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1 Grain Size Analysis of Loess and Glacial Sediments at Stony Brook University Timothy D. Clare Stony Brook University, Stony Brook, NY [email protected] Abstract Previous studies of grain size distributions within sediments on Long Island have produced puzzling results. The data obtained during these studies showed consistent bimodal grain size distributions in the 10-40 micrometer and 300-400 micrometer ranges. These sediments, which are of glacial or eolian origin, also displayed unusual poor sorting. In many cases pebbles have been found within the loess layer. These pebbles may be the result of cryoturbation, but may also have another origin. The research discussed within this paper sought to obtain similar bimodal grain size distributions in varying depths of sediment collected on Long Island, and provide a possible explanation for this occurrence. It is the hypothesis of this author that the poor sorting and bimodal grain size distributions observed in sediments on Long Island may be the result of a shockwave, associated with an extraterrestrial collision over the Laurentide Ice Sheet approximately 13,000 years ago. This impact has been proposed by others (Firestone et al., 2007; Wittke et al., 2013) as a possible trigger for the onset of the Younger Dryas cooling period, which began abruptly ~12.9 ka. 11 different sediment samples were collected in this study from 3 sites along an exposed streambed in Stony Brook, NY. The results obtained correlate well those previously found in other studies. Bimodal grain size distributions existed in 9 of the 11 samples with primary modes in the fine-medium silt range (8-40 um) and a secondary mode in the medium sand range (250- 400 um). Pebbles with diameters up to 5cm were also found within samples of loess. The data was obtained using very replicable methods and further research into the topic should be considered. While the data obtained within this study does not prove an impact theory, it does support the hypothesis that a tremendous shockwave produced by an extraterrestrial collision disturbed and resorted sediments on Long Island ~13 ka. Introduction The purpose of the research described in this paper is to determine whether bimodal grain size distributions in the 10-30 and 300-400 micrometer range existed in loess and glacial sediments on Long Island. Soil samples were taken from a well-exposed streambed on the campus of Stony Brook University in Stony Brook, New York. The stream site provided accessible and well-defined sections of loess, glacial till, and glacial outwash, all of which were sampled. This study also sought to discover whether or not pebbles existed within the loess layer and provide possible explanations for this occurrence. Previous research on this topic is limited, especially on Long Island, providing little data to compare results to. However Olaofe (2012) and Zhong (2000) conducted similar research on glacial sediments on Long Island and displayed that bimodal grain size distributions existed within their samples. These modes were consistently in the fine-medium silt and medium- coarse sand size ranges. Olaofe (2013) also describes the unusual poor sorting of glacial sediment on Long Island, where coarser sediments, including pebbles, exists within the loess. If the loess were a true, undisturbed eolian sediment layer one would expect a well sorted grading from coarse to finer sediments. That is not observed on Long Island. Kay (1931) describes and provides possible evidence for a distinct pebble band on the surface of the Iowan till in northeastern Iowa, similar to what we observe on Long Island. Kay (1931) believes the pebble band is either residual coarse material from the Iowan till that was quickly weathered or eroded material from the till that had been broken down over a longer period of time. Neither explanation seems plausible, however, as violent windstorms during the period immediately following deposition would have covered the till with finer sediments. It also does not address why such poorly sorted coarse material is found mixed within a layer that should either be outwash or loess. Leverett (1932) also describes this pebble layer on the Iowan till in Minnesota, determining that; the pebbly concentrate on Iowan drift prior to deposition of the Peorian loess was a hasty process requiring a negligible amount of time, an assumption that is open to serious question (Leverett, 1932). At this period of time an impact theory was not documented as a possible cause for the pebble layer. Firestone et al. (2007) provide the first evidence of several low-density

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Page 1: Grain Size Analysis of Loess and Glacial Sediments at ... loess/Clare .pdfGrain Size Analysis of Loess and Glacial Sediments at Stony Brook University Timothy D. Clare Stony Brook

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Grain Size Analysis of Loess and Glacial Sediments at

Stony Brook University

Timothy D. Clare

Stony Brook University, Stony Brook, NY

[email protected]

Abstract

Previous studies of grain size distributions within sediments on Long Island have produced puzzling results. The

data obtained during these studies showed consistent bimodal grain size distributions in the 10-40 micrometer

and 300-400 micrometer ranges. These sediments, which are of glacial or eolian origin, also displayed unusual

poor sorting. In many cases pebbles have been found within the loess layer. These pebbles may be the result of

cryoturbation, but may also have another origin. The research discussed within this paper sought to obtain

similar bimodal grain size distributions in varying depths of sediment collected on Long Island, and provide a

possible explanation for this occurrence. It is the hypothesis of this author that the poor sorting and bimodal

grain size distributions observed in sediments on Long Island may be the result of a shockwave, associated with

an extraterrestrial collision over the Laurentide Ice Sheet approximately 13,000 years ago. This impact has

been proposed by others (Firestone et al., 2007; Wittke et al., 2013) as a possible trigger for the onset of the

