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Mobility of metals over the redox boundary in Peru Basin sediments
Deep-Sea Research II 48 (2001) 3549–3567
Mobility of metals over the redox boundary in Peru Basin sediments Jens Stummeyer, Vesna Marchig* Bundesanstalt fur . Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany
Abstract Downcore variations and phase distribution of metals in the Quaternary sediments from six areas of the Peru Basin were investigated. The sediments are characterised by a 5–20 cm thick, oxic layer at the water/ sediment interface formed under the influence of dissolved oxygen in the bottom water. In deeper sediment sections, suboxic conditions are found due to the degradation of organic matter. Variations in the downcore abundance of metals (e.g., Fe, Mn, Co, Ni, Cu, Zn, V, Mo, Cr) were determined by bulk chemical methods. A significant accumulation of manganese is observed at the depth of the manganese redox boundary, caused by diagenetic remobilisation under suboxic conditions and reprecipitation under oxic conditions. To investigate their phase distributions, partitioning of elements in selected samples over the sediment column was carried out in sequential leaching experiments. A comparison between the surface layer and deeper sediment layers shows a distinct change in the element distribution between operationally defined host phases (carbonates, oxy-hydroxides, detrital components). Above the redox boundary at the sediment surface, transition metals are enriched in the reducible Mn-oxide fraction. When the horizontal position of the Mn-redox boundary moves upward, influenced by naturally occurring enhancement of surface-water productivity, these elements are mobilised and available to the manganese nodule accretion process if the redox boundary reaches the level of growth of the nodules. In contrast to the naturally induced mobilisation of metals, a technical impact (e.g., mining of manganese nodules) would be a rapid event occurring under completely different redox conditions. Metals would be released into the bottom water, an oxic environment, where scavenging of dissolved metals and resedimentation takes place in relatively short periods of time. Presumably, a long-distance transport of contaminants will not happen. r 2001 Elsevier Science Ltd. All rights reserved.
*Corresponding author. E-mail address: [email protected] (V. Marchig). 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 5 6 - X
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1. Introduction The nodule field of the Peru Basin, located in the eastern equatorial Pacific, shows a very dense coverage with manganese nodules, with a maximum abundance of 50 kg/m2 near the calcite compensation depth (CCD) between 4000 and 4200 m (v. Stackelberg, 1997). Accretion rates of the nodules are up to two orders of magnitude higher than elsewhere in the Pacific (Reyss et al., 1982). The nodules are mostly diagenetic in origin; the observed high accretion rates are assumed to result from the remobilisation of metals from the Quaternary sediments and their reprecipitation at the sediment/water interface under the influence of dissolved oxygen. This reprecipitation causes a dark brown colour of the semi-liquid layer at the top of the sediments (5–20 cm depth) in contrast to the light green–grey colour of the underlying sediments. The highest accretion rates are found at the bottom of nodules embedded in the semi-liquid layer at a level immediately above the redox boundary. The well-investigated diagenetic remobilisation of manganese and iron in the sediments and the formation of an oxic top layer with embedded nodules (Reyss et al., 1982; Dymond et al., 1984; v. Stackelberg, 1987, 1997) will be expanded in this paper with the behaviour of other metals (e.g., Mn, Fe, Co, Ni, Cu, Zn) in the diagenetic remobilisation/reprecipitation process under changing redox conditions. For these investigations, six sediment cores were selected, four within the nodule field of the Peru Basin and two adjacent to the nodule field with a thin oxic layer on the top of the sediment and no manganese nodules. The positions of the selected cores are mapped in the appendix (CD-ROM). Table 1 lists some important parameters of the selected cores.
2. Geologic setting The area of investigation is located on the northern Nazca Plate, which was generated on the Galapagos Rise spreading centre 20–6 million years ago (Mamerickx et al., 1980; Mayes et al., 1990). The Galapagos Rise subsided the spreading activity 9 million years B P at its southern end; the northernmost part ceased spreading 6 million years B P (Rea, 1978). Since then the Nazca Plate has been created at the East Pacific Rise and has drifted towards South America with a slightly changed spreading direction. The change of the spreading direction is probably the reason for the reactivation of volcanic activity on old fault zones (Wiedicke and Weber, 1996), which gave rise to hydrothermal circulation that can still be observed on scattered positions (Marchig et al., 1999). The sediment cover is fairly thin in the investigated area, from a maximum of 150 m in the north where the sedimentation is influenced by higher biogeneous production of the equatorial zone to several tens of meters in the south where the sediments are located below the calcite compensation depth (Fellerer, 1980). At a few positions, basalt was reached (by coring) only 15–17 m beneath the seafloor, but this basalt is not the primary basement, but rather a product of reactivated late Miocene volcanism (Marchig et al., 1999). The general observation that the sedimentation rates decrease from north to south following the increase of water depth (Erlandson et al., 1981; Naar and Hey, 1989) indicates that the main factor governing the sedimentation rate is the dissolution of calcite. The direction of plate
Area
Coordinates
Core with corresponding short core
Water depth (m)
Thickness of Quaternary sediment (m)
Content of CaCO3 (quat. Sed.) (%)
Manganese nodule typeb
Manganese nodules (kg/m2)
6: Marginal part of the equatorial high production zone 5: SEDIPERU 2:DISCOL 2: DISCOL > CCD 1b: Eastern area 1a: Eastern area with influence of upwelling
516.500 S, 90114.840 W
243KL, 287KG
3940
18.5c
33.4715.2
F
0
6134.410 S, 901 31.130 W 717.790 S, 88134.200 W 716.400 S, 88122.700 W 6134.290 S, 86111.430 W 5130.080 S, 85122.370 W
184KL, 183KG 206KL, 204KG 217KL, 193KG 278KA, 252KG 244KA, 255KG
4104 4060 4294 4105 4085
12 7 14.3 6 11
21.7712.9 10.378.1 5.173.9 7.776.8 14.877.4
B A A A F
16.8 23.0 2.4 21.4 0
a
CCD=calcite compensation depth. AFbig cauliflower-like nodules (scarce), BFsmall ellipsoidal nodules (frequent). c Extrapolated from sedimentation rate. b
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Table 1 Parameters of the six investigated cores from the study areasa
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movement towards the ESE does not transport the sediment into areas of lower primary productivity; the positions of the cores have moved more or less parallel to the isolines of biogenic production, at least for 6 million years. Thus, there is no biogenic change in sedimentation from west to east, although the nature of the inorganic part is changing towards the South American continent, being more and more influenced by continental detritus (Marchig et al., 2001).
3. Core locations Essential data about the six selected core locations are presented in Table 1. Samples were recovered using the following tools: box corer (KG), piston corer (KL) and long box corer (KA). The box corer (KG-samples) was used at the same locations as the piston corer (KL-samples) to recover nearly undisturbed top sediment with nodules in their original positions. Core 243KL, with corresponding short core 287KG, is located nearest to the equatorial high productivity zone compared to the other investigated cores (CD-ROM; Area 6: marginal part of the equatorial high productivity zone). Carbonate and opaline silica in the sediments are high in this area (Weber et al., 2000). The length of the core is 12.85 m, the deepest sample has an age of 1.19 million years. Extrapolated from the average sedimentation rate in this area, the thickness of the Quaternary part of the sediment would be about 18.5 m, which would represent the thickest Quaternary section of all investigated cores. The corresponding short core from the same position does not contain manganese nodules on the sediment, as it is located outside of the Peru Basin nodule field. Core 184KL with corresponding short core 183KG is located in the SEDIPERU area (CDROM; Area 5), the site with the most homogeneous coverage of manganese nodules. The oxic semi-liquid layer is also thicker. Carbonate contents as well as the thickness of the Quaternary section are lower. The corresponding short core contains 16.8 kg/m2 of small ellipsoidal manganese nodules. Core 206KL from the DISCOL area (CD-ROM; Area 2) contains a shorter Quaternary section and less carbonate. The corresponding short core 204KG contains 23 kg/m2 of big cauliflower-like manganese nodules. Core 217KL, with corresponding short core 193KG, was obtained in a trough in the DISCOL area, significantly below the calcite compensation depth (CD-ROM; Area 2). As expected, the carbonate content is very low. The thickness of the Quaternary sediment is unexpectedly high, which can be explained by focusing of fine grained material from adjacent morphologic highs. 2.4 kg/m2 of big cauliflower-like nodules were sampled from the sea floor. Core 278KA, with corresponding short core 252KG, also has a shorter Quaternary sequence section and lower carbonate content. It was sampled significantly closer to the South American continent compared to the other cores investigated, but still inside the Peru Basin nodule field (CD-ROM; Area 1b). Consequently, the short core at this position was covered with 21.4 kg/m2 of big cauliflower-like nodules. Core 244KA, with corresponding short core 255KG, was deposited under the influence of enhanced productivity due to upwelling (CD-ROM; Area 1a). It has elevated carbonate content
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and a thicker Quaternary sediment section. Opaline silica does not show a significant increase (Weber et al., 2000). This core is located outside of the manganese nodule field.
