Ecological implications of changing sources and accumulation rates of organic carbon during the last 300 years
in Hvaler, ytre Oslofjorden
A geochemical and micropalaeontological study
Sigrid Marketta Aasgaard
Thesis submitted for the degree of Master in Environmental Geoscience
60 credits
Departement of Geology
Faculty of mathematics and natural sciences
UNIVERSITY OF OSLO
Ecological implications of changing sources and accumulation rates of organic carbon during the last 300
years in Hvaler, ytre Oslofjorden
A geochemical and micropalaeontological study
Sigrid Marketta Aasgaard
© 2020 Sigrid Marketta Aasgaard
Ecological implications of changing sources and accumulation rates of organic carbon during the last 300 years in Hvaler, ytre Oslofjorden
http://www.duo.uio.no/
Printed: Reprosentralen, University of Oslo
Abstract
Hvaler is an estuary in SE Norway, and is influenced by the largest river in Norway. The geochemistry and foraminiferal assemblages of two overlapping cores from the area was analysed. The cores were dated, and analysed for grain size distribution, heavy metal concentrations, TC, TN, TOC,δ13C,δ15N, and changes in foraminiferal assemblages. Based on extrapolated dating, the sediment cores span >300 years. The heavy metal pollution in the cores reflects the emissions of the industry in the area, reaching concentration maxima in sediments from 1972. The heavy metal concentrations are declining towards the present. The or- ganic matter in the sediments is of mainly marine origin, but is becoming slightly more terrestrially influenced over time. The foraminiferal assemblages were ini- tially characterized by an oscillating pattern ofHyalinea balthica andAdercotryma glomerata/wrighti. In the last ∼100 years, the accumulation rates of TOC have increased, accompanied by an increase in Stainforthia fusiformis and Textularia earlandi, which are species tolerant to organic carbon enrichment. The invasion of Nonionella sp. T1 was first observed in the cores in 2016. The species seems to replace Stainforthia fusiformis, which went from dominating, to comprising only a small fraction of the assemblage in the surface sediments.
Forord
Jeg ønsker spesielt å takke Elisabeth og Silvia, for god hjelp og støtte, spesielt når det kom til tolkninger, litteraturforslag og artsbestemmelse av foraminiferer. Jeg takker Aud og Anouk, for gode inspill underveis.
En stor takk til Mufak, for endeløs tålmodighet, vennlighet og hjelp med labresul- tater. Jeg takker Bill for isotopresultater.
Jeg takker crewet på toktet: kaptein Sindre, Jan, Leif-Arild og Tor, for å ha vært hjelpsomme og vennlige.
Til slutt må en spesiell takk rettes til min kjære mor og far, og min kjære samboer, som har vært oppmuntrende og støttende. En ekstra takk til far og samboer, for å ha lest oppgaven igjen og igjen, og kommet med gode tilbakemeldinger.
Denne masteroppgaven ble skrevet under Covid-19 pandemien, som desverre be- grenset noen av analysemulighetene, ettersom Universitetet og labene var stengt.
Contents
1 Introduction 1
1.1 The study area . . . 4
1.2 Pollution history . . . 7
2 Materials and methods 9 2.1 The cruise . . . 9
2.2 Sample collection and treatment . . . 9
2.2.1 Sediment cores . . . 9
2.2.2 Surface samples . . . 9
2.3 Freeze drying and calculation of water content . . . 10
2.4 Grain size analyses . . . 10
2.5 Dating and sediment accumulation rate . . . 11
2.6 Heavy metal analyses . . . 11
2.7 Carbon, nitrogen and stable isotope analysis . . . 12
2.7.1 Sample preparation . . . 12
2.7.2 TOC63, TIC . . . 13
2.7.3 TC, TN and C/N . . . 14
2.7.4 Stable isotope analyses . . . 14
2.7.5 Mixing equation . . . 15
2.8 Micropalaeontological analyses . . . 15
2.8.1 Dry picking . . . 15
2.8.2 Wet picking . . . 16
2.8.3 Calculation of diversity, sensitivity and ecological quality indices . 16
3 Results 19
3.1 Core description . . . 19
3.2 Water content . . . 19
3.3 Grain size analysis . . . 20
3.4 Dating and sediment accumulation rate . . . 22
3.5 Heavy metals . . . 25
3.6 Carbon, nitrogen and stable isotopes . . . 28
3.6.1 TC, TN and C/N . . . 28
3.6.2 TOC63 and TOC . . . 29
3.6.3 Stable isotope ratios . . . 31
3.6.4 Mixing equation . . . 33
3.7 Accumulation rates of organic carbon . . . 34
3.8 Micropalaeontological analyses . . . 35
3.8.1 Abundance of foraminifera . . . 35
3.8.2 Diversity, sensitivity and ecological quality indices . . . 37
3.8.3 Pyrite found in foraminifera . . . 41
4 Discussion 42 4.1 Depositional environment and accumulation rate . . . 42
4.2 Heavy metal pollution history . . . 42
4.3 Sources and accumulation rates of organic carbon over time . . . 45
4.3.1 Determining the sources of organic carbon over time . . . 46
4.3.2 Mixing equation . . . 49
4.4 Ecological changes . . . 50
4.4.1 The reliability of the fossil record . . . 50
4.4.2 Foraminifera and heavy metal pollution . . . 52
4.4.3 Ecological changes . . . 52
4.4.4 Invasion of a species resembling Nonionella stella . . . 54
4.5 Establishing reference conditions . . . 56
5 Conclusions 57
References 58
6 Appendix 65
A CTD 65
B Sample information 66
C Dating report 67
D Grain size table 73
E Foraminifera counts 75
F AMBI-groups 77
G Bubble plots 78
H Linear regression of δ15N and δ13C 81
I Taxonomical reference list 82
1 Introduction
Estuaries are coastal areas influenced by river discharge, characterized by stratified water masses and lower salinities. Estuaries typically have high biological activity (Alve, 1995).
Estuarine sediments can contain a varied mixture of terrigenous and marine organic matter (Thornton and Mcmanus, 1994; Peters et al., 1978; Wada et al., 1987; Peterson and Howarth, 1987).
Benthic foraminifera are protists living on and in sediments. Different species have different preferences and tolerances when it comes to environmental conditions, like temperature, water depth, salinity, oxygen concentration and food availability (Alve, 1994; Alve, 2010; Duffield et al., 2015; Alve and Nagy, 1990; Bergsten et al., 1996).
Many foraminifera form calcareous or agglutinated tests which are preserved in the sedimentary record.
Silled fjords are a type of estuary, and can have restricted deep water circulation, which makes the bottom waters predisposed to low oxygen conditions. This natural effect can be exacerbated by cultural eutrophication (Dolven et al., 2013). The process of eutrophication is when a water body becomes overly enriched with nutrients. Eutroph- ication can occur from natural reasons, so when it is caused by human activity it is called cultural eutrophication. The sources of nutrients are mainly fertilizers or sewage, containing nitrogen and phosphorous. The excess nutrients can spur algal bloom, which causes an increased flux of organic matter to the sediments. When too much organic matter is deposited, the decomposition processes consume more oxygen than the dif- fusion and water circulation can resupply, which leads to hypoxic (low oxygen levels) or anoxic (oxygen depletion) conditions in the water (Veileder, 2018; Bernhardt and Schlesinger, 2013, and references therein).
The diversity of foraminifera in present day assemblages was observed to vary along concentrations of O2 in the water. Lower concentrations accompanied lower diversities.
