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

Silicon as a permanent-carbon sedimentation tracer

A procedure to quantify permanent carbon (C) sedimentation rates was required to compare these rates to methane (CH4) and carbon dioxide (CO2) water–air emission rates measured during reservoir C flux studies. Therefore, a new method to estimate C burial rates using silicon (Si) as a tracer was devised and applied. Burial rates in 8 tropical reservoirs were measured. Ages of these 8 reservoirs varied between 3.7 and 49 years. Each reservoir was surveyed 3 times during 1 year. Median burial rate was 78 (min 12, max 516; n = 66) mg C m-2 d-1. Trapped C (Ct) rates were also measured; the resulting median was 845 mg C m-2 d-1 (min 179, max 19 064; n = 40). Burial efficiency (comparison between C burial rate and Ct rate) was ~10%. Carbon burial efficiency of the 8 reservoirs showed strong dependence on bottom water temperature, efficiency being halved for each 3.4 °C increase in annual average temperature of reservoir bottom water. This finding strongly supported the adequacy of the Si-tracer method for rate measurements of carbon burial in sediments. Simultaneous with our new Si-tracer method we conducted traditional lead 210 isotope (210Pb) dating. The resulting median was 133 (min 11, max 441; n = 15) mg C m-2 d-1. Compared to the Si-tracer median, the 210Pb-dating technique resulted in a higher C median burial rate because the sampling sites that lacked sediment (and therefore contributed a null burial rate) were, in retrospect, erroneously disregarded

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

Especiação de metais-traço em sedimentos de florestas de manguezais com Avicennia e Rhizophora Metal speciation under Rhizophora and Avicennia mangles

<abstract language="eng">Speciation studies of Fe, Cr, Co, Ni and Cu on reactive fraction (adsorved on oxides, hydroxides, carbonates and clay minerals) and pyrite were performed in Avicennia schaueriana and Rhizophora mangle sediments from Amapá shoreline-Brazil. The soil under Avicennia showed a higher heavy metal concentration in reactive fraction than under Rhizophora. The soil under Rhizophora showed low heavy metal bioavailability, having an increasing association with pyrite across sediment section

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