New Stratigraphic Evidence from the Equatorial Indian Ocean across the Eocene-Oligocene Transition: A Carbonate Deposition Response to Orbital Forcing

The Eocene-Oligocene Transition (EOT, ∼34 Ma) is the most profound global cooling event of the Cenozoic, marking a fundamental transformation from a predominantly ice-free to an icehouse climate state. Although the EOT is well documented globally, its sedimentary records, chronostratigraphic framework and driving mechanisms in the low-latitude equatorial Indian Ocean, a region influenced by the Tethyan realm, remain poorly constrained. The unique tectonic setting of this region renders it a critical window for understanding the responses of the global ocean-atmosphere system to Paleogene climatic changes. Here, based on ODP Sites 709 and 711, we present the first high-resolution chemostratigraphic and cyclostratigraphic framework for the EOT in the pelagic equatorial Indian Ocean. Through new bulk carbonate carbon and oxygen isotope analyses, we identify the characteristic positive isotopic excursions that define the EOT, including the Oi-1 glacial maximum. This chemostratigraphic framework is integrated with existing calcareous nannofossil record to establish a solid chronostratigraphy. A key finding is that carbonate deposition in this region began to increase significantly prior to the main EOT event, an early response signal often obscured by intense dissolution in other deep-sea settings. Furthermore, spectral analysis of XRF-derived Ca data reveals that the sedimentary record across the EOT is dominated by the 405-kyr long eccentricity cycle. Notably, the onset of the EOT is coupled with an eccentricity minimum, suggesting a critical role for orbital forcing in triggering this major climate transition. These findings not only provide an essential stratigraphic anchor for future paleoceanographic studies in the Indian Ocean but also raise fundamental questions about the drivers of carbonate deposition. The pre-EOT increase in carbonate burial, coupled with the positive 𝛿𝛿13C excursion, strongly implies either a significant enhancement in primary productivity or a major shift in ocean chemistry that preceded the main phase of Antarctic glaciation.