Artículos

 

Trace metal distribution in muddy-patch sediments from the northern Portuguese Shelf

 

Distribución de metales traza en sedimentos de zonas de lodo de la plataforma costera norte de Portugal

 

Maria-João Madureira¹, Carlos Vale^1* and Nuno Fonseca¹

 

¹ Institute for Fisheries and Sea Research (IPIMAR), Av. Brasília, 1449-006 Lisboa, Portugal. *E-mail: cvale@ipimar.pt

 

Recibido en diciembre de 2001;
aceptado en octubre de 2002.

 

Abstract

Three sediment cores were collected in the muddy patches of the northern Portuguese Shelf located offshore the Douro and Minho rivers. Bivalves and worms were found in the upper and sub-surface layers of two cores. Vertical distributions of dissolved manganese and iron, iron oxides and acid volatile sulphide (AVS) in the bioturbated cores differ dramatically from those in the core where macrofauna was not observed. Also, Fe, Zn, Cu, Pb and Cd simultaneously extracted with AVS (1 M HCl) showed pronounced increases in concentrations deeper in the sediments. Where bioturbation was not visible the profiles of extractable Cu, Pb and Cd are consistent with the precipitation of downward diffusing dissolved metals. However, this redistribution did not affect the metal-total vertical profiles. Reactive metals exhibited multiple peaks in the irrigated sub-surface layers where AVS and dissolved Fe showed successive maxima and minima with the depth. We interpreted these profiles as the net result of dissolved oxygen being pumped into the sediment and the formation of several oxic/anoxic interfaces. At these depths trace metals are mobilized in the oxidized layers, diffused a few millimetres towards the layers where sulphide is present, and precipitated. The consequence of this internal redistribution was the broad maxima of total metal concentrations in irrigated sediment depths.

Key words: trace metals, sulphides, early diagenesis, northern Portuguese Shelf.

 

Resumen

Se muestrearon tres testigos de sedimento en manchas de lodo procedentes de la plataforma continental del norte de Portugal localizadas frente a los ríos Duero y Miño. En las capas superficial y subsuperficial de dos de los testigos se encontraron bibalvos y gusanos. Las distribuciones verticales de hierro y manganeso disueltos, óxidos de hierro y sulfuros volátiles ácidos (AVS) en los testigos bioperturbados son drásticamente diferentes de aquellos en los que no se observó macrofauna alguna. De igual forma, el Fe, Zn, Cu, Pb y Cd extraídos simultáneamente con los AVS (HCl 1M), mostraron incrementos marcados en la concentración en zonas más profundas del sedimento. Donde la bioperturbación no fue visible, los perfiles de Cu, Pb y Cd extraíbles concuerdan con la precipitación y los metales disueltos con difusión descendente. Sin embargo, esta redistribución no afectó los perfiles verticales de metales totales. Los metales reactivos mostraron picos múltiples en capas irrigadas sub-superficiales donde los AVS y el Fe disuelto presentaron máximos y mínimos sucesivos con la profundidad. Interpretamos estos perfiles como el resultado neto del bombeo de oxígeno disuelto hacia el sedimento y la formación de varias interfases óxicas-anóxicas. En estas profundidades, los metales traza son movilizados en las capas oxidadas, y difundidos unos pocos milímetros hacia las capas donde hay sulfuro y éste precipita. La consecuencia de esta redistribución interna determina el máximo de las concentraciones totales de metales en profundidades con sedimentos irrigados.

Palabras clave: metales traza, sulfuros, diagenesis temprana, plataforma norte de Portugal.

 

Introduction

The interpretation of the sedimentary metal record requires a clear understanding of metal mobility and preservation during the early diagenesis of sediments (Berner, 1980). Several works have shown evidence of the release of transition metals associated with the degradation of biogenic detritus in oxic/suboxic sediment layers (Klinkhammer et al., 1982; Gobeil et al., 1987; Gobeil and Silverberg, 1989; Shaw et al., 1990). The concentration-depth profiles are maintained by a coupling of this regeneration with varying degrees of interaction between dissolved metal and sediment below the interface (Klinkhammer et al., 1982). Most of the well-defined profiles were registered in marine sediment deposits consisting of sharply defined zones, within which only certain redox reactions can occur (Froelich et al., 1979). In coastal sediments, the vertical chemical zonation in both pore water and solid fraction tends to be less clear, due to the presence and activity of benthic organisms (Aller, 1977) and seasonal variations in organic carbon input to the sediment and in temperature (Aller, 1994; Thamdrup et al., 1994). These variations can cause a fluctuation in the vertical position of the oxic/anoxic boundary within the sediment, and create zones where metals are alternately oxidized and reduced, and consequently, metal sulphides are alternately precipitated and dissolved (Gobeil et al., 1997; Brugmann et al., 1998). Modifications have also been reported in deep-sea sediments due to an overall change in the ocean productivity during the last millennium (Pedersen et al., 1986) and during the sapropel formation in the North Atlantic (Thomson et al., 1993).

