Mineralogical and geochemical influences on sediment color of Amazon wetlands analyzed by the CIELAB system.

Descriptions of sedimentary facies, geochemical and mineralogical data of sediment cores were used to evaluate the difference in results between visual analysis based on HVC and instrumental analysis based on CIELAB to recognize sediment color. The relationship between instrumental color measurements and geochemical/mineralogical components was also established, to evaluate the applicability of this nondestructive technique in sedimentological studies of Amazon wetlands. The comparison between HVC and CIELAB systems revealed a generally biased visual perception. Additionally, the CIELAB data are directly related to mineralogical and chemical composition of sediments, including quartz, pyrite, iron oxyhydroxides and total organic carbon (TOC) content. The sedimentological interpretation suggests the alternation of flooded and waterlogged conditions in the wetland sediments. At the Maraba site, the data suggest an autochthonous influence with peat and pyrite formation. The coastal wetland sites at Marajo and Amapa represent the development of a typical tidal flat setting with pyrite and iron oxyhydroxids formation during extended flooding and prolonged exposure, respectively. Thus, color measurements by spectrophotometric analyses can more objectively discriminate among sediments within a core sequence than visual assessments/comparisons. Additionally, it is more readily and more inexpensively than other techniques, and the results may then be used in paleoenvironmental interpretation.