210Pb-derived ages for the reconstruction of terrestrial contaminant history into the Mexican Pacific coast: Potential and limitations

Marine Pollution Bulletin 59 (2009) 134–145

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210

Pb-derived ages for the reconstruction of terrestrial contaminant history into the Mexican Pacific coast: Potential and limitations A.C. Ruiz-Fernández a,*, C. Hillaire-Marcel b,1 a b

Universidad Nacional Autónoma de México, Instituto de Ciencias del Mar y Limnología, Calz. Joel Montes Camarena s/n, 82040 Mazatlán, Sin., Mexico Centre de Recherche en Géochimie et Géodynamique (GEOTOP), Université du Québec à Montréal, C.P. 8888 Centre-ville, Montréal, Quebec, Canada H3C3P8

a r t i c l e

i n f o

Keywords: 210 Pb 137 Cs Marine coastal sediments Pacific coast of Mexico Mudflats Land-use changes

a b s t r a c t 210 Pb is widely used for dating recent sediments in the aquatic environment; however, our experiences working in shallow coastal environments in the Pacific coast of Mexico have demonstrated that the potential of 210Pb for reliable historical reconstructions might be limited by the low 210Pb atmospheric fallout, sediment mixing, abundance of coarse sediments and the lack of 137Cs signal for 210Pb corroboration. This work discusses the difficulties in obtaining adequate sedimentary records for geochronological reconstruction in such active and complex settings, including examples of 210Pb geochronologies based on sediment profiles collected in two contrasting areas coastal areas (mudflats associated to coastal lagoons of Sinaloa State and the continental shelf of the Gulf of Tehuantepec), in which geochemical data was used to support the temporal frame established and the changes in sediment supply recorded in the sediment cores which were related to the development of land-based activities during the last century. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Heavy metals and nutrients often have a strong affinity for particle surfaces of fine sediments; therefore, scavenging by suspended particulate matter and subsequent sedimentation creates a repository of valuable historical information on the temporal trend of pollutants input into aquatic ecosystems (Goldberg et al., 1977). If there is a valid accumulation rate, those records can provide specific data about rates of sediment accumulation and the anthropogenic fluxes of pollutants (Axelsson and El-Daoushy, 1989). In the absence of long-term monitoring data, the sedimentary record can be used to provide retrospective information on the past characteristics of the aquatic environment. The use of marine sediments to provide information on changing sediment fluxes is well established (e.g. Emeis et al., 2000; Smith, 2001) and the most widely used method for dating recent sediments in marine or lacustrine environments is based on the examination of 210Pb profiles (Koide et al., 1973). The 210Pb is a natural radionuclide ðT 1=2 ¼ 22:26 yrÞ by product of 226Ra decay; it is supplied to the aquatic environment by atmospheric deposition, although in can be also result from the decay of dissolved 226Ra in the open ocean (where production of 210Pb from 226Ra can exceed the atmospheric

* Corresponding author. Tel.: +52 669 9852845; fax: +52 669 9826133. E-mail addresses: [email protected] (A.C. Ruiz-Fernández), [email protected] (C. Hillaire-Marcel). 1 Fax: +1 514 9873635. 0025-326X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpolbul.2009.05.006

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Pb contribution by factors of 2–20; Cochran, 1992). 210Pb has shown to be an ideal tracer for dating aquatic sediments deposited during the last 100 years, a period of time during which appreciable environmental changes occurred due to industrialization. The 210Pb chronology is often corroborated by 137Cs measurements. 137Cs is an artificial radionuclide ðT 1=2 ¼ 30:14 yrÞ present in the environment mainly due to global scale fallout from the atmospheric testing of thermonuclear weapons, which began shortly after the initial test in the early 1950s, peaked in 1963 and then rapidly declined following implementation of the international treaty banning such tests (DeLaune et al., 1978). Both radioactive tracers are useful in that they provide two independent means of measuring the same burial processes occurring within a given sediment core. 137Cs is an impulse-marker and the sedimentation rate is calculated from the maximum activity depth (1963); whereas the 210Pb-derived sedimentation rates are obtained from the profiles generated by radioactive decay of the 210Pb buried in the sediments, which is supplied from atmospheric fallout and the water column to the surface of the sedimentary column at a supposedly constant rate (Lynch et al., 1989). 210 Pb and 137Cs chronologies provide the temporal frame to interpret the records of sedimentation changes promoted by anthropogenic impact or climatic variations. When combined with historical data, these records are of great interest because they offer a possibility for retrospective studies about environmental changes, beyond the time-scale of any existing monitoring program. However, finding unaltered sedimentary records in the coastal marine environment is a difficult task. Waves, tide and

A.C. Ruiz-Fernández, C. Hillaire-Marcel / Marine Pollution Bulletin 59 (2009) 134–145

wind currents, benthonic fauna and anthropogenic activities (i.e. dredging, fishing, aquaculture) in the coastal zone are some of the main factors that contribute to obliterate the sedimentary records in the coastal lagoons, producing anomalous 210Pb profiles that are useless for historical reconstruction. On the other hand, process such as: (a) reduction of sediment load due to rivers impoundment, (b) increased accretion due to erosion on the continent promoted by land use changes or (c) transport of old sediments (depleted in 210Pb) produced by tillage in agriculture fields, are also factors than produce non-monotonous 210Pb profiles that are difficult to interpret. Another problem reportedly observed in sediment cores collected in the coastal areas of the Mexican Pacific is the absence of 137Cs signal (Páez-Osuna and Mandelli, 1985; Ruiz-Fernández et al., 2001, 2002, 2004, 2007) which has been explained on the basis of: (a) a low 137Cs atmospheric flux in the region due to atmospheric circulation patterns and the dry weather in the Northern Mexican Pacific coast, since 137Cs fluxes are directly correlated with the amount of precipitation (Mitchell et al., 1990; Aoyama and Hirose, 2003; Sanchez-Cabeza et al., 2007); and (b) the greater solubility of 137Cs in seawater (Stanners and Aston, 1981). The low 137 Cs inventories associated with areas of reduced receipt of fallout introduce measurement problems in terms of both detection limits and the long count times required to obtain result with an acceptable degree of precision. Walling and He (1999) has recognized that, because of the limitations in the use of 137Cs in such areas of the world, there is a need to explore the use of alternative tracers.

