� Corresponding

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0265-931X/$ - see

doi:10.1016/j.jenv

author. Tel.: +52-6-985-28-45; fax: +52-6-982-61-33.

: caro@ola.icmyl.unam.mx (A.C. Ruiz-Fernandez).

front matter # 2004 Elsevier Ltd. All rights reserved.

rad.2004.03.024

Journal of Environmental Radioactivity 76 (2004) 161–175

www.elsevier.com/locate/jenvrad

210Pb geochronology and trace metal fluxes(Cd, Cu and Pb) in the Gulf of Tehuantepec,

South Pacific of Mexico

Ana Carolina Ruiz-Fernandez a,�, Federico Paez-Osuna a,Marıa Luisa Machain-Castillo b, Elsa Arellano-Torres c

a Mazatlan Academic Unit, Institute of Marine Sciences and Limnology—UNAM, Calz. Joel Montes

Camarena s/n, 82040 Mazatlan, Sin., Mexicob Marine and Environmental Geology Academic Unit, Institute of Marine Sciences and

Limnology—UNAM, Circuito Ciudad Universitaria s/n, 04510 Mexico D.F., Mexicoc Posgraduate Program in Marine Sciences and Limnology, Institute of Marine Sciences and

Limnology—UNAM, Circuito Ciudad Universitaria s/n, 04510 Mexico D.F., Mexico

Received 5 June 2003; received in revised form 17 January 2004; accepted 28 January 2004

Abstract

Distributions of Al, Cd, Cu, Fe, Li, Mn and Pb were analyzed in a sediment core col-lected in the Gulf of Tehuantepec, an important fisheries region located in the South Pacificof Mexico, where data on metal accumulation and accretion rates were previously almostnonexistent. Depth profiles of metal concentrations were converted to time-based profiles byusing a 210Pb-derived vertical accretion rate, estimated to be 0.05 cm year�1 on the average.Sediments were dated up to 8 cm depth, corresponding to a layer of ca. 140 years old. Thehistorical changes of metal accumulation along the sediment core have shown a moderateenrichment of Cd, Cu and Pb concentrations at present, of about threefold the correspond-ing background concentrations. Chronological trace metal records showed that metal fluxeshave increased over the last 20 years, reaching the maximum values at present of 2.5, 22.5and 45.8 (lg cm�2 year�1) for Cd, Pb and Cu, respectively. These increments in metal fluxesare likely influenced by the development of anthropogenic land-based activities since overthis period of time oil production activities in the region have had a significant development.# 2004 Elsevier Ltd. All rights reserved.

Keywords: 210Pb-derived sedimentation rates; Trace metals accumulation; Gulf of Tehuantepec

A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175162

1. Introduction

The Gulf of Tehuantepec is located in the Mexican Tropical Pacific,approximately between 14

v300 to 16

v120 N and 92

v000 to 96

v000 W, repre-

senting the southern oceanic boundary of the Mexican Exclusive EconomicZone. It has a continental shelf with a width of about 120 km and anapproximated radial extension of 200 km (Lavin et al., 1992). The region istropical warm with a mean annual temperature of 27

vC, showing minimal

variations during the year (Garcıa, 1981). It is a very productive and dynamicsystem that supports a great variety of commercial fisheries, including shrimpand tuna fisheries, from the coastline to the open ocean. It has been specu-lated that the location of the fishing resources in open waters is stronglyrelated to wind circulation patterns (the presence of anticyclone gyres thatpropagate toward the west), while close to the coast the accumulation oforganisms has been mostly related to the propagation of internal wavestoward the coastline. However, it is also recognized that there is a stronginfluence of the intense wind-induced upwelling promoted by the northernwinds (called ‘‘Tehuantepecers’’) produced by cold continental polar massesduring the winter months, which dominate the oceanic circulation and changesthe thermohaline characteristics of the Gulf (Mc Creary et al., 1989; Lavinet al., 1992; Monreal and Salas, 1998). Moreover, the area experiences signifi-cant tectonic activity due to active subduction. The continental crust in thearea has been subjected to continuous uplift, producing plutonic and meta-morphic outcrops, which form terrestrial detritus that is carried to the sea,reflecting the composition of the source rocks (Carranza-Edwards et al., 1998).The environmental impact caused by human and industrial activities has beenscarcely studied at the site, although high pollution by oil-derived hydro-carbons and heavy metals, along with the presence of herbicides and insecti-cides residues, have already been reported in waters of the coastal bodiesconnected to Gulf of Tehuantepec (Tapia-Garcıa et al., 1998).This paper presents a basic biogeochemical characterization of a sediment core

