Geochemical and Physical Controls on Vadose Zone Hydrology of Holocene Carbonate Sands, Grand Bahama Island

Abstract

This paper explores the relationship between vadose zone hydrology and geochemical changes in mixed mineralogy carbonate sands from a Bahamian coastal dune of Holocene age. Cores were taken from two sites: at site A, a shallow humic Entisol is developed beneath open scrub vegetation, while at site B a deeper, more organic-rich Inceptisol has formed beneath a mature hardwood coppice. X-ray diffraction analysis reveals significant contrasts in mineralogy both within and between the two sites, with partial stabilization of high-Mg calcite and aragonite, to low-Mg calcite. Stabilization is greater at site B, and is accompanied by a significant increase in total porosity. Diagenetic changes in pore-size distribution have implications for residence times of percolating water, as determined using measurements of moisture retention characteristics using pressure plate apparatus, and hydrological models of unsaturated zone moisture flux. The diagenetically more mature sands from site B have a 50–100 per cent higher moisture retention, although unsaturated hydraulic conductivity is also higher, particularly at greater suctions. The increase in water retention is likely to enhance further rates of mineral-controlled reactions, while development of an organic-rich soil also enhances the geochemical drive for dissolution. Carbonate diagenesis thus appears to be strongly linked to vadose zone hydrology, and the interactions identified here have important consequences for the nature and long-term rates of mineral stabilization. © 1997 by John Wiley & Sons, Ltd.