Iron isotopes and ocean evolution

The main driver for biological activity in the oceans far from land is the availability of iron, and this helps control the burial of organic carbon and hence aspects of global climate.  At low Fe concentrations, as they have been since the oxygenation of the surface environment from 2 billion years ago, iron is cycled in the marine environment in a matter of a few hundred years.  So, ocean water responds very quickly, in geological terms, to changes in the source of any dissolved iron.  There are two main sources, discharge of hydrothermal fluids from the oceanic lithosphere and delivery of river water and dust derived from the continents.  Of the last, riverine sources probably end up in near-shore sediments and only dust contributes significantly to deep ocean water.  The slowly growing nodules and crusts, composed mainly of iron and manganese compounds, on the ocean floor can chart variations in the relative proportions of these sources, because their growth produces zonation.  Measurements of d56Fe in various materials show that the two sources are different in isotopic composition (Beard, B.L. et al. 2003.  Iron isotope constrains on Fe cycling and mass balance in oxygenated Earth oceans. Geology, v. 31, p. 629-632).  While continent derived materials exude iron that is essentially the same as that in terrestrial volcanic rocks (d56Fe ~0.0‰), ocean-floor hydrothermal activity is significantly depleted in 56Fe (‰56Fe ~ -0.38‰).  From 6 Ma to 1.7 Ma iron-manganese crusts record iron with a dominant hydrothermal origin, but during the glaciation-dominated period since 1.7 Ma the contribution of continent-derived dusts becomes overwhelming – cooling forces drying on a global scale.  Because hydrothermal contributions probably stay much the same over very long periods, because of the sluggishness of plate tectonics, iron isotopes in deep marine sediments, such as Fe-Mn crusts,  may be important tracers for glacial events in the distant past, such as the glaciations during the Neoproterozoic and Palaeozoic. Interestingly, the largest iron-rich deposits on the planet, the BIFs that peaked during Archaean and Palaeoproterozoic times, record far larger excursions in iron isotopes than any other.  The very low d56Fe values of some BIFs (down to – 2.4‰) probably signify the dominance of sea-floor sources, although a non-oxidising atmosphere would have mobilised dissolved iron from the continents too, which explains the range in BIFs up to +1.0‰.

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