FY 2001

Crustal helium in deep Pacific waters

Well, R., J. Lupton, and W. Roether

J. Geophys. Res., 106(C7), doi: 10.1029/1999JC000279, 14,165–14,177 (2001)

We have analyzed deep helium isotope data from World Ocean Circulation Experiment section P17 along 135°W in the Pacific Ocean. Combining these data with neon data, we have derived the isotopic ratio (R_t) of the nonatmospheric part of helium (terrigenic helium). The calculated R_t values are significantly lower than helium isotope ratios from mid-ocean ridge (MOR) sources (R_MOR ≈ 8 R_a; R_a := (³He/⁴He)_air), which are generally presumed to dominate the isotope characteristics of helium of nonatmospheric origin. For the depths most influenced by the southern East Pacific Rise (EPR) helium plume the isotopic ratio of terrigenic helium is ~7.4 R_a. However, both the northern EPR plume and the Juan de Fuca Ridge (JdFR) plume show significantly lower R_t values of ~6.8 R_a. We find minimum R_t values of ~5.6 R_a at about 20°N to 30°N 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 ⁴He out of deep-sea sediments and the oceanic crust which causes a significant deviation of R_t toward values lower than R_MOR. Using our calculated terrigenic ⁴He concentrations and isotope ratios R_t and the helium isotope ratios of the two end-members (MOR helium and crustal helium), we derive concentrations and ocean bottom fluxes of crustal ⁴He (⁴He_crust). Maximum concentrations of ⁴He_crust (up to ~30 pmol kg⁻¹) are found at about 20°N to 30°N between 2000 m and 4000 m along the P17 section. The mean crustal ⁴He flux is obtained as 1.0 (±0.4) 10⁹ atoms m⁻²_Earth s⁻¹.