Carbonate Compensation Depth and Carbonate Carbon Flux in the Indian Ocean over the Cenozoic

The Indian Ocean, a crucial component of the global thermohaline circulation with a carbonate saturation state intermediate between the Atlantic and Pacific oceans, plays a vital role in climate variability. It serves as a major sink for atmospheric carbon dioxide (CO₂), sequestering approximately 20% of the world's anthropogenic carbon. However, a major gap exists in understanding the deep carbon cycle of the Indian Ocean because the evolution of deep-sea carbonate carbon reservoirs, as a key contributor to the long-term global carbon cycle, remains unknown across this ocean over the Cenozoic. Here, we present new regional carbonate compensation depth (CCD) reconstructions incorporating dynamic topography and eustasy impacts to quantify the storage and fluxes of carbonate carbon to the Indian seafloor since the early Cenozoic. The CCD is defined as the water depth at which carbonate supply from the surface is balanced with its dissolution, leading to the absence of carbonate components below the CCD. Due to the complexity of carbonate distribution across the Indian Ocean, we model the Cenozoic CCD across six regions: western North Indian, western and eastern equatorial Indian, western and eastern South Indian, and the Indian sector of the Southern Ocean. Utilizing updated age models and backtracking with lithology-specific decompaction from 118 deep-sea drill sites (DSDP, ODP, and IODP expeditions), we compute the CCD through a linear reduced major-axis regression of the carbonate accumulation rate (CAR) versus paleo-water depth. The regression analysis is carried out in 0.5 My time intervals. Our results illustrate distinct CCD patterns across the Indian Ocean, fluctuating regionally by ~1.5–2.5 km over the Cenozoic. The western equatorial Indian shows a long-term deepening trend from ~2.7 km at 44 Ma to ~4.9 at present, while the eastern equatorial maintains a deep CCD fluctuating between ~4.2 km and ~4.8 km since 19 Ma. The relatively shallow CCD of the Indian sector of the Southern Ocean, between ~2–4 km since 43 Ma, experiences pronounced variability across the Indian Ocean, indicating significant oceanographic changes and the complexity of diverse factors influencing the carbonate system in this high-latitude region. The highly variable CCDs across the Indian Ocean result in substantial regional heterogeneity in carbonate carbon flux corresponding to distinct oceanography characteristics such as deep-water carbonate chemistry and gradients of carbonate rain rate. The regional CCD models for the Indian Ocean are utilized to estimate the evolution of deep-sea carbonate carbon reservoir across the entire Indian during the Cenozoic in the context of the long-term global carbon cycle.