UNIVERSITY OF GOTHENBURG
Department of Earth Sciences
Geovetarcentrum/Earth Science Centre
Measurements of total
nitrogen in sedimentary
rocks from Kinnekulle,
southwestern Sweden
Josefine Jahansson
ISSN 1400-3821
Mailing address
Geovetarcentrum
S 405 30 Göteborg
Address
Geovetarcentrum
Guldhedsgatan 5A
B920
Bachelor of Science thesis
Göteborg 2016
Telephone
031-786 19 56
Telefax
031-786 19 86
Geovetarcentrum
Göteborg University
S-405 30 Göteborg
SWEDEN
Abstract
The significance of geological nitrogen (N), as a source of input to ecosystems, has previously
been neglected in biogeochemical models. However, sedimentary and metasedimentary rocks
constitute a large pool of N, which is essential when it comes to biological productivity. There
are two ways of incorporated N relevant to this project. The first type is organic N deposited
with sediments that later during lithification become sedimentary rock. The second type is
ammonium (inorganic N), which can substitute potassium in silicate minerals. The primary
purpose of this study was to measure the total N content and quantify stable isotope values
δ15N for sedimentary rocks at Kinnekulle, southwestern Sweden. The second purpose was to
discuss if the oil content is a controlling factor as to whether the alum shale has high contents
of N or not. And the third purpose was to discuss the potential environmental impacts that N
in sedimentary rocks could have for the study area. The instrument used was an elemental
analyzer coupled with an isotope-ratio mass spectrometer (IRMS) and in order to control the
accuracy of the results, a sample of muscovite with known ammonium content was analyzed.
The sedimentary rock types that were analyzed include sandstone, alum shale, limestone and
shale from the Cambrian to the Silurian time period (541-419.2 Ma). The rock type with the
highest amount of incorporated N was the alum shale. The result from three sample locations
and a comparison with previous N measurements of the alum shale presented a lateral and
vertical variation of N within the unit. The average N content of the alum shale at Kinnekulle
was 0.19% and 0.27%N. The muscovite sample had lower N content than expected indicating
the possible underestimation of inorganic N in all analyzed samples. The potential direct
environmental implications as a result of elevated geologic N content can be intesified by a
lower pH-level, over fertilization during farming and this can lead to potential mobilization of
heavy metals deriving from the bedrock.
Keywords: biogeochemistry, nitrogen, sedimentary rocks, alum shale, IRMS
Sammanfattning
Det geologiska kvävets (N) betydelse som källa till ekosystem, har i regel blivit förbisedd i
biogeokemiska modeller trots att sedimentära och metasedimentära bergarter utgör en stor
pool av N. För biologisk produktivitet är N oumbärligt och det finns två olika typer av
geologiskt N som är relevant för föreliggande studie. Den första typen är organiskt N
deponerad med sediment som senare bildar den sedimentära bergarten. Den andra typen är
ammonium (oorganiskt N) som kan byta ut kalium i kiselmineral. Studiens primära syfte var
att uppmäta den totala N-halten samt stabila isotopvärden δ15N för sedimentära bergarter vid
Kinnekulle, sydvästra Sverige. Det andra syftet var att diskutera huruvida bergarter med
innehåll av olja kan kontrollera N-halten i alunskiffern. Det tredje syftet var att diskutera den
potentiella miljöpåverkan N-halten kan ha för den studerade platsen. Det instrument som
användes var en elemental analyzer kopplad till en isotope-ratio mass spectrometer (IRMS)
och för att kontrollera resultatet analyserades även ett externt prov av muskovit med en redan
känd N-halt. De sedimentära bergarter som analyserades var sandsten, alunskiffer, kalksten
och lerskiffer från tidsperioderna Kambrium till Silur (541-419,9 Ma). Alunskiffern var den
sedimentära bergarten som innehöll mest N. Resultatet från tre olika platser för provtagning,
jämfört med tidigare mätning av N från alunskiffern vid Kinnekulle påvisade en lateral samt
vertikal variation av N inom bergarten. De medelvärden för alunskiffern vid Kinnekulle var
0.19% och 0.27%N. Provet med muskovit hade en lägre N-halt än förväntat vilket indikerar
en möjlig underskattning av oorganiskt N i alla resultat. Potentiella miljöeffekter som följer
höga nivåer av geologiskt N kan bli intensifierad av lägre pH-nivåer och övergödning vid
jordbruk vilket i sin tur kan leda till en ökad mobilitet för tungmetaller med ursprung från
bergarten.
1
Table of Contents
Abstract ...................................................................................................................................... 1
Sammanfattning ......................................................................................................................... 1
Table of Contents ....................................................................................................................... 2
1. Introduction ............................................................................................................................ 3
1.1 Nitrogen cycle .................................................................................................................. 3
1.2 Stable Isotopes .................................................................................................................. 4
1.3 Geological nitrogen and previous work ........................................................................... 5
1.4 Effect on ecosystems ........................................................................................................ 6
2. Study area ............................................................................................................................... 6
2.1 Stratigraphy ...................................................................................................................... 8
2.1.1Cambrian (541-485.4 Ma) .......................................................................................... 8
2.1.1.1. Sandstone .............................................................................................................. 8
2.1.1.2. Alum shale ............................................................................................................. 9
2.1.2. Ordovician (485.4-443.8 Ma)................................................................................. 10
2.1.2.1 Limestone ............................................................................................................. 10
2.1.3 Silurian (443.8-419.2 Ma) ....................................................................................... 10
2.1.3.1. Shale .................................................................................................................... 10
3. Method ................................................................................................................................. 11
3.1. Field work ..................................................................................................................... 11
3.2. Preparation .................................................................................................................... 12
3.3. Analysis of rock samples with an IRMS ....................................................................... 12
4. Results .................................................................................................................................. 12
4.1 Analysis of the stratigraphy............................................................................................ 13
4.2 Analysis of muscovite .................................................................................................... 14
4.3 The standard reference material ..................................................................................... 15
5. Discussion ............................................................................................................................ 16
5.1. Occurrence of nitrogen in the stratigraphy and a methodical evaluation ...................... 16
5.2. Alum shale..................................................................................................................... 17
5.3. Environmental aspect of nitrogen in the alum shale ..................................................... 18
5.4. Outlook and future research .......................................................................................... 19
6. Conclusion ............................................................................................................................ 19
Acknowledgements .................................................................................................................. 20
References ................................................................................................................................ 21
Appendix .................................................................................................................................. 23
2
1. Introduction
Nitrogen (N) is one of the most controlling factors when it comes to biological productivity.
