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Confusion and contamination of 8.2 ka cold climate
records caused by the Storegga tsunami in the
Nordic Seas
Stein Bondevik ( Stein.Bondevik@hvl.no )
Western Norway University of Applied Sciences
Bjørg Risebrobakken ( bjri@norceresearch.no )
NORCE Climate & Environment, Bjerknes Centre for Climate Research
Steven Gibbons ( steven.gibbons@ngi.no )
Norwegian Geotechnical Institute https://orcid.org/0000-0002-7822-0244
Tine Rasmussen ( tine.rasmussen@uit.no )
UiT the Arctic University of Norway
Finn Løvholt ( nn.lovholt@ngi.no )
Norwegian Geotechnical Institute
Article
Keywords:
Posted Date: September 26th, 2023
DOI: https://doi.org/10.21203/rs.3.rs-3082245/v2
License: This work is licensed under a Creative Commons Attribution 4.0 International License.
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Additional Declarations: There is NO Competing Interest.
1
Confusion and contamination of 8.2 ka cold climate
records caused by the Storegga tsunami in the Nordic
Seas
Stein Bondevik1, Bjørg Risebrobakken2, Steven J. Gibbons3, Tine L.
Rasmussen4 and Finn Løvholt3
1 Department of Environmental Sciences, Western Norway University of Applied
Sciences, P.O. Box 133, NO-6851 Sogndal, Norway.
2 NORCE Climate & Environment, Bjerknes Centre for Climate Research, Bergen,
Norway.
3 Norwegian Geotechnical Institute (NGI), P.O. Box. 3930 Ullevål Stadion, N-0806
Oslo, Norway.
4 Department of Geosciences, UiT the Arctic University of Norway, Tromsø,
Norway.
Corresponding author:
stein.bondevik@hvl.no
Keywords
8.2 ka event, Storegga tsunami, climate change, marine geology,
Norwegian Sea
Abstract
Since the end of the last ice age, no cold snap rivals the one dated
to 8200 years ago. Its oceanic response has been reconstructed in part
from sediments in the Norwegian Sea and North Sea. Here we show that
these sediments have been reworked by currents generated by the
Storegga tsunami, dated to the coldest decades of the 8.2 ka event. From
a new simulation of the Storegga tsunami we calculated the maximum
flow velocity to be 2–5 m/s on the shelf offshore western Norway and in
the shallower parts of the North Sea, and up to about 1 m/s down to a
water depth of 1000 m. We re-investigated sediment core MD95-2011,
from which a large and abrupt 8.2 ka cooling had been inferred, and
found the cold-water foraminifera to be re-deposited and 11,000 years of
age. Oxygen isotopes of the recycled foraminifera and the content of sand
2
grains, thought to be dropped from ice bergs, might have led to an
interpretation of a too large and dramatic climate cooling. Our
simulations imply that large parts of the sea floor in the North Sea and
Norwegian Sea might have been reworked by currents during the
Storegga tsunami.
3
Introduction
Tsunamis can disturb the seabed and rework offshore sediments.
Investigations after the 2011 Tohoku tsunami in Japan showed
redeposited mud and sorted sand layers on the Sendai shelf down to
~100 m water depth (Ikehara et al., 2021). Resuspended sediments were
also carried offshore and continued down the slope to deep water, as
turbidity currents (Arai et al., 2013). The Storegga slide in the Norwegian
Sea (Bryn et al., 2005) triggered a giant tsunami (Bondevik et al., 2005).
Deposits from this tsunami have been extensively mapped onshore in
Scotland (e.g. Dawson et al., 1988; Smith et al., 2004), Norway (e.g.
Bondevik et al., 1997), Shetland (Bondevik et al., 2003; Smith et al.,
2004), and the Faroe Islands (Grauert et al., 2001). Most likely, currents
in the Storegga tsunami must also have disturbed the seabed and
reworked offshore sediments, but to what extent?
