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Lead in sediments and suspended particulate matter of the German Bight: natural versus anthropogenic origin
Applied Geochemistry 17 (2002) 621–632 www.elsevier.com/locate/apgeochem
Lead in sediments and suspended particulate matter of the German Bight: natural versus anthropogenic origin J. Hinrichs, O. Dellwig, H.-J. Brumsack* Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, PO Box 2503, D-26111 Oldenburg, F.R.Germany Received 15 January 2001; accepted 1 September 2001 Editorial handling by M. Novak
Abstract Sediments (568) and suspended particulate matter (SPM, 302 samples) of the southern German Bight and the adjacent tidal flat areas were analysed for selected major elements (Al, Fe, K), trace metals (Mn, Pb), and 206Pb/207Pb ratios using XRF, ICP–OES, ICP–MS. For selected samples a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e.g. Mn/Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. Enrichment factors versus average shale (EFS) reveal elevated Pb contents for all investigated sediments and SPM in the following order: Holocene tidal flat sediments (HTF, human-unaffected) 5 mg l1) < offhore SPM (< 5 mg l1). Besides pollution, RTF contain elevated amounts of natural Pb-rich materials (K-feldspars and heavy minerals) due to a man-made high-energy environment (dike building) in comparison to HTF. 206Pb/207Pb ratios of RTF (1.192 0.019) are similar to the local geogenic background, determined from HTF (1.207 0.008). In contrast, Pb isotope ratios of nearshore SPM (1.172 0.007) and offshore SPM (1.166 0.012) show a distinct shift towards the anthropogenic/atmospheric signal of 1.11–1.14. This difference between RTF and SPM supports the assumption of low deposition rates of fine material in the intertidal systems. As the 206Pb/207Pb ratios of SPM do not reach the pure anthropogenic signal, the adsorbed Pb fraction was examined (leaching). However, the leachates also contained large amounts of geogenic Pb (SPM 40%, recent sediments 60%). The authors assume that the uptake of natural Pb occurs in nearshore waters, presumably in the turbid intertidal systems. Possible sources for dissolved Pb are mobilisation during weathering (geogenic signal) and dissolution of oxihydroxide coatings with subsequent release from porewaters, and unspecific riverine input. Comparatively small parts of SPM leave the coastal water mass and reach the open North Sea. This process therefore leads to a decontamination of the tidal flat sediments. Due to more pronounced atmospheric input, the offshore SPM becomes enriched in anthropogenic Pb as indicated by decreasing 206 Pb/207Pb ratios with increasing distance from the coast. # 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction The heavy metal Pb has frequently been used as a sensitive pollution indicator (e.g. Balls, 1985; Ve´ron et al., 1992; Tappin et al., 1995) as this volatile element is
* Corresponding author. Tel.: +49-441-798-3584; fax: +49441-798-3404. E-mail address: [email protected] (H.-J. Brumsack).
released during high-temperature processes and combustion of leaded fuel. Being enriched in industrialised countries, Pb is distributed by atmospheric and riverine input (e.g. Ottley and Harrison, 1991; Turner et al. 1991; Grousset et al., 1999). In numerous studies Pb has been determined in sediments, suspended particulate matter (SPM) and waters of the southern North Sea and adjacent tidal flat areas and rivers (e.g. Duinker and Nolting, 1977; Schwedhelm and Irion, 1985; Kersten et al., 1988; Haarich et al., 1993; Koopmann et al., 1993; Puls
0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00124-X
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et al., 1997a). But total Pb concentrations alone may be insufficient for separating pollution from the natural background as the latter is often highly variable due to natural processes (Dellwig, 1999). However, ICP–MS analyses allow the determination of Pb isotope ratios that may distinguish between natural and anthropogenic Pb (e.g. Kersten et al. 1992; Ritson et al., 1994; Monna et al. 1997; Ve´ron and Church, 1997; Monna et al. 2000). The major goal of this study is to combine both parameters, i.e., concentrations and isotope ratios, to differentiate between natural and anthropogenic Pb in sediments and SPM of the German Bight and coastal areas. As Pb is a very particle reactive element (Balls, 1988; Krause et al., 1993) in particular, the anthropogenic Pb is thought to be adsorbed on particle surfaces. Therefore, leaching experiments are carried out to assess the contribution of adsorbed Pb. Besides anthropogenic Pb, it is supposed also that an adsorbed geogenic fraction may be involved that is released, e.g. during weathering.
2. Geographical setting Fig. 1 shows the sampling locations of sediments and suspended particulate matter (SPM) in the southern German Bight and the adjacent tidal flat areas (East Frisian Islands, Jade Bay). Holocene tidal flat sediments (HTF) were recovered from 6 drill cores (locations Wangerland and Schweiburg), whereas recent tidal flat sediments (RTF) are represented by surface sediments (0–2 cm) from the Spiekeroog Island back-barrier intertidal system and drill core surface samples from Jade Bay (Arngast). For assessing the anthropogenic influence, we use the Holocene sediments deposited between 5.5 and 2.2 ka BP (Mid-Holocene) as a reference for background values unaffected by human activity (e.g. dike building, pollution). The age limits are based on calibrated 14C determinations of peat layers below and above the tidal flat sediments (Dellwig, 1999; Dellwig et al., 2000).
Fig. 1. Map of the study area showing sampling sites of sediment cores (asterisks), suspended particulate matter (SPM, filled circles), and SPM time series stations (open circles). The bold line indicates the separation between nearshore (SPM<5 mg l1) and offshore samples (SPM >5 mg 1). Thin lines denote salinity. The light grey arrow represents the general flow pattern in the German Bight (Hainbucher et al., 1987; Puls et al., 1997a, b).
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Additionally the authors investigated 3 short cores (22–26 cm length) from the Helgoland Island mud hole area (MH) covering at maximum the last 50 a (sedimentation rates 5–18 mm a1; Baumann, 1991; Dominik et al., 1978; Fo¨rstner and Reineck, 1974). SPM samples were collected from surface water during several cruises (FS Victor-Hensen 97-08; FS Heincke 98-09, 99-08; FK Senckenberg 98-07, 99-03, 99-09, 00-01, 00-04). While most data points shown in Fig. 1 refer to single samples (filled circles), time series samples were collected close to the Spiekeroog back-barrier intertidal system and near Helgoland Island (open circles). SPM is divided into nearshore (SPM concentrations >5 mg l1) and offshore SPM (<5 mg l1) to account for its different geochemical composition. The bold line in Fig. 1 marks the transition of both sampling areas.
3. Material and methods A total number of 568 sediment samples (HTF 377, RTF 119, MH 72) were analysed for selected major (Al, Fe, K) and trace elements (Mn, Pb) by XRF (Philips PW 2400, equipped with a Rh-tube) using fused lithium tetraborate glass beads. Sampling resolution was 5–10 cm on Holocene and Arngast drill cores, 1 cm on short cores from the Helgoland Island mud hole area (MH), and 0.5 cm on recent surface sediments from the Spiekeroog backbarrier intertidal area. The samples were stored in PEbags, sealed, and immediately frozen. At the laboratory the samples were freeze-dried and homogenised in an agate mortar. The ground powder was used for all subsequent geochemical analyses. During the cruises listed above (see Section 2), 302 SPM samples were collected. Onboard ship, 0.2–4 l of seawater were filtered through pre-weighed Millipore1 filters (0.45 mm, for multi-element analyses) and Whatman1 glass fibre filters (0.7 mm, for TC and TIC analyses). The filters were rinsed with 18 M water, dried at 60 C and re-weighed for the determination of total suspended material retained on the filters. For elemental analysis, the Millipore1 filters were digested completely in closed PTFE autoclaves (Heinrichs et al., 1986) at 180 C in a mixture of HNO3, HClO4 (purified by subboiling distillation) and HF (suprapure). Al, Fe, K, and Mn were analysed by ICP–OES (Perkin Elmer Optima 3000XL), while Pb was determined by ICP–MS (Finnigan MAT Element). 206Pb/207Pb isotopic ratios were determined on acid digestions of 57 sediment samples and 289 SPM samples by ICP–MS (Finnigan MAT Element). Total digestions were used without further purification, just dilution to Pb concentrations of about 5 mg l1. No mass bias correction was necessary since the instrument was tuned to give best accuracy for the NIST-Standard SRM 981. Total aquisition time was 3 min, consisting of 60 scans.
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For selected samples (sediments 19; SPM nearshore 3, offshore 1) a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e.g. Mn/Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. International (JF-1; K-feldspar) and in-house (TW-TUC; shale) standards were used to check the extent of dissolution of the mineral matrix by the acid attack. According to the methods described by, e.g. Monna et al. (2000) and Huerta-Diaz and Morse (1990), 250 mg sample were treated with 2 ml 1 M HCl, shaken for 16 h at ambient temperature, centrifuged, decanted, and washed 3 times with 1 ml 18 M H2O. The solutions were also analysed for the aforementioned elements and 206Pb/207Pb isotopic ratios. Precision and accuracy were checked by parallel analysis of international reference materials (GSD-3, GSD5, GSD-6, NIST-Standard SRM 981), as well as inhouse standards (see Appendix A). For all methods, filter and procedural blanks were determined and used for correcting the sample values. Traces of Pb were detected in the Millipore filters, while shipboard contamination was insignificant. The average Pb concentration of the blank filters is 1.2 0.9 mg kg1.
