Reply to bg-2018-513-RC3 (Anonymous Referee)

Abstract

This is a very nice and detailed study of organic matter in recent, relatively unaltered cherts. Indeed, a good case is made for variable maturity as a result of localized hydrothermal circulation. I have some points of criticism (mostly focusing on the interpretation of the Raman spectral analyses), but these are not critical. There are some issues (as described below) that need to be clarified better, and some references to literature on these issues should be made. Overall, this manuscript can be published after only minor revisions. Comment from referee: 1) A laser power of 1 mW was used during Raman spec- C1 BGD Interactive commentPrinter-friendly version Discussion paper troscopy. These kerogen fractions are very immature, with derived temperatures as low as 40 C. For such unaltered, fragile material, a laser power of 1mW is quite high. Did the authors test if the laser actually affects the kerogen during analysis? For instance causing alteration, or worse, cause combustion?. This should be demonstrated, by a comparison analysis using lower laser power (e.g. 0.1 mW).Author's response: We agree, and we are fully aware of this problem. In our study, laser energy and exposure time were optimized on representative organic-bearing test spots prior to analyzing the actual spots selected for presentation in the manuscript. With the resulting protocol the degradation of organics (during laser irradiation) was found to be minor.Changes planned: We will describe the laser power test in the "Materials and Methods" section (2.6 Raman Spectroscopy).Comment from referee: 2) The very low temperature of alteration (as low as 40C), and the presence of biomarkers for specific groups of prokaryotes, suggests that the Raman spectra of the organic fractions do not only reflect degree of alteration, but also could reflect the type of biologic precursor. For instance, this is suggested by Qu et al. (2015, Astrobiology, 15, 825-841) for carbonaceous fractions found in e.g. the Rhynie chert and the Bitter Springs chert. This should at least be expressed as a possibility, that the Raman-based geothermometer (I don't know if Schito et al., 2017, actually address this issue) is influenced by the type of biomass.Author's response: We agree, this is certainly an important point.Changes planned: We will refer to the study by Qu et al. (2015) and include the information that the obtained low temperature Raman data possibly reflect both, thermal maturity and the specific type of biological precursor. Comment from referee: 3) The Raman spectra that are presented in Fig. 2 are not of high quality. There is a very low signal to noise ratio. The presented peak-fitting C2 BGD Interactive commentPrinter-friendly version Discussion paper protocol, however, is quite sophisticated and requires a high-quality spectrum. It should be explained in detail then, what the uncertainties actually are of fitting these peaks to the range of Raman spectra that were obtained. Also, in general, the calibration of lowtemperature Ramanbased geothermometers is qu...