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In this work we present a summary of the most frequent problems we found in our previous experiences trying to establish 210 Pb-derived geocronologies of the environmental changes recorded in the sediment cores collected in reportedly impacted coastal areas at the Pacific coast of Mexico. We present examples from two contrasting coastal environments: (1) very shallow coastal lagoons (mean water depth ranging from 0.5 to 3 m) with muddy sediments, that receive pollutants from diverse point and non-point sources; and (2) at the open ocean, on the continental shelf of the Gulf of Tehuantepec, where sediments are predominantly coarse and point sources of pollutants are not evident. Besides the irregular shape of the 210Pb profiles, the absence of a secondary radioactive tracer to corroborate 210Pb-derived dates made interpretation even more complicated; however, with the support of geochemical information (i.e. trace metals and nutrients concentrations, and C and N isotopic composition of the organic matter) these sedimentary records become an appropriate playground to evidence the impact that agriculture and urbanization have had in these areas during the 20th century. 2. Materials and methods 2.1. Study area Sediment cores collected to reconstruct the chronology by using Pb were collected in the coastal lagoons of Chiricahueto, Ohuira and Estero de Urías, in Sinaloa State, and in the continental shelf of Tehuantepec Gulf (Fig. 1). 210

Fig. 1. Location of sampling areas in Sinaloa and Oaxaca States, on the Pacific coast of Mexico.

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2.1.1. Coastal zone of Sinaloa State The State of Sinaloa is located in the Gulf of California, Northwestern Mexico (22°31’ to 26°56’N and 105°24’ to 109°27’W) and has 656 km of coastline and 221,600 ha of coastal lagoons. The weather in the coastal zone varies from very dry, very warm at the Northern region (mean annual temperatures ranges from 22 to 26 °C, and total annual precipitation from less than 400 to 600 mm) to warm sub-humid with summer rains at the South (from 22 to 28 °C, with a total annual precipitation varying from 700 to 1000 mm) with a mean annual evapotranspiration of 2100 mm (INEGI, 2008). Sinaloa economy is mostly based on agroindustry, fisheries, aquaculture and tourism. It is the national leader in food production (it is the main producer and exporter of shrimp, tuna, corn and vegetables) but to consolidate such an important economical background have had important environmental costs, since it has been recognized that the coastal lagoons neighboring the most developed economic areas of Sinaloa state are showing signs of infilling and eutrophication that have been related with the dumping of untreated effluents from agriculture croplands, aquaculture facilities and human settlements (PáezOsuna et al., 2008). The coastal lagoons of Sinaloa included in this study are shallow and brackish coastal environments which support local fishing activities, including wild and cultivated shrimp for export; that have been included among the aquatic systems experiencing desiccation of lowlands and infilling, as well as water quality problems (Arriaga-Cabrera et al., 1998). Ohuira lagoon (125 km2 and average depth of 3 m) and Chiricahueto lagoon (94 km2, <0.5 m depth) are surrounded by the two most important agriculture valleys of Sinaloa (El Fuerte and Culiacan, respectively) which are characterized by intensive and highly technified croplands, where monocropping, severe tillage, irrigation and, heavy application of fertilizers and pesticides, are common practices. The cultivated area of El Fuerte and Culiacan Valleys are 281,100 and 281,400 ha, respectively (250,000 and 172,381 ha of irrigated fields correspondingly; OEIDRUS, 2008). The most important crops in both valleys are corn, sorghum, sugar cane and beans. On the other hand, the Estero de Urías lagoon (18 km2, 0.5–2 m depth) is surrounded by the shipping terminal and the industrial zone (food processing and thermoelectric plants) of Mazatlan City; and ca. 3 km of the proximal section to the sea is constantly dredged to preserve an artificial navigation channel of 10 m depth. It also receives runoff of agriculture wastes but in a minor extent than the other two coastal lagoons, since the cultivated area is considerably smaller (41,700 ha in total, with 5500 ha of irrigated fields) with the main crops being sorghum with 34,000 ha and corn with 2300 ha (OEIDRUS, 2008). 2.1.2. Tehuantepec Gulf It is located in the Mexican Tropical Pacific, approximately from 14°300 to 16°120 N and from 92°000 to 96°000 W. It extends over 500 km on the coastal zone of Oaxaca and Chiapas States and has a mouth 160 km wide. The weather is tropical warm sub-humid with summer rains (mean annual temperature oscillating between

22 to 28 °C; and mean annual total precipitation between 800 to 2000 mm). The economical development in the region has been mostly based on the oil industry activities, and has included major transformations such as the construction of the industrial harbor of Salina Cruz and the Salina Cruz oil refinery, which produces the 24% of the refined oil products of Mexico (SENER, 2008). 2.2. Sampling The sediment cores from the coastal lagoons were collected manually with a plastic tube (7 cm i.d.) in tidal saltmarsh areas covered by saltwort plant patches (Salicornia pacifica and Batis maritima), whereas the sediment core from the coastal zone of the Gulf of Tehuantepec was collected using a Reineck-type corer and subsampled using a plastic tube (10 cm i.d.) on board of O/V El Puma during cruise Tehua-II. The sampling information is presented in Table 1. No signs of sediment shortening or deformation were observed in none of the cores. The coastal lagoon cores were extruded and sectioned at 1 cm intervals, whereas TehuaII-21 core was subsampled at 0.3 cm intervals down to 10 cm and at 1 cm intervals for the rest of the core. The observations derived from these studies might be limited by the representativeness of a single core to reconstruct the environmental history at each site; however, the cores provided good examples of retrospective studies of the land-sea interactions in such coastal areas where environmental data is relatively scarce. Sediment samples were freeze-dried, ground to a powder with a porcelain mortar and pestle and stored in polyethylene bags, except for the aliquots for grain size analysis. Water content data were used to calculate the porosity (Berner, 1971) and in situ dry sediment bulk density to estimate the mass depth (g cm2), which would be used instead of depth (cm) in dating calculations to compensate sediment compaction due to burial (Robbins, 1978). 2.3. Laboratory analysis Total 210Pb (210Pbtot) activities in the cores were estimated by measuring the activity of its daughter product 210Po (Schell and Nevissi, 1983) assuming secular equilibrium between the two isotopes. About 0.25 g of sediment samples were spiked with 209Po as a recovery tracer and were acid digested (1:5:4 HF + HNO3 + HCl) in closed PFA SavillexTM containers on a hotplate (120 °C, overnight). The digestate was evaporated to dryness and the digestion residue was converted to a chloride salt by repeated evaporation with 12 M HCl, and then dissolved in 0.5 M HCl in presence of ascorbic acid as a reducing agent for Fe3+. Po isotopes were spontaneously deposited on a spinning Ag disc (Hamilton and Smith, 1986) and the activity was measured by a-spectrometry using ORTEC silicon surface barrier detectors coupled to a PC running under MaestroTM data acquisition software. 137 Cs activities, as well as the 210Pb fraction supported by 226Ra (210Pbsup), were analyzed by c-ray spectrometry using a Canberra HPGe well-detector at the Centre GEOTOP, University of Quebec in Montreal. Weighed sediment samples were stored into glass vial

