.gif)
Topics Covered
Background
Particle Size Analysis –
Important Geological Information
Particle
Characterization and the Origin of Materials
Correlating Particle
Size to Environment
The Relationship between
Grain Size and Current Velocity
Ocean Sediments
Effects of Geologic
History
Lake Sediments
Tracing Climate Change
Using Grain Size Variations in Sediments
Sediment Formation and
Types of Sediment
Terrigenous
Sediments
Biogenic Sediments
Authigenic Sediments
Volcanogenic
Particles
Cosmogenous
Particles
Environmental
Applications – Contaminants in Sediments
Ocean and Lake Floor
Sediments
Sediments Disturbed by
Bio-Organisms
The Effect of Deforestation on
Sediment Loads
Limitations of
Traditional Particle Analysis Techniques
Advantages of Modern
Particle Size Analysis Methods
Laser Diffraction Particle Size
Analysis
Sample Preparation and Testing
MethodsDigital Image
Analysis
Background
Horiba Scientific-Particle
measurement instruments offer advanced optics, powerful algorithms, and
flexible software, combined with advanced sample handling systems and a full
range of options.
HORIBA's
commitment to particle characterization instruments brings you the widest range
of solutions to your particle analysis needs including:
Particle Size Analysis – Important Geological Information
Particle size
analysis of a geologic area can reveal critical information about the
region’s formation, history and climate. Particle characterization is an
important tool for studying changes in geology and climate over time, mode of
formation, and current environmental effects related to pollution transport,
erosion, and sediment transport.
Particle Characterization and the Origin of Materials
Sediments are geologic materials that are
formed in one place, moved to another, and deposited. The study of ocean and
lake sediments provides a wealth of information about the geological history of
a particular area. The characterization of sediments involves a number of
parameters including size, composition, shape, spatial arrangement of grains,
and the mode of formation (the origin of the material).
Geologists commonly use the Wentworth Scale
(a geometric scale based on 1mm, decreasing in diameter by ½). The Phi Scale is
a commonly-used modification that allows the use of simple whole numbers for
class boundaries by applying the logarithmic transformation: phi = -log2d, where
d is the particle diameter in millimeters. The geometric size scale offers a
simple relationship to physical transport properties, specifically current
velocity.
Correlating Particle Size to Environment
The principal factors controlling
sedimentation are particle
size and deposition-site
energy conditions. Generally, the particle size of a deposit is proportional to the energy
level present at the time of deposition. Thus, high energy beaches are composed
of coarse sand and, conversely, quiet lagoons are composed of fine
mud.
The Relationship between Grain Size and Current Velocity
Grain size and current velocity determine
whether a particle will be eroded, transported, or deposited. The Hjülstrom
diagram (Fig. 1) represents a well-known relationship between grain size and
current velocity.
Figure 1
These basic principles allow us to measure
grain size and density for an ancient deposit and infer the energy of the
formation event for that geologic area. For example, deposits in West Texas and
Central Mexico trace their origin to a 65 million year old
tsunami.
Ocean Sediments
Study of ocean-floor sediments allows us to
learn specific information about the formation of these areas. Continental
shelves are shallow and close to continental sediment supply. The deep sea
shelves are far from continental sediment supply and see virtually zero wave
energy.
The expectation is that, due to the greater
energy in the shallow water, larger material will be moved in shallow depths,
but not out into the ocean, so we would expect grain size to decrease off-shore
across shelves. Indeed, reality matches expectation in that most continental
shelves are covered with coarse sand.
Effects of Geologic History
Sea level has varied in the past with the
rise and fall of continental glaciers. Up to a mile of ice covered Canada,
Siberia, and Scandinavia. The water in this ice came from the oceans, thus
lowering sea levels. Eventually, as sea levels rose again, coarse material was
deposited across the oceanic shelves. This “relic” sediment, accumulated at an
earlier time and under different conditions, comprises up to 70% of all shelf
sediment.
As these shelves were near sea level and
exposed to the energy of the waves, a number of other factors can play a role.
Tidal forces mobilize the sediment once or twice daily. Waves can also erode
shelf sediment, thereby permitting transportation through additional currents.
