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Heavy metal distributions in Peru Basin surface sediments in relation to historic, present and disturbed redox environments
Deep-Sea Research II 48 (2001) 3757–3777
Heavy metal distributions in Peru Basin surface sediments in relation to historic, present and disturbed redox environments Andrea Koschinsky* Freie Universitat . Berlin, FR Rohstoff- und Umweltgeologie, Malteserstr. 74-100, D-12249 Berlin, Germany
Abstract Heavy metal distributions in deep-sea surface sediments and pore water profiles from five areas in the Peru Basin were investigated with respect to the redox environment and diagenetic processes in these areas. The 10–20-cm-thick Mn oxide-rich and minor metal-rich top layer is underlain by an increase in dissolved Mn and Ni concentrations resulting from the reduction of the MnO2 phase below the oxic zone. The mobilised associated metals like Co, Zn and Cu are partly immobilised by sorption on clay, organic or Fe compounds in the post-oxic environment. Enrichment of dissolved Cu, Zn, Ni, Co, Pb, Cd, Fe and V within the upper 1–5 cm of the oxic zone can be attributed to the degradation of organic matter. In a core from one area at around 22–25 cm depth, striking enrichments of these metals in dissolved and solid forms were observed. Offset distributions between oxygen penetration and Mn reduction and the thickness of the Mn oxide-rich layer indicate fluctuations of the Mn redox boundary on a short-term time scale. Within the objectives of the German ATESEPP research programme, the effect of an industrial impact such as manganese nodule mining on the heavy metal cycle in the surface sediment was considered. If the oxic surface were to be removed or disturbed, oxygen would penetrate deep into the formerly suboxic sediment and precipitate Mn2+ and metals like Ni and Co which are preferably scavenged by MnO2. The solid enrichments of Cd, V, and other metals formed in post-oxic environments would move downward with the new redox boundary until a new equilibrium between oxygen diffusion and consumption is reached. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The present work is part of the ATESEPP (impacts of potential technical interventions on the deep-sea ecosystem of the southeastern Pacific off Peru) research programme. During research *Fax: +49-030-83870742. E-mail address: [email protected] (A. Koschinsky). 0967-0645/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 6 6 - 2
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cruise SO 106 in the Peru Basin (January/February 1996) sediment cores were sampled from five selected sites around 4000 m water depth between 5–71S and 85–911W (Schriever et al., 1996; Thiel and Forschungsverbund Tiefsee-Umweltschutz, 2001) with the intention to include different depositional environments in this region. As a consequence of variations of organic matter flux in the five ATESEPP areas, differing depths of redox zonations in the upper layers of the sediments were expected. For most heavy metals associated with the sediments, redox conditions are the most important determining factor for how the metals are distributed between the solid and dissolved phases and whether they can mobilise and migrate in the pore waters. In an early work, Bonatti et al. (1971) had presented diagenetic metal distributions in a hemipelagic core at 21450 N and 851200 W, which is somewhat north of the ATESEPP areas. They reported Mn, Ni and Co enrichments in the upper oxic sediment layer and Cr, V and U enrichments in the lower reduced zone. The general geochemical and mineralogical variability of surface sediment in the Peru Basin was shown in previous and recent work (Schriever et al., 1996; Weber et al., 2000; Marchig et al., 2001). The upper layer of the Peru Basin deep-sea sediment is characterised by Mn oxide in wt% concentrations. It is well known from other investigations that Mn oxide forms the carrier phase for many other heavy metals like Co and Ni (Balistrieri and Murray, 1986; Koschinsky, et al., 2001b). With increasing depth, the oxic zone changes to suboxic, causing reduction and dissolution of Mn oxide. This phenomenon is reflected in the sediment and pore water profiles of Mn and other heavy metals. In order to determine the magnitude of dissolved and particulate heavy metal concentrations in the Peru Basin surface sediments, one multicorer (MC) sample was chosen from each of the five sites mentioned above that had also been investigated for dissolved oxygen, anion and ammonia concentrations for redox characterisation (Haeckel et al., 2001). The distribution of Mn, Fe and other heavy metals (Co, Ni, Cu, Zn, Pb, Cd, Ba, Mo, V and U) in the sediment column and associated pore waters was studied in the upper 30 cm of the MC cores. Knowledge of the diagenetic situation and redox environment in the Peru Basin was used to discuss potential effects of an industrial-scale disturbance of the system such as manganese nodule mining. In the normal deep-sea environment, a quasi-equilibrium between the bottom water and the sediment surface with its diagenetic processes has been reached. However, if as a result of a naturally or technically induced disturbance, the upper sediment layer is removed and resuspended into the bottom water, this equilibrium would be disturbed and heavy metals in their dissolved form and bound in or on the particulate sediment phases would be discharged. The fate of the heavy metals in the surface . sediment after disturbance is discussed in the light of the modelled scenarios presented by Konig et al. (2001).
2. Analytical methods and materials 2.1. Pore water analyses Sampling of pore waters during the SO 106 research cruise has been described by Haeckel et al. (2001). Pore water samples and the respective solid sediment samples were taken at approximately 1-cm intervals. Dissolved trace metals (Zn, Cd, Pb, Cu, Co, Ni, Mo, V and U) in the acidified (pH 2, HC1 Suprapure) pore water samples were determined by sensitive stripping voltammetric
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methods described by Koschinsky et al. (2001c). Ba concentrations were measured by ICP-AES. Fe and Mn data were taken from Haeckel et al. (2001). 2.2. Sediment analyses A 0.2 g sample of homogenised, ground and oven-dried (1051C) sediment was digested in Teflon pressure bombs at 1801C with a mixture of 3 ml HF (48%)+3 ml HCl (30%)+1 ml HNO3 (65%). After evaporating twice with diluted nitric acid, the residue was taken up in 0.5 M nitric acid to a total volume of 50 ml. Al, Mg, Ca, Fe, Mn, P, Ba, Sr, Ti, Ni, Co, Zn, Cu, V and Mo were measured with a Leeman ICP-OES spectrometer. Cd and Pb were measured by graphite furnace atomic absorption spectrometry with a Perkin Elmer instrument. Unfortunately, no reliable data for U were obtained with the methods available. 2.3. Sample material A detailed description of the sediment characteristics from the five selected ATESEPP areas (Table 1 and map, see Thiel, 2001) has been given elsewhere (Schriever et al., 1996; Weber et al., 2000; Haeckel et al., 2001; Marchig et al., 2001). Oxygen penetration depths between 5 cm (ATESEPP areas 1a, 1b, 5, and 6) and 10 cm (ATESEPP area 2), and nitrate penetration depths varying between 20 cm (area 1a) and 250 cm (area 2; Haeckel et al., 2001) reveal the range of oxic and suboxic conditions in the cores from the five areas.
3. Results 3.1. Description of the sediment and pore water profiles The major elements in all five sediment cores are Ca (up to 20%, often low in the upper 10 cm and then increasing below that), and Fe and Al in the range of 1–5 wt%. Si was not determined. Mn lies mostly between 1.5 and 2 wt% in the upper MnO2-rich layer and is constantly lower than 0.5% below this layer, representing a rather constant level of Mn in detrital minerals or Mn carbonate. Mg, Ba, Ti and P (all typically below 1 wt%) have rather uniform concentration profiles in all cores. In contrast, all minor metals show a strong small-scale variability that mostly correlates either with Mn (association with the Mn oxide phase) or with Al (possibly indicating
Table 1 Location of the sediment cores from the Peru Basin Area 1a 1b 2 5 6
Core no. 145 245 275 166 283
MC MC MC MC MC
Water depth (m) 4084 4107 4152 4128 3940
Latitude 0
5130.03 S 6134.270 S 7104.290 S 6134.060 S 5106.100 S
Longitude 85122.360 W 86111.410 W 88127.920 W 90127.980 W 90114.840 W
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Fig. 1. Bulk sediment major and minor/trace element concentrations versus depth in core 283 MC from area 6 (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
terrigenous input control). Organic carbon ranges between 0.3 and 1.3 wt% (see depth profiles by Haeckel et al., 2001). Core 283 MC of area 6 (Fig. 1) represents a typical example; MnO2 reduction appears to be complete below 13 cm, with Cu, Ni, Mo, Cd (and Co) decreasing at this depth as well. In the pore water samples, heavy metal concentrations were always significantly higher than bottom water concentrations (which are at the sub- or lower ppb level), except for Mo, V and U which were in the range of bottom water concentrations of a few ppb (Koschinsky, 1996). This is consistent with other investigations of deep-sea sediment pore waters (e.g., Klinkhammer, 1980; Sawlan and Murray, 1983; Shaw et al., 1990). Mn2+ concentrations below the Mn redox boundary were several ppm. Zn, Cu and Ni ranged between 5 and 100 ppb, with a few isolated peaks of higher concentrations. Cd and Pb mostly showed values between 0.5 and 10 ppb. Lowest concentrations between 0.01 and 6 ppb were measured for Co. Mo, V and U mostly lay between 1 and 20 ppb, with a few exceptions, especially for V, that exceeded 100 ppb. Many metals are often enriched in the upper cm and at isolated points within the profile. Ba is the element with the least variation with concentrations between 30 and 50 ppb. In the example of 283 MC (Fig. 2), the Mn profile shows a regular steep increase between 7 and 11 cm, correlating only with Ni, Mo and V values are several times higher in the upper 10 cm than in the lower part. A strong correlation of Pb and Cd is found throughout the profile. Distinct metal peaks attract notice in the upper cm of the profile (Fig. 2). The sediment and pore water profiles of the other areas (Figs. 3–10) demonstrate a certain variability between the different locations, like different thicknesses (between 10 and 20 cm) of the Mn oxide-rich layer, but more or less follow the same general pattern. Only 145 MC from area 1a shows a marked additional metal enrichment in both solid and pore water phases in the lower part of the profile (Fig. 11). Below the rather uniform middle part of the profile, at around 25 cm there occur local solid phase maxima for some elements that are most pronounced for Cu, V and Cd. Cd concentrations rise up to 50 ppm within a broad peak. This striking variation is not reflected in any of the other major elements measured. Around 22 cm, there are also very sharp increases of pore water Fe, Zn, Cu, Ni and Co, and Mn also shows small peaks superimposed on the regular profile. For pore water Cd and V, the somewhat wider peaks occur around 25 cm, and Mo slowly increases below 25 cm. We interpret these metal enrichments in core
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Fig. 2. Heavy metal pore water concentrations versus depth in core 283 MC from area 6 (concentrations given in ppb=mg/l).
Fig. 3. Bulk sediment major and minor/trace element concentrations versus depth in core 145 MC from area 1a (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
Fig. 4. Heavy metal pore water concentrations versus depth in core 145 MC from area 1a (concentrations given in ppb=mg/l).
145 MC as further diagenetic signals that are restricted to area la; they were not detected in the upper 30 cm of the cores from the other four areas, which are located further off the South American continent.
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Fig. 5. Bulk sediment major and minor/trace element concentrations versus depth in core 245 MC from area 1b (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
Fig. 6. Heavy metal pore water concentrations versus depth in core 245 MC from area 1b (concentrations given in ppb=mg/l).
Fig. 7. Bulk sediment major and minor/trace element concentrations versus depth in core 275 MC from area 2 (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
3.2. Statistical evaluation of relationships To get an overview of the relationships between the large number of parameters determined, we performed a factor analysis with the Stat View 4.5 software for Macintosh. Data for dissolved
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Fig. 8. Heavy metal pore water concentrations versus depth in core 275 MC from area 2 (concentrations given in ppb=mg/l).
Fig. 9. Bulk sediment major and minor/trace element concentrations versus depth in core 166 MC from area 5 (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
Fig. 10. Heavy metal pore water concentrations versus depth in core 166 MC from area 5 (concentrations given in ppb=mg/l).
oxygen, nitrate, nitrite, phosphate and silicate were taken from Haeckel et al. (2001). After checking the suitability of the data set for factor analysis, a principal component analysis with Varimax rotation was run. An eigenvalue o1 was chosen as the break-off test for the factors. Separate analysis of the sediment data (Table 2) grouped Ti, Fe, V, Mg, Pb (and P) into one factor. This probably reflects the high abundance of smectite of up to 65%
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Fig. 11. Comparison of pore water and bulk sediment metal concentrations in profile 145 MC; sediment data (given in wt% or ppm=mg/kg) are presented on a carbonate-free basis to exclude effects by carbonate dilution. Pore water data . are given in ppb=mg/l. The ancient Fe redox boundary was determined at about 20–23 cm (Konig et al., 1997).
