# Geochemical and mineralogical characteristics of fjord sediments in Arctic Svalbard: insights into Holocene glacial activity and weathering variability

Kwangchul Jang, Germain Bayon, Youngkyu Ahn, Young Jin Joe, Eun Jin Son, Sae Yun Kwon, Jung-Hyun Kim, Christoph Vogt, Matthias Forwick, Eunji Byun, et al.

<figure><img src="image_1.png" /></figure>

## To cite this version:

Kwangchul Jang, Germain Bayon, Youngkyu Ahn, Young Jin Joe, Eun Jin Son, et al., Geochemical and mineralogical characteristics of fjord sediments in Arctic Svalbard: insights into Holocene glacial activity and weathering variability. Journal of Sedimentary Research, 2025, 95 (2), pp.223-238. 10.2110/jsr.2024.088. hal-05105688

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Submitted on 31 Oct 2025

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# Geochemical and mineralogical characteristics of fjord sediments in Arctic Svalbard: insights into Holocene glacial activity and weathering variability

Jang Kwangchul<sup>1,2</sup>, Bayon Germain<sup>3</sup>, Ahn Youngkyu<sup>1</sup>, Joe Young Jin<sup>1</sup>, Son Eun Jin<sup>4</sup>, Kwon Sae Yun<sup>4</sup>, Kim Jung-Hyun<sup>1</sup>, Vogt Christoph<sup>5</sup>, Forwick Matthias<sup>6</sup>, Byun Eunji<sup>2,*</sup>, Nam Seung-II<sup>1</sup>

## Abstract :

The Svalbard archipelago at the border of the Arctic Ocean experiences substantial glacier retreat due to global warming, resulting in a retreat of glacier termini from the marine to terrestrial settings. However, the impact of these transitions on marine environments remains poorly understood. To improve our understanding of how Arctic Svalbard responds to fluctuations in climate and glaciers, we reconstructed past glacimarine environments in Hornsund, southern Svalbard, over the last 4,900 years. By tracking sediment provenance using detrital neodymium isotopes and quartz-to-carbonate ratios, we revealed a progression from nearly open-marine conditions with minimal glacial activity during the middle Holocene to gradual seaward glacial expansion into the Neoglacial period (4,000 years B.P. to 790 C.E.). Glaciers likely retreated landward during the Medieval Warm Period (MWP; 790 to 1,470 C.E.), punctuated by significant read advances at least twice. The Little Ice Age (LIA) witnessed multiple glacial advances into the marine environment, followed by an overall glacial retreat in modern times. Geochemical analysis of authigenic sediment components indicates pronounced enrichments of middle rare earth elements, particularly during rapid glacial advances such as the MWP and LIA, which we interpret as enhanced glacial erosion and accelerated sulfide oxidation. Paleo-analogues from the middle Holocene, characterized by minimal marine-terminating glaciers and reduced sulfide weathering, suggest potential future declines in sulfur-related bioessential nutrient fluxes into Arctic coastal environments and subsequently to marine ecosystems.

<footnote n="1"><sup>1</sup> Division of Glacier and Earth Sciences, Korea Polar Research Institute, 21990 Incheon, Republic of Korea</footnote>

<footnote n="2"><sup>2</sup> Department of Earth System Sciences, Yonsei University, 03722 Seoul, Republic of Korea</footnote>

<footnote n="3"><sup>3</sup> Université Brest, CNRS, Ifremer, Geo-Ocean, F-29280 Plouzané, France</footnote>

<footnote n="4"><sup>4</sup> Division of Environmental Science and Engineering, Pohang University of Science and Technology, 37673 Pohang, Republic of Korea</footnote>

<footnote n="5"><sup>5</sup> Crystallography and Geomaterials Research, FB05 Geosciences and MARUM, University of Bremen, Klagenfurter Strasse 2-4, 28359 Bremen, Germany</footnote>

<footnote n="6"><sup>6</sup> Department of Geosciences, UiT The Arctic University of Norway, NO-9037 Tromsø, Norway</footnote>

<footnote>* Corresponding author : Eunji Byun, email address : eb@yonsei.ac.kr</footnote>
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INTRODUCTION

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Recent global warming has triggered unprecedented shifts in climate and environmental conditions (e.g., Diffenbaugh et al., 2017; Trenberth et al., 2014), particularly in the Svalbard archipelago – a rapidly warming region at the margin of the Arctic Ocean (Nordli et al., 2014). Rising temperatures have led to significant landscape changes in Svalbard, including negative glacier mass balance over approximately 30 years before 2,000 C.E. (Hagen et al., 2003) and a reduction in the glaciated area by up to 7% between pre-1,980 and 2,010 C.E. (Nuth et al., 2013). The recent acceleration of glacier mass loss (Kohler et al., 2007; Zemp et al., 2019) implies that many marine-terminating glaciers in Svalbard will turn into land-terminating glaciers in the near future. Among the most affected areas is the Hornsund Fjord in southern Svalbard, which has witnessed substantial glacial retreat over decades (e.g., Błaszczyk et al., 2019; Saferna et al., 2023) and even centuries (e.g., Pälli et al., 2003). Projections suggest that, if the current retreat trend of glaciers (e.g., Hornbreen and Hambergbreen) persists, the glaciated strait of eastern Hornsund may vanish in the near future (Grabiec et al., 2018), potentially isolating Sørkapp Land from the rest of Spitsbergen (Fig. 1). Given these drastic environmental changes, accurate forecasts of future environmental shifts in response to global warming are crucial for effective adaptation (e.g., Füssel, 2007). However, the accuracy of such predictions is hampered by limited environmental observations, which typically cover only a few hundred years. Past environmental reconstructions represent a crucial methodological approach, basing upon measured climate and environmental datasets to test the reliability of climate-model predictions (Braconnot et al., 2012; Lee et al., 2023).

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Svalbard fjord sediments serve as an invaluable archive for reconstructing past glacial environments in response to climate fluctuations. Over the late Quaternary, Svalbard was
covered by glaciers during cold intervals, but it also experienced major deglacial events during
warm intervals (e.g., Hughes et al., 2016). Varying glacier extents have also been documented
throughout the Holocene (Bartels et al., 2017; Forwick and Vorren, 2009; Hald et al., 2004;
Salvigsen and Høgvard, 2006). For instance, glaciers reached their minimum extents during the
early and/or middle Holocene (review in Farnsworth et al., 2020) and underwent substantial
regrowth, referred to as the Neoglacial period, reaching their maximum postglacial extents (e.g.,
Werner, 1993). High-resolution sedimentary records for the Holocene thus offer valuable
insights into the environmental response to various glacial states during periods dominated by
either marine- or land-terminating glaciers or even in the nearly complete absence of glaciers.
Since glacial activity has significant impact on land surface processes, such as physical abrasion
and bedrock (or regolith) weathering (e.g., Anderson et al., 2000), Svalbard fjord sediments
deposited during the Holocene can archive useful information about weathering processes and
other environmental changes linked to past glacial dynamics. Such sedimentary records can
provide glimpses of potential future environmental conditions resulting from global warming,
hence emphasizing the importance of reconstructing high-resolution past environments in
Svalbard.

In this study, we investigate glacimarine environmental changes in Hornsund over the
last 4,900 years, based on neodymium (Nd) isotope and bulk mineralogical data from a sediment
core. By identifying the Nd isotope signature and mineralogical properties of potential sediment
sources, based on reported datasets for various surface sediments in Hornsund, we are able to
trace changes in sediment provenance and corresponding glacier configuration. Additionally, we
report abundances of rare earth elements (REEs) for Fe-(oxyhydr)oxide phases leached from
bulk sediments, which provide particular insight into sulfide oxidation processes linked to glacial
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weathering. We show that sustained sulfide oxidation prevailed throughout the entire period,
being most likely more intense at times of glacial advances.

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STUDY AREA

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The Svalbard archipelago is located at the border between the Arctic and the N. Atlantic
Oceans, encompassing all islands situated between 74-81° N and 15-35° E, including
Spitsbergen, Barentsøya, Edgeøya, and Nordaustlandet (Fig. 1). Over the late Quaternary period,
this archipelago has undergone repeated glacier advances and retreats in response to climate
variability. For example, during the last glacial maximum, Svalbard was almost entirely
glaciated (Hughes et al., 2016), whereas currently, approximately 57% of its land area remains
glaciated (Nuth et al., 2013).

