Relative sea level in inner Nordfjord at 8150 cal. a BP

Relative sea level in inner Nordfjord at 8150 cal. a BP Supporting information to Vasskog et al. Introduction The most precise way of determining past relative sea level in Scandinavia is considered to be the isolation basin approach (e.g. Hafsten, 1983; Kaland, 1984; Anundsen, 1985; Kjemperud, 1986; Svendsen and Mangerud 1987; Romundset et al., 2010; Long et al., 2011). No complete Holocene sea-level curves have been constructed for Nordfjord using this method, however, and the only reliable evidence of past sea level at the Nerfloen outlet is a raised shoreline at ~65 m above present mean sea level (msl). It is thought to have formed shortly after withdrawal of the Scandinavian Ice Sheet from inner Nordfjord around 11,000 cal. yr BP, and represents the Marine Limit in the area (Rye et al., 1997). Theoretical relative sea-level (RSL) curves for inner Nordfjord One possible way of estimating Holocene relative sea-level (RSL) changes in inner Nordfjord is to project the local Marine Limit (65 m, ~11,000 cal. a BP) onto the shoreline diagram from Sunnmøre and Sør-Trøndelag (Svendsen and Mangerud, 1987) and construct a theoretical RSL curve from the point in the diagram where the 11,000 cal. a shoreline intersects an altitude of 65 m. This approach has been used in inner Nordfjord by previous investigators (e.g. Lyså et al., 2010). From the Svendsen and Mangerud (1987) composite shoreline diagram (hereafter referred to as the shoreline diagram ) we find that the Marine Limit in inner Nordfjord plots ~90 km from the outer coast along the projection plane. Thus we may construct a theoretical RSL curve from the shoreline diagram using the age and elevation of shorelines crossing a horizontal line at 90 km. The curve resulting from this approach is shown in Figure S1A. There are, however, certain intrinsic assumptions connected to this approach. Most importantly it requires that isobase directions have remained more-or-less parallel through time, and although Svendsen and Mangerud (1987) show that this has probably been the case for their study area, it may not be true for inner Nordfjord or areas further south. With the new data obtained through this study (cf. the main paper) we are able to test this assumption. The theoretical curve constructed from the shoreline diagram based on the Marine Limit in inner Nordfjord predicts that Nerfloen (29 m above msl) should have been isolated from the sea around 6800 cal. a BP. This is clearly wrong, as our new data show that the

isolation occurred at ~9350 cal. a BP, i.e. more than 2500 years earlier than suggested by the theoretical curve. If we construct a curve at 78 km in the Svendsen and Mangerud (1987) shoreline diagram we obtain a curve that intersects the isolation of Nerfloen, but this curve underestimates the altitude of the Marine Limit in inner Nordfjord by about 15 m (Figure S1A). We may therefore conclude that the shoreline diagram is not able to produce reliable RSL curves for inner Nordfjord, and that the isobase directions have not been parallel through time in inner Nordfjord (Figure S2). Figure S1. A: Relative sea-level curves constructed at 90 km and 78 km in the composite shoreline diagram from Sunnmøre/Sør-Trøndelag (Svendsen and Mangerud, 1987). Note the steeper fall in the RSL curve drawn for Nerfloen. B: Eustatic sea level from Peltier and Fairbanks (2006) and residual curves (glacio-isostatic adjustment and geoidal change) calculated from the RSL curves in A. Shaded red areas represent a qualitative uncertainty estimate.

Residual signals, glacio-isostatic adjustment, and isobase directions By subtracting eustatic sea-level from relative sea-level curves we obtain a residual signal that reflects glacio-isostatic adjustment (GIA) and geoidal changes (e.g. Lohne et al., 2007). In Figure S1B residual signals are shown for the two theoretical RSL curves and for the two RSL data points from inner Nordfjord. It shows that for a period following deglaciation glacio-isostatic rebound must have occurred faster in inner Nordfjord than in areas to the north that has experienced the same amount of emergence during the last 11,000 years, i.e. the 90 km point in the shoreline diagram (Figure S2). A further implication is that at some point after the isolation of Nerfloen (~9350 cal. a BP) the situation must have switched so that glacio-isostatic rebound was lower in inner Nordfjord relative to the 78 km and 90 km RSL sites (Figure S1). By combining the two RSL data points from inner Nordfjord with the shoreline diagram it is possible to draw two approximate isobases: a 65 m isobase for ~11,000 cal. a BP running through Nerfloen and intersecting the shoreline diagram at 90 km, and a 29 m isobase for ~9350 cal. a BP intersecting the shoreline diagram at 78 km (Figure S2), which demonstrate that isobases from these two time slices were not parallel. This skewing of isobase directions is most probably a result of differing deglaciation histories between Nordfjord and Sunnmøre/Sør-Trøndelag. In Nordfjord (and areas further south) a major re-advance of the Scandinavian Ice Sheet margin occurred during the Younger Dryas chronozone (YD) (Rye et al., 1997), whereas in the Sunnmøre/Sør-Trøndelag region there is little evidence for a significant YD re-advance (Mangerud, 1980). As a result, the final deglaciation occurred much later in inner Nordfjord (~11,000 cal. a BP; Rye et al., 1997) compared to the area covered by the shoreline diagram (~14,800-14,000 cal. a BP; Svendsen and Mangerud, 1987), and we propose that this is the reason that the rate of emergence was relatively faster in inner Nordfjord between 11,000 and 9350 cal. a BP. Altitude of the Nerfloen outlet at 8150 cal. a BP In order to obtain a minimum run-up estimate for the Storegga tsunami in inner Nordfjord, we need to know how high the Nerfloen outlet was elevated above sea level when the tsunami struck at ~8150 cal. a BP. On their own our two data points from inner Nordfjord cannot tell us how RSL changed between 9350 cal. a BP and 8150 cal. a BP; however, by studying the regional GIA-signal it is possible to infer a qualitative estimate of uplift rates and thereby calculate a plausible range of RSL change (emergence) for this period by accounting for the contemporaneous change in eustatic sea level (Peltier and Fairbanks 2006; Figure S1).