4. Results and discussion 4.1. Elemental composition of sediments and SPM The analytical results of investigation, i.e. Al2O3, Fe2O3, K2O, Mn, Pb concentrations, Al-normalised values, and 206Pb/207Pb ratios of Holocene tidal flat sediments (HTF), recent tidal flat sediments (RTF), Helgoland Island mud hole sediments (MH) and suspended particulate matter (SPM) are summarised in Table 1. An overview on the geochemistry of the study area is given by Dellwig et al., 2000. Fig. 2 shows the enrichment factors versus average shale (AS; Wedepohl, 1971) of Pb, K, Fe, and Mn for sediments and SPM calculated from: EFS ¼ ðelement=A1Þsample =ðelement=A1ÞAS All sediment and SPM samples investigated are enriched in Pb to varying degrees with lowest EFS values for HTF and highest values for offshore SPM. In a first approximation the higher EFS of RTF compared to HTF is due to anthropogenic Pb input. Comparable data are scarce in the literature as most authors limit their studies to different size fractions and digestion methods (e.g. Fo¨rstner and Reineck, 1974; Schwedhelm and Irion, 1985; Koopmann et al., 1993; Irion, 1994). For instance, Schwedhelm and Irion (1985) reported elevated Pb contents of on average 62 mg kg1 in the < 2 mm fraction of sediments from the East Frisian intertidal system. They calculated a mean enrichment factor
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Table 1 Analytical results (mean and standard deviations SD): A12O3, Fe2O3, K2O, Mn, Pb concentrations, Al-normalised values, and 206 Pb/207Pb ratios of Holocene tidal flat sediments, recent tidal flat sediments, Helgoland Island mud hole sediments and suspended particulate matter (SPM)a Holocene tidal flat sediment na Al2O3 Fe2O3 K2O Mn Pb Fe/Al K/Al Mn/Al104 Pb/Al104 206 Pb/207Pb a
377 (22) 8.10 3.52 1.96 433 14 0.56 0.39 101 3.4 1.207
SD
1.84 1.21 0.28 187 4 0.10 0.05 34 0.7 0.008
Recent tidal flat sediment 119 (23) 3.56 0.82 1.29 124 13 0.29 0.56 64 6.7 1.192
SD
0.77 0.38 0.17 59 4 0.07 0.05 26 1.5 0.019
Mud hole sediment 72 (12) 6.86 2.61 1.76 312 24 0.50 0.41 87 6.9 1.184
SD
1.34 0.62 0.22 70 6 0.04 0.03 17 2.4 0.009
Nearshore SPM 242 (229) 9.98 5.01 1.79 1334 67 0.68 0.29 263 13 1.1720
SD
1.63 0.70 0.25 529 24 0.15 0.06 125 6 0.007
Offshore SPM
SD
60 (60) 7.89 2.53 3.69 1.16 1.40 0.48 1327 562 131 154 0.63 0.15 0.29 0.07 346 183 40 52 1.166 0.012
n is the total sample number, the values in parentheses indicate the number of digestions made.
Fig. 2. Enrichment factors versus average shale (EFS) for Pb, K, Fe, Mn analysed in sediments (Holocene tidal flat, HTF; recent tidal flat, RTF; Helgoland Island mud hole, MH) and SPM. Average shale (AS) data after Wedepohl (1971).
versus the natural background (15 mg kg1) of about 4 (range 2–9). This is comparable to a value of 6.4 in the same size fraction of the present study area (HTF <2 mm: 15 mg kg1, O. Dellwig, unpublished data; RTF <2 mm: 96 mg kg1, Hild, 1997). Schwedhelm and Irion (1985) conclude that the Pb enrichment in tidal flat sediments is exclusively caused by atmospheric pollution. However, regarding the EFS values (Fig. 2) the human unaffected HTF are also subject to a minor Pb enrichment that has to be attributed to a natural Pb-rich compound. This finding necessitates careful interpretation of geochemical data with respect to the assignment of natural and anthropogenic heavy metal contributions. Thus, background values are considerably variable in intertidal systems. For example, background values are low in sheltered back-barrier intertidal systems (HTF < 2 mm: 15 mg kg1 Pb) in comparison to
more estuarine influenced intertidal areas (HTF < 2 mm: 28 mg kg1 Pb; O. Dellwig, unpublished data). Tidal flat sediments consist mainly of quartz, organic material, carbonates, clay minerals and feldspars (van Straaten, 1954). The latter two are the most important natural contributors of Pb because Pb can replace K in the mineral lattice due to similar ionic radii. Clays contain on average 22 mg kg1 Pb (average shale; Wedepohl, 1971) which is the sum of Pb incorporated into the mineral lattice as well as Pb adsorbed on mineral surfaces (see below). K-feldspars usually have higher Pb contents. For instance, K-feldspars from granites and pegmatites from Southern Norway, UK, and Germany average 82 mg kg1 Pb (range 25–177 mg kg1; Wedepohl, 1978). Therefore, the slight enrichment of Pb in HTF may be explained by a higher contribution of Kfeldspars as is indicated by elevated EFS values for K. The same is true for RTF that show distinctly higher EFS for K indicating a higher contribution of K-feldspars in comparison to HTF. This difference is likely caused by an increase in depositional energy in the intertidal systems from the Holocene to the recent situation leading to a pronounced lack in fine grained material (clay) and enrichments in heavy minerals in RTF (Dellwig et al., 2000). According to Flemming and Nyandwi (1994) and Flemming and Davies (1994) the energy increase results from modern dike building leading to steeper than normal energy gradients in the intertidal zones. Therefore, the silt fraction, which is enriched in feldspars (LittleGadow, 1978), contributes to a larger extent to the bulk sediment composition when compared with the Holocene situation. An additional effect of the increased energy conditions in RTF are elevated levels of heavy minerals (Dellwig et al., 2000). This sediment fraction, which accounts for 4% of the total sediment, is
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also enriched in Pb. Concentrations average 140 mg kg1 Pb in heavy mineral separates of the Spiekeroog back-barrier intertidal system (Hild, 1997). Nevertheless, the Pb/K-ratio of RTF (12.2104) is higher than that of HTF (8.6104) pointing to a further Pb source. For this reason, the authors assume that the enrichment of Pb in RTF is the result of two anthropogenic effects: (i) changing energy conditions leading to a larger fraction of Pb-rich feldspars and heavy minerals and (ii) pollution. In MH samples the Pb input from pollution appears to be more pronounced as the high EFS values for Pb are only accompanied by K enrichments on the same magnitude as HTF (Pb/K=8.6104). In SPM, highest EFS for Pb were observed, probably caused exclusively by pollution. The slight depletion in K may be the result of partial K replacement by seawater Na. Iron and Mn both show similar behaviour for the investigated materials. The most pronounced feature of these two elements is the depletion in RTF (and MH) in comparison to the shale-like HTF. Iron and Mn are mobilised under reducing conditions (Dehairs et al., 1989; Tappin et al., 1995; Hinrichs et al., 2000) in the adjacent tidal flats and North Sea bottom sediments. Subsequently divalent Fe and Mn ions become oxidised in the water column and form oxihydroxide coatings on SPM, which explains the observed Fe and Mn enrichments in SPM. Additionally, the precipitation process may largely control adsorption (Schoemann et al., 1998) of other metals, e.g. Pb, on particles. Especially in the studied coastal high turbitidy area, SPM is expected to play an essential role in Pb transport. As Pb is known to be highly particle-reactive (Bru¨gmann et al., 1985; Balls, 1988), it may be scavenged from solution onto particles during formation of oxihydroxide coatings. The following scenario can explain the different behaviour of Fe and Mn in HTF and RTF. At present the higher depositional energy in intertidal systems causes a stronger separation of coarse grained sediments and fine material suspended in the water column in comparison to the Holocene (Dellwig et al., 2000). The authors assume that the prevailing sands promote a rapid exchange of porewaters with seawater. This likely leads to a flux of dissolved Mn and Fe out of the reducing tidal flat sediments (Hinrichs et al., 2000). After oxidation of the reduced Fe and Mn species they attach to SPM, possibly co-precipitating Pb. While at present SPM is mainly exported to the open North Sea, this process was presumably less pronounced in the Holocene, and more of the Mn and Fe-bearing particles were re-deposited locally in the intertidal flats. Additionally a stronger fixation of Fe and Mn during early diagenesis as pyrite and rhodochrosite in microenvironments can be assumed for HTF. This assumption is confirmed by different S and carbonate (IC) contents in HTF and RTF (Hild, 1997; Dellwig, 1999). While S and IC contents in
