Contamination of 8.2 ka cold climate records by the Storegga tsunami in the Nordic Seas

Domain: earth_science
Published: 2024-04-04
Editorial outcome: major_revision

Abstract
The 8200-year BP cooling event is reconstructed in part from sediments in the Norwegian and North Seas. Here we show that these sediments have been reworked by the Storegga tsunami – dated to the coldest decades of the 8.2 ka event. We simulate the maximum tsunami flow velocity to be 2–5 m/s on the shelf offshore western Norway and in the shallower North Sea, and up to about 1 m/s down to a water depth of 1000 m. We re-investigate sediment core MD95-2011 and found the cold-water foraminifera in the 8.2 ka layer to be re-deposited and 11,000 years of age. Oxygen isotopes of the recycled foraminifera might have led to an interpretation of a too large and dramatic climate cooling. Our simulations imply that large parts of the seafloor in the Norwegian and North Seas probably were reworked by currents during the Storegga tsunami.

Introduction
Tsunamis can disturb the seabed and rework offshore sediments. Investigations after the 2011 Tohoku tsunami in Japan showed re-deposited mud and sorted sand layers on the Sendai shelf down to ~100 m water depth1. Resuspended sediments were also carried offshore and continued down the slope to deep water, as turbidity currents2. The Storegga slide in the Norwegian Sea3 triggered a giant tsunami4. Deposits from this tsunami have been extensively mapped onshore in Scotland e.g. refs. 5,6, Norway e.g. ref. 7, Shetland 6,8, and the Faroe Islands9. 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 seafloor and are a function of surface elevation (wave amplitude) and water depth. The maximum horizontal flow velocity, u max, in a propagating tsunami can then be approximated to the maximum surface elevation η max through the expression:

u max = η max \sqrt{g/d} (1)

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 Hjulström10, relating 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 fine silt to fine pebbles (5 μm to 6 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 7300±20 14C years BP11 and calibrated to 8080–8180 cal yr BP (8140±55 years ago, Supplementary Methods 1.1). The moss had still small amounts of intact chlorophyll, and thus must have been killed during the tsunami. From analysis of the growth pattern of these green moss stems it has been suggested that the tsunami happened in late autumn, possibly October–November12. 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 event11.

Here we ask the following question: Could the 8.2 ka cold event, as reconstructed from marine sediment cores in the Norwegian and North Seas, 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 1050 m – one of the few North Atlantic marine sediment cores that recorded a distinct and dramatic cooling, the 8.2 ka event13,14. 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 Norwegian and North Seas from computer simulations of the landslide and ensuing tsunami using a linear shallow water model e.g. refs. 4,15. The Storegga Slide was modeled as a cohesive clay-rich debris flow. The subsequent tsunami was simulated from the time-dependent changes in water depth caused by the moving landslide16. 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 deposits17 (See methods for details).

The simulated maximum flow velocity representing the main part of the water column reflects the maximum wave amplitude and water depth. On the Norwegian shelf and in the North Sea, we calculate maximum velocities greater than 1 m/s at depths shallower than 250 m (Fig. 1, Supplementary Fig. 1). 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 back wall 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). For the sites of interest, the simulated currents are uniform throughout the main part of the water column, with a thin boundary layer typically ranging from 1 to 10 m (see Methods). The simulated velocities shown in Fig. 1 apply almost down to the seafloor.

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 climate event13,14. 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 rates18. Currents at the core location are at present too weak (<1 cm/s) to influence the bottom sediments19. However, during the Storegga tsunami, the simulations show a peak 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)20. 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 3 °C of the surface ocean water. This anomaly is not seen in the oxygen isotopes of N. incompta14.

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. pachyderma 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. pachyderma 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 cal yr BP. The high δ18O value of this species (2.95‰) is comparable to the Early Holocene or Younger Dryas values in the same core14 and in agreement with their radiocarbon ages.

The 8.2 ka layer fines upwards and the radiocarbon dates show a hiatus at the lower boundary that indicate erosion. The radiocarbon dates show a jump in ages across the layer and this jump indicates a time gap in the stratigraphy (Supplementary Fig. 4). We thus included a hiatus at the lower boundary (535 cm) in the age-depth modeling. 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. 3a–d). The counts of mineral grains >150 μm, 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.

