DPhil research provides isotopic evidence for changes in the zinc cycle during Oceanic Anoxic Event 2

DPhil research provides isotopic evidence for changes in the zinc cycle during Oceanic Anoxic Event 2

A study led by researchers from the Earth Sciences Department at the University of Oxford sheds new light on the complex interplay between nutrient cycles and ocean anoxia during times of major environmental change in the Earth’s history.

This study by graduate student Tim Sweere (St Edmund Hall 2014) was recently published in Geology and is the result of collaborations with his supervisors Alex Dickson, Hugh Jenkyns, Don Porcelli, and Gideon Henderson at the University of Oxford, and external collaborators from the University of New Mexico and Shell.

Global warming can be associated with ocean deoxygenation due to changes in the solubility of oxygen, changes in ocean circulation, but may also lead to the same result indirectly, by impacting marine life. High oceanic nutrient availability in warming worlds can sustain high rates of plankton production in the surface ocean, which leads to increased ocean deoxygenation at depths where this organic material is oxidised.

Oceanic Anoxic Events provide an extreme case study for the complex interplay between global warming and changes in ocean environments and are used by scientists to assess how these processes interact. By carefully analysing marine sediments from such a period of warming about 94 million years ago, the scientists reveal major changes in ocean chemistry associated with this event. These marine sediments, now present as the white chalk cliffs in southern England and limestones from mountains in southern Italy and southern Mexico, provide an archive for geochemical information as they are largely made up of calcifying organisms that lived in the ocean during this time. The researchers used new analytical techniques to analyse the zinc-isotope composition of these rocks.

Zinc is essential to marine life and taken up by plankton in the surface ocean, but its cycling in the ocean is also strongly controlled by the availability of oxygen in the ocean. By analysing the zinc-isotope composition of rocks from the same age, but different locations, the researchers show that the global ocean zinc cycle experienced dramatic change during periods of global warming and cooling. These changes in the zinc cycle were largely controlled by intervals of ocean (de)oxygenation as a result of global climate change and were likely also impacted by extensive volcanism during this time.

Such environmental perturbations have an impact on marine organisms and have led to the extinction of several species during the studied interval. The availability of (micro)nutrients, like zinc, has previously been suggested to have had an impact on blooms and extinctions of marine organisms during this time. The new study provides direct geochemical evidence of changes in the (micro)nutrient cycles that supports this view.

This is the first study to show coeval changes in the zinc-isotope composition of marine sediments from different locations, which implies that this relatively new tool may also be useful to reconstruct global environmental change during other periods in Earth’s history.

Paper: Isotopic evidence for changes in the zinc cycle during Oceanic Anoxic Event 2 (Late Cretaceous), by Tim C. Sweere, Alexander J. Dickson, Hugh C. Jenkyns, Don Porcelli, Maya Elrick, Sander H.J.M. van den Boorn and Gideon M. Henderson was published in Geology. DOI:doi.org/10.1130/G40226.1

Image: The Raia del Pedale section, used in this study of zinc-isotope stratigraphy, is exposed along the road-cut through the mountain in the foreground.  Campania, Italy.