Accumulation of organic carbon in five Brazilian lakes during the Holocene

A comparative study was made of organic carbon accumulation in five lacustrine environments in Brazil (largge open water lake, ponds with floating meadows and marshes), The most representative cores were used to calculate the accumulation of total organic carbon (TOC) in the sediment over the past 12,000 cal years BP. Carbon accumulation rates were determined using the TOC concentration, sediment <<insitu>> density, C-14 calibrated ages and the interpolated sedimentation rate (cm year 1) or sediment accumulation rate (g m (-2) year (-1)). The C/N ratio and microscopic observations were used to characterise the sedimentary organic matter, Recent TOC accumulation in the lakes was estimated at 7-41 g m(-2) year (-1) by averaging the last 500 years. This variation roughly correlated with the present-day ratio of watershed area vs. lake area, indicating that nutrient input is important for the modem accumulation of organic matter. The accumulation rates were greatest in lakes with floating macrophytes. The largest lake (Dom Helvecio), with a strong thermal stratification, low primary production and low carbon accumulation, had relatively well preserved organic matter when its low watershed area/lake area ratio was considered. The general increase in carbon accumulation during the Holocene followed the expansion of lakes and the development of a wetter climate, due to a growing penetration of the Atlantic and Amazonian wet air mass over Brazil driven by insolation changes. This change in carbon accumulation was not controlled by the higher water and nutrient inputs alone, more organic matter was decomposed,when lake levels were low at the beginning of the Holocene. Some lakes were also influenced by higher inputs of allochthonous organic matter during the early and middle Holocene. These inputs included high concentrations of charcoal fragments. Lake morphology and its changes as the lake level rose, as well as the nature of primary producers, strongly influenced the carbon accumulation rates, Our understanding of how climate changes are linked to changes in the lacustrine environment should not be based on measuring organic matter alone, but requires a good description of the organic matter and other limnological parameters. (C) 2002 Elsevier Science B.V. All rights reserved.