Sediment core LINK14 from a trough on the eastern shelf of the Faroe Islands (Fig. 1, Supplementary Table 1, Supplementary Methods 2.2), has an ‘8.2 ka layer’ interpreted by Rasmussen and Thomsen22 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. The age–depth models of the radiocarbon ages (Supplementary Table 3) in the sediment core (Supplementary Figs. 6 and 7) suggest that the age of the layer is between 7930 and 8470 yr BP (2σ range).

Our tsunami simulations show that the sediment core site of LINK14 would experience quite strong currents of up to about 1.8 m/s in the water column (Fig. 2b). Considering the boundary layer thickness, this current is roughly reduced to about 1.2 m/s one meter above the seafloor (Supplementary Table 2). These currents could hence transport sand from the shallower banks into the trough. Both the Sundborg diagram23 and the equations by Miller et al.24 show that such strong currents would erode sediments with grain sizes of fine silt to fine pebbles (Supplementary Table 2).

Core 28–03 in the Norwegian Channel (Fig. 1; Supplementary Table 1, Supplementary Methods 2.3) also show a distinct and sudden 8.2 ka event25 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 Figs. 8 and 9). The strongest simulated currents are 0.5 m/s one meter above the seafloor (Fig. 2c) and are aligned parallel to the Norwegian Channel (Fig. 1). According to Supplementary Table 2 these currents could stir up fine-grained sediments (silt and sand) in the trench26 and possibly rework and redeposit the foraminifera.

The simulations also reveal strong currents on the shelf around Iceland. Andrews and Giraudeau27 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.28 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 molluscs29. The authors suggested that the peak in C. lobatulus might be due to reworking. We think it is likely that this erosion and reworking was caused by the Storegga tsunami.

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 (Supplementary 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 simulation of Storegga slide and tsunami
The tsunami generated by the Storegga slide is modeled in two stages. First, the dynamics and time-evolution of the slide itself are simulated using the two-layer depth-averaged BingCLAW model17, developed for cohesive clay-rich landslides. Secondly, the tsunami propagation, driven by the time-dependent changes in the water depth modeled by BingCLAW, is simulated using the GloBouss tsunami model e.g30–32. The simulations used ocean depths as derived by Hill et al. 33. 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 h 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 heights4, as guided by the sensitivity study of Kim, et al.17. The landslide simulation in the current study uses a volume of 3200 km3, an initial yield strength, τi, of 12 kPa, a residual yield strength, τr, of 3 kPa, a remolding coefficient, Γ, of 0.0005, and an added mass parameter, cm, of 0.1. All other parameters are held to the fixed values employed by Kim et al.17.

In this study we have used a landslide volume of 3200 km3, close to the original volume estimate34. Recently Karstens et al.35 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.4. The landslide is strongly linked to the properties of the slide material at the time, which are uncertain17. 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 al.35 the location of the slide material is not much altered compared to the original reconstruction.

Estimating critical velocity for erosion
Flow velocities in a tsunami are mainly uniform throughout the water column apart from in a boundary layer near the seafloor. In deep water the boundary layer for a tsunami is relatively thin compared with the water depth. For the Storegga Slide tsunami the boundary layer will typically be about 1–10 m thick36, and will vary with the current velocity, wave period, and surface roughness.

