Bull Environ Contam Toxicol DOI 10.1007/s00128-014-1222-9
The Last Two Centuries of Lead Pollution in the Southern Gulf of Mexico Recorded in the Annual Bands of the Scleractinian Coral Orbicella faveolata Guillermo Horta-Puga • Jose´ D. Carriquiry
Received: 27 August 2013 / Accepted: 1 February 2014 Ó Springer Science+Business Media New York 2014
Abstract Lead (Pb) pollution history (1855–2001 A.D.) of the southern Gulf of Mexico (SGM) was reconstructed from the geochemical record contained in the annual bands of the hermatypic coral Orbicella faveolata from the Veracruz Reef System, Mexico. Pb concentrations ranged from 5.5 lg/g in 1889–23.6 lg/g in 1992, with an average of 10.0 ± 4.1 lg/g. These high concentrations are evidence of a highly polluted environment. High statistical correlations were observed between the annual Pb coral time-series and both, the production of alkyl-lead gasoline in Mexico during the second half of the twentieth century (r = 0.86, p \ 0.001), and the industrial production of lead in North America for the 1900–1940 years period (r = 0.73, p \ 0.001). Hence, this research provides evidence that these two processes generated Pb-rich aerosols that were atmospherically transported, increasing the environmental levels of Pb in the SGM. Keywords Lead pollution Veracruz Reefs Pb-aerosols Atmospheric transport Lead (Pb) is a metallic element extensively used since antiquity, and its environmental concentration has been increasing steadily and significantly because of human
G. Horta-Puga (&) J. D. Carriquiry Instituto de Investigaciones Oceanolo´gicas, Universidad Auto´noma de Baja California, Carr. Tijuana-Ensenada km 107, 22800 Ensenada, Baja California, Mexico e-mail: [email protected] Present Address: G. Horta-Puga FES Iztacala, UBIPRO, Universidad Nacional Auto´noma de Me´xico, Av. de Los Barrios 1, Los Reyes Iztacala, Tlalnepantla, 54090 Mexico, Mexico
influence. In recent times the main sources of lead into the environment have been the aerosols derived from emissions during mineral processing of the non-ferrous metallurgic industry and the combustion of alkyl-lead gasoline, which are transported far away from their sources by atmospheric circulation which is responsible for dispersing and increasing their environmental levels at a global scale (Murozumi et al. 1969; Nriagu 1989; Candelone et al. 1995). Biogenic carbonates, especially the skeleton of scleractinian corals, has proven to be a useful archive of marine pollution (Shen and Boyle 1987; Medina-Elizalde et al. 2002; Desenfant et al. 2006; Kelly et al. 2009; Prouty et al. 2013), and although the history of lead pollution in the tropical western Atlantic in the last two centuries is well known (Shen and Boyle 1987; Lazareth et al. 2000; Desenfant et al. 2006; Kelly et al. 2009), it is not the case for the Gulf of Mexico. The southern Gulf of Mexico (SGM) is considered a highly polluted oceanic region, because of high environmental levels of lead (Albert and Badillo 1991; Villanueva and Botello 1992; Villanueva and Pa´ez-Osuna 1996), although they have been decreasing in the last decade (Horta-Puga et al. 2013). In this area of the tropical western Atlantic, there is a well-developed reef ecosystem, the Veracruz Reef System (VRS) that lies in front off Veracruz, the most important city and cargo port in the SGM. The VRS also is influenced by river discharge, atmospheric fallout, and surface oceanic currents that are able to carry contaminants generated by human activities, including lead, from the adjacent mainland and distant oceanic regions (Carriquiry and Horta-Puga 2010; HortaPuga et al. 2013). Thus, the aim of the present study was to use the geochemical information contained in the annual density bands of the scleractinian coral Orbicella faveolata from the VRS in order to reconstruct the history of lead pollution in the SGM during the last two centuries.
