Trace metals in sediment cores from the Campeche shelf, Gulfof Mexico

J.V. MacõÂ as-Zamoraa,*, J.A. Villaescusa-Celayaa, A. MunÄ oz-Barbosa a,G. Gold-Bouchot b

aInstituto de Investigaciones OceanoloÂgicas U.A.B.C., Apdo 453 Ensenada, Baja California, Mexico, 22800bCINVESTAV-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 varia-tions in their vertical and/or horizontal sedimentary distributions. Possible links to petroleum sources were investigated as this areacontains very productive oil explotation and transportation activities. Sediments were collected undisturbed using a vegematic typebox 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 theyhave been associated to oil. Several metals were measured and representative distributions are presented. Horizontal distributionsappear 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 wasused as a normalizing agent and the carbonate content was acting as a diluter of total trace metal content.# 1999 Elsevier ScienceLtd. All rights reserved.

Keywords: Gulf of Mexico; Pollution studies; Trace metals in sediments; Core samples

1. Introduction

Crude oils represent a complex mixture of bothorganic and inorganic components. Trace metals areone group of elements amongst the inorganic compo-nent present in crudes. These elements have been foundin di�erent proportions in di�erent crudes. Althoughother metals such as Fe, and Zn may be importantlypresent, frequently Ni and V are found in the largestconcentrations in crude. For example, values presentedfor some Venezuelan crude oils (Simoza et al., 1985) aswell as Middle East crude oils (Al-Shahristani and Al-Atyia, 1972) have reported the predominance for V andNi metal content. Vanadium concentrations can be ashigh as 2000 ppm (Tissot and Welte, 1984, cited inManning and Gize, 1993). It is expected that, uponextraction of the crude oil and accidental deposition onsurface sediments, bacterial decomposition, dissolutionand oxidation of most of the organic components and

remineralization of the organic matrix, trace metals areincorporated in the sediment load, increasing the back-ground 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 deCampeche, in the Gulf of Mexico, we expected thataccidental releases of oil in this area could re¯ect in alarger than normal presence of V and Ni as well as othermetals in the sedimentary column.This study was carried out to determine the hori-

zontal and vertical distribution of trace metals in sedi-ments of the Campeche Shelf. This geographical arearepresents one of the most important oil producingareas for Mexico, with over 70% of domestic produc-tion. The reported annual production of oil reaches 109million barrels (Soto and Escobar-Briones, 1995). Thereare seven oil producing platforms as well as two longpipelines transporting oil to two opposite places, one onthe coast and one on Cayo Arcas island (Fig. 1).Frequent accidental discharges of oil in this area havebeen reported (Leahy et al., 1990), the largest of thembeing the blow-out of Ixtoc well in 1979. We expected

0269-7491/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0269-7491(98)00153-5

ENVIRONMENTAL

POLLUTION

Environmental Pollution 104 (1999) 69±77

* Corresponding author. Tel.:+52-61-74-46-01; fax:+52-61-74-53-

03; e-mail: vmacias@bahia.ens.uabc.mx.

that some indication of these and other less severeevents might have left their imprint on relatively recentsedimentary materials, which could be observablethrough the use of trace metals ¯uctuations as indica-tors of these unusual events. It has been stated thatsediments, under some circumstances, can preserve andintegrate a record of trace elements introduced, eitherthrough natural, or anthropogenic activities (Regnierand Wollast, 1993). However, the challenge becomesone of distinguishing the natural from manmade sour-ces.The area is located in the southern part of the Gulf of

Mexico (Fig. 1). This oceanographic region is char-acterized by a shallow platform, the so called CampecheBank representing the submarine extension of theYucatan Peninsula, and which consists of carbonaceousmaterial and lacks terrigenous material inputs to thesea. It is also characterized by the Yucatan Current orthe loop current, ¯owing from the south between theYucatan Peninsula and Cuba. Inside the Gulf, the cur-rent is a�ected by the widening of the area and ashallower topography. As a consequence, cyclonicmeandering currents have been reported (Carranza-Edwards et al., 1993). The meandering currents are saidto be a�ected by seasonal tropical storms (Lewis, 1992).The warm waters (mostly above 20�C) are oligotrophicwith low nutrients as well as chlorophyll content when

compared to other environments such as the CaliforniaCurrent or the Sea of Cortez. Although upwellingevents produced by topographical features have beenreported (Furnas and Smayda, 1987) producing upwardmixing of near bottom water, those water are of tropicalorigin with poor nutrient content. Most productivity ofthese waters has been reported to be associated to theinput of riverine and estuarine materials during rainyseasons. This introduction of water and terrigenousmaterial accounts for a large proportion of dissolvednutrients 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 westof our working area. This system includes also a coastallagoon (the Terminos lagoon) which serves as a tem-porary trap for sediments, part of these are exported tothe sea during rainy seasons (Pa ez-Osuna et al., 1987).This study was carried out to look for possible in¯u-

ences of oil activities on the trace metals load on sedi-ment in the area, as well as to any relationship ofpossible metal enrichment on benthonic organisms dis-tributions. We present our results on the ®rst part of theobjectives.

