Globally enhanced calcification across the coccolithophore Gephyrocapsa complex during the mid-Brunhes interval

Evolutionary or adaptative changes in Noelaerhabdaceae coccolithophores occurred in parallel with major changes in carbonate export and burial during scenarios of low orbital eccentricity, with a ∼400 kyr recurrence, during the Pleistocene. Coeval with these conditions of enhanced proliferation, here we report that the calcification of specimens was enhanced at a global scale and across multiple species or morphotypes within the Gephyrocapsa complex during the Mid-Brunhes (MB) interval. This acme of increased production of organic and inorganic carbon by Gephyrocapsa, suggests that such global changes may originate from a common driver. Increased seawater alkalinity, with an appropriately long residence time, is proposed as environmental trigger on the selection of a wide variety of highly calcified and prolific Gephyrocapsa taxa. This new perspective highlights the role of orbital forcing in phytoplankton evolution or adaptation, via a global environmental driver in the form of seawater carbon chemistry. Our results fit with earlier proposals appealing for an intensified biological pump and respiration dissolution during this interval. We hypothesize that the Gephyrocapsa acme may play a double-edged role, by increasing shallow respiration dissolution rates, limiting the removal of alkalinity by burial, which may help to recycle alkalinity and maintain constant levels at the ∼400 kyr scale. This idea suggests the potential capacity of the Noelaerhabdaceae coccolithophore acmes to modify the typical behaviour of carbonate compensation in the ocean and that the changes in coccolithophore calcification may be indicative of changes in ocean carbonate chemistry and the operation of the global carbon cycle in the past.