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Oxygen Isotope Composition of Fjord and River Water in the Sognefjorden Drainage Area, Western Norway. Implications for Paleoclimate Studies
Estuarine, Coastal and Shelf Science (2000) 50, 441–448 doi:10.1006/ecss.1999.0581, available online at http://www.idealibrary.com on
Oxygen Isotope Composition of Fjord and River Water in the Sognefjorden Drainage Area, Western Norway. Implications for Paleoclimate Studies G. Mikalsen and H. P. Sejrup Department of Geology, University of Bergen, Allegt. 41, N-5007 Bergen, Norway Received 7 June 1999 and accepted in revised form 29 October 1999 During two cruises and one field expedition from 1996 to 1998, 32 water samples from 10 stations in Sognefjorden and tributary fjords and six samples from rivers draining into the fjord were obtained for oxygen isotope measurements. In addition, CTD profiles were obtained from all of the fjord locations. The river samples show a large gradient in the oxygen isotopic composition from the outer fjord region to the rivers at the fjord head. The difference is more than 6‰ with highest values recorded at the fjord mouth decreasing inward along the fjord. The low oxygen isotope values are found in areas with a high altitude from the watershed. The gradient in the river water is explained by distance to the moisture source, topographic effects and temperature of precipitation. The isotopic composition of the fjord waters suggest a mixing line for Sognefjorden of 0·31‰ for a salinity change of 1 which is slightly higher than the North Sea mixing line (Israelson & Buchardt, 1991), and lower than the North Atlantic mixing line (Craig & Gordon, 1965). The river samples are not used in the calculation of the mixing because of the large differences in the isotopic composition of river water along the fjord, and large seasonal differences of precipitation. 2000 Academic Press
Keywords: oxygen isotopes; mixing line; fjords; western Norway
Introduction The need for obtaining high resolution climatic proxy records has triggered an increasing interest for fjord sediments as climate archives. This is also the case for western Norwegian fjords, partly because of the probability for obtaining high resolution paleoceanographic/paleoclimatic records, and partly because their location close to the Norwegian Current, that offer possible link between the marine and terrestrial paleoclimate records. Stable oxygen isotope measurements on benthonic foraminifera are a promising method for reconstructing climate change in the fjord environment. From such studies it is possible to estimate temperature or salinity changes through time (e.g. Sarnthein et al., 1995). To do this we need to know the controlling factors of the oxygen isotope composition of fjord water. As the Holocene (last c. 10 000 years) is of particular interest for such studies it can be assumed that the global ice volume effect on sea water 18O is negligible at least since 7–8 ka BP. Information on how the mixing of oceanic water and fresh water influences the oxygen isotope 0272–7714/00/040441+08 $35.00/0
composition of fjord water is needed for such studies to be quantitative. So far, no such studies are published from the western Norwegian fjords. Oxygen isotope mixing lines have been constructed for North Atlantic waters (Craig & Gordon, 1965) and the North Sea (Israelson & Buchardt, 1991). Craig and Gordon (1965) found for the North Atlantic a linear mixing line that gave 0·61‰ change in isotopic composition for a change in salinity of 1. Israelson and Buchardt (1991) proposed a mixing line for the North Sea (0·25‰ change in isotopic composition for a change in salinity of 1), that is much lower than the North Atlantic mixing line. In this paper a mixing line for Songefjorden (western Norway) is presented, which perhaps is applicable to other western Norwegian fjords. This is based on 32 oxygen isotope measurements at depths between 20 to 638 m at 10 stations in the Sognefjorden fjord system. Six measurements of fresh water from rivers draining into Sognefjorden will also be discussed in the light of the location of the samples relative to topography and distance from the coast, and the 2000 Academic Press
442 G. Mikalsen and H. P. Sejrup 40° 30° 20° 10° 0° NLAN
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F 1 (a) A location map showing Sognefjorden and major oceanic surface currents (NC=Norwegian current, CC=Coastal Current) in the North Atlantic and Nordic Seas. =location of Lista (b) Sognefjorden drainage basin with sample locations in the fjords ( ) and from rivers ( ). [=Location of Ygleelvi (Henriksen et al. 1996).
isotopic composition of fresh water draining into the fjord. Physiographical setting The general physiography of western Norway is characterized by high mountains that drop off seawards to the strand flat (Nesje & Sulebak, 1994). The western Norwegian fjords cut far into the country and are deep (maximum depth of Sognefjorden is c. 1300 m) surrounded by steep slopes high mountains. These are normally in the inner parts of the fjord between 1300 to 1800 m high. The highest total vertical relief in Sognefjorden is up to 2600 m. Rivers enter the fjord at the fjord head or in tributary fjords. Some of the rivers are fed by glaciers and carry older water into the fjord. The largest freshwater input is during late spring and
summer when snow melts in the mountain areas surrounding the fjord. Precipitation in western Norway is normally transported with south-westerly winds from the North Sea and North Atlantic. During times with easterly winds almost all precipitation falls on the eastern side of the water divide and little is contributed to the water in the western Norwegian fjords. Sognefjorden is the largest fjord in Norway, c. 200 km long and more than 1300 m deep. The fjord is separated from the shelf areas by three deep sills where the shallowest is more than 160 m deep. The Sognefjorden drainage system covers 12 339 km2 (Nesje & Sulebak, 1994) reaching from the coast to the water divide between the eastern and western part of Norway (Figure 1). The highest mountains in the drainage system reach up to c. 2400 m above sea-level.
