# Thallium cycling and boundary exchange in a continental margin basin

Kasper Olesen, Sune Nielsen, Chadlin Ostrander, Nicol Udy, Donald Canfield

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Kasper Olesen, Sune Nielsen, Chadlin Ostrander, Nicol Udy, Donald Canfield. Thallium cycling and boundary exchange in a continental margin basin. Geochimica et Cosmochimica Acta, 2025, 399, pp.64-81. 10.1016/j.gca.2025.04.005. hal-05344391

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# Thallium cycling and boundary exchange in a continental margin basin

Kasper P. Olesen<sup>a,*</sup>, Sune G. Nielsen<sup>b,c,d</sup>, Chadlin M. Ostrander<sup>e</sup>, Nicol Udy<sup>a</sup>, Donald E. Canfield<sup>a</sup>

## ARTICLE INFO

Associate editor: Brian Kendall

**Keywords:**

Thallium isotopes
Thallium cycling
Boundary exchange
Continental margin
Diagenesis

## ABSTRACT

Thallium (Tl) isotopes are a robust tracer of ancient ocean oxygenation through their link to the burial flux of deep-sea manganese oxide minerals. Yet, the details of Tl geochemistry at the continental margin remain largely unexplored, hindering quantitative application of the palaeoredox proxy. To develop our understanding of Tl diagenesis, phase dynamics, and mass transfer at the continent-ocean interface, we collected sediments from three sites in the perennially oxic Skagerrak sea as well as sediments and suspended particles from five adjacent rivers. The three Skagerrak sediment sites are biogeochemically diverse, with variable oxygen penetration (9–18 mm) and C-oxidation occurring predominantly through either sulfate, iron, or manganese reduction. Sequential extractions and porewater analyses reveal a highly homogeneous abundance and phase association pattern of Tl across all sample types and sites. Sediment and particle Tl resides almost exclusively in acid-leachable aluminous clays and residual crystalline phases. We interpret these phases to be terrigenous, except at the highly Mn-enriched site S9 where we find up to ~45 % of Tl is authigenic. This claim is supported by Tl isotopic compositions of acid-leachable sediment (e<sup>205</sup>Tl<sub>HNO3</sub>), which average -2.5 ± 0.9‰, within uncertainty of the average upper crust value of -2.0 ± 0.5‰ and a regional riverine particle delivery value of -2.1 ± 1.2‰.

Incubations of Skagerrak sediments with added bottom water Tl reveal a fractionation factor (α) of ~1.0007, amounting to an isotope ratio difference of ~7‰. A comparable fractionation of 2.4 to 4.5‰ is calculated for authigenic Tl removal in the Skagerrak sediments. However, this fractionation is lower than that observed in deep sea pelagic clays and hydrogenetic Mn crusts and nodules (α ~1.0021), yet significantly higher than expected for mature diagenetic Mn oxides like triclinic birnessite and todorokite (α ~1.00005). Thallium cycling is largely decoupled from Mn diagenesis at our marine sites and only associates weakly with the active sedimentary Mn recycling. Rather, effluxing of dissolved Tl is observed at all sites. We employ a diagenetic reaction-transport model to quantify dissolved Tl fluxes and compare these fluxes with independent assessments of Tl mass balance based on authigenic/terrigenous Tl proportions. We find that within the oxic Skagerrak basin, sediments may be both true sources and, if Mn-enriched, true sinks of Tl, with extrapolated flux magnitudes relevant to marine Tl mass balances. Our study presents a much-needed description of Tl diagenesis in diffusive margin sediments and suggests that hitherto unrecognized sources of originally terrigenous Tl to the marine environment may occur at the continent-ocean boundary, either via the release of labile clay-bound Tl directly from riverine particles or during sediment diagenesis.

## 1. Introduction

The geochemical behavior of thallium (Tl) in modern marine environments has received much recent attention. The development of sedimentary Tl isotope signatures as a robust proxy for past ocean

oxygen levels has opened new avenues to further our understanding of the dynamic oxygenation of Earth's oceans and atmosphere (e.g., Fan et al., 2020; Heard et al., 2023; Newby et al., 2021; Ostrander et al., 2017). However, credible interpretation of Tl isotope signals and quantification of past ocean oxygenation signals via environmental

<footnote><sup>a</sup> Department of Biology, Nordcee, University of Southern Denmark, Odense, Denmark</footnote>

<footnote><sup>b</sup> Department of Geology & Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA, Lets, USA</footnote>

<footnote><sup>c</sup> NIRVANA Laboratories, Woods Hole Oceanographic Institution, Woods Hole, MA, USA</footnote>

<footnote><sup>d</sup> Centre de Recherches Pétrographiques et Géochimiques, CNRS, Université de Lorraine, Nancy, France</footnote>

<footnote><sup>e</sup> Department of Geology & Geophysics, University of Utah, Salt Lake City, UT, USA</footnote>

<footnote>* Corresponding author.
E-mail address: kasperolesen@biology.sdu.dk (K.P. Olesen).</footnote>
models requires an accurate understanding of Tl source and sink vectors, and the environmental relevance of reactions that fractionate Tl isotopes.

Thallium is predominantly present as the monovalent species $Tl^+$ and $TlCl^0$ in the modern oceans. It is well mixed in the oceans with a residence time of $\tau \sim 20$ kys, has an average concentration of 64.5 pM, and shows a conservative-type depth distribution in ocean waters (Flegal and Patterson, 1985; Kaplan and Mattigod, 1998; Matthews and Riley, 1970; Nielsen et al., 2017; Owens et al., 2017; Rehkämper and Nielsen, 2004; Turner et al., 2010). Known Tl sources to the oceans include, in order of decreasing importance, volcanic degassing, riverine dissolved loads, black smoker hydrothermal fluids, and mineral aerosols (Baker et al., 2009; Nielsen et al., 2011; Nielsen et al., 2009; Nielsen et al., 2017; Owens et al., 2017). The marine cycling of Tl is controlled by a strong affinity for reaction with Mn oxides (Rehkämper et al., 2002) and Fe sulfides (Owens et al., 2017). Thallium is also adsorbed onto and fixed in clays, likely due to its size and valency similarity with the alkali metals (Jacobson et al., 2005; Martin et al., 2018; Turner et al., 2010; Voegelin et al., 2022; Wick et al., 2020). Various pathways of passive and active interaction with organic material have also been observed and might impact the marine Tl mass balance. Thus, Tl can be removed from ambient water during algal blooms and incorporate in algal cytosols (Flegal et al., 1986; Mori et al., 2021; Zhang and Rickaby, 2020).

Adsorption of $Tl^+$ to Mn oxides and subsequent oxidation to $Tl^{3+}$ is associated with a strong isotopic fractionation in which the heavier mass $^{205}\text{Tl}$ isotope is preferentially enriched in the oxides (Nielsen et al., 2013; Schauble, 2007). The maximum fractionation factor of $\alpha \approx 1.0021$ is observed in deep sea hydrogenetic Fe-Mn crusts and is primarily associated with Tl sorption to hexagonal birnessite (Nielsen et al., 2013; Peacock and Moon, 2012; Phillips et al., 2023; Rehkämper et al., 2002; Wick et al., 2019). The ratio of $^{205}\text{Tl}$ to $^{203}\text{Tl}$ is expressed as $\epsilon^{205}\text{Tl}$ normalized to the NIST 997 standard where:

$$ \epsilon^{205}\text{Tl}(\%\text{ccc}) = \left( \frac{\left( \frac{^{205}\text{Tl}}{^{203}\text{Tl}} \right)_{\text{sample}}}{\left( \frac{^{205}\text{Tl}}{^{203}\text{Tl}} \right)_{\text{NIST997}}} - 1 \right) \cdot 10^4 \quad (1) $$

Burial of Mn oxides with high $\epsilon^{205}\text{Tl}$ values drives the modern ocean to a lower $\epsilon^{205}\text{Tl}$ value of $-6\%\text{ccc}$, relative to the known Tl sources to the oceans: volcanic gases ($\epsilon^{205}\text{Tl} = -2\%\text{ccc}$), rivers ($\epsilon^{205}\text{Tl} = -2.5\%\text{ccc}$), hydrothermal fluids ($\epsilon^{205}\text{Tl} = -2\%\text{ccc}$), and mineral aerosols ($\epsilon^{205}\text{Tl} = -2\%\text{ccc}$) (Nielsen et al., 2017; Owens et al., 2017). In anoxic/euoxic settings (Fan et al., 2020; Ostrander et al., 2024; Owens et al., 2017) and some hypoxic settings (Wang et al., 2022), dissolved Tl is transferred to sediments without isotopic fractionation, likely through quantitative reaction with dissolved or solid sulphide phases. As a result, sediments from these settings can capture the globally homogenous seawater $\epsilon^{205}\text{Tl}$ value and track the extent of Mn oxide burial at the time of deposition (Owens et al., 2017). Stabilisation of Mn oxides in sediments requires the presence of dissolved $O_2$ in overlying bottom waters (Calvert and Pedersen, 1996), meaning that ancient seawater $\epsilon^{205}\text{Tl}$ values can indirectly track changes to ancient oceanic $O_2$ inventories.

Despite considerable research on marine Tl cycling and its application as a palaeoredox proxy, there is a notable gap in our understanding of Tl fluxes at the land-ocean interface. The chemical transfer of matter between terrestrial materials and seawater can occur through many pathways in estuarine, marine and sedimentary environments. Such transfer is collectively termed boundary exchange (Jeandel and Oelkers, 2015; Oelkers et al., 2011). The lack of knowledge regarding boundary exchange of Tl imparts an inherent but undisclosed uncertainty on the current marine Tl mass balance. Our current understanding of continental Tl delivery to the ocean is focused on dissolved riverine Tl, and our knowledge of fluxes associated with Tl diagenesis, both on the continental shelves and in deeper waters, is very limited. The magnitude of the riverine Tl source to the oceans is based on a compilation of dissolved Tl concentrations in rivers (Nielsen et al., 2005). However, the

capability of non-conservative estuarine Tl cycling (as identified by Anagboso et al. (2013); Böning et al. (2017); Nielsen et al. (2005) and Turner et al. (2010)), and post-delivery Tl release from riverine particles to affect continental Tl source fluxes, is not well constrained. Additionally, a diffusive efflux of Tl from continental margin sediments was previously inferred to be a significant source of Tl to the oceans, based on Mn nodule Tl/Mn ratios and estimates of global continental shelf Mn diffusive effluxes (Nielsen et al., 2017). This inference was based on the assumption that the Tl affinity for Mn oxides controls the diagenetic behavior of Tl. However, recent investigations of North Sea coastal sediments impacted by pore water advection reveal a pattern of tidally controlled non-conservative Tl behavior and Tl-Mn anticorrelations in both pore waters and the water column (Ahrens et al., 2021; Böning et al., 2018), indicating that this assumption may not be valid.

