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Trace metals in sediment cores from the Campeche shelf, Gulf of Mexico
ENVIRONMENTAL POLLUTION
Environmental Pollution 104 (1999) 69±77
Trace metals in sediment cores from the Campeche shelf, Gulf of Mexico J.V. MacõÂas-Zamora a,*, J.A. Villaescusa-Celaya a, A. MunÄoz-Barbosa a, G. Gold-Bouchot b a
Instituto de Investigaciones OceanoloÂgicas U.A.B.C., Apdo 453 Ensenada, Baja California, Mexico, 22800 b CINVESTAV-I.PN., Unidad MeÂrida. Apdo. Postal 73-Cordemex. MeÂrida, YucataÂn, Mexico, 97310 Received 1 April 1998; accepted 4 August 1998
Abstract Trace metals in sediment cores from the Gulf of Mexico, in the Campeche shelf area were studied to investigate possible variations in their vertical and/or horizontal sedimentary distributions. Possible links to petroleum sources were investigated as this area contains very productive oil explotation and transportation activities. Sediments were collected undisturbed using a vegematic type box corer. The upper 10 cm. were collected and cut in sections of 2 cm each. Incomplete, open vessel sediment digestion and atomic absorption was used for trace element determination. Particular interest was placed on vanadium and nickel concentrations as they have been associated to oil. Several metals were measured and representative distributions are presented. Horizontal distributions appear to conform to predominant current circulation patterns reported and closeness to sources of sedimentary materials. Vertical distributions are discussed. The vertical distributions were strikingly constant. Correlation among metals are also presented. Fe was used as a normalizing agent and the carbonate content was acting as a diluter of total trace metal content. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Gulf of Mexico; Pollution studies; Trace metals in sediments; Core samples
1. Introduction Crude oils represent a complex mixture of both organic and inorganic components. Trace metals are one group of elements amongst the inorganic component present in crudes. These elements have been found in dierent proportions in dierent crudes. Although other metals such as Fe, and Zn may be importantly present, frequently Ni and V are found in the largest concentrations in crude. For example, values presented for some Venezuelan crude oils (Simoza et al., 1985) as well as Middle East crude oils (Al-Shahristani and AlAtyia, 1972) have reported the predominance for V and Ni metal content. Vanadium concentrations can be as high as 2000 ppm (Tissot and Welte, 1984, cited in Manning and Gize, 1993). It is expected that, upon extraction of the crude oil and accidental deposition on surface sediments, bacterial decomposition, dissolution and oxidation of most of the organic components and * Corresponding author. Tel.:+52-61-74-46-01; fax:+52-61-74-5303; e-mail: [email protected].
remineralization of the organic matrix, trace metals are incorporated in the sediment load, increasing the background levels of metal content of the local sediment. Given the large activities of oil exploration, extraction and transportation in the area known as the Sonda de Campeche, in the Gulf of Mexico, we expected that accidental releases of oil in this area could re¯ect in a larger than normal presence of V and Ni as well as other metals in the sedimentary column. This study was carried out to determine the horizontal and vertical distribution of trace metals in sediments of the Campeche Shelf. This geographical area represents one of the most important oil producing areas for Mexico, with over 70% of domestic production. The reported annual production of oil reaches 109 million barrels (Soto and Escobar-Briones, 1995). There are seven oil producing platforms as well as two long pipelines transporting oil to two opposite places, one on the coast and one on Cayo Arcas island (Fig. 1). Frequent accidental discharges of oil in this area have been reported (Leahy et al., 1990), the largest of them being the blow-out of Ixtoc well in 1979. We expected
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compared to other environments such as the California Current or the Sea of Cortez. Although upwelling events produced by topographical features have been reported (Furnas and Smayda, 1987) producing upward mixing of near bottom water, those water are of tropical origin with poor nutrient content. Most productivity of these waters has been reported to be associated to the input of riverine and estuarine materials during rainy seasons. This introduction of water and terrigenous material accounts for a large proportion of dissolved nutrients and organic materials introduced to the area (Soto and Escobar-Briones, 1995). The main entry of terrigenous material is due to the input of the Grijalva and Usumacinta rivers to the west of our working area. This system includes also a coastal lagoon (the Terminos lagoon) which serves as a temporary trap for sediments, part of these are exported to the sea during rainy seasons (PaÂez-Osuna et al., 1987). This study was carried out to look for possible in¯uences of oil activities on the trace metals load on sediment in the area, as well as to any relationship of possible metal enrichment on benthonic organisms distributions. We present our results on the ®rst part of the objectives. Fig. 1. Study area, sampling stations and isobats (m) in the Campeche shelf.
