Landscape formation and soil genesis in volcanic parent materials in humid tropical lowlands of Costa Rica

The influence of volcanism on landscape genesis, and formation of soils on volcanic parent material was studied in the Atlantic lowland of Costs Rica. This lowland is a subduction basin of tectonic origin, in which thick alluvial and marine sediments are accumulated. At its southwestern side it is bordered by active volcanoes. The climate of the area is hot and humid throughout the year, with a constant mean air temperature of about 25°C and a welldistributed mean annual rainfall of about 3500 to 5500 mm.Landscape formation was found to be strongly influenced by volcanism. I investigated two particular phenomena: (1) the formation of beach ridges and (2) landscape dynamics in the middle and lower parts of the alluvial plains.The formation of beach ridges along the Caribbean coast, ranging in age between about 100 and 5000 years, appears to be related to discontinuous sand supply by eruptions of the volcanoes which border the study area. This is indicated by small but consistent and statistically significant differences in chemical composition between sediments of individual beach ridges, which cannot be explained by textural differences. The differences are thought to be caused by variations in magma composition of different eruptions in the volcanic hinterland. Si0 2 contents of the ridge sediments vary between about 52% and 58%, other elements show variations corresponding to magmatic differentiation. Petrographically this is expressed in variations in the amount of andesitic rock fragments and pyroxene grains. Chemical composition of beach ridge sediments is similar in composition to the erupted products, in spite of the removal of part of the mobile elements and mixing with weathered sediments.Late Holocene sand deposits fill up former river channels and cover adjacent overbanks in the central fluvial plains of the study area. Two facies types are distinguished. The first is an up to several km long, 3 to 10 m deep, and 10 to 80 m wide coarse sandy channel fill facies, which shows an overall fining upward sequence both in grain size and structure. This facies has a pebbly base, small coarse sandy large troughs in the middle part and sandy trough cross-beds in the upper part. The second facies is a wing-shaped, less than 1.5 m thick, sandy overbank facies which shows small cross-beds and ripple-marks and extends laterally up to 300 m from the channel. The deposits testify to episodic supply of huge amounts of loose material on the upper slopes of the volcanoes that border the area, as a consequence of eruptions. Complete choking of the channel and concomitant shifts of the river course appears to occur only during extreme rainfall events. Of a total surface area of about 300 km 2, two smaller areas covered with this kind of channel fill deposits related to eruptions of Turrialba volcano about 2000 yrs BP are described, as well as a similar but much younger deposit related to the 1963-1965 eruptions of Irazú volcano. In the distal part of watersheds, thick (up to 1 m or more) crevasse splay sediments appear to be a less voluminous manifestation of fluvial sedimentation triggered by volcanic activity- One of such deposits is described, possibly related to the 1864-1866 eruptions of Turrialba volcano. In the fluvial plain short periods of highly active sedimentation and landscape formation are alternated with longer, rather inactive periods.Soil formation has been studied in relation with time in a Holocene (< 5000 yr) soil chronosequence on sandy, andesitic, ocean beach ridges along the Caribbean coast . All soils are under tropical rainforest. Tropopsamments are present on the 3 younger beach ridges and Hapludands are on the older ones. Drainage conditions change by subsidence from excessively drained in the two youngest soils to imperfectly drained in the two oldest soils. The parent materials of all soils are sands with similar mineralogical composition: andesitic rock fragments, plagioclase, and pyroxene dominate, with minor amounts Of Opaque minerals. None of the parent materials contained>13% (v/v) volcanic glass. It has been found that under these conditions Andisols form within 2000 years. Imperfect drainage caused mottling and accumulation of iron-coatings, as well as the formation of a thin O- horizon in the two oldest profiles. The increase in fine material and the accumulation of organic matter cause an increase of CEC and andic properties, and a decrease in bulk density and pH with soil age. Depth of biological influence increases with soil age, but soil faunal activity is hampered in the oldest three profiles, probably by imperfect drainage. Due to extreme leaching, the sum of exchangeable bases is less than 2 cmol+.kg -1in the B-horizons of the older soils, notwithstanding the presence of a considerable amount of weatherable primary minerals.Using micromorphological, mineralogical, and chemical analyses, weathering and neoformation of minerals was investigated in the same chronosequence. Weathering and neoformation of minerals with increasing soil age is characterized by: (i) increasing pellicular and linear alteration of sand grains and (ii) decrease of the sand fraction and concomitant increase of finer material. Andesitic rock fragments weather more rapidly than plagioclase and pyroxene mineral grains. The alteration rates of the latter two are similar. Clay content in the about 2000-yr-old soil is several times higher than in soils developed on rhyolitic parent materials of similar age in New Zealand. Formation of allophane with Si/Al ratios ranging from 1.9 to 3.8 takes places mainly in the B horizons. Aluminium-humus complexes, allophane, and Al oxides and hydroxides are mainly formed in the A horizons. Small amounts of gibbsite were noticed in soils older than 2000 yr. Small amounts of 2:1 and 1:1 clay minerals present in the clay fraction of all soils are thought to be inherited from the parent material, which contained sand-sized bodies of clay and andesitic rock fragments with clay pseudomorphs, both consisting of 2:1 and 1:1 clay minerals.Furthermore, an attempt was made to quantify aspects of soil formation in eight soil profiles developed on volcanic parent material in the area. Assuming Ti to be immobile, gains and losses of major elements in were calculated from total element contents. Initially soil formation (0 to 0.5 ka) in Tropopsamments involves dilation of the sandy deposits by incorporation of organic matter and formation of structure and biopores, without detectable gains or losses of elements. In 2 to 5 ka old sandy Hapludands, primary minerals are still abundant, but up to 20% of the mineral soil consists of X-ray amorphous materials. Dilation continues and losses of Mg, Ca, Na and K, and to a lesser degree Si, have become measurable. In a < 18 ka old, stony Melanudand primary minerals and, especially, volcanic glass, become depleted in the fine-earth fraction. A mixture of short-range order material, metal-humus complexes, gibbsite and kaolin minerals dominate in the A horizon, while gibbsite, halloysite, and short- range order material are the most important secondary minerals at greater depths. Dilation of the soil mass prevails in the A horizon, while collapse has occurred in the B horizon. Considerable amounts of mobile elements are lost: 50 to 85% of Si, Mg, Ca, Na, and K have been leached from the soil profile. The strongly collapsed Haploperox,>50 to 450 ka old, thought to have formed from Andisols, are devoid of primary minerals (except opaques) in the upper meters, and are dominated by gibbsite, kaolin minerals and iron (hydr)oxides. Under the prevailing environmental conditions, weathering and neoformation of primary volcanic minerals lead to almost complete losses of basic cations, probably in a time period between 20 and 50 ka. Si and P mineral reserves are depleted considerably, but part is still present after such long time periods.