To address this knowledge gap, we conducted a study of Tl phase associations and isotope dynamics in the Skagerrak basin in the eastern North Sea. We sampled three sites within the basin dominated by sulfate, iron and manganese reduction for both sediments and pore waters. In addition, we sampled five rivers, representing ~60% of the direct riverine water discharge to the Skagerrak and some to the nearby Kattegat, for waters, bottom sediments, and suspended particles. To assess the boundary exchange of Tl in the Skagerrak system and its impact on marine Tl mass and isotopic mass balance, we performed a sequential extraction protocol on all particles and sediments. In addition, we measured $\epsilon^{205}\text{Tl}$ values of sediments, particles, and the water column, and performed sediment incubations to assess the in-situ isotopic fractionation of Tl expressed under the variable biogeochemical conditions sampled.

## 2. Study area and sampling

### 2.1. Skagerrak basin

The Skagerrak is a hydrographically and biogeochemically highly variable ~32,300 km² continental margin sea in the eastern part of the North Sea, connected with the Baltic Sea through the Kattegat and the Danish straits (Fig. 1). It is characterized by a significant depth range with expanses of shallow (<100 m) sandy sediments extending northward from the coast of Jutland, Denmark, and dropping into the deep (up to 700 m) mud-dominated Norwegian trench. The Norwegian trench is silled off towards the North Sea at a depth of 270 m, giving the Skagerrak bathymetry a “fjord like” appearance (Longva and Thorsnes, 1997; Stevens et al., 1996). The entirety of the Skagerrak proper remains oxygenated throughout the year (Andersson, 1996; Aure and Dahl, 1993). Water circulation is dominated by Atlantic and North Sea currents entering the Skagerrak in the west and southwest, turning northwards upon contact with the eastern coastline and exiting the Skagerrak as the Norwegian current in the north. These dynamics create a counterclockwise current pattern with a central gyre developing over the Norwegian trench (Fonselius, 1990; Gyllencreutz and Kissel, 2006; Longva and Thorsnes, 1997; Rodhe, 1987).

Significant sediment loads are carried with North Sea currents and deposit in the Skagerrak (Eisma and Kalf, 1987), especially in the southern Skagerrak and along the slopes of the Norwegian trench (Van Weering et al., 1987; Van Weering et al., 1993). The fine fraction of this sediment is dominated by material discharged by the continental European rivers Elbe and the Rhine (Kuijpers et al., 1993a). Thus, the Skagerrak is sometimes viewed as the main depocenter of the North Sea (Van Weering et al., 1987) and a large body of literature exists that describes the dynamics of this sediment transport (Kuijpers et al., 1993a; Kuijpers et al., 1993b; Lepland and Stevens, 1996b; Rodhe and Holt, 1996; Stevens et al., 1996). For example, Sediment transport in the bottom nepheloid layer is widespread within the Skagerrak (Van Weering et al., 1993). However, the total sediment accumulation in the Skagerrak, based on $^{210}\text{Pb}$-derived sedimentation rates, exceeds estimates of suspended material delivered with Atlantic and North Sea
<figure><img src="image_3.png" /><figcaption>Fig. 1. Sampling locations in the Skagerrak basin and for the 5 rivers sampled, shown with rough outlines of river course and drainage areas.</figcaption></figure>

currents by at least a factor of two (Van Weering et al., 1987). The fraction unaccounted for has previously been linked to resuspension events and sediment erosion in the southern North Sea, and the possibility of an underestimated riverine source has not been considered (Van Weering et al., 1987).

Of the sediment depositing in the Skagerrak, 15 % has typically been attributed to a combination of local terrigenous material and Baltic outflow. Most riverine material has been assumed to deposit in silled fjords along the coast (Longva and Thorsnes, 1997) and to our knowledge few studies deal with the role of riverine material in the Skagerrak proper. Lepland and Stevens (1996a) and Gyllencreutz and Kissel (2006) identified that sediments on the northern slopes of the Norwegian trench can be associated with local continental material based on their magnetic properties. Helland (1996) found that a sediment trap station 10 km off the coast of Norway in the Northeastern Skagerrak collected polychlorinated biphenyls specifically associated with runoff from the river Glomma. Bengtsson and Stevens (1997) and Pederstad et al. (1993)

found indications that illite, specifically trioctahedral illite, vermiculite, and illite-vermiculite mixed layer clays are derived from local rivers and that the abundance of these clays decrease from the fjords and westward in the Skagerrak. Thus, river influence on Skagerrak sediments may be more important than typically described, especially moving from west to east in the basin.

The Skagerrak basin and adjacent fjords are well studied for their diverse, and at times peculiar, biogeochemical conditions (Canfield et al., 1993a; e.g., Canfield et al., 1993b; Dale et al., 2008; Goldberg et al., 2012; Knab et al., 2008; Munksby et al., 2002; Rajendran et al., 1992; Rysgaard et al., 2001; Ståhl et al., 2004; Thamdrup and Dalsgaard, 2000). The water column is completely oxygenated, and anoxia does not develop. In the deepest part of the Norwegian trench, an area with extensive Mn enrichments (up to 3.14 wt%) is found where dissimilatory Mn reduction accounts for 90 % of sediment C-oxidation (Canfield et al., 1993a). The high Mn enrichments are most likely produced by internal Mn cycling within the Skagerrak basin (Rajendran et al., 1992). Further
southwards in shallower waters, iron and then sulfate reduction become the dominant sediment C-oxidation pathways (Canfield et al., 1993a).

## 2.2. Sampling

### 2.2.1. River sampling

Between the 5th and 7th of April 2023 we sampled the five rivers Otra (Norway, NO), Nidelva (NO), Drammenselva (NO), Glomma (NO) and Göta Ålv (Sweden, SE), roughly covering the range of lithologies and major river types draining into the Skagerrak and Kattegat basins (sample locations and drainage basins shown in Fig. 1, with river details in Table 1). Sample sites were selected as close to river mouths as possible yet upstream from coastal cities and associated industrial areas. This was not possible for Drammenselva where samples were collected in an urban setting.

River water was collected for suspended particulate matter using a General Oceanics © external-spring trace-metal grade Niskin bottle, which was thoroughly washed, internally leached with 0.1 M HCl and rinsed before first use. The Niskin bottle was submerged from bridges using a PTFE covered steel wire and stainless-steel weights attached to the Niskin bottle. To minimize potential contact of sampled water with weights, the weights were suspended from the Niskin bottle with 1 m wire. This sampling procedure was inapplicable in the rivers Nidelva and Glomma. Here water was collected using an HCl leached metal-free bucket thrown into the rivers from extended boating docks. The lid was attached firmly to the bucket using latex gloves, and while still submerged, to ensure that surface water was not sampled.

Waters were filtered immediately after sampling using 0.45 µm 142 mm Pall Supor Polyethersulfone (PES) filters and a 0.1 M HCl leached PTFE in-line filter holder that was pressurized with a mechanical pump connected through acid leached tubing and 0.2 µm Cellulose Acetate (CA) filters. The PES filter type was selected for minimal metal contamination and all filters were cleaned overnight in 10 % HCl at 60 °C (Cutter et al., 2017). After this, filters were thoroughly rinsed, dried under a clean lab laminated airflow atmosphere, preweighed and stored in closed HCl washed plastic petri dishes until use. After filtration of river water, 100 mL MQ (Milli-Q, 18.2 MΩ·cm) water was filtered through and the filter was swiftly removed from filter holder with Teflon tweezers, placed in a petri dish, and kept frozen until use.

Additional small aliquots of water were sampled from boat bridges by submerging an acid leached syringe and filtering the water into acid washed HDPE bottles. Aliquots for metal analysis were acidified to 2 % HNO₃ and aliquots for dissolved organic carbon measurements were frozen.

River sediments were collected with a metal-free centrifuge tube attached to a 4 m steel rod. Sediments were frozen immediately after sampling.

### 2.2.2. Sediment sampling and onboard sample treatment

In December 2021 we sampled the three hydrographically and bio-geochemically distinct sites S4, S6 and S9 within the eastern Skagerrak. Sampling was conducted onboard the R/V Skagerak (sample locations on Fig. 1). Sediment cores were retrieved using a box-corer and sub-sampled with PC and PVC core tubes. Cores were stored cold in

overlying bottom waters that were aerated continuously using an aquarium pump. Cores that were not subsampled or analyzed onboard were kept continuously aerated for later incubation experiments at the University of Southern Denmark, DK.

Oxygen penetration depths (OPD) and total oxygen uptake rates (TOU) were measured on triplicate cores from each site. OPD was measured using glass microelectrodes (Revsbech, 1989). The cores were subsequently incubated dark and sealed, with a stirring magnet. Dis-solved O₂ concentrations were measured regularly until depleted using an optode patch (Klimant et al., 1995) with a Firesting® O₂-meter.

For trace metal and isotopic analyses, undisturbed sediment cores with well-preserved sediment-water interfaces were sliced within 24 h of retrieval inside a N₂-filled glovebag using acid washed plastics. At each site, bottom water was removed with an acid-washed syringe, filtered and acidified to 2 % HNO₃ before the core was inserted into the glove bag. Each sediment horizon was homogenized and subsampled into metal-free centrifuge tubes and acid-washed Eppendorf tubes. Sediment in centrifuge tubes was centrifuged onboard at 3000 rpm for 10 min to separate the porewater. The porewater was subsequently removed, filtered using 0.2 µm CA filters, and acidified to 2 % HNO₃ inside an N₂-filled glovebag. Residual sediments from porewater extractions and sediments in Eppendorf tubes were stored frozen. For sediment physical properties, one additional core from each site was sliced under normal atmosphere, and the sediment was transferred to zip lock bags and frozen.

Water samples from each site were collected at a range of depths using a rosette water sampler with mounted Niskin bottles. The water was filtered using 0.2 µm CA filters and acidified to 2 % HNO₃ for metal analyses. At site S9, 1L of filtered surface and bottom water was acidified to 1 M HCl for isotopic analyses.

All acids used for cleaning and acidification were bought trace metal grade. HNO₃ was additionally distilled in-house.

