PIXE Study Of Lacustrine Sediments Including A Sediment
Core From Lake Maicuru, Para, Brazil
I. I. Kravchenko, R. F. Kelly, F. E. Dunnam, H. A. Van Rinsvelt
Department of Physics, University of Florida, P.O.Box118440, Gainesville, FL 32611
J. H. Curtis
Department of Geological Sciences, University of Florida, Gainesville, FL 32611
P. E. De Oliveira
Universidade Guarulhos, Sao Paulo, Brazil
Abstract. PIXE (Particle Induced X-ray Emission) measurements are reported on downcore sediments from Lake
Maicuru, a remote lake in the Amazon Basin. We validate the method by comparison of previously obtained wet
chemistry/ICP elemental analysis of sediment from four different lakes with elemental analysis obtained by PIXE
measurement. Statistical analysis of results from both methods suggests that PIXE analysis works well for heavier
elements (Fe, K, and Ca) but poorly on lighter elements (Na, Mg, and P). The sample preparation procedure developed
to deal with the hydrophobic nature of the Maicuru sediments is discussed. Changes in the downcore elemental
distribution of Ti, Fe, Mn, and Zn in the lake Maicuru sediments are remarkably similar. The control of this parameter is
uncertain but is probably due to climatic changes and water availability and weathering. The good comparison of PIXE
and ICP for heavier elements and the consistent results on the downcore study suggests that the PIXE technique holds
promise for geochemical and paleoenvironmental studies.
Traditional geochemical studies of heavier
elements typically require complete sample digestion
using very strong acid (e.g. hydrofluoric acid) in
addition to further wet chemistry techniques prior to
measurement techniques such as atomic absorption
spectrometry or inductively coupled plasma
spectrometry. PIXE analysis avoids nearly all wet
chemistry allowing relatively rapid throughput of
samples with fewer chances for contamination and a
reduction in exposure to potentially dangerous
chemicals.
INTRODUCTION
Sediments from lakes located in remote regions can
record past climatic and environmental changes that
have not been distorted by human activities. Many
techniques are employed in extracting information
from the sediment cores including: radiocarbon dating,
isotopic analysis (O, C, N, H), grain size analysis,
pollen grain analysis, diatom analysis, etc. We
propose that PIXE (Particle Induced X-ray Emission)
analysis of sediments from lakes can yield information
about changes in erosion rates that taken together with
other paleoenvironmental evidence can provide insight
into past climatic conditions. This is especially true in
cases where elements in question are low in
concentration.
The concentration of both heavy and light elements
in lacustrine sediments is of interest to researchers in
many fields including geology, paleoclimatology,
environmental studies, anthropology, and archaeology.
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
419
and analyzed for each sample. Analyses of the PIXE
spectra were carried out using Pixfit [1] and final
element weight concentrations were averaged for each
sample.
For example, the history of acid rain in areas impacted
by emissions from power plants and automobiles can
be traced by examining Al concentration in sediment
of lakes downwind of the sources. A second example
is examining eutrophication of lakes using light
elements such as P. Heavier elements such as Pb, Cu,
and Ar have been measured in lake sediments to study
history of smelting of metals, herbicide and pesticide
usage, and gasoline additives.
SAMPLE PREPARATION
To quantify the spectral data the internal standard
method was chosen. Yttrium was used as the dopant
by which all other elemental concentrations in the
AGV-1 USGS geological standard were normalized
[2]. Obtained in such a way, sensitivity factors for
each studied element were utilized to calculate
absolute elemental concentrations, in mg/kg (PPM), in
samples that were laced with yttrium following
identical doping procedure. The biggest challenge in
preparing the Maicuru samples was obtaining a
uniform distribution of yttrium over the sample. The
specimens were powders with hydrophobic properties.
Simply dropping the yttrium directly onto the powder
created wet lumps resulting in a non-uniform Y
distribution in the target. To solve the problem the
following multi-step sample preparation procedure
was devised:
We present PIXE analysis results of the topmost 1
meter of a 5.15 meter long sediment core from Lake
Maicuru, Para, Brazil (0°30’S, 54°15’W) that covers
the period from more than 50,000 years before present
to the present. The sediment core has previously been
dated with 23 radiocarbon dates on terrestrial material
(seeds, twigs, and charcoal).
