# Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK

Heather Goring-Harford, Jessica Klar, Hannah Donald, Christopher Pearce, Douglas Connelly, Rachael James

► To cite this version:

Heather Goring-Harford, Jessica Klar, Hannah Donald, Christopher Pearce, Douglas Connelly, et al.. Behaviour of chromium and chromium isotopes during estuarine mixing in the Beaulieu Estuary, UK. Earth and Planetary Science Letters, 2020, 536, pp.116166. 10.1016/j.epsl.2020.116166. hal-03400756

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<figure><img src="image_1.png" /></figure>
Behaviour of chromium and chromium isotopes during estuarine mixing in the
Beaulieu Estuary, UK

Heather J. Goring-Harford¹*, Jessica K. Klar²,³, Hannah K. Donald¹, Christopher R. Pearce⁴,
Douglas P. Connelly⁴, Rachael H. James¹

¹ School of Ocean and Earth Science, National Oceanography Centre, University of Southampton Waterfront Campus, Southampton, SO14 3ZH, UK.

²Université de Perpignan Via Domitia, Centre de Formation et de Recherche sur les Environnements Méditerranéens (CEFREM), UMR 5110, 52 Avenue Paul Alduy, F-66860 Perpignan Cedex, France

³CNRS, CEFREM, UMR 5110, 52 Avenue Paul Alduy, F-66860 Perpignan Cedex, France

⁴ Marine Geoscience, National Oceanography Centre, University of Southampton Waterfront Campus, Southampton, SO14 3HZ, UK.

*Corresponding Author

Email address: [h.j.goring-harford@soton.ac.uk](mailto:h.j.goring-harford@soton.ac.uk)
**Abstract**

Rivers are the principal source of chromium (Cr) to seawater and the Cr isotopic signatures of ancient marine sediments are widely considered to provide a record of the presence or absence of oxidative weathering processes on land. This assumes, however, that the δ⁵³Cr value of river water is faithfully transferred to the oceans and is not modified in the estuarine mixing zone. To test this assumption we have determined the concentration and δ⁵³Cr values of inorganic Cr (Cr(III)+Cr(VI)), and also Cr speciation for water samples collected within the estuarine mixing zone of the Beaulieu River, UK. The δ⁵³Cr values of dissolved inorganic Cr ranged from -0.59 to 1.68‰, Cr(VI) concentrations from 0.39 to 1.83 nmol kg⁻¹ and Cr(III) concentrations from 0.11 to 3.21 nmol kg⁻¹. Both Cr(VI) concentrations and δ⁵³Cr values increased linearly as a function of salinity, while Cr(III) concentrations decreased linearly with salinity. Thus δ⁵³Cr, Cr(III) and Cr(VI) all showed conservative behaviour in the estuarine mixing zone, and the δ⁵³Cr signature of Beaulieu River water was modified only by mixing between the river and seawater endmembers. The calculated average δ⁵³Cr value of the river water endmember (-0.39 ± 0.08‰) was, however, lower than the range that has been observed in other rivers, which we attribute to input of organically-bound Cr(III) released by anoxic weathering processes. This is supported by the fact that Cr recovered by UV irradiation was found to have low δ⁵³Cr values (-0.11 to -0.75‰). While input of Cr from anoxic weathering processes is unlikely to be an important source of Cr to the oceans today, this suggests that processes other than oxidative weathering may have an influence on the δ⁵³Cr values of estuarine and coastal waters on the local scale. The δ⁵³Cr value of the coastal seawater endmember (1.6 ± 0.4‰) was also higher than the range observed in the deep open ocean, due to *in situ* biogeochemical cycling of Cr. These factors need to be considered in the interpretation of marine sedimentary δ⁵³Cr records.

**Keywords:** chromium isotopes, chromium speciation, chromium cycling, estuarine mixing, anoxic weathering
1. Introduction

Chromium (Cr) is a redox sensitive element with two stable oxidation states, Cr(VI) and Cr(III),

at Earth surface conditions. It has four stable isotopes, ⁵⁰Cr, ⁵²Cr, ⁵³Cr and ⁵⁴Cr, which can

fractionate during oxidation and reduction reactions (Zink et al., 2010; Døssing et al., 2011;

Kitchen et al., 2012). Most ground water and seawater samples analysed to date are enriched

in heavy Cr isotopes (i.e. they have relatively high δ⁵³Cr values, where

$$ \delta^{53}\text{Cr} = \left( \frac{(^{53}\text{Cr}/^{52}\text{Cr})_{\text{sample}} - (^{53}\text{Cr}/^{52}\text{Cr})_{\text{NBS979 standard}}}{(^{53}\text{Cr}/^{52}\text{Cr})_{\text{NBS979 standard}}} \right) \times 1000; \text{ Ellis et al. 2002; Bonnand et al. 2013;} $$

Scheiderich et al. 2015; Holmden et al. 2016; Paulukat et al. 2016; Pereira et al. 2015; Goring-

Harford et al. 2018; Moos and Boyle 2018; Bruggmann et al. 2019) compared to crustal rocks

(δ⁵³Cr = -0.12 ± 0.10‰; Schoenberg et al. 2008). This has been attributed to the preferential

release of heavy Cr isotopes during the oxidative weathering of Cr(III), followed by transfer of

the Cr(VI) that forms to the oceans via rivers (Frei et al. 2009), and to the preferential removal

of light Cr isotopes during biologically-mediated redox reactions in seawater (Scheiderich et

al., 2015; Goring-Harford et al., 2018; Moos and Boyle, 2018).

Records of ancient seawater δ⁵³Cr values, preserved in the authigenic fraction of marine

sediments, are considered to provide evidence for the evolution of atmospheric oxygen, and

consequently life, on Earth (e.g. Frei et al. 2009; Crowe et al. 2013; Planavsky et al. 2014).

Critically, this interpretation relies on the assumption that the oxidative weathering signal is

faithfully transferred from rivers to seawater, as rivers are the main source of Cr to the oceans

(>90% of the total Cr input; Jeandel and Minster 1987; Bonnand et al. 2013; Reinhard et al.

2013; McClain and Maher 2016; Sun et al. 2019). The δ⁵³Cr range previously reported for

unpolluted river water is -0.3 to 1.7‰ (n=49; Figure 1; Frei et al. 2014; D'Arcy et al. 2016; Wu

et al. 2017; Andronikov et al. 2019), which is similar to that of seawater (0.02 to 1.7‰; n=156;

Bonnand et al. 2013; Scheiderich et al. 2015; Holmden et al. 2016; Paulukat et al. 2016;

Pereira et al. 2015; Goring-Harford et al. 2018; Moos and Boyle 2018; Bruggmann et al. 2019).

However, the mean value for river water (0.49 ± 0.45‰) is higher than the mean value for

seawater (1.07 ± 0.35‰), which implies that δ⁵³Cr values may be altered after oxidative
28 weathering occurs, for example by redox reactions of Cr in surface ocean waters (Scheiderich
29 et al., 2015; Goring-Harford et al., 2018; Bruggmann et al., 2019). Although other potential
30 mechanisms for Cr isotope fractionation in natural waters have yet to be investigated in detail,
31 it is likely that refinements to the model proposed by Frei et al. 2009 are required to properly
32 employ Cr as a proxy for atmospheric oxygenation (see also Toma et al. 2019).