## 3. Methods

### 3.1. Solid phase analyses and sequential extraction protocol

Sediment physical properties (density, water content, porosity) were quantified on an aliquot of frozen, wet, homogenized sediment that was thawed before use. Density, ρ, was determined by filling a pre-weighed cut-off syringe with a known volume and re-weighing. Water content, as mass fraction M_H2O, was determined as the mass loss after a 24-hour heating at 110 °C. Porosity, φ, was then calculated using equation (2), assuming a density of water of 1 g/mL.

$$ \varphi = \frac{M_{H2O}}{\rho_{H2O}} \cdot \left( \frac{M_{H2O}}{\rho_{H2O}} + \left( \frac{1 - M_{H2O}}{\rho_{H2O}} \right) \right)^{-1} \cdot \rho_{Sed}^{-1} \quad (2) $$

Freeze-dried sediment was analysed for total organic carbon (TOC) using the following protocol. Sediment aliquots were first treated with 0.5 M HCl at 50 °C to liberate all inorganic carbon, mainly CaCO₃. The treatments ran for at least 48 h or until no more bubbles of CO₂ were observed forming. The samples were rinsed with MQ water, centrifuged twice, and then dried at 60 °C for 72 h. The remaining sediment was

**Table 1**

Compilation of data from sampled rivers. Discharge and catchment area from given references. DOC, dissolved organic carbon; TDN, total dissolved nitrogen; SPM, suspended particulate matter; TDS, total dissolved solids.

<table><thead><tr><th>River</th><th>Sample Date</th><th>DOC<br>mg/L</th><th>TDN<br>mg/L</th><th>SPM<br>mg/L</th><th>TDS<br>mg/L</th><th>[Tl]<br>pM</th><th>Discharge<br>m³·s<sup>-1</sup></th><th>Catchment<br>km²</th><th>References</th></tr></thead><tbody><tr><td>Otra</td><td>05-04-2023</td><td>4.0</td><td>0.2</td><td>0.9</td><td>14</td><td>32</td><td>149</td><td>3539</td><td>(Fonselius, 1990)</td></tr><tr><td>Nidelva</td><td>05-04-2023</td><td>4.2</td><td>0.6</td><td>1.0</td><td>15</td><td>34</td><td>125</td><td>3907</td><td>(Fonselius, 1990)</td></tr><tr><td>Drammenselva</td><td>06-04-2023</td><td>3.6</td><td>0.5</td><td>2.2</td><td>49</td><td>15</td><td>313</td><td>16,020</td><td>(Fonselius, 1990)</td></tr><tr><td>Glomma</td><td>07-04-2023</td><td>4.7</td><td>0.6</td><td>10.6</td><td>61</td><td>18</td><td>708</td><td>41,284</td><td>(Fonselius, 1990)</td></tr><tr><td>Göta</td><td>07-04-2023</td><td>5.1</td><td>0.6</td><td>11.2</td><td>83</td><td>23</td><td>565</td><td>50,229</td><td>(Swedish Meteorological and Hydrological Institute 2010; Göta älvs vattenvårdsförbund 2015)</td></tr></tbody></table>
homogenized and subsampled, and organic carbon was analysed using a Thermo 2000 Elemental analyzer. A calibration curve was made from Casein protein with known carbon content and an in-house reference was analysed to assess measurement accuracy. The average analytical relative standard deviation for TOC determination was 3.7 % and the reference material was reproduced to 98 %.

We carried out a sequential extraction protocol modified from Neaman et al. (2004), Owens et al. (2017), Poulton and Canfield (2005) and Wick et al. (2020), to assess the phase associations of major and trace elements in river particles, river sediments and Skagerak sediments. The extraction protocol, including the acid grade used and the associated references, is summarized in Table 2. For marine and river sediments, ~0.5 g of wet homogenized sediment was weighed into 50 mL centrifuge tubes. For river particulates, particles were removed from filters with HCl-washed metal-free cell scrapers and MQ water. The particles in MQ solution were centrifuged, the supernatant was collected and the residual sediment was freeze dried. Between 1 to 25 mg of dry river particles were weighed into HCl washed 5 mL Eppendorf tubes.

Sediments and particles were initially leached for 90 min in 1 M ammonium acetate with a pH of 6.9 to quantify the pools of exchangeable cations, i.e. Tl surface-bound to secondary clays (Wick et al., 2020). Between this and the following steps, the sediments were centrifuged for 10 min at 4000RPM, and the supernatants were removed by pipetting. The second extraction step constituted a 120-minute leach in 0.1 M hydroxylamine HCl in 1 M ammonium acetate at a pH of 6.2. This step targets Mn Oxides with supposed minimal leaching of Fe oxides and minimal readsorption of liberated metals (Neaman et al., 2004; Wick et al., 2020). The residue was then subjected to a 2-hour dithionite extraction to target the bulk of Fe oxides (Poulton and Canfield, 2005). The dithionite was prepared as 50g/L in a 0.2 M/0.35 M sodium citrate/ acetic acid buffer. A nitric acid leach was modified from Owens et al. (2017) to mimic the leach typically employed for Tl isotope analysis (see below, section 3.4), where sediment is leached in 3 mL 2 M HNO₃ for 12–18 h at 130 °C. This leach is employed to extract pyrite-bound Tl with supposed minimal liberation of the detrital silicate fraction of Tl and is typically used to assess the “authigenic” ε²⁰⁵Tl signature (henceforth TlHNO₃). In our modification, we transferred the extracted sediments to Teflon® bombs and leached them twice in 3 mL 2 M HNO₃ at 130 °C with a 60-minute holding time in a CEM Mars 6 microwave digestion system. A final bulk digestion was run to extract residuals using the microwave system. Samples were digested at 200 °C for 30 min using HNO₃, HF, HCl in a 3:2:1 ratio. After cooling, 4 % boric acid was added in 3.3× the digestion acid volume, and the samples were rerun on the microwave for 15 min at 170 °C. After this the supernatant was removed and diluted for analysis.

All extraction supernatants were acidified to 2 % HNO₃ (distilled, NORMATOM®, VWR chemicals) immediately after removal and diluted accordingly for major and trace element analyses. Element concentrations were determined in triplicate by Quadrupole-ICP mass spectrometry on an Agilent Technologies, USA, 7900 instrument at University of Southern Denmark, with 8x or 25x times Ar gas dilution to

accommodate high matrix introduction. Measurements were performed using He collision cell technology, and plasma suppression was corrected for by normalizing to an on-line addition of Scandium, Rhodium, Indium, and Iridium to the sample. Calibration standards were matrix matched with appropriate concentrations of extractants. Concentrations were backcalculated to dry weight basis using the measured sediment water content (Table S1). The precision of repeat measurements of an internal standard, and detection limits, are given in Table S2. A discussion of the extraction specificity is given in the supplemental material, figure S1.

## 3.2. Pore water and river water analysis

Acidified porewaters and river waters were diluted and analysed in triplicate using a quadrupole ICP-MS, with similar methodology to the sequential extractions, but with 4x Ar gas dilution to accommodate the high salinity of the samples and without matrix matching of the calibration standards. Measurement precision and accuracy was assessed by repeated analyses of the NIST 1643f trace metals in water certified reference in 250 times dilution and the CASS-6 Nearshore seawater certified reference (Table S3).

River water was measured in-situ for total dissolved solid contents using a HACH HQ40D multi meter kit. Filtered and subsequently frozen river water was thawed and analysed for Dissolved Organic Carbon (DOC) and Total Dissolved Nitrogen (TDN) using a Shimadzu TOC-L equipped with a TNM-L Module.

## 3.3. Sediment incubations

To quantify the isotope fractionation accompanying Tl uptake by the sediments, duplicate intact sediment cores from each Skagerrak site were incubated with added TlCl to original Skagerrak bottom waters. Cores were incubated at 5 °C in the dark, protected from atmospheric deposition, and waters were mixed and oxygenated twice daily for the length of the experiment using HNO₃-washed glass frits and 0.2 µm filtered ambient air. The incubations were terminated when sampling showed substantial depletion of Tl in the bottom water, or no later than 8½ days after initiation. Oxygen penetration depths were measured with glass microelectrodes (Revsbech, 1989) at the initiation and end of the experiments assuring that the sediment oxycline did not move significantly during the incubation time.

The experiments were initiated by adding TlCl (Opripur®, Merck) to final concentrations of ~0.02 µM in the bottom water, ie. ~350x the Tl concentration in ambient seawater (Matthews and Riley, 1970). This concentration was deemed enough to develop a large diffusive gradient into the sediment. This Tl concentration also allowed small volume subsampling with sufficient Tl for isotopic analysis, and thus minimized changes to the water volume during the experiment. Finally, this concentration was below concentrations found toxic to marine micro- and macroalgae, invertebrates, and vertebrates (e.g., Puddu et al., 1988; Rickwood et al., 2015; Turner and Furniss, 2012; Zhang and Rickaby, 2020).

Drawdown of Tl during the incubation period was quantified as pseudo first-order reactions and rate constants were estimated by fitting the concentration vs. time evolution to the two-parameter exponential function:

$$[Tl] = [Tl]_0 \cdot e^{-kt} \quad (3)$$

Effective fractionation factors of Tl isotopes during reactive drawdown, α<sub>effective</sub>, were calculated using the Rayleigh equations for kinetic or non-steady state isotope effects (Scott et al., 2004). Here linear regressions were fitted to ln(R/R₀) vs. ln(C/C₀) evolution, where R is the solution ²⁰⁵Tl/²⁰³Tl ratio and C is the solution Tl concentration at progressive time points, while R₀ and C₀ are the initial ratios and concentrations respectively. From these regressions the fractionation factors were

<table><caption>**Table 2**

Description of sequential extractions.</caption><thead><tr><th>Extractant</th><th>Target phases</th><th>Method</th><th>References</th></tr></thead><tbody><tr><td>1 M Ammonium Acetate</td><td>Clay surfaces, weak carbonates</td><td>90 min, Ambient temp</td><td>Wick et al., (2020)</td></tr><tr><td>0.1 M Hydroxylamine HCl</td><td>Mn Oxides, stronger carbonates</td><td>120 min, Ambient temp</td><td>Neaman et al., (2004) and Wick et al., (2020)</td></tr><tr><td>Dithionite 50 g/L</td><td>Fe-oxides</td><td>120 min, Ambient temp</td><td>Poulton and Canfield (2005)</td></tr><tr><td>2 M Nitric Acid</td><td>Sulfides, labile OM, secondary clays</td><td>2x1 hr, 130 °C, microwave</td><td>Owens et al., (2017)</td></tr><tr><td>HNO3-HF-HCl 3:2:1</td><td>Residual – Detrital</td><td>20 min, 180 °C, microwave</td><td></td></tr></tbody></table>
calculated as slope = $\frac{\alpha}{1000} + 1$ (Scott et al., 2004).

