# Constraints on the source of reactive phases in sediment from a major Arctic river using neodymium isotopes

Christina S. Larkin, Alexander M. Piotrowski, Ruth S. Hindshaw, Germain

Bayon, Robert G. Hilton, J. Jotautas Baronas, Mathieu Dellinger, Ruixue

Wang, Edward T. Tipper

► To cite this version:

Christina S. Larkin, Alexander M. Piotrowski, Ruth S. Hindshaw, Germain Bayon, Robert G. Hilton, et al.. Constraints on the source of reactive phases in sediment from a major Arctic river using neodymium isotopes. Earth and Planetary Science Letters, 2021, 565, 116933 (11p.). 10.1016/j.epsl.2021.116933. hal-04203477

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**Constraints on the source of reactive phases in sediment from a**

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**major Arctic river using neodymium isotopes**

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Christina S. Larkin<sup>a,1*</sup>, Alexander M. Piotrowski<sup>a</sup>, Ruth S. Hindshaw<sup>a,2</sup>, Germain

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Bayon<sup>b</sup>, Robert G. Hilton<sup>c</sup>, J. Jotautas Baronas<sup>a</sup>, Mathieu Dellinger<sup>c</sup>, Ruixue Wang<sup>a</sup>,

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Edward T. Tipper<sup>a</sup>

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Affiliations:

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<sup>a</sup>Department of Earth Sciences, University of Cambridge, UK

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<sup>b</sup>IFREMER, Marine Geosciences Unit, Brest, France.

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<sup>c</sup>Department of Geography, Durham University, UK

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*Corresponding author: csl42@cam.ac.uk

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<sup>1</sup>Present address: School of Ocean and Earth Science, National Oceanography

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Centre, University of Southampton, UK

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<sup>2</sup>Present address: Norconsult AS, Kjørboveien 22, 1338 Sandvika, Norway

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**Abstract**
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27 Riverine suspended particulate matter (SPM) is essential for the delivery of
28 micronutrients such as iron (Fe) to the oceans. SPM is known to consist of multiple
29 phases with differing reactivity, but their role in the delivery of elements to the
30 oceans is poorly constrained. Here we provide new constraints on the source and
31 composition of reactive phases in SPM from the Mackenzie River, the largest
32 sediment source to the Arctic Ocean. Sequential leaching of SPM shows that river
33 sediments contain labile Fe phases. We estimate the labile Fe flux is substantial
34 (0.21(+0.06,-0.05) Tg/yr) by quantifying Fe concentrations in weak leaches of the
35 SPM. The labile Fe phase hosts a considerable amount of rare earth elements
36 (REE), including neodymium (Nd). We demonstrate that the labile Fe phase and
37 dissolved load have radiogenic Nd isotope ratios that are identical within uncertainty,
38 but up to 8 epsilon units distinct from the silicate phase. We interpret this as
39 evidence for dynamic cycling between Fe-oxide phases in SPM and the river water,
40 demonstrating the high reactivity of the labile Fe phase. Nd isotope and elemental
41 molar ratios suggest that a significant amount of labile Fe- and Nd-bearing phases
42 are derived from Fe-oxides within the sedimentary source rock rather than silicate
43 mineral dissolution. Thus, sedimentary rock erosion and weathering provides an
44 important source of labile Fe, manganese (Mn) and by extension potentially other
45 trace metals. Our results imply that both past and future environmental change in the
46 Arctic, such as permafrost thaw, may trigger changes to the supply of reactive trace
47 metals. These results demonstrate that a re-evaluation of sediment reactivity within
48 rivers is required where uplifted sedimentary rocks are present.

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50 **Key words:** iron oxides; bioavailable; neodymium isotopes; Arctic
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# 1. Introduction

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54 Rivers are conduits for the transfer of particulate and dissolved material derived from
55 weathering to the oceans (Gaillardet et al., 1999). Weathering plays a critical role in
56 the cycling of elements at the Earth's surface and is a key regulator of the Earth's
57 climate over geological timescales (Walker et al., 1981). Rivers also transport
58 essential nutrients to the oceans sustaining marine primary productivity (e.g.,
59 Deutsch and Weber, 2012). Changes to the flux of nutrients that limit productivity in
60 the oceans have been linked to large scale perturbations in the carbon cycle and
61 past climate change (Martin, 1990; Vincent and Berger, 1985). The flux of
62 suspended particulate matter (SPM) sustains the supply of certain elements to the
63 oceans and plays an important role in bio-geochemical cycling (Jeandel and Oelkers,
64 2015; Jones et al., 2012).

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66 Micronutrients (e.g., Fe), the rare earth elements (REE, particularly Nd) and many
67 other trace elements have low solubility in natural waters. In the dissolved load (<0.2
68 µm) these elements are predominately associated with nanoparticulate and colloidal
69 phases (Gaillardet et al., 2014, and references therein). Their concentrations in river
70 dissolved loads are low (ppb to ppt, Gaillardet et al., 2014); concentrations in SPM
71 are several orders of magnitude higher (Jeandel and Oelkers, 2015). Salt-induced
72 flocculation of many such nonmobile elements in estuaries further removes most of
73 these low concentration elements prior to reaching the oceans (Elderfield et al.,
74 1990).

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76 Basalts and volcanic particulates are important sources of key bio-limiting nutrients

77 (e.g. Fe) as they are easily weatherable and enriched in those elements (Jones et

78 al., 2012). However, the importance of sedimentary rocks has not been similarly

79 evaluated. Marine sediments contain phases, including Fe-Mn oxyhydroxides with
80 elevated REE concentrations, which are formed from precipitation and exchange

81 with seawater and pore fluids (e.g., Chester and Hughes, 1967). Marine sedimentary

82 rocks tectonically emplaced on the continents retain some of these characteristics

83 (Hindshaw et al., 2018; Jang et al., 2020). This results in a reservoir of inherited
84 phases which can be more reactive than silicate minerals and may supply a greater

85 amount of bioavailable Fe to riverine sediments (Hindshaw et al., 2018). For
86 instance, amorphous Fe (oxy)hydroxides (e.g. ferrihyrite) contained within riverine

87 sediments are easily reducible and potentially bioavailable (Bhatia et al., 2013,
88 Hawkings et al., 2018).

