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Antarctic Ice Key to the Carbon Cycle

Source: Laufkotter, C., A. A. Stern, J. G. John, C. A. Stock, and J. P. Dunne (2018), Glacial iron sources stimulate the Southern Ocean carbon cycle. Geophysical Research Letters, doi:10.1029/2018GL079797

The ocean helps regulate the global climate system by taking up carbon dioxide from the atmosphere. The ocean surrounding Antarctica, called the Southern Ocean, accounts for a particularly large proportion of the total global ocean carbon uptake. One of the ways that the ocean absorbs CO2 is with the help of tiny microalgae called phytoplankton. Phytoplankton take up carbon dioxide from the atmosphere through photosynthesis, just like trees and other land plants. Therefore, understanding the processes that determine the distribution of phytoplankton in the Southern Ocean has implications for the global carbon cycle.

One of the factors limiting phytoplankton growth in the Antarctic is the nutrient iron. Therefore, adding iron often stimulates large growth events called phytoplankton blooms. There are several different sources of iron to the region including dust, sediments, and ice. Which of these sources is the most important in determining where blooms occur? The answer is unclear, in part due to lack of observational data. A recent study led by Charlotte Laufkotter at Princeton University seeks to quantify the role of iron from glacial melt and icebergs in the Southern Ocean carbon cycle.

An icebreaker collecting data in the Southern Ocean (NSF photo via Wikimedia Commons).

Taking measurements in the Southern Ocean is challenging due to its remote location and harsh weather. Because there is so little available data, Laufkotter’s study used a complex model to try to understand how glacial ice impacts biological productivity and oceanic carbon uptake in the Antarctic. Models, like the one used in this study, that integrate physical, chemical, and biological processes are becoming increasingly important as we try to untangle the complex feedbacks that drive changes in ecosystems and climate.

Icebergs can fertilize blooms by supplying iron, a nutrient necessary for phytoplankton growth (Wikimedia Commons)

In order to run a model, it is necessary to specify the initial values of each variable. The model in this study was run four times with different glacial iron concentrations to represent the range of observational estimates. The resulting iron and chlorophyll (which is an indicator of phytoplankton biomass) values from each of the model solutions were then analyzed. In particular, the distributions from each simulation were compared to measured iron and chlorophyll from research cruises in the Southern Ocean and satellite chlorophyll data. These comparisons suggest that the observed present-day patterns in biological productivity are consistent with the model results from the high glacial iron input scenario.

Furthermore, the model suggests that glacial melt and icebergs supports the biological production that accounts for about 30% of the total organic matter (decomposing biological material) exported to the deep ocean in the Antarctic annually. This export corresponds to a carbon uptake of 0.14 Pg per year (equal to the weight of about 28 million elephants!). Laufkotter and colleagues do point out that the observational data used to evaluate the four model simulations is sparse in space and time. Therefore, it is crucial to improve the coverage of measurements in the Antarctic and continue monitoring these changes in the future. Still, these results suggest that accurately accounting for iron inputs from ice melt is necessary to predict the strength of the Southern Ocean carbon sink – and how this might change in the future.


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