nai

Project Progress

Archean carbonate minerals are often assumed to be direct precipitates of seawater, therefore acting as proxies to infer conditions of the ancient earth. More specifically, banded iron formation (BIF) carbonates record some of the largest excursions yet observed in C, O, and Fe isotope compositions for marine sedimentary rocks (Figure 1), and the origin of these variations have been highly debated. Large excursions towards highly negative δ¹³C values have been interpreted to reflect an ocean stratified in C isotope compositions for inorganic carbon (Beukes and Klein, 1990; Klein, 2005; Winter and Knauth, 1992), or, alternatively, microbial oxidation of organic matter during authigenic mineral formation (Becker and Clayton, 1972; Baur et al., 1985; Fischer et al., 2009). Low-δ¹⁸O values in Precambrian carbonates and cherts have been interpreted to record high seawater temperatures (e.g., Knauth, 2005), or to reflect seawater that had negative δ¹⁸O values (e.g., Kasting et al., 2006). The origin of variations in Fe isotope compositions of Precambrian carbonates and shales has been debated, and has included interpretations that the isotopic compositions reflect changing ocean δ⁵⁶Fe values (Rouxel et al., 2005; von Blanckenburg et al., 2008), or that the δ⁵⁶Fe values represent diagenetic processes that record microbial dissimilatory iron reduction (DIR) (Yamaguchi et al., 2005; Beukes and Gutzmer, 2008; Johnson et al., 2008 a, b).

Two studies have been completed on this project, the first of which involved Fe, C, and O isotope compositions (Heimann et al., 2010) of Fe-carbonates from the 2.5 Ga Kuruman Iron Formation and underlying platform carbonates (calcite/dolomite) from the Gamohaan Formation, Transvaal Craton, South Africa. The second study focused on the Rb-Sr isotope compositions (Ludois et al., submitted) of the same material previously analyzed for Fe, C, and O isotope compositions, as well as Nd isotope and REE variations of interbedded shales. The Kuruman IF is correlative with the Hamersley Basin BIFs of NW Australia, but were subjected to lower grades of metamorphism, and therefore more likely to record unmodified isotope compositions in the carbonate minerals. Previous isotopic studies of these carbonates demonstrated large Fe isotope variability (δ⁵⁶Fe= +1 to -1‰), low δ¹³C values (-20 to -1‰), and low δ¹⁸O values of ~21‰ in Fe-rich carbonates, relative to compositions expected for precipitation from seawater. Testing the validity of seawater proxies such as BIF carbonate minerals provides important tests of proposals that the ocean was stratified in δ13C values, had high ocean temperatures, or that atmospheric CO₂ was high. In addition, these results provide insight into the cause of large excursions in δ⁵⁶Fe values in the Neoarchean and Paleoproterozoic, which has been subject to great debate. Based on the C, O, and Fe isotope compositions, Heimann et al. (2010) inferred that the Fe carbonates in the Kuruman BIF were not in isotopic equilibrium with seawater when they formed, but instead reflect Fe carbonates that formed in the sedimentary pile prior to lithification, where dissimilatory iron reduction (DIR) mobilized isotopically light aqueous Fe, leaving the residual material isotopically heavy. This study provides important evidence for DIR as a metabolic pathway involved in BIF formation in the Neoarchean, and does not support the model proposed by Rouxel et al., (2005) that large Fe isotope excursions reflect changing ocean δ⁵⁶Fe values.

Our second study (Ludois et al., submitted) involved Rb-Sr isotope determinations on the same material previously analyzed for C, O, and Fe isotope composition by Heimann et al., (2010). Strontium is an important tracer of past ocean chemistry because Sr is isotopically homogenous in the oceans at any point in time, reflecting its long residence time, and, importantly, there is no significant fractionation during mineral precipitation, unlike stable isotope systems such as C, O, and Fe. Our results show that most Fe-carbonates have extremely radiogenic measured and initial ⁸⁷Sr/⁸⁶Sr ratios that plot far from the Sr isotope composition of Neoarchean seawater (⁸⁷Sr/⁸⁶Sr ~ 0.702-0.705) (Figure 2). The low ⁸⁷Rb/⁸⁶Sr ratios for most of the carbonates analyzed results in little age correction to the measured ⁸⁷Sr/⁸⁶Sr ratios. More importantly, there is no correlation between initial ⁸⁷Sr/⁸⁶Sr ratios and ⁸⁷Rb/⁸⁶Sr ratios, precluding physical clay (high Rb) contamination as an explanation for the radiogenic Sr isotope compositions. The origin of the high ⁸⁷Sr/⁸⁶Sr ratios in Fe-rich carbonate is likely a result of incorporation of radiogenic Sr derived from dispersed clay particles through ion-exchange in the soft sediment during authigenic mineral formation, prior to lithification. Such a process also explains the low δ¹³C and δ¹⁸O values of these Fe carbonates, as well as their wide range in δ⁵⁶Fe values, over small scales (Figure 3).

Combining the data from the studies provides strong evidence iron-rich carbonates cannot be assumed to be direct precipitates of seawater. The fact that the isotopic compositions of C, O, Fe, and Sr are decoupled indicate that these elements do not follow the same pathways during carbonate formation, and suggest that multiple chemical and isotopic proxies are needed to infer a seawater origin of carbonates. For example, REE contents of carbonates are a commonly used approach for inferring ancient seawater chemistry, but the results of this work demonstrates that this cannot be assumed. These results demonstrate that multiple isotopic systems, including those not directly cycled by life, provide important constraints on interpreting isotopic biosignatures in ancient rocks.