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Grain size analysis as a means to establish factors for determination of ecological settings

Keith Chojnacki

Stony Brook University, Stony Brook, NY

Abstract

Numerous studies have been performed to analyze the loess found on Long Island. The processes that

formed it are however, still poorly understood. The purpose of this study was to look at grain size

distribution of loess as a determining factor of ecological settings found on Long Island. Soil samples

were taken from four different sites: Stony Brook University (SBU), Suffolk County Farm (SCF), North

Street (NS), and the Dwarf Pine Plains (DPP). Each of the samples taken from varying depths were

analyzed using the both the Mastersizer 2000 and a Gradistat program. The sample revealed an

abundance of silt and gravel at both the SBU and SCF locations but found the NS location to include

medium sand along with silt and some gravel and the DPP location is predominantly very coarse sand

with some silt. There appears to be a correlation between the ecological settings and grain sizes at the

SBU and SCF sites as well as a correlation between the NS and DPP sites. With there being similarities

between sites it can be reasoned that the processes that lead to the deposition of sediments are similar

if not the same. The presence of a diamict however, leads one to believe that the processes that lead to

the deposition of these sediments are more complex. They may be the result of an impact event such as

the one thought to have caused the Younger Dryas approximately 13,000ya. With factors such as

precipitation and temperature are the same at all four sites, it can be argued that the grain size of the

loess found at these sites is the determining factor of ecological settings across Suffolk County.

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Introduction

Loess is a geologically recent deposit of silt or material which is usually yellowish or brown in

color and consisting of tiny mineral particles brought by wind to the places where they now lie. It is

found world-wide in places such as northern China, central Europe, and parts of Russia. The thickest

deposits of Loess are found in the Midwest along the Missouri River. Recently though, the discussion of

the existence of ‘pebbly loess’ on Long Island has arose. By definition, one would expect the wind-

blown sediments to be well sorted and lack sediments larger than 2mm in diameter. But an analysis of

loess at various sites found in Suffolk County, New York show that the loess is actually poorly sorted and

contains large clasts ranging in sizes up to 10mm. This is not characteristic of typical loess. Another

characteristic of Loess is that it helps to form very fertile soil. In areas where loess is found, one would

see flourishing farms, forests and various other ecosystems. During a recent study, grain size analysis

was performed on soil taken from four different sites in Suffolk County. The results of the analysis

showed that each of the four sites contains vastly different grain sizes. Among the sites are different

ecosystems as well. This is in contrast to the findings of a previous study by Champey in the Pine Barren

region. It concluded that there weren’t any distinct physical characteristics that accounted for

differences in plant settings and the differences were most likely due to abiotic and biotic interactions

and not the grain sizes (Champey). The data in a study by Adler et al show similarities in grain sizes and

distribution in the Dwarf Pine Plains region with the predominance of gravelly sands with silt sized

particles present (Adler et al). Considering all things, grain size seems to be a characteristic to consider

when discussing a factor contributing to the selection of ecological settings.

Some factors that determine types of ecosystems include temperature, rainfall, altitude, pollution,

sunlight, and wind. With all of these things equal between the four sites this studies purpose was to see

if there is a correlation between the soil grain sizes near the surface of the sites and types of ecosystems

that inhabit them. Even though the four sites are found within the same climate, the ecosystems are

vastly different. It is believed that the various grain sizes are responsible.

Methods

Samples were collected from four different sites across Suffolk County, New York. The four sites include

Suffolk Country Farm (SCF) (40°49'40.02"N 72°55'31.44"W), Stony Brook University Campus (SBU) (40°

54'51.78"N 73° 7'44.64"W), North Street (NS)(40°52'15.96"N 72°50'17.58"W), and the Dwarf Pine Plains

(DPP)( 40°52'18.12"N 72°39'17.40"W) (Figure 1).

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At each of the four locations, the research team dug at sites that appeared to be undisturbed. At each

site soil samples were taken at varying depths and placed into labeled bags. Samples were collected

from each of the labeled regions shown in the figures below. Layers were determined based upon

differences in apparent coloring.