### 3.4. Tl isotope analysis

The Skagerrak sediments remaining after porewater extraction (section 2.2.2) were freeze dried and pulverized using a Fritsch pulverisette with Zr oxide bowl and ball. River sediments and particles were freeze dried and pulverized using an agate mortar and pestle. For isotope analysis we used approximately 80–100 mg of sediments and the USGS SCo-1 standard, whereas we used approximately 1–25 mg of dried river particles (see section 3.1). Samples were leached in 3 mL 2 M double-distilled HNO₃ heated to 130 °C for 14–15 h. This method was applied to selectively leach authigenic Tl associated with pyrite and/or Mn (oxy) hydroxides (TlHNO₃) and avoid detrital Tl (Nielsen et al., 2011; Ostrander et al., 2017; Owens et al., 2017; Wang et al., 2022). After the acid treatment, the leachate was separated from the residual sediment by two cycles of centrifugation, decanting of the supernatant and washing in MQ water. The supernatant was then repeatedly treated with Aqua regia at 120 °C, and dried and treated with HNO₃ + H₂O₂ at 120 °C to rid samples of organic matter.

Thallium was purified from extraction leachates as well as from in-situ and incubated seawater samples using ion exchange chromatography protocols as described in detail in previous work (Nielsen et al. (2004); Rehkämper and Halliday (1999)). In short, the acid-treated supernatants were dried down and then fluxed in 5 mL 1 M HCl, and seawater samples were acidified to 1 M HCl. Water saturated with bromine was added to all samples prior to Tl purification to oxidize all Tl⁺ to Tl³⁺, which ensures that Tl is retained on the column. Thallium purification was achieved with two consecutive passes through ion-exchange columns loaded with AG1X8 anion exchange resin (BioRad). Dilute (2 M and 0.5 M) HNO₃ with brominated water was used to retain oxidized Tl³⁺ on the columns while eluting the sample matrix. Finally, 0.1 M HCl containing ~5 % by weight of dissolved SO₂ gas was then used to elute and collect reduced Tl⁺. This procedure has been shown to recover ~100 % of Tl, while quantitatively removing Pb, allowing later Pb addition to monitor instrumental drift (Nielsen et al., 2011; Prytulak et al., 2013).

Thallium isotope compositions were measured in duplicate (river particulates in singles) using a Thermo Finnigan Neptune multiple collector inductively coupled plasma mass spectrometer (MC-ICPMS) located at the Woods Hole Oceanographic Institution Plasma Mass Spectrometry Facility. We employed both standard sample bracketing with SRM NIST 997 and external normalization to added SRM NIST 981 Pb to correct for mass bias. This procedure also allows for conversion of ²⁰⁵Tl/²⁰⁸Pb ratios to concentrations of acid-leached Tl. The USGS SCo-1 reference material was processed with each sample batch to monitor measurement precision and accuracy. This material yielded an ε²⁰⁵TlHNO₃ value of -2.8 ± 0.4‰, which is consistent with previously reported values (ε²⁰⁵TlHNO₃ = -2.8 ± 0.4‰; Ostrander et al. (2017)). All reported sample errors are in 2SD and are equal to the individual sample reproducibility or our reproducibility of the SCo-1 standard, whichever was greater.

### 3.5. Tl Reaction model

We quantified the net magnitude of diagenetic dissolved Tl fluxes at the Skagerrak sites through the use of a diagenetic transport-reaction model (Boudreau, 1997). This model estimates the rates of Tl precipitation and dissolution R<sub>Tl</sub>. In addition to precipitation and dissolution, Tl porewater profiles are shaped by molecular diffusion, biodiffusion (particle mixing/bioturbation) and bioirrigation, which at steady state is described by the one-dimensional mass conservation equation (4):

$$ \left( \frac{\partial \phi |Tl|_{\text{porewater}}}{\partial t} \right)_x = \frac{\partial}{\partial x} \left( \phi (D_e + D_B) \frac{\partial |Tl|_{\text{porewater}}}{\partial x} \right) + \phi \alpha \left( |Tl|_{\text{bottomwater}} - |Tl|_{\text{porewater}} \right) + R_{Tl} = 0 \quad (4) $$

where φ is sediment porosity, t is time (s), x is depth (cm), D_e is the effective diffusion coefficient (cm²·s⁻¹) as calculated by eqn (5), D_b is the biodiffusion coefficient (cm²·s⁻¹), α is the irrigation coefficient (s⁻¹), not to be confused with isotopic fractionation factors, and R<sub>Tl</sub> is the net rate of Tl precipitation/dissolution. Biodiffusion coefficients for sites S6 and S9 (Table 5) were estimated using the empirical relationship with water depth described by Middelburg et al. (1997). Depth-dependent irrigation coefficients, α, were calculated for each depth interval at each site using equation (5) (Dale et al., 2008), where α₀ is the surficial non-local exchange coefficient of the solute, and b is a depth-attenuation coefficient.

$$ \alpha = \alpha_0 \cdot e^{-b \cdot x} \quad (5) $$

Values of α₀ and b for sites S6 and S9 (Table 5) were modelled from porewater DIC profiles at the Skagerrak sites AU1 and AU2 by Kristensen et al. (2018). These sites are biogeochemically comparable to S9 and S6 respectively.

## 4. Results

### 4.1. River geochemistry and particle composition

The five rivers had little suspended particulate matter (SPM) at the time of sampling in early April 2023, ~1–11 mg/L (Table 1 and figure S2). For these five rivers, SPM contents correlate with the river size, with greater SPM associated with larger catchments (R² = 1.0) and greater discharge (R² = 0.9) as well as higher dissolved solid contents (R² = 0.8). In contrast, dissolved Tl concentrations correlate inversely with river discharge (R² = 0.5). Dissolved organic carbon contents are relatively homogeneous, 3.6–5.1 mg/L, despite other parameters varying by up to one order of magnitude.

Fig. 2 shows the relative enrichments (pie areas normalized to enrichment factors) and proportions of Mn and Tl leached in the various extractions for both Skagerrak sediment and river suspended and bed-load material. Fig. 3 shows the enrichment factor for Tl in both river and Skagerrak sediment (divided into the top layer and average for the sediment) partitioned into the oxide and exchangeable leaches, the HNO₃ leach, and the remaining residual Tl. The enrichment factors were calculated by normalization to upper continental crust values (expressed as Metal enrichment factors where Me<sub>EF</sub> = (Me / Al)<sub>sample</sub> / (Me / Al)<sub>UCC</sub> (Rudnick and Gao, 2014). Manganese exhibits a range of enrichment factors from 0.6 to 5.2 across the river sample population, and Tl<sub>EF</sub> exhibit a range from 0.7 to 2.2 (Table S4). High metal enrichments in suspended particles from the smallest rivers Otra and Nidelva are driven by low Al concentrations. If excluding these, the Mn<sub>EF</sub> and Tl<sub>EF</sub> ranges narrow to 1 to 2.2 and 0.7 to 1.2, respectively.

Both Tl and Mn are dominantly extracted by both the HNO₃ leach and the residual. The sum of Tl leached by ammonium acetate, hydroxylamine HCl and dithionite never exceeds 25 % of total Tl. Generally for the rivers, we find that more Tl is leached by HNO₃ in the suspended particles than in the bedload. A part of this trend is likely an analytical artefact, as the leach specificity and digestion yield were not completely consistent between the low- and high-sample-amount analyses of the S4-5 internal standard (see supplementary text and figure S1). The Mn phase associations are more variable, and appreciable amounts of Mn are leached with ammonium acetate, hydroxylamine HCl and dithionite in all rivers. In the rivers Otra and Nidelva, hydroxylamine HCl Mn contents dominates the SPM. Manganese contents are generally enriched in the SPM relative to bedloads, apart from at the Glomma river, where there is a substantial hydroxylamine HCl-bound
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Sequential extraction results for Tl and Mn plotted as pie charts area-normalized to upper crustal Me/Al ratios (Rudnick and Gao, 2014). There is a larger variation in Mn phase behavior than Tl phase behavior, and a general decoupling of the two elements. The small rivers Otra and Nidelva have strong metal enrichments in the suspended matter, driven by low Al concentrations.</figcaption></figure>

Mn enrichment in the bedload material.

## 4.2. Manganese and thallium diagenesis in Skagerrak sediments

The cores from the three Skagerrak sites display different biogeochemical conditions. For example, we see both increasing proportions and concentrations of extractable Mn oxides from site 4 to 9 (Figs. 2 and 4–6) as expected from previous studies (Canfield et al., 1993a; Canfield et al., 1993b). All sites were significantly bioturbated, and sediments appeared homogeneous on the cm-scale with no laminations. Oxygen penetration depths increase, and total oxygen uptake rates decrease through sites S4, S6 and S9 (Figs. 4–6, Table S5). Fig. 2 shows Mn and Tl enrichment relative to upper continental crust and the sequential extraction distribution in the top 0.5 cm of sediment, while Figs. 4–6 shows the downcore Mn and Tl concentrations of the sequential extractions and the concentrations of porewater Tl and Mn at each site (Table S6). At all sites there are diagenetic enrichments of porewater Mn and Tl, and solid phase Mn. However, a solid phase diagenetic enrichment of Tl is only apparent at site S9.

At S4, SO₄²⁻ reduction is the dominant metabolic pathway of organic matter mineralization (Canfield et al., 1993b), and we see a minor Mn enrichment in the top 1 cm of sediment (Fig. 4). This enrichment is specifically found in the hydroxylamine HCl extraction. The overall Mn enrichment in the top 1 cm, however, remains less than crustal average

value with an MnEF = 0.8 (Table S4). Some Mn reduction is evident from elevated porewater Mn²⁺ concentrations extending down to >10 cm, but overall, Mn reduction is a minor pathway of organic carbon mineralization at this site (Canfield et al., 1993b). This is the only site where the Mn porewater gradient extends to the sediment-water interface (SWI). The dominant carriers of Mn are phases liberated by the HNO₃ leach and the residual digestion.

Similar, but exacerbated, Mn dynamics are observed at the Fe-reduction dominated S6 (Fig. 5). Mn enrichments are exclusively associated with phases leached with hydroxylamine HCl, and in the top 0.5 cm of sediment, the MnEF is 2.3. Porewater accumulation of Mn is observed below the surficial solid phase Mn enrichment, and the gradient does not extend to the SWI, indicating near-quantitative reoxidation and precipitation.

At S9, the most Mn enriched sediment is found at 1.75 cm depth with MnEF of 64. The Mn enrichment is almost exclusively associated with the hydroxylamine HCl leach, and porewater Mn is effectively reoxidized at >1 cm depth similar to at S6. At this site, Mn reduction is the dominant metabolic pathway of carbon mineralization, and Canfield et al. (1993a) found that it accounts for >90 % of total sediment C oxidation. Our results are consistent with this, and S9 thus remains an ideal site to study diagenetic Mn – trace metal interactions.