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Table 2 Factor loadings of principal component analyses of the pore water data, solid sediment data, and the complete data set of pore water and sediment samples; PW=pore water, S=sediment Oblique solution primary pattern matrix Factor 1
Factor 2
Factor 3
Factor 4
Factor 5
Factor 6
Factor 7
Pore water data Communality: final estimate between 0.73 for Pb and 0.93 for Fe; sampling adequacy: between 0.44 for Cd and 0.80 for O2; sum of variance prop.=0.82 Depth 0.81 0.08 0.35 0.03 0.09 Mn (PW) 0.68 0.01 0.16 0.61 0.19 Fe (PW) 6.92E4 0.96 0.11 0.05 0.10 Zn (PW) 0.04 0.48 0.04 0.31 0.66 Cd (PW) 0.31 0.18 0.16 0.41 0.70 Pb (PW) 0.27 0.01 1.76E4 0.06 0.77 Cu (PW) 0.29 0.33 0.05 0.02 0.75 Ni (PW) 0.17 0.86 0.01 0.26 0.03 Co (PW) 0.03 0.96 0.03 0.02 0.02 Mo (PW) 0.46 0.41 0.85 0.07 0.26 V (PW) 0.10 0.03 0.58 0.42 0.35 U (PW) 0.30 0.04 0.85 0.13 0.02 Ba (PW) 0.02 0.09 0.01 0.89 7.35E4 O2 0.63 0.05 0.03 0.10 0.52 NO2 0.81 0.05 0.05 0.38 0.01 NO3 0.70 0.18 0.15 0.38 0.05 NH4 0.62 2.23E4 0.63 0.11 0.13 PO4 0.51 0.11 0.65 0.12 0.04 SiO4 0.84 0.07 0.08 0.19 0.18 Sediment data Communality final estimate between 0.83 for Cu and 0.97 for Ti; sampling adequacy: between 0.67 for Mn and 0.92 for Mg (only Cd 0.38); sum of variance prop.=0.89 Depth 0.03 0.19 0.05 0.22 1.00 Al (S) 0.08 0.13 0.97 0.05 0.12 Mg (S) 0.86 0.07 0.15 0.05 0.31 Fe (S) 1.02 0.09 0.12 0.12 0.09 Ca (S) 1.10 0.11 0.19 0.10 4.15E4 Mn (S) 0.38 0.54 0.01 0.15 0.68 Ba (S) 0.12 0.95 0.54 0.24 0.03 Ti (S) 1.16 0.40 0.13 0.01 0.10 P (S) 0.46 0.47 5.27E4 0.15 0.17 Cu (S) 0.12 0.37 0.45 0.12 0.17 Ni (S) 0.04 0.54 2.59E4 0.20 0.60 Co (S) 0.36 1.02 0.14 0.03 0.05 Zn (S) 0.39 0.10 0.67 0.04 0.16 V (S) 0.93 0.40 3.16E4 0.54 0.04 Mo (S) 2.81E4 0.04 0.73 0. 05 0.35 Pb (S) 0.59 0.56 0.07 0.05 0.30 Cd (S) 0.01 0.15 0.08 0.97 0.07
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Table 2 (continued) Oblique solution primary pattern matrix Factor 1
Factor 2
Factor 3
Complete data set (sediment and pore water data) Communality: final estimate between 0.72 for Mo (S) and (PW) and 0.82 for Ca (S); sum of variance prop. =1.00 Depth 0.41 0.03 0.22 Mn (PW) 0.69 0.31 0.07 Fe (PW) 0.07 0.02 0.12 Zn (PW) 0.04 0.26 0.72 Cd (PW) 1.49E3 0.15 0.21 Pb (PW) 0.12 0.11 0.88 Cu (PW) 0.02 0.09 0.83 Ni (PW) 0.20 0.15 0.03 Co (PW) 2.06E3 0.04 0.03 Mo (PW) 0.11 0.13 0.08 V (PW) 0.08 0.06 1.72E3 U (PW) 0.27 0.10 0.08 Ba (PW) 0.26 0.73 0.08 O2 0.23 9.40E5 0.69 NO2 0.11 0.02 0.17 NO3 0.77 1.49E3 0.12 NH4 0.14 0.13 0.35 PO4 0.10 0.03 0.03 SiO4 0.53 0.09 0.35 Al (S) 0.03 0.83 0.05 Mg (S) 0.08 0.88 0.07 Fe (S) 0.08 0.93 3.24E3 Ca (S) 2.35E5 0.94 0.03 Mn (S) 0.94 0.02 0.06 Ba (S) 0.82 0.05 0.13 Ti (S) 0.20 0.98 0.01 P (S) 0.57 0.54 0.19 Cu (S) 0.64 0.46 3.24E3 Ni (S) 0.83 0.23 0.06 Co (S) 0.88 0.03 0.14 Zn (S) 0.16 0.78 0.04 V (S) 0.25 0.89 0.02 Mo (S) 0.40 0.58 0.05 Pb (S) 0.46 0.66 0.09 Cd (S) 0.06 2.68E3 0.09 Corg (S) 0.56 0.45 0.01
Factor 4
Factor 5
Factor 6
Factor 7
0.97 for Cd (S); sampling adequacy: between 0.42 for NO2 0.12 0.02 0.97 0.40 0.04 0.02 0.25 0.88 0.97 0.22 0.09 0.13 0.18 0.11 0.21 0.16 0.11 0.09 0.13 0.04 0.11 0.08 0.03 0.03 0.03 0.03 0.09 0.10 0.03 0.05 0.01 2.40E3 0.11 0.05 0.11 0.11
0.09 0.04 0.02 0.03 0.92 0.03 0.06 0.06 0.02 0.11 0.83 0.19 0.15 0.02 0.12 0.17 0.04 0.13 0.04 0.18 0.12 0.08 0.16 0.08 0.08 0.01 0.08 0.14 0.15 0.06 0.03 0.32 0.20 0.12 0.94 0.31
0.22 0.27 0.03 0.20 0.19 0.02 0.01 1.07E4 0.01 0.81 0.26 0.28 0.24 0.27 0.67 0.17 0.72 0.03 0.39 2.01E3 0.05 0.01 0.04 0.06 0.36 0.10 0.16 4.73E3 0.08 0.23 0.09 0.14 0.24 0.26 0.01 0.11
0.61 0.18 0.05 0.12 0.01 0.13 0.14 0.03 0.10 0.06 0.11 0.67 0.14 0.09 0.51 0.13 0.24 0.86 0.24 0.11 0.22 0.17 0.17 0.08 0.04 0.27 0.08 0.21 0.05 0.38 0.22 0.10 0.01 0.12 0.16 0.30
(Grupe et al., 2001). Ca representing carbonate shows a strong negative loading to this factor. Factor 2 stands for the Mn oxide phase and combines Co, Ba, Mn, Ni, Pb (and P and Cu). In factor 3, Al, Mo, Zn and Cu obviously represent a detrital fraction with a negative relationship to Ba which is probably a biogenic signal. Ba is enriched by some xenophyophoriae (Hopwood et al.,
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1997) that are numerous in the investigated areas and had been collected and analysed (Koschinsky et al., 2001a). Cd and V form another factor which is due to correlating peaks within the profiles, and Mn and Ni have a characteristic relationship with depth in factor 5, which is due to the redox sequence. Analysis of the pore water data (Table 2) shows that the most pronounced feature is represented by the redox sequence with positive factor loadings for depth, nitrite, nitrate, oxygen, ammonia (and Mo), and negative loadings for Mn, silicate and phosphate. Factor 2 represents the striking peaks of Fe, Co, Ni (Zn and Mo) within the profiles. The anionic components Mo, U, V and phosphate, and ammonia are presented by factor 3. Ba, Mn (Cd and V) are grouped by factor 4 that describes biogenic components, and Pb, Cu, Cd and Zn are correlated with oxygen by a fifth factor, which is determined by their increased concentrations in the upper cm of several cores. In order to check if there are definite relationships between dissolved and solid phase parameters, the entire set of pore water and sediment data was next submitted to factor analysis (Table 2). The first factor describes the dominant role of the oxic MnO2-rich surface layer, and nitrate as the only pore water component. Dissolved Mn and silicate have a negative loading of this factor. The second important factor combines the elements present in the detrital clay fractions (dominated by illite and smectite), inversely phased by Ca (as Ca carbonate). Factors 3– 7 describe component groups that have been identified similarly in the separate factor analyses. Except for solid and pore water Mn, no clear relationships between pore water and solid sediment data were found. The relationships are similar to those found in the separate sediment and pore water factor analyses.
4. Discussion The distribution of heavy metals in the solid and pore water phases of the Peru Basin surface sediments is controlled by the composition of the particles (e.g. calcite, clay and Mn oxide portions), the input of particulate and dissolved material from the water column, biogenic and physical redistribution, and (especially important for redox-sensitive metals) diagenetic redistribution according to the model of suboxic diagenesis of Froehlich et al. (1979). The thickness of the different redox zonations can change with time, and the solid element distribution does not always correspond exactly to the present redox zonation. As our sediment profiles from the Peru Basin show, some metals do not follow the strict pattern of redox zonation, as they may have several reaction characteristics and/or only a small fraction may be diagenetically reactive. There are also differences between the cores from the five areas, resulting from local variations of sedimentation or small-scale variability within the areas. Similar differences have been found by Graybeal and Heath (1984) for sediments from two different sites in the eastern Pacific. Surface sediments also can be significantly influenced by microenvironments formed around biogenic structures like burrows and tubes, as shown by Aller and Aller (1986); besides the effect of homogenising the upper cm of the sediment through bioturbation, local microenvironments differing significantly from the regular pattern can be created. The pore water composition is particularly sensitive to environmental variations, while the solid phase has a certain capacity to ‘‘buffer’’ environmental changes. In the following section, we discuss the complex pattern of heavy metal distributions in the Peru Basin surface sediments in the light of: (1) the present-day redox
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zonation (Haeckel et al., 2001), (2) the dynamics of the redox zonation on short-term (seasonal– . interannual) and (3) long-term (glacial–interglacial) time scales (Konig et al., 2001).
4.1. Present-day conditions According to leaching experiments (Koschinsky et al., 2001b), up to 80% of total Mn is diagenetically mobile and, therefore, the Mn distribution perfectly reflects the local redox conditions. The characteristic pattern of Mn distribution around the oxic/suboxic boundary is found in all of the five investigation areas (see also pore water O2 data in Haeckel et al., 2001). According to the leaching experiments, those metals associated with the Mn oxide phase in the surface layer, such as Co, Ni, Cu and Zn, generally follow the solid profile pattern of Mn (which is reflected by a common factor of these elements in the factor analysis). This is especially manifest for Ni in profile 145 MC (Fig. 11); Ni is also the only metal that shows a significant correlation of its dissolved species with dissolved Mn. This close similarity of Ni with Mn in deep-sea sediments also has been found by Heggie et al. (1986) and Thomson et al. (1993). Dissolved Co, Zn and Cu, as well as all other metals, do not show a definite relation to pore water Mn, as is confirmed by the lack of relationship with Mn in the factor analysis (Table 2), although in their solid state most of them are associated with the Mn oxide phase. This behaviour has also been described for Ni, Co and Cu by Jarvis and Higgs (1987) and has been attributed to adsorption of the mobilised metals on organic matter or clay minerals like smectite or immobilisation by Fe compounds. Dissolved Cu, Zn, Ni, Co, Pb, Cd, Fe and V are significantly enriched in the top 1–2 cm of all profiles (which is manifested in a common factor together with oxygen in the factor analysis). These dissolved surface enrichments originate from the degradation and release from organic matter (Sawlan and Murray, 1983; Heggie et al., 1986; Shaw et al., 1990). A certain diffusion into the bottom water can be assumed for these metals, and further below a strong fixation on the Mn oxide phase prevails. Dissolved V is more stable in the oxidised surface layer and is not significantly adsorbed by Mn oxide. Therefore the V peak is wider (0–5 cm depth). In the five ATESEPP areas, early diagenesis presently reaches the states of oxygen consumption, denitrification and Mn(IV) reduction, but there is no evidence of Fe(III) reduction, sulphate reduction and methanogenesis (Fig. 13c; Haeckel et al., 2001). A closer examination of the typical diagenetic Mn depth profiles reveals that the distribution patterns of solid phase Mn on the one hand, and pore water O2 and Mn2+ on the other, are offset in virtually all of the cores (Fig. 12): In core 145 MC, the solid phase Mn is enriched in the uppermost 11 cm of the sediment, and a marked peak of Mn enrichment at the base of this layer must result from an oxic/suboxic boundary at that depth. In contrast, the oxic zone was only 5 cm thick at the moment of core retrieval, and there was release of Mn2+ to the pore water at the depth of the solid phase enrichment peak starting from 6 cm depth. Ni exactly follows this offset observed for Mn (Fig. 12). The same phenomenon is found in core 283 MC with a 6 cm deep oxic layer and a 13 cm thick MnO2 - and Ni-rich layer. Analogously, at the other three investigation sites the surface layer of solid phase Mn enrichment (15–20 cm) was thicker than the oxic layer (5–10 cm) at the moment of core retrieval.