2. Methods and materials
that some indication of these and other less severe events might have left their imprint on relatively recent sedimentary materials, which could be observable through the use of trace metals ¯uctuations as indicators of these unusual events. It has been stated that sediments, under some circumstances, can preserve and integrate a record of trace elements introduced, either through natural, or anthropogenic activities (Regnier and Wollast, 1993). However, the challenge becomes one of distinguishing the natural from manmade sources. The area is located in the southern part of the Gulf of Mexico (Fig. 1). This oceanographic region is characterized by a shallow platform, the so called Campeche Bank representing the submarine extension of the Yucatan Peninsula, and which consists of carbonaceous material and lacks terrigenous material inputs to the sea. It is also characterized by the Yucatan Current or the loop current, ¯owing from the south between the Yucatan Peninsula and Cuba. Inside the Gulf, the current is aected by the widening of the area and a shallower topography. As a consequence, cyclonic meandering currents have been reported (CarranzaEdwards et al., 1993). The meandering currents are said to be aected by seasonal tropical storms (Lewis, 1992). The warm waters (mostly above 20 C) are oligotrophic with low nutrients as well as chlorophyll content when
Sediment samples were collected during the ®rst phase of Xaman-Ek cruises, on board of the R/V Justo Sierra in February of 1992. This was a part of a larger program designed to study possible eects of oil activities on benthic communities in the area. The sampling plan included 26 stations located on ®ve segments labeled A±E with a dierent number of stations on each segment (Fig. 1). Samples were collected using a box corer of 40 by 40 cm with a 50 cm depth. The box corer used was a Kessler-Sandia MK II of stainless steel vegematic type sampler. Subsamplers inside of the box corer were used for dierent purposes. In particular, the subsampler for trace metal analysis was Te¯on coated to avoid contamination of samples. The depth of the cores selected was expected to include at least a history of the last 50 years, in which most exploration and commercial exploitation of oil ®elds has taken place. The core sediments were obtained from the Te¯oncoated square subcorer of 1010 cm. Each sample was sliced every 2 cm to a ®nal depth of 10 cm. As part of the quality control, two true replicates were also collected for only the top 2 cm surface sediments. All surface concentration numbers presented for the top 2 cm are averages of these three replicates. The samples were placed inside previously cleaned plastic bottles and frozen on board. All samples were latter freeze-dried, grounded and sieved through a 0.5 mm sieve to remove
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large particles and pieces of plants and animals at the Marine Geochemistry Laboratory in Merida, Yucatan, of the Center for Research and Advanced Studies (CINVESTAV). We did not select any particular grain size to carry the chemical analysis. The metal extraction technique for sediments closely follows that of Breder (1982). In general, it consisted of an open-vessel digestion of 0.5 grams of sediment with re¯ux. Samples were placed in 150 ml beakers. Six millilitres of concentrated HNO3 and 2 ml of HCl were added to each beaker and covered with clean watch glasses. The samples were placed on heating plates inside a fume hood and the temperature was raised to 80 C. The reaction was allowed to proceed for 5 h. After cooling, all samples were quantitatively transferred to graduated centrifuge tubes and the ®nal volume was made-up to exactly 20 ml. All glassware had been previously decontaminated with the use of a 2% solution of Micro1 detergent, followed by cleaning with a 30% HNO3 solution and rinsed with de-ionized water. As part of the quality control process, one procedural blank and one standard reference material (BCSS-1) was included in each batch of 12 samples. The results for BCSS-1 are shown in Table 1. Additionally, in the ®eld, the labeling of samples was done ``blindly'', which meant that we did not know the origin of any sample until all results were obtained. In most cases, the digestion of standard reference material was more dicult than the actual samples because of the large carbonate content which was easily dissolved by the acid treatment. Trace metal analysis was carried-out with an atomic absorption from Thermo Jarrel Ash (TJA) model Smith-Hieftje 12. For Ni, Cu, Zn, V and Fe, we used air-acetylene ¯ame. For Cr., the ¯ame was nitrous oxide-acetylene. The analysis of Cd and Pb metals was made in a graphite furnace TJA model CTF 188, using Smith-Hieftje background correction and aerosol deposition TJA Fastac II. In addition, for the Pb analysis, we used a matrix modi®er solution containing 500 mg/ml of Pd±Mg (Bulska et al., 1990; Hinds and Jackson, 1990). To obtain the spatial distribution for the surface sediments (0±2 cm), we used an exploratory analysis procedure of the data. The measured data were taken as regionalized variables and they were used to generate Table 1 Recoveries obtained for the standard reference material BCSS-1 of the National Research Council of Canada, under the mixed acid treatment conditions used Metal Expected Measured
Ni
V
Cu
Zn
Cd
Mn Fe2O3
Pb
Cr
55.3 93.4 18.5 119 0.25 229 4.70 22.7 123 3.6 4.9 2.7 12 0.4 215 .14 3.4 14 44.3 71 17.1 100.6 0.258 150 6.11 21.3 58
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spatial contours. For interpolation a kriging algorithm was used with a linear variogram (Weherens et al., 1993). To classify sampling stations, we used an R mode cluster analysis based on a complete agglomeration analysis with Euclidean distance coecients. The matrix was previously standardized using Z function. This procedure was accomplished with the SYSTAT statistical package (Wilkinson, 1990). 3. Results and discussions Most metals showed a horizontal distribution with a tendency to increase towards the south-west in the study area. Nickel presented a regional average of 23.0 mg gÿ1 with an interval from 0.56 to 76.9 mg gÿ1, with the largest values for the stations A4 and A5. For Cu, the interval of concentrations varied from 3.82 to 18.7 mg gÿ1, with an average of 7.53 mg gÿ1. Cu was the element that showed the smallest variation in the sediments of the region (the ratio maximum/minimum was 4.6). For Zn, as well as Ni, the largest values were found at stations A4 and A5. Zn was the element with the largest variation for the area, the smallest concentration was of only 0.04 mg gÿ1 in station D3 (4±6 cm), and the largest was 79.6 mg gÿ1 for station A5 (4±6 cm). The regional media for these stations was 18.5 mg gÿ1. Chromium showed also a small variability with a media concentration of 39.8 mg gÿ1 and a smallest value of 3.0 mg gÿ1 and a largest of 100 mg gÿ1. Cadmium also showed small variability its interval of concentrations found was from 0.01 to 0.7 mg gÿ1 with a median value of 0.09 mg gÿ1. Generally speaking, cadmium presented the largest concentrations in transects A and B and the smallest concentration close to the Yucatan platform (transects C and D). However, the concentration of 0.7 mg gÿ1 found for station B6 was considered as ``anomalous'' given that Cd was not larger than 0.2 for all regions. Similarly to other metals, Lead showed the largest concentrations in the south-west area. The average concentration was 4.3 mg gÿ1, while its interval was from 0.22 mg gÿ1 in station D4 (0±2 cm) to 20.2 mg gÿ1 for station A5 (2±4 cm). The average value for vanadium was 47.78 mg gÿ1 with a minimum of 15.6 mg gÿ1 and a maximum of 117.5 mg gÿ1 present at the A5 (4±6 cm)station. Manganese had an average for the area of 110.7 mg gÿ1 with a minimum of 12.5 mg gÿ1 at station E2(0±2 cm) and a maximum of 448.9 mg gÿ1, again at station A5 (0±2 cm). Finally, iron reported here as percentage of Fe2O3, presents similar trends to those of other metals. In general, the largest concentrations were found in transect A. The average concentration was of 1.84% with a wide range of variation going from below detection levels close to the Yucatan Peninsula (station D4, level 6±8 cm) to as much as 7.9% found at station A5 (4±6 cm).
