Molybdenum isotopes in late Archean carbonate rocks: Implications for early Earth oxygenation

We present carbonate Mo isotope data of the 2.64-2.5 Ga old Ghaap Group (Transvaal Supergroup, South Africa) to contribute to the debate on early Earth oxygenation. Mo fractionation in carbonates has just recently been introduced as a potential new proxy for ocean redox chemistry and it was suggested that under favorable conditions non-skeletal carbonates closely monitor the ambient seawater delta(98/95) Mo composition. The record presented here is characterized by three phases of moderately to strongly fractionated delta(98/95) Mo with values of up to 1.6 parts per thousand, interrupted by two shifts towards detrital levels. This near-continuous presence of heavy delta(98/95) Mo suggests a quasi-permanent availability of free atmospheric O-2, because fractionation relative to the continental background signature occurs exclusively when Mo is present as dissolved molybdate ions (MoO42-). We compare our carbonate data to a previously published black shale Mo record on samples from the same drill core. The general interpretation of both records is identical: a rise in O-2 preceded the commonly accepted transition to a consistently oxygenated atmosphere by similar to 150 Ma. However, in the Ghaap Group carbonates fractionated Mo is found at a time when the coeval black shales still indicate detrital sedimentation. A combination of detritus admixture and/or redox controlled Mo isotope fractionation during shale formation in suboxic to anoxic waters is suggested as cause for a period characterized by a persistent offset between lighter shale- and heavier carbonate delta(98/95) Mo. The apparent offset can then be attributed to the different geochemical behavior of the two archives with respect to the local depositional conditions (redox- vs. non-redox-dependent). Such changes in the sedimentary environment might also explain why the carbonates do not reproduce rapid fluctuations found throughout the shale record but show a continuous signature with scarce outliers. Thus, by means of the combined archives the conspicuous delta(98/95) Mo scatter found in the shale record could be attributed to variable depositional conditions during its formation rather than global oxic/anoxic fluctuations. Simultaneously, the carbonates provide a better approximation of the long-term delta(98/95) Mo seawater evolution. This study introduces the Mo isotopic composition of carbonates as a promising new tool for paleo-redox studies, in particular when combined with black shales. (C) 2010 Elsevier B.V. All rights reserved.