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Fig. 12. Demonstration of the offset of solid Mn and Ni, dissolved Mn and Ni and oxygen concentrations in profiles 145 MC (area 1a) and 283 MC (area 6); solid Mn and Ni concentrations in wt%, dissolved Mn and Ni concentrations in mg/l, and O2 concentrations in mmol/l (taken from Haeckel et al., 2001); (- - -) represents the boundary of oxygen penetration, (. . . . . . . .) represents the depths of the Mn oxide-rich layer.
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Fig. 13. Schematic presentation of the historic development of sediment and pore water metal distributions in Peru Basin surface sediment (area 1a): While during the glacial Mn and Fe reduction took place within the high reactivity surface layer due to the strong depositional flux (a), the Mn and Fe redox boundaries were deeper but coincided at the transition to the interglacial (b) because oxygen and nitrate penetration were both confined to the depth of the reactive Fe(II) layer. Oxic and sub-oxic metal peaks developed along this boundary. During burial of the reactive Fe(II) layer, the sub-oxic enrichments passively moved down with this layer while the oxic metal enrichments remained confined to the oxic layer which is again separated from the nitrate penetration depth (c).
4.2. Short-term fluctuations The offset between oxygen penetration, solid and dissolved Mn, and Ni is explained by vertical oscillations of the oxic/suboxic redox boundary, which are also documented in these sediments by transitional pore water NO 3 profile shapes and the downcore relation between the pore water . profiles and the solid phase Fe(II)/Fe(III) record (Konig et al., 2001). The redox zonation NO 3 fluctuates on the seasonal–interannual time scale due to variations in the depositional flux of organic matter, which relate to the ENSO phenomenon in the eastern tropical Pacific Ocean . (Konig et al., 2001). Also Gobeil et al. (1997) discussed offset distributions of dissolved and solid Mn and the diagenetic separation of Mn and Cd in the light of fluctuations of the redox boundary caused by changes in the flux of organic carbon to the seafloor. They found solid Mn concentrated in a narrow surface layer, often with a pronounced peak at the deepest point of this layer, and low solid Cd concentrations at the surface followed by a Cd peak at about 5–20 cm depth. According to their model, the repeated process of dissolution, migration and reprecipitation of Mn
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transports this element upward and concentrates it into a thin layer at the upper limit of the redox excursion. In the cores from the Peru Basin, such Mn peaks also appear at the bottom of the Mn oxide enrichment (Figs. 1 and 12 and Figs. 3,5,7 and 9), and can be explained by the upward pumping and concentration of Mn in a narrow layer. However, Gobeil et al. (1997) were dealing with continental margin sediments where the oxic zone is underlain by a sulphidic zone in which Cd is precipitated as Cd sulphide. We found only a single peak of Cd at some distance below the oxic zone in core 145 MC from area la (Fig. 11). Whereas in the cores of the other four ATESEPP areas no sulphide was detected, in area 1a (which receives higher organic input) sulphide was . found in measurable concentrations (Konig et al., 1996). Since none of the measured major elements follows the peak of Cd at around 25 cm depth in core 145 MC, Cd sulphide is the most probable form to explain the Cd enrichment. Furthermore, the enrichment of Cu, Ni, Zn, Co, and eventually Pb, below 23 cm depth (Fig. 11), can be explained by the precipitation of sulphides of these metals. Rosenthal et al. (1995) have shown that the precipitation of CdS in suboxic sediments is a very efficient immobilisation mechanism even at trace level sulphide concentrations. Our observations in core 145 MC support the assumption of these authors that this phenomenon is not restricted to Cd, but may also appear for other chalcophilic metals (Cu, Zn, Pb, and possibly Co and Ni). 4.3. Long-term geochemical evolution Distinct peaks of solid phase metal concentrations such as in the downcore patterns of Cu, V and Cd in core 145 MC (Fig. 11) have been described by several authors as resulting from the slow and continual downcore expansion of the oxic zone in pelagic surface sediments (Colley et al., 1984; Wilson et al., 1986; Jarvis and Higgs, 1987; Thomson et al., 1993, 1998). These authors describe heavy metal (im)mobilisations as the results of local reducing conditions that are produced by turbidites in a previously oxic sediment column. The turbidites form a geochemical barrier for the diffusion of oxidising components (oxygen and nitrate) into deeper layers. The different characteristics of the heavy metals within the diagenetic sequence lead to peaks in the sediment in the following succession from oxic to post-oxic: Mn with Ni, Cu, Co and Zn, followed by Fe, Cu, Cd, Pb, V, Zn, U (Thomson et al., 1993, 1998). In contrast to Mn and associated metals, U and V are mobilised under oxic conditions as U(VI) and V(V), diffuse downward, and are fixed under reducing conditions as U(IV) and V(IV) oxides. Besides this anoxic enrichment, elevated V in the oxic section was attributed to a certain V surface complexation by sediment phases. The sequence of metal enrichment observed for core 145 MC (Fig. 11) is very similar to those observed for the redox boundaries produced by organic-rich turbidites. Though we can exclude the existence of turbidites in our samples, a similar geochemical barrier is present by . reoxidisable Fe(II) in clay minerals in the sediment of area 1a (Konig et al., 1997). These similarities in metal distributions between the sediments characterised by turbidites and the Peru Basin sediments indicate that different evolutational histories can produce similar geochemical effects. . According to the geochemical model of the Peru Basin deep-sea floor by Konig et al. (2001), a continual downcore expansion of the oxic zone has occurred in Peru Basin surface sediments during glacial/interglacial transitions in response to decreasing depositional fluxes of organic matter (Fig. 13). Changing sedimentation signals from glacial to postglacial have already been
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reported by Thomson et al. (1984). Moreover, previous investigations of the Peru Basin bulk sediment Fe(II)/Fe(III) record have shown that the diagenetic stage of Fe reduction was reached in the sea floor at investigation site ATESEPP-1a (the site of core 145 MC), and large fractions of . the structural Fe in deep-sea clays were chemically reduced (Konig et al., 1997, 1999). Thus reactive Fe(II) layers (RFe(II)Ls), i.e. layers rich in reoxidisable clay Fe(II) that act as geochemical barriers with respect to pore water O2 and NO 3 ; were formed below the uppermost . (o5 cm) layers of the sea floor (Konig et al., 1999). Therefore, the expansion of the oxic zone was delayed by the burn-down process of the RFe(II)L until the oxic zone had reached its post-glacial . equilibrium width (Konig et al., 2001). The slow downcore migration of the oxic/suboxic boundary into the RFe(II)L resulted in the formation of solid phase Cd and other metal peaks by presumably the same mechanism as described by Rosenthal et al. (1995) for the burn-down of the oxic/suboxic boundary in northeast Atlantic turbidites. Thomson et al. (1996) documented a characteristic sequence of metal enrichment peaks, namely Mn+Mo, Fe+As+P, Se+V, (Cd), and U in downcore succession, and interpreted this as reconcentrations of a diagenetic fraction of each element on a redox gradient around the active oxic/suboxic boundary. The prolonged near-stasis of the redox boundary, a necessary prerequisite for peak formation, has in their case also been caused by the change in sediment accumulation regime between glacial and interglacial times. At greater depth in the sediment sequence, Thomson et al. (1996) found only the suboxic peak fraction (Se, V, Cd and U) left at a previous glacial/interglacial transition (oxygen isotope stages 6/5), due to the fact that the oxic peak fraction (Mn, Mo and Fe) had moved upcore along with the oxic zone. Also in our core 145 MC, the suboxic fraction (Cu, V and Cd) is separated from the . oxic fraction (Mn and Mo, Fig. 3 and Fig. 11). This is in accordance with the model of Konig et al. (2001) which claims that the post-glacial equilibrium width of the oxic zone (ca. 10 cm) was reached several thousand years ago. Since then, the Cu, V and Cd concentration peaks have been detached from the oxic/suboxic boundary and therefore remained unchanged, and their downcore movement has only been due to burial (Fig. 13). Presently there is no active redox boundary at the depth of the peaks, and there is no indication that the diagenetic stage of Fe reduction is reached.
4.5. Consequences of a strong disturbance of the surface sediment In case of a commercial use of the deep-sea such as manganese nodule mining, at least 10–20 cm of the surface sediment would be heavily disturbed or removed. This would affect the whole oxic Mn-rich surface layer of the Peru Basin sediments. Similar effects could be produced by natural events like turbidites or slumps. Periodically appearing ‘‘benthic storms’’ with current velocities of up to 35 cm/s (Gross et al., 1988) also can remove parts of the sediment surface. In a worst-case scenario disturbance, the interface of oxic seawater and oxic surface sediment would be substituted by an interface between the oxic seawater and suboxic sediment. When no Mn oxiderich surface layer is present, scavenging of upward diffusing metals like Mn, Co, Ni, Zn and Cu is reduced and significant loss of dissolved pore water components into the bottom water could occur. The ratio of lost-to-bottom-water and scavenged-on-particles metal rates will be influenced by the steepness of the redox gradients present in the new situation and the geochemical and mineralogical composition of the sediment (see also Fritsche et al., 2001).
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Penetration of oxygen from the oxygen-rich bottom water will initiate Mn precipitation and . coprecipitation or sorption of associated metals like Ni and Co. Model calculations by Konig et al. (2001) have shown that oxygen diffusion would be fast and intense. However, diffusion experiments with oxic and suboxic sediment layers and heavy metal solutions and modelling of Mn fluxes after the disturbance have shown that significant amounts of Mn can still diffuse into the overlying water despite the oxic character of the water (Fritsche et al., 2001). Since Mn redox reactions are strongly kinetically controlled (Grill, 1982) and the input on the new sediment surface (resedimentation of suspended sediment or input of fresh organic matter) is an unknown but decisive factor, it is difficult to make a clear statement on the time scale for the formation of a new Mn oxide-rich surface layer. Mn oxidation is probably especially slow in the deep sea due to the low temperature and low microbial activity compared to shallow marine systems. From our experimental results, it appears that the presence of Mn oxide is an important component for scavenging Mn (possibly via autocatalytic oxidation); the absence of Mn oxide leads to a slower and less efficient removal of Mn and other metals from the oxic seawater phase (Koschinsky et al., 2001b). The formation of a Mn oxide-rich surface layer comparable to the original oxic surface layer, that acts as an efficient barrier for Mn diffusion, will probably require much more time than diffusion of oxygen into the formerly suboxic sediment. In ATESEPP area 1a, oxygen would penetrate the sediment to about 20 cm depth (compared to 5 cm at present), taking into account that the reactive Fe(II) in the clay structures below 20 cm . depth forms a geochemical barrier (Konig et al., 2001). This would also restrict the diagenetic reactions as well as upward and downward migration of the redox-sensitive metals in the deeper sediment layers. In ATESEPP area 2, oxygen would even penetrate between 125 and . 150 cm deep into the sediment, compared to about 10 cm at present (Konig et al., 2001). This drastic enlargement of the oxic surface layer would be followed by precipitation of Mn oxide and associated metals in this layer, fed from the high pore water concentrations of Mn in the formerly post-oxic sediment and diffusive transport from deeper sediment layers (Fritsche et al., 2001). This upward diffusion of dissolved metals into the new oxic layer would probably also initiate the slow formation of new manganese nodules. Post-oxic enriched metals like Cd, V and U (and eventually including some Zn, Cu, Co, Ni and Pb) would redissolve in the newly oxic environment, diffuse downward and precipitate in deeper post-oxic layers. Probably proportions of the remobilised metals also would move upward, be fixed in the Mn oxide-rich layer and/or diffuse into the bottom water. If also the upper part of the sub-oxic layer is resuspended, stronger metal release, especially of Mn, from this material has to be taken into account. More probable than the complete removal of the sediment surface layer during deep-sea mining, however, is a strong intermixing of the oxic surface layer, eventually with more consolidated parts of deeper sediment, and the coverage of the surface with resedimented oxic sediment. The uplifting of sub-oxic or anoxic sediment into the oxic surface layer would initiate a succession of redox reactions, and oxidation of the sub-oxic sediment would be promoted by the input of oxygen-rich bottom water during the disturbance and by the increased diffusive oxygen penetration through the poorly compacted resedimented layer. Due to the roughness of the mixed and resedimented material, the active surface for material exchange between bottom water and sediment would be increased. The reactions of the heavy metals might be similar to that described for the worst-case scenario, but less pronounced.
