Germanium/silicon and trace element geochemistry of silicate weathering and mineral aerosol deposition

Deposition of mineral aerosols ("dust") in both terrestrial and marine environments can have important biogeochemical consequences. Abiologic processes governing soil development include losses due to weathering, and gains from atmospheric additions. We take advantage of a well-characterized Hawaiian soil chronosequence to quantify trace element fluxes to, from, and within the soil system. We find substantial mobility of Zr, Hf, and Th under intense weathering, but find that Nb and Ta are "immobile". The conservative behavior of Nb allows us to quantify the behavior of other elements. Dust is an important source of many elements, including Th, U, rare earth elements, Sr, and P. This is important as P availability limits new forest production at some of our sites. Weathering of dust derived from the atmosphere may help alleviate nutrient limitation in old Hawaiian forests.; Ge/Si ratios are a potentially useful tracer of silicate weathering and silica cycling given an understanding of the processes that fractionate the pair. We find that soils developed on both Hawaiian basalt and granitic rocks in Puerto Rico have Ge/Si ratios higher than the parent material. The mechanism for Ge/Si fractionation appears to be preferential incorporation of Ge into secondary aluminosilicates during weathering. This behavior is consistent with a thermodynamic model of Ge/Si fractionation.; A simple Ge/Si mass balance between atmospheric dust and marine sediments, combined with laboratory dissolution experiments suggests that 10 to 20% of the total Si in dust is released to seawater during settling through the water column. This process could account for a significant additional source of dissolved silica to the oceans that has not been previously recognized. The atmospheric silica flux may be 20 to 40% of the riverine flux at present. We argue that the paleoceanographic Ge/Siopal record reflects increased silica delivery, driven by an increased dust flux, during glacial maxima. High glacial dust fluxes may increase silica availability to diatoms, relieving silica limitation in "low silicate, high nitrate, low chlorophyll" regions of the ocean. Because diatoms contribute a large share of oceanic export productivity, this mechanism could be important for regulating glacial-interglacial pCO2.