Fig. 2A-SBU section, 124 cm depth Fig. 2B- SCF section, 40.64cm depth

Fig. 2C- NS section, depth 60.96cm Fig. 2D- DPP section, 68.58cm depth

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After samples were collected, they were brought back to Stony Brook University and prepped for

analysis using the following method outlined by Timothy Clare (2013):

After collection, the samples were taken into the lab and laid out on clean sheets of white paper to dry

for 24 hours. The next day each sample was prepped for grain-size analysis using the following

procedure:

1. Large pebbles and organic matter removed from sample

2. 10 minute paper folding method to remix sample, as sediments will naturally sort overnight

3. 10 grams of sample is weighed out into a small cup (cup is weighed first)

4. 10 g sample is sifted through a 1mm sieve

5. Larger particles (≥1𝑚𝑚) are removed, weighed and put aside

6. Particles <1𝑚𝑚 are weighed and placed into a small plastic bottle

*Steps are repeated for each sample

7. Each bottle is then filled with a (NaPO3)6 solution that acts as a dispersant

8. Bottle is shaken vigorously for 30 seconds after dispersant is added

9. Samples are left to sit for 24 hours

Once samples are prepared in the manner detailed above, they can be run through Mastersizer 2000

particle size analyzer. As detailed by the manufacturer Malvern (2014), the Mastersizer 2000 uses laser

diffraction to measure the size of particles by measuring the intensity of light scattered as a laser beam

passes through a dispersed particulate sample. The instrument software analyzes the data and

calculates the size of the particles that created the scattering pattern.

The samples were prepared for the Mastersizer 2000 in the following manner detailed by Clare (2013):

1. (NaPO3)6 solution is run through the machine as a background for the particulate sample

2. Sample is shaken for 30 seconds using a vibration machine to complete mix the particles

3. Small amount of sample is drawn out of bottle with a pipette and dropped into solution to an

obscuration of ~15% (measured by computer software)

4. Sample is passed through Mastersizer 3 times

The Mastersizer instrument software averages the three tests to yield a mean, mode, and percent of silt

versus sand and clay for each of the samples. For the purposes of this study, this information was then

input into a program called Gradistat to run grain size statistics for each sample. Gradistat provides

rapid grain size statistics by arithmetic, geometric and logarithmic moments and methods (Blott and

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Pye, 2001). Gradistat assumes sieves are used to measure the amount of sediment retained in a

number of size fractions, which serves as the input data for the program (Blott and Pye, 2001). Sample

statistics including mean, mode(s), sorting, skewness, kurtosis, and a range of cumulative percentile

values (Blott and Pye, 2001). According to Blott and Pye (2001), the following parameters are used to

define a grain size distribution from gradistat: the average size of grains, sorting, skewness, and kurtosis.

Sorting refers to the spread of the sizes around the average. The preferential spread of grains to one

side of the average is known as the skewness. Lastly, kurtosis refers to the degree of concentration of

grains relative to the average or is considered the measure of “peakedness” in a distribution curve (Blott

and Pye, 2001). A normal distribution, or symmetrical curve is considered mesokurtic. An excessively

peaked distribution curve, better sorted in the central portion of the graph rather than the outer tails is

leptokurtic. If the opposite holds true, and sorting is better on the tail ends, the sample is platykurtic.

Results

For the purposes of the study, only the upper two layers (A and B) are used for each of the four sites.

Stony Brook Campus

Figure 3A- SBU A

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According to the phi values on figure 3A, in the upper layer at SBU there are two modes present which

represent the presence of silt and gravel. Figure 3B shows the same modes for both silt and gravel.

Stony Brook Ecology

The setting for this site is a deciduous forest that is predominantly Acer rubrum, Quercus alba,

and Vibernum acerifolium that is adjacent to a stream bed (Chojnacki, 2013). There is the presence of

leaf litter on the forest floor that is mainly leaves from the previously mentioned Acer rubrum and

Quercus alba.

Figure 3B- SBU B

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Suffolk County Farm

Figure 4A- SCF A

Figure 4B- SCFB

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As shown in figure 4A, there are three distinct modes present that represent silt, coarse sand and

pebbles. However, in figure 4B, only two modes are present. Those are representative of silt and

gravel.

