Marine Pollution Bulletin 194 (2023) 115396 7 3.1.4. Gallium (Ga) The Ga mass fractions (see Fig. 2D, the distribution map in ESM Fig. A10, a detailed distribution map for area N-4 in ESM Fig. A11 and a high-resolution boxplot in ESM Fig. A12) range between 8.6 mg kg 1  0.5 mg kg 1 and 27 mg kg 1  14 mg kg 1. The majority of the analyzed samples feature Ga mass fractions within the range of North Sea sedi- ments as published by Klein et al. (2022a) (14 mg kg 1  1 mg kg 1 to 23 mg kg 1  5 mg kg 1), which is close to the crustal abundance of 17.5 mg kg 1  0.7 mg kg 1 (Rudnick and Gao, 2003). Slightly higher mass fractions are found in area N-4 (2016) for two samples with mass fractions of 25 mg kg 1  1 mg kg 1 and 27 mg kg 1  14 mg kg 1. 3.1.5. Indium (In) The In mass fractions (see Fig. 2E the distribution map in ESM Fig. A13, and a high-resolution boxplot in ESM Fig. A14) range between 65 ?g kg 1  9 ?g kg 1 and 270 ?g kg 1  30 ?g kg 1. In mass fractions of all samples are significantly higher than the crustal abundance of 56 ?g kg 1  8 ?g kg 1 (Rudnick and Gao, 2003). However, median mass fractions of most areas are within the variability of North Sea sediments of 73 ?g kg 1  4 ?g kg 1 to 237 ?g kg 1  13 ?g kg 1 as published by Klein et al. (2022a), except for area N-3 (2018, 2019 and 2021) showing median mass fractions slightly below the reference range. Area N-2 shows increased mass fractions in 2021 compared to previous years, but mass fractions from 2022 are in the range of 2018 and 2019 again. Area N-2 and N-3 show lower variability than area N-4. All mass fractions fall below the PNEC of 5051 mg kg 1 (ECHA, 2020) by far. 3.2. Sr isotope ratios Isotope amount ratios of Sr were analyzed in the sediment samples taken in 2018 and 2019. The data is given in ESM Table A6 and distri- bution maps of the isotope ratio n(87Sr)/n(86Sr)2 can be found in ESM Fig. A15. For both years n(87Sr)/n(86Sr) ranged between 0.71247  0.00014 and 0.71767  0.00014. Fig. 3A shows n(87Sr)/n(86Sr) plotted against the inverse Sr mass fraction. The OWF area N-4 north of Heli- goland features significantly heavier isotope ratios and higher Sr mass fractions than the OWF areas N-2, N-3 and N-6 in the western area of the German Bight. The isotope-amount ratio n(87Sr)/n(86Sr) can be used as a measure for the age and/or type of rocks and sediments as well as their origin: The ratio n(87Sr)/n(86Sr) varies as the relative abundance of the radiogenic isotope 87Sr increases by the radioactive decay of 87Rb. Therefore, n(87Sr)/n(86Sr) depends on the geological age and initial amount of Rb in the source materials and therefore its origin. Moreover, Sr isotope-amount ratios may depend on the grain size of the analyzed sediment sample. Finer sediment fractions are more radiogenic than sandy sediments, due to an enrichment of Rb-rich minerals in the grain size fraction (Bayon et al., 2021). As can be seen in Fig. 3A n(87Sr)/n (86Sr) plotted against the inverse Sr mass fractions results in samples grouped along a linear regression, thus indicating the mixing of at least two different sediment types with different geological origin. Sediment samples with higher isotope amount ratios are in good accordance with literature data for sediments from the Elbe estuary (Reese et al., 2019.) and the lower Weser (Deng et al., 2021) (both measured in the <63 ?m size fraction), thus, suggesting a significant proportion of sediments originating from the North Sea’s tributaries. However, future analysis of further endmembers are required for an in-depth source tracing. Together with the share of the <20 ?m grain size fraction (Fig. 3B) the four areas feature different characteristics: Sediments from area N-2 have generally low amounts of the fine grain size fraction (<8 %) and low n(87Sr)/n(86Sr) values (0.71247  0.00014 to 0.71516  0.00017). Sediments of area N-3 feature a similar range of n(87Sr)/n(86Sr) values (0.71273  0.00015 to 0.7150  0.0004) with higher amounts of the <20 ?m grain size fraction (4 % to 15 %). Sediments of area N-4 show significantly heavier isotope ratios (0.71421  0.00015 to 0.71767  0.00014) together with a broad variation in the <20 ?m grain size fraction (0.6 % to 19 %). The highest amounts of the <20 ?m grain size fraction can be found in the sediments of area N-6 (7 % to 22 %) which are accompanied by a narrow range in n(87Sr)/n(86Sr) (0.71462  0.00015 to 0.71484  0.00015). 