Similar Tropical Upper-Ocean Temperatures in the Late Miocene and Pleistocene Interglacials

The late Miocene cooling (LMC; ~7–5.4 Ma) represents a major global climate transition associated with declining atmospheric CO₂, large-scale aridification, and reorganization of terrestrial ecosystems. While cooling at high and mid latitudes during this interval is well documented, temperature changes in the tropical oceans appear muted. Existing tropical sea-surface temperature (SST) reconstructions based on alkenone unsaturation (UK’37​) are limited by proxy saturation at ~29 °C, potentially leading to an underestimation of tropical warmth. Here, we investigate tropical upper-ocean temperature evolution during the late Miocene using coccolith clumped isotope (∆47) thermometry, which reflects habitat-depth temperatures rather than regressed SSTs.

We present new coccolith ∆47 temperature records from ODP Site 926 (Ceara Rise) spanning the late Miocene and compare them to late Pleistocene glacial and interglacial intervals from nearby ODP Site 925. Coccolith-enriched sediment fractions were carefully isolated and screened prior to ∆47 analysis. Results indicate muted tropical cooling of ~3 °C during the LMC, consistent with global temperature compilations. Notably, reconstructed late Miocene upper-ocean temperatures (~20–25 °C) are similar to those observed during Pleistocene interglacials, despite fundamentally different climate states characterized by Antarctic-only glaciation in the late Miocene and bihemispheric glaciation in the Pleistocene.

These findings suggest that muted tropical cooling during the late Miocene is not solely an artefact of alkenone saturation.

This study underlines the potential of coccolith clumped isotopes to provide constraints on upper-ocean temperatures, avoiding uncertainties associated with regressing proxy signals to SST. However, changes in coccolithophore depth habitat and water-column stratification remain key uncertainties. Ongoing paired coccolith–foraminifera ∆47 and foraminifera δ¹⁸O analyses will improve interpretations of tropical ocean temperatures and vertical gradients across contrasting climate states.