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5. Conclusions The surface sediment of the Peru Basin in its present state is represented by a 5–20 cm thick Mn oxide-rich top layer associated with increased Ni, Co, Mo and sometimes also Zn, Cu, Pb and Cd concentrations. The pore water can have significant metal peaks in the uppermost 1–2 cm which result from the degradation of organic matter (Fig. 13c). At present, the 20–30 cm core length covers only the Mn redox boundary in the diagenetic sequence. An offset of several cm between the depths of oxygen penetration and increase of dissolved Mn and Ni concentrations on the one hand (which at present is at 5–10 cm), and the thickness of the Mn oxide- and Ni-rich sediment layer (10–20 cm) indicates that the oxic/sub-oxic redox boundary oscillates on a short-term time scale. Only Ni more or less follows the strong increase in concentrations of dissolved Mn below the Mn redox boundary. Co, Cu, Zn, Mo, Cd and Pb are obviously re-immobilised readily after release from the Mn oxide fraction by sorption on organic, clay or Fe compounds. A strong enrichment of these metals and V was observed below 22 cm in core 145 MC from area 1a, where the ancient Fe redox front is reached at 20–22 cm depth. Since the diagenetic stage of Fe reduction is not reached at present in the Peru Basin sediments, the formation of these peaks can only be explained in a historic context, as depicted in Fig. 13. In case of a strong disturbance of the surface sediment as during manganese nodule mining, the present redox situation would be destroyed. The zone with oxic metal enrichments would be resuspended, removed and eventually resedimented, but metal release from the oxic sediment is improbable. Loss of dissolved heavy metals from the sub-oxic pore water to the bottom water can . occur until a new and stable oxic front at the sediment surface has formed. According to Konig et al. (2001), oxygen would proceed deep into the sediment layer within a few years, unless the ancient Fe redox boundary with reoxidisable Fe(II) in clay structures prohibits further oxygen penetration. From the surface downwards, slow formation of MnO2 associated with proportions of Co, Ni, Mo and other metals would start. As long as the redox boundary moves, V, Cd, U and other peaks formed in the post-oxic environment would dissolve in the newly oxic environment and follow the redox boundary downward to reprecipitate in deeper layers.
Acknowledgements Research cruise SO 106, during which the sediment cores were sampled, and the project GECOMET (project no. 03 G 0106 G) were funded by the German Bundesministerium fur . Bildung und Forschung. The responsibility for the contents of this manuscript is with the author. I thank C. Arndt, M. Weigelt and B. Alberts for their analytical contributions, and H. Thiel for comments on the manuscript. M. Haeckel provided his pore water data of Mn, Fe and Ba in the multicorer samples. This manuscript benefitted substantially from the co-operation with the . FEMILIEU project (no. 03 G 0106 H) and intense discussions with I. Konig on the interpretation of the heavy metal distributions in the light of her model developed within the ATESEPP research programme. I would like to express my special thanks to her. Also the helpful reviews of J. Thomson and T. Kuhn and the English corrections of E. Foell in the final version greatly improved the manuscript.
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Appendix Heavy metal concentrations in pore water and in sediment presented in the text also can be found in Annex 2.10 on CD-Rom: Fig. 3: Fig. 4: Fig. 5: Fig. 6: Fig. 7: Fig. 8: Fig. 9: Fig. 10:
Bulk sediment major and minor/trace element concentrations versus depth in core 145 MC from area 1a; Heavy metal pore water concentrations versus depth in core 145 MC from area 1a; Bulk sediment major and minor/trace element concentrations versus depth in core 245 MC from area 1b; Heavy metal pore water concentrations versus depth in core 245 MC from area 1b; Bulk sediment major and minor/trace element concentrations versus depth in core 275 MC from area 2; Heavy metal pore water concentrations versus depth in core 275 MC from area 2; Bulk sediment major and minor/trace element concentrations versus depth in core 166 MC from area 5; Heavy metal pore water concentrations versus depth in core 166 MC from area 5.
The results of the principal component analyses of the pore water data, solid sediment data, and the complete data set of pore water and sediment samples (Table 2) are also presented in Annex 2.10 on CD-Rom.
References Aller, J.Y., Aller, R.C., 1986. Evidence for localized enhancement of biological activity associated with tube and burrow structures in deep-sea sediments at the HEBBLE site, western North Atlantic. Deep-Sea Research I 33, 755–790. Balistrieri, L.S., Murray, J.W., 1986. The surface chemistry of sediments from the Panama Basin: the influence of Mn oxides on metal adsorption. Geochimica et Cosmochimica Acta 50, 2235–2243. Bonatti, E., Fisher, D.E., Joensuu, O., Rydell, H.S., 1971. Postdepositional mobility of some transition elements, phosphorus, uranium and thorium in deep sea sediments. Geochimica et Cosmochimica Acta 35, 189–201. Colley, S., Thomson, J., Wilson, T.R.S., Higgs, N.C., 1984. Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the North East Atlantic. Geochimica et Cosmochimica Acta 48, 1223–1235. Fritsche, U., Koschinsky, A., Winkler, A., 2001. The different diffusive transport behaviours of some metals in layers of Peru Basin surface sediments. Deep-Sea Research II 48, 3653–3681. Froehlich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B., Maynard, V., 1979. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochimica et Cosmochimica Acta 43, 1075–1090. Gobeil, C., MacDonald, R.W., Sundby, B., 1997. Diagenetic separation of cadmium and manganese in suboxic continental margin sediments. Geochimica et Cosmochimica Acta 61, 4647–4654. Graybeal, A.L., Heath, G.R., 1984. Remobilization of transition metals in surficial pelagic sediments from the eastern Pacific. Geochimica et Cosmochimica Acta 48, 965–975. Grill, E.V., 1982. Kinetic and thermodynamic factors controlling manganese concentrations in oceanic waters. Geochimica et Cosmochimica Acta 46, 2435–2446. Gross, T.F., Williams, A.J., Grant, W.D., 1988. A deep-sea sediment transport storm. Nature 331, 421–518. Grupe, B., Becker, H.J., Oebius, H.U., 2001. Geotechnical and sedimentological investigations of deep-sea sediments from a manganese nodule field of the Peru Basin. Deep-Sea Research II 48, 3593–3608.
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. Haeckel, M., Konig, I., Riech, V., Weber, M.E., Suess, E., 2001. Pore water profiles and numerical modeling of biogeochemical processes in Peru Basin deep-sea sediments. Deep-Sea Research II 48, 3713–3736. Heggie, D., Kahn, D., Fischer, K., 1986. Trace metals in metalliferous sediments, MANOP site M: interfacial pore water profiles. Earth and Planetary Science Letters 80, 106–116. Hopwood, J.D., Mann, S., Gooday, A.J., 1997. The crystallography and possible origin of barium sulfate in deep sea rhizopod protists (Xenophyophoria). Journal of the Marine Biological Association, United Kingdom 77, 969–987. Jarvis, I., Higgs, N., 1987. Trace-element mobility during early diagenesis in distal turbidites: late Quaternary of the Madeira Abyssal Plain, N Atlantic. In: Weaver, P.P.E., Thomson, J. (Eds.), Geology and Geochemistry of Abyssal Plains. Geological Society Special Publications no. 31, Oxford, pp. 179–213. Klinkhammer, G.P., 1980. Early diagenesis in sediments from the eastern equatorial Pacific II. Pore water metal results. Earth and Planetary Science Letters 49, 81–101. . Konig, I., D.ahlmann, A., Domeyer, B., Greinert, J., Haeckel, M., Siemensen, M., 1996. FEMILIEU. In: Schriever, G., Koschinsky, A., Bluhm, H. (Eds.), Cruise Report ATESEPP: Impacts of Potential Technical Interventions on the Deep-sea Ecosystem of the Southeast Pacific off Peru. Berichte aus dem Zentrum fur . Meeres- und Klimaforschung, Reihe E, pp. 155–169. . Konig, I., Drodt, M., Suess, E., Trautwein, A.X., 1997. Iron reduction through the tan-green color transition in deepsea sediments. Geochimica et Cosmochimica Acta 61, 1679–1683. . Konig, I., Haeckel, M., Drodt, M., Suess, E., Trautwein, A.X., 1999. Reactive Fe(II) layers in deep-sea sediments. Geochimica et Cosmochimica Acta 63, 1517–1526. . Konig, I., Haeckel, M., Lougear, A., Suess, E., Trautwein, A.X., 2001. A geochemical model of the Peru Basin deep-sea floor – and the response of the system to technical impacts. Deep-Sea Research II 48, 3737–3756. Koschinsky, A., 1996. GECOMET. In: Schriever, G., Koschinsky, A., Bluhm, H. (Eds.), Cruise Report ATESEPP: Impacts of Potential Technical Interventions on the Deep-sea Ecosystem of the Southeast Pacific off Peru. Berichte aus dem Zentrum fur . Meeres- und Klimaforschung, Reihe E, pp. 131–154. Koschinsky, A., Borowski, C., Halbach, P., 2001a. Reactions of the heavy metal cycle on industrial activities in the deep sea: an ecological assessment. International Review of Hydrobiology, submitted. Koschinsky, A., Fritsche, U., Winkler, A., 2001b. Sequential leaching of Peru Basin surface sediment for the assessment of aged and fresh metal associations and mobility. Deep-Sea Research II 48, 3683–3699. Koschinsky, A., Gaye-Haake, B., Arndt, C., Maue, G., Spitzy, A., Winkler, A., Halbach, P., 2001c. Experiments on the influence of sediment disturbance on the biogeochemistry of the deep-sea envionrment. Deep-Sea Research II 48, 3629–3651. Marchig, V., von Stackelberg, U., Hufnagel, H., Durn, G., 2001. Compositional changes of surface sediments and variability of manganese nodules in or Peru Basin. Deep-Sea Research II 48, 3523–3547. Rosenthal, Y., Lam, P., Boyle, E.A., Thomson, J., 1995. Authigenic cadmium enrichments in suboxic sediments: precipitation and postdepositional mobility. Earth and Planetary Science Letters 132, 99–111. Sawlan, J.J., Murray, J.W., 1983. Trace metal remobilization in the interstitial waters of red clay and hemipelagic marine sediments. Earth and Planetary Science Letters 64, 213–230. Schriever, G., Koschinsky, A., Bluhm, H., 1996. Cruise Report ATESEPP: Impacts of Potential Technical Interventions on the Deep-sea Ecosystem of the Southeast Pacific off Peru. Berichte aus dem Zentrum fur . Meeresund Klimaforschung, Reihe E, 195pp. Shaw, T.J., Gieskes, J.M., Jahnke, R.A., 1990. Early diagenesis in differing depositional environments: the response of transition metals in pore water. Geochimica et Cosmochimica Acta 54, 1233–1246. Thiel, H., 2001. Use and protection of the deep seaFan introduction. Deep-Sea Research II 48, 3427–3431. Thiel, H., Forschungsverbund Tiefsee-Umweltschutz, 2001. Evaluation of the environmental consequences of polymetallic nodule mining based on the results of the TUSCH Research Association. Deep-Sea Research II 48, 3433–3452. Thomson, J., Wilson, T.R.S., Culkin, F., Hydes, D.J., 1984. Non-steady state diagenetic record in eastern equatorial Atlantic sediments. Earth and Planetary Science Letters 71, 23–30. Thomson, J., Higgs, N.C., Croudace, I.W., Colley, S., Hydes, D.J., 1993. Redox zonation of elements at an oxic/postoxic boundary in deep-sea sediments. Geochimica et Cosmochimica Acta 57, 579–595.
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Thomson, J., Higgs, N.C., Colley, S., 1996. Diagenetic redistribution of redox-sensitive elements in northeast Atlantic glacial/interglacial transition elements. Earth and Planetary Science Letters 139, 365–377. Thomson, J., Jarvis, I., Green, D.R.H., Green, D.A., Clayton, T., 1998. Mobility and immobility of redox-sensitive elements in deep-sea turbidites during shallow burial. Geochimica et Cosmochimica Acta 62, 643–656. Weber, M.E., von Stackelberg, U., Marchig, V., Wiedicke, M., Grupe, B., 2000. Variability of surface sediment in the Peru Basin: dependence on water depth, productivity, bottom water flow, and seafloor topography. Marine Geology 163, 169–184. Wilson, T.R.S., Thomson, J., Hydes, D.J., Colley, F., Culkin, F., S"ırensen, J., 1986. Oxidation fronts in pelagic sediments. Diagenetic formation of metal rich layers. Science 232, 972–975.
