CONSTRAINTS ON ARCHEAN OXYGEN FROM MANTLE ISOTOPE SYSTEMATICS

Various redox proxies in ancient sediments suggest that atmospheric O₂ was low before 2.3 Ga. However, these arguments are vulnerable to the episodic nature of the sedimentary record; they can be challenged as representing atypical periods of time or unusual depositional settings.

An answer to such ambiguities may lie in a surprising location: The Earth's mantle. Solid earth geochemists have long noted the "Pb paradox", the observation that ²⁰⁶Pb/²⁰⁴Pb vs. ²⁰⁷Pb/²⁰⁴Pb in MORB and OIB defines a linear array with an "age" of 1.5 -- 2.0 Ga rather than the Geochron age of 4.55 Ga. Solutions involve ongoing creation of mantle reservoirs enriched in U/Pb (HIMU) compared to the bulk Earth. Integration of U-Pb systematics into numerical simulations of mantle convection show that the paradox is solved if HIMU regions were absent in the Archean [1]. This condition is satisfied if there was a post-Archean increase in the flux of U relative to Pb to the mantle via subduction. The potential connection to Earth's surface redox state lies in the fact that most of the U in the downgoing slab is derived from U dissolved in seawater, in the form of authigenic U in sediments and in hydrothermally-altered basalts. This non-detrital U is transported from the crust to the oceans dissolved in rivers. Because the mobility of U, in contrast to Pb, increases with rising O₂, the Pb paradox can be solved if the Archean atmosphere was insufficiently oxidizing to mobilize U; a rise in O₂ after 2.3 Ga would have increased the flux of U relative to Pb to the seafloor and ultimately into the mantle  [1, 2 and references therein]. A similar argument can explain ²³²Th/²³⁸U and ²⁰⁸Pb/²⁰⁶Pb systematics [2]. Recent measurements of redox-sensitive metals in Precambrian sediments support this scenario [3].

Although it is not the only solution to the Pb paradox [1], this redox scenario requires that low O₂ was the norm throughout the Archean. Hence, if this solution proves correct, sedimentary evidence of low O₂ in the Archean cannot be ascribed to unusual depositional settings or transient episodes of anoxia. Models of mantle geochemistry may provide a valuable time-integrated perspective on surface redox evolution that bridges the gaps of the sedimentary record.

1.     Xie and Tackley, JGR 109: B11204, 2004

2.     Elliott et al., EPSL 169: 129, 1999

3.     Scott et al., this conference