Oxidative Weathering and Euxinia in the Late Archean (Invited)
Abstract
A large body of evidence points to a sharp rise in the concentration of atmospheric oxygen during the Paleoproterozoic between 2.45 and 2.32 billion years ago (Ga), but the history of deep ocean oxygenation is less well known. The deposition of banded iron formation (BIF) during the Archean and early Proterozoic (~3.8 - 1.8 Ga) has been taken to imply that deep ocean water masses were anoxic and rich in dissolved ferrous iron (Fe2+) derived from high temperature weathering of seafloor basalt under low oceanic sulfate (SO42-) concentrations. Reducing and iron-rich (ferruginous) deep ocean conditions are thought to have persisted for most of Earth’s early history, although a notable lack of BIF between 2.4 - 2.0 Ga has rendered deep ocean chemistry during this period obscure. In any case, the cessation of BIF deposition at ~1.8 Ga is generally linked to the oxygenation of the atmosphere through the eventual removal of Fe2+ from the ocean either as ferric (hydr)oxides or as pyrite in euxinic (anoxic and sulfidic) basins. A corollary of the latter model is that oxidative delivery of sulfate to the ocean was not sufficient to remove reactive iron, via microbial sulfide production, before ~1.8 Ga. However, recent studies of the late Archean Mt. McRae Formation suggest that oxidative sulfur cycling may have preceded the Paleoproterozoic rise in atmospheric oxygen and that conditions sufficient to authigenically enrich molybdenum (Mo) in marine sediments existed at ~2.5 Ga. On the modern Earth, significant enrichment of Mo into sediments occurs following the conversion of soluble molybdate (MoO42-) to particle-reactive thiomolybdates (MoO4-xSx2-) in stable sulfidic environments, indicating that the Mo enrichments seen in the Mt. McRae shale may have resulted from the accumulation of free sulfide in the water column. Here, we present iron speciation data for the late Archean Mt. McRae Shale that provide evidence for a euxinic water column at ~2.5 Ga. Sulfur isotope data compiled from the same stratigraphic section, combined with a consideration of sulfide dissolution kinetics, suggest that euxinic conditions could have been stimulated by an increase in oceanic sulfate concentrations attendant to oxidative weathering of sulfide minerals under a still O2-poor atmosphere. Variability in local organic matter flux, which would have limited microbial sulfide production, likely confined euxinic conditions to mid-portions of the water column on the basin margin. These findings indicate that euxinic conditions may have been common on a variety of spatial and temporal scales both prior to and immediately following the rise in atmospheric oxygen, hinting at previously unexplored texture and variability in deep ocean chemistry during the Archean and Paleoproterozoic.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.B12A..03R
- Keywords:
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- 0325 ATMOSPHERIC COMPOSITION AND STRUCTURE / Evolution of the atmosphere;
- 0424 BIOGEOSCIENCES / Biosignatures and proxies;
- 0444 BIOGEOSCIENCES / Evolutionary geobiology;
- 1050 GEOCHEMISTRY / Marine geochemistry