As a nutrient, nitrogen is essential for most living organisms and plays an important role in
the ecosystems (Schlesinger, 2005). Nitrogen deriving from rocks has proven to be a potential
source of input to ecosystems that have previously been overlooked, and can therefore be one
important part of the nitrogen cycle (Holloway & Dahlgren, 2002). This project aims to give
insight into how nitrogen in sedimentary rocks can be measured from Kinnekulle, Sweden,
and discuss the possible impact nitrogen might have on local ecosystems.
1.1 Nitrogen cycle
In order to understand the biogeochemical cycle, the included nitrogen cycle in particular, it is
important to consider the global geological evolution of element transport. The most
important processes controlling the sedimentation rates are weathering of bedrock and
transporting the exhumed material to a basin where sedimentation can occur. The transported
material and biogenic sediment over time can then become sedimentary rocks through a
process called lithification. As this global geological cycle transports material, the elements
within can be the key to a healthy or an unhealthy environment. The biogeochemical cycle
manages the biological uptake (immobilization) and release of elements (mineralization) that
can control the local environmental conditions (Ingri, 2012).
There are many processes within the nitrogen cycle and several models that describe the
complex system. Key processes transforming the nitrogen throughout the cycle and within the
biogeochemical cycle are fixation, immobilization, mineralization and nitrification and
denitrification (fig.1). The nitrogen mineralization controls the phase where organic nitrogen
becomes inorganic nitrogen when microbes release the excessive nitrogen as ammonium
(NH4+). Immobilization of nitrogen is the next step where microbes take up this inorganic
nitrogen. The cycle continues with nitrification where ammonium (NH4+) transforms into
nitrite (NO2-) and, ultimately, to nitrate (NO3-). Further, the denitrification process converts
nitrate into nitrous oxide (N2O) and dinitrogen gas (N2) (Paul & Paul, 2007).
3
Figure 1. Schematic sketch of the nitrogen cycle, showing the terrestrial ways of nitrogen transport and
transformation (Paul & Paul, 2007.)
1.2 Stable Isotopes
All nitrogen reservoirs naturally contain the two stable isotopes 14N and 15N. The percent of
natural abundance for the stable isotopes 14N and 15N are 99.63% and 0.3663% respectively
(Dawson, Mambelli, Plamboeck, Templer & Tu, 2002). What defines a stable isotope is the
variation of neutrons in relation to the number of protons in the element core. The different
number of neutrons does not alter the chemical properties of the element but rather the
physical. The physical difference is expressed as a change of mass of the element (Fry, 2006).
The ratio of the isotopes, compared with an atmospheric standard (AIR), gives a δ-value that
can work as a signature for each reservoir (fig 2) (Dawson et al., 2002). The stable isotopes
will not decay during normal environmental conditions but can be altered through different
processes such as thermal alteration (Younger, 2007). What controls the ratio of stable
isotopes is the fractionation process where a mixed pool of isotopes separates due to different
reaction rates (Fry, 2006). The use of stable isotopes is extensive in the field of
biogeochemistry. The astonishing capacity of stable isotopes, to function as a “signature” for
a specific environment can help scientists to make progress in terms of understanding
ecosystems and nitrogen fractionation (Dawson et al., 2002).
4
Figure 2. 15N in different pools of nitrogen within the nitrogen cycle and their isotopic signature
expressed in δ‰ (Fry, 2006).
1.3 Geological nitrogen and previous work
Geological nitrogen, meaning the nitrogen contained in rocks, can exist in different forms.
There are two types of incorporated nitrogen in rock relevant to this project. The first type is
organic matter deposited in situ with the sedimentary material that makes up the sedimentary
rock. The second form of geologic nitrogen is where ammonium ions replaces potassium due
to the same ionic radius in silicate minerals, such as phyllosilicates, through diagenesis
(during and after lithification) and thus elevates the nitrogen content in rocks (Holloway &
Dahlgren, 2002) and (Busigny, Cartigny, Philippot & Javoy, 2003)
During metamorphism of sedimentary bedrock, when the bedrock is altered by temperature
and/or pressure, the nitrogen concentration and isotopic signature can change.
Metamorphosed sedimentary rocks such as mica schists do not contain as much organic
nitrogen as sedimentary rocks, in general, but the inorganic nitrogen incorporated in the
mineral structure still remains (Holloway & Dahlgren, 2002).
The different methods used in order to measure the total nitrogen in rocks have produced
more or less successful results. A common method called the Kjeldahl extraction is
documented to not completely recover the nitrogen content compared to a combusting
method. Furthermore, the use of different acids in the Kjeldahl method has proven to give
inconsistent results (Holloway & Dahlgren, 2002). Work by Johansson, Sundius, &
Westergård (1943), on the alum shale at Kinnekulle where this project took place, show
nitrogen contents between 0.27 and 0.44% (Appendix 1), the choice of method was the
Kjeldahl method as well as the Dumas method for that study.
5
Saint-Denis and Goupy, (2004), compared the two methods and concluded that the Dumas
method is a faster approach and at least as accurate as Kjeldahl. The Dumas method quantifies
the nitrogen by burning the sample in a ceramic crucible letting the sample oxidize. The
nitrogen oxides become nitrogen by a catalyst and an electrical signal relative to the total
nitrogen content display the concentration.
The article “Ammonium quantification in muscovite by infrared spectroscopy” by Busigny et
al. (2003), measure the ammonium concentration in muscovite provided by Thomas Zack,
University of Gothenburg. The result is compared with the results in this study. More research
on sedimentary rock assembled by Holloway and Dahlgren (2002) is also discussed and can
be found in Appendix 2.
1.4 Effect on ecosystems
It has been observed by Morford, Houlton and Dahlgren (2011) that soil overlying nitrogen
rich bedrock tends to have elevated nitrogen content compared to soil underlain by nitrogenpoor bedrock. Sedimentary and metasedimentary rocks constitute a big pool of nitrogen and
can serve as a source of input to streams via chemical weathering (Montross, McGlynn,
Montross & Gardner, 2013).
