Composition of recent coral reef-derived sediments across the remote Chagos Archipelago

The production of carbonate sediments by coral reef organisms is essential for the formation and maintenance of tropical beaches and is especially important for low-lying islands. Since global coral bleaching events have substantial impacts on reef communities, even in areas characterised by minimal local human impacts, reef islands are highly vulnerable to anthropogenic climate change. Thus, with the ongoing degradation of coral reef ecosystems and
sea-level rise, better understanding of reef-derived sediment supply has become increasingly important. We therefore analyse the spatial distribution of the main sediment components across reef zones around six islands in the uninhabited northern atolls of the Chagos Archipelago, central Indian Ocean, and compare them with adjacent beach sediments. At three of these islands, local seabird guano-derived nutrient runoff has been found to enhance the productivity of some reef organisms. As this may influence their contribution to the sediment,
we analyse spatial variations in guano-derived nutrient impacts (i.e. sediment organic carbon and nitrogen contents, and δ15N ratios) and compare sediment composition around islands with high seabird densities to rat-infested islands with few birds. Initial results reveal elevated δ15N ratios around islands with high seabird densities, with peaks in the island and beach samples and declining values with increasing distance to shore. Quantifying the contribution of
carbonate producers to the sediments show a high variability in the abundance of the main components between different atolls. At sites in Salomon Atoll, the sediment across reef zones is mainly formed by coral fragments (around 50–72%), followed by molluscs (≈7–13%) and foraminifera (≈5–11%), and in minor amounts by Halimeda segments (0.7–15%), particularly on the outer reef slope (15–25 m depth). Sites in Peros Banhos Atoll are characterised by a
slightly higher contribution from Halimeda (3.5–21%), while at the Great Chagos Bank these calcifying green algae segments locally represent the main sediment component. When comparing the abundance of these components in reef and beach sediments, beach sediments across all atolls are primarily composed of corals and molluscs fragments, with additional contributions by foraminifera in some lagoon sites. Halimeda segments, however, are
underrepresented in beach sediments compared to the adjacent reef sediments. Detailed analyses of benthic foraminifera assemblages across the different atolls show a higher contribution to the lagoonal than seaward sediments, and a slight increase in abundance near islands with high seabird densities compared to islands with few birds. Ongoing analyses will further resolve whether seabird nutrient inputs influence the contribution of other carbonate producers to local sediments.