This relationship makes foraminifera a suitable parameter for environmental monitoring (Bouchet et al., 2012). Diversity and ecological quality indices used to quantify the eco- logical status of macrofauna assemblages were intercalibrated to be used om foraminifera by Alve, Silvia Hess, et al., 2019.
Hypoxia and anoxia are environmental stressors for the foraminifera, and only a few species tolerate hypoxic to anoxic conditions (Alve, 1994). The relationship between or- ganic carbon enrichment to the sediments, and low oxygen concentrations in the water, is combined with foraminiferal assemblages occuring in sediments with different concen-
trations of organic carbon, to describe a spectrum of low to high tolerance to organic carbon enrichment. The result is a sensitivity index, Foram-AMBI (Alve, Korsun, et al., 2016), which is based on the analogous macrofauna sensitivity index AMBI (Borja et al., 2000).
The Water Framework Directive (WFD) commits all member countries to achieve "good"
or higher quality status for all water bodies (European Parliament, 2008). Some water bodies are naturally in moderate to bad status classes (Dolven et al., 2013). In order to distinguish these areas of from those with bad environmental conditions due to hu- man influences the reference conditions must be established. The sedimentary record of a location contains information about the depositional environment, geochemistry and ecology over time (Philip A Meyers, 1997; Dolven et al., 2013). From an environmental monitoring perspective, sediment cores can help in trackin the effects of human influ- ences. If a sediment core is deep enough to reach reference conditions, current conditions can be compared with conditions from centuries ago, giving a timeline of the onset and severity of human impacts in a study area (Dolven et al., 2013; Polovodova Asteman et al., 2015; Silvia Hess et al., 2020). Comparing foraminiferal assemblages in sediment cores predating times of increasing human influences, can give answers to whether or not the present conditions reflect the natural conditions (Dolven et al., 2013; Polovodova Asteman et al., 2015; Silvia Hess et al., 2020).
The sensitivity and diversity indices are utilized to quantify the ecological quality status (table 1).
Table 1: Status class intervals for heavy metals, TOC63and oxygen from Veileder, 2018, and H’log2_f, ES100_f and NQI_f from Alve, Silvia Hess, et al., 2019.
Changes in foraminiferal assemblages have been used to interpret the effects of climate change, as assemblages reflect the varying temperatures and food sources, as a result of changing patterns of palaeoproductivity (Polovodova Asteman et al., 2013; Irina Polovodova Asteman, Risebrobakken, et al., 2018). Some species thrive on fresh pyh- todetritus, while some others are content with degraded organic matter (Duffield et al., 2015; Alve, 2010).
The aim of this thesis is to investigate whether there has been a change over time in the sources and accumulation rates of organic carbon in the study area, and if it has affected the benthic foraminiferal assemblages.
This thesis is based on results from two overlapping sediment cores. The data sets include grain size analysis, dating, sediment accumulation rates, heavy metal concen- trations, concentrations of carbon and nitrogen, isotope ratios of the sediments (C13/C12 and N15/N14), the abundance and composition of benthic foraminifera, and change in ecological quality status. The latter are quantified using an ecological quality index (NQIf), diversity indices (ES100 and H’log2), and a sensitivity index (Foram-AMBI) (table 1).
1.1 The study area
Singlefjorden is located in the Hvaler estuary. The estuary is in the eastern parts of the outer Oslofjord (figure 1). The inner part of the area is sheltered from the sea, and heavily influenced by the river Glomma (Helland, Åberg, et al., 2002; Holtan, 1996).
In the estuary, the study area is located to the east of the mouth of the river, and to the north of the mouth of Ringsdalsfjorden and Iddefjorden. Singlefjorden is connected to the outer marine basin. The water depths between the outer area and Singlefjorden range from ∼ 60 m to >160 m (Kartverket, 2019).
Figure 1: The label shows the water depth at the sampling location. The outlets of Glomma (near Fredrikstad) and Tista (near Halden) are marked with arrows. The map was created using QGIS 3.2.1, and data from WMS-servers: Natural Earth Raster and vector data n.d.; Kartverket, 2020b; Kartverket, 2020a
The river Glomma drains into the estuary and marine basin outside of the group of islands (figure 1). Glomma is Norway’s largest river, with a length of 619 km (Thorsnæs, 2020), and a catchment area of 42 443 km2 (NVE, 2020), which is approximately 13%
Norway’s surface area (Thorsnæs, 2020). The soil in the lower parts of the catchment consists mainly of marine deposits, which were deposited during the end of the previous ice age (NGU, 2020; Holtan, 1996).
The water masses closest to the mouth of the river are characterised by estuarine circu- lation, with stratification of fresh and salt water. The lighter fresh water flows out and over the saline seawater, while saltwater flows inwards as a compensation current (Staal- strøm and Skogan, 2015). The layer of fresh water becomes thinner as it flows outwards into the sea. The sources of freshwater into the estuary are the rivers Glomma and Tista (figure 1). The inner Hvaler area including Singlefjorden is mostly influenced by inputs from Glomma, and less so from the Skagerrak/Kattegat area (Bjerkeng, 1997). The average tidal amplitude since 1991 is 27 cm, thus the estuary is microtidal (Kartverket, n.d.).
Due to the varying river discharge, the thickness and salinity of the surface water layer changes over the year. In periods with higher discharge, the thicker layer of less saline surface displaces the underlying more saline water. When measured in a transect sea- ward, from the mouth of the river to the middle of the estuary, the salinity increases and the thickness of the surface layer decreases. The thickness and reach of the surface layer varies seasonally with the discharge patterns (Helland, 2001).
The sampling location in Singlefjord is to the east of the transect described above. At the time of sample collection (25.06.2019), the salinity of the surface waters was 8-25 in the first 10 m, 25-32 from 10 to 20 m, and 34 at the bottom (89 m water depth). The CTD-output can be found in the appendix, figure 23 on page 65.
The predominant circulation patterns in Skagerrak are characterized by deep Atlantic water entering from the southwest, the Baltic Current entering from Kattegat in the southeast, and the Norwegian Coastal Current moving in surface waters along the southern coast of Norway. Skagerrak surface water has salinities ranging from 20 to 32, Norwegian coastal water has salinities ranging from 25 to 32, and Mixed Skagerrak Water has salinities ranging from 32 to 35 (Danielssen et al., 1997). Based on salinities, the bottom water in Singlefjorden could receive inputs of Mixed Skagerrak Water.
Instances of hypoxia have been recorded in some locations in the estuary area (Naustvoll et al., 2017; Rygg et al., 2000). From the 1980’s to the mid 1990’s, okxygen conditions were worst near the mouth of the river, in the deepest part of Løperen, and in Rings- dalsfjorden (locations: figure 1), with oxygen concentrations in Ringsdalsfjorden being as low as <1 ml/l ("bad" status class) in the interval 1990-94 (Magnusson and Sørensen, 1996). The cause could be a combination of cultural eutrophication, as well as natu- rally restricted bottom water circulation. The sill at the mouth of Ringsdalsfjorden and Iddefjorden is 9-10 m deep, which cuts off the deep water in the basin outside, from cir- culating into the fjord system (Polovodova Asteman et al., 2015). In the inner parts of
Iddefjorden, bottom water renewal during winter did not happen at all during the winter of 2016, due to a thicker than usual layer of low salinity water, which when combined with the effects of the fjordic bathymetry inhibited deep water circulation (Naustvoll et al., 2017). In 2016, oxygen concentrations of <2.5 ml/l ("poor" status class) were observed in Ringsdalsfjorden and Iddefjorden (Naustvoll et al., 2017). However, the situation is observed to be different in Singlefjorden, with oxygen concentrations vary- ing from 4 to 6.5 ml/l in 1990 to 1994 (Magnusson and Sørensen, 1996), and 4.2 to 5.8 in 2016 ("good" to "high" status class) (Naustvoll et al., 2017). The deep water depths connecting Singlefjorden to the Skagerrak area seem to allow for good deep water exchange, giving high oxygen concentrations in the water.