Through leaching experiments it is possible to calculate an approximate amount of available
elements from one rock sample during certain conditions. The pioneering study by Montross,
et al. (2013), traced the source of nitrogen in streams to the local bedrock in southwest
Montana, USA with help of δ15N.
If bedrock with a high content of nitrogen leaches out nitrogen to the overlying soil, the soil
can contain more nitrogen than demanded by ecosystems. The consequence is oversaturated
soils where available nitrate and hydrogen can spread through surficial water and groundwater
in the watershed. Excessive hydrogen, released when ammonium (inorganic nitrogen) is
oxidized, is a known problem in soil due to the acidic affect it might have on ecosystems
(Holloway & Dahlgren, 2002). Ecosystems that are acidic with the right redox potential can
mobilize heavy metals and be destructive if the mobilized elements are bioavailable (Siegel,
2002).
Previously, only natural processes controlled the nitrogen input but today a supplementary
source of nitrogen to ecosystems is from an anthropogenic source (Schlesinger, 2005).
Nitrogen is used as a fertilizer and can consequently affect the ecosystems considerably if the
ecosystems already have high background nitrogen contents (Holloway & Dahlgren, 2002).
2. Study area
Kinnekulle, the area where this project took place, is located in the southwestern part of
Sweden, close to lake Vänern (fig.3). The site is an appreciated location for research and has
been for hundreds of years. The chosen location was of particular interest for this project due
to the impressive sedimentary stratigraphy of Kinnekulle. The stratigraphy spans from the
Cambrian to the Permian period (541-419.2 Ma). The sedimentary stratigraphy serves as a
record of the past environments and can be seen as a detailed document of the global journey
Sweden and Kinnekulle has taken. The plateau mountain of Kinnekulle is well preserved due
to the diabase “cover” that can be found on top of the entire 250m high stratigraphy. The
diabase protected the underlying sedimentary bedrock from weathering and erosion; this is
why the stratigraphy remains in spite of its age (Erlström, 2014).
6
A classification of the different sedimentary layers (that will be referred to as units in this
project) has previously been done (fig4) (Johansson et al., 1943).
A stratigraphy similar to Kinnekulle can be found about 60km to the west. The location called
Halle- and Hunneberg (fig 3) does not have a stratigraphy as diverse but contains alum shales
which were of importance for this study.
Figure 3. Map of southern Sweden with the location of Kinnekulle (K) and Hunneberg (H) marked in the red box
(Andersson, Dahlman, Gee, & Snäll, 1985).
7
Figure 4. Schematic sketch of Kinnekulle with its bedrock geology. The legend is showing the order of bedrock.
The map is from (www.lansstyrelsen.se, 2016) but edited for this project.
2.1 Stratigraphy
A brief and simplified description of the sedimentary rock types relevant to this project
follows in a chronologic order.
2.1.1Cambrian (541-485.4 Ma)
2.1.1.1. Sandstone
Sandstone is a porous rock that can hold water and oil making the sandstone economically
valuable and well researched worldwide. Out of all the sedimentary rocks on earth, sandstone
makes up about 10-20% in total. The most common mineral in sandstone is quartz (SiO2).
Quartz which is a very resilient mineral derives from previously weathered rock such as
granite, gneiss and sedimentary rocks (Prothero & Schwab, 2014). The sandstone from
Kinnekulle represents sediment deposited at a shoreline and at shallow depths during the
Cambrian time period (see fig 5). The sandstone, at the bottom of the Kinnekulle stratigraphy,
consists mainly of quartz grains and has a white to a green-yellow colour (Johansson et al.,
1943).
8
2.1.1.2. Alum shale
The next layer in the stratigraphy is the alum shale. There are several traits that make up the
alum shale layer; high contents of organic matter, uranium, sulphides and oil among others.
The high content of organic matter in the form of kerogen varies within the unit (Johansson,
et al, 1943). Kerogen can become oil in reduced environments where carbon dioxide and
water is extracted (Erlström, 2014).The alum shale at Kinnekulle contain up to 130 ppm
uranium. The uranium concentration increases in the upper sections of the stratigraphy, as
does the oil yield (Andersson, et al., 1985). The importance of the oil content will be
discussed further in section 5 but oil is a large source of nitrogen according to Montross et al.
(2013) and references therein. The oil yield in Halle- and Hunneberg was, as said by
Andersson et al. (1985), 0%, suggesting a lower content of oil. (table 1).
The depositional environment for the alum shale was a marine environment, on the ocean
floor, and the elevated content of uranium indicates that the depositional environment was
anoxic (Andersson et al., 1985) and (Ingri, 2012). The entire Alum shale unit is 20-25m thick
at Kinnekulle and is composed of algae and marine organisms from the Cambrian period.
There was no organic life on land during this period, and so the organic material comes from
the marine environment exclusively (Erlström, 2014).
The alum shale at Kinnekulle has favourable conditions to form biogenic gas and can
therefore contain carbon dioxide, nitrogen gas and hydrogen sulfide. The gas can reach the
land surface and travel through cracks and layers of the alum shale (Erlström, 2014).
The unit of alum shale at Kinnekulle is divided into several subunits according to age and is
interlayered with stinkstones, a calcareous rock (Leventhal, 1991). The mineral content is
dominated by phyllosilicates (minerals with a crystal structure arranged in sheets); this makes
the alum shale layered and easy to break apart. The relatively high levels of sulphides which,
during weathering of the shale, release sulphuric acid have the ability to affect the pH level in
the local environment and are an important aspect of the alum shale (Casserstedt, 2014).
Possible implications for ecosystems were introduced in section 1.4 and will be discussed
further in the section 5.
Table 1. The average organic matter and oil yield (%) at Halle- and Hunneberg (fig 6.2) and Kinnekulle (fig
6.1). The data is copied from (Andersson et al., 1985)
Area
Kinnekulle
HalleHunneberg
Billingen
Shale
(billion
tonnes)
1
1
Organic matter
(%)
Organic matter
(million tonnes)
Oil (%)
Oil million tonnes
14
13
140
100
3.4
0
20
0
12
13
1600
1.5
200
9
2.1.2. Ordovician (485.4-443.8 Ma)
1.1.2.1. Limestone
Limestone, or carbonate rocks, contain high quantities of carbonate (CO32-) and precipitate in
shallow marine environments (fig. 5). There is a low production of carbonate rocks today
compared to time periods such as the Ordovician when the climate was notably warmer and
shallow marine seas covered the lands due to a higher sea level (Prothero & Schwab, 2014).