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Hornsund is the southernmost fjord in the western part of Spitsbergen. This fjord is approximately 32 km long in an east-west direction and up to 10 km wide. It comprises one main basin and five sub-basins, Brepollen, Samarinvågen, Austre and Vestre Burgerbukta, and Isbjörnhamna (Fig. 1), all affected by tidewater glaciers. The main basin is > 250 m deep, while water depths in the sub-basins range from 55 m in Isbjörnhamna to 180 m in Vestre Burgerbukta.
The hydrography in Hornsund is governed primarily by the interplay between advected regional seawater and local glacial meltwater (Beszczynska-Moller et al., 1997). The warm Atlantic Water and the cold Arctic Water, transported by the West Spitsbergen Current and the East Spitsbergen Current, respectively, contribute to the oceanic water masses in the region (Fig. 1).
Glacial meltwater runoff not only causes significant hydrological perturbation but also leads to rapid sediment accumulation (Błaszczyk et al., 2019; Görlich, 1986).

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The bedrock geology around Hornsund exhibits significant diversity (Dallmann and Elvevold, 2015) (Fig. 1). The catchment areas of western Hornsund are characterized by Proterozoic metamorphic rocks, including phyllites, schists, quartzites and marbles, while those around the central fjord correspond to Cambrian to Ordovician carbonate rocks such as marbles, limestones, and dolostones. On the other hand, the bedrock geology surrounding eastern Hornsund is composed predominantly of upper Paleozoic to Lower Cretaceous sedimentary rocks. In the easternmost glacial catchment area, Paleogene sedimentary rocks also prevail.

Three main periods of glacier advance have been identified during the Holocene. The oldest, known as the Grönfjorden Stage, commenced before 8 kyr B.P. (Marks and Pekala, 1986). Subsequently, the Revdalen stage occurred around 3.0-2.5 kyr B.P. (Karczewski et al., 1981). After glacial retreat, a renewed glacier advance followed during the Little Ice Age (LIA), dated to 0.6-0.1 kyr B.P. (Lindner and Marks, 1993 and references therein). In Hornsund, the maximum glacier extent during the LIA reached near the western boundary of Brepollen and the southern boundary of Burgerbukta (Błaszczyk et al., 2013). In comparison, during the Revdalen stage, the glacier extended beyond the western boundaries of Isbjörnhamna to the north and Gåshamna to the south (Lindner and Marks, 1993; Lindner et al., 1984). It is noteworthy that the glacier expansion event in Hornsund has been reported to occur before the LIA between ~ 2.0 and 1.5 kyr B.P., based on ¹⁰Be dating of moraines (Philipps et al., 2017).

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# MATERIALS AND METHODS

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## Sample Collection

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The gravity core HH19-878-GC (76°57.543' N, 15°41.886' E, water depth 125 m, 405 cm long) was retrieved from the western parts of the central basin of Hornsund during the Korea-Norway joint expedition of RV Helmer Hanssen (UiT The Arctic University of Norway) in 2019 (Fig. 1). The wet bulk density and magnetic susceptibility of the whole core were measured in 1-cm intervals using a GEOTEK Multi-Sensor Core Logger at UiT. After splitting the core, a half-core section was used for acquiring x-radiographs and line-scan images using a GEOTEK Standard X-ray CT System and an Avaatech X-ray fluorescence core scanner, respectively, at UiT (cf. Joe et al., 2022). Another half-core section was subsampled at 1 cm intervals and then freeze-dried for additional analyses.

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## Grain-Size Distribution

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The grain-size analysis was conducted at 5 cm intervals using a Malvern Mastersizer 3000 laser diffraction particle size analyzer at Korea Polar Research Institute (KOPRI). Before the analysis, ~ 130 mg of sediments were treated with 35% hydrogen peroxide for 24 hours to eliminate the organic matter, followed by thorough rinsing with deionized water (18.2 MΩ, Millipore). Mean grain sizes were calculated with GRADISTAT v 8.0 (Blott and Pye, 2001) based on the method of Folk and Ward (1957). Grains larger than 1 mm were counted at 5 cm intervals from x-radiographs. Additionally, the proportion of grains > 63 µm was determined at 10 cm intervals via manual wet sieving.

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AMS $^{14}$C Age Dating

We collected marine mollusc shells and mixed benthic foraminifers from sediments > 63 µm at these depths: 4-5, 54-55, 59-60, 79-80, 104-105, 114-115, 139-140, 149-150, 179-180, 214-215, 249-250, 309-310, 359-360, 389-390 and 399-400 cm (n = 15). These samples were analysed at the BETA Analytic Laboratory (Miami, USA) or the Alfred Wegener Institute (Bremen, Germany) for AMS $^{14}$C dating (Table 1). All $^{14}$C dates were used to establish the age-depth relationship at 0.5 cm intervals through Bayesian age-depth models, BACON v. 3.2.0 (Blaauw and Christen, 2011). In this model, $^{14}$C dates were initially converted to calendar ages based on the Marine20 calibration curve (Heaton et al., 2020), applying a reservoir age offset of -61 ± 37 years in western Svalbard (Pieńkowski et al., 2022). The age of the core surface was established as 2019 C.E.. While we generally adhered to the default settings of BACON, we made a slight adjustment to the Student's t-distribution parameters, t.a. and t.b., changing from 3 and 4 to 33 and 34, respectively. This modification aligns with previous studies conducted in Svalbard (Jang et al., 2021, 2023; Joe et al., 2022), resulting in a less smoothed age-depth relationship. The weighted mean age at each depth provided by BACON was used as the final age estimate in this study, along with the uncertainty at the 95% confidence level calculated based on Markov Chain Monte Carlo iterations.

Rare Earth Element Concentrations and Neodymium Isotopes

Rare earth element (REE) concentrations and neodymium isotope compositions were measured from two different components within sediments, defined as “authigenic” and “detrital” fractions, at every 5 to 50 cm intervals. The authigenic fraction primarily corresponds to hydrogenous Fe-(oxyhydr)oxides extracted from bulk sediments with a mixed solution of 0.02
179 M hydroxylamine hydrochloride in 25% acetic acid (v/v) buffered to pH of 4.5 (referred to as
180 HH solution from herein) for 1 hour (Jang et al., 2021). To minimize the potential dissolution of
181 reactive terrigenous materials during leaching, the reagent-to-sediment ratio was maintained at
182 approximately 1 (Jang et al., 2017; Wilson et al., 2013). The remaining detrital fraction,
183 conversely, comprised materials devoid of authigenic components. To remove any authigenic
184 remnants from the sediments, an extra chemical leaching process was conducted using the HH
185 solution, with a higher reagent-to-sediment ratio of approximately 10, for over 24 hours.
186 Following this leaching process, the residual detrital fractions were fused with a mixture of
187 sodium peroxide and sodium hydroxide in a furnace at ~ 650 °C for 15 minutes (Bayon et al.,
188 2009).

189 The concentrations of REEs in the aliquots of both authigenic and detrital fractions were
190 determined with a Thermo Scientific Element XR sector field Inductively Coupled Plasma Mass
191 Spectrometer at the Pôle Spectrométrie Océan (Brest, France), using the Tm addition method
192 (Barrat et al., 1996). Isobaric interferences were corrected using oxide formation rates
193 determined via analysis of mono-elemental solutions (Ba-Ce, Nd-Pr, and Sm-Eu-Gd-Tb). The
194 internal error for all measurements was consistently below 5%. The accuracy was evaluated by
195 analysing various geological certified reference materials (AGV-1; AN-G; BCR-1; DR-N; UB-
196 N; WS-E; and IF-G, i.e., an iron-ore deposit), which provided concentrations consistent with
197 reference values (< 13%).

198 Before isotopic analyses, pure fractions of Nd were isolated with a two-stage column
199 separation protocol using both TRU (50-100 µm, Eichrom) and Ln resins (50-100 µm, Eichrom),
200 following a method adapted from Pin and Zalduegui (1997). The purified Nd fractions from both
201 authigenic and detrital fractions were then loaded onto Re filaments for isotopic analysis using a
thermal ionization mass spectrometer (TIMS, Triton, Thermo Scientific) at KOPRI. Mass
fractionation during analysis was corrected by normalizing measured <sup>143</sup>Nd/<sup>144</sup>Nd ratios to
<sup>144</sup>Nd/<sup>146</sup>Nd = 0.7219. Long-term replicate analyses of the JNdi-1 standard yielded <sup>143</sup>Nd/<sup>144</sup>Nd
= 0.512121 ± 0.000008 (2 SD, n = 14), consistent with the reference value (Tanaka et al., 2000).
The inferred external reproducibility is 0.2 ε<sub>Nd</sub> units, (ε<sub>Nd</sub> =
[(<sup>143</sup>Nd/<sup>144</sup>Nd)<sub>sample</sub>/(<sup>143</sup>Nd/<sup>144</sup>Nd)<sub>CHUR</sub>-1] × 10<sup>4</sup>, (<sup>143</sup>Nd/<sup>144</sup>Nd)<sub>CHUR</sub> ~ 0.512638 from Jacobsen and
Wasserburg, 1980). The accuracy was assessed using certified reference materials (BCR-2,
BHVO-2), yielding <sup>143</sup>Nd/<sup>144</sup>Nd ratios consistent within uncertainty with the literature values
(Weis et al., 2006). Duplicate analyses are also consistent with each other within analytical
uncertainty.