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90 Since Fe-oxides are enriched in REEs, their origin can be traced using radiogenic Nd
91 isotopes (¹⁴³Nd/¹⁴⁴Nd, expressed as εNd), a source tracer in riverine and marine

92 sediments (Goldstein and Jacobsen, 1988; Goldstein and Hemming, 2003). Since
93 there is limited fractionation of the Sm/Nd ratio in particulate phases during

94 weathering and the decay constant is long compared to the timescales of recent
95 weathering (Babechuk et al., 2014), εNd is only controlled by changes in source. Nd

96 in the dissolved load typically has εNd greater (more radiogenic) than in the SPM in
97 rivers with abundant sedimentary rocks in their catchments (Goldstein and

98 Jacobsen, 1987; Hindshaw et al., 2018). This offset is attributed to dissolved Nd
99 sourced from the preferential weathering of marine precipitates and other reactive

100 components contained within the parent rock (Goldstein and Jacobsen, 1987).
Nonetheless, a clear link between these reactive phases and dissolved riverine
chemistry has not been shown.

103

With knowledge of the source composition, εNd is a valuable tool for tracing the
origin of reactive phases from sedimentary rock weathering and their transport in
river systems. We apply this tracer to a large Arctic river system. The Mackenzie
River, as the first and third largest source of suspended sediment and water,
respectively, to the Arctic Ocean (Macdonald et al., 1998, Holmes et al., 2002),
provides a good representation of basin-scale fluxes. It is dominated by shale
weathering with a low abundance of volcanic or mafic rocks and limited influence
from the weathering of crystalline rocks (Millot et al., 2003, Fig. 1B). This makes the
Mackenzie an ideal location to investigate the mobilisation of reactive, micronutrient-
bearing phases during shale weathering on a large scale.

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In this study we characterize the εNd of the dissolved load (<0.2 µm and
ultrafiltrates) and suspended sediment (sequential extractions and residue) in the
Mackenzie and key tributaries over two years (2017, 2018) sampled within a few
days of the peak discharge after ice-break up. We show that the εNd is within
uncertainty between the dissolved load and easily leachable phases in the SPM. We
use REE patterns to constrain the mineralogy of the leachable phases. We quantify
the concentrations of major and some trace elements in leachable phases and
estimate the flux of labile Fe from the Mackenzie to the Arctic Ocean.

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## 2. Materials and Methods

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126 Full analytical and procedural methods are given in Supplementary Text 1 and
127 summarised briefly here. All analysis was carried out at the University of Cambridge,
128 Department of Earth Sciences.

129

130 **2.1 Study area and sample collection**

131

132 The Mackenzie River system is large (Fig. 1, area 1.78x10⁶ km²). The weathering of
133 sedimentary rocks dominates the dissolved and SPM loads of the river (Calmels et
134 al., 2007; Horan et al., 2019; Huh et al., 2004; Millot et al., 2003) despite the large
135 surface area of crystalline rocks (29.2% of the basin geology, Fig. 1b). Less than 1%
136 of the basin is underlain by volcanic rocks (Fig. 1b). The river is characterised by a
137 sharp peak in discharge each year after the seasonal melting (freshet) of river ice in
138 May-July (Fig. 2). Samples were collected at this period of maximum flux.

139

140 The Mackenzie mainstem and two large tributaries which enter at the delta, the Peel
141 River and the Arctic Red River, were sampled at 3 main sites (Fig. 1) in early June in
142 2017 and 2018. Suspended sediment samples were taken at different depths in the
143 channel using a 5L Van-Dorn type depth sampler (following Hilton et al., 2015).
144 Discharge was quantified using an acoustic-Doppler current profiler (ADCP, Rio
145 Grande II (1200 kHz), Teledyne Instruments, Fig. 2, Supplementary Table S4).
146 Additionally, in 2018 a small Peel River tributary and meltwater from the surface
147 Arctic Ocean (seawater diluted by sea and river ice melt) were sampled. Dredged
148 bed load, bank sand and rock fragments were also collected.

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150 River samples were filtered at 0.2 µm, collecting both water and sediment. Filtered
151 waters collected for major cation and REE concentrations were acidified using
152 distilled HNO₃ to pH 2 in acid cleaned bottles. In 2018, ultrafiltration was carried out
153 on <0.2 µm filtrates from the Mackenzie (Middle Channel) and the Peel River, using
154 dialysis membranes with molecular cut off weights of 10 kDa and 1 kDa
155 (Supplementary Text 1.2). Approximately 10 L of <0.2 µm water was collected for the
156 analysis of Nd isotopes using a Fe co-precipitation method (Hindshaw et al., 2018).

157

158 To characterize the longer-term integrated composition of the Mackenzie River SPM
159 leachate compositions from 5 offshore sediment cores from the Mackenzie delta and
160 one from deeper in the Beaufort Sea were analysed (Natural Resources Canada,
161 Geology Survey of Canada's Marine Geoscience Collection, Supplementary Table
162 S5). Two silicate residue samples were analysed from two of the shallowest core
163 sites. The sedimentation rate at shallow sites in the Canadian Beaufort sea is high
164 (on the order of 2-3 m/ka, O'Regan et al., 2018), indicating that the range and depth
165 sampled within the cores (~10 cm) represents sediment that is likely several
166 decades old, and homogenises inter-annual and seasonal variability.

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168 2.2 Sequential extractions

169

170 Approximately 50-100 mg of dry sediment was leached sequentially to extract: 1)
171 exchangeable phases (1M NH₄Cl), 2) reactive Fe-Mn (oxy)hyroxides (a weak acid-
172 reductive leach of 5 mM hydroxylamine hydrochloride (HH)-3 mM Na-EDTA-1.5%
173 acetic acid buffered to a pH of ~4 with NaOH), 3) calcite and additional carbonates
174 (1.7M acetic acid), 4) dolomite, and any remaining authigenic components (e.g.,
175 crystalline Fe-Mn oxides, 1M HCl). The focus of this study is the weak acid-reductive
176 leach, hereafter referred to as the HH leach, targeting amorphous reactive Fe-Mn
177 oxides. This is a more dilute HH leach than previous methods (e.g. Bhatia et al.,
178 2013; Chester and Hughes, 1967) carried out on non-decarbonated sediments,
179 following Blaser et al., 2016. This method minimizes dissolution of silicate phases
180 and avoids the loss of other reactive Fe phases during removal of carbonates. Due
181 to the buffering of this reaction in the presence of varying amounts of carbonate, it
182 represents a minimum estimate of the reactive Fe-Mn oxyhydroxides rather than a
183 fully quantitative leach. This leach will mobilise reactive carbonate phases and
184 potentially some REEs associated with phosphates. Residue sediment was digested
185 following lithium borate fusion (Supplementary Text S1.3).

186

187 **2.3 Major and trace concentration analysis**

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189 Major and selected trace element (e.g. Fe, Mn) concentrations on acidified waters
190 were determined by ICP-OES (Agilent 5100, with a precision and accuracy of better
191 than 10% based on repeated analysis of certified water standards (Supplementary
192 Text 1.4.1).

