Tracing the stepwise warming trend in the tropical Indian Ocean through a 40-year record of oxygen isotope composition in Maldives corals

Sea surface temperature (SST) across the tropical Indian Ocean is warming at a significantly higher rate than other tropical oceans over the past two decades. However, the cause of the abnormal warming remains unclear due to the short duration of instrumental observations as well as sparse long-term paleoclimate proxy records (e.g., from corals) within this region. Before coral-based records can be compiled to infer long-term environmental change in the region, an assessment of the suitability and robustness of the various coral archive types for evaluating warming must be made. Here, we present a 40-year coral stable oxygen isotopic ratio (δ¹⁸O) record derived from both dome-shaped (i.e., massive) and microatoll corals (Porites sp.) from the southern Maldives, located in the equatorial Indian Ocean. We found a consistent δ¹⁸O reproducibility in both dome-shaped and microatoll corals, demonstrating the viability of utilising a range of coral morphotypes for reconstructing long-term ocean conditions and climate change. Statistical analysis indicates that our coral δ¹⁸O values are primarily influenced by SST rather than sea surface salinity (SSS). Notably, the δ¹⁸O record exhibits a stepwise shift, with average δ¹⁸O values of -4.93 ‰ and -5.01 ‰ for the periods 1978–1999 and 2000–2019, respectively. This shift in δ¹⁸O corresponds with the increase in regional SST, despite its relatively small magnitude of approximately 0.3 °C. Furthermore, the negative shift in δ¹⁸O values after 2000 coincides with the transition to La Niña-like conditions and the negative Interdecadal Pacific Oscillation (IPO) phase in the tropical Pacific Ocean. Under these conditions, the geostrophic transport of the Indonesian Throughflow (ITF) was enhanced, which likely contributed to warming in the tropical Indian Ocean by bringing in more warm water from the Pacific. Therefore, the consistent shifts between our Maldives coral δ¹⁸O anomalies and instrumental SST anomalies support the argument that an intensified ITF may have contributed to the abnormal warming in the tropical Indian Ocean over the past two decades. Our findings suggest that microatoll corals in well-flushed open ocean environments can provide robust climatic proxy data comparable to dome-shaped corals. By compiling these records, we show the primary impact of SST on coral δ¹⁸O values in the tropical Indian Ocean, evidenced by a notable shift closely aligned with regional SST fluctuations around 2000.