The “greenhouse” climates of the Paleocene and Eocene have formed the focus for many proxy and modelling studies in recent decades, as they are the closest geological analogues for our future warmer anthropogenic world. Yet, the long-term evolution of ocean temperature and carbonate chemistry at orbital-resolution, especially at low latitudes, are still poorly constrained. Here we present new orbital-resolution foraminiferal trace metal (Mg/Ca & B/Ca) records spanning the late Paleocene to early Eocene (~58–53 Ma) from a new splice between ODP Site 758 and IODP Site U1443, Ninetyeast Ridge, northern Indian Ocean. We generated coupled Mg/Ca and B/Ca records from well-preserved mixed layer and thermocline-dwelling planktic foraminifera, and benthic foraminifera deposited at a shallow palaeo-water depth (~1500 m), to construct temperature change and carbonate chemistry (related to pH and DIC concentration) across a water column depth transect above Ninetyeast Ridge. Our new trace metal records are the first long-term orbital-resolution records of their kind from the poorly studied Indian Ocean. We estimate both the magnitude of long-term warming and associated carbonate chemistry change from the late Paleocene–early Eocene, as well as the magnitude of change on orbital (405- & 100-kyr) timescales. In addition, a portion of the Paleocene-Eocene Thermal Maximum is resolved in our records, providing a critical minimum constraint for the magnitude of temperature and carbonate chemistry change within the low-latitude Indian Ocean during this hyperthermal event.

A transient period of climate change, characterized by a global warming of ~2.5–5°C followed by a cooling to pre-excursion conditions, occurred during the last 300 kyr of the Maastrichtian (~66.34–66.02 Ma). This instability may have played a role in destabilizing marine and terrestrial ecosystems, priming the system for abrupt extinction at the K-Pg boundary, likely triggered by a large bolide impact. This pre-K-Pg warming event has often been linked to the main phase of Deccan Trap volcanism, however large uncertainties associated with radio-isotopic dating methods of basalts, along with low sedimentation rates and hiatuses in many studied sedimentary sequences, have long hampered a definitive correlation. To complement recent advances in dating of the traps, we have generated the first complete and highest resolution (2.5–4 kyr) benthic stable δ13C and δ18O record for the final million years of the Maastrichtian using the epifaunal foraminifera species Nuttallides truempyi from ODP Site 1262, Walvis Ridge, South Atlantic, calibrated to an updated orbitally-tuned age model. We then compare our data to other previously published geochemical data from other sites in the high, middle, and low latitudes. Our data confirms that the onset of the warming event coincides with the onset of the main phase of Deccan volcanism, strongly suggesting a causal link. Furthermore, spectral analysis of our extended late Maastrichtian-Early Eocene record suggests that the onset of the warming event corresponds to a 405-kyr eccentricity minima, in contrast to many transient warming events (hyperthermals) of the Paleogene, suggesting a control by orbital forcing alone is unlikely. A peculiar feature of the event, compared to other hyperthermals, is a muted carbon cycle response during warming, which may be related to the comparatively heavier δ13C signature of volcanogenic CO2 (–7‰), compared to other sources of light carbon invoked to explain Paleogene hyperthermals. The warming event coincided with minor extinctions of thermocline-dwelling foraminifera, along with dwarfing and blooms of the opportunistic disaster genera Guembelitria, suggesting that Deccan-induced climatic instability may have played a role in priming high-stress ecosystems which were tipped over a threshold into mass extinction during bolide impact.