Table 1 Sampling data for the cores collected in the mudflats of coastal lagoons in Sinaloa and the coastal zone of Tehuantepec Gulf. Sampling site

Location

Sampling date

Core lenght (cm)

Sampling depth

Chiricahueto lagoon

24° 35’N 107° 30’W 23° 09’N 106° 20’W 25° 41’N 108° 53’W 15° 60’N 94° 48’W

September, 2001

40

No water column

March, 2002

35

No water column

June, 2004

30

No water column

October, 2004

18

67 m

Estero de Urías lagoon Ohuira lagoon Gulf of Tehuantepec

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the coastal zone of Sinaloa have demonstrated how complex could be the sedimentary dynamics in a coastal lagoon. Fig. 2 shows the 210 Pbxs depth profiles obtained from a previous study developed in the coastal lagoon system of Altata-Ensenada del Pabellón (RuizFernández et al., 2001) were the 210Pb profile trends were very difficult to interpret and useless to provide the intended chronological evidence of environmental impact. Such disrupted 210Pb profiles were explained as a result of the combination of low 210 Pb fallout, the accumulation of 210Pbxs-depleted sediment transported from agriculture fields, bioturbation, physical mixing and scouring, due to waves and currents produced by winds and tides in such shallow environments. After this previous experience, it was evident that in order to improve the probability of recovering undisturbed sediments in the coastal lagoons of Sinaloa, the coring sites should be selected in areas more protected from tide and wind currents. Taking into consideration the experience provided by McCaffrey and Thompson (1980) in salt marshes, the sediment cores were collected in the mud flat areas covered by halophyte vegetation (Salicornia pacifica and Batis maritima) that generally surround the coastal lagoons in Sinaloa State, assuming that this vegetation protects sediments from the erosive effect of tides and waves, ensuring a better preservation of the sedimentary record. Mudflats are formed in the areas where tidal processes dominate and there is abundant supply of fine-grained sediment; this type of mud deposit is characterized by small thickness, low accumulation rate, and high degree of continuity in sedimentation; and therefore, they can be very useful to study environmental evolution (Gao, 2005). 210 Pbtot activities in the core collected in the three coastal lagoons were considerably low (maximum value obtained was 33 Bq kg1) but consistent with previous work developed in the area (Ensenada del Pabellon lagoon system, 33–50 Bq kg1, RuizFernández et al. 2001; Culiacan River Estuary, 18–38 Bq kg1; Ruiz-Fernández et al., 2002). Low 210Pb activities might result from different factors, including high contents of coarse sediments that might have diluted 210Pb specific activities; however, sediments collected for this study were mostly composed by silty-clays (<90%; Fig. 3a–c). Other factors could be regional characteristics such as low atmospheric 210Pb fluxes and/or low production of 210 Pb from decay of parent 226Ra dissolved in shallow waters; or more local features such as physical transport processes (i.e. sediment focusing) and biological activity (dilution by organic matter fluxes or mixing by bioturbation). 210Pb atmospheric flux may vary from place to place depending on factors such as rainfall regimen (Sanchez-Cabeza et al., 2007). or the geographical location (Appleby, 1998). This variability is strongly related to the amount and seasonality of precipitation, since wet deposition of 210Pbxs

(8  40 mm, 1 mL capacity) and hermetically sealed for at least 21 days prior to analysis. 226Ra specific activities on Tehua II-21 core (and a few sections of the coastal lagoon cores) were based on the measurement of its decay chain descendant 214Pb, assuming secular equilibrium between both radionuclides. Replicate analyses (n = 12) of the standard reference material IAEA-300 (Radionuclides in Baltic Sea sediment) confirmed the reliability of activities determined for 210Pb and 137Cs. Accuracy and precision of 210Po analysis were 99% and 4.6%, respectively. The excess 210 Pb (210Pbxs) activities were determined by subtracting the 210 Pbsup fraction from 210Pbtot for each depth interval. Grain size was determined by standard methods of sieving and pipetting analysis (Folk, 1974). For Ohuira core, organic carbon (OC) and total nitrogen (N) contents were measured using a FISONS NA2000 Element Analyzer after removal of the carbonate fraction in silver capsules using 1.5 M HCl. The average standard deviation of each measurement was determined by replicate analyses of the IAEA standard NBS19 (0.07% for OC and 0.009% for N). Precision of both methods was estimated to be 4% (±1r here and henceforth for other methods). Stable isotopic analyses of organic matter (13C/12C and 15N/14N) were carried out on the same samples using a Finnigan Delta Plus mass spectrometer, which was directly coupled to a Fisons NA2000 EA by means of a CONFLO interface for continuous flow measurements. Stable nitrogen and carbon isotope ratios are reported in the conventional d-notation with respect to atmospheric N2 (air) and PDB (Pee Dee Belemnite) carbonate standard, respectively. Uncertainties were lower than ±0.2‰ as determined from routine replicate measurements of the reference sample standard IAEA-NBS 19 for the d13C and standard 1AEA-N-1 for the d15N. Metal analyses for cores collected in Chiricahueto and Estero de Urías lagoons, as well as in Tehuantepec Gulf, were performed by atomic absorption spectrophotometry (Al, Cu and Fe by flame, and Hg by cold vapour). About 0.5 g of dry sediment samples were total digested (Loring and Rantala, 1992) in closed SavillexTM containers on a hot plate (120 °C overnight). Accuracy and analytical precision of metal analyses were evaluated through six replicates of the certified reference material IAEA-356. Metal recovery varied between 90% and 110% and analytical precision from 4% to 10%. 3. Results and discussion 3.1. Coastal lagoons of Sinaloa 3.1.1. 210Pb activities Our first attempts to obtain 210Pb-derived historical records of environmental impacts generated by the agriculture industry in

a

b

210

Pb (Bq kg-1)

0

10

20

30

Pb (Bq kg-1)

0

40

0

c

210

10

20

30

40

0 0

10

10

50

30

60 70 80

40

20

30

40

30 40 50 60

50

90 100

10

20 20

Depth (cm)

40

Depth (cm)

Depth (cm)

30

Pb (Bq kg-1)

0

10 20

210

60

70 80

Fig. 2. Total 210Pb profiles for coastal sediment cores collected at the lagoon system Altata-Ensenada del Pabellon, in Sinaloa State: (a) Estero El Brinco, (b) Chiricahueto wetlands, (c) El Castillo fisheries ground. Uncertainties are <4%. Dashed lines represent the average value of supported 210Pb (20–30 Bq kg1). Modified from Ruiz-Fernández et al. (2001).