33 Fractionation of Cr isotopes during transport from rivers to the ocean has the potential to
34 significantly modify δ53Cr values before preservation in marine sediments (Bonnand et al.,
35 2013; Paulukat et al., 2015; D'Arcy et al., 2016). Rivers typically contain between 2nM and
36 20nM of dissolved Cr (e.g. Bonnand et al. 2013 and references therein), though rivers draining
37 mafic lithologies or land contaminated with industrial waste can contain much higher levels
38 (100-2000nM; McClain and Maher 2016; Novak et al. 2014; Paulukat et al. 2015).
39 Chromium(VI) is the thermodynamically stable form of Cr in oxygen-replete waters (Elderfield,
40 1970), but rivers commonly contain both Cr(III) and Cr(VI) in varying proportions (Cranston
41 and Murray, 1980; Dolamore-Frank, 1984; Kieber and Helz, 1992; Abu-Saba and Flegal, 1995;
42 Gardner and Ravenscroft, 1996; Comber and Gardner, 2003; Saputro et al., 2014; McClain
43 and Maher, 2016), likely due to complexation of Cr(III) with organic molecules, which stabilises
44 it in solution (Dolamore-Frank, 1984; Kaczynski and Kieber, 1994; Abu-Saba and Flegal, 1995;
45 Buerge and Hug, 1998; Icopini and Long, 2002). Chromium(VI) shows conservative behaviour
46 in many estuaries, meaning its concentration is simply determined by the relative proportion
47 of freshwater to seawater (Cranston and Murray, 1980; Mayer and Schick, 1981; Dolamore-
48 Frank, 1984). However, in some estuaries Cr(VI) appears to be partially reduced to Cr(III) in
49 the presence of organic molecules or photochemically produced Fe(II) (Kieber and Helz, 1992;
50 Saputro et al., 2014; D'Arcy et al., 2016). The Cr(III) that is produced can be removed (i) at
51 low salinities due to the flocculation of organic materials (Cranston and Murray, 1980; Mayer
52 and Schick, 1981; Campbell and Yeats, 1984), or (ii) by adsorption onto sedimentary particles
53 (Mayer and Schick, 1981; Abu-Saba and Flegal, 1995; Jonas and Millward, 2010). As Cr(III)
54 is expected to be enriched in lighter isotopes (Ellis et al., 2002; Zink et al., 2010; Døssing et
al., 2011; Kitchen et al., 2012), this may drive δ<sup>53</sup>Cr values in the remaining Cr(VI) that is delivered to the oceans to higher values (Bonnand et al., 2013; Paulukat et al., 2015; D'Arcy et al., 2016).

To date, Cr isotope behaviour has only been studied in the Connecticut estuary, USA (Sun et al., 2019). This study revealed that there were no systematic variations in δ<sup>53</sup>Cr (or Cr concentration) as a function of salinity, although Cr speciation was not determined. Given the diverse range of Cr reactions that can occur in estuarine mixing zones, further investigation is needed to assess their potential for modification of δ<sup>53</sup>Cr. This study reports the results of a systematic investigation of δ<sup>53</sup>Cr values and Cr speciation with respect to salinity in the Beaulieu River and estuary (UK).

## 2. Sampling location

The Beaulieu River is located in the New Forest National Park (UK), and drains into the strait between mainland England and the Isle of Wight known as the Solent (Figure 2). The protected status of the National Park means that its catchment is sparsely populated and there is little industry, so levels of pollution in the Beaulieu River are minimal compared to most UK rivers.

The New Forest primarily consists of heathlands, woodlands, bogs and wetlands. Surficial deposits are predominantly the Paleogene Barton and Headon Group sands, although Quaternary alluvia, clays and silts are found around the river and promote the growth of forest patches (Gilkes 1968). There are also tidal flats around the mouth of the estuary. The upper limit of the estuary is marked by a sluice gate, upstream of which is a small lake (the Mill Dam; Figure 2) that sometimes contains a small component of seawater. The depth of the river does not usually exceed ~1m whereas the depth of the estuary changes considerably with the state of the tide, varying between ~0.1 and 4m. The freshwater residence time in the estuary is ~7 days (Fang, 1995), which is similar to the residence time of water in the Solent (~6.25 days; Dyer and Lasta King 1975).
Concentrations of dissolved oxygen are high (up to 370µmol L⁻¹) throughout the river, and pH varies between 6.5 and 7.8 (Hopwood et al. 2014). Due to the nature of the vegetation in the catchment, the river waters have high levels of dissolved organic carbon (DOC), and estuarine waters can have DOC concentrations of >1mg L⁻¹ even at high salinities (S = >24) (Moore et al. 1979). Concentrations of dissolved iron (dFe) in the river are highly variable (8-21µM), and they fall significantly as the river water mixes with seawater, because Fe is incorporated into organic flocculates or forms Fe oxyhydroxide precipitates (Holliday and Liss, 1976; Fang, 1995; Hopwood et al., 2014). Up to ~50% of Fe at low salinities (S = <5) is present as Fe(II) (Hopwood et al., 2014), which is an effective reductant for Cr(VI) (Døssing et al., 2011; Kitchen et al., 2012).

The total dissolved concentration of Cr in the Solent adjacent to the Beaulieu estuary (also known as Southampton Water) is ~1.3-1.8nM (Bonnand et al., 2013), although higher concentrations (up to 5nM) have been recorded in the Beaulieu River itself (Dolamore-Frank, 1984). In a previous study, dissolved Cr appeared to behave conservatively during estuarine mixing, although loss of Cr(VI) from solution was suggested at high salinities, possibly as a result of reduction of Cr(VI) by DOC and removal of the Cr(III) that formed (Dolamore-Frank, 1984). Nevertheless, concentrations of particulate Cr were low even though dissolved concentrations of Cr(III) were relatively high, indicating that adsorption of this Cr(III) onto particulate material was not an important process in the estuary (Dolamore-Frank, 1984). The δ⁵³Cr value of total dissolved Cr in Southampton Water at salinities of 30 and higher appears to be consistent over time at 1.50 ± 0.02‰, (n = 7; Bonnand et al. 2013; Goring-Harford et al. 2018) though the Cr isotopic compositions of river and estuarine waters of the Beaulieu have not hitherto been measured.

## 3. Methods

### 3.1. Sampling

All containers and sampling/filtration equipment were acid cleaned before use. A river water endmember sample was taken from the King's Hat Enclosure on 20th March 2016 and the
108 estuary was sampled around high tide on 22nd March 2016 from the R.V. *Bill Conway*. A rigid-
109 inflatable boat (RIB) was used to access the shallowest waters, but the low to intermediate
110 salinities ($S = 1-14$) were inaccessible due to low rainfall in the preceding week, and the rapid
111 mixing between freshwater and seawater at the sluice gate. A second set of samples targeting
112 a lower salinity range was therefore taken on 5th October 2016 from the river, Mill Dam and
113 upper part of the estuary waters during the early stages of the flood tide. Rainfall in the week
114 preceding each sampling date were 0.0mm for March and 8.8mm for October (measured by
115 a Davis Vantage Pro Plus weather station; http://www.tottonweather.co.uk/data-
116 summary/).

117 All samples were recovered from 0.1-0.5m depth below the surface using a 1L bottle either by
118 hand or by deploying a weighted metal free bottle holder. The 1L aliquots were homogenised
119 in a larger container (5-6L total sample volume) and a subsample of this was taken
120 immediately to measure salinity, pH and temperature (WTW Measurement Systems Inc. 340i
121 handheld multimeter).

