Crustal helium in deep Pacific waters

We have analyzed deep helium isotope data from World Ocean Circulation Expe riment section P17 along 135 degreesW in the Pacific Ocean. Combining these data with neon data, we have derived the isotopic ratio (R-t) of the nonat mospheric part of helium (terrigenic helium). The calculated R-t values are significantly lower than helium isotope ratios from midocean ridge (MOR) s ources (R-MOR approximate to 8R(a); R-a:= (He-3/He-4)(air)), which are gene rally presumed to dominate the isotope characteristics of helium of nonatmo spheric origin. For the depths most influenced by the southern East Pacific Rise (EPR) helium plume the isotopic ratio of terrigenic helium is similar to7.4 R-a. However, both the northern EPR plume and the Juan de Fuca Ridge (JdFR) plume show significantly lower R-t values of similar to6.8 R-a. We find minimum R-t values of similar to5.6 R-a at about 20 degreesN to 30 deg reesN from 2000 m depth to the ocean floor. We conclude that in addition to the EPR and JdFR helium sources (having MOR isotope characteristics) there is a flux of radiogenic He-4 out of deep-sea sediments and the oceanic cru st which causes a significant deviation of R-t toward values lower than R-M OR Using our calculated terrigenic He-4 concentrations and isotope ratios R -t and the helium isotope ratios of the two end-members (MOR helium and cru stal helium), we derive concentrations and ocean bottom fluxes of crustal 4 He (4He(crust)). Maximum concentrations of He-4(crust) (up to similar to 30 pmol kg(-1)) are found at about 20 degreesN to 30 degreesN between 2000 m and 4000 m along the P17 section. The mean crustal He-4 flux is obtained as 1.0 (+/-0.4) 10(9) atoms m(Earth)(-2) s(-1).