collected in the area and a reconnaissance of metals accumulated over time. Back-ground concentrations of trace metals Cd, Cu and Pb were established, metalenrichment was evaluated by using enrichment and anthropogenic factors (EF andAF), and the 210Pb-derived accumulation rate was used to assess trace metal fluxvariability during the last 140 years.

2. Methods

2.1. Sampling

The gravity core 10T, 10 cm in diameter and 1.88 m in length, was extractedfrom the Gulf of Tehuantepec (15

v27.220 N and 94

v22.860 W, water depth

~240 m) on board of O/V El Puma, during the oceanographic cruise PAC-

163A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175

MEX 1 in May–June 1999 (Fig. 1). The core was split in halves lengthwise,

sealed and stored horizontally at 4vC. No lamination or evidence of bioturba-

tion signs was observed. Sediments for geochemical characterization, trace

metal and radioisotope analysis (210Pb, 137Cs, 226Ra) were subsampled at 1 cm

depth intervals.

2.2. Laboratory analysis

Grain size was analyzed by wet sieving through a 63-lm stainless steel sieve, to

estimate the amount of sand (>63 lm) and mud (<63 lm) in the sample. Carbon-

ate content (CaCO3) and organic carbon (OC) were analyzed by colorimetric pro-

cedures (Stuardo and Villarroel, 1976; Loring and Rantala, 1992). Metal analyses

were performed by atomic absorption spectrophotometry (Al, Fe and Mn by

flame, and Cd, Cu, Li, and Pb by graphite furnace) after microwave digestion of

the sediments in a mixture of HNO3 þHClþHF (Loring and Rantala, 1992).

Accuracy of the metal analyses was evaluated by using samples of standard refer-

ence materials (IAEA-356 and BCSS-1) and analytical precision by using six repli-

cates of a homogenized sample. Recoveries (%) varied according to the metal as

follows: 89–101 for Al, 96–116 for Cd, 91–93 for Cu, 81–104 for Fe, 54–89 for Mn

and 105–109 for Pb and the variation coefficients were estimated to be minor to

10%. 210Pb, 137Cs and 226Ra were simultaneously analyzed by gamma spectrometry

Fig. 1. Sampling area. Core 10T was collected at 15v27.220 N and 94

v22.860 W, at 240 m depth.

A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175164

at the University of Southern California, with analytical uncertainties varying from2% to 20% for 210Pb and from 10% to 12% for 226Ra.

3. Results and discussion

3.1. 210Pb geochronology

The total 210Pb activity concentrations (210Pbtot) were measured up to 8 cmdepth, where the activity found was comparable to the 226Ra values obtained alongthe same depth interval (13.3–16.7 Bq kg�1). 210Pbtot activities ranged from 28 to753 Bq kg�1, and showed an almost monotonic decline with depth indicating arather undisturbed environment, where bioturbation or physical mixing was con-sidered negligible. Sediments at core 10T are assumed to have been deposited with-out much subsequent mixing, because Cd, Cu and Pb profile distributions do notappear to coincide (Fig. 2a). For a profile that is substantially controlled by mixingand not by sediment accumulation, no such differences would be expected if it is

le distributions of (a) 210Pbtot and226Ra; (b) Cd, Cu and Pb and (c) 210Pbxs in

Fig. 2. Profi the surficial

section (8 cm depth) of core 10T.