Younger Dryas cooling period, which began abruptly ~12.9 ka. 11 different sediment samples were collected in

this study from 3 sites along an exposed streambed in Stony Brook, NY. The results obtained correlate well

those previously found in other studies. Bimodal grain size distributions existed in 9 of the 11 samples with

primary modes in the fine-medium silt range (8-40 um) and a secondary mode in the medium sand range (250-

400 um). Pebbles with diameters up to 5cm were also found within samples of loess. The data was obtained

using very replicable methods and further research into the topic should be considered. While the data

obtained within this study does not prove an impact theory, it does support the hypothesis that a tremendous

shockwave produced by an extraterrestrial collision disturbed and resorted sediments on Long Island ~13 ka.

Introduction

The purpose of the research described in this paper is to determine whether bimodal grain size distributions in

the 10-30 and 300-400 micrometer range existed in loess and glacial sediments on Long Island. Soil samples

were taken from a well-exposed streambed on the campus of Stony Brook University in Stony Brook, New

York. The stream site provided accessible and well-defined sections of loess, glacial till, and glacial outwash,

all of which were sampled. This study also sought to discover whether or not pebbles existed within the loess

layer and provide possible explanations for this occurrence. Previous research on this topic is limited,

especially on Long Island, providing little data to compare results to. However Olaofe (2012) and Zhong

(2000) conducted similar research on glacial sediments on Long Island and displayed that bimodal grain size

distributions existed within their samples. These modes were consistently in the fine-medium silt and medium-

coarse sand size ranges. Olaofe (2013) also describes the unusual poor sorting of glacial sediment on Long

Island, where coarser sediments, including pebbles, exists within the loess. If the loess were a true, undisturbed

eolian sediment layer one would expect a well sorted grading from coarse to finer sediments. That is not

observed on Long Island. Kay (1931) describes and provides possible evidence for a distinct pebble band on

the surface of the Iowan till in northeastern Iowa, similar to what we observe on Long Island. Kay (1931)

believes the pebble band is either residual coarse material from the Iowan till that was quickly weathered or

eroded material from the till that had been broken down over a longer period of time. Neither explanation

seems plausible, however, as violent windstorms during the period immediately following deposition would

have covered the till with finer sediments. It also does not address why such poorly sorted coarse material is

found mixed within a layer that should either be outwash or loess. Leverett (1932) also describes this pebble

layer on the Iowan till in Minnesota, determining that; the pebbly concentrate on Iowan drift prior to deposition

of the Peorian loess was a hasty process requiring a negligible amount of time, an assumption that is open to

serious question (Leverett, 1932). At this period of time an impact theory was not documented as a possible

cause for the pebble layer. Firestone et al. (2007) provide the first evidence of several low-density

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extraterrestrial objects exploding over North America ~12.9 ka and possibly destabilizing the Laurentide ice

sheet. The evidence Firestone et al. (2007) provide include a carbon-rich black layer at 50 Clovis sites in North

America, an iridium layer and the mysterious Carolina Bays found throughout several regions of North

America. Firestone’s hypothesis may hold clues to the pebble layer found on the Iowan till, the pebbles within

loess on Long Island, and the poor sorting of glacial sediments on Long Island. That being, a tremendous

shockwave produced from such a collision or airburst may have significantly altered layering within the young

sediments. This shockwave was likely not strong enough to displace material great distances, but could have

tossed material upward allowing it to resettle very quickly in a poorly sorted fashion. Kundic (2005) has shown

using Muscovite Ar/Ar that the loess on Long Island has a mode of 300-400 ma, and therefore must be from

eroded Acadian terrane to the North. The most likely source of the loess on Long Island, according to Kundic

(2005), was the exposed bed of the exposed Lake Connecticut, beginning at ~17 ka. A shockwave caused by an

extraterrestrial impact over the Laurentide Ice Sheet likely only disturbed the loess and underlying glacial

sediment and may provide the best explanation for the origin of pebbles. Wittke et al. (2013) also support this

theory, providing further evidence for an airburst or collision that includes a possible Younger Dryas boundary

field marked by the presence of microspherules and nanodiamonds.

The purpose of the research discussed in this paper is to test the hypothesis that loess and glacial sediments on

Long Island demonstrate bimodal grain size distributions in the 20-40 and 300-500 micrometer range.

