THE GLOBAL BERYLLIUM 10 CYCLE

The cosmogenic radionuclide Be-10 has generated much interest because of its potential as a tracer in the environment and applications to geology, archaeology, glaciology, and oceanography. Nevertheless, for Be-10 to be useful as a tool in the Earth sciences its geochemical cycle as outlined below needs to be understood more fully. Beryllium 10 (t1/2 = 1.5 x 10(6) years) is mainly produced in the atmosphere by spallation of oxygen and nitrogen induced by secondary neutrons formed by cosmic ray interactions with the atmosphere, but some is produced in situ on the surface of the Earth. Deposition of Be-10 onto the surface of the Earth depends primarily on precipitation. Deposited Be-10 is made up of several components, primarily Be-10 produced in the stratosphere and in the troposphere and Be-10 recycled from dust and soil particles, and secondarily Be-10 recycled from the ocean as hygroscopic nuclei and from cosmic dust. Even though paleoprecipitation dominated Be-10 deposition at any one location in the past, cosmic ray flux and major changes in the Earth's magnetic field also influenced Be-10 deposition. The Be-10 deposited on land will either be fixed in soils or be carried away in overland flow through the fluvial system, or locked in ice. Most of the beryllium is transported in the sediment load and that which stays in solution shows a strong pH dependence and is highly mobile in organic-rich continental waters. Beryllium 10 from sediments and river water is quickly deposited in the nearshore sediment along the coastlines along with a small amount of Be-10 that is released and dispersed to the deep sea. In the open sea, most of the beryllium is in solution and the rest resides on particulate matter, much of which is of biogenic origin. Beryllium 10 that is added to the sea may be scavenged by such particles, but as they settle out into deeper waters the organic matter may oxidize and calcareous organisms may slowly dissolve, releasing Be-10 back into solution, though fecal pellets may carry much of the Be-10 to the seafloor. Slow-growing manganese nodules absorb some Be-10 directly from the surrounding water, but pelagic and slope sediments act as the ultimate sinks for Be-10 as the residence time for Be-10 in the sediments approaches that of the mean life of Be-10, 2.18 x 10(6) years. Nevertheless, a small portion of Be-10 is subducted or accreted at the world's trenches, and it has been used as a tracer for the study of island-arc volcanism.