Oxygen isotope mixing line, western Norway 443
The hydrographical conditions along the western Norwegian coast is influenced by Atlantic water (>35 salinity, Helland-Hansen & Nansen, 1909; Hopkins, 1991) flowing northwards into the Nordic Seas (the Norwegian Current). A branch of this current flows southwards in the North Sea (Figure 1). Coastal water flows northwards along the coast (the Coastal Current) (North Sea Task Force, 1993). The coastal current (<35 salinity, Hopkins, 1991) is a mixture of Atlantic water, water out-flow from the Baltic Sea, and precipitation and river runoff from the Norwegian mainland. Within the fjord the hydrological conditions are dominated by a low salinity surface water (Figure 2) (commonly less than 50 m thick) caused by precipitation and river runoff. Under this (termed the intermediate layer with salinities of less than 35) is a mixture of surface water and deep fjord water. This intermediate water can under certain wind conditions have a connection to the coastal current (Svendsen, 1981). The deep fjord water has its origin from open ocean water and has salinities close to or above 35 (Hermansen, 1974). This water is believed to originate from cooling of the coastal current water during extreme cold winters, or Atlantic water being raised above sill level by favourable wind conditions. The connection between the basin waters in Sognefjorden and the oceanic waters (Atlantic or Coastal waters) is believed to be good. All basin water is exchanged on an average rate between 5 to 10 years (Hermansen, 1974). Materials and methods Water samples were collected during cruises on University of Bergen’s RV Hans Brattstøm in September 1996 and February 1998 (cruise HB111-96 and HB118-98). Water from rivers was collected during a field expedition in June 1998. Salinity was measured with an accuracy of 0·02 with a CTD SD204 on cruises HB111-96. On cruise HB118-98 a CTD OTS probe was used. The CTDs were run slowly with a maximum speed of 0·5 m s 1. Water samples for isotopic measurements was sampled with the CTDs rosette at different depths in the water column. The river samples were sampled from the surface river water 0·5 to 2 km upsteam from the mouth of the river (Figure 1). Oxygen isotope ratios were measured at the GMS laboratory at the University of Bergen. Water samples were equilibrated with CO2 at 20 C in an automated Finnigan preparation line, before automatically transfer to a Finnigan DELTA-E mass spectrometer for isotope analysis. The analytical uncertainty is reported to be 0·1‰ for the oxygen isotope measurements
on water samples (Johannessen, pers. comm.). The samples were measured relative to the VSMOW standard, and are referenced relative to VSMOW. Data and calculations River water 18O Figures 1 and 3 show that the rivers, from which water has been sampled, have large differences in size and elevation of drainage areas. The results of the oxygen isotope measurements show a decreasing west-east gradient in the fjord (Figure 3). Hence the results show a gradient from 9·24‰ in Ytredalselva (elva=river) at Vadheim to 14·5‰ in Haugselvi (elvi=river) at Skjolden. All river samples (except in Ytredalselva at Vadheim) have to some extent an input of glacial melt water. The glaciers reappeared in western Norway c. 5000 years ago (Nesje & Kvamme, 1991). However, since our river measurements are representing winter precipitation (snow melting in the mountains) we believe that the isotopic composition has not changed much since. The most likely explanation for the observed gradient is a combination of the continental effect (distance from source) and temperature of the precipitation. As the air is transported, continental precipitation depletes the clouds of heavy isotopes (Dansgaard, 1964). The high inland mountains will also force the clouds to precipitate at colder temperatures at higher elevations. There is also a continental temperature effect during winter, when the coastal areas are warmer than the inland caused by the coastal exposure to the Norwegian current which transports warm water northwards. The continental effect and the temperature effects add up to produce a gradient from the coast and inland. The low values at the fjord head are similar to values as far inland as Pechora in northern Russia (Rozanski et al., 1993). Fjord water 18O The sampling depth, isotopic measurements and salinity of the fjord water samples are presented in Table 1 and Figure 2. The salinity range (CTD profiles) of the fjord waters from 20 m depth and down to the bottom is between 32·4 to just above 35 for all stations except the silled tributary fjords (stations 111-8, 111-9 and 111-15). The results suggest a mixing line of 0·31‰ change in 18O per unit change in salinity [Figure 4(c)]. This indicates an average freshwater input of 10·49‰. Equation 1 expresses a mixing line constructed from water samples within the fjord, x=salinity, R2 =0·88.
300
400
Salinity HB111-9
200
0
100
200
300
400
Salinity HB111-8
36 33 30 27 24 21 18 15 12
100
0
200
400
600
800
34 32 30
Outer
28
0
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800 1000
0
20
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100 150 200 250 300
34 32 30
26
32 30 0
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800 1000
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100 200 300 400 Depth in metres
0
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40 80 120 Depth in metres
160
34 Salinit HB111-15
Salinity HB118-1
34
36 Depth in metres HB111-1
36 33 30 27 24 21 18 15
0
28
36
28
36 33 30 27 24 21 18 15 12 9 6
36 Salinity HB111-11
Salinity HB118-5
36
Outer
Inner
36 33 30 27 24 21 18 15 12
0
Tributary fjords
36 33 30 27 24 21 18 15
Outer
Main fjord
Salinity HB111-5
Inner
Salinity HB111-4
Salinity HB111-3
444 G. Mikalsen and H. P. Sejrup
32
30
34 32 30 28
500
F 2. Salinity profiles from the fjord stations. Dark dots indicate salinity and sample depth for the measured samples. For location see Figure 1. All values are listed in Table 1.