2. Methods and materials

Sediment samples were collected during the ®rstphase of Xaman-Ek cruises, on board of the R/V JustoSierra in February of 1992. This was a part of a largerprogram designed to study possible e�ects of oil activ-ities on benthic communities in the area. The samplingplan included 26 stations located on ®ve segmentslabeled A±E with a di�erent number of stations on eachsegment (Fig. 1). Samples were collected using a boxcorer of 40 by 40 cm with a 50 cm depth. The box corerused was a Kessler-Sandia MK II of stainless steelvegematic type sampler. Subsamplers inside of the boxcorer were used for di�erent purposes. In particular, thesubsampler for trace metal analysis was Te¯on coatedto avoid contamination of samples.The depth of the cores selected was expected to

include at least a history of the last 50 years, in whichmost exploration and commercial exploitation of oil®elds has taken place.The core sediments were obtained from the Te¯on-

coated square subcorer of 10�10 cm. Each sample wassliced every 2 cm to a ®nal depth of 10 cm. As part ofthe quality control, two true replicates were also col-lected for only the top 2 cm surface sediments. All sur-face concentration numbers presented for the top 2 cmare averages of these three replicates. The samples wereplaced inside previously cleaned plastic bottles and fro-zen on board. All samples were latter freeze-dried,grounded and sieved through a 0.5 mm sieve to remove

Fig. 1. Study area, sampling stations and isobats (m) in the Campeche

shelf.

70 J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77

large particles and pieces of plants and animals at theMarine 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 forsediments closely follows that of Breder (1982). In gen-eral, it consisted of an open-vessel digestion of 0.5grams of sediment with re¯ux. Samples were placed in150 ml beakers. Six millilitres of concentrated HNO3

and 2 ml of HCl were added to each beaker and coveredwith clean watch glasses. The samples were placed onheating plates inside a fume hood and the temperaturewas raised to 80�C. The reaction was allowed to proceedfor 5 h. After cooling, all samples were quantitativelytransferred to graduated centrifuge tubes and the ®nalvolume was made-up to exactly 20 ml. All glasswarehad been previously decontaminated with the use of a2% solution of Micro1 detergent, followed by cleaningwith a 30% HNO3 solution and rinsed with de-ionizedwater. As part of the quality control process, one pro-cedural blank and one standard reference material(BCSS-1) was included in each batch of 12 samples. Theresults 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 anysample until all results were obtained.In most cases, the digestion of standard reference

material was more di�cult than the actual samplesbecause of the large carbonate content which was easilydissolved by the acid treatment.Trace metal analysis was carried-out with an atomic

absorption from Thermo Jarrel Ash (TJA) modelSmith-Hieftje 12. For Ni, Cu, Zn, V and Fe, we usedair-acetylene ¯ame. For Cr., the ¯ame was nitrousoxide-acetylene. The analysis of Cd and Pb metals wasmade in a graphite furnace TJA model CTF 188, usingSmith-Hieftje background correction and aerosoldeposition TJA Fastac II. In addition, for the Pb analy-sis, we used a matrix modi®er solution containing 500mg/ml of Pd±Mg (Bulska et al., 1990; Hinds and Jack-son, 1990).To obtain the spatial distribution for the surface

sediments (0±2 cm), we used an exploratory analysisprocedure of the data. The measured data were taken asregionalized variables and they were used to generate

spatial contours. For interpolation a kriging algorithmwas used with a linear variogram (Weherens et al.,1993). To classify sampling stations, we used an R modecluster analysis based on a complete agglomerationanalysis with Euclidean distance coe�cients. The matrixwas previously standardized using Z function. Thisprocedure was accomplished with the SYSTAT statis-tical package (Wilkinson, 1990).

