School of Engineering and Science - Jacobs University

modern and Archean oceans, as well as for Eu, which indicates a greater contribution to
Archean seawater from high-T hydrothermal fluids (e.g., Figs. 4,5). It should be noted
that the small negative Ce anomaly in the shallow water Holocene reef carbonates
relative to world seawater is not inconsistent, as very shallow seawater often displays
smaller negative Ce anomalies compared to bulk seawater (e.g., de Baar et al., 1983).
The fact that Archean IFs commonly have seawater-like REY distributions is not
consistent with REY scavenging by Fe-oxyhydroxides in a manner similar to that
observed in modern oceans. As shown in Fig. 8, adsorption-desorption reactions at
equilibrium strongly fractionate the REY between metal-oxides and seawater. If
Archean IFs precipitated from seawater as Fe(III) minerals, then their observed REY
distributions would require quantitative scavenging of all REY in solution, perhaps in a
manner analogous to the laboratory co-precipitation of REE-Fe-oxide phases from
seawater in early studies (e.g., Goldberg et al., 1963). However, the exact nature of the
primary IF precipitate remains unresolved, though data presented in Chapter 4 suggests
that initial deposition of Fe(II) mineral phases may have been important. Regardless, the
seawater-like REY distributions in Archean IFs, and the lack of other suitable archives,
indicates that IFs offer the best opportunity for discerning the Nd isotopic composition
of >2.7 Ga seawater.
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