4. Sampling and analytical methods Sampling was performed with 20 m-long piston corers. The cores were subsampled at intervals of 10 cm for XRF bulk analysis. Additionally, at the same positions a box corer was used (50 50 50 cm3) to gain undisturbed surface sediment with manganese nodules in their original positions. The corresponding short cores from this box corer were subsampled every 5 cm for chemical analyses and the determination of element mobility over the redox boundary at 5–20 cm. . At some positions, a long box corer was used for pore water investigations (Konig et al., 2001). The long box corer was sampled for chemical analyses at 5 cm intervals. Sediment samples were freeze-dried and milled to less than 40 mm particle size in an agate mill. Bulk chemical analyses were performed with X-ray fluorescence using Philips PW 1400 and PW 1480 wavelength dispersive spectrometers. Prior to quantitative determination of 42 major and trace elements, samples were fused with Li-metaborate at 12001C for 20 min. The resulting glassy disks ensure a uniform distribution of the elements. Quality of the results was controlled with more than 100 certified reference materials (CRM). An analytical precision better than 70.5% relative for major elements and 1–10 mg/kg for trace elements could be achieved. The determination of element mobility from selected Quaternary sediment sections was performed with a three-step sequential leaching procedure. Procedures from the literature were modified according to the carbonate content and the content of iron and manganese oxides (Chester and Hughes, 1967; Tessier et al., 1979; Kheboian and Bauer, 1987; Koschinsky, 1994; Koschinsky and Halbach, 1995). The resulting sequential leaching procedure was performed as follows: 1. Weak-acid leachable fraction (including exchangeable cations and carbonates). 0.5 g of powdered dried sample were shaken with 20 ml of a 0.1 M acetic acid solution for 12 h overnight. In case of a carbonate content above 20%, the first step was performed twice to ensure complete dissolution. After centrifugation the residue was washed with water. 2. Easily reducible fraction (including Mn-oxides and co-precipitated elements). 20 ml of a 0.1 M hydroxylamine–hydrochloride solution (NH2OH/HCl; pH=2) were added to the residue of step 1. The mixture was shaken for 12 h at room temperature. The residue was again centrifuged and washed with water. 3. Residual fraction (Fe-oxyhydroxide phases and other elements leachable with HCl/HNO3). The residue of step 2 was treated with 4 ml of boiling HCl/HNO3 (3 : 1). All reagents used were of analytical grade. Possible contamination of the resulting solutions was checked using reagent blanks. The solutions of the leaching steps were diluted with water to a volume of 50 ml. Element concentrations (Fe, Mn, Co, Ni, Cu, Zn, V, Cr, Mo, Pb, Cd, Al, Mg, Ca, Sr, Ba, Be) were analysed with a Jobin–Yvon ICP-OES (optical emission spectrometer), type JY 166 Ultrace. Standard deviations of the leaching procedure and the ICP-OES measurements were generally better than 10%, determined with multiple extraction of selected samples. The
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recovery of easily soluble elements (e.g., Ca and Mn) ranged between 90% and 110% compared to XRF bulk analyses.
5. Results and discussion To investigate downcore variations of metal abundance in the Quaternary sediment sections (Fe, Mn, Co, Ni, Cu, Zn, V, Mo, Cr), element/aluminium ratios versus sediment depth were plotted for the six areas (see Appendix). In Figs. 1–3, the Fe/Al, Mn/Al, and transition metal/Al ratios (sum of Co, Ni, Cu, and Zn in mg/kg) analysed by XRF are plotted. To ensure the comparison between different areas, the carbonate content resulting from the XRF analyses of CaO was subtracted and the element concentrations were normalised to the non-carbonate content. The calculated values of the Peru Basin sediments are compared to the average element concentrations of shales given in Table 2 (Wedepohl, 1970, 1991; Rose et al., 1979) to estimate element enrichment or depletion. 5.1. Area 6: Sediments from the marginal part of the equatorial high productivity zone The sediments of this area (core 243KL; short core 287KG) show the highest Fe/Al ratio (Fe/ AlE0.8) compared with the other areas investigated (Fig. 1). The Fe/Al-ratio is characterised by a considerable short-time variability, but it is relatively constant over the whole Quaternary sediment section. The input of iron into the sediment is controlled by the productivity of the water column, and it is not influenced by diagenetic remobilisation. The short distance variations of the element/Al ratios (Figs. 1–3) within the sampled depth are higher compared to those found in other areas. These variations are caused by changes of the material input into the sediment, concentrating or diluting elements according to water-column productivity. Manganese shows a very large surface-layer peak due to diagenetic remobilisation and upward transport within the pore water (Fig. 2). The transition metals Co, Ni, Cu, and Zn (details see appendix; sum given in Fig. 3) follow the manganese distribution at the surface layer with a significant surface enrichment. The high element/Al ratios and the variation of transition metal profiles reflect variable biogenic input of material due to changes of the productivity conditions at this site. Despite suboxic sediment conditions and diagenetic remobilisation, no manganese nodules were recovered in this area. 5.2. Area 5: SEDIPERU In the sediments of the SEDIPERU area (cores 184KL and 183KG), a surface-layer accumulation of manganese and less significant of the transition metals Co, Ni, Cu, and Zn are observed (Figs. 2 and 3). Compared to the average concentration of shales, iron is enriched in the Quaternary sediment section also (Fig. 1), but no diagenetic remobilisation is noted. The surface accumulation of manganese (Mn/AlE0.6) is less pronounced compared to the area influenced by high productivity. In the top sediment layer at this site, 16.8 kg/m2 of small ellipsoidal manganese nodules were recovered.
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Fig. 1. Downcore distribution of iron in Peru Basin sediments.
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Fig. 2. Downcore distribution of manganese in Peru Basin sediments.
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Fig. 3. Downcore distribution of transition metals (Co, Ni, Cu, Zn) in Peru Basin sediments.
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Table 2 Average element concentration of shales Element
mg/kg
El/Al
Al Fe Mn Co Ni Cu Zn P Co+Ni+Cu+Zn V Mo Cr
88000 48000 850 19 68 45 95 227 130 2.6 90
E0.55 E0.001 E0.00022 E0.00077 E0.00051 E0.0011 E0.0026 E0.0015 E0.00003 E0.001
5.3. Area 2: DISCOL In the DISCOL area in water depths above the CCD (cores 206KL and 204KG: water depth 4060 m), a similar trend of the element distribution in the Quaternary sediments can be observed. Iron is enriched downcore (Fig. 1), but there is also no evidence for diagenetic removal. No shorttime variation of the distribution of Fe, Mn, and transition metals is found, so that sedimentation is supposed to happen under quite constant conditions. No enrichment of the transition metals (Mn, Co, Ni, Zn) can be found in the oxic surface layer. Over the whole sampled sediment, the element/Al ratios are not significantly higher than the average shale values. Only the surface accumulation of manganese indicates a diagenetic remobilisation process in the Quaternary sediment. In cores sampled in water depths below the CCD (cores 217KL and 193KG: water depth 4294 m), the surface enrichment of metals and the correlation between their profiles in the sediment is less significant (Figs. 1–3). There are some remarkable downcore peaks of Co, Ni and Cu (see appendix), and the profiles of these elements follow each other very closely, but there is no correlation to the manganese distribution so that their variation may be controlled by a similar process that does not follow the manganese diagenetic remobilisation.