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Materials and Methods A live and healthy coral colony of O. faveolata was cored in July 2003, using a submersible hydraulic drill from the back reef slope (7 m depth) of the Anegada de Adentro reef (Fig. 1), in the VRS (19°130 4500 N, 96°030 4600 W), the same colony used for the reconstruction of Ba pollution in the VRS (Carriquiry and Horta-Puga 2010). Later in the lab, the core was longitudinally sectioned generating 0.8 mm-thick coral slabs. These were X-rayed to reveal the annual growth bands in the skeleton. The chronology of the core was determined using the x-radiographs as templates, and the position of each annual density band was carefully delineated on a transparency film, and the year of deposition of each annual band was dated from the top 2001 annual band backwards. Also, to avoid dating errors associated to the method, the annual growth rates were measured manually from the end of a highdensity band to next, except for the annual bands corresponding to the years 1918, 1919, 1942 and 1982, because of their position in the top/end of core sections (broken parts), but that did not preclude its use in the chemical analysis of Pb. Each annual band was traced on the coral slab, and later cutout using a variable speed rotary tool equipped with a diamond cutting wheel. Small blocks of *1 g were obtained from each annual cut-out. These blocks were washed and rinsed several times with a HNO3 0.2 M solution and deionized water, respectively, in an ultrasonic cleaner to eliminate contamination associated with the cutting process. Later, each block was crushed and sieved to retain the 280–700 lm fraction (Shen and Boyle 1988). In order to eliminate trace elements adsorbed to the surface of carbonate crystals, the powdered samples were washed with a HNO3 5 mM solution and rinsed (in an ultrasonic cleaner) with deionized water for five consecutive times, following a cleaning protocol similar to that used by Guzma´n and Jarvis (1996). Two-hundred mg from each thoroughly cleaned annual sample was digested in 2 mL of HNO3 2.5 M. An aliquot of 10 lL, equivalent to *1 mg of coral CaCO3, was obtained from each digested solution and later it was taken to a final volume of 5 mL with HNO3 (2 %). The final solution had a concentration of Ca *76 lg/L ([CaCO3]coral & 0.2 mg/mL), and calibration standards were prepared with a matrix matching solution in order to avoid interferences during the instrumental analysis (de Villiers et al. 2002). Lead concentrations were simultaneously determined along with the Ba/Ca molar ratio by the intensity ratio method (Carriquiry and Horta-Puga 2010), using an ICP-OES (Thermo-Jarell Ash Iris/APÒ) coupled to an ultrasonic nebulizer (CETAC U-5000AT ?Ò). The ICP-OES was optimized in order to achieve robust plasma operating conditions (MgII(280.270)/MgI(285.213) [ 5), the lowest detection limit (3.0 ng/mL), and the highest possible precision (*4.0 %) (Brenner and Zander 2000; Villaescusa-Celaya and Carriquiry 2004). To assure the quality of Pb instrumental analysis,
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only high-purity or purified reagents (Pb-free) were used; all quality control blanks always gave a non-detectable concentration of Pb. The whole chemical treatment and instrumental analysis of coral samples was done in a class 100 clean room at the Geochemistry Lab in the Universidad Auto´noma de Baja California. A synthetic reference solution that matched the chemical matrix of samples with a known concentration of Pb gave a percentage of recovery of 107 %. Considering the final dilution factor used during the instrumental analysis, and the instrumental detection limit (3.0 ng/mL), the lowest Pb concentration that could be confidently quantified in annual samples was 4.5 lg/g (Pb/Coral CaCO3). Lead concentrations in annual bands are reported in mass units (lg/g).
Results and Discussion The growth record of the scleractinian coral O. faveolata from the VRS spanned from 1835 to 2002, which is the same used for the Ba/Ca molar ratios historical reconstruction (Carriquiry and Horta-Puga 2010). However, reliable Pb concentrations were only obtained for the 1855–2001 period, a continuous record of 147 years (Fig. 2). The average annual growth rate (linear extension) for this period was 11.2 ± 2.3 mm/year, which is higher than previous reports for this species (Da´valos-Dehullu et al. 2008), which also suggests a colony that has been growing healthily. The measured range of lead concentrations is high: from 5.5 lg/g in 1889 and 1945 to 23.6 lg/g in 1992, with an average of 10.0 ± 4.1 lg/g. The record shows three periods of high concentrations with peaks in 1864 (8.1 lg/g), 1924 (11.4 lg/g), and 1992 (23.6 lg/g). The range of Pb concentrations are equivalent to a molar ratio (Pb/Ca) from 2.6 lmol/mol to 11.3 lmol/mol with an annual average of 5.0 lmol/mol, which is one of the highest concentrations recorded in scleractinian corals like in Porites evermanni (9.5 lg/g) from Hawaii (Miao et al. 2001), Siderastrea radians (32 lg/g) from the Caribbean of Central America (Guzma´n and Jime´nez 1992), and in Montastraea annularis (62.4 lg/g) from the VRS (HortaPuga and Ramı´rez-Palacios 1996). For the VRS there are some other records of high environmental concentrations of Pb, up to 42 lg/g in inter-reef surface sediments (Rosales-Hoz et al. 2007), 179 lg/L in rainwater in the city of Veracruz (Albert and Badillo 1991), and more recently 0.2 lg/g in macroalgae (Horta-Puga et al. 2013). Hence, our results are evidence that the SGM has been polluted by Pb at least during the last two centuries, just like some other reef areas that have been documented to be strongly influenced by anthropogenic activities, like the Caribbean coast of Central America (Guzma´n and Jime´nez 1992), Hawaii (Miao et al. 2001), and Bermuda (Prouty et al. 2013).
