I5 15 1 Introduction Geochemical Tracers have always been an important adjunct to physical oceanography The distribution of dissolved oxygen and to some extent of the nutrients nitrate phosphate and silica played a very important role in defining the major water masses of the ocean see Sverdrup Johnson and Fleming 1942 for a review of this subject Many attempts also have been made to harness the loss of dissolved oxygen from the water column as a measure of the rates of oceanic mixing processes e g Riley 1951 Wyrtki 1962 These latter pursuits however have been of only marginal success because of our lack of knowledge of the consumption rate of 02 within the sea The big breakthrough in geochemical tracing came after World War II with the discovery of the cosmicray produced isotopes 14C and 3H A further impetus to this field came with the realization in the mid 1950s that the ocean was receiving significant amounts of Tracers and Ocean Circulation W S Broecker 90 Sr 13 7 Cs 3 H 14 C etc from nuclear testing Because the distributions of radioisotopes offered information not so highly dependent on assumptions regarding the rates at which biological processes proceed in the ocean the emphasis in chemical oceanography moved quickly away from the traditional chemical tracers to the radiotracers Only quite recently has interest in the chemically used compounds in the sea been renewed Three reasons can be given for this renaissance 1 Radiocarbon is transported in particulate matter as well as in solution hence the contributions of the two processes must be separated if the distribution of 14C is to be used for water transport modeling This separation is based on the distribution of ECO2 concentration of total dissolved inorganic carbon alkalinity and dissolved 02 in the ocean 2 The concentrations of nitrate and phosphate can be combined with that of dissolved oxygen to yield the quasi conservative properties PO and NO As reviewed below such properties are needed in modeling to unscramble the mixtures found in the deep sea 3 With the advent of a sediment trapping and other means for the direct measurement of the fluxes of particulate matter into the deep sea b devices designed to measure the fluxes of materials from the sea floor and c better means for the measurement of plant productivity interest has been renewed in generating models capable of simultaneously explaining the distribution of the chemical species the distribution of the radiospecies and the flux measurements In this chapter I shall emphasize the development of radioisotope tracing as I feel that it constitutes the major contribution of geochemistry to our understanding of ocean circulation over the past four decades i e since the writing of The Oceans I will mention the 434 W S Broecker I 1 I use of the classical chemical tracers only where they bear on the interpretation of the radioisotope data Over the last decade the Geochemical Ocean Sections Study GEOSECS has determined the distribution of the radioisotope tracers on a global scale Attempts to model the previously existing 14C results Bolin and Stommel 1961 Arons and Stonmmel 1967 made clear the inadequacy of this data set Henry Stommel therefore brought together a number of geochemists interested in this problem and encouraged them to think big to work together and to produce a global set of very accurate 14C data Because of its massive scope and of the measurement accuracy achieved the GEOSECS data set has become dominant in the field of marine geochemistry While previously existing radioisotope data for review see Burton 1975 were of great importance in the development of thinking with regard to the interpretation of tracer results and in the separation of the natural and the bomb test contributions to 14C and 3H the new data set eclipses what we had in 1969 when this program began Thus I shall refer frequently to these new results in the sections that follow At the time this chapter was written the GEOSECS field program had been completed Maps showing the ship tracks and station positions are given in figure 15 1 The laboratory analyses for the Atlantic and Pacific phases of the program are complete Those for the Indian Ocean are still in progress The mammoth job of making scientific use of this data set has just begun Many years will pass before the meat of this effort will appear in print nent constitutes about 20 of the total in surface waters and is negligible in deep water 3 He the daughter product of 3H is also a tracer It is produced within the sea by the decay of its parent it also leaks into the deep sea from the mantle These components produce an excess over atmospheric solubility within the sea As will be shown below the contributions of these sources can usually be separated The applicability of any given isotope depends on its half life in the case of steady state tracers or its temporal input function in the case of the transient tracers It also depends on the geographic distribution of the input function The differences from tracer to tracer are sufficiently large that the information obtained from the distribution of one isotope is not redundant with that obtained from another tracer In the sections that follow the geochemistry of each of these tracers is reviewed Natural radiocarbon 14C is produced in the atmosphere by the interaction of 14 N atoms and the neutrons produced by cosmic rays The current production rate is estimated to be about 2 2 atoms cm 2s Lingenfelter and Ramaty 1970 This production is balanced by the beta decay of radiocarbon atoms In its 8200 year mean lifetime the average 14C atom can penetrate the active carbon reservoirs atmosphere terrestrial biosphere ocean ice soils Some 14 C atoms become Table 15 1 Ocean Tracers Currently in Usea Isotope 15 2 Water Transport Tracers The efforts in the field of radioisotope tracing can be divided into two categories those that are aimed at a better understanding of the dynamics of the ventilation of and mixing within the ocean s interior and those that are aimed at a better understanding of the origin movement and fate of particulate matter within the sea While many of the tracers we use are influenced by both processes a division can be made into a group primarily distributed by water transport and into a group primarily distributed by particulate transport table 15 1 I will simplify my task by discussing here only the water transport tracers The water transport tracers can be
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