The muddy patches of Douro and Minho are located in the NW Iberian Margin, around 20 km northwest offshore the Douro and Minho rivers, respectively. The deposit of fine particles transported from the continent is facilitated by the non-uniform topography of the area (Dias and Nittrouer, 1984). The sedimentation rate in the area was estimated between 0.1 and 0.3 cm/year (Drago et al., 1998). Elemental analyses of upper sediment layers revealed no relationships with the fluvial sources and relatively uniform metal concentrations (Araújo et al., 1994; Barbosa et al., 1999). Post-depositional changes in the sediment chemical composition have not been investigated. In this paper we present data on the distribution of copper, lead, cadmium, manganese and iron in sediment cores from the Douro and Minho muddy patches. In some cores, maxima of reactive and total trace metals were registered in sub-surface layers where sulphide concentrations changed pronouncedly. We describe the relationships between iron, manganese and trace metal distribution and discuss the possibility of the vertical distributions reflecting changes in the redox conditions within the sediment.

 

Material and methods

Sampling

Three sediment cores were sampled in two muddy patches located offshore the Douro and Minho rivers, in the northern Portuguese Shelf (fig. 1). The samples were collected using a multi-corer aboard the research vessel Mestre Costeiro in 1998. Cores E and G, 30 cm long, were collected from the Douro muddy patch and core J, 15 cm long, from the Minho patch. The cores, not disturbed by the sampling, were sub-sampled on board into 0.5-cm horizontal layers in the first 10 cm and then in 1-cm intervals. Deeper sediments were sectioned in 2-cm layers. A 1-g sediment aliquot was separated immediately for sulphide and amorphous iron determinations. All these operations were done quickly in order to minimize changes resulting from the exposure of the sediment slices to the atmosphere. Samples were stored in leakproof polycarbonate tubes, completely filled in a glove box under nitrogen, and kept in a refrigerated chamber. In the laboratory, samples were centrifuged for 30 min at 3000 rpm and 4°C to separate the pore water from the solid phase. The resulting pore water was filtered through 0.45-µm Millipore filters, acidified to pH 2 (1% equivalent volume of ultrapure HNO₃) and stored at 4°C. The sediment samples and the cakes of centrifuged sediment were placed in plastic bags and kept frozen at -20°C.

Chemical analysis

Total dissolved iron and manganese in pore water samples were measured by atomic absorption spectroscopy (AAS), using direct aspiration into an air-acetylene flame Perkin Elmer 4000 device. Sediment aliquots were freeze-dried, homogenized by grinding, and digested with a mixture of acids according to the method described by Rantala and Loring (1977). The digestates were then analyzed by atomic absorption spectroscopy using direct aspiration in a N2O-acetylene flame (Al, Si, Ca, Mg), an air-acetylene flame (Fe, Mn, Zn) and a pyrolytic graphite furnace equipped with a L'vov platform (Cu, Pb and Cd). Precision errors were: Al, 2.3%; Si, 1.6%; Ca, 4.7%; Mg, 3.6%; Fe, 2.3%; Mn, 1.0%; Zn, 2.0%; Cu, 2.0%; Pb, 4.0%; and Cd, 2.0% (P = 0.05). Aliquots of wet sediment samples were used for sulphide determinations. Sulphides from acid volatile sulphide (AVS) were released in 1 M HCl, purging the system with nitrogen gas, and trapping the H2S in 1 M NaOH solution (Henneke et al., 1991; Madureira et al., 1997). Hydrogen sulphide trapped in the basic solution was determined by voltametric methods (Luther et al., 1985), using a Methrom device equipped with a 693 VA processor and a 694 VA stand. The recovery of the standard sulphide solution was 97% and the detection limit of the method was 0.01 µM. The simultaneously extracted iron and zinc, released from the AVS by acidification of the sediment with 1 M HCl, were analyzed directly in the sample by AAS. The simultaneously extracted copper, lead and cadmium were analyzed directly by atomic absorption using a graphite furnace as previously described. The amorphous iron hydroxides were obtained by means of an ascorbate extraction according to the method described by Anschutz et al. (1998), using an aerobic solution at pH 8. Iron concentration in the obtained solution was determined by AAS. International certified standards (MESS-1 and BCSS-1) were used to control the accuracy of the procedure for total metal determinations. The values obtained were not statistically different from the certified standards (P < 0.01).