To estimate the critical velocity for erosion at the three sediment core sites LINK14, 28-03, and MD95-2011 (Fig. 1, Supplementary Table 1) we first consider the boundary layer thickness influenced by bottom friction for each site using the results from the simulations by Williams and Fuhrman36. From visual inspection of those simulations, we found that the current speed 1 m above the seafloor was reduced by a factor of about 0.7 (core site LINK14) to 0.95 (core site MD95-2011) of the uniform water speed in the water column. Next, we found the largest and smallest erodible sediment grain sizes by reading off the grain sizes in the Sundborg diagram23 for each current velocity and by calculations using the equations in Miller et al.24; see Supplementary Table 2 for details. The methods provide similar results, and both estimated higher velocities than needed to erode the grain sizes in the 8.2 ka layer at the sediment core sites LINK14 and 28-03.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is a well-documented paper which casts doubt on the credibility of records purporting to show the\n8.2 ka event in cores from the Nordic Seas. The data are of high quality, well set out in figures and\ntables, and entirely credible. Is clear that for evidence of the 8.2 ka event cores from other locations\nmust be employed. The Storegga slide caused a well-documented tsunami for which bottom velocities\nare calculated here and demonstrated to be adequate to resuspend shallow sediments which then flow\ndown slope in the form of turbidity currents contaminating any climatic records of that time. This is a\nsignificant and important result as negative data is not always accorded the significance that is due to\nit.\nI do not think the words ‘Confusion and’ are needed in the title as ‘contamination’ is quite sufficient.\nOne can reasonably conclude that one will lead to the other.\nIn places I find the authors to be rather too tentative, for example on line 44 ‘most likely’ could be\nremoved, also ‘could’ on line 167, and ‘likely’ rather than ‘possible’ on lines 203 and 240.\nThere are two aspects that need revision by the authors.\n1. They are using three terms for the water surface perturbation due to the Storegga slide; ‘surface\nelevation’, ‘wave amplitude’, and ‘wave height’. In figure 2 the caption is ‘wave height’ but the\nreference to it on line 115 is ‘wave amplitude’. Normally in wave theory wave height is twice the wave\namplitude. The authors need to sort out the terminology here. Is ‘surface elevation’ wave height ?\n2. The authors should not be using the archaic Hjulstrom curve for critical erosion conditions. This was\nimproved by his student Sundborg (1956), but the most commonly used curve based on analysis of\nlarge amounts of data is that due to Miller et al (1977). These authors give an equation for the flow\nvelocity measured at 1 m above the bed for the coarse end of the grainsize spectrum (D >2 mm) as U\n= 160D^0.45 giving a speed range of 0.6 to 1.2 m/s for 1 to 5 mm coarse sand-fine gravel. This, as\nwith the condition based on Yalin (1977) which they cite, is also based on Shields curve. The period of\nthe tsunami wave is sufficiently long (>20s) that the flow may be considered unidirectional and thus\nthe wave velocity of Komar & Miller (1973, 1975) is not needed.\nReferences.\nKomar, P.D & M.C. Miller, 1973. The threshold of sediment movement under oscillatory water waves.\nJournal of Sedimentary Petrology, 43, 1101-1110.\nKomar, P.D & M.C. Miller, 1975. Comparison between threshold of sediment motion under waves and\nunidirectional currents with a discussion of practical evaluation of threshold. Journal of Sedimentary\nPetrology, 43, 362-367\nSundborg, A, 1956. The River Klarälven; a study in fluvial processes. Geografiska Annaler, 38, 125-\n316.\nMiller, M.C., McCave, I.N. and Komar, P.D. 1977. Threshold of sediment motion under unidirectional\ncurrents. Sedimentology, 24, 507-527.", "reviewer_2": "The manuscript by Bondevik et al combines numerical modeling and sediment core records to argue\nthe sediment core records had been more likely contaminated and falsely used to refer the 8.2ka cold\nclimate due to the rework of sediments by the Storegga landslide tsunami. In that regard, this\nmanuscript provides rational evidences after re-examining offshore and nearshore cores, which are\nfurther explained using model simulation of the Storegga tsunami, specifically using the tsunami-\ninduced current speed. The methods used in this study are technically correct, and they favorably\nsupport the quality of the data obtained with appropriate techniques. The conclusions arrived from this\nstudy may generate potential impact on how the 8.2ka cold climate event was defined and what\nmagnitude of impact it had posed regionally and globally. The conclusion that “a strong 8.2 ka\nanomaly with evidence of re-working should rather be considered as a deep-water Storegga tsunami\ndeposit” may be overstated as there have been evidences other than the sediments supporting the 8.2\nka code event. However, I admit that interpretation of geological core materials relating to specific\ncauses is outside the scope of my expertise, and I am unable to assess fully. My comments below are\nmostly focused on numerical modeling of the tsunami and sediment transport.