MIT 12 000 - Geochemical Tracers and Ocean Circulation

Unformatted text preview:

I5GeochemicalTracers and OceanCirculationW. S. Broecker15.1 IntroductionTracers have always been an important adjunct tophysical oceanography. The distribution of dissolvedoxygen 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 Sver-drup, Johnson, and Fleming (1942) for a review of thissubject]. Many attempts also have been made to har-ness the loss of dissolved oxygen from the water col-umn as a measure of the rates of oceanic mixing proc-esses (e.g., Riley, 1951; Wyrtki, 1962). These latterpursuits, however, have been of only marginal successbecause of our lack of knowledge of the consumptionrate of 02 within the sea.The big breakthrough in geochemical tracing cameafter World War II with the discovery of the cosmic-ray-produced isotopes 14C and 3H. A further impetus tothis field came with the realization in the mid 1950sthat the ocean was receiving significant amounts of90Sr, 13 7Cs, 3H, 14C, etc., from nuclear testing. Becausethe distributions of radioisotopes offered informationnot so highly dependent on assumptions regarding therates at which biological processes proceed in theocean, the emphasis in chemical oceanography movedquickly away from the traditional chemical tracers tothe radiotracers. Only quite recently has interest in thechemically used compounds in the sea been renewed.Three reasons can be given for this renaissance:(1) Radiocarbon is transported in particulate matter aswell as in solution; hence the contributions of the twoprocesses must be separated if the distribution of 14Cis to be used for water-transport modeling. This sepa-ration is based on the distribution of ECO2(concentra-tion of total dissolved inorganic carbon), alkalinity, anddissolved 02 in the ocean.(2) The concentrations of nitrate and phosphate can becombined with that of dissolved oxygen to yield thequasi-conservative properties "PO" and "NO". As re-viewed below, such properties are needed in modelingto unscramble the "mixtures" found in the deep sea.(3) With the advent of (a) sediment trapping and othermeans for the direct measurement of the fluxes of par-ticulate matter into the deep sea, (b) devices designedto measure the fluxes of materials from the sea floor,and (c) better means for the measurement of plant pro-ductivity, interest has been renewed in generatingmodels capable of simultaneously explaining the dis-tribution of the chemical species, the distribution ofthe radiospecies, and the flux measurements.In this chapter I shall emphasize the development ofradioisotope tracing, as I feel that it constitutes themajor contribution of geochemistry to our understand-ing of ocean circulation over the past four decades (i.e.,since the writing of The Oceans). I will mention the434W. S. Broecker_I 1 ___·I_ __ _ _ _use of the classical chemical tracers only where theybear on the interpretation of the radioisotope data.Over the last decade, the Geochemical Ocean Sec-tions Study (GEOSECS) has determined the distribu-tion of the radioisotope tracers on a global scale. At-tempts to model the previously existing 14C results(Bolin and Stommel, 1961; Arons and Stonmmel, 1967)made clear the inadequacy of this data set. HenryStommel therefore brought together a number of geo-chemists interested in this problem, and encouragedthem to think big, to work together, and to produce aglobal set of very accurate 14C data.Because of its massive scope and of the measurementaccuracy achieved, the GEOSECS data set has becomedominant in the field of marine geochemistry. Whilepreviously existing radioisotope data (for review seeBurton, 1975) were of great importance in the devel-opment of thinking with regard to the interpretationof tracer results and in the separation of the naturaland the bomb-test contributions to 14C and 3H, the newdata set eclipses what we had in 1969 when this pro-gram began. Thus I shall refer frequently to these newresults in the sections that follow.At the time this chapter was written the GEOSECSfield program had been completed. Maps showing theship tracks and station positions are given in figure15.1. The laboratory analyses for the Atlantic and Pa-cific phases of the program are complete. Those for theIndian Ocean are still in progress. The mammoth jobof making scientific use of this data set has just begun.Many years will pass before the meat of this effort willappear in print.15.2 Water-Transport TracersThe efforts in the field of radioisotope tracing can bedivided into two categories: those that are aimed at abetter understanding of the dynamics of the ventilationof, and mixing within, the ocean's interior, and thosethat are aimed at a better understanding of the origin,movement, and fate of particulate matter within thesea. While many of the tracers we use are influencedby both processes, a division can be made into a groupprimarily distributed by water transport and into agroup primarily distributed by particulate transport(table 15.1). I will simplify my task by discussing hereonly the water-transport tracers.The water-transport tracers can be subdivided ac-cording to their mode of origin. 9Sr, 137Cs, 85Kr, and thefreons are entirely anthropogenic in origin and henceare "transient tracers." 39Ar, 22 2Rn, 22 8Ra, and 32Si areentirely natural in origin and hence are steady-statetracers. 14C and 3H are in part natural and in part an-thropogenic in origin. In the case of 3H the man-madecomponent dominates. For 14C the man-made compo-nent constitutes about 20% of the total in surfacewaters and is negligible in deep water.3He, the daughter product of 3H, is also a tracer. It isproduced within the sea by the decay of its parent; italso leaks into the deep sea from the mantle. Thesecomponents produce an excess over atmospheric sol-ubility within the sea. As will be shown below, thecontributions of these sources can usually be separated.The applicability of any given isotope depends on itshalf-life (in the case of steady-state tracers), or its tem-poral-input function in the case of the


View Full Document

MIT 12 000 - Geochemical Tracers and Ocean Circulation

Documents in this Course
Load more
Download Geochemical Tracers and Ocean Circulation
Our administrator received your request to download this document. We will send you the file to your email shortly.
Loading Unlocking...
Login

Join to view Geochemical Tracers and Ocean Circulation and access 3M+ class-specific study document.

or
We will never post anything without your permission.
Don't have an account?
Sign Up

Join to view Geochemical Tracers and Ocean Circulation 2 2 and access 3M+ class-specific study document.

or

By creating an account you agree to our Privacy Policy and Terms Of Use

Already a member?