Figure S2. A: Overview map of southern Norway. B: Map of the Norwegian coast from Nordfjord to Sør- Trøndelag showing the locations, isobases, and shoreline diagram projection planes discussed in the text, redrawn from Svendsen and Mangerud (1987) in order to include Nordfjord. The YD ice sheet margin in Nordfjord is redrawn from Rye et al. (1997). Presumably, the rate of GIA in inner Nordfjord has followed a more-or-less similar temporal pattern as the areas situated northwards along the Main Line isobases, although the absolute rate must have been somewhat higher during the early Holocene and correspondingly lower towards the late Holocene. Thus we find it likely that the rate of uplift started to slow significantly around 10,200 cal. a BP, similarly to what can be seen in the residual curves from Sunnmøre/Sør-Trøndelag (Figure S1B). In other words, a continued high rate of uplift between 9350 cal. a BP and 8150 cal. a BP would imply unrealistically low uplift rates in the later half of the Holocene, and we have therefore used the residual signal from the 90 km Sunnmøre/Sør-Trøndelag curve as a likely maximum rate of uplift for Nerfloen during this interval. There is no evidence of an ingression in the sediment cores from Nerfloen and Oppstrynsvatnet, and we have therefore used the 78 km residual curve as a minimum estimate for the uplift rate, seeing that this is close to the rate required to keep the outlet of Nerfloen

above sea level for the period between 9350 and 8150 cal. a BP (i.e. close to the rate of eustatic sea-level rise). From these assumptions we estimate a mean uplift rate of 11.7 ± 1.7 mm a -1 for the Nerfloen outlet during the ~1200 years following the lake s isolation, which amounts to a mean rate of emergence of 2.5 ± 1.7 mm a -1 after subtraction of eustatic sea level rise (Figure S1). Our conclusion is therefore that the Nerfloen outlet was most likely elevated between 1 and 5 m above high tide when the Storegga tsunami struck inner Nordfjord. Uncertainties in the 8150 cal. yr BP Nerfloen RSL estimate We cannot rule out completely that Nerfloen was inundated by a RSL rise (i.e. the Tapes transgression) prior to the Storegga tsunami, seeing that it is possible that erosion from the tsunami may have removed sedimentary evidence of such an event. We find this unlikely, however, based on the regional GIA history. For a transgression to occur between the isolation of Nerfloen and the tsunami event, an unrealistically rapid change in the uplift rate would be required between 9350 and 8150 cal. a BP (cf. Figure S1B). In the sediment core named STP111 (Figure 4 and 7, main paper) the lacustrine section between the isolation contact and the lower boundary of the tsunami deposit is about 30 cm long, which corresponds to a period of ~1350 years using the mean sedimentation rate of the upper lacustrine part of the core (unit C), and this is longer than the ~1200 year period inferred to have passed between the isolation and the tsunami from the more well-dated core STP107. From this we may conclude that the sedimentation rate in the lower lacustrine section of STP111 was most probably higher than the mean sedimentation rate of unit C in the same core, but also that there was likely very little erosion connected to the tsunami inundation at this coring site. As no ingression contact is observed in STP111, the conclusion that the lakes were not inundated by the Tapes transgression is strengthened. We do not know at which point in the tidal cycle the tsunami struck, which adds some additional uncertainty to our minimum run-up estimate. We assume that the isolation contact in Nerfloen represents the time when the outlet was raised above local high-tide, which is generally the case for all RSL estimates based on the isolation basin approach (see e.g. discussion in Lohne et al., 2007). At present the difference between astronomical high and low tide in Stryn is 233 cm. This means that if the tsunami occurred at extreme low tide, it would require an additional run-up of about 2.5 m in order to inundate Nerfloen. Theoretically, tide levels may have been somewhat different at 8150 cal. a BP as compared to the present. A lower eustatic sea-level in the North Sea may have influenced the tidal amplitude along the west coast of Norway somewhat (Hall and Davies, 2004), and a certain

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