122 River sediment pore waters were sampled by inserting 80mm diameter plastic core liners into
123 the river bed at King's Hat Enclosure in April 2017. Cores were immediately transported back
124 to the National Oceanography Centre Southampton (NOCS) and placed in a constant
125 temperature laboratory at 10°C (to approximately match the temperature of the river water)
126 before processing.

3.2. Determination of Cr concentration and Cr isotopic composition of river and estuarine waters

129 All acids used in sample processing were thermally distilled, while the other chemicals were
130 Romil UpA, Fluka TraceSELECT Ultra, or Aristar® Ultra grade and contributed negligible Cr
131 to the total procedural blank.

132 Water samples were filtered within 24 hours of collection in a class 100 clean laboratory using
133 pre-rinsed Sartorius Sartobran 300 capsule filters (0.45µm). A $ ^{50}Cr + ^{54}Cr(III) $ double spike was
added to samples being processed for isotopic analysis to account for any fractionation
incurred during sample processing (full details of the spike are given in Goring-Harford et al
2018). At least 100ng of Cr, at a concentration of ~30 ppb in the analyte, was required for
isotopic analysis. Because the Cr concentration of river waters and seawater is much lower
(typically ~0.1ppb), it was necessary to pre-concentrate Cr. To do this we employed a Fe(II)
co-precipitation method (Bonnand et al. 2013; Scheiderich et al. 2015; Goring-Harford et al.
2018). Samples collected in March 2016 were preserved by acidification to pH <2 using 2ml
L-¹ of sub-boiled HCl prior to Fe(II) precipitation (except for the aliquots taken for Cr(III)
analysis). The October 2016 samples were precipitated with Fe(II) on the day of filtration
(within ~8 hours). Both approaches are expected to successfully capture all inorganic Cr, as
Cr(III) adsorption to container walls is prevented both by low pH conditions prior to spiking and
by prompt co-precipitation after collection; adsorption is not thought to be important for Cr(VI)
(Gaillardet et al. 2003).

The pH of the river water samples (~3L volume) was adjusted to pH 8-9 approximately 24
hours after the double spike was added. Total dissolved inorganic chromium (CrT, defined as
Cr(III) + Cr(VI)) was then removed from solution using an Fe(II) hydroxide precipitate (made
from 2mM ammonium Fe(II) sulphate, Sigma-Aldrich batch 04728LI, 10ml L⁻¹ of sample) that
converts any Cr(VI) to Cr(III) and quantitatively removes the Cr(III) in the form of a Fe(III)-
Cr(III) precipitate (Shigematsu et al., 1977; Cranston and Murray, 1978; Dolamore-Frank,
1984; Jeandel and Minster, 1984). The precipitate was then separated by vacuum filtration
using acid cleaned Millipore Omnipore filters (1 µm). A two-stage ion exchange
chromatography procedure was carried out to remove the Fe and residual salts using the
Biorad AG1-X8 and AG50-X12 resins respectively (Bonnand et al., 2013), and any remaining
organic material was oxidised using H₂O₂. The Cr isotopic composition of the purified Cr
sample-spike mixture was determined using a Thermo Fisher Neptune multicollector
inductively coupled plasma mass spectrometer (MC-ICP-MS) in medium resolution mode.
Newton-Raphson deconvolution and isotope dilution calculations were used to calculate
δ⁵³Cr_T and Cr_T respectively. Raw data were corrected for instrumental drift and the total
procedural blank contribution, which mainly came from the Fe precipitate and typically
constituted <10% of the total Cr analysed (blank δ⁵³Cr_T = -0.34 ± 0.32‰ 2SD, n = 6). Repeat
analyses of the NBS979 standard, normalised to the daily average NBS979 value, yielded an
analytical reproducibility of ±0.04‰ (n = 344, 2SD). The external reproducibility of co-
precipitated samples, determined from multiple analyses of the OSIL salinity standard (n = 6),
was better than ±0.11‰ (Goring-Harford et al., 2018).

It is important to note that this technique is not expected to collect organically bound dissolved
Cr (CrORG), as this fraction is thought to be resistant to reduction and adsorption (Nakayama
et al., 1981; Abu-Saba and Flegal, 1995). By contrast, previously published analyses of δ⁵³Cr
in river waters have been obtained from samples that were pre-concentrated by evaporation;
a technique considered to capture CrT+ORG (Frei et al., 2014; Paulukat et al., 2015; D'Arcy et
al., 2016). Nevertheless, a wide range of organic complexes with different binding strengths
are present in the Beaulieu River (Hopwood et al., 2015), and our Fe co-precipitation method
may capture weakly complexed CrORG, thus CrT values reported in this study are considered
to be operationally defined. Filtered aliquots of two samples (B1 and B2) were analysed after
UV irradiation for ~3 hours in LDPE bottles using a UV box containing 4 x G20T10 lamps, in
an attempt to oxidise any CrORG and assess the potential contribution of this fraction to the
total dissolved Cr pool.

3.3. Determination of Cr speciation in river and estuarine waters

Dissolved Cr(III) concentrations were measured using a modified version of the Fe co-
precipitation technique, wherein Fe(III) hydroxide was used in place of Fe(II) hydroxide to
collect Cr(III) only. Cr(III) is strongly adsorbed to Fe(III) hydroxide, whereas Cr(VI) is not,
allowing effective separation of the two species (Shigematsu et al., 1977; Cranston and
Murray, 1978; Dolamore-Frank, 1984; Jeandel and Minster, 1984). As for CrT, Cr(III) values
are considered operationally defined because there is potential to collect some weakly bound
CrORG. A 500 mL sub-sample for Cr(III) analysis was taken from the bulk 5-6L sample and
filtered, spiked and treated with the Fe(III) hydroxide precipitate (3 mL) either in a class 100 clean laboratory at NOCS or on board the R.V. *Bill Conway*. The Fe(III) solution was prepared in the same manner as the Fe(II) solution described in Section 3.2, except it was allowed to oxidise for 1 week with occasional shaking. As the proportions of Cr(III) and Cr(VI) in natural water samples may change rapidly after sample collection (Kingston et al., 1998), co-precipitation was done within 4 hours of sample collection. Blank solutions of Milli-Q water were processed alongside the samples.

The $^{50}$Cr+$^{54}$Cr double spike was replaced with a $^{53}$Cr(III) single spike for Cr(III) analysis and isotope dilution calculations were made using the $^{52}$Cr/$^{53}$Cr ratio. The lowest analytical error occurs when the $^{52}$Cr/$^{53}$Cr ratio is 0.1 - 3.5 (Bedson, 2007); all samples had ratios of 0.1 - 1.1. Despite the lack of isobaric interferences on $^{52}$Cr and $^{53}$Cr, large quantities of residual Fe and salts will interfere with ICP-MS analysis so an ion exchange procedure similar to that described above was used to purify the Cr fraction. $^{52}$Cr/$^{53}$Cr ratios were then measured on an Element 2 ICP-MS. The accuracy of this procedure was tested by processing Cr(III) standards (NBS979) and mixed Cr(III) + Cr(VI) standards (NBS979 and potassium chromate, Fluka TraceCERT®) in the same way as samples. Cr(VI) concentrations were subsequently calculated by subtracting Cr(III) from $Cr_T$.