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HTF are high (S 0.7%, IC 1.2%), RTF are comparatively poor in both parameters (S 0.2%, IC 0.5%). 4.2. 206Pb/207Pb ratios and Pb concentrations of sediments and SPM The results discussed so far require a distinction between natural and anthropogenic Pb. 206Pb/207Pb ratios are a useful tool for identifying Pb pollution, therefore these ratios were determined in sediment and SPM samples. Two end members have to be considered: (i) the local geogenic background established from the pre-industrial HTF (206Pb/207Pb 1.207 0.008) and (ii) the atmospheric signal of the study area. The latter was determined from natural concentrators (Rachold et al., 1992) of local dust (mosses and spider webs) with lower 206 Pb/207Pb ratios of 1.142 0.004 (H.-J. Sach, personal communication). The local geogenic value is in accordance with continental European sources with a 206Pb/207Pb ratio of approximately 1.18–1.20 (Elbaz-Poulichet et al., 1986). The local atmospheric end member is in the upper range of values for aerosols from the North Sea (206Pb/207Pb 1.11–1.14) influenced by the highly industrialised UK and continental Europe (Kersten et al., 1992). The anthropogenic signal of the study area is influenced by inputs of volatile Pb released during high temperature industrial processes and combustion of leaded fuel (Erel et al., 1997; Hansmann and Ko¨ppel, 2000). Most Pb ores are at present imported from Australian and Canadian Precambrian sources with low Pb isotopic ratios (Kersten et al., 1992). The anthropogenic Pb is distributed through the atmosphere and subsequently enters the terrestrial and marine environment by wet and dry deposition (Ottley and Harrison, 1991; Rojas et al., 1993). Although the combustion of leaded fuel has decreased recently, the atmospheric Pb isotope signal is still significantly lower than the European geogenic signal (Do¨ring et al., 1997). A further contributor of Pb are particles from North Atlantic surface water which show 206 Pb/207Pb ratios of 1.18 (Ve´ron et al., 1994). However, according to Eisma and Irion (1988) Atlantic water is considered to be separated from the sampling area by the eastward flow of coastal water masses as can be seen from the salinity distribution in Fig. 1. Fig. 3 provides an overview of the 206Pb/207Pb ratios determined in sediments and SPM from the study area. In general, the SPM shows lower Pb isotope ratios than the sediments due to atmospheric input of anthropogenic Pb and subsequent scavenging by particles. The similarity of RTF values (1.192 0.019) with the local geogenic background (HTF) confirms the assumption that anthropogenic Pb is of minor importance in RTF. By contrast, nearshore SPM (1.172 0.007) shows a more pronounced anthropogenic contribution. The low variation in nearshore samples reflects a comparable Pb-
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Fig. 3. 206Pb/207Pb ratios of sediments (HTF, RTF, MH; grey boxes), SPM (white boxes), and literature data for atmospheric input (bold-lined box, values from Kersten et al., 1992) in the study area. For symbols see Fig. 1. Data points connected by grey lines belong to transects sampled on the cruises S00-04 (1), S99-03 (2), VH97-08 (3=Weser, 4=Elbe).
isotopic composition of the entire coastal area sampled. Thus, Pb isotope ratios of SPM samples from the backbarrier tidal flat, Jade Bay, estuarine and coastal sites (SPM > 5 mg l1) show no significant differences. The discrepancy between RTF and SPM evidences the low SPM deposition in the intertidal flats due to the present increase in depositional energy (e.g. Flemming and Nyandwi, 1994; Dellwig et al., 2000). Therefore, it is assumed that large parts of the fine material remain suspended in the water column and may be transported off the coastal regions. The offshore SPM samples show slightly lower isotope ratios. A candidate deposition site is the nearby Helgoland Island mud hole area (MH) where the sedimentation rate has increased since the onset of dike-building and land reclamation in the 11th century. Most estimates of the recent sedimentation rate range from 5 to 18 mm a1 (e.g. Fo¨rstner and Reineck, 1974; Dominik et al., 1978; Baumann, 1991), while von Haugwitz et al. (1988) propose a significantly lower average sedimentation rate of 3.2 mm a1 between 8 and 1.5 ka BP. On one hand, a
shift is observed towards lower Pb isotope ratios for MH sediments (1.184 0.009) when compared with the other sediments under study. On the other hand, Pb isotope ratios are significantly higher than in offshore SPM. Although some deposition of polluted particles is seen in MH samples, it is presumed that a significant fraction of the SPM is transported further offshore. Therefore, the main area of deposition for the fine-fraction seems to be the western Skagerrak/Norwegian Channel, where presently about 50–70% of the total mass accumulation of the North Sea occurs (Eisma and Kalf, 1987). Fig. 4 presents the relationship between 206Pb/207Pb ratios and Pb contents expressed as Al-normalised values. In general a trend of increasing Pb/Al ratios is accompanied by a shift towards lower isotopic ratios due to higher contents of anthropogenic (excess) Pb. HTF samples show a small-scale variability in isotope ratios reflecting the natural range, while RTF samples show a much larger scatter. Some RTF samples are enriched in Pb that is not derived from pollution as indicated by their geogenic Pb isotope ratios. This possibly reflects
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Pbxs;sample ¼ Pbsample;total Pbsample;background
Fig. 4. Scatter plot of 206Pb/207Pb ratios versus Pb/Al ratios of sediments and SPM. Note that the x-axis is logarithmic. The arrows indicate Pb/Al ratios of average shale (Wedepohl, 1971) and the local geogenic background estimated from HTF.
the presence of elevated amounts of K-feldspars and heavy minerals in comparison to HTF. According to Friese (1990), analyses of pure K-feldspars yielded Pb isotope ratios similar to the HTF (mean 1.203, range 1.191–1.225). Nevertheless, some RTF samples contain significant amounts of fine material rich in excess Pb leading to lower Pb isotope ratios. The MH samples show a linear correlation between both parameters (r=0.95; regression line not shown in Fig. 4 due to the use of a log scale) which indicates that these sediments might result from a mixture of Holocene sediment material and nearshore SPM. Offshore SPM seems to contribute to a smaller extent as this material contains distinctly higher amounts of Pb. Although the input of leaded fuel has decreased in the last decade, the MH show a trend towards lower isotope ratios and increasing Pb contents in the uppermost few cm of the cores which were deposited in the last years. This finding points at a still existing large reservoir for anthropogenic Pb in the environment as a contaminant of SPM. Thus, all SPM samples are enriched in Pb with the highest variability seen for offshore SPM. This variability strongly depends on the sample location, however, a general trend of decreasing Pb isotope ratios and increasing Pb contents with increasing distance from the coast is observed. Fig. 5a and b show the results of 4 transects (Fig. 3) from the coast to the open North Sea for SPM contents, salinity, 206Pb/207Pb ratios, Pb/Al ratios, and particulate Pb in excess of the natural background (Pbxs). The latter parameter was calculated from: Pbsample;background ¼ A1sample ðPb=AlÞHTF
The excess value (dry weight) was then converted into volume specific molar units by multiplication with the total SPM values and a conversion factor to account for the correct units. With increasing distance from the coast, salinities increase while SPM concentrations decrease. One exception is transect 4 that is influenced by material derived from the small islands Neuwerk and Scharho¨rn (compare Fig. 1). The profiles shown in Fig. 5b confirm the aforementioned relationship between the 206 Pb/207Pb and Pb/Al ratios of SPM with respect to the distance from the coast. Thus, with increasing distance from the coast the particles are more enriched in Pb from anthropogenic sources as seen from the decreasing Pb isotope ratios. The observed trends are in accordance with results of Krause et al. (1993) who found seawards decreasing 206Pb/207Pb ratios for a transect from the river Elbe towards the German Bight. However, considering the Pb concentration per water volume as expressed by Pbxs a reverse trend is seen when compared with Pb/Al ratios. On one hand, the offshore particles contain higher amounts of Pb (high Pb/Al ratios), on the other hand the Pb inventory per water volume decreases due to lower SPM concentrations. The coastal waters contain the largest amounts of Pb per water volume but the Pb/Al ratios are distinctly lower than for offshore waters. This means that a single nearshore particle is less contaminated with Pb than offshore, but because of high SPM concentrations the total Pb inventory is highest nearshore. The observed spatial distribution is likely the result of several effects: (i) Pb uptake in nearshore waters, presumably in the most turbid intertidal systems, from the dissolved phase. Possible sources are Pb mobilised during weathering (geogenic signal) and dissolution of oxihydroxide coatings with subsequent release from porewaters, and unspecific riverine input indicated by low salinities (Fig. 5a); (ii) settling of coarser particles (more pronounced geogenic signal) during transport towards the open North Sea; and (iii) the uptake of atmospheric/anthropogenic Pb by clay minerals and phytoplankton. The biogenic uptake is more pronounced in the remote areas because of higher abundance of plankton as indicated by offshore POC enrichments in comparison to nearshore SPM (Krause et al., 1993; Dellwig et al., 2000). 4.3. Dominance of anthropogenic Pb? Leaching experiments (1 N HCl) provide information on Pb fixation in sediments and SPM. The results for SPM shown in the following figures represent the mean of 3 nearshore samples. As only one offshore sample was available for the leaching procedure, this was excluded. Lead is predominantly bound in alumo-silicate structures
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Fig. 5. Transects (tracks shown in Fig. 3) displaying (a) SPM concentration and salinity; (b) 206Pb/207Pb ratios, Pb/Al ratios, and Pbxs concentrations versus distance from nearshore station. For Pbxs calculation, see text.
