Mineralogical and geochemical influences on sediment color of Amazon wetlands analyzed by visible spectrophotometry

Based on sedimentological and geochemical data, this work relates spectrophotometric measurements with sediment composition and its application in palaeoecological studies of Amazon wetlands. The CIELAB values are directly related to mineralogical and chemical composition, mostly involving quartz, iron oxyhydroxides and sulfides (e.g. pyrite), and total organic carbon. Total organic carbon contents between 0.4-1%, 1-2%, 3-5% and 15-40% were related to L* (lightness) data of 27, 26-15, 7-10 and 7 or less, respectively. The CIELAB values of a deposit in Maraba, Para, were proportional to variations in quartz and total organic carbon contents, but changes in zones of similar color, mainly in the +a* (red) and +b* (yellow) values of deposits in Calcoene, Amapa and Soure, Para, indicate a close relationship between total organic carbon content and iron oxyhydroxides and sulfides. Furthermore, the Q7/4 diagram (ratio between the % reflectance value at 700 nm to that at 400 nm, coupled with L*) indicated iron-rich sediments in the bioturbated mud facies of the Amapa deposit, bioturbated mud and bioturbated sand facies of Soure deposit, and cross-laminated sand and massive sand facies of the Maraba core. Also, organic-rich sediments were found in the bioturbated mud facies of the Amapa deposit, lenticular heterolithic and bioturbated mud facies of the Soure deposit, and laminated mud and peat facies of the Maraba deposit. At the Maraba site, the data suggest an autochthonous influence with peat formation. The coastal wetland sites at Marajo and Amapa represent the development of a typical tidal flat setting with sulfide and iron oxyhydroxides formation during alternated flooding and drying.