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Abstract

At the peak of the Cretaceous hothouse, Earth’s climate — with ice-free poles, warm bottom waters, and elevated sea levels — differed markedly from today. This greenhouse state was characterized by weak latitudinal temperature gradients, high atmospheric CO2 concentrations, and enhanced seafloor spreading rates. Eustatic sea level was, on average, 75–250 m higher than present, and large, quasi-permanent ice sheets were likely absent, although small-scale glaciations have been proposed. From the Turonian to the Maastrichtian, a long-term cooling trend led toward the cooler climate of the Cenozoic. The coldest interval of the Cretaceous occurred during its final seven million years, from the late Campanian through the Maastrichtian. This study focuses on short-term climate fluctuations during the transitional phase characterized by warmer-than-present conditions, with a focus on the Maastrichtian. Here, different key aspects were examined, namely bottom-water and sea-surface temperatures changes, the development of small-scale ice sheets, as well as environmental and biotic changes. The first part explores the mid-Maastrichtian event (MME), a warming phase superimposed on the long-term cooling trend of the Maastrichtian. High-resolution benthic foraminiferal δ13C and δ18O, Mg/Ca-derived bottom-water temperatures, and CaCO3 wt% data from IODP Site U1403 (North Atlantic) for the time interval between ∼69.4 and 67.5 Ma reveal a dynamic deep-ocean circulation paced by orbital precession. These fluctuations are interpreted as the result of changing bottom water source regions. The data suggest that Large Igneous Province volcanism not only triggered the MME but also drove a reorganization of ocean circulation. A marked transition around 68 Ma, reflected by a shift towards the dominance of Northern Component Water in the North Atlantic, signals the end of low-latitude bottom-water formation that had persisted throughout much of the Cretaceous. The second part addresses the question of dynamic Antarctic glaciation during the latest Cretaceous. Although the Cretaceous is traditionally viewed as an ice-free greenhouse world, recent proxy and model evidence challenge this assumption. A high-resolution δ18Osw record from ODP Sites 1209 and 1210 (Shatsky Rise, western Pacific) provides strong evidence for a dynamic Antarctic ice sheet during the late Maastrichtian. Ice-volume estimates range from 0–19 % of modern Antarctic ice-sheet volume during interglacials to 55–80 % during glacials, with associated glacioeustatic sea-level falls of up to 50 m. The MME coincides with an inferred ice-free Antarctic continent. These findings challenge the traditional paradigm of an ice-free Late Cretaceous. The third study reconstructs sea-surface temperatures (SSTs) from well-preserved planktic foraminifera at ODP Sites 1209 and 1210 (Shatsky Rise, western Pacific), covering 69.4–67 Ma. SSTs ranged from 32–34 °C, exceeding modern tropical values and surpassing existing proxy reconstructions. A pronounced increase in the latitudinal SST gradient from ∼0.27 °C/°latitude to ∼0.47 °C/°latitude by ∼68 Ma reflects intensified high-latitude cooling while low latitudes stayed warm. This steepening gradient supports the plausibility of polar ice-sheet formation during the Maastrichtian. The final part examines environmental and biotic changes in the tropical Pacific, a region of ecological significance with limited existing records. High-resolution stable isotope data from ODP Sites 1209 and 1210, combined with bottom and sea surface temperatures, enabled the reconstruction of δ18Oivf-sw (sea-surface salinity proxy) and Δδ13C (proxy for surface-water productivity). SSS variability was controlled by the balance of precipitation and evaporation and by upwelling intensity. The Δδ13C record indicates that the biological pump was modulated by ∼20 kyr ENSO-like variability, with enhanced productivity during precession maxima and weakened pump strength during minima. These newly generated high-resolution records offer a novel perspective on the climatic and oceanic variability of the Maastrichtian. Their exceptional temporal resolution and multi-proxy approach allow for an unprecedented reconstruction of environmental conditions during this pivotal time in Earth’s history. Together, they reveal dynamic interactions between ocean circulation, temperature gradients, and ice-volume changes that were previously unresolved. By capturing short-term variability superimposed on long-term trends, these records challenge long-standing assumptions about the stability of greenhouse climates and provide critical insights into the mechanisms driving climate evolution during the final stage of the Cretaceous.