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

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, 1998a, 1998b, 2000). Based on a modeled absolute-age chronology which was derived from a previously reported average ridge formation rate, this study aims to demonstrate the application of an existing chronology for calculation of new rates of soil development in the ridges. The soil development rates will be based on soil pH measurements, horizon thickness, and color development combined with estimated soil ages.

STUDY SITE

Sturgeon Bay Beach Ridges

The Sturgeon Bay beach ridge plain (45[degrees] 43'N, 84[degrees]55'W) is located in the Wilderness State Park in Emmet County, at the northwestern tip of the Lower Peninsula, Michigan, United States (Figure 1). The beach ridge plain contains about 60 shoreline-parallel ridges, arching along Sturgeon Bay. The ridges have been formed after the mid-Holocene Nipissing (5,000-5,500 years before present; Larson and Schaetzl 2001) lake-stage high stand (Lichter 1995). The ridges are up to 3 km long and 0.5-1.5 m high. They vary from irregular, hummocky dune ridges close to the present shoreline to regular and continuous ridges. The ridges are separated by ridge-parallel swales, which are dry in some parts, but can also be narrow ponds, especially in the central and inner part of the beach ridge plain (Figure 2). Based on aerial photographs, maps, and field observations, new ridges and swales appear to be forming along the Sturgeon Bay shoreline (Figure 3). Judging from the lack of coarse grains and the hummocky surface, aeolian influence on the ridges is strong. Some ridges are heavily distorted by later aeolian activity and some ridges are completely covered by parabolic dunes, especially in the west-central and inner parts of the beach ridge plain. Previous studies estimated the age of the youngest ridge in the sequence around AD 1970 (Lichter 1997) and the innermost ridges in the sequence to around 375 BC (Lichter 1998a). An average ridge formation rate of 35.7 years/ridge has also been calculated (Lichter 1997).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Climate

The beach ridge plain is situated in a snow-temperate humid climate with warm summers and cold winters, corresponding to the Koppen-Geiger zone Dfb (Kottek et al. 2006). The mean annual temperature is 5.8[degrees]C and the mean annual precipitation during the same period is 715 mm, both means of 1971' 2000 data from Cross Village, 6 km southwest of study site (National Climatic Data Center 2002). The soil temperature and moisture regimes are frigid (Natural Resources Conservation Service), The well-aerated state of the studied soils, combined with climate data (National Climatic Data Center 2002), indicates an udic soil moisture regime, i.e., a net precipitation surplus and no pronounced dry season.

Vegetation

At the dune ridges adjacent to the present shoreline the vegetation is open dune vegetation dominated by beach grass (Ammophila breviligulata) vegetation with willow (Salix spp.) scrubs, grading towards mainly juniper (Juniperus communis) and bearberry (Arctostaphylos uva-ursi) scrubs when moving inland. The ridges are forested from ridge 3-4 and further inland. The forest is dominated by various coniferous tree species such as white and red pine {Pinus strobus and Pinus resinosa), white spruce (Picea glauca) and white cedar (Thuja occidentalis), interspersed with red oaks (Quercus rubra) and paper birch (Betula papyrifera). The forest floor is covered by various grasses and ferns, as well as patchy wintergreen (Chimaphila umbellata), but in many areas unvegetated litter dominates. These observations correspond with detailed accounts from previous studies (Lichter 1998b).

Texture and Soil Types

The texture of the soils on the ridges is in the sand fraction (USDA nomenclature), analogous to the offshore sediment (Lichter 1998a). According to the USDA-NRCS soil survey (Natural Resources Conservation Service), the area is dominated by the poorly drained Roscommon soil series (Soil Taxonomy classification: Mixed, frigid Mollic Psammaquents) and the drier Eastport series (mixed, frigid Spodic Udipsamments) interspersed with muck soils and ponds. The soils grade into slightly more podzolized Deer Park series (mixed, frigid Spodic Udipsamments) on the parabolic dunes at the eastern edge of the beach ridge plain. A previous study reported that the soil development grades from undifferentiated parent material at the Lake Michigan shoreline to soils with well-defined albic and spodic horizons (Lichter 1998a).

METHODS

Field methods

Soil probes were used to generate a brief soil description and provide sample material for further analyses. Samples were collected from the crests of 15 ridges, one sample from each of the 11 ridges closest to the present shoreline and one sample each from ridges 15, 20, 30, and 40 (Figure 1). All sites were examined to a depth of at least 30 cm below the upper boundary of the Chorizon, which in practice equaled maximal depths of 50 to 75 cm below the surface, depending on the solum thickness. At each field site, soil horizon thicknesses were noted and the soil horizon colors were determined using a Munsell Soil Color chart. Soil samples were collected from all Chorizons, as well as E and Bw horizons at Ridges 30 and 40. A brief description of the local vegetation was also noted, along with the GPS position as received from a Magellan eXplorist 200 GPS. The GPS positions were used to measure the distance from each sampling site along a shoreline-perpendicular line to the present shoreline, using a georeferenced aerial photograph.

