Biogeochemical Cycling and Authigenic Mineral Formation in Amazon Shelf Sediments

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9422470 Savin The Amazon River delivers about 1.2 billion metric tons of sediment annually to the eastern tropical continental shelf of South America. The potentially reactive layer of amorphous iron, manganese, and aluminum oxides coating much of this sediment is exposed on the continental shelf to seawater, mixed with labile organic matter derived from marine plankton, and then deposited in shelf sediments where it undergoes a variety of diagenetic (burial-related) biogeochemical reactions. Having documented a variety of such diagenetic processes in their past work, the PIs will now look in more detail at the variety of minerals in Amazon Shelf deposits and attempt to distinguish those of continental origin from those which have been formed in situ by diagenetic reaction. Mineralogical analysis will involve transmission electron microscopy, X-ray diffraction, and electron microprobe to determine particle structures and composition -especially to newly-precipated minerals from those of continental origin and to document the formation and/or dissolution of surface coating over originally deposited particles. Oxygen, strontium, and boron isotope analyses will be used help establish the origin (continental or marine) of the variuos mineral phases. This research is geophysically provocative for two main reason. Some authigenic Amazon Shelf minerals like siderite (a reduced iron carbonate) are found in ancient sedimentary deposits whose paleoenvironmental characteristics are unclear and have long been in dispute; determining the origin of such minerals in present-day deposits could help solve such problems. Secondly, the Amazon River is a major contributor of fresh water, solutes, and sediments to the world ocean; thus understanding the fate of materials delivered to its delta is important for understanding a large part of global marine geochemistry.