Suffolk County Farm Ecology

The setting for this site was a section of deciduous forest that is adjacent to farmland. The area where

the samples were taken was away from the farm in an undisturbed portion of the forest. This forest

consisted of Acer rubrum, Prunus serotina and Quercus alba.

North Street and Dwarf Pine Plains

Figure 5- Ecology Maps: The Nature Conservacy,

2003

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North Street

Figure 6A- NS A

Figure 6B- NS B

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The grain size analysis in figure 6A shows three distinct modes which are representative of silt, medium

sand, and gravel. Figure 6B however only shows two modes of silt and medium sand.

North Street Ecology

The ecological setting for this site is shown the ecological map provided in figure 5 which classifies this

region as an Oak-Pine forest. The predominant plants here include Quercus alba, Pinus rigida, and

Quercus prenoides. This setting also includes a nearby river.

Dwarf Pine Plains

Figure 7A (top)- DPP A, Figure 7B (bottom)- DPP B

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Figure 8- Soil texture diagrams for Stony Brook site, (top left) Suffolk County Farm, (top right) North Street, (bottom left) and Dwarf Pine Plains (bottom

right). (Dominguez, 2014)

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Figure 9- Soil analysis of Oak-Pine Forest, Brookhaven National Lab and Air

National Guard. (Champey)

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Figure 10- Grain size distributions of Long Island Pine Barren soil samples (Adler).

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The grain size analysis of the DPP A site shows two distinct modes that represents both silt and very

coarse sand. Figure 7B also shows two modes but is predominantly very coarse sand and little of silt.

Dwarf Pine Plains Ecology

The ecological setting for this site is described as Pitch Pine forest, as shown in the ecology maps shown

in figure 5. The predominant plants of this site are Pinus rigida and Quercus prenoides. This site, like

the North Street site, are both found within the boundary of the Long Island Pine Barrens.

Discussion

Based on the data that was collected during this study, it is evident that the grain size of loess may be

the determinant for the various ecological settings discussed in both western and eastern Suffolk

County. There appears to be a transition from predominantly all the way to very coarse sand as we

move west the east along the four sites (figure 8). Although each of these sites are only miles apart, the

western and eastern ends differ drastically on the ecological scale, despite the fact that they are

exposed to the same climatic conditions. Starting in the west at the SBU site, we find modes that are of

silt and gravel. These grain sizes found in the upper layers of soil are consistent with the requirements

for seed germination of the dominant plant species found there. Those include the V. acerifolium, Q.

alba, and A. rubrum, all of which prefer a leaf litter layer and plenty of moisture (Hutnick, 1961) which

would be present here due to the predominance of silt found at the SBU site. The Quercus alba prefers

looser soils Minckler, 1961) which is also found at the SBU site due to the presence of gravel with silt.

This would provide the conditions necessary for such an ecosystem to exist.

Progressing eastward to the SCF site, there are similar ecological settings. Much like the SBU site, there

are Quercus alba and Acer rubrum found in abundance at the SCF. This would make sense since the

grain analysis shows similar composition. Just like at the SBU site, the SCF site is found to be

predominantly both silt and gravel. The fact that the two sites are so strikingly similar it would be safe

to assume that the processes that deposited the loess were the same, likely glacial processes that

created the moraine and tunnel valley where we find these two sites. Although that it is likely glacial

processes responsible for the deposition, there are other processes that may have contributed to the

nature of the sediment. This makes sense considering the classification of these sediments as a diamict.

A diamict is defined as non-sorted, or poorly sorted, unconsolidated sediment containing a wide range

of particle sizes (Flint, 1971: Frakes, 1978; Eyles et al., 1983). The processes that created this diamict

may be the result of an impact event that occurred on the Laurentide Ice Sheet that caused the Younger

Dryas event that took place approximately 13,000ya (Firestone, 2007). From the SCF moving further

eastward, changes in ecological settings become more apparent. This may be due to the fact that the

processes that deposited the sediments were different. Both the NS and DPP sites are on what are

believed to be outwash plains. This would explain the fact that we find more sand here and less gravel

and pebbles.