Consistent modes in these ranges within varying sediment layers may support the hypothesis that an

extraterrestrial impact at the onset of the Younger Dryas may have disturbed these sediments and affected

sorting. The sand (300-500 micrometers) was likely carried in the glacial load and deposited during melting or

outwash from streams as the ice sheet retreated. The loess, silt (20-40 micrometers), is likely eolian sediment

transported by high-speed katabatic winds from North. It is my hypothesis that at the time of deposition of the

loess, it was well sorted and bedded on top of the sand, pebbles and glacial till. Some sort of disturbance must

have occurred to produce the bimodal grain size distributions studied in this research. The technique used to

test grain size in this study was collection of sediment from several layers of exposed loess and glacial till, and

analysis with the Malvern Mastersizer 2000 particle size analyzer. The procedures used in this study are

efficient and produced good data. The only area of concern within the study may be the reliability of the

Mastersizer 2000 machine, which by all accounts seems to produce sound and reliable data.

Figure 1. Stony Brook University stream site (40°54′ 51.96" N 73°07' 44.10" 𝑊). SB1 – 4 samples were

taken from exposed layers of loess and till. SB2 – 3 samples taken from exposed layers of loess and till. SB3 –

3 samples taken from exposed layers of loess and till, and 1 sample of outwash taken from a 2 foot hole.

SB 1 SB 3 SB 2

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Methods Samples were collected for analysis from three sites in a well-exposed section of loess and glacial sediments

along a small stream on the campus of Stony Brook University (40°54′ 51.96" N 73°07' 44.10" 𝑊) [Figure

1]. Each site had well-exposed sections of loess and till from which samples were taken. At the third site,

labeled as SB 3 in this study, a 24 in. hole was dug at the base of the section and samples were taken which

represent glacial outwash. Each section was first scraped clean with a small hand shovel, as to prevent

contamination, and samples were then taken with a small trowel and placed into well-labeled plastic bags. A

total of 11 samples were taken from the 3 sites, at varying depths, representing the different types of glacial

sediment, as described below.

Samples – Stony Brook University Stream Site (40°54′ 51.96" N 73°07' 44.10" 𝑊) [Figure 2]

Name Depth Section Description

SB 1A 20” Loess Light-Brown silt, with no sand or pebbles

SB 1B 40” Loess Tan silt with pebbles, little sand

SB 1C 50” Till Reddish-Brown silty sand (coarse) with pebbles

SB 1D 60” Till Dull-Brown coarse sand with pebbles, some silt

SB 2A 10” Loess Tan silt with sand and pebbles

SB 2B 30” Till Light-Brown coarse sand with many pebbles

SB 2C 60” Till Dull-Brown silty sand with pebbles

SB 3A 10” Loess Tan sandy silt with few pebbles

SB 3B 30” Loess Tan silt with few pebbles

SB 3C 50” Till Red-Brown sand, no pebbles

SB 3D 80” Outwash Light-Brown gravel-sand with pebbles, some silt

SB 1 – Samples A – D shown from left to right:

SB2 – Samples A – C shown from left to right:

Figure 2. Sediment samples from Stony Brook University stream site

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SB3 – Samples A – D shown from left to right:

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

The following day the samples are prepared for grain size analysis. Grain size was measured in this study using

the Mastersizer 2000 particle size analyzer. The Mastersizer 2000 is a small machine hooked into a computer

unit that uses laser diffraction to measure particle size. As a dispersed particulate sample is passed through the

machine, the intensity of light scattering is measured as a laser beam passes through the sample. The

Mastersizer software then analyzes the data to calculate the size of the particles that created the scattering

pattern (Malvern.com, 2013). The soil samples were prepared for analysis in the Mastersizer 2000 using the

following method:

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

After the three tests have been run on a particulate sample the Mastersizer computer program produces an

average of the three tests; calculating a mean, mode and percentage of grain sizes for each sample. The results

of this study are shown in the following section.

Results

Every sample analyzed in this study, save samples SB 1B and SB 3B, displayed bimodal grain size

distributions. The primary mode for each sample fell in the fine-medium silt range (8-31 um), with a secondary

mode in the medium sand range (250-500 um). Samples SB 3C and SB 3D produced opposite results with

primary modes in the medium sand range and secondary modes in the very fine silt range (2-8 um). The

samples taken from SB 1 [Figure 3] displayed common modes from the top layer of loess down through the till.

SB 1A had a primary mode of 36 um (very fine sand) and a secondary mode of ~450 um (medium sand). SB

1B had a primary mode of 25 um (medium silt), with no secondary mode. SB 1C had a primary mode of 16 um

(medium silt) and a secondary mode of ~475 um (medium sand). SB 1D had a primary mode of 20 um

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Figure 3. Grain size distributions and modes of samples SB 1A – 1D.

Figure 4. Comparison of grain size distributions of the four samples taken from site SB1.

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Figure 5. Grain size distributions and modes of samples SB 2A – 2C.

Figure 6. Comparison of grain size distributions of samples SB 2A – 2C.

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(medium silt) and a very small secondary mode of 500 um (medium sand). A graph comparing the four

samples taken at site SB 1 is provided on the following page [Figure 4].

