HR: 1340h
AN: V43C-1435    [Abstracts]
TI: Evolution of Redox State of Shallow and Deep Seawater
AU: * Komiya, T
EM: tkomiya@geo.titech.ac.jp
AF: Tokyo Institute of Technology, Meguro-ku, Tokyo, 152-8551 Japan
AB: Redox state of seawater and atmosphere of early Earth is still controversial. Generally speaking, many previous works indicated that oxygen was free even in the shallow seawater before 2.7 Ga, and gradually increased because of emergence of oxygen-producing lives since then. However, it is still poorly known the detailed secular change of redox state of shallow and deeper part of the seawater, respectively. It is well known that carbonates are deposited equilibrated with ambient seawater in microbial or abiotic environment. Therefore, the distribution, composition, and mineralogy of the carbonate rocks and minerals give constraints on physical and chemical properties of paleoseawater. This work presents in-situ analyses of major, trace and rare earth elements of well-preserved carbonate minerals in shallow and deep-sea deposits with EPMA and LA-ICP-MS. We focus on carbonates with original depositional textures because of elimination of post-depositional alteration. The shallow marine deposits include sedimentary carbonates in 3.0 Ga Pongola, 2.7 Ga Tumbiana, 2.5 Ga Wittenoom and Campbellrand, 2.4 Ga Mooidraai, 2.3 Ga Kazput, 2.2 Ga Duck Creek, 1.9 Ga Slave, 1.0 Nepal, 0.58 Ga Altai and modern Solomon Islands, and amygdaloidal carbonates within hot-sport basalts in 3.5 Ga North Pole, 2.7 Ga Belingwe, Mount Roe and Maddina, 2.5 Ga Hamersley, and modern OIB. The deep-sea carbonates include amygdaloidal carbonates within mid-oceanic and mature rift-type basalts in 3.5 Ga North Pole, 2.7 Ga Belingwe, 2.5 Ga Hamersley, 1.9 Ga Glengarry and modern MORB. Deep-sea carbonates have LREE-enriched pattern with faint Ce and Eu anomalies between 3.5 and 1.9 Ga, but modern equivalents have significant negative Ce anomaly. In contrast, negative Ce anomalies in shallow carbonates were frequently deviated from those in deep-sea carbonate with the equivalent ages. The negative Ce anomalies increase since 2.78 Ga Mount Roe Basalt, but they decreased until 2.72 Ga, again. They significantly increased after 2.6 Ga, but vanished just after 2.4 Ga global glaciation, again. They gradually increased between 2.2 and 1.0 Ga, but vanished just after 0.8 Ga global glaciation, again. They suddenly increased since 0.6 Ga. The evidence implies complicated secular change of redox state even in shallow water as well as in spite of presence of free-oxygen in atmosphere, paleoseawater was anoxic until Proterozoic in deep-sea environment.
DE: 8125 Evolution of the Earth
DE: 3670 Minor and trace element composition
DE: 1065 Trace elements (3670)
DE: 1635 Oceans (4203)
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting