Widespread and Persistent Deposition of Iron Formations for Two Billion Years

Abstract

Composed of chemical precipitates rich in iron and silica, Precambrian iron formations from marine sedimentary records may reveal biogeochemical processes over the first half of Earth history. The limited record of early Archean rock suggests that preservation biases the iron formation record. Like ophiolites, which provide a sparse record of past ocean floor, iron formations deposited on oceanic crust ought to also be rare and preserved only when accreted onto cratons. To correct for potential preservation bias, we scaled masses of iron formations to the areal extent of basement rock of similar age and found that the resultant record is consistent with persistent deposition of iron formations across much of the deep ocean for two billion years. Widespread and long‐term iron formations imply that ferrous iron was available in the deep ocean for billions of years and that the requisite (bio)geochemical mechanisms to produce iron formations were present by 3.8 Ga. Plain Language Summary Why did the ancient ocean deposit an increasing volume of iron over time, until these “Iron Formations” largely disappeared? Does this pattern reflect a growing titration of oxygen, a surge in iron from mantle superplumes, or the prerequisite of developing continental crust? Or is this distribution simply from the diminishing record of ancient rocks? We normalize the iron formation record over time by scaling it to crustal preservation, and the preservation‐scaled record suggests that there may have been widespread and unchanging deposition of these iron formations across the ancient oceans for two billion years—implying that the (bio)chemical process responsible for making iron formations operated from ~3.8 to ~1.8 billion years ago.