Limestones have an alkaline property that can raise the pH-level. The calcium carbonate can
work as a buffering system in acidic water and is used today to solve certain environmental
problems (Ingri, 2012).
A typical unit of limestone at Kinnekulle from this period contain fossils. The most prominent
fossil, giving the Ordovician limestone its name at Kinnekulle is the Orthoceratite. The
Orthoceratite limestone is subdivided into four groups, the lower red, the lower gray, the
upper red and the upper grey. The colour classification is a simplified system that can only be
applied at Kinnekulle (Geologiska instutitionen & Stockholms Universitet, 1994).
2.1.3 Silurian (443.8-419.2 Ma)
2.1.3.1. Shale
The shale, also called fissile mudrock normally contains sand, silt and clay (Prothero &
Schwab, 2014). What differentiates the shale from the alum shale is the lower organic
content, enrichment of metallic trace elements such as manganese (Mn) and that the shale is
formed in an oxygen rich environment (Ingri, 2012). The Silurian shale in the local
stratigraphy, also containing fossils, is divided into two subunits in the local stratigraphy. The
lower, and older, shale contains fossils while the overlying younger shale does not (Johansson
et al., 1943). The minerals in the shale are dominantly phyllosilicates such as clay minerals
similar to the alum shale (Prothero & Schwab 2014).
Figure 5 An edited figure from (Ingri, 2012) representing the element differentiation in the geochemical cycle
and a simplified sketch of the depositional environment for sedimentary rocks.
10
3. Method
There is not an established method to quantify nitrogen in rocks but several different have
been successful, the use of an elemental analyser being one of them. When analyzing rock
samples, the quantification of nitrogen content is often left out due to the use of instruments
lacking the ability to measure nitrogen. This inconvenience could be one reason for the nonexisting standard methodology and recognized standard material. The method I have used in
terms of preparation of samples is approximately the same as the recent work of Morford et
al. (2015).
3.1. Field work
Rock samples were collected on two occasions (2016-02-02 and 2016-04-18). The samples
were chosen based on locality and appearance, avoiding too weathered material so the results
would be as accurate as possible. We followed the known stratigraphy at Kinnekulle, Sweden,
spanning from Cambrian to Permian units. One or two samples from each unit were collected
resulting in about 25 samples. Each sample was given an ID and put into a paper bag. The
second excursion had the purpose to sample more of the most interesting units with elevated
content of nitrogen, known after the first analysis of rock samples from each unit in the
stratigraphy. In order to get a better overview of the nitrogen content in the Alum shale a
piece from Hunneberg, southwestern Sweden (fig 6.2) was sampled from the same geological
period as the alum shale at Kinnekulle.
Figure 6.1 and 6.2. Figure 6.1 Schematic sketch of Kinnekulle with the legend showing the stratigraphy and the
numbers indicating where samples were taken. The sketch was edited for this project but is originally from
(www.lansstyrelsen.se, 2016) Figure 6.2 Schematic sketch of Hunneberg. The sketch was edited for this project
and the number indicate where the alum shale sample were taken (Andersson, Dahlman, Gee, & Snäll, 1985)
11
3.2. Preparation
The preparation, following Morford et al. (2015), included several steps with special attention
put into avoiding sources of nitrogen contamination.
A diamond saw was used to cut away weathered surfaces (surfaces that have been altered by
the local conditions such as wind, precipitation and biological activity). It is important to
remove weathered surfaces so the nitrogen content reflects the true content of the sample.
After removing all weathered surfaces, the samples were put in a hydrogen peroxide (H2O2)
bath for 24 hours. The samples were then boiled in the acid for 30 minutes at 150°C in order
to eliminate potential nitrogen contamination and organic surfaces. Next, the samples were
dried for 24 hours and then crushed and pulverized using a swing mill. The samples,
contained in glass vials, were then dried once again so all water could evaporate and were
then weighed into tin capsules. A detailed laboratory method description is attached as
appendix 4.
3.3. Analysis of rock samples with an IRMS
The analysis of the samples took place in the lab IsoGot at the department of Earth Science,
University of Gothenburg. I used an elemental analyser coupled to an isotope-ratio mass
spectrometer (20-22, Sercon Ltd., Crewe UK) which calculated the total nitrogen content as
well as the stable isotopes. The IRMS measures the amount of stable isotopes of nitrogen in
this case. The sample was heated up to just over 1000 °C. The sample then combusted and the
nitrogen were ionized. Next, a magnetic field separated the nitrogen based on mass letting the
δ 15N be quantified. To eliminate the risk of nitrogen being carried over to the following
samples, blanks were put in between the combusting samples. As reference material, soil with
low organic matter (B2152, standard OAS 30g 242117) was used. The equation (Eq. 1), to
quantify δ 15N, is based on the ratio (R) between the heavy (15N) and the light (14N) isotope in
the rock sample divided by the ratio of the standard, in this case being air (AIR).
(1)
3.4 Analysis of muscovite
In order to control the recovery of inorganic nitrogen a muscovite sample containing a known
amount of nitrogen, already analyzed in the paper by Busigny et al. (2003), sample “59-4#1”,
was analyzed. The sample where weighed into tin capsules à 30 and 60 mg in a total of six
samples.
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4. Results
The result is divided into three parts, first the results of the entire stratigraphy, second an
analysis of the mineral muscovite and third a graph visualizing the reference material showing
the consistence of the organic nitrogen content.
4.1 Analysis of the stratigraphy
The sandstone, limestone and the Silurian shale had low contents of nitrogen. The Alum shale
from Kinnekulle (ID: 3, 4) show an elevated nitrogen content compared with the alum shale
from Hunneberg which had considerably lower content. The Ordovivian limestone sequence
shows some variation but do not contain a significant amount of nitrogen. Figure 7 is a visual
representation of the nitrogen content in the correct stratigraphic order. Note the elevated
nitrogen content in the alum shale from Kinnekulle.
As the first analysis of the stratigraphy showed elevated nitrogen content in the alum shales at
Kinnekulle, more samples were collected in order to establish the high total nitrogen content
an also the isotopic signature for each sample location.