5.4. Area 1a, b: Sediments from the eastern areas These areas (cores 278KA and 252KG no influence of upwelling, and 244KA and 255KG under influence of upwelling) are strongly influenced by detrital material from the South American continent (Figs. 1–3). The Fe/Al ratios are very low and do not show any downcore variation. The uniformity of the profiles indicates that the composition of the detrital component has been constant over the period of sedimentation and no diagenetic remobilisation has occurred. Mn/Al ratios are also low with little surface accumulation. The ranges of the concentration profiles of the transitions metals are close to average shale levels.
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The element/Al ratios in the Quaternary sediment sections of all sampled locations follow a general trend that indicates the influence of the South American continent on the eastern part of the Peru Basin (locations 1a, b, and 2), with its input of detrital components presumably supplied by regional currents. Low element/Al ratios, with little variation at depth, are observed in the east. In contrast, the western part of the Peru Basin (Areas 5 and 6) gets its supply from pelagic zones of the Pacific deep-sea floor, enriched in authigenic clay minerals. Downcore variations of the element/Al ratios presumably reflect productivity changes of the water column (Marchig et al., 2001). 5.5. Chemical partitioning of selected metals The distribution of 10 elements (Ca, Sr, Mg, Al, Fe, Mn, Co, Ni, Cu, Zn: see CD-ROM) in selected depths of the Quaternary sediments were investigated over the operationally defined phases in the sequential leaching procedure. The different host phases of the investigated elements are defined as follows: 1. Carbonates, carbonate-bond, and adsorbed elements. 2. Mn-oxyhydroxides and co-precipitated elements. 3. Acid soluble, residual fraction (Fe-oxyhydroxides, clay minerals, detrital fraction (partially)). In Figs. 4 and 5, mean values of the elements Fe, Mn, Co, Ni, Cu, and Zn are plotted from different depths of the Quaternary sediment sections, representing the influence of different redox conditions on the transition-metal phase distribution. In Fig. 4, the element distribution in deeper sediment sections under suboxic conditions (samples see Table 3) are given. As expected, 90% of the calcium occurs as carbonate, except in samples recovered below the CCD in the DISCOL area. From 50–70% of the strontium is incorporated in the carbonate phase of the sediments, with highest levels in areas under enhanced productivity (see CD-ROM). Al and Fe are part of the residual (detrital) sediment phase with no significant productivity influence. About 60% of the manganese occurs in an oxyhydroxide phase even under suboxic conditions in deeper sediment sections. A minor amount of the transition metals Co, Ni, Cu, and Zn is bonded to the easily reducible phase by adsorption or incorporation into the crystal lattice of the Mn-oxyhydroxides, but 50–80% of these metals still remain in the residual phase and are not available for processes of diagenetic remobilisation. The third step of the leaching procedure is supposed to leach elements from parts of the crystal lattice of most silicates and clay minerals. In this step, the largest parts of Al, Ba, Cr (not shown) and Fe are dissolved. In Fig. 5, the distribution of elements in the sequential leaching steps is plotted only for samples of the oxic, semi-liquid layer from the water/sediment interface (samples see Table 3). The partitioning of Ca, Sr, and Mg, occurring as carbonate phases, as well as Al, Ba, Cr (not shown) and Fe in the (operationally defined) residual phase, is quite similar compared to the phase distribution of these elements in deeper suboxic sediment sections. The influence of changing redox-conditions with the sample depth is very small on these elements in contrast to the same influence on the transition metals Co, Ni, Cu, Zn and especially Mn. These elements are significantly enriched in the reducible fraction (Fig. 5). Up to 90% of the manganese content is fixed in an oxide/hydroxide phase together with the transition metals (Co>Ni>Cu>Zn). The
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Fig. 4. Average phase distribution of metals in suboxic sediments.
distinct change of the phase distribution at levels of the manganese redox boundary at the top of the sediments (5–20 cm) indicates the mechanism of diagenetic remobilisation/reprecipitation as follows. In deeper sediment sections, suboxic conditions are induced by the decomposition of organic material. Manganese is reduced to a low valence state (Mn(II)) and is transported upward in the
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Fig. 5. Average phase distribution of metals in the oxic top sediment layer.
. sediment pore water by diffusion (Calvert and Pedersen, 1993; Konig et al., 2001). Near the sediment surface the influence of oxic bottom water changes the redox milieu of the sediment surface layer from anoxic to oxic. Mn(II) is thermodynamically unstable in the presence of oxygen and is oxidised to insoluble Mn(III) and Mn(IV) phases. The surface accumulation of Mnoxyhydroxide phases is shown in Fig. 6.
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Table 3 Samples treated in the sequential leaching procedure Area
Core
Sampling depth (cm)
Equatorial, high production SEDIPERU DISCOL DISCOL > CCD Eastern area Eastern area (upwelling)
243KL/287KG 184KL/183KG 206KL/204KG 217KL/193KG 278KA/252KG 244KA/255KG
0–40/100–150 0–60/210–300 0–30/100–150 0–15/250–370/1450–1520 0–40/150–185 0–45/90–125
Other elements such as the transition metals Co, Ni, Cu, and Zn are adsorbed or coprecipitated to the Mn-oxyhydroxide phases. Under present conditions, these precipitated elements are fixed within the boundary under oxic conditions and are actually not available for the manganese nodule accretion process. Redox-controlled diagenesis in the Quaternary sections of Peru Basin sediments depends on the supply of organic material from the water column and its breakdown in the sediments (Finney et al., 1988; Dymond et al., 1984). In the sediments of the areas investigated, organic carbon content varies inversely to the carbonate content (see Marchig et al., 2001). The content of CaCO3 is controlled by carbonate dissolution, depending on the core location in relation to the CCD. The variation of carbonate with depth in the cores reflects the known Pacific dissolution/preservation cycles, but is larger in cores with higher average CaCO3 contents. In cores sedimented deeper in the Peru Basin, only relics of former carbonate maxima are preserved. The opposite trend in organic carbon downcore variations can be explained with the higher permeability of foraminiferal ooze. Organic matter reaching the seafloor can react with oxic bottom water until it is cut off from the sea water by a layer of impermeable sediment. Due to the fact that foraminiferal oozes are more permeable than clayey sediments, it requires a thicker layer to preserve the organic matter from seawater influence. Therefore, carbonate-poor sediments can preserve a larger amount of organic matter safe from oxidation. The organic carbon variations are superimposed on a general decrease with depth in all six cores. The decrease cannot be explained with a better preservation towards the sediment surface. The reason must be an increase of the primary production and therefore a higher input of organic matter to the sediment. After the complete consumption of all available oxygen, the buried organic matter is oxidised in 3+ 2 and Mn4+. As a result of the consumption of a second step by oxidants like NO 3 , SO4 , Fe these oxidants, the relevant sediment sections are reduced, and the manganese is obviously dissolved and transported with the pore water to the top of the sediment column as Mn2+, where it is again oxidised and reprecipitated under the influence of oxygen-rich seawater. The oxidation can occur only at the sediment/water interface, depending on the actual horizontal position of the redox boundary. The transition zone between dark brown, manganese-rich top and green–grey reduced underlying sediment is characterised by traces of bioturbation, well visible because of the contrasting colours; brown within the bioturbation zone and green–grey in the surrounding (Suckow et al., 2001). In the acetic-acid dissolution step, a highly significant correlation between Ca and Sr is observed. The result of an extrapolation of the strontium concentration to 100% CaCO3 shows
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Fig. 6. Manganese concentration and phase distribution in Peru Basin sediments.