\nUsing numerical models, the manuscript carefully investigated the current speed generated by the\nStoregga tsunami at a few sediment cores previously obtained offshore and nearshore. The initiation\nand movement of the sediments are quantitatively explained using the model results. The manuscript\nclearly shows the sediments indicating the 8.2 ka cold climate had been reworked during the\nStoregga tsunami event. However, the sediment deposition and erosion at these cores are not clearly\ndiscussed and sufficiently illustrated, and can be improved from the following aspects:\n• The time series of wave height and flow speed (not velocity) in Fig. 2 and Supplementary Fig. 1\nclearly shows significant drawdown of water either as the leading wave (the East of Storegga\nbackscarp) or following the first wave crest, and the largest flow speeds are mostly associated with\nthese receding waves (certainly a normal phenomenon during a landslide tsunami). Wouldn’t these\nreceding waves result in more erosion than deposition, particularly in the nearshore area, and as a\nresult affect how one interprets the causes of the sediment layers indicating the 8.2 Ka event? What\nroles do these receding flows play in correlating the sediment core records to the 8.2 ka event? That\nsaid, it will be helpful to include vectorized flow velocities in those time series plots and offer more\ninsightful discussions.\n• The manuscript offers only very brief discussions on “Estimating critical velocity for erosions”. The\ncomparisons between the modeled velocity and the critical velocities could have been provided in a\ntable in the supplement. I would also strongly suggest the authors to take a more rigorous step:\nconduct model simulation on sediment transport (assuming such a modeling tool is available to\nauthors) to approximate the amounts and deposition and erosion distributions of the sediments for the\nStoregga tsunami. This additional modeling is expected to clarify the sedimentation process at the\ncore sites, and endow more solid and more complete conclusions. For this reason, I consider this is a\nlack of sufficient detail in the present manuscript to fully support their conclusions, a standard\nrequired by Nature Communications.\n• One more detail the manuscript can provide is how the bottom friction was taken into account in the\nmodel simulation, and how much it affects the flow velocities.\nBased on my comments above, my overall recommendation is a minor-major revision depending how\nmuch more work the authors are willing to invest in this study."}, "author_response": "Response to the review of our manuscript:\nConfusion and contaminafion of 8.2 ka cold climate records caused by the Storegga\ntsunami in the Nordic Seas\n[Text in blue is our response, text in black is the reviewers]\nWe thank you for the review and the opportunity to resubmit the paper for possible publicafion in\nNature Communicafions! Both reviewers are happy with the documentafion and evidence we provide\nthat the record of the well-known 8.2 ka cold climate event, as inferred from marine sediment cores\nin the Norwegian and Nordic Seas, is contaminated and disturbed by the Storegga tsunami. This is the\nmain point of our paper. We are very pleased that the reviewers find our paper “well-documented\nand of high quality, well set out in figures and tables, and enfirely credible” (reviewer 1).\nThe reviewers and editor ask us to provide more discussion and modeling of sediment deposifion and\nerosion. We have revised our approach and provide a modificafion to the modeling based on the\nrequest from both reviewers. To this end, we have used the sediment model as suggested by\nreviewer #1. In addifion, we have invesfigated the effect of the boundary layer to the tsunami current\nby consulfing results from sophisficated modelling approaches considering the local shear profile at\nthe sea floor that influences the erosion (mofivated by comments from reviewer #2). The revised\napproach clearly improves the modelling, and we are hence thankful for the reviewer’s suggesfions. It\nfurther consolidates the findings already reported in the first draft of this paper, with only slightly\nrevised values for the erodible grain sizes.\nHence, the revision we have carried out, based on the reviewer’s comments is important because it\nclearly shows the potenfial for re-sedimentafion and movement of sediment grains. On the other\nhand, it is beyond the scope of this paper to model in detail the erosion and sedimentafion paftern,\nthis should and must be left for further studies. In addifion, we stress that erosion potenfial is largely\nbased on empirically based thresholds, and we point clearly to current velocifies that exceed these\nthresholds in this paper.