Major environmental changes recorded by lacustrine sedimentary organic matter since the Last Glacial Maximum near the Equator (Lagoa do Caçó, NE Brazil).
Résumé
Sediment samples collected along a 6-m core, drilled in the deepest part of the Lagoa do Caçó (NE Brazil), have been investigated in order to determine source(s) and degradation conditions of the organic matter (OM) with special emphasis on paleoenvironmental implications. Bulk organic geochemistry (Rock-Eval pyrolysis, C/N determination, δ13C and δ15N measurement) and petrography combined with sedimentological evidence and radiocarbon dates allowed to identify four major intervals documenting major environmental changes that occurred during the last 20,000 years. The first interval, dating back to the end of the Last Glacial Maximum (LGM), contains well-preserved OM derived from higher plants. This material was most probably produced in an ephemeral palustrine system and rapidly buried by sands. This level is thought to have been deposited under relatively arid climate conditions associated with strong but episodic rainfalls. Between 19,240 and 17,250 Cal years BP, the climate appears to have been more humid and seasonality more pronounced as suggested by the presence of a permanent lake. After a drastic environmental change dating back to 17,250 Cal years BP, the sediment became truly lacustrine with restricted mineral input and highly degraded higher plant-derived organic matter. After that, a stepwise improvement in the preservation of OM occurred, as revealed by several pronounced shifts in the Rock-Eval TpS2 signal. These changes could document abrupt climatically driven changes during the Late Glacial. Finally, around 5610 Cal years BP, environmental conditions, approaching those prevailing today were established. Minor climatic changes during the Holocene were probably buffered by a high water table which might explain the lack of paleoenvironmental fluctuations.
Domaines
Géochimie
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