At all sites, Tl has shallow porewater peaks and is dominantly found in phases liberated by the HNO₃ leach and the residual digest. Only at S9
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Tl sequential extraction results partitioned into oxides and exchangeable leaches (NH₄Ac + Hydroxylamine HCl + Dithionite), HNO₃ leach and residual digest, and plotted as enrichment factors (Rudnick and Gao, 2014). The plot shows the general homogeneity of Tl abundances and phase associations in the Skagerrak basin and rivers, and the low-Al driven Tl enrichments of the small mountainous rivers Otra and Nidelva.</figcaption></figure>

do we find an obvious Tl enrichment in the surface sediment. This Tl enrichment is associated with a phase liberated by the hydroxylamine HCl leach. Solid phase Tl concentrations increase more than two-fold from S4 through S9, but when normalized to Al the increase remains minimal with Tl<sub>EF</sub> of ~0.9, ~0.8 and ~0.9 at S4, S6 and S9, respectively. Porewater Tl accumulates at depths where Mn reduction occurs at all three sites, but Tl is not effectively removed within the sediment. Rather, Tl gradients extend to the SWI at all three sites. Thallium diagenesis thus appears linked to Mn but with important decouplings. At both S4 and S9 there is a significant increase in HNO<sub>3</sub> leached Tl with depth.

### 4.3. Tl isotope incubation experiments

Fig. 7 shows the evolution of all incubations as a double natural log plot of Tl isotope ratios against Tl concentrations. Isotope ratios are shown as ratios of R/R₀ and Tl concentrations are shown as ratios of C/C₀. Large decreases in C/C₀ concurrent with decreases in R/R₀ show that Tl was removed from solution while the isotopic signature of the remaining dissolved Tl became lighter during incubations of sediments from sites S6 and S9. This removal follows pseudo first-order reaction models, and using the methods presented in section 3.3 we calculate that Tl is drawn down with rate constants of kS6 = 1.21 × 10⁻⁵s⁻¹ and kS9 = 5.7 × 10⁻⁵s⁻¹, and the ²⁰⁵Tl isotope is enriched in the sediment with fractionation factors αeffective of 1.00021 and 1.00015 for S6 and S9, respectively. During incubations of sediments from S4 no change in isotopic composition was observed. The evolution of concentrations and isotope ratios can be found on standard plots for individual sites in

Fig. S3.

### 4.4. ε²⁰⁵Tl variation in sediments and river particles

The ε²⁰⁵Tl<sub>HNO3</sub> of Skagerrak sediments, riverine particulates and riverine bedloads all fall within a range of -3.2‰ to -1.6‰ (Table 3 and Fig. 8). For all samples, apart from S4-3, S4-4, S4-9, S4-11 and Göta River SPM, ε²⁰⁵Tl<sub>HNO3</sub> values are within analytical uncertainty of the average upper crustal value of ε²⁰⁵Tl = -2 ± 0.5‰ (Nielsen et al., 2005). However, the individual marine sites are statistically distinct from one another (Analysis of Variance, ANOVA, p < 0.05) and the mean ε²⁰⁵Tl<sub>HNO3</sub> signature increases from site S4 through S9. At all the sites, a surficial heavy inflection can be identified, but this feature is only significant at the heavily Mn-enriched site S9, where the initial offset (~0.65‰) is also preserved with depth.

## 5. Discussion

### 5.1. Phase dynamics and diagenesis of Tl in the Skagerrak system

#### 5.1.1. Homogeneous Tl phase associations

The Tl deposited in the Skagerrak basin is strikingly similar to that transported in south Scandinavian rivers, in terms of both abundance and phase associations (Figs. 2 and 3). This similarity spans diverse river morphologies. Despite this similarity, the deposited sediment may not be dominantly derived from those rivers, but rather from Atlantic and North sea sources (section 2.1, e.g., Longva and Thorsnes (1997)). The
S4

Manganese

Thallium

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

Fig. 4. Sedimentary profiles of Mn and Tl sequential extraction results and porewater concentrations at site S4. Sequential extraction results are shown in same color profile as in Fig. 2, i.e. green is 1 M ammonium acetate leach, purple is 0.1 M hydroxylamine HCl leach, red is 50 g/L dithionite leach, yellow is 2 M HNO3 leach and black is residual digest. The grey shaded area is the sum of all extractions. Oxygen penetration profiles in % atmospheric saturation are shown shaded in Manganese porewater profiles.

S6

Manganese

Thallium

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

Fig. 5. As in figure 4, but with results from site S6.
S9

Manganese

Thallium

<figure><img src="image_8.png" /><figcaption>Fig. 6. As in figure 4, but with results from site S9.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 7. Results of ε²⁰⁵Tl sediment core incubation for all sites plotted as 1000 · ln(R/R₀) against ln(C/C₀) and with linear regressions and slopes for sites S6 and S9 where reactive isotope fractionation was observed. R/R₀ is the ²⁰⁵Tl/²⁰³Tl ratio of the overlying water column normalized to initial, and C/C₀ the Tl concentration of the overlying water column normalized to initial.</figcaption></figure>

similarity in Tl phase association across rivers and Skagerrak sediments could suggest a general, or at least regional, homogeneity in the phase associations of particulate Tl.

Particulate Tl in the sampled rivers is almost completely associated with the nitric acid leach and the extraction residual. The nitric acid leach has been shown to primarily target pyrite-associated Tl with authigenic ε²⁰⁵Tl values in ancient organic rich sediments (Nielsen et al., 2011; Owens et al., 2017). However, this interpretation cannot be applied in the case of Skagerrak river particulates as these are extensively exposed to oxygenating conditions. It is also unlikely that a Tl-sulfide association can explain the bulk of marine sediment HNO₃-extracted Tl, as porewaters are oxygenated above the first two sampling depths at all sites.

Instead, we find a close correlation between Tl, Al, and Fe, but not with Ca in the nitric leach (Fig. S4). These relationships suggest a dominant association with clay minerals for the TlHNO₃ fraction, regardless of whether the clays are marine authigenic, riverine authigenic, or detrital (with the latter two grouped under the term “terrigenous”). In soils developing from Tl-rich source rocks, the bulk of Tl has been found to structurally incorporate as Tl(I) in micaceous clays (Voegelin et al., 2015; Wick et al., 2020). The details of this retention mechanism are well understood. Thallium has a high affinity for edge-association with illite and interlayer-association with vermiculite, and lesser but still important affinities for planar surfaces of illite and muscovite (Martin et al., 2018; Voegelin et al., 2022). In the Skagerrak, illite was shown by Bengtsson and Stevens (1997) to constitute > 50 %
<table><caption>**Table 3**

Tl isotope signatures and isotope dilution concentrations of seawater and HNO₃-leached sediments and river particles.</caption><thead><tr><th>Sample ID</th><th>Type</th><th>Sediment depth/Water depth<br>cm/m</th><th>ε<sup>205</sup>Tl<sub>HNO3</sub></th><th>Analytical 2SD<br>±</th><th>Tl<sub>HNO3</sub> ID<br>ng/g</th></tr></thead><tbody><tr><td>S4-1</td><td>Marine Sediment</td><td>0.25</td><td>-2.9</td><td>0.1</td><td>115</td></tr><tr><td>S4-2</td><td>Marine Sediment</td><td>0.75</td><td>-2.7</td><td>0.4</td><td>203</td></tr><tr><td>S4-3</td><td>Marine Sediment</td><td>1.25</td><td>-3.0</td><td>-</td><td>182</td></tr><tr><td>S4-4</td><td>Marine Sediment</td><td>1.75</td><td>-3.0</td><td>0.1</td><td>161</td></tr><tr><td>S4-7</td><td>Marine Sediment</td><td>4.5</td><td>-2.9</td><td>0.1</td><td>146</td></tr><tr><td>S4-9</td><td>Marine Sediment</td><td>7</td><td>-3.0</td><td>0.5</td><td>120</td></tr><tr><td>S4-11</td><td>Marine Sediment</td><td>11</td><td>-3.1</td><td>0.3</td><td>117</td></tr><tr><td>S4-12</td><td>Marine Sediment</td><td>13</td><td>-2.9</td><td>0.6</td><td>116</td></tr><tr><td>S4-13</td><td>Marine Sediment</td><td>16</td><td>-2.9</td><td>0.4</td><td>123</td></tr><tr><td>S6-1</td><td>Marine Sediment</td><td>0.25</td><td>-2.5</td><td>0.5</td><td>272</td></tr><tr><td>S6-2</td><td>Marine Sediment</td><td>0.75</td><td>-2.4</td><td>0.7</td><td>279</td></tr><tr><td>S6-3</td><td>Marine Sediment</td><td>1.25</td><td>-2.8</td><td>0.1</td><td>209</td></tr><tr><td>S6-4</td><td>Marine Sediment</td><td>1.75</td><td>-2.56</td><td>0.02</td><td>281</td></tr><tr><td>S6-5</td><td>Marine Sediment</td><td>2.5</td><td>-2.6</td><td>0.2</td><td>271</td></tr><tr><td>S6-8</td><td>Marine Sediment</td><td>5.5</td><td>-2.7</td><td>0.3</td><td>286</td></tr><tr><td>S6-9</td><td>Marine Sediment</td><td>7</td><td>-2.7</td><td>0.4</td><td>252</td></tr><tr><td>S6-10</td><td>Marine Sediment</td><td>9</td><td>-2.69</td><td>0.06</td><td>226</td></tr><tr><td>S6-12</td><td>Marine Sediment</td><td>12</td><td>-2.5</td><td>0.1</td><td>195</td></tr><tr><td>S6-14</td><td>Marine Sediment</td><td>17</td><td>-2.6</td><td>0.2</td><td>201</td></tr><tr><td>S9-1</td><td>Marine Sediment</td><td>0.25</td><td>-2.3</td><td>0.1</td><td>348</td></tr><tr><td>S9-2</td><td>Marine Sediment</td><td>0.75</td><td>-1.7</td><td>0.4</td><td>417</td></tr><tr><td>S9-4</td><td>Marine Sediment</td><td>1.75</td><td>-1.70</td><td>0.02</td><td>386</td></tr><tr><td>S9-6</td><td>Marine Sediment</td><td>3.5</td><td>-1.8</td><td>0.2</td><td>316</td></tr><tr><td>S9-8</td><td>Marine Sediment</td><td>5.5</td><td>-1.9</td><td>0.2</td><td>225</td></tr><tr><td>S9-9</td><td>Marine Sediment</td><td>7</td><td>-2.0</td><td>0.1</td><td>255</td></tr><tr><td>S9-10</td><td>Marine Sediment</td><td>9</td><td>-1.83</td><td>0.08</td><td>293</td></tr><tr><td>S9-11</td><td>Marine Sediment</td><td>11</td><td>-2.0</td><td>0.2</td><td>379</td></tr><tr><td>S9-12</td><td>Marine Sediment</td><td>13</td><td>-2.1</td><td>0.3</td><td>333</td></tr><tr><td>S9-14</td><td>Marine Sediment</td><td>17</td><td>-1.96</td><td>0.02</td><td>356</td></tr><tr><td>S9 WC 1</td><td>Water Column</td><td>685</td><td>-5.2</td><td>0.2</td><td></td></tr><tr><td>S9 WC 5</td><td>Water Column</td><td>10</td><td>-5.65</td><td>0.07</td><td></td></tr><tr><td>Drammenselva Bdl</td><td>River Sediment</td><td>-</td><td>-2.2</td><td>-</td><td></td></tr><tr><td>Glomma Bdl</td><td>River Sediment</td><td>-</td><td>-1.7</td><td>-</td><td></td></tr><tr><td>Göta Bdl</td><td>River Sediment</td><td>-</td><td>-1.6</td><td>-</td><td></td></tr><tr><td>Drammenselva SPM</td><td>River Particles</td><td>-</td><td>-2.1</td><td>-</td><td></td></tr><tr><td>Glomma SPM</td><td>River Particles</td><td>-</td><td>-2.0</td><td>-</td><td></td></tr><tr><td>Göta SPM</td><td>River Particles</td><td>-</td><td>-3.2</td><td>-</td><td></td></tr><tr><td>Sco-1 (N = 5)</td><td>Reference material</td><td>-</td><td>-2.8</td><td>0.4</td><td></td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Sediment 2 M HNO₃ leached ε²⁰⁵Tl profiles of sites S4, S6 and S9 show a general isotopic uniformity with an apparent slight heaving from site S4 to S9 and a general convergence towards crustal values. Only S9 is found to have an appreciable authigenic Tl enrichment with Mn oxide associated isotopic fractionation.</figcaption></figure>

