Shifts in Phytoplankton Composition and Stepwise Climate Change During the Middle Miocene

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. Plain Language Summary How will marine plankton communities respond to a much warmer world than today? How fast or slow would such changes be? We can learn valuable lessons from the fossil record of coccolithophores that represent a prominent phytoplankton group in both past and modern oceans. Changes in the composition of fossil assemblages show that species tracked past climate change on submillion year scales in the Atlantic Ocean. When ocean temperatures were ~7°C warmer during the middle Miocene (~16 million years ago), tropical species reached into the midlatitudes (42°N) of the North Atlantic. When the Antarctic ice sheet grew larger and the oceans cooled after ~14.7 million years ago, a group known as the reticulofenestrids diversified and became the most common everywhere, also in the tropics. Apparently, this group maintained high fitness in an “icehouse” world with greater latitudinal temperature contrasts, more vigorous ocean circulation, and higher nutrient availability in the photic zone. Indeed, their cosmopolitan distribution implies broad temperature and other environmental tolerances, similar to the group's latest descendant, the modern species Emiliania huxleyi that also displays a wide genetic and ecophenotypic diversity