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Fig. 2. Horizontal distribution of metals concentrations (mg gÿ1). (a) Ni, (b) Zn, (c) Pb and (d) Cu, respectively.
Fig. 3. Linear regresion for iron concentration vs each metal. Broken lines represent the range at 95% limit of con®dence. Dotted lines represent 95% con®dence level band for all data.
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Fig. 4. Vertical distribution concentrations (in mg/gÿ1 dry weight) for all metals at two dierent stations. (a) For station 5A. (b) For station B6. Filled circles represent the raw metal concentration. Open circles represent the iron-normalized data.
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Fig. 5. Areal clasi®cation according cluster analysis for all surface sediments atributes (metal concentrations, carbonate content, organic matter). Shading was done according to the three groups generated.
To our knowledge there appears to be little information on metal concentration studies for the Sonda de Campeche area. We found one study for the Laguna de Terminos in Campeche (PaÂez-Osuna et al., 1987). A second study reported on the distribution of Ni, V and hidrocarbons content in recent sediment (VaÂzquez et al., 1991), but their work area is located in a region north of our location. A third work reported on the composition of mayor elements in suspended matter in the sonda the Campeche area (Carranza-Edwards et al., 1993). However, little direct comparison to our results could be established. The concentrations found in this analysis can be considered low with respect to those reported for other areas. For example, O'Connor and Cantillo (1992) report a compilation of studies on the surveillance of pollution in the U.S. In their study, the average values for Ni, Cu, Zn, Cr, Cd and Pb are 34, 35, 140, 110, 0.48 y 43 mg gÿ1 respectively. These authors also report on world levels for average concentrations (after elimination of highly polluted localities) for Ni, Cu, Zn, Cr, Cd, and Pb as 26, 28, 91, 61, 1.1 and 34 mg gÿ1 respectively. The average values found in this study for the enlisted metals were 21.5, 7.5, 18.5, 39.8, 0.10 and 4.30 mg gÿ1 respectively. In Fig. 2 (a, b, c and d) we show the horizontal distributions of trace metals for surface sediments (0±2 cm)
Fig. 6. Box and whiskers diagram showing the 3 dierent zones based on carbonate (a) and iron (b) content.
for Ni, Zn, Pb and Cu. Other metals trends follow similar patterns (data not shown). Similarly, the horizontal distributions at other depths showed approximately the same distribution patterns. A common feature is the longitudinal gradient for all metals, with lower values towards the Yucatan platform and higher values in the south-west area. This is exactly opposite to that found for carbonates in sediments (Gold-Bouchot, 1994). These distribution suggest that the metal distributions are being controlled to a large extent by the closeness to the source of terrigenous material. It also suggest that the metal distribution gradients may also be aected by the circulation patterns produced as a result
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Fig. 7. Ni/V ratio for surface sediments in the Campeche shelf.
of a cyclonic circulation (Vasquez, 1993) forced by wind in the Campeche bay. In Fig. 3, we show the linear regressions between the concentrations of each metal (Ni, V, Cu, Zn, Cr, Cd, Pb and Mn) versus Fe2O3 concentration. We have used iron as a normalizing tool because of two reason. First, as shown in Fig. 3 (a±h), iron is largely controlling the distribution of most metals (Cd is the only exception with an r2 of 0.10). Second, given that our digestion procedure did not include HF for total digestion, recoveries for Aluminum, which has been extensively used for normalization purposes (Windown et al., 1989; Regnier and Wollast, 1993), were very low. The broken lines represent the range at 95% limit of con®dence. The dotted lines represent the 95% con®dence band for all data (Windown et al., 1989). Values found above these lines represent enrichment for the metals at a particular station and depth level. The linear regressions also suggest, particularly in the cases of Cr, Ni and maybe Zn, the separation of data points within generally three sets, those with low metal content, those with a relatively large metal content and a transitional group conforming to previously suggested zonation (Carranza-Edwards et al., 1993.). In Fig. 4(a and b), we show vertical distribution of all metals for only two stations. In Fig. 4(a), the vertical distribution for station 5A, and in Fig. 4(b) for station B6. In both cases, the ®lled circles represent the raw
