Calibrating Individual Foraminifera Analysis for Climate reconstruction in the Western Indian Ocean: Assessing Seasonal and Interannual Variability

The Indian Ocean is affected by seasonal and interannual climate variability, with large consequences on the water cycle over adjacent landmasses. The northern Indian Ocean influences the African and Asian monsoon systems, and its seasonal and interannual variability interacts with El Niño-Southern Oscillation (ENSO) in the Pacific Ocean, potentially triggering Indian Ocean Dipole (IOD) development. Future projections suggest that Indian Ocean modes of climate variability could be enhanced under warming scenarios, but a lack of past records precludes a deeper understanding of how the Indian Ocean modes of variability could change over evolving boundary conditions.

Due to low sediment accumulation rates and bioturbation in marine cores, bulk analyses cannot usually record past climate variability at seasonal or interannual timescales. In contrast, individual foraminifera analyses (IFA) can theoretically estimate the total variance in a population of foraminifera that experienced upper ocean variability at sub-centennial timescales. Geochemical records (δ¹⁸O, Mg/Ca) based on IFA have been used to reconstruct past climate variance including seasonality, ENSO in the Pacific Ocean, and IOD in the eastern Indian Ocean. However, using variance in a foraminifera population to pinpoint which mode of variability is the ultimate driver remains a challenge.

Here we model the range of variability of different key sites in the Indian Ocean to seasonal vs interannual climate variability using ORAS-5 re-analysis temperature and salinity data spanning the period from 1958 to 2018, and to assess the impact of changing the amplitude of these modes of variability on total δ¹⁸O variance of model foraminifera populations. In light of these results, we then analyzed IFA (δ¹⁸O and morphology) in three core-top samples, targeting two planktonic foraminifera species with different seasonalities and depth habitats: Ocean Drilling Program Site 722 in the Arabian Sea upwelling region, monsoon-influenced Site MD77-191 near the southern tip of India, and Site MD96-2060 core offshore Tanzania in the western Indian Ocean. Results will allow us to better evaluate how to interpret IFA in different oceanographic biomes of the Indian Ocean, ultimately leading to a better understanding of past climate extremes embedded in paleoclimate record.