An overview of five decades of studies of cosmic ray produced nuclides in oceans

The concentrations of a large number of naturally occurring radioactive and stable nuclides, those produced in nuclear reactions caused by cosmic radiation and by energetic particles produced in radioactive disintegrations. and in nuclear decays of naturally occurring long-lived nuclides, have been measured in the past five decades in the marine environment. In many cases these nuclides serve as tracers for the study of physical, chemical and biological processes in the oceans. In this article, an overview of their sources and source functions is presented, and the potential and demonstrated usefulness of these nuclides with special emphasis on those produced by cosmic rays. The concentrations of cosmogenic nuclides in the oceans are nominally in the range of 10y5]10y1 dpmrl, with isotoperelement ratios in the range of 10y18]10y12. Major technical advances in the past two decades have allowed their measurements to be done fairly reliably. The data base on the distribution of these tracers in the oceans is therefore growing steadily. Several radiotracers successfully provide chronology of sediments, corals and manganese nodules, but learning about large scale ocean circulation is another matter. We discuss that merely our ability to make measurements of a tracer in the marine environment is not sufficient to use it as an effective tracer for delineating the important oceanic variables. Tracer data must be examined in terms of ocean models. Constructing ocean models is an iterative process between data acquisition and model building, forcing model outputs to become compatible with the observations. The oceanic processes are very complex, exhibiting significant spatial and temporal variability on a wide range of scales. For the tracer data to be useful in developing meaningful coupled atmosphere]ocean circulation models, which may be considered as the goal of tracer studies, one would require three-dimensional tracer data with sufficient resolution in the horizontal direction. The latter are not available, except for 14C, where a considerable data base is growing as a result of recent WOCE World Ocean Circulation Experiment. expeditions. It is gratifying to see that the cosmogenic radiotracer field is evolving from an academic curiosity in the 1950]1980s, to its present-day form wherein it aims to become an integral part of realistic atmosphere]ocean global circulation models.