Heavy metal distributions in Peru Basin surface sediments in relation to historic, present and disturbed redox environments Andrea Koschinsky* Freie Universitat . Berlin, FR Rohstoff- und Umweltgeologie, Malteserstr. 74-100, D-12249 Berlin, Germany
Abstract Heavy metal distributions in deep-sea surface sediments and pore water profiles from five areas in the Peru Basin were investigated with respect to the redox environment and diagenetic processes in these areas. The 10–20-cm-thick Mn oxide-rich and minor metal-rich top layer is underlain by an increase in dissolved Mn and Ni concentrations resulting from the reduction of the MnO2 phase below the oxic zone. The mobilised associated metals like Co, Zn and Cu are partly immobilised by sorption on clay, organic or Fe compounds in the post-oxic environment. Enrichment of dissolved Cu, Zn, Ni, Co, Pb, Cd, Fe and V within the upper 1–5 cm of the oxic zone can be attributed to the degradation of organic matter. In a core from one area at around 22–25 cm depth, striking enrichments of these metals in dissolved and solid forms were observed. Offset distributions between oxygen penetration and Mn reduction and the thickness of the Mn oxide-rich layer indicate fluctuations of the Mn redox boundary on a short-term time scale. Within the objectives of the German ATESEPP research programme, the effect of an industrial impact such as manganese nodule mining on the heavy metal cycle in the surface sediment was considered. If the oxic surface were to be removed or disturbed, oxygen would penetrate deep into the formerly suboxic sediment and precipitate Mn2+ and metals like Ni and Co which are preferably scavenged by MnO2. The solid enrichments of Cd, V, and other metals formed in post-oxic environments would move downward with the new redox boundary until a new equilibrium between oxygen diffusion and consumption is reached. r 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction The present work is part of the ATESEPP (impacts of potential technical interventions on the deep-sea ecosystem of the southeastern Pacific off Peru) research programme. During research *Fax: +49-030-83870742. E-mail address: [email protected] (A. Koschinsky). 0967-0645/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 6 6 - 2
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cruise SO 106 in the Peru Basin (January/February 1996) sediment cores were sampled from five selected sites around 4000 m water depth between 5–71S and 85–911W (Schriever et al., 1996; Thiel and Forschungsverbund Tiefsee-Umweltschutz, 2001) with the intention to include different depositional environments in this region. As a consequence of variations of organic matter flux in the five ATESEPP areas, differing depths of redox zonations in the upper layers of the sediments were expected. For most heavy metals associated with the sediments, redox conditions are the most important determining factor for how the metals are distributed between the solid and dissolved phases and whether they can mobilise and migrate in the pore waters. In an early work, Bonatti et al. (1971) had presented diagenetic metal distributions in a hemipelagic core at 21450 N and 851200 W, which is somewhat north of the ATESEPP areas. They reported Mn, Ni and Co enrichments in the upper oxic sediment layer and Cr, V and U enrichments in the lower reduced zone. The general geochemical and mineralogical variability of surface sediment in the Peru Basin was shown in previous and recent work (Schriever et al., 1996; Weber et al., 2000; Marchig et al., 2001). The upper layer of the Peru Basin deep-sea sediment is characterised by Mn oxide in wt% concentrations. It is well known from other investigations that Mn oxide forms the carrier phase for many other heavy metals like Co and Ni (Balistrieri and Murray, 1986; Koschinsky, et al., 2001b). With increasing depth, the oxic zone changes to suboxic, causing reduction and dissolution of Mn oxide. This phenomenon is reflected in the sediment and pore water profiles of Mn and other heavy metals. In order to determine the magnitude of dissolved and particulate heavy metal concentrations in the Peru Basin surface sediments, one multicorer (MC) sample was chosen from each of the five sites mentioned above that had also been investigated for dissolved oxygen, anion and ammonia concentrations for redox characterisation (Haeckel et al., 2001). The distribution of Mn, Fe and other heavy metals (Co, Ni, Cu, Zn, Pb, Cd, Ba, Mo, V and U) in the sediment column and associated pore waters was studied in the upper 30 cm of the MC cores. Knowledge of the diagenetic situation and redox environment in the Peru Basin was used to discuss potential effects of an industrial-scale disturbance of the system such as manganese nodule mining. In the normal deep-sea environment, a quasi-equilibrium between the bottom water and the sediment surface with its diagenetic processes has been reached. However, if as a result of a naturally or technically induced disturbance, the upper sediment layer is removed and resuspended into the bottom water, this equilibrium would be disturbed and heavy metals in their dissolved form and bound in or on the particulate sediment phases would be discharged. The fate of the heavy metals in the surface . sediment after disturbance is discussed in the light of the modelled scenarios presented by Konig et al. (2001).
2. Analytical methods and materials 2.1. Pore water analyses Sampling of pore waters during the SO 106 research cruise has been described by Haeckel et al. (2001). Pore water samples and the respective solid sediment samples were taken at approximately 1-cm intervals. Dissolved trace metals (Zn, Cd, Pb, Cu, Co, Ni, Mo, V and U) in the acidified (pH 2, HC1 Suprapure) pore water samples were determined by sensitive stripping voltammetric
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methods described by Koschinsky et al. (2001c). Ba concentrations were measured by ICP-AES. Fe and Mn data were taken from Haeckel et al. (2001). 2.2. Sediment analyses A 0.2 g sample of homogenised, ground and oven-dried (1051C) sediment was digested in Teflon pressure bombs at 1801C with a mixture of 3 ml HF (48%)+3 ml HCl (30%)+1 ml HNO3 (65%). After evaporating twice with diluted nitric acid, the residue was taken up in 0.5 M nitric acid to a total volume of 50 ml. Al, Mg, Ca, Fe, Mn, P, Ba, Sr, Ti, Ni, Co, Zn, Cu, V and Mo were measured with a Leeman ICP-OES spectrometer. Cd and Pb were measured by graphite furnace atomic absorption spectrometry with a Perkin Elmer instrument. Unfortunately, no reliable data for U were obtained with the methods available. 2.3. Sample material A detailed description of the sediment characteristics from the five selected ATESEPP areas (Table 1 and map, see Thiel, 2001) has been given elsewhere (Schriever et al., 1996; Weber et al., 2000; Haeckel et al., 2001; Marchig et al., 2001). Oxygen penetration depths between 5 cm (ATESEPP areas 1a, 1b, 5, and 6) and 10 cm (ATESEPP area 2), and nitrate penetration depths varying between 20 cm (area 1a) and 250 cm (area 2; Haeckel et al., 2001) reveal the range of oxic and suboxic conditions in the cores from the five areas.
3. Results 3.1. Description of the sediment and pore water profiles The major elements in all five sediment cores are Ca (up to 20%, often low in the upper 10 cm and then increasing below that), and Fe and Al in the range of 1–5 wt%. Si was not determined. Mn lies mostly between 1.5 and 2 wt% in the upper MnO2-rich layer and is constantly lower than 0.5% below this layer, representing a rather constant level of Mn in detrital minerals or Mn carbonate. Mg, Ba, Ti and P (all typically below 1 wt%) have rather uniform concentration profiles in all cores. In contrast, all minor metals show a strong small-scale variability that mostly correlates either with Mn (association with the Mn oxide phase) or with Al (possibly indicating
Table 1 Location of the sediment cores from the Peru Basin Area 1a 1b 2 5 6
Core no. 145 245 275 166 283
MC MC MC MC MC
Water depth (m) 4084 4107 4152 4128 3940
Latitude 0
5130.03 S 6134.270 S 7104.290 S 6134.060 S 5106.100 S
Longitude 85122.360 W 86111.410 W 88127.920 W 90127.980 W 90114.840 W
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Fig. 1. Bulk sediment major and minor/trace element concentrations versus depth in core 283 MC from area 6 (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
terrigenous input control). Organic carbon ranges between 0.3 and 1.3 wt% (see depth profiles by Haeckel et al., 2001). Core 283 MC of area 6 (Fig. 1) represents a typical example; MnO2 reduction appears to be complete below 13 cm, with Cu, Ni, Mo, Cd (and Co) decreasing at this depth as well. In the pore water samples, heavy metal concentrations were always significantly higher than bottom water concentrations (which are at the sub- or lower ppb level), except for Mo, V and U which were in the range of bottom water concentrations of a few ppb (Koschinsky, 1996). This is consistent with other investigations of deep-sea sediment pore waters (e.g., Klinkhammer, 1980; Sawlan and Murray, 1983; Shaw et al., 1990). Mn2+ concentrations below the Mn redox boundary were several ppm. Zn, Cu and Ni ranged between 5 and 100 ppb, with a few isolated peaks of higher concentrations. Cd and Pb mostly showed values between 0.5 and 10 ppb. Lowest concentrations between 0.01 and 6 ppb were measured for Co. Mo, V and U mostly lay between 1 and 20 ppb, with a few exceptions, especially for V, that exceeded 100 ppb. Many metals are often enriched in the upper cm and at isolated points within the profile. Ba is the element with the least variation with concentrations between 30 and 50 ppb. In the example of 283 MC (Fig. 2), the Mn profile shows a regular steep increase between 7 and 11 cm, correlating only with Ni, Mo and V values are several times higher in the upper 10 cm than in the lower part. A strong correlation of Pb and Cd is found throughout the profile. Distinct metal peaks attract notice in the upper cm of the profile (Fig. 2). The sediment and pore water profiles of the other areas (Figs. 3–10) demonstrate a certain variability between the different locations, like different thicknesses (between 10 and 20 cm) of the Mn oxide-rich layer, but more or less follow the same general pattern. Only 145 MC from area 1a shows a marked additional metal enrichment in both solid and pore water phases in the lower part of the profile (Fig. 11). Below the rather uniform middle part of the profile, at around 25 cm there occur local solid phase maxima for some elements that are most pronounced for Cu, V and Cd. Cd concentrations rise up to 50 ppm within a broad peak. This striking variation is not reflected in any of the other major elements measured. Around 22 cm, there are also very sharp increases of pore water Fe, Zn, Cu, Ni and Co, and Mn also shows small peaks superimposed on the regular profile. For pore water Cd and V, the somewhat wider peaks occur around 25 cm, and Mo slowly increases below 25 cm. We interpret these metal enrichments in core
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Fig. 2. Heavy metal pore water concentrations versus depth in core 283 MC from area 6 (concentrations given in ppb=mg/l).
Fig. 3. Bulk sediment major and minor/trace element concentrations versus depth in core 145 MC from area 1a (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
Fig. 4. Heavy metal pore water concentrations versus depth in core 145 MC from area 1a (concentrations given in ppb=mg/l).
145 MC as further diagenetic signals that are restricted to area la; they were not detected in the upper 30 cm of the cores from the other four areas, which are located further off the South American continent.
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Fig. 5. Bulk sediment major and minor/trace element concentrations versus depth in core 245 MC from area 1b (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
Fig. 6. Heavy metal pore water concentrations versus depth in core 245 MC from area 1b (concentrations given in ppb=mg/l).
Fig. 7. Bulk sediment major and minor/trace element concentrations versus depth in core 275 MC from area 2 (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
3.2. Statistical evaluation of relationships To get an overview of the relationships between the large number of parameters determined, we performed a factor analysis with the Stat View 4.5 software for Macintosh. Data for dissolved
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Fig. 8. Heavy metal pore water concentrations versus depth in core 275 MC from area 2 (concentrations given in ppb=mg/l).
Fig. 9. Bulk sediment major and minor/trace element concentrations versus depth in core 166 MC from area 5 (concentrations given in wt% and ppm=mg/kg, presented on a carbonate-free basis).
Fig. 10. Heavy metal pore water concentrations versus depth in core 166 MC from area 5 (concentrations given in ppb=mg/l).
oxygen, nitrate, nitrite, phosphate and silicate were taken from Haeckel et al. (2001). After checking the suitability of the data set for factor analysis, a principal component analysis with Varimax rotation was run. An eigenvalue o1 was chosen as the break-off test for the factors. Separate analysis of the sediment data (Table 2) grouped Ti, Fe, V, Mg, Pb (and P) into one factor. This probably reflects the high abundance of smectite of up to 65%
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Fig. 11. Comparison of pore water and bulk sediment metal concentrations in profile 145 MC; sediment data (given in wt% or ppm=mg/kg) are presented on a carbonate-free basis to exclude effects by carbonate dilution. Pore water data . are given in ppb=mg/l. The ancient Fe redox boundary was determined at about 20–23 cm (Konig et al., 1997).
