Strontium isotope constraints on the seasonal variation of the provenance of base cations in rain water at Kawakami, Central Japan

Abstract

The 87Sr/ 86Sr ratios in monthly precipitation in the forested basin at Kawakami, central Japan, varied seasonally from 0.709 to 0.711 in spring to as low as 0.7062 ± 0.0004 in autumn over nine years from 1987 to 1995. The seasonal variation can be explained in terms of the mixing of three sources of Sr: sea salt ( 87Sr/ 86Sr∼ 0.70917), soluble eolian minerals originating from deserts in continental Asia (∼ 0.711), and biogenic materials growing on soils derived from the volcanic rock substrates in the vicinity ( ∼ 0.706). It is estimated that the contribution of sea-salt Sr into the Kawakami rain Sr is relatively constant (10 ± 5%) and that more than half of the Sr is of biogenic origin throughout the year except during spring rains when 50% of the Sr is due to the dissolution of Ca-minerals from Asian dusts. The dominant contribution of eolian components to spring rains is consistent with the high concentration of 3H, which is typical of air masses from Asia.