Coral reef and mangrove interplay during Mid- and Late-Holocene sea-level changes in Belitung Island, Indonesia

Tubagus Solihuddin, Gino de Gelder , Dwi Amanda Utami, Frida Sidik, Rima Rachmayani, Marfasran Hendrizan, Sri Yudawati Cahyarini, Meggi Rhomadona Purnama, Mary Elliot, Laurent Husson

Coral reefs and mangrove forests are susceptible coastal ecosystems, particularly in light of intense human-induced pressures, including the anticipated sea-level rise in the 21st century. Here, we explore the chronology and interconnectedness between coral reef and mangrove interplay in response to Holocene sea-level changes, taking Belitung Island, Indonesia, as a case study. We describe the regressive stratigraphy of an outcrop in a drained tin mine and collected a series of 3 m long cores from three study sites, which suggest that the nearshore reef establishment was taking place at ∼2 m elevation below the present mean sea level by approximately ∼6.1–5.7 kyr BP. After a phase of vertical reef accretion until ca. 4.3–3.8 kyr BP, the reef system—from the forereef to onshore beaches— changed to a horizontally prograding system under conditions of relative sea-level fall. The shoreline accordingly prograded seaward, leaving kilometer-wide platforms constructed by the antecedent Mid-Holocene coral reefs, thereby providing accommodation space for terrestrial sediment deposition and mangrove forests that thrived from the Late Holocene onwards. The case study of Belitung Island permits us to derive general behavior laws based on the deterministic role of geological history in the development of coastal ecosystems. By shedding light on the intricate relationship between coral reefs, mangroves, and sea level changes over geological timescales, these results outline the need to analyze the past evolution of these coupled systems to predict their future evolution.