193

194 Leachate phases and dissolved sediment residues were similarly analysed for major
195 and some trace element concentrations by ICP-OES, with matrix-matched calibration
196 lines. External reproducibility (typically better than 10%) was monitored using
197 certified standards (Supplementary Text 1.4.1).

198
199 Mass balance was verified by comparison of the sum of concentrations in the
200 leachates and residue with bulk sediment and a leached USGS shale standard with
201 certified values (Supplementary Table 8). The sum reproduced expected values
202 usually within 10%, and always within 20%.

203

204 **2.4 REE Analysis**

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206 REE concentrations on filtered waters were measured by isotope dilution
207 (Supplementary Text 1.5). Nd (alongside REE) concentrations in sediments and
208 sediment leachates were determined using ICP-OES (Agilent 5100, Nd
209 concentrations > ~4 ppb only) and using a Thermo Element-XR ICP-MS with a
210 matrix-matched calibration line. Reproducibility was monitored using certified
211 standards and values were within ±10%. Two residue samples and all sequential
212 extraction steps from one SPM sample were determined by isotope dilution, as
213 described for filtered water samples (Supplementary Text 1.5).

214

215 **2.5 Nd and Sr isotopes**

216

217 Nd was separated from the leachate and residue solutions using established
218 chromatographic procedures; the light REE's were separated using TRUspec resin,
219 and Nd from Sm using LNspec resin. Radiogenic Sr isotope ratios (⁸⁷Sr/⁸⁶Sr) were
220 determined on a subset of water, sediment leachate and sediment samples. Sr was
221 isolated using SrSpec resin (following Hindshaw et al, 2018). Nd was separated from
222 water samples using two stages of cation exchange (Biorad AG50W-X8) and
223 LnSpec resin (Supplementary Text 1.1).
224

225 Nd and Sr isotopes were measured on a Thermo Neptune Plus MC-ICP-MS.

226 ¹⁴⁶Nd/¹⁴⁴Nd was normalised to 0.7219 using an exponential correction. Samples
227 were corrected to the accepted value of reference standard JNdI-1;

228 ¹⁴³Nd/¹⁴⁴Nd=0.512115 (Tanaka et al., 2000) which was repeatedly analysed at the
229 sample concentration throughout each measurement session. Two times the
230 standard deviation (2σ) on JNdI-1 for each measurement session is quoted as the
231 analytical uncertainty. εNd was calculated in parts per 10,000 relative to the
232 chondritic uniform reservoir (CHUR), ¹⁴³Nd/¹⁴⁴NdCHUR=0.512638 (Jacobsen and

233 Wasserburg, 1980). For Nd isotopes, long-term reproducibility was monitored using
234 rock standards (Supplementary Text 1.4.2). Unless otherwise stated, where Nd
235 isotope replicates (measurement or full procedural) were carried out, the value
236 reported is the average of the replicates and the associated 2σ, if higher than the
237 measurement session uncertainty. For Sr isotopes, ⁸⁵Rb was monitored to correct for
238 Rb interferences on ⁸⁷Sr and Kr interferences were corrected for by measurement of
239 the baseline in a blank solution (on-peak zeros). Values were normalised to ⁸⁶Sr/⁸⁸Sr
240 = 0.1194 using an exponential correction. For Sr isotopes, samples were analysed in
241 duplicate, and the uncertainty quoted is the associated 2σ. Repeated measurements
242 of the standard NBS 987 yielded ⁸⁷Sr/⁸⁶Sr= 0.710275±46 ppm (2σ, n=20).

243

244 **3. Results**

245

246 **3.1 Dissolved Nd**

247
Nd was partitioned between nano-particulate and colloidal phases and the 'truly' dissolved (<1 nm; ≈1 kDa) phase (Table 1). Approximately 70% of Nd in the <0.2 µm filtrate is hosted in colloids or nano-particulates (Table 1). In the Mackenzie River (Middle Channel) εNd values between the <0.2 µm and <1 kDa fractions agreed within 0.43 epsilon units (Table 1). This demonstrates that nano-particulate and colloidal phases and the 'truly' dissolved load have the same source and so the <0.2 µm εNd value is representative of the 'truly' dissolved composition, consistent with previous observations in large rivers (Merschel et al., 2017).

## 3.2 Paired sediment and dissolved εNd

As the <0.2 µm fraction is indicative of the 'truly' dissolved εNd, it is possible to use these measurements to compare with leachable phases and assess their reactivity.

We therefore refer to the <0.2 µm as the dissolved load. The dissolved load is always more radiogenic than the sediment residue, with the offset ranging from 1.4 to 8.2 epsilon units (Fig. 3, average = 3.2). This is consistent with, and in some cases far exceeds, the offset observed by Goldstein and Jacobsen, 1987, for large rivers with significant marine sedimentary source rocks in their catchments. In contrast, εNd in the dissolved load is in close agreement with HH leachates targeting reactive Fe-Mn oxides with a linear regression within error of the 1:1 line (Fig. 3).

The dissolved vs. residue εNd linear regression is systematically offset from the 1:1 line, with a weaker correlation (Fig. 3).

Residue εNd compositions are in close agreement with previous measurements of sediment already reported for the Mackenzie River (Vonk et al., 2015). Dissolved
273 εNd reported herein for the Mackenzie River at Tsiigehtchic (-13.4 ±0.2 (2σ, n=1,

274 2018) and -12.3±0.3 (2σ, n=2, 2017)) is consistent with the only previous dissolved

275 measurement (2003, -12.9±0.3 (Zimmermann et al., 2009)).

276

277 SPM in the Middle Channel of the Mackenzie River is hydrodynamically sorted, due

278 to faster settling velocities of coarser sedimentary particles (e.g. Bouchez et al.,

279 2011). This is illustrated by Al/Si, a proxy for grain size (Fig. 4), indicating the

280 dominance of finer sediments towards the surface and coarser at the bottom.

281 Despite this hydrodynamic sorting, there is no appreciable variation of εNd with

282 depth in either the sediment residue or HH leachate (Fig. 4). Dissolved εNd from the

283 surface and base of the water column demonstrate that the water is well mixed.

284

285 There are systematic differences in dissolved εNd between 2017 and 2018. The

286 furthest downstream site (Mackenzie, Middle Channel) dissolved composition

287 changes from εNd=-10.71±0.41 (2σ, n=3, 2017) to -12.97±0.15 (2σ, n=1, 2018),

288 likely driven by changes in the Peel River which shows similar differences, changing

289 from εNd=-9.83±0.16 (2σ, n=1, 2017) to -11.47±0.15 (2σ, n=1, 2018). This could

290 represent inter-annual εNd variability, as is argued to be the case for changes in

291 organic carbon and dissolved nutrient sources on the same sample set (Schwab et

292 al., 2020), but additional data would be needed to determine if this is the case.