Oxygen isotope mixing line, western Norway 445 –17 North side
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Metres above sea level
dals
° røyelv A
a Vikj
1000
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75
Oxygen isotopes (VSMOW)
–15 South side
100 125 150 km from open ocean
175
200
225
–6 250
F 3. An indication of the height of the watershed on the north and south side of the Sognefjorden (height above sea level on left) as a profile from the coast to the inner fjord. The isotopic values from the river samples (thin line) are also plotted (values on right). [=Isotope values of Ygleelvi (Henriksen et al., 1996). =Isotope values for the precipitation along the western Norwegian coast, at Lista (Rozanski et al., 1993; Yurtsever & Gat, 1981).
y= 10·492‰+0·31x
(1)
From Equation 1 and Figure 3 it can be noted that the average isotopic composition of the freshwater end member is comparable to outer fjord runoff. If we combine the results from the water column with the results obtained from rivers [Figure 4(a and b)], (Equation 2) it can be seen that there is a small change in the mixing line to 0·37‰ for each salt unit change and the average isotopic composition of precipitation/runoff changes to 12·51‰. Equation 2 expresses the mixing line constructed from water samples within the fjord and from river samples, x=salinity, R2 =0·96. y= 12·512‰+0·37x
(2)
Clearly the river data changes the average isotopic composition of the freshwater end member and the mixing line. This may be a result of, (a) the fjord water samples in some areas being more strongly influenced by some rivers than others. If the sampling stations are unevenly distributed in the fjord this will affect the isotopic composition of the calculated freshwater input. However, in this study we regard the vertical and horizontal sampling as evenly spaced. (b) A second explanation may be the sampling strategy of the rivers. We do not have control on how much these rivers (that are sampled) contribute to the mixing in the fjord. There are many large rivers in the Sognefjord drainage system which were not sampled (Figure 1). (c) There may be a strong influence from the coastal waters, at least in the upper 200 m (but
below the surface waters) in the main fjord and above sill level in the tributary fjords (e.g. Gade, 1976). This may influence the low salinity values in the equations. Mixing line for Sognefjorden Figures 3 and 4(a) show both the variability in the oxygen isotope composition of the rivers along the fjord and the decreasing values toward the fjord head. The river water samples reflect mostly winter precipitation because of snow melting in the mountain areas reducing sampling. The reason for the decrease inland has partly to do with the continental effect. The distance between the sampling locations are only c. 150 km, however the distance where the precipitation fall (drainage areas) is up to c. 200 km. Another factor is topography and its effect on temperature. Since the coastal relief is lower than the relief further inland, the precipitation at the coast will precipitate under higher temperatures than in the mountain areas at the water divide. Henriksen et al. (1996) documented oxygen isotope values of c. 11·5‰ for the water in the river Ygleelvi in the central part of Sognefjorden (Figures 1 and 3), and measurement of the precipitation along the Norwegian coast, at Lista (e.g. Rozanski et al., 1993; Yurtsever & Gat, 1981) show a annual mean value of 6·74‰, with 5·35‰ annual variations. Both these values seem to support the observations of a decreasing gradient inland (Figure 3). A meteorological station at Fjærland (Figure 1) has measured the oxygen isotope composition of the precipitation in 1992–1993 (Henriksen et al., 1996). This
446 G. Mikalsen and H. P. Sejrup T 1. All river and fjord water samples
Sample station 111-1 111-1 111-1 111-1 111-1 111-3 111-3 111-3 111-3 111-4 111-4 111-4 111-4 111-5 111-8 111-8 111-8 111-8 111-9 111-9 111-9 111-11 111-11 111-11 111-15 111-15 118-1 118-1 118-1 118-5 118-5 118-5 A r røyelvi (Barsnesfjord) Jostedalselvi (Gaupne) Fortundalselva (Skjolden) Haugselvi (Skjolden) Ytredalselva (Vadheim) Vikja (Vik)
Position
Water depth
Salinity
Oxygen isotop values (VSMOW)
61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.26.0N 07.22.6E 61.26.0N 07.22.6E 61.26.0N 07.22.6E 61.26.0N 07.22.6E 61.19.5N 07.21.3E 61.19.5N 07.21.3E 61.19.5N 07.21.3E 61.19.5N 07.21.3E 61.14.3N 07.22.0E 61.12.3N 07.05.9E 61.12.3N 07.05.9E 61.12.3N 07.05.9E 61.12.3N 07.05.9E 61.14.65N 07.07.35E 61.14.65N 07.07.35E 61.14.65N 07.07.35E 61.03.7N 06.25.5E 61.03.7N 06.25.5E 61.03.7N 06.25.5E 61.04.5N 05.38.1E 61.04.5N 05.38.1E 61.08.48N 05.45.69E 61.08.48N 05.45.69N 61.08.48N 05.45.69E 61.09.25N 06.35.90E 61.09.25N 06.35.90E 61.09.25N 06.35.90E 61.16.2N 07.09.5E 61.24.5N 07.16.0E 61.29.8N 07.40.0E 61.30.0N 07.36.2E 61.15.5N 05.49.3E 61.04.2N 06.36.5E
20 50 200 400 460 20 100 200 325 20 100 300 371 638 20 50 100 200 20 50 80 20 100 190 75 120 40 90 140 40 80 120 0 0 0 0 0 0
32 748 34 475 35 129 35 167 35 165 32 453 34 846 34 941 35 055 32 608 34 944 35 029 35 022 35 030 32 724 33 916 34 109 34 066 32 790 33 064 33 119 32 604 34 899 35 097 33 500 33 554 33 508 34 451 34 846 33 756 34 581 34 752 0 0 0 0 0 0
0·33 0·24 0·37 0·53 0·36 0·39 0·36 0·39 0·40 0·38 0·25 0·12 0·39 0·41 0·50 0·06 0·09 0·01 0·44 0·24 0·46 0·27 0·34 0·03 0·21 0·17 0·13 0·11 0·24 0·03 0·16 0·29 12·35 13·82 14·04 14·50 9·24 11·17
show large differences in the oxygen isotope composition related to season (from max 1·17‰ to min 17·4‰, mean value 8·58) with low values recorded during winter and highest values during summer. The meteorological station (Norsk Meteorologisk Institutt, 1992, 1993) shows that most of the precipitation is received during the months (winter time) with low oxygen isotope values. The oxygen isotope mixing line calculation based on the rivers and fjord samples gives a mixing line of 0·37‰ for each salt unit change (Figure 4). If only the fjord water samples are considered, a lower line of 0·31‰ per salt unit is obtained (Figure 4). The variability in oxygen isotope composition in the river
water along the fjord (>6‰) and the large differences related to seasonal variations in the precipitation (more than 15‰), make us believe that the water measurements within the fjord waters are the most suitable for calculating an oxygen isotope mixing line for this fjord. We suggest an oxygen isotope mixing line of 0·31‰ for each salt unit for Sognefjorden including tributary fjords, and assume this to be a linear correlation. This fjord mixing line differs from other calculations (by 0·06‰) for the North Sea (Israelson & Buchardt, 1991). We have demonstrated that the approach of using endmembers (highest to lowest salinity) for calculations of mixing lines is not satisfactory for the fjord environment. In this
Oxygen isotope mixing line, western Norway 447
0
runoff along the fjord, and mixing of Atlantic and Coastal waters.