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TOC, δ<sup>13</sup>C<sub>org</sub>, and N<sub>org</sub>

The total organic carbon (TOC) contents and its isotopic ratios (δ<sup>13</sup>C<sub>org</sub>) were determined using a Thermo Delta V Isotope Ratio Mass Spectrometer connected to a Thermo Flash 2000 Elemental Analyzer at KOPRI. Before the analysis, freeze-dried samples, collected at every 5 cm intervals, were powdered using an agate mortar and pestle. Subsequently, the samples were acidified with 2M hydrochloric acid for 24 hours to remove inorganic carbonate. The decarbonated samples were thoroughly rinsed with deionized water (18.2 MΩ, Millipore) and then freeze-dried for loading onto the analyzer. The reported δ<sup>13</sup>C<sub>org</sub> values are expressed in per mil notation relative to the Vienna Pee Dee Belemnite international standard. Duplicate analyses of TOC and <sup>13</sup>C<sub>org</sub> show average standard deviations of less than 0.2% and 0.15‰, respectively.

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Regarding organic nitrogen (N<sub>org</sub>) content, bulk sediment samples, also collected at 5 cm
intervals, were powdered and divided into two aliquots. The first aliquots were used to determine

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total nitrogen content, while the second aliquots were used to measure inorganic nitrogen content
after the complete removal of organic nitrogen using a KOBr-KOH solution (Silva and Bremner,
1966). Contents of total and inorganic nitrogen were measured using the Thermo Flash 2000
Elemental Analyzer at KOPRI. The N<sub>org</sub> content was then calculated by subtracting the
inorganic-nitrogen content from the total-nitrogen content, with a reproducibility of less than
0.2%.

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Bulk Mineral Assemblage

Mineralogical analysis of bulk powdered sediments at 5 cm intervals in core HH19-878-GC (n = 82) was performed using a Philips X'Pert Pro multipurpose diffractometer at the University of Bremen, Germany (for detailed setting, see Jang et al., 2023). The Philips software X'Pert HighScore™ was used primarily to identify the main mineral assemblages, while Apple MacIntosh X-ray diffraction interpretation software MacDiff 4.25 was used for sheet silicates (Petschick et al., 1996). Mineralogical quantification was carried out using the QUAX full-pattern method based on a reference material database (Vogt et al., 2002).

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In this study, we focused exclusively on the abundance of quartz, calcite, dolomite, and Fe-(oxyhydr)oxides. The reported abundance of Fe-(oxyhydr)oxide in this study includes crystalline Fe-(oxyhydr)oxide, such as hematite, goethite, and lepidocrocite, but specifically excludes magnetite and amorphous Fe-(oxyhydr)oxides. Analytical uncertainties, determined through tests on artificial mixtures including the Reynolds Cup (Raven and Self, 2017), were found to be better than 1-3% for well-crystallized minerals like quartz and calcite, and 10% for Fe-(oxyhydr)oxide (Vogt et al., 2002). The full XRD results can be accessed at the Korea Polar Data Center (https://kpdc.kopri.re.kr/).
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# RESULTS

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## Sediment Lithology

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Core HH19-878-GC consists primarily of brown-colored sandy mud (Fig. 2). Significant changes in sediment color and primary structures are absent. Bioturbation is prevalent throughout the entire core, with varying degrees and features. The wet bulk density fluctuates between 1.62 and 1.86 g/cc (average $1.73 \pm 0.05$, 1 SD, n = 398), while magnetic susceptibility varies from $8.14 \times 10^{-5}$ ($10.0 \pm 0.78 \times 10^{-5}$, 1 SD, n = 376) (Fig. 2 and Supplementary Table S1). The grain-size distribution exhibits a general-coarsening trend downcore, with a mean grain size ranging from 7.0 to 16.5 µm (Fig. 2 and Supplementary Table S1). Notably, x-radiographs show intermittent occurrences of grains larger than 1 mm size, including pebbles and boulders (hereafter referred to as ice-rafted debris, IRD; compared with Forwick and Vorren, 2009), with their sizes and amounts displaying considerable variability (Fig. 2).

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## Chronology

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Measured radiocarbon dates are listed in Table 1 and Figure 2. Age inversions occurred repeatedly at depths of 59-60, 79-80, 114-115, 139-140, 179-180, and 389-390 cm, regardless of material types used in the analysis. These anomalies are typically interpreted as sediment reworking (Flink et al., 2018). The age-depth relationship established by BACON indicates that core HH19-878-GC spans approximately 4.9 kyr (Fig. 3 and Supplementary Table S2). The average sedimentation rate is estimated at ~ 0.08 cm/yr, slightly lower than that of nearby core HF_2011 (76°57.238' N, 15°41.782' E, water depth 135 m from Pawłowska et al., 2016; for location, see Fig. 1).

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Geochemical and Mineralogical Data

**Nd isotopes.---** Detrital εNd values in core HH19-878-GC do not show any significant correlation with the authigenic εNd values (r = -0.13, n = 21) (Fig. 4 and Supplementary Table S3). The detrital εNd values vary from -15.5 to -12.5 (-13.9 ± 0.9, 1 SD, n = 21), showing a saw-tooth pattern with short-lived εNd excursions towards both unradiogenic and radiogenic values between approximately 500 C.E. and 1,600 C.E.

On the other hand, authigenic εNd values vary within a narrower range from -10.6 to -9.6 (-10.2 ± 0.2, 1 SD, n = 21) (Fig. 4 and Supplementary Table S3). These εNd values are slightly higher than the regional εNd signature for seawater on the western Svalbard shelf (i.e. the Sørkapp Current, with -11.6 < εNd < -10.6 from Laukert et al., 2017), but they are comparable to or lower than the reported εNd values of authigenic fractions in surface sediments collected from Brepollen, the innermost part of Hornsund (-10.4 < εNd < -9.0 from Jang et al., 2020). Overall, no distinct temporal trend is observed in authigenic εNd values, given the analytical uncertainty.

**REE concentrations.---** The sedimentary REE abundances in core HH19-878-GC are expressed as concentrations (µg/g) relative to the initial mass of bulk sediments (Supplementary Table S4). Detrital sediments show REE concentrations approximately two orders of magnitude higher than those in authigenic sediments. For instance, lanthanum (La) concentrations in detrital sediments range from 19.7 to 54.5 µg/g, corresponding abundances in authigenic sediments vary from 0.37 to 0.66 µg/g. Likewise, gadolinium (Gd) and ytterbium (Yb) concentrations in detrital sediments fall within the ranges of 2.45-5.56 µg/g and 1.32-3.10 µg/g, respectively, while authigenic sediments exhibit much lower concentrations (0.25-0.42 µg/g for Gd and 0.05-0.09 µg/g for Yb).
µg/g for Yb). Notably, the REE abundances in detrital sediments at this site display flat patterns when normalized to world river average silt values (WRAS; Bayon et al., 2015) (Fig. 5).

Conversely, the WRAS-normalized REE patterns in corresponding authigenic sediments show strong mid-REE enrichments relative to light- and heavy-REE (Fig. 5). The different degrees of mid-REE enrichments can be quantified using the “concavity index (CI)”, calculated as $Gd_N/(La^6_N \times Yb^7_N)^{1/13}$ (Bayon et al., 2020), which results in calculated CI values of 0.9-1.0 and 3.0-4.6, for detrital and authigenic sediments, respectively.

**Organic geochemistry (TOC, δ¹³Corg and Norg).---** The average contents of TOC and Norg are 1.54 ± 0.08 wt.% (1 SD, n = 81) and 0.22 ± 0.08 wt.% (1 SD, n = 42), respectively (Fig. 6 and Supplementary Table S4). Both contents display general upward-decreasing trends although TOC contents display a somewhat scattered distribution. The combined ratios of TOC/Norg show wide fluctuations between 12 and 43. These ratios generally increase upward, reaching a maximum at 1,900 C.E., and then decrease back to 22. Conversely, δ¹³Corg values generally decrease upward, although their variability is relatively narrower in the range between -25.0 and -23.6. Note the absence of pronounced peaks in the uppermost part of the core.

