The abundance and composition of modern phytoplankton are primarily related to equator‐to‐pole temperature gradients and global ocean circulation, which in turn determine the availability of nutrients in the photic zone. The nutricline is found at greater depths in warm, tropical waters, whereas more vigorous surface mixing in higher latitudes (seasonally) enhances nutrient availability and primary productivity. Ocean temperatures were ~7°C higher during the middle Miocene Climatic Optimum (MCO; ~16.9–14.7 million years ago, Ma), which was followed by Antarctic glaciation and global cooling during the middle Miocene Climate transition (MMCT; 14.7–13.8 Ma). Four decades ago, Haq (1980, https://doi.org.10.2307/1485353) already related migration patterns of low‐latitude versus high‐latitude calcareous nannoplankton in the Atlantic Ocean to major climatic fluctuations during the Miocene. Here, we detail and discuss the macroevolutionary patterns and processes across the middle Miocene (~16.5–11 Ma) at five deep sea sites on a north‐south transect in the Atlantic Ocean (57°N to 28°S). We show that the major cooling step toward the modern “icehouse” world impacted coccolithophore communities at all latitudes. Contrary to previous observations suggesting that tropical sites showed little change and that midlatitudes were the most sensitive recorders of climate change across the MMCT, we show that all sites recorded a marked diversification and increase in abundance of reticulofenestrids. Global cooling and related increased meridional overturning circulation are implicated as likely forcings for this macroevolutionary step toward establishing modern coccolithophore communities that are dominated by eurythermal and eurytrophic species such as Emiliania huxleyi.

<p>Numerous hyperthermal events have been documented through the Paleocene-Eocene transition. The best known hyperthermal event is the Paleocene-Eocene Thermal Maximum (PETM; around 56Ma), a period that led to surface and bottom water warming of about 5°C within a few millennia at tropical latitudes. It is therefore considered as one of the best analogues of current global warming. The PETM is also characterized by an abrupt 3-4 per mil negative δ<sup>13</sup>C excursion in deep marine core sediments and by a thin clay-rich layer associated with the PETM onset, most often interpreted as carbonate dissolution due to the shoaling of the CCD. The duration represented by these clays and carbonates is of peculiar interest to constrain the exported carbonate production dynamics of surface ocean and its dissolution throughout the water column. This is key to produce realistic carbon budgets across hyperthermal events.</p> <p>To this end, we generated a new 4 Ma (57.5-53.5) record of extraterrestrial <sup>3</sup>He-derived sedimentation rates from pelagic sediments recording at least 10 hyperthermal events at ODP Site 1209 (North Pacific). Our main results indicate that carbonate sedimentation dropped drastically during the PETM onset (minimum of 0.02 cm/ka) and recovered rapidly during the recovery phase of the event (around 0.7 cm/ka). Surprisingly, the sedimentation rate is low (0.3 cm/ka) after the recovery until the Eocene Thermal Maximum 2 (ETM2; around 54Ma). After this major event, the sedimentation rate increased abruptly (0.7 cm/ka) over the last 500 ka of the studied interval due to the overabundance of <em>Zygrhablithus</em> <em>bijugatus</em> a large rod-shaped nannofossil whose ecology is poorly understood yet.</p> <p>Comparisons between the new record of extraterrestrial <sup>3</sup>He-derived sedimentation rate and dissolution proxies from this and previous studies lead us to challenge the widely accepted model previously proposed for hyperthermal events, which assumes that the CaCO<sub>3</sub> accumulation is mainly controlled by dissolution.</p>

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