 

Results and discussion

Sediment description

The visual inspection of the collected cores revealed an abundant fraction of fine-grained material, as commonly observed in the muddy patches of the northern Portuguese Shelf (Drago et al., 1998). During the opening procedure, several macrobenthic organisms were found in sub-surface layers of core G (Annelida, Polychaeta, Ophiuroidea and bivalve shells) and core J (high density of small bivalves). Bioturbation was not evident in core E.

Major, minor and trace elemental composition

The aluminium concentrations and the major, minor and trace metal ratios to Al of cores E, G and J are presented in table 1. Aluminium concentrations in the three cores were relatively uniform with the depth, generally varying between 6% and 7%. These values are in agreement with the grain size distribution that indicates a stable deposition of fine-grained material in these muddy patches (Drago, 1995). The Fe/Al ratios ranged also in a narrow interval, with a mean value of 0.36 ± 0.031. Values of the Si/Al, Ca/Al and Mg/Al ratios were less uniform, with core G presenting higher Si/Al and lower Mg/Al and Ca/Al ratios. Moreover, Ca/Al and Mg/Al ratios in the 1-cm layer were half of the values found in deeper sediments, which indicates that the material is more siliceous and contains less carbonates. The Mn/Al and Zn/Al ratios were similar in the three cores and did not vary considerably with depth. The slight increase of the Mn/Al ratio in the first 0.5-cm sediment layer of core G could be the result of diagenetic processes (Sundby et al., 1981). A steep Cu/Al ratio gradient was also found in the first 1-cm layer, which can be due to the presence of freshly deposited organic matter derived from the water column. The sharp decrease of the Cu/Al ratio in the first centimetre may also indicate an intense degradation of organic matter. This agrees with the model proposed by Klinkhammer et al. (1982), with most of the copper arriving to the sea floor being regenerated near the interface and released to the water column. This tendency was not registered in the other cores, probably because of lower carbon input to the sediment at those stations. Whereas Cd/Al ratios were almost constant along core E, multiple peaks were recorded in core G and a broad enhancement, between 2 and 4 cm depth, was observed in core J. Increase of total cadmium concentration with the sediment depth is reported by Gobeil et al. (1987, 1997), and it is attributed to post-mobilization and subsequent precipitation as sulphide. Also, Pb/Al ratios were less uniform in core G and most of their values exceeded those found in cores E and J.

Dissolved iron and manganese, iron oxides and AVS

The vertical distributions of total dissolved iron and manganese in sediment pore waters and of AVS in the three cores are shown in figure 2. The manganese and iron profiles in core E are characterized by regular shapes with maximum values below the sediment-water interface. The concentration peaks indicate the oxide reduction near the surface and the subsequent decreases with the removal of these elements from the solution with depth. The vertical distribution of the iron extracted by the ascorbate method elucidates the depth where iron amorphous oxides are formed (Anschutz et al., 1998). Whereas in core E iron precipitated just below the sediment-water interface, the iron oxide peak in core G was formed in the sub-surface layers where several maxima and minima of dissolved iron are present. The values of iron oxides in core J were uniform with depth. The AVS levels increased with depth as dissolved iron in pore waters decreased, which confirms the removal of iron from solution. Manganese and iron in pore waters of cores G and J showed irregular shapes with successive maxima and minima. The manganese maximum occurred in sub-surface layers and iron increased in deeper sediment layers, between 5 and 13 cm. Whereas manganese concentrations were similar in the three cores, iron in pore waters of core E exceeded ten times the values found in core G. The AVS values were low (<0.01 µmol g^-1) in the first 5 cm and then increased, showing multiple peaks in core G. Levels of AVS were uniformly low (<0.01 µmol g^-1) along core J, except in two layers, around the 8 and 13 cm depths, where concentrations were ten times lower than values recorded in cores G and E.

