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.

The biogeochemistry of ferruginous lakes and past ferruginous oceans

“…The most extensive IF deposition occurred from 2.7 to 2.4 Ga, with a spike again at 1.9 to 1.8 Ga (Figure 1; Bekker et al, 2010). However, when bias is removed by scaling iron content with crustal preservation, a recent analysis suggested that IF deposition likely persisted at near constant rate from 3.8 to 1.8 Ga (Johnson and Molnar, 2019).…”

“…"ferruginous") conditions in the deep ocean were spatially extensive and temporally pervasive, and continued through periods of Earth's history not typified by IF deposition (e.g. Canfield et al, 2008;Clarkson et al, 2016;Johnson and Molnar, 2019;Johnston et al, 2010;März et al, 2008;Planavsky et al, 2011;Poulton et al, 2015;Poulton and Canfield, 2011). However, the very low concentrations of iron in the current ocean point to fundamentally different redox conditions in the modern (oxic) as compared to past (anoxic) oceans.…”

Ferric iron triggers greenalite formation in simulated Archean seawater

“…Chemically precipitated sediments record the (bio)geochemistry of their aqueous environments (Derry and Jacobsen, 1990), and deeper marine strata from the Archean Eon (4.0-2.5 Ga) are dominated by iron formations (silica-rich chemical sediments with >15% iron) (Simonson and Hassler, 1996). The long-term, widespread deposition of iron formations suggests anoxic conditions and high iron concentrations in seawater, yet the accumulation of iron-rich sediments was likely triggered by a chemical change or (bio) geochemical process that induced mineral precipitation (Holland, 1984;Bekker et al, 2014;Johnson and Molnar, 2019).…”

A New Depositional Framework for Massive Iron Formations After the Great Oxidation Event

“…When examining the geologic record, the largest volumes of preserved IFs span the Late Archean to Paleoproterozoic, ending rather abruptly after 1.87 Ga (Gole and Klein, 1981; Trendall, 2002; age from Fralick et al., 2002). This record could reflect continuous IF deposition that is no longer evident due to preservation bias (Johnson & Molnar, 2019), or cessation of massive IF deposition after the GOE followed by brief IF resurgence ∼1.88 Ga (e.g., Bekker et al., 2014; Konhauser et al., 2017; Lyons et al., 2014).…”