The oxygen concentrations in the deep waters of Singlefjorden have been measured semi- regulary since 1990s (Magnusson and Sørensen, 1996; Miljødirektoratet, n.d.), where the trends show seasonal patterns of highest concentrations in early the late winter, and the lowest concentrations in late summer. The avereage concentrations have declines the last 20 years, although the average concentrations in recent years are still in the "high" status class (Miljødirektoratet, n.d.). In 2016, when there was moderate deep-water exchange, the oxygen concentrations were classified as moderate (Naustvoll et al., 2017).
During the previous century, the mean temperature in Norway has risen, and the amount of precipitation has increased (Meteorologisk Institutt, n.d.). Correspondingly, the av- erage discharge from the Glomma river has increased over time (figure 2) (Naustvoll et al., 2017; NVE, n.d.). In the 1890’s the yearly mean discharge was 675 m3 s-1. This was measured at Sarpsfoss (NVE, n.d.). The mean in the period 2010 to 2020 was 760 m 3 s-1(figure 2). This was measured at Solbergfoss (NVE, n.d.). The discharge from Glomma varies throughout the year, due to two main pulses of water from snowmelt dur- ing spring and summer (Holtan, 1996; Helland, 2001; NVE, n.d.). During periods of low discharge, the values are around 400-500 m3 s-1, while the discharge can reach 1500-2000 m3 s-1during periods of spring and summer flooding (NVE, n.d.). Two other freshwater sources in the area, Tista and Enningdalselva in Iddefjorden, contribute much less fresh water than Glomma to the estuary. In 1997 their combined mean yearly discharge was 30 m3 s-1 (Berge et al., 1997).
Figure 2: Note the gap in data from 1905 to 1964.
1.2 Pollution history
The river Glomma has been used as a source of hydropower for the wood processing industry, as well as a means of transport for raw materials. The wood processing industry was prevalent in the area already from the late 1500’s (Borregaard 1955; Hansen, 1944).
When the royal privileges were no longer needed in order to run a sawmill in 1860 (Mardal, 2014), the number of sawmills increased (Dehli, 1973).
The most impactful industry in the Halden area was Saugbrugsforeningen, a union of 30 saw-mills along the Tista river, founded in 1859. An associated sulfite plant was built in 1908, and a paper mill in 1915. The pollution from the combined industries consists of the organic material from wood fibers, heavy metals from the roasting of sulfite ore, and the use of mercury as a slimicide in the paper making industry. The shutting down of plants, and improvements to discharge management reduced the emissions from the late 1970’s (Holtan, 1996; Knutzen et al., 1978; Polovodova Asteman et al., 2015).
In the Fredrikstad and Sarpsborg-area, the most impactful entities are Borregaard in- dustries Ltd. (which over time has involved wood processing plants, paper mills and chemical plants) and Peterson Greaker A/S (which produced paper and associated sul- fite plant), and Kronos Titan A/S (which is a chemical plant). Regulation of emissions from Kronos Titan lead to a sharp decline in heavy metal pollution in the late 1980’s,
and the closing of the sulfite plant at Greaker in 1980 lead to a decrease of emissions by 90% (Holtan, 1996).
With a rapidly increasing population during the last century and a half, the need for proper sanitation increased as well. The percentage of households in the Hvaler region with WC rose from 77% in 1970, to 95% in 1990. The were several sewage treatment plants built in the region after the 1970’s, leading to a decrease in phosporous emissions from sewage in the 1980’s (Holtan, 1996).
The use of nitrogen and phosphorous fertilizers in Norway increased in the years follow- ing World War II. Discharges of phosphorous and nitrogen to the marine waters from the Swedish border, down to the southern tip of Norway, decreased by 33% and 11%
respectively, from 1985 to 2010 (SSB, 2012). The discharges of nitrogen and phospho- rous from the Glomma river have increased over the period 1970-1995, with a high peak (1000 tonnes P/y, 18 000 tonnes N/y) around early to mid 1980, while the discharges of phosphorous and nitrogen to the study area from sewage and industrial sources has declined since the 1980’s (Holtan, 1996).
Currently, the largest anthropogenic sources of nutrients (nitrogen and phosphorus) to the study area are estimated to be agriculture and sewage. The changes in emissions of nitrogen are increasing in the outer Oslofjord area over time (1990-2015) with statis- tically significant amounts at a 95% confidence level (Naustvoll et al., 2017). The dis- charges of phosphorous and nitrogen in the Glomma river were 499 and 15 182 tonnes/y, respectively. Increased emissions of nitrogen and phosphorous to the coastal waters has been associated with increased river discharge (Holtan, 1996).
The Hvaler area is popular for recreation, with 4300 cabins in 2019 (Henriksen, 2019), increasing from 1500 in 1980 (Holtan, 1996). In 1990, there were 30 000 leisure boats with a hull size of 15 feet registered in the area bordering the Glomma estuary, and this number was estimated to grow in the order of 2000 annually (Helland and Bakke, 2002). Based on this, it is safe to assume that the number of leisure boats is far greater today. Leisure boats are permitted to dump their untreated sewage directly into the sea as long as it is done 300 m from the shore (Lovdata, 2013).
2 Materials and methods
2.1 The cruise
A cruise was conducted using the research vessel Trygve Braarud from 24.06.2019 to 26.06.2019, at the locations shown in figure 1, on page 4.
2.2 Sample collection and treatment
2.2.1 Sediment cores
The cores in this study are collected from the deepest part of the Singlefjorden basin.
They were collected using a Gemini corer and an Abdullah corer. The Gemini corer has an inner diameter of 8 cm, and core liner length of 79 cm. The Abdullah corer is longer and narrower, with an inner diameter of 56 mm, and a core liner length of 2 m. One core from each coring device were selected for subsampling. The cores were chosen based on having an undisturbed appearance, with minimal bioturbation, intact stratigraphy and sediment/water-interface.
The Gemini-core was subsampled in its entirety, while sediment from the top of the Abdullah core was intentionally discarded, in order to leave an overlap in core depth of 20 cm between the two cores. The Abdullah core was collected to extend the sedimen- tary record from the Gemini core. The cores were sliced using a thin metal plate, an acrylic cylinder and a sediment core stand with a piston mechanism that could grad- ually push the sediment core up and out of the core liner. The slicer and measuring cylinder were rinsed thoroughly between each slice in order to reduce the cross contam- ination. Smearing inside the core liner was unavoidable, and must have lead to some cross contamination between the slices.
The cores were sliced in increments of 1 cm from 0 to 20 cm core depth, in increments of 2 cm from 20 cm to 80 cm core depth, and in increments 5 cm from 80 cm to 122 cm core depth. The sediment slices were placed in labelled containers. The samples were frozen immediately after slicing.
2.2.2 Surface samples
In addition to the two long cores, the sediment surface (0-1 cm) of 4 Gemini cores was collected. The sediment was immediately treated with a mixture of 2 g rose Bengal (rB) and 70% ethanol (Schönfeld et al., 2012) in order to both preserve the organisms, and
dye the living assemblage. Due to time constrains, only one of the surface samples was analysed for living (rB stained) foraminifera.