3. Results and discussions

Most metals showed a horizontal distribution with atendency to increase towards the south-west in the studyarea. Nickel presented a regional average of 23.0 mg gÿ1

with an interval from 0.56 to 76.9 mg gÿ1, with the larg-est values for the stations A4 and A5. For Cu, theinterval of concentrations varied from 3.82 to 18.7 mggÿ1, with an average of 7.53 mg gÿ1. Cu was the elementthat showed the smallest variation in the sediments ofthe region (the ratio maximum/minimum was 4.6). ForZn, as well as Ni, the largest values were found at sta-tions A4 and A5. Zn was the element with the largestvariation for the area, the smallest concentration was ofonly 0.04 mg gÿ1 in station D3 (4±6 cm), and the largestwas 79.6 mg gÿ1 for station A5 (4±6 cm). The regionalmedia for these stations was 18.5 mg gÿ1. Chromiumshowed also a small variability with a media concentra-tion of 39.8 mg gÿ1 and a smallest value of 3.0 mg gÿ1

and a largest of 100 mg gÿ1. Cadmium also showedsmall variability its interval of concentrations found wasfrom 0.01 to 0.7 mg gÿ1 with a median value of 0.09 mggÿ1. Generally speaking, cadmium presented the largestconcentrations in transects A and B and the smallestconcentration close to the Yucatan platform (transectsC 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 con-centrations in the south-west area. The average con-centration was 4.3 mg gÿ1, while its interval was from0.22 mg gÿ1 in station D4 (0±2 cm) to 20.2 mg gÿ1 forstation A5 (2±4 cm). The average value for vanadiumwas 47.78 mg gÿ1 with a minimum of 15.6 mg gÿ1 and amaximum of 117.5 mg gÿ1 present at the A5 (4±6cm)station. Manganese had an average for the area of110.7 mg gÿ1 with a minimum of 12.5 mg gÿ1 at stationE2(0±2 cm) and a maximum of 448.9 mg gÿ1, again atstation A5 (0±2 cm).Finally, iron reported here as percentage of Fe2O3,

presents similar trends to those of other metals. In gen-eral, the largest concentrations were found in transectA. The average concentration was of 1.84% with a widerange of variation going from below detection levelsclose to the Yucatan Peninsula (station D4, level 6±8cm) to as much as 7.9% found at station A5 (4±6 cm).

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 Ni V Cu Zn Cd Mn Fe2O3 Pb Cr

Expected 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

Measured 44.3 71 17.1 100.6 0.258 150 6.11 21.3 58

J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77 71

Fig. 2. Horizontal distribution of metals concentrations (mg gÿ1). (a) Ni, (b) Zn, (c) Pb and (d) Cu, respectively.

72 J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77

Fig.3.Linearregresionforironconcentrationvseach

metal.Broken

linesrepresenttherangeat95%

limitofcon®dence.Dotted

linesrepresent95%

con®dence

levelbandforalldata.

J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77 73

Fig.4.Verticaldistributionconcentrations(inmg/gÿ1

dry

weight)forallmetalsattwodi�erentstations.(a)Forstation5A.(b)ForstationB6.Filledcirclesrepresenttherawmetalconcentration.

Open

circlesrepresenttheiron-norm

alizeddata.

74 J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77

To our knowledge there appears to be little informationon metal concentration studies for the Sonda de Cam-peche area. We found one study for the Laguna deTerminos in Campeche (Pa ez-Osuna et al., 1987). Asecond study reported on the distribution of Ni, V andhidrocarbons content in recent sediment (Va zquez et al.,1991), but their work area is located in a region north ofour location. A third work reported on the compositionof mayor elements in suspended matter in the sonda theCampeche area (Carranza-Edwards et al., 1993). How-ever, little direct comparison to our results could beestablished.The concentrations found in this analysis can be

considered low with respect to those reported for otherareas. For example, O'Connor and Cantillo (1992)report a compilation of studies on the surveillance ofpollution in the U.S. In their study, the average valuesfor Ni, Cu, Zn, Cr, Cd and Pb are 34, 35, 140, 110, 0.48y 43 mg gÿ1 respectively. These authors also report onworld levels for average concentrations (after elimina-tion 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 enlistedmetals 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 dis-

tributions of trace metals for surface sediments (0±2 cm)

for Ni, Zn, Pb and Cu. Other metals trends followsimilar patterns (data not shown). Similarly, the hor-izontal distributions at other depths showed approxi-mately the same distribution patterns. A commonfeature is the longitudinal gradient for all metals, withlower values towards the Yucatan platform and highervalues in the south-west area. This is exactly opposite tothat found for carbonates in sediments (Gold-Bouchot,1994). These distribution suggest that the metal dis-tributions are being controlled to a large extent by thecloseness to the source of terrigenous material. It alsosuggest that the metal distribution gradients may also bea�ected by the circulation patterns produced as a result

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.

Fig. 6. Box and whiskers diagram showing the 3 di�erent zones based

on carbonate (a) and iron (b) content.