of the clay fraction throughout, and illite concentrations were positively correlated with local terrigenous particulate delivery. We interpret that illite is a dominant vector for both particulate Tl transport from the south Scandinavian mainland and a dominant vector for particulate Tl delivery to Skagerrak sediments, whether this delivery and deposition

are linked or not. We infer that clay-bound Tl is leached, at least partially, with the HNO₃ acid extraction we employed here.

### 5.1.2. Diagenetic Tl and Mn decoupling

We find strong evidence that Tl and Mn diagenesis is essentially
decoupled in all Skagerrak sites. Manganese diagenesis follows the ca-
nonical pattern (Calvert and Pedersen, 1996; Froelich et al., 1979),
where dissimilatory reduction leads to increased porewater concentra-
tions in the anoxic part of the sediment and upwards transport (both
through diffusion, biodiffusion and bioirrigation) of dissolved Mn, fol-
lowed by oxidation and reprecipitation as Mn(oxy)hydroxides. This
recycling forms an effective sedimentary manganese oxide trap near the
sediment-water interface. In all 3 sites, these Mn enrichments are
entirely associated with the hydroxylamine HCl leach, and only at site
S4 is the Mn enrichment close enough to the sediment surface that the
porewater Mn gradient extends to the SWI. Thallium deviates funda-
mentally from this pattern, as porewater Tl gradients are produced and
extend to the SWI at all sites. Simultaneously, surficial Tl enrichments
can only be unambiguously identified at S9 where they are strongly
associated with the hydroxylamine HCl leach, ie. the Mn(oxy)hydrox-
ides. At S4 and S6, porewater Tl enrichments in the upper few cm are
small albeit significant. Furthermore, at these sites, the porewater Tl
enrichments cannot be attributed to the dissolution of any particular
phase in the sediments, based on the sequential extraction results. Thus,
the affinity of Tl for Mn(oxy)hydroxides doesn't control the aggregate of
Tl diagenetic behaviour. Even at the extremely Mn-rich site S9 where
Mn-associated Tl is released through the reduction of Mn oxides, Tl does
not completely readsorb onto the surfaces of Mn oxides, but rather dif-
fuses out of the sediment. This decoupling of Mn and Tl during
diagenesis underlines that benthic Tl fluxes cannot be calculated via
association with Mn.

The origin of the porewater Tl release is elusive. Divergence between
Tl and Mn behaviour as presented here was also identified in shoreline
advective sediments (Ahrens et al. (2021) and in the tidally influenced
water columns in the German Wadden Sea (Böning et al. (2018). Ahrens
et al. (2021) identified Tl accumulation in porewaters prior to Mn
accumulation and inferred the major sources of Tl to be release associ-
ated with OM degradation and release through reoxidation of previously
precipitated Fe-sulphides with strong affinity for Tl. Neither of these
processes likely play a role in the Skagerrak shelf sediments. We find no
indication of Tl release associated with OM degradation from TL-TOC
relationships (Fig. S5), and no indication of a significant Tl association
with iron sulphides as the majority of Tl leached by HNO3 is clay-derived
as explored above. We surmise that S4 and S6 porewater Tl peaks are
predominantly driven by Tl release from an aluminous clay phase.
Thallium release from clays should drive a decrease in measured TlHNO3
with sediment depth. However, this decrease is likely small and seem-
ingly masked by secular variability and measurement resolution.
Competition with NH4+ for clay edge-sites is shown to strongly control
the solubility of Tl in soil porewaters (Wick et al. (2020)), and we
speculate that OM degradation and derived NH4+ build-up in porewater
may be a driver of clay-associated Tl release in marine sediments. This
hypothesis should be tested in future research.

5.2. Reactive fractionation of Tl in S6 and S9 sediments

During sediment core incubations, Tl was removed from the bottom
water due to the forced diffusive gradients into the sediments. These
experiments therefore reveal something about the capability of the
sediment to reactively remove and fractionate Tl, but nothing about in-
situ flux directions and magnitudes. Additionally, as the reaction frac-
tionating Tl isotopes occurs within the pore space of the sediments, the
fractionation factor expressed in the water column above is diminished
due to diffusive limitations to reactant mixing (Brandes and Devol,
1997; Clark and Johnson, 2008). Hence the fractionation factors esti-
mated in section 4.3 can be termed effective fractionation factors, and
the intrinsic fractionation factors associated with reactions in the sedi-
ment can be determined by equation (6) (Clark and Johnson, 2008):

$$
\alpha_{\text{effective}} = \sqrt{\alpha_{\text{intrinsic}}} \cdot \left( \frac{1 + \frac{L}{\lambda}}{1 + \frac{L}{\lambda} \sqrt{\alpha_{\text{intrinsic}}}} \right) \qquad (6)
$$

here, L is the distance between the SWI and the top of the reaction zone where isotopic fractionation occurs. We assume L to be the distance from the SWI to the top of sediment zone with the highest $e^{205}\text{Tl}$, i.e. 0.5 cm for both S6 and S9 (Fig. 8). The parameter $\lambda$ is the e-folding distance of concentration decrease in the sediment described by equation (7), and the parameters $D_e$ and k are the effective diffusion coefficient and rate constant of the reaction, respectively.

$$
\lambda = \sqrt{\frac{D_e}{k}} \qquad (7)
$$

Effective diffusion coefficients are calculated using equation (8),
from the porosity φ of the top 1 cm of sediment (Table S1) and the tracer
diffusion coefficient Dt of Tl adjusted to the viscosity of water at 6 °C (Li
and Gregory, 1974).

$$
D_e = D_t \cdot \phi^2 \qquad (8)
$$

Intrinsic fractionation factors associated with reactive removal of
water column Tl into the sediments are determined to be 1.0007 at both
S6 and S9. The reactive removal is thus most likely driven by adsorption
onto Mn oxides, as this is the only known Tl isotope fractionating
mechanism with substantial positive fractionation factors (Nielsen et al.,
2017).

5.3. Quantification of authigenic Tl and evidence for boundary exchange

Identifying the primary origin of diagenetically liberated Tl in the
Skagerrak sediments is a prerequisite for disentangling internal marine
cycling from boundary exchange, a possible net source of Tl, to the
oceans. In other words, is the Tl in the sediments authigenic from the
marine environment or terrigenous? Fig. 9 shows Tl plotted against Al
for the sum of all leaches prior to the residual digest. This resulting trend
encompasses, in principle, all authigenic Tl as well as some terrigenous
Tl, as indicated by the association with Al as discussed above. A clear
terrigenous background can be identified for the Skagerrak sediments,
with a Tl enrichment at S9. We interpret this enrichment to be truly
authigenic, i.e. derived from seawater, and thus a product of internal
marine Tl cycling. We calculate the fraction of leached Tl that can be
considered marine authigenic as the deviation from the linear regression
through S4 and S6 (Table 4) and find between 17–41 % authigenic Tl at
S9.

River particles and bedload materials are also enriched in Tl relative to the terrigenous background found in Skagerrak sediments. A similar calculation can be performed to assess the river sediment and particle excess Tl/Al compared to the linear regression. This calculation yields Tl enrichments of between -7 and +100 % (average = 52 %). However, what this enrichment actually means is somewhat ambiguous.

The riverine enrichment relative to the Skagerrak sediment back-
ground could mean that considerable terrigenous Tl is lost from river
particulates during transport and interaction with saline seawater in
estuaries or as they undergo sediment diagenesis. This would imply a
terrigenous source of Tl to the water column. However, the Tl depletions
in the background Skagerrak sediments relative to river materials might
also be caused by a primary sediment source in the North Sea rather than
from local rivers. In this case, the comparison between river sediments
and Skagerrak sediments would reveal little about the possible boundary
exchange between particulate river-derived Tl and the ocean. As noted
in the introduction, evaluations of Skagerrak sediment sources estimate
that 15 % of sediments are locally derived and 85 % are derived from the
North sea (see section 2.1, Longva and Thorsnes (1997)). However,
strong correlations between marine sediment illite contents and local
<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Total leached (all leaches bar the residual digest) Tl vs. Al of Skagerrak sediments outline a clear detrital background with all S9 samples offset by true authigenic Tl enrichment (ie. derived from the ocean). River sediments and particles are colored yellow and green respectively and are altogether enriched relative to the Skagerrak sediment background. This offset can be due Skagerrak sediments not primarily being sourced from the rivers (Longva and Thorsnes, 1997) or by a boundary exchange process where particulate Tl is released to the environment during transport.</figcaption></figure>

<caption>**Table 4**</caption>

Calculation of authigenic ε²⁰⁵Tl signatures for S9 based on Tl v. Al relationships of total leach (i.e., all extractions bar the residual digest). Seawater ε²⁰⁵Tl is sample S9 WC 1 and terrigenous ε²⁰⁵TlHNO₃ is the mean of bedload and suspended particles of rivers Drammenselva, Glomma and Göta.

