Mangrove sediments reveal drivers of Late Holocene sea-level change in the equatorial Pacific Ocean

Proxy relative sea level (RSL) reconstructions offer context for historical observations and future projections. Unlike many sites in the equatorial Pacific Ocean where Late Holocene RSL fell from a high stand, the subsiding volcanic islands of Pohnpei and Kosrae in Micronesia experienced continuous RSL rise, allowing mangrove sediment accumulation. Through sedimentological, geochemical, and palynological techniques alongside radiocarbon dating, we reconstruct RSL over the past ∼3000 years. On Pohnpei, RSL rose 3.6 ± 0.17 m since 1270 BCE and on Kosrae RSL rose 1.9 ± 0.33 m since 310 BCE. Our findings indicate subsidence rates of 1.3 mm/y for Pohnpei and 1.1 mm/y for Kosrae, consistent with contemporary rates, but greater than expected based on current understanding of the lifespan of ocean islands. These new estimates do not significantly alter existing twenty-first-century RSL projections for Pohnpei but given the similarity in predicted and reconstructed RSL histories between Pohnpei and Kosrae, we anticipate that for planning purposes, future RSL rise at Kosrae will resemble that of Pohnpei. Our work integrates these reconstructions into an updated global mean sea-level curve, emphasizing the value of Late Holocene proxy data from under-represented regions. • We use mangrove sediment to reconstruct relative sea level over the past 3,000 years in Micronesia. • On Pohnpei and Kosrae, relative sea level rose at rates similar to contemporary island subsidence. • The rate of relative sea-level rise is greater than expected for the age of the ocean islands. • We update the Late Holocene global mean sea level curve.