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Fig. 1 Veracruz Reef System, southern Gulf of Mexico. Collection site marked with a black square
Fig. 2 Historical Pb concentration in O. faveolata from the VRS, southern Gulf of Mexico (black line), and in the sclerosponge Ceratoporella nicholsoni (gray line) from the Bahamas (Lazareth et al. 2000)
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The concentration of Pb in the annual bands of O. faveolata shows a general increasing trend since the middle of the nineteenth century to the end of the record at the beginning of the twenty-first century, as Lazareth et al. (2000) found in sclerosponges from Bahamas (Fig. 2), and as also has been recorded in the tropical western Atlantic area (Shen and Boyle 1987; Medina-Elizalde et al. 2002; Desenfant et al. 2006; Kelly et al. 2009; Prouty et al. 2013). And, as in other regions of the tropical western Atlantic, as will be discussed below, the higher Pb environmental concentrations in the SGM could be attributed to: (1) the atmospheric dispersion of the aerosols generated during the smelting of lead ores for the production of metallic Pb and other non-ferrous metals for the industry in the nineteenth and early twentieth centuries (Murozumi et al. 1969; Nriagu 1989; Lima et al. 2005); and (2) the massive use of alkyl-lead gasoline in continental Mexico since the 1940s until when it phased-out during the 1990s (Wu and Boyle 1997; Medina-Elizalde et al. 2002). In the nineteenth century the Upper Mississippi Valley region was the main producer of Pb, and the main source of Pb into the environment in North America (Lima et al. 2005). In that period, the efficiency in the recovery of fumes and dusts (Pb-aerosols) during the smelting and refining of ores and scraps in the silver and lead processing plants were low. The loss of Pb from ores was estimated to be *2 % during the 1750–1880 period, which was reduced gradually to *0.5 % in 1920, 0.06 % in 1960, and less than 0.0034 % in recent times (Murozumi et al. 1969; USEPA 1998). Winds can transport Pb-aerosols far away from their source regions and, particularly, in the SGM during the winter season up to 30 events of high-pressure systems occur, which move large masses of cold air from the NW to the SE, across the continental USA and into the Gulf of Mexico (Hsu 1988; Carrillo et al. 2007), and this could be the mechanism responsible for transporting Pbrich aerosols originated in continental North America into the SGM. Lima et al. (2005) demonstrated that the concentration of environmental Pb in eastern North America increased after 1840s due mainly to ore processing. In the middle nineteenth century the annual industrial Pb production was \25 tons 9103 in the 1850–1870 period, and \100 tons 9103 by 1880 (Shen and Boyle 1987). In spite of that Pb production was ten times lower than in the early twentieth century, as the recovery of dust and fumes was very low at this period, it is possible that large quantities of Pb-aerosols were transported south into the SGM, and also west to Bermuda and Bahamas by the mid-latitude westerlies (Shen and Boyle 1987), contributing to the increase in the environmental concentration of Pb in the tropical western Atlantic during the middle nineteenth century (Fig. 2), as is evidenced by the observed concentration peaks in the coral record from the SGM (1860s),
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Fig. 3 Historical Pb concentration in O. faveolata from the VRS (black line), and the US Pb industrial production (gray line) for the 1900–1940 period (USGS 2008)
and the sclerosponge record (early 1870s) from the Bahamas (Lazareth et al. 2000). Soon after, by the 1880s, the release of industrial Pb-rich aerosols decreased (Murozumi et al. 1969), due to more efficient recovery processes, with the concomitant reduction of the environmental levels of Pb, as also can be seen in Fig. 2. However, no further evidence supports this assumption. By the 1890s the coral Pb concentrations began to increase until the 1920s, with a second peak in 1924, and soon after concentrations decrease to lower values in the early 1940s. This pattern also seems to be associated to the Pb production in the USA. Figure 3 shows the US Pb production (USGS 2008) and the Pb coral time-series in O. faveolata from the VRS, for the 1900–1940 period. The correlation between both time-series is high (r = 0.73, p \ 0.001) allowing a 1-year lag in the coral record, in order to compensate for the timelag existing between the emission of Pb-aerosols into the atmosphere, and the time they reach the SGM to become recorded in the coral skeletal growth bands. Moreover, both series show a similar trend of variation with a peak in the 1920s decade, although not in the same year, that also was recorded with slight timing differences at each locality, in the Pb time-series in corals from Bermuda, and Puerto Rico and Venezuela (Shen and Boyle 1987; Reuer et al. 2003; Desenfant et al. 2006; Kelly et al. 2009), and sclerosponges from Bahamas (Lazareth et al. 2000). Thus, evidencing a cause-effect relationship between the source (Pb-aerosols emitted in continental North America) and the coral concentration record of Pb for this time period in the SGM. By the middle twentieth century Pb-aerosol emissions from industrial primary lead production became negligible, and alkyl-lead gasoline arose as the main source of Pb into the environment (Murozumi et al. 1969; Wu and Boyle 1997). Figure 4 shows the annual alkyl-lead gasoline production in Mexico, the nearest continental area, for the
Bull Environ Contam Toxicol Fig. 4 Historical Pb concentration in O. faveolata from the VRS (black line); historical Pb/Ca molar ratios (gray line) in Orbicella annularis from the Mexican Caribbean (Medina-Elizalde et al. 2002); and Mexican production of leaded gasoline (open triangles) for the 1938–1997 period (INEGI 2010)
1938–1997 period (INEGI 2010), and the Pb record from the VRS. Although the coral Pb record has high variability, 1r = 4.7 lg/g, which is threefold higher than the variability (1r = 1.4 lg/g) for the previous period (1855–1939), it follows the same increasing trend until the phase-out of alkyl-lead gasoline in Mexico, when both variables began to decrease, just like the coral Pb record from the Mexican Caribbean (Medina-Elizalde et al. 2002). The maximum concentration of Pb in the coral was recorded just 1 year after the peak of production of alkyllead gasoline in Mexico in 1991, which suggests a time lag between the timing of the release of Pb to the atmosphere and its incorporation into the coral skeleton. Correlation between both time-series is high (r = 0.86, p \ 0.001, simple linear correlation analysis), so, it can be concluded that the combustion of alkyl-lead gasoline in Mexico was the main source of Pb in the SGM during the 1938–1997 year period, as also was recorded in lacustrine sediments from the lake Verde (*18°360 N, 95°200 W), some 110 km to the southeast of the VRS (Soto-Jime´nez et al. 2006). Also it allows explaining the timing differences of the different Pb peaks observed at different localities of the tropical western Atlantic subjected to equivalent influences of the alkyl-lead gasoline consumption in North America (Shen and Boyle 1987; Lazareth et al. 2000; Desenfant et al. 2006; Kelly et al. 2009). Although the input of Pb into the environment from the combustion of alkyl-lead gasoline increased steadily through time, the coral Pb record shows some periods of declining concentrations, which depart from the expected increasing trend. In the case of the VRS, the Jamapa River is a major source of suspended solids (Carriquiry and Horta-Puga 2010). Terrigenous sediments from the watershed, along with previously deposited Pb-aerosols derived
from alkyl-lead gasoline (Soto-Jime´nez et al. 2006), are carried to the coastal zone (Horta-Puga et al. 2013). As river flow volume vary concomitantly with precipitation, environmental Pb levels in the VRS tend to decrease on those years with lower precipitation rates (Fig. 5). This process helps to explain some of the high variability and departures from the increasing trend observed in the geochemical record.
