Coastal vegetated habitats can be important sinks of organic carbon (C) and mitigate global warming by sequestering significant quantities of atmospheric CO and storing sedimentary C for long periods, although their C burial and storage capacity may be affected by on-going sea level rise and human intervention. Geochemical data from published Pb-dated sediment cores, collected from low-energy microtidal coastal wetlands in El Salvador (Jiquilisco Bay) and in Mexico (Salada Lagoon; Estero de Urias Lagoon; Sian Ka'an Biosphere Reserve) were revisited to assess temporal changes (within the last 100years) of C concentrations, storage and burial rates in tropical salt marshes under the influence of sea level rise and contrasting anthropization degree. Grain size distribution was used to identify hydrodynamic changes, and δC to distinguish terrigenous sediments from those accumulated under the influence of marine transgression. Although the accretion rate ranges in all sediment records were comparable, C concentrations (0.2-30%), stocks (30-465Mgha, by extrapolation to 1m depth), and burial rates (3-378gmyear) varied widely within and among the study areas. However, in most sites sea level rise decreased C concentrations and stocks in sediments, but increased C burial rates. Lower C concentrations were attributed to the input of reworked marine particles, which contribute with a lower amount of C than terrigenous sediments; whereas higher C burial rates were driven by higher mass accumulation rates, influenced by increased flooding and human interventions in the surroundings. C accumulation and long-term preservation in tropical salt marshes can be as high as in mangrove or temperate salt marsh areas and, besides the reduction of C stocks by ongoing sea level rise, the disturbance of the long-term buried C inventories might cause high CO releases, for which they must be protected as a part of climate change mitigation efforts.

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Extensive waste deposits (tailings) and ash from the ignition oven of the abandoned gold mine of mining district El Triunfo (MD-ET) in Baja California Sur, Mexico have released trace elements into the sediments of the Hondo-Las Gallinas-El Carrizal arroyo, which connects to the Pacific Ocean through an evaporitic basin. Migration of these elements through the arroyo is mainly caused by winds or tropical hurricanes that occur sporadically during the summer and cause the otherwise dry arroyo to overflow. To evaluate the concentration and distribution of the elements As, Hg, Pb, and Zn along the 48 km arroyo, surface sediments were collected from 26 sites, ranging from close to the MD-ET to the mouth of the arroyo at the Pacific Ocean. Concentrations in tailings and ash were for As 8890 and 505 000 mg kg(-1); for Hg 0.336 and 54.9 mg kg(-1); for Pb 92,700 and 19,300 mg kg(-1); and for Zn 49,600 and 1380 mg kg(-1). The average of the Normalized Enrichment Factor (Av-NEF) in surface sediments, calculated using background levels, indicates that the sediments are severely contaminated with As and Zn (Av-NEF = 22), Pb (Av-NEF = 24) and with a moderate contamination of Hg (Av-NEF = 7.5). The anthropogenic influence of those elements is reflected in the arroyo sediments as far as 18 km away from the MD-ET, whereas the samples closest to the discharge into the Pacific Ocean show a natural to moderate enrichment for As and Zn and low or no enrichment for Hg and Pb. A principal components analysis identified four principal components that explained 90% of the total variance. Factor 1 was characterized by a high positive contribution of the anthropogenic source elements, especially As, Pb, and Zn (37%), whereas Factor 2 was strongly correlated with the oxy-hydroxides of Fe and Mn (27%). Factor 3 was correlated with Li (16%) and Factor 4 with Al (10%), which indicates more than one source of lithogenic composition, though they played a minor role in the distribution of the elements.

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La causa principal de la contaminación en sistemas costeros proviene de las actividades humanas próximas al litoral marino o a los estuarios hacia donde, además, los ríos transportan los contaminantes. Así, en la costa gallega los procesos industriales han afectado a las rías aumentando la presencia de metales como se recoge en una reciente revisión (Prego y Cobelo-García 2003). Ocasionalmente los contaminantes pueden llegar desde el océano al litoral cuando ocurren accidentes en el mar principalmente en zonas de intenso tráfico marítimo de buques y en las áreas cercanas a las refinerías de petróleo (Clark 2001) tales como el corredor del Cabo Finisterre y la Ría de La Coruña en el noroeste de España (Ros 1996). Múltiples estudios publicados (Patin 1999) atienden a los efectos de los contaminantes orgánicos vertidos al medio marino por el transporte del petróleo y sus derivados. Entre ellos se encuentran ejemplos de accidentes ocurridos en la plataforma continental adyacente a La Coruña (Pastor et al. 2001). Sin embargo, hasta el momento se ha prestado escasa atención