ORGANIC CARBON DEPOSITION IN THE PARAGUAY RIVER FLOODPLAIN DURING THE MID-HOLOCENE

Abstract

The deposition of organic carbon during the Holocene in the wetlands of Pantanal was analyzed through sediment cores obtained from the lakes Negra and Castelo, located on the west border of Pantanal, Mato Grosso do Sul state, Brazil. The sediments in these lakes hold information about the environmental history of Pantanal since the Late Pleistocene. The cores were collected with a vibro-coring device (two long cores) and with a gravity-coring device (two short cores). The sediments were dated either by 14C analysis (holocenic sediments) or 210Pb analysis (sediments of the last 100 years). Loss-on-ignition analysis of organic matter and CHN Carlo Erba analysis of organic carbon suggested a greater deposition of organic carbon during mid-Holocene (6500 cal years B.P.) in all samples. According to the core from Negra lake, the sedimentation started at about 11500 cal years B.P., under strong influence of the Paraguay river. The sediment analysis of the core from Castelo Lake indicates that the lake remains linked to Paraguay river to the present days. An abrupt transition from sandy inorganic sediments to organic sediments (initiation of lacustrine sedimentation) in Castelo lake took place about 6500 cal years B.P. These facts, in addition to age inversions observed within this period, suggest that the deposition organic matter was preceded by an erosive period caused by an increment in humidity. This was not observed with Negra lake, reinforcing the hypothesis that marginal levees were constructed in an earlier period. The Negra lake has not dried up completely for a long time, thus preserving organic matter deposits of early to mid-Holocene.