『Abstract
Fe isotopes can provide new insight into redox-dependent biogeochemical
processes. Precambrian iron formations (IF) are deserving targets
for Fe isotope studies because they are composed predominantly
of authigenic Fe phases and record a period of unprecedented iron
deposition in Earth's history. We present Fe isotope data for
bulk samples from 24 Archean and Proterozoic IF and eight Phanerozoic
Fe oxide-rich deposits. These data reveal that many Archean and
early Paleoproterozoic iron formations were a sink for isotopically
heavy Fe, in contrast to later Proterozoic and Phanerozoic Fe
oxide-rich rocks. The positive δ56Fe values in IF are
best explained by delivery of particulate ferric oxides formed
in the water column to the sediment-water interface. Because IF
are a net sink for isotopically heavy Fe, there must be a corresponding
pool of isotopically light Fe in the sedimentary record. Earlier
work suggested that Archean pyritic black shales were an important
part of this light sink before 2.35 billion years ago (Ga). It
is therefore likely that the persistently and anomalously low
δ56Fe values in shales are linked with the deposition
of isotopically heavy Fe in IF in the deeper parts of basins.
IF deposition produced a residual isotopically high dissolved
Fe pool that was captured by pyritic Fe in shales. Local dissimilatory
Fe reduction in porewater and associated diagenetic reactions
resulting in pyrite and carbonate precipitation may have further
enhanced Fe isotope heterogeneity in marine sediments, and an
‘iron shuttle’ may have transported isotopically light Fe from
shelf sediments to the basin. Nevertheless, water-column processing
of hydrothermally delivered Fe likely had the strongest influence
on the bulk iron isotope composition of Archean and Paleoproterozoic
iron formations and other marine sediments.』
1. Introduction
2. Analytical methods
3. Sample details
4. Results
5. Discussion
5.1. Iron-formation depositional processes
5.2. Comparison with previous Fe isotope studies of IF
5.3. iron isotope mass balance in the Earth's early oceans
6. Conclusions
Acknowledgments
Appendix A. Supplementary data
References