293

294 The average HH leach εNd from sediment cores was -11.3±1.9 (2σ, n=6), consistent

295 with the same fraction in riverine sediments (Fig. 3) with the leachate always more

296 radiogenic compared to the silicate residue (two cores, <10m deep, average offset =

297 2.7±1.2, 2σ, n=2). Shallow (<100 m) seawater in the Beaufort Sea is more
298 radiogenic than these leachate compositions (εNd = ~6 to -9, Porcelli et al., 2009).

299 Therefore, the offset between residue and leachate is preserved and exported in sediment to the shelf. εNd in the HH leachate at the deepest and least proximal core site (εNd = -9.55, 1054 m water depth) overlaps with εNd measured in deep (> 1000 m) seawater in the Beaufort Sea, which is less radiogenic (εNd between -9 and -11, Porcelli et al., 2009).

304

305 **3.3 Sediment partitioning and characterisation**

306

307 The partitioning of selected elements amongst all leachates relative to the bulk sediment is shown in Fig. 5. In the NH₄Cl leach, targeting exchangeable phases, Fe and Nd were below detection. This is consistent with a previous study which reported low concentrations of REEs in a river sediment exchangeable fraction (Adebayo et al., 2018). Elemental concentrations in the NH₄Cl leach which were not below detection are reported in Tipper et al., 2021. The HH leach contains on average 3%, 8% and 30% of the total sediment Fe, Nd and Mn respectively (Fig. 5). Ca, Mg and Sr are also high in the HH leach (Fig. 5) consistent with significant carbonate lithologies present within the Mackenzie Basin (Millot et al., 2003). On average 60% of the Fe and Nd are hosted within the silicate residue.

317

318 REE on the HH leach show a middle REE (MREE) enrichment (Fig. 6a). A MREE enrichment of this magnitude is typical of an Fe-Mn oxyhydroxide signature (Fig. 6a) and is consistent with that previously reported for similar sediment extractions (e.g. Leybourne and Johannesson, 2008).

322
REE in the SPM leachate phases were partitioned relative to the bulk (sum of leachates and residue; Middle Channel, 2017, Fig 6b). 15% of REEs relative to the total sediment are mobilised during the HH leaching step. A smaller fraction (<1%) of REEs are released during the subsequent acetic acid leach step (Fig. 6b), despite the large amount of Ca and Mg (22% of total sediment Ca, 11% of total sediment Mg). Less Fe and Mn are mobilised in the acetic acid leach (1% of total Fe, 5% of total Mn), relative to the HH leach (4% of total Fe, 42% of total Mn). This demonstrates that the REE are not associated with carbonate phases.

$^{87}$Sr/$^{86}$Sr on the dissolved load and sediment residues (Fig. 6c) are in agreement with previously published data for this river (e.g. Millot et al., 2003). $^{87}$Sr/$^{86}$Sr values indicate carbonate and silicate sources in the HH leach and residue respectively. Sr isotopes do not indicate extensive leaching into the silicate detrital fraction. Although there is scatter in the leachable εNd compositions (Fig. 6c), on stronger leaching εNd is not substantially more radiogenic than the HH leach.

## 4. Discussion

εNd values from paired dissolved load and SPM, alongside the characterisation of elemental concentrations in sequential extractions, are used to trace both the source and the reactivity of different phases within the SPM. Firstly, we seek to assess the reactivity of the phases in the HH leach and then characterise sources. We then estimate the flux of labile Fe from the Mackenzie to the Arctic Ocean and address the implications of this study in terms of the oceanic Nd budget.
348 **4.1 Reactivity of the labile phases in SPM**

349

350 The reactivity of phases hosted in the HH leachate are traced with εNd and
351 elemental partitioning. Significant fractions of Fe, Mn, Ca, Mg, and REEs relative to
352 the bulk sediment are present in what is an extremely weak and buffered leach (5
353 mM HH, 1.5% acetic acid, Fig. 5, Fig. 6). Although subsequent leaches use stronger
354 reactants and are not buffered, they release smaller amounts of Nd compared to the
355 HH leach (Fig. 6b). This suggests that some of these reactive phases are
356 susceptible to acid-reductive dissolution with small changes in pH or redox
357 conditions (which may occur in the delta or offshore). Nd and REEs are depleted in
358 carbonate rocks and are unlikely to be directly incorporated into biogenic calcite (e.g.
359 Tachikawa et al., 2014). Less Nd is released in the 1.7 M acetic acid leach,
360 compared to the HH leach, confirming that during the HH leaching step the Nd
361 released is not associated with carbonates.

362

363 Although an MREE enrichment in the HH leach is typical of an Fe-Mn oxyhydroxide
364 phase, this could also be consistent with some phosphate phases (such as biogenic
365 apatite, Fig. 6a., Leybourne and Johannesson, 2008, and references therein).
366 Secondary precipitation of phosphates could also result in an MREE enrichment
367 (Leybourne and Johannesson, 2008), but the lack of an LREE enrichment (low
368 HREE/LREE, Fig. 6a) in HH leaches makes this unlikely (Köhler et al., 2005).

369

370 The dissolved εNd composition is always similar to the HH leachate composition
371 (Fig. 3). This similarity is prevalent in different sampling localities and tributaries,
372 which differ in sediment and water characteristics. This points to a fundamental
process that is shared between these catchments and suggests that this observation is not unique to the time of year sampled (freshet). In rivers, the SPM is supplied by bank erosion and hillslope processes (Hilton et al., 2015), which are likely to be decoupled in space and time from the hydrological pathways that deliver river water (Vonk et al., 2019). Therefore, the consistency in εNd between dissolved and HH leach εNd is most likely explained by rapid and recent cycling within riverine implying equilibrium between the dissolved load and Fe-oxide phases in the HH leach.

Dissolved and HH leachate εNd are always similar at each site in the Mackenzie River, both upstream and downstream of major confluences with the Peel and Arctic Red Rivers (Tsiigehtchic to Middle Channel, Fig. 1a). Previous discharge and sediment flux estimates demonstrate that water and suspended sediment do not mix in the same proportions downstream of the Peel confluence, with the Peel supplying 17% of the sediment load (Carson et al., 1998), but only 7% of the total water discharge (Schwab et al., 2020 and references therein). This implies the potential for rapid cycling between the dissolved load and HH leach phases during transit between the upstream and downstream sites, despite only limited changes in pH and Nd concentration (supplementary Table S4 and S10).