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Table 2 Factor loadings of principal component analyses of the pore water data, solid sediment data, and the complete data set of pore water and sediment samples; PW=pore water, S=sediment Oblique solution primary pattern matrix Factor 1
Factor 2
Factor 3
Factor 4
Factor 5
Factor 6
Factor 7
Pore water data Communality: final estimate between 0.73 for Pb and 0.93 for Fe; sampling adequacy: between 0.44 for Cd and 0.80 for O2; sum of variance prop.=0.82 Depth 0.81 0.08 0.35 0.03 0.09 Mn (PW) 0.68 0.01 0.16 0.61 0.19 Fe (PW) 6.92E4 0.96 0.11 0.05 0.10 Zn (PW) 0.04 0.48 0.04 0.31 0.66 Cd (PW) 0.31 0.18 0.16 0.41 0.70 Pb (PW) 0.27 0.01 1.76E4 0.06 0.77 Cu (PW) 0.29 0.33 0.05 0.02 0.75 Ni (PW) 0.17 0.86 0.01 0.26 0.03 Co (PW) 0.03 0.96 0.03 0.02 0.02 Mo (PW) 0.46 0.41 0.85 0.07 0.26 V (PW) 0.10 0.03 0.58 0.42 0.35 U (PW) 0.30 0.04 0.85 0.13 0.02 Ba (PW) 0.02 0.09 0.01 0.89 7.35E4 O2 0.63 0.05 0.03 0.10 0.52 NO2 0.81 0.05 0.05 0.38 0.01 NO3 0.70 0.18 0.15 0.38 0.05 NH4 0.62 2.23E4 0.63 0.11 0.13 PO4 0.51 0.11 0.65 0.12 0.04 SiO4 0.84 0.07 0.08 0.19 0.18 Sediment data Communality final estimate between 0.83 for Cu and 0.97 for Ti; sampling adequacy: between 0.67 for Mn and 0.92 for Mg (only Cd 0.38); sum of variance prop.=0.89 Depth 0.03 0.19 0.05 0.22 1.00 Al (S) 0.08 0.13 0.97 0.05 0.12 Mg (S) 0.86 0.07 0.15 0.05 0.31 Fe (S) 1.02 0.09 0.12 0.12 0.09 Ca (S) 1.10 0.11 0.19 0.10 4.15E4 Mn (S) 0.38 0.54 0.01 0.15 0.68 Ba (S) 0.12 0.95 0.54 0.24 0.03 Ti (S) 1.16 0.40 0.13 0.01 0.10 P (S) 0.46 0.47 5.27E4 0.15 0.17 Cu (S) 0.12 0.37 0.45 0.12 0.17 Ni (S) 0.04 0.54 2.59E4 0.20 0.60 Co (S) 0.36 1.02 0.14 0.03 0.05 Zn (S) 0.39 0.10 0.67 0.04 0.16 V (S) 0.93 0.40 3.16E4 0.54 0.04 Mo (S) 2.81E4 0.04 0.73 0. 05 0.35 Pb (S) 0.59 0.56 0.07 0.05 0.30 Cd (S) 0.01 0.15 0.08 0.97 0.07
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Table 2 (continued) Oblique solution primary pattern matrix Factor 1
Factor 2
Factor 3
Complete data set (sediment and pore water data) Communality: final estimate between 0.72 for Mo (S) and (PW) and 0.82 for Ca (S); sum of variance prop. =1.00 Depth 0.41 0.03 0.22 Mn (PW) 0.69 0.31 0.07 Fe (PW) 0.07 0.02 0.12 Zn (PW) 0.04 0.26 0.72 Cd (PW) 1.49E3 0.15 0.21 Pb (PW) 0.12 0.11 0.88 Cu (PW) 0.02 0.09 0.83 Ni (PW) 0.20 0.15 0.03 Co (PW) 2.06E3 0.04 0.03 Mo (PW) 0.11 0.13 0.08 V (PW) 0.08 0.06 1.72E3 U (PW) 0.27 0.10 0.08 Ba (PW) 0.26 0.73 0.08 O2 0.23 9.40E5 0.69 NO2 0.11 0.02 0.17 NO3 0.77 1.49E3 0.12 NH4 0.14 0.13 0.35 PO4 0.10 0.03 0.03 SiO4 0.53 0.09 0.35 Al (S) 0.03 0.83 0.05 Mg (S) 0.08 0.88 0.07 Fe (S) 0.08 0.93 3.24E3 Ca (S) 2.35E5 0.94 0.03 Mn (S) 0.94 0.02 0.06 Ba (S) 0.82 0.05 0.13 Ti (S) 0.20 0.98 0.01 P (S) 0.57 0.54 0.19 Cu (S) 0.64 0.46 3.24E3 Ni (S) 0.83 0.23 0.06 Co (S) 0.88 0.03 0.14 Zn (S) 0.16 0.78 0.04 V (S) 0.25 0.89 0.02 Mo (S) 0.40 0.58 0.05 Pb (S) 0.46 0.66 0.09 Cd (S) 0.06 2.68E3 0.09 Corg (S) 0.56 0.45 0.01
Factor 4
Factor 5
Factor 6
Factor 7
0.97 for Cd (S); sampling adequacy: between 0.42 for NO2 0.12 0.02 0.97 0.40 0.04 0.02 0.25 0.88 0.97 0.22 0.09 0.13 0.18 0.11 0.21 0.16 0.11 0.09 0.13 0.04 0.11 0.08 0.03 0.03 0.03 0.03 0.09 0.10 0.03 0.05 0.01 2.40E3 0.11 0.05 0.11 0.11
0.09 0.04 0.02 0.03 0.92 0.03 0.06 0.06 0.02 0.11 0.83 0.19 0.15 0.02 0.12 0.17 0.04 0.13 0.04 0.18 0.12 0.08 0.16 0.08 0.08 0.01 0.08 0.14 0.15 0.06 0.03 0.32 0.20 0.12 0.94 0.31
0.22 0.27 0.03 0.20 0.19 0.02 0.01 1.07E4 0.01 0.81 0.26 0.28 0.24 0.27 0.67 0.17 0.72 0.03 0.39 2.01E3 0.05 0.01 0.04 0.06 0.36 0.10 0.16 4.73E3 0.08 0.23 0.09 0.14 0.24 0.26 0.01 0.11
0.61 0.18 0.05 0.12 0.01 0.13 0.14 0.03 0.10 0.06 0.11 0.67 0.14 0.09 0.51 0.13 0.24 0.86 0.24 0.11 0.22 0.17 0.17 0.08 0.04 0.27 0.08 0.21 0.05 0.38 0.22 0.10 0.01 0.12 0.16 0.30
(Grupe et al., 2001). Ca representing carbonate shows a strong negative loading to this factor. Factor 2 stands for the Mn oxide phase and combines Co, Ba, Mn, Ni, Pb (and P and Cu). In factor 3, Al, Mo, Zn and Cu obviously represent a detrital fraction with a negative relationship to Ba which is probably a biogenic signal. Ba is enriched by some xenophyophoriae (Hopwood et al.,
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1997) that are numerous in the investigated areas and had been collected and analysed (Koschinsky et al., 2001a). Cd and V form another factor which is due to correlating peaks within the profiles, and Mn and Ni have a characteristic relationship with depth in factor 5, which is due to the redox sequence. Analysis of the pore water data (Table 2) shows that the most pronounced feature is represented by the redox sequence with positive factor loadings for depth, nitrite, nitrate, oxygen, ammonia (and Mo), and negative loadings for Mn, silicate and phosphate. Factor 2 represents the striking peaks of Fe, Co, Ni (Zn and Mo) within the profiles. The anionic components Mo, U, V and phosphate, and ammonia are presented by factor 3. Ba, Mn (Cd and V) are grouped by factor 4 that describes biogenic components, and Pb, Cu, Cd and Zn are correlated with oxygen by a fifth factor, which is determined by their increased concentrations in the upper cm of several cores. In order to check if there are definite relationships between dissolved and solid phase parameters, the entire set of pore water and sediment data was next submitted to factor analysis (Table 2). The first factor describes the dominant role of the oxic MnO2-rich surface layer, and nitrate as the only pore water component. Dissolved Mn and silicate have a negative loading of this factor. The second important factor combines the elements present in the detrital clay fractions (dominated by illite and smectite), inversely phased by Ca (as Ca carbonate). Factors 3– 7 describe component groups that have been identified similarly in the separate factor analyses. Except for solid and pore water Mn, no clear relationships between pore water and solid sediment data were found. The relationships are similar to those found in the separate sediment and pore water factor analyses.
4. Discussion The distribution of heavy metals in the solid and pore water phases of the Peru Basin surface sediments is controlled by the composition of the particles (e.g. calcite, clay and Mn oxide portions), the input of particulate and dissolved material from the water column, biogenic and physical redistribution, and (especially important for redox-sensitive metals) diagenetic redistribution according to the model of suboxic diagenesis of Froehlich et al. (1979). The thickness of the different redox zonations can change with time, and the solid element distribution does not always correspond exactly to the present redox zonation. As our sediment profiles from the Peru Basin show, some metals do not follow the strict pattern of redox zonation, as they may have several reaction characteristics and/or only a small fraction may be diagenetically reactive. There are also differences between the cores from the five areas, resulting from local variations of sedimentation or small-scale variability within the areas. Similar differences have been found by Graybeal and Heath (1984) for sediments from two different sites in the eastern Pacific. Surface sediments also can be significantly influenced by microenvironments formed around biogenic structures like burrows and tubes, as shown by Aller and Aller (1986); besides the effect of homogenising the upper cm of the sediment through bioturbation, local microenvironments differing significantly from the regular pattern can be created. The pore water composition is particularly sensitive to environmental variations, while the solid phase has a certain capacity to ‘‘buffer’’ environmental changes. In the following section, we discuss the complex pattern of heavy metal distributions in the Peru Basin surface sediments in the light of: (1) the present-day redox
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zonation (Haeckel et al., 2001), (2) the dynamics of the redox zonation on short-term (seasonal– . interannual) and (3) long-term (glacial–interglacial) time scales (Konig et al., 2001).
4.1. Present-day conditions According to leaching experiments (Koschinsky et al., 2001b), up to 80% of total Mn is diagenetically mobile and, therefore, the Mn distribution perfectly reflects the local redox conditions. The characteristic pattern of Mn distribution around the oxic/suboxic boundary is found in all of the five investigation areas (see also pore water O2 data in Haeckel et al., 2001). According to the leaching experiments, those metals associated with the Mn oxide phase in the surface layer, such as Co, Ni, Cu and Zn, generally follow the solid profile pattern of Mn (which is reflected by a common factor of these elements in the factor analysis). This is especially manifest for Ni in profile 145 MC (Fig. 11); Ni is also the only metal that shows a significant correlation of its dissolved species with dissolved Mn. This close similarity of Ni with Mn in deep-sea sediments also has been found by Heggie et al. (1986) and Thomson et al. (1993). Dissolved Co, Zn and Cu, as well as all other metals, do not show a definite relation to pore water Mn, as is confirmed by the lack of relationship with Mn in the factor analysis (Table 2), although in their solid state most of them are associated with the Mn oxide phase. This behaviour has also been described for Ni, Co and Cu by Jarvis and Higgs (1987) and has been attributed to adsorption of the mobilised metals on organic matter or clay minerals like smectite or immobilisation by Fe compounds. Dissolved Cu, Zn, Ni, Co, Pb, Cd, Fe and V are significantly enriched in the top 1–2 cm of all profiles (which is manifested in a common factor together with oxygen in the factor analysis). These dissolved surface enrichments originate from the degradation and release from organic matter (Sawlan and Murray, 1983; Heggie et al., 1986; Shaw et al., 1990). A certain diffusion into the bottom water can be assumed for these metals, and further below a strong fixation on the Mn oxide phase prevails. Dissolved V is more stable in the oxidised surface layer and is not significantly adsorbed by Mn oxide. Therefore the V peak is wider (0–5 cm depth). In the five ATESEPP areas, early diagenesis presently reaches the states of oxygen consumption, denitrification and Mn(IV) reduction, but there is no evidence of Fe(III) reduction, sulphate reduction and methanogenesis (Fig. 13c; Haeckel et al., 2001). A closer examination of the typical diagenetic Mn depth profiles reveals that the distribution patterns of solid phase Mn on the one hand, and pore water O2 and Mn2+ on the other, are offset in virtually all of the cores (Fig. 12): In core 145 MC, the solid phase Mn is enriched in the uppermost 11 cm of the sediment, and a marked peak of Mn enrichment at the base of this layer must result from an oxic/suboxic boundary at that depth. In contrast, the oxic zone was only 5 cm thick at the moment of core retrieval, and there was release of Mn2+ to the pore water at the depth of the solid phase enrichment peak starting from 6 cm depth. Ni exactly follows this offset observed for Mn (Fig. 12). The same phenomenon is found in core 283 MC with a 6 cm deep oxic layer and a 13 cm thick MnO2 - and Ni-rich layer. Analogously, at the other three investigation sites the surface layer of solid phase Mn enrichment (15–20 cm) was thicker than the oxic layer (5–10 cm) at the moment of core retrieval.
