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Reef islands accumulate as shallow landforms on or adjacent to reef complexes and are mainly composed of reef-derived carbonate sediment. Due to their unconsolidated and low-lying nature, reef islands are exposed to hydrodynamic processes, whereby they naturally experience erosion and accumulation. In order to compensate deficits of sediments lost during erosion, a sufficient supply of suitable carbonate sediment is required for accumulation. Under changing environmental conditions, the reefs are exposed to multiple stress factors that, among other things, can alter carbonate production and thus also threaten the stability of the reef islands. Hence, it is of importance to understand how past changes in the production of carbonate sediment have affected the evolution of reef islands systems, in order to assess potential future scenarios. Here we reconstruct the Holocene and recent sedimentological dynamics from one inhabited Indonesian reef island on the outer shelf of the Spermonde Archipelago, directly bordering the Strait of Makassar. We investigate the carbonate facies from sediment obtained from deep push cores up to 8 meters below surface and from shallow, hand-drilled boreholes. Targeted radiocarbon dating helps to reveal the temporal understanding of facies evolution and formation dynamics. Our data show that the island complex started to form in the mid-Holocene, around 6,400 years cal BP. The carbonate sediment from this time is almost exclusively composed of sand-sized coral fragments. Over time, the sediment composition becomes more diverse, with increased abundances of mollusks and algae, likely reflecting the ecological evolution of the reef. The shallow samples close to mean relative sea-level show radiocarbon ages of 4,000 to 3,000 years cal BP, suggesting that the initial island formed around this time and accreted in the following millennia. Of note, the proportions of the green algae Halimeda is even further increased in the youngest sediments. Overall, this suggests that the island has been able to continue growing despite changes in the ecosystems that provide the sediment, and that the increased production of green algae may have even promoted island accretion in general. Our study provides valuable insights into the dynamics of reef island development in the context of evolving reef systems and extends the understanding of Holocene island formation in the study area.

Reef islands in monsoonal regions undergo constant erosion and accumulation, making seasonally shifting morphologies part of their nature. Additionally, sea-level rise alongside climate change is thought to be incisive and challenge coastal communities. With multiple pressures acting on sediment-generating coral reef ecosystems, changes in sediment supply may force further response of these dynamic landforms. Here we present new sedimentological data from a larger reef island in the Spermonde Archipelago, Indonesia. By evaluating the subsurface data in the context of the island’s morphological behavior, we reconstruct its agile past and present. Based on remote sensing data complemented by reports of local citizens, we find the inhabited island to have tripled its surface area in the past century, however also losing more than 10% of surface area in the recent decades. The deeper sediments of the island are dominated by coral fragments, the youngest and uppermost sediments indicate the green algae Halimeda as dominating material contributor. Our study thereby (1) underlines the highly and far-reaching dynamics of reef islands and (2) suggests their adaptive potential to altering material budgets.

We surveyed the elevation and age (14C) of paleo sea-level indicators in five islands of the Spermonde Archipelago. We describe 24 new sea-level index points from fossil microatolls, and we compare our dataset with both previously published proxies and sea-level predictions from a set of 54 Glacial Isostatic Adjustment (GIA) models, using different assumptions on both ice melting histories and mantle structure and viscosity. We then investigate the implications of our data and models in terms of vertical land movements in the study area, with two main results.First, data from the heavily populated island of Barrang Lompo are significantly lower (ca. 80 cm) than those at all the other islands. In absence of instrumental data (e.g., GPS or tide gauges) in any of the islands, we advance the hypothesis that this difference may be due to groundwater extraction and loading of buildings on Barrang Lompo, that would cause this island to subside at rates in the order of ~3-11 mm/a.Second, Common Era data (0-400 a BP) seem to indicate that the islands in the archipelago may be affected by tectonically-driven vertical land motions in the order of -0.88±0.61 mm/a (1-sigma), albeit slight uplift cannot be excluded. Different assumptions on vertical land motions affect, in turn, the assessment of which GIA model shows the best match with Late Holocene (ca. 4-5 ka) sea level data. Tectonic stability or slight uplift would favor iterations of ANICESELEN (De Boer et al., 2014), while subsidence would cause the sea level data to fit better with iterations of ICE-6G (Peltier et al., 2015).ReferencesDe Boer, Bas, Paolo Stocchi, and Roderik Van De Wal. A fully coupled 3-D ice-sheet-sea-level model: algorithm and applications." Geoscientific Model Development 7.5 (2014): 2141-2156.Peltier, W. R., D. F. Argus, and R. Drummond. Space geodesy constrains ice age terminal deglaciation: The global ICE‐6G_C (VM5a) model. Journal of Geophysical Research: Solid Earth 120.1 (2015): 450-487.AcknowledgmentsThis project is funded by SEASCHANGE (RO-5245/1-1) and HAnsea (MA-6967/2-1) from the Deutsche Forschungsgemeinschaft (DFG), part of the Special Priority Program (SPP)-1889 "Regional Sea Level Change and Society". Parts of this study are under review in Climate of the Past (https://www.clim-past-discuss.net/cp-2019-63/)

Indonesia is a country composed of several thousand islands, many of them small, low-lying and densely inhabited. These are, in particular, subject to high risk of inundation due to future relative sea level changes. The Spermonde Archipelago, off the coast of Southwest Sulawesi, consists of more than 100 small islands. This study presents a dataset of 24 sea-level index points from fossil microatolls, surveyed on five islands in the Spermonde Archipelago and compares these new results with published data from the same region and with relative sea level predictions from different Glacial Isostatic Adjustment (GIA) models. The newly surveyed fossil microatolls are located around the islands of Tambakulu, Suranti (both ~ 60 km offshore of Makassar city), Bone Batang and Kodingareng Keke (both located in the center of the Archipelago) and Sanrobengi (located ~ 20 km south-southwest of Makassar). Results from the near- and mid-shelf islands indicate that relative sea level between 4 to 6 ka BP was less than one meter above present sea level. The only exception to this pattern is the heavily populated island of Barrang Lompo, where we record a significant subsidence when compared to the other islands. These new results support the conclusions from a previous dataset and are relevant to constrain late Holocene ice melting scenarios. Samples from the two outer islands (Tambakulu and Suranti) yielded ages spanning the Common Era that represent, to our knowledge, the first reported for the entire Southeast Asian region.