165A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175

assumed that those trace metals trace the same sediment materials. Attempts weremade to measure 137Cs activities in core 10T, but, as previously reported for othercores collected in the Mexican Pacific coast (Paez-Osuna and Mandelli, 1985;Ruiz-Fernandez et al., 2001), values obtained did not exceed background levels.226Ra activities were used as an estimate of supported 210Pb background levels(210Pbsup) and, by subtracting the corresponding value of 210Pbsup from 210Pbtot ateach layer of the core, excess 210Pb (210Pbxs) values were found to range from 738.3to 15.0 Bq kg�1. 210Pbxs activities plotted on a logarithmic scale against depthresulted in a linear profile (Fig. 2b) from where sedimentation rate can be obtainedby a least squares fit procedure (Oldfield and Appleby, 1984). Ignoring compactionand assuming that: (a) the rate of deposition of 210Pbxs from the atmosphere isconstant; (b) 210Pbxs is quickly removed from solution onto particulate matter, so210Pbxs activity in sediments is essentially that due to overhead fallout from theatmosphere; (c) the flux of 210Pbxs via the water column to the sediment is alsoconstant and (d) the 210Pbxs, once incorporated in the sediments, does not diffusevia the pore water of the sediment (Schell and Nevissi, 1983), downcore ageswere calculated from the constant initial concentration (CIC) model by using theequation

t ¼ k�1lnCo=Cz

where Co and Cz are the values of 210Pbxs at the sediment–water interface and atdepth z, respectively, and k is the 210Pbxs radioactive decay constant (0.03114year�1).Sediment core 10T was dated up to 8 cm depth, corresponding to a layer of 140

years old. The average sedimentation rate obtained was 0:05� 0:01 cm year�1 and,assuming a particle density of 2.55 g cm�3 as the average density for clayey sedi-ments (Berner, 1971), the mean accumulation rate was calculated to be

0:15� 0:02 g cm�2 year�1. The sedimentation rate obtained from core 10T is com-parable with those obtained for other oceanic regions in the Gulf of California,Mexico, also characterized by a high biological productivity, tectonic activities andupwelling phenomena, i.e. the basins of Guaymas, Carmen and Pescadero (0.05,0.03 and 0.05 cm year�1) although those rates were estimated from cores collectedat deeper sites (>800 m depth) (Paez-Osuna and Osuna-Lopez, 1990). However,there is a growing body of data indicating that upper slope and shelf sedimentaccumulation rates based on 210Pb-geochronology fall mostly in the range of0.1–1.0 cm year�1; and exceptions outside this range are typically associated withlarge rivers, such as the Amazon, Ganges–Brahmaputra and Changjiang, wheresedimentation rates may be an order of magnitude higher (Wheatcroft and Drake,2003).

3.2. Sediment characterization

The core was almost homogeneously constituted by fine-grained sediments (siltsand clays) which accounted for �85% of the total sediment mass. However, a bandof coarser sediments, consisting of 30% sand, was observed at the section corre-

A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175166

sponding to 1.4 m depth. This boundary has been also distinguished in the profilesof other geochemical parameters analyzed such as OC, CO3, Mn and Li, as will bedescribed below. The sediments also included a significant amount of biogenicmaterial, mainly formed by benthonic and planktonic foraminifers, micromolluskand shell fragments. The OC content ranged from 1.3 to 28 mg g�1, showing twosections noticeably different (Fig. 3a): one at the upper part of the core that resem-bled hypothetical diagenetic profiles, with OC content declining exponentiallydowncore from surface to 1.4 m depth; and another one, below this depth, wheresediments are of low and almost constant OC content (SD, 2:36� 0:68) whichlikely represents the nonmetabolizable organic fraction of the sedimentary organicmatter (Berner, 1964).Typical exponential decay of OC concentrations, related to destruction of

organic components during diagenesis, can be described by the first-order G model(Berner, 1964; Zimmerman and Canuel, 2000):

Gi ¼ Goe�kt þ G1

where k is the decomposition constant at any time t, Go represents the degradableelement concentration present at the sediment surface and G1 is the asymptoticconcentration at depth and represents the refractory fraction. The first-order modelwas converted to the linear form y ¼ slopeðxÞ þ intercept obtained from therelationship ln(Gi/Go) versus depth x, where Gi represents the OC concentration atzith depth of the sedimentary column. The slope of the regression line obtainedequals the product of multiplying the first-order rate constant of decomposition (k)and the sedimentation rate (w) derived from the 210Pb geochronology (0.05cm year�1). The intercept obtained from the regression equation lnðG1=GoÞ ¼ð�0:0045�depthÞ � 0:1959 is equivalent to the logarithm of the nondegraded OCfraction, with G1 corresponding to the OC concentration at the sediment–waterinterface, from where the metabolized fraction can be estimated by the difference(1� G1=Go).

