Iron is an essential fuel for life in the oceans. The influence of this element on biogeochemistry — and nitrogen cycling in particular — varies across environments and time.
Marine iron cycling
Iron is a vital micronutrient in the marine environment, and variations in the supply and transformation of this element can alter ocean productivity. In this web focus, we present a collection of articles that examine the modern marine iron cycle and assess how iron cycling has varied through time.
The recent expansion of observational data has changed our understanding of the ocean iron cycle and its linkages with nutrients such as carbon and nitrogen.
External metal inputs to oceans affect ocean productivity and metal cycling. A synthesis of researchreveals that internal processes such as metal retention, recycling and remineralizationare also important.
Large iron deposits formed episodically in the Archaean oceans. Experimental data and geochemical modelling suggest that green rust was an important contributor to the formation of these deposits and the Archaean iron cycle in general.
Fixed nitrogen is lost from oxygen minimum zones. Experimental data from an anoxic lake show that the presence of Fe(II) limits this loss, suggesting that ancient anoxic and iron-rich oceans may not have been nitrogen limited.
The largest known hydrothermal plume moves dissolved iron halfway across the Pacific. In situ measurements show that dissolved and particulate iron transport is facilitated by reversible exchange of dissolved iron onto organic compounds.
Dust-borne nutrients can enhance productivity in the surface ocean. Two years of sediment trap data reveal that dust enhances carbon export to depth by increasing surface nitrogen fixation, productivity and carbon sinking rates in the North Atlantic.
Atmospheric oxygen was maintained at low levels throughout huge swathes of Earth's early history. Estimates of phosphorus availability through time suggest that scavenging from anoxic, iron-rich oceans stabilized this low-oxygen world.
Dissolved iron is mysteriously pervasive in deep ocean hydrothermal plumes. An analysis of gas, metals and particles from a 4,000 km plume transect suggests that dissolved iron is maintained by rapid and reversible exchanges with sinking particles.
From the archives
Volcanic eruptions at ocean ridges produce large volumes of glass that is rapidly leached by seawater. Geochemical calculations suggest that this process helps to explain the deposition of carbonates at the end of extreme ice ages.
Deeper ocean waters were anoxic during the Neoproterozoic. Geochemical data suggest a transition from sulphidic to iron-rich mid-depth waters about one billion years ago, coincident with increased iron influx from the supercontinent Rodinia.
Pyrite formation has been considered a key iron sink in organic-rich marine sediments. Analyses of sediments from the Ivory Coast–Ghana Marginal Ridge demonstrate that iron can be buried at greater rates during green-clay formation.
Dissolved oxygen in the mid-depth tropical Pacific Ocean has declined. Simulations with a combination of atmosphere and ocean models suggest that anthropogenic pollution can interact and amplify climate-driven impacts on ocean biogeochemistry.
The release of carbon dioxide during biological carbonate production counters carbon uptake by phytoplankton. The carbon chemistry of sinking particles in the Southern Ocean suggests that iron availability stimulates this carbonate counter pump.
Nutrient input from icebergs can fertilize productivity in the ocean. Ten years of satellite measurements reveal that giant icebergs could be responsible for up to 20% of carbon export to depth in the Southern Ocean.
Earth’s initial oxygenation took several hundred million years. Experiments and geochemical modelling suggest that early photosynthetic marine microbes may have been repeatedly stressed by Fe(II) delivered by submarine volcanism.
Bioavailable iron is released from anoxic sediments, such as those that underlie the Peruvian upwelling zone. Analyses of iron levels in sediments from this region suggest that iron release occurs in a relatively narrow range of redox conditions, and that the amount of iron released to the upwelling waters has varied over the past 140,000 years.
Marine sediments deposited beneath the eastern Pacific upwelling margin are a substantial sink for silica. The geochemistry of these sediments suggests that periods of intense upwelling result in iron limitation, which enhances the export of silica from the surface to the deep ocean and sediments.