School of Engineering and Science - Jacobs University

ABSTRACT
The neodymium (Nd) isotopic composition of circa three billion-year-old (Ga)
seawater is inferred from studies of banded iron-formations (IF) from South Africa.
Iron-formations from the 2.9 Ga Pongola Supergroup display negative Є Nd values that
are indistinguishable from local shale, suggesting that Nd derived from weathering of
continental crust dominated the Nd isotopic composition of shallow seawater. In
contrast, Є Nd (t) in IF from the 2.95 Ga Pietersburg greenstone belt is clearly
distinguishable from local clastic material, and indicates that where Nd was not sourced
from local crust, seawater could possess more radiogenic Є Nd values of +1. The
Pietersburg IF Є Nd (t) values are therefore interpreted to represent bulk Archean seawater
not affected by local crustal Nd signatures. This Є Nd value of +1 is very similar to
published Є Nd (t) estimates for Archean seawater, when only those IFs which display rare
earth element ratios (REE) common to seawater are considered. Screening the data in
such a manner clearly identifies seawater precipitates, and relatively constant Є Nd (t) of
+1 to +2 for Archean IFs suggest these values are a good average for 2.5-3.8 Ga
seawater.
If bulk Archean seawater typically displayed Є Nd (t) of +1, then hydrothermal
alteration of mafic oceanic crust is considered the likely process for delivering relatively
radiogenic Nd to seawater. Data from this study and previous work suggest significant
high temperature (T) fluid input to seawater between 2.5-3.8 Ga, as supported by
positive Eu anomalies in IFs, and the absence of negative Ce anomalies in any Archean
IFs indicates that seawater was more reducing than modern seawater. Modern high-T
fluids altering mafic oceanic crust possess both positive Є Nd values and Eu anomalies,
and it is possible that these reducing, Fe-rich fluids exerted a greater control on Archean
seawater chemistry than is observed today.
However, the negative Є Nd (t) values for the Pongola IF data indicate that locally,
shallow seawater could be isotopically distinct, and possess an Nd isotopic signature
identical to local crust. If world seawater was isotopically homogenous with respect to
Nd between 2.9-3.0 Ga, these data indicate a significant shift from positive Є Nd values in
seawater during deposition of the Pietersburg IFs, to negative Є Nd values during
deposition of the Pongola IFs. This is considered unlikely, and the Nd isotopic data are
rather interpreted to reflect that, similar to modern oceans, ~3.0 Ga world oceans were
composed of water masses that could be isotopically distinct with respect to Nd.
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