Conclusions This is the first study of the history of annual Pb pollution in the SGM, including the last two centuries (1855–2001), recovered by analyzing a core of the hermatypic coral O. faveolata from the VRS, one of the longest records for the tropical western Atlantic. The concentrations of Pb along the entire coral record are higher than those reported from other coastal localities, and have been steadily increasing, at least since the middle nineteenth century, evidencing that the coastal area of the SGM has been a highly polluted environment. The main sources of Pb in the SGM have been the aerosols derived from the industrial production of lead in North America, and the combustion of leaded gasoline in Mexico. These aerosols, which are atmospherically dispersed, have been contributing during the last two centuries to increase the concentration of Pb above natural background levels in the SGM, and have imprinted its signature in the geochemical record of corals in the VRS, as is evidenced by the high statistical correlation among time-series. However, in order to determine the very source of Pb in the SGM, it will be necessary to study the Pb isotopic composition, and the time of its changes. Due to the enforcement of environmental regulations, like
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Bull Environ Contam Toxicol Fig. 5 Historical Pb concentration in O. faveolata from the VRS (black line); and total annual precipitation in the VRS area (gray line), for the 1940–1997 period (unpublished data from Centro de Prevision del Golfo de Mexico, Comision Nacional del Agua)
the ban of the use of alkyl-lead gasoline, and to more efficient processes for the recovery of dust and fumes with Pb-aerosols in the ore processing and metallurgical industries, the environmental concentrations of Pb have been decreasing in the last two decades, as also is evidenced for the SGM by our results, and from the study of Pb concentrations in macroalgae from the VRS (Horta-Puga et al. 2013). Finally, hermatypic corals from the SGM, like those from other geographical areas, have proven to be very useful recorders of past environmental conditions allowing the temporal and spatial reconstruction of pollution, their sources, and fate of Pb in the ocean. Acknowledgments We greatly thank the help of Julio VillaescusaCelaya and Pedro Castro (UABC, IIO) for their support at all the stages of laboratory work. We also acknowledge the fieldwork support provided by the ‘Acuario de Veracruz’, especially to Alberto ´ ngel Roma´n. Also thanks to Claire Lazareth for Rı´os and Miguel A providing us the sclerosponge Pb time-series data. The comments of two anonymous reviewers contributed to improve this manuscript. This study was financed in part by a Grant from the National Council of Science and Technology of Mexico (CONACYT: 46814) to JD Carriquiry; and a scholarship from PASPA-DGAPA-UNAM to G Horta-Puga.