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strontium content of 1400 mg/kg in the calcite phase. The uniformity in the chemical composition of the calcites from all investigated areas is an indication of their homogeneous composition. In contrast to the positive correlation between Ca and Sr, there is no correlation between Ca and Mg, because much of the magnesium is desorbed from clay minerals. The second step of leaching with NH2OH in acid medium (reducible fraction) dissolves metals present as oxides or adsorbed/incorporated to the oxide–hydroxide phases of the sediment. Mainly transition metals are leached in this step. Among these only manganese is enriched in oxides in comparison to the low manganese content in silicates. Pb, Co, Ni, Cu, and Zn occur as cations in an oxidative environment and are adsorbed and enriched on the negatively charged manganese hydroxide surface (Cronan, 1980). Under similar conditions vanadium and molybdenum occur as anions and therefore, are enriched on positively charged iron hydroxide surfaces. Only between 2–4% of iron is leachable by the acid-reducing medium in the second step of the procedure. But due to the high bulk iron content in the sediments, the amount of iron occurring as hydroxide is probably sufficient to scavenge and enrich vanadium and molybdenum. In the dissolution step with concentrated acids, the largest parts of Al, Fe, Ba, and Cr are dissolved. High amounts of the silicates in the investigated sediments from the northern Nazca Plate (Marchig et al., 2001) are clay minerals, authigenic montmorillonite with exchange of Al with Fe3+, nontronite, illite, kaolinite and chlorite. The last three are detrital and enriched in the east. The presence of Al and Fe in the acid extracts is explained by the partial dissolution of clay minerals and the dissolution of crystalline Fe-oxides under the conditions of extraction. In the sediments Ba occurs as barite, which is also partially soluble. The Cr species are unknown, but their predominant amount is bonded to acid-soluble silicates. 5.6. Composition of the reducible sediment phase and manganese nodules Table 4a and b show the chemical composition of manganese nodules and crusts from the Peru Basin (v. Stackelberg, 1997) in contrast to the composition of the reducible phase of sediments from the water/sediment interface. From the tables it can be seen that the oxide phase composition from the top of SEDIPERU area sediments is similar to the composition of dendritic diagenetic nodule layers. The sediment oxide phase composition of the DISCOL area differs significantly from the diagenetic layers, which are characteristic for large nodules. The reducible leaching results agree with the growth mechanism hypotheses for different nodule types. The dense-laminated diagenetic layers of nodules are formed by a second mobilisation step of metals from an oxidation zone due to a change in the horizontal position of the manganese redox boundary in the sediment column. Easily mobilisable metals (especially Mn) are depleted, and hardly mobilisable elements like Fe together with parts of Co, Ni, Cu, and Zn are left in the sediment. This mechanism causes an enrichment of manganese in diagenetic layers of big nodules and a depletion of manganese in the remaining sediment oxide phase. Dymond et al. (1984) described the moving of the horizontal position and the thickness of the redox boundary in deep-sea sediments under the influence of climatically induced labile carbon fluxes into the sediment. A high-labile carbon flux, caused by enhanced productivity of the water column in cold periods, increases the iron and manganese reduction rate at the top of the
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Table 4 Chemical compositiona Mn/Fe
Mn/Co
Mn/Cu
Mn/Ni
Mn/Zn
79 11 10 2
3935 1520 632 129
307 67 49 54
156 32 38 25
794 210 284 244
(b) Reducible sediment phases (top layers) Area Equatorial, high production 9,5 SEDIPERU 12,9 DISCOL 4 DISCOL > CCD 4 Eastern area 5 Eastern area (upwelling) 6,2
873 471 428 304 697 955
152 180 72 43 83 91
59 60 39 38 45 53
885 258 174 140 212 243
(a) Mn-nodules and crusts Growth structure H d(b) d+h Crust
a
hFdense, laminated, dFdendritic, bFlaminated to columnar.
sediments and decreases the thickness of the oxidised layer. At present, the thickness of the redox boundary varies from 5 to 15 cm in the different investigated areas (Marchig et al., 2001, Figure 7), with lowest levels under the influence of equatorial high productivity and upwelling conditions, obviously caused by an enhanced input of organic material. The diagenetic remobilisation/reprecipitation process should produce manganese-rich oxides under low carbon flux conditions. Such remobilisation dissolves only manganese, leaving iron and trace elements in a reduced, manganese-depleted environment. This could be the reason for the local enrichment of trace elements, especially transition metals (Co, Ni, Cu, Zn) in the reduced parts of the Quaternary sediments. However, because only limited parts of the cores were investigated by sequential leaching methods, no general conclusion can be made about such a mechanism of trace enrichment in deeper reduced sediment sections. As it is evident that the position of the redox boundary changes from time to time, nodule layers rich in manganese should be ones that are precipitated from remobilisation under low organic flux conditions in a narrow redox (Eh ) range.
6. Conclusions Depth profiles of metals in Quaternary sediment sections of six cores from the Peru Basin suggest that manganese and other transition metals are mobilised from suboxic sediment sections and precipitated under oxic conditions near the water/sediment interface. The supposed mechanism of the diagenetic remobilisation/reprecipitation process is compatible with data from element partitioning by sequential leaching, showing the phase distribution of metals enriched at
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the water/sediment interface under oxic conditions. Transition metals occur in oxide–hydroxide sediment phases and are available for incorporation into manganese nodules during time periods when the horizontal position of the redox boundary and the surfaces of manganese nodules are located in the same depth. Anthropogenic impacts such as deep-sea mining of manganese nodules will disturb the position of the redox boundary. The top sediment layers will be stirred and resuspended into the bottom water. In contrast to the mobilisation of metals from sediments by naturally induced variations of the redox conditions, mining will be rapid. Metals will be released into the bottom water under oxic conditions. Metal concentrations of Peru Basin sediments, their variation with depth and their phase distribution are important parameters to consider in the evaluation of technical . impacts on the deep sea environment (Rolinski et al., 2001; Konig et al., 2001; Haeckel et al., 2001). Acknowledgements The material for this publication was obtained during a cruise of R.V. Sonne, funded by the German Bundesministerium fur . Bildung und Forschung (Project No. 03G0106). We appreciate the helpful discussion and review of the manuscript by D. S. Cronan. The responsibility for the contents of this publication is solely with the authors.
References Calvert, E.E., Pedersen, T.F., 1993. Geochemistry of recent oxic and anoxic marine sediments: implications for the geological record. Marine Geology 113, 67–88. Chester, R., Hughes, M.J., 1967. A chemical technique for the separation of Fe–Mn-minerals, carbonate minerals and adsorbed trace elements from pelagic sediments. Chemical Geology 2, 249–262. Cronan, D.S., 1980. Underwater Minerals. Academic Press, London, 362pp. Dymond, J., Lyle, M., Finney, B., Piper, D.Z., Murphy, K., Conrad, C., Pisias, N., 1984. Ferromanganese nodules from MANOP sites H, S, and RFcontrol of mineralogical and chemical composition by multiple accretionary processes. Geochimica Cosmochimica Acta 48, 931–949. Erlandson, D., Hussong, D., Campell, J., 1981. Sediments and associated structure of the Northern Nazca Plate. In: Kulm, L., Dymond, J., Dash, E.J., Hussong, D.M., Roderick, R. (Eds.), Nazca Plate: Crustal Formation and Andean Convergence. Geological Society of America, Memoir 154, 295–314. Fellerer, R., 1980. Manganknollen Forschungskampagne SOPAC II: Fahrtbericht zu den Ausfahrten SO 11/1 und SO 11/2. Finney, B.P., Lyle, M.W., Heath, G.R., 1988. Sedimentation at MANOP site H (eastern equatorial pacific) over the past 400.000 years: climatically induced redox variations and their effects on transition metal cycling. Paleoceanography 3 (2), 169–189. . Haeckel, M., Konig, I., Riech, V., Weber, W.E., Suess, E., 2001. Pore water profiles and numerical modelling of biogeochemical processes in Peru Basin deep-sea sediments. Deep-Sea Research II 48, 3713–3736. Kheboian, C., Bauer, C.F., 1987. Accuracy of selective extraction procedures for metal speciation in model aquatic sediments. Analytical Chemistry 59 (10), 1417–1423. . 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.
