Iron isotopes have become particularly useful in recent years for tracing biogeochemical cycling in the oceans. Iron is an important micronutrient for living species in the ocean, particularly for the growth of phytoplankton. Iron is estimated to limit phytoplankton growth in about one half of the ocean. As a result, the development of a better understanding of sources and cycling of iron in the modern oceans is important. Iron isotopes have been used to better constrain these pathways through data collected by the GEOTRACES program, which has collected iron isotopic data throughout the ocean. Based on the variations in iron isotopes, biogeochemical cycling and other processes controlling iron distribution in the ocean can be elucidated.
For example, the combination of iron concentration and iron isotope data can use to determine the sources of oceanic iron. In the South Atlantic and in the Southern Ocean, isotopically light iron is observed in intermediate waters (200 - 1,300 meters), whereas isotopically heavy iron is observed in surface waters and deep waters (> 1,300 meters). To first order, this demonstrates that there are different sources, sinks, and processes contributing to the iron cycle in varying water masses. The isotopically light iron in intermediate waters suggests that the dominant iron sources include remineralized organic matter. This organic matter is isotopically light because phytoplankton preferentially take up light iron. In the surface ocean, the isotopically heavy iron represents the external sources of iron, such as dust, which is isotopically heavy relative to IRMM-014, and the sink of light isotopes as a result of their preferential uptake by phytoplankton. The isotopically heavy iron in the deep ocean suggests that the iron cycle is dominated by the abiotic, non-reductive release of iron, via desorption or dissolution, from particles. Isotopic analyses similar to the one above are utilized throughout all of the world's oceans to better understand regional variability in the processes which control iron cycling. These analyses can then be synthesized to better model the global biogeochemical cycling of iron, which is particularly important when considering primary production in the ocean.