Simultaneously extracted metals with AVS

The concentrations of iron, zinc, copper, lead and cadmium extracted from the sediments with a 1 M HCl solution showed pronounced increases below 1 cm in core E and below 10 cm in core G (fig. 3). In core E, concentrations of all the analyzed metals increased sharply below 1 cm as iron is removed from pore waters, and exhibited maximum values between 2 and 4 cm, where AVS show their first increases in the sediment column (0.4 to 1.9 µmol g^-1). The trace metal maxima did not occur at the same depth and the shape of their curves varied slightly from metal to metal. Iron, zinc and copper showed maximum values at the same depth, while lead and cadmium had a second peak right below them. Peaks of extractable copper, lead and cadmium are sharp and values were two to three times those found at the sediment surface. The iron maximum was broader in all the cores analyzed. In core G, the maximum was registered deeper in the sediment, just below the amorphous iron oxide peak and in the zone of irregular AVS concentrations. A sharp increase of zinc was recorded at the same sediment depth, while maximum values of copper, lead and cadmium occurred 0.5 cm above. The profiles in core J were not clear and only iron showed a broad maximum where the levels of AVS varied abruptly (fig. 3). Maximum concentrations of trace metals in core G exceeded largely the values found in the other two cores analyzed.

Mobilization of trace metals in oxic/anoxic interfaces

Where bioturbation was not visible (core E) the profiles of copper, lead and cadmium extracted with a 1 M HCl solution are consistent with the precipitation of downward diffusing dissolved metals. This implies that trace metals remobilize in the oxic surface layer of the muddy patches. Several works have proposed the regeneration of most transition metals near the sediment-water interface (Klinkhammer et al., 1982; Shaw et al. , 1990). The release of cadmium in the oxic layers of St. Lawrence sediments and its downward diffusion influences the sediment depth distribution (Gobeil et al., 1997). Lead profiles both in pore waters and the extractable solid fraction evidence its remobilization inside the sediments (Gobeil and Silverberg, 1989). The close correspondence found in our study between the iron, copper, lead, cadmium and AVS distributions is in complete agreement with sulphide precipitation. The precipitation of cadmium and lead slightly below copper sulphides could be interpreted as a higher availability in pore waters, or diffusion to the layers where metals meet dissolved sulphide produced deeper by sulphate reduction. Because only 18% of copper and 30% of cadmium were involved in this diagenetic process the total metal concentrations were not affected substantially.

The contrast between the trace metal distributions in cores G and E is striking. The peaks of reactive trace metals in the core bioturbated by macrobenthic organisms (G) occurred deeper in the sediments where AVS presented their first maximum-minimum oscillation with depth. This distribution pattern is extended at least until 30 cm depth. A thicker benthic layer and irrigation mean that dissolved oxygen is pumped deeper into the anoxic zone creating several oxic/anoxic interfaces. The irregular AVS profile and the sharp peak of iron oxides corroborate the dramatic changes occurred inside the sediments. Because inhabitant organisms create these interfaces, this situation is probably in non-steady-state conditions. The net result of several redox boundaries is to mobilize trace metals between oxic/suboxic zones and layers where sulphide is present in a larger sediment volume. This mobilization explains the higher quantities of zinc, copper and cadmium associated with monosulphides in the bioturbated core G. Presumably the intensity of these exchanges is enough to influence the total metal distribution, in the solid fraction of the sediments.

 

Conclusions

The trace metal distribution recorded in sediment cores from muddy patches of the northern Portuguese Shelf points to the importance of bioturbation on the redistribution and fractionation of metals in coastal sediments. Irrigation and sediment bioturbation causes irregularities in metal fractionation as response to inter-layers of metal oxides and sulphides.

 

Acknowledgements

The authors wish to thank Teresa Drago for the description of the cores.

 

References

Aller, R.C. (1977). Diagenetic processes near the sediment-water interface of Long Island Sound. II. Fe and Mn. Adv. Geophys., 22: 351-415.         [ Links ]

Aller, R.C. (1994). The sedimentary Mn cycle in Long Island Sound: Its role as intermediate oxidation and the influence of bioturbation, O₂, and C_org flux on diagenetic reaction balances. J. Mar. Res., 52: 259-295.         [ Links ]

Anschutz, A., Zhong, S., Sundby, B., Mucci, A. and Gobeil, C. (1998). Burial efficiency of phosphorus and the geochemistry of iron in continental margin sediments. Limnol. Oceanogr., 43(1): 53-64.         [ Links ]

Araújo, M.F., Dias, J.M.A. and Jouanneau, J.-M. (1994). Chemical characterization of the main fine sedimentary deposit at the northwestern Portuguese shelf. GAIA, 9: 59-65.         [ Links ]

Barbosa, T., Araújo, M.F.D., Jouanneau, J.-M., Gouveia, M.A., Weber, O. and Dias, J.M.A. (1999). Geochemistry of sediments from the Portuguese shelf. Actas do II Congresso Ibérico de Geoquímica, XI Semana da Geoquímica, Lisbon, Portugal, 14-17 June, pp. 437-440.         [ Links ]