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a

0

20

Grain size (%) 40 60

80

100

b

80

100

c

0

0

5

5

4

20 25

Depth (cm)

15

15 20 25

30

28 32

10

4.0 3.5

Pbxs

y = -0.42x + 3.23 r = 0.99

1.0

0

2

4

6

8

10

12

0

3.5

-1

2.5

Accum rate= 0.12 g cm -2 yr-1 Sediment rate= 0.33 cm yr -1 y = -0.27x + 3.46 r = 0.98

-2 -3 -4

2.0 1.5

0.0 -0.5 -1.0

2

8

10

-2

-5

0.5

0

Mass depth (g cm )

f

4.0

3.0 210

1.5

2.0

Accum rate= 0.08 g cm -2 yr-1 Sedim rate= 0.10 cm yr -1

ln

Pbxs

2.5

ln

3.0

Mass depth (g cm-2)

e

Pbxs

8

210

6

ln

4

100

24

35 -2 Mass depth (g cm )

80

20

40

2

Grain size (%) 40 60

16

35

0

20

12

30

d

0

8

10 Depth (cm)

Depth (cm)

Grain size (%) 40 60

20

0

10

210

0

-6

1.0

-7

0.5

-8

y = -0.25x - 0.37 r = 0.90 Accum rate= 0.13 g cm -2 yr-1 Sedim rate= 0.32 cm yr -1

4

6

y = -1.04x + 2.33 r = 0.97 Accum rate= 0.03 g cm -2 yr-1 Sedim rate= 0.06 cm yr-1

210

Fig. 3. Grain size distribution and ln Pbxs profiles in the sediment cores collected at the lagoons of Chiricahueto (3a, 3d), Estero de Urías (3b, 3e) and Ohuira (3c, 3f). Symbols in the grain size distribution profiles represents: clays (4), silts (h) and sands (d). 210Pbxs uncertainties errors are <5%.

(precipitation and scavenging) is more efficient than dry deposition, excepting at desert areas (Preiss et al., 1996). On the other hand, the availability of 222Rn depends on the continent surface or land area covered by oceans or ice and the origin of the dominant air mass, which will be enriched or depleted in 210Pb depending on its continental or oceanic origin. Due to the origin of the air masses, there is a consistent west to east increase in 210Pb fallout within the major continents, superimposed on a baseline 210Pb flux of 30–40 Bq m2 yr1 (Appleby, 1998). The 210Pb fluxes modeled by Turekian et al. (1977) for the mid-Northern Hemisphere (15– 55°N), in a longitudinal scale, predicted the minimum worldwide values for North America. Additionally, since the western part of the continent is mostly influenced by the westerlies (which has an oceanic origin and hence, depleted in 210Pb) the lowest 210Pb fluxes will be found in the western areas (Preiss et al., 1996). Since Sinaloa State is located within a semi-arid region, at the Western coast of North America and under the influence of westerlies during most of the year, the low levels of 210Pbtot found in the cores are most probably a consequence of the low flux of atmospheric 210Pb in the area. 210 Pbsup values obtained from the gamma measurements were consistent among the three areas and no big variability was observed along the cores (5 ± 2 Bq kg1); however, although this value is comparable with the 210Pbsup values estimated from the mean 210Pbtot activity (Bq kg1) in the asymptotic section of the profile for each coastal lagoon core (7.98 ± 0.52 for Chiricahueto, 8.30 ± 0.48 for Ohuira and 2.68 ± 0.06 for Estero de Urías) these latter were preferred for a better compliance with the method of analysis, since the analytical uncertainties from the gamma measurements were at least twofold higher than those obtained from the alpha determinations. The 210Pbxs profiles from Ohuira to Estero de Urías lagoons showed subsurficial peak values (Fig. 3d–f) and in order to eliminate variations promoted by changes in the composition of the

sediments, the 210Pbxs specific concentrations were normalized to content of clays, silts, calcium carbonate and organic matter (data not shown here; Ruiz-Fernández et al., 2009a), but the 210Pb profiles in almost all the sediment cores showed increased values, but exactly the same depth-distribution. Only the normalized 210 Pb profile from Estero de Urías core showed an important change in depth distribution in comparison with the original 210 Pb profile; however, the 210Pb-derived sedimentation rates, using normalized and raw 210Pb data were equivalent. The significant correlation (r > 0.90, P < 0.05) between the 210Pbxs activities and depth (in g cm2) indicated that the Constant Flux Constant Sedimentation (CFCS) model (Robbins, 1978) was appropriated to obtain a mean accumulation (g cm2 yr1) and sedimentation rates (cm yr1) for Chiricahueto core (0.08 and 0.10, respectively, 100 years between surface and 11 cm depth) and for Estero de Urías cores (0.12 and 0.33 correspondingly, 100 years between surface and 35 cm depth). In the case of Ohuira core, as 210Pbxs depth profile showed two sections with different slopes (from surface to 6 cm depth, and from 6 to 16 cm depth) two accumulation and sedimentation rates were obtained: 0.03 g cm2 yr1 and 0.06 cm yr1 for the older sections of the core, and 0.13 g cm2 yr1 and 0.32 cm yr1 for the most recent layers, for a total record of 106 ± 11 years in 12 cm. These sedimentation rates (cm yr1) are comparable with the mean sedimentation rate found at other mudflats in the world (Table 2) characterized by microtidal regimes. 3.1.2. Geochemical data to corroborate 210Pb geochronologies 137 Cs activities in all coastal lagoon cores were below the minimum detectable activity. In the absence of 137Cs data to corroborate the 210Pb-derived geochronologies, we propose the use of geochemical data, such as nutrients (C, N) and trace metals (Cu, Hg) concentrations, as well as d13C and d15N profiles, to evidence environmental modifications resulting from land-use changes

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A.C. Ruiz-Fernández, C. Hillaire-Marcel / Marine Pollution Bulletin 59 (2009) 134–145 Table 2 Sedimentation rates at different mudflat areas in the world. Mudflat area Grådyb, Wadden Sea, Denmark Juvre Dyb, Wadden Sea, Denmark Knudedyb, Wadden Sea, Denmark Kongsmark, Lister Dyb, Denmark Ho Bugt Bay, Denmark Venice Lagoon, Italy Chiricahueto Lagoon, Mexico Ohuira Lagoon, Mexico Estero de Urias Lagoon, Mexico San Francisco Bay, USA Wanggang Estuary, China Authie, Pas-de-Calais, France Seine, Normandy, France Bay of Fundy, Canada Horrid Hill, Medway estuary, UK Vasiere Nord, Seine, France Montportail-Brouage, Marennes-Oléron Bay, France Humber estuary (Skeffling), UK

