Geological inheritance controls reef–mangrove responses to Holocene sea-level change

Coral reef–mangrove systems record the coupled effects of relative sea-level change, sediment supply, and coastal surface processes, yet their long-term interactions remain poorly constrained. Here, we reconstruct the Holocene evolution of a reef–mangrove system on Belitung Island (Indonesia) by integrating sedimentary archives, geochronology, paleo–sea-level indicators, and numerical modeling. We document regressive coastal stratigraphy exposed in a drained tin mine and analyze a suite of ~3 m sediment cores collected along a nearshore-to-onshore transect. Radiocarbon dating of corals indicates nearshore reef initiation at ~2 m below present mean sea level between ~6.1 and 5.7 ka BP, followed by vertical reef accretion until ~4.3–3.8 ka BP. Fossil oysters provide independent paleo–sea-level constraints, recording a prolonged mid-Holocene relative sea-level highstand (~6.5–4 ka) at ~3 m elevation, followed by a relatively abrupt ~2 m fall and subsequent smaller-amplitude fluctuations. Optical stimulated luminescence (OSL) dating shows that mangrove colonization and terrestrial sedimentation initiated during Late Holocene shoreline progradation. We combine the coral radiocarbon and mangrove OSL ages within a Bayesian inversion framework coupled to a reef-growth model to reconstruct system evolution. We use model results to compare fluctuating mid-to-late Holocene relative sea-level scenarios with single-peaked highstand, and their respective effects on reef/mangrove architecture. These findings highlight how geological inheritance and non-monotonic boundary conditions govern sedimentary and ecological responses in tropical coastal systems, with implications for anticipating future landscape responses to sea-level change on the regional scale.