Relative sea-level change in South Florida during the past ~5000 years

A paucity of detailed relative sea-level (RSL) reconstructions from low latitudes hinders efforts to understand the global, regional, and local processes that cause RSL change. We reconstruct RSL change during the past ~5 ka using cores of mangrove peat at two sites (Snipe Key and Swan Key) in the Florida Keys. Remote sensing and field surveys established the relationship between peat-forming mangroves and tidal elevation in South Florida. Core chronologies are developed from age-depth models applied to 72 radiocarbon dates (39 mangrove wood macrofossils and 33 fine-fraction bulk peat). RSL rose 3.7 m at Snipe Key and 5.0 m at Swan Key in the past 5 ka, with both sites recording the fastest century-scale rate of RSL rise since ~1900 CE (~2.1 mm/a). We demonstrate that it is feasible to produce near-continuous reconstructions of RSL from mangrove peat in regions with a microtidal regime and accommodation space created by millennial-scale RSL rise. Decomposition of RSL trends from a network of reconstructions across South Florida using a spatio-temporal model suggests that Snipe Key was representative of regional RSL trends, but Swan Key was influenced by an additional local-scale process acting over at least the past five millennia. Geotechnical analysis of modern and buried mangrove peat indicates that sediment compaction is not the local-scale process responsible for the exaggerated RSL rise at Swan Key. The substantial difference in RSL between two nearby sites highlights the critical need for within-region replication of RSL reconstructions to avoid misattribution of sea-level trends, which could also have implications for geophysical modeling studies using RSL data for model tuning and validation. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version Authors were supported by National Science Foundation Awards (OCE-1702587, OCE-1831450, and OCE-2002437 to Kopp and Ashe; OCE-2002431, OCE-1458921, OCE-1831382, and OCE-1942563 to Kemp; OCE-1458903 to Engelhart). RPM is supported by the US Fish and Wildlife Service’s State Wildlife Grants Program (#F13AF00982). BPH is supported by the Singapore Ministry of Education Academic Research Fund MOE2019-T3-1-004 and MOE-T2EP50120-0007, the National Research Foundation Singapore, the Singapore Ministry of Education under the Research Centers of Excellence Initiative.