(a)
Oxygen isotope (VSMOW)
Y = 0.37x – 12.51 R2 = 0.97
–4
Implications for paeloclimate/oceanographic studies
–8
–12
0
6
12
18 Salinity
24
30
36
1
Oxygen isotope (VSMOW)
(b)
0
–1 32
34 Salinity
36
1 (c)
Oxygen isotope (VSMOW)
Y = 0.31x – 10.68 R2 = 0.89
0
–1 32
34 Salinity
36
F 4. (a) Salinity vs 18Owater for all samples (Fjord and rivers). (b) An enlargement of Figure 4(a) in the range between 32 to 36 salinity. (c) Salinity vs 18Owater from the fjord samples.
study it is assumed that the mixing of oceanic and fresh waters has a linear relationship for the oxygen isotope signal. This is not necessarily true in a fjord environment because of the 18O variations in river
The oxygen isotope composition of calcareous organisms such as foraminiferas, molluscs, ostracods etc. that calcify these tests in isotopic equilibrium with the adjacent water masses or with a known isotopic vital effect is a promising paleoclimate proxy. To be able to quantify temperature and salinity variations in the adjacent water masses a detailed mixing line is required. The mixing line present here is probably not valid in the surface water masses, because no samples above 20 m water depth have been obtained and due to local isotopic variations in freshwater input. We believe the mixing line is valid for intermediate and basin waters in western Norwegian fjords and hence should be applied in future studies of western Norwegian fjords. The low gradient mixing line (0·31‰ for each salt unit) shows that salinity changes are harder to detect in the fjords than in the North Atlantic. Hermansen (1974) showed that there has been a parallel trend between temperature and salinity in the deeper parts of Sognefjorden from 1930 to 1970 and that larger amplitudes are recorded in temperature (c. 0·5 C) and smaller amplitude changes in salinity (c. 0·08). This gives an oxygen isotopic variation of 1‰ for temperature and 0·0248 for salinity. Also in the outer part of Sognefjorden (Sognesjøen, Figure 1) oceanographic measurements has been performed since the 1930s (Aure & Østensen, 1993), and show the same relationship between temperature and salinity as the deep fjord. However, the amplitude of the signals are larger of more frequent water exchange, and show 2·5 C (0·575‰ of stable oxygen isotopes) and 0·3 in salinity (0·093‰ in oxygen isotopes). This, and the parallel trend between temperature and salinity indicates that oxygen isotope records retrieved for paleoclimatic/paleoceanographic reconstructions from Sognefjorden (western Norwegian fjords) are most likely reflecting a minimum temperature signal.
Acknowledgements We would like to thank the crew of RV Hans Bratstrøm for their help, Odd Hansen for laboratory assistance; Dorthe Klitgaard-Kristensen, Eystein Jansen and reviewer Denise Smythe Wright for comments on the manuscript; and Jane Karin Ellingsen and Else Hansteen Lier for the help with the map. The project
448 G. Mikalsen and H. P. Sejrup
has been funded through the Norwegian Research Consul and the EU (funded ENAM2 project. References Aure, J. & Østensen, Ø. 1993 Sognesjøen. St nr: 67, 1935–92, 6104 N 0450.4 E. Hydrografiske normaler og langtidsvariasjoner i Norske kyst farvann. Fisken og Havet 6, 36–43. Craig, H. & Gordon, L. I. 1965 Deuterium and oxygen 18 variations in the oceans and the marine atmosphere. In Stable Isotopes in Oceanographic Studies and Paleotemperatures, Spoleto, July, 23th–26th. Consigilo Nazionale Delle Richerche, Laboratorio di Geologia Nucleare, Pisa, pp. 9–130. Dansgaard, W. 1964 Stable isotopes in precipitation. Tellus 16, 436–469. Gade, H. 1976 Transport mechanisms in fjords. In Freshwater on the Sea (Skreslet, S., Leinebø, R., Matthews, J. B. L. & Sakshaug, E., eds), The Association of Norwegian Oceanographers, pp. 51–56. Helland-Hansen, B. & Nansen, F. 1909 The Norwegian Sea. Its physical oceanography. Based upon the Norwegian researches 1900–1904. The Norwegian Report on Norwegian Fisheries and Marine Investigations 2, 1–359. Henriksen, H., Rye, N. & Soldal, O. 1996 Groundwater transit times in a small coastal aquifer at Esebotn, Sogn of Fjordane, western Norway. Nores Geologiske Undersøkelser, Bull 431, 5–17. Hermansen, H. O. 1974 Sognefjordens hydrografi og vannutveksling. Candidate’s Science Thesis, Geophysical Ins, University of Bergen. 56 pp. Hopkins, T. S. 1991 The GIN Sea—A synthesis of its physical oceanography and literature review 1972–1985. Earth and Planetary Letters 30, 175–318.
