School of Engineering and Science - Jacobs University

Chapter 1. Introduction
1.1. Statement of research goal
The research presented in this thesis relates to physical and chemical processes
occurring in seawater during the first 2 billion years (2 Ga) of Earth’s history, and
particularly addresses seawater chemistry ca. 3.0 Ga. As samples of seawater from this
time period are not available, except perhaps as microscopic fluid inclusions in some
geologic samples, the research fundamentally rests upon archives of seawater chemistry
that exist in the rock record. For studies of the Archean oceans (ca. 2.5-4.0 Ga) archives
of seawater chemistry are limited due to the scarcity of the rock record. However,
banded iron-formations (IFs) are common and extensive constituents of the Archean
geologic record, and the work described here exclusively deals with IFs and Fe-rich
sediments associated with IF deposition. Iron-formations were formally defined by
James (1954) as “a chemical sediment, typically thin-bedded or laminated, containing
15 per cent or more iron of sedimentary origin, commonly but not necessarily
containing layers of chert”. These chemical sediments are clearly of marine origin (see
Simonson, 2003, for details), and due to their ubiquitous occurrence during the
Archean, IFs are likely one of the best seawater archives prior to ~2.7 Ga.
One question regarding Archean oceans is the extent to which seawater chemistry
was controlled by weathering of oceanic or continental crust. Different conclusions have
been drawn, with some workers supporting weathering of continental crust as a
dominant control on seawater chemistry (e.g., Miller and O’Nions, 1985), while others
have supported hydrothermal alteration of oceanic crust as a primary control (Jacobsen
and Pimentel-Klose, 1988a, 1988b; Veizer et al., 1989; Alibert and McCulloch, 1993;
Bau et al. 1997a). The research presented here attempts to resolve these discrepant
conclusions through new trace metal and isotopic data for ca. 3.0 Ga IFs. A secondary
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