2.3 Freeze drying and calculation of water content
The frozen sediment core samples were weighed in order to calculate the wet sediment weight. Only a small subset of the samples were out of the freezer at once, and then put back into the freezer once they were weighed, in order to keep them from thawing.
When all of the samples were weighed, and thoroughly frozen, they were freeze dried.
The dry samples were weighed, and the weights of the wet and dry samples were used to calculate the water content of each sample (1).
%W ater content= Mw−Md
Mw ∗100 (1)
Where Mwis the wet sediment sample weight, and Mdis the dry sediment sample weight.
2.4 Grain size analyses
A small portion of sample (approximately 0.065 g) from each interval was analysed using a Beckman Coulter LS12 320. When portioning out sediment for analysis, objects (shell fragments, sticks) appearing to be larger than 2 mm were avoided, as larger objects could cause a blockage in the instrument. The sample was prepared for analysis by adding approximately 5 ml of 5% Calgon in a beaker, and placing the beaker in an ultrasonic cleaner bath for 3-5 minutes, so that any lumps of sediment and fecal pellets were dispersed. For each analysis, a small amount of sample was added to the instrument, until the obscuration in the device was optimal. A laser is pointed at the senor, and as the sample is pumped through the instrument, the sediment grains pass over the sensor, casting a shadow. The grain size fractions are determined by analysing the pattern of the shadows cast by the sediment grains. The shadow patterns correspond to different grain sizes.
The instrument was rinsed (to insure that there is no remaining sediment from the previous sample) and the background obscuration of the tap water was measured before each sample.
Sediment from each core interval was analysed in at least two replicates, to ensure that the samples were representative. If the discrepancy between the outputs of the analysis was too large between to replicates (based on overall fit between grain size distribution
curves), a third or fourth analysis was done, in order to get more representative data, and make up for potential unevenness.
When all the intervals were done, the results of all of the replicates were averaged and saved in spreadsheets.
2.5 Dating and sediment accumulation rate
Approximately 4 g (± 0.1 g) from each interval of the cores, including the overlap be- tween the Gemini and the Abdullah core, were sent to the Environmental Radioactivity Research Centre (ERRC) at the University of Liverpool for dating. The dating report from ERRC is included in the appendix, page 67.
The cores were dated using 210Pb and 137Cs. The backround level of 210Pb in the sediments is "supported" by the decay of 226Ra. Younger sediments have an excess of "unsupported" 210Pb. The sediments are dated based on the relationship between
"supported" and "unsupported"210Pb. 137Cs-peaks are used to identify the early 1960s, which was the time of maximum fallout from the atmospheric testing of nuclear weapons.
Not every interval of the Gemini core was analyzed by the researchers at ERRC. They analyzed approximately every other interval. To save time and labour, I also chose to analyse approximately every other interval, when it came to heavy metal concentration and carbon and nitrogen content. The intervals they chose, and I chose are exactly the opposite ones, so in order to compare directly in a single interval I used R to interpolate the dating results in the gaps, using the "na.approx" function.
Along with the dating, the report from ERRC included an extrapolation of the dating results, going back to 1802, which is estimated to be at 77 cm core depth.
Assuming the accumulation rate remained consistent downwards from this point, the age for the entire core can be extrapolated using a linear model (using year and core depths as the covariates) and the "predict" function.
2.6 Heavy metal analyses
Sediment core intervals analysed for metals were chosen to get an even distribution of data-points, as well as higher resolution data in the shallowest intervals. Approximately every other interval is analysed in the middle, while every interval is analysed in the top and bottom of the cores (samples spacing: table 4, page 66). The sediments were analysed for Cr, Cd, Hg, Cu, Zn, Pb and Ni.
Approximately 1 g of sediment was portioned out from the original sample, and pulver- ized in an agate mortar. The utensils (plastic spoon, mortar and pestle, spatula) were cleaned with ethanol between each sample in order to avoid cross contamination. 50 ml centrifuge tubes were labelled and weighed, before 1 g of the pulverized sediment was placed into them.
20 ml of 7.22 M HNO3 was added to the centrifuge tubes. The samples were placed in an autoclave at a temperature of 120°C and a pressure of 1.2 bar, in order to dissolve the metals in the sediments. The samples were then centrifuged for 10 minutes at 3000 rpm.
0.2 ml of the solute was extracted from the 50 ml centrifuge tube using a pipette, and transferred to a 15 ml centrifuge tube. The solute was diluted 50 times, using 10 ml 1%HNO3. The method is based on “NS 47700:1994 Determination of metals by atomic absorption spectrophotometry, atomization in flame”.
The diluted samples were analysed using a ICP-MS (Aurora Elite, equipped with a Cetac ASX-250 autosampler and an ESI oneFAST sample introduction system), at the Geoscience Department of University of Oslo.
The resulting concentrations were adjusted for acid dilution factor in Excel.
2.7 Carbon, nitrogen and stable isotope analysis
TC is the total amount of carbon in the sediment, TOC is the total organic carbon content, TIC is the total inorganic carbon content, and TN is the total nitrogen content.
These paramters were analysed after heavy metals, and the metal concentration gave an indication of where a more sparse sampling distribution could still give an accurate picture of the development in the sediment cores. This resulted in sparser sampling rate in the deepest intervals. The spacing of samples can be found in the appendix, page 66.
2.7.1 Sample preparation Non-acid treated samples
1.5 g of freeze dried sediment sample was pulverized in an agate mortar. The utensils (spoon, mortar and pestle, spatula) were cleaned with ethanol between each sample in order to avoid cross contamination. 50 ml centrifuge tubes were labelled and weighed, before 1 g of the previously mentioned pulverized sediment was placed into them. The remaining 0.5 g was portioned into labelled glass vials.
Acid treatment
15 ml of 10%HCl was added to the 50 ml centrifuge tubes containing 1 g of pulverized sample, while working under a fume hood. The acid is added to dissolve the inorganic carbon in the sediment. The samples were placed on a shaking rack (approximately 240 rpm for 2 hours) to ensure that the sediment and acid were thoroughly mixed, and that all of the inorganic carbon had reacted.
The acid treated samples were centrifuged (3000 rpm, 10 minutes), and then the acid was carefully decanted. The tubes were filled with 40 ml distilled water (MilliQ), and the sediment was resuspended using a glass mixing rod. This process of centrifuging and washing with distilled water was repeated 3 times, until the sediment is assumed to have reached a neutral pH.
The samples were dried at40°Cin an oven. The dry samples were weighed while still in the tube. This was done to ensure all of the sample is weighed, because some clung to the walls of the centrifuge tube, and was too difficult to remove. After weighing, the sediment was re-pulverized in an agate mortar. The mortar and spatula were cleaned with ethanol before each sample to avoid cross contamination. The pulverized samples were placed in labelled glass vials. The method is based on NS-EN 13137, 2001: Determination of total organic carbon (TOC) in waste, sludge and sediments.
2.7.2 TOC63, TIC TOC63
A portion of acid treated and re-pulverized sample, with weights in the range of 13.4 mg to 15.5 mg, was transferred from the glass jars into tin capsules, using a scale with 0.001 mg readability. The dimensions of the tin capsules are 8x5 mm. The capsules with sediment were folded along two axes using two pairs of tweezers. The work was con- ducted while wearing gloves, in order to avoid sebum and skin particles to contaminate the samples. The tweezers were cleaned with tissue paper and ethanol.
The samples in the tin capsules were analysed for TOC in a Thermo scientific FlashSmart CHNS/O Analyzer with Multi-Valve Control (MVC), at the Geoscience department of the University of Oslo.
