THE IMPACT OF HYDROCHEMICAL FLUX AND BIOCYCLING ON SOIL MASS BALANCE AND WEATHERING IN HAWAII

Elemental mass balance calculations provide a convenient way of interpreting losses and gains attributable to weathering. They are however subject to variation in soil biogeochemical processes at the landscape scale. In the absence of significant physical erosion, elemental losses are usually assumed to indicate leaching of elements suggesting a dependence of weathering on effective moisture and more generally on climate. This is true but the dependence is strongly non-linear. In Hawaii, element balances are influenced by water balance with some surprising results. First, on moderately old lava flows (150 ky), there is a strong leaching threshold at the point where water balance changes from seasonal to year-round leaching. A second more selective threshold occurs at low rainfall where leaching is ineffective but plant pumping of bioavailable elements leads to element concentration at the soil surface. Even in the absence of water erosion, the dead biomass and surface soil organic matter is subject to wind erosion leading to significant long-term losses of elements that are usually measured as indicators of weathering intensity (e.g. Ca, Si). Arid zone elemental losses lead to an interesting paradox: water flux is low and element loss though measurable does not contribute directly to dissolved load. In intermediate rainfall zones leaching losses are lower than at high rainfall because evapotranspiration removes much of the moisture, at the same time vegetation remains vigorous and more of the biocycled elements are returned to soil without being lost through wind erosion. In this intermediate zone, primary minerals are broken down but elements are conserved within profiles. These relationships among rainfall, leaching and biocycling are time dependent and are far less strongly expressed on young lava flows and more strongly expressed on old ones. Hawaii provides an unparalleled opportunity to quantify the impact of climate one weathering and soil development. I will analyze the influence of climate on weathering using data gathered from several climosequences on different age lava flows.