Sedimentation rate (cm yr1) Microtidal regime 0.26 0.7 0.4 0.5–0.8 0.4–1.2 0.05–0.25 0.08 0.06–0.32 0.12 Mesotidal regime 2.2 3.0 Macrotidal regime 18 15 >1.0 0.35–0.85 2.3–3.0 0.26–0.95 0.6–0.8

and the industrial activities in the region, for which the historical data is well establish. 3.1.2.1. Enrichment factors of Cu and Hg in Chiricahueto. In order to identify trace metal enrichments during the period encompassed by 210Pb-derived geochronologies, an enrichment factor (EF) was calculated using Al as a normalization tracer, according to the following formula (Glasby and Szefer, 1997):

EF ¼

½AlX sample X ½Al preind

where [X/Al]sample is the ratio of concentration of the element X to concentration of Al in the sample at a given depth, and [X/Al]pre-ind is the background level established for each metal in the cores. The EF calculations assumed that the chemistry which determines dissolution and sorption of metals is the same through the sedimentary column, and the enrichment observed was not referred to natural concentration levels, but to values representative of the beginning of 20th century. In general, EF of unity or thereabouts indicate that the element is incorporated in the sediment dominantly as lithogenous material, whereas EF much greater than unity indicate anthropogenic pollution (Glasby and Szefer, 1997). Release of metals to the environment by anthropogenic activities has produced historically increased fluxes of the contaminants to the coastal zone, that are implied in the Cu and Hg enrichment factor profiles of Chiricahueto core (Fig. 4a and b). They showed a fluctuating but generally increasing tendency since early 1930s with peak values from early 1950s. Both profiles exhibited a rather non-steady trend, perhaps due to metal contributions from diffuse and point sources, or may be related to the year-to-year variations on the pluvial patterns of the region and the occasional occurrence of heavy rains during the storm season. The similarities in both enrichment factor profiles, as well as the significant correlation between Cu and Hg concentrations (P < 0.05) suggested that these pollutants are derived from a single source, most likely agriculture wastes from the Culiacan Valley. Hg was traditionally used in agricultural chemicals as a fungicide, mildewcide or pesticide; and is contained in minor amounts in phosphate-rock fertilizers, lime and animal manure applied to agricultural lands (Dreher and Follmer, 2004); whereas Cu has been historically used as fungicides (as copper oxychloride) and herbicides (copper sulfate) and seed disinfectants (copper chloride) as well as purposely added to regular blend fertilizers to meet the demand of plant growth for these ele-

Reference Pedersen and Bartholdy (2006) Pedersen and Bartholdy (2006) Pedersen and Bartholdy (2006) Andersen et al. (2000) Madsen et al. (2005) McClennen and Housley (2006) This study This study This study Watson (2004) Wang et al. (2005) Deloffre et al. (2007) Deloffre et al. (2007) Kostaschuk et al. (2008) Cundy et al. (2007) Cundy et al. (2007) Gouleau et al. (2000) Andersen et al. (2000)

ments (He et al., 2005). The initial expansion of Culiacan Valley agriculture activities started with the establishment of the irrigation districts and the construction of the main irrigation channels in central Sinaloa in the decade of 1930, (Morales-Zepeda, 2008), which coincides with the first leap in the EF values for both metals in Chiricahueto core; and with the beginning of operation of Sanalona dam in 1948 (which was set up to support the irrigation of the Culiacan Valley) this agricultural area increased threefold (from 35,000 to 94,000 ha) in only this single year (Ruiz-Fernández et al., 2002). The peak values for Cu and Hg EF in 1950 are likely the result of exacerbated trace metals runoff after clear cutting of vegetation for agriculture field creation. 3.1.2.2. Hg enrichment factor profile in Estero de Urías lagoon. The Estero de Urías sediment core evidenced significant Hg enrichment since late 1960s; however, from late 1980s to the most recent sediment layer of the core, EF values steadily increased to reach values as high as 70 (Fig. 4c). The sampling site where Estero de Urías core was collected is rather isolated into a dense mangrove forest where no direct discharges of pollutants were observed; therefore, the most probable source of this Hg pollution is atmospheric deposition. The electricity sector is considered the largest uncontrolled industrial source of Hg emissions in Canada and contributes with a large share of Canadian nitrogen oxides and sulfur dioxide emissions (EC, 2009). In Mexico, thermoelectric plants are recognized as the 11th most important Hg atmospheric source, with emissions accounting for 0.13 Mg yr1. This is related to the quality of the heavy fuel oil burned in most Mexican thermal power plants, which is produced from Maya crude oil, known to be high in sulfur and trace metals (Acosta-Ruiz and Powers, 2003). The thermoelectric power generation plant of Mazatlan (José Aceves Pozos) was built in 1966 with an original capacity of 39,000 KW; but since then, two additional units have been installed, one in 1976 (to reach the capacity of 316,000 KW; Alvarez-León, 1980) and the last one in 1981, to attain the current total operation capacity of 616,000 MW (SENER, 2006), to produce about 3476 GWh yr1. There are not previous studies about Hg emissions from this power generation plant so far; however, it has been already identified as a highly pollutant facility, which releases high concentrations of SO2 (181 lg m3) accounting for a highly contaminant emissions of about 65,300 ton yr1 (SEMARNAT/CEPAL, 2004). The Hg EF chronology profile shows that Hg concentrations recorded in the sediment core from Estero de Urías lagoon started to

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Fig. 4. Trace metals enrichment factor profiles in the coastal lagoons sedimentary records of Sinaloa: (a) Cu and (b) Hg enrichment factors from Chiricahueto; and (c) Hg in Estero de Urías.

increase since late 1960s, very probably as a result of the Hg fallout promoted by the emissions of the Mazatlan thermoelectric plant that started operations in 1966; however, this Hg enrichment is more conspicuous from late 1980s, most likely due to the significant increase of the production capacity of the power facility. A time-lag of about 7 ± 1 years was observed between the 210Pb-derived Hg enrichment chronology and the historical data of the power generation plant during the 1980s. One possible explanation explored was the possibility of Hg enrichment on the sediment surface due to redox diagenesis (Hg sorption to diagenetic iron and manganese oxyhydroxides near sediment core surface); however, diagenesis was ruled out since other redox-sensitive elements that were measured in the same sediment core (such as Cd, Cu or Pb; Ruiz-Fernández et al., 2009a) showed exactly the reverse trend toward the surface sediments and a negligible enrichment all along the core (1 < EF < 2). Discontinuities between historical data and the sediment record have been previously observed in other studies elsewhere (i.e. Plater and Appleby, 2004). Besides down-core variations in sediment composition (grain size, organic content) and potential influences from diagenesis, another possible reason considered is the balance between deposition and erosion controlled by natural or anthropogenic changes in the morphology of the intertidal zone. In the case of Estero de Urías lagoon (as for many other water bodies associated to coastal cities), as the proximal area to the sea of Estero de Urías lagoon serves as the main entrance to Mazatlan Harbor (for the transit of small fishing boats up to tourist