Israelson, C. & Buchardt, B. 1991 The isotopic composition of oxygen and carbon in some fossil and recent bivalve shells from East Greenland. LUNDQUA Report 33, 117–123. Nesje, A. & Kvamme, M. 1991 Holocene glacier and climate variations in western Norway: Evidence for early Holocene glacier demise and multiple Neoglacial event. Geology 19, 610–612. Nesje, A. & Sulebak, J. R. 1994 Quantification of late Cenozoic erosion and denudation in the Sognefjord drainage basin, western Norway. Norsk Geografisk Tidsskrift 48, 85–92. Norsk Meteorologisk Institutt. 1992 Daglige nedbørhøyder for 1992, Skarestad, Fjærland. Norsk Meteorologisk Institutt. 1993 Daglige nedbørhøyder for 1993, Skarestad, Fjærland. North Sea Task Force. 1993 North Sea Subregion 1. Assessment Report, 44 pp. Rozanski, K., Aragua´ s-Aragua´ s, L. & Gonfiantini, R. 1993 Isotopic patterns in modern global precipitation. Geophysical Monograph 78, 1–36. Sarnthein, M., Jansen, E., Weinelt, M., Arnold, M., Duplessy, J. C., Erlenkeuser, H., Flatøy, A., Johannessen, G., Johannessen, T., Jung, S., Koc, N., Laberie, L., Marslin, M., Pflaumann, U. & Schulz, H. 1995 Variations in Atlantic surface ocean paleoceanography, 50–80N: A time-slice record of the last 30 000 years. Paleoceanography 10, 1063–1094. Svendsen, H. 1981 Wind-induced variations of circulation and water level in coupled fjord-coast system. In (Sætre, R. & Mork, M., eds), University of Bergen, Bergen, pp. 229–262. Yurtsever, Y. & Gat, J. R. 1981 Atmospheric waters. In Stable Isotope Hydrology—Denterium and Oxygen in the Water Cycle (Gat, J. R. & Gonfiaritini, R., eds), IAEA Technical Reports Series no. 210 Wien, pp. 103–142.
Oxygen Isotope Composition of Fjord and River Water in the Sognefjorden Drainage Area, Western Norway. Implications for Paleoclimate Studies G. Mikalsen and H. P. Sejrup Department of Geology, University of Bergen, Allegt. 41, N-5007 Bergen, Norway Received 7 June 1999 and accepted in revised form 29 October 1999 During two cruises and one field expedition from 1996 to 1998, 32 water samples from 10 stations in Sognefjorden and tributary fjords and six samples from rivers draining into the fjord were obtained for oxygen isotope measurements. In addition, CTD profiles were obtained from all of the fjord locations. The river samples show a large gradient in the oxygen isotopic composition from the outer fjord region to the rivers at the fjord head. The difference is more than 6‰ with highest values recorded at the fjord mouth decreasing inward along the fjord. The low oxygen isotope values are found in areas with a high altitude from the watershed. The gradient in the river water is explained by distance to the moisture source, topographic effects and temperature of precipitation. The isotopic composition of the fjord waters suggest a mixing line for Sognefjorden of 0·31‰ for a salinity change of 1 which is slightly higher than the North Sea mixing line (Israelson & Buchardt, 1991), and lower than the North Atlantic mixing line (Craig & Gordon, 1965). The river samples are not used in the calculation of the mixing because of the large differences in the isotopic composition of river water along the fjord, and large seasonal differences of precipitation. 2000 Academic Press
Keywords: oxygen isotopes; mixing line; fjords; western Norway
Introduction The need for obtaining high resolution climatic proxy records has triggered an increasing interest for fjord sediments as climate archives. This is also the case for western Norwegian fjords, partly because of the probability for obtaining high resolution paleoceanographic/paleoclimatic records, and partly because their location close to the Norwegian Current, that offer possible link between the marine and terrestrial paleoclimate records. Stable oxygen isotope measurements on benthonic foraminifera are a promising method for reconstructing climate change in the fjord environment. From such studies it is possible to estimate temperature or salinity changes through time (e.g. Sarnthein et al., 1995). To do this we need to know the controlling factors of the oxygen isotope composition of fjord water. As the Holocene (last c. 10 000 years) is of particular interest for such studies it can be assumed that the global ice volume effect on sea water 18O is negligible at least since 7–8 ka BP. Information on how the mixing of oceanic water and fresh water influences the oxygen isotope 0272–7714/00/040441+08 $35.00/0
composition of fjord water is needed for such studies to be quantitative. So far, no such studies are published from the western Norwegian fjords. Oxygen isotope mixing lines have been constructed for North Atlantic waters (Craig & Gordon, 1965) and the North Sea (Israelson & Buchardt, 1991). Craig and Gordon (1965) found for the North Atlantic a linear mixing line that gave 0·61‰ change in isotopic composition for a change in salinity of 1. Israelson and Buchardt (1991) proposed a mixing line for the North Sea (0·25‰ change in isotopic composition for a change in salinity of 1), that is much lower than the North Atlantic mixing line. In this paper a mixing line for Songefjorden (western Norway) is presented, which perhaps is applicable to other western Norwegian fjords. This is based on 32 oxygen isotope measurements at depths between 20 to 638 m at 10 stations in the Sognefjorden fjord system. Six measurements of fresh water from rivers draining into Sognefjorden will also be discussed in the light of the location of the samples relative to topography and distance from the coast, and the 2000 Academic Press
442 G. Mikalsen and H. P. Sejrup 40° 30° 20° 10° 0° NLAN
a nd
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F 1 (a) A location map showing Sognefjorden and major oceanic surface currents (NC=Norwegian current, CC=Coastal Current) in the North Atlantic and Nordic Seas. =location of Lista (b) Sognefjorden drainage basin with sample locations in the fjords ( ) and from rivers ( ). [=Location of Ygleelvi (Henriksen et al. 1996).