EXPERIMENTAL
PIXE spectra were obtained with 2.5 MeV protons
from the NEC tandem 1.7 MV accelerator at the
University of Florida. The beam was defined by a
series of graphite collimators and defocused in such a
way that its diameter on the target was approximately
5 mm. The beam intensity was kept below 20 nA in
order to avoid overheating and charge build-up on the
sample. The preset charge passed through the target
was typically up to 20 µC. Characteristic X-rays were
detected in a 30 mm2 x 3 mm thick Kevex Si(Li)
detector located inside the vacuum chamber, in the
horizontal plane, and making an angle of 135o with the
incident beam direction. The solid angle of the
detector was defined by means of a high purity
aluminum collimator and data were obtained with a
660 µm thick Mylar absorber in front of the detector.
Some PIXE experiments were conducted with a
“funny” filter that was the same absorber with a
coaxial opening 1% of the total filter area. A Kevex
4525P amplifier/pulse processor was used in
conjunction with the detector. The resolution obtained
with the detector-pulse processor combination is 180
eV for the Mn Kα line.
Step 1. The samples were dried overnight at 85°C
in a laboratory oven. This resulted in a reduction of
weight of up to 5% due to moisture loss.
Step 2. The weighed samples were placed in a
Teflon vessel and 100 µl of a 5% solution of a
nonionic surfactant (Fisher Triton-X-100) made with
deionized ultrafiltered (DIUF) water was added. The
samples were returned to the oven for approximately
one hour until the solution was absorbed by the
sample. This process was repeated until the sample
was completely wetted, i.e. additional solution soaked
uniformly into material, instead of remaining as a drop
on top of the material. Some samples did not require
surfactant, but all samples were processed identically.
Step 3. After the samples were wetted, Yttrium
Plasma Standard Solution (Alfa Aesar Specpure,
Y2O3 in 5% HNO3, Y1000µg/ml) was added in such
an amount that the yttrium concentration was roughly
1000 ppm.
Aluminized Mylar was chosen as backing for the
PIXE targets. Fixation of the sample was achieved
with a 1% solution of polystyrene in benzene in which
some amount of powdered sample (typically 0.5 gram
per 4 ml of the solvent) was suspended. PIXE runs on
blank targets (polystyrene without sample material on
the substrate) revealed no contaminants other than a
trace amount of Zn. At least three targets were made
Step 4. Following oven drying overnight at 85°C,
the samples were shaken on a Micro-Dismembrator
until the consistency was that of a fine powder.
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Potassium
Concentration K by ICP (ppm)
Concentration Mg by ICP (ppm)
4 00 000
6000
4000
2000
R = 0.534
0
0
150 0
0
8000
Concentration P by ICP (ppm)
6000 0
R = 0.106
10000
50 00 0
1000
12000
40 00 0
2000
14000
10 00 0
C o ncentratio n N a b y PIXE (ppm)
3000
30 00 0
25 00 0
20 000
15000
1000 0
R = 0.260
4000
1 00 0
5000
5000
50 0
10000
Sodium
6000
0
C oncentration P b y PIXE (ppm)
15000
5 000
Concentration Ca by ICP (ppm)
Phosphorus
20000
0
C o ncen tratio n Mg b y PIXE (ppm)
Magnesium
25000
0
R = 0.902
0
30 000
0
Concentration Fe by ICP (ppm)
100000
20000
0
200000
3 00 00 0
R = 0.979
300000
0
5000
400000
2 00 00 0
10000
500000
10000 0
15000
40 00 0
0
10 000
0
3000 0
R = 0.990
C o ncentratio n C a by PIXE (ppm)
10000
20000
7500
20000
25000
5000
30000
30000
2 50 0
40000
Calcium
35000
10 000
C oncentration K b y PIXE (ppm)
50000
20000
C oncentratio n Fe b y PIXE (ppm)
Iron
60000
Concentration Na by ICP (ppm)
Figure 1. Statistical Analysis of ICP vs. PIXE Results for Six Elements.
compare the results of the ICP studies with PIXE
results, PIXE measurements were conducted with a
“funny” filter to enhance detection of lower energy Xrays from light elements below K in the periodic table.
ICP vs. PIXE results for six elements are plotted,
employing R-squared regression, and are presented in
Figure 1. The correlation is high between the two
measurement techniques for heavier elements (Fe, K
and Ca), but is low for lighter elements (Na, Mg, and
P). A number of factors can be responsible for the fact
that the trends are not promising for some elements.