\nBelow we address point by point the comments and crifique raised by the reviewers.\nReviewer 1\nI do not think the words ‘Confusion and’ are needed in the fitle as ‘contaminafion’ is quite sufficient.\nOne can reasonably conclude that one will lead to the other.\nWe agree and have now deleted the word ‘Confusion’ from the fitle. The fitle is changed to:\n“Contaminafion of 8.2 ka cold climate records caused by the Storegga tsunami in the Nordic Seas”\nIn places I find the authors to be rather too tentafive, for example on line 44 ‘most likely’ could be\nremoved, also ‘could’ on line 167, and ‘likely’ rather than ‘possible’ on lines 203 and 240.\nThere are two aspects that need revision by the authors.\n1. They are using three terms for the water surface perturbafion due to the Storegga slide; ‘surface\nelevafion’, ‘wave amplitude’, and ‘wave height’. In figure 2 the capfion is ‘wave height’ but the\nreference to it on line 115 is ‘wave amplitude’. Normally in wave theory wave height is twice the wave\namplitude. The authors need to sort out the terminology here. Is ‘surface elevafion’ wave height ?\nIn the capfion to Fig. 2 we have changed wave height to sea surface elevafion.\nIn line 115 we use the word “amplitude” because it is the amplitude of the wave we are referring to.\nHere is the sentence: However, during the Storegga tsunami the simulafions show a current speed of\n39 cm/s corresponding to a wave amplitude of 3.8 m (Fig. 2a). In Fig. 2a the amplitude is 3.8 m – it is\nthe distance from the zero-line to the wave top. We could also use “surface elevafion” here, but we\nthink that using the word “wave amplitude” we indicate to the reader that it is the top point of the\nfirst wave we address, instead of saying “surface elevafion”.\nIs ‘surface elevafion’ wave height?\nNo. The sea surface elevafion is the height of the sea surface at any given fime. It is not the same as\nwave height or wave amplitude. We have taken care to make sure that all occurrences of the terms\nare now the correct ones.\n2. The authors should not be using the archaic Hjulstrom curve for crifical erosion condifions. This\nwas improved by his student Sundborg (1956), but the most commonly used curve based on analysis\nof large amounts of data is that due to Miller et al (1977). These authors give an equafion for the flow\nvelocity measured at 1 m above the bed for the coarse end of the grainsize spectrum (D >2 mm) as U\n= 160D^0.45 giving a speed range of 0.6 to 1.2 m/s for 1 to 5 mm coarse sand-fine gravel. This, as\nwith the condifion based on Yalin (1977) which they cite, is also based on Shields curve. The period of\nthe tsunami wave is sufficiently long (>20s) that the flow may be considered unidirecfional and thus\nthe wave velocity of Komar & Miller (1973, 1975) is not needed.\nWe thank the reviewer for this comment. Based on the suggesfion, we have adopted the curve by\nMiller et al. (1977). This has further been combined with an esfimate of the boundary layer that\nreduces the velocity at 1 m height by a factor of 0.7-0.95 of the uniform water velocity. We have\ndescribed this in the first paragraphs of the method secfion and also added a table with details in the\nsupplements (Supplementary Table 2). However, this much improved method only slightly modifies\nthe results and reaffirms the conclusions from the first version of the manuscript.\nReviewer #2 (Remarks to the Author):\nThe manuscript by Bondevik et al combines numerical modeling and sediment core records to argue\nthe sediment core records had been more likely contaminated and falsely used to refer the 8.2ka cold\n2\nclimate due to the rework of sediments by the Storegga landslide tsunami. In that regard, this\nmanuscript provides rafional evidences after re-examining offshore and nearshore cores, which are\nfurther explained using model simulafion of the Storegga tsunami, specifically using the tsunami-\ninduced current speed. The methods used in this study are technically correct, and they favorably\nsupport the quality of the data obtained with appropriate techniques. The conclusions arrived from\nthis study may generate potenfial impact on how the 8.2ka cold climate event was defined and what\nmagnitude of impact it had posed regionally and globally.\nThe conclusion that “a strong 8.2 ka anomaly with evidence of re-working should rather be\nconsidered as a deep-water Storegga tsunami deposit” may be overstated as there have been\nevidences other than the sediments supporfing the 8.2 ka code event. However, I admit that\ninterpretafion of geological core materials relafing to specific causes is outside the scope of my\nexperfise, and I am unable to assess fully. My comments below are mostly focused on numerical\nmodeling of the tsunami and sediment transport.\nWe thank the reviewer for the comments to our manuscript. Partly based on the comments below,\nand partly based on reviewer #1 comments, we have slightly revised our approach to assess the\nerosion condifions on the sea floor. In the revised paper, we include other empirical sediment erosion\nmodels, and also more carefully evaluate the flow field above the seafloor. Yet, the conclusions from\nthe first version of the paper are sfill retained with the new approach (see above comments to\nreviewer #1).