Laboratory Methods

Soil pH was measured in a 1:1 soil and demineralized water solution, using a Denver UP-5 pH meter. AH soil samples were measured three times 3-3 hours after water addition. Each of the 15 sampled pedons was assigned to an (estimated) Soil Taxonomy soil family. POD (Podzolization) index values were calculated for each pedon; the POD index being defined as:

POD index = [sigma] ([DELTA]V.[2.sup.[DELTA]H]) (1)

where [DELTA]V is the difference in E and B horizon color value and AH is the difference in hue, measured as the number of Munsell soil color chart pages. The index number is calculated by comparing each B horizon with the E horizon with the brightest color (highest value) and summing all index numbers for each soil profile (Schaetzl and Mokma 1988). The POD index was chosen, as it relates directly to podzolization (rather than a variety of different soil development parameters) and allows rapid comparison with index values from other studies, unlike the frequently used Profile Development Index (Harden 1982), where different soil parameters and scales may he selected for each study. A POD index value of 0 represents no visible podzolic horizon differentiation, while Typic Haplorthods have a mean index value of 11.6 (Schaetzl and Mokma 1988).

Chronology

All ridge age estimates have been calculated by multiplying an average ridge formation rate of 35.7 from Lichter (1997) with the appropriate ridge number. The youngest ridge, Ridge 1, was dated to AD 1970 by pioneer tree ring counts (Lichter 1997). Considering that a new proto-ridge in the making was found during fieldwork in 2006, the age of Ridge 1 matches the average ridge formation rate of--36 years perfectly. The chronology is based on the assumption that each beach ridge has the same age along its length (i.e., that laterally equivalent portions of the ridge formed simultaneously). The calculated absolute ages of all sampled ridges are plotted in Figure 4a, and the derived shore displacement rate (1.07 m/y) is plotted in Figure 4b. Margins of error on the estimated ages are hard to assess, but the average ridge formation rate is based on a good fit ([r.sup.2] = 0.98) of radiocarbon dates with an estimated error margin of [+ or -] ~10% (Lichter 1997), so the calibration of relative ages would likely have a comparable error margin. Some periods of ridge formation may see rates deviating markedly from the average rate, adding to the modeled absolute age error, but the average rate would be representative for most ridges over a long time span (cf. Nielsen 2008). In addition, the radiocarbon age controls reported by Lichter (1997) reveal that ridge formation rates did not fluctuate markedly during the 1,500-year time span covered by the ridges in this study.

[FIGURE 4 OMITTED]

RESULTS AND DISCUSSION

Soil Development Parameters

Solum thickness (the thickness of all horizons above the C horizon; see also Table 1), soil pH, and corresponding soil ages are displayed in Figure 5. From Figure 5a, it can be seen that the solum thickness increases from virtually zero in the youngest ridges to ~50 cm in the oldest ridges, yielding an average solum formation rate of 0.33 mm/y. This rate is slightly faster than, but still comparable to, the solum development rates derived from the data reported in similar studies of podzolized beach ridge sequences in the Great Lakes region (Barrett 2001; VandenBygaart and Protz 1995). These rates range from 0.16 mm/y to 0.20 mm/y. However, other studies indicate faster rates; from data in one study (Bormann et at. 1995), solum development rates in Alaskan podzolic soils can be derived. These rates range from 0.64-1.72 mm/y in the youngest soils of the study (~50 years) to 0.38-0.54 mm/y in the oldest soils (320-350 years), indicating that the solum development rate decreases with increasing soil age. Similarly, solum development rates in podzolic soils from Denmark (Nielsen 2004) range from 0.45 [+ or -] 0.01 mm/y (mean [+ or -] SE) in soils ~1700 years old, but in soils with an age of ~2800 years, the rates decrease to 0.30 [+ or -] 0.05 mm/y.

[FIGURE 5 OMITTED]
TABLE 1. Horizon thicknesses in cm.

