Assessing the potential of Individual Foraminifera Analyses ( δ 18 O) to reconstruct variability, seasonality and extremes in the tropical Indian Ocean

Abstract. The Indian Ocean plays a critical role in the global climate system by regulating heat and moisture transport, influencing major fluxes that drive atmospheric circulation. Individual foraminiferal analyses (IFA) of the stable oxygen isotope composition (δ18O) of their test provides a powerful approach to reconstruct past climate variability beyond changes in mean conditions, with the potential to capture seasonal to interannual climatic extremes associated with phenomena such as the Indian monsoon and the Indian Ocean Dipole (IOD). However, field-based calibrations of this proxy remain sparse in the Indian Ocean, especially in the western part of the basin, limiting our understanding of how IFA records regional hydrographic signals. In this study, we combine a forward-modelling approach with IFA in Indian ocean core-top sediment samples to evaluate how δ18O variability responds to changes in temperature and salinity linked to seasonal and interannual variability. Using ocean reanalysis data, we simulate surface and thermocline conditions at several sites representative of different oceanographic provinces across the Indian Ocean and generate synthetic core-top IFA datasets through random sampling. We evaluate the sensitivity of results by artificially amplifying or reducing seasonal and interannual climate variability. Forward-modelled data are then compared with IFA data from core-tops at four key sites across the Indian Ocean. Our results show that surface-ocean individual foraminiferal δ18O changes in the western and central Indian Ocean are mainly controlled by temperature seasonality related to monsoon dynamics, whereas in the eastern Indian Ocean, seasonality is less pronounced and interannual variability is more dominant. On the other hand, thermocline δ18O variability for all sites is primarily associated with interannual variability and temperature changes, highlighting the potential of thermocline species to record interannual temperature variability. Forward-modelled IFA closely match real datasets from Late Holocene core-tops for surface and thermocline depths. This framework provides a basis for interpreting IFA in the Indian Ocean in terms of underlying climate processes and offers perspectives for reconstructing past seasonal and interannual climate variability in this important region.