metal concentration versus depth and the open circles represent the Fe-normalized metal concentration. The fact that the small vertical variations of metal concentrations are smoothed-out, particularly for station 5A, by the normalization with respect to the iron content of the sediment, suggest that there is no presence of a diagenetic process or, that it is not observable for the ®rst 10 cm. With the exception of Cd in station A5 (8± 10 cm), and in station B6 (2±4), all metals remain remarkably constant versus depth. The largely homogeneous concentrations found could also be due to extensive bioturbation or to a well oxygenated sediment, probably due to the large sediment porosity expected resulting from its large carbonate content, or to a combination of both. For the B6 station however, some vertical changes are clearly and consistently present for all iron normalized data. It is concebible that ¯uctuations in riverine loads and/or ¯uctuations of circulation regimes might result in changes in dilution of metal loads to the sediments. These are surprisingly consistent for all metals, suggesting a common source and a common event, resulting in a simultaneous change in all metal concentrations at around the 4±6 cm depth. Both, the spatial distributions of carbonates an iron suggest an in¯uence of terrigenous material deposited in the coastal areas specially the areas close to the Tabasco and Veracruz states. This material is most probably introduced by the pluvial input of the Grijalva and Usumacinta rivers (Carranza-Edwards et al., 1993; Gold-Bouchot, 1994). Our results support this as the probable origin for most of the metal load for the area. The fact that the metal bearing inorganic detritus carried by rivers is recognized as the main source of metal load argues against local oil related anthropogenic inputs. In the case of Cd, its distribution is being strongly in¯uenced by the values that we considered outliers. It is worth noting that a parallel study conducted for dating sediments based on the 210Pb technique (Wartel and Salinas, 1996), resulted in an estimated sedimentation rate for the area of between 0.8 to 1.4 mm/yearsÿ1. This estimate places the average rate for the area at 1.1 mm/yearsÿ1 yielding, for the 10 cm depth cores a period of approximately 90 years. It is clear that the faster sedimentation rate is expected to occur for the area closer to the rivers and a smaller sedimentation rate for the carbonaceous shelf closer to the Yucatan peninsula and further away from terrigenous sources. When all stations are grouped by cluster analysis, 3 groups were obtained (Fig. 5). This is consistent with the three area zonation proposed by others (CarranzaEdwards et al., 1993). One area with a high carbonate content, located at the southeast on the main Campeche shelf; one with relatively higher terrigenous material content located at the southwest and near the rivers,
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and one transitional zone in the middle of these two regions. These regions are signi®catively dierent as can also be seen from Fig. 6 (a and b) when the stations are plotted based on their carbonate and iron contents. Finally, there is little if any, evidence in this study to believe that the area has had a large anthropogenic metal input. There is but one indication that suggests some in¯uence of oil activities. In Fig. 7, we show the distribution for the Ni/V concentration ratio showing the maximum values of this ratio close to stations near the restricted oil exploitation areas. However, for example, the V/Ni ratios for 5 oil exploration wells in the area, has been reported to vary from 3.25 to 6.78 (Bertrand et al., 1994). In our study, there were only a few values within this range for our data points, furthermore, the trends could also be explained, at least partially, as generated by the same circulation and depositional patterns with a maximum in the vicinity of the oil production activities and not necessarily deriving from them.
Acknowledgements We would like to thank CINVESTAV, Merida for their invitation to participate in this project. We would also like to thank CONACYT (Contract 1284M9204), and to the European Community (Contract CI 1*-CT91-0890 (HSMU)) for providing the funds for the project. Finally, we would like to thank several anonymous reviewers for helping in improving this manuscript.
References Al-Shahristani, H., Al-Atyia, M.J., 1972. Vertical migration in Iraqui oil ®elds: evidence based on vanadium and nickel concentrations. Geochim. Cosmochim. Acta 36, 929±938. Bertrand, C., CanÄipa, N., Carrillo, T., Espinosa, M., GarcõÂa, F., Lucach, S., Olivo, E., 1994. Geochemical methods applied to the characterization and correlation of oils and source rocks of Mexico. First Joint AAPG/AMGP Research Conference. Mexico. Breder, R., 1982. Optimization studies for reliable trace metal analysis in sediments by atomic absorption spectrophotometric methods. Frezenius Z. Anal. Chem. 313, 395±402. Bulska, E., Grobenski, Z., Schlemmer, G., 1990. In situ removal of contamination from palladium-magnesium chemical modi®er with an electrothermal atomizer for the determination of cadmium in serum. J. Anal. Atomic Spectrosc. 5, 203±204. Carranza-Edwards, A., Rosales Hoz, L., Monreal-GoÂmez, A., 1993. Suspended sediments in the southeastern Gulf of Mexico. Mar. Geol. 112, 257±269.
