231Pa in seawaterby desolvated micro-nebulization Inductively coupled plasma magnetic
sector mass spectrometry. Mar Chem 76(1–2):99–112. https://​doi.​org/​10.​1016/​s0304-​4203(01)​00050-0
Deng F, Thomas AL, Rijkenberg MJA, Henderson GM (2014) Controls on
seawater 231Pa, 230Th and 232Th concentrations along the flow paths ofdeep waters in the Southwest Atlantic. Earth Planet Sci Lett 390:93–102.
https://​doi.​org/​10.​1016/j.​epsl.​2013.​12.​038
Edmonds HN, Edwards RL, Moran SB, Hoff JA, Smith JN (1998) Protactinium-231 and thorium-230 abundances and high scavenging rates in thewestern Arctic Ocean. Science 280(5362):405–407. https://​doi.​org/​10.​1126/​scien​ce.​280.​5362.​405
Edwards RL, Cheng H, Murrell MT, Benjamin TM (1997) Protactinium dating ofcarbonates by thermal ionization mass spectrometry: implications forZhang et al. Progress in Earth and Planetary Science(2023) 10:65

Siddall M, Henderson GM, Edwards NR, Frank M, Müller SA, Stocker TF, Joos F(2005) 231Pa/230Th fractionation by ocean transport, biogenic particle flux
and particle type. Earth Planet Sci Lett 237(1–2):135–155. https://​doi.​org/​
10.​1016/j.​epsl.​2005.​05.​031
Thomas AL, Henderson GM, Robinson LF (2006) Interpretation of the231
Pa/230Th paleocirculation proxy: new water-column measurementsfrom the southwest Indian Ocean. Earth Planet Sci Lett 241(3–4):493–504.
https://​doi.​org/​10.​1016/j.​epsl.​2005.​11.​031
Thomas AL, Henderson GM, McCave IN (2007) Constant bottom waterflow into the Indian Ocean for the past 140 ka indicated by sediment231
Pa/230Th ratios. Paleoceanography 22(4):PA4210. https://​doi.​org/​10.​1029/​2007p​a0014​15
Walter HJ, Rutgers van der Loeff MM, Hoeltzen H (1997) Enhanced scavenging
of 231Pa relative to 230Th in the South Atlantic south of the Polar Front:
implications for the use of the 231Pa/230Th ratio as a paleoproductivity
proxy. Earth Planet Sci Lett 149(1–4):85–100. https://​doi.​org/​10.​1016/​
s0012-​821x(97)​00068-x
Walter HJ, Geibert W, Rutgers van der Loeff MM, Fischer G, Bathmann U (2001)
Shallow vs. deep-water scavenging of and in radionuclide enrichedwaters of the Atlantic sector of the Southern Ocean. Deep-Sea Res
48(2):471–493. https://​doi.​org/​10.​1016/​s0967-​0637(00)​00046-7

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