The values were corrected for the weight before and after the removal of TIC (acid treatment).
The TOC values are adjusted for grain size using equation 2 (Veileder, 2018).
T OC63 =T OCmg/g + 18∗(1−p < 63µm) (2)
Due to the small grain sizes in the sediment samples, this step had a small effect on the values, but was done in order to assign status classes to the concentrations of organic carbon, based on the Norwegian guidelines for classification of environmental status in water (Veileder, 2018).
TIC
The total inorganic carbon is calculated by subtracting the weight-corrected and grain size-adjusted TOC from the TC.
2.7.3 TC, TN and C/N
The remaining 0.5 g of sediment, which was pulverized but not acid treated, were used for TC, TN and stable15N isotope analyses.
TC and TN were analysed the same way as descibed for TOC, the only difference being that the samples used were not acid treated.
2.7.4 Stable isotope analyses
δ13C and δ15N are measures of the difference between the ratios of stable isotopes (C13/C12 and N15/N14) in the sample compared to a reference material with a know isotopic signature, given by the equation:
δ15N or δ13C(h) = ( Rsample
Rstandard −1)×1000 (3)
For analysis of δ13C, a portion of acid treated and re-pulverized sample, with weights ranging from 6.4 mg to 3.3 mg, was transferred from the glass vials and into tin capsules (5x8 mm), using a scale with 0.001 mg readability. The amount of sample needed for each interval was calculated based on the measured content of carbon, in order to analyse approximately the same total amount for each interval. The capsules with sediment were folded along two axes using two pairs of tweezers. The work was conducted while wearing gloves, in order to avoid sebum and skin particles to contaminate the samples.
The tweezers were cleaned with tissue paper and ethanol.
The δ15N samples were prepared the same way as the δ13C samples, with the exception of the amounts used. The sample amounts for δ15N ranged from 17.4 mg to 12.4 mg.
The samples were analysed using Thermo Fisher Scientific EA IsoLink IRMS System, which consists of a Thermo Fisher Scientific Flash Elemental Analyzer and a Thermo Fisher Scientific DeltaV Isotope Ratio Mass Spectrometer.
During the analysis, the internal lab reference materials were analyzed every 12th sam- ple, and used to normalize the data to the VPDB scale forδ13C, and the AIR scale for δ15N analysis. Triplicates were analysed at the bottom, middle and top of the core, and the results were averaged.
2.7.5 Mixing equation
The mixing equation (eq. 5), is used to to determine the proportions of terrestrial vs.
marine OM in sediment samples, based on inputting values for δ13C, δ15N and C/N (Shultz and Calder, 1976; Thornton and Mcmanus, 1994). A sample is compared to end-members, which are representative values (C/N,δ13C) of the terrestrial and marine OM in an area.
X =FtXt+FmXm (4)
Equation 4 rearranged:
Ft= X−Xm Xt−Xm
(5) Ft is the fraction of terrestrial organic carbon, Fm is the fraction of marine organic carbon. The sum of Ft and Fm is 1, so Fm is 1-Ft. Xm is the marine end-member, and Xt the terrestrial end-member. X is the sample which is being compared to the terrestrial and marine end-members.
2.8 Micropalaeontological analyses
2.8.1 Dry picking
An initial trial run with 5 g of sediment from 3 samples was washed through 500µm and 63µm sieves. The 3 sample intervals were from the top, middle and bottom of the core.
The fractions of washed sediment were dried at40°C, and weighed.
The washed and dried samples were hand picked for foraminifera, until 250-350 indi- viduals were identified and counted. Using fewer individuals is associated with higher errors in the calculation of diversity indices, while picking more gives diminishing re- turns in error reduction (Bouchet et al., 2012). The picking was done by using a small damp brush, and two foraminifera slides. One slide was coated with glue, and one was not. A small amount of sample was placed on the slide without glue, and foraminifera
tests were picked out with the tip of the damp brush, and placed on the glue slide. The moisture from the brush reactivated the glue on the slide. Picked sediment was placed in a separate labelled jar. The picked and unpicked portions of the washed sediment were weighed. The weight are used to calculate the absolute abundances (tests per gram of dry sediment).
5 g of sediment was concluded to be more than enough to find a sufficient amount of foraminifera, and the subsequent foraminiferal samples consisted of 2 g of dry unwashed sediment.
Due to the time consuming nature of foraminifera picking, a smaller amount of intervals was analysed, compared to the other parameters in this thesis. This will give a lower resolution of the data.
2.8.2 Wet picking
The surface sample was washed through 500µm and 63µm sieves. The washed sample was divided into 8 approximately equal parts using a modified Elmgren wet splitter (Elmgren, 1973), and placed in glass vials. The samples were left to settle, before decanting off excess water using a pipette. The jars were filled with 70% ethanol, in order to preserve the sample.
One 1/8 of the surface sample was picked for living (stained) foraminifera, using a Duffield-Bogorov tray (C. J. Duffield and Alve, 2014). This amount of sample was more than enough to obtain >250 individuals. The foraminifera were glued to fauna slides.
2.8.3 Calculation of diversity, sensitivity and ecological quality indices After obtaining the counts of the different species of foraminifera (Appendix: table 6, page 75), the relative (%) and absolute abundances (tests g-1 dry sediment) were cal- culated using Excel spreadsheets, in addition to the benthic foraminiferal accumulation rate (tests cm-2 y-1).
Foram-AMBI
The Foram-AMBI (Alve, Korsun, et al., 2016) is a marine biotic index specifically meant to be used on foraminifera datesets, based on the AZTI Marine Biotic Index (AMBI) (Borja et al., 2000), meaning that the value gives information about how sensitive the foraminiferal assemblage is to enrichment of organic carbon, and the subsequent decrease in oxygen concentrations in the sediments. Foraminifera species assigned to group I are sensitive to organic carbon enrichment, while species in group II are indifferent, and occur in broader ranges of organic carbon concentrations. Group III-species are tolerant
to enrichment of organic carbon. Species assigned to group IV and group V are 2nd and 1st order opportunists, and show clear positive responses to organic carbon enrichmet (Alve, Korsun, et al., 2016).
Foram-AMBI calculated using equation 6 (Alve, Korsun, et al., 2016) in Microsoft Excel.
Foraminifera assigned to a high ecological group are tolerant, while those assigned to a low ecological group are sensitive. The tolerant species are weighted heavily in the formula (eq. 6), giving a higher AMBI-value when the foraminiferal assemblage has a high proportion of tolerant species.
AM BI = {(0×%G I) + (1.5×%G II) + (3×%G III) + (4.5×%G IV) + (6×%G V)}
100 (6)
Diversity indices
ES100 and H’log2 were calculated using Primer 6.
ES100, Hurlbert’s diversity (Hurlbert, 1971) ES100 is a measure of diversity, and specif- ically a measure of how many different species you are expected to find in a random sample of 100 individuals.
ES100 =
S
X
i
(1
N−Ni 100
N 100
) (7)
N is the number of individuals, S is the number of species, and Ni is the number of individuals belonging to the species i.
H’log2 Shannon-Wiener diversity (Shannon and Weaver, 1963) It is a measure of the richness of the species in the assemblage, as well as the distribution.
H0log2 =
S
X
i=1
(Ni
N × −log2 Ni
N ) (8)
Ni is the number of individuals of the species i, N is the total number of individuals, and S is the total number of species.
NQIf
The Norwegian Quality Index is an ecological quality index suited for use on foraminifera (Alve, Silvia Hess, et al., 2019). It is calculated by combining AMBI and ES100, as shown in eq. 9.
