COMPONENT SEPARATION OF OCEANIC HELIUM

A new procedure to quantify the components of oceanic helium (''terrig enic'' He-3 and( 4)He released from the ocean floor and ''tritiugenic' ' He-3 from tritium decay) is described. Terrigenic He and nonatmosphe ric He-3 (i.e., terrigenic and tritiugenic combined) are obtained in t erms of measured concentrations of the He isotopes and also of neon (N e) (which improves the separation considerably), assuming terrigenic H e to vanish in the mixed layer. For the subsequent separation of terri genic and tritiugenic He-3, additional information is required and He- 3 due to natural tritium represents a complication. The procedure is a pplied to data from a hydrographic section in the South Atlantic (19 d egrees S, 1991) and one in the Eastern Mediterranean (1987). The 1 sig ma data precisions and a systematic error accounting for uncertainties in mixed-layer He are approximately 0.3%. Sections of the new represe ntations of oceanic He and He-3 and comparisons to the nearest classic al quantities (i.e. He-3, He) are presented. In the South Atlantic the He-3 distribution reflects the hydrographic structure. East of 20 deg rees W the average He-3/He-4 ratio of the terrigenic He below 800 m is 4.5 +/- 0.8 times the atmosphere ratio, which implies a substantial c ontribution of crustal He. In the upper waters, tritiugenic He-3 (0.5 tritium units, +/- 20%) is separated from terrigenic He-3. In the East ern Mediterranean, tritiugenic He-3 is quantified throughout the water column in the presence of substantial levels of terrigenic He; the re lease rate of terrigenic He from the sea floor is found to be 3.1 +/- 1.2 10(10) atoms m(-2) s(-1), similar to the rate for continental crus t, with a mantle He contribution of 5 +/- 1.2% only. Recommendations f or future work are to reduce the mentioned systematic error and the un certainty margins of the He and Ne solubilities and of H-3 due to natu ral tritium.