From coral to Halimeda: Green algae as an important factor for Holocene reef island formation in Southeast Asia

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.