Fig. 3. Depth distribution of (a) organic carbon and (b) modeled organic carbon decay in core 10T.

167A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175

From the first-order equation obtained for the uppermost section of the core10T (Fig. 3b), it was found that the OC had: (a) an apparent decay rate constantof 2:3� 10�4 year�1; (b) a half-life of 3014 years; (c) a residence time of 4167 yearsand that (d) 18% of the initial concentration has degraded at the sediment–waterinterface. The OC first-order decay constant derived here is higher by one to sev-eral orders of magnitude than those previously reported for other sites in the open

ocean (2 4� 10�5 year�1, Heath et al., 1977; 0:8 1:4� 10�6 year�1, Waples and

Sloan, 1980; 1:7 21:1� 10�6 year�1, Muller and Mangini, 1980; 1� 10�6 1� 10�14,Emerson and Hedges, 1988) but smaller to those found by Paez-Osuna (1988) at

the Sea of Cortes (1:53 14:5� 10�3 year�1) where the OC half-life was estimatedto range between 10 and 330 years. However, this high variability in OC decay ratevalues is not really surprising since rate constants of OC remineralization may varyconsiderably depending on lithology, OC concentration levels and depositionalenvironment (Whelan and Farrington, 2003). According to Canfield (1994), thesediment deposition rate is the most important factor influencing organic materialpreservation and, in general, sedimentation rates higher than 0.04 cm year�1 wouldenhance preservation in both oxic and anoxic environments.The CaCO3 content varied widely, ranging from 1 to 39 mg g�1, showing sub-

surficial peaks and the smallest values at the lower section of the core (Fig. 4a).Aluminum, Li and Fe, which are commonly associated with clay minerals, showedsmall variations with depth (3:1� 0:5%, 12:3� 2:1 lg g�1 and 2:8� 0:9 mg g�1) inthe upper section of the core (above 1.2 m depth), suggesting a terrigenous inputwith a rather constant mineralogical sediment composition, although it was notice-able that none of these elements were significantly correlated. This lack of corre-lation could be explained on the basis of the mineralogical composition of thesedimentary lithogenic matrix since Al is a major constituent of fine-grained alumi-nosilicates (including immature physically weathered materials, such as feldspars)and Li, which is preferentially retained during the alteration of clay minerals (Huhet al., 1998), is mainly found in primary micas, ferromagnesium minerals and sec-ondary clay minerals, but not in feldspars (Aloupi and Angelidis, 2001). The sig-nificant inverse correlations (P < 0:05) found between Fe and both OC and CaCO3

have indicated that catchment material input has been slightly diluted with locallyderived biogenic material.Below 1.4 m depth, Li variability increased significantly and has shown a signifi-

cant correlation (P < 0:05) with Al and Fe, likely indicating that sediments foundat the bottom of core 10T not only have a higher variability in CaCO3 and mudcontent, but also a different mineralogical composition regarding the sedimentsfound above this depth. This package of sediments could be the result of a slumpdeposit, likely created by the frequent seismic events in the region.The Mn depth profile showed a rather chaotic distribution (484� 301 lg g�1)

with the lowest values below the layer of 1.4 m depth (Fig. 4b). Fe and Mn areknown to be diagenetically mobile in aquatic systems, depending on redox con-ditions, although more dramatic changes are usually observed for Mn behavior,due to the increased sensitivity of Mn to redox change (Cochran et al., 1998). The

A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175168

low concentrations of Mn and OC found at the bottom of the core could beexplained on the basis of a suboxic zone below 1.4 m depth, which could represent

Fig. 4. Profile distributions of (a) Al, Fe, Li andMn; (b) CaCO3 andmuds and (c) Cd, Cu and Pb, in core 10T.