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Coral reefs of Southern Gulf of Mexico. Texas A&M University Press, College Station TX, pp 34–40 Carriquiry JD, Horta-Puga G (2010) The Ba/Ca record of corals from the Southern Gulf of Mexico: contributions from land-use changes, fluvial discharge and oil-drilling muds. Mar Pollut Bull 60:1625–1630 Da´valos-Dehullu E, Herna´ndez-Arana H, Caricart-Ganivet JP (2008) On the causes of density banding in skeletons of corals of the genus Montastraea. J Exp Mar Bio Ecol 365:142–147 de Villiers S, Graves M, Elderfield H (2002) An intensity ratio calibration method for the accurate determination of Mg/Ca and Sr/Ca of marine carbonates by ICP-AES. Geochem Geophys Geosyst 3:1001. doi:10.1029/2001GC000169 Desenfant F, Veron AJ, Camoin GF, Nyberg J (2006) Reconstruction of pollutant lead invasion into the tropical North Atlantic during the twentieth century. Coral Reefs 25:473–484 Guzma´n HM, Jarvis KE (1996) Vanadium century record from Caribbean reef corals: a tracer of oil pollution in Panama. Ambio 25:523–526 Guzma´n HM, Jime´nez CE (1992) Contamination of coral reefs by heavy metals along the Caribbean coast of Central America (Costa Rica and Panama). Mar Pollut Bull 24:554–561 Horta-Puga G, Ramı´rez-Palacios R (1996) Niveles de plomo en esqueleto del coral arrecifal Montastrea annularis. In: Botello AV, Rojas-Galaviz JL, Benı´tez JA, Za´rate-Lomelı´ D (eds) Golfo de Me´xico, contaminacio´n e impacto ambiental: Diagno´stico and tendencias. Universidad Auto´noma de Campeche, EPOMEX Serie Cientı´fica 5, Campeche, pp 363–367 Horta-Puga G, Cha´zaro-Olvera S, Winfield I, Avila-Romero M, MorenoRamı´rez M (2013) Cadmium, copper and lead in macroalgae from the Veracruz Reef System, Gulf of Mexico: spatial distribution and rainy season variability. Mar Pollut Bull 68:127–133 Hsu SA (1988) Coastal meteorology. Academic Press, San Diego CA INEGI (2010) Instituto Nacional de Estadı´stica Geografı´a de Me´xico. Estadı´sticas histo´ricas de Me´xico 2009, Coleccio´n Memoria, Tomo I. Aguascalientes, Me´xico Kelly AE, Reuer MK, Goodkin NF, Boyle EA (2009) Lead concentrations and isotopes in corals and water near Bermuda, 1780-2000. Earth Planet Sci Lett 283:93–100 Lazareth CE, Willenz P, Nevez J, Keppens E, Dehairs F, Andre´ L (2000) Sclerosponges as a new potential recorder of environmental changes: lead in Ceratoporella nicholsoni. Geology 28: 515–518
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The Last Two Centuries of Lead Pollution in the Southern Gulf of Mexico Recorded in the Annual Bands of the Scleractinian Coral Orbicella faveolata Guillermo Horta-Puga • Jose´ D. Carriquiry
Received: 27 August 2013 / Accepted: 1 February 2014 Ó Springer Science+Business Media New York 2014
Abstract Lead (Pb) pollution history (1855–2001 A.D.) of the southern Gulf of Mexico (SGM) was reconstructed from the geochemical record contained in the annual bands of the hermatypic coral Orbicella faveolata from the Veracruz Reef System, Mexico. Pb concentrations ranged from 5.5 lg/g in 1889–23.6 lg/g in 1992, with an average of 10.0 ± 4.1 lg/g. These high concentrations are evidence of a highly polluted environment. High statistical correlations were observed between the annual Pb coral time-series and both, the production of alkyl-lead gasoline in Mexico during the second half of the twentieth century (r = 0.86, p \ 0.001), and the industrial production of lead in North America for the 1900–1940 years period (r = 0.73, p \ 0.001). Hence, this research provides evidence that these two processes generated Pb-rich aerosols that were atmospherically transported, increasing the environmental levels of Pb in the SGM. Keywords Lead pollution Veracruz Reefs Pb-aerosols Atmospheric transport Lead (Pb) is a metallic element extensively used since antiquity, and its environmental concentration has been increasing steadily and significantly because of human
G. Horta-Puga (&) J. D. Carriquiry Instituto de Investigaciones Oceanolo´gicas, Universidad Auto´noma de Baja California, Carr. Tijuana-Ensenada km 107, 22800 Ensenada, Baja California, Mexico e-mail: [email protected] Present Address: G. Horta-Puga FES Iztacala, UBIPRO, Universidad Nacional Auto´noma de Me´xico, Av. de Los Barrios 1, Los Reyes Iztacala, Tlalnepantla, 54090 Mexico, Mexico
influence. In recent times the main sources of lead into the environment have been the aerosols derived from emissions during mineral processing of the non-ferrous metallurgic industry and the combustion of alkyl-lead gasoline, which are transported far away from their sources by atmospheric circulation which is responsible for dispersing and increasing their environmental levels at a global scale (Murozumi et al. 1969; Nriagu 1989; Candelone et al. 1995). Biogenic carbonates, especially the skeleton of scleractinian corals, has proven to be a useful archive of marine pollution (Shen and Boyle 1987; Medina-Elizalde et al. 2002; Desenfant et al. 2006; Kelly et al. 2009; Prouty et al. 2013), and although the history of lead pollution in the tropical western Atlantic in the last two centuries is well known (Shen and Boyle 1987; Lazareth et al. 2000; Desenfant et al. 2006; Kelly et al. 2009), it is not the case for the Gulf of Mexico. The southern Gulf of Mexico (SGM) is considered a highly polluted oceanic region, because of high environmental levels of lead (Albert and Badillo 1991; Villanueva and Botello 1992; Villanueva and Pa´ez-Osuna 1996), although they have been decreasing in the last decade (Horta-Puga et al. 2013). In this area of the tropical western Atlantic, there is a well-developed reef ecosystem, the Veracruz Reef System (VRS) that lies in front off Veracruz, the most important city and cargo port in the SGM. The VRS also is influenced by river discharge, atmospheric fallout, and surface oceanic currents that are able to carry contaminants generated by human activities, including lead, from the adjacent mainland and distant oceanic regions (Carriquiry and Horta-Puga 2010; HortaPuga et al. 2013). Thus, the aim of the present study was to use the geochemical information contained in the annual density bands of the scleractinian coral Orbicella faveolata from the VRS in order to reconstruct the history of lead pollution in the SGM during the last two centuries.