Table 2 Showing the samples’ age, rock type, number of samples from Kinnekulle, avarage total content of
nitrogen in percent (N%). The ID refer to figures 6.1 and 6.2.
ID
Time period
Rock type
No. of
Samples
3
Nitrogen
(mg N kg-1)
34
Average
N%
0.0034
Standard
deviation
0.0006
1
Cambrian(541-485.4
Ma)
Cambrian(541-485.4
Ma)
Cambrian(541-485.4
Ma)
Cambrian(541-485.4
Ma)
Sandstone
Alum shale
(Hunneberg)
Alum shale
4
112
0.0112
0.0045
2
2678
0.2678
0.0078
4
1881
0.1881
0.0105
Ordovician (485.4443.8 Ma)
Ordovician (485.4443.8 Ma)
Alum shale
with
stinkstones
Lower red
limestone
Lower grey
limestone
2
55
0.0055
0.0058
6
57
0.0057
0.0036
7
Ordovician (485.4443.8 Ma)
Upper red
limestone
5
95
0.0095
0.0021
8
Ordovician (485.4443.8 Ma)
Upper grey
limestone
4
114
0.0114
0.0125
9
Ordovician (485.4443.8 Ma)
Limestone
3
120
0.0120
0.0024
10
Silurian(443.8-419.2
Ma)
11. Silurian(443.8419.2 Ma)
Shale
5
62
0.0062
0.0026
Shale
3
69
0.0069
0.0007
2
3
4
5
6
11
13
Figure 7. The nitrogen content (%) in the analyzed sedimentary rock types, note the peak of nitrogen for the
alum shale.
4.2 Analysis of muscovite
Table 3 presents the muscovite sample that were analysed in order to control whether the
inorganic nitrogen were fully recovered in the previous analysis. The nitrogen content is
relatively consistent regardless of the sample weight but the isotopic signature is most
realistic for the samples that weighed 60mg compared with previous work. The nitrogen
content is less than half of the true content (0.015 NH4+ %) indicating a potential low nitrogen
recovery during the combustion process which could affect all previous results.
14
Table 3 An analysis of muscovite with a known Ammonium (NH4+) content (0.015 NH4+%) in order to test the
nitrogen recovery. The muscovite (Mica) comes from Busigny et al. 2003, provided by Thomas Zack, University
of Gothenburg.
Muscovite
Weight (mg)
Nitrogen
(mg N kg-1)
N (%)
No. 1
No. 2
No. 3
No. 4
No. 5
No. 6
Average
Standard deviation
40.768
39.208
40.495
61.187
60.873
61.181
-
43
51
61
41
46
38
0.0043
0.0051
0.0061
0.0041
0.0046
0.0038
0,0046
0,0008
4.3 The standard reference material
The following graph visualizes the steady nitrogen quantification with the weight from 10-30
mg of a standard soil with low content of organic matter. The result correlates with the true
content of the soil indicating a good nitrogen recovery of organic nitrogen.
_________________________________________________________________________
0,2500
N content (%)
0,2000
0,1500
0,1000
0,0500
0,0000
0
5
10
15
20
Sample weight (mg)
25
30
35
Figure 8. Soil with low organic content versus weight of sample. Showing the nitrogen content of the reference
material, standard soil with low organic matter content, with different sample weights.
15
5. Discussion
The following sections will discuss previous work in comparison with the results of this
project and the reliability of the results. Other subjects that will be discussed are the possible
implications for the investigated location and further projects that could improve the method
and our understanding of the role of nitrogen in the Kinnekulle area.
5.1. Occurrence of nitrogen in the stratigraphy and a methodical evaluation
The analysis of the rock samples demonstrates how the nitrogen content can vary both
between rock types and within each unit in the stratigraphy (table. 2). The sandstone,
limestone and shale show low contents of nitrogen while the alum shale from Kinnekulle
contains elevated nitrogen content. Comparing the results of the analysis to Morford et al.
(2016), found in appendix 3, the values seem reasonable. The average nitrogen content in
sandstone of 0.0034%N at Kinnekulle, compared with a sandstone from W.San Joaquin
Valley in USA (Holloway & Dahlgren, (2002), and references therein) with 0.0033%N as the
lowest measured content, demonstrates the credibility of my results. The Ordovician
limestone and its subunits show a range between 0.0055-0.0120 %N reflecting the low
nitrogen content within the same range as the Cretaceous limestones (0.0075 -0,0155 %N)
from Moford et al. (2016). In the limestone sequence, the upper grey unit (ID:8) has an
elevated nitrogen content compared to lower units in the limestone stratigraphy but still holds
a small to insignificant quantity of nitrogen. The Silurian shale had unexpectedly low contents
compared to the Cambrian alum shale. The Silurian shale had about 0.0062-0.0069N% and
when compared with the shale and mudrocks from Moford et al. (2016) there is a difference
in order of magnitudes. The Silurian shale and the alum shale are both rich in phyllosilicates
where nitrogen could take a place in the crystal structure. One possible reason why there is
such a big difference in nitrogen content is that the Silurian shale was formed further off shore
as opposed to the alum shale where organic material could have been deposited to a greater
extent (fig 5).
The nitrogen-rich alum shale at Kinnekulle, with nitrogen content between 0.19-0.27 %N,
compared with the Australian oil shale from Holloway and Dahlgren, (2002) support the
hypothesis that the oil content within the alum shale can control the nitrogen content. The
correlation of results throughout the stratigraphy are promising although each sedimentary
depositional environment, when it comes to incorporated nitrogen, should be different and
should therefore be evaluated separately and this also holds true for δ15N.
Even though the results are comparable with previous studies, it is important to test whether
the results are trustworthy or not. For this reason I analyzed an external sample of muscovite
from Busigny et al. (2003), with a known ammonium content of 0.015 NH4+ % (table 4).
Muscovite is a common rock mineral included in sedimentary rocks and is thus relevant to
this project. The analyzed muscovite had relatively constant nitrogen values although the
values were consistently less than half of the true ammonium content, ca 0.0046%N. This
means that the nitrogen recovery in my analysis could be less than desired during the
combustion process. The probable reason for the low inorganic recovery of nitrogen is a not
complete combustion of the sample. This means that the inorganic nitrogen is fixed in the
minerals and cannot be released, perhaps the temperature was too low or that there is a need
for a catalysis aiding the combustion process. The consequence could be that all my results, in
general, are underestimations of inorganic nitrogen content if only the organic nitrogen is
completely recovered. The rock sample that I think could contain significantly more nitrogen
fixed in the silicate minerals is the Silurian shale due to the abundance of phyllosilicates.
