Croatica Chemica Acta, Vol. 79 No. 1, 2006.
Original scientific paper
Assessing Past Environmental Changes through Sediment Records in a Hypersaline Lagoon
Regina L. Santelli
Angela de L. R. Wagener
Klaus Wagener
Sambasiva Patchineelam
Abstract
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 process
Keywords
carbon isotopes; oxygen isotopes; climate change; Southeastern Brazil; Vermelha Lagoon
Hrčak ID:
2623
URI
Publication date:
22.4.2006.
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