Although the mechanism for equilibration of Nd between HH leach phases and the dissolved load is unclear, since the weakly adsorbed exchangeable phases are removed in the leaching step prior to the HH leach, this rapid cycling is likely driven by dissolution of Nd from phases such as Fe-Mn oxyhydroxides. The concentration of Nd in the HH leach (ppb to ppm, Supplementary Table S9 and S11) is several orders of magnitude higher than the dissolved concentration (Supplementary Table
398 S10), suggesting that phases within the HH leach may drive the dissolved

399 composition.

400

401 Our findings demonstrate dynamic cycling between the dissolved load and phases

402 soluble in the HH leach, such as Fe-Mn oxyhydroxides. The similarity of the

403 dissolved load and HH leach phases and decoupling from the silicate residue

404 suggests that the HH leach phases are labile and likely bioavailable. These results

405 imply not only rapid cycling of Nd but potentially other immobile elements contained

406 within the HH leach.

407

408 **4.2 Marine authigenic phases as a source of radiogenic Nd in river waters**

409

410 The mass balance of sources of radiogenic Nd that contribute to higher εNd in the

411 dissolved and HH fractions relative to silicates provides information on the relative

412 supply of Nd from different phases. Here we discuss possible sources of radiogenic

413 Nd that could explain both the HH leaches and dissolved load. Average global

414 seawater εNd (~-8.8, Lacan et al., 2012) and modern core top authigenic fractions

415 (Haley et al., 2017) are more radiogenic than the upper continental crust (-11.4±2.5,

416 Goldstein et al., 1984). Therefore, when marine sediments are emplaced on the

417 continents, they can provide a distinct source of radiogenic Nd to the weathering

418 reactor. Measurements of εNd on Fe-Mn oxides and other inherited authigenic

419 phases of marine origin hosted in sedimentary rocks show that they are on average

420 more radiogenic than associated silicates (Jang et al., 2020, Hindshaw et al., 2018).

421 They can be reactive and so result in a more radiogenic dissolved and leachate
422 composition when compared to the bulk source rock in rivers (Bayon et al., 2020;

423 Goldstein and Jacobsen, 1987; Hindshaw et al., 2018; Jang et al., 2020).

424

425 Preferential weathering of inherited marine authigenic phases is the most likely

426 reason for decoupled εNd between dissolved and silicate residue sediment in the

427 Mackenzie. Radiogenic Nd in Fe-Mn oxyhydroxides of Mackenzie River SPM is likely

428 derived from the weathering of these phases through physical transportation, and

429 equilibration with waters. These radiogenic phases become mixed with Fe-oxides

430 resulting from the weathering of silicate minerals, with εNd inherited from the source

431 silicate. Nonetheless, it is possible that alternate sources contribute labile and

432 radiogenic Nd, and these are addressed below.

433

434 **4.3 Alternative sources of radiogenic Nd to river waters**

435 Accessory minerals enriched in REEs and garnet can have more radiogenic εNd

436 than their bulk source rocks (e.g. Rickli et al., 2017). However, radiogenic release of

437 Nd during incongruent weathering of crystalline rocks is minimal (Dausmann et al.,

438 2019). Another possible source of radiogenic Nd is the preferential weathering of

439 volcanic rocks, but they are rare in the Mackenzie basin (Fig 1b, Millot et al., 2003).

440

441 Several studies indicate the dominance of shale and carbonate weathering on the

442 Mackenzie dissolved load, with no evidence of large volcanic or crystalline rock

443 contributions (Horan et al., 2019; Huh et al., 2004; Millot et al., 2003). εNd

444 decoupling between dissolved and silicate residue is at a maximum in a small Peel

445 River tributary that, to the best of our knowledge, does not have crystalline or

446 volcanic rocks in its catchment and drains almost exclusively shales. Moreover,
447 preferential weathering of a specific rock type would not explain the coupling of the
448 dissolved load with the HH leach, and their decoupling from the silicate residue, in all
449 sampling sites, with differing source lithologies.

450 Phosphates in sedimentary rocks are enriched in REEs and may be partially
451 mobilised in the HH leach. Phosphates in the main source rock type (shales) are
452 likely to have formed during marine diagenesis, so can be considered as inherited
453 authigenic phases, with a comparable εNd to concurrent Fe-Mn oxides. Fe-oxides
454 are ubiquitous in all sediments and sedimentary rocks. Given the range in source
455 rock age (Proterozoic to Cenozoic, Wheeler, 1996) and depositional environment,
456 Fe-Mn oxides are the most probable universal constituent consistent with the high
457 fraction of Fe and Mn in the HH leach, whereas phosphates may not always be
458 present in sufficient abundance. Equilibration between the dissolved load and
459 phosphates hosted in the HH leach would not likely result in the observed REE
460 patterns, as secondary phosphates are typically LREE enriched (Fig. 6a, Köhler et
461 al., 2005). Therefore, it is unlikely that phosphate minerals play a significant role, and
462 the more positive Nd is derived from the preferential weathering of inherited Fe-Mn
463 oxides.

464

465 While Fe-Mn oxides are insoluble in oxygenated and neutral range pH which typify
466 large rivers including the Mackenzie (Tank et al., 2016) mobilization of amorphous
467 and nano-particulate Fe can occur, and in acidic environments dissolution of Fe-
468 oxides may occur. Extensive pyrite oxidation provides the majority of dissolved
469 sulfate in the Mackenzie (Calmels et al., 2007). A product of the oxidative weathering
470 of pyrite is Fe-oxide, and if instantaneous buffering of this reaction by dissolution of
471 carbonate minerals does not occur, it will result in an acidic pH alongside high sulfate
472 concentrations (Horan et al., 2019). This is seen in one tributary of the Peel River

473 where high sulfate concentrations (4660 µM) and relatively low pH of ~6 are coupled

474 to elevated Nd (950 ppt), high Fe (1 µM) and Mn (7.8 µM) concentrations in the

475 dissolved load (<0.2 µm, Supplementary Tables S9 and S12) with the highest

476 observed offset between the residue and HH leach εNd (8 epsilon units,

477 Supplementary Table S10).

478

479 Nd is not likely to be incorporated directly into pyrite but will be present in

480 ‘contaminant’ phases in pyrite or in other phases within a sedimentary rock

481 succession, namely Fe-Mn oxyhydroxides or phosphates (Raiswell and Plant, 1980).

482 High Nd and Mn dissolved concentrations indicate the dissolution of reactive Fe-Mn

483 oxides within the source rock (shale), with differing εNd to silicate sources, coupled

484 to the oxidative weathering of pyrite.