isotopic composition of fresh water draining into the fjord. Physiographical setting The general physiography of western Norway is characterized by high mountains that drop off seawards to the strand flat (Nesje & Sulebak, 1994). The western Norwegian fjords cut far into the country and are deep (maximum depth of Sognefjorden is c. 1300 m) surrounded by steep slopes high mountains. These are normally in the inner parts of the fjord between 1300 to 1800 m high. The highest total vertical relief in Sognefjorden is up to 2600 m. Rivers enter the fjord at the fjord head or in tributary fjords. Some of the rivers are fed by glaciers and carry older water into the fjord. The largest freshwater input is during late spring and
summer when snow melts in the mountain areas surrounding the fjord. Precipitation in western Norway is normally transported with south-westerly winds from the North Sea and North Atlantic. During times with easterly winds almost all precipitation falls on the eastern side of the water divide and little is contributed to the water in the western Norwegian fjords. Sognefjorden is the largest fjord in Norway, c. 200 km long and more than 1300 m deep. The fjord is separated from the shelf areas by three deep sills where the shallowest is more than 160 m deep. The Sognefjorden drainage system covers 12 339 km2 (Nesje & Sulebak, 1994) reaching from the coast to the water divide between the eastern and western part of Norway (Figure 1). The highest mountains in the drainage system reach up to c. 2400 m above sea-level.
Oxygen isotope mixing line, western Norway 443
The hydrographical conditions along the western Norwegian coast is influenced by Atlantic water (>35 salinity, Helland-Hansen & Nansen, 1909; Hopkins, 1991) flowing northwards into the Nordic Seas (the Norwegian Current). A branch of this current flows southwards in the North Sea (Figure 1). Coastal water flows northwards along the coast (the Coastal Current) (North Sea Task Force, 1993). The coastal current (<35 salinity, Hopkins, 1991) is a mixture of Atlantic water, water out-flow from the Baltic Sea, and precipitation and river runoff from the Norwegian mainland. Within the fjord the hydrological conditions are dominated by a low salinity surface water (Figure 2) (commonly less than 50 m thick) caused by precipitation and river runoff. Under this (termed the intermediate layer with salinities of less than 35) is a mixture of surface water and deep fjord water. This intermediate water can under certain wind conditions have a connection to the coastal current (Svendsen, 1981). The deep fjord water has its origin from open ocean water and has salinities close to or above 35 (Hermansen, 1974). This water is believed to originate from cooling of the coastal current water during extreme cold winters, or Atlantic water being raised above sill level by favourable wind conditions. The connection between the basin waters in Sognefjorden and the oceanic waters (Atlantic or Coastal waters) is believed to be good. All basin water is exchanged on an average rate between 5 to 10 years (Hermansen, 1974). Materials and methods Water samples were collected during cruises on University of Bergen’s RV Hans Brattstøm in September 1996 and February 1998 (cruise HB111-96 and HB118-98). Water from rivers was collected during a field expedition in June 1998. Salinity was measured with an accuracy of 0·02 with a CTD SD204 on cruises HB111-96. On cruise HB118-98 a CTD OTS probe was used. The CTDs were run slowly with a maximum speed of 0·5 m s 1. Water samples for isotopic measurements was sampled with the CTDs rosette at different depths in the water column. The river samples were sampled from the surface river water 0·5 to 2 km upsteam from the mouth of the river (Figure 1). Oxygen isotope ratios were measured at the GMS laboratory at the University of Bergen. Water samples were equilibrated with CO2 at 20 C in an automated Finnigan preparation line, before automatically transfer to a Finnigan DELTA-E mass spectrometer for isotope analysis. The analytical uncertainty is reported to be 0·1‰ for the oxygen isotope measurements
on water samples (Johannessen, pers. comm.). The samples were measured relative to the VSMOW standard, and are referenced relative to VSMOW. Data and calculations River water 18O Figures 1 and 3 show that the rivers, from which water has been sampled, have large differences in size and elevation of drainage areas. The results of the oxygen isotope measurements show a decreasing west-east gradient in the fjord (Figure 3). Hence the results show a gradient from 9·24‰ in Ytredalselva (elva=river) at Vadheim to 14·5‰ in Haugselvi (elvi=river) at Skjolden. All river samples (except in Ytredalselva at Vadheim) have to some extent an input of glacial melt water. The glaciers reappeared in western Norway c. 5000 years ago (Nesje & Kvamme, 1991). However, since our river measurements are representing winter precipitation (snow melting in the mountains) we believe that the isotopic composition has not changed much since. The most likely explanation for the observed gradient is a combination of the continental effect (distance from source) and temperature of the precipitation. As the air is transported, continental precipitation depletes the clouds of heavy isotopes (Dansgaard, 1964). The high inland mountains will also force the clouds to precipitate at colder temperatures at higher elevations. There is also a continental temperature effect during winter, when the coastal areas are warmer than the inland caused by the coastal exposure to the Norwegian current which transports warm water northwards. The continental effect and the temperature effects add up to produce a gradient from the coast and inland. The low values at the fjord head are similar to values as far inland as Pechora in northern Russia (Rozanski et al., 1993). Fjord water 18O The sampling depth, isotopic measurements and salinity of the fjord water samples are presented in Table 1 and Figure 2. The salinity range (CTD profiles) of the fjord waters from 20 m depth and down to the bottom is between 32·4 to just above 35 for all stations except the silled tributary fjords (stations 111-8, 111-9 and 111-15). The results suggest a mixing line of 0·31‰ change in 18O per unit change in salinity [Figure 4(c)]. This indicates an average freshwater input of 10·49‰. Equation 1 expresses a mixing line constructed from water samples within the fjord, x=salinity, R2 =0·88.
