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Soil development on a l,500-year-old beach ridge plain, Sturgeon Bay, NW Lower Michigan.

Article Excerpt
ABSTRACT

This study examines the development of the beach ridge plain at Sturgeon Bay, Michigan, and the soil formation in the ridges. A preexisting average ridge formation rate of 35.7 years was used to establish an absolute age chronology, spanning about 1,500 years back in time. Soil development rates in the ridges were assessed through solum thickness, soil pH measurements, soil color determinations, Soil Taxonomy classification and podzolization index value calculations based on the soil colors. A rapid pH decrease of two pH units was observed in 150 years of soil development, followed by stagnation around phi 5. The solum thickness increase of 0.33 mm/y, podzolization indices of O-l and soil classifications indicate slow soil formation compared to podzolic soils in other temperate environments, hut typical for soils in the Great Lakes region.

INTRODUCTION

Soils comprise the surface on which we live, providing building materials and the substrate for agricultural production. Hence, soils are an invaluable asset to society. The development of soil characteristics is equally important, as it may affect the value of this asset through changes in soil fertility, physical stability, etc. Whereas most soil forming processes take place at a [10.sup.3]-[10.sup.6] year time scale (Buol et al. 1989), some processes such as acidification have much faster rates (Chadwick and Chorover 2001), leading to substantial changes in soil pH, even at a decadal time scale (e.g., Bronick and Mokma 2005). Rapid changes are especially common in unbuffered, sandy soils, such as the subjects of this study.

Nevertheless, calculation of both long- and short-term soil formation rates is necessary in order to assess the change of soil characteristics as a whole. The quantification of local, natural soil development rates is needed for a better assessment of soil changes induced by human activities. In order to study natural soil development, one has to embark on the difficult task of finding soils with little or no alteration due to human activities. Even if relatively well-preserved soils are found, natural soil development remains inherently difficult to study, mostly because long term soil development rates occur on timescales exceeding those of most scientific studies. The longest running experiment plot in the world at Rothamsted, UK, with continuous field plot studies since 1843 (Rothamsted Research Station), encompasses only a small fraction of the millennial time scale involved in soil development.

The difficulties of studying slow soil processes have forced soil scientists to study long-term soil development through a more indirect approach, known as soil chronosequences, which builds on the CLORPT soil formation equation (jenny 1941). This equation states that soil formation is the sum of five and possibly more soil forming factors: Climate (CL), organisms including human activity (O), relief (R), parent material (P), and time (T). Chronosequences arc, in essence, an attempt to isolate the time factor (T) in that formula by keeping all other factors constant (Jenny 1941)- This is done by examining samples from soils with similar soil formation conditions, except for soil age, and plotting the soil properties versus age. In reality, true soil chronosequences are rare, if not non-existent, as soil formation factors vary over small distances and through time. Instead, so-called non-strict soil chronosequences with minimal variation in soil formation factors (except soil age) are employed for chronosequence studies (Huggett 1998).

An impressive array of different geomorphic surfaces has been used in soil chronosequence studies, spanning from series of end moraines over alluvial terraces to mine spoils (Yaalon 1975; Birkeland 1990; Huggett 1998 and references therein). One particularly useful set of geomorphic surfaces is a sandy beach ridge plain, which is a closely-spaced, semi-parallel sequence of wave- or swash-formed ridges or debris lines, usually reinforced by aeolian deposition (Otvos 2000). If the ridges have been relatively undisturbed by human activities, they offer a good set of geomorphic surfaces for soil chronosequence studies of natural soil formation in sandy soils, often spanning several thousand years of soil history. Consequently, beach ridges have been employed in numerous chronosequence studies. The numerous beach ridge plains flanking the Great Lakes have been the subject of several soil chronosequence studies (VandenBygaart and Protz 1995; Lichter 1998a; Barrett 2001), in addition to a number of studies conducted on other serial shoreline deposits (e.g., Barrett and Schaetzl 1992).

The subject of this study, the Sturgeon Bay beach ridge plain, has been intensively studied (Lichter 1995, 1997,...

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