For long wave tsunamis, such as the one generated by the Storegga
Slide, the currents are uniform from the surface to near the
water/sediment interface at the sea floor and are a function of surface
elevation (wave amplitude) and water depth. The maximum horizontal
flow velocity,
umax,
in a propagating tsunami can then be approximated to
the maximum surface elevation
ηmax
through the expression:
u
max
=
η
max
d
∙
g
∙
d
where
d
is water depth and
g
is the gravitational constant. If the
maximum sea surface elevation is 5 m and the water depth is 250 m, the
maximum horizontal velocity will be approximately 1 m/s. According to
the classical diagram of Hjulström (1935), that relates flow speed to
erosion, transportation and deposition of different grain sizes, a velocity
of 1 m/s will erode and entrain a wide range of grain sizes, from very fine
silt to fine pebbles (5μm to 7 mm).
The Storegga tsunami has been dated to the coldest decades of the
8.2 ka climatic event. Radiocarbon measurements of green moss
fragments, picked out of the onshore tsunami deposits, were dated to
4
7300±20 14C years BP (Bondevik et al., 2012) and calibrated to 8080–
8180 cal yr BP (8140±30 years ago, see age calibration in Supplementary
Information). The moss had still small amounts of intact chlorophyll, and
thus must have been killed during the tsunami. According to the
correlation with the Greenland ice cores, the date assigns the Storegga
tsunami to the coldest part of the 160-year-long interval of the 8.2 ka
event (Bondevik et al., 2012).
Here we ask the following question:
Could the 8.2 ka cold event, as
reconstructed from marine sediment cores in the North Sea and
Norwegian Sea, be contaminated or confused with sediments re-
deposited by the Storegga tsunami?
We answer by simulating the
maximum flow velocity of the Storegga tsunami in the Norwegian Sea
and North Sea and re-investigate a sediment core from the Vøring
plateau, about 200 km offshore Norway in a water depth of 1048 m – one
of the few North Atlantic marine sediment cores that recorded a distinct
and dramatic cooling, the 8.2 ka event (Risebrobakken et al., 2003;
Tegzes et al., 2014). Our short answer to the question is yes.
Results
Computer simulations of the Storegga Slide and tsunami
We obtained flow velocities generated by the Storegga tsunami in the
North Sea and Norwegian Sea from computer simulations of the landslide
and ensuing tsunami using a linear shallow water model (e.g. Bondevik et
al., 2005; Harbitz, 1992). The Storegga Slide was modelled as a cohesive
clay-rich debris flow. The following tsunami waves were simulated from
the time-dependent changes in water depth caused by the moving
landslide (Løvholt et al., 2017). We chose parameters of the landslide
rheology that best matched the run-out of the landslide and the observed
run-up heights of the tsunami deposits (Kim et al., 2019) (See methods for
details).
The simulated maximum flow velocity of the Storegga Slide tsunami
reflects the maximum wave amplitude and water depth. On the
5
Norwegian shelf and in the North Sea we calculate maximum velocities
larger than 1 m/s at depths shallower than 250 m (Fig. 1, Supplementary
Figs. 1a, b, d). On the shallowest shelves, around Shetland, the Faroe
Islands and in Western Norway, maximum velocities are between 2 and 5
m/s. At many places with depth down to 1000 m we also simulated
velocities above 1 m/s, especially in vicinity of the Storegga Slide (Fig 1).
The strongest simulated currents were due to the initial drawdown of
water from the Norwegian shelf towards the backwall of the Storegga
Slide; the currents here reached 5–10 m/s and the sea surface waters
lowered by 15–30 m (Supplementary Fig. 1b).
6
Fig. 1| Maximum flow velocity of the Storegga tsunami as
calculated for each grid point during the 10 h simulation time.
White triangles show location of marine sediment cores of the “8.2
event”. Purple triangles (a, b, c and d) are additional locations of
simulated time series (Supplementary Fig. 1). The white outline of the
Storegga Slide is the run-out of landslide debris (Haflidason et al., 2004),
the continuation of turbidites is not shown. Pixels in red-brown have
maximum velocity > 5 m/s and < 20 m/s. Green circles are onshore
locations of Storegga tsunami deposits (Bondevik, 2019). Depth contours
are paleo-bathymetry used in the simulations. Countries are shown with
their present-day coastlines.
