Ocean deoxygenation

deoxygenation theme button
Organic-rich mudrock lamination, Toarcian OAE, Yorkshire, UK.

Background: One of the understudied consequences of climate change at present day is a decrease in the concentration of dissolved oxygen in the oceans (deoxygenation) and this is a major threat to marine biodiversity. Today, deoxygenated areas extend to 7% of the global area and affect more than 400 marine biological systems. Every decade since the 1960s the area of the seafloor affected by deoxygenation has doubled.

Research by other groups on present-day and recent de-oxygenation is providing valuable information on the short-term influence but the geological record of deoxygenation events, provides the only record of the long-term changes and consequences. These long term affects are the focus of work within the Open University's team investigating palaeoenvironmental change.

Oceanic deoxygenation occurs because as the oceans warm the amount of dissolved oxygen decreases and there is an increase in ocean stratification. In addition, deoxygenation, particularly in coastal areas, is driven by changes in the ecosystem (eutrophication) this includes the bloom of phytoplankton. Eutrophication occurs because of increased nutrients from anthropogenic inputs such as effluent and agricultural run-off as well as natural causes.

COEToarcian OAE sampling 2
Sampling organic-rich mudrocks, Yorkshire, UK. All our work involves careful sampling at the 1-5cm resolution.

What we are researching and why: Members of the team researching palaeoenvironmental change at the Open University are investigating the chemical and biological effects of several different episodes of oceanic de-oxygenation that have occurred during Earth history. The reasons for looking at these very old events in Earth history are that we can:

  • determine how they influenced the biota over long time-scales (1000s of years)
  • discover how long it took for the onset and, importantly, how long it took for the Earth to recover to a new steady state
  • assess the feedback mechanisms were that returned the Earth to a new steady state
  • provide documentation of a spectrum of events of different magnitudes, including extreme end members
  • discover whether species went extinct and or new species evolved due to the extreme conditions.
COEBonarelli Italy2
The black layer represents the Bonarelli OAE (Italy).

In particular, members of the team study an event that occurred 183 million years ago during the Jurassic. This event is known as the Toarcian Oceanic Anoxic Event (OAE). We also work on the Palaeocene-Eocene thermal maximum (PETM), the Kimmeridge Clay Formation (Kimmeridgian), Monterey Formation (Mid-Miocene), Triassic -Jurassic boundary and several OAEs that occurred during the Cretaceous (Bonarelli OAE and Selli OAE).

Key papers that members of the team have published on this theme

Effect of de-oxygenation on shellfish (Gold open access paper): 

Mass-extinction linked to oceanic de-oxygenation:

Development and application of an isotopic proxy to assess the lateral extent globally of deoxygenation (See also the geochemistry):

High-resolution timescale and driving mechanism for the Toarcian OAE

Evidence for the negative weathering feedback that returned the Earth to a new steady state hundreds of thousands of years after the onset of the Toarcian OAE (see also geochemistry).

What happened in the Arctic Ocean during the PETM

Ongoing research

Toarcian OAe
The dark coloured rocks near the base of the cliff represent the Toarcian OAE (Port Mulgrave, Yorkshire, UK)

Angela Coe has ongoing projects with Bryony Caswell (University of Liverpool) on the marine benthos from de-oxygenation events.

Angela Coe and Anthony Cohen are preparing further manuscripts following on from their NERC standard grant which applied isotopic proxies to deposits representing the Palaeocene-Eocene thermal maximum (PETM) and showed that small areas of the oceans became anoxic at that time. This work is in conjunction with Alex Dickson who is now at Oxford University.

David Kemp is working on extending the astronomical timescale of the Toarcian OAE. Gareth Izon, who recently finished a PhD at the Open University and is now at St Andrews University examined the extent of the Cretaceous OAEs using Mo-isotopes.

KENNEDY SEM Tasminitid
Scanning electron microscope image of Tasminitid from the Toarcian OAE.

Current PhD students, Alice Kennedy (supervised by Angela Coe and Will Gosling) and Felicity Rhodes (supervised by Anthony Cohen, Angela Coe and Manuela Fehr (ETH Zurich)) are working on the effect of Toarcian OAE on microfossils and  the developing the Re-isotope proxy to measure the extent and timing of oceanic deoxygenation during the Toarican respectively. Alice and Felicity will shortly also be joined by Matthew Saker-Clark who will be looking at the record of the Toarcian OAE in Morocco.

Marie-Laure Bagard and Marc Davies are employed on the NERC grant RESPIRE (RESponse of Global Ocean Oxygenation to Early Cainozoic Climate Extremes) to work with Anthony Cohen on generating a multi-proxy reconstruction of global ocean oxygenation changes during the time interval 56-25 million years. They will further develop and combine the Mo, Re and U stable isotope proxies for deoxygenation (see isotope geochemistry) by determining their compositions in past seawater from selected marine sedimentary deposits from the Arctic Ocean and Central Europe (former Tethys Ocean).

Phillip Jardine, Phil Sexton and Neil Edwards have also work on the PETM.