Abstract
AbstractThe 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.
Highlights
IntroductionThe physicochemical properties of the surface oceans (including sea surface temperature, light, and nutrient availability) directly influence marine photosynthetic algae and primary productivity
The physicochemical properties of the surface oceans directly influence marine photosynthetic algae and primary productivity
Contrary to previous observations suggesting that tropical sites showed little change and that midlatitudes were the most sensitive recorders of climate change across the middle Miocene Climate transition (MMCT), we show that all sites recorded a marked diversification and increase in abundance of reticulofenestrids
Summary
The physicochemical properties of the surface oceans (including sea surface temperature, light, and nutrient availability) directly influence marine photosynthetic algae and primary productivity. Climate change can significantly contribute to changes in the abundance and composition of phytoplankton communities, which, in turn, affect global biogeochemical cycles. Rising ocean temperatures (Levitus et al, 2005) affect ocean circulation and the physical properties of the surface water and will likely cause poleward shifts in phytoplankton thermal niches (Thomas et al, 2012). Marine algae that are able to sustain photosynthesis and growth under low‐nutrient concentrations, such as the coccolithophores (calcifying haptophytes), are expected to HENDERIKS ET AL. In order to better understand the long‐term impacts of such ecological shifts, we need to study the fossil record of past warm climates
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