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3.1.2.3. d13C and d15N in Ohuira lagoon. Organic carbon and nitrogen stable isotope ratios are natural source indicators of sedimentary particulate organic matter. They can be used to identify organic matter provenance in estuarine and near-shore marine environments (Thornton and MacManus, 1994) allowing to discriminate the origin of pollutants associated to the sedimentary organic material. Sedimentary organic matter found in Ohuira core fits in the reported ranges of d15N (1.2 to +10.6‰; Cloern et al., 2002) for coastal and estuarine–marine organic matter, although d13C values were higher than expected for estuarine-marine organic matter (17 to 28‰; Cloern et al., 2002). The d13C background levels in Ohuira core were observed in the sediments older than

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cruises), a variety of engineering interventions have been done to modify coastal features within the lagoon in order to optimize port services. Other important activities coexisting in this lagoon are artisanal fishing and shrimp farming; extensive clearance of mangrove forest has taken place in the surroundings of the distal area of the lagoon in order to build shrimp farm ponds, and a weir was constructed in the middle of the lagoon (1987), by an inshore fishing cooperative, with the purpose to increase fish catching. Although there are not studies available to prove it, all these modifications must have created important hydrodynamic changes in Estero de Urías lagoon, including increments in inundation height and residence time of the water inside the lagoon, increasing the chances of pollutants scavenging that are better recorded in our sediment core from the end of the decade of 1980.

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Fig. 5. Geochronological reconstruction of C and N isotopic composition from Ohuira lagoon core and its relationship with agriculture development in the surrounding environment: (a) d13C, (b) d15N and (c) cultivated area growth with time.

A.C. Ruiz-Fernández, C. Hillaire-Marcel / Marine Pollution Bulletin 59 (2009) 134–145

100 years (Fig. 5a), but above this region of the profile, contrasting trends can be observed with values decreasing 3‰ between 7.5 and 11.5 cm depth (1900–1959) and afterwards increasing again 4‰ towards the most recent layers; whereas the d15N profile showed a steady increasing trend from the bottom to the surface of the core (Fig. 5b). The increasing d13C values observed in the most surficial layers of Ohuira core could be the result from the input of a recent 13C-enriched organic matter source. d13C values as high as 11‰ have been previously observed in coastal tidal flats sediments, and have been related to the input of relatively coarse detritus from C4 grasses (Mook and Tan, 1991). According to Van der Merwe and Vogel (1978), even a discrete terrestrial organic matter input, introducing a C4 plant component from cultivated fields (as corn, sorghum or sugar cane, which are cultivated in El Fuerte valley) could shift the d13C values as high as 12.5‰. Post-1930 increments in d13C values are most likely the result of the gradual increment in corn, sorghum and sugarcane production wastes as cropland extensions at El Fuerte Valley (Fig. 5c). Only between 1994 and 2004, sorghum and sugar cane crops passed from 9160 to 55,807 and from 448,560 to 1,006,034 tons per year, respectively (INEGI, 1994, 2005). According to Meyers (1997), the observed d15N trend could be the result of the reduction of freshwater supply after the partial diversion of a river. According to the plot of d15N and d13C (Fig. 6), the Ohuira sediment core is showing the transition between two kind of environments: (a) from the bottom of the core up to 11.5 cm depth (year 1900), a wetter environment in which estuarine organic matter has a more terrestrial character; and (b) from 9.5 cm depth (early 1930s) to the surface, a dryer environment in which estuarine organic matter has a more marine character. d15N and d13C values increasing together can be produced by dryer conditions (runoff diminution) with bloom maxima migrating upstream (as the saline plume arrives further up in the estuarine zone) and decreased size of the bloom due to lower nutrient flux to the site (Bratton et al., 2003). C and N isotopic changes are undoubtedly related with the development of the agroindustry in the surrounding area of Ohuira Lagoon. The period of gradual decreasing d13C values in the core between 1900 and 1930 most likely corresponded with the development of more saline conditions in the lagoon as a consequence of the gradual reduction of fresh water promoted by the beginning of the water management maneuvers to develop the agriculture fields of El Fuerte Valley (the Tastes channel was built in 1892 to

Fig. 6. Information on organic matter sources through the relationships between: (a) d15N and d13C (modified from Ruiz-Fernández et al. 2007).

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derive water from El Fuerte river to Los Mochis desert). The d15N values increase is likely due to the decreasing input of isotopically light land-plant detritus carried by the river waters to the estuary (Meyers and Lallier-Vergès, 1999) where marine algae is more abundant. d15N increasing values are most likely also the result of the reduction in fresh water supply to Ohuira lagoon as previously observed in the d13C profile; but it seems that d15N is more sensitive as a proxy for fresh water supply reduction than d13C, that seems to better record the influence of the C4 plants wastes transported to the lagoon. The increasing d15N after 1930 (when d13C profile switches its trend) is sustained by the continuous reduction in water supply. After Tastes channel construction, in early 1940s, several other minor channels were connected through 500 pumps, used to irrigate 18,000 ha; and by 1947, the SICAE channel was built and the irrigated area abruptly doubled from 18,000 to 36,000 ha. With the construction of Miguel Hidalgo dam in 1956, the irrigation capacity increased from 55,000 to 240,000 ha (Gill, 2003). More recently, with the promulgation of the Mexican National Water Law (1992) that explicitly declares the sustainable development as it main objective, the Sinaloa Government has encouraged the agriculture producers to incorporate dripping irrigation systems in order to improve water use efficiency (Arredondo Salas and Wilson, 2004). This technology has been quickly adopted in the main agriculture valleys of Sinaloa due to improvement in benefit-cost ratio of crop cultivation, as compared to surface irrigation. However, it has also meant an even more important reduction to freshwater supply to the coastal wetlands that neighbor these vast agriculture areas, provoking concern due to desiccation of lowlands and infilling (Arriaga-Cabrera et al., 1998). 3.2. The Gulf of Tehuantepec continental shelf Cs and 210Pb activities Pb method in the marine environment is feasible only where accumulation rates are sufficiently rapid (>0.03 cm yr1) and this considerations restrict 210Pb dating of marine sediments to relatively near-shore environments (Appleby and Oldfield, 1992). The first attempt to obtain a 210Pb-derived chronological frame to study the influence of anthropogenic land-based activities in the Gulf of Tehuantepec provided a very well preserved sedimentary record (core 10T, collected at 240 m depth; Ruiz-Fernández et al., 2004) but with such a slow mean sedimentation rate (0.05 ± 0.01 cm yr1) that the reconstruction of the anthropogenic fluxes of trace metals were confined only in the two uppermost layers of the core (2 cm depth, 40 years). Therefore, in order to perform a finer reconstruction, it was obvious that the sediment core should be collected in a shallower depth; however, shallow near-shore environments are often highly energetic and consequently, characterized by a high content of coarse sediments, which are usually considered unsuitable for geochronology reconstruction studies, since sandy sediments tend to present very low 210 Pb activities (Madsen et al., 2007). 210 Pbtot activities ranged from 15.6 to 118.9 Bq kg1, which is high in comparison to 210Pbtot activities measured in any other site in Mexico so far, but much lower than those previously found for core 10T (28–753 Bq kg1), most likely due to the coarser nature of the sediments contained in the Tehua II-21 core, in comparison with the clayey sediments from core 10T. 210Pbsup obtained from cspectrometry (10.8 ± 1.6 Bq kg1) was lower than the 210Pbsup obtained from the average value of asymptotic section of the 210Pb profile derived from alpha measurements (15.6 ± 1.0 Bq kg1). We have chosen this last value to estimate excess specific 210Pb activities because: (a) alpha-derived data have smaller uncertainties (3%) in comparison with gamma data (around 10%); (b) are more in agreement with values previously obtained for the Gulf of Tehuantepec by Ruiz-Fernández et al. (2004); and (c) the dates 3.2.1.