N QI_f = 0.5
1−AM BI_f 7
+ 0.5
ES100_f 35
(9) The resemblance between the assemblage in pairs of samples was determined using the Bray-Curtis similarity coefficient in Primer 6. The analysis was based on the relative abundances. The data was pre-treated by square root transforming. Un-transformed data formed tighter cluster, were trends were not as clearly visible, so only transformed data was used in the final analyses. Based on the similarity analysis, a dendrogram and several MDS cluster- and bubble plots were created.
3 Results
3.1 Core description
The sediment of the cores was soft and fine grained. In the short cores (several nearly identical cores were collected and inspected, but not sub-sampled), the colour of the sediments was brown in the upper 10 cm, brownish-gray the next 5 cm, then gray and dark gray in the bottm. The gray intervals showed some dark lamina, especially in the lower parts.
3.2 Water content
Figure 3
The plot (figure 3) of the percent of water content shows a fairly smooth decline from the top to the bottom of the core. This is consistent with what is expected, as the sediments become more compacted as they are buried deeper. The absence of spikes in the data indicates the core is undisturbed and that the stratigraphy is likely intact. The overlap between the two cores shows approximately similar values.
3.3 Grain size analysis
The median grain sizes are slightly larger upwards in the core, with the median particle diameter changing from 3.8 µm in the lower parts of the core, to 6.4 µm in the upper parts of the core (figure 4a). Generally, the grain size distribution in the cores changes only slightly (figure 5). The grain size distributions from each interval shows a slight coarsening upwards in the cores, with the graph "shifting" towards larger grain sizer for shallower intervals (figure 5), a larger percentage of silt (from 70% to 80%, figure 4c), and correspondingly a smaller percentage of clay (29% to 16%, figure 4b).
(a) (b) (c) Figure 4
Figure 5: The legend shows core depth
For more details, a table with grain size information is included in the appendix (page 73).
3.4 Dating and sediment accumulation rate
The calculated sedimentation rate (table 2) is stable at 0.29 g cm-2y-1 from the surface down to 12.5 cm core depth (2002). The rate then drops to 0.15 g cm-2y-1 until 33 cm core depth (1961). The calculated rate stays the same until 49 cm core depth (1909), which is the last data point before the extrapolations begin. The extrapolations are based on the assumption that the sedimentation rate stayed the same further downcore.
Based on these assumptions, the already extrapolated date was extrapolated for the entire core, although these numbers are very uncertain (figure 6). The results are found in table 2, where numbers in bold are from the original report, non-bold and non-itallic are interpolated, and numbers in itallics are extrapolated and/or interpolated. In figures, the differing uncertainties associated with the dating results is reflected by the line type
and colour. Dotted lines and gray colour are the most uncertain dates, while solid black lines are the most certain.
Figure 6
Table 2: Interpolated and extrapolated dating results and sediment accumulation rate.
The borders in the table frame values calculated in the report, extrapolated in the report, and extrapolated in this thesis.
3.5 Heavy metals
In figure 7, the lowest intervals of the cores have stable concentrations of heavy metals in the sediment over time, up until a peak at 82.5 cm core depth (extrapolated date
∼1790). After this peak, the concentrations still remain somewhat stable, but at higher concentrations than in the deeper sediments. There are two concentration maxima, one at 35 cm core depth, (dated to 1955), and at 29 cm core depth which is dated to the year 1972. The concentrations have been declining from the mid 1970’s with the execption of two smaller peaks at 15.5 and 9.5 cm core depth (1998 and 2007).
Figure 7: Note the different scale on x-axis for Zn, Cd and Hg.
The results for Hg are semiquantitative and not reliable as actual concentrations. This is due to the large amount of energy needed to ionize Hg, and ICP-MS only measures ions.
If a small proportion of Hg atoms are ionized, the concentration will not be realistic.
However, the results can give information about the change in proportion to itself. The trend in the result for Hg is similar to the other metals, with clear reference conditions up until 55 cm core depth (1880’s) (figure 7)
Using the status classes defined in Norwegian guidelines for classification of environmen- tal status in water (Veileder, 2018), the change in heavy metal pollution status class over time can be visualized (table 3).
Table 3: Heavy metal environmental classification (Veileder, 2018) (the years in bold are the measured or extrapolated values from the original dating report from ERRC (appendix, page 67). The years in itallic are interpolated and/or extrapolated based on the ERRC-report.
3.6 Carbon, nitrogen and stable isotopes
3.6.1 TC, TN and C/N TC
There is an increase upwards in the cores, from approximately 18.5 mg/g in the lower part, to 28 mg/g in the upper parts. The increase appears to be more rapid from approximately 1900 (figure 8a)
TN
There is an increase in the concentration of nitrogen upwards in the cores (figure 8b).
The concentrations increase from 2.3 mg/g in the lower parts of the core, to 3.2 mg/g in the surface.
C/N
The C/N-ratio is the ratio of total carbon to total nitrogen. The ratio appears stable in the bottom of the core, until the increase in TOC from the late 1800s to early 1900s, which subsequently leads to a higher C/N ratio (figure 8c). The value changes from approximately 7.7 in the lower parts of the core, to 8.8 in the upper parts.
(a) (b) (c) Figure 8
3.6.2 TOC63 and TOC TIC
There is an outlier in the lowest sample from the Gemini core, as well as a slightly smaller spike at 19.5 cm core depth. Overall, the concentrations increase slightly, from approximately 4.3 mg/g in the bottom of the core, to 5.0 mg/g in the upper parts of the core.
TOC63
The concentrations of TOC63 increased upwards in the cores, from approximately 14 mg/g in the lower parts, to 23.5 mg/g in the upper parts (figure 9b).
There is an outlier in the lowest data point from the Gemini core, as well as a small dip at 19.5 cm core depth. These spikes and dips are the inverse of the ones found in the TIC, because they are inversely related. These values, and the possible errors in them,
affect the TIC, because TIC is calculated based on TOC and TC.
As mentioned in the methods, during acid treatment, the samples are shaken on a shaking rack. Some of the sample was flung up on the sides of the centrifuge tubes, and might not have reacted fully, compared to the sample that was submerged in acid throughout the process. After the acid treatment, the samples were weighed without their lids, so if some of the sediment had splashed onto the lid, it would skew the results.
For the lower spike at 67 cm core depth, the reason for the TOC being high, is not due to the sample weights being anomalous, but rather due to the actual measured value of TOC being high. For the spike at 19.5 cm, the measured TOC is not unusually high or low, but the TOC correction factor (weight after acid treatment divided by weight before acid treatment) is unusually low, which causes the weight adjusted TOC to be low. This is due to the weight before acid treatment being higher than the surrounding samples, while the weight after acid treatment is proportional to the surrounding samples. This suggests either that there was more inorganic carbon in this sample, that was removed during acid treatment, or that some of the sample was lost, or that a mistake was made during weighing. The spikes are not present in the TC results, which suggests that they are caused by one of the steps in the acid treatment process.
(a) (b)
Figure 9: The colours in figure (b) reflect the status classes defined in Veileder, 2018, summarized in table 1 on page 3.
3.6.3 Stable isotope ratios δ13C
The δ13C values become slightly lighter upwards in the core. The value changes from -22.99h from the lower parts of the cores, to -23.58h in the upper parts of the cores (figure 10a).
δ15N
The δ15N values steadily increase, but remain under 5h up until the beginning of the 1800’s. The lowest values are 4.80-4.90 h at the bottom of the core.
