Marine Pollution Bulletin 194 (2023) 115396 4 2.3. Sample preparation The frozen sediment samples were freeze-dried (Christ Gefrier- trocknungsanlagen, Osterode, Germany) and wet-sieved over a cascade of sieves (Atechnik, Leinburg, Germany). The <20 ?m fraction of the sediments was obtained by continuous flow centrifugation (Contifuge Stratos, Thermo Scientific, Waltham, USA) in conjunction with a tita- nium rotor (Continous Flow Rotor 3049, Thermo Scientific) after pass- ing the last sieve with a mesh size of 20 ?m. Dried fine-grain sediment aliquots of 50 mg were digested in tripli- cates with 5 mL HNO3, 2 mL HCl and 1 mL HBF4 for 300 min at 180 C either with a MARS Xpress or a MARS 6 microwave (CEM Corp., Kamp Lintfort, Germany) in 55 mL pre-cleaned TFM digestion vessels following the protocol described by Zimmermann et al. (2020). The marine sediment reference materials GBW 07311 and GBW 07313 were treated similarly and digested in duplicates per digestion batch (12 samples in triplicates). The reference material was quantitatively digested by the presented digestion method resulting in clear, particle free digests. For the analysis of Sr isotope ratios aliquots of the digested sediment were transferred to a pre-cleaned 50 mL DigiTUBE (SCP Science, Quebec, Canada), evaporated to dryness at 75 C and re-dissolved in 1 mL of 2 mol L 1 HNO3. Matrix separation was performed following an automated separation procedure using the fully automated sample preparation system prepFAST-MC® (Elemental Scientific, Omaha, USA). A self-packed column with 3 mL bed volume (ESI part. no. CF-3000) packed with DGA Resin (DN-B100-S, TrisKem International, Bruz, France) was utilized. The separation protocol is based on Retzmann et al. (2017) and Zimmermann et al. (2019b). All steps of the separation procedure can also be found in ESM Table A1. 2.4. Instrumental analysis 2.4.1. Grain size analysis The grain size distribution of each sediment sample was determined by laser diffraction (Analysette 22 NanoTec, Fritsch, Idar-Oberstein, Germany). 2.4.2. Multi-element analysis Determination of elemental mass fractions in the sediment digests was performed using an inductively coupled plasma tandem mass spectrometer (ICP-MS/MS) (Agilent 8800, Agilent Technologies, Tokyo, Japan) coupled to an ESI SC-4 DX FAST autosampler (Elemental Sci- entific, Omaha, Nebraska, USA) (Profrock and Prange, 2012). A list of measured isotopes and their detection modes can be found in ESM Table A2. A detailed description of all ICP-MS/MS operating parameters and used cell gas modes can be found in ESM Table A3. The instrument was tuned daily using a tune solution containing Li, Co Y, Ce and Tl at a concentration of 10 ?g L 1. Quantification was performed by external calibration covering a concentration range from 0 ?g L 1 to 10,000 ?g L 1 for Mg, Al, K, Ca, Ti, Fe, Mn, Ba and P and 0 ?g L 1 to 100 ?g L 1 for all other analytes. Solutions and blanks were prepared on a daily basis from custom made multi-element standards (Inorganic Ventures, Christiansburg, USA). Wash blanks were measured after each sample triplicate to monitor and avoid potential carry-over effects. 2.4.3. Isotopic analysis The analysis of Sr isotope ratios was performed according to the method described by Retzmann et al. (2017). A multi collector induc- tively coupled plasma mass spectrometer (MC ICP-MS) (Nu Plasma II, Nu Instruments Ltd., Wrexham, UK) was used for the measurement. The instrument was equipped with an APEX omega membrane desolvation system (Elemental Scientific, Omaha, Nebraska, USA) in combination with a PFA micro flow nebulizer (Elemental Scientific, Omaha, Nebraska, USA) as sample introduction system. All measurements were performed in static measurement mode with low mass resolution. Data collection was accomplished over a period of 600 s with an integration time of 10 s, resulting in 60 measurements per sample. Sr concentrations of samples and bracketing standards (NIST SRM 987) were matched within 10 % in terms of the obtained signal intensities prior to anal- ysis. Sr fractions and the corresponding isotopic standard were doped with Zr (Merck-Millipore) as internal standard for correction of instru- mental isotopic fractionation (IIF) (Horsky et al., 2016). 2.5. Data evaluation and presentation 2.5.1. Multi-element analysis Multi-element data were processed using MassHunter version 4.4 or higher (Agilent Technologies, Tokyo, Japan) and a custom written Excel© spreadsheet. The isobaric interference of 115Sn on 115In was corrected for by peak stripping as implemented in MassHunter using the signal of 118Sn and the isotopic abundances provided by IUPAC’s Commission on Isotopic Abundances and Atomic Weights (de Laeter et al., 2003). The Limits of detection (LOD) and Limits of quantification (LOQ) were calculated according to DIN 32645 (2008) and DIN ISO 11843-2 (2006). Combined uncertainties were estimated using a Kragten spreadsheet approach (Kragten, 1994) taking into account reproduc- ibility, repeatability and measurement precision for each sample. The significant number of digits of elemental mass fractions are given ac- cording to GUM and EURACHEM guidelines, whereby the uncertainty determines the significant number of digits to be presented with the value (EURACHEM/CITAC, 2012). 2.5.2. Isotopic analysis The IIF was corrected for by following an internal inter-elemental approach combining standard sample bracketing and external calibra- tion to account for time dependent and matrix dependent IIF variation between the samples as described elsewhere (Horsky et al., 2016; Retzmann et al., 2017). Isobaric interferences of residual 87Rb? on 87Sr? were subtracted via peak stripping. The detailed calculation approach for Sr isotope ratio analysis and data processing including all equations and constants as well as reference values used for interference and IIF correction are given elsewhere (Retzmann et al., 2017). Total combined uncertainty budgets were determined considering sample inhomogeneity, precision of the isotope ratio measurement for samples and standards and within-run repeatability of the measured isotope ratio in the bracketing standards as proxy for instrument sta- bility (Horsky et al., 2016; Reese et al., 2019). 2.5.3. Data assessment criteria In order to obtain comparable and standardized elemental mass fractions in sediments, different normalization approaches are commonly applied. Isolation of the fine grain fraction by (wet-) sieving (size fraction <20 ?m or <63 ?m) can be regarded as a physical normalization reducing differences in the granulometric composition. The wet-sieving was performed in a closed set-up using 1 L of MilliQ water. Leaching of analytes into the water used for sieving is negligible (between 5  10 5 % and 5  10 9 % for all analytes) (Nham, 2017). Coarser particles, which usually do not bind anthropogenic contami- nants and would therefore dilute their elemental mass fractions, are removed from the sample (Ackermann et al., 1983). To ensure compa- rability with the long-term reports of the BSH on the assessment of metal Table 1 Overview of sample stations (n) per campaign and area. Area N-2 Area N-3 Area N-4 Area N-6 LP20160725 (2016) – – 18 – AT261 (2018) 14 20 18 – AT275 (2019) 15 22 33 10 LP2020629 (2020) – 3 16 – AT004 (2021) 2 3 10 8 AT010 (2022) 3 3 11 1 A. Ebeling et al.