The samples taken from site SB 2 [Figure 5] also displayed common modes, again primarily in the fine silt

range with a secondary mode in the medium sand range. Sample 2A had a primary mode of 19 um (medium

silt) and secondary mode of 350 um (medium sand). Sample 2B had a primary mode of 10 um (fine silt) and a

secondary mode of 400 um (medium sand). Sample 2C had a primary mode of 13 um and a secondary mode of

350 um (medium sand). The grain size distributions of these three samples are compared on a graph in Figure

6.

The samples taken from site SB 3 [Figure 7] produced somewhat more ambiguous results with the two samples of loess producing only a primary mode in the very fine sand range. The samples of till and outwash produced bimodal grain size distribution, but with primary modes in the medium sand range and secondary modes in the fine silt range. Sample SB 3A had a primary mode of 27 um (medium silt) and a very small secondary mode of 1250 um (very coarse sand). Sample SB 3B had a primary mode of 48 um (very fine sand) and no secondary mode. Sample SB 3C had a primary mode of 365 um (medium sand) and a secondary mode of 5 um (fine silt). Sample SB 3D had a primary mode of 383 um (medium sand) and a secondary mode of 3 um (fine silt). Again, the grain size distributions of the samples from site SB 3 are compared graphically in Figure 8. Discussion The results from this study support the hypothesis that the loess and glacial sediments on Long Island have bimodal grain size distributions. The results also show that as depth increases into the sediment layer common modes and distributions exist, and the grain sizes are more poorly sorted than the processes that delivered them should allow. The modes in grain size distribution from the samples in this study were primarily in the fine-medium silt range and the medium sand range. The medium sand is likely a product of deposition by streams flowing from the retreating and melting ice sheet, or was material carried at the base of the glacier and deposited along with the till. The medium sand must have been deposited immediately following the retreat of the ice sheet and well before the deposition of loess. The fine-medium sand found in these samples is likely eolian sediment derived from exposed Acadian bedrock to the North. The most likely source being the exposed bed of what is today the Long Island Sound (Kundic, 2005). This sediment was picked up, carried, and deposited by high-speed katabatic winds blowing down from the area of high pressure that exists over an ice sheet. The silt, loess, was carried only a short distance and deposited on top of till and outwash. Heavier, larger grained, loess would likely have settled out first, while finer silt would only have been deposited when the winds subsided. This would logically produce well-sorted bedding throughout a column of sediment. However, the data in this study shows that the loess and glacial material collected have an unusual sorting of fine and larger sediments. The data obtained shows a mix of fine and medium silt and medium sand throughout the various layers sampled. This pattern exists from the loess layer down into the till. As observed in the photographs of the samples provided, pebbles were also present within the loess. The data and observable evidence suggest that the sediments on Long Island were possibly at some time disturbed. This supports the theory of a shockwave produced by an extraterrestrial impact ~12.9 ka, that although not strong enough to displace material was powerful enough to sufficiently shake the sediment and mix varying sized particles. While the data produced in this study does not prove an impact at the onset of the Younger Dryas, it certainly adds another piece of evidence in the support of this theory. Along with the occurrence of Carolina Bays on Long Island, the poor sorting and bimodal grain size distributions of sediments point to a fact that something significant happened to the area ~13 ka. Additional research into this topic may unravel further clues about this hypothesis. While further similar studies of grain size distributions at different locations on Long Island would be helpful in producing

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Figure 7. Grain size distributions of samples SB 3A – 3D.

Figure 8. Comparison of grain size distributions of samples SB 3A – 3D

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comparable data. The proof of an extraterrestrial collision ~13 ka may lie in the study of Carolina Bays and possible impact materials in these areas, including microspherules and nanodiamonds. Bennington, J.B., Hakiman, A. (2001) Grain size analysis of cretaceous and Pleistocene sedimentary facies exposed at Caumsett State Park, Lloyd Neck, New York. Firestone, R.B. et al. (2007) Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc Natl Acad Sci USA, v. 104(41). p. 16016 – 16021. Folk, R.L. (1964) A review of grain-size parameters. Sedimentology, v. 6. P. 73-93. Kay, G. (1931) Origin of the pebble band on Iowan till. The Journal of Geology, v. 39(4). P. 377-380. Kundic, V. (2005) Age and provenance of Long Island loess, Masters Thesis, Stony Brook University, 61 p. Olafoe, A. (2012) The effects of acid rain on soil pH and the investigation of the relationship between soil pH and depth. Wittke, J.H. et al. (2013) Evidence for deposition of 10 million tonnes of impact spherules across four continents 12,800 years ago, Proc Natl Acad Sci USA, www.pnas.org/ggi/doi/10.1073/pnas. p. 2088-2097. Zhong, J. (2000) Grain size analysis and provenance of Long Island loess.