\nThe conclusion that “a strong 8.2 ka anomaly with evidence of re-working should rather be\nconsidered as a deep-water Storegga tsunami deposit” may be overstated as there have been\nevidences other than the sediments supporfing the 8.2 ka cold event.\nWe do not say that the 8.2 ka cold event does not exist! Our finding and conclusion are that what has\nbeen taken as the 8.2 cold climate event deposits in the Norwegian and North Seas are rather\ndeposits from the Storegga tsunami.\nUsing numerical models, the manuscript carefully invesfigated the current speed generated by the\nStoregga tsunami at a few sediment cores previously obtained offshore and nearshore. The inifiafion\nand movement of the sediments are quanfitafively explained using the model results. The manuscript\nclearly shows the sediments indicafing the 8.2 ka cold climate had been reworked during the\nStoregga tsunami event. However, the sediment deposifion and erosion at these cores are not clearly\ndiscussed and sufficiently illustrated, and can be improved from the following aspects:\n• The fime series of wave height and flow speed (not velocity) in Fig. 2 and Supplementary Fig. 1\nclearly shows significant drawdown of water either as the leading wave (the East of Storegga\nbackscarp) or following the first wave crest, and the largest flow speeds are mostly associated with\nthese receding waves (certainly a normal phenomenon during a landslide tsunami). Wouldn’t these\nreceding waves result in more erosion than deposifion, parficularly in the nearshore area, and as a\nresult affect how one interprets the causes of the sediment layers indicafing the 8.2 Ka event? What\nroles do these receding flows play in correlafing the sediment core records to the 8.2 ka event? That\nsaid, it will be helpful to include vectorized flow velocifies in those fime series plots and offer more\ninsighfful discussions.\nIt is not the scope of this arficle to conduct a fime dependent analysis of the detailed pafterns of\nerosion and deposifion of the Storegga tsunami. Rather, we aftempt to analyze whether the current\nspeed has the potenfial to erode the sea floor. This is done by comparing the modeled wave current\n3\nwith state-of-the-art empirical models for erosion, and our analysis shows clearly that the current\nspeeds exceed the erosion potenfial. We agree that this suggesfion would be an interesfing analysis.\nIt would however add significant addifional complicafions and it is not the target of this paper.\n• The manuscript offers only very brief discussions on “Esfimafing crifical velocity for erosions”. The\ncomparisons between the modeled velocity and the crifical velocifies could have been provided in a\ntable in the supplement.\nYes, good idea, we have added such a table in the supplement, Supplementary Table 2.\nI would also strongly suggest the authors to take a more rigorous step: conduct model simulafion on\nsediment transport (assuming such a modeling tool is available to authors) to approximate the\namounts and deposifion and erosion distribufions of the sediments for the Storegga tsunami. This\naddifional modeling is expected to clarify the sedimentafion process at the core sites, and endow\nmore solid and more complete conclusions. For this reason, I consider this is a lack of sufficient detail\nin the present manuscript to fully support their conclusions, a standard required by Nature\nCommunicafions.\nAs stressed above, fime dependent sediment transport modelling is not the main scope here. The\nmain purpose is to assess whether the current velocifies have substanfial erosion potenfial at the\nsediment core sites. On the other hand, we have revised our modelling approach conducfing the\nfollowing two steps: Based on reviewer #1's comment, we have included the erosion models of Miller\net al. (1977). As a second step, we have analyzed the effect of the boundary layer thickness for\nesfimafing the velocity at the 1 m height above the seafloor needed as input to the erosion models.\nIn our opinion this is the most important modificafion as we now more carefully evaluate the flow\nfield right above the seabed. We find that the speed used for esfimafing the erosion (one meter\nabove the seafloor) is reduced by factors of 0.7-0.95 from the current speed shown in Fig. 2. This only\nslightly modifies the results and reaffirms the conclusions from the first version of the manuscript.\n• One more detail the manuscript can provide is how the boftom fricfion was taken into account in\nthe model simulafion, and how much it affects the flow velocifies.\nAs this is deep-water tsunami propagafion, the fricfion force on the wave is of minor importance and\nit is hence omifted in the simulafions.\nBased on my comments above, my overall recommendafion is a minor-major revision depending how\nmuch more work the authors are willing to invest in this study.