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Furnas, M.J., Smayda, T.J., 1987. Inputs of thermocline waters and nitrate onto the Campeche Bank. Continental Shelf Res. 7(2), 161± 175. Gold-Bouchot, G., 1994. In¯uencia a largo plazo de los hidrocarburos del petroleo sobre la meiofauna de la Sonda de Campeche, MeÂxico. Primer Reporte de Avance a CONACyT. Hinds, M., Jackson, M., 1990. Comparison of palladium nitrate and chloride as chemical modi®er for the determination of lead in solutions and soil slurries by electrothermal atomic absorption spectrometry. J. Anal. Atomic Spectrosc. 5, 199±202. Leahy, J.G., Somerville, C.C., Cunningham, G., Adamantiades, A., Byrd, J.J., Colwell, R.R., 1990. Hydrocarbon mineralization in sediments and plasmid incidence in sediment bacteria from the Campeche Bank. Appl. Environ. Microbiol. 56(6), 1565±1570. Lewis, J.K., 1992. The physics of the Gulf of Mexico. J. of Geophys. Res. 97(C2), 2141±2142. Manning, D.A.C., Gize, A.P., 1993. The role of organic matter in ore transport processes. In: Engels, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Principles and Applications. Plenum Press, New York. pp. 547±563. O'Connor, T.P., Cantillo, A.Y., 1992. Trace element contaminants in sediments from the NOAA National Status and Trends Programme compared to data from throughout the world. Chem. Ecol. 7, 31±50. PaÂez-Osuna, F., ValdeÂz-Lozano, D.S., Alexander, H.M., FernaÂndezPeÂrez, H., 1987. Trace metals in the ¯uvial system of the Terminos Lagoon. MeÂxico. Mar. Pollut. Bull. 18, 294±297. Regnier, P., Wollast, R., 1993. Distribution of trace metals in suspended matter of the Scheldt estuary. Mar. Chem. 43, 3±19. Simoza, R., CarrioÂn de Rosa Brussin, N., Torres, R.J., Eyzaguirre, C.L., 1985. DistribucioÂn de V, Ni, Zn, Mo, Ca, Ca, K, Fe y Cu en los crudos de la regioÂn sur de la cuenca oriental de Venezuela, VI Congreso GeoloÂgico Venezolano, Memoria. Proceedings of the 6th Venezuelan Geological Congress, 3, pp. 2088±2116. Soto, L.A., Escobar-Briones, E., 1995. Coupling mechanisms related to benthic production in the SW Gulf of Mexico. In: Eleftheriou, A., Ansell, A.D., Smith, C.J. (Eds.), Biology and Ecology of Coastal Waters: Proceedings of the 28th European Marine Biology Symposium. Fredensborg, Denmark: Olsen and Olsen. 233±242. Tissot, B.P., and Welte, D.H., 1984. Petroleum Formation and Occurrence. Springer-Verlag, Berlin. Vasquez, A. M. 1993. Bay of Campeche cyclone, PhD Thesis. Texas A. M. University. VaÂzquez, F., Sanchez, M., Alexander, H., Delgado, D., 1991. Distribution of Ni, V, and Petroleum hidrocarbons in recent sediments from the Veracruz Coast. Mexico. Bull. Environ. Contamin. Toxicol. 46, 774±781. Wartel, S., Salinas, E.I.C., 1996. Sedimentological Research. In: Longterm in¯uences on meiobenthic faunal associations relevant to oil and gas development activities on the Campeche Shelf, Yucatan (Mexico). (3rd. Report Joint Researc Proposal. Contribution Contract No. Cl*-CT91-0890 (HSMU)). Weherens, R., Van Hoof, P., Buydens, L., Kateman, G., Vossen, M., Mulder, W.H., Bakker, T., 1993. Sampling of aquatic sediments. Design of a decision-support system and a case study. Anal. Chim. Acta 271, 11±24. Wilkinson, L., 1990. SYSTAT: The system for statistics. SYSTAT Inc., Evanston, IL, pp. 677. Windown, H.L., Schropp, S.J., Calder, F.D., Ryan, J.D., Smith, R.G., Jr., Burney, L.C., Lewis, F.G., Rawlinsons, C.H., 1989. Natural trace metal concentrations in estuarine and coastal marine sediments of the southeastern United States. Environ. Sci. Technol. 23, 314±320.
Environmental Pollution 104 (1999) 69±77
Trace metals in sediment cores from the Campeche shelf, Gulf of Mexico J.V. MacõÂas-Zamora a,*, J.A. Villaescusa-Celaya a, A. MunÄoz-Barbosa a, G. Gold-Bouchot b a
Instituto de Investigaciones OceanoloÂgicas U.A.B.C., Apdo 453 Ensenada, Baja California, Mexico, 22800 b CINVESTAV-I.PN., Unidad MeÂrida. Apdo. Postal 73-Cordemex. MeÂrida, YucataÂn, Mexico, 97310 Received 1 April 1998; accepted 4 August 1998
Abstract Trace metals in sediment cores from the Gulf of Mexico, in the Campeche shelf area were studied to investigate possible variations in their vertical and/or horizontal sedimentary distributions. Possible links to petroleum sources were investigated as this area contains very productive oil explotation and transportation activities. Sediments were collected undisturbed using a vegematic type box corer. The upper 10 cm. were collected and cut in sections of 2 cm each. Incomplete, open vessel sediment digestion and atomic absorption was used for trace element determination. Particular interest was placed on vanadium and nickel concentrations as they have been associated to oil. Several metals were measured and representative distributions are presented. Horizontal distributions appear to conform to predominant current circulation patterns reported and closeness to sources of sedimentary materials. Vertical distributions are discussed. The vertical distributions were strikingly constant. Correlation among metals are also presented. Fe was used as a normalizing agent and the carbonate content was acting as a diluter of total trace metal content. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Gulf of Mexico; Pollution studies; Trace metals in sediments; Core samples
1. Introduction Crude oils represent a complex mixture of both organic and inorganic components. Trace metals are one group of elements amongst the inorganic component present in crudes. These elements have been found in dierent proportions in dierent crudes. Although other metals such as Fe, and Zn may be importantly present, frequently Ni and V are found in the largest concentrations in crude. For example, values presented for some Venezuelan crude oils (Simoza et al., 1985) as well as Middle East crude oils (Al-Shahristani and AlAtyia, 1972) have reported the predominance for V and Ni metal content. Vanadium concentrations can be as high as 2000 ppm (Tissot and Welte, 1984, cited in Manning and Gize, 1993). It is expected that, upon extraction of the crude oil and accidental deposition on surface sediments, bacterial decomposition, dissolution and oxidation of most of the organic components and * Corresponding author. Tel.:+52-61-74-46-01; fax:+52-61-74-5303; e-mail: [email protected].
