“Give me half a tanker of iron and I’ll give you the next ice age.” – Dr. John Martin
While Dr. Martin said these words in jest, there is real science behind them. By now, human-induced climate change is universally accepted (minus a few naysayers) and we owe the changing climate to the increased amounts of carbon dioxide we put into the atmosphere. While we scramble to find out how to deal with climate change, the earth has long had a way of handling carbon dioxide. Plants have the important task of taking in carbon dioxide and releasing oxygen through photosynthesis (Fig. 1). But the amount of carbon dioxide taken in by a plant depends on how active it is and whether or not it has other critical resources. Resources such as nitrogen, phosphorous, and iron, usually limit plant productivity. These resources are also important to the health and function of marine plants, like microscopic phytoplankton. Globally, the oceans create about half of the oxygen we breathe, and in doing so, they are taking in a lot of carbon dioxide. Marine habitats could be more productive and take in more carbon dioxide if they had access to other nutrients, hence Dr. Martin suggesting that if we were to put a lot of iron in the ocean, productivity would be so high that carbon dioxide would decrease to levels found in ice ages of the past.
The Southern Ocean (the one surrounding Antarctica) is classified as a high nutrient, low chlorophyll region, meaning there are high nutrient concentrations but not a lot of phytoplankton. The reason behind this is that this ocean lacks iron, and without it, phytoplankton are nutrient limited. Iron usually enters the Southern Ocean via atmospheric dust, but sometimes comes from glacial melt water rich in sediments from the continent. It has been observed that areas in the water around floating sea ice have higher chlorophyll concentrations due to the iron released in the melt water. However, the amount of melt water is controlled by large icebergs (those bigger than 18 km long!) and their impact on Southern Ocean productivity has yet to be studied on a large scale. Here, researchers from the UK took on this challenge using satellite images to determine how icebergs impacted the size of productive areas and the duration of productivity.
Large icebergs are born in a process called glacial calving (Fig. 2). This occurs when huge chunks of glaciers break off and float out to sea. Depending on where the glacier is on the continent determines how much iron and nutrients are trapped in ice. Once an iceberg is calved it floats in the sea, moving with the currents. These icebergs slowly melt, releasing the trapped iron and sediments. These researchers wanted to see what kind of impact these icebergs had on productivity over a long period of time and began looking at satellite images of ocean color. From these satellite images, researchers can determine how productive an area is based on the concentration of chlorophyll (more chlorophyll means more phytoplankton). These images are kind of like heat maps for phytoplankton productivity/ abundance where warmer colors indicate a higher concentration (Fig. 3 and Fig. 4). Researchers began tracking large icebergs and quantifying the chlorophyll concentrations around them and in their wake from 2003-2013. What they found was pretty amazing.
Researchers saw that chlorophyll concentrations in the wake of icebergs were ten times higher than normal (Fig. 5). The high concentrations persisted for at least a month. High chlorophyll concentrations were found to spread on average 500 km from the iceberg itself and in some cases was enhanced as far as 1000 km from the iceberg! Researchers did find differences in the amount of productivity enhancement and duration based on where the iceberg was calved.
So there are higher chlorophyll concentrations, how does that help fight climate change? The higher chlorophyll concentrations associated with large icebergs means that there are more phytoplankton, and where there is more phytoplankton there is more photosynthesis, and where there is more photosynthesis more carbon dioxide is being taken out of the atmosphere. In this scenario, more carbon is then exported and stored in the sea floor, in fact, more than twice the amount of carbon is moved to the ocean bottom. It was also estimated that carbon export resulting from icebergs accounts for 10-20% of all carbon export in the Southern Ocean.
As climate change continues to warm the world, iceberg calving is happening at a faster rate. But as this study shows, more icebergs mean greater fertilization of the Southern Ocean and enhanced productivity. Therefore, these icebergs, despite being a result of a warming climate, are fighting back by enabling more carbon uptake and storage by phytoplankton.
Check out this video of calving!