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Unlike modern Mn(IV)-dominated manganese nodules, ancient manganese deposits primarily contain Mn(II) (manganous) mineral phases. The general assumption is that these manganous minerals formed post-depositionally from a Mn(IV) precursor phase during diagenesis and metamorphism. However, recent studies have shown that aqueous Mn(II) may combine with either CO 3 2– (aq) or H 4 SiO 4(aq) to nucleate and precipitate as primary manganous phases within anoxic waters. These findings thus make it difficult to assess what comprised the primary mineral assemblage of ancient Mn deposits, which in turn, has implications for understanding the redox fabric of the past marine water column. Here, we investigate the paragenesis and oxidation state of various Mn minerals (oxides, carbonates, and silicates) from the late Devonian Xialei and middle to late Permian Zunyi manganese deposits in South China. Through characterization of the micro-scale distribution, composition, and speciation of different Mn-bearing mineral phases, we provide evidence to support an oxidative mechanism during Mn cycling at the time of deposition. We document the presence of a residual oxide phase (hausmannite; Mn 3 O 4) that represents a primary to early formed Mn(III)-bearing oxide preserved in Mn(II)-carbonate nodules. Additionally, we observe spherulitic textures comprised of mixed-valence Mn(II,III,IV)-silicates enclosed in diagenetic Mn(II)-carbonate laminations. Our results allow for the construction of a paragenetic model for these deposits beginning with the reduction of a primary Mn(IV) phase and the subsequent precipitation of intermediate Mn(III)-oxide or -silicate mineral phases. Following continued exposure to reducing conditions during early diagenesis, Mn(III)-oxides were then replaced by Mn(II)-carbonates. Similarly, Mn(III)-silicates were transformed into various Mn(II)-silicates during late-stage diagenesis and metamorphism. This model suggests that the accumulation of various divalent Mn mineral phases in ancient sediments may, therefore, be indicative of deposition under a water column with a sufficient oxidizing potential to induce Mn(II) oxidation. [ABSTRACT FROM AUTHOR]

Understanding the process of CO 2 degassing during precipitation of calcite from a Ca2+–HCO 3 − solution is crucial for interpreting isotope compositions in the calcite precipitates. Unlike diffusion-controlled outgassing, i.e., dissolved CO 2 escaping from the solution via diffusion, degassing caused by precipitation of calcite is accompanied by a large carbon isotope fractionation between CO 2 (g) and HCO 3 − due to breaking of C O bond, with an equilibrium fractionation of ca. −9‰ at ambient temperature. Such a magnitude of fractionation has a great influence on carbon isotope compositions (δ13C) of DIC (dissolved inorganic carbon) reservoir in the solution and thus on δ13C of calcite precipitated from it. However, knowledge on isotope fractionation is limited during precipitation-associated degassing of CO 2 in a supersaturated solution where rapid calcite precipitation drives CO 2 degassing out of isotopic equilibrium. Here we show the data of water chemistry and carbon isotope compositions of DIC and carbonate precipitates in a mountain stream at Baishuitai, China. Results from numerical models show there exist large degrees of carbon isotope disequilibrium during precipitation-associated degassing of CO 2 via HCO 3 − dehydration and dehydroxylation. Average carbon isotope fractionation between CO 2 (g) and HCO 3 − ( ε C O 2 (g) - HC O 3 - ) calculated from our dataset is about −20‰ which is much lower than the equilibrium value. Moreover, an inverse correlation between ε C O 2 (g) - HC O 3 - and precipitation rate was observed, indicating higher precipitation rates cause ε C O 2 (g) - HC O 3 - farther from equilibrium value. By compiling the data from this study and literatures, we infer that disequilibrium isotope fractionation of carbon between CO 2 (g) and HCO 3 − may be common during the growth of speleothem and travertine from a solution supersaturated with respect to calcite. The rate dependence of ε C O 2 (g) - HC O 3 - has special implications for speleothem archives from ventilated caves. As partial pressure of CO 2 in cave atmosphere evolves with ventilation, variable precipitation rates of calcite will cause inconstant degrees of disequilibrium isotope fractionation between CO 2 and DIC and thus perturb the time-series of speleothem's δ13C records for paleo-environmental reconstruction. [ABSTRACT FROM AUTHOR]

Abstract: Travertine δ18O values can be used to reconstruct paleo-temperatures if the oxygen isotope fractionation factors between travertine and water are accurately understood. For this purpose, the δ18O values of pool travertine and its parent water, and the deposition rates of the calcite were investigated at Baishuitai (Yunnan, SW China) over the course of the full hydrological year, April 23 2006–April 25 2007. The results show that the travertine–water isotope fractionation factors are close to the commonly accepted equilibrium line of Kim and O’Neil (1997). This differs from the results obtained by Yan et al. (2012) who found that the oxygen isotope fractionation factors in the travertine-depositing pools were close to the line suggested as equilibrium relationship by Coplen (2007). The average calcite deposition rate (2.30mgcm−2 d−1) in the present study is six times larger than that (0.38mgcm−2 d−1) in Yan et al. (2012). If slower calcite precipitation leads to equilibrium oxygen isotopic fractionation, then the results of this study support the results of Coplen (2007) that indicate that the equilibrium fractionation factor may be greater than the commonly accepted one derived by Kim and O’Neil (1997). The relationship between oxygen isotope fractionation factor and calcite deposition rate in our study also agrees with the results of Dietzel et al. (2009) who found that the kinetic-isotope effect favors preferential incorporation of 16O in solid calcite as the calcite deposition rate increases. There was a threshold for calcite precipitation rate control on oxygen isotopic equilibrium. In our case of travertine-depositing pools, when the calcite deposition rate was lower than 0.38mgcm−2 d−1, oxygen isotopic equilibrium between calcite and water was attained. Therefore, calcite deposition rate is a potentially important consideration when using δ18O in natural carbonates as a proxy for terrestrial and ocean temperature. [Copyright &y& Elsevier]