The use of chromium reduction in the analysis of organic carbon and inorganic sulfur isotope compositions in Archean rocks

One of the most serious issues with deciphering the evolution of organisms and their biogeochemical environments from the ancient rock record is the difficulty in obtaining well-preserved samples. Although not much can be done to avoid diagenetic and metamorphic alteration when they have occurred, alteration due to weathering can be avoided by working on drill core samples. This implies however that the amount of sample is limited, which may in turn restrain the number of possible chemical and isotopic analyses that can be performed. In order to save sample we show here that the chemical protocol used for the sulfur sulfide extraction (for later sulfur isotope analyses) is also suitable to decarbonate samples (for later organic carbon isotope analyses). In the case of carbonated rocks, both sulfur sulfide extraction and decarbonation require high amounts of sample so that coupling them may save a significant amount of sample and time. In addition it allows both organic carbon (TOC and δ13C) and sulfur isotope composition measurements to be performed on the exact same powder, which is essential when trying to understand couplings between S and C cycles in heterogeneous samples. We thus tested the efficiency of the acidic chromium solution, commonly used to extract sulfur from sulfide, for sample decarbonation on various Archean rocks. Our results show that no significant carbon isotope fractionation is caused by this new decarbonation protocol, even for the samples with low organic carbon content. The chromium solution seems to be perfectly adapted for the analysis of organic matter in the ancient rock record, at least when the rock samples have experienced low greenschist facies metamorphism. Further tests will be needed to verify if this protocol can also be used for less mature organic matter.