**Mineralogical abundance.---** Calcite contents determined by XRD range from 2.5 to 11.3 wt.% (6.9 ± 2.5 wt.%, 1 SD, n = 82) (Fig. 6 and Supplementary Table S5), being generally higher in the lower part of the core and gradually decreasing upward. Dolomite contents fluctuate from 0 to 10.9 wt.%, with an average content of 3.0 ± 2.1 wt.% (1 SD, n = 82). This mineral is also more abundant in the lower part of the core than in the upper part. However, the upward decreasing trend of dolomite is less pronounced than calcite.
Quartz contents do not display any significant trend downcore. The average quartz content in core HH19-878-GC is 25.2 ± 3.8 wt. (%) (1 SD, n = 82), ranging between 19.1 and 33.9 wt.%, except for one outlier at ~ 1,750 C.E. with a content of 42.0 wt.% (Fig. 6 and Supplementary Table S5). The quartz-to-carbonate ratio, which represents the siliciclastic-to-carbonate mineral ratios in Hornsund after Görlich (1986) (for details, see section *Potential Sediment Sources in Hornsund*), shows a general upward-increasing trend. In this study, carbonate minerals are defined as the sum of calcite and dolomite (Görlich, 1986).

The contents of Fe-(oxyhydr)oxides range between 0 to 5.98 wt.%. They are generally almost absent throughout the entire core, as also reflected in the average content of 0.3 ± 1.0 wt. (%) (1 SD, n = 82) (Fig. 6 and Supplementary Table S5). Nevertheless, three prominent peaks are observed at ~ 1,750 C.E., ~ 1,200 C.E., and ~ 910 C.E..

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DISCUSSION

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*Stratigraphy*

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In this study, we defined three stratigraphic units in core HH19-878-GC based on their respective geochemical and lithological characteristics, and age (Fig. 2). Unit A, which extends from the core base to 200 cm, was deposited between 4.9 kyr B.P. and ~ 790 C.E., covering the period from the latest stage of the middle Holocene to the early stage of the late Holocene. This unit is characterized by a general rise in detrital εNd values and is composed predominantly of bioturbated sandy mud with horizontal burrows. Mean grain size is generally the largest among the stratigraphic units (average 11.2 ± 2.2 μm, 1 SD, n = 41) and tends to decrease upward. In

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contrast, while IRD is rarely recognized at the base, scattered outsized clasts and an IRD layer (e.g., ~310 cm) become more apparent in the upper part of the unit.

Unit B, defined from 200 to 80 cm (~790 to 1,470 C.E.), predominantly corresponds to the Medieval Warm Period (MWP). This unit shows significant fluctuations in detrital $\epsilon_{Nd}$ values, with three maximum peaks reaching as high as -12.5 (Fig. 2). This unit comprises primarily sandy mud, characterized by the alteration of massive and bioturbated zones with thin vertical tubes. Unit B shows a general reduction in mean grain size compared to Unit A, mainly due to higher clay contents, with an average of 8.2 ± 1.1 µm (1 SD, n = 24). Notably, the sand content fluctuates largely between 5.8 and 44.8 wt.% (average 16.7 ± 12.0 wt.%, 1 SD, n = 12), marked by two prominent peaks. Additionally, numerous coarse clasts, especially IRD-enriched layers, are frequently identified in Unit B.

Unit C, located between 80 cm and the core top (after 1,470 C.E.), was deposited mainly during and after the Little Ice Age (LIA) (Fig. 2). In this unit, detrital $\epsilon_{Nd}$ values are generally lower compared to the other units, reaching as low as -15.5. Unit C consists of bioturbated sandy mud with vertical tubes. There is no significant change in mean grain size, which ranges between 7.4 and 8.2 µm (8.2 ± 0.6, 1 SD, n = 17). Some coarse clasts are sporadically identified, generally decreasing in number towards the top.

## Potential Sediment Sources in Hornsund

The reconstruction of sediment provenance changes in polar regions can provide unique insights into past glacier configurations, given the close relationship between regional sediment supply and glacial activity (e.g., Farmer et al., 2006; Phillips and Grantz, 2001). Here, we use
358 detrital εNd values and bulk mineral assemblages to trace provenance variations downcore and
359 their link to glacial activity in Hornsund for the past c. 4.9 kyr. The detrital εNd of bulk sediments
360 is particularly useful for fingerprinting different sediment sources eroded from nearby catchment
361 areas via diverse transport pathways and processes (e.g., Horikawa et al., 2015; Jang et al., 2021;
362 Wilson et al., 2018). Given that the diverse bedrock geology around Hornsund may complicate
363 the discriminative use of Nd isotopes for reconstructing provenance, we also use bulk mineral
364 assemblages as source indicators (e.g., Görlich, 1986; Jang et al., 2021; Vogt and Knies, 2009).
365 In particular, εNd values and bulk mineral assemblages from surface sediments, especially those
366 collected near glacier fronts or sediment outlets, serve as the representative value of each source
367 region (i.e., end-member) (e.g., Horikawa et al., 2010). This approach is preferred over using
368 bedrock values due to the heterogeneity of Svalbard bedrock and the diversity of their εNd values,
369 within a single catchment area (Hindshaw et al., 2018; Jang et al., 2020).

370 The sub-basins of Hornsund (i.e., Brepollen, Samarinvågen, Austre and Vestre
371 Burgerbukta, and Isbjörnhamna) are currently affected by tidewater glaciers, thereby serving as
372 primary depocenters for terrigenous sediments (Fig. 1). As a result, each sub-basin is typically
373 recognized as a single-source basin, influenced predominantly by suspension settling from local
374 meltwater plumes (Görlich, 1988; Moskalik et al., 2018). In contrast, in the ice-distal location of
375 the main basin, sediment composition is governed by the mixing of sedimentary materials from
376 various sources, each represented by individual sub-basins in conjunction with external inputs
377 drained into the fjord (Görlich, 1986). In this context, proxy records for provenance in fjord
378 surface sediments can be used to establish the geochemical characteristics of each source.
A compilation of detrital $\varepsilon_{Nd}$ values for surface sediments in Hornsund based on previous data (Jang et al., 2020; Kim et al., 2023) illustrates basin-specific geochemical variability (Fig. 7A), which we use to identify three potential source areas (A, B, and C) for Hornsund sediments.

1) Group A represents the innermost source, distinguished by significantly high detrital $\varepsilon_{Nd}$ values. This source is expressed in the surface sediments of Brepollen, the innermost part of Hornsund, exhibiting detrital $\varepsilon_{Nd}$ values as high as -10.1 (Fig. 7A). The predominance of Mesozoic and Cenozoic sedimentary rocks in the outcrops surrounding Brepollen (Dallmann and Elvevold, 2015) indicates their presumed role as primary source rocks. This inference could be further supported by similarly higher $\varepsilon_{Nd}$ values in the surface sediments of Storfjorden, where catchment areas are dominated by similar rock formations (Dallmann and Elvevold, 2015) (Fig. 7A).

2) Group B is defined as the central source, identified primarily by detrital $\varepsilon_{Nd}$ in surface sediments from Hornsund central sub-basins, such as Samarinvågen, Austre Burgerbukta, and Vestre Burgerbukta. In general, the detrital $\varepsilon_{Nd}$ in these regions is lower, reaching values as low as -24.9 near the glacier front in Vestre Burgerbukta (Fig. 7A). Despite a higher detrital $\varepsilon_{Nd}$ value of -12.8 near the glacier front in Austre Burgerbukta, the detrital $\varepsilon_{Nd}$ values of surface sediments in the region where the sediment inputs from Austre and Vestre Burgerbukta merge are notably lower, ranging from -23.7 to -20.7 (Fig. 7A). This observation implies minor sediment contribution from Austre Burgerbukta to the main basin, suggesting that Group B sediments are mostly tagged with relatively lower $\varepsilon_{Nd}$ values. Considering the geology of bedrock terranes adjacent to central Hornsund sub-basins, Group B sediments are most likely derived from the erosion of Paleozoic sedimentary rocks, including Cambrian and Ordovician carbonate rocks (Dallmann and Elvevold, 2015).
3) Group C is characterized by high εNd values, whose provenance is inferred from a general seaward detrital εNd increase of surface sediments from the main basin to the outer fjord. Notably, the detrital εNd value in the outer fjord exceeds the core-top value at site HH19-878-GC by 2-epsilon (ε) units, providing a distinctive marker for identification (Fig. 7A). Considering the prevailing eastward oceanic current along the southern coast governed by the Coriolis force (Jakacki et al., 2017; Pawłowska et al., 2017), it is likely that sediments with radiogenic detrital εNd signatures can be transported from outside Hornsund into the main basin by surface water circulation, thereby contributing to the advection of sediments carrying higher detrital εNd values. While the potential source rocks of Group C remain uncertain, the prevailing oceanic current following coastal lines may point towards late Paleozoic to Mesozoic sedimentary rocks in the southwestern catchments of Hornsund.