169A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175

the redox boundary in core 10T. In natural aquatic systems with reducing sedi-ments overlain by oxygen-rich waters, high Mn concentrations are found in thesurface oxic sediments since Mn2+ diffuses upwards from the reduced sedimentsto be reoxidized and precipitated as Mn oxides. Some Mn2+ might also diffusedownwards, precipitating as Mn carbonate or mixed Mn–Ca carbonates in sedi-ments with highly alkaline pore waters due to sulfate reduction without requiringelevated dissolved Mn2+ and in sediments with sulfidic pore waters, Mn may alsocoprecipitate with FeS (Russell and Morford, 2001). Reduced conditions and thesignificantly low Mn content found below the redox boundary at 1.4 m depth canbe the result of the utilization of Mn oxides for the destruction of OC in the sub-oxic zone (Chester, 2000) and the low OC concentrations found below this depthmight represent the refractory organic matter that remained after degradation ofhydrolysable and oxygen sensitive organic matter, or that could not be oxidizedsince redox conditions were not propitious anymore.However, regardless of the changes in redox conditions that might have occurred

at this depth of core 10T, there still remains the possibility that these sediments areof a different nature, as a result of a submarine slumping. Phenomena like thishave been reported elsewhere for zones characterized by the occurrence of intensebottom currents and upwelling, and this mass movement was explained as a likelyresult of increased sedimentation and/or earthquakes (Goldstrand, 1998), whichcould also be the case in the Gulf of Tehuantepec, as previously described above.

3.3. Trace metal concentrations

The trace metal profiles Cd, Cu and Pb from core 10T are presented in Fig. 4c.The three profiles showed a high variability, but also an increasing trend withmaximum surficial peaks (as previously seen in detail in Fig. 2a). Cd depth distri-bution along the entire core 10T presented a more chaotic profile than the othertwo metals, which could be explained on the basis of: (a) diagenetic mobility, sinceCd tends to migrate from the sediments to form chloride complexes in the seawatercolumn (Cochran et al., 1998) or enrichment under mildly reducing conditions(Van Geen et al., 1995); (b) a low particle reactivity (in comparison to other metalssuch as Cu and Zn), as demonstrated by laboratory manipulations of slurries ofsediments and pure metal oxides (Leckie et al., 1980) and (c) variations in biologi-cal scavenging, since Cd, as for some other trace metals, tends to accumulate inmarine plankton, specially foraminifers. Phytoplankton may strip a significantamount of dissolved Cd from the water column during spring or upwelling blooms(Sanudo-Wilhelmy and Flegal, 1996) which eventually contribute to enrich thesediments.The background levels of Cd, Cu and Pb were determined as the mean concen-

tration obtained from those layers of the core where no excess of 210Pb was found(between 0.1 and 1.4 m depth) prior to the effects of atmospheric pollution. Thevalues obtained (mg kg�1) were 7.3 for Cd, 138.9 for Cu and 92.2 for Pb. Theseconcentrations are high in comparison with those reported for other zones at theMexican Western Coast, such as the Gulf of California (Paez-Osuna, 1988).

A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175170

According to Carranza-Edwards et al. (1998), the continental crust in the Gulf ofTehuantepec has been subjected to continuous uplift, exposing plutonic and meta-morphic outcrops, which weather to form terrestrial detritus that is carried to thesea, and that reflect the composition of the source rocks.The lithogenic contribution of the metals analyzed in the core 10T was evaluated

by an enrichment factor calculated according to the formula (Salomons andForstner, 1984):

EF ¼ ðMþs =AlsÞ

� �= ðMþ

c =Alc� �

where M+ and Al represent the trace metal and aluminum concentrations, respect-ively; and, the subscripts s and c refer to sample and Earth’s crust values, respect-ively. The crustal values considered were obtained from Martin and Meybeck(1979). EFs of unity or thereabouts indicate that the element is incorporated in thesample dominantly as lithogenous material; whereas, EFs much greater than unityindicate that the element is enriched in the sample relative to its concentration inthe Earth’s crust, e.g. because of organic and other authigenic material (Guo et al.,2000). The EF values obtained varied from 1 to 4 for Fe, <1 to 3 for Mn, �1 forLi, 6 to 36 for Cu, 6 to 38 for Pb and 21 to 322 for Cd, indicating an almost con-stant supply of Li, a slight increment of Mn and Fe and a significant enhancementof Cd, Cu and Pb concentrations over natural crustal values, that were more evi-dent from surface to 2 cm depth, corresponding to the last 20 years.When levels of trace metals in the sediments are enriched relative to the crust