16
Figure 7 visualize the difference of N% throughout the stratigraphy and even though the
inorganic nitrogen was only recovered less than half of the true content the relative nitrogen
content is still significantly higher for the alum shale after a potential “correction” for each
unit.
The isotopic values had not been studied for the muscovite sample and could therefore not be
compared with my results. The δ15N values in all the units in the stratigraphy, as well as the
muscovite, were not included in the results due to inconsistent values. However, as previously
stated, it is hard to compare the δ 15N values between locations due to the different
biogeochemical processes that control the fractionation process of the light and heavy stable
isotopes.
Throughout the analysis the soil standard with low organic matter (figure 8) is consistent and
dependable. The nitrogen content in the standard material is independent of the sample size
indicating that the analysis is working. The soil contains organic nitrogen but not inorganic
nitrogen fixed in minerals. My rock samples have a different composition then, which might
burn differently in the combustion process compared to the soil. Perhaps a change of
temperature for the combusting process could be considered, to optimize the release of
inorganic nitrogen fixed in minerals. Another approach worth mentioning is the one suggested
by Bräuer and Hahne (2005), where vanadium oxide (V2O5) work as a catalyst increasing the
nitrogen recovery, both total nitrogen and δ15N.
Another potential source of error brought to attention by Holloway and Dahlgren (2002), is
the potential loss of fixed ammonium when preparing the samples for analysis with a
hydrogen peroxide bath. It is difficult to know for sure whether this has affected the fixed
nitrogen content in my analysis due to the risk of low recovery of inorganic nitrogen.
5.2. Alum shale
The alum shale at Kinnekulle had the highest amount of nitrogen, so I focused more on that
layer in the stratigraphy. The samples of alum shale were sampled at three different places
(fig 6.1 and 6.2) and each location had its own nitrogen content indicating that there could be
a lateral as well as a vertical difference in nitrogen within the Alum shale unit. The highest
nitrogen content was attained from the Alum shale in Råbäck (ID:3 in fig 6.1). The sample
showed a nitrogen content similar to the studies by Johansson et al. 1943. However, the
sample rock (ID:3) was more weathered than the other alum shale samples (ID:2 and 4) which
could have affected the results by adding organic material. It is hard to find a good piece of
Alum shale since the rock type often splits into sheets and has cracks that can be filled with
water from either precipitation or ground water increasing the rate of weathering and
leaching.
Previous analysis by Johansson et al. (1943), of the alum shale at Kinnekulle (appendix 1)
showed nitrogen content between 0.27-0.44 N%. When compared to the results in this study,
(0.19-0.27) N% for the alum shale at Kinnekulle and 0.01%N for the alum shale sampled at
Hunneberg, the previous work shows higher nitrogen content. The study was conducted in
1930 and little information of the method and analysis is provided. Since the study is old it is
hard to get further details except that two methods were used, the Kjeldahl and the Dumas
method briefly explained in section 1.5.
To know for sure whether the results are accurate or not is difficult, but since the results of
both methods are similar to one another it could mean that the analysis is somewhat
17
dependable. Table 1 from Andersson et al. (1985), shows that the amount of alum shale at
Kinnekulle is up to one billion tonnes. A rough calculation of the total amount of nitrogen in
the alum shale unit at Kinnekulle, from my analysis, multiplied with the total mass alum shale
is about 1.9 to 2.7 million tonnes of nitrogen stored within the alum shale.
While the Alum shale at Kinnekulle had high total nitrogen content, the Alum shale from
Hunneberg (ID:2) did not. This was surprising considering the elevated contents in
Kinnekulle. Furthermore the analyzed sample was very black suggesting high organic
content. It would however be interesting to investigate that particular sample to greater extent.
Erlström (2014) discuss the geochemical properties in the alum shale available in Sweden.
The report shows the extensive variation in the alum shale between the different locations and
may therefore explain my low results of nitrogen at Hunneberg and the high at Kinnekulle.
Another argument why the nitrogen content is low at Hunneberg can be seen in table 1 from
Andersson et al. (1985), in which there is a corresponding low oil yield at Hunneberg. The
0% oil yield does not mean that the oil is absent within the alum shale but suggests a possible
lower content. My hypothesis, that the oil content is the most controlling factor whether the
alum shale contains high contents of nitrogen or not, is supported by the fact that oil is one of
the largest nitrogen reservoirs (Holloway and Dahlgren, 2002). This would mean that one
could assume high contents of nitrogen where a large amount of oil is present. A way to test
the hypothesis would be to analyze alum shale samples from Billingen, about 30 km southeast
of Kinnekulle. If the hypothesis is correct the nitrogen content should be in between the
values attained from Hunneberg and Kinnekulle due to the oil yield and organic matter
presented in table1.
5.3. Environmental aspect of nitrogen in the alum shale
One conclusion that can be drawn from the analysis and comparisons with other studies is that
there is an elevated concentration of nitrogen in the alum shale at Kinnkulle. The effect,
destructive or not, with elevated concentrations of nitrogen could be carried out as a
continuation of this project. I can merely discuss the possible geochemical outcomes as a
consequence of the nitrogen content. The geochemical properties such as the alkaline effect of
the limestone and the acidic effect of the alum shale are briefly described in section 2.1.1-2.13
and should be accounted for when considering possible environmental impacts in the area.
The nitrogen content in the alum shale could, if released from the rock, pollute the soils and
water within the area. Through leaching and acid released from sulphides, the adjacent
environment could become acidic. The pH level of the soil is of importance because of its
potential negative effect on the local agriculture, notably because acidic conditions can
release heavy metals from the soil and rocks, which in turn can become accessible to biota. As
previously mentioned the alum shale contain the element uranium and in an oxidised acidic
environment (pH <3), uranium can become mobile (Ingri, 2012).