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Koschinsky, A., 1994. Geochemische Krustenprofile und sequentielle Laugungsversuche an Manganerzkrusten aus dem Zentralpazifik zur Kl.arung von Genese und Elementassoziationen. Berliner Wissenschaftliche Abhandlungen, Reihe A, Band 158; Berlin. Koschinsky, A., Halbach, P., 1995. Sequential leaching of marine ferromanganese precipitates: genetic implications. Geochimica Cosmochimica Acta 59 (24), 5113–5132. Mamerickx, J., Anderson, R.N., Menard, H.W., Smith, S.M., 1980. Morphology and tectonic evolution of the east central pacific. Geological Society of America Bulletin 86, 111–118. Marchig, V., v. Stackelberg, U., Wiedicke, M., Durn, G., Milovanovic, D., 1999. Hydrothermal activity associated with off-axis volcanism in the Peru Basin. Marine Geology 159, 179–203 Marchig, V., von Stackelberg, U., Hufnagel, H., Durn, G., 2001. Compositional changes of surface sediments and variability of manganese nodules in the Peru Basin. Deep-Sea Research II 48, 3523–3547. Mayes, C.L., Lavwer, L.A., Sandwell, D.T., 1990. Tectonic history of South Pacific. Journal of Geophysical Research 95 (6), 8543–8567. Naar, D.F., Hey, R.N., 1989. Recent Pacific-Easter-Nazca Plate motions. AGU, pp. 9–30. Rea, D.K., 1978. Evolution of the East Pacific Rise between 31S and 131S since the middle miocene. Geophysical Research Letters 5 (7), 561–13564. Reyss, J.L., Marchig, V., Ku T.L., 1982. Rapid growth of a deep sea manganese nodule. Nature, 295, 5848, 401–403. Rolinski, S., Segschneider, J., Sundermann, . J., 2001. Long-term propagation of tailings from deep-sea mining under variable conditions by means of numerical simulations. Deep-Sea Research II 48, 3469–3485. Rose, A.W., Hawkes, H.E., Webb, J.S., 1979. Geochemistry in mineral exploration. Academic Press, New York. v. Stackelberg, U., 1987. Pumice and buried manganese nodules from the equatorial north pacific ocean. Geologisches Jahrbuch D87, 229. v. Stackelberg, U., 1997. Growth history of manganese nodules and crusts of the Peru Basin. In: Nicholson, K., Hein, J.R., Buhn, . B., Dasgupta, S. (Eds.), Manganese Mineralization: Geochemistry and Mineralogy of Terrestrial and Marine Deposits. Special Publication No. 119, Geological Society. pp, 153–176. Suckow, A., Treppke, U., Wiedicke, M., Weber, M.E., 2001. Bioturbation coefficients of deep-sea sediments from the Peru Basin determined by gamma spectrometry of 210Pbexc. Deep-Sea Research II 48, 3569–3592. Tessier, A., Campbell, P.G.C., Bisson, M., 1979. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 51 (7), 844–851. Weber, M.E., v. Stackelberg, U., Marchig, V., Wiedicke, M., Grupe B., 2000. Variability of surface sediments in the Peru Basin: Dependence on water depth, Productivity, bottom water flow, and seafloor topography. Marine Geology 163, 169–184. Wedepohl, K.H., 1970. Environmental influences on the chemical composition of shales and clays. In: Ahrens, L.H., Press, F., Runcorn, S.K., Urey, H.C. (Eds.), Physics and chemistry of the earth. Pergamon Press, Oxford, pp. 307–333. Wedepohl, K.H., 1991. The composition of the upper earth’s crust and the natural cycles of selected metals. In: Merian, E. (Ed.), Metals and their compounds in the environment. Verlag Chemie, Weinheim. Wiedicke, M., Weber, M.E., 1996. Small-scale variability of seafloor features in the northern Peru basin: results from acoustic survey methods. Marine Geophysical Research 18, 507–526.
Mobility of metals over the redox boundary in Peru Basin sediments Jens Stummeyer, Vesna Marchig* Bundesanstalt fur . Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany
Abstract Downcore variations and phase distribution of metals in the Quaternary sediments from six areas of the Peru Basin were investigated. The sediments are characterised by a 5–20 cm thick, oxic layer at the water/ sediment interface formed under the influence of dissolved oxygen in the bottom water. In deeper sediment sections, suboxic conditions are found due to the degradation of organic matter. Variations in the downcore abundance of metals (e.g., Fe, Mn, Co, Ni, Cu, Zn, V, Mo, Cr) were determined by bulk chemical methods. A significant accumulation of manganese is observed at the depth of the manganese redox boundary, caused by diagenetic remobilisation under suboxic conditions and reprecipitation under oxic conditions. To investigate their phase distributions, partitioning of elements in selected samples over the sediment column was carried out in sequential leaching experiments. A comparison between the surface layer and deeper sediment layers shows a distinct change in the element distribution between operationally defined host phases (carbonates, oxy-hydroxides, detrital components). Above the redox boundary at the sediment surface, transition metals are enriched in the reducible Mn-oxide fraction. When the horizontal position of the Mn-redox boundary moves upward, influenced by naturally occurring enhancement of surface-water productivity, these elements are mobilised and available to the manganese nodule accretion process if the redox boundary reaches the level of growth of the nodules. In contrast to the naturally induced mobilisation of metals, a technical impact (e.g., mining of manganese nodules) would be a rapid event occurring under completely different redox conditions. Metals would be released into the bottom water, an oxic environment, where scavenging of dissolved metals and resedimentation takes place in relatively short periods of time. Presumably, a long-distance transport of contaminants will not happen. r 2001 Elsevier Science Ltd. All rights reserved.
*Corresponding author. E-mail address: [email protected] (V. Marchig). 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 5 6 - X
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1. Introduction The nodule field of the Peru Basin, located in the eastern equatorial Pacific, shows a very dense coverage with manganese nodules, with a maximum abundance of 50 kg/m2 near the calcite compensation depth (CCD) between 4000 and 4200 m (v. Stackelberg, 1997). Accretion rates of the nodules are up to two orders of magnitude higher than elsewhere in the Pacific (Reyss et al., 1982). The nodules are mostly diagenetic in origin; the observed high accretion rates are assumed to result from the remobilisation of metals from the Quaternary sediments and their reprecipitation at the sediment/water interface under the influence of dissolved oxygen. This reprecipitation causes a dark brown colour of the semi-liquid layer at the top of the sediments (5–20 cm depth) in contrast to the light green–grey colour of the underlying sediments. The highest accretion rates are found at the bottom of nodules embedded in the semi-liquid layer at a level immediately above the redox boundary. The well-investigated diagenetic remobilisation of manganese and iron in the sediments and the formation of an oxic top layer with embedded nodules (Reyss et al., 1982; Dymond et al., 1984; v. Stackelberg, 1987, 1997) will be expanded in this paper with the behaviour of other metals (e.g., Mn, Fe, Co, Ni, Cu, Zn) in the diagenetic remobilisation/reprecipitation process under changing redox conditions. For these investigations, six sediment cores were selected, four within the nodule field of the Peru Basin and two adjacent to the nodule field with a thin oxic layer on the top of the sediment and no manganese nodules. The positions of the selected cores are mapped in the appendix (CD-ROM). Table 1 lists some important parameters of the selected cores.
2. Geologic setting The area of investigation is located on the northern Nazca Plate, which was generated on the Galapagos Rise spreading centre 20–6 million years ago (Mamerickx et al., 1980; Mayes et al., 1990). The Galapagos Rise subsided the spreading activity 9 million years B P at its southern end; the northernmost part ceased spreading 6 million years B P (Rea, 1978). Since then the Nazca Plate has been created at the East Pacific Rise and has drifted towards South America with a slightly changed spreading direction. The change of the spreading direction is probably the reason for the reactivation of volcanic activity on old fault zones (Wiedicke and Weber, 1996), which gave rise to hydrothermal circulation that can still be observed on scattered positions (Marchig et al., 1999). The sediment cover is fairly thin in the investigated area, from a maximum of 150 m in the north where the sedimentation is influenced by higher biogeneous production of the equatorial zone to several tens of meters in the south where the sediments are located below the calcite compensation depth (Fellerer, 1980). At a few positions, basalt was reached (by coring) only 15–17 m beneath the seafloor, but this basalt is not the primary basement, but rather a product of reactivated late Miocene volcanism (Marchig et al., 1999). The general observation that the sedimentation rates decrease from north to south following the increase of water depth (Erlandson et al., 1981; Naar and Hey, 1989) indicates that the main factor governing the sedimentation rate is the dissolution of calcite. The direction of plate
Area
Coordinates
Core with corresponding short core
Water depth (m)
Thickness of Quaternary sediment (m)
Content of CaCO3 (quat. Sed.) (%)
Manganese nodule typeb
Manganese nodules (kg/m2)
6: Marginal part of the equatorial high production zone 5: SEDIPERU 2:DISCOL 2: DISCOL > CCD 1b: Eastern area 1a: Eastern area with influence of upwelling
516.500 S, 90114.840 W
243KL, 287KG
3940
18.5c
33.4715.2
F
0
6134.410 S, 901 31.130 W 717.790 S, 88134.200 W 716.400 S, 88122.700 W 6134.290 S, 86111.430 W 5130.080 S, 85122.370 W
184KL, 183KG 206KL, 204KG 217KL, 193KG 278KA, 252KG 244KA, 255KG
4104 4060 4294 4105 4085
12 7 14.3 6 11
21.7712.9 10.378.1 5.173.9 7.776.8 14.877.4
B A A A F
16.8 23.0 2.4 21.4 0
a
CCD=calcite compensation depth. AFbig cauliflower-like nodules (scarce), BFsmall ellipsoidal nodules (frequent). c Extrapolated from sedimentation rate. b
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Table 1 Parameters of the six investigated cores from the study areasa
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movement towards the ESE does not transport the sediment into areas of lower primary productivity; the positions of the cores have moved more or less parallel to the isolines of biogenic production, at least for 6 million years. Thus, there is no biogenic change in sedimentation from west to east, although the nature of the inorganic part is changing towards the South American continent, being more and more influenced by continental detritus (Marchig et al., 2001).