and adsorbed on particle surfaces, while carbonates and organic matter contain only low amounts of Pb (Monna et al., 2000). It would be expected that the adsorbed component would be distinguished from the mineralbound Pb by the leaching procedure applied. As a reference for clay rich material an in-house shale standard (26 mg kg1 Pb) was used and for K-feldspars the JF-1 standard (33 mg kg1 Pb). In Fig. 6 the percentage of Al, Pb, Fe, and Mn observed in the leachate compared to the bulk concentration are presented for HTF, RTF, MH, nearshore SPM, shale and K-feldspar. The acid attack does not affect the alumo-silicate matrix as can be seen from the low Al content in the leachate (mean <5%) which confirms that the procedure only liberates the adsorbed metals. For Pb increasing percentages are observed in the order HTF
and resembles the shale standard (87%) indicating that in both materials Pb is almost exclusively adsorbed on particle surfaces. By contrast, in the K-feldspar Pb is mostly fixed in the mineral lattice as can be inferred from the low percentage of Pb in the leachate (4%). In general Fe and Mn are similarly distributed to Pb indicating that Fe and Mn oxihydroxides play a significant role in transferring Pb to particle surfaces by co-precipitation (scavenging). In Fig. 7a, the results of 206Pb/207Pb analyses in the leachate for HTF, RTF, MH, and SPM are shown. As the leachates represent the adsorbed Pb, we assume that this fraction is the main carrier for anthropogenic Pb. While the isotopic ratio of HTF leachate (1.207) is identical to its bulk value due to a lack of anthropogenic influences, RTF and MH show a significant shift towards lower ratios (both 1.174) when compared with the bulk sediment samples (RTF: 1.192; MH: 1.184). Lowest 206Pb/207Pb ratios were observed for nearshore SPM leachates (1.159), indicating elevated amounts of atmospheric/anthropogenic Pb. This contribution is slightly higher in the offshore sample (not shown) with a value of 1.142. While the offshore sample almost reaches the pure anthropogenic 206Pb/207Pb signal of
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Fig. 6. Percentage of Al, Pb, Fe, Mn in leachates (1 N HCI) of sediments, nearshore SPM, shale (in-house standard TW– TUC) and K-feldspar (international standard JF-1).
1.11–1.14 (Kersten et al., 1992), the ratios of the nearshore leachates are distinctly higher. Although one sample is not representive, the difference in nearshore and offshore SPM seems to be significant. Therefore, the adsorbed Pb-fraction contains both, an anthropogenic as well as a significant geogenic Pb component derived from weathering of detrital material. These two components can be estimated (Fig. 7b) from the Pb isotope ratios of the leachates and the two 206Pb/207Pb end members (geogenic 1.207, anthropogenic 1.11–1.14, compare range in Fig. 7b). In the modern sediments (RTF and MH), the anthropogenic component of adsorbed Pb amounts to approximately 40%. In contrast, SPM shows a distinctly higher percentage of this fraction (58%) because of a stronger influence of atmospheric input. This means that a sig-
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nificant fraction of adsorbed Pb is derived from natural geogenic sources even in SPM. It should be noted that the percentage of leachable Pb differs largely in sediments and SPM (RTF, MH 50–70%; SPM 90%). To represent the amounts of geogenic and anthropogenic Pb in the bulk materials, the analytical results of bulk Pb, Pb isotopes and leaching experiments for sediments and SPM are summarised in Fig. 8. The heights of the bars represent total Pb, though differences to average Pb values given in Table 1 result from the lower sample numbers chosen for the leaching experiments. The percentage leached (Fig. 6) is used to infer mineral bound and adsorbed Pb concentrations. The latter fraction has been subdivided into geogenic and anthropogenic Pb based on the Pb isotope ratios of the leachates (Fig. 7a) and the end member isotopic compositions (Fig. 7b). As absolute Pb concentrations are presented, one has to consider dilution effects caused by, e.g. quartz and organic matter. The mineral bound Pb varies only slightly in the sediments. The quartz-rich RTF sediments show a similar concentration compared to HTF. Thus, the dilution effect of quartz in RTF is likely compensated by the occurrence of Pb-rich K-feldspars and heavy minerals. In contrast, the mineral bound Pb of MH likely results from an enhanced clay content. SPM nearly reaches the shale level as the dilution with quartz is negligible and dilution with organic matter is also relatively low POC=4.5–6.6%; Dellwig et al., 2000). Highest concentrations of adsorbed geogenic Pb are observed in nearshore SPM (Fig. 8), although the percentage of adsorbed geogenic Pb is lowest in SPM (Fig. 7b). A similar value is observed for the offshore sample (64 mg kg1). This fraction is distinctly lower in
Fig. 7. (a) 206Pb/207Pb ratios in leachates (1N HC1) of sediments and SPM. (b) Percentage of geogemc and anthropogenic Pb in leachates calculated from the anthropogenic (mean 206Pb/207Pb ratio=1.125) and geogenic (206Pb/207Pb=1.21) end members. The error bars indicate the range of the anthropogenic contribution (1.11–1.14) reported by Kersten et al. (1992).
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fine material, leads to its decontamination by export of Pb-rich SPM.
5. Conclusions
Fig. 8. Absolute concentrations of mineral bound Pb as well as adsorbed geogenic and anthropogenic Pb in sediments and SPM. For calculation of fractions see text.
the sediments. While HTF and RTF show similar concentrations (4–5 mg kg1), the adsorbed geogenic Pb is about 3-fold higher in MH. The high geogenic Pb inventory of SPM is due to enhanced uptake of Pb released during weathering of host rocks and sediments in the nearshore low salinity coastal water mass (compare Section 4.2). On a short time scale, most of the SPM may be deposited and resuspended in the intertidal area, thus approximating an equilibrated system. But on a longer time scale, a small fraction of SPM rich in geogenic Pb leaves the nearshore mixing zone and reaches the open North Sea. Once the geogenic Pb is adsorbed in the nearshore area, it possibly remains on particles as indicated by the similar nearshore and offshore concentrations of adsorbed geogenic Pb. For this reason, the elevated amount of adsorbed geogenic Pb in MH is presumably caused by some deposition of SPM. Adsorbed anthropogenic Pb shows similar behaviour, with the exception that no anthropogenic signal can be seen in HTF. In the modern sediments, natural Pb prevails over anthropogenic Pb. Due to atmospheric input a dominance of anthropogenic Pb occurs in SPM. The atmospheric influence is more pronounced offshore (adsorbed anthropogenic Pb 239 mg kg1) because of a long residence time of SPM in the German Bight (5 month in summer; Puls et al., 1997b). Therefore, SPM collects the atmospheric Pb in addition to the natural enrichment of Pb. Irrespective of the Pb source (anthropogenic and/or natural), the man-made high energy environment in the intertidal systems, which diminishes the deposition of
All investigated sediments (Holocene and recent tidal flat, Helgoland Island mud hole area) and suspended particulate matter samples (nearshore and offshore) of the Southern North Sea are enriched in Pb compared with average shale. Recent tidal flat sediments are distinctly higher in Pb than human-unaffected Holocene sediments. Besides pollution, the elevated Pb contents in recent tidal flat sediments result from the enhanced abundance of Kfeldspars and heavy minerals due to a man-made increase in depositional energy caused by dike-building. 206 Pb/207Pb ratios reveal only slight differences for Holocene and recent tidal flat sediments, whereas SPM shows a pronounced shift towards an anthropogenic signal with increasing distance from the coast. This indicates that only small amounts of SPM are presently deposited in the intertidal systems caused by the present high-energy environment. Leaching experiments show that large amounts of Pb are adsorbed on mineral surfaces (recent sediments 50– 70%, SPM 90%). The adsorbed fraction comprises both, anthropogenic as well as geogenic Pb. The proportions of these fractions, estimated from 206Pb/207Pb ratios, reveal comparatively high amounts of adsorbed geogenic Pb for recent sediments (60%) and SPM (40%). Irrespective of its origin the Pb-rich SPM is partly exported from the intertidal systems into the open North Sea. Without the anthropogenic change of energy conditions, more Pb-rich SPM from the North Sea would be deposited in the intertidal flat area. Therefore, the process of SPM export likely leads to a decontamination of the tidal flat sediments.
Acknowledgements The authors wish to thank H. Streif and J. Barckhausen (Geological Survey of the Federal State of Lower Saxony, Germany) for the supply of Holocene sediment material and A. Hild for some recent tidal flat sediments. Further thanks are due to the crews of F.K. Senckenberg, F.S. Heincke, and F.S. Victor Hensen as well as to R. Reuter and B. Warning. Two anonymous reviewers are thanked for their constructive comments. This study was funded by the German Science Foundation (DFG) through grants No. Scho 561/3-1, 4-1 and forms part of the interdisciplinary special research program ‘‘Bio-geochemical changes over the last 15,000 years—continental sediments as an expression of changing environmental conditions’’.