485

486 The HH leach of a shale fragment from this catchment has comparable εNd to the

487 bedload HH leachate and dissolved load (Fig. 3, Supplementary Table S10).

488 Although the mineralogical source of Nd and Fe is different (Fe from pyrite and Nd

489 from other phases) these observations imply that the mobilization of Fe and Nd are

490 coupled during the oxidative weathering of pyrite. This suggests that oxidative

491 weathering of pyrite may provide an important source of reactive Nd and Fe bearing

492 phases, with radiogenic εNd. Once the REEs are in solution, they can be

493 incorporated into secondary Fe-oxides upon increasing pH, when Fe-oxides bearing

494 REEs will precipitate out of solution. Close coupling between riverine Nd and Fe has

495 been noted in previous studies (e.g. Elderfield et al., 1990; Ingri et al., 2000).

496
497 **4.4 Constraints on the contribution of different sources of labile Fe and Nd**

498

499 Using a simple mass balance model it is possible to estimate the fraction of Nd and
500 Fe sourced from inherited authigenic phases contained within the sedimentary
501 source rocks.

502

503 Dissolved εNd is set by exchange with labile HH leach phases in the SPM. The labile
504 εNd is considered to be a mixture of Fe-oxides derived from modern-day weathering
505 of silicate minerals (εNd<sub>sil</sub>) and Nd derived from inherited authigenic phases (εNd<sub>auth</sub>,
506 equation 1, Figure 7).

$$
507 \qquad \varepsilon \mathrm{Nd}_{\mathrm{labile}}=f_{\mathrm{Nd}, \mathrm{auth}} \varepsilon \mathrm{Nd}_{\mathrm{auth}}+f_{\mathrm{Nd}, \mathrm{sil}} \varepsilon \mathrm{Nd}_{\mathrm{sil}} \quad (1)
$$

508 where εNd<sub>labile</sub> is the average of the leachate and dissolved composition, and f is the
509 fraction of Nd in either the inherited authigenic or silicate phases where f<sub>Nd,auth</sub> + f<sub>Nd,sil</sub> = 1.

511

512 Equation 1 was solved for f<sub>Nd,auth</sub>, with εNd<sub>sil</sub> given by the residue silicate composition
513 measured for sample set. εNd<sub>labile</sub> is the average of the HH leachate composition and
514 (if measured) the dissolved composition for each sample set. εNd<sub>auth</sub> is unknown and
515 given the range of ages and types of sedimentary rocks in the Mackenzie basin, will
516 be variable. εNd<sub>auth</sub> was considered over the range εNd<sub>auth</sub>=0 to εNd<sub>auth</sub>= εNd<sub>labile</sub>
517 sampled from a synthetic uniform distribution to account for the uncertainty of this
518 end-member. εNd<sub>labile</sub> and εNd<sub>sil</sub> were measured at each sampling location and
519 considered as normal distributions. We used Monte-Carlo methods to estimate the
520 error when solving for f<sub>Nd,auth</sub> (calculating f<sub>Nd,auth</sub> for 100,000 values of each variable).
521 f<sub>Nd,auth</sub> was calculated for 4 individual sample sets (2017 and 2018 Middle Channel,
522 Arctic Ocean and in shelf sediment cores, Fig. 8, Supplementary Table S13). Taking
523 the interquartile range from the sum of all four sample sets it estimated that, at the
524 mouth of the Mackenzie River a minimum of 23-50% of labile Nd is derived from
525 inherited authigenic phases (Fig. 8).

526

527 Given this significant estimate for f<sub>Nd,auth</sub>, it is likely that a similarly high fraction of
528 other immobile elements such as Fe are also sourced from inherited authigenic
529 phases. The fraction of labile Fe sourced from inherited authigenic phases (including
530 from pyrite and inherited Fe-oxides) was estimated using mass balance:

531

$$
\left(\frac{\mathrm{Nd}}{\mathrm{Fe}}\right)_{\text {labile }}=f_{\mathrm{Fe,sil}}\left(\frac{\mathrm{Nd}}{\mathrm{Fe}}\right)_{\text {sil}}+f_{\mathrm{Fe,auth}}\left(\frac{\mathrm{Nd}}{\mathrm{Fe}}\right)_{\text {auth}}
$$

532 where $f_{\text{Fe,auth}} + f_{\text{Fe,sil}} = 1$, and $(\frac{\text{Nd}}{\text{Fe}})_i$ is the molar ratio of Nd to Fe in each phase.

533 This equation was solved for f<sub>Fe,auth</sub>, the fraction of labile Fe sourced from inherited
534 authigenic phases. (Nd/Fe)auth is unknown, and the largest source of error and was

535 based on the range from HH leaches of a shale fragment and coarse bank sediment
536 ((Nd/Fe) x 1000 = 0.63 to 2.4). (Nd/[Fe]sil) was taken as the average of the measured

537 silicate residue values which are within error of average upper continental crustal
538 estimates ((Nd/Fe) x 1000 = 0.33±0.1, Supplementary Text 3). (Nd/Fe)labile is the measured

539 HH leachate composition at each sampling site. Using Monte Carlo methods to
540 estimate errors, f<sub>Fe,auth</sub> is estimated to be between 11-29% (interquartile range, from
541 sum of Middle Channel samples sets, 2017 and 2018, Supplementary Table S14).

542

543 Despite the uncertainty it is clear that the SPM in the Mackenzie must host a
544 significant fraction of labile Nd and Fe which is derived from inherited authigenic
phases in sedimentary rocks. Accordingly, the weathering of sedimentary rocks, especially marine shales, must supply a significant amount of reactive immobile elements (Nd, Fe) in the Mackenzie basin.

## 4.5 The flux of labile, potentially bioavailable, Fe

We have demonstrated that the SPM in the Mackenzie River has a labile Fe phase.

To estimate its supply to the Arctic Ocean, we use the suspended sediment flux, which is well documented for the Mackenzie in the years 1974-1994. There is significant inter-annual variability ranging from 81 Tg/yr to 224 Tg/yr (Carson et al., 1998) with a consistent seasonal peak in the summer months following freshet, the time period over which our samples were collected (Fig. 2).

