Geomorphological Relationships of Tropospherically Derived Quartz in the Soils of the Hawaiian Islands

Abstract

AbstractThe percentage of quartz in surface soils over quartz‐free mafic (basic) rocks of the Hawaiian Islands varies with the amount and source of annual rainfall and with landscape elevation and age. On the Island of Oahu, the quartz content of the soil A horizon varies in the range 0.2 to 0.7% in Molokai soil at 40 m elevation (and 750 mm of rainfall), 1.1 to 1.6% in Wahiawa soil at 300 m elevation (and 1,250 mm of rainfall), 13 to 22% in Paaloa soil at 380 m elevation (and 2,000 mm of mainly Trade Wind rainfall), and 1 to 45% in Olokui soil at 450–1,250 m elevation (and 1,750 to 5,000 mm of rainfall). The pronounced increase in quartz percentage from Wahiawa to Paaloa soil reflects the somewhat greater rainfall, but particularly the greater proportion of Trade Wind rainfall and the greater landscape age of the Paaloa sites on old interfluves. With higher rainfall and site age of Olokui soil on mountain tops, the quartz contents are higher; where erosion and denudation have kept the landscape younger on ridges and saddle crests, the contents are lower. Quartz contents, varying from 0.1 to 17% with rainfall and landscape age, were also determined in soils of the Islands of Kauai, Molokai, Maui, and Hawaii.The close similarity of three parameters of the quartz (particle size distribution—70% in the 10‐2µm fraction; particle morphology—angular rather than euhedral; and oxygen isotope abundance—∂18O = averaging 17.6 ‰) in soils of several islands are remarkably similar to those of pelagic sediments of the north central Pacific Ocean and aerosolic dusts of the Northern Hemisphere. This similarity is attributed to their common eolian origin from arid continental areas of the Northern Hemisphere. Dust carried by the circumpolar Westerly Winds is scrubbed by clouds formed by orographic‐convective rise of air from the Trade Winds in the Hawaiian Islands and then is deposited on soils by rainfall. The percentage of quartz in surface soils on the oldest landscapes approaches that in the north central Pacific pelagic sediments. The percentage of quartz in most Hawaiian soils, however, is much lower as a consequence of dilution by mixing with indigenous quartz‐free soil, weathering products, occasional volcanic ash showers, and soil organic matter. Mass wasting appears to have removed the primary landscapes from the basalt dome. The thickness of quartz‐rich surface horizons since accumulated is therefore only a fraction of a meter rather than the several meters thickness of pelagic sediments laid down during periods corresponding to the known island ages. The quartz‐enriched (and micaceous vermiculite‐enriched) surface soil horizons constitute an example of an ombitrophic ecologic environment in which soil‐borne nutrients are provided abundantly from the atmosphere. This environment is especially likely to receive pollution from radioactive elements and toxic chemicals such as DDT, lead from gasoline, and related manderived materials.