Contribution of diatoms to the reconstruction of the Palaeogene climate

by Cécile Figus |

The Palaeogene (~66 to ~23 million years ago [Ma]) is a period characterised by an initial extreme warmth, followed by gradual climate cooling, and punctuated by numerous short-term eventsknown as ‘hyperthermals’. These hyperthermal events are associated, among other characteristics, with an abrupt warming of 100,000 to 2 million years, completed in 1,000 to 100,000 years, a negative carbon isotope excursion (CIE), and an increase in atmospheric CO₂ and ocean acidification. The best-known hyperthermal event is the Palaeocene-Eocene Thermal Maximum (PETM; ~56 Ma), which is often considered analogous to present-day climate warming due to the sudden rise in temperature and release of CO₂ into the atmosphere. Still poorly understood, the PETM is thought to have been triggered by the release of thousands of petagrams of carbon over ~10,000 years, in the form of methane and carbon dioxide, into the atmosphere and oceans. One of the most important questions surrounding the PETM is what mechanism led to a climate recovery phase after the rapid warming. Several hypotheses have been put forward: the production of marine phytoplankton favoured a reduction in CO₂ after carbon release; an increase in the coupling between climate and silicate weathering, implying that the silicate weathering flux was produced with a lower pCO₂; or a lack of landforms caused by global tectonic uplift rendered silicate weathering almost inactive at the end of the Palaeocene (~66 to ~56 Ma). These different scenarios are all linked to the carbon cycle through the CO₂ sink created by silicate weathering and the organic oceanic carbon pump, in which diatoms play a major role.

Diatoms are currently the main primary producers and exporters of organic carbon and silica in the oceans. However, their importance in the carbon and silicon cycles during the Palaeogene is unknown due to their vulnerability to diagenetic processes and the lack of global-scale data on marginal marine environments. To reconstruct fluctuations in marine opal flux and diatom assemblages during the Palaeogene, I have compiled data on diatomites and deep-sea diatom-bearing sediments, which I have compared with different palaeoceanographic reconstruction models and geochemical proxies.

Figure 1. Rosaforma paillesiae Figus & Witkowski (2024)

Figure 2. Rhomboneis shtapovii Figus & Witkowski (2024)

This ongoing study has also given me the opportunity to describe two fossil taxa from the middle Eocene—Rosaforma paillesiae Figus & Witkowski (2024) (Fgure 1) and Rhomboneis shtapovii Figus & Witkowski (2024) (Figure 2)—and to recently take part in Workshop 1 of the 21st Century Drilling MagellanPlus series, where I was selected to be the diatom expert for the biostratigraphy team. This project allows me to extend my work to Miocene diatoms from the South Atlantic.