remineralization of the organic matrix, trace metals are incorporated in the sediment load, increasing the background levels of metal content of the local sediment. Given the large activities of oil exploration, extraction and transportation in the area known as the Sonda de Campeche, in the Gulf of Mexico, we expected that accidental releases of oil in this area could re¯ect in a larger than normal presence of V and Ni as well as other metals in the sedimentary column. This study was carried out to determine the horizontal and vertical distribution of trace metals in sediments of the Campeche Shelf. This geographical area represents one of the most important oil producing areas for Mexico, with over 70% of domestic production. The reported annual production of oil reaches 109 million barrels (Soto and Escobar-Briones, 1995). There are seven oil producing platforms as well as two long pipelines transporting oil to two opposite places, one on the coast and one on Cayo Arcas island (Fig. 1). Frequent accidental discharges of oil in this area have been reported (Leahy et al., 1990), the largest of them being the blow-out of Ixtoc well in 1979. We expected
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compared to other environments such as the California Current or the Sea of Cortez. Although upwelling events produced by topographical features have been reported (Furnas and Smayda, 1987) producing upward mixing of near bottom water, those water are of tropical origin with poor nutrient content. Most productivity of these waters has been reported to be associated to the input of riverine and estuarine materials during rainy seasons. This introduction of water and terrigenous material accounts for a large proportion of dissolved nutrients and organic materials introduced to the area (Soto and Escobar-Briones, 1995). The main entry of terrigenous material is due to the input of the Grijalva and Usumacinta rivers to the west of our working area. This system includes also a coastal lagoon (the Terminos lagoon) which serves as a temporary trap for sediments, part of these are exported to the sea during rainy seasons (PaÂez-Osuna et al., 1987). This study was carried out to look for possible in¯uences of oil activities on the trace metals load on sediment in the area, as well as to any relationship of possible metal enrichment on benthonic organisms distributions. We present our results on the ®rst part of the objectives. Fig. 1. Study area, sampling stations and isobats (m) in the Campeche shelf.
2. Methods and materials
that some indication of these and other less severe events might have left their imprint on relatively recent sedimentary materials, which could be observable through the use of trace metals ¯uctuations as indicators of these unusual events. It has been stated that sediments, under some circumstances, can preserve and integrate a record of trace elements introduced, either through natural, or anthropogenic activities (Regnier and Wollast, 1993). However, the challenge becomes one of distinguishing the natural from manmade sources. The area is located in the southern part of the Gulf of Mexico (Fig. 1). This oceanographic region is characterized by a shallow platform, the so called Campeche Bank representing the submarine extension of the Yucatan Peninsula, and which consists of carbonaceous material and lacks terrigenous material inputs to the sea. It is also characterized by the Yucatan Current or the loop current, ¯owing from the south between the Yucatan Peninsula and Cuba. Inside the Gulf, the current is aected by the widening of the area and a shallower topography. As a consequence, cyclonic meandering currents have been reported (CarranzaEdwards et al., 1993). The meandering currents are said to be aected by seasonal tropical storms (Lewis, 1992). The warm waters (mostly above 20 C) are oligotrophic with low nutrients as well as chlorophyll content when
Sediment samples were collected during the ®rst phase of Xaman-Ek cruises, on board of the R/V Justo Sierra in February of 1992. This was a part of a larger program designed to study possible eects of oil activities on benthic communities in the area. The sampling plan included 26 stations located on ®ve segments labeled A±E with a dierent number of stations on each segment (Fig. 1). Samples were collected using a box corer of 40 by 40 cm with a 50 cm depth. The box corer used was a Kessler-Sandia MK II of stainless steel vegematic type sampler. Subsamplers inside of the box corer were used for dierent purposes. In particular, the subsampler for trace metal analysis was Te¯on coated to avoid contamination of samples. The depth of the cores selected was expected to include at least a history of the last 50 years, in which most exploration and commercial exploitation of oil ®elds has taken place. The core sediments were obtained from the Te¯oncoated square subcorer of 1010 cm. Each sample was sliced every 2 cm to a ®nal depth of 10 cm. As part of the quality control, two true replicates were also collected for only the top 2 cm surface sediments. All surface concentration numbers presented for the top 2 cm are averages of these three replicates. The samples were placed inside previously cleaned plastic bottles and frozen on board. All samples were latter freeze-dried, grounded and sieved through a 0.5 mm sieve to remove
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large particles and pieces of plants and animals at the Marine Geochemistry Laboratory in Merida, Yucatan, of the Center for Research and Advanced Studies (CINVESTAV). We did not select any particular grain size to carry the chemical analysis. The metal extraction technique for sediments closely follows that of Breder (1982). In general, it consisted of an open-vessel digestion of 0.5 grams of sediment with re¯ux. Samples were placed in 150 ml beakers. Six millilitres of concentrated HNO3 and 2 ml of HCl were added to each beaker and covered with clean watch glasses. The samples were placed on heating plates inside a fume hood and the temperature was raised to 80 C. The reaction was allowed to proceed for 5 h. After cooling, all samples were quantitatively transferred to graduated centrifuge tubes and the ®nal volume was made-up to exactly 20 ml. All glassware had been previously decontaminated with the use of a 2% solution of Micro1 detergent, followed by cleaning with a 30% HNO3 solution and rinsed with de-ionized water. As part of the quality control process, one procedural blank and one standard reference material (BCSS-1) was included in each batch of 12 samples. The results for BCSS-1 are shown in