the other hand, the Chirp reflections have very similar characteristics to pelagic sediments we have worked on elsewhere (Mitchell, 1995b; Mitchell and ...
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Red Sea isolation history and sea level variations suggested by PlioPleistocene seismic reflection sequences - response to a reviewer comment A comment posted on researchgate.net questions whether the aragonite layers may instead have a tectonic origin, such as through fault movements leading to restriction of flow over Hanish Sill. This was addressed in part of our replies to reviewer comments. In case this is a wider concern or the issues discussed below are more generally interesting, we are including a slightly modified part of these replies alongside our article (please excuse the excessive number of self-citations).
Reviewer 3 has not elaborated on how he/she anticipates tectonics to have caused the reflections so we have had to consider all possibilities here. Concerning tectonics local to the data collection sites, in reviewing the Chirp data, we have found some isolated evidence for shallow slumping of deposits around Thetis Deep in these data, which could be interpreted as due to seismically triggered slope failure. Potentially, they relate to halokinetic movements, as there is clear evidence for salt flowage in multibeam data (Augustin et al., 2014; Mitchell et al., 2010). However, the character of the reflections away from those slumps is very unlike earthquakegenerated stratigraphy ("seismites" (Bull et al., 2005)), as they are smoothly continuous laterally, paralleling the seabed reflection, without major stratigraphic disruptions commonly seen in seismites. Folding of the evaporite surface from flowage is gentle in the seismic data, so it does not alter the reflection sequences. On the other hand, the Chirp reflections have very similar characteristics to pelagic sediments we have worked on elsewhere (Mitchell, 1995b; Mitchell and Huthnance, 2013; Tominaga et al., 2011). We have studied carbonates deposited on tectonically active mid-ocean ridges since the early 1990s (Mitchell, 1993, 1995a, b, 1996, 1998) and find no similarity between these Red Sea draping sediments and turbidites typical of remobilised deposits, which are typically flat lying (Mitchell et al., 1998). Overall, these deposits appear to have been merely rafted atop the mobile Miocene evaporites, with deformation locally only around faults or where the deposits have been oversteepened. Whether variations in Red Sea salinity, δ18O and sediment physical properties were affected by tectonic movements in the region of Hanish Sill (or volcanic processes, as the active Hanish Island lies adjacent to the sill) is an interesting question. The following map shows the Bouguer gravity anomaly field for this region, which forms part of a separate study. Hanish Sill (star symbol on map) lies over a 20 mGal positive anomaly lying between two trends of lows (one down the centre of the Red Sea here and one sub-perpendicular to it running parallel to Hanish Island). It seems that Hanish Sill overlies dense material that probably lies in the footwalls of the two sets of faults, if they are normal or have normal components, which seems likely given the extensional environment (although, as we mention in the article, the recent GPS work has revealed no present-day extension across the strait). Modest uplift of the sill was originally suggested by salinity impacts on benthic faunas (Rohling et al., 1998) and confirmed by the δ18O modelling of Siddall et al. (2003); this uplift could correspond with modest footwall uplift.
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There is insufficient information on fault displacement history and volcanic history of the Hanish Sill region to evaluate the sill modification history independently with data from there (and realistically given the political situation in the region and the location in sea lanes it is unlikely new data will be collected in the near future). Nevertheless, the inversion of the high-amplitude δ18O variations in sediments from the Red Sea has been shown to produce a relative sea level record (corresponding with water level over Hanish Sill) up to 500 ka (Grant et al., 2014; Rohling et al., 2010; Rohling et al., 2009; Rohling et al., 2013; Siddall et al., 2003; Siddall et al., 2004) that is extremely close (when corrected for isostasy and the steady uplift mentioned) to estimates of the eustatic variation produced independently (de Boer et al., 2012; Elderfield et al., 2012). We emphasize that the Red Sea RSL is derived from two well-known components (effect of evaporation on oxygen isotopes and the exchange fluxes in straits) so the Hanish Sill water depth is the only unknown (and hence derived) parameter. The fidelity of that model sea level record is strong evidence in our view that Hanish Sill has been stable (only modestly uplifting) as far back as 500 ka. The following figure from Rohling et al. (2014) compares relative sea level (RSL) at Hanish Sill (black) with RSL at the Gibraltar Sill (red) derived from δ18O data from the Mediterranean. If these variations were a result of coincident tectonic movements of the sills of the two basins, also coincident with variations in the deep-water δ18O data, this would be an incredible coincidence.
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Given the lack of continuous samples older than 500 ka, strictly speaking we cannot rule out sudden tectonic or volcanic changes as having led to the sedimentary properties responsible for Chirp reflection R5 or other deeper weaker reflections (we have updated the text to acknowledge this possibility). Nevertheless, as illustrated by the model in Figure 11, the sediment depth of R5 does coincide with the younger edge of MIS 16 if one assumes a simple continuous sedimentation at a comparable rate of deposition to the later sediments (MIS10 to 16 only 60% larger than MIS10 to present), so the simplest explanation is that it too is linked to sea level changes. To re-emphasize these points further, the following figure shows (upper graph) the Bintanja et al. and Rohling et al. sea level models (the latter is the Red Sea RSL corrected for the long-term steady uplift and, with the scale correction, for hydroisostasy). The difference graph shows features with 10s of ky in durations. It seems unlikely to us that these were created by a geological process at Hanish Sill (faults extending but then immediately reversing?!). More likely, small chronological differences of the two models, particularly during deglacials, analytical uncertainty and the smoothed character of the Bintanja et al. model, which is an artifact of their method, have created these differences.
