Soil Development

Bronwen Wang, USGS, Alaska Science Center
Chien Lu Ping, University of Alaska, Fairbanks
Gary Michaelson, University of Alaska, Fairbanks

The newly erupted volcanic site provides a rare opportunity to initiate long term monitoring of volcanic ash weathering, soil formation and vegetation succession and their role in the recolonization of Kasatochi Island by seabirds. This element of the study is designed to address the following short and long term questions: 1) What are the initial effects of the ash on pH? 2) What elements are released in early stages of weathering? 3) What are the effects of plant colonization and succession on soil evolution?, 4) What is the relative importance of marine- and atmospheric-derived nitrogen to the soil nitrogen pool?, and 5) How do these biochemical properties affect the recolonization of plants and animals?

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Soils form through the weathering of rocks, sediments, or other geologic material. Soils derived from volcanic ash exhibit unique physical and chemical properties, such as low bulk density, high water retention and phosphate sorption.

Soils develop rapidly on volcanic ash because of the large surface area and high porosity of the ash. The humid and relatively mild climate of the Aleutian region may be optimal for soil development. The interaction between surface and ground water and sulfur compounds may produce acidic conditions. High acidity may enhance the initial chemical weathering of the ash. As vegetation and microbial communities reemerge, humic substances will become incorporated into the incipient soil. Rapid humus accumulation is typical of volcanic soils.

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Following the evolution of soil on Kasatochi volcano is inherently long-term but it is important to characterize the process in its early stages with short-term assessments. Sampling sites were determined by the landforms on the island (e.g., ridgetops, shoulder slopes, side slopes, etc.), the results of our geologic studies, and previous surveys of seabirds, vegetation and soils. Data loggers were deployed to obtain physical and chemical characteristic (such as soil temperature, soil moisture, pH, specific conductivity) with time. Sites were equipped with soil cation/anion collector membranes to monitor the release of elements as the ash weathers on monthly, yearly and multi-year time scales.

In the lab we are conducting leaching experiments on the initial ash materials to assess the leachable acidity and the influence of water, nitrogen, carbonic acid, and organic acids on both the elemental releases from the ash and changes in extractable iron and aluminum. This will then be correlated with time necessary for these kinds of input-influences to happen in the field to predict soil evolution.

Volcanic ash clouds typically contain sulfur and halogen gases and metal volatiles that become adsorbed to fine ash particles during ash transport and fallout. After the ash accumulates on the ground or falls in the ocean it becomes susceptible to reaction with rainfall, surface water and seawater, and the adsorbed sulfur and halogen compounds, which are generally soluble, produce leachates. Ash leachate has potential ecological significance because it may contain high concentrations of arsenic, chlorine, fluorine, mercury, lead, sulfate and selenium that are toxic to marine and terrestrial organisms and may inhibit growth of certain species of plants. In marine environments however, a number of studies have shown that dissolution of adsorbed salts and aerosols from volcanic ash can increase the nutrient availability and this may cause a rise in marine productivity. These nutrients and trace metals can increase primary productivity in the ocean and thus volcanic ash and ash-leachates may act as natural fertilizers.

The August 7, 2008 Kasatochi eruption produced a significant sulfur dioxide (SO₂) cloud that was tracked by satellite around the globe. Observations in late August 2008 indicated sulfur precipitates on the surficial ash deposits and many gas escape features associated with the release of gasses trapped within the ash and debris mantle. Although we have not been able to obtain leachate data from field measurements, the nature of the eruption and fine-grained character of the surfical deposits suggests that ash leachates generated by rainfall and surface runoff at Kasatochi will likely be at least moderately acidic.

The erosion of the ash and formation of ash leachate presents a dual problem. On the one hand, it is possible that the nearshore zone will be compromised by high amounts of suspended sediment and associated toxic elements and acidic solutions. On the other hand, dissolution of adsorbed constituents on the ash could increase the bioavailability of some key nutrients and increase primary marine productivity. To address this, we collected and analyzed ash samples from the flanks of Kasatochi volcano, and suspended sediment and nutrient samples from the nearshore zone to determine whether the volcanic ash leachates inhibit or promote ecological recovery. We are integrating these results with those obtained from the analysis of rill/gully erosion to determine potential long-term effects of volcanic ash in the marine environment.