  This chapter investigates the complexities surrounding the iron
biogeochemical cycle from the Archean to present, with a focus on assessing the
balance between iron sources and sinks during long periods of Earth's history
with relatively invariable redox conditions, when steady state can be safely
assumed. Currently, the residence time of iron in the ocean may be as short as
approximately 5 years. The input flux of iron is highly sensitive to redox
cycling in sediments, while its removal primarily occurs through dispersed
processes of oxidation and precipitation. In the Archean, we find a significant
imbalance between continental and hydrothermal inputs, which collectively
contribute between 61,500 to 263,000 Gg/yr of dissolved iron to the oceans, and
the most obvious sinks such as iron formations (IFs), which sequester up to
~43,000 Gg/yr of iron. A possible solution to this imbalance involves the
dispersed abiotic precipitation and removal of iron as silicates, sulfides, and
carbonates in marine basins. Additionally, we calculate the residence time of
dissolved iron in the Archean oceans to be between 6 kyr and 3 Myr, which is
significantly longer than the ocean mixing timescale. Our estimates indicate
that under the anoxic Archean atmosphere, the iron cycle was more protracted
than today, and the isotopic compositions and concentrations of dissolved iron
were likely more uniform. Distinct water bodies were likely confined to limited
areas or specific, dynamic systems with intense iron turnover, such as regions
where deep-sea upwelling currents brought hydrothermal iron to photic zones
rich in biotic or abiotic oxidants.
