Procjena prošlih promjena u okolišu proučavanjem sedimenata u hiperslanoj laguni

Carbon and oxygen isotopes, organic matter elemental composition, and mineralogy of carbonates were used as proxies to investigate environmental changes recorded in the carbonate-rich sediment of the hypersaline lagoon "Vermelha", Rio de Janeiro. Isotopic and other data suggest that two striking shifts in environmental conditions occurred about 1900 and 3200 years ago. Carbonates δ(18O) and δ(13C) were found in the range of +1.5‰ to +4.5 ‰ (PDB), and –2 ‰ to –11‰ (PDB), respectively. In organic matter, δ(13C) ranged from –22 ‰ to –15 ‰ (PDB). Calcites and dolomites are present in the studied core; their relative abundance seems to be related to the observed changes. Calcites are enriched up 38 % in MgCO3 (mole fraction, x). As distinct from calcites with lower magnesium contents, Mg-calcite with x(Mg) > 20 % shows an additional peak at 0.2929 nm in the X-ray diffractogram. Samples that contain Mg-calcite or 100 % dolomite show similar δ(13C) values. There are strong indications for an early formation of dolomite already in the microbial mat region. 18O and 2H determinations in lagoon water confirm evaporation as the water balance controlling processU radu je opisana uporaba izotopa ugljika i kisika, elementnog sastava organske tvari i mineralogije karbonata kao faktora u istraživanju promjena na okolišu utvrđenih u karbonatom bogatim sedimentima hiperslane lagune "Vermelha" kod Rio de Janeira u Brazilu. Izotopski i drugi podaci ukazuju na dvije značajne promjene okolišnih uvjeta, koje su se dogodile prije 1900 odnosno 3200 godina. U karbonatima, vrijednosti δ(18O) i δ(13C) izmjerene su u rasponu od +1,5 ‰ do +4,5 ‰, odnosno od –2 ‰ do –11 ‰ (standard Pee Dee Belemnite – PDB). Kod organske tvari, vrijednost δ(13C) varira u rasponu od –22 ‰ do –15 ‰ (PDB). U istraživanoj su sedimentnoj jezgri utvrđeni kalcit i dolomit, a njihova relativna zastupljenost povezana je s utvrđenim okoliš nim promjenama. Kalciti sadrže do 38 % MgCO3 (molni udjeli, x). Za razliku od kalcita s nižim sadržajem magnezija, magnezijev kalcit (x(Mg) > 20 %) pokazuje dodatnu liniju na rendgenskom difraktogramu kod 2929 nm. Uzorci koji sadrže magnezijev kalcit ili čisti dolomit pokazuju slične vrijednosti δ(13C). Postoje snažne indicije za stvaranje dolomita već u području mikrobnih pokrova. Vrijednosti 18O i 2H u vodi lagune ukazuju na to da je isparavanje proces koji kontrolira bilancu vode u sustavu

Sediment mixing and accumulation in a mangrove ecosystem: Evidence from 210Pb, 234Th and 7Be.

210Pb, 234Th and 7Be activities were measured to establish sediment accumulation rates, estimate sediment mixing rates, and determine the depth of the sediment mixed layer in the Sepetiba Bay mangrove ecosystem near Rio de Janeiro City, Brazil. Three sediment cores were collected from Enseada das Garças, a typical exposed tidal flat region with a sequence of sedimentary features. The seaward edge of this sequence is a mud flat with the landward portion covered with Spartina alterniflora followed by mangrove vegetation. An additional core was collected on an overwash island near Barra de Guaratiba, which is cover with mangroves without a mud flat or Spartina alterniflora sequence. Sediment accumulation rates were determined to range up to 1.8 cm/yr with the Spartina alterniflora having the maximum rate. Mixing rates were estimated for the Spartina alterniflora core at 40 cm2/yr based on 210Pb and 7Be from the upper mixed region of the core. The 234Th activity in this core suggested that either mixing or the input of 234Th were not in steady state. The sediment mixed region depth ranged from 4 cm to greater than 30 cm. At the Enseada das Garças site the mixing depth decreased in the landward direction (i.e. mud flats \u3e 30 cm, Spartina alterniflora 11 cm, mangroves 4 cm). Along with this decrease in sediment mixing depth was a shift from physical to biological mixing. The Barra de Guaratiba core had a sediment mixed layer of 13 cm as a result of physical and intense biological activity

Sediment accumulation rates along the inner eastern Brazilian continental shelf.

