Biogeochemistry of coastal marine sediments

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The interest in the environmental chemistry of metals stems from the fundamental importance of metals to living organisms and the realization that man's activities have altered the natural cycle of metals. Metals enter the environment via different pathways and are removed by deposition on the sea floor and burial. Before metals are buried so deep that they are removed permanently and become part of the geological record, they participate in chemical and biochemical reactions, mediated by the organisms that inhabit the sedimentary environment. This proposal will examine organism-sediment interactions in sediment where the dominant macro-organisms are either invertebrates or plants. The focus will be on metal-binding organic compounds produced by organisms, and the complexes they form with metal ions. These complexes have different solubility, mobility, availability, and toxicity than uncomplexed metals. Invertebrate organisms construct burrows and galleries, extending the contact between sediment and overlying water, and stimulating the exchange of complexed and uncomplexed metals between sediment and water. Intertidal vascular plants do not influence exchange of metals with the water column, but their hollow roots transport oxygen, which oxidizes reduced sediment. Root also release metal-binding organic acids. The growth and decay of roots produce profound changes in metal speciation and cause intense cycling of metals between sediment, pore water, and plant tissue. The proposed research will use electrochemical techniques in combination with a solid-state voltammetric microelectrode to determine concentrations, properties, and fluxes of metals and metal-complexes. The fact that invertebrates and roots are hidden from view is a major impediment to understanding organism-sediment interactions. We will visualize the distributions of roots and invertebrates with CAT-scan analysis of intact cores. The 3-D images will guide the positioning of microelectrodes and let us measure oxygen, sulfide, iron, and manganese near surfaces of burrow and roots. 3-D images of root distributions will let us estimate how root biomass and oxygen flux evolve during the growth season.