Isotopic constraints on ocean redox at the end of the Eocene

Alexander J Dickson,Marie-Laure Bagard,Joachim A.R Katchinoff,Marc Davies,Simon W Poulton,Anthony S Cohen

doi:10.1016/j.epsl.2021.116814

Abstract

A multi-million-year decrease in global temperatures during the Eocene was accompanied by large reorganisations to ocean circulation, ocean chemistry and biological productivity. These changes culminated in the rapid growth of grounded ice on Antarctica during the Eocene–Oligocene climate transition (EOT), ∼34 million years ago. However, while it is likely that environmental perturbations of this magnitude altered the oceanic oxygen inventory, the sign and magnitude of the response is poorly constrained. We show that euxinic, hydrographically restricted conditions developed in the Austrian Molasse Basin during the EOT. The isotopic compositions of molybdenum and uranium captured by sediments accumulating in the Molasse Basin at this time reveal that the global extent of sulfidic conditions during the EOT was not appreciably different to that of the Early Eocene greenhouse world. Our results suggest that the early Cenozoic oceans were buffered against extreme long-term changes in oxygenation.

Highlights

The spread of deoxygenated zones in the modern ocean threatens the stability of marine ecosystems and nutrient cycling
Detrital sediments have little to no effect on Mo and U trends: enrichment factors (EF) of Mo and U, calculated by normalising Mo/Al and U/Al ratios to Upper Continental Crust averages of 0.0000135 and 0.000027 respectively (Rudnick and Gao, 2003), follow the major trends in the raw concentrations (Fig. 2)
We suggest that the expansion of anoxic depositional conditions documented in the Molasse Basin (MB) during the Eocene–Oligocene climate transition (EOT), and perhaps in the wider Para-Tethys Sea (Veto, 1987), may have partly balanced any increase in U and Mo burial into more oxygenated sediments in the open ocean

Summary

Introduction

The spread of deoxygenated zones in the modern ocean threatens the stability of marine ecosystems and nutrient cycling. In the early part of the Cenozoic Era, the Earth moved from a greenhouse world with temperatures ∼10 ◦C warmer than pre-industrial during the early Eocene Climatic Optimum (EECO, ∼50–53 Ma) (Cramwinckel et al, 2018; Zachos et al, 2008) to a glaciated world in the early Oligocene, with the inception of an extensive Antarctic ice sheet during the Eocene–Oligocene Transition (EOT, ∼33.7 Ma) (Lear et al, 2008; Katz et al, 2008) This ∼20million-year climatic transition presents an opportunity to study how the main drivers of seawater oxygen concentration [O2] interacted over multi-million year timescales. Before ∼50 Ma, deepwater production was concentrated in the Southern Ocean (Hohbein et al, 2012; Nunes and Norris, 2006) where early Cenozoic surface temperature decreases were large (Bijl et al, 2009; Hollis et al, 2009; Liu et al, 2009)

Methods

Results

Conclusion