Box cores were collected close to river mouths along the eastern Brazilian shelf at water depths of 10-30 m. One core was taken from more than 1000 m depth at the shelf slope. 210Pb and 226Ra activities were measured to establish sediment accumulation rates. Seven of the 10 cores exhibited an exponential decrease with depth of excess 210Pb activities. The sediments from the sheltered Sudeste Channel off Caravelas revealed the highest sediment accumulation rate of 0.81 cm yr-1. The sediments at the shelf slope seaward of the Rio Doce revealed the lowest accumulation rate of 0.13 cm yr-1. Sediment accumulation rates increased towards the Caravelas Bank. Current patterns and the morphology of the seabed favor sediment deposition in this area

A historical evaluation of anthropogenic impact in coastal ecosystems by geochemical signatures

Anthropogenic activities in Brazilian coastal areas were successfully used as a support for 210Pb dating (Guarapina Lagoon, Marambaia Sound and data previously published on Guaratuba Bay). The calculated 210Pb sedimentation rates were 1.4 ± 0.27, 6.1 ± 0.54 and 4.7 ± 0.21 mm/year, respectively. In the Guarapina Lagoon, an increase in the organic matter concentration was well correlated with a construction of an artificial channel in the 1950\u27s. In Marambaia Sound, temporal changes in grain-size and heavy metal concentrations corresponded to hydrological diversions in the 1950\u27s and the beginning of industrial activities since the 1960\u27s. In Guaratuba Bay, a 1963 137Cs fallout peak was found and Hg fluxes throughout the sediment profile that were correlated with human activities within the drainage basin, such as building of reservoirs and intensification of agricultural practices since the 1960\u27s

Recent sediment accumulation in a mangrove forest and its relevance to local sea-level rise (Ilha Grande, Brazil).

An accumulation rate in a well-developed mangrove forest has been associated with relative sea-level rise on an island off the coast of Rio de Janeiro State. This rate was calculated by 210Pb dating models from a single sediment core. Results indicate an accumulation rate of approximately 1.7 mm/y for the past approximately 100 years. This rate is almost identical to the ongoing eustatic mean rise in global sea level, indicating a tectonically stable mangrove habitat. Organic C (OC), total N, δ13C(OC), and δ15N values from selected core intervals suggest a constant source of accumulating vegetal debris, dominated by C3-type vegetation with insignificant input of marine-derived organic matter, and a stable subaerial mangal habitat

Baseline Studies on Water and Sediments in the Copper Mining Region of Salobo-3A, Carajas: Amazon, Brazil

A survey was performed in the hydrological system of the Salobo 3A region, in the Carajás Mining Province, to obtain baseline data on the water chemistry, on the copper concentration and copper speciation, and to evaluate the first environmental impacts derived from anthropogenic activities. Measurements included copper concentration and speciation and a number of essential parameters to characterize water and sediment matrices. The natural background Cu concentration in Salobo waters is around 50-60 nmol L-1 while in the vicinities of the mining complex it reaches more than 300 nmol L-1. In sediments, concentrations ranged between 61 to 1.744 mug g-1. The mining activities at pilot scale resulted in substantial change in the copper distribution between particulate and dissolved phases, as well as in the dissolved copper speciation. Concentration of major water components is low compared to North America and world river average

Recent 137Cs deposition in sediments of Admiralty Bay

Cesium-137, radium-226 and lead-210 profiles of a 25 cm sediment core give an indication of recent changes in land–ocean interactions at a polar coastal environment (Admiralty Bay, King George Island, Antarctica). The linear sedimentation accumulation rate at the study site calculated from the unsupported 210Pb profile was 6.7 mm/year from 1965 to 2005. A 3.5-fold increase in 137Cs concentrations was observed in the top layer of this sediment core. This sharp increase seems to indicate a recent redistribution of fallout radionuclides previously deposited on soil, vegetation and snow. These results imply enhanced land–ocean interactions at this site likely as a result of climate change. Because our results are based on a single core, additional investigations are needed to confirm our observations