Biogeochemical Processes in Amazon Shelf Sediments

Abstract

Each year the Amazon River delivers approximately 1 billion metric tons of sediment to the equatorial Atlantic. A portion of this debris is highly weathered and contains abundant reactive Fe, Mn, Al, and Si oxides. The associated terrestrial organic matter is biologically refractory. Upon entering the Amazon Shelf region, particles are exposed to seawater brine, mixed with labile planktonic organic matter, and incorporated into the seabed where a variety of geochemical reactions take place. Major research efforts within the AmasSeds Project include elucidating the types and rates of these diagenetic reactions, their governing factors, and their influence on the properties of overlying water and preserved deposits. Modern views of early diagenetic processes in shallow marine sediments have largely derived from studies of temperate estuaries and shelves, particularly along the coasts of North America and Europe. In these environments, sedimentary organic matter is decomposed through the general sequential series of biogeochemical oxidants (O2, NO3, Mn, Fe, SO4, CO2), although reactions are typically dominated over large vertical intervals by sulfate reduction or methanogenesis. Oxygen is often relegated to a largely indirect role as an oxidant of anaerobic metabolites. Other potential oxidants of organic matter such as NO3, Mn and Fe oxides, are of minor importance. The chemical properties of sediments and patterns of authigenic mineral formation in deposits reflect these reaction paths, as well as diagenetic transport regimes strongly influenced by the bioturbation activities of benthic organisms. In contrast, early studies of Amazon Shelf sediments demonstrated extensive zones of suboxic reactions associated with Fe,Mn reduction and oxidation cycles, and little evidence of macrobenthic activity. Unlike other shelf deposits of similar organic content, sulfate depletion is limited, dissolved sulfide is undetectable, and sulfide minerals are minor in the upper few meters. High concentrations of dissolved Fe2+ and Mn2+ in pore water (0.1-1 mM) are found throughout the upper 0.5-2 m of Amazon deposits compared with vertical intervals 0.01-0.05-m thick in many shelves. These suboxic zones extend across the Amazon Shelf for similar to 100 km, encompassing an area of at least 40,000 km2 off Brazil alone. Because Fe and Mn oxides and their reduction products are carrier phases for numerous minor and trace elements, their redox cycling can critically influence oceanic chemistry. The input of highly weathered debris and massive physical mobility/reworking of sediments are apparently critical factors determining these patterns.