Rivers flowing across what is now a shelf either deposited sand, gravel, or
deltas, or eroded slope canyons, increasing deposition in deep ocean
areas.
Lake Sediments
Grain-size variations in lake sediments
reflect changes in the processes and energy of sediment transport. Particle
sizes are closely linked to turbulence, wave energy, and proximity to shoreline;
increased grain sizes generally correspond to higher energy conditions of
sediment production or transport.
Tracing Climate Change Using Grain Size
Variations in Sediments
Variability of sediments in lake core samples
suggests that grain-size variation in sediments is an effective proxy for
environmental change in the area. The absence of precise ages for the cores may
allow only tentative correlations of the fluctuations to climatic
events.
Making assumptions about sediment
accumulation rate, different sediment levels can be correlated to a time period.
If this assumption is valid, increases in sand size or content may indicate a
period of lower lake levels corresponding to an arid or warm period. Conversely,
decreases in sand content may reflect periods of wet and cold
climates.
Prevalence of ostracodes can also be used to
infer lake-level changes. Ostracodes are small, shelled crustacea commonly
preserved in sediments. Because many ostracode species have narrow ecological
limits controlled by temperature, salinity, oxygen, food and other factors, they
can provide an important tool for paleoceanographic
reconstruction.
Sediment Formation and Types of
Sediment
Sediments originate from one of five general
sources. Each can be identified by specific chemical and physical
characteristics, including particle size.
Terrigenous Sediments
Terrigenous sediments are derived from the
land (terra). Rocks weather to small particles and are transported to the ocean.
This is called erosion. Much of it is deposited in river
deltas.
Biogenic Sediments
Biogenic sediments are the shells and
skeletal remains of living organisms. Only the “hard parts” are preserved,
typically CaCO3 and silica. These skeletons dominate the sediment in
many places.
Authigenic Sediments
Authigenic sediments are formed in place by
hydrothermal deposits at mid-ocean ridges and vents. Water circulates though the
crust, dissolving minerals and bringing dissolved ions to the ocean floor. This
water cools and the minerals precipitate out, leaving mineral rich sediments.
The most common are ferro-manganese nodules that have large potential economic
value (Mn, Co, Ni, Cu and other trace metals).
Volcanogenic Particles
Volcanogenic particles (ash) are produced by
most volcanic eruptions, but can be transported large distances by wind. Major
eruptions can affect sediments on a global scale.
Cosmogenous Particles
Cosmogenous particles are produced from
fragmented meteorites and products of their impacts. Although a small portion of
the total, they are important tracers of “events”.
Environmental Applications –
Contaminants in Sediments
Particle size of sediments is a primary factor in
determining how efficiently it retains contaminants. Finer sediments will trap
these contaminants for a longer period of time. Larger particles have greater
interstitial spaces, allowing the contaminants to be washed out and continue in
the water stream, having a continued effect on the
biosystem.
Ocean and Lake Floor Sediments
A profile of ocean or lake floor sediments is
important to study the conditions necessary to suspend bottom material and to
measure the transport of suspended sediment between different areas. Accurate
modelling of the transport and fate of both nutrients and anthropogenic
pollutants requires knowledge of the concentration and the particle size
distribution of suspended particulates. Re-suspension events have the capacity
to inject considerable amounts of particulate material (along with their
associated nutrients and/or pollutants) into the water.
Sediments Disturbed by Bio-Organisms
Also, changes in the size distribution can be
evidence of bio-organisms that disturb the sediment, causing a breakdown in
sediment size. This can be related back to the amount of nutrients available to
the organisms.
The Effect of Deforestation on Sediment Loads
Increasing sediment loads entering lakes and
rivers owing to widespread deforestation and erosion are increasing the need for
understanding of the effects of influent sediment composition on a biosystem.
Experiments have been conducted investigating the effects of exposure to
sediments of differing particle size ranges on survival of plant and animal life
in lakes and rivers.
It was found that survival rates decreased
with decreasing sediment particle size. This suggests that runoff from areas
that produce fine-grained sediments have greater detrimental effects on the
ecosystem and require greater attention.