\n4"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review of the resubmission of “contamination of 8.2 K a cold climate records caused by the Storegga\ntsunami in the Nordic seas”, by S. Bondevik et al.\nOverall the authors have responded satisfactorily to (both) reviewers comments. The authors make a\nstrong case for not interpreting data from the Nordic seas and North Sea around the time of the 8.2\nevent as evidence for that event. There are plenty of cores from outside this region which demonstrate\nthe impact of the event on the marine realm.\nThere are only minor matters to highlight.\nlines\n31. ... ‘might have been’ , better ‘probably were’.\n55-8 and 223-4. It is not clear why the authors persist in citing the antique diagram of Hjulstrom\n(other than Nordic solidarity) which is incorrect in both its designation of the deposition field and in\nplotting the erosion resistance of fine sediment as a function of grainsize - see work of Dade et al.\nPost-war (WW II) and modern treatments of critical erosion do not mention Hjulstrom, for example\nthe books of Yalin, Raudkivi, Mehta and Nielsen.\n88. (102) I think the authors need to specify where their maximum flow velocity occurs, either at the\nseafloor or is it at the top of the boundary layer (the latter).\n155-7. pachyderma\n369, if they persist, needs date.\n397, 413, 423, capitalisation,\nReferences.\nDade, W.B., Nowell, A.R.M. and Jumars, P.A., 1992. Predicting erosion resistance of muds. Mar. Geol.,\n105, 285-297.\nDade, W.B., and Nowell, A.R.M., 1991, Moving muds in the marine environment, In Proceedings\nCoastal Sediments ’91, Water Resources Division Am. Soc. Civil. Eng., pp. 54-71.\nMehta, A.J., 2023. An introduction to hydraulics of fine sediment transport. Second edition, World\nscientific, Singapore, 1027 pp.\nNielsen, P., 1992. Coastal bottom boundary layers and sediment transport. World scientific, Singapore,\n324 pp.\nRaudkivi, AJ., 1976. Loose boundary hydraulics. 2nd edition, Pergamon press Oxford, 397 pp.\nYalin, M. S., 1977, Mechanics of Sediment Transport,2nd Edition, Pergamon Press.", "reviewer_2": "The revised manuscript is a much-improved version from the original. The authors’ responses to my\ncomments are mostly reasonable, particularly to my request regarding the critical velocity for\nerosions. Following my suggestions, the authors added a table and enriched the discussions on this\nproblem. It has led to much clearer explanation of the simulation results.\nI agree with the authors that the time-dependent sediment modeling and analysis may be beyond the\nscope of the present study. The revised modeling approach by including the erosion models of Mill et\nal. (1977) represents a good alternative.\nHowever, I am not fully convinced by the authors’ response regarding the friction factor that it is not\nimportant in the present simulations. Yes, friction factor might not be important in deep water, but\nthey may play a role in sediment transport in shallow water, especially the flow speed at the points, a\n(190 m), b (45 m), and d (120 m), shown and discussed in Figure S1. There are no descriptions or\ndiscussions on the friction factor used in the model in the manuscript. So, the model did not consider\nthe bottom friction at all?"}, "author_response": "Response to the review of our resubmifted manuscript:\nContaminafion of 8.2 ka cold climate records caused by the Storegga tsunami in the\nNordic Seas\n[Text in blue is our response, text in black is the reviewers]\nWe are grateful that the reviewers of our revised manuscript are safisfied with our response and\nchanges to the original manuscript. Reviewer #1 says: “Overall the authors have responded\nsafisfactorily to both reviewers’ comments” and reviewer #2 says: “The revised manuscript is a much-\nimproved version from the original. Following my suggesfions, the authors added a table and\nenriched the discussions on this problem. It has led to much clearer explanafion of the simulafion\nresults. The revised modeling approach by including the erosion models of Mill et al. (1977)\nrepresents a good alternafive.\nHowever, there are sfill a few things and quesfions raised by the two reviewers that we are happy to\nclarify and rephrase in our 2nd revision of our manuscript.\nBelow we address point by point to the comments and quesfions by the reviewers:\nReviewer 1\n(line) 31. ... ‘might have been’ , befter ‘probably were’.\nThanks for poinfing this out, we have changed the text accordingly.\n(line) 55-8 and 223-4. It is not clear why the authors persist in cifing the anfique diagram of Hjulstrom\n(other than Nordic solidarity) which is incorrect in both its designafion of the deposifion field and in\nplofting the erosion resistance of fine sediment as a funcfion of grainsize - see work of Dade et al.\nPost-war (WW II) and modern treatments of crifical erosion do not menfion Hjulstrom, for example\nthe books of Yalin, Raudkivi, Mehta and Nielsen.