From the middle to late 1800’s the values increase, and reach 5.5 h at the turn of the 20th century (1900).
The values continue increasing upwards in the core. The highest values are found in the surface, were theδ15N value is 6 h.
(a) δ13C (b)δ15N
Figure 10: The values of δ13C and δ15N in the sediment cores
3.6.4 Mixing equation
The terrestrial and and marine end-members used were based on results from the study area, where the authors present values forδ13C and C/N ratios from the mouth of the river in the estuary, and from the marine basin outside of the estuary(Helland, Åberg, et al., 2002). The marine end-members used in the calculation were δ13C -21.20h, and C/N 7.84. The terrestrial end-members were δ13C -27h, and C/N 11.45 (Helland, Åberg, et al., 2002).
Figure 11: The estimated percentage of carbon of terrestrial origin
When results from this thesis is input in the mixing equation (eq. 5, page 15) with the end-members described above, the δ13C and C/N ratios show a decrease in the
percentage of terrestrially derived organic matter downwards in the core.
3.7 Accumulation rates of organic carbon
The accumulation rates of TOC63, TC and TN all show an increase around 35 cm core depth (1955), and appear to be stabilizing somewhat at 12.5 cm core depth (2002).
Figure 12
3.8 Micropalaeontological analyses
3.8.1 Abundance of foraminifera
As seen in figure 13b, the relative abundance of Stainforthia fusiformis (blue) increases over time (∼15% to >50%), while Cassidulina laevigata (red) steadily decreases (>10%
to ∼1.5%). Nonionella sp. T1 (purple) is completely absent until 2.5 cm core depth (which corresponds to the year 2016), and is dominant in the surface sample (37%).
Stainforthia fusiformis appears to be "replaced" with Nonionella stella, and declines sharply in abundance in the surface sample (3%). Textularia earlandi (green, figure 13b) is steadily increasing in abundance upwards in the core (∼5% to >20%), until it drops in the surface interval(2.6%). Bulimina marginata (yellow, figure 13b) comprises 10%of the assemblage throughout most of the core, until 40 cm core depth (late 1930’s), when it begins to decline. There is a spike in the relative abundance ofB. marginata at 87.5 cm core depth (∼1770).
(a) (b) Triangles indicate living (rB-stained)
Figure 13: The arrows in figure a) correspond to the years in figure b).
In the plot of absolute abundance, Hyalinea balthica is highly abundant in the lowest interval of the core (extrapolated date: ∼1650), and shows a patterns of successive de- clines and peaks. The next peak is at 79 cm core depth (extrapolated date: 1800), and the last is at 49 cm core depth, which is dated to 1909. Adercotryma glomerata/wrighti andBulimina marginata seem to develip inversely toHyalinea balthica in the lower part of the cores. Adercotryma glomerata/wrighti varies in absolute abundance over time, with an increase from the deepest core interval up until 87.5 cm core depth (∼1760), before it declines at 79 cm core depth (∼1800). The absolute abundance increases and remains fairly stable until 49 cm core depth (1909), where it declines again and remains stable until 6.5 cm core depth (2011), where there is a spike. From 1909, Stainforthia fusiformis and Textularia earlandi increase in abundance, while the others decline. To- wards the surface,H. balthica increases in abundance again. Since a dry surface interval was not picked, the drop inTextularia earlandi in the surface is not recorded in the ab- solute abundance plot. In order to calculate the absolute abundance, the dry sediment weights are needed, as the absolute abundance is tests per gram dry sediment. Picking a dry surface sample (non-stained) would mean counting dead and living (at the time of sampling) foraminifera from the same interval. This is not recommended, as it will not accurately show the current status of the surface (Bouchet et al., 2012).
Several species with relatively low relative abundances throughout the cores, were sud- deny more abundant in the rb-stained surface layer. Most prominent among them were Liebusella göesi (generally 0.4-2%, 11.5% in the surface), Uvigerina peregrina (gener- ally 0.4%, 6.7% in the surface), Brizalina sp., and Leptohalysis scottii (0.3-2%, 6.3%).
Bubble plots feautring the individual species can be found in the appendix, from page 78.
The percentage of agglutinated tests varies greatly, especially in the lower intervals. The percreases from approximately 7%to 35%from the bottom to the top of the core (figure 14a)
The benthic foraminiferal accumulation rate increases sharply from ca. 50 cm core depth (∼1900) up until present (figure 14b).
(a) (b) BFAR Figure 14
3.8.2 Diversity, sensitivity and ecological quality indices
There is a sharp decrease in AMBI value in the surface of the core, from 4 to 2.5 (figure 15a). The general trend in AMBI is that the value increases upwards in the core, from a value of approximately 2.3 in the lower parts, to a value of 4 in the upper parts (with the exception of the surface samples).
Since AMBI is calculated based on what proportions of the assemblage that belong to different ecological groups, the development of the relative abundances of individuals belonging to these groups changes in a similar pattern upwards in the core (figure 15b) The abundance of EG1 species drops, while the abundance of EG5 species increases.
The abundances of EG2 remains fairly constant (with some variation), except for the
surface interval. The abundance of EG3 shows some variation, but overall seems to be declining, up until the surface intervals, where it becomes the most abundant. The largest change is seen in the interval at 59 cm core depth (extrapolated date: 1880), where EG1 steadily declines.
(a) (b)
Figure 15
The diversity indices show a similar development, with mostly stable values from the bottom of the core and upwards, then a dip in 1972, and then a spike (figure 16) After this point the values diverge, with ES100 remaining stable, and H’log2 increasing, and NQIf increases. Broadly speaking, the values seem to have declined slightly over time.
The values are always in the "high" or "good" ecological status classes.
(a) (b) (c)
Figure 16: Diversity indices with colour coding corresponding to class boundaries defined by Alve, Silvia Hess, et al., 2019
The dendrogram (figure 17) shows that the biggest grouping is between the surface sample and the rest of the core. This is due to Nonionella stella suddenly becoming domintant, while it is completely absent further down in the core. Further down, the data is separated into before and after 1972. The branches of the dendrogram are no longer "chronologial" in the oldest interval. This is perhaps a reflection of the assem- blages showing clearer trends from ca. 65-60 cm core depth (∼1850-1870), while the older and deeper intervals show more variation, instead of gradual growth and decline (figures 13).
Figure 17: Dendrogram based on relative abundances
The MDS plot shows the same trends, with the surface sample plotting alone by itself, and the groups are split in clusters of 65 similarity before and after 1972(figure 18).
Figure 18: Cluster diagram based on relative abundances
Bubbleplots of the speciesLiebusella göesi, Uvigerina peregrina, Brizalina skagerraken- sis, Brizalina spathulata and Leptohalysis scotti can be found in the appendix (from page 78)
3.8.3 Pyrite found in foraminifera
In four intervals (indicated by arrows in figure 19), there was growth of a gray metallic mineral assumed to be pyrite in the foraminifera tests. The mineral growth was the most prominent in the deepest interval (120-122 cm core depth), with a thick (relative to the thickness of the foraminiferal tests) metallic coating on the inside of the tests. In test fragments the coating could be seen from the inside of the test, and pyrite could be seen in clusters of small crystals, appearing rounded and bead-like.
Figure 19: For foraminifera legend, see figure 13 on page 35
4 Discussion
4.1 Depositional environment and accumulation rate
The increase of precipitation and river discharge is reflected in the increased sediment accumulation rate and higher percentages of larger grain sizes, compared to deeper and older sediments (figure 4, page 21 and figure 5, page 22).