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Fig. 12. Demonstration of the offset of solid Mn and Ni, dissolved Mn and Ni and oxygen concentrations in profiles 145 MC (area 1a) and 283 MC (area 6); solid Mn and Ni concentrations in wt%, dissolved Mn and Ni concentrations in mg/l, and O2 concentrations in mmol/l (taken from Haeckel et al., 2001); (- - -) represents the boundary of oxygen penetration, (. . . . . . . .) represents the depths of the Mn oxide-rich layer.
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Fig. 13. Schematic presentation of the historic development of sediment and pore water metal distributions in Peru Basin surface sediment (area 1a): While during the glacial Mn and Fe reduction took place within the high reactivity surface layer due to the strong depositional flux (a), the Mn and Fe redox boundaries were deeper but coincided at the transition to the interglacial (b) because oxygen and nitrate penetration were both confined to the depth of the reactive Fe(II) layer. Oxic and sub-oxic metal peaks developed along this boundary. During burial of the reactive Fe(II) layer, the sub-oxic enrichments passively moved down with this layer while the oxic metal enrichments remained confined to the oxic layer which is again separated from the nitrate penetration depth (c).
4.2. Short-term fluctuations The offset between oxygen penetration, solid and dissolved Mn, and Ni is explained by vertical oscillations of the oxic/suboxic redox boundary, which are also documented in these sediments by transitional pore water NO 3 profile shapes and the downcore relation between the pore water . profiles and the solid phase Fe(II)/Fe(III) record (Konig et al., 2001). The redox zonation NO 3 fluctuates on the seasonal–interannual time scale due to variations in the depositional flux of organic matter, which relate to the ENSO phenomenon in the eastern tropical Pacific Ocean . (Konig et al., 2001). Also Gobeil et al. (1997) discussed offset distributions of dissolved and solid Mn and the diagenetic separation of Mn and Cd in the light of fluctuations of the redox boundary caused by changes in the flux of organic carbon to the seafloor. They found solid Mn concentrated in a narrow surface layer, often with a pronounced peak at the deepest point of this layer, and low solid Cd concentrations at the surface followed by a Cd peak at about 5–20 cm depth. According to their model, the repeated process of dissolution, migration and reprecipitation of Mn
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transports this element upward and concentrates it into a thin layer at the upper limit of the redox excursion. In the cores from the Peru Basin, such Mn peaks also appear at the bottom of the Mn oxide enrichment (Figs. 1 and 12 and Figs. 3,5,7 and 9), and can be explained by the upward pumping and concentration of Mn in a narrow layer. However, Gobeil et al. (1997) were dealing with continental margin sediments where the oxic zone is underlain by a sulphidic zone in which Cd is precipitated as Cd sulphide. We found only a single peak of Cd at some distance below the oxic zone in core 145 MC from area la (Fig. 11). Whereas in the cores of the other four ATESEPP areas no sulphide was detected, in area 1a (which receives higher organic input) sulphide was . found in measurable concentrations (Konig et al., 1996). Since none of the measured major elements follows the peak of Cd at around 25 cm depth in core 145 MC, Cd sulphide is the most probable form to explain the Cd enrichment. Furthermore, the enrichment of Cu, Ni, Zn, Co, and eventually Pb, below 23 cm depth (Fig. 11), can be explained by the precipitation of sulphides of these metals. Rosenthal et al. (1995) have shown that the precipitation of CdS in suboxic sediments is a very efficient immobilisation mechanism even at trace level sulphide concentrations. Our observations in core 145 MC support the assumption of these authors that this phenomenon is not restricted to Cd, but may also appear for other chalcophilic metals (Cu, Zn, Pb, and possibly Co and Ni). 4.3. Long-term geochemical evolution Distinct peaks of solid phase metal concentrations such as in the downcore patterns of Cu, V and Cd in core 145 MC (Fig. 11) have been described by several authors as resulting from the slow and continual downcore expansion of the oxic zone in pelagic surface sediments (Colley et al., 1984; Wilson et al., 1986; Jarvis and Higgs, 1987; Thomson et al., 1993, 1998). These authors describe heavy metal (im)mobilisations as the results of local reducing conditions that are produced by turbidites in a previously oxic sediment column. The turbidites form a geochemical barrier for the diffusion of oxidising components (oxygen and nitrate) into deeper layers. The different characteristics of the heavy metals within the diagenetic sequence lead to peaks in the sediment in the following succession from oxic to post-oxic: Mn with Ni, Cu, Co and Zn, followed by Fe, Cu, Cd, Pb, V, Zn, U (Thomson et al., 1993, 1998). In contrast to Mn and associated metals, U and V are mobilised under oxic conditions as U(VI) and V(V), diffuse downward, and are fixed under reducing conditions as U(IV) and V(IV) oxides. Besides this anoxic enrichment, elevated V in the oxic section was attributed to a certain V surface complexation by sediment phases. The sequence of metal enrichment observed for core 145 MC (Fig. 11) is very similar to those observed for the redox boundaries produced by organic-rich turbidites. Though we can exclude the existence of turbidites in our samples, a similar geochemical barrier is present by . reoxidisable Fe(II) in clay minerals in the sediment of area 1a (Konig et al., 1997). These similarities in metal distributions between the sediments characterised by turbidites and the Peru Basin sediments indicate that different evolutational histories can produce similar geochemical effects. . According to the geochemical model of the Peru Basin deep-sea floor by Konig et al. (2001), a continual downcore expansion of the oxic zone has occurred in Peru Basin surface sediments during glacial/interglacial transitions in response to decreasing depositional fluxes of organic matter (Fig. 13). Changing sedimentation signals from glacial to postglacial have already been
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reported by Thomson et al. (1984). Moreover, previous investigations of the Peru Basin bulk sediment Fe(II)/Fe(III) record have shown that the diagenetic stage of Fe reduction was reached in the sea floor at investigation site ATESEPP-1a (the site of core 145 MC), and large fractions of . the structural Fe in deep-sea clays were chemically reduced (Konig et al., 1997, 1999). Thus reactive Fe(II) layers (RFe(II)Ls), i.e. layers rich in reoxidisable clay Fe(II) that act as geochemical barriers with respect to pore water O2 and NO 3 ; were formed below the uppermost . (o5 cm) layers of the sea floor (Konig et al., 1999). Therefore, the expansion of the oxic zone was delayed by the burn-down process of the RFe(II)L until the oxic zone had reached its post-glacial . equilibrium width (Konig et al., 2001). The slow downcore migration of the oxic/suboxic boundary into the RFe(II)L resulted in the formation of solid phase Cd and other metal peaks by presumably the same mechanism as described by Rosenthal et al. (1995) for the burn-down of the oxic/suboxic boundary in northeast Atlantic turbidites. Thomson et al. (1996) documented a characteristic sequence of metal enrichment peaks, namely Mn+Mo, Fe+As+P, Se+V, (Cd), and U in downcore succession, and interpreted this as reconcentrations of a diagenetic fraction of each element on a redox gradient around the active oxic/suboxic boundary. The prolonged near-stasis of the redox boundary, a necessary prerequisite for peak formation, has in their case also been caused by the change in sediment accumulation regime between glacial and interglacial times. At greater depth in the sediment sequence, Thomson et al. (1996) found only the suboxic peak fraction (Se, V, Cd and U) left at a previous glacial/interglacial transition (oxygen isotope stages 6/5), due to the fact that the oxic peak fraction (Mn, Mo and Fe) had moved upcore along with the oxic zone. Also in our core 145 MC, the suboxic fraction (Cu, V and Cd) is separated from the . oxic fraction (Mn and Mo, Fig. 3 and Fig. 11). This is in accordance with the model of Konig et al. (2001) which claims that the post-glacial equilibrium width of the oxic zone (ca. 10 cm) was reached several thousand years ago. Since then, the Cu, V and Cd concentration peaks have been detached from the oxic/suboxic boundary and therefore remained unchanged, and their downcore movement has only been due to burial (Fig. 13). Presently there is no active redox boundary at the depth of the peaks, and there is no indication that the diagenetic stage of Fe reduction is reached.
4.5. Consequences of a strong disturbance of the surface sediment In case of a commercial use of the deep-sea such as manganese nodule mining, at least 10–20 cm of the surface sediment would be heavily disturbed or removed. This would affect the whole oxic Mn-rich surface layer of the Peru Basin sediments. Similar effects could be produced by natural events like turbidites or slumps. Periodically appearing ‘‘benthic storms’’ with current velocities of up to 35 cm/s (Gross et al., 1988) also can remove parts of the sediment surface. In a worst-case scenario disturbance, the interface of oxic seawater and oxic surface sediment would be substituted by an interface between the oxic seawater and suboxic sediment. When no Mn oxiderich surface layer is present, scavenging of upward diffusing metals like Mn, Co, Ni, Zn and Cu is reduced and significant loss of dissolved pore water components into the bottom water could occur. The ratio of lost-to-bottom-water and scavenged-on-particles metal rates will be influenced by the steepness of the redox gradients present in the new situation and the geochemical and mineralogical composition of the sediment (see also Fritsche et al., 2001).
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Penetration of oxygen from the oxygen-rich bottom water will initiate Mn precipitation and . coprecipitation or sorption of associated metals like Ni and Co. Model calculations by Konig et al. (2001) have shown that oxygen diffusion would be fast and intense. However, diffusion experiments with oxic and suboxic sediment layers and heavy metal solutions and modelling of Mn fluxes after the disturbance have shown that significant amounts of Mn can still diffuse into the overlying water despite the oxic character of the water (Fritsche et al., 2001). Since Mn redox reactions are strongly kinetically controlled (Grill, 1982) and the input on the new sediment surface (resedimentation of suspended sediment or input of fresh organic matter) is an unknown but decisive factor, it is difficult to make a clear statement on the time scale for the formation of a new Mn oxide-rich surface layer. Mn oxidation is probably especially slow in the deep sea due to the low temperature and low microbial activity compared to shallow marine systems. From our experimental results, it appears that the presence of Mn oxide is an important component for scavenging Mn (possibly via autocatalytic oxidation); the absence of Mn oxide leads to a slower and less efficient removal of Mn and other metals from the oxic seawater phase (Koschinsky et al., 2001b). The formation of a Mn oxide-rich surface layer comparable to the original oxic surface layer, that acts as an efficient barrier for Mn diffusion, will probably require much more time than diffusion of oxygen into the formerly suboxic sediment. In ATESEPP area 1a, oxygen would penetrate the sediment to about 20 cm depth (compared to 5 cm at present), taking into account that the reactive Fe(II) in the clay structures below 20 cm . depth forms a geochemical barrier (Konig et al., 2001). This would also restrict the diagenetic reactions as well as upward and downward migration of the redox-sensitive metals in the deeper sediment layers. In ATESEPP area 2, oxygen would even penetrate between 125 and . 150 cm deep into the sediment, compared to about 10 cm at present (Konig et al., 2001). This drastic enlargement of the oxic surface layer would be followed by precipitation of Mn oxide and associated metals in this layer, fed from the high pore water concentrations of Mn in the formerly post-oxic sediment and diffusive transport from deeper sediment layers (Fritsche et al., 2001). This upward diffusion of dissolved metals into the new oxic layer would probably also initiate the slow formation of new manganese nodules. Post-oxic enriched metals like Cd, V and U (and eventually including some Zn, Cu, Co, Ni and Pb) would redissolve in the newly oxic environment, diffuse downward and precipitate in deeper post-oxic layers. Probably proportions of the remobilised metals also would move upward, be fixed in the Mn oxide-rich layer and/or diffuse into the bottom water. If also the upper part of the sub-oxic layer is resuspended, stronger metal release, especially of Mn, from this material has to be taken into account. More probable than the complete removal of the sediment surface layer during deep-sea mining, however, is a strong intermixing of the oxic surface layer, eventually with more consolidated parts of deeper sediment, and the coverage of the surface with resedimented oxic sediment. The uplifting of sub-oxic or anoxic sediment into the oxic surface layer would initiate a succession of redox reactions, and oxidation of the sub-oxic sediment would be promoted by the input of oxygen-rich bottom water during the disturbance and by the increased diffusive oxygen penetration through the poorly compacted resedimented layer. Due to the roughness of the mixed and resedimented material, the active surface for material exchange between bottom water and sediment would be increased. The reactions of the heavy metals might be similar to that described for the worst-case scenario, but less pronounced.
