Calcareous nannoplankton represent a major component of oceanic phytoplankton, ranging in size from 0.25 to 30 μm. The first records are from the Late Triassic. Their calcareous skeletons can be found in fine-grained pelagic sediments in high concentrations and the biomineralization of coccoliths is a globally significant rock-forming process. This heterogeneous group includes coccoliths, discoasters and nannoconids. They are crucial elements for our understanding of past and present oceans. Their skeletons take up chemical signals from the sea water, in particular isotopes of oxygen and carbon. In the Jurassic and Cretaceous oceans, the calcareous nannoplankton was the most efficient rock-forming group, for that reason the characterization of calcareous nannofloras in OAE (Oceanic Anoxic Events) intervals are used to improve our understanding of the marine ecosystem and biological processes such as photosynthesis (biological pump) and biomineralisation (carbonate pump) that affect the organic and inorganic carbon cycle, as well as adsorption of atmospheric CO2 in the oceans (Erba, 2013).
Coccolithophores are unicellular marine golden-brown algae differing from other Chrysophyta in having two flagella and a third flagella-like appendage called a haptonema. They also posses calcified scales, called coccoliths, at some stage in their life as a protective armour that eventually falls to the ocean floor to build deep-sea ooze and fossil chalks. Declines in the abundance of nannofossils through several past global warming events were linked to biocalcification crises caused by climate change and ocean acidification. Now, a new study presents a global record of ‘ghost’ nannofossils that reveals that nannoplankton were more resilient to past warming events than traditional fossil evidence would suggest.
The new study focused on the Toarcian Oceanic Anoxic Event (T-OAE ∼183 mya) considered as one of the most severe of the Mesozoic era. This event is associated with a major negative carbon isotope excursion, mass extinction, marine transgression and global warming caused by massive volcanism in the Southern Hemisphere. Previous studies had indicated that during the peak of this event calcareous nannofossils collapsed due to ocean acidification. But the new research have found that these fossils had been overlooked due to their tiny size and their mode of preservation.
After their death, nannofossils were buried in soft sediment at the bottom of the sea, while their imprints were preserved in the surfaces of other organic matter, such as pollen or spores. These imprints—or “ghost”—nannofossils were found in sediments through the TOAE in the UK, Germany, Japan and New Zealand, but also from two similar global warming events in the Cretaceous: the early Aptian Oceanic Anoxic Event (OAE1a, 120 Ma) from Sweden, and the Oceanic Anoxic Event 2 (OAE2, 94 Ma) from Italy. These findings provide new tools to understand how the calcareous nannoplankton respond to warming events.
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