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The NS site is dominated by Quercus alba, Pinus rigida, and Quercus pleniodes. One would expect not to

see Q. alba present but the presence of silt in the upper layers of this region are still very good

conditions for seed germination. Species that were not seen west of here are the P. rigida and Q.

plenoides. Both of these species, especially the P. rigida, prefer sites with little or no leaf litter and soils

with a sandy or gravelly texture (Little, 1953). The difference in grain size appears to be the determining

factor here in the transition between ecosystems with climatic conditions remaining the same. Moving

further eastward to the final site the ecological transition continues.

At the DPP site we find a predominance of very coarse sand. We also see the absence now of Quercus

alba but Pinus rigida is now the dominant plant species. This is possibly due to the change in grain size.

Quercus alba require an adequate amount of moisture for seedlings to survive whereas the Pinus rigida

require little to no leaf litter and well drained soils such as the DPP site. Although the DPP and NS sites

have their differences, they also have similarities. Those would be the transition to a predominance of

larger grain sizes at or near the surface and thus both have Pinus rigida inhabiting them. With these

similarities, one would assume again that the processes that resulted in the deposition of sediments

would be the same. These findings have both similarities and differences compared to studies done

previously by Champey and Adler in the Pine Barren region. The study performed by Champey shows

that samples analyzed from both an Oak-Pine and Dwarf Pine setting similar to those performed in this

study at North Street and Dwarf Pine Plains. The results of the Champey study show do not show the

bimodal nature of sediment size that this study does. The Champey results show a greater distribution

of sand grains ranging from very fine to very coarse without the presence of much silt (figure 9). This

study shows the presence of two distinct modes at very coarse sand and silt. It may be the presence of

the silt allowing for the moisture retention necessary for the survival of the oak seeds. Based on the

data, Champey concluded that physical characteristics of the soil were not responsible for the ecological

settings, but rather a combination of both biotic and abiotic factors such as wind pruning and

competition (Champey). From the data obtained by Adler et al, they concluded that the analyzed soil

samples from the Long Island Dwarf Pine Plains may be classified as slightly gravelly (pebbly) sands, with

up to nearly 10% silt-sized and finer particles (figure 10) (Adler et al). These finding are similar to those

obtained in this current study. The similarities shown in grain sizes could support the idea that soil

composition help to determine the ecological settings in Suffolk County.

The contrasting findings between this current study and Champey may be due to the fact that this study

incorporates physical characteristics of soils outside the Pine Barren region. It may be necessary for

further sampling to be done to draw any concrete conclusions in relating the importance of grain sizes in

determining ecological settings across Suffolk County. Differences in findings may also be because

analysis of the soil was also performed differently. The results of this study used the Mastersizer 2000

for grain size analysis whereas the previous study utilized sieves for analysis. It is possible that the more

precise measurements of the Mastersizer could be the difference in grain size analysis. It is also possible

that flaws in this current study exist. It would be beneficial to take additional samples to be analyzed

from the same areas and extend the samples to other regions east of the areas sampled. It is apparent

that further investigation is necessary to support any findings in this study.

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Conclusion

Ecological settings are typically determined by a multitude of factors such as wind, temperature,

precipitation and pH of soil. But when all of these factors are the same and we find different ecological

settings within miles of each other, another factor must be as play. Based on the results of this study it

is apparent that the grain size of the soil should be considered as a factor in the determination of

ecological settings on Long Island. The four sites that were used in this study show a progression from a

silty loam to a sandy loam moving west to east (figure 8). The change in the grain sizes found at each

site is what allows for the successful germination of seedlings at those sites. Also, although it is likely

glacial processes that are responsible for the differences in sediment sizes, there may be other

processes at play that would explain the diamict nature of the loess at these sites.

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References

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Central Suffolk County, N.Y." Web. 20 Aug. 2014.

Clare, T.D., 2013, Grain Size Analysis of Loess and Glacial Sediments atStony Brook University, Stony

Brook University, 9p.

Eyles, N., Eyles, C.H., and Miall, A.D., 1983, Lithofacies types and vertical profile models; an alternative

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