Reconstructions of relative sea-level from geological archives such as coastal sediment offer insight into sea-level changes on a range of geographic (local, regional, and global) and time (years to millennia) scales. These reconstructions can be leveraged to constrain the long-term, pre anthropogenic components of sea-level change, which must be quantified to produce projections of future sea-level rise needed to build resilience in coastal communities. Observational records are too short to perform this task and existing reconstructions spanning the past ~5000 years are primarily located at sites around the North Atlantic Ocean. There is a scarcity of records from the tropical Pacific Ocean, in part due to a lack of sites with suitable coastal sediment. Confounding the lack of data in the tropical Pacific Ocean is the heightened vulnerability of small island nations to the consequences of sea level rise. This project will produce near-continuous reconstructions of relative sea-level change during the past ~5000 years using sediment that was deposited in mangrove swamps in the Federated States of Micronesia (FSM; Chuuk, Pohnpei, Kosrae). Incorporation of the results into a statistical framework will allow regional-scale projections of relative sea level change in FSM during the 21st and 22nd centuries to be developed. These projections will be shared with stakeholders in FSM (e.g., federal and state government agencies and non-governmental organizations) to help plan for future sea level rise. Multi-disciplinary training will also be provided to undergraduate and graduate students and a postdoctoral scientist.Mangrove swamps have a systematic and quantifiable relationship to tidal elevation, as do micro-organisms such as foraminifera that live on the sediment surface in mangroves and leave a fossil record. This ecological preference for specific elevations makes mangrove sediment and foraminifera valuable sea-level proxies. As relative sea level rises, mangrove swamps accumulate sediment to preserve their elevation in the tidal frame. Over 100s to 1000s of years, multi-meter sequences of sediment are deposited that can be collected and studied to reconstruct relative sea level. In much of the tropics, however, long-term sea-level fall prohibits the accumulation of sediment. Crucially, long-term tectonic subsidence in FSM has resulted in near-continuous accumulation of mangrove peat over the past ~5000 years. This project will recover sediment cores in FSM and establish a history of sediment accumulation through radiocarbon dating and recognition of pollution and land-use changes of known age in downcore elemental and isotopic profiles. Foraminifera preserved within the buried sediment will be used to estimate the height of former sea-level through quantitative comparison of fossil assemblages with modern analogs. The new reconstructions will be combined with an existing (and updated) database of late Holocene relative sea-level reconstructions to estimate global mean sea-level trends using an existing spatio-temporal model. This model will also facilitate partitioning of the relative sea level signal into its constituent parts to better understand the causes of local, regional, and global trends on short (decadal) and long (multi century) timescales. Using a statistical framework that incorporates multiple sources of uncertainty and accounts for simultaneous and co varying contributions from regional scale processes (e.g., meltwater fingerprints and/or ocean dynamics), this research will produce relative sea-level projections for FSM that will be shared directly with key local stakeholders.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
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