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Supplementary Information: Materials for Radiometric Dating
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Consistent with all other studies of island evolution [e.g. Woodroffe et al., 2007; Kench et al.,
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2012; Kayanne et al., 2012] we radiometrically dated carbonate material to provide a
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temporal framework for the deposition of island sediments on Jabat. Radiometric dating of
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skeletal carbonate sediments can be problematic as a disparity may exist between the time of
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death of the organism and time of final deposition, which may occur after an undefined
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period of transport, breakdown, and mixing. In such instances, samples yield the earliest
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possible time of deposition. Consequently, interpretation of island accumulation using this
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dating approach must be undertaken with caution. Careful selection of samples, with respect
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to the degree of abrasion, is necessary to minimise the time lag between organism death and
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deposition and in order to evaluate which sediments may be more prone to time lags.
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In this study three types of skeletal materials were selected for analysis. Each type of skeletal
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material has different reliability with respect to the temporal lag between organism death and
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deposition.
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First, a set of five in situ coral samples were selected for analysis. These samples were
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retrieved via drilling of the reef flat surface and base of cores (Figure 2b, S3). The reef flat
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samples were interpreted as in situ corals based on the ‘intact’ condition of corallites, the
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orientation of corallite structures, orientation of the growth form within cores, and clear and
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horizontal boundary between the coral and underlying basement (Fig. S2). Dates on these
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corals provide an accurate indicator of the death of the organism and the in situ condition
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ensures there has been no post-mortem transport. In this study ages on in situ corals provide a
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temporal framework on final stages of reef growth.
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Second, seven cobble-size coral clasts were dated. In each case cobbles had intact corallite
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structures and clasts were not rounded. As shown by Ford and Kench [2012] corallite
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structures are readily worn down in the initial stages of transport in littoral systems. Such
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characteristics indicate the clasts have not undergone significant post-mortem littoral
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transport. Furthermore clasts dated showed an absence of secondary erosion and boring as
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may be expected if cobbles had been lying inert on the deeper fore reef for a considerable
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period of time between death and transport onto the reef platform. Consequently, it is
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thought that deposition of these clasts occurred rapidly in a high energy event or sequence of
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events and that the time between organism death and deposition is short. In this study ages on
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the coral cobbles provides an age framework for deposition of the underlying gravel
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basement within the island.
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Third, 13 ages on sand-size materials were obtained. Woodroffe et al. [2007] review the
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difficulties in dating sand-size skeletal grains and concluded that single constituent dating
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may provide a more reliable indicator of the time of deposition. In their study Woodroffe et
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al. [2007] used foraminifera and molluscan grains to provide a chronological account of the
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accumulation of Warraber Island. The premise of this approach is to use those components
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within deposits that appear to be intact and have undergone very little abrasion. However, the
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selection of single grains can also lead to considerable error in defining a time of deposition.
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Samples selected for analysis in this study were rich in the foraminifer Calcarina sp. These
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components were preferentially selected for dating as they preserved near intact tests as
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opposed to coral and coralline algal grains. Of the three types of material chosen for dating it
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is expected that temporal lags would be more apparent for the sand-size sediments. It is
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important to stress that ages on sand-size materials are used in this study to define a general
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window within which the accumulation of sand-size materials occurred on Jabat, rather than
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provide a definitive chronology of sedimentation. However, several pieces of evidence
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suggest that the ages can be used to construct a maximum age of accumulation of sand-size
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sediments within the island. First, foraminifera selected for analysis generally had near
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pristine tests. Second, results reveal a consistent ages and a lack of inversions.
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References
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Ford, M. R. and P. S. Kench (2012), The durability of bioclastic sediments and implications
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for coral reef deposit formation, Sedimentology, 59(3), 830-842.
Kayanne, H.T., T. Yasukochi, T. Yamaguchi, and H. Yamano (2011), Rapid settlement of
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Majuro atoll, central Pacific, following its emergence at 2000 years Cal BP, Geophys Res
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Let, 38, L20405, doi:10.1029/2011GL049163.
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Kench, P.S., S.G. Smithers, and R.F. McLean (2012), Rapid reef island formation and
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stability over an emerging reef flat: Bewick Cay, northern Great Barrier Reef, Australia,
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Geology, 40, 347–350.
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Woodroffe, C.D., B. Samasorn, Q. Hua, and D. Hart (2007), Incremental accretion of a sandy
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reef island over the past 3,000 years indicated by component-specific radiocarbon dating,
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Geophys Res Lett, 34, L03602, doi:10.1029/2006GL028875.