If the soil and water is acidic I propose that the condition could be “counterbalanced” by the
alkaline limestone higher up in the Kinnekulle stratigraphy. This means that the overlying
limestone can work as a buffering tool when precipitation transports alkaline water down
slope in the watershed neutralizing the pH-level. It would be interesting to measure the pH in
the soil and water and correlate the changes of pH during wet and dry periods to see if the
potential buffering by the limestones could occur. This is, of course, only relevant if the soil is
acidic in the first place. High amounts of nitrogen are not only responsible for negative effects
18
but could also be beneficial for land being farmed on the alum shale, working as a “natural”
fertilizer. The only concern could be the pH-level which should be supervised in order to
avoid mobilization of heavy metals like uranium.
5.4. Outlook and future research
I propose a future project where soil and plant samples, in close contact with the Alum shale,
will be analyzed. The isotopic signature could then be compared with the values attained in
this study. Additional nitrogen measurements of the water in the watershed could provide an
isotopic correlation and a valuable parameter for a potential mass balance calculation. If the
isotopic ratio is similar to present results one can assume that the nitrogen in the soil derives
from the Alum shale. It is of importance to know the “natural” nitrogen input in order to
establish a background value to make sure guidelines are kept. This means that external
fertilizers or contamination could be detected based on the isotopic signature in the soil.
The isotopic values attained in this study (not shown in this report) vary and should be viewed
separately for each sample location. The span of the isotopic ratio for the alum shale in the
area was moderately large and can cover several other nitrogen pools (fig 2). One challenge is
that the δ15N value is close to zero (atmospheric value, see fig 2), similar to the atmospheric
isotopic value making it hard to distinguish the nitrogen from the rock and the atmosphere.
If environmental management is required to decrease excessive nitrogen, several solutions
proposed by Holloway and Dahlgren (2002), could be of interest. One solution is to cultivate
the area with organic life that has a high C:N ratio. One such organism is the oak tree with its
big rooting system. Kinnekulle has several oak trees and could therefore already attend to the
possible excessive available nitrogen-issue. But if not, it could be an idea to prevent
oversaturation.
6. Conclusion
The result shows that there is an elevated content of nitrogen (0.19-0.27%N), in the Alum
shale at Kinnekulle Sweden. The isotopic measurements were not included in this report but
should be looked at separately for each sample location in order to correlate with each local
isotopic signature. The alum shale from Kinnekulle contained significantly more nitrogen
than the alum shale from Hunneberg. I hypothesise that the oil content is the controlling factor
whether the alum shale contains high amounts of nitrogen or not. Previous results, compared
with the results in this study, show that the elemental analyser might not give complete
nitrogen recovery. The reason could be the used reference material not behaving like the rock
sample during the combustion process due to the lack of inorganic nitrogen. The sandstone,
limestone and shale had low concentrations of nitrogen and their δ 15N values could therefore
not be accounted for due to their low certainty. Although the total nitrogen content is low, my
results correlate well with the database from Moford et al. (2016) suggesting they could be
dependable depending on the amount of inorganic nitrogen incorporated in the rock that
might be underestimated. Further projects should be focused on ameliorating the method
including the recovery of nitrogen compared to a standard more similar to the rock samples. A
first step would be a standard marine sediment and perhaps to create an in-house standard at
the University of Gothenburg. Another approach would be to use vanadium oxide as a
catalyst and possibly then get a better nitrogen recovery. Personally, I would like to see an
environmental impact assessment of the environment connected to the alum shale. If the
nitrogen recovery of inorganic nitrogen is underestimated the local environment could be
exposed to more nitrogen than the results in this report show. The environmental impact
19
assessment should be implemented in order to rule out destructive effects and establish the
natural background nitrogen level in plants, soil and water now that values are attained for the
rocks.
Acknowledgements
I would like to thank my advisor Dr. Tobias Rütting (associate professor) and technical
advisor Dr. Louise Andresen (researcher) at the department of Earth Science at the University
of Gothenburg for giving me the opportunity to pursue this project and for their support. I also
want to thank my examiner Thomas Zack and opponent Axel Hellman. I owe a thank you to
Ulf Wiktander at the county government of Västra Götaland who provided me with valuable
information about Kinnekulle. I want to thank Magnus Knutsson for letting us sample fresh
alum shale on his property. I would also like to thank the helpful staff and students at the
department of Earth Science, University of Gothenburg with a special thanks to Andreas
Karlsson, Axel Sjöqvist and Johanna Pihlblad. My final appreciation goes to my Canadian
support Dr. Anne-Marie Ryan at Dalhousie University and Rebecca Aucoin.
20
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Morford, S., Houlton, B., & Dahlgren, R. (2016). Geochemical and tectonic uplift controls on
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22
Appendix
The following tables are from previous studies of nitrogen in rocks.
Appendix 1. Previous by Johansson, Sundius, & Westergård, 1943, showing the total nitrogen content for the
Alum Shale at Kinnekulle based on two different methods, the Kjeldahl method and the Dumas method.
Method
Dumas
Kjeldahl
No. 1 (N%)
0.34
0.27
No. 2 (N%)
0.41
0.37
No. 3 (N%)
0.44
0.36
No. 4 (N%)
0.39
0.34
Appendix 2. An assembly of previous studies of nitrogen in sedimentary rocks from Holloway and Dahlgren
(2002)
Appendix 3. A selection from the database by Moford et. Al (2016) of previous analysis of metasedimentary
(Mica schist and Phylliste) and sedimentary bedrock. The total nitrogen content and isotopic values δ15N can be
seen to the right.
Sample
ID
Latitude
Longitude
Geologic Age
A02
[WGS84
D.D.]
40,26075
[WGS84
D.D.]
-123,25181
Early Cretaceous
A05
40,27423
-123,28075
Early Cretaceous
A08
40,26715
-123,26354
A17
40,28638
A18
Total Nitrogen
d15N
N
[mg/kg]
[permil]
%
Mica schist
1085
1,68
0,1085
Mica schist
1007
1,43
0,1007
Early Cretaceous
Mica schist
663
1,56
0,0663
-123,25051
Early Cretaceous
Mica schist
804
3,28
0,0804
40,28056
-123,24286
Early Cretaceous
Mica schist
1113
2,26
0,1113
A19
40,26681
-123,22711
Early Cretaceous
Mica schist
1210
2,08
0,121
GNSW00
23
GNSW00
24
42,4534
-124,16638
-
Mica schist
739
0,26
0,0739
42,4534
-124,16638
-
Mica schist
762
0,92
0,0762
23
Rock Type
GNSW00
32
GNSW00
35
GNSW00
36
GNSW00
37
GNSW00
39
GNSW00
44
GNSW00
45
GNSW00
47
GNSW00
20
GNSW00
09
GNSW00
13
GNSW00
27
GNSW00
38
GNSW00
43
GNSW00
46
GNSW00
41
CH10
41,91315
-122,90229
Jurassic(?)
