Topographic zonation and polycyclic pedogenesis in the northern atolls of the Chagos Archipelago, Indian Ocean

Abstract

We conducted soil surveys on two islands in the Peros Banhos and Salomon atolls of the northern Chagos Archipelago, Indian Ocean. We found muted but consistent topographic and soil zonation from the ocean shores to the lagoons. The main elements of the zonation are: berms of coral boulders and rubble along the heads of the ocean-side beaches; rubble-strewn soils inland of the berm; and pale sands with shallower topsoils and few coral clasts on slight rises and declivities over the rest of the islands. The ocean-side rubbly soils have interstitial coarse sand and are the most fertile on the islands, with dense tangled stands of unmanaged coconuts, profuse litter, and deep humic topsoils. Topsoils are shallower and less humic in the pale sands inland. Sand size decreases from ocean to lagoon, but increases with depth in most profiles. Water tables are often <2 m deep, and many soils have faint pale brownish mottling in the lower subsoils. There is a low tabular outcrop of bare Holocene coral sandstone on one of the islands. It is incised by shallow grikes that are partly infilled with silty muck, as are some small depressions in the central parts of both islands. The pedogenic environment appears to be dynamic, with storm surges depositing fresh sand, eroding coastlines, and infilling inter-island channels. Some soils have buried humic topsoils, stone layers, sand size inversions, and slight changes in sand colour, which are attributed to polycyclic pedogenesis. Some topsoils have elevated levels of total Zn, which is thought to be derived from long distance volcanic ash. Our data indicate that the soils are of low nutrient fertility. Total N and available P do not attain the strikingly eutric levels found in some atoll soils. The low fertility is attributed to the predation of seabirds by inadvertently introduced black rats. This precludes soil enrichment with marine-derived nutrients by guano deposition.