## 3.4. Analysis of dissolved iron concentrations

Total dissolved iron concentrations in the <0.45µm fraction were measured using the ferrozine method (Stookey, 1970). Approximately 5mL of each sample was filtered through pre-cleaned 0.45µm syringe filters (Millipore Millex) within 4 hours of collection. 3mL aliquots of the filtered water were immediately pipetted into vials pre-loaded with 0.2mL of 5mM ferrozine and 0.2mL of 10mM ascorbic acid. A series of Fe standards (0-50µM, made with ammonium Fe(II) sulphate) were treated in the same way. Under these conditions all Fe ($Fe(II) + Fe(III))$ is converted to Fe(II) that reacts to form a purple complex with ferrozine. The method collects inorganic Fe as well as organically complexed Fe, except for the most strongly bound Fe complexes (Hopwood et al., 2014). Fe concentrations were determined using a Unicom 8625
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UV/Visible spectrometer set to measure absorbance at 562nm. A 4cm cell was used to
maximise sensitivity and the detection limit was 0.13µM.

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**3.5. Processing of push cores and sampling of pore waters**

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Profiles of dissolved oxygen were measured for two of the four push cores using an optical oxygen meter (Pyroscience FirestingO2 with OX50 needle-type sensor) mounted on a micromanipulator (Pyroscience MU1). The uppermost 5cm of sediment from each core was then centrifuged to extract the pore water. Pore waters were combined and centrifuged once more; co-precipitation with Fe was not necessary. The pore waters were then vacuum filtered through pre-cleaned 0.45µm membrane filters (Whatman Polycarbonate). Organic material was oxidised by addition of 0.2mL H₂O₂ and refluxing in 1mL of aqua regia at 130°C for ~24 hours. Any precipitate was digested by refluxing in 0.5mL of hydrogen fluoride at 170°C for 4 days. The samples were then dissolved in 3mL of 7M HCl and dried before being passed through the ion chromatography procedure. The total procedural blank was ~3 ng, almost entirely contributed by the filter (2.4 ± 0.8 ng, n = 3). This is less than 1% of the Cr processed and is therefore considered negligible.

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4. Results

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**4.1. Method validation**

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It is essential that equilibration between natural Cr in the sample and Cr in the spike is achieved in order to produce accurate data, because the yield of Cr is <100% (9 – 60% in samples with S ≤ 10, and 60 - 85% in samples with S ≥ 10). Equilibration is effective in seawater samples (Jeandel and Minster, 1984; Bonnand et al., 2013; Scheiderich et al., 2015; Goring-Harford et al., 2018), but to verify that this was also the case for low salinity samples, we tested an archived Beaulieu river water sample (S = 0) that had not been acidified for Cr<sub>T</sub>, Cr(III) and Cr<sub>T+ORG</sub> using the <sup>53</sup>Cr spike, yielding concentrations of 0.63 nmol kg<sup>-1</sup>, 0.37 nmol kg<sup>-1</sup> and 0.73 nmol kg<sup>-1</sup> respectively. To obtain an independent Cr<sub>T+ORG</sub> value, two more aliquots were simply dried down and treated with H<sub>2</sub>O<sub>2</sub> to eliminate organic matter before analysis on the Element 2. The Cr<sub>T+ORG</sub> values of these samples (0.71 ± 0.09 nmol kg<sup>-1</sup> 2SD,
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n=2) were within error of the spiked and co-precipitated sample. Thus, this investigation demonstrates that: (i) spike equilibration is effective in low salinity waters containing a mixture of Cr(III) and Cr(VI); (ii) CrORG is effectively released by UV irradiation; and (iii) all Cr species have been determined by our analyses. Apart from its atomic mass, the ⁵⁰+⁵⁴Cr spike is identical to the ⁵³Cr spike, so it is reasonable to assume that spike equilibration was also effective for CrT and δ⁵³CrT analyses.

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For speciation analyses, the average total procedural blank for the samples that were partly processed on board the R.V. *Bill Conway* was 0.08 ± 0.02nmol (2SD, n=2). This is within error of our long term total procedural blank for samples processed in the clean laboratory (0.09 ± 0.06nmol 2SD; n=13), implying that the samples were not compromised by handling outside of the clean laboratory. Several authors have shown that Fe(III) co-precipitation is an effective technique for measuring Cr(III) in natural waters, with or without employing an isotopic spike (Shigematsu et al., 1977; Cranston and Murray, 1978; Dolamore-Frank, 1984; Jeandel and Minster, 1984). To check that the method is capable of producing accurate Cr(III) concentrations with our ⁵³Cr spike, Cr(III) and mixed Cr(III) + Cr(VI) standards in Milli-Q water were tested (Table 2). The Cr(III) standards gave values within ±4% of the expected value, demonstrating that little Cr(III) was lost to container walls before sample-spike equilibration occurred. Mixed Cr(III) + Cr(VI) standards had a slightly higher uncertainty of ±10% (Table 1). The higher uncertainty of the mixed standards may reflect speciation changes prior to addition of the Fe precipitate, as this was not done until ~24 hours after spiking. In an effort to circumvent this issue, the Fe precipitate was added to river water and estuary samples within 4 hours of sample collection; nevertheless, the uncertainty of Cr(III), Cr(VI) and CrT measurements are conservatively reported in this study as ± 10%. In support of this, sample B13 was processed twice and yielded an average Cr(III) concentration of 0.25nmol kg⁻¹ ± 7%.

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## 4.2. Cr and δ⁵³CrT variation in estuarine waters

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The range of CrT values for samples collected in March 2016 (1.64 to 1.89 nmol kg⁻¹; Table 2) was similar to that measured previously for this location (1.64 to 5.05nmol kg⁻¹; Bonnand et al.

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269 2013; Dolamore-Frank 1984), whereas the October samples had very low CrT (0.66 to 1.03 nmol kg⁻¹). On both sampling dates CrT decreased slightly with salinity. A negative correlation 270 was observed between Cr(III) and salinity (Figure 3C; R² = 0.95, p < 10⁻⁷ for March, R² = 0.57, 271 p < 0.1 for October), whereas Cr(VI) showed a positive correlation (Figure 3D; R² = 0.87, p < 272 10⁻⁵ for March, R² = 0.24, p < 0.4 for October). Approximately 15% of CrT was found to be 273 present as Cr(III) in high salinity waters (S = >29), compared with up to 55% in low salinity 274 waters (S = <5). The δ⁵³CrT values (-0.59 to 1.68‰) increased linearly with increasing salinity 275 (Figure 3E; R² = 0.98, p < 10⁻¹⁰ for March, R² = 0.84, p < 0.02 for October). Assuming that the 276 relationship between salinity and δ⁵³CrT is linear, even at very low salinity (see Section 5.1), 277 then the extrapolated δ⁵³CrT value of the river endmember at S = 0 was the same in March 278 (δ⁵³CrT-CALCULATED = -0.43 ± 0.06‰) and October (δ⁵³CrT-CALCULATED = -0.34 ± 0.13‰). 279

## 4.3. Cr associated with organic material

281 Tests performed using the ⁵³Cr single spike method demonstrated that UV irradiation did not 282 contribute any significant Cr to the total procedural blank, as the average UV irradiated blank 283 (0.06 ± 0.03nmol, n = 3) was within error of the long term procedural blank (0.09 ± 0.06nmol; 284 n=13).