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Materials and Methods A live and healthy coral colony of O. faveolata was cored in July 2003, using a submersible hydraulic drill from the back reef slope (7 m depth) of the Anegada de Adentro reef (Fig. 1), in the VRS (19°130 4500 N, 96°030 4600 W), the same colony used for the reconstruction of Ba pollution in the VRS (Carriquiry and Horta-Puga 2010). Later in the lab, the core was longitudinally sectioned generating 0.8 mm-thick coral slabs. These were X-rayed to reveal the annual growth bands in the skeleton. The chronology of the core was determined using the x-radiographs as templates, and the position of each annual density band was carefully delineated on a transparency film, and the year of deposition of each annual band was dated from the top 2001 annual band backwards. Also, to avoid dating errors associated to the method, the annual growth rates were measured manually from the end of a highdensity band to next, except for the annual bands corresponding to the years 1918, 1919, 1942 and 1982, because of their position in the top/end of core sections (broken parts), but that did not preclude its use in the chemical analysis of Pb. Each annual band was traced on the coral slab, and later cutout using a variable speed rotary tool equipped with a diamond cutting wheel. Small blocks of *1 g were obtained from each annual cut-out. These blocks were washed and rinsed several times with a HNO3 0.2 M solution and deionized water, respectively, in an ultrasonic cleaner to eliminate contamination associated with the cutting process. Later, each block was crushed and sieved to retain the 280–700 lm fraction (Shen and Boyle 1988). In order to eliminate trace elements adsorbed to the surface of carbonate crystals, the powdered samples were washed with a HNO3 5 mM solution and rinsed (in an ultrasonic cleaner) with deionized water for five consecutive times, following a cleaning protocol similar to that used by Guzma´n and Jarvis (1996). Two-hundred mg from each thoroughly cleaned annual sample was digested in 2 mL of HNO3 2.5 M. An aliquot of 10 lL, equivalent to *1 mg of coral CaCO3, was obtained from each digested solution and later it was taken to a final volume of 5 mL with HNO3 (2 %). The final solution had a concentration of Ca *76 lg/L ([CaCO3]coral & 0.2 mg/mL), and calibration standards were prepared with a matrix matching solution in order to avoid interferences during the instrumental analysis (de Villiers et al. 2002). Lead concentrations were simultaneously determined along with the Ba/Ca molar ratio by the intensity ratio method (Carriquiry and Horta-Puga 2010), using an ICP-OES (Thermo-Jarell Ash Iris/APÒ) coupled to an ultrasonic nebulizer (CETAC U-5000AT ?Ò). The ICP-OES was optimized in order to achieve robust plasma operating conditions (MgII(280.270)/MgI(285.213) [ 5), the lowest detection limit (3.0 ng/mL), and the highest possible precision (*4.0 %) (Brenner and Zander 2000; Villaescusa-Celaya and Carriquiry 2004). To assure the quality of Pb instrumental analysis,
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only high-purity or purified reagents (Pb-free) were used; all quality control blanks always gave a non-detectable concentration of Pb. The whole chemical treatment and instrumental analysis of coral samples was done in a class 100 clean room at the Geochemistry Lab in the Universidad Auto´noma de Baja California. A synthetic reference solution that matched the chemical matrix of samples with a known concentration of Pb gave a percentage of recovery of 107 %. Considering the final dilution factor used during the instrumental analysis, and the instrumental detection limit (3.0 ng/mL), the lowest Pb concentration that could be confidently quantified in annual samples was 4.5 lg/g (Pb/Coral CaCO3). Lead concentrations in annual bands are reported in mass units (lg/g).
Results and Discussion The growth record of the scleractinian coral O. faveolata from the VRS spanned from 1835 to 2002, which is the same used for the Ba/Ca molar ratios historical reconstruction (Carriquiry and Horta-Puga 2010). However, reliable Pb concentrations were only obtained for the 1855–2001 period, a continuous record of 147 years (Fig. 2). The average annual growth rate (linear extension) for this period was 11.2 ± 2.3 mm/year, which is higher than previous reports for this species (Da´valos-Dehullu et al. 2008), which also suggests a colony that has been growing healthily. The measured range of lead concentrations is high: from 5.5 lg/g in 1889 and 1945 to 23.6 lg/g in 1992, with an average of 10.0 ± 4.1 lg/g. The record shows three periods of high concentrations with peaks in 1864 (8.1 lg/g), 1924 (11.4 lg/g), and 1992 (23.6 lg/g). The range of Pb concentrations are equivalent to a molar ratio (Pb/Ca) from 2.6 lmol/mol to 11.3 lmol/mol with an annual average of 5.0 lmol/mol, which is one of the highest concentrations recorded in scleractinian corals like in Porites evermanni (9.5 lg/g) from Hawaii (Miao et al. 2001), Siderastrea radians (32 lg/g) from the Caribbean of Central America (Guzma´n and Jime´nez 1992), and in Montastraea annularis (62.4 lg/g) from the VRS (HortaPuga and Ramı´rez-Palacios 1996). For the VRS there are some other records of high environmental concentrations of Pb, up to 42 lg/g in inter-reef surface sediments (Rosales-Hoz et al. 2007), 179 lg/L in rainwater in the city of Veracruz (Albert and Badillo 1991), and more recently 0.2 lg/g in macroalgae (Horta-Puga et al. 2013). Hence, our results are evidence that the SGM has been polluted by Pb at least during the last two centuries, just like some other reef areas that have been documented to be strongly influenced by anthropogenic activities, like the Caribbean coast of Central America (Guzma´n and Jime´nez 1992), Hawaii (Miao et al. 2001), and Bermuda (Prouty et al. 2013).
