Aqueous geochemistry of the rare earth elements in marine anoxic basins

Abstract

Life in the oceans mainly occurs in the upper tens of meters of the watercolumn, where sunlight penetrates. This sunlight is used by phytoplankton to combine carbon and nutrients to organic matter, which subsequently serves zooplankton and higher life forms as food. When the plankton dies it slowly settles to the seafloor, meanwhile being decomposed by bacteria. This decomposition requires oxygen, which is extracted directly from the surrounding seawater. If this oxygen would not be constantly replenished, the oceans would soon be completely devoid of oxygen (anoxic). Oxygen is replenished slowly by diffusion and more readily by advection i.e. by supply of oxygen-rich water. If advection is locally impeded, then mere diffusion is usually not sufficient to balance the consumption of oxygen and an anoxic basin may be formed. This is the case for instance in the Bannock Basin, eastern Mediterranean, where a volume of brine does not mix with the overlying seawater, and in the Black Sea, where seawater that is supplied at depth through the Bosporus mixes poorly with freshwater that is supplied at the surface by several major rivers. Very interesting is the behaviour of certain trace metals in the waters of anoxic basins, in particular at the interface between the oxygen-poor and the overlying oxygenrich waters. Manganese and iron are present in oxygen-rich waters mainly as poorly soluble oxides adsorbed onto particulate matter. When the particulate matter settles across the interface, manganese and iron are reduced to a valency that does not form poorly soluble oxides. As a result, the concentrations of dissolved manganese and iron strongly increase across the interface. This leads to upward diffusion of dissolved manganese and iron into the oxygen-rich waters, where they are once more oxidized and adsorbed onto particulate matter. This cycling closely resembles a process whereby dissolved trace metals are removed from the seawater in the upper part of the watercolumn by adsorption onto particulate matter and released at depth as the particulate matter is decomposed. This process, known as 'scavenging', is the major mechanism for the transport of trace metals from the seawater to the sediment and constitutes an important component of the cycling of trace metals in the oceans. Scavenging is difficult to study, since it occurs on a worldwide scale and on timescales of hundreds to thousands of years. By studying the cycling of trace metals at the interface between oxygen-poor and oxygen-rich waters in anoxic basins, a similar process that is however localized and occurs on considerably shorter timescales, much can indirectly be learned about scavenging. The rare earth elements or lanthanides are very well Suited for this purpose. Their cycling in the oceans seems to be governed by the same mechanisms that govern the cycling of many other trace metals and is in particular closely associated with the cycling of manganese. At the interface between oxygen-poor and oxygen-rich waters in anoxic basins they show a passive cycling that seems to be driven by the cycling of manganese and possibly also by that of iron. Moreover, the element cerium shows an active cycling that is caused by its own oxidation-reduction chemistry, a property that is unique within the rare earth element series. The rare earth elements form a chemically coherent group, yet their chemical properties are not completely the same. Consequently, the cycling of each rare earth element is subtly different from that of all others. Since the chemical properties of the rare earth elements depend in a gradual and more or less predictable way on atomic number, the mechanisms that govern the cycling of the rare earth elements in the ocean and at the interface between oxygen-poor and oxygen-rich waters in anoxic basins can be studied and described in terms of relative rather than absolute behaviour. In fact, the behaviour of the rare earth elements as a group forms a frame of reference for the behaviour of each rare earth element separately. By studying the cycling of the rare earth elements, information can indirectly be obtained about the cycling of trace metals like manganese and iron, for which such a frame of reference is not available