Limitations of Traditional Particle
Analysis Techniques
Traditional particle size measurement
techniques include sieves for the larger size ranges, usually above 63µm (230
mesh size). Sieves are limited in resolution (number of sieves = number of data
channels), are slow and operator intensive, and are limited for measuring the
smaller size classes. Pipette or sedimentation is used for the finer fractions.
This is also a slow technique with significant operator dependency of the
results.
Both are affected by particle shape
influences. Particles pass through a sieve on the second smallest dimension, so
a needle-like particle will be reported as the smaller dimension, not the
length. Flat particles, like clay, will sediment in an orientation that gives
the greatest hydrodynamic resistance, like a leaf falling. This will be reported
as a much finer particle than the average of all dimensions. Care must be taken
when interpreting results or correlating historical data to new analytical
techniques.
Advantages of Modern Particle Size
Analysis Methods
Modern automated analytical techniques used
for sizing sediments include laser diffraction and digital image processing.
These new techniques are fast, easy, operator independent, have a much broader
range, and have a much higher resolution with many more data
channels.
The higher resolution of these techniques
allows for significantly more information to be obtained from a sample. Small
changes in the mode may not be picked up by widely spaced sieves, but are easily
resolved along with more details of the total size distribution. The
significantly greater speed of these techniques allows a much greater number of
samples to be analyzed, providing more detailed information about an area of
interest.
Laser Diffraction Particle Size
Analysis
Laser diffraction measures light scattered
from the particle as it passes through the measurement cell. The angle of
scatter is related to the size of the particles. The measurement is essentially
instantaneous, although total analysis times are on the order of seconds for
most samples. The HORIBA LA-950 has proven popular for this application
because of its wide size range (0.01-3000µm), speed, stability, and ease of
use.
Figure 2. LA-950SlurrySampler
In addition, the software is able to display
the results directly in the Phi Scale or in sieve size channels to correlate to
historical data (Fig. 3).
Figure 3. Lake sediment sample measured on the
LA-950
The large number of samples necessary to get
a comprehensive profile of an area has made automation options popular. The
Slurry Sampler can accommodate 15 samples and completely automates the sample
analysis task. Once configured properly, this automation can also significantly
improve reproducibility by removing any remaining operator
dependency.
Sample Preparation and Testing Methods
Depending on the source, samples may need to
be pretreated with 30% H2O2 to remove organic materials or
1 Molarity NaOH to remove biogenic silica (diatoms). Samples are generally
dispersed in deionized water for analysis. Finer grades with clay fractions may
require additional surfactant (usually 0.1% sodium hexametaphosphate) and
ultrasonic treatment to disperse agglomerates.
Owing to the wide range of sizes seen in a
given sample, care must be taken that sufficient sample concentration is used
across all sizes. Particularly with the larger particle size ranges, the number
of particles will be very low when compared to an equivalent volume or mass of
smaller particles. The resultant light transmittance values may not represent
the total sample breadth.
There must be sufficient sample so that if
the large particles were removed and analyzed separately, there would be enough
material to provide sufficient detector signal to get an accurate measurement.
This may give laser transmittance values outside of the default range. Method
development will need to include tests at different sample concentrations and
secondary confirmation using other methods, like microscopy or
sieving.
Digital Image Analysis
Digital image processing uses CCD (digital)
cameras and image analysis software to record a picture of the particles falling
in front of a light source, and then analyze the images to provide a wide range
of size and shape information. HORIBA’s dynamic
image analysis solution, the Camsizer, measures dry samples over a size
range of 30µm – 30mm, allowing analysis of even extremely coarse grades of
material.
The Camsizer’s
ability to report shape parameters also allows the user to correlate
measurements to traditional techniques such as sieving. For samples containing
finer grades including clay, the Camsizer’s
data is often combined with laser diffraction measurements, frequently the HORIBA LA-300,
to provide complete size range coverage.
Figure 4. Images from the
Camsizer
Figure 5. Camsizer with
Autosampler
For a complete set of
references, please refer to Particle Size Analysis of Soils and Sediments –
Applications Note by Horiba Scientific-Particle
Products
For more information on this source please visit Horiba Scientific-Particle
Products