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obtained using this value are in good agreement with the 137Cs geochronology (as described below). The deviations from linearity of the logarithmic 210Pbxs activity profile (Fig. 7a) suggested that sediment accumulation has varied during the period recorded in the core; and therefore, a 210Pbxs derived geochronology was set up using the Constant Rate of 210Pb Supply (CRS) model in which non-linearities are interpreted assuming a constant net rate of supply of 210Pbxs from sea-waters to the sediment, irrespective of changes which may have occurred in the net dry mass sedimentation rate (Appleby and Oldfield, 1992). The Tehua II-21 record provided a geochronology of 100 years (surface to 13 cm depth, 18 g cm2 mass depth), with accretion and sediment mass accumulation rates varying from 0.03 to 0.21 cm yr1 and 0.05 to 0.29 g cm2 yr1, respectively. Sediments in Tehua II-21 core were mostly sandy (76.1–89.0%) but in contrast with the sediment cores collected in the coastal lagoons of Sinaloa (previously described), it provided a well resolved 137 Cs peak (7b); although 137Cs activities found were such small that, in order to obtain a minimum uncertainty of 10%, the gamma-detector counting time was more than one week per sample. The 137Cs peak value (1.70 ± 0.2 Bq kg1) was found at 6.75 cm depth; assuming that this sediment layer was at surface in 1963, the average post-1963 sedimentation rate can be calculated as follows:

of the expected dates (1962–1964), corroborating the accuracy of the 210Pb-derived dates for the core Tehua II-21 (Fig. 7b). The most conspicuous change in the trend of the sedimentation rate profile is observed from early 1950s, and they were associated to landuse changes, including deforestation for agriculture development and industrialization. 3.2.2. Geochemical data to corroborate 210Pb geochronologies 3.2.2.1. Al and Fe as tracers for terrigenous input. In order to corroborate that the changes in 210Pb-derived sedimentation rates observed in the Tehuantepec Gulf core were the result of higher erosion in the continent, we show here un example of the use of Fe and Al as tracers for detrital contribution from the continent (other terrigenous indexs are discussed in Ruiz-Fernández et al., 2009b). Al is considered a conservative constituent of terrigenous materials and is often used as tracer for terrigeneous input. It is considered conservative in that it has a uniform flux from crystal rock sources and so compensate for variations in sedimentation rate changes or in the input rates of various diluents (Loring and Rantala, 1992). Fe is naturally introduced to the ocean due to continental weathering of silicate rocks and, subsequently, river input of dissolved weathering products. Fe and Al are major constituents of a large variety of mineral structures such as feldspars, clay minerals and amorphous aluminosilicate gels (Bortleson and Lee, 1972). Both metals are used as terrigenous tracers because they are enriched in the earth’s crust (average abundance in the continental crust: Al, 8.23%; Fe, 5.63%; Taylor, 1964). Previous work in the literature have shown that the ratio Fe/Al can be useful to determine the provenance of detritic particles transported either by the wind (Wu et al., 2008) or by the rivers (Aucour et al., 2003). Fe/Al ratio is not expected to change during transport and the differences in Fe/Al mostly result from changes in mineral composition (Wu et al., 2008). Windblown dust exhibits an average Fe/Al ratio of 0.28 (Pye, 1987), the continental crust

SR ¼ d=ðt 0 —1963Þ where SR is an average sedimentation rate (cm yr1), t0 is the year when sediment was collected, d is the depth (cm) of the 137Cs peak from the sediment surface. The average sedimentation rate obtained was 0.16 cm yr1. The 137Cs-derived accretion rate is comparable to the mean accretion rate obtained from the 210Pb chronology between surface and 6.75 cm depth (0.16 ± 0.04 cm yr1); and the 210Pb-derived date for this depth was 1963 ± 2 years, which lies within the range

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A.C. Ruiz-Fernández, C. Hillaire-Marcel / Marine Pollution Bulletin 59 (2009) 134–145