285 Water samples B1 and B2 irradiated with UV light had higher dissolved Cr concentrations 286 (CrT+ORG) compared to non-irradiated aliquots, indicating that at least 18% of the Cr in low to 287 mid salinity waters was organically bound (Table 3). The UV-irradiated samples also had lower 288 δ⁵³CrT+ORG compared to δ⁵³CrT in non-irradiated samples, which implies that the CrORG fraction 289 has a lower δ⁵³Cr composition than the CrT fraction. The δ⁵³Cr value of CrORG is given by:

290 $$ \delta^{53}\text{Cr}_{\text{ORG}} = \frac{(\delta^{53}\text{Cr}_{\text{T+ORG}} - (F_{\text{T}} \times \delta^{53}\text{Cr}_{\text{T}}))}{F_{\text{ORG}}} $$

291 Where FT = CrT/CrT+ORG and FORG = CrORG/CrT+ORG. B1 and B2 yield δ⁵³CrORG values of - 292 0.75‰ and -0.11‰, respectively (Table 3).
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## 4.4. Cr concentration and Cr isotopic composition of river sediment pore waters

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The sediment core-top waters contained 230-260µM of dissolved O₂, and the pH was 6.9 at approximately 1cm depth below the river bed (measured in one core only). Oxygen profiles (Figure 4) revealed a shallow oxygen penetration depth of 2.9 ± 1.7mm (2SD) below the sediment-water interface. CrT concentrations of pore waters from the two cores were 100 and 120nmol kg⁻¹ and δ⁵³CrT+ORG values were 0.06 ± 0.05‰ and 0.13 ± 0.05‰ respectively. These analyses included any CrORG in the pore waters as the Fe co-precipitation method was not used.

301

# 5. Discussion

302

## 5.1. Behaviour of Cr and Cr isotopes during estuarine mixing

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Concentrations of CrT in samples collected in October were significantly lower than in March, and compared to previous measurements (Dolamore-Frank, 1984). Dilution of surface water Cr may have played a part as there was more rainfall (8.8 mm) in October, however this cannot explain a reduction of ~50% in CrT. Another explanation is that a greater proportion of dissolved Cr(III) was contained in the organic fraction (which is not captured by our Fe co-precipitation method) due to the release of dissolved organic material in the autumn following the decomposition of deciduous tree leaves; this would have affected even the high salinity sample (B20, S = 29.6) because it was taken from a well vegetated part of the river (Figure 2). Organic molecules from leaf litter leachates have been shown to strongly bind copper in the early stages of decomposition because microbes transform or consume the most effective organic ligands (proteins and polyphenol-like compounds) over time (Cuss and Gueguen, 2012). It is possible that a similar situation applies to Cr. In support of this, large amounts of foam were observed on the river surface in October, suggesting that concentrations of organic surfactants were high.

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Figure 3A provides evidence for the removal of dissolved iron at low salinities in the Beaulieu Estuary as observed in previous studies (Holliday and Liss, 1976; Fang, 1995), which is thought to reflect the incorporation of Fe into flocculated organic particles (Sholkovitz et al.,
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1978; Moore et al., 1979; Hopwood et al., 2014). However, no relationship was observed between Cr<sub>T</sub> and dFe concentrations at low salinities (R<sup>2</sup> = 0.01 for October), suggesting that inorganic Cr species were unaffected by the flocculation of organic material during the transition from river to estuary.

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Concentrations of Cr(VI), Cr(III) and δ<sup>53</sup>Cr<sub>T</sub> exhibited linear relationships with salinity on both sampling dates, whilst Cr<sub>T</sub> showed little variation with salinity because the river and seawater endmembers had similar Cr<sub>T</sub> concentrations (Figure 3B-E). Re-release of Cr from particulate material at high salinities has previously been observed in both the Beaulieu estuary (Dolamore-Frank, 1984) and the Connecticut estuary (Sun et al., 2019), but there was no increase in Cr<sub>T</sub> that would suggest a similar mechanism in this case. Furthermore, the relationship between δ<sup>53</sup>Cr<sub>T</sub> and salinity was much stronger (R<sup>2</sup> = 0.98 and 0.84 for March and October respectively; Figure 3E) than that between δ<sup>53</sup>Cr<sub>T</sub> and Cr<sub>T</sub> (R<sup>2</sup> = 0.24 and 0.00 for March and October respectively; Figure 3F), strongly suggesting that salinity, rather than reduction and removal of isotopically light Cr(III), exerts the main control on δ<sup>53</sup>Cr<sub>T</sub>. The distributions of Cr(VI), Cr(III) and δ<sup>53</sup>Cr<sub>T</sub> in the Beaulieu estuary are therefore thought to be principally controlled by simple mixing between a high Cr(III)/low Cr(VI), low δ<sup>53</sup>Cr<sub>T</sub> river water endmember and a low Cr(III)/high Cr(VI), high δ<sup>53</sup>Cr<sub>T</sub> seawater endmember. This is supported by our mixing model (Figure 5; see Sections 5.2 and 5.3), where river and seawater endmember values for Cr(III), Cr(VI) and δ<sup>53</sup>Cr<sub>T</sub> for March have been calculated by linear extrapolation, and intermediate values calculated by adding the proportions contributed by the endmembers; there is good agreement between model and measured values. These findings contrast with those from the Connecticut estuary, where Cr removal onto suspended particulate material and into sediments at low to moderate salinities appeared to be associated with Cr isotope fractionation (Sun et al., 2019).

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It is possible that removal of Cr(III) and/or Cr(VI) occurs at very low salinities (<0.5), because sample B1 (which was taken from further upstream than the other samples), had a much higher Cr<sub>T</sub> concentration (5.05nmol kg<sup>-1</sup>) than the other samples. Reduction of Cr(VI) and

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subsequent removal of the Cr(III) that forms would drive the $\delta^{53}Cr_T$ composition of the remaining dissolved Cr to a higher value because the fractionation factors for Cr(VI) reduction by Fe(II) or DOC, the most abundant reductants in the Beaulieu River system, are large ($\Delta_{Cr(VI)-Cr(III)} = 3.6$ to $4.2\%$ and $3.1\%$ respectively; Døssing et al. 2011; Kitchen et al. 2012). Figure 5A models the $\delta^{53}Cr_T$ value of remaining dissolved Cr in the Beaulieu estuary for March, assuming that all of the Cr(III) in sample B1 ($3.21\text{nmol kg}^{-1}$, 64% of the $Cr_T$) was produced by reduction of Cr(VI), and that this Cr(III) was subsequently removed at $S = <0.5$, followed by conservative mixing at higher salinities. It is clear that this would result in much higher $\delta^{53}Cr_T$ values than those observed in the estuary, which instead fall along a conservative mixing line with a river water endmember $\delta^{53}Cr_{T-CALCULATED}$ value of -0.39‰ (see Section 5.2). Therefore, if Cr removal does occur, it must be associated with no, or minimal, isotopic fractionation.