Mineralogical abundances can further improve our ability to discriminate between potential sediment sources, in particular between Group A and Group C, which are both characterized by relatively high εNd values. Previous investigations on sediment mineralogy in Hornsund have revealed basin-specific mineralogical features, such as a quartz-feldspar province in Brepollen and a carbonate-feldspar province in the outer fjord (Görlich, 1986). In this context, we explore the spatial variability of quartz-to-carbonate ratios in Hornsund surface sediments (data from Görlich, 1986; Jang et al., 2023; Vogt and Jang, 2023) (Fig. 7B). The surface sediments in Brepollen show high quartz-to-carbonate ratios, ranging between approximately 14 and 327, significantly surpassing the ratios in the outer fjord ranging from about 1.9 to 5.3. Such a stark contrast between the ratios in the representative depocenters affected by Groups A and C highlights that quartz-to-carbonate ratios effectively complement the use of detrital εNd for tracing sediment provenance.
It is important to note that the Isbjörnhamna catchment area could also serve as a source of sediment in Hornsund, as inferred from its large estimated sediment yield of approximately 2,000 t km⁻² yr⁻¹ (Moskalik et al., 2018). However, the absence of any detrital εNd data for surface sediments in Isbjörnhamna makes it challenging to determine whether this new source can be integrated into adjacent source groups like Groups B and C, or if it should be classified as a distinctive end-member. An early investigation of sediment mineralogy in Isbjörnhamna (Görlich, 1986) demonstrated that the surface sediments in Isbjörnhamna exhibited relatively low carbonate contents, mostly reflecting the absence of carbonate rocks in the surrounding catchment areas. As a result, the quartz-to-carbonate ratios of surface sediments in inner Isbjörnhamna (6.8 ± 2.6, 1 SD, n = 4) tend to be higher than those for Samarinvågen (1.7 ± 0.4, 1 SD, n = 4), Vestre Burgerbukta (1.8 ± 0.3, 1 SD, n = 4) and the outer fjord (3.6 ± 2.4, 1 SD, n = 2) (data from Görlich, 1986; Jang et al., 2023; Vogt and Jang, 2023) (Fig. 7B). This particular mineralogical feature suggests that the Isbjörnhamna catchment can be considered as a distinctive potential source (Group D) in this study, even though its corresponding detrital εNd value remains uncertain.

To summarize, we identify four potential sources for Hornsund sediments. Firstly, the Brepollen catchment area is proposed as the innermost source (Group A), with relatively high detrital εNd values (up to -10.1) and quartz-to-carbonate ratios (up to 327; Fig. 8). Secondly, the catchment areas around Vestre Burgerbukta and Samarinvågen correspond to the central sources (Group B), supplying sediments with unradiogenic detrital εNd values (as low as -24.9) and low quartz-to-carbonate ratios (down to ~ 1; Fig. 8). Thirdly, sediments can also be transported by advected oceanic inflow from outside the fjord (Group C). This outermost and external source is characterized by a relatively higher detrital εNd value of -11.9 and a low to intermediate quartz-
to-carbonate ratio (3.6 ± 2.4, 1 SD, n = 2) (Fig. 8). Lastly, the Isbjörnhamna catchment can also be considered a local source (Group D) associated with carbonate-depleted sediments, hence with intermediate to high quartz-to-carbonate ratios compared to Groups B and C (Fig. 8).

451

452

*Changes in Sediment Sources at Site HH19-878-GC over the Last 4.9 kyr*

The variability of detrital εNd in core HH19-878-GC over the last 4.9 kyr does not strongly correlate with quartz-to-carbonate ratios (Fig. 9), possibly reflecting various mixing processes between multiple sources and/or the potential influence of grain size on sediment composition (Görlich, 1988). Nevertheless, the collective insights gained from both proxy records are used below to infer the provenance of sediments deposited at site HH19-878-GC.

Initially, the innermost region (Group A) with high quartz-to-carbonate ratios can be excluded as a potential sediment source at site HH19-878-GC. The quartz-to-carbonate ratios at this site fall predominantly within the range of 1.0 to 6.1, with a single outlier reaching up to 12.6 around 1,750 C.E. (3.0 ± 1.6, 1 SD, n = 82). These mineralogical ratios closely align with the expected ranges from three potential sources situated around the central to the outer fjord (i.e., Groups B, C, and D). On the other hand, the quartz-to-carbonate ratios at site HH19-878-GC are an order of magnitude lower than those found in surface sediments in Brepollen, despite certain paired detrital εNd values being comparable to those observed in the same sediments (Fig. 8). This comparative analysis of measured bulk mineralogy strongly suggests that the innermost region (Group A) played a minor role in supplying sediment to site HH19-878-GC over the last 4.9 kyr. Despite evidence that the glaciers around Brepollen have retreated rapidly over the last decades (e.g., Błaszczyk et al., 2019; Saferna et al., 2023) or centuries (e.g., Pälli et al., 2003),
their minor contribution to detrital sedimentation at site HH19-878-GC can largely be attributed
to the efficient trapping of supplied sediments in the near-source zone, i.e., the glacier front.

Indeed, the sedimentation rate in Hornsund tends to decrease exponentially seaward, with the
greatest efficiency of sediment removal in tidewater-glacier systems (Görlich, 1986; Görlich et
al., 1987). A minor fraction of sediments originating from the inner Hornsund may survive
beyond the near-source zone; however, their influence on the sediment components at site
HH19-878-GC could be further diluted by contributions from more proximal sources around
Isbjörnhamna. This interpretation is reinforced by distinctive basin-specific detrital εNd patterns
and the quartz-to-carbonate ratios in surface sediments (Fig. 7).

Secondly, the overall trend observed in our proxy records proposes that sediment
deposition at site HH19-878-GC is influenced primarily by the external (Group C) and local
(Group D) regions, with an episodic contribution from central Hornsund (Group B). As
mentioned earlier, the detrital εNd values at this site indicate significant variability in the upper
part of the core, where vertical bioturbation is prevalent. While burrower activity may have
partially reworked sediments, potentially contributing to the geochemical fluctuations (e.g., Aller
et al., 2010), evidence for both high sedimentation rates and concurrent variability in bulk
mineralogy within the same sediment interval suggests that changes in sediment sources are
likely to represent the primary cause for the observed saw-tooth εNd pattern (Fig. 9).
Additionally, despite limited Nd isotope data, the bottom part of the core, influenced by
horizontal bioturbation, also exhibits pronounced fluctuations in the quartz-to-carbonate ratio,
hence suggestive of multiple shifts in sediment provenance over the last 4.9 kyr, with terrestrial
inputs being gradually enhanced in the most recent period (Fig. 9). These trends are consistent
with sediment mixing between two primary sources, each characterized either by high detrital εNd
and low quartz-to-carbonate ratio or vice versa. In this scenario, Group C from outside Hornsund
may contribute to a rise in detrital εNd and a reduction in quartz-to-carbonate ratios, thereby
establishing itself as one of the primary sources. On the other hand, Group B, in central
Hornsund, appears to exert a lesser influence on sediment deposition at site HH19-878-GC. This
is because the other primary source in this binary mixing system must be characterized by a
relatively lower detrital εNd value and a higher quartz-to-carbonate ratio, while Group B has the
potential to induce declines in both proxy records. Instead, Group D around Isbjörnhamna
emerges as a compelling additional primary source, the only remaining candidate among
potential sediment sources in Hornsund (for details, see section *Potential Sediment Sources in Hornsund*). This local source is indeed characterized by an intermediate to high quartz-to-
carbonate ratio (Görlich, 1986) and is likely to have relatively lower detrital εNd value, which can
counterbalance the influence of Group C in this binary mixing system. Nevertheless, we admit
that binary mixing does not fully account for the sediment transport system in Hornsund over the
last 4.9 kyr. Low detrital εNd values were intermittently aligned with low mineralogical ratios at
specific time intervals (e.g., ~1.5 and ~1.0 kyr B.P.; Fig. 9), which suggests episodic sediment
transport from central Hornsund. In this context, we focus on assessing the relative sediment
contribution not between each source but between internal sources (Groups B and D) and the
external source (Group C), based solely on detrital εNd values.

Taken together, the downcore evolution of detrital εNd at site HH19-878-GC hence
indicates a comprehensive shift in sediment provenance in Hornsund over the past 4.9 kyr,
transitioning predominantly from marine to terrestrial sources. This interpretation is supported by
an upward rise in TOC/N<sub>org</sub> and a simultaneous decline in δ<sup>13</sup>C<sub>org</sub> at site HH19-878-GC (Fig. 9),
which collectively suggest an increasing influx of land-derived terrestrial organic matter relative
516 to marine organic matter (Knies and Martinez, 2009; Winkelmann and Knies, 2005). This
517 evidence from organic proxies further supports enhanced terrestrial meltwater discharge in
518 Hornsund since the middle Holocene.