0.84 (fluvial input) and hydrothermal sources 1.61 (Taylor and Mc Lennan, 1985). Fe/Al ratios have been used to identify changes in the provenance of the sediments, supposedly as a result of the increasing input of terrigenous in the marine environment (Klump et al., 2000; Stuut et al., 2007) and have even been useful to indicate the seasonality in the source of the material (Martinez et al., 2007). Before 1950s, Fe/Al ratio profile showed values ranging between 0.12 and 0.19, and from 1950s to 2004, between 0.20 and 0.28 (Fig. 7c). This change indicates the possibility of two different sources of sediments in Tehua II-21 core, with the higher Fe/Al ratios most likely due to an increased input of the terrigenous matter (mainly windblown dust) resulting of enhanced continental erosion, promoted by land use changes associated with the urban and industrial development of the coastal zone adjacent to Tehuantepec Gulf: i.e. conversion of forests into pasture lands, deforestation due to urbanization, roads construction and industrialization, all increasing as the population expands. During the decade of 1940 there was an important development in the area, including the improvement of Salina Cruz harbor, the establishment of several factories (cement, limestone and pop-soda), the impoundment of several rivers and the construction of the Transisthmus and Panamerican roads (Renaud Orozco and Segura, 2000). Previous studies in other marine areas the world such as the Potomac Estuary (Oldfield et al., 1989), the Baltic Sea (Emeis et al., 2000), the Adriatic sea (Oldfield et al., 2003) and the pericontinental shelf of Hong Kong (Owen and Lee, 2004) in which other type of continental transport tracers were used (i.e. pollen) have also found evidences that the changes in marine sedimentation reflect increased material input from land, related to deforestation and expanding human impact in periods characterized by major land use changes. 4. Summary and conclusions The Pacific coast of Mexico is a very difficult area for 210Pb reconstruction studies due to several reasons: shallow coastal lagoon sediments are often disturbed by natural processes (i.e. wind and tide currents); the region has very low 210Pb atmospheric fluxes and the 137Cs activities (as secondary tracer to corroborate 210 Pb dates) are often under background analytical levels. In order to develop a strategy to reconstruct recent geochronologies of the environmental changes in such complex environments, in the case of the coastal lagoons, we propose to improve the 210Pb profile integrity by collecting the sediment cores at mudflat intertidal areas, covered by halophyte vegetation. In the examples provided in this work, some of the cores presented subsurficial 210Pb peak values that could not be eliminated by normalization to grains size (or other geochemical parameters), and the signal of 137Cs for 210 Pb-derived dates corroboration was very weak or not detectable. In other to provide an option to validate 210Pb geochronologies we propose the use of geochemical data (trace metal enrichment factors and C and N isotopic composition profiles) which in these cases, provided sound information about environmental changes for which historical information is well known, i.e. the operation history of the thermoelectric plant that neighbor the Estero de Urias lagoon supported the observed changes in enrichment factors of Hg; or the temporal evolution of the cropland area at the two most important agriculture valleys of Sinaloa surrounding the Chiricahueto and Ohuira lagoons supported the increments in enrichment Hg and Cu fluxes to the sediments. The sedimentation rates found for the coastal lagoons studied varied between 0.06 and 0.33 cm yr1 (which are in the range of other microtidal areas in the world) and at the Gulf of Tehuantepec they varied from 0.03 to 0.21 cm yr1.

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With this contribution we pretend to demonstrate that nonmonotonous 210Pb profiles, sandy sediments and the lack of 137Cs corroboration data are not necessarily conditions that preclude the use of 210Pb dating in such naturally complex coastal environments; and we strongly recommend to always support 210Pb dating surveys with geochemical data, that besides providing the possibility of an additional temporal frame as previously explained, can also help to explain sedimentological process that could be misinterpreted (such as increasing sediment rates, higher primary production rates, reduction of water supply, to mention some examples) because at the end, the atypical profiles of every sediment core has a tale to tell. Acknowledgments Financial support for these studies was provided by the grants CONACyT-SEMARNAT-2002-C01-0161, CONACyT 45841-F and DGAPA-UNAM IX242504 and IN103605. Mobility support to ACRF to participate in the Ocean Sciences Meeting 2008 was provided by the UNAM-CIC International Academic Exchange Program. Thanks are due to M.C. Ramírez-Jáuregui, G. Ramírez-Reséndiz, H. Bojórquez-Leyva, L.H. Pérez-Bernal, V. Montes-Montes and G. González-Chávez for their technical assistance; as well to the crew of O/V El Puma for their support during Tehua II oceanographic cruise. Very special thanks to S. Bojórquez-Sánchez and P.G. Mellado-Vázquez for their invaluable support in the laboratory activities. This is contribution No. 2009-0012 from GEOTOP-UQAM. References Acosta-Ruiz, G., Powers, B., 2003. Preliminary atmospheric emissions inventory of mercury in Mexico. In: Proceedings of the Twelfth International Emission Inventory Conference ‘‘Emission Inventories – Applying New Technologies, San Diego, April 29–May 1, 2003. Alvarez-León, R., 1980. Hidrología y zooplancton de tres esteros. Anales del Centro de Ciencias del Mar y Limnología. . Andersen, T.J., Mikkelsen, O.A., Møller, A.L., Pejrup, M., 2000. Deposition and mixing depths on some European intertidal mudflats based on 210Pb and 137Cs activities. Continental Shelf Research 20, 1569–1591. Aoyama, M., Hirose, K., 2003. Temporal variation of 137Cs water column inventory in the North Pacific since the 1960s. Journal of Environmental Radioactivity 69 (1– 2), 107–117. Appleby, P.G., 1998. Dating of sediments and determination of sedimentation rate. In: Proceedings of a Seminar held in Helsinki, 2–3 April, 1997. Appleby, P.G., Oldfield, F., 1992. Application of lead-210 to sedimentation studies. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium-Series Disequilibrium. Applications to Earth, Marine and Environmental Sciences. Clarendon Press, Oxford, pp. 731–783. Arredondo Salas, S.M., Wilson, P.N., 2004. A farmer-centered analysis of irrigation management transfer in Mexico. Irrigation and Drainage Systems 18 (1), 89– 107. Arriaga-Cabrera, L., Vázquez-Domínguez, E., González-Cano, J., Jiménez Rosenberg, R., Muñoz López, E., Aguilar Sierra, V., 1998. Regiones marinas prioritarias de México. CONABIO, . Aucour, A.M., Tao, F.-X., Moreira-Turcq, P., Seyler, P., Sheppard, S., Benedetti, M.F., 2003. The Amazon River: behavior of metals (Fe, Al, Mn) and dissolved organic matter in the initial mixing at the Rio Negro/Solimoes confluence. Chemical Geology 197, 271–285. Axelsson, V., El-Daoushy, F., 1989. Sedimentation in the Edsviken bay studied by the X-ray and the Pb-210 methods. Geografiska Annaler 71A. Berner, A., 1971. Principles of Chemical Sedimentology. McGraw-Hill, New York. p. 240. Bortleson, G.C., Lee, G.F., 1972. Recent sedimentary history of Lake Mendota, Wis. Environmental Science and Technology 6, 799–808. Bratton, J.F., Colman, S.M., Seal, R.R., 2003. Eutrophication and carbon sources in Chesapeake Bay over the last 2700 yr: human impacts in context. Geochimica et Cosmochimica Acta 67 (18), 3385–3402. Cloern, J.E., Canuel, E.A., Harris, D., 2002. Stable carbon and nitrogen isotope composition of aquatic and terrestrial plants of the San Francisco Bay Estuarine System. Limnology and Oceanography 47 (3), 713–729. Cochran, J.K., 1992. The oceanic chemistry of the uranium- and thorium-series nuclides. In: Ivanovich, M., Harmon, R.S. (Eds.), Uranium-Series Disequilibrium: Applications to Earth, Marine and Environmental Sciences. Clarendon Press, Oxford, pp. 334–391.

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