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Global biomass production is fundamentally affected by the hydrological cycling of elements at the Earth's surface. Continental weathering processes are the major source for most bioessential elements in marine environments and therefore affect primary productivity. In addition, critical zone biomass depends on energy and chemical exchange reactions in weathering profiles. The latter reservoirs are in turn influenced by different climatic conditions that control weathering and pore water parameters like pH and Eh, these then regulate mineral break down rates that dictate the mobility and mass flux of elements. Two Deccan Trap basalt-weathering profiles of contrasting age and alteration intensity provide a natural laboratory for investigating the effects of rock alteration on Si and Cr and their isotopic systematics. The sub-recent Chhindwara profile has progressed to a moderate degree of alteration (saprolite), whilst the Paleogene Bidar example displays an extremely altered laterite. The Chhindwara profile shows a near uniform Cr and Si concentration and isotopic composition, whereas the Bidar profile is characterised by an intense loss of Si, a large

The presence of excess 234 U in seawater is a compelling argument for active delivery of solutes from the continents to the oceans. Previous studies found, however, that the complementary 234 U 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 234 U 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 234 U deficit attributable to chemical weathering. The U and 234 U 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 234 U inventory is likely sourced from a thin active saprolite zone.

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