Solution to Decades-Old Puzzle of Ice Age Ocean Chemistry

The Southern Ocean, southeast of Tasmania. Credit: IMAS

A mystery that has baffled scientists for more than two decades is being solved by new study on the chemistry of the oceans during ice eras.

The question is how much CO2 entered the ocean throughout the ice ages that can be attributed to the “biological pump,” where atmospheric carbon is absorbed by phytoplankton and deposited to the bottom when organisms die and sink.

The mystery must be solved in order to increase the accuracy of climate models and get a better knowledge of how ocean systems may respond to future climate change.

Led by IMAS and University of Liverpool scientists and published today (October 10, 2019) in Nature Communications, the study found ice age phytoplankton in the tropics absorbed high levels of CO₂ due to fertilization by iron-rich dust blowing into the ocean.

Lead author Dr. Pearse Buchanan said that until now models had only been able to explain a portion of the CO₂ that entered ice age oceans via the biological pump.

“During past ice ages, carbon levels were lower in the atmosphere and higher in the oceans than today, but scientific models aren’t able to account for all of the additional CO₂ that entered the ocean,” Dr. Buchanan said.

“The leading hypothesis has been that iron-rich dust blown from glacial landscapes stimulated phytoplankton growth in high latitudes, but this only explained around one-third of the extra CO₂ absorbed through the biological pump: the other two-thirds was effectively “missing.”

“We used an ocean model to look at the response to iron-rich dust of phytoplankton in tropical waters, particularly a group of phytoplankton called “nitrogen fixers.”

“These are able to biochemically “fix” nitrogen from the atmosphere, much like nitrogen-fixing bacteria that help legume crops thrive in nutrient-poor soil.

“Marine nitrogen fixers are known to be important in the marine nitrogen cycle, and now we’ve shown they’re also critically important in the marine carbon cycle.

“When we added iron to our ocean model, nitrogen fixers thrived, and their growth and subsequent sinking to the deep ocean can account for much of the missing CO₂,” Dr. Buchanan said.

IMAS Associate Professor Zanna Chase said this solution was first proposed in 1997 but had gained little traction over the last two decades.

“The beauty of this approach is that it can explain almost all of the additional CO₂ that phytoplankton transported into the oceans during the last Ice Age,” Associate Professor Chase said.

“The increased activity of the biological pump in the tropics complemented that happening in colder waters, drawing higher levels of CO₂ into the oceans and locking it away in the deep ocean.

“This pathway for carbon to the deep ocean is reduced today because less fertilizing iron is being circulated by the wind and phytoplankton growth, including that of nitrogen fixers, is correspondingly limited, although there are signs that it has strengthened within the Pacific since the industrial revolution.

“Taking account of these links between the cycles of iron, nitrogen, and carbon in our ocean and climate change models will make them better able to explain ocean processes and predict future changes.

“But how iron fertilization of phytoplankton will evolve is currently uncertain, undermining our ability to predict the ocean’s role in drawing CO₂ out of the atmosphere in the coming centuries,” Associate Professor Chase said.

Reference: “Marine nitrogen fixers mediate a low latitude pathway for atmospheric CO₂ drawdown” by Pearse J. Buchanan, Zanna Chase, Richard J. Matear, Steven J. Phipps and Nathaniel L. Bindoff, 10 October 2019, Nature Communications.
DOI: 10.1038/s41467-019-12549-z