Th/U and U series systematics of saprolite: importance for the oceanic 234U excess

The presence of excess U-234 in seawater is a compelling argument for active delivery of solutes from the continents to the oceans. Previous studies found, however, that the complementary U-234 deficit on the continents is surprisingly modest, which would require protracted U loss from a large continental weathering pool. Our new compilation and statistical analysis of the published data, coupled with a mass balance calculation demonstrates that the apparent small U-234 deficit in the continental weathering pool implied by previous studies is insufficient to balance the observed oceanic excess. Our new data for a saprolite weathering profile developed on Deccan basalt reveal a very strong overall loss of U (elevated Th/U) with a strong U-234 deficit attributable to chemical weathering. The U and U-234 deficits reported here from a geologically recent saprolite confirm the importance of the early stages of chemical weathering at the weathering front in the supply of nutrients to the oceans. Thus, as much as half the oceanic U-234 inventory is likely sourced from a thin active saprolite zone.