Mica schist
823
0,23
0,0823
42,4534
-124,16638
-
Mica schist
919
-0,80
0,0919
42,4534
-124,16638
-
Mica schist
849
0,48
0,0849
42,4534
-124,16638
-
Mica schist
1109
0,21
0,1109
41,23709
-123,67493
Mica schist
568
0,47
0,0568
41,90614
-122,9316
Middle to Late
Jurassic
Jurassic(?)
Mica schist
672
0,19
0,0672
41,34435
-123,75842
Early Cretaceous
Mica schist
695
0,58
0,0695
41,90037
-122,93245
Jurassic(?)
Mica schist
668
0,35
0,0668
42,40536
-124,19967
-
Phyllite
867
0,47
0,0867
39,39946
-123,03216
Early Cretaceous
Phyllite
788
0,55
0,0788
40,221
-122,98496
Early Cretaceous
Phyllite
802
0,10
0,0802
42,43801
-124,25315
-
Phyllite
542
1,03
0,0542
42,4534
-124,16638
-
Phyllite
409
0,09
0,0409
41,35421
-123,75523
Early Cretaceous
Phyllite
1047
1,30
0,1047
41,34435
-123,75842
Early Cretaceous
Phyllite
983
0,45
0,0983
42,4534
-124,16638
-
Phyllite
823
0,74
0,0823
36,49178
-120,57807
Mudstone
792
4,89
0,0792
CH11
36,49178
-120,57807
Mudstone
1312
4,34
0,1312
CH12
36,54167
-120,6017
Mudstone
1091
3,92
0,1091
CH2
36,53967
-120,55502
Mudstone
1698
7,77
0,1698
CH5
36,53967
-120,55502
Mudstone
1524
8,30
0,1524
CH7
36,53967
-120,55502
Mudstone
1687
7,68
0,1687
GNSW00
11
GNSW00
12
GNSW00
87
CH6
39,34776
-122,67566
Shale
693
1,55
0,0693
39,33238
-122,7141
Shale
889
1,85
0,0889
40,35122
-122,85956
Paleocene to
Oligocene
Paleocene to
Oligocene
Paleocene to
Oligocene
Oligocene to
Pliocene
Oligocene to
Pliocene
Oligocene to
Pliocene
Jurassic to
Cretaceous
Jurassic to
Cretaceous
Early Cretaceous
Shale
616
0,67
0,0616
36,49178
-120,57807
Shale
1328
4,79
0,1328
CH13
36,49178
-120,57807
Shale
885
3,25
0,0885
CH1
36,49178
-120,57807
Shale
955
5,84
0,0955
GNSW01
47
GNSW01
48
GNSW01
49
GNSW01
50
GNSW01
51
40,46538
-123,65344
Shale
669
1,19
0,0669
39,32109
-122,90996
Shale
795
-0,56
0,0795
40,28581
-124,0544
Shale
636
2,03
0,0636
39,38961
-123,42103
Shale
630
0,90
0,063
39,86422
-123,7234
Paleocene to
Oligocene
Paleocene to
Oligocene
Paleocene to
Oligocene
Jurassic to
Cretaceous
Jurassic to
Cretaceous
Paleocene to late
Eocene
Jurassic to
Cretaceous
Late Cretaceous to
Pliocene
Shale
609
1,47
0,0609
24
GNSW02
72
GNSW02
74
GNSW02
75
GNSW01
08
GNSW01
09
GNSW01
10
GNSW00
02
GNSW00
03
CH3
38,9164
-121,0145
38,9164
-121,0145
38,9164
-121,0145
39,83367
-123,63607
39,83367
-123,63607
39,31134
-122,93468
39,38961
-123,42103
39,52783
-123,4018
36,4966
-120,53478
Early Proterozoic to
Cretaceous
Early Proterozoic to
Cretaceous
Early Proterozoic to
Cretaceous
Late Cretaceous to
Pliocene
Late Cretaceous to
Pliocene
Jurassic to
Cretaceous
Jurassic to
Cretaceous
Late Cretaceous to
Pliocene
Paleocene to
Oligocene
Limestone
155
-1,76
0,0155
Limestone
75
-0,24
0,0075
Limestone
145
2,73
0,0145
Limestone
170
0,66
0,017
Sandstone
132
-2,29
0,0132
Sandstone
218
1,41
0,0218
Sandstone
213
1,63
0,0213
Sandstone
118
2,38
0,0118
Sandstone
224
1,56
0,0224
Appendix 4. A detailed laboratory description
Cleaning of rocks with H2O2 and physical cleaning / crushing
Materials:
Gloves (nitrogen free), fumehood
Hot plate
H2O2 30 % to dilute
1 glass beaker for each sample
Tool to crush, brass plate and a Swing mill
Ethanol to clean tools
Sample preparation
1. In the laboratory, any rock weathering rind was removed using a lapidary slab saw.
2.
Label each baker with rock number and H2O2 and your name.
3.
Submerge the rock into 5% hydrogen peroxide for 24 h at room temperature (22 ± 1°C,
darkness) to remove surficial organic contaminants (in fume hood)
4.
Heat beaker with sample etc. to 150°C to decompose the hydrogen peroxide.
5.
Samples were dried in an oven for 24 hours at 110°C and then crushed with a metal
hammer on a plate of brass to particle sizes <20 mm in diameter cleaned with ethanol
and deionized water in between samples.
6.
Next, samples were pulverized in a swing mill with a quartz sample prior, in between
and after each sample to avoid cross contamination. The metal dish was also cleaned
with ethanol in between each sample. To get the rock powder out of the metal dish, a
piece of aluminum foil were carefully “brushing” it of onto a bigger piece of aluminum
foil and then put into beaker (preferably glass).
7.
The samples were then dried for 24 hours at 110°C
25