## 5.2. Controls on the Cr concentration and Cr isotopic composition of the river water endmember

The variation in $Cr_T$ and $\delta^{53}Cr_T$ values is higher in the lower salinity samples that were collected from upstream of the sluice gate ($S \le 14.5$; Figure 3). In particular, samples B1 and B17 have high $\delta^{53}Cr_T$, high $Cr_T$ and a low proportion of Cr(III) relative to samples of similar salinity that may be indicative of input of Cr from a different source. Nevertheless, overall there is a linear relationship between $\delta^{53}Cr_T$ and salinity, so the average $\delta^{53}Cr_T$ value of the river water endmember can be estimated by extrapolating the regression line to $S = 0$. The estimated $\delta^{53}Cr_T$ value of the river water endmember, based on both the March and October data, is $-0.39 \pm 0.08\%$, lower than the range determined for most other rivers to date (Figure 1). It is also lower than the $\delta^{53}Cr$ value of crustal rocks ($-0.12 \pm 0.10\%$; Schoenberg et al. 2008), and the $\delta^{53}Cr_{T+ORG}$ values of pore waters in Beaulieu River bed sediments ($0.06$ to $0.13\%$). Although Cr isotope fractionation during oxidative weathering is poorly characterised, it is expected to produce aqueous Cr(VI) that has a higher $\delta^{53}Cr$ value than the crustal value (e.g. Frei et al. 2014; D'Arcy et al. 2016), thus oxidative weathering cannot be the principal control on the $\delta^{53}Cr_T$ value of the Beaulieu River.
Chromium can also be released into solution via dissolution of sedimentary Fe-Cr (oxy)hydroxides under anoxic conditions (Rigaud et al., 2013). The oxic-anoxic boundary is located close to the sediment-water interface in the Beaulieu River (Figure 4), thus given the high concentration of dissolved Cr<sub>T+ORG</sub> in the sediment pore waters (100-120nmol kg<sup>-1</sup>), diffusion of dissolved Cr(III) derived from the dissolution of Fe-Cr (oxy)hydroxides could be an important source of Cr into the Beaulieu river. Chromium contained in (oxy)hydroxides is expected to be enriched in lighter Cr isotopes (Døssing et al., 2011; Kitchen et al., 2012), so this mechanism may also account for the low δ<sup>53</sup>Cr<sub>T</sub> values in the Beaulieu river endmember. Analyses of sediment pore waters from the Beaulieu do not support this scenario, however, as their δ<sup>53</sup>Cr<sub>T+ORG</sub> values (0.06‰ and 0.13‰) were higher than that of the river water endmember (δ<sup>53</sup>Cr<sub>T-CALCULATED</sub> = -0.39‰; measured values as low as -0.59‰). Instead, we suggest that the δ<sup>53</sup>Cr<sub>T</sub> value of Beaulieu River water is primarily controlled by the presence of organic ligands (Figure 6). Mobile organic Cr(III) species are likely to form in the soil solution (Icopini and Long, 2002; Beck et al., 2008; Rigaud et al., 2013), and may be released into river waters. Formation of these organic Cr(III) complexes can occur via inorganic or biological reduction of Cr(VI), which may be derived from oxidation of Cr(III) in the uppermost oxic sediments, or from Cr(VI) that has formed via the diffusion and oxidation of dissolved Cr(III) originating from the underlying anoxic zone. Reduction of Cr(VI) by both mechanisms is associated with large isotopic fractionation factors (Δ<sub>Cr(VI)-Cr(III)</sub> = 1.8 - 4.5‰ for various bacteria and 3.1‰ for organic molecules; Sikora et al. 2008; Han et al. 2012; Kitchen et al. 2012; Basu et al. 2014). Thus, organic Cr(III) complexes produced in this way are likely to have a low δ<sup>53</sup>Cr value, consistent with our findings for δ<sup>53</sup>Cr<sub>ORG</sub> in the Beaulieu (-0.8 to -0.1‰; Table 3). Dissociation of these low δ<sup>53</sup>Cr(III) bearing complexes by photochemical reactions (Kieber and Helz, 1992) would also explain why δ<sup>53</sup>Cr<sub>T</sub> values were low and the proportion of dissolved Cr(III) was high in low salinity waters, and particularly as we sampled very shallow waters on clear, sunny days. In support of this hypothesis, it is notable that the δ<sup>53</sup>Cr<sub>T+ORG</sub> value of the UV irradiated sample B1 (0.06‰) was very similar to that of the pore waters (0.06-0.13‰) that were sampled in very close proximity; it appears that the river water directly reflects the pore
402

water Cr source when CrORG is taken into account. The changing proportion of Cr released
into solution *via* this process *versus* Cr released due to oxidative weathering may also explain
the higher scatter in the data at low salinities.

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## 5.3. Controls on the Cr concentration and Cr isotopic composition of the seawater endmember

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The estimated δ⁵³CrT value of the coastal seawater endmember at S = 35 (1.6 ± 0.4‰; calculated using March and October data) is similar to those previously measured in shelf waters (typically 1.3 to 1.5‰, S = 30 to 34; Bonnand et al. 2013; Scheiderich et al. 2015; Goring-Harford et al. 2018), but higher than the values typically measured in oxic deep water masses (1.0 to 1.2‰; Bonnand et al. 2013; Scheiderich et al. 2015; Goring-Harford et al. 2018). This likely reflects internal cycling of Cr within coastal and shelf waters, e.g. the reduction of Cr(VI) to Cr(III) in surface waters followed by removal onto biogenic particles, and the release of Cr from sedimentary sources (Scheiderich et al., 2015; Goring-Harford et al., 2018), Because the volume of the Beaulieu River is far smaller than that of Southampton Water and its CrT is low, the low δ⁵³CrT of the Beaulieu River does not have a significant impact Southampton Water δ⁵³CrT values. In other estuarine systems, however, high salinity shelf waters (which typically have similar Cr concentrations to Southampton Water, ~2 nM; Scheiderich et al. 2015; Goring-Harford et al. 2018) may be significantly impacted by riverine δ⁵³CrT if there is a larger riverine Cr input. Figure 7 shows how δ⁵³CrT values in high salinity waters would be modified by a river with similar δ⁵³CrT to the Beaulieu and a higher CrT of 15nM (similar to, for example, the Humber River; Comber and Gardner 2003). In this situation, the riverine input would still be significantly impacting δ⁵³CrT at S = 34 (1.18‰), causing it to be over 0.4‰ lower than the seawater endmember (S = 35, δ⁵³CrT = 1.60‰), whereas for a river like the Beaulieu (with much lower CrT), the difference is <0.1‰. Thus, rivers with high CrT may influence the δ⁵³Cr value of high salinity estuarine waters, and possibly even the adjacent continental shelf waters if the river and seawater endmembers also have very different δ⁵³Cr values.

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## 5.4. Implications for the δ<sup>53</sup>Cr redox proxy

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Whilst our data provide no insight as to the behaviour of organically bound Cr in estuaries, significant quantities of Cr<sub>ORG</sub> are unlikely to be transferred to the open ocean because DOC concentrations are extremely low in the open ocean (~40-100µmol C L<sup>-1</sup>, compared to up to ~5mmol C L<sup>-1</sup> in the Beaulieu River; e.g. Miller and Zepp 1995; Hansell and Carlson 1998).