<table><thead><tr><th>Sample depth (cm)</th><th>Tl<sub>Authigenic</sub> (% Tl)</th><th>Seawater ε<sup>205</sup>Tl<br>Detrital ε<sup>205</sup>Tl<br>ε<sup>205</sup>Tl<sub>Authigenic</sub></th><th>-5.2<br>-2.1<br>Δ<sup>205</sup>Tl<sub>SW-Authigenic</sub></th></tr></thead><tbody><tr><td>0.25</td><td>39</td><td>-2.8</td><td>2.4</td></tr><tr><td>0.75</td><td>44</td><td>-1.0</td><td>4.2</td></tr><tr><td>1.75</td><td>39</td><td>-1.0</td><td>4.2</td></tr><tr><td>3.5</td><td>36</td><td>-1.2</td><td>4.0</td></tr><tr><td>5.5</td><td>24</td><td>-1.0</td><td>4.2</td></tr><tr><td>7</td><td>21</td><td>-1.4</td><td>3.8</td></tr><tr><td>9</td><td>27</td><td>-0.7</td><td>4.5</td></tr><tr><td>11</td><td>37</td><td>-1.7</td><td>3.5</td></tr><tr><td>13</td><td>42</td><td>-2.2</td><td>3.0</td></tr><tr><td>20</td><td>46</td><td>-1.8</td><td>3.4</td></tr></tbody></table>

riverine sediment origin have also been identified (Bengtsson and Stevens, 1997; Pederstad et al., 1993). With our finding that Tl is dominantly associated with illite-type clays in both riverine materials and marine sediments, the main Tl source could be local rivers even if the bulk sediment is not. We are thus unable to reject either: 1) a different sediment source or 2) boundary exchange Tl loss from river material during transport as the cause of marine sediment Tl depletions relative to river material.

Consistent, however, with a loss of terrestrial-derived Tl to the marine environment, several studies on the geochemistry of Tl in estuarine environments find evidence of non-conservative Tl behaviour. Böning et al. (2017) present data on particulate Tl in the Weser estuary and find that reactive particle Tl decreases steadily with increasing salinity, while Nielsen et al. (2005) found a strong positive deviation from conservative mixing in the Kalix and Amazon estuaries suggesting a source of Tl in this environment. Furthermore, Turner et al. (2010) found a dramatically increased capability of estuarine sediments to adsorb Tl in river water compared to seawater. In combination with the results of our study, these observations warrant a reinvestigation of the relevance of Tl particulate reactivity at the fluvial-marine boundary.

### 5.4. Controls on ε²⁰⁵Tl

The Tl isotope signature of Skagerrak sediments as measured on “authigenic” (i.e., 2 M HNO₃) leaches appears predominantly controlled by crustal input. Small expressions of other influences occur at sites S4 and S9. These two sites are statistically distinct from the population of riverine sediments measured (Students t-Test, p < 0.05) with slightly lower and higher ε²⁰⁵TlHNO₃ values, respectively. If the ε²⁰⁵Tl value of the terrigenous TlHNO₃ depositing in the Skagerrak is represented by the population of riverine ε²⁰⁵TlHNO₃ values, then the most likely explanation for the lower ε²⁰⁵Tl values at S4 is that a minor fraction of the clay-associated TlHNO₃ at this site is in fact of marine origin (e.g. marine authigenic clays with ε²⁰⁵Tl ~ -6‰). The higher ε²⁰⁵TlHNO₃ values at S9 reflects isotopic enrichment related to Mn oxide adsorption, as discussed in section 5.3.

The observation of authigenic Tl at S9 allows a quantification of the isotopic offset Δ²⁰⁵TlSW-Authigenic between seawater ε²⁰⁵TlSW and ε²⁰⁵Tlauthigenic using the simple mass balance equation (9):

$$ε^{205}Tl_{authigenic} = ε^{205}Tl_{measured} - \frac{(1 - F_{authigenic} \cdot ε^{205}Tl_{terrigenous})}{F_{authigenic}} \quad (9)$$

Using the bottom water ε²⁰⁵Tl value of -5.2‰ (Table 3), a terrigenous ε²⁰⁵Tl value of -2.1‰ (average of riverine particulate matter), and fractions of leached Tl that is authigenic Fauthigenic, we find ε²⁰⁵Tlauthigenic values between -2.8‰ and -0.7‰, and seawater offsets Δ²⁰⁵TlSW-Authigenic between + 2.4‰ and + 4.5‰ (Table 4). These values are comparable to the intrinsic fractionation factors of reactive Tl removal as observed in our sediment incubation experiments (section 5.2., α = 1.0007), and thus provide a self-coherent line of evidence for the isotopic fractionation expressed during Tl adsorption to Mn(oxy)hydroxides in Skagerrak sediments.

These findings add nuance to the complex issue of what drives the variability of Tl isotope fractionation onto Mn oxides observed in nature. In the dataset of Rehkämper et al. (2002), Δ²⁰⁵TlSW-Mnoxide demonstrably differs between hydrogenetic- (ε²⁰⁵Tl = +10.4‰ to + 14.3‰), deep sea diagenetic- (ε²⁰⁵Tl = +0.5‰ to + 8.9‰), and shallow water diagenetic (ε²⁰⁵Tl = -0.2‰ to -5.4‰) Mn nodules and crusts. Attempts to define fractionation factors for Tl adsorption onto hexagonal birnessite in lab experiments yield a large range of α values, from 1.0002 to 1.0015, that are inversely correlated with the mass of Tl sorbed per mass of birnessite. This inverse correlation most likely stems from the
saturation of an oxidative vacancy site that strongly fractionates Tl isotopes, and the increased importance of adsorption to a less fractionating edge site, when $m_{Tl-sorbed}/m_{birnessite}$ increases (Nielsen et al., 2013; Wick et al., 2019). With depth and diagenetic maturation, Mn oxides transform to more stable minerals like triclinic birnessite and todorokite (Atkins et al., 2014; Jung et al., 2020; Yang et al., 2018). These minerals exhibit much smaller Tl fractionation factors, ranging from 0.99999 to 1.00010 for triclinic birnessite and 1.00002 to 1.00005 for todorokite, due to their lack of Tl(I) oxidizing adsorption sites (Peacock and Moon, 2012; Phillips et al., 2023).

We speculate that the muted Tl isotope fractionations expressed in S6 and S9 compared to deep sea hydrogen genetic Mn crusts and nodules is caused by diagenetic Mn(III) saturation. Indeed, Wick et al. (2019) showed that Mn(III) saturation of vacancy sites on $\delta$-MnO$_2$, a lab analogue to hexagonal birnessite, limits the capability for oxidative adsorption of Tl(I). This process of Mn(III) saturation is analogous to saturation of oxidative adsorption sites with Tl itself, which causes lowered Tl isotope fractionation (Nielsen et al., 2013). Saturation of sorption sites with Mn(III) can occur through partial reduction of hexagonal birnessite or by adsorption of porewater Mn(III), which are both common processes in the shallow diagenetic environment (Flynn and Catalano, 2019; Madison et al., 2013; Wick et al., 2019). Regardless of these interpretations, the uniform fractionation factor ($\alpha$ = 1.0007) observed between S6 and S9, despite the lack of fractionation expressed in the sediments at S6, implies that a Mn(oxy)hydroxide able to oxidatively adsorb Tl, e.g., hexagonal birnessite, likely remains a driver of Tl isotope fractionation in Skagerrak sediments during diagenesis.

## 5.5. Skagerrak sediment Tl budget

We calculate a budget for dissolved Tl efflux and authigenic particle deposition which shows that S6 is a net source of terrigenous Tl to the overlying water column, while S9 is a net sink. The direction of Tl flux at S4 is ambiguous (Table 6). These budgets assume that dissolved Tl fluxing out of the sediment must be liberated from either particles carrying a marine authigenic Tl component, or a riverine authigenic component. Thus, if a dissolved efflux of Tl exceeds the mass accumulation rate of marine authigenic particulate Tl, the excess Tl must be sourced from a terrigenous Tl component. In that case the diagenetic liberation constitutes a net input of Tl to the oceans.

The flux of dissolved Tl in or out of the sediment is the sum of molecular diffusion, biodiffusion and bioirrigation. Depth profiles of net rates of Tl dissolution/precipitation, $R_{Tl}$, were computed using the PROFILE script (Berg et al., 1998) as described in section 3.5, for sites S6 and S9 and are shown in Fig. 10 together with measured and modelled [Tl]$_{porewater}$ concentrations. The reaction rate profiles fit well to the data, $R^2$ for S6 = 0.96 and $R^2$ for S9 = 0.89. The model for S6 yields 4 distinct zones of $R_{Tl}$. This is most likely an artefact of the [Tl] shoulder at ~5 cm depth and not a confident interpretation of deep Tl dissolution. As the rates of Tl precipitation/dissolution modelled extend to depths of invariant porewater Tl concentrations, any downwards Tl flux below the horizon of Tl dissolution is quantified as Tl precipitation by the model. We therefore assume that the model describes a diagenetic steady-state condition, and the net production/dissolution (integrated over the modelled depth) is equal to the flux of dissolved Tl across the SWI, $Tl_{flux}$,

<caption>**Table 5**</caption>

Variables used to model net Tl production using the diagenetic transport-reaction model (eqn (7)), and their literature sources.

<table><caption>**Table 6**</caption><thead><tr><th>Site</th><th>S6</th><th>S9</th><th>Units</th><th>Source</th></tr></thead><tbody><tr><td>D<sub>b</sub>, Biodiffusion coefficient</td><td>4.45</td><td>3.4</td><td>cm<sup>-2</sup>·year<sup>-1</sup></td><td>Middelburg et al., (1997)</td></tr><tr><td>α<sub>0</sub>, Surficial non-local exchange coefficient</td><td>0.24</td><td>0.04</td><td>day<sup>-1</sup></td><td>Kristensen et al., (2018)</td></tr><tr><td>b, depth attenuation coefficient</td><td>2.44</td><td>1.92</td><td>cm<sup>-1</sup></td><td>Kristensen et al., (2018)</td></tr></tbody></table>

Thallium flux calculations of all sites. The net Tl flux at each site is the total diffusive efflux subtracted the authigenic deposition, and thus represents terrigenous Tl. The total efflux of dissolved Tl is the net production modelled across the porewater profiles using the diagenetic transport-reaction model described in section 5.5.1, for sites 6 and 9. For site 4 the total dissolved efflux is taken as the diffusive Tl flux calculated using the surficial Tl gradient and Ficks 1st law. Authigenic Tl deposition rates are calculated using sedimentation rates from Van Weering et al., (1987) and authigenic Tl concentrations. Authigenic Tl concentrations are taken as the sum of Tl leached with ammonium acetate and hydroxylamine HCl for sites 4 and 6, and as the excess Tl relative to the terrigenous Tl/Al line described in section 5.3 for site 9.

