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SI APPENDIX Giant Boulders and Last Interglacial Storm

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SI APPENDIX Giant Boulders and Last Interglacial Storm SI APPENDIX Giant boulders and Last Interglacial storm intensity in the North Atlantic Authors: A. Rovere, E. Casella, D. L. Harris, T. Lorscheid, N.A.K. Nandasena, B. Dyer, M.R. Sandstrom, P. Stocchi, W.J. D’Andrea, M.E. Raymo Please address correspondence to: [email protected] [email protected] Contents Glossary and Synopsis of results ...................................................................................................................... 4 Description of the study area ............................................................................................................................. 6 Bathymetry and Topography .............................................................................................................................. 9 Boulder size, density and volume .................................................................................................................... 11 Hydrodynamic models ....................................................................................................................................... 16 Perfect Storm (1991) ......................................................................................................................................... 20 Hurricane Andrew (1992) .................................................................................................................................. 24 Hurricane Sandy (2012) .................................................................................................................................... 30 XBeach (1D model) ........................................................................................................................................... 33 XBeach model set up ........................................................................................................................................ 34 Cow and Bull ....................................................................................................................................................... 35 Modern analog ................................................................................................................................................... 37 Threshold flows for boulder transport ............................................................................................................. 37 LIG Relative sea level ....................................................................................................................................... 42 Additional discussions ....................................................................................................................................... 49 References .......................................................................................................................................................... 53 Index of Tables Table S1. Names of relevant topographic features, meteorological events or other definitions used both in the main text and in the Supplementary Text. Table S2. Summary of results obtained in this study. Table S3. Workflow, parameters and commands used in Agisoft Photoscan to calculate 3D scenes from drone and pole photographs. Table S4. Results of boulder density calculations. Table S5. Calculations of paleo RSL from the elevation of RSL indicators measured in the field and from GIA models. For more details of the formulas used to calculate RSL and δRSL from the elevation of a RSL indicator, see Rovere et al. (2016). All measures are in meters. Details on the GIA models can be found in Lorscheid et al (2017). 2 Index of Figures Fig.S1 Study area. Fig.S2 Results of topographic surveys. Fig.S3 SfM workflow. Fig.S4 Results of the SfM workflow. Fig.S5 Orthorectified photographs of the Cow and Bull boulders Fig.S6 2D model grids. Fig.S7 Input to the 2D wave model for the 1991 Perfect Storm. Fig.S8 2D wave model results for the 1991 Perfect Storm. Fig.S9 Input to the 2D wave model for the 1992 Hurricane Andrew. Fig.S10 Additional model results for Hurricane Andrew. Fig.S11 2D wave model results for the 1992 Hurricane Andrew. Fig.S12 Input to the 2D wave model for the 2012 Hurricane Sandy. Fig.S13 2D wave model results for the 1992 Hurricane Sandy. Fig.S14 Examples of water levels and maximum wave-generated flow velocity against the Cow and Bull cliffs in different RSL scenarios (0-3-6-9-12-15m) and for each modeled event. Fig.S15 Flow velocities produced by each swell on the face of the Cow and Bull cliff in all RSL scenarios Fig.S16 Flow velocities calculated at the modern analog. Fig.S17 Modern analog flow velocities used for the calculation of the coefficient of lift. Fig.S18 Probability density function