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Appendix Precision and accurracy of analytical methods Element
Method
Precision SD 2 (%)
Accuracy (%)
Al Fe K Mn Pb Al Fe K Mn Pb 206 Pb/207Pb
XRF
1.3 1.1 1.5 1.6 5.6 4.9 4.9 8.8 4.4 12.4 0.32
1.1 0.9 1.6 4.1 2.7 4.0 2.6 15.2 2.6 5.1 <0.1
ICP–OES
ICP–MS
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Lead in sediments and suspended particulate matter of the German Bight: natural versus anthropogenic origin J. Hinrichs, O. Dellwig, H.-J. Brumsack* Institute of Chemistry and Biology of the Marine Environment (ICBM), Carl von Ossietzky University of Oldenburg, PO Box 2503, D-26111 Oldenburg, F.R.Germany Received 15 January 2001; accepted 1 September 2001 Editorial handling by M. Novak
Abstract Sediments (568) and suspended particulate matter (SPM, 302 samples) of the southern German Bight and the adjacent tidal flat areas were analysed for selected major elements (Al, Fe, K), trace metals (Mn, Pb), and 206Pb/207Pb ratios using XRF, ICP–OES, ICP–MS. For selected samples a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e.g. Mn/Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. Enrichment factors versus average shale (EFS) reveal elevated Pb contents for all investigated sediments and SPM in the following order: Holocene tidal flat sediments (HTF, human-unaffected)
1. Introduction The heavy metal Pb has frequently been used as a sensitive pollution indicator (e.g. Balls, 1985; Ve´ron et al., 1992; Tappin et al., 1995) as this volatile element is
* Corresponding author. Tel.: +49-441-798-3584; fax: +49441-798-3404. E-mail address: [email protected] (H.-J. Brumsack).
released during high-temperature processes and combustion of leaded fuel. Being enriched in industrialised countries, Pb is distributed by atmospheric and riverine input (e.g. Ottley and Harrison, 1991; Turner et al. 1991; Grousset et al., 1999). In numerous studies Pb has been determined in sediments, suspended particulate matter (SPM) and waters of the southern North Sea and adjacent tidal flat areas and rivers (e.g. Duinker and Nolting, 1977; Schwedhelm and Irion, 1985; Kersten et al., 1988; Haarich et al., 1993; Koopmann et al., 1993; Puls
0883-2927/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00124-X
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et al., 1997a). But total Pb concentrations alone may be insufficient for separating pollution from the natural background as the latter is often highly variable due to natural processes (Dellwig, 1999). However, ICP–MS analyses allow the determination of Pb isotope ratios that may distinguish between natural and anthropogenic Pb (e.g. Kersten et al. 1992; Ritson et al., 1994; Monna et al. 1997; Ve´ron and Church, 1997; Monna et al. 2000). The major goal of this study is to combine both parameters, i.e., concentrations and isotope ratios, to differentiate between natural and anthropogenic Pb in sediments and SPM of the German Bight and coastal areas. As Pb is a very particle reactive element (Balls, 1988; Krause et al., 1993) in particular, the anthropogenic Pb is thought to be adsorbed on particle surfaces. Therefore, leaching experiments are carried out to assess the contribution of adsorbed Pb. Besides anthropogenic Pb, it is supposed also that an adsorbed geogenic fraction may be involved that is released, e.g. during weathering.
2. Geographical setting Fig. 1 shows the sampling locations of sediments and suspended particulate matter (SPM) in the southern German Bight and the adjacent tidal flat areas (East Frisian Islands, Jade Bay). Holocene tidal flat sediments (HTF) were recovered from 6 drill cores (locations Wangerland and Schweiburg), whereas recent tidal flat sediments (RTF) are represented by surface sediments (0–2 cm) from the Spiekeroog Island back-barrier intertidal system and drill core surface samples from Jade Bay (Arngast). For assessing the anthropogenic influence, we use the Holocene sediments deposited between 5.5 and 2.2 ka BP (Mid-Holocene) as a reference for background values unaffected by human activity (e.g. dike building, pollution). The age limits are based on calibrated 14C determinations of peat layers below and above the tidal flat sediments (Dellwig, 1999; Dellwig et al., 2000).
Fig. 1. Map of the study area showing sampling sites of sediment cores (asterisks), suspended particulate matter (SPM, filled circles), and SPM time series stations (open circles). The bold line indicates the separation between nearshore (SPM<5 mg l1) and offshore samples (SPM >5 mg 1). Thin lines denote salinity. The light grey arrow represents the general flow pattern in the German Bight (Hainbucher et al., 1987; Puls et al., 1997a, b).
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Additionally the authors investigated 3 short cores (22–26 cm length) from the Helgoland Island mud hole area (MH) covering at maximum the last 50 a (sedimentation rates 5–18 mm a1; Baumann, 1991; Dominik et al., 1978; Fo¨rstner and Reineck, 1974). SPM samples were collected from surface water during several cruises (FS Victor-Hensen 97-08; FS Heincke 98-09, 99-08; FK Senckenberg 98-07, 99-03, 99-09, 00-01, 00-04). While most data points shown in Fig. 1 refer to single samples (filled circles), time series samples were collected close to the Spiekeroog back-barrier intertidal system and near Helgoland Island (open circles). SPM is divided into nearshore (SPM concentrations >5 mg l1) and offshore SPM (<5 mg l1) to account for its different geochemical composition. The bold line in Fig. 1 marks the transition of both sampling areas.
3. Material and methods A total number of 568 sediment samples (HTF 377, RTF 119, MH 72) were analysed for selected major (Al, Fe, K) and trace elements (Mn, Pb) by XRF (Philips PW 2400, equipped with a Rh-tube) using fused lithium tetraborate glass beads. Sampling resolution was 5–10 cm on Holocene and Arngast drill cores, 1 cm on short cores from the Helgoland Island mud hole area (MH), and 0.5 cm on recent surface sediments from the Spiekeroog backbarrier intertidal area. The samples were stored in PEbags, sealed, and immediately frozen. At the laboratory the samples were freeze-dried and homogenised in an agate mortar. The ground powder was used for all subsequent geochemical analyses. During the cruises listed above (see Section 2), 302 SPM samples were collected. Onboard ship, 0.2–4 l of seawater were filtered through pre-weighed Millipore1 filters (0.45 mm, for multi-element analyses) and Whatman1 glass fibre filters (0.7 mm, for TC and TIC analyses). The filters were rinsed with 18 M water, dried at 60 C and re-weighed for the determination of total suspended material retained on the filters. For elemental analysis, the Millipore1 filters were digested completely in closed PTFE autoclaves (Heinrichs et al., 1986) at 180 C in a mixture of HNO3, HClO4 (purified by subboiling distillation) and HF (suprapure). Al, Fe, K, and Mn were analysed by ICP–OES (Perkin Elmer Optima 3000XL), while Pb was determined by ICP–MS (Finnigan MAT Element). 206Pb/207Pb isotopic ratios were determined on acid digestions of 57 sediment samples and 289 SPM samples by ICP–MS (Finnigan MAT Element). Total digestions were used without further purification, just dilution to Pb concentrations of about 5 mg l1. No mass bias correction was necessary since the instrument was tuned to give best accuracy for the NIST-Standard SRM 981. Total aquisition time was 3 min, consisting of 60 scans.
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For selected samples (sediments 19; SPM nearshore 3, offshore 1) a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e.g. Mn/Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. International (JF-1; K-feldspar) and in-house (TW-TUC; shale) standards were used to check the extent of dissolution of the mineral matrix by the acid attack. According to the methods described by, e.g. Monna et al. (2000) and Huerta-Diaz and Morse (1990), 250 mg sample were treated with 2 ml 1 M HCl, shaken for 16 h at ambient temperature, centrifuged, decanted, and washed 3 times with 1 ml 18 M H2O. The solutions were also analysed for the aforementioned elements and 206Pb/207Pb isotopic ratios. Precision and accuracy were checked by parallel analysis of international reference materials (GSD-3, GSD5, GSD-6, NIST-Standard SRM 981), as well as inhouse standards (see Appendix A). For all methods, filter and procedural blanks were determined and used for correcting the sample values. Traces of Pb were detected in the Millipore filters, while shipboard contamination was insignificant. The average Pb concentration of the blank filters is 1.2 0.9 mg kg1.