3. Core locations Essential data about the six selected core locations are presented in Table 1. Samples were recovered using the following tools: box corer (KG), piston corer (KL) and long box corer (KA). The box corer (KG-samples) was used at the same locations as the piston corer (KL-samples) to recover nearly undisturbed top sediment with nodules in their original positions. Core 243KL, with corresponding short core 287KG, is located nearest to the equatorial high productivity zone compared to the other investigated cores (CD-ROM; Area 6: marginal part of the equatorial high productivity zone). Carbonate and opaline silica in the sediments are high in this area (Weber et al., 2000). The length of the core is 12.85 m, the deepest sample has an age of 1.19 million years. Extrapolated from the average sedimentation rate in this area, the thickness of the Quaternary part of the sediment would be about 18.5 m, which would represent the thickest Quaternary section of all investigated cores. The corresponding short core from the same position does not contain manganese nodules on the sediment, as it is located outside of the Peru Basin nodule field. Core 184KL with corresponding short core 183KG is located in the SEDIPERU area (CDROM; Area 5), the site with the most homogeneous coverage of manganese nodules. The oxic semi-liquid layer is also thicker. Carbonate contents as well as the thickness of the Quaternary section are lower. The corresponding short core contains 16.8 kg/m2 of small ellipsoidal manganese nodules. Core 206KL from the DISCOL area (CD-ROM; Area 2) contains a shorter Quaternary section and less carbonate. The corresponding short core 204KG contains 23 kg/m2 of big cauliflower-like manganese nodules. Core 217KL, with corresponding short core 193KG, was obtained in a trough in the DISCOL area, significantly below the calcite compensation depth (CD-ROM; Area 2). As expected, the carbonate content is very low. The thickness of the Quaternary sediment is unexpectedly high, which can be explained by focusing of fine grained material from adjacent morphologic highs. 2.4 kg/m2 of big cauliflower-like nodules were sampled from the sea floor. Core 278KA, with corresponding short core 252KG, also has a shorter Quaternary sequence section and lower carbonate content. It was sampled significantly closer to the South American continent compared to the other cores investigated, but still inside the Peru Basin nodule field (CD-ROM; Area 1b). Consequently, the short core at this position was covered with 21.4 kg/m2 of big cauliflower-like nodules. Core 244KA, with corresponding short core 255KG, was deposited under the influence of enhanced productivity due to upwelling (CD-ROM; Area 1a). It has elevated carbonate content
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and a thicker Quaternary sediment section. Opaline silica does not show a significant increase (Weber et al., 2000). This core is located outside of the manganese nodule field.
4. Sampling and analytical methods Sampling was performed with 20 m-long piston corers. The cores were subsampled at intervals of 10 cm for XRF bulk analysis. Additionally, at the same positions a box corer was used (50 50 50 cm3) to gain undisturbed surface sediment with manganese nodules in their original positions. The corresponding short cores from this box corer were subsampled every 5 cm for chemical analyses and the determination of element mobility over the redox boundary at 5–20 cm. . At some positions, a long box corer was used for pore water investigations (Konig et al., 2001). The long box corer was sampled for chemical analyses at 5 cm intervals. Sediment samples were freeze-dried and milled to less than 40 mm particle size in an agate mill. Bulk chemical analyses were performed with X-ray fluorescence using Philips PW 1400 and PW 1480 wavelength dispersive spectrometers. Prior to quantitative determination of 42 major and trace elements, samples were fused with Li-metaborate at 12001C for 20 min. The resulting glassy disks ensure a uniform distribution of the elements. Quality of the results was controlled with more than 100 certified reference materials (CRM). An analytical precision better than 70.5% relative for major elements and 1–10 mg/kg for trace elements could be achieved. The determination of element mobility from selected Quaternary sediment sections was performed with a three-step sequential leaching procedure. Procedures from the literature were modified according to the carbonate content and the content of iron and manganese oxides (Chester and Hughes, 1967; Tessier et al., 1979; Kheboian and Bauer, 1987; Koschinsky, 1994; Koschinsky and Halbach, 1995). The resulting sequential leaching procedure was performed as follows: 1. Weak-acid leachable fraction (including exchangeable cations and carbonates). 0.5 g of powdered dried sample were shaken with 20 ml of a 0.1 M acetic acid solution for 12 h overnight. In case of a carbonate content above 20%, the first step was performed twice to ensure complete dissolution. After centrifugation the residue was washed with water. 2. Easily reducible fraction (including Mn-oxides and co-precipitated elements). 20 ml of a 0.1 M hydroxylamine–hydrochloride solution (NH2OH/HCl; pH=2) were added to the residue of step 1. The mixture was shaken for 12 h at room temperature. The residue was again centrifuged and washed with water. 3. Residual fraction (Fe-oxyhydroxide phases and other elements leachable with HCl/HNO3). The residue of step 2 was treated with 4 ml of boiling HCl/HNO3 (3 : 1). All reagents used were of analytical grade. Possible contamination of the resulting solutions was checked using reagent blanks. The solutions of the leaching steps were diluted with water to a volume of 50 ml. Element concentrations (Fe, Mn, Co, Ni, Cu, Zn, V, Cr, Mo, Pb, Cd, Al, Mg, Ca, Sr, Ba, Be) were analysed with a Jobin–Yvon ICP-OES (optical emission spectrometer), type JY 166 Ultrace. Standard deviations of the leaching procedure and the ICP-OES measurements were generally better than 10%, determined with multiple extraction of selected samples. The
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recovery of easily soluble elements (e.g., Ca and Mn) ranged between 90% and 110% compared to XRF bulk analyses.
5. Results and discussion To investigate downcore variations of metal abundance in the Quaternary sediment sections (Fe, Mn, Co, Ni, Cu, Zn, V, Mo, Cr), element/aluminium ratios versus sediment depth were plotted for the six areas (see Appendix). In Figs. 1–3, the Fe/Al, Mn/Al, and transition metal/Al ratios (sum of Co, Ni, Cu, and Zn in mg/kg) analysed by XRF are plotted. To ensure the comparison between different areas, the carbonate content resulting from the XRF analyses of CaO was subtracted and the element concentrations were normalised to the non-carbonate content. The calculated values of the Peru Basin sediments are compared to the average element concentrations of shales given in Table 2 (Wedepohl, 1970, 1991; Rose et al., 1979) to estimate element enrichment or depletion. 5.1. Area 6: Sediments from the marginal part of the equatorial high productivity zone The sediments of this area (core 243KL; short core 287KG) show the highest Fe/Al ratio (Fe/ AlE0.8) compared with the other areas investigated (Fig. 1). The Fe/Al-ratio is characterised by a considerable short-time variability, but it is relatively constant over the whole Quaternary sediment section. The input of iron into the sediment is controlled by the productivity of the water column, and it is not influenced by diagenetic remobilisation. The short distance variations of the element/Al ratios (Figs. 1–3) within the sampled depth are higher compared to those found in other areas. These variations are caused by changes of the material input into the sediment, concentrating or diluting elements according to water-column productivity. Manganese shows a very large surface-layer peak due to diagenetic remobilisation and upward transport within the pore water (Fig. 2). The transition metals Co, Ni, Cu, and Zn (details see appendix; sum given in Fig. 3) follow the manganese distribution at the surface layer with a significant surface enrichment. The high element/Al ratios and the variation of transition metal profiles reflect variable biogenic input of material due to changes of the productivity conditions at this site. Despite suboxic sediment conditions and diagenetic remobilisation, no manganese nodules were recovered in this area. 5.2. Area 5: SEDIPERU In the sediments of the SEDIPERU area (cores 184KL and 183KG), a surface-layer accumulation of manganese and less significant of the transition metals Co, Ni, Cu, and Zn are observed (Figs. 2 and 3). Compared to the average concentration of shales, iron is enriched in the Quaternary sediment section also (Fig. 1), but no diagenetic remobilisation is noted. The surface accumulation of manganese (Mn/AlE0.6) is less pronounced compared to the area influenced by high productivity. In the top sediment layer at this site, 16.8 kg/m2 of small ellipsoidal manganese nodules were recovered.