Furthermore, organically bound Cr in the Beaulieu River is <20% of the total Cr, even though it has far higher DOC concentrations than most other rivers (Moore et al., 1979). Thus, Cr is most likely to be transferred to the ocean in its inorganic forms, with the composition of the dissolved Cr(III) and Cr(VI) species controlling the ‘baseline’ δ<sup>53</sup>Cr value of seawater on a global scale (though modifications will occur in the ocean itself; Scheiderich et al. 2015; Goring-Harford et al. 2018; Bruggmann et al. 2019). Our study shows that inorganic forms of dissolved Cr behave conservatively during estuarine mixing in the Beaulieu and that Cr(VI) is not reduced to Cr(III). If this were true for all estuaries, the global average riverine δ<sup>53</sup>Cr value would be faithfully transferred to the ocean. However, our study also reveals that anoxic, rather than oxidative, weathering processes are likely to be the principal source of Cr to the Beaulieu River, and that the δ<sup>53</sup>Cr value of the river water endmember is controlled by complexation with organic ligands. The operation of fractionating processes beside oxidative weathering potentially complicates the interpretation of seawater δ<sup>53</sup>Cr values in terms of the level of atmospheric oxygenation, although we note that on a global scale, the input of Cr from oxidative weathering in mafic catchments is likely to be far more important source of Cr to the oceans than from organic rich, non-mafic catchments (McClain and Maher, 2016).

Finally, in a scenario where riverine δ<sup>53</sup>Cr values are dissimilar to those of seawater, and especially where the riverine Cr concentration is high, δ<sup>53</sup>Cr values of shelf waters may be significantly influenced by input of riverine Cr (Figure 7). As a result, shelf sediments are likely to be poor candidates for redox proxy studies, because δ<sup>53</sup>Cr values will reflect a mixture of local and global scale processes.
455 6. Conclusions

456 In contrast to most river systems that have relatively high δ⁵³Cr values due to release of Cr(VI)
457 by oxidative weathering processes, dissolved Cr in the Beaulieu River is characterised by low
458 δ⁵³Cr_T values (-0.59 to 0.24‰). We suggest that in this DOC-rich river system, pore water
459 Cr(VI) is partly back-reduced to Cr(III) by organic or Fe(II) reductants, and the formation of
460 organic complexes keeps the Cr(III) in solution. Our results also demonstrated that Cr(III),
461 Cr(VI) and Cr isotopes behave conservatively in the estuarine mixing zone and, even in the
462 presence of high concentrations of dFe and DOC, we found no evidence for reduction of Cr(VI)
463 to Cr(III).

464 Knowledge of the behaviour of Cr and Cr isotopes in rivers and during estuarine mixing is
465 essential for the interpretation of the Cr isotope redox proxy. We have shown that oxidative
466 weathering is not the only source of dissolved Cr in organic-rich river systems, and that input
467 of Cr from anoxic weathering processes contributes Cr with a distinctly lower δ⁵³Cr value (~ -
468 0.4‰). At the global scale, however, such organic-rich rivers are unlikely to be an important
469 source of dissolved Cr to the oceans. While inorganic riverine δ⁵³Cr values appear to be
470 unmodified during estuarine mixing, the estimated δ⁵³Cr value of the seawater endmember
471 (1.6 ± 0.4‰) is higher than the range reported to date for most deep water masses (1.0-1.2‰),
472 which supports existing evidence that cycling of Cr within coastal and shelf seas may modify
473 the δ⁵³Cr value of seawater (Scheiderich et al., 2015; Goring-Harford et al., 2018). Internal
474 biogeochemical cycling of oceanic Cr therefore needs to be taken into consideration in the
475 interpretation of the seawater δ⁵³Cr record preserved in authigenic marine sediments in terms
476 of oxidative weathering. We also show that even in estuaries where Cr behaves
477 conservatively, δ⁵³Cr values of shelf waters can be modified by input of riverine Cr. The
478 sedimentary record of seawater δ⁵³Cr in these settings may therefore reflect local, rather than
479 global, processes.
480 Acknowledgements

481 This work was partly supported by NERC grant NE/H006443/1. HJGH was supported by a

482 PhD studentship funded by the Graduate School of the National Oceanography Centre

483 Southampton (NOCS), the University of Southampton and the NERC National Capability

484 programme. We thank Graham Blythe and Kevin Padley for assistance with fieldwork and

485 Andy Milton for technical support for the Element and Neptune mass spectrometers.
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733 **Figure captions**

734 Figure 1. δ<sup>53</sup>Cr data for unpolluted rivers that drain into the ocean (¹D'Arcy et al. 2016; ²Wu et al. 2017; ³Frei et al. 2014). Horizontal grey bar shows δ<sup>53</sup>Cr values for silicate crust; orange bar shows typical δ<sup>53</sup>Cr and Cr concentration of seawater (Bonnand et al., 2013; Scheiderich et al., 2015; Paulukat et al., 2016).

738

739 Figure 2. Sampling locations. Darker blue areas are intertidal flats.

740

741 Figure 3. Relationship between salinity and (A) dFe, (B) Cr<sub>T</sub>, (C) Cr(III) concentration, (D) Cr(VI) concentration, and (E) δ<sup>53</sup>Cr<sub>T</sub> (where error bars represent the external reproducibility; 0.11‰). (F) Relationship between Cr<sub>T</sub> and δ<sup>53</sup>Cr<sub>T</sub>. Error bars for Cr<sub>T</sub>, Cr(VI) and Cr(III) are 10%. The March 20<sup>th</sup> sample (B1) is excluded from A and from the trend lines in B-E because it is thought to be an outlier (see text for details).

746

747 Figure 4. Combined O₂ profile for river sediment push cores (n=3). Solid horizontal lines show the standard deviation from the mean value (none for -2.0 to -0.2mm as only one set of measurements taken). Dotted line is the sediment-water interface and dashed line is the average oxic-anoxic boundary depth.

751

752 Figure 5. (A) Modelled evolution of the δ<sup>53</sup>Cr value of Cr remaining in solution after reductive removal of Cr(VI) in sample B1 at S = <0.5, as a function of salinity. Sample B1 and B14 δ<sup>53</sup>Cr and Cr<sub>T</sub> values used to represent river and seawater endmembers respectively. River endmember δ<sup>53</sup>Cr for conservative mixing line = -0.39‰ (see text for details). (B) Modelled evolution of Cr(VI)/Cr(III) for conservative mixing. River (S = 0) and seawater (S = 35) endmember Cr(VI) and Cr(III) values extrapolated from March data.

758

759 Figure 6. Schematic diagram showing potential mechanisms for Cr release and redox transformations within sediments/soils in the Beaulieu River catchment (¹Frei et al. 2014; ²D'Arcy et al. 2016; ³Wu et al. 2017).

762

763 Figure 7. Modelled evolution of the δ<sup>53</sup>Cr value of Cr during conservative mixing between a seawater endmember with Cr<sub>T</sub> = 1.6 nM and δ<sup>53</sup>Cr = 1.6‰, and river water endmembers with various Cr<sub>T</sub> and δ<sup>53</sup>Cr values.

