Changing resilience of oceans to climate change
An international team of scientists led by ETH Zurich 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 5°C 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 humans are adding nutrient pollution of the oceans – both of which could drive more rapid and expansive oxygen loss.
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.