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References cited Augustin, N., Devey, C.W., van der Zwan, F.M., Feldens, P., Tominaga, M., Bantan, R., Kwasnitschka, T., 2014. The transition from rifting to spreading in the Red Sea. Earth Planet. Sci. Lett. 395, 217-230. Bull, J.M., Minshull, T.A., Mitchell, N.C., Dix, J.K., Hardardottir, J., 2005. Magmatic and tectonic history of Iceland's western rift zone at Lake Thingvallavatn. Geol. Soc. Am. Bull. 117, 1451-1465. de Boer, B., van de Wal, R.S.W., Lourens, L.J., Bintanja, R., 2012. Transient nature of the Earth's climate and the implications for the interpretation of benthic δ18O records. Palaeogeog., Palaeoclim. Palaeoecol. 335-336, 4-11. Elderfield, H., Ferretti, P., Greaves, M., Crowhurst, S., McCave, I.N., Hodell, D., Piotrowski, A.M., 2012. Evolution of ocean temperature and ice volume through the Mid-Pleistocene Climate Transition. Science 337, 704-709. Grant, K.M., Rohling, E.J., Bronk Ramsey, C., Cheng, H., Edwards, R.L., Florindo, F., Heslop, D., Marra, F., Roberts, A.P., Tamisiea, M.E., Williams, F., 2014. Sealevel variability over five glacial cycles. Nature Communications Art. 5076, doi:10.1038/ncomms6076. Mitchell, N.C., 1993. A model for attenuation of backscatter due to sediment accumulations and its application to determine sediment thickness with GLORIA sidescan sonar. J. Geophys. Res. 98, 22477-22493. Mitchell, N.C., 1995a. Characterising the extent of volcanism at the Galapagos Spreading Centre using Deep Tow profiler records. Earth Planet. Sci. Lett. 134, 459-472. Mitchell, N.C., 1995b. Diffusion transport model for pelagic sediments on the MidAtlantic Ridge. J. Geophys. Res. 100, 19,991-920,009. Mitchell, N.C., 1996. Creep in pelagic sediments and potential for morphologic dating of marine fault scarps. Geophys. Res. Lett. 23, 483-486. Mitchell, N.C., 1998. Sediment accumulation rates from Deep Tow profiler records and DSDP Leg 70 cores over the Galapagos Spreading Centre, in: Cramp, A.,
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MacLeod, C.J., Lee, S.V., Jones, E.J.W. (Eds.), Geological Evolution of Ocean Basins: Results From the Ocean Drilling Program, Geol Soc spec publ. Geological Society, London, pp. 199-209. Mitchell, N.C., Allerton, S., Escartin, J., 1998. Sedimentation on young ocean floor at the Mid-Atlantic Ridge, 29˚N. Mar. Geol. 148, 1-8. Mitchell, N.C., Huthnance, J.M., 2013. Geomorphological and geochemical evidence (230Th anomalies) for cross-equatorial currents in the central Pacific. Deep-Sea Res. I. 78, 24-41. Mitchell, N.C., Ligi, M., Farrante, V., Bonatti, E., Rutter, E., 2010. Submarine salt flows in the central Red Sea. Geol. Soc. Am. Bull. 122, 701-713. Rohling, E.J., Braun, K., Grant, K., Kucera, M., Roberts, A.P., Siddall, M., Trommer, G., 2010. Comparison between Holocene and Marine Isotope Stage-11 sea-level histories. Earth Planet. Sci. Letts. 291, 97-105. Rohling, E.J., Foster, G.L., Grant, K.M., Marino, G., Roberts, A.P., Tamisiea, M.E., Williams, F., 2014. Sea-level and deep-sea-temperature variability over the past 5.3 million years. Nature 508, 477-482. Rohling, E.J., Grant, K., Bolshaw, M., Roberts, A.P., Siddall, M., Hemleben, C., Kucera, M., 2009. Antarctic temperature and global sea level closely coupled over the past five glacial cycles. Nature Geosci. 2, 500–504. Rohling, E.J., Grant, K.M., Roberts, A.P., Larrasoaña, J.-C., 2013. Paleoclimate variability in the Mediterranean and Red Sea regions during the last 500,000 years: Implications for hominin migrations. Current Anthropology 54, S183-S201. Siddall, M., Rohling, E.J., Almogi-Labin, A., Hemleben, C., Meischner, D., Schmeizer, I., Smeed, D.A., 2003. Sea-level fluctuations during the last glacial cycle. Nature 423, 853-858. Siddall, M., Smeed, D.A., Hemleben, C., Rohling, E.J., Schmeizer, I., Peltier, W.R., 2004. Understanding the Red Sea response to sea level. Earth Panet. Sci. Lett. 225, 421-434. Tominaga, M., Lyle, M., Mitchell, N.C., 2011. Seismic interpretation of pelagic sedimentation regimes in the 18–53 Ma eastern equatorial Pacific: Basin-scale sedimentation and infilling of abyssal valleys. Geochem. Geophys. Geosys. 12, Paper Q03004, doi:03010.01029/02010GC003347.