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5. Conclusions The surface sediment of the Peru Basin in its present state is represented by a 5–20 cm thick Mn oxide-rich top layer associated with increased Ni, Co, Mo and sometimes also Zn, Cu, Pb and Cd concentrations. The pore water can have significant metal peaks in the uppermost 1–2 cm which result from the degradation of organic matter (Fig. 13c). At present, the 20–30 cm core length covers only the Mn redox boundary in the diagenetic sequence. An offset of several cm between the depths of oxygen penetration and increase of dissolved Mn and Ni concentrations on the one hand (which at present is at 5–10 cm), and the thickness of the Mn oxide- and Ni-rich sediment layer (10–20 cm) indicates that the oxic/sub-oxic redox boundary oscillates on a short-term time scale. Only Ni more or less follows the strong increase in concentrations of dissolved Mn below the Mn redox boundary. Co, Cu, Zn, Mo, Cd and Pb are obviously re-immobilised readily after release from the Mn oxide fraction by sorption on organic, clay or Fe compounds. A strong enrichment of these metals and V was observed below 22 cm in core 145 MC from area 1a, where the ancient Fe redox front is reached at 20–22 cm depth. Since the diagenetic stage of Fe reduction is not reached at present in the Peru Basin sediments, the formation of these peaks can only be explained in a historic context, as depicted in Fig. 13. In case of a strong disturbance of the surface sediment as during manganese nodule mining, the present redox situation would be destroyed. The zone with oxic metal enrichments would be resuspended, removed and eventually resedimented, but metal release from the oxic sediment is improbable. Loss of dissolved heavy metals from the sub-oxic pore water to the bottom water can . occur until a new and stable oxic front at the sediment surface has formed. According to Konig et al. (2001), oxygen would proceed deep into the sediment layer within a few years, unless the ancient Fe redox boundary with reoxidisable Fe(II) in clay structures prohibits further oxygen penetration. From the surface downwards, slow formation of MnO2 associated with proportions of Co, Ni, Mo and other metals would start. As long as the redox boundary moves, V, Cd, U and other peaks formed in the post-oxic environment would dissolve in the newly oxic environment and follow the redox boundary downward to reprecipitate in deeper layers.
Acknowledgements Research cruise SO 106, during which the sediment cores were sampled, and the project GECOMET (project no. 03 G 0106 G) were funded by the German Bundesministerium fur . Bildung und Forschung. The responsibility for the contents of this manuscript is with the author. I thank C. Arndt, M. Weigelt and B. Alberts for their analytical contributions, and H. Thiel for comments on the manuscript. M. Haeckel provided his pore water data of Mn, Fe and Ba in the multicorer samples. This manuscript benefitted substantially from the co-operation with the . FEMILIEU project (no. 03 G 0106 H) and intense discussions with I. Konig on the interpretation of the heavy metal distributions in the light of her model developed within the ATESEPP research programme. I would like to express my special thanks to her. Also the helpful reviews of J. Thomson and T. Kuhn and the English corrections of E. Foell in the final version greatly improved the manuscript.
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Appendix Heavy metal concentrations in pore water and in sediment presented in the text also can be found in Annex 2.10 on CD-Rom: Fig. 3: Fig. 4: Fig. 5: Fig. 6: Fig. 7: Fig. 8: Fig. 9: Fig. 10:
Bulk sediment major and minor/trace element concentrations versus depth in core 145 MC from area 1a; Heavy metal pore water concentrations versus depth in core 145 MC from area 1a; Bulk sediment major and minor/trace element concentrations versus depth in core 245 MC from area 1b; Heavy metal pore water concentrations versus depth in core 245 MC from area 1b; Bulk sediment major and minor/trace element concentrations versus depth in core 275 MC from area 2; Heavy metal pore water concentrations versus depth in core 275 MC from area 2; Bulk sediment major and minor/trace element concentrations versus depth in core 166 MC from area 5; Heavy metal pore water concentrations versus depth in core 166 MC from area 5.
The results of the principal component analyses of the pore water data, solid sediment data, and the complete data set of pore water and sediment samples (Table 2) are also presented in Annex 2.10 on CD-Rom.
References Aller, J.Y., Aller, R.C., 1986. Evidence for localized enhancement of biological activity associated with tube and burrow structures in deep-sea sediments at the HEBBLE site, western North Atlantic. Deep-Sea Research I 33, 755–790. Balistrieri, L.S., Murray, J.W., 1986. The surface chemistry of sediments from the Panama Basin: the influence of Mn oxides on metal adsorption. Geochimica et Cosmochimica Acta 50, 2235–2243. Bonatti, E., Fisher, D.E., Joensuu, O., Rydell, H.S., 1971. Postdepositional mobility of some transition elements, phosphorus, uranium and thorium in deep sea sediments. Geochimica et Cosmochimica Acta 35, 189–201. Colley, S., Thomson, J., Wilson, T.R.S., Higgs, N.C., 1984. Post-depositional migration of elements during diagenesis in brown clay and turbidite sequences in the North East Atlantic. Geochimica et Cosmochimica Acta 48, 1223–1235. Fritsche, U., Koschinsky, A., Winkler, A., 2001. The different diffusive transport behaviours of some metals in layers of Peru Basin surface sediments. Deep-Sea Research II 48, 3653–3681. Froehlich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B., Maynard, V., 1979. Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: suboxic diagenesis. Geochimica et Cosmochimica Acta 43, 1075–1090. Gobeil, C., MacDonald, R.W., Sundby, B., 1997. Diagenetic separation of cadmium and manganese in suboxic continental margin sediments. Geochimica et Cosmochimica Acta 61, 4647–4654. Graybeal, A.L., Heath, G.R., 1984. Remobilization of transition metals in surficial pelagic sediments from the eastern Pacific. Geochimica et Cosmochimica Acta 48, 965–975. Grill, E.V., 1982. Kinetic and thermodynamic factors controlling manganese concentrations in oceanic waters. Geochimica et Cosmochimica Acta 46, 2435–2446. Gross, T.F., Williams, A.J., Grant, W.D., 1988. A deep-sea sediment transport storm. Nature 331, 421–518. Grupe, B., Becker, H.J., Oebius, H.U., 2001. Geotechnical and sedimentological investigations of deep-sea sediments from a manganese nodule field of the Peru Basin. Deep-Sea Research II 48, 3593–3608.
3776
A. Koschinsky / Deep-Sea Research II 48 (2001) 3757–3777
. Haeckel, M., Konig, I., Riech, V., Weber, M.E., Suess, E., 2001. Pore water profiles and numerical modeling of biogeochemical processes in Peru Basin deep-sea sediments. Deep-Sea Research II 48, 3713–3736. Heggie, D., Kahn, D., Fischer, K., 1986. Trace metals in metalliferous sediments, MANOP site M: interfacial pore water profiles. Earth and Planetary Science Letters 80, 106–116. Hopwood, J.D., Mann, S., Gooday, A.J., 1997. The crystallography and possible origin of barium sulfate in deep sea rhizopod protists (Xenophyophoria). Journal of the Marine Biological Association, United Kingdom 77, 969–987. Jarvis, I., Higgs, N., 1987. Trace-element mobility during early diagenesis in distal turbidites: late Quaternary of the Madeira Abyssal Plain, N Atlantic. In: Weaver, P.P.E., Thomson, J. (Eds.), Geology and Geochemistry of Abyssal Plains. Geological Society Special Publications no. 31, Oxford, pp. 179–213. Klinkhammer, G.P., 1980. Early diagenesis in sediments from the eastern equatorial Pacific II. Pore water metal results. Earth and Planetary Science Letters 49, 81–101. . Konig, I., D.ahlmann, A., Domeyer, B., Greinert, J., Haeckel, M., Siemensen, M., 1996. FEMILIEU. In: Schriever, G., Koschinsky, A., Bluhm, H. (Eds.), Cruise Report ATESEPP: Impacts of Potential Technical Interventions on the Deep-sea Ecosystem of the Southeast Pacific off Peru. Berichte aus dem Zentrum fur . Meeres- und Klimaforschung, Reihe E, pp. 155–169. . Konig, I., Drodt, M., Suess, E., Trautwein, A.X., 1997. Iron reduction through the tan-green color transition in deepsea sediments. Geochimica et Cosmochimica Acta 61, 1679–1683. . Konig, I., Haeckel, M., Drodt, M., Suess, E., Trautwein, A.X., 1999. Reactive Fe(II) layers in deep-sea sediments. Geochimica et Cosmochimica Acta 63, 1517–1526. . Konig, I., Haeckel, M., Lougear, A., Suess, E., Trautwein, A.X., 2001. A geochemical model of the Peru Basin deep-sea floor – and the response of the system to technical impacts. Deep-Sea Research II 48, 3737–3756. Koschinsky, A., 1996. GECOMET. In: Schriever, G., Koschinsky, A., Bluhm, H. (Eds.), Cruise Report ATESEPP: Impacts of Potential Technical Interventions on the Deep-sea Ecosystem of the Southeast Pacific off Peru. Berichte aus dem Zentrum fur . Meeres- und Klimaforschung, Reihe E, pp. 131–154. Koschinsky, A., Borowski, C., Halbach, P., 2001a. Reactions of the heavy metal cycle on industrial activities in the deep sea: an ecological assessment. International Review of Hydrobiology, submitted. Koschinsky, A., Fritsche, U., Winkler, A., 2001b. Sequential leaching of Peru Basin surface sediment for the assessment of aged and fresh metal associations and mobility. Deep-Sea Research II 48, 3683–3699. Koschinsky, A., Gaye-Haake, B., Arndt, C., Maue, G., Spitzy, A., Winkler, A., Halbach, P., 2001c. Experiments on the influence of sediment disturbance on the biogeochemistry of the deep-sea envionrment. Deep-Sea Research II 48, 3629–3651. Marchig, V., von Stackelberg, U., Hufnagel, H., Durn, G., 2001. Compositional changes of surface sediments and variability of manganese nodules in or Peru Basin. Deep-Sea Research II 48, 3523–3547. Rosenthal, Y., Lam, P., Boyle, E.A., Thomson, J., 1995. Authigenic cadmium enrichments in suboxic sediments: precipitation and postdepositional mobility. Earth and Planetary Science Letters 132, 99–111. Sawlan, J.J., Murray, J.W., 1983. Trace metal remobilization in the interstitial waters of red clay and hemipelagic marine sediments. Earth and Planetary Science Letters 64, 213–230. Schriever, G., Koschinsky, A., Bluhm, H., 1996. Cruise Report ATESEPP: Impacts of Potential Technical Interventions on the Deep-sea Ecosystem of the Southeast Pacific off Peru. Berichte aus dem Zentrum fur . Meeresund Klimaforschung, Reihe E, 195pp. Shaw, T.J., Gieskes, J.M., Jahnke, R.A., 1990. Early diagenesis in differing depositional environments: the response of transition metals in pore water. Geochimica et Cosmochimica Acta 54, 1233–1246. Thiel, H., 2001. Use and protection of the deep seaFan introduction. Deep-Sea Research II 48, 3427–3431. Thiel, H., Forschungsverbund Tiefsee-Umweltschutz, 2001. Evaluation of the environmental consequences of polymetallic nodule mining based on the results of the TUSCH Research Association. Deep-Sea Research II 48, 3433–3452. Thomson, J., Wilson, T.R.S., Culkin, F., Hydes, D.J., 1984. Non-steady state diagenetic record in eastern equatorial Atlantic sediments. Earth and Planetary Science Letters 71, 23–30. Thomson, J., Higgs, N.C., Croudace, I.W., Colley, S., Hydes, D.J., 1993. Redox zonation of elements at an oxic/postoxic boundary in deep-sea sediments. Geochimica et Cosmochimica Acta 57, 579–595.
A. Koschinsky / Deep-Sea Research II 48 (2001) 3757–3777
3777
Thomson, J., Higgs, N.C., Colley, S., 1996. Diagenetic redistribution of redox-sensitive elements in northeast Atlantic glacial/interglacial transition elements. Earth and Planetary Science Letters 139, 365–377. Thomson, J., Jarvis, I., Green, D.R.H., Green, D.A., Clayton, T., 1998. Mobility and immobility of redox-sensitive elements in deep-sea turbidites during shallow burial. Geochimica et Cosmochimica Acta 62, 643–656. Weber, M.E., von Stackelberg, U., Marchig, V., Wiedicke, M., Grupe, B., 2000. Variability of surface sediment in the Peru Basin: dependence on water depth, productivity, bottom water flow, and seafloor topography. Marine Geology 163, 169–184. Wilson, T.R.S., Thomson, J., Hydes, D.J., Colley, F., Culkin, F., S"ırensen, J., 1986. Oxidation fronts in pelagic sediments. Diagenetic formation of metal rich layers. Science 232, 972–975.