derived from the solution of Eq.3 and Eq.4 for the two modern analog boulders. Fig.S19 Probability (cumulative density) of boulder transport vs flow velocity. Fig.S20 RSL indicators in forereef environment. Fig.S21 Sedimentary RSL indicators. Fig.S22 Ensamble of GIA model predictions for Eleuthera throughout MIS 5e. 3 Glossary and Synopsis of results In Table S1 we summarize the terminology used throughout the paper and this SI Appendix file. Table S1. Names of relevant topographic features, meteorological events or other definitions used both in the main text and in the Supplementary Text. Name Description Glass Window This is the topographic name of the isthmus connecting the central and the northern parts Bridge of the island of Eleuthera. In its narrowest part, the strip of land connecting the two sides of the island is only few meters wide. The ‘Cow and The ‘Cow and Bull’ are the local names of two among seven mega-boulders identified Bull’ boulders along the Glass Window Bridge cliffs and interpreted as deposited during ‘paleo- superstorms’ (1). These boulders are located on the top of the Glass Window Bridge cliffs, and correspond to Boulder n.1 (the Bull) and Boulder n.2 (the Cow) reported by Hearty (1997) (1) (see their Table 2 and their Fig.7). Modern analog The modern analog boulders reported in this study were measured north of the ‘Cow and boulders Bull’ mega-boulders, in proximity of the Glass Window bridge that gives the name to the entire area. These boulders are smaller than the ‘Cow and Bull’, and historical accounts report that they have been ripped from the cliffs by the waves of Hurricane Andrew, August 1992. Superstorm The ‘paleo-superstorm’ term has been introduced in the last interglacial climate debate by Hansen et al. (2015) (2) to indicate strong late-Eemian (MIS 5e) storminess. This definition derives from the findings of Hearty (1997) (1), who suggested that chevron ridges, runup deposits and giant boulders in the Bahamas and Bermuda were deposited by ‘massive storms […] much larger than those occurring during the Holocene’ (1). This concept was also stressed in a follow-up paper by Hearty et al. (1998) (3), where the authors affirm that chevron ridges, runup deposits and boulders were created by ‘larger and more frequent cyclonic storms in the North Atlantic than those seen today’ (3). In all these papers, “superstorms” are related to modern winter cyclonic storms generating near-hurricane wind forces (e.g. the 1991 ‘Perfect Storm’). The hypothesis that “superstorms” might intensify in future warmer climates is related to increased water vapor due to the warming of low- latitude oceans, eventually combined with a ‘cooler North Atlantic Ocean from AMOC slowdown and an increase in mid- latitude eddy energy causing more severe baroclinic storms. Increased high pressure due to cooler high-latitude ocean […] can make blocking situations more extreme, with a steeper pressure gradient between the storm’s low- pressure center and the blocking high, thus driving stronger North Atlantic storms’ (2). In another recent paper, Hearty and Tormey (2017) (4) reinforce the notion that the field evidence from the Bahamas and Bermuda has been emplaced by “superstorms”. Structure-from- Structure from motion (SfM) is a range of imaging techniques used to estimate three- Motion dimensional structures from sequences of two-dimensional images (5) Boulder Equations deriving minimum wave flow velocity necessary to transport boulders either by transport lifting or rolling from balances of forces and moments, cliff type and pre-transport settings equations of boulders (6) Wave flow The flow velocity as calculated by SWAN or XBeach. The parameter extracted from the velocity model to represent flow velocity is the u vector of the Eulerian velocity in cell center. 4 In Table S2 we present an overview of the results of this study, which are explained in detail in the subsections below. The results of our 1D and 2D models are available in Pangea at the following link: https://doi.pangaea.de/10.1594/PANGAEA.880687. Table S2. Summary of results obtained in this study. Boulder size, density and volume Axes [m] a b c The Cow 9.4 5.2 3.8 186 2.06 383 The Bull 12.5 6.9 5.2 449 2.06 925 Modern 2.6 2 1.2 6.2 2.06 13 analog 1 Modern 6.7 1.7 1.4 15.9 2.06 33 analog 2 Hydrodynamic models Relative sea Significant wave height (Hs) [m] Smoothed peak period (Tp) [s] level [m] RSL 1991 1992 2012 1991 1992 2012 0 4.7 7.4 9.2 17.0 11.0 12.1 3 4.7 7.7 9.4 17.0 11.5 12.1 6 4.7 8.0 9.5 16.9 11.7 12.1 9 4.7 8.2 9.7 16.9 11.8 12.1 12 4.7 8.6 9.8 16.9 11.9 12.1 15 4.8 10.2 10.1 16.8 12.1 12.1 Relationship between RSL and Hs / Tp (Used in XBeach) Event This field is derived from the third-order polynomial interpolation of the data shown in the section above (see also Fig.S8, S11 and S13) Hs = 0.00007(RSL)3 - 0.0012(RSL)2
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