(EF > 1) and do not correlate significantly with Al, as in the case for Cu, Cd andPb, it could indicate that these elements were significantly enriched relative to crus-tal material and that their concentrations were not directly controlled by continen-tal weathering, that is, they were not associated with continentally derivedaluminosilicate minerals. However, it is important to take into consideration thatAl cannot always be successfully used as a conservative element, especially whensediments are mainly composed of immature physically weathered materials, suchas feldspars of variable grain size.The historical changes of the metal enrichment along the sediment core were

quantitatively calculated by an AF as follows:

AF ¼ Cs=Cd

where Cs referred to the trace metal concentrations at surface and Cd to the valuedetermined as pre-anthropogenic concentration at depth in the sediment column.Higher than unity values of AF mean that there is an enrichment of a given elementin a top (recent) segment in respect to deep one of sediment core (background), i.e.deposited in pre-civilization area (Szefer, 2002). The AF levels found could be arran-ged in two groups: (a) one for Fe, Mn and Li, that varied from 0.4 to 1.9, 0.01 to1.6, and 0.6 to 1.4, respectively, indicating a slight enrichment over time and (b) asecond one for Cd, Cu and Pb that ranged from 0.3 to 3.6, 0.4 to 3.4 and 0.4 to 2.7,correspondingly, that showed a moderate enrichment. Based on the AF valueshigher than unity, it may suggest that an anthropogenic source was mainly respon-sible for the presence of these elements (Szefer, 2002) and even when Cd in sedimen-

171A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175

tary records is more difficult to interpret in terms of anthropogenic impact, it wasnoticeable that levels of enrichment for the three metals analyzed were comparable.The interpretation of the trace metal enrichment on the upper layers of a core

can be problematic since there are several factors that can interfere in theinterpretation of data. The most important deals with obtaining an undisturbedprofile since deformation of the sediments due to inadequate coring techniques andbioturbation and physical mixing are artifacts commonly found. Another problemdifficult to deal with is the role of diagenesis since it may lead to false trace metalprofiles that could be associated to anthropogenic pollution, but, in reality, mayactually result from a combination of depositional and post-depositional processes.For instance, the apparent enrichment of Cu, Cd and Pb at the sediment core sur-face could rather be a result of the cycling of Fe and Mn in the sediments underspecific redox conditions or remobilization after OC diagenesis. It is well knownthat organic matter, as well as Fe and Mn oxyhydroxides, either as bulk phases oras coatings of mineral particles, are the main adsorbents of metals in sediments(Benjamin and Leckie, 1980; Tessier et al., 1982; Farley et al., 1985; Dzombak andMorel, 1990; Turner and Millward, 2002); however, it has been also observed that,in anoxic and suboxic basins, whereas Fe and Mn are mobilized into the water col-umn, anoxic waters remarkably influence the trapping of Cd, Cu and Pb in thesediment phase since they can be fixed in the sediments as sulfides (Borg andJonsson, 1996; Szefer, 2002).Regarding the possibility that Pb, Cu and Cd have migrated through the sedi-

ment column and precipitated at the oxygen-rich surface layer, taking into accountthat Fe and Mn are the most redox-sensitive metals, in case of redox remobiliza-tion, both metals would have shown a clear pattern of enrichment at the surface ofthe core, even more evident than those exhibited by Cd, Cu and Pb (Fig. 5a).Besides, the AF estimated for Fe and Mn would have had at least about the samedegree of enrichment showed by Pb, Cu and Cd, which is not true. Additionally, ifFe and Mn had been remobilized through the sedimentary column and had trap-

5. Trace metal flux (Cd, Cu and Pb) geochronologies in core 1

Fig. 0T.

A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175172

ped the other trace metals (Cd, Cu and Pb) on the surface, Fe or Mn would have

exhibited a significant relationship with the concentration of Pb, Cu and Cd at

least in the upper part of the core, as previously observed by Sokolowski et al.

