Carbon cycling and environmental transitions: insights from sediment core analyses in sepetiba bay mangrove
Authors: Vieira Pereira Valleria , Hamacher Claudia , dos Reis Antonio Tadeu , Oliveira Farias Cassia , Passos Araújo Michelle , Gomes Soares Mário Luis , Barbosa dos Santos Sonia , de Oliveira Chaves Filipe , Guizan Silva Cleverson , Coimbra Abreu Ana Clara , Bouloubassi Ioanna 
Published: March 2024

To comprehend the historical distribution of organic matter (OM) leading to its role as a source in the soils of coastal bays is crucial for unraveling environmental changes and the consequent biogeochemical processes that occurred during the evolution of these regions. Fluctuations in the concentration of stable isotopes of carbon and nitrogen, coupled with radiocarbon dating of sediment cores, enable the reconstruction of environmental characteristics prevalent at the time of sedimentation, contributing to the development of a better understanding of the environmental evolution. In order to comprehend the contributions to the preserved OM in sediments on the eastern portion of Sepetiba Bay (BS), Rio de Janeiro, Brazil, two field campaigns were conducted in the year of 2022 (Jan/2022 and Feb/2022) to collect three sediment cores using a corer along a transect perpendicular to the coast. The cores, measuring 250 cm, 360 cm, and 540 cm were collected from a mangrove forest (G2), a tidal flat (G4), and a Sarcocornia marsh (G5), respectively. In the laboratory, the cores were sectioned at 5 cm intervals, from which every 5 cm were sampled and taken for elemental and isotopic determinations (organic carbon, total nitrogen, elemental carbon-to-nitrogen ratio [(C/N)], and isotopic ratios of carbon (&#948;13C) and nitrogen (&#948;15N)). Malacological material (bivalve and gastropod shells) found in the cores was identified, and samples were selected for radiocarbon dating. Elemental and isotopic determinations were performed using a Thermo Scientific Flash 2000 CHN Elemental Analyzer. The identified shells mainly belonged to the Anomalocardia brasiliana species, a characteristic bivalve of shallow clay-sand bottoms. All three cores showed maximum ages between 6500- and 7000-years Cal, based on 14C dating. Isotopic and elemental analysis values revealed records indicating alternating dominance of marine and terrestrial OM sources along the cores. At this time, authors estimate, based on curves created using radiocarbon dating, that the relative sea level was 5m higher than it is today, subjecting the area to inundation. Starting from 6000 years BP, the sea level begins to steadily fall until reaching its current level. Relating these facts to possible environmental conditions and &#120517;13C data, it can be considered a possibility that this environment transitioned from being flooded to exposed intermittently, and the shift in sediment input from fluvial to estuarine increased salinity and enabled the establishment of coastal vegetation, like mangrove. As can be inferred on the &#120517;13C data, going further into the depth we can see a most marine signature for de 13C, which can indicate the increasing of marine contribution to the sediment. &#160;These results may be associated with the evolution of BS and the gradual closure of the sandy barrier, leading to the establishment of estuarine characteristics in a more isolated environment, transitioning from a predominance of marine OM sources to fluvio-estuarine influences. Additionally, the influence of shorter-duration dry and wet periods was observed, correlating with variations in &#948;13C values obtained throughout the sedimentary record.