4. Results and discussion 4.1. Elemental composition of sediments and SPM The analytical results of investigation, i.e. Al2O3, Fe2O3, K2O, Mn, Pb concentrations, Al-normalised values, and 206Pb/207Pb ratios of Holocene tidal flat sediments (HTF), recent tidal flat sediments (RTF), Helgoland Island mud hole sediments (MH) and suspended particulate matter (SPM) are summarised in Table 1. An overview on the geochemistry of the study area is given by Dellwig et al., 2000. Fig. 2 shows the enrichment factors versus average shale (AS; Wedepohl, 1971) of Pb, K, Fe, and Mn for sediments and SPM calculated from: EFS ¼ ðelement=A1Þsample =ðelement=A1ÞAS All sediment and SPM samples investigated are enriched in Pb to varying degrees with lowest EFS values for HTF and highest values for offshore SPM. In a first approximation the higher EFS of RTF compared to HTF is due to anthropogenic Pb input. Comparable data are scarce in the literature as most authors limit their studies to different size fractions and digestion methods (e.g. Fo¨rstner and Reineck, 1974; Schwedhelm and Irion, 1985; Koopmann et al., 1993; Irion, 1994). For instance, Schwedhelm and Irion (1985) reported elevated Pb contents of on average 62 mg kg1 in the < 2 mm fraction of sediments from the East Frisian intertidal system. They calculated a mean enrichment factor
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Table 1 Analytical results (mean and standard deviations SD): A12O3, Fe2O3, K2O, Mn, Pb concentrations, Al-normalised values, and 206 Pb/207Pb ratios of Holocene tidal flat sediments, recent tidal flat sediments, Helgoland Island mud hole sediments and suspended particulate matter (SPM)a Holocene tidal flat sediment na Al2O3 Fe2O3 K2O Mn Pb Fe/Al K/Al Mn/Al104 Pb/Al104 206 Pb/207Pb a
377 (22) 8.10 3.52 1.96 433 14 0.56 0.39 101 3.4 1.207
SD
1.84 1.21 0.28 187 4 0.10 0.05 34 0.7 0.008
Recent tidal flat sediment 119 (23) 3.56 0.82 1.29 124 13 0.29 0.56 64 6.7 1.192
SD
0.77 0.38 0.17 59 4 0.07 0.05 26 1.5 0.019
Mud hole sediment 72 (12) 6.86 2.61 1.76 312 24 0.50 0.41 87 6.9 1.184
SD
1.34 0.62 0.22 70 6 0.04 0.03 17 2.4 0.009
Nearshore SPM 242 (229) 9.98 5.01 1.79 1334 67 0.68 0.29 263 13 1.1720
SD
1.63 0.70 0.25 529 24 0.15 0.06 125 6 0.007
Offshore SPM
SD
60 (60) 7.89 2.53 3.69 1.16 1.40 0.48 1327 562 131 154 0.63 0.15 0.29 0.07 346 183 40 52 1.166 0.012
n is the total sample number, the values in parentheses indicate the number of digestions made.
Fig. 2. Enrichment factors versus average shale (EFS) for Pb, K, Fe, Mn analysed in sediments (Holocene tidal flat, HTF; recent tidal flat, RTF; Helgoland Island mud hole, MH) and SPM. Average shale (AS) data after Wedepohl (1971).
versus the natural background (15 mg kg1) of about 4 (range 2–9). This is comparable to a value of 6.4 in the same size fraction of the present study area (HTF <2 mm: 15 mg kg1, O. Dellwig, unpublished data; RTF <2 mm: 96 mg kg1, Hild, 1997). Schwedhelm and Irion (1985) conclude that the Pb enrichment in tidal flat sediments is exclusively caused by atmospheric pollution. However, regarding the EFS values (Fig. 2) the human unaffected HTF are also subject to a minor Pb enrichment that has to be attributed to a natural Pb-rich compound. This finding necessitates careful interpretation of geochemical data with respect to the assignment of natural and anthropogenic heavy metal contributions. Thus, background values are considerably variable in intertidal systems. For example, background values are low in sheltered back-barrier intertidal systems (HTF < 2 mm: 15 mg kg1 Pb) in comparison to
more estuarine influenced intertidal areas (HTF < 2 mm: 28 mg kg1 Pb; O. Dellwig, unpublished data). Tidal flat sediments consist mainly of quartz, organic material, carbonates, clay minerals and feldspars (van Straaten, 1954). The latter two are the most important natural contributors of Pb because Pb can replace K in the mineral lattice due to similar ionic radii. Clays contain on average 22 mg kg1 Pb (average shale; Wedepohl, 1971) which is the sum of Pb incorporated into the mineral lattice as well as Pb adsorbed on mineral surfaces (see below). K-feldspars usually have higher Pb contents. For instance, K-feldspars from granites and pegmatites from Southern Norway, UK, and Germany average 82 mg kg1 Pb (range 25–177 mg kg1; Wedepohl, 1978). Therefore, the slight enrichment of Pb in HTF may be explained by a higher contribution of Kfeldspars as is indicated by elevated EFS values for K. The same is true for RTF that show distinctly higher EFS for K indicating a higher contribution of K-feldspars in comparison to HTF. This difference is likely caused by an increase in depositional energy in the intertidal systems from the Holocene to the recent situation leading to a pronounced lack in fine grained material (clay) and enrichments in heavy minerals in RTF (Dellwig et al., 2000). According to Flemming and Nyandwi (1994) and Flemming and Davies (1994) the energy increase results from modern dike building leading to steeper than normal energy gradients in the intertidal zones. Therefore, the silt fraction, which is enriched in feldspars (LittleGadow, 1978), contributes to a larger extent to the bulk sediment composition when compared with the Holocene situation. An additional effect of the increased energy conditions in RTF are elevated levels of heavy minerals (Dellwig et al., 2000). This sediment fraction, which accounts for 4% of the total sediment, is
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also enriched in Pb. Concentrations average 140 mg kg1 Pb in heavy mineral separates of the Spiekeroog back-barrier intertidal system (Hild, 1997). Nevertheless, the Pb/K-ratio of RTF (12.2104) is higher than that of HTF (8.6104) pointing to a further Pb source. For this reason, the authors assume that the enrichment of Pb in RTF is the result of two anthropogenic effects: (i) changing energy conditions leading to a larger fraction of Pb-rich feldspars and heavy minerals and (ii) pollution. In MH samples the Pb input from pollution appears to be more pronounced as the high EFS values for Pb are only accompanied by K enrichments on the same magnitude as HTF (Pb/K=8.6104). In SPM, highest EFS for Pb were observed, probably caused exclusively by pollution. The slight depletion in K may be the result of partial K replacement by seawater Na. Iron and Mn both show similar behaviour for the investigated materials. The most pronounced feature of these two elements is the depletion in RTF (and MH) in comparison to the shale-like HTF. Iron and Mn are mobilised under reducing conditions (Dehairs et al., 1989; Tappin et al., 1995; Hinrichs et al., 2000) in the adjacent tidal flats and North Sea bottom sediments. Subsequently divalent Fe and Mn ions become oxidised in the water column and form oxihydroxide coatings on SPM, which explains the observed Fe and Mn enrichments in SPM. Additionally, the precipitation process may largely control adsorption (Schoemann et al., 1998) of other metals, e.g. Pb, on particles. Especially in the studied coastal high turbitidy area, SPM is expected to play an essential role in Pb transport. As Pb is known to be highly particle-reactive (Bru¨gmann et al., 1985; Balls, 1988), it may be scavenged from solution onto particles during formation of oxihydroxide coatings. The following scenario can explain the different behaviour of Fe and Mn in HTF and RTF. At present the higher depositional energy in intertidal systems causes a stronger separation of coarse grained sediments and fine material suspended in the water column in comparison to the Holocene (Dellwig et al., 2000). The authors assume that the prevailing sands promote a rapid exchange of porewaters with seawater. This likely leads to a flux of dissolved Mn and Fe out of the reducing tidal flat sediments (Hinrichs et al., 2000). After oxidation of the reduced Fe and Mn species they attach to SPM, possibly co-precipitating Pb. While at present SPM is mainly exported to the open North Sea, this process was presumably less pronounced in the Holocene, and more of the Mn and Fe-bearing particles were re-deposited locally in the intertidal flats. Additionally a stronger fixation of Fe and Mn during early diagenesis as pyrite and rhodochrosite in microenvironments can be assumed for HTF. This assumption is confirmed by different S and carbonate (IC) contents in HTF and RTF (Hild, 1997; Dellwig, 1999). While S and IC contents in
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HTF are high (S 0.7%, IC 1.2%), RTF are comparatively poor in both parameters (S 0.2%, IC 0.5%). 4.2. 206Pb/207Pb ratios and Pb concentrations of sediments and SPM The results discussed so far require a distinction between natural and anthropogenic Pb. 206Pb/207Pb ratios are a useful tool for identifying Pb pollution, therefore these ratios were determined in sediment and SPM samples. Two end members have to be considered: (i) the local geogenic background established from the pre-industrial HTF (206Pb/207Pb 1.207 0.008) and (ii) the atmospheric signal of the study area. The latter was determined from natural concentrators (Rachold et al., 1992) of local dust (mosses and spider webs) with lower 206 Pb/207Pb ratios of 1.142 0.004 (H.-J. Sach, personal communication). The local geogenic value is in accordance with continental European sources with a 206Pb/207Pb ratio of approximately 1.18–1.20 (Elbaz-Poulichet et al., 1986). The local atmospheric end member is in the upper range of values for aerosols from the North Sea (206Pb/207Pb 1.11–1.14) influenced by the highly industrialised UK and continental Europe (Kersten et al., 1992). The anthropogenic signal of the study area is influenced by inputs of volatile Pb released during high temperature industrial processes and combustion of leaded fuel (Erel et al., 1997; Hansmann and Ko¨ppel, 2000). Most Pb ores are at present imported from Australian and Canadian Precambrian sources with low Pb isotopic ratios (Kersten et al., 1992). The anthropogenic Pb is distributed through the atmosphere and subsequently enters the terrestrial and marine environment by wet and dry deposition (Ottley and Harrison, 1991; Rojas et al., 1993). Although the combustion of leaded fuel has decreased recently, the atmospheric Pb isotope signal is still significantly lower than the European geogenic signal (Do¨ring et al., 1997). A further contributor of Pb are particles from North Atlantic surface water which show 206 Pb/207Pb ratios of 1.18 (Ve´ron et al., 1994). However, according to Eisma and Irion (1988) Atlantic water is considered to be separated from the sampling area by the eastward flow of coastal water masses as can be seen from the salinity distribution in Fig. 1. Fig. 3 provides an overview of the 206Pb/207Pb ratios determined in sediments and SPM from the study area. In general, the SPM shows lower Pb isotope ratios than the sediments due to atmospheric input of anthropogenic Pb and subsequent scavenging by particles. The similarity of RTF values (1.192 0.019) with the local geogenic background (HTF) confirms the assumption that anthropogenic Pb is of minor importance in RTF. By contrast, nearshore SPM (1.172 0.007) shows a more pronounced anthropogenic contribution. The low variation in nearshore samples reflects a comparable Pb-
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Fig. 3. 206Pb/207Pb ratios of sediments (HTF, RTF, MH; grey boxes), SPM (white boxes), and literature data for atmospheric input (bold-lined box, values from Kersten et al., 1992) in the study area. For symbols see Fig. 1. Data points connected by grey lines belong to transects sampled on the cruises S00-04 (1), S99-03 (2), VH97-08 (3=Weser, 4=Elbe).
