Lacustrine Organic Matter Elemental and Isotopic Compositions as Markers of Environmental and Paleoenvironmental Changes: Examples from Lagoa do Caco (Maranhao State, Brazil)

Organic matter is important to the reconstruction of paleoenvironmental changes from lacustrine sediments. Organic matter and its allochtonous and autochtonous fractions provide information about the evolution of ecosystems in the lake catchment and in the sedimentation basin and about physical and chemical water column conditions. Most studies that use organic matter as a marker of paleoenvironmental changes have been limited to relative descriptions of lake evolution because they lack calibration to modern conditions. We present the results of our study of modern sedimentation of organic matter in Lagoa do Caco and their application to reconstruction of late-glacial environmental changes in northeastern Brazil. We measured C/N ratios and carbon and nitrogen isotopic compositions of organic matter in surficial sediments collected along four transverse transects and one longitudinal transect of this lake. Each transverse profile starts from a margin dominated by emergent macrophytes, crosses the central part of the lake, and finishes on the opposite margin. The elemental and isotopic results characterise the different depth-related lake environments and enable interpretation of variations in sediment bulk organic matter and its properties. C/N and del 13C values decrease and del 15N values increase from the lake edge to 4m water depth before stabilizing as algae replace macrophytes as the predominant sources of organic matter. We applied these results to reconstruct a 20-ky sediment-core history of lake level changes based on organic matter properties that reflects the evolution of regional late-glacial and Holocene climate.