Trace element evolution of seawater through geological time as revealed by Precambrian iron formations

Iron oxyhydroxide minerals have precipitated directly from seawater
throughout geological time. While such occurrences today are generally
restricted to settings strongly influenced by hydrothermalism, during
the Precambrian (prior to 541 Ma) iron oxyhydroxide muds accumulated
over large areas of the ocean floor to be preserved as iron formation
(IF). The Precambrian IF record is extensive, spanning every continent
and dating as far back as Earth’s oldest known sedimentary deposits.
As nearly pure chemical sediments, the precipitates that formed IF
captured elemental and isotopic signatures of evolving ancient seawater
by sorption and co-precipitation reactions, rendering IF a rich record
of ancient Earth's evolving marine geochemistry. That record is explored
here using a database of >3000 chemical and isotopic analyses of IF
compiled from literatures sources as well as new analyses. Using
phosphorus as an example, partitioning of trace elements between
seawater and precursor IF minerals is examined in terms of how modern
observation, laboratory experiment, and the IF rock record itself can
each be employed to relate trace element concentrations in ancient iron
oxides to the waters from which they formed. Secular trends (or lack
thereof) in the IF record of selected micronutrients (Ni, Zn, Co) will
then be explored with particular regard to the hypothesis that shifting
marine trace element availability guided the evolution of biological
demand. Finally, established and emerging trace element proxies for the
redox evolution of the ocean-atmosphere system (Cr, Mo, U) will be
examined in the context of the IF record and recent studies suggesting
free oxygen on the early Earth well before the ca. 2.5 Ga Great
Oxidation Event.