J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77 75

of a cyclonic circulation (Vasquez, 1993) forced by windin the Campeche bay.In Fig. 3, we show the linear regressions between the

concentrations of each metal (Ni, V, Cu, Zn, Cr, Cd, Pband Mn) versus Fe2O3 concentration. We have usediron as a normalizing tool because of two reason. First,as shown in Fig. 3 (a±h), iron is largely controlling thedistribution of most metals (Cd is the only exceptionwith an r2 of 0.10). Second, given that our digestionprocedure did not include HF for total digestion,recoveries for Aluminum, which has been extensivelyused for normalization purposes (Windown et al., 1989;Regnier and Wollast, 1993), were very low. The brokenlines represent the range at 95% limit of con®dence. Thedotted lines represent the 95% con®dence band for alldata (Windown et al., 1989). Values found above theselines represent enrichment for the metals at a particularstation and depth level. The linear regressions also sug-gest, 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 relativelylarge metal content and a transitional group conformingto previously suggested zonation (Carranza-Edwards etal., 1993.).In Fig. 4(a and b), we show vertical distribution of all

metals for only two stations. In Fig. 4(a), the verticaldistribution for station 5A, and in Fig. 4(b) for stationB6. In both cases, the ®lled circles represent the raw

metal concentration versus depth and the open circlesrepresent the Fe-normalized metal concentration. Thefact that the small vertical variations of metal con-centrations are smoothed-out, particularly for station5A, by the normalization with respect to the iron con-tent of the sediment, suggest that there is no presence ofa 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 remainremarkably constant versus depth. The largely homo-geneous concentrations found could also be due toextensive bioturbation or to a well oxygenated sedi-ment, probably due to the large sediment porosityexpected resulting from its large carbonate content, orto a combination of both. For the B6 station however,some vertical changes are clearly and consistently pre-sent for all iron normalized data. It is concebible that¯uctuations in riverine loads and/or ¯uctuations of cir-culation regimes might result in changes in dilution ofmetal loads to the sediments. These are surprisinglyconsistent for all metals, suggesting a common sourceand a common event, resulting in a simultaneouschange in all metal concentrations at around the 4±6 cmdepth.Both, the spatial distributions of carbonates an iron

suggest an in¯uence of terrigenous material deposited inthe coastal areas specially the areas close to the Tabascoand Veracruz states. This material is most probablyintroduced by the pluvial input of the Grijalva andUsumacinta rivers (Carranza-Edwards et al., 1993;Gold-Bouchot, 1994). Our results support this as theprobable origin for most of the metal load for the area.The fact that the metal bearing inorganic detritus car-ried by rivers is recognized as the main source of metalload argues against local oil related anthropogenicinputs.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 (Warteland Salinas, 1996), resulted in an estimated sedimenta-tion rate for the area of between 0.8 to 1.4 mm/yearsÿ1.This estimate places the average rate for the area at 1.1mm/yearsÿ1 yielding, for the 10 cm depth cores a periodof approximately 90 years. It is clear that the fastersedimentation rate is expected to occur for the areacloser to the rivers and a smaller sedimentation rate forthe carbonaceous shelf closer to the Yucatan peninsulaand further away from terrigenous sources.When all stations are grouped by cluster analysis, 3

groups were obtained (Fig. 5). This is consistent withthe three area zonation proposed by others (Carranza-Edwards et al., 1993). One area with a high carbonatecontent, located at the southeast on the main Campecheshelf; one with relatively higher terrigenous materialcontent located at the southwest and near the rivers,

Fig. 7. Ni/V ratio for surface sediments in the Campeche shelf.

76 J.V. MacõÂas-Zamora et al. / Environmental Pollution 104 (1999) 69±77

and one transitional zone in the middle of these tworegions. These regions are signi®catively di�erent as canalso be seen from Fig. 6 (a and b) when the stations areplotted 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 anthropogenicmetal input. There is but one indication that suggestssome in¯uence of oil activities. In Fig. 7, we show thedistribution for the Ni/V concentration ratio showingthe maximum values of this ratio close to stations nearthe restricted oil exploitation areas. However, forexample, the V/Ni ratios for 5 oil exploration wells inthe area, has been reported to vary from 3.25 to 6.78(Bertrand et al., 1994). In our study, there were only afew values within this range for our data points, fur-thermore, the trends could also be explained, at leastpartially, as generated by the same circulation anddepositional patterns with a maximum in the vicinity ofthe oil production activities and not necessarily derivingfrom them.

Acknowledgements

We would like to thank CINVESTAV, Merida fortheir invitation to participate in this project. Wewould also like to thank CONACYT (Contract 1284M-9204), and to the European Community (Contract CI1*-CT91-0890 (HSMU)) for providing the funds forthe project. Finally, we would like to thank severalanonymous reviewers for helping in improving thismanuscript.

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