First, light elements below K in the periodic table
produce lower energy characteristic X-rays in an
energy region possessing a higher continuous X-ray
background. This drastically reduces PIXE sensitivity,
leading to greater statistical errors in concentration
calculations. Another source of errors during PIXE
measurements is non-uniform sample material
distribution over the target surface, which makes it
difficult to estimate the significant absorption of very
soft x-rays in the sample. On the other hand, the
preparation method for the ICP samples may have left
some elements in mineral form. For example K could
RESULTS AND DISCUSSION
Method Validation
As a first step, we validate the method of using
PIXE to determine elemental concentrations in
lacustrine sediments by comparing previously obtained
results from ICP studies with new results using PIXE.
We compared 7 samples from 4 lakes. The basins
studied include Lake Bainbridge (Galapagos, Equator),
Lake Valencia (Venezuela) San Jose Chulchaca
(Yucatan, Mexico), and Lake Titicaca (Bolivia/Peru).
The sediment of these lakes has highly different
chemistry allowing comparison of a large range of
elemental concentrations.
ICP measurements employed an inductively
coupled plasma spectrometer (Jarrell-Ash Model
9000) following combustion at 550°C for 2 hours and
digestion in 1N HCL for 1 hour [3]. In order to
421
before present. This makes determining of causes of
elemental concentration changes more difficult.
However, the trends of elemental concentration of Ti,
Mn, Fe and Zn with time are remarkably similar,
suggesting a common control on the elemental
concentration.
Climate in this region of South
America was drier in the last glacial period and the
two oldest data points at the record have the greatest
concentration. Independent indication of this drying is
the hiatus in sedimentation in the record (during which
time it is assumed that the lake was dry). During the
deglaciation from about 15,000 to 10,000 years before
present, water availability increased and the lake filled.
This increase in water availability is evidenced in the
core by a general trend to lower concentration of
heavy elements, beginning at about 16,000 years
before present. The slight increase in elemental
concentration of heavy elements at about 11,500 years
before present could correspond with the Younger
Dryas period during which climate briefly returned to
cold, dry, glacial conditions. During the Holocene
(10,000 years before present to the present) heavy
element concentrations have decreased likely due to
increased moisture availability and increased organic
carbon input that dilutes other elements.
have been left in minerals such as clays that are not
nitric acid soluble. Better results might be expected in
instances where HF is used in the wet chemistry
technique for ICP analysis.
Downcore Results Of Maicuru Sediments
Our second objective was measurement of
sediments from Lake Maicuru to evaluate the
feasibility of using PIXE analysis on lacustrine
sediment samples. The 13 sediment samples
corresponded to depth from 0 cm to 100 cm in the
sediment core. A typical sediment PIXE spectrum is
presented in Fig.2. Due to the poor reliability of data
for light elements mentioned above, only heavier
elements (Fe, K, and Ca) from the Maicuru sediment
core will be discussed. Spectra for heavier elements
were obtained with the regular Mylar filter. Very high
levels of Ti are observed in the lake deposits as a result
of extensive tropical weathering of the local bedrock.
Figure 3 presents downcore variations of such
elements as Ti, Mn, Fe and Zn. Unfortunately a hiatus
exists in the Lake Maicuru sediment record from
approximately 16,000 to 28,000 radiocarbon years
Figure 2. A Typical PIXE Spectrum from 0 cm Depth of Sediments from Lake Maicuru.
422
Figure 3. Downcore Distributions of Ti, Fe, Mn, and Zn in Lake Maicuru Sediments.
CONCLUSIONS
REFERENCES
The PIXE method has been used to study lake
sediments
and
explore
its
feasibility
in
paleoclimatology and geochemistry. The method is
promising: PIXE and ICP results compare favorably
for heavier elements and wet chemistry is eliminated
from the sample preparation. Consistent results on the
downcore study suggest that the technique holds
promise for geochemical and paleoenvironmental
studies.
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Accelerators in Research and Industry-1997, edited by J.
L. Dugan and I. L. Morgan, AIP Conference Proceedings
475, New York: American Institute of Physics, 1997, pp.
555-558.
2. Carlsson, L. E., Akselsson, K. R., Nucl.Instr.Meth.
181, 531-537 (1981).
3. Andersen, J. M., “An ignition method for
determination of total phosphorus in lake
sediments.” Water Research 10, 329-331 (1976).
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