7
Re-investigation of the 8.2 ka cold event layer in the Norwegian Sea
We re-investigated sediment core MD95-2011 that had revealed very
distinct changes identified as a response to the 8.2 ka climatic event
(Risebrobakken et al., 2003; Tegzes et al., 2014). The core was retrieved
from the inner part of the Vøring plateau on the slope towards the shelf
break (Fig. 1), in an area of high sedimentation rates (Rumohr et al.,
2001). Currents at the core location are at present too weak (< 1cm/s) to
influence the bottom sediments (Voet et al., 2010). However, during the
Storegga tsunami the simulations show a current speed of 39 cm/s
corresponding to a wave amplitude of 3.8 m (Fig. 2a).
The proxies of the sediment core show a well-defined anomaly
dated to around 8100 years ago. The anomaly, in a two-centimeter-thick
layer, from 533–535 cm, shows a significant change in grain size, in the
number of foraminifera, in the species of foraminifera, and in oxygen
isotope values (Fig. 3).
The mixture of foraminiferal species in this layer indicates re-
sedimentation. The abundances of both the cold polar species,
Neogloboquadrina pachyderma
and the warm Atlantic species
Neogloboquadrina incompta
increase 12–14 times (Fig. 3b). We also find
a large number of benthic foraminifera that belong to environments of
shallower water and stronger currents than presently found at the core
site (Fig. 3c) (Mjelde, 2002). Oxygen isotope values (δ18O) of
N.
pachyderma
increase from 2.25 to 2.95 ‰ at this level (Fig. 3d) which
would indicate a cooling of 3oC of the surface ocean water. This anomaly
is not seen in the oxygen isotopes of
N. incompta
(Risebrobakken et al.,
2003).
8
Fig. 2 | Simulation of wave height (blue line) and flow velocity
(dotted) during the Storegga tsunami at the location of marine
sediment cores with an 8.2 ka layer. Paleo-water depths in brackets.
For core locations, see Fig. 1. a Core site MD95-2011 at the Vøring
plateau in the Norwegian Sea. b Core site LINK14 east of the Faroe
Island and c Core site Troll, 28-03 in the Norwegian channel.
9
TABLE 1. RADIOCARBON MEASUREMENTS OF PLANKTONIC FORAMINIFERA OF THE
«8.2 ka EVENT» LAYER IN CORE MD95-2011
Lab.no
Depth
(cm)
Species
Weight
(mg)
δ13C
(‰
PDB)
14C age BP
(yr)
Calibrated yr
BP
(2 σ-range)*
Above the 8.2 ka layer
ETH-
130635
531–
532
N. incompta
0.7
–3.80
7810 ± 100
7980–8500
ETH-
130636
531–
532
N.pachyderm
a
0.7
–1.26
7600 ±
140†
7710–8350
Within the 8.2 ka layer
ETH-
130637
533–
534
N.
pachyderma
1.0
–2.55
10,220 ±
140
10,990–
11,880
TUa-8053
534–
535
N.
pachyderma
8.3
1.0§
9900 ± 60
10,690–
11,170
Poz-8236
533–
534
N. incompta
5.0
8530 ± 160
8670–9520
TUa-8054
534–
535
N. incompta
8.1
1.0§
8375 ± 55
8680–9190
Below the 8.2 ka layer
ETH-
130638
536–
537
N. incompta
0.7
–2.88
7860 ± 110
8010–8580
ETH-
130639
536–
537
N.pachyderm
a
0.67
–2.32
8080 ± 100
8290–8900
* Radiocarbon ages were calibrated to calendar years with the dataset Marine20
(Heaton et al. 2020) using a local marine reservoir correction of R = –145 ± 35.
† Not used in the age modelling. Measured on the leached fraction of the foraminifera
sample, main fraction too small to measure.
§ Assumed value.