Ridge no. Horizon thickness (cm)

 Oiea A E and EB BE and Bs

1 N/A N/A N/A N/A
2 N/A N/A N/A N/A
3 N/A 5 N/A N/A
4 N/A 16 N/A N/A
5 N/A 7 N/A N/A
6 N/A 10 5 N/A
7 N/A 11 N/A N/A
8 N/A 11 N/A N A
9 N/A 11 N/A N/A
10 N/A 11 N/A N/A
11 N/A 11 N/A N/A
15 N/A 12 N/A N/A
20 9 N/A 10 N/A
30 7 N/A 18 26
40 5 N/A 12 27


Soil pH, at a fixed depth of 15-25 cm below the surface, follows a non-linear development, dropping about 2 pH units after ~150 years of soil development, and subsequently remaining at a more or less stable level around pH 4.8 (Figure 5b). In addition, samples from deeper-lying Bs and C horizons in Ridge 30 and 40 illustrate that soil pH increases with depth. The swift drop in young soils has been reported elsewhere (e.g., Crocker and Dickson 1957; Bormann et al- 1995), but chronosequences often Sack the youngest soils (Bronick and Mokma 2005) in order to report this pH drop in detail. A pH development trend tike this can he attributed to ongoing acidification in a non-carbonate, coarse-grained, buffered soil and subsequent stabilization by aluminum compound buffering (Lofts et al. 2001). The increasing pH with depth is interpreted as a sign of the acidification originating from the soil surface, i.e., from organic-acid production during litter decomposition (Lundstrom et al. 2000) and rainwater with carbonic acid. Interestingly enough, the pH drop between ridge 3 and 4 corresponds to the change from open dune vegetation and scrubs to dense forest (Figure 2).

The rapid acidification, sandy parent material, predominantly coniferous or mixed forest surface vegetation types, and the discovery of Eastport sands in the oldest ridges with slightly developed podzolic horizons (Table 2) all lead to the conclusion that podzolization is the predominant soil forming process in the study area. This observation is supported by a study from the northeastern tip of the Lower Peninsula of Michigan (i.e., the study site and the surrounding area), which found that the dry, sandy soils in the region have potential for strong podzolization (Schaetzl and Isard 1991). These conclusions allow a calculation of podzolization index values, such as the POD index (Schaetzl and Mokma 1988), in order to quantify the degree of podzolization. POD index values for all profiles are 0, except Ridge 30 with an index value of 1. This matches the Soil Taxonomy classifications well; Ridges 1-20 are classified as Shelldrake sand (frigid, uncoated Typic Quartzipsamments), while Ridges 30 and 40 contain Eastport sand (mixed, frigid Spodic Quartzipsamments; Table 2). Paralleling solum thickness data, this illustrates the very slight soil development across the Sturgeon Bay ridge plain, with little soil development in most soils (except acidification and surface accumulation of organic matter) and slightly expressed podzolic horizons in the two oldest ridges.
TABLE 2. Soil profile descriptions. Classifications according to Soil
Taxonomy (Soil Survey Staff 2006). Pedons for ridges 1 and 2 are
frigid, uncoated Typic Quartzip-samments like the Shelldrake series,
but lack pedogenetically altered horizons. The Shelldrake series is
frigid, uncoated Typic Quartzipsamments. The Eastport series is mixed,
frigid Spodic Udipsamments.

