25th International Meeting on Organic Geochemistry IMOG 2011

P-456
Post-glacial paleoclimatic changes in northeastern Brazil
inferred from depth-sensitive depositional records of organic
matter in sediments of Lagoa do Caçó
Abdelfettah Sifeddine 1,2 , Philip Meyers 3 , Renato Campello Cordeiro 2 , Ana Luiza
Spadano Albuquerque 2 , Marcello Bernardes 2 , Marie-Pierre Ledru 1 , Bruno Turcq 1 , Jorge
Joao Abrao 2
1 Institut de Recherche pour le Développement, France (IRD), LOCEAN, UMR 7159 CNRS-IRD-Univ. P. & M.
Curie-MNHN), Bondy, France, 2 Departamento de Geoquímica, Universidade Federal Fluminense, Niteroi,
Brazil, 3 Department of Geological Sciences, The University of Michigan, Ann Arbor, United States of
America (corresponding author:pameyers@umich.edu)
Elemental and isotopic compositions of organic matter
in surficial sediments from five transects across
Lagoa do Caçó (Brazil) have been analyzed to
identify the depth-related processes that affect the
production, preservation, and deposition of
sedimentary organic matter in this shallow tropical
lake. Each of four transverse transects begins at a
margin dominated by aquatic macrophytes
(Eleocharis), crosses the central deep part of the lake,
and finishes in the opposite macrophyte-dominated
margin. In each transect, TOC concentrations, C/N
ratios, and � 13 C values decrease between 0 to 4 m,
whereas � 15 N values increase. The parameters
remain stable in sediment from water depths from 4 m
to the center of the lake at 10 m. The depth-related
patterns reflect differences in both the delivery and
the deposition of organic matter in the lake. Organic
matter is abundantly produced in the marginal area by
emersed and submerged macrophyte vegetation that
diminishes with depth and disappears at 4 meters.
After the disappearance of macrophytes, organic
matter is principally produced at low rates by openlake
phytoplankton. Drawdown of dissolved oxygen is
high in the lake margins, but it is low in the
oligotrophic open waters of the lake. Preservation of
organic matter is consequently better in sediments of
the lake margins than in deep waters. The depthrelated
pattern of organic matter delivery and
deposition in the sediments of Lagoa do Caçó, in
which water levels are sensitive to groundwater
fluctuations, was combined with pollen data to
reconstruct changes in the post-glacial
paleohydrology of this lake system. A sediment core
from the lake margin and another from the center of
the lake record different histories of organic matter
accumulation that reflect changes in lake level over
the last 21 kyrs. At the end of the last glacial
maximum, a predominantly dry regional climate was
interrupted by short humid phases as reflected by a
succession of very thin layers of sand and organic
matter. The subsequent late glacial climate was
relatively wet and included two rapid lake-level
increases accompanied by forest expansion. The two
moist phases were separated by a phase where the
lake level remained stable and a cool climate
―Podocarpus‖ forest developed. These humid climate
periods differed significantly from the presently warm
tropical conditions. We conclude that post-glacial to
Holocene climate moisture has been controlled by
intensifications or positional shifts of the Intertropical
Convergence Zone that affected lake levels, which in
turn have left an imprint on the elemental and isotopic
compositions of sediment organic matter.
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