4. Discussion The following discussion of selected elements is based on work by Reese et al. (2020) who suggested Cd, Ga, In, Pb and Zn as the most promising tracers for potential emissions from galvanic anodes. In general, the dataset demonstrated a high variability likely due to the complex hydrographic dynamics and sediment movement in the North Sea as well as different further sources (e.g. riverine inputs) and natural variation for metals. The data is provided in ESM Table A5 together with the mass fractions of 50 other elements analyzed in the samples from the different sampling locations. 4.1. Temporal variation of elemental mass fractions The tracers Cd, Ga, In, Pb and Zn do not indicate a clear accumula- tion in the investigated areas, due to the short time series and high variability of the data. However, here we will discuss some features observed in the data. Median Cd mass fractions for areas N-2, N-3 and N-4 were the highest for samples taken 2021 and 2022 compared to previous years sampled within this study, suggesting either increased Cd inputs or remobiliza- tion of older, contaminated sediments. Yet, all Cd values measured in the present study are below the NOAA ERL, therefore, no significant effects on the marine environment caused by Cd is to be expected at the current stage. The majority of the sampled OWFs were built between 2013 and 2017. Each area consists of 150 to 200 offshore wind turbines. There- fore, considering five years and 150 offshore wind turbines, a release of 14 g to 420 g of Cd by galvanic anodes can be estimated (Reese et al., 2020). Consequently, the potential increase of Cd mass fractions is un- likely solely caused by galvanic anodes of OWFs. The increase might be related to sources like remobilization of older sediments (Carpenter et al., 2016; Forster, 2018), inputs by river discharges, e.g. the Elbe, Rhine and Weser rivers (Deng et al., 2021; Klein et al., 2022b; Reese et al., 2019) or dissolution of Zn galvanic anodes used for corrosion protection of ship hulls (OSPAR, 2009). These anodes contain 100-times more Cd than Al-based anodes, with Cd being the third most abundant metal after Zn and Al (Reese et al., 2020). Similar to Cd, sediment samples from area N-2 (2018, 2019), area N- 3 (2019) and area N-4 (2016–2020) feature Pb mass fractions above the reported variability of North Sea sediments. Indeed, median mass frac- tions tend to be lower and within the expected range of North Sea sed- iments for samples taken in recent years (2021 and 2022) for all areas, thus, rather indicating decreasing Pb mass fractions. Nevertheless, mass fractions of Pb in the analyzed samples exceed the NOAA ERL, like in most parts of the German Bight (OSPAR, 2010; von der Au et al., 2022). For Al-based anodes a release of 390 g to 580 g of Pb can be estimated for an OWF area of 150 monopiles within five years (Reese et al., 2020). As Pb is a particle active element, inputs of dissolved Pb will quickly adsorb onto suspended particulate matter and thus accumulate in the sediment or might be dispersed or transported also over larger distances or even will find its way into the marine food chain via filter feeding organisms (Kremling et al., 1999). Pb is introduced into the marine environment by a wide variety of other mostly anthropogenic sources, such as industrial applications (Boyle et al., 2014) or combustion processes of fossil fuels (Komarek et al., 2008; Larsen et al., 2012). Also a remobilization from 2 IUPAC-recommended notation for SI-traceable isotope-amount ratios r ? n (iE)/n(jE), with the amount of substance n, and the isotopes iE and jE of the element E. The notation iE/jE (e.g. 87Sr/86Sr) is commonly used interchange- ably, even though lacking information on the type of quantity (microscopic vs. macroscopic) (see also Coplen, 2011). A. Ebeling et al.