Crustal helium in deep Pacific waters

Abstract

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 (Rt) of the nonatmospheric part of helium (terrigenic helium). The calculated Rt values are significantly lower than helium isotope ratios from midocean ridge (MOR) sources (Rmor ≈ 8Ra; Ra:= (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 Ra. However, both the northern EPR plume and the Juan de Fuca Ridge (JdFR) plume show significantly lower Rt values of ∼6.8 Ra. We find minimum Rt values of ∼5.6 Ra 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 Rt toward values lower than RMor. Using our calculated terrigenic 4He concentrations and isotope ratios Rt 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 (4Hecrust). Maximum concentrations of 4Hecrust (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-2Earth S-1.