Ridge Soil horizons Predominant vegetation; USDA
no. groundcover classification

1 C Dunegrasses Unaltered parent
 material

2 C, 2Ab Low scrubs; dunegrasses Unaltered parent
 material with
 buried horizons

3 A, C Open spruce forest, low Shelldrake
 scrubs series

4 A, C Dense spruce forest Shelldrake
 series

5 A, C Open oak/pine/spruce Shelldrake
 forest series

6 A, E, C Open oak/pine forest; Shelldrake
 grasses series

7 A, C Open oak/spruce forest; Shelldrake
 wintergreen, oak litter series

8 A, C Open oak/spruce forest; Shelldrake
 ferns/oak litter series

9 A, C Open oak/spruce forest; Shelldrake
 mosses, reindeer lichen series

10 A, C Red pine forest; mosses, Shelldrake
 wintergreen, ferns series

11 A, C Spruce forest; mosses, Shelldrake
 needle litter series

15 A, C Oak/spruce forest; Shelldrake
 wintergreen series

20 Oiea, E, C White pine/cedar forest; Shelldrake
 needle litter series

30 Oiea, E, EB, Pine/cedar forest; Eastport series

 Bs, C wintergreen, scrubs,
 ferns

40 Oiea, E, BE, Pine/cedar/birch Eastport series
 forest;

 Bs, C needle litter


This soil development pattern, however, is inconsistent with the findings of other local studies (Lichter 1998a; Barrett 2001); the latter studied a similar beach ridge sequence just across Lake Michigan, near Naubinway, MI. Both studies reported discernible E and B horizons at soil ages around 200 years, whereas this study does not encounter such horizon development in the Sturgeon Bay ridges until a soil age of ~ 1,100 years. Other studies of early podzolization in coastal dune sands with coniferous trees have encountered podzolic horizon development within 100 years in Jutland, Denmark (Stutzer 1998), in 300-500 years at the Falkland Islands (Wilson 2000) and in 400-500 years in Northwest Finland (Jauhiainen 1973). In British Columbia, one study reported that a complete podzol profile could develop in 350 years (Singleton and Lavkulich 1987), while another study found that podzolic horizons developed in 40 years under a Mid-Michigan pine plantation on hitherto undifferentiated sands (Bronick and Mokma 2005). On the other hand, a study from Emmet County, Michigan (same area as this study), reported that 4,000-10,000 years were required for formation of podzolic horizons (Barrett and Schaetzl 1992). Similarly, a study discovered E-horizons in 3,000 year old podzolized lake sediments in Emmet and Cheboygan counties, Michigan, but 2,250 year old sediments in the same area lacked these horizons (Franzmeier and Whiteside 1963). A study on outwash sediments near the Michigan-Wisconsin border concluded that podzolic horizons did not occur until 7,000 years of soil development (Ewing and Nater 2002). As for the present study's POD index values of 0-1, these values correspond well with a study reporting no POD index values above 1 in 2,000-3,000 year old Michigan soils (Schaetzl and Mokma 1988). Adding to this consistency, solum development rates and soil pH development (mentioned above) are both consistent with studies in the Great Lakes Region or similar settings.

Although the soil development rates from the present study are bracketed by results from nearby studies, a fairly large inconsistency in the minimum age of podzolic horizon development persists between the present study and another local study (Lichter 1998a). The differing ages indicate large spatial differences in podzolization development, supported by the wide range of podzolization rates found in the literature, even within Michigan. The difference in soil development rates can be explained by (i) vegetation differences between the sampling sites of the two studies, (ii) parent material heterogeneity, and (iii) different sampling methods. However, the vegetation found in this study is comparable to the vegetation assemblies reported in Lichter (1998b). As for the second explanation, limestone boulders were found at the north shore of the Wilderness State Park. Limestone may delay the podzolization process by buffering the soil pH, but such buffering was dissolved within a few centuries of soil development in both studies. Parent material heterogeneity could also be caused by varying aeolian deposition in the study area. Parabolic dunes, parent material sorting, and the buried A horizon at Ridge 2 (Table 2) all indicate that aeolian deposition has affected the Sturgeon Bay ridges. An unevenly distributed, younger aeolian blanket across the Sturgeon Bay sequence would reset the soil development at some locations by burying the original sola in undifferentiated materials. Future optical stimulated luminescence (OSL) dating of the Sturgeon Bay sequence would clarify if the ridges are buried by substantially younger aeolian layers in some places, as these aeolian layers would have OSL dates considerably younger than the ages of neighboring ridges and underlying layers (cf. Nielsen et al. 2006). Ground-penetrating radar (GPR) transects might also help clarifying the ridge composition (cf. Nielsen 2008). However, without such analyses, the only obvious explanation for the differing soil development will be the difference in sampling methods. The soil probes can blur the recognition of thin horizons, which would be found in excavated soil pits. In addition, soil chemical analyses can help to differentiate thin eluvial horizons otherwise masked by the organic materials in the overlying A horizons.

CONCLUSIONS

The soil development across the Sturgeon Bay beach ridge plain is relatively slow; solum thickness increases at a rate of about 0.33 mm/y, while Soil Taxonomy classifications indicate slight podzolic horizon development in Ridge 30 and 40 (~1,100 and ~1,400 years old, respectively). Earlier podzolization chronosequence studies from Michigan suggested a wide scatter in podzol formation rates (50-10,000 years), even within small regions. The results from this study lie within that interval, as podzolic horizon development, based on the Sturgeon Bay beach ridge sequence, occurs within 1,100 years. While the podzolic horizon development rate differs from previously published rates from the Sturgeon Bay ridges, the soil pH development, exhibiting a steep drop from pH ~7 to ~5 after about 150 years of soil development, is in good agreement with other soil development studies; the same applies to solum thickness development rates, which are in line with the results from other studies in the Great Lakes region.

Finally, with a good, pre-existing chronology, beach ridge plains like this are very suitable for a wide range of subsequent investigations, illustrating the potential of collaboration between soil geomorphologists, Quaternary geologists, and other geomorphologists in order to provide chronologies for geomorphic surfaces like this, with possible spin-off projects in other disciplines.

ACKNOWLEDGMENTS

I would like to thank Trevor C. Hobbs and Bradley E. Blumer for assistance during the fieldwork. I also thank Randall J. Schaetzl, Michigan State University for valuable comments on the manuscript, and the Department of Geography, Michigan State University for providing field equipment and laboratory facilities. This study was carried out during a stay as a visiting research scholar at the Department of Geography, Michigan State University. Finally, two anonymous reviewers are thanked for their insightful comments and constructive suggestions for improvement of the paper.

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ASGER H. NIELSEN

University of Copenhagen
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Author:Nielsen, Asger H.
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Date:Sep 22, 2008
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