(2001) in Vistula river, which is not the case, since by using the more recent

sections of the core (ca. 140 years), we have found that Pb, Cu and Cd are not

related at all with Mn or Fe.Cd, Cu and Pb were significantly correlated (P < 0:05) likely as the result of a

common origin, i.e. land-based anthropogenic activities taking place in the coastal

zone. Previous studies conducted at the coastal zone of the Gulf of Tehuantepec

have hinted at a potential pollution by trace metals and hydrocarbons as a result

of sewage discharges, agriculture effluents and oil production activities (Tapia-

Garcıa et al., 1998). The oil refinery activities of the region started with the estab-

lishment of the petrochemical complex ‘‘Antonio Dovalı Jaime’’ in 1979, which

currently has an operation capacity of 330,000 barrels per day of the crude oil that

is transported to the refinery from oil wells through several kilometers of oleoducts

(Anonymous, 2003). It has been reported that the Superior lagoon has received

agriculture effluents for many years, containing the residues of several thousands of

tons of phosphate fertilizers that have been applied for the culture of sugarcane,

which also contain significant amounts of Cd associated with the phosphatic min-

eral (Benıtez, 1994); additionally, it seems that the lagoon receives the urban wastes

of the 70% of the total population of the basin, as well as considerable amounts of

hydrocarbons released from several and frequent accidental oil spills (Tapia-Garcıa

et al., 1998). The Ventosa bay, the Port of Salina Cruz and the Tehuantepec river

were also included among the most affected points of the region, showing high con-

centrations of Cr, Ni and V in water, above the maximum limits established by

Mexican legislation (Pica-Granados et al., 1994). Considering that the coastal

water bodies of Salina Cruz, Ventosa bay, Superior lagoon and Tehuantepec river

have considerable water exchange with the sea, and that the coastal current circu-

lation pattern will enhance the permanence of these water masses in the area, it

would not be that difficult to ascertain that the levels of metal enrichment found in

the sediments of the core 10T are likely related to the anthropogenic activities tak-

ing place at the coastal zone of the Gulf of Tehuantepec.Metal fluxes (Cd, Cu and Pb) excess nutrient inventories were calculated as fol-

lows (Cochran et al., 1998):

Fi ¼ Siqi Mþ½ �ixs

where Fi is the metal flux for the ith depth interval (lg cm�2 year�1), Si is the210Pb-derived accumulation rate for the ith interval (g cm�2 year�1), and ½Mþ�ixs isthe excess metal concentration (above background level) for the ith interval

(lg g�1). The 210Pb-derived geochronology allowed the estimation of fluxes of Cd,

Cu and Pb (lg cm�2 year�1) for the last 140 years, which varied as follows: from

0.1 to 2.5 for Cd, 2.9 to 45.8 for Cu and 1.7 to 22.5 for Pb (Fig. 5b).

173A.C. Ruiz-Fernandez et al. / J. Environ. Radioactivity 76 (2004) 161–175

4. Conclusions

The geochemical features observed in core 10T described a relatively steadyenvironment characterized by high concentration level of metals, which appearedto be mostly controlled by (a) the geological setting (represented by the detriticmatrix that reflects the composition of the source rock) and (b) a moderateanthropogenic impact. The 210Pb geochronology was useful to obtain an averagesedimentation rate of 0.05 cm year�1 and provided an estimate of the metal fluxesfor the last 140 years, showing that during the last 20 years, fluxes of Cd, Cu andPb have increased by 25, 15 and 13 times, respectively, in relation to the pre-anthropogenic fluxes. However, due to the typically slow oceanic sedimentaryrates, the resolution of the core did not allow the reconstruction of a detailed tracemetal accumulation history. It is therefore recommended to try other radioactivetracers, such as 14C or 230Th, in order to obtain a geochronology that helps tounderstand the geochemical and palaeoceanographic process in the area, over alonger timescale.

Acknowledgements

The authors are indebted to Dr. Donn Gorsline and Dr. T.L. Ku at the Universityof Southern California, for the 210Pb activity data. G. Ramırez-Resendiz providedassistance in the edition of figures and M.C. Ramırez-Jauregui and V. Montes inbibliographic research. The authors are grateful for critical and constructive com-ments on the original manuscript given by Paul Martin and two anonymousreviewers.

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