519

520 Reconstruction of Paleoenvironmental Changes in Hornsund Since the Middle Holocene

521 **Middle Holocene (before 4 kyr B.P.).---** The temporal variability in sediment
522 provenance at site HH19-878-GC reflects the overall environmental changes in Hornsund since
523 the middle Holocene. The initial clue for environmental conditions during the latest stage of the
524 middle Holocene can be obtained from the bottom part of Unit A, which is characterized by
525 bioturbated mud deposits with low amounts of IRD. The bioturbated mud unit in Svalbard fjords,
526 including Hornsund, typically represents ice-distal glacimarine sediments (Forwick and Vorren,
527 2009; Görlich, 1986; Ó Cofaigh and Dowdeswell, 2001). The limited occurrence of IRDs in this
528 unit further suggests lower glacial activity (e.g., Baeten et al., 2010; Forwick and Vorren, 2009;
529 Hald et al., 2004), pointing towards conditions akin to nearly open-marine conditions during the
530 middle Holocene. This period has been recognized not only as the global Holocene thermal
531 maximum (~ 6.5 kyr B.P.; Kaufman and Broadman, 2023), but also as a time when Svalbard
532 glaciers attained their minimum extent or even vanished (Farnsworth et al., 2020; Fjeldskaar et
533 al., 2018; van der Bilt et al., 2015). The development of some marine beaches in the Bogstranda
534 region, situated east of Hansbreen in Hornsund (Fig. 1), indicates glacier retreat linked to sea
535 ingress during the middle Holocene (Lindner and Marks, 1993). This is supported by
536 radiocarbon dating of mollusc shells collected from the Hornbreen forefield in Brepollen, which
537 suggested that glacier recession at that time led to the opening of the glaciated strait between
538 Sørkapp Land and the rest of Spitsbergen (Fig. 1C), facilitating marine environments in
Hornsund during most of the early and middle Holocene, c. 10.9-3.9 kyr B.P. (Osika et al., 2022).

The landward retreat of glaciers in Hornsund, including Hansbreen, would have likely allowed the coast to remain ice-free during the middle Holocene. As a result, the suspension settling and iceberg calving from this terrestrial source would be restricted, thereby resulting in lower clay and poor IRD flux at site HH19-878-GC, respectively (Fig. 9). Consequently, the advected oceanic inflow may have overwhelmed the terrestrial meltwater discharge, potentially contributing to high detrital $\epsilon_{Nd}$ values and low quartz-to-carbonate ratios at site HH19-878-GC during the middle Holocene (Fig. 9).

**Neoglacial: early stage of the late Holocene (~ 4 kyr B.P. to ~ 790 C.E.).**--- From c. 4 kyr B.P., a noticeable increase in the frequency of IRD was observed from the bottom to the upper part of Unit A. This trend can be attributed to enhanced iceberg calving, suggesting an overall environmental transition toward higher glacial activity during the early stage of the late Holocene. While the potential influence of sea-ice IRD transportation cannot be excluded (e.g., Forwick and Vorren, 2009; Joo et al., 2019), the presence of outsized clasts up to 5 cm in diameter most likely reflects the involvement of icebergs (e.g., Gilbert, 1990; Knies et al., 2001). In this context, the identification of an IRD-enriched layer at 2.2 kyr B.P. may have resulted from the collapse of marine-terminating glaciers (e.g., Elliot et al., 2001), potentially offering empirical evidence for the presence of such glaciers in Hornsund (Figs. 2, 6) (e.g., Elliot et al., 2001).

On this basis, we infer that glaciers in Hornsund began to regrow and expand seaward during the early stage of the late Holocene. This time interval broadly coincides with increased
glacial activity in Svalbard during the Neoglacial period (review in Farnsworth et al., 2020),
likely caused by summer insolation decline (Laskar et al., 2004) and ocean cooling (Farnsworth
et al., 2020; Hald et al., 2004; Rasmussen et al., 2012). Neoglacial glacier growth and readvances
have been observed in the northern and western parts of Svalbard, including the catchment areas
of Linnévatnet (Svendsen and Mangerud, 1997) and Hajeren lakes (van der Bilt et al., 2015), as
well as Billefjorden (Baeten et al., 2010) and Tempelfjorden (Forwick et al., 2010). Similarly, in
Hornsund, the widespread glacial advance has been well documented through age determinations
of outwash sands, tills, and moraines across the region (Lindner and Marks, 1993; Lindner et al.,
1984). The absence of dated mollusc shells from 3.9 to 1.3 kyr B.P. in the terrestrial parts of
Hornbreen forefield also suggests a potential reclosure of the glaciated strait between Sørkapp
Land and the rest of Spitsbergen in Brepollen (Osika et al., 2022), providing additional support
for the Neoglacial glacier readvance in the Hornsund region.

Independent support can be derived from the application of Fe-(oxyhydr)oxides as a
proxy for meltwater discharge events from adjacent glaciers (Jang et al., 2023) (Fig. 9).
Interestingly, the abundance of Fe-(oxyhydr)oxide minerals at site HH19-878-GC peaked around
2.2 kyr B.P., albeit in small quantities. As glacially derived iron accumulates primarily at the
interface between meltwater and seawater (Boyle et al., 1977; Raiswell et al., 2018; Schroth et
al., 2014; Zhang et al., 2015), these modest enrichments potentially indicate that glacier fronts
extended closer to the study site after the middle Holocene, at least until 2.2 kyr B.P..

Collectively, we suggest that enhanced glacial activity prevailed in Hornsund during the
Neoglacial period, particularly linked to the development of marine-terminating glaciers. This
transition would have led to increased contribution of terrestrial meltwater discharge, explaining
the overall decrease in both detrital εNd and δ¹³Corg, concomitant with higher clay contents (Fig. 9).

### Medieval Warm Period: middle stage of the late Holocene (~790 to 1,470 C.E.).---

Unit B, spanning approximately from 790 to 1,470 C.E., corresponds broadly to the Medieval Warm Period (MWP). While the MWP is conventionally set between 950 and 1,200 C.E. (Mann et al., 2009; Miller et al., 2010), it is generally considered to encompass the 1,000 to 1,600 C.E. period, especially in Hornsund (Pawłowska et al., 2016). During the MWP, various regions, particularly around the North Atlantic sector of the Arctic, experienced warmer climate conditions compared to the early 20th century (review in Hughes and Diaz, 1994). Although the MWP is not recognized as a global climate event (Broecker, 2001; Hughes and Diaz, 1994), ice-core records from western Svalbard indicate that Svalbard also encountered general warmth before 1,200 C.E., comparable to the late 1990s (Divine et al., 2011). Notably, dating investigations of both mollusc shells and moraine boulders have suggested that this warming event most likely contributed to the overall retreat of glaciers in Hornsund inland, leading to the reopening of the glaciated strait in Brepollen from ~1.3 kyr B.P. (650 C.E.) to before ~0.7 kyr B.P. (1,250 C.E.) (Osika et al., 2022). Similarly, low and stable IRD fluxes at core HF_2011 indicate limited glacial activity in Hornsund between 1,000 and 1,800 C.E. (Pawłowska et al., 2016). The multiple detrital εNd maxima observed in Unit B at site HH19-878-GC also support a general glacial retreat inland in Hornsund between 790 and 1,470 C.E. (Fig. 9), with higher εNd values being suggestive of enhanced sediment contribution from outer Hornsund brought by regional surface waters relative to glacial sediment delivered by local meltwater discharge.
However, the detrital εNd values fluctuated and reached minimum values twice during that interval (Fig. 9), indicating the alternation of successive periods of reduced and enhanced glacial activity. Notably, Unit B of core HH19-878-GC stands for its abundant outsized clasts and IRD-enriched layers, occurring more frequently than in the underlying stratigraphic units (Fig. 2). These findings strongly imply a pattern of marked glacial fluctuations, with alternating advances and retreats of marine-terminating glaciers in Hornsund between 790 and 1,470 C.E.. Fe-(oxyhydr)oxide minerals provide additional evidence for episodic glacial advances. The occurrence of two prominent peaks of Fe-(oxyhydr)oxides slightly precedes the occurrence of low detrital εNd values (Fig. 9). These peaks of Fe-(oxyhydr)oxide minerals may have resulted from episodical advances of local glaciers, such as Hansbreen, into deeper waters in Hornsund between 790 and 1,470 C.E. (cf. Jang et al., 2023).

