THE EFFECT OF DUST DEPOSITION ON SOIL FERTILITY ON A TROPICAL OCEAN ISLAND, SAN CRISTOBAL, GALAPAGOS

Acidic tropical soils have low base cation contents and carbon storage potential, and can develop poorly crystalline clays that absorb important nutrients. Many authors have noted that in some tropical island settings (e.g., Hawaii, Barbados) aerosols derived from arid areas with little vegetation are an important contributor to continued soil fertility by adding base cations, and unweathered primary minerals or crystalline secondary minerals back into the soil system. This study quantifies how much aerosol inputs are found at an equatorial study site where the intertropical convergence zone may prevent aerosol , and determines how these aerosol inputs are cycled through the surface soil by using Sr and Pb isotopes. Galápagos is an ideal location to study how dust affects equatorial sites because of strict regulations that prevent the use of many NPK fertilizers and of well-constrained climate data collected on San Cristóbal Island for the last several decades. Using the radioisotopes Cesium-137 and Lead-210, we calculated the relative aerosol inputs on a climosequence across the island. In the rainy highlands, Cs-137 inventories are higher than at low, dry sites. We calculated inventories across depth profiles and show that aerosol Pb-210 was deposited at all of our study sites, with the greatest accumulation in the highlands, corroborating the Cs-137 flux data. We used strontium and lead isotopes to identify potential sources of aerosol inputs to the Galápagos archipelago. Using 87/86Sr ratios, we hypothesize that seawater (87/86Sr = 0.709 [1]) is an important contributor of cations to wet, highland soils (0.709), and that lower-elevation, dry soils (0.705) do not receive many cations from seawater. Using our strontium data in conjunction with previously measured 87/86Sr ratios for the bedrock (0.703 [2]), we will test if East Asian loess and South American ash are important sources for aerosol inputs. We will use radiogenic lead isotopes to further refine the source of inputs; the radiogenic lead isotopes will allow for assessment of aerosol sources due to low oceanic [Pb]. We used X-ray diffractometry to identify any potential airborne mineral components, such as unweathered primary minerals like quartz and feldspar.

[1] Faure & Mensing, 2005 [2] White, et al., 1993