300
400
Salinity HB111-9
200
0
100
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300
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Salinity HB111-8
36 33 30 27 24 21 18 15 12
100
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200
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600
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34 32 30
Outer
28
0
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800 1000
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34 32 30
26
32 30 0
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100 200 300 400 Depth in metres
0
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40 80 120 Depth in metres
160
34 Salinit HB111-15
Salinity HB118-1
34
36 Depth in metres HB111-1
36 33 30 27 24 21 18 15
0
28
36
28
36 33 30 27 24 21 18 15 12 9 6
36 Salinity HB111-11
Salinity HB118-5
36
Outer
Inner
36 33 30 27 24 21 18 15 12
0
Tributary fjords
36 33 30 27 24 21 18 15
Outer
Main fjord
Salinity HB111-5
Inner
Salinity HB111-4
Salinity HB111-3
444 G. Mikalsen and H. P. Sejrup
32
30
34 32 30 28
500
F 2. Salinity profiles from the fjord stations. Dark dots indicate salinity and sample depth for the measured samples. For location see Figure 1. All values are listed in Table 1.
Oxygen isotope mixing line, western Norway 445 –17 North side
–16
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i
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Metres above sea level
dals
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a Vikj
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Oxygen isotopes (VSMOW)
–15 South side
100 125 150 km from open ocean
175
200
225
–6 250
F 3. An indication of the height of the watershed on the north and south side of the Sognefjorden (height above sea level on left) as a profile from the coast to the inner fjord. The isotopic values from the river samples (thin line) are also plotted (values on right). [=Isotope values of Ygleelvi (Henriksen et al., 1996). =Isotope values for the precipitation along the western Norwegian coast, at Lista (Rozanski et al., 1993; Yurtsever & Gat, 1981).
y= 10·492‰+0·31x
(1)
From Equation 1 and Figure 3 it can be noted that the average isotopic composition of the freshwater end member is comparable to outer fjord runoff. If we combine the results from the water column with the results obtained from rivers [Figure 4(a and b)], (Equation 2) it can be seen that there is a small change in the mixing line to 0·37‰ for each salt unit change and the average isotopic composition of precipitation/runoff changes to 12·51‰. Equation 2 expresses the mixing line constructed from water samples within the fjord and from river samples, x=salinity, R2 =0·96. y= 12·512‰+0·37x
(2)
Clearly the river data changes the average isotopic composition of the freshwater end member and the mixing line. This may be a result of, (a) the fjord water samples in some areas being more strongly influenced by some rivers than others. If the sampling stations are unevenly distributed in the fjord this will affect the isotopic composition of the calculated freshwater input. However, in this study we regard the vertical and horizontal sampling as evenly spaced. (b) A second explanation may be the sampling strategy of the rivers. We do not have control on how much these rivers (that are sampled) contribute to the mixing in the fjord. There are many large rivers in the Sognefjord drainage system which were not sampled (Figure 1). (c) There may be a strong influence from the coastal waters, at least in the upper 200 m (but
below the surface waters) in the main fjord and above sill level in the tributary fjords (e.g. Gade, 1976). This may influence the low salinity values in the equations. Mixing line for Sognefjorden Figures 3 and 4(a) show both the variability in the oxygen isotope composition of the rivers along the fjord and the decreasing values toward the fjord head. The river water samples reflect mostly winter precipitation because of snow melting in the mountain areas reducing sampling. The reason for the decrease inland has partly to do with the continental effect. The distance between the sampling locations are only c. 150 km, however the distance where the precipitation fall (drainage areas) is up to c. 200 km. Another factor is topography and its effect on temperature. Since the coastal relief is lower than the relief further inland, the precipitation at the coast will precipitate under higher temperatures than in the mountain areas at the water divide. Henriksen et al. (1996) documented oxygen isotope values of c. 11·5‰ for the water in the river Ygleelvi in the central part of Sognefjorden (Figures 1 and 3), and measurement of the precipitation along the Norwegian coast, at Lista (e.g. Rozanski et al., 1993; Yurtsever & Gat, 1981) show a annual mean value of 6·74‰, with 5·35‰ annual variations. Both these values seem to support the observations of a decreasing gradient inland (Figure 3). A meteorological station at Fjærland (Figure 1) has measured the oxygen isotope composition of the precipitation in 1992–1993 (Henriksen et al., 1996). This
446 G. Mikalsen and H. P. Sejrup T 1. All river and fjord water samples
Sample station 111-1 111-1 111-1 111-1 111-1 111-3 111-3 111-3 111-3 111-4 111-4 111-4 111-4 111-5 111-8 111-8 111-8 111-8 111-9 111-9 111-9 111-11 111-11 111-11 111-15 111-15 118-1 118-1 118-1 118-5 118-5 118-5 A r røyelvi (Barsnesfjord) Jostedalselvi (Gaupne) Fortundalselva (Skjolden) Haugselvi (Skjolden) Ytredalselva (Vadheim) Vikja (Vik)
Position
Water depth
Salinity
Oxygen isotop values (VSMOW)