Thus, our finding suggests two scenarios for the development of the glacimarine environments in Hornsund during the MWP. First, the MWP may have persisted for a shorter duration than previously defined in Hornsund (e.g., ~1,000 to 1,600 C.E. according to Pawłowska et al., 2016). Alternatively, the warming during this period might not have led to the prevalent glacial retreat in Hornsund. In any case, it is probable that the peak warmth occurred during the interval between two glacial advances, as inferred from the low detrital εNd values (i.e., 940 to 1,380 C.E.). Supporting this hypothesis, marine records from core HR-3 (76°57.36' N, 15°12.13' E, water depth 145 m; for location, see Fig. 1) in outer Hornsund reveal significant increases in IRD fluxes at two specific intervals around 1,075 and 1,330 C.E. (Majewski et al., 2009). These IRD peaks coincide with declining δ¹⁸O values of benthic foraminifers, suggesting substantial calving events associated with the advances of Hansbreen glacier into marine environments at those times (Majewski et al., 2009). Likewise, glacier advances or surges have
been widely identified in the inner Hornsund around 1,140 C.E. (Marks and Pekala, 1986) and in
other Svalbard fjords between approximately 1,060 and 1,430 C.E. (Evans and Rea, 2005;
Farnsworth et al., 2017; Hald et al., 2001; Larsen et al., 2018; Lovell et al., 2018; Lyså et al.,
2018). Collectively, it is inferred that either the MWP persisted over a short duration, or Svalbard
glaciers exhibited a less pronounced response to warmth during that period.

Glacimarine environments in Hornsund between 790 and 1,470 C.E. exhibited significant
variability, characterized by rapid fluctuations in glacial dynamics. While glaciers generally
retreated landward, including during the peak warmth of the MWP, they also advanced at least
twice. These dynamic shifts likely intensified the influence of terrestrial inputs, as evidenced by
general low δ¹³Corg values and high clay contents (Fig. 9).

### Little Ice Age and modern: late stage of the late Holocene (after ~ 1,470 C.E.).---
Unit C, deposited over the past ~ 600 years, broadly covers the Little Ice Age (LIA, 1,400-1,700 C.E. from Mann et al., 2009; 1,600-1,900 C.E. from Pawłowska et al., 2016), and the modern period. While Unit C shares similar characteristics with Unit B, IRD is primarily dispersed rather than concentrated into distinct layers (Fig. 2). Glacial activity in Hornsund appears to have persisted at higher levels, influencing sediment deposition at site HH19-878-GC after approximately 1,470 C.E.. The scattered IRD may indicate the potential impact of both marine-terminating glacier calving and sea-ice drifting. However, the prevailing low detrital εNd values suggest a greater contribution from local glaciers (i.e., Hansbreen) rather than sea-ice from the Barents-Kara seas (-8.3 < εNd < -9.4 from Tütken et al., 2002). A previous investigation on the lithological composition of IRD collected from outer Hornsund yielded a similar conclusion, emphasizing that the IRD in this region originates predominantly from marine-terminating
glaciers and, to a small degree, from sea-ice (Majewski et al., 2009). Therefore, in this section, we discuss the environmental reconstruction over the last ~ 600 years, focusing primarily on glacier behavior.

The fluctuations in detrital $\epsilon_{Nd}$ values between -12.5 and -15.5 in Unit C at site HH19-878-GC reveal highly unstable environmental conditions in Hornsund during the last six centuries (Fig. 9). In detail, high detrital $\epsilon_{Nd}$ values at the boundary to Unit B most likely indicate reduced supply of terrigenous sediment due to limited glacial activity in the Hornsund region during or shortly after the MWP. However, detrital $\epsilon_{Nd}$ abruptly decreases thereafter, reaching values as low as -15.5 around 1,580-1,620 C.E. (Fig. 9). This $\epsilon_{Nd}$ shift towards lower values indicates increased sediment supply from the catchment areas around Isbjörnhamna, implying glacial activity peaking during the onset of the LIA. Glaciers such as Hansbreen would have expanded into the fjord basin, consistent with the peak accumulation of Fe-(oxyhydr)oxide minerals following low $\epsilon_{Nd}$ at site HH19-878-GC (e.g., Jang et al., 2023) (Fig. 9). Given the subsequent up-and-down variations in detrital $\epsilon_{Nd}$ values, albeit with smaller variability (Fig. 9), glaciers in Hornsund appear to have experienced retreat and readvance since then. In particular, the second glacial advance over the last ~ 600 years would have occurred around 1,900 C.E., which broadly coincided with the end of the LIA. The most recent increase in detrital $\epsilon_{Nd}$ values appears to correspond with the recent glacial retreat (e.g., Błaszczyk et al., 2019; Pälli et al., 2003). However, it is important to note that supply from terrestrial sources continued.

In summary, glacial dynamics during and after the LIA can be characterized by at least two cycles of retreat and advance. Based on detrital $\epsilon_{Nd}$, the first glacial advance likely occurred around 1,580-1,620 C.E., followed by the second-largest advance around 1,900 C.E.. These findings are consistent with the multi-step glacial advance recorded in other Svalbard areas
during the LIA (Røthe et al., 2015; van der Bilt et al., 2015; Werner, 1993). However, exact
timings remain to be determined. High-resolution investigations will offer the potential to
precisely reconstruct the intricate glacial dynamics in Hornsund during the LIA.

*Changes in Fe-S Cycling in Response to Glacial Dynamics*

The origin of authigenic Fe-(oxyhydr)oxide phases in sediments can be inferred from REE abundances and shale-normalized patterns. Leached fractions from riverine and marine sediments worldwide typically display mid-REE enrichments relative to light- and heavy-REE, typically exhibiting CI values between 1.2 and 2.5 (Jang et al., 2024 and references therein). The mid-REE enrichments in leached authigenic fractions reflect their preferential uptake upon Fe-(oxyhydr)oxide precipitation (Martin et al., 2010) and/or the fact that they are sourced from upward-migrating diagenetic fluids enriched in mid-REE in the marine environment (Haley et al., 2004). However, authigenic fractions leached from sediments draining sedimentary basins dominated by black shales (Bayon et al., 2020) or from glacial catchments (Jang et al., 2024) display distinctively higher mid-REE enrichments that depart significantly from CI values for typical marine Fe-(oxyhydr)oxides. Recently, Jang et al. (2024) have shown that such high CI values in Fe-(oxyhydr)oxide fractions from glacial environments were linked to pyrite oxidation following glacial erosion and weathering. Glacial weathering is known to be dominated by the oxidation of sulfide minerals (Torres et al., 2017). Following glacial erosion and oxidative sulfide weathering, the release of sulfuric acid can drive the preferential dissolution of apatite, a mid-REE-enriched phosphate mineral. The resulting Fe-(oxyhydr)oxide phases, precipitated from glacial meltwaters, may incorporate a characteristic mid-REE-enriched signature (Jang et
al., 2024). The CI values in leached fjord sediments can hence be used to infer glacial activity and related chemical weathering processes in surrounding catchments.

At site HH19-878-GC, the CI values of authigenic sediments vary between 3.0 and 4.6 (Fig. 9), markedly surpassing the typical range observed in leached sediments worldwide, as well as those of corresponding detrital sediments (CI ~ 1; Fig. 5). As discussed above, such high CI values exceeding 2.5 are found exclusively in leached fractions of sediments derived from watersheds dominated by intense sulfide weathering (Bayon et al., 2020; Jang et al., 2024). Therefore, at site HH19-878-GC, these mid-REE enrichments are clearly related to sulfide oxidation and the subsequent precipitation of glacially derived Fe-(oxyhydr)oxide phases that can be transported to the marine environment. Likewise, core HH17-1085-GC retrieved from the continental shelf off northern Svalbard also displays high CI values, ranging between 2.3 and 3.8 (average CI ~ 2.8 ± 0.3, 1 SD, n = 29; Jang et al., 2024).