Bull Environ Contam Toxicol
Fig. 1 Veracruz Reef System, southern Gulf of Mexico. Collection site marked with a black square
Fig. 2 Historical Pb concentration in O. faveolata from the VRS, southern Gulf of Mexico (black line), and in the sclerosponge Ceratoporella nicholsoni (gray line) from the Bahamas (Lazareth et al. 2000)
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The concentration of Pb in the annual bands of O. faveolata shows a general increasing trend since the middle of the nineteenth century to the end of the record at the beginning of the twenty-first century, as Lazareth et al. (2000) found in sclerosponges from Bahamas (Fig. 2), and as also has been recorded in the tropical western Atlantic area (Shen and Boyle 1987; Medina-Elizalde et al. 2002; Desenfant et al. 2006; Kelly et al. 2009; Prouty et al. 2013). And, as in other regions of the tropical western Atlantic, as will be discussed below, the higher Pb environmental concentrations in the SGM could be attributed to: (1) the atmospheric dispersion of the aerosols generated during the smelting of lead ores for the production of metallic Pb and other non-ferrous metals for the industry in the nineteenth and early twentieth centuries (Murozumi et al. 1969; Nriagu 1989; Lima et al. 2005); and (2) the massive use of alkyl-lead gasoline in continental Mexico since the 1940s until when it phased-out during the 1990s (Wu and Boyle 1997; Medina-Elizalde et al. 2002). In the nineteenth century the Upper Mississippi Valley region was the main producer of Pb, and the main source of Pb into the environment in North America (Lima et al. 2005). In that period, the efficiency in the recovery of fumes and dusts (Pb-aerosols) during the smelting and refining of ores and scraps in the silver and lead processing plants were low. The loss of Pb from ores was estimated to be *2 % during the 1750–1880 period, which was reduced gradually to *0.5 % in 1920, 0.06 % in 1960, and less than 0.0034 % in recent times (Murozumi et al. 1969; USEPA 1998). Winds can transport Pb-aerosols far away from their source regions and, particularly, in the SGM during the winter season up to 30 events of high-pressure systems occur, which move large masses of cold air from the NW to the SE, across the continental USA and into the Gulf of Mexico (Hsu 1988; Carrillo et al. 2007), and this could be the mechanism responsible for transporting Pbrich aerosols originated in continental North America into the SGM. Lima et al. (2005) demonstrated that the concentration of environmental Pb in eastern North America increased after 1840s due mainly to ore processing. In the middle nineteenth century the annual industrial Pb production was \25 tons 9103 in the 1850–1870 period, and \100 tons 9103 by 1880 (Shen and Boyle 1987). In spite of that Pb production was ten times lower than in the early twentieth century, as the recovery of dust and fumes was very low at this period, it is possible that large quantities of Pb-aerosols were transported south into the SGM, and also west to Bermuda and Bahamas by the mid-latitude westerlies (Shen and Boyle 1987), contributing to the increase in the environmental concentration of Pb in the tropical western Atlantic during the middle nineteenth century (Fig. 2), as is evidenced by the observed concentration peaks in the coral record from the SGM (1860s),
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Fig. 3 Historical Pb concentration in O. faveolata from the VRS (black line), and the US Pb industrial production (gray line) for the 1900–1940 period (USGS 2008)
and the sclerosponge record (early 1870s) from the Bahamas (Lazareth et al. 2000). Soon after, by the 1880s, the release of industrial Pb-rich aerosols decreased (Murozumi et al. 1969), due to more efficient recovery processes, with the concomitant reduction of the environmental levels of Pb, as also can be seen in Fig. 2. However, no further evidence supports this assumption. By the 1890s the coral Pb concentrations began to increase until the 1920s, with a second peak in 1924, and soon after concentrations decrease to lower values in the early 1940s. This pattern also seems to be associated to the Pb production in the USA. Figure 3 shows the US Pb production (USGS 2008) and the Pb coral time-series in O. faveolata from the VRS, for the 1900–1940 period. The correlation between both time-series is high (r = 0.73, p \ 0.001) allowing a 1-year lag in the coral record, in order to compensate for the timelag existing between the emission of Pb-aerosols into the atmosphere, and the time they reach the SGM to become recorded in the coral skeletal growth bands. Moreover, both series show a similar trend of variation with a peak in the 1920s decade, although not in the same year, that also was recorded with slight timing differences at each locality, in the Pb time-series in corals from Bermuda, and Puerto Rico and Venezuela (Shen and Boyle 1987; Reuer et al. 2003; Desenfant et al. 2006; Kelly et al. 2009), and sclerosponges from Bahamas (Lazareth et al. 2000). Thus, evidencing a cause-effect relationship between the source (Pb-aerosols emitted in continental North America) and the coral concentration record of Pb for this time period in the SGM. By the middle twentieth century Pb-aerosol emissions from industrial primary lead production became negligible, and alkyl-lead gasoline arose as the main source of Pb into the environment (Murozumi et al. 1969; Wu and Boyle 1997). Figure 4 shows the annual alkyl-lead gasoline production in Mexico, the nearest continental area, for the