The labile, potentially bioavailable, Fe concentration was considered to be equal to the HH leach Fe concentration measured on SPM from the Mackenzie (Middle Channel, 2017 and 2018), which approximated to a normal distribution (1466±350 µg/g (±1σ, n=16)). The uncertainty was estimated using Monte-Carlo methods (n=100000), similar to that used by Hilton et al., 2015. The sediment flux was considered as a uniform distribution, taking into account all potential values at equal probability (81 Tg/yr to 224 Tg/yr, Carson et al., 1998), a maximum approximation of error. The median (±interquartile range) flux of labile Fe associated with SPM in the Mackenzie River was calculated to be 0.21(+0.06,-0.05) Tg/yr. We do not include dissolved (<0.2 µm) Fe concentrations in this estimate as they are negligible in comparison to Fe hosted in the reactive sediment phase (estimated dissolved Fe flux represents <0.03% of the labile sediment Fe flux, Supplementary Text 4).
570

571 The calculated flux of potentially bioavailable Fe in the SPM of the Mackenzie is substantial; comparable to that from the Greenland Ice Sheet (~0.3 Tg/yr, Bhatia et al., 2013). Greenland Ice Sheet fluxes are estimated using the sum of strong SPM leaching and dissolved Fe followed by the application of an estuarine loss factor (90%, Bhatia et al., 2013).

576

577 Much of the SPM from the Mackenzie is rapidly buried offshore or trapped in the delta, and the loss due to estuarine processes is unknown. Even if ~90% of the bioavailable Fe in the SPM is removed or trapped, the resulting flux to the oceans is still significant (~0.02 Tg/yr). Moreover, benthic release of this particulate associated Fe, alongside Nd, on the Mackenzie shelf may play an important role in delivering a higher proportion of this labile phase to the Arctic Ocean.

583

584 For every two moles of sulfate released from the oxidative weathering of pyrite in the Mackenzie basin, 1 mole of Fe will be incorporated into an Fe-oxide. Using the pyrite-derived sulfate flux of Calmels et al., 2007 it is estimated that the oxidative weathering of pyrite will result in the production of ~3.6 Tg/yr of Fe, as Fe-oxides. This is around an order of magnitude greater than the labile Fe flux associated with SPM that we estimate herein (0.2 Tg/yr). This implies a large accumulation of Fe-oxides derived from the oxidative weathering of pyrite within the critical zone in the Mackenzie basin, which are not exported to labile phases in the SPM.

592

593 **4.6 Nd isotopic end-member compositions and inputs to the Arctic ocean**

594
The Mackenzie River is a large point source of sediment and water to the Arctic Ocean; therefore, it is an important finding that labile Nd has an isotopic composition that is distinct from the bulk SPM. This finding is potentially applicable to all riverine catchments with marine sedimentary source rocks. It is equally noteworthy that there is variability in dissolved εNd between sampling years. Although our data is similar to the only previous dissolved data point on the Mackenzie (Zimmermann et al., 2009), it extends the range with implications for studies involving εNd in the Arctic Ocean (e.g. Deschamps et al., 2019). Despite variability the HH leachates from shallow marine core sites reflect the average dissolved composition of the Mackenzie over decadal timescales. Therefore, such leachates from shelf sediment may provide a more useful integrated εNd end-member. This isotopically distinct, labile, phase is maintained offshore, meaning that any labelling of seawater via 'boundary exchange' processes on the shelf will have a significantly more radiogenic composition than the bulk sediment. This suggests that bulk shelf sediment and continental margin εNd may be systematically offset from the likely oceanic source (Jeandel et al., 2007) as it is the reactive (leachable) compositions which will characterise any shelf inputs.

5 Conclusions

This study highlights the potential for sedimentary rock weathering in the Arctic to provide a critical source of labile Fe. As approximately half of the rocks weathered on land draining into the Arctic Ocean are shales (Amiotte Suchet et al., 2003), similar observations to this study would be expected in other Arctic rivers. The on-going state of permafrost thaw in the Mackenzie basin has resulted in increased slumping in the Peel catchment which has exposed fresh pyrite minerals and resulted in
620 increased sulfuric acid weathering (Zolkos et al., 2018). Increased exposure of pyrite
621 may also lead to increased mobility and dissolution of REEs (and Fe) within this
622 catchment. However, it is also important to note that delivery of bioavailable Fe from
623 the Mackenzie basin to the Arctic Ocean will have changed in the past, including
624 during periods of glaciation and glacial retreat in this region. Melt water and ice-
625 rafting could have allowed for more direct delivery of un-weathered labile Fe phases
626 to the Arctic Ocean.

627

628 Recycled sedimentary components within source rocks may provide labile
629 micronutrients and other trace elements, a process which is likely applicable globally,
630 and not just in the Arctic. Fe, and the supply of other micronutrients that limit primary
631 productivity, are thought to be capable of having long-term and far-reaching effects
632 on global climate, by way of their potential for increasing productivity and organic
633 carbon burial in the oceans. Therefore, when considering increases in nutrient
634 availability in the past, the source lithology and its reactivity must be taken into
635 consideration. We highlight the impact of source lithology on the budget of labile
636 trace elements (Nd) and micronutrients (Fe) in rivers, which has implications for both
637 present and past fluxes to the oceans and paves the way for restructuring how
638 riverine SPM is viewed in terms of its reactivity when sedimentary rocks are present
639 in the source catchment.

640

641 Acknowledgements

642

643 M. Greaves, G. Hughes, H. Chapman, M. Bickle and K. Relph are thanked for their
644 assistance in the lab. E. Amos and M. Schwab are thanked for assisting with
645 fieldwork. E. Stevenson is thanked for her help and advice on Arctic fieldwork and

646 the co-precipitation method. M. Murphy and C. Arendt are thanked for sharing their

647 methods for ultra-filtration and $FeCl_3$ cleaning respectively. Y. Plancherel and E.

648 Stevenson are thanked for supplying filtered Severn River water used as an internal

649 standard. K. Jarrett and the Geological Survey of Canada, Marine Geoscience

650 Collection are thanked for supplying sediment core subsamples. The authors both

651 thank and acknowledge the assistance of the Aurora College Research Institute,

652 Inuvik. Samples were collected under research licenses 15288 and 16106. C.S.L

653 was funded by a NERC studentship (NE/L002507/1) with support for travel to

654 Canada in 2018 from the University of Cambridge Department of Earth Sciences

655 Leave to Work Away Research Fund. R.G.H acknowledges funding from the

656 European Research Council (Starting Grant ROC-CO₂, 678779) and a NERC UK-

657 Canada Arctic Partnership Bursaries Program. E.T.T acknowledges funding from

658 NERC Standard Grant 703 NE/P011659/1. The manuscript was greatly improved by

659 comments from two helpful anonymous reviewers and the editor L. Derry.