<table><thead><tr><th>Site</th><th>S4</th><th>S6</th><th>S9</th><th>Units</th></tr></thead><tbody><tr><td>Tl dissolved efflux</td><td>1.0·10<sup>-8</sup></td><td>1.1·10<sup>-8</sup></td><td>9.3·10<sup>-8</sup></td><td>mmol·cm<sup>-2</sup>·year<sup>-1</sup></td></tr><tr><td>Tl exchangeable + Mn oxide deposition</td><td>-1.2</td><td>-5.9</td><td>-1.3</td><td>mmol·cm<sup>-2</sup>·year<sup>-1</sup></td></tr><tr><td>Tl True Authigenic deposition</td><td>10<sup>-8</sup></td><td>10<sup>-9</sup></td><td>10<sup>-7</sup></td><td>mmol·cm<sup>-2</sup>·year<sup>-1</sup></td></tr><tr><td>Net flux</td><td>-1.9</td><td>5.6</td><td>-1.3</td><td>mmol·cm<sup>-2</sup>·year<sup>-1</sup></td></tr><tr><td></td><td>10<sup>-9</sup></td><td>9</td><td>10<sup>-8</sup></td><td>year<sup>-1</sup></td></tr></tbody></table>

$d|Tl|$ = $R_{Tl\_net}$. For the S4 dissolved Tl flux we use the diffusive efflux calculated using Ficks 1st law (Equation (10)), as we were unable to fit the diagenetic transport-reaction model satisfactorily. This yields total dissolved effluxes of 1.0, 1.1 and $9.2·10^{-8}$ mmol·cm$^{-2}$·year$^{-1}$, for S4, S6 and S9 respectively, presented in Table 6.

$$J = D_e \frac{d|Tl|}{dx} \qquad (10)$$

To calculate the Tl budgets with respect to the marine Tl mass balance at the Skagerrak sites, we subtract rates of authigenic particle deposition from dissolved Tl effluxes. At S4 and S6 we calculate the authigenic Tl particle deposition from the Tl content of $NH_4Ac$ and hydroxylamine HCl leaches, as we assume these leaches extract phases that are most likely marine authigenic. The calculation employs sedimentation rates from Van Weering et al. (1987) and surficial sediment densities shown in Table S1. The detection limits were rather high during analysis of the hydroxylamine HCl leach, and the background thus masks the likely lower true Tl concentrations. Thus, we here use the calculated background equivalent concentrations (BEC) as a best estimate for this leach. At worst, using BEC overestimates the assessment of authigenic particle delivery by a factor of ~2.5 for S4 and a factor of ~2 for S6 (Table S4). At S9 we calculate the authigenic Tl particle deposition from the authigenic Tl concentrations as described in section 5.2 and Fig. 9. Table 6 shows the results of these calculations.

We find that for S9 authigenic ocean-derived Tl exceeds the dissolved efflux and thus represents a net sink of Tl. In contrast, S6 is a net source of Tl to the oceans, likely through the hitherto unrecognized process of Tl liberation from terrigenous clay particles. However, the interpretation of S4 is ambiguous as the transport-reaction modelling didn't yield a robust dissolved flux estimate, the authigenic Tl particle flux to the sediments is below the detection limit of our method, and as marine clays may be an authigenic source of $Tl_{HNO_3}$ as evidenced by the low $\epsilon^{205}Tl_{HNO_3}$ values at this site compared to S6. We therefore consider it too uncertain to determine whether S4 constitutes a net source or sink of Tl to seawater. Within the uncertainty of our data both are possible.

## 5.6. Implications for the marine Tl mass balance and $\epsilon^{205}Tl$ palaeoredox proxy

Continental margin sediments were previously thought to be a net source of Tl to the global oceans, via the release of Tl during reductive Mn oxide dissolution (Owens, 2020; Rehkämper and Nielsen, 2004). However, recent discoveries that Tl does not dominantly associate with Mn oxides in some coastal (Ahrens et al., 2021) and margin sediments
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Tl porewater profiles of S6 and S9 overlain with modelled Tl precipitation/dissolution rates and the model Tl porewater profiles necessary to produce these under the given porosity, biodiffusion and bioirrigation constraints. The S6 model fit has an R² of 0.96 and the S9 model fit has an R² of 0.89. We interpret the model prediction of a “deep” dissolution zone at S6 to be an artefact of the shoulder on the porewater concentration profile and not a diagenetic steady state phenomenon.</figcaption></figure>

(this study) could challenge this assumption. So, too, could evidence of
strong authigenic Tl retention in many oxygenated margin sediments
(Wang et al., 2022).

Our results add new merit to the idea that margin sediments could be
a net source of Tl to the oceans, albeit not via release of Tl previously
held in Mn oxides. As specifically observed at site S6 and potentially also
site S4 in this study, effluxing Tl may be sourced from terrigenous clay
phases and occur at magnitudes relevant for the global Tl mass balance.

The clay-derived Tl efflux is only recognizable and quantifiable with sediment porewater profiles, which to our knowledge is only published for marine sediments beyond the coastal zone in our study (other studies with published marine porewater Tl concentrations: Ahrens et al. (2021); Böning et al. (2018)). Indeed, sites with such a terrigenous efflux, and the magnitude of the flux, will be indistinct when only considering sediment Tl<sub>HNO3</sub> concentrations and ε<sup>205</sup>Tl<sub>HNO3</sub> values, as these parameters will be converging with those of the upper continental crust. We note that while crustal isotopic signatures may also occur at sites with an authigenic Tl burial flux, 25 % of samples in the global core-top dataset (N = 85) of Wang et al. (2022) have ε<sup>205</sup>Tl values between -4‰ and -1‰, very close or slightly lighter than the upper crustal average of -2.0 ± 1.0‰ (Nielsen et al., 2017). These sites are

potential Tl sources to the ocean and this calls for an evaluation of the
global scale of diagenetic terrigenous Tl efflux.

In addition, our finding that HNO3 acid leaches may liberate substantial terrigenous Tl, highlights a necessity to correctly establish whether TlHNO3 contents are authigenic when using ε205TlHNO3 values in palaeoredox studies. Many ε205TlHNO3 palaeoredox studies investigate TOC-rich shales with high Tl contents, where Tl was removed in association with pyrite from a sulfidic water column (Nielsen et al., 2011). In such cases, terrigenous Tl is likely an insignificant fraction of authigenic Tl-dominated TlHNO3 contents. In sediments where Tl contents are relatively low (e.g., Tl/Al ratios close to 0.1104 of the upper continental crust abundance (Rudnick and Gao, 2014)), or where the oxygen levels of the depositional environment are ambiguous, extra care is warranted. Methods to determine the fidelity of ε205TlHNO3 signals in cases of low TlHNO3 contents have been presented by Wang et al. (2022), although it is unknown if these can quantitatively account for a potential terrigenous Tl component.

**6. Conclusion**

Our examination of Tl dynamics in the Skagerrak system has
revealed unrecognized aspects of Tl cycling at the continent-ocean boundary, with important implications for our general understanding of marine Tl geochemistry and the marine Tl mass balance.

Thallium is largely decoupled from Mn in terms of both sediment delivery and diagenesis in Skagerrak continental margin sediments. Significant Mn enrichments do not necessarily equate to significant Tl enrichments, and while Mn effectively reoxidizes upon O₂ exposure after dissimilatory reduction, released Tl does not efficiently (re)adsorb to Mn oxides, but rather diffuses out. Instead, we find that Tl in both riverine and marine sedimentary environments is extensively associated with terrigenous aluminous clays that can be mobilized with 2 M HNO₃. Thus, the 2 M HNO₃ leach cannot be equated with authigenic Tl in continental margin sediments where a terrigenous particle flux is to be expected.

This clay association of TlHNO₃ causes a strong continental control on sedimentary ε²⁰⁵TlHNO₃ values, highlighting a need for care when interpreting authigenic ε²⁰⁵Tl values in margin environments. Regardless, we find that Tl adsorbs to Mn oxides in two Skagerrak sites with isotopic fractionation factors α ~1.0002-1.0007, determined independently from both isotopic incubations and sediment ε²⁰⁵Tl analysis. These fractionation factors are much smaller than those of deep-sea hydrogenous Mn nodules and crusts and much larger than those associated with diagenetically matured Mn oxide minerals like triclinic birnessite and todorokite. They are, however, comparable to fractionation factors observed in shallow water diagenetic Mn nodules.

Importantly, our study has potential implications for the marine Tl budget as we find that boundary exchange of riverine particulates constitutes a novel vector of Tl transfer from land to sea. This boundary exchange is expressed as a net flux of Tl out of the sediment caused by the liberation of terrigenous clay-associated Tl during diagenesis. Such a flux is identified in at least one, possibly two, of the three sites investigated, although the global flux associated with such environments is presently highly uncertain. Additionally, we find that riverine materials are enriched with between 36 and 241 ng/g Tl in comparison to the marine sediments. This enrichment likely constitutes an additional boundary exchange flux occurring during particle transport, which is coherent with productive deviations from non-conservative mixing of Tl in estuaries published in the literature. Ultimately, we conclude that continental margin sediments are not always sinks but may be important sources of continentally derived Tl to the global oceans.

### CRediT authorship contribution statement

**Kasper P. Olesen:** Writing – review & editing, Writing – review & editing, Visualization, Validation, Investigation, Formal analysis, Conceptualization.
**Sune G. Nielsen:** Writing – review & editing, Supervision, Resources, Conceptualization.
**Chadlin M. Ostrander:** Writing – review & editing, Investigation, Funding acquisition.
**Nicol Udy:** Validation, Methodology, Investigation.
**Donald E. Canfield:** Writing – review & editing, Supervision, Resources, Methodology, Investigation, Funding acquisition, Conceptualization.

### Data availability

Data is available through figshare at https://doi.org/10.6084/m9.figshare.c.7578389.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

We thank M. Auro, J. Blusztajn and J. Kaare-Rasmussen for assistance with isotopic analyses at Woods Hole Oceanographic Institution.

T. Frickmann is thanked for assistance with sequential extractions and ICPMS analyses, Z. Kofoed for TOC analyses and A. Glud and H. Jensen for assistance with incubation experiments and cruise preparations at the University of Southern Denmark. The crew of the R/V Skagerak, P. Hall and H. Olsson from the University of Gothenburg and C. Nantke, D. Otto and A. Schell from the Leibniz-Institute for Baltic Sea Research are thanked for assistance with cruise preparations and sampling. We also thank editor Brian Kendall and two anonymous reviewers for constructive comments. K.P.O acknowledges support from the William Demant, Augustinus and Torben and Alice Frimodt Foundations for travel grants. C.M.O. and S.G.N. acknowledge support from National Science Foundation grant EAR-2129034. D.E.C. acknowledges support from the Villum Foundation grant 16518.

## Appendix A. Supplementary material

Supplementary figures referenced in the article, and a discussion of the sequential extraction sediment and particle specificity, can be found in the supplementary materials document. All data reported in this study along with quality control are reported in the six supplementary tables attached.

Supplementary material to this article can be found online at https://doi.org/10.1016/j.gca.2025.04.005.

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