Bull Environ Contam Toxicol Fig. 4 Historical Pb concentration in O. faveolata from the VRS (black line); historical Pb/Ca molar ratios (gray line) in Orbicella annularis from the Mexican Caribbean (Medina-Elizalde et al. 2002); and Mexican production of leaded gasoline (open triangles) for the 1938–1997 period (INEGI 2010)
1938–1997 period (INEGI 2010), and the Pb record from the VRS. Although the coral Pb record has high variability, 1r = 4.7 lg/g, which is threefold higher than the variability (1r = 1.4 lg/g) for the previous period (1855–1939), it follows the same increasing trend until the phase-out of alkyl-lead gasoline in Mexico, when both variables began to decrease, just like the coral Pb record from the Mexican Caribbean (Medina-Elizalde et al. 2002). The maximum concentration of Pb in the coral was recorded just 1 year after the peak of production of alkyllead gasoline in Mexico in 1991, which suggests a time lag between the timing of the release of Pb to the atmosphere and its incorporation into the coral skeleton. Correlation between both time-series is high (r = 0.86, p \ 0.001, simple linear correlation analysis), so, it can be concluded that the combustion of alkyl-lead gasoline in Mexico was the main source of Pb in the SGM during the 1938–1997 year period, as also was recorded in lacustrine sediments from the lake Verde (*18°360 N, 95°200 W), some 110 km to the southeast of the VRS (Soto-Jime´nez et al. 2006). Also it allows explaining the timing differences of the different Pb peaks observed at different localities of the tropical western Atlantic subjected to equivalent influences of the alkyl-lead gasoline consumption in North America (Shen and Boyle 1987; Lazareth et al. 2000; Desenfant et al. 2006; Kelly et al. 2009). Although the input of Pb into the environment from the combustion of alkyl-lead gasoline increased steadily through time, the coral Pb record shows some periods of declining concentrations, which depart from the expected increasing trend. In the case of the VRS, the Jamapa River is a major source of suspended solids (Carriquiry and Horta-Puga 2010). Terrigenous sediments from the watershed, along with previously deposited Pb-aerosols derived
from alkyl-lead gasoline (Soto-Jime´nez et al. 2006), are carried to the coastal zone (Horta-Puga et al. 2013). As river flow volume vary concomitantly with precipitation, environmental Pb levels in the VRS tend to decrease on those years with lower precipitation rates (Fig. 5). This process helps to explain some of the high variability and departures from the increasing trend observed in the geochemical record.
Conclusions This is the first study of the history of annual Pb pollution in the SGM, including the last two centuries (1855–2001), recovered by analyzing a core of the hermatypic coral O. faveolata from the VRS, one of the longest records for the tropical western Atlantic. The concentrations of Pb along the entire coral record are higher than those reported from other coastal localities, and have been steadily increasing, at least since the middle nineteenth century, evidencing that the coastal area of the SGM has been a highly polluted environment. The main sources of Pb in the SGM have been the aerosols derived from the industrial production of lead in North America, and the combustion of leaded gasoline in Mexico. These aerosols, which are atmospherically dispersed, have been contributing during the last two centuries to increase the concentration of Pb above natural background levels in the SGM, and have imprinted its signature in the geochemical record of corals in the VRS, as is evidenced by the high statistical correlation among time-series. However, in order to determine the very source of Pb in the SGM, it will be necessary to study the Pb isotopic composition, and the time of its changes. Due to the enforcement of environmental regulations, like
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Bull Environ Contam Toxicol Fig. 5 Historical Pb concentration in O. faveolata from the VRS (black line); and total annual precipitation in the VRS area (gray line), for the 1940–1997 period (unpublished data from Centro de Prevision del Golfo de Mexico, Comision Nacional del Agua)
the ban of the use of alkyl-lead gasoline, and to more efficient processes for the recovery of dust and fumes with Pb-aerosols in the ore processing and metallurgical industries, the environmental concentrations of Pb have been decreasing in the last two decades, as also is evidenced for the SGM by our results, and from the study of Pb concentrations in macroalgae from the VRS (Horta-Puga et al. 2013). Finally, hermatypic corals from the SGM, like those from other geographical areas, have proven to be very useful recorders of past environmental conditions allowing the temporal and spatial reconstruction of pollution, their sources, and fate of Pb in the ocean. Acknowledgments We greatly thank the help of Julio VillaescusaCelaya and Pedro Castro (UABC, IIO) for their support at all the stages of laboratory work. We also acknowledge the fieldwork support provided by the ‘Acuario de Veracruz’, especially to Alberto ´ ngel Roma´n. Also thanks to Claire Lazareth for Rı´os and Miguel A providing us the sclerosponge Pb time-series data. The comments of two anonymous reviewers contributed to improve this manuscript. This study was financed in part by a Grant from the National Council of Science and Technology of Mexico (CONACYT: 46814) to JD Carriquiry; and a scholarship from PASPA-DGAPA-UNAM to G Horta-Puga.
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