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Abstract: 210Pb 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. [Copyright &y& Elsevier]

210Pb geochronologies of Cd, Cu, Hg, and Pb fluxes were obtained from the intertidal mudflat sediments of the coastal lagoons Chiricahueto, Estero de UrIas, and Ohuira in the Mexican Pacific. The Cu and Hg sediment concentrations at the three lagoons fell within the ranges of 6-76 μg g-1 and 0.1 to 592 ng g-1, respectively; Chiricahueto and Esterb de UrIas sediments had comparable Cd and Pb concentrations within the ranges of 0.2-2.1 μg g-1 and 10-67 μg g-1, respectively; whereas in Ohuira lagoon, Cd concentrations were lower (0.1-0.5 μg g-1) and Pb concentrations were higher (115- 180 μg g-1) than in the other lagoons. The metal fluxes (μg cm-2 y-1) for the three lagoons fell within the ranges of 0.02-0.15 for Cd, 0.7-6.0 for Cu, 0.001-0.045 for Hg, and 0.7-20 for Pb. The Hg pollution in Estero de Urlas was attributed to the exhausts of the thenrioelectric plant of Mazatlan and the metal enrichment in Chiricahueto and Ohuira was related to the agrochemical wastes from the croplands surrounding these lagoons. [ABSTRACT FROM AUTHOR]

Abstract: Short-lived radionuclides (210Pb and 137Cs) were used to document sedimentation regime changes over the last ∼100 years, from analysis of a single sedimentary sequence collected in the coastal zone of the Gulf of Tehuantepec, South Pacific of Mexico. Sedimentation rates found in the core Tehua II-21 varied from 0.03 to 0.21cmyr−1, whereas mass accumulation rates ranged from 0.05 to 0.29gcm−2 yr−1. 137Cs data validate results obtained from 210Pb measurements and confirm significant changes in the sedimentation regime between 1950 and 1970. We associate these alterations to land use changes, including deforestation for agriculture development and industrialization, as well as the regimentation of the Tehuantepec River from the early 1960s to the late 1970s. This interval has been marked by a major demographic expansion that followed the industrial development of the area. [Copyright &y& Elsevier]

This article presents information related to component analysis of the metal pollution from urban wastes in the Culiacán river which is located in the Pacific coastal plain province of Mexico. Sediment core was characterized by an almost homogeneous content of sands (90%) and rather low contents of organic carbon (OC) and carbonates (0.3- 3.0 % and 0.1-3.1%, respectively). Aluminium and lithium contents showed rather small variations, indicating that sediments accumulated within the core had a similar source.

Six sediment push-cores were collected at a coastal lagoon system affected by urban and agriculture wastes. The sediments were analyzed for 228Th, 230Th, 232Th, 210Pb, 226Ra, and 137Cs. 137Cs activities were at background level for all samples. The 210Pbtot activities found in the area varied from 0.5 to 4.5 dpm g–1 with 210Pbsup levels ranging between 1.2–1.8 dpm g–1. Cores CHI and EPC showed flat profiles depleted of 210Pbxs, indicating the absence of recent sedimentation. Core CAI shows a flat 210Pbxs profile that seems to be bioturbated. Cores ERC and BRI show chaotic profiles with layers totally depleted in 210Pbxs, likely caused by resuspension triggered by storm conditions. High 228Th/232Th values observed at core ERC suggest that the resuspension event occurred less than 10 years ago. The contaminated sediment of the lagoon are frequently resuspended, re-oxygenated, and therefore the contaminating trace metals will continue to be easily remobilized in the food chain. [ABSTRACT FROM AUTHOR]

Abstract: Qualitative and quantitative analysis of recent organic-walled dinoflagellate cysts (dinocysts) was performed on surface sediment samples and a core from the continental shelf of the Gulf of Tehuantepec, Mexico, in order to document the spatial distribution of dinocyst assemblages in relation to upwelling and primary productivity, and to assess the environmental history of the last century. The analyses of surface sediment samples show a close relation between dinocyst assemblages and productivity on a regional scale. Polysphaeridium zoharyi and heterotrophic taxa (notably Brigantedinium spp.) dominate in the high productivity zone, whereas Spiniferites delicatus and other phototrophic taxa are more abundant in the lower productivity zone. Sediment in an eighteen cm long gravity core (dated using 210Pb and 137Cs) provided a record of the last century at annual to decadal resolution, thus yielding a unique opportunity to examine variations in dinocyst assemblages associated with environmental changes. Cyst concentrations in the core range between 477 and 2300 cysts g−1, giving cyst fluxes between 68 and 494 cysts cm−2 yr−1. Twenty-three phototrophic and heterotrophic cyst taxa were identified. Brigantedinium spp., P. zoharyi and Bitectatodinium spongium are dominant, and are associated with the seasonal upwelling that characterizes the area. Cysts of potentially toxic species such as P. zoharyi (the cyst of Pyrodinium bahamense var. compressum and/or bahamense) occur throughout the core. Despite slight variations in relative abundances of the taxa in the assemblages, there is no evidence for eutrophication following industrial development of the adjacent coastal zone. Core samples were also analyzed for benthic foraminiferal content in order to determine possible effects of high, upwelling-induced productivity on bottom water oxygenation. The benthic foraminiferal assemblages are dominated by Hanzawaia concentrica (over 50%), with less abundant Uvigerina excellens, Cancris spp., Planulina ornata, Quinqueloculina lamarckiana, Epistominella sandiegoensis, Nonionella basispinata, Cassidulina modeloensis and Textularia foliacea. The benthic foraminiferal assemblages are characteristic of oxygen concentrations above 1 ml l−1, indicating that possible changes in productivity did not significantly affect bottom water oxygen concentrations over the last 100 years. [Copyright &y& Elsevier]

This paper summarizes the geochemical investigations about the origin and loading history of some trace metals (Ag, Cu and Zn) and nutrients (N and P) in the coastal lagoon complex of Altata-Ensenada del Pabellón, Mexico, by using the radioactive chronometers 210Pb and 228Th and the stable isotopes of C and N. The examination of sediment cores collected at different locations in the lagoon system identified a slight enrichment in metals and nutrients in some points, which was mainly associated to organic matter accumulation. Stable C and N isotope ratios revealed wastewater inputs to the lagoon system and the 210Pb geochronology showed that anthropogenic impact started 50 years ago, with the beginning of the agriculture development and the associated urban growth of the surrounding area. Several atypical 210Pb and 228Th/232Th profiles demonstrated that biological and physical disturbances are common phenomena in these environments, that frequently mask the pollution records; and therefore, considering that the contaminated sediments at some locations in the lagoon system are frequently resuspended and re-oxygenated, the pollutants will continue to be easily remobilized in the food chain.

210Pb geochronology and sediment profiles of carbon, phosphorus and nitrogen were used to study time dependent changes in nutrients fluxes to Culiacan River Estuary. Results indicate that the release of urban sewage and agriculture wastes transported through Culiacan River has produced historically increased carbon, phosphorus and nitrogen fluxes to the study area. C:N:P elemental ratios showed that increments in the nutrients input begins simultaneously for C, N and P in 1948 with the clearing of the catchment for agriculture; although excess of nutrients input increased most importantly around the 1970s to roughly follow the rapidly growing population of Culiacan City. C/N ratios, delta13C and delta15N suggested that nutrient enrichment is mostly influenced by sewage delivered through Culiacan River.