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Fig. 1. Downcore distribution of iron in Peru Basin sediments.
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Fig. 2. Downcore distribution of manganese in Peru Basin sediments.
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Fig. 3. Downcore distribution of transition metals (Co, Ni, Cu, Zn) in Peru Basin sediments.
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Table 2 Average element concentration of shales Element
mg/kg
El/Al
Al Fe Mn Co Ni Cu Zn P Co+Ni+Cu+Zn V Mo Cr
88000 48000 850 19 68 45 95 227 130 2.6 90
E0.55 E0.001 E0.00022 E0.00077 E0.00051 E0.0011 E0.0026 E0.0015 E0.00003 E0.001
5.3. Area 2: DISCOL In the DISCOL area in water depths above the CCD (cores 206KL and 204KG: water depth 4060 m), a similar trend of the element distribution in the Quaternary sediments can be observed. Iron is enriched downcore (Fig. 1), but there is also no evidence for diagenetic removal. No shorttime variation of the distribution of Fe, Mn, and transition metals is found, so that sedimentation is supposed to happen under quite constant conditions. No enrichment of the transition metals (Mn, Co, Ni, Zn) can be found in the oxic surface layer. Over the whole sampled sediment, the element/Al ratios are not significantly higher than the average shale values. Only the surface accumulation of manganese indicates a diagenetic remobilisation process in the Quaternary sediment. In cores sampled in water depths below the CCD (cores 217KL and 193KG: water depth 4294 m), the surface enrichment of metals and the correlation between their profiles in the sediment is less significant (Figs. 1–3). There are some remarkable downcore peaks of Co, Ni and Cu (see appendix), and the profiles of these elements follow each other very closely, but there is no correlation to the manganese distribution so that their variation may be controlled by a similar process that does not follow the manganese diagenetic remobilisation.
5.4. Area 1a, b: Sediments from the eastern areas These areas (cores 278KA and 252KG no influence of upwelling, and 244KA and 255KG under influence of upwelling) are strongly influenced by detrital material from the South American continent (Figs. 1–3). The Fe/Al ratios are very low and do not show any downcore variation. The uniformity of the profiles indicates that the composition of the detrital component has been constant over the period of sedimentation and no diagenetic remobilisation has occurred. Mn/Al ratios are also low with little surface accumulation. The ranges of the concentration profiles of the transitions metals are close to average shale levels.
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The element/Al ratios in the Quaternary sediment sections of all sampled locations follow a general trend that indicates the influence of the South American continent on the eastern part of the Peru Basin (locations 1a, b, and 2), with its input of detrital components presumably supplied by regional currents. Low element/Al ratios, with little variation at depth, are observed in the east. In contrast, the western part of the Peru Basin (Areas 5 and 6) gets its supply from pelagic zones of the Pacific deep-sea floor, enriched in authigenic clay minerals. Downcore variations of the element/Al ratios presumably reflect productivity changes of the water column (Marchig et al., 2001). 5.5. Chemical partitioning of selected metals The distribution of 10 elements (Ca, Sr, Mg, Al, Fe, Mn, Co, Ni, Cu, Zn: see CD-ROM) in selected depths of the Quaternary sediments were investigated over the operationally defined phases in the sequential leaching procedure. The different host phases of the investigated elements are defined as follows: 1. Carbonates, carbonate-bond, and adsorbed elements. 2. Mn-oxyhydroxides and co-precipitated elements. 3. Acid soluble, residual fraction (Fe-oxyhydroxides, clay minerals, detrital fraction (partially)). In Figs. 4 and 5, mean values of the elements Fe, Mn, Co, Ni, Cu, and Zn are plotted from different depths of the Quaternary sediment sections, representing the influence of different redox conditions on the transition-metal phase distribution. In Fig. 4, the element distribution in deeper sediment sections under suboxic conditions (samples see Table 3) are given. As expected, 90% of the calcium occurs as carbonate, except in samples recovered below the CCD in the DISCOL area. From 50–70% of the strontium is incorporated in the carbonate phase of the sediments, with highest levels in areas under enhanced productivity (see CD-ROM). Al and Fe are part of the residual (detrital) sediment phase with no significant productivity influence. About 60% of the manganese occurs in an oxyhydroxide phase even under suboxic conditions in deeper sediment sections. A minor amount of the transition metals Co, Ni, Cu, and Zn is bonded to the easily reducible phase by adsorption or incorporation into the crystal lattice of the Mn-oxyhydroxides, but 50–80% of these metals still remain in the residual phase and are not available for processes of diagenetic remobilisation. The third step of the leaching procedure is supposed to leach elements from parts of the crystal lattice of most silicates and clay minerals. In this step, the largest parts of Al, Ba, Cr (not shown) and Fe are dissolved. In Fig. 5, the distribution of elements in the sequential leaching steps is plotted only for samples of the oxic, semi-liquid layer from the water/sediment interface (samples see Table 3). The partitioning of Ca, Sr, and Mg, occurring as carbonate phases, as well as Al, Ba, Cr (not shown) and Fe in the (operationally defined) residual phase, is quite similar compared to the phase distribution of these elements in deeper suboxic sediment sections. The influence of changing redox-conditions with the sample depth is very small on these elements in contrast to the same influence on the transition metals Co, Ni, Cu, Zn and especially Mn. These elements are significantly enriched in the reducible fraction (Fig. 5). Up to 90% of the manganese content is fixed in an oxide/hydroxide phase together with the transition metals (Co>Ni>Cu>Zn). The
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Fig. 4. Average phase distribution of metals in suboxic sediments.
distinct change of the phase distribution at levels of the manganese redox boundary at the top of the sediments (5–20 cm) indicates the mechanism of diagenetic remobilisation/reprecipitation as follows. In deeper sediment sections, suboxic conditions are induced by the decomposition of organic material. Manganese is reduced to a low valence state (Mn(II)) and is transported upward in the
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Fig. 5. Average phase distribution of metals in the oxic top sediment layer.
. sediment pore water by diffusion (Calvert and Pedersen, 1993; Konig et al., 2001). Near the sediment surface the influence of oxic bottom water changes the redox milieu of the sediment surface layer from anoxic to oxic. Mn(II) is thermodynamically unstable in the presence of oxygen and is oxidised to insoluble Mn(III) and Mn(IV) phases. The surface accumulation of Mnoxyhydroxide phases is shown in Fig. 6.