The grain size distribution is stable downwards in the core, which supports the working assumption that the sediment accumulation rate has remained constant. The change in sediment accumulation rate in the upper parts of the core is associated with an increase in larger grain sizes. However, the gradual increase of larger grain sizes begins at 45 cm core depth, while the sediment accumulation rate sharply increases at 32 cm core depth, so the two parameters are not in perfect agreement.
4.2 Heavy metal pollution history
The two peaks found in the concentrations of Zn, Cu, and Pb match the heavy metal pollution development recorded in Ringsdalsfjorden (outer Iddefjorden) (Knutzen et al., 1978; John Berge and Helland, 1993; Polovodova Asteman et al., 2015). In Ringsdals- fjorden, the peaks are dated to 1965 and 1975. In the core from Singlefjorden, the dating results (table 2, page 24, and appendix, page 67) place intervals with the highest heavy metal concentrations were deposited in the years 1955 and 1972, which is earlier than what would be expected.
The Pb-concentration history from the middle of the estuary (station 5, Helland, Åberg, et al., 2002), does not show the two peak pattern. This might be caused by different timing of discharge maxima of Pb in the two locations, or that the sampling resolution was not fine enough to reveal a potential two peak pattern. The concentration maxima of Pb was dated to be in 1955. In a core from the outer basin (station 9, Helland, Åberg, et al., 2002), the sediment accumulation rate was too high for the core to reach pre-industrial levels (lowest concentration measured was 30 mg/kg). The concentration maxima was reached in sediments dating to 1980. The background levels of Pb in Singlefjorden were 18-20 mg/kg, measured below 87.5 cm core depth. The sources of lead to the estuary are the surrounding industry, as well as atmospheric depostition, due to the use of leaded gasoline (Helland, Åberg, et al., 2002; Holtan, 1996).
The two peaks of highest concentrations co-occur for all metals analysed in this study (except for Cd, which does not exhibit a distinct peak-pattern). In Ringsdalsfjorden the peak for Hg predates the peaks in Zn, Cu and Pb. This is due to particularly high
discharges of Hg in in the years 1964 to 1968 at Saugbrugsforeningen in Halden, where Hg was used as a slimicide in the paper making industry.
Figure 20
. Based on the comparison of the heavy metal pollution development in the Hvaler estuary, the outer Hvaler basin (Helland, Åberg, et al., 2002), and Ringsdalsfjorden (Outer Iddefjorden) (Knutzen et al., 1978; John Berge and Helland, 1993; Polovodova Asteman et al., 2015), it seems that the dating is progressively more off downwards in the core. Singlefjorden is farther away from the sources of pollution in Halden, Fredrikstad and Sarpsborg, and the peaks in heavy metal concentrations would be expected to appear slightly later, rather than sooner in a more distant location. The uncertainties for the interval dated to be from 1972 are ±3 years, so the results from all locations seem to be in agreement
For the second lower peak from 1955 (35 cm core depth), the uncertainty is±5 years(appendix, page 70), and overall the dates agree fairly well, although the trend of the dating of the
Singlefjorden cores skewing "old" is becoming more apparent. The increase of Hg- concentrations higher than those found in background levels was measured in the sed- iment layers dated to be from the mid 1920’s in Outer Iddefjorden by Knutzen et al., 1978. The same increase in the Singlefjorden cores occurs in intervals dating back to approximately 1900, which is a 20 year discrepancy. Last non-extrapolated date in the Singlefjorden cores is 1909 at 49 cm core depth, which has an uncertainty of ±16 years (appendix, page 70).
The increase in Pb-concentrations compared to what appears to be reference conditions were measured in the Hvaler estuary in sediments dated to 1900 (Helland, Åberg, et al., 2002). The corresponding interval is not dated in the Singlefjord cores, because the uncertainties were too large at this core depth. The dating extrapolation places the interval in approximately 1860, which gives a 40 year discrepancy. Based on these comparisons, it seems like the deeper intervals in the Singlefjorden cores are not as old as the extrapolation results imply.
The heavy metal pollution history of different locations with varying influence from different sources cannot be expected to be fully analogous. The concentration maxima found in sediment cores will not necessarily pinpoint specific discharge maxima, due to the effects of time averaging and smearing. Due to the different depositional behavior of the heavy metals studied, no single sediment core can be representative of the estuary as a whole (Helland, 2001).
As mentioned in the introduction, the pollution industry in the area is mainly influenced by the industry in Fredrikstad, Sarpsborg and Halden. Borregaard and associated in- dustries (Svovelsyrefabrikken, textile factory) has been a prominent contributor to the heavy metal pollution over time (Holtan, 1996). Zn was used in rayon production at Borregaard. The maxima of Zn emissions to Glomma were in 1968 (Holtan, 1996). The Borregaard Chloro-alkali plant, which used mercury as a catalyst, was reported to dis- charge 50 kg per year 1978 to 1987. A 800 kg accidental leak occured in 1985, but it does not appear to have created a peak in the heavy metal concentrations in Singlefjor- den. Mercury absorbs to organic matter, and has a lower specific gravity than mineral particles. A study of the transportation and deposition of heavy metals in the estuary found that mercury would settle out in higher concentrations at locations farther away from the mouth of the river (Helland, 2001). There was a gradual decline in emissions from the chloro-alkali plant from the middle of the 1980’s to the mid 1990’s (Holtan, 1996).
The heavy metal pollution from Saugbrugsforeningen in Halden, with the exception of Hg, is mainly associated with the discharges of wasteproducts from the roasting of sulphide ore ("kisaske"), which were largest in the years 1970-1975. The composition of the "kisaske" used is assumed to have varied over time (Knutzen et al., 1978).
If the extrapolated dates of the sediments in the deeper parts of the cores are correct, a possible explanation for the the small peak in metals in the late 1700’s could be the Storofsen flood, which was one of the largest floods in recent history (NVE, 2016). There was a steady increase in mining industry in the catchment area of the Glomma river in the 1700s (Nissen, 1976), and the flood could have washed mining waste into the estuary.
However, other parameters do not show anomalies at this level. Co-occuring peaks are not found in the grain size data (figure 4, page 21), or in TOC63 and δ13C or C/N ratios. However, the early metal peak is followed by a peak in the absolute abundance of Stainforthia fusiformis, which could indicate rapid changes in the environment and an influx of organic carbon.
4.3 Sources and accumulation rates of organic carbon over time
The effect of decomposition and diagenesis
Only a small fraction of the organic matter in a water body is actually preserved in the sediment, as the OM is degraded during sinking. Once the remaining fraction is de- posited, decomposition continues in the sediments, and is most active in the bioturbated zone. The more labile fractions are preferentially degraded, and the resulting sediment can therefore end up giving the wrong picture of what was originally deposited (Philip A Meyers, 1997). In studies aiming to determine the sources of organic to sediments, Philip A. Meyers, 1994 concludes that C/N andδ13C reliably retain information of or- ganic matter sources, while Thornton and Mcmanus, 1994 concludes that only δ13C is suitable for this purpose, as the the isotopic signature is less susceptible to be change due to biochemical alterations. δ13N signatures can changed as a results of biogenic transformation, when the nitrogen compounds are recycled.
Using sediment traps in conjunction with analyzing surface sediments give information about what is lost during organic material sinking and early diagenesis. A study from the Hvaler estuary compared characteristics of suspended particulate matter (SPM), as well as material from sediment traps and bottom sediment samples. They found that decomposition had not changed the material (at least not to the point that the sources of OC could not be determined), and that their results showed a change in the source of organic carbon (Helland, Åberg, et al., 2002).