We radiocarbon-dated the planktonic foraminifera in the 8.2 ka
layer and found them to be much older than 8200 yr BP (Table 1). From
the lowest centimeter of the layer (534–535 cm) we dated the cold-water
species
N. pacyderma
to 10,690–11,170 cal yr BP and the warm-water
species
N. incompta
to 8680–9190 cal yr BP (Table 1 and Fig. 3b). From
the next cm up-core, within the layer,
N. pacyderma
was dated to 10,990–
11,880 and
N. incompta
to 8670–9520 cal yr BP. All these ages are older
than the radiocarbon dates below the layer, and especially the dates of
N.
pachyderma
are about 2500–3000 years older than 8200 year BP. The
high δ18O value of this species (2.95 ‰) is comparable to the Early
10
Holocene or Younger Dryas values in the same core (Risebrobakken et
al., 2003) and in agreement with their radiocarbon ages.
11
Fig. 3| Data from core MD95-2011 plotted according to age. a
Percentage of grains > 63 μm and the number of grains > 150 μm per
gram sediments. b Number of planktonic foraminifera, the blue line is for
the cold-water species
Neogloboquadrina pachyderma
and the red line is
12
for the warm-water species
Neogloboquadrina incompta.
New 14C dates
(this study) are plotted. c Percentage of benthic foraminifera that could
indicate re-deposition, shelf species
Trifarina angulosa
and
Elphidium
excavatum
(Mjelde, 2002)
.
d Oxygen isotopes of
Neogloboquadrina
pachyderma
. e Green curve shows the modelled distribution of the ages
of onshore green moss fragments within the Storegga tsunami, grey
curve is the original distribution (Bondevik et al., 2012). (See
Supplementary Information for age modelling).
The 8.2 ka layer fines upwards and has an erosive lower boundary.
The radiocarbon dates show a jump in ages across the layer and this jump
indicates a time gap in the stratigraphy. We thus included a hiatus at the
lower boundary (535 cm) in the age-depth modelling. The best fit shows a
time gap or hiatus of 400 years (Supplementary Fig. 5). According to the
sedimentation rates this corresponds to as much as 20 cm of erosion
(stippled lines in Fig. 3 a–d). The counts of mineral grains > 150μm, grain
size and amounts of foraminifera, both planktonic and benthic (Fig. 3a,
b), shows that the lowest centimeter (534–535 cm) is coarser grained
than the next centimeter higher up (533–534 cm), indicating that the
layer is normally graded.
All data support the hypothesis that the 8.2 ka layer in MD95-2011
is a turbidite. The erosive lower boundary, the upward fining of grains,
the large amounts of redeposited older foraminifera and especially the
high number of benthic foraminifera that thrive in shallower water
confirm our suggestion that this layer is a fine grained turbidite deposited
from the shelf break. Although the core site experienced currents directly
from the propagating tsunami (Fig. 2a), we conclude that the 8.2 ka layer
was deposited from a turbidity current released from sediments eroded
and transported to the shelf break in the tsunami backwash.
Discussion
Layers similar to the sand layer in MD95-2011 were found offshore
Japan after the Tohoku tsunami in 2011. A fine-grained soft sediment
layer covered wide areas of the deep sea outside the Sendai shelf after
13
the Tohoku tsunami (Ikehara et al., 2021; Usami et al., 2017). Evidence of
the turbidity currents came from an ocean bottom pressure sensor,
deployed at a water depth of 1052 m. The sensor was caught in the
turbidity current and transported 1 km downslope three hours after the
earthquake (Arai et al., 2013). The interpretation is that the tsunami
backflow carried sediments to the shelf and this cloud of suspended
sediments rushed down the slope because of excess density. The moving
suspension cloud grew into turbidity currents incorporating sea floor
sediments and the ocean bottom pressure sensor. After 1 km downslope
movement the 60 cm wide, 42-kilogram, sensor settled and, when the
cloud came to rest, the sediments settled: sand first followed by finer
grains. We think it is possible that similar sheet-like turbidity currents
could have been initiated along the shelf areas in the Norwegian Sea due
to suspension induced by the Storegga tsunami backwash.