isotopic composition of the entire coastal area sampled. Thus, Pb isotope ratios of SPM samples from the backbarrier tidal flat, Jade Bay, estuarine and coastal sites (SPM > 5 mg l1) show no significant differences. The discrepancy between RTF and SPM evidences the low SPM deposition in the intertidal flats due to the present increase in depositional energy (e.g. Flemming and Nyandwi, 1994; Dellwig et al., 2000). Therefore, it is assumed that large parts of the fine material remain suspended in the water column and may be transported off the coastal regions. The offshore SPM samples show slightly lower isotope ratios. A candidate deposition site is the nearby Helgoland Island mud hole area (MH) where the sedimentation rate has increased since the onset of dike-building and land reclamation in the 11th century. Most estimates of the recent sedimentation rate range from 5 to 18 mm a1 (e.g. Fo¨rstner and Reineck, 1974; Dominik et al., 1978; Baumann, 1991), while von Haugwitz et al. (1988) propose a significantly lower average sedimentation rate of 3.2 mm a1 between 8 and 1.5 ka BP. On one hand, a
shift is observed towards lower Pb isotope ratios for MH sediments (1.184 0.009) when compared with the other sediments under study. On the other hand, Pb isotope ratios are significantly higher than in offshore SPM. Although some deposition of polluted particles is seen in MH samples, it is presumed that a significant fraction of the SPM is transported further offshore. Therefore, the main area of deposition for the fine-fraction seems to be the western Skagerrak/Norwegian Channel, where presently about 50–70% of the total mass accumulation of the North Sea occurs (Eisma and Kalf, 1987). Fig. 4 presents the relationship between 206Pb/207Pb ratios and Pb contents expressed as Al-normalised values. In general a trend of increasing Pb/Al ratios is accompanied by a shift towards lower isotopic ratios due to higher contents of anthropogenic (excess) Pb. HTF samples show a small-scale variability in isotope ratios reflecting the natural range, while RTF samples show a much larger scatter. Some RTF samples are enriched in Pb that is not derived from pollution as indicated by their geogenic Pb isotope ratios. This possibly reflects
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Pbxs;sample ¼ Pbsample;total Pbsample;background
Fig. 4. Scatter plot of 206Pb/207Pb ratios versus Pb/Al ratios of sediments and SPM. Note that the x-axis is logarithmic. The arrows indicate Pb/Al ratios of average shale (Wedepohl, 1971) and the local geogenic background estimated from HTF.
the presence of elevated amounts of K-feldspars and heavy minerals in comparison to HTF. According to Friese (1990), analyses of pure K-feldspars yielded Pb isotope ratios similar to the HTF (mean 1.203, range 1.191–1.225). Nevertheless, some RTF samples contain significant amounts of fine material rich in excess Pb leading to lower Pb isotope ratios. The MH samples show a linear correlation between both parameters (r=0.95; regression line not shown in Fig. 4 due to the use of a log scale) which indicates that these sediments might result from a mixture of Holocene sediment material and nearshore SPM. Offshore SPM seems to contribute to a smaller extent as this material contains distinctly higher amounts of Pb. Although the input of leaded fuel has decreased in the last decade, the MH show a trend towards lower isotope ratios and increasing Pb contents in the uppermost few cm of the cores which were deposited in the last years. This finding points at a still existing large reservoir for anthropogenic Pb in the environment as a contaminant of SPM. Thus, all SPM samples are enriched in Pb with the highest variability seen for offshore SPM. This variability strongly depends on the sample location, however, a general trend of decreasing Pb isotope ratios and increasing Pb contents with increasing distance from the coast is observed. Fig. 5a and b show the results of 4 transects (Fig. 3) from the coast to the open North Sea for SPM contents, salinity, 206Pb/207Pb ratios, Pb/Al ratios, and particulate Pb in excess of the natural background (Pbxs). The latter parameter was calculated from: Pbsample;background ¼ A1sample ðPb=AlÞHTF
The excess value (dry weight) was then converted into volume specific molar units by multiplication with the total SPM values and a conversion factor to account for the correct units. With increasing distance from the coast, salinities increase while SPM concentrations decrease. One exception is transect 4 that is influenced by material derived from the small islands Neuwerk and Scharho¨rn (compare Fig. 1). The profiles shown in Fig. 5b confirm the aforementioned relationship between the 206 Pb/207Pb and Pb/Al ratios of SPM with respect to the distance from the coast. Thus, with increasing distance from the coast the particles are more enriched in Pb from anthropogenic sources as seen from the decreasing Pb isotope ratios. The observed trends are in accordance with results of Krause et al. (1993) who found seawards decreasing 206Pb/207Pb ratios for a transect from the river Elbe towards the German Bight. However, considering the Pb concentration per water volume as expressed by Pbxs a reverse trend is seen when compared with Pb/Al ratios. On one hand, the offshore particles contain higher amounts of Pb (high Pb/Al ratios), on the other hand the Pb inventory per water volume decreases due to lower SPM concentrations. The coastal waters contain the largest amounts of Pb per water volume but the Pb/Al ratios are distinctly lower than for offshore waters. This means that a single nearshore particle is less contaminated with Pb than offshore, but because of high SPM concentrations the total Pb inventory is highest nearshore. The observed spatial distribution is likely the result of several effects: (i) Pb uptake in nearshore waters, presumably in the most turbid intertidal systems, from the dissolved phase. Possible sources are Pb mobilised during weathering (geogenic signal) and dissolution of oxihydroxide coatings with subsequent release from porewaters, and unspecific riverine input indicated by low salinities (Fig. 5a); (ii) settling of coarser particles (more pronounced geogenic signal) during transport towards the open North Sea; and (iii) the uptake of atmospheric/anthropogenic Pb by clay minerals and phytoplankton. The biogenic uptake is more pronounced in the remote areas because of higher abundance of plankton as indicated by offshore POC enrichments in comparison to nearshore SPM (Krause et al., 1993; Dellwig et al., 2000). 4.3. Dominance of anthropogenic Pb? Leaching experiments (1 N HCl) provide information on Pb fixation in sediments and SPM. The results for SPM shown in the following figures represent the mean of 3 nearshore samples. As only one offshore sample was available for the leaching procedure, this was excluded. Lead is predominantly bound in alumo-silicate structures
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Fig. 5. Transects (tracks shown in Fig. 3) displaying (a) SPM concentration and salinity; (b) 206Pb/207Pb ratios, Pb/Al ratios, and Pbxs concentrations versus distance from nearshore station. For Pbxs calculation, see text.
