Past and Present Dynamics of the Iron Biogeochemical Cycle

TL;DR

This chapter explores the historical and current dynamics of Earth's iron biogeochemical cycle, emphasizing sources, sinks, and residence times from the Archean era to today, highlighting the cycle's complexity and long-term evolution.

Contribution

It provides new estimates of iron fluxes, residence times, and discusses mechanisms balancing iron sources and sinks across Earth's history.

Findings

Archean iron residence time between 6 kyr and 3 Myr.

Significant imbalance between continental and hydrothermal iron inputs in the Archean.

Iron cycle was more prolonged and uniform in the Archean than today.

Abstract

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.