Sediment core (LINK14) from a trough on the eastern shelf of the
Faroe Islands (Fig. 1, Supplementary Table 1), has an ‘8.2 ka layer’
interpreted by Rasmussen and Thomsen (2010) as deposited directly from
the Storegga tsunami currents. A fine sand layer (63–150 μm) between
114 and 116 cm core depth, shows an increase in larger-sized planktic
and benthic foraminifera. Small planktic species are absent. The peculiar
composition of the fauna, indicative of warmer sea surface temperatures,
led the authors to conclude that this layer was subject to some kind of
sorting effect, possibly from currents in the Storegga tsunami. Our
tsunami simulations show that the sediment core site would experience
quite strong currents of 1.2 and 1.75 m/s (Fig. 2b). These currents could
transport sand from the shallower banks into the trough. Both
Hjulström's diagram (Hjulström, 1935) and Weiss (2008) show that such
strong currents would erode sediments with grain sizes of fine silt to fine
pebbles.
Core 28–03 in the Norwegian Channel (Fig. 1; Supplementary Table
1) also show a distinct and sudden 8.2 ka event (Klitgaard-Kristensen et
14
al., 1998) that we think could be contaminated from currents in the
Storegga tsunami. The layer assigned to the 8.2 ka event is 3–4 cm thick
(~341–344 cm core depth), of clay-rich silts, and the age of the upper
boundary is ca. 8200 cal yr BP (Supplementary Information). The
strongest simulated currents are 0.7 m/s (Fig. 2c) and are aligned parallel
to the Norwegian Channel (Fig. 1). According to Hjulström (1935) these
currents could stir up fine-grained sediments (clayey silts) in the trench
(Klitgaard-Kristensen et al., 2002) and possibly rework and redeposit the
foraminifera.
The simulations also reveal strong currents on the shelf around
Iceland (Supplementary Figs. 1 and 2). Andrews and Giraudeau (2003)
found high amounts of reworked Neogene (Late Tertiary) coccoliths in a
sediment core from a shallow basin at 164 m depth, dated to 8.2 ka BP.
The peak of coccoliths was a unique signature of the 5.5 m long sediment
core and was inferred to be the result of erosion of Neogene outcrops and
re-sedimentation. Farther off the Icelandic coast, at 440 m water depth,
there might also be indications of re-sedimentation in core MD99-2275
(Supplementary Figs. 2 and 3). Knudsen et al. (2008) found a distinct
minimum in magnetic susceptibility followed by a peak of the epifaunal
foraminifera
Cibicides lobatulus
between 8200 and 8000 cal yr BP.
Radiocarbon ages of
C. lobatulus
are about 1500 years older than the
molluscs (Knudsen pers. comm., 2023). The authors suggested that the
peak in
C. lobatulus
might be due to reworking. We think it is possible
that this erosion and reworking was caused by the Storegga tsunami.
Conclusion
Large parts of the ocean floor between Norway, Iceland, and Greenland
could have experienced Storegga tsunami currents strong enough to
rework and move sediments. According to our simulations, a maximum
flow velocity of 25 cm/s or larger, capable of moving grains up to 1 mm,
was simulated down to about 1000 m water depth between 58° and 74° N
15
(Supplemental Fig. 3). In addition, turbidity currents could have been
released from the shelf breaks forming turbidites – as demonstrated for
core MD95-2011 at the Vøring plateau.
The previous climate reconstructions of a large and abrupt 8.2 ka
cooling from marine sediment cores in this area are thus faulty and
should be discarded. Instead, sea floor sediments showing a strong 8.2 ka
anomaly with evidence of re-working should rather be considered as a
deep-water Storegga tsunami deposit.
Methods
Computer simulations
Estimating critical velocity for erosion
We estimated the critical velocity for erosion at the two sediment
core sites LINK14 and 28-03 (Fig. 1). First, we plotted the maximum
simulated current velocities for each site in the Hjulström's diagram
(Hjulström, 1935), to see if the current velocities where higher than the
threshold for erosion of the grain sizes in the 8.2 ka layer. Secondly, we
used the procedure described by Weiss (2008) based on Shields diagram
(Yalin, 1977), and solved numerically the non-linear equation 3 in Weiss'
paper for the critical velocity for setting grains in motion. We noted that
the critical velocity for erosion were slightly higher using Hjulström's
diagram than Shields diagram (Weiss, 2008). Both methods estimated
higher velocities than the threshold for erosion of the grain sizes in the
8.2 ka layer at the two sediment core sites.