In Hornsund, the most prominent peaks in CI values occurred during the MWP and the LIA (Fig. 9). A broad positive correlation between detrital εNd and CI values since the MWP (r = 0.48, n = 17) could imply that the observed increase in CI values coincided with a shift in provenance involving higher contribution from sulfide-bearing shale deposits. However, before the MWP, CI values remain relatively stable despite of varying detrital εNd values at site HH19-878-GC. Thus, we propose instead that the observed CI increase during the MWP and LIA is related to accelerated sulfide oxidation linked with more intense glacial weathering (understood as chemical weathering in glacial catchments; cf. Jang et al., 2020). Rapid glacial advances likely played a key role in this scenario, triggering active glacial abrasion and subsequent widespread exposure of newly eroded rock substrates (e.g., Calmels et al., 2007; Stachnik et al., 2022). A similar case can be found in core HH17-1085-GC from the continental shelf off northern
Svalbard, albeit with lower temporal resolution for CI values (n = 29 over the last 16.3 kyr; Jang et al., 2024). In this core, significant increases in CI values occurred exclusively during the late Holocene, coinciding with glacier readvances (Jang et al., 2021). Since coal mining in Svalbard began in the early 1900s, acid mine drainage may have also contributed to significant mid-REE enrichment (Jang et al., 2024), possibly coinciding with the second, more recent, CI peak during the LIA (Fig. 9). Although most coal mining activities in Svalbard took place around Isfjorden, Kongsfjorden, and Van Mijenfjorden—areas distant from Hornsund—the extraordinary high TOC/N<sub>org</sub> ratio, reaching up to 43, could hint at the possibility that yet unknown coal mining activity occurred in adjacent areas or that remote transport pathways of mining-related particulates episodically influenced local sedimentation (cf. Winkelmann and Knies, 2005), hence supporting the influence of anthropogenic impact on the observed high CI values in modern era. We acknowledge that bioturbation (as evidenced by the presence of vertical tubes in Units B and C at site HH19-878-GC) may possibly have contributed, at least partly, to the observed CI increase, by promoting diagenetic sulfide oxidation (Canfield et al., 1993; Thamdrup et al., 1994; Wehrmann et al., 2014; Wehrmann et al., 2017). However, despite intense vertical bioturbation in the modern period (Fig. 2), CI values tend to decrease (Fig. 9), likely in response to recent glacier retreat (e.g., Błaszczyk et al., 2019; Pälli et al., 2003). Therefore, we conclude that the observed CI increase since the MWP was driven primarily by enhanced glacial weathering.

The inferred relationship between sulfide oxidation and glacial activity has implications for the future evolution of sulfur and other bioessential elements in the Arctic coastal environments. Over the past decades, Svalbard glaciers have significantly retreated, as evidenced by negative glacier mass balance (Hagen et al., 2003) and/or the reduced glaciated area (Nuth et
al., 2013). The recent acceleration of glacier mass loss (Kohler et al., 2007; Zemp et al., 2019) suggests that many marine-terminating glaciers in Svalbard will eventually become land-terminating glaciers in the near future. Drawing an analogy to the diminished CI values during the middle Holocene when the tidewater glaciers mostly disappeared in Hornsund, a potential shift from marine to land-terminating glaciers in the near future may reduce the bioessential elemental input driven by sulfide oxidation associated with glacial weathering, with negative feedbacks in marine ecosystems (Jørgensen et al., 2021). Further investigations is required to fully understand the implications of such environmental changes on sub-Arctic marine ecosystems and biogeochemical processes.

752

CONCLUSIONS

753

In this study, we present multi-proxy records from an approximately 400-cm-long sediment core retrieved from outer Hornsund. This core enables us to reconstruct environmental changes over the past 4,900 years, including nearly open-marine conditions during the middle Holocene, the gradual formation of marine-terminating glaciers during the Neoglacial period (~4,000 years B.P. to 790 C.E.), rapid glacial fluctuations during the MWP and LIA, and currently ongoing deglaciation. Additionally, the use of REE in authigenic sediments indicates significant mid-REE enrichments in Hornsund, which we attribute to prevailing sulfide oxidation driven by glacial weathering in surrounding catchments. Our REE data show that enhanced pyrite oxidation occurred during glacial advances, such as during specific intervals of the MWP and LIA. Assuming parallel conditions between the middle Holocene and future climate projections, we infer that sulfide oxidation and associated nutrient fluxes into Arctic coastal environments will weaken in the future in response to reduced glacier extent. Further paleoenvironmental

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investigations in Hornsund are needed to improve our understanding and predictive capabilities regarding future environmental responses to ongoing climate warming.

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MSIT)(NRF-2021M1A5A1075512, NRF-2017R1A6A1A07015374, and RS-2024-00336841). We would like to thank all the participants of the cruises on RV *Helmer Hanssen* for their support of the core sampling, as well as D. Kim, Y. Son, and I. Nam for laboratory assistance. C.V. thanks the XRD lab team (J. Birkenstock, E.M. Schmidt, R.X. Fischer) for smoothly running the XRD machines. M.-L. Rouget is warmly thanked for assistance during ICP-MS analyses in Brest. Lastly, we appreciate the Editor and two anonymous reviewers for their constructive feedbacks on this manuscript.

The data used in the study can be found in five supplementary tables and are also available in the Korea Polar Data Center via doi:10.22663/KOPRI-KPDC-00002463 with permission.

Aller, R.C., Madrid, V., Chistoserdov, A., Aller, J.Y., and Heilbrun, C., 2010, Unsteady diagenetic processes and sulfur biogeochemistry in tropical deltaic muds: implications for

ACKNOWLEDGMENTS

DATA AVAILABILITY

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<figure><img src="image_3.png" /><figcaption>Figure 1. Map of A) the Arctic Ocean, B) Svalbard, and C) Hornsund. The sampling site is marked with a yellow diamond, while comparative sites are indicated with greenish blue diamonds. Various bedrock ages around Hornsund are represented by different colors (Dallmann and Elvevold, 2015). The bathymetric information is based on IBCAO 4.0 bathymetric grid (Jakobsson et al., 2020). Basemap datasets © Norwegian Polar Institute (https://geodata.npolar.no/).</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2. Three stratigraphic units of core HH19-878-GC defined based on lithological and geochemical characteristics, and age. Linescan image, x-radiograph, lithological log, AMS ¹⁴C ages (for error, see Table 1), wet bulk density, magnetic susceptibility, grain-size composition, mean grain size, sand fraction (> 63 µm), the number of grains greater than 1 mm, detrital εNd values and time period are shown for comparison. Note that the horizontal axis is reversed for detrital εNd.</figcaption></figure>

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<figure><img src="image_5.png" /><figcaption>Figure 3. Bayesian age-depth model constructed using Bacon v. 3.2.0 at site HH19-878-GC.</figcaption></figure>

Top-left panel displays Markov Chain Monte Carlo iterations (n = 4,000; left), while middle and right panels at the top represent the prior (green) and posterior (gray) distributions for accumulation mean and memory properties, respectively. In the bottom panel, blue symbols indicate the calibrated ¹⁴C ages, and the red dotted line illustrates the weighted mean age along with the uncertainty at the 95% confidence level (gray bands).
<figure><img src="image_6.png" /></figure>

Figure 4. Detrital (black) and authigenic εNd records (red) at site HH19-878-GC in Hornsund.

Error bars indicate ± 2σ uncertainties.
<figure><img src="image_7.png" /><figcaption>Figure 5. Shale-normalized (WRAS; Bayon et al., 2020) REE patterns for the detrital (diamonds) and authigenic (circles) phases of core HH19-878-GC. While general flat patterns are prevalent in detrital phases (average CI ~ 1), strong mid-REE enrichments are prominent in authigenic phases (average CI ~ 3.7). The degree of mid-REE enrichments in authigenic sediments varies depending on stratigraphic units. Note that the REE data were also normalized by Gd<sub>WRAS</sub> for clarity.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 6. Records of organic chemistry and bulk mineralogy at site HH19-878-GC in Hornsund. The contents of total organic carbon (TOC) and organic nitrogen ($N_{org}$), $\delta^{13}C_{org}$ values, TOC/$N_{org}$ ratios, the contents of calcite, dolomite, quartz and Fe-(oxyhydr)oxides are shown. Note that the horizontal axis is reversed for $\delta^{13}C_{org}$.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Figure 7. Spatial variability in A) the measured detrital εNd values and B) quartz-to-carbonate ratios (wt.%/wt.%) of surface sediments collected from Hornsund. Data marked with blue symbols serve as the representative values for each source, as described in Figure 8.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Figure 8. Potential sources for Hornsund sediments and their geochemical (detrital εNd) and mineralogical characteristics (quartz-to-carbonate ratios). The plot suggests that sediment deposition at site HH19-878-GC appears to have been influenced by both internal meltwater discharge (blue arrows) and external oceanic inflow (green arrows). Terrestrial inputs likely originate from the Isbjörnhamna regions (Group D), with additional contributions from areas around Burgerbukta and Samarinvågen (Group B).</figcaption></figure>

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<figure><img src="image_11.png" /><figcaption>Figure 9. Downcore variability in proxy records at site HH19-878-GC. The records include detrital εNd, quartz-to-carbonate ratio, Fe-(oxyhydr)oxide contents, δ¹³Corg values, TOC/Norg ratios, IRD flux (number of grains greater than 1 mm per cm²·kyr), clay contents, and concavity index. Note that the horizontal axis is reversed for detrital εNd and δ¹³Corg.</figcaption></figure>

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