61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.01.35N 04.48.4E 61.26.0N 07.22.6E 61.26.0N 07.22.6E 61.26.0N 07.22.6E 61.26.0N 07.22.6E 61.19.5N 07.21.3E 61.19.5N 07.21.3E 61.19.5N 07.21.3E 61.19.5N 07.21.3E 61.14.3N 07.22.0E 61.12.3N 07.05.9E 61.12.3N 07.05.9E 61.12.3N 07.05.9E 61.12.3N 07.05.9E 61.14.65N 07.07.35E 61.14.65N 07.07.35E 61.14.65N 07.07.35E 61.03.7N 06.25.5E 61.03.7N 06.25.5E 61.03.7N 06.25.5E 61.04.5N 05.38.1E 61.04.5N 05.38.1E 61.08.48N 05.45.69E 61.08.48N 05.45.69N 61.08.48N 05.45.69E 61.09.25N 06.35.90E 61.09.25N 06.35.90E 61.09.25N 06.35.90E 61.16.2N 07.09.5E 61.24.5N 07.16.0E 61.29.8N 07.40.0E 61.30.0N 07.36.2E 61.15.5N 05.49.3E 61.04.2N 06.36.5E
20 50 200 400 460 20 100 200 325 20 100 300 371 638 20 50 100 200 20 50 80 20 100 190 75 120 40 90 140 40 80 120 0 0 0 0 0 0
32 748 34 475 35 129 35 167 35 165 32 453 34 846 34 941 35 055 32 608 34 944 35 029 35 022 35 030 32 724 33 916 34 109 34 066 32 790 33 064 33 119 32 604 34 899 35 097 33 500 33 554 33 508 34 451 34 846 33 756 34 581 34 752 0 0 0 0 0 0
0·33 0·24 0·37 0·53 0·36 0·39 0·36 0·39 0·40 0·38 0·25 0·12 0·39 0·41 0·50 0·06 0·09 0·01 0·44 0·24 0·46 0·27 0·34 0·03 0·21 0·17 0·13 0·11 0·24 0·03 0·16 0·29 12·35 13·82 14·04 14·50 9·24 11·17
show large differences in the oxygen isotope composition related to season (from max 1·17‰ to min 17·4‰, mean value 8·58) with low values recorded during winter and highest values during summer. The meteorological station (Norsk Meteorologisk Institutt, 1992, 1993) shows that most of the precipitation is received during the months (winter time) with low oxygen isotope values. The oxygen isotope mixing line calculation based on the rivers and fjord samples gives a mixing line of 0·37‰ for each salt unit change (Figure 4). If only the fjord water samples are considered, a lower line of 0·31‰ per salt unit is obtained (Figure 4). The variability in oxygen isotope composition in the river
water along the fjord (>6‰) and the large differences related to seasonal variations in the precipitation (more than 15‰), make us believe that the water measurements within the fjord waters are the most suitable for calculating an oxygen isotope mixing line for this fjord. We suggest an oxygen isotope mixing line of 0·31‰ for each salt unit for Sognefjorden including tributary fjords, and assume this to be a linear correlation. This fjord mixing line differs from other calculations (by 0·06‰) for the North Sea (Israelson & Buchardt, 1991). We have demonstrated that the approach of using endmembers (highest to lowest salinity) for calculations of mixing lines is not satisfactory for the fjord environment. In this
Oxygen isotope mixing line, western Norway 447
0
runoff along the fjord, and mixing of Atlantic and Coastal waters.
(a)
Oxygen isotope (VSMOW)
Y = 0.37x – 12.51 R2 = 0.97
–4
Implications for paeloclimate/oceanographic studies
–8
–12
0
6
12
18 Salinity
24
30
36
1
Oxygen isotope (VSMOW)
(b)
0
–1 32
34 Salinity
36
1 (c)
Oxygen isotope (VSMOW)
Y = 0.31x – 10.68 R2 = 0.89
0
–1 32
34 Salinity
36
F 4. (a) Salinity vs 18Owater for all samples (Fjord and rivers). (b) An enlargement of Figure 4(a) in the range between 32 to 36 salinity. (c) Salinity vs 18Owater from the fjord samples.
study it is assumed that the mixing of oceanic and fresh waters has a linear relationship for the oxygen isotope signal. This is not necessarily true in a fjord environment because of the 18O variations in river
The oxygen isotope composition of calcareous organisms such as foraminiferas, molluscs, ostracods etc. that calcify these tests in isotopic equilibrium with the adjacent water masses or with a known isotopic vital effect is a promising paleoclimate proxy. To be able to quantify temperature and salinity variations in the adjacent water masses a detailed mixing line is required. The mixing line present here is probably not valid in the surface water masses, because no samples above 20 m water depth have been obtained and due to local isotopic variations in freshwater input. We believe the mixing line is valid for intermediate and basin waters in western Norwegian fjords and hence should be applied in future studies of western Norwegian fjords. The low gradient mixing line (0·31‰ for each salt unit) shows that salinity changes are harder to detect in the fjords than in the North Atlantic. Hermansen (1974) showed that there has been a parallel trend between temperature and salinity in the deeper parts of Sognefjorden from 1930 to 1970 and that larger amplitudes are recorded in temperature (c. 0·5 C) and smaller amplitude changes in salinity (c. 0·08). This gives an oxygen isotopic variation of 1‰ for temperature and 0·0248 for salinity. Also in the outer part of Sognefjorden (Sognesjøen, Figure 1) oceanographic measurements has been performed since the 1930s (Aure & Østensen, 1993), and show the same relationship between temperature and salinity as the deep fjord. However, the amplitude of the signals are larger of more frequent water exchange, and show 2·5 C (0·575‰ of stable oxygen isotopes) and 0·3 in salinity (0·093‰ in oxygen isotopes). This, and the parallel trend between temperature and salinity indicates that oxygen isotope records retrieved for paleoclimatic/paleoceanographic reconstructions from Sognefjorden (western Norwegian fjords) are most likely reflecting a minimum temperature signal.
Acknowledgements We would like to thank the crew of RV Hans Bratstrøm for their help, Odd Hansen for laboratory assistance; Dorthe Klitgaard-Kristensen, Eystein Jansen and reviewer Denise Smythe Wright for comments on the manuscript; and Jane Karin Ellingsen and Else Hansteen Lier for the help with the map. The project
448 G. Mikalsen and H. P. Sejrup
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