Computer simulation of Storegga slide and tsunami
The tsunami generated by the Storegga slide is modelled in two
stages. First, the dynamics and time-evolution of the slide itself are
simulated using the two-layer depth-averaged BingCLAW model (Kim et
al., 2019), developed for cohesive clay-rich landslides. Secondly, the
tsunami propagation, driven by the time-dependent changes in the water
16
depth modelled by BingCLAW, is simulated using the GloBouss tsunami
model (e.g. Løvholt et al., 2008; Løvholt et al., 2010; Løvholt et al., 2015).
The simulations used ocean depths as derived by Hill et al. (2014) that
account for changes in bathymetry since 8150 yr BP, covering a region
from 12.5°W to 16.6°E and 53.3°N to 70.0°N on a grid with
approximately 2 km spacing. Both landslide and tsunami simulations
were run for a duration of 10 hours which was deemed sufficient to
capture the evolution of the water wave over all geographical regions of
interest.
The landslide dynamics and runout are controlled by several
parameters describing the rheology of the flow. We chose the landslide
parameters that gave the best match to the observed tsunami run-up
heights (Bondevik et al., 2005), as guided by the sensitivity study of Kim
et al. (2019). The landslide simulation in the current study uses a volume
of 3200 km3, an initial yield strength,
τ
i
, of 12 kPa, a residual yield
strength,
τ
r
, of 3 kPa, a remoulding coefficient, Γ, of 0.0005, and an
added mass parameter,
c
m
, of 0.1. All other parameters are held to the
fixed values employed by Kim et al. (2019).
In this study we have used a landslide volume of 3200 km3, close to
the original volume estimate (Haflidason et al., 2004). Recently Karstens
et al. (2023) suggested that the volume of the Storegga slide should be
reduced to 1300–2300 km3. We emphasize that such a reduced volume is
not expected to change the conclusions found herein for the following
reasons: Firstly, the simulated waves and currents are calibrated towards
observed tsunami run-up heights (e.g. Bondevik et al., 2005). The
landslide is strongly linked to the properties of the slide material at the
time, which are uncertain (Kim et al., 2019). By reducing the strength
properties of the slide material, we can likely reproduce the same
amplitudes and currents of the tsunami with such a smaller landslide
volume. Secondly, the wave generation is dominated by the slide material
from farthest up the slope, in shallow water. In the study by Karstens et
17
al. (2023) the location of the slide material is not much altered compared
to the original reconstruction.
Data availability and Resources/Acknowledgements
The source code for the BingCLAW program that computes the evolution
of the viscoplastic debris flow is available from
https://github.com/norwegian-geotechnical-institute/BingCLAW_5.6.1
(last accessed 2023/08/23). BingCLAW requires the CLAWPACK software
library and all its dependencies. CLAWPACK is available for download
from http://www.clawpack.org/ (version 5.6.1 required: last accessed
2023/08/23). The source code to the GloBouss software for calculating
the tsunami is found on https://github.com/geirkp/geirkp.github.io (last
accessed 2023/08/23). Details of the implementation of the numerical
model are found in the document
https://github.com/geirkp/geirkp.github.io/blob/master/SUP/globouss/rap
port.pdf
We are grateful to Jon Hill for providing a file of the paleobathymetry
from his 2014 paper on which the tsunami simulation was calculated.
The map of the output from the tsunami simulation is generated using
GMT software (Wessel et al., 2019) available from https://www.generic-
mapping-tools.org/ (last accessed 2023/08/23).
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Acknowledgements
Maarten Blaauw answered questions about the age-depth modelling.
Morten Hald suggested us to radiocarbon date the foraminifera at the 8.2
ka spike. Radiocarbon measurements were supported by Research
council of Norway, project no. 325333.
Author contributions
S.B. conceived the idea of the paper and wrote the first draft of the
manuscript. B.R led the re-investigation of sediment core MD95-2011 and
picked foraminifera for radiocarbon dating. S.G. simulated the currents
and surface elevation of the Storegga tsunami under the guidance of F.L.