Table 1. Additionally, in the ®eld, the labeling of samples was done ``blindly'', which meant that we did not know the origin of any sample until all results were obtained. In most cases, the digestion of standard reference material was more dicult than the actual samples because of the large carbonate content which was easily dissolved by the acid treatment. Trace metal analysis was carried-out with an atomic absorption from Thermo Jarrel Ash (TJA) model Smith-Hieftje 12. For Ni, Cu, Zn, V and Fe, we used air-acetylene ¯ame. For Cr., the ¯ame was nitrous oxide-acetylene. The analysis of Cd and Pb metals was made in a graphite furnace TJA model CTF 188, using Smith-Hieftje background correction and aerosol deposition TJA Fastac II. In addition, for the Pb analysis, we used a matrix modi®er solution containing 500 mg/ml of Pd±Mg (Bulska et al., 1990; Hinds and Jackson, 1990). To obtain the spatial distribution for the surface sediments (0±2 cm), we used an exploratory analysis procedure of the data. The measured data were taken as regionalized variables and they were used to generate Table 1 Recoveries obtained for the standard reference material BCSS-1 of the National Research Council of Canada, under the mixed acid treatment conditions used Metal Expected Measured
Ni
V
Cu
Zn
Cd
Mn Fe2O3
Pb
Cr
55.3 93.4 18.5 119 0.25 229 4.70 22.7 123 3.6 4.9 2.7 12 0.4 215 .14 3.4 14 44.3 71 17.1 100.6 0.258 150 6.11 21.3 58
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spatial contours. For interpolation a kriging algorithm was used with a linear variogram (Weherens et al., 1993). To classify sampling stations, we used an R mode cluster analysis based on a complete agglomeration analysis with Euclidean distance coecients. The matrix was previously standardized using Z function. This procedure was accomplished with the SYSTAT statistical package (Wilkinson, 1990). 3. Results and discussions Most metals showed a horizontal distribution with a tendency to increase towards the south-west in the study area. Nickel presented a regional average of 23.0 mg gÿ1 with an interval from 0.56 to 76.9 mg gÿ1, with the largest values for the stations A4 and A5. For Cu, the interval of concentrations varied from 3.82 to 18.7 mg gÿ1, with an average of 7.53 mg gÿ1. Cu was the element that showed the smallest variation in the sediments of the region (the ratio maximum/minimum was 4.6). For Zn, as well as Ni, the largest values were found at stations A4 and A5. Zn was the element with the largest variation for the area, the smallest concentration was of only 0.04 mg gÿ1 in station D3 (4±6 cm), and the largest was 79.6 mg gÿ1 for station A5 (4±6 cm). The regional media for these stations was 18.5 mg gÿ1. Chromium showed also a small variability with a media concentration of 39.8 mg gÿ1 and a smallest value of 3.0 mg gÿ1 and a largest of 100 mg gÿ1. Cadmium also showed small variability its interval of concentrations found was from 0.01 to 0.7 mg gÿ1 with a median value of 0.09 mg gÿ1. Generally speaking, cadmium presented the largest concentrations in transects A and B and the smallest concentration close to the Yucatan platform (transects C and D). However, the concentration of 0.7 mg gÿ1 found for station B6 was considered as ``anomalous'' given that Cd was not larger than 0.2 for all regions. Similarly to other metals, Lead showed the largest concentrations in the south-west area. The average concentration was 4.3 mg gÿ1, while its interval was from 0.22 mg gÿ1 in station D4 (0±2 cm) to 20.2 mg gÿ1 for station A5 (2±4 cm). The average value for vanadium was 47.78 mg gÿ1 with a minimum of 15.6 mg gÿ1 and a maximum of 117.5 mg gÿ1 present at the A5 (4±6 cm)station. Manganese had an average for the area of 110.7 mg gÿ1 with a minimum of 12.5 mg gÿ1 at station E2(0±2 cm) and a maximum of 448.9 mg gÿ1, again at station A5 (0±2 cm). Finally, iron reported here as percentage of Fe2O3, presents similar trends to those of other metals. In general, the largest concentrations were found in transect A. The average concentration was of 1.84% with a wide range of variation going from below detection levels close to the Yucatan Peninsula (station D4, level 6±8 cm) to as much as 7.9% found at station A5 (4±6 cm).
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Fig. 2. Horizontal distribution of metals concentrations (mg gÿ1). (a) Ni, (b) Zn, (c) Pb and (d) Cu, respectively.
Fig. 3. Linear regresion for iron concentration vs each metal. Broken lines represent the range at 95% limit of con®dence. Dotted lines represent 95% con®dence level band for all data.
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Fig. 4. Vertical distribution concentrations (in mg/gÿ1 dry weight) for all metals at two dierent stations. (a) For station 5A. (b) For station B6. Filled circles represent the raw metal concentration. Open circles represent the iron-normalized data.
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Fig. 5. Areal clasi®cation according cluster analysis for all surface sediments atributes (metal concentrations, carbonate content, organic matter). Shading was done according to the three groups generated.
To our knowledge there appears to be little information on metal concentration studies for the Sonda de Campeche area. We found one study for the Laguna de Terminos in Campeche (PaÂez-Osuna et al., 1987). A second study reported on the distribution of Ni, V and hidrocarbons content in recent sediment (VaÂzquez et al., 1991), but their work area is located in a region north of our location. A third work reported on the composition of mayor elements in suspended matter in the sonda the Campeche area (Carranza-Edwards et al., 1993). However, little direct comparison to our results could be established. The concentrations found in this analysis can be considered low with respect to those reported for other areas. For example, O'Connor and Cantillo (1992) report a compilation of studies on the surveillance of pollution in the U.S. In their study, the average values for Ni, Cu, Zn, Cr, Cd and Pb are 34, 35, 140, 110, 0.48 y 43 mg gÿ1 respectively. These authors also report on world levels for average concentrations (after elimination of highly polluted localities) for Ni, Cu, Zn, Cr, Cd, and Pb as 26, 28, 91, 61, 1.1 and 34 mg gÿ1 respectively. The average values found in this study for the enlisted metals were 21.5, 7.5, 18.5, 39.8, 0.10 and 4.30 mg gÿ1 respectively. In Fig. 2 (a, b, c and d) we show the horizontal distributions of trace metals for surface sediments (0±2 cm)
Fig. 6. Box and whiskers diagram showing the 3 dierent zones based on carbonate (a) and iron (b) content.
