Mangrove Paleoecological Responses During Holocene Sea Level Changes: A Multiproxy Aapproach

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Mangroves (coastal marine forests composed of a number of different tree species) exist in the tropics and sub-tropics at the transition between the ocean and terrestrial environment; a setting that is particularly sensitive to environmental changes such as sea-level rise. Many coastal environments depend on mangrove habitats to reduce erosion, buffer the influence of hurricanes and provide habitats for marine organisms. There is concern about the response of mangroves to future sea-level rise scenarios. However, laboratory and field experiments involving mangroves provide a limited temporal and spatial perspective of mangrove responses to environmental change. Our proposed research presents a novel approach to study past mangrove responses to environmental change throughout the Holocene (the last ~10,000 years), a period during which sea-level is known to have risen. The continuous deposits of mangrove peat present in Belize are the largest in the world and provide an exceptional resource to study past mangrove ecosystems over this period. A number of peat cores have already been collected and will be analyzed during this proposed project. We propose analyzing cores from three different sites to develop a robust picture of regional past mangrove ecology. One core (~8 m in depth) is from Twin Cays and a second core (~8 m in depth) is from Tobacco Range. These sites are ~12 km off of the mainland coast of Belize and are therefore less liable to be influenced by mainland tropical plant species. In addition, by analyzing a core (~6 m in depth) taken from coastal mangroves on the mainland, we will be able to compare and contrast coastal and island mangrove responses to Holocene sea-level changes. Ecosystem changes will be established by studying multiple lines of evidence (multiple proxies of the past vegetation) preserved in these peat cores, which are dated using radiocarbon dating. Peat cores will be analyzed for proxies that include counts of fossil pollen to track changes in vegetation composition, together with bulk and compound specific stable isotopic (C and N) techniques that will monitor physiological changes in mangroves with time. By analyzing a suite of proxies at each site, we will be able to develop detailed histories describing important aspects of these ecosystems, including factors such as species composition, plant morphology and structure, nutrient status and availability, peat accumulation rates, and physiological stress. By analyzing multiple sites, and multiple cores within a site, we will be able to distinguish between site-specific and regional trends, which will allow us to determine a robust assessment of overall ecosystem changes. In turn, these data will be compared with the framework of Holocene sea-level change for the region. We believe that our sites are among the best available to develop long-term records of mangrove ecosystems. Ultimately, knowledge of how these ecosystems responded to past environmental change is important in assessing how tropical ecosystems will function in response to future environmental changes.