HR: 0800h
AN: GC51C-1062    [Abstracts]
TI: Biogeochemical Indicators of Paleoenvironmental Changes in a Humid Region of Amazonia (Lagoa da Pata, Sao Gabriel da Cachoeira)
AU: * Cordeiro, R C
EM: rccordeiro@geoq.uff.br
AF: Departamento de Geoquimica Universidade Federal Fluminense, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AU: Turcq, B
EM: bturcq@geoq.uff.br
AF: IRD, Programa de Geoquimica, UFF, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AU: Mendes, L G
EM: leonmendes@ig.com.br
AF: Departamento de Geoquimica Universidade Federal Fluminense, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AU: Azeredo, J B
EM: jazeredo@ig.com.br
AF: Departamento de Geoquimica Universidade Federal Fluminense, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AU: Moura, M S
EM: monikemoura@ig.com.br
AF: Departamento de Geoquimica Universidade Federal Fluminense, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AU: de Oliveira, A R
EM: apaulixtinha@terra.com.br
AF: Departamento de Geoquimica Universidade Federal Fluminense, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AU: Monteiro, F
EM: fabiofmsg@pop.com.br
AF: Departamento de Geoquimica Universidade Federal Fluminense, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AU: Santelli, R E
EM: santelli@geoq.uff.br
AF: Departamento de Geoquimica Universidade Federal Fluminense, Morro do valonguinho s/n, Niteroi, RJ 24020-007 Brazil
AB: Analysis of four sediment core collected in Lagoa da Pata lake show significant changes in the environmental history of Amazonia during the last 40,000 years. The Lagoa da Pata Lake (0ø17'S, 66ø40'W) is located on the top of the Morro dos Seis Lagos, an isolated hill in the extensive forested plane surface of high Rio Negro basin. The climate in the region presents mean annual precipitation around 3000 mm without dry season. Three distinct sections are clearly identified in the LPT III, LPT IV and LPT V cores. They consist of upper and lower organic-rich layers, separated by a clastic layer which represents a short period of rapid accumulation around 18 ka BP. Analysis of the organic matter composition by d13C and d15N, total Carbon and Nitrogen, sedimentary chlorophyll concentration and organic matter petrography show three different phases related to organic matter deposition. Between, at least 46,000 and 31,000 14C yr B.P., the lake presented a high water level attested by high accumulation rates of organic carbon. Values of d13C (around -32%) associated to high C/N ratio demonstrate a lignocellulosic material contribution in this phase; the second phase between the 31000 yr 14C B.P. and 18000 14C B.P. is characterised by a lower productivity attested by low sedimentary chlorophyll and total organic carbon values. The d13C values increase with medium values around -28% that may correspond to an input of organic matter rich in C4 plants (grasses). The sedimentation rate decreases significantly. High charcoal fluxes were observed in this period. At around 18000 14C yr BP, there appears to have been a sudden input of clastic material. This is represented by a sandy facies, which exhibits lower carbon contents. From 18000 14C yr B.P. to the present time occurred an increase in lacustrine productivity marked by an increase in carbon and chlorophyll derivate accumulation rates. The C/N values dropped indicting an algae organic matter contribution. It probably corresponds to a lake level rise increasing the production of the system. Increases in charcoal deposition were observed during the upper and medium Holocene. This project is part of the PROFIX/CNPq (processo 54086/01-5) and CNPq (Brazil)-IRD(France) project "PALEOTROPICA" .
DE: 9360 South America
DE: 9604 Cenozoic
DE: 3344 Paleoclimatology
DE: 1890 Wetlands
DE: 1040 Isotopic composition/chemistry
SC: Global Climate Change [GC]
MN: 2004 AGU Fall Meeting