Coral reef and mangrove interplay during Holocene sea-level changes, 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 reefs and mangroves in response to Holocene sea level changes, taking Belitung

Island, Indonesia, as a case study. We described the regressive stratigraphy of an out-

crop in a drained tin mine and collected a series of 3-m-long cores from three study

sites, which suggests 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 kilometre-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 allows us to derive general behaviour laws based on the determin-

istic role of geological history in the development of coastal ecosystems. By shedding

light on the complex connection between coral reefs, mangroves and sea level

changes over geological timescales, these results outline the need to analyse the past

evolution of mangrove forests and coral reefs as a unique system to predict their

future evolution.