THE LIGNIN GEOCHEMISTRY OF SEDIMENTARY AND AQUATIC HUMIC SUBSTANCES (FULVIC ACIDS, BRAZIL)

Lignins are structural polymers unique to vascular plants and serve as unambiguous terrestrial biomarkers. This dissertation describes the application of lignin analytical techniques to dissolved (freshwater) and sedimentary humic substances. Humic substances contain lignin components which are demonstrated to be source-specific and process-dependent geochemical tracers.; All humic substances from soils and coastal (but not pelagic) sediments have clearly recognizable lignin components, comprising up to 8% of the carbon. Lower yields of recognizable lignin were found in humic substances from soils than from sediments, apparently due to greater lignin degradation in aerobic, well-drained environments. Compositional patterns of lignin-derived phenols, including higher yields of acidic phenols and selective loss of methoxylated units, support this hypothesis and also suggest that, contrary to present theory, fulvic acids are diagenetically more altered than corresponding humic acids.; A major portion of the lignin in humic substances from coastal sediments is likely extracted from coexisting, chemically intact plant fragments. In fact, fresh plant material also yields significant levels of humic substances by classical extraction techniques. In recent (2000 yr) continental shelf sediments lignin and carbon levels in humic and fulvic acid fractions decrease with depth relative to bulk levels. Thus humic substances are not produced in sediments but appear to be geochemically labile.; Aquatic humic substances, particularly humic acid fractions, from small temperate streams and the Amazon River contain significant levels of lignins which are indicative of the dominant local vegetation. Over most of the Amazon River, dissolved humic substances behave conservatively. However, humic acids from the Rio Negro, a blackwater tributary, appear to adsorb onto suspended sediment below the confluence with the Amazon River. The annual flux of dissolved lignin in the Amazon River at Obidos is estimated to be 1.2 x 10('11) g/yr. The fulvic acid fraction, which represents half of the dissolved lignin flux, could be expected to impart a characteristic terrestrial signature to dissolved organic matter in the ocean.