for Ni, Zn, Pb and Cu. Other metals trends follow similar patterns (data not shown). Similarly, the horizontal distributions at other depths showed approximately the same distribution patterns. A common feature is the longitudinal gradient for all metals, with lower values towards the Yucatan platform and higher values in the south-west area. This is exactly opposite to that found for carbonates in sediments (Gold-Bouchot, 1994). These distribution suggest that the metal distributions are being controlled to a large extent by the closeness to the source of terrigenous material. It also suggest that the metal distribution gradients may also be aected by the circulation patterns produced as a result
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Fig. 7. Ni/V ratio for surface sediments in the Campeche shelf.
of a cyclonic circulation (Vasquez, 1993) forced by wind in the Campeche bay. In Fig. 3, we show the linear regressions between the concentrations of each metal (Ni, V, Cu, Zn, Cr, Cd, Pb and Mn) versus Fe2O3 concentration. We have used iron as a normalizing tool because of two reason. First, as shown in Fig. 3 (a±h), iron is largely controlling the distribution of most metals (Cd is the only exception with an r2 of 0.10). Second, given that our digestion procedure did not include HF for total digestion, recoveries for Aluminum, which has been extensively used for normalization purposes (Windown et al., 1989; Regnier and Wollast, 1993), were very low. The broken lines represent the range at 95% limit of con®dence. The dotted lines represent the 95% con®dence band for all data (Windown et al., 1989). Values found above these lines represent enrichment for the metals at a particular station and depth level. The linear regressions also suggest, particularly in the cases of Cr, Ni and maybe Zn, the separation of data points within generally three sets, those with low metal content, those with a relatively large metal content and a transitional group conforming to previously suggested zonation (Carranza-Edwards et al., 1993.). In Fig. 4(a and b), we show vertical distribution of all metals for only two stations. In Fig. 4(a), the vertical distribution for station 5A, and in Fig. 4(b) for station B6. In both cases, the ®lled circles represent the raw
metal concentration versus depth and the open circles represent the Fe-normalized metal concentration. The fact that the small vertical variations of metal concentrations are smoothed-out, particularly for station 5A, by the normalization with respect to the iron content of the sediment, suggest that there is no presence of a diagenetic process or, that it is not observable for the ®rst 10 cm. With the exception of Cd in station A5 (8± 10 cm), and in station B6 (2±4), all metals remain remarkably constant versus depth. The largely homogeneous concentrations found could also be due to extensive bioturbation or to a well oxygenated sediment, probably due to the large sediment porosity expected resulting from its large carbonate content, or to a combination of both. For the B6 station however, some vertical changes are clearly and consistently present for all iron normalized data. It is concebible that ¯uctuations in riverine loads and/or ¯uctuations of circulation regimes might result in changes in dilution of metal loads to the sediments. These are surprisingly consistent for all metals, suggesting a common source and a common event, resulting in a simultaneous change in all metal concentrations at around the 4±6 cm depth. Both, the spatial distributions of carbonates an iron suggest an in¯uence of terrigenous material deposited in the coastal areas specially the areas close to the Tabasco and Veracruz states. This material is most probably introduced by the pluvial input of the Grijalva and Usumacinta rivers (Carranza-Edwards et al., 1993; Gold-Bouchot, 1994). Our results support this as the probable origin for most of the metal load for the area. The fact that the metal bearing inorganic detritus carried by rivers is recognized as the main source of metal load argues against local oil related anthropogenic inputs. In the case of Cd, its distribution is being strongly in¯uenced by the values that we considered outliers. It is worth noting that a parallel study conducted for dating sediments based on the 210Pb technique (Wartel and Salinas, 1996), resulted in an estimated sedimentation rate for the area of between 0.8 to 1.4 mm/yearsÿ1. This estimate places the average rate for the area at 1.1 mm/yearsÿ1 yielding, for the 10 cm depth cores a period of approximately 90 years. It is clear that the faster sedimentation rate is expected to occur for the area closer to the rivers and a smaller sedimentation rate for the carbonaceous shelf closer to the Yucatan peninsula and further away from terrigenous sources. When all stations are grouped by cluster analysis, 3 groups were obtained (Fig. 5). This is consistent with the three area zonation proposed by others (CarranzaEdwards et al., 1993). One area with a high carbonate content, located at the southeast on the main Campeche shelf; one with relatively higher terrigenous material content located at the southwest and near the rivers,
J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77
and one transitional zone in the middle of these two regions. These regions are signi®catively dierent as can also be seen from Fig. 6 (a and b) when the stations are plotted based on their carbonate and iron contents. Finally, there is little if any, evidence in this study to believe that the area has had a large anthropogenic metal input. There is but one indication that suggests some in¯uence of oil activities. In Fig. 7, we show the distribution for the Ni/V concentration ratio showing the maximum values of this ratio close to stations near the restricted oil exploitation areas. However, for example, the V/Ni ratios for 5 oil exploration wells in the area, has been reported to vary from 3.25 to 6.78 (Bertrand et al., 1994). In our study, there were only a few values within this range for our data points, furthermore, the trends could also be explained, at least partially, as generated by the same circulation and depositional patterns with a maximum in the vicinity of the oil production activities and not necessarily deriving from them.
Acknowledgements We would like to thank CINVESTAV, Merida for their invitation to participate in this project. We would also like to thank CONACYT (Contract 1284M9204), and to the European Community (Contract CI 1*-CT91-0890 (HSMU)) for providing the funds for the project. Finally, we would like to thank several anonymous reviewers for helping in improving this manuscript.
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