\nPlease note that we revised our interpretafion of the erosion based on the boundary layer thickness\n(William and Fuhrman, 2016) in the previous revision. We replaced the grain size values from\nHjulström with those from Sundborg and included the equafions from Miller et al., as suggested in\nthe previous review. Thus, we have assessed several different erosion models and not just one.\nWe know that the Hjulström diagram has its flaws and weaknesses, especially for the fine grain sizes,\nbut for pedagogical reasons the diagram of Hjulström visualize the relafionship between sediment\nerosion, transportafion, and deposifion in an easy understandable way. Therefore, in our revised\nversion we keep the Hjulström reference in lines 55-58, as an introducfion to the topic, but delete\nHjulström from line 223-234. The lafter was a mistake by us in the previous revision; we are not using\nHjulström’s diagram to esfimate erosion of grains, but rather Sundborg’s diagram and the Miller et al.,\nequafion (see table S2).\n88. (102) I think the authors need to specify where their maximum flow velocity occurs, either at the\n1\nseafloor or is it at the top of the boundary layer (the lafter).\nYes, we agree that this needs clarificafion. The paragraph dealing with the flow velocity is that\nbeginning at line 88 on the previous submission. To make it clear that the simulated velocity applies\nonly down to the top of the boundary layer, we have changed the sentence (line 88) to: “The\nsimulated maximum flow velocity, represenfing the main part of the water column, reflects the\nmaximum wave amplitude and water depth”. (New text underlined.) In the paragraph following line\n88 it becomes clear that the simulated velocity is the top of the boundary layer.\nWe have also added this explanafion to the figure text in line 102. Here it now says: Fig. 1| Maximum\nflow velocity of the water column down to about 1–10 m above the sea floor during the Storegga\ntsunami. (New text underlined.)\nAnd similar for figure text to Fig. 2:\nFig. 2 | Simulated tsunami surface elevafion (blue line) and flow velocity down to about 1–10 m\nabove the sea floor (dofted) during the Storegga tsunami at the locafion of marine sediment cores\nwith an 8.2 ka layer.(New text underlined.)\n155-7. pachyderma\nThanks for poinfing out this misspelling. We have changed text accordingly.\n369, if they persist, needs date.\nThis was a typographical error and has now been fixed. Thank you.\n397, 413, 423, capitalisafion,\nWe have checked the capitalizafion of these three references and, as far as we can see, they are\nconsistent with the published fitles’ bibliographical details. We will however address any issues\nflagged in typesefting.\nReviewer 2\nHowever, I am not fully convinced by the authors’ response regarding the fricfion factor that it is not\nimportant in the present simulafions. Yes, fricfion factor might not be important in deep water, but\nthey may play a role in sediment transport in shallow water, especially the flow speed at the points, a\n(190 m), b (45 m), and d (120 m), shown and discussed in Figure S1.\nWe very much agree that the fricfion is important in interprefing the effect of the boundary layer. For\nthis we considered the boundary layer thickness influenced by boftom fricfion for each sediment core\nsite using the results from the simulafions by Williams and Fuhrman (2016), see Methods secfion. We\nfirst computed the current velocity in the mid water column in the tsunami simulafions. We then\ncorrected for the boundary layer effect and fricfion when determining the effecfive velocity for the\nerosion at each core site. Boftom fricfion does not affect much the propagafion of the first cycles of\n2\nthe wave propagafion (see e.g. Davies et al., 2020). To remedy, we have changed the figure text to Fig.\nS1:\nSupplementary Fig. 1. Simulafion of wave height and flow speed of the water column down to\nabout 1-10 m above the sea floor at locafions surrounding Storegga. (New text underlined.)\nThere are no descripfions or discussions on the fricfion factor used in the model in the manuscript.\nSo, the model did not consider the boftom fricfion at all?\nThe propagafion model for predicfing the flow velocity throughout the main part of the water does\nnot include fricfion as it is not important for the deep-water propagafion. However, the fricfion is\ntaken into account in the interpretafion of the erosion at the discussed sediment core sites, through\nboundary layer values from Williams and Fuhrman (2016), see Methods secfion.\nReferences:\nDavies, G., Romano, F., and Lorito, S. (2020). Global dissipation models for simulating tsunamis at far-\nfield coasts up to 60 hours post-earthquake: multi-site tests in Australia.Frontiers in Earth Science,8,\n598235.\n3"}, {"round": 3, "reviewer_comments": {"reviewer_2": "It's good to know that the authors did consider friction in the modeling of each core site and added\nclarification in the texts. I have no more comments. My recommendation is to accept the manuscript\nfor publication."}, "author_response": ""}]

Decision letter