and adsorbed on particle surfaces, while carbonates and organic matter contain only low amounts of Pb (Monna et al., 2000). It would be expected that the adsorbed component would be distinguished from the mineralbound Pb by the leaching procedure applied. As a reference for clay rich material an in-house shale standard (26 mg kg1 Pb) was used and for K-feldspars the JF-1 standard (33 mg kg1 Pb). In Fig. 6 the percentage of Al, Pb, Fe, and Mn observed in the leachate compared to the bulk concentration are presented for HTF, RTF, MH, nearshore SPM, shale and K-feldspar. The acid attack does not affect the alumo-silicate matrix as can be seen from the low Al content in the leachate (mean <5%) which confirms that the procedure only liberates the adsorbed metals. For Pb increasing percentages are observed in the order HTF
and resembles the shale standard (87%) indicating that in both materials Pb is almost exclusively adsorbed on particle surfaces. By contrast, in the K-feldspar Pb is mostly fixed in the mineral lattice as can be inferred from the low percentage of Pb in the leachate (4%). In general Fe and Mn are similarly distributed to Pb indicating that Fe and Mn oxihydroxides play a significant role in transferring Pb to particle surfaces by co-precipitation (scavenging). In Fig. 7a, the results of 206Pb/207Pb analyses in the leachate for HTF, RTF, MH, and SPM are shown. As the leachates represent the adsorbed Pb, we assume that this fraction is the main carrier for anthropogenic Pb. While the isotopic ratio of HTF leachate (1.207) is identical to its bulk value due to a lack of anthropogenic influences, RTF and MH show a significant shift towards lower ratios (both 1.174) when compared with the bulk sediment samples (RTF: 1.192; MH: 1.184). Lowest 206Pb/207Pb ratios were observed for nearshore SPM leachates (1.159), indicating elevated amounts of atmospheric/anthropogenic Pb. This contribution is slightly higher in the offshore sample (not shown) with a value of 1.142. While the offshore sample almost reaches the pure anthropogenic 206Pb/207Pb signal of
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Fig. 6. Percentage of Al, Pb, Fe, Mn in leachates (1 N HCI) of sediments, nearshore SPM, shale (in-house standard TW– TUC) and K-feldspar (international standard JF-1).
1.11–1.14 (Kersten et al., 1992), the ratios of the nearshore leachates are distinctly higher. Although one sample is not representive, the difference in nearshore and offshore SPM seems to be significant. Therefore, the adsorbed Pb-fraction contains both, an anthropogenic as well as a significant geogenic Pb component derived from weathering of detrital material. These two components can be estimated (Fig. 7b) from the Pb isotope ratios of the leachates and the two 206Pb/207Pb end members (geogenic 1.207, anthropogenic 1.11–1.14, compare range in Fig. 7b). In the modern sediments (RTF and MH), the anthropogenic component of adsorbed Pb amounts to approximately 40%. In contrast, SPM shows a distinctly higher percentage of this fraction (58%) because of a stronger influence of atmospheric input. This means that a sig-
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nificant fraction of adsorbed Pb is derived from natural geogenic sources even in SPM. It should be noted that the percentage of leachable Pb differs largely in sediments and SPM (RTF, MH 50–70%; SPM 90%). To represent the amounts of geogenic and anthropogenic Pb in the bulk materials, the analytical results of bulk Pb, Pb isotopes and leaching experiments for sediments and SPM are summarised in Fig. 8. The heights of the bars represent total Pb, though differences to average Pb values given in Table 1 result from the lower sample numbers chosen for the leaching experiments. The percentage leached (Fig. 6) is used to infer mineral bound and adsorbed Pb concentrations. The latter fraction has been subdivided into geogenic and anthropogenic Pb based on the Pb isotope ratios of the leachates (Fig. 7a) and the end member isotopic compositions (Fig. 7b). As absolute Pb concentrations are presented, one has to consider dilution effects caused by, e.g. quartz and organic matter. The mineral bound Pb varies only slightly in the sediments. The quartz-rich RTF sediments show a similar concentration compared to HTF. Thus, the dilution effect of quartz in RTF is likely compensated by the occurrence of Pb-rich K-feldspars and heavy minerals. In contrast, the mineral bound Pb of MH likely results from an enhanced clay content. SPM nearly reaches the shale level as the dilution with quartz is negligible and dilution with organic matter is also relatively low POC=4.5–6.6%; Dellwig et al., 2000). Highest concentrations of adsorbed geogenic Pb are observed in nearshore SPM (Fig. 8), although the percentage of adsorbed geogenic Pb is lowest in SPM (Fig. 7b). A similar value is observed for the offshore sample (64 mg kg1). This fraction is distinctly lower in
Fig. 7. (a) 206Pb/207Pb ratios in leachates (1N HC1) of sediments and SPM. (b) Percentage of geogemc and anthropogenic Pb in leachates calculated from the anthropogenic (mean 206Pb/207Pb ratio=1.125) and geogenic (206Pb/207Pb=1.21) end members. The error bars indicate the range of the anthropogenic contribution (1.11–1.14) reported by Kersten et al. (1992).
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fine material, leads to its decontamination by export of Pb-rich SPM.
5. Conclusions
Fig. 8. Absolute concentrations of mineral bound Pb as well as adsorbed geogenic and anthropogenic Pb in sediments and SPM. For calculation of fractions see text.
the sediments. While HTF and RTF show similar concentrations (4–5 mg kg1), the adsorbed geogenic Pb is about 3-fold higher in MH. The high geogenic Pb inventory of SPM is due to enhanced uptake of Pb released during weathering of host rocks and sediments in the nearshore low salinity coastal water mass (compare Section 4.2). On a short time scale, most of the SPM may be deposited and resuspended in the intertidal area, thus approximating an equilibrated system. But on a longer time scale, a small fraction of SPM rich in geogenic Pb leaves the nearshore mixing zone and reaches the open North Sea. Once the geogenic Pb is adsorbed in the nearshore area, it possibly remains on particles as indicated by the similar nearshore and offshore concentrations of adsorbed geogenic Pb. For this reason, the elevated amount of adsorbed geogenic Pb in MH is presumably caused by some deposition of SPM. Adsorbed anthropogenic Pb shows similar behaviour, with the exception that no anthropogenic signal can be seen in HTF. In the modern sediments, natural Pb prevails over anthropogenic Pb. Due to atmospheric input a dominance of anthropogenic Pb occurs in SPM. The atmospheric influence is more pronounced offshore (adsorbed anthropogenic Pb 239 mg kg1) because of a long residence time of SPM in the German Bight (5 month in summer; Puls et al., 1997b). Therefore, SPM collects the atmospheric Pb in addition to the natural enrichment of Pb. Irrespective of the Pb source (anthropogenic and/or natural), the man-made high energy environment in the intertidal systems, which diminishes the deposition of
All investigated sediments (Holocene and recent tidal flat, Helgoland Island mud hole area) and suspended particulate matter samples (nearshore and offshore) of the Southern North Sea are enriched in Pb compared with average shale. Recent tidal flat sediments are distinctly higher in Pb than human-unaffected Holocene sediments. Besides pollution, the elevated Pb contents in recent tidal flat sediments result from the enhanced abundance of Kfeldspars and heavy minerals due to a man-made increase in depositional energy caused by dike-building. 206 Pb/207Pb ratios reveal only slight differences for Holocene and recent tidal flat sediments, whereas SPM shows a pronounced shift towards an anthropogenic signal with increasing distance from the coast. This indicates that only small amounts of SPM are presently deposited in the intertidal systems caused by the present high-energy environment. Leaching experiments show that large amounts of Pb are adsorbed on mineral surfaces (recent sediments 50– 70%, SPM 90%). The adsorbed fraction comprises both, anthropogenic as well as geogenic Pb. The proportions of these fractions, estimated from 206Pb/207Pb ratios, reveal comparatively high amounts of adsorbed geogenic Pb for recent sediments (60%) and SPM (40%). Irrespective of its origin the Pb-rich SPM is partly exported from the intertidal systems into the open North Sea. Without the anthropogenic change of energy conditions, more Pb-rich SPM from the North Sea would be deposited in the intertidal flat area. Therefore, the process of SPM export likely leads to a decontamination of the tidal flat sediments.
Acknowledgements The authors wish to thank H. Streif and J. Barckhausen (Geological Survey of the Federal State of Lower Saxony, Germany) for the supply of Holocene sediment material and A. Hild for some recent tidal flat sediments. Further thanks are due to the crews of F.K. Senckenberg, F.S. Heincke, and F.S. Victor Hensen as well as to R. Reuter and B. Warning. Two anonymous reviewers are thanked for their constructive comments. This study was funded by the German Science Foundation (DFG) through grants No. Scho 561/3-1, 4-1 and forms part of the interdisciplinary special research program ‘‘Bio-geochemical changes over the last 15,000 years—continental sediments as an expression of changing environmental conditions’’.
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Appendix Precision and accurracy of analytical methods Element
Method
Precision SD 2 (%)
Accuracy (%)
Al Fe K Mn Pb Al Fe K Mn Pb 206 Pb/207Pb
XRF
1.3 1.1 1.5 1.6 5.6 4.9 4.9 8.8 4.4 12.4 0.32
1.1 0.9 1.6 4.1 2.7 4.0 2.6 15.2 2.6 5.1 <0.1
ICP–OES
ICP–MS
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