765
<figure><img src="image_2.png" /></figure>

Glenariff River¹

Connecticut River²

Parana River³

Uruguay River¹

Beaulieu (this study)
<figure><img src="image_3.png" /></figure>

● March 2016

■ October 2016

○ Sluice gate

☆ King's Hat Enclosure

☆ Bonnand et al. 2013
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

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

<figure><img src="image_7.png" /></figure>

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

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /></figure>

O₂ (µmol L⁻¹)

Anoxic boundary
(<5 µmol L⁻¹ O₂)
<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>
<figure><img src="image_13.png" /></figure>

$$ \delta^{53}\text{Cr} = -0.59 \text{ to } 0.24\text{‰} $$

$$ \delta^{53}\text{Cr} = -0.40 \text{ to } -0.10\text{‰}^{1,2,3} $$

$$ \delta^{53}\text{Cr} = 0.06 \text{ to } 0.13\text{‰} $$
<figure><img src="image_14.png" /></figure>
<table><caption>Table 1. Results of analysis of Cr(III) concentration by Fe(III) pre-concentration and addition of ⁵³Cr-enriched spike.</caption><thead><tr><th>Standard type</th><th>Calculated Cr(III)<br>(nmol kg<sup>-1</sup>)</th><th>Measured Cr(III)<br>(nmol kg<sup>-1</sup>)</th><th>Difference (%)</th></tr></thead><tbody><tr><td rowspan="6">Cr(III) only</td><td>11.00</td><td>11.23</td><td>2.0</td></tr><tr><td>10.77</td><td>11.19</td><td>3.8</td></tr><tr><td>10.58</td><td>10.48</td><td>-2.0</td></tr><tr><td>10.04</td><td>10.44</td><td>1.9</td></tr><tr><td>10.49</td><td>10.78</td><td>+2.8</td></tr><tr><td>10.45</td><td>10.23</td><td>-2.1</td></tr><tr><td rowspan="4">Cr(III) + Cr(VI)<br>(1:1 mix)</td><td>4.75</td><td>5.19</td><td>8.6</td></tr><tr><td>4.81</td><td>5.23</td><td>8.0</td></tr><tr><td>5.77</td><td>5.25</td><td>-9.7</td></tr><tr><td>5.77</td><td>5.23</td><td>-10.0</td></tr></tbody></table>
<table><caption>Table 2. Composition of water samples from the Beaulieu River and Beaulieu estuary. BDL = Below Detection Limit.</caption><thead><tr><th>Sampling date</th><th>Sample</th><th>Salinity (S)</th><th>pH</th><th>Temperature (°C)</th><th>δ<sup>53</sup>Cr (‰)</th><th>2SD</th><th>Cr<sub>T</sub> (nmol kg<sup>-1</sup>)</th><th>Cr(III) (nmol kg<sup>-1</sup>)</th><th>Cr(VI) (nmol kg<sup>-1</sup>)</th><th>dFe (µM)</th></tr></thead><tbody><tr><td>20<sup>th</sup> March 2016</td><td>B1</td><td>0.0</td><td>7.68</td><td>8.6</td><td>0.24</td><td>0.02</td><td>5.05</td><td>3.21</td><td>1.83</td><td>12.63</td></tr><tr><td rowspan="14">22<sup>nd</sup> March 2016</td><td>B2</td><td>14.5</td><td>7.81</td><td>10.4</td><td>0.57</td><td>0.04</td><td>1.89</td><td>1.02</td><td>0.87</td><td>0.68</td></tr><tr><td>B3</td><td>17.5</td><td>7.95</td><td>9.6</td><td>0.66</td><td>0.01</td><td>1.77</td><td>0.88</td><td>0.89</td><td>0.17</td></tr><tr><td>B4</td><td>20.0</td><td>8.10</td><td>11.3</td><td>0.89</td><td>0.00</td><td>1.70</td><td>0.70</td><td>1.01</td><td>0.30</td></tr><tr><td>B5</td><td>19.3</td><td>8.09</td><td>10.2</td><td>0.79</td><td>0.06</td><td>1.66</td><td>0.81</td><td>0.85</td><td>0.17</td></tr><tr><td>B6</td><td>21.1</td><td>8.09</td><td>10.3</td><td>0.89</td><td>0.02</td><td>1.74</td><td>0.67</td><td>1.07</td><td>0.59</td></tr><tr><td>B7</td><td>15.9</td><td>7.97</td><td>11.1</td><td>0.59</td><td>0.04</td><td>1.76</td><td>0.92</td><td>0.84</td><td>0.39</td></tr><tr><td>B8</td><td>18.2</td><td>8.05</td><td>10.3</td><td>0.72</td><td>0.02</td><td>1.81</td><td>0.72</td><td>1.09</td><td>0.34</td></tr><tr><td>B9</td><td>18.9</td><td>8.05</td><td>11.4</td><td>0.74</td><td>0.06</td><td>1.78</td><td>0.85</td><td>0.92</td><td>BDL</td></tr><tr><td>B10</td><td>24.3</td><td>8.11</td><td>10.7</td><td>1.07</td><td>0.11</td><td>1.78</td><td>0.53</td><td>1.25</td><td>BDL</td></tr><tr><td>B11</td><td>26.0</td><td>8.08</td><td>10.3</td><td>1.29</td><td>0.06</td><td>1.74</td><td>0.45</td><td>1.29</td><td>BDL</td></tr><tr><td>B12</td><td>28.2</td><td>8.12</td><td>10.3</td><td>1.38</td><td>0.04</td><td>1.75</td><td>0.49</td><td>1.26</td><td>BDL</td></tr><tr><td>B13</td><td>30.4</td><td>8.14</td><td>10.3</td><td>1.44</td><td>0.02</td><td>1.77</td><td>0.25</td><td>1.52</td><td>BDL</td></tr><tr><td>B14</td><td>31.8</td><td>8.14</td><td>9.9</td><td>1.68</td><td>0.04</td><td>1.64</td><td>0.26</td><td>1.39</td><td>BDL</td></tr><tr><td rowspan="5">5<sup>th</sup> October 2016</td><td>B15</td><td>10.2</td><td>7.40</td><td>14.7</td><td>0.02</td><td>0.06</td><td>0.84</td><td>0.21</td><td>0.63</td><td>1.37</td></tr><tr><td>B16</td><td>0.5</td><td>7.04</td><td>13.3</td><td>-0.59</td><td>0.05</td><td>0.66</td><td>0.28</td><td>0.38</td><td>7.55</td></tr><tr><td>B17</td><td>2.8</td><td>7.68</td><td>14.7</td><td>0.14</td><td>0.01</td><td>1.03</td><td>0.49</td><td>0.54</td><td>4.22</td></tr><tr><td>B18</td><td>3.1</td><td>7.31</td><td>13.7</td><td>-0.16</td><td>0.07</td><td>0.87</td><td>0.48</td><td>0.39</td><td>4.76</td></tr><tr><td>B19</td><td>8.8</td><td>7.37</td><td>15.7</td><td>0.00</td><td>0.01</td><td>0.99</td><td>0.28</td><td>0.70</td><td>1.57</td></tr><tr><td>B20</td><td>29.6</td><td>8.17</td><td>17.1</td><td>0.98</td><td>0.04</td><td>0.70</td><td>0.11</td><td>0.59</td><td>BDL</td></tr></tbody></table>
<table><caption>Table 3. Results for UV irradiated samples.</caption><thead><tr><th>Sample</th><th>Cr<sub>T</sub> + Cr<sub>ORG</sub><br>(nmol kg<sup>-1</sup>)</th><th>Cr<sub>ORG</sub><br>(nmol kg<sup>-1</sup>)</th><th>Cr<sub>ORG</sub><br>(%)</th><th>δ<sup>53</sup>Cr Cr<sub>T</sub> +<br>Cr<sub>ORG</sub><br>(‰)</th><th>2SD</th><th>δ<sup>53</sup>Cr Cr<sub>ORG</sub><br>calculated<br>(‰)</th></tr></thead><tbody><tr><td>B1</td><td>6.17</td><td>1.13</td><td>18</td><td>0.06</td><td>0.01</td><td>-0.75</td></tr><tr><td>B2</td><td>2.52</td><td>0.63</td><td>25</td><td>0.40</td><td>0.01</td><td>-0.11</td></tr></tbody></table>