Changing resilience of oceans to climate change

Date:
January 15, 2021
Source:
University of Exeter
Summary:
Oxygen levels in the ancient oceans were surprisingly resilient
to climate change, new research suggests.



FULL STORY ========================================================================== Oxygen levels in the ancient oceans were surprisingly resilient to
climate change, new research suggests.


========================================================================== Scientists used geological samples to estimate ocean oxygen during a
period of global warming 56 million years ago -- and found "limited
expansion" of seafloor anoxia (absence of oxygen).

Global warming -- both past and present -- depletes ocean oxygen,
but the new study suggests warming of 5DEGC in the Paleocene Eocene
Thermal Maximum (PETM) led to anoxia covering no more than 2% of the
global seafloor.

However, conditions are different today to the PETM -- today's rate of
carbon emissions is much faster, and we are adding nutrient pollution
to the oceans - - both of which could drive more rapid and expansive
oxygen loss.

The study was carried out by an international team including researchers
from ETH Zurich, the University of Exeter and Royal Holloway, University
of London.

"The good news from our study is that the Earth system was resilient to seafloor deoxygenation 56 million years ago despite pronounced global
warming," said lead author Dr Matthew Clarkson, of ETH Zurich.



========================================================================== "However, there are reasons why things are different today.

"In particular, we think the Paleocene had higher atmospheric oxygen
than today, which would have made anoxia less likely.

"Additionally, human activity is putting more nutrients into the
ocean through fertilisers and pollution, which can drive oxygen loss
and accelerate environmental deterioration." To estimate ocean oxygen
levels during the PETM, the researchers analysed the isotopic composition
of uranium in ocean sediments, which tracks oxygen concentrations.

Surprisingly, these barely changed during the PETM.



==========================================================================
This sets an upper limit on how much ocean oxygen levels could have
changed.

Computer simulations based on the results suggest a maximum ten-fold
increase in the area of seafloor devoid of oxygen -- taking the total
to no more than 2% of the global seafloor.

This is still significant, at around ten times the modern area of anoxia,
and there were clearly detrimental impacts and extinctions of marine
life in some parts of the ocean.

Co-author Professor Tim Lenton, Director of Exeter's Global Systems
Institute notes: "This study shows how the resilience of the Earth's
climate system has changed over time.

"The order of mammals we belong to -- the primates -- originated in
the PETM.

Unfortunately, as we primates have been evolving for the last 56 million
years, it looks like the oceans have been getting less resilient."
Professor Lenton added: "Although the oceans were more resilient than
we thought at this time in the past, nothing should distract us from
the urgent need to reduce emissions and tackle the climate crisis today."

========================================================================== Story Source: Materials provided by University_of_Exeter. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Matthew O. Clarkson, Timothy M. Lenton, Morten B. Andersen,
Marie-Laure
Bagard, Alexander J. Dickson, Derek Vance. Upper limits
on the extent of seafloor anoxia during the PETM from
uranium isotopes. Nature Communications, 2021; 12 (1) DOI:
10.1038/s41467-020-20486-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/01/210115091351.htm

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