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Table 3 Samples treated in the sequential leaching procedure Area
Core
Sampling depth (cm)
Equatorial, high production SEDIPERU DISCOL DISCOL > CCD Eastern area Eastern area (upwelling)
243KL/287KG 184KL/183KG 206KL/204KG 217KL/193KG 278KA/252KG 244KA/255KG
0–40/100–150 0–60/210–300 0–30/100–150 0–15/250–370/1450–1520 0–40/150–185 0–45/90–125
Other elements such as the transition metals Co, Ni, Cu, and Zn are adsorbed or coprecipitated to the Mn-oxyhydroxide phases. Under present conditions, these precipitated elements are fixed within the boundary under oxic conditions and are actually not available for the manganese nodule accretion process. Redox-controlled diagenesis in the Quaternary sections of Peru Basin sediments depends on the supply of organic material from the water column and its breakdown in the sediments (Finney et al., 1988; Dymond et al., 1984). In the sediments of the areas investigated, organic carbon content varies inversely to the carbonate content (see Marchig et al., 2001). The content of CaCO3 is controlled by carbonate dissolution, depending on the core location in relation to the CCD. The variation of carbonate with depth in the cores reflects the known Pacific dissolution/preservation cycles, but is larger in cores with higher average CaCO3 contents. In cores sedimented deeper in the Peru Basin, only relics of former carbonate maxima are preserved. The opposite trend in organic carbon downcore variations can be explained with the higher permeability of foraminiferal ooze. Organic matter reaching the seafloor can react with oxic bottom water until it is cut off from the sea water by a layer of impermeable sediment. Due to the fact that foraminiferal oozes are more permeable than clayey sediments, it requires a thicker layer to preserve the organic matter from seawater influence. Therefore, carbonate-poor sediments can preserve a larger amount of organic matter safe from oxidation. The organic carbon variations are superimposed on a general decrease with depth in all six cores. The decrease cannot be explained with a better preservation towards the sediment surface. The reason must be an increase of the primary production and therefore a higher input of organic matter to the sediment. After the complete consumption of all available oxygen, the buried organic matter is oxidised in 3+ 2 and Mn4+. As a result of the consumption of a second step by oxidants like NO 3 , SO4 , Fe these oxidants, the relevant sediment sections are reduced, and the manganese is obviously dissolved and transported with the pore water to the top of the sediment column as Mn2+, where it is again oxidised and reprecipitated under the influence of oxygen-rich seawater. The oxidation can occur only at the sediment/water interface, depending on the actual horizontal position of the redox boundary. The transition zone between dark brown, manganese-rich top and green–grey reduced underlying sediment is characterised by traces of bioturbation, well visible because of the contrasting colours; brown within the bioturbation zone and green–grey in the surrounding (Suckow et al., 2001). In the acetic-acid dissolution step, a highly significant correlation between Ca and Sr is observed. The result of an extrapolation of the strontium concentration to 100% CaCO3 shows
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Fig. 6. Manganese concentration and phase distribution in Peru Basin sediments.
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strontium content of 1400 mg/kg in the calcite phase. The uniformity in the chemical composition of the calcites from all investigated areas is an indication of their homogeneous composition. In contrast to the positive correlation between Ca and Sr, there is no correlation between Ca and Mg, because much of the magnesium is desorbed from clay minerals. The second step of leaching with NH2OH in acid medium (reducible fraction) dissolves metals present as oxides or adsorbed/incorporated to the oxide–hydroxide phases of the sediment. Mainly transition metals are leached in this step. Among these only manganese is enriched in oxides in comparison to the low manganese content in silicates. Pb, Co, Ni, Cu, and Zn occur as cations in an oxidative environment and are adsorbed and enriched on the negatively charged manganese hydroxide surface (Cronan, 1980). Under similar conditions vanadium and molybdenum occur as anions and therefore, are enriched on positively charged iron hydroxide surfaces. Only between 2–4% of iron is leachable by the acid-reducing medium in the second step of the procedure. But due to the high bulk iron content in the sediments, the amount of iron occurring as hydroxide is probably sufficient to scavenge and enrich vanadium and molybdenum. In the dissolution step with concentrated acids, the largest parts of Al, Fe, Ba, and Cr are dissolved. High amounts of the silicates in the investigated sediments from the northern Nazca Plate (Marchig et al., 2001) are clay minerals, authigenic montmorillonite with exchange of Al with Fe3+, nontronite, illite, kaolinite and chlorite. The last three are detrital and enriched in the east. The presence of Al and Fe in the acid extracts is explained by the partial dissolution of clay minerals and the dissolution of crystalline Fe-oxides under the conditions of extraction. In the sediments Ba occurs as barite, which is also partially soluble. The Cr species are unknown, but their predominant amount is bonded to acid-soluble silicates. 5.6. Composition of the reducible sediment phase and manganese nodules Table 4a and b show the chemical composition of manganese nodules and crusts from the Peru Basin (v. Stackelberg, 1997) in contrast to the composition of the reducible phase of sediments from the water/sediment interface. From the tables it can be seen that the oxide phase composition from the top of SEDIPERU area sediments is similar to the composition of dendritic diagenetic nodule layers. The sediment oxide phase composition of the DISCOL area differs significantly from the diagenetic layers, which are characteristic for large nodules. The reducible leaching results agree with the growth mechanism hypotheses for different nodule types. The dense-laminated diagenetic layers of nodules are formed by a second mobilisation step of metals from an oxidation zone due to a change in the horizontal position of the manganese redox boundary in the sediment column. Easily mobilisable metals (especially Mn) are depleted, and hardly mobilisable elements like Fe together with parts of Co, Ni, Cu, and Zn are left in the sediment. This mechanism causes an enrichment of manganese in diagenetic layers of big nodules and a depletion of manganese in the remaining sediment oxide phase. Dymond et al. (1984) described the moving of the horizontal position and the thickness of the redox boundary in deep-sea sediments under the influence of climatically induced labile carbon fluxes into the sediment. A high-labile carbon flux, caused by enhanced productivity of the water column in cold periods, increases the iron and manganese reduction rate at the top of the
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Table 4 Chemical compositiona Mn/Fe
Mn/Co
Mn/Cu
Mn/Ni
Mn/Zn
79 11 10 2
3935 1520 632 129
307 67 49 54
156 32 38 25
794 210 284 244
(b) Reducible sediment phases (top layers) Area Equatorial, high production 9,5 SEDIPERU 12,9 DISCOL 4 DISCOL > CCD 4 Eastern area 5 Eastern area (upwelling) 6,2
873 471 428 304 697 955
152 180 72 43 83 91
59 60 39 38 45 53
885 258 174 140 212 243
(a) Mn-nodules and crusts Growth structure H d(b) d+h Crust
a
hFdense, laminated, dFdendritic, bFlaminated to columnar.
sediments and decreases the thickness of the oxidised layer. At present, the thickness of the redox boundary varies from 5 to 15 cm in the different investigated areas (Marchig et al., 2001, Figure 7), with lowest levels under the influence of equatorial high productivity and upwelling conditions, obviously caused by an enhanced input of organic material. The diagenetic remobilisation/reprecipitation process should produce manganese-rich oxides under low carbon flux conditions. Such remobilisation dissolves only manganese, leaving iron and trace elements in a reduced, manganese-depleted environment. This could be the reason for the local enrichment of trace elements, especially transition metals (Co, Ni, Cu, Zn) in the reduced parts of the Quaternary sediments. However, because only limited parts of the cores were investigated by sequential leaching methods, no general conclusion can be made about such a mechanism of trace enrichment in deeper reduced sediment sections. As it is evident that the position of the redox boundary changes from time to time, nodule layers rich in manganese should be ones that are precipitated from remobilisation under low organic flux conditions in a narrow redox (Eh ) range.
6. Conclusions Depth profiles of metals in Quaternary sediment sections of six cores from the Peru Basin suggest that manganese and other transition metals are mobilised from suboxic sediment sections and precipitated under oxic conditions near the water/sediment interface. The supposed mechanism of the diagenetic remobilisation/reprecipitation process is compatible with data from element partitioning by sequential leaching, showing the phase distribution of metals enriched at
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the water/sediment interface under oxic conditions. Transition metals occur in oxide–hydroxide sediment phases and are available for incorporation into manganese nodules during time periods when the horizontal position of the redox boundary and the surfaces of manganese nodules are located in the same depth. Anthropogenic impacts such as deep-sea mining of manganese nodules will disturb the position of the redox boundary. The top sediment layers will be stirred and resuspended into the bottom water. In contrast to the mobilisation of metals from sediments by naturally induced variations of the redox conditions, mining will be rapid. Metals will be released into the bottom water under oxic conditions. Metal concentrations of Peru Basin sediments, their variation with depth and their phase distribution are important parameters to consider in the evaluation of technical . impacts on the deep sea environment (Rolinski et al., 2001; Konig et al., 2001; Haeckel et al., 2001). Acknowledgements The material for this publication was obtained during a cruise of R.V. Sonne, funded by the German Bundesministerium fur . Bildung und Forschung (Project No. 03G0106). We appreciate the helpful discussion and review of the manuscript by D. S. Cronan. The responsibility for the contents of this publication is solely with the authors.
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