Carbon Cycling at the Dawn of the Cenozoic

Speaker: Dr Michael Henehan (Bristol)

Dr Henehan is an isotope geochemist primarily interested in the global carbon cycle, Earth’s climate regulation, and the influence of biota on both.

Title: Carbon Cycling at the Dawn of the Cenozoic

Venue: Seminar rooms or join online via Zoom link

Abstract: The Paleocene – the first epoch of the Cenozoic (66–56 Ma) – is an intriguing, and often puzzling, time interval sandwiched between the charismatic events of the Paleocene-Eocene Thermal Maximum (PETM) and Cretaceous-Palaeogene (K-Pg) boundary. The epoch is relatively understudied, but spans numerous notable climatic and biogeochemical phenomena. In the oceans, calcifying plankton communities had to gradually recover after the K-Pg bolide impact, after which benthic marine carbonate δ¹³C values record one of the largest positive excursions in of the last 100 Myr¹ in the form of the Paleocene Carbon Isotope Maximum. On land, an as-yet-unexplained extreme step-change in global weathering regime is indicated by marine carbonate δ⁷Li values². More generally, despite benthic foraminiferal oxygen isotopes that suggest a greenhouse climate much warmer than today³, scant proxy estimates of atmospheric CO₂ in the Paleocene mostly indicate low CO₂ levels more similar to those seen during the relatively colder late Neogene⁴.

In this talk, I will summarise some of the recent headway we have made in understanding geochemical cycling and ocean chemistry at the dawn of the Cenozoic, immediately after the K-Pg impact. Following this, I will present new benthic and planktic foraminiferal boron and lithium isotope data that address some of the outstanding puzzles of this formative period for the Cenozoic carbon cycle. Our new, higher boron-derived atmospheric CO₂ estimates for this interval resolve some of the apparently anomalous³ behaviour of the Paleocene climate system, and in doing so provide a better understanding of the climatic baseline to the PETM.

At the end of the talk, I’ll introduce future research plans at Bristol – specifically my 5-year ERC Consolidator Grant (now a UKRI Frontier Research Grant) PETRARCH, with the help of which I hope to push some of these quantitative proxy approaches back to the peak warmth of the Cretaceous period to understand what it takes to trigger a global Ocean Anoxic Event.

Literature:

1. Friedrich, O., Norris, R. D. & Erbacher, J. Evolution of middle to Late Cretaceous oceans–A 55 m.y. record of Earth’s temperature and carbon cycle.Geology40, 107–110 (2012).
2. Misra, S. & Froelich, P. N. Lithium Isotope History of Cenozoic Seawater: Changes in Silicate Weathering and Reverse Weathering.Science 335, 818–823 (2012).
3. Westerhold, T.et al. An astronomically dated record of Earth’s climate and its predictability over the last 66 million years. Science 369, 1383–1387 (2020).
4. Beerling, D. J. & Royer, D. L. Convergent Cenozoic CO₂history. Nat. Geosci. 4, 418–420 (2011).

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