660

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878

879

880

881

882 **Figure Captions**

883

884 **Figure 1.** Area of study A. Sample locations and elevation (GDEM, resolution 30 Arc

885 second). B. Bedrock geology of the Mackenzie Basin (shaded black line),

886 categorized by rock type (Wheeler et al., 1996). White circles indicate the location of

887 sampling sites identified in A.
888

889 **Figure 2.** Water discharge from the Mackenzie River (2017 and 2018). Daily discharge (lines) from Environment Canada gauging station at Tsiigehtchic, retrieved from the ArcticGro database (https://www.arcticrivers.org/data, Supplementary text S2). ADCP Discharge at the Middle Channel and Tsiigehtchic (this study, light blue, 2017, dark blue 2018).

894

895 **Figure 3.** εNd on coupled HH leachates (circles), dissolved (<0.2µm only shown) and silicate residue sediment (triangles) at all sampling localities (2017 and 2018).
896
897 Probability density of sediment residue (green) and leachate (pink) shown, difference in modes (dashed grey line) is indicated (black arrow). Open symbols are bank sediment and coloured symbols, SPM, coloured diamonds are sediment core data.
898
899 Coloured squares are a shale fragment HH leachate and residue (pink and green respectively), which was not used in the linear regression. Error bars are 2σ analytical uncertainty (where visible or smaller than the symbol size). 1:1 line (dashed line) is shown.
900
901
902
903
904

905 **Figure 4.** Two sediment depth profiles from the Mackenzie River (Middle Channel, 2017). Water depth is normalised to the deepest sample. Error bars are 2σ analytical uncertainty. Dissolved compositions are from waters obtained at the surface and at depth.

909

910 **Figure 5.** Probability density of the fraction of a suite of elements in leachates relative to the total (sum of leachates and residue). Data is from all sampling sites across both years. The fraction of the HH leachate, acetic acid leachate, and the
913 sum of all leaching steps are shown. Symbols are data points that form the
914 probability distribution.

915

916 **Figure 6. Characterisation of sequential extractions A. MREE/MREE***

917 HREE/LREE cross-plot, following Martin et al., 2010, alongside a non-exhaustive
918 literature compilation (plotted for reference, Supplementary Text S2). HREE= Σ(Tm,
919 Yb, Lu) LREE= Σ(La, Pr, Nd), MREE= Σ(Gd, Tb, Dy), MREE*=(HREE+LREE)/2. All
920 concentrations are normalised to PAAS (after Pourmand et al., 2012). Foraminiferal
921 coatings and marine leachates are extractions thought to represent Fe-Mn
922 oxyhydroxides. **B. A REE partitioning in a Mackenzie (Middle Channel, 2017) SPM
923 sequential extraction. Left hand side axis is the REE concentration, right hand side
924 axis fraction of total (sum of leachates and residue) REE concentration (bars) in
925 each phase. C. ⁸⁷Sr/⁸⁶Sr against εNd on Mackenzie (Middle Channel, 2017) SPM
926 sequential extractions and water (⁸⁷Sr/⁸⁶Sr data point from Tipper et al., 2021). 2σ
927 analytical uncertainty is shown, unless smaller than the symbol size.

928

929 **Figure 7.** Schematic indicating the mechanisms presented herein that may lead to
930 more radiogenic εNd in the HH leachates of SPM and the dissolved load of the
931 Mackenzie river, when compared to the SPM silicate residue.

932

933 **Figure 8.** Probability density of calculated f<sub>Nd,auth</sub> (equation 1) for 4 sampling sites
934 generated using Monte Carlo methods. Median value (white line) and interquartile
935 range (dashed lines) are shown. Input compositions for this calculation are listed in
936 Table S13.

937
938

939

940

941

942

943

944

945

946

947

948

949

950

951

952

953
access/download;Figure;Figure

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>
<figure><img src="image_4.png" /><figcaption>Click here to access/download:Figure;Figure 2.pdf</figcaption></figure>
<figure><img src="image_5.png" /></figure>

$R^2=0.7525$
A figure

<figure><img src="image_6.png" /></figure>

B

Click here to access/download;Figure;Figure 4.pdf
<figure><img src="image_7.png" /></figure>

Click here to access/download;Figure;Figure 5.pdf

Fraction of total
<caption>Figure 6</caption>

Click here to
access/download;Figure;Figure 6.pdf

A

<figure><img src="image_8.png" /></figure>

B

<figure><img src="image_9.png" /></figure>

C

<figure><img src="image_10.png" /></figure>
Figure

Click here to
access/download;Figure;Figure 8.pdf

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>
Sedimentary Rock

<figure><img src="image_13.png" /></figure>

Dissolved

εNd

=

HH

leach

εNd

=

εNd<sub>labile</sub>
<figure><img src="image_14.png" /><figcaption>**Table 1.** εNd and Nd concentrations on different filtrates from the Mackenzie River and a major tributary (2018).</figcaption></figure>

<table><thead><tr><th>River</th><th>Filter Size/MWCO</th><th>εNd</th><th>Error (2σ)<sup>1</sup></th><th>[Nd] ppt</th><th>Error (2σ)</th><th>% of &lt;0.2µm</th></tr></thead><tbody><tr><td>Mackenzie</td><td>&lt;0.2µm</td><td>-12.97</td><td>0.15</td><td>20.7<sup>2</sup></td><td>0.7 (n=2)</td><td></td></tr><tr><td>River (Middle Channel)</td><td>10kda</td><td>-12.41</td><td>0.51</td><td>10.4<sup>3</sup></td><td>1.4 (n=2)</td><td>50%</td></tr><tr><td></td><td>1kda</td><td>-13.40</td><td>0.34</td><td>5.9<sup>3</sup></td><td>0.4 (n=2)</td><td>29%</td></tr><tr><td></td><td>&lt;0.2µm</td><td>-11.47</td><td>0.15</td><td>12.1</td><td>n/a (n=1)</td><td></td></tr><tr><td>Peel River</td><td>10kda</td><td>-11.61</td><td>0.34</td><td>7.1<sup>2</sup></td><td>0.1 (n=2)</td><td>59%</td></tr><tr><td></td><td>1kda</td><td>n.d</td><td>n.d</td><td>4.1<sup>3</sup></td><td>0.8 (n=2)</td><td>34%</td></tr></tbody></table>

<footnote n="1">¹Analytical uncertainty, ²Average and 2σ of two replicate measurements during different analytical sessions, ³Average and 2σ of two full procedural replicates (including filtration)</footnote>
**Author contributions**

C.S.L, E.T.T., A.M.P, R.G.H and R.S.H designed the study. C.S.L, R.G.H, M.D, J.J.B carried out the sample collection and fieldwork. R.G.H and E.T.T and C.S.L obtained funding for fieldwork and analysis. C.S.L and R.W carried out method development to analyse the samples. C.S.L analysed the samples and wrote the manuscript under the supervision of E.T.T and A.M.P with input and guidance from all co-authors.