Berner, R.A. (1980). Early Diagenesis: A Theoretical Approach. Princeton Univ. Press, New Jersey, 241 pp.         [ Links ]

Brügmann, L., Hallberg, R., Larsson, C. and Löffler, A. (1998). Trace metal speciation in sea and pore water of the Gotland Deep, Baltic Sea, 1994. Appl. Geochem., 13: 359-368.         [ Links ]

Dias, J.M.A. and Nittrouer, C.A. (1984). Continental shelf sediments of northern Portugal. Cont. Shelf Res., 3(2): 147-165.         [ Links ]

Drago, T. (1995). La vasiére ouest-Douro sur la plataforme continentale nord-portugaise. Rôle, fonctionment, evolution. Ph.D. thesis, Univ. Bordeaux I (unpublished).         [ Links ]

Drago, T., Oliveira, A., Magalhães, F., Cascalho, J., Jouanneau, J.-M. and Vitorino, J. (1998). Some evidences of northward fine sediment transport in the northern Portuguese continental shelf. Oceanol. Acta, 21(2): 223-231.         [ Links ]

Froelich, P.N., Klinkhammer, G.P., Bender, M.L., Luedtke, N.A., Heath, G.R., Cullen, D., Dauphin, P., Hammond, D., Hartman, B. and Maynard, V. (1979). Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic: Suboxic diagenesis. Geochim. Cosmochim. Acta, 43: 1075-1090.         [ Links ]

Gobeil, C. and Silverberg, N. (1989). Early diagenesis of lead in Laurentian Trough sediments. Geochim. Cosmochim. Acta, 53: 1889-1895.         [ Links ]

Gobeil, C., Silverberg, N., Sundby, B. and Cossa, D. (1987). Cadmium diagenesis in Laurentian Trough sediments. Geochim.Cosmochim. Acta, 51: 589-596.         [ Links ]

Gobeil, C., Macdonald, R.W. and Sundby, B. (1997). Diagenetic separation of cadmiun and manganese in suboxic continental margin sediments. Geochim. Cosmochim. Acta, 21: 4647-4654.         [ Links ]

Henneke, E., Luther, G.W. and DeLange, G. J. (1991). Determination of inorganic sulphur speciation with polarographic techniques: Some preliminary results for recent hypersaline anoxic sediments. Mar. Geol., 100: 115-123.         [ Links ]

Klinkhammer, G., Heggie, D.T. and Graham, D.W. (1982). Metal diagenesis in oxic marine sediments. Earth Planetary Sci. Lett., 61: 211-219.         [ Links ]

Luther, G.W., Giblin, A.E. and Varsolona, R. (1985). Polarographic analysis of sulfur species in marine porewaters. Limnol. Oceanogr., 30: 727-736.         [ Links ]

Madureira, M.J., Vale, C. and Simões Gonçalves, M.L., (1997). Effect of plants on sulphur geochemistry in the Tagus salt-marshes sediments. Mar. Chem., 58: 27-37.         [ Links ]

Pedersen, T.F., Vogel, J.S. and Southon, J.R. (1986). Copper and manganese in hemipelagic sediments at 21^oN, East Pacific Rise: Diagenetic contrast. Geochim. Cosmochim. Acta, 50: 2019-2031.         [ Links ]

Rantala, R.T.T. and Loring, D.H. (1977). A rapid determination off 10 elements in marine suspended particulate matter by atomic absortion. Atom. Absorpt. Newslett., 16: 51-52.         [ Links ]

Shaw, T.J., Gieskes, J.M. and Jahnke, R.A. (1990). Early diagenesis in differing depositional environments: The response of transition metals in pore water. Geochim. Cosmochim. Acta, 54: 1233-1246        [ Links ]

Sundby, B., Silverberg, N. and Chesslet, R. (1981). Pathways of manganese in an open estuarine system. Geochim. Cosmochim. Acta, 45: 293-307.         [ Links ]

Thamdrup, B., Fossing, H. and Jergensen, B. B. (1994). Manganese, iron, and sulfur cycling in a coastal marine sediment, Aarhus Bay, Denmark. Geochim. Cosmochim. Acta, 58(23): 5115-5129.         [ Links ]

Thomson, H., Higgs, N.C., Croudace, I.W., Colley, S. and Hydes D.J. (1993). Redox zonation of elements at an oxic/postoxic boundary in deep sea sediments. Geochim. Cosmochim. Acta, 57: 579-595.         [ Links ]