<p><strong>Abstract.</strong> Organic matter in Archean hydrothermal cherts may provide an important archive for molecular traces of earliest life on Earth. The geobiological interpretation of this archive, however, requires a sound understanding of organic matter preservation and alteration in hydrothermal systems. Here we report on organic matter (including molecular biosignatures) enclosed in hydrothermally influenced cherts of the Pleistocene Lake Magadi (Kenya; High Magadi Beds and Green Beds) &#8211; important analogs for Archean cherts. The Magadi cherts contain low organic carbon (<&#8201;0.4&#8201;wt.%) that occurs in form of finely dispersed clots, layers, or encapsulated within microscopic carbonate rhombs. Both, extractable (bitumen) and non-extractable organic matter (kerogen) was analyzed. The bitumens contain immature <q>biolipids</q> like glycerol mono- and diethers (e.g., archaeol and extended archaeol), fatty acids and &#8211; alcohols indicative for, inter alia, thermophilic cyanobacteria, sulfate reducers, and haloarchaea. However, co-occurring <q>geolipids</q> such as <i>n</i>-alkanes, hopanes, and polycyclic aromatic hydrocarbons (PAHs) indicate that a fraction of the bitumen has been thermally altered to early or peak oil window maturity. This more mature fraction likely originated from defunctionalization of dissolved organic matter and/or hydrothermal petroleum formation at places of higher thermal flux. Like the bitumens, the kerogens also show variations in thermal maturities, which can partly be explained by admixture of thermally pre-altered macromolecules. However, findings of archaea-derived isoprenoid moieties in some of the kerogens indicate that a fast sequestration of microbial lipids into kerogen must have occurred while hydrothermal alteration was active. We posit that such early sequestration may enhance the survival of molecular biosignatures during in-situ hydrothermal (and post-depositional) alteration through deep time. Furthermore, the co-occurrence of organic matter with different thermal maturities in the Lake Magadi cherts suggests that similar findings in Archean hydrothermal deposits could partly reflect original environmental conditions, and not exclusively post-depositional overprint or contamination. Our results support the view that kerogen in Archean hydrothermal cherts may contain important information on early life. Our study also highlights the suitability of Lake Magadi as an analog system for hydrothermal chert environments on the Archean Earth.</p>
Recent studies have shown that biosignatures of ancient microbial life exist in mineral coatings in deep bedrock fractures of Precambrian cratons, but such surveys have been few and far between. Here, we report results from southwestern Sweden in an area of 1.6–1.5 Ga Paleoproterozoic rocks heavily reworked by the 1.14–0.96 Ga Sveconorwegian orogeny, a terrane previously scarcely explored for ancient microbial biosignatures. Calcite‐pyrite‐adularia‐illite‐coated fractures were analyzed for stable isotopes via Secondary Ion Mass Spectrometry (δ13C, δ18O, δ34S) and in situ Rb/Sr geochronology via Laser‐ablation inductively coupled plasma mass spectrometry. The Rb/Sr ages for calcite‐adularia and calcite‐illite show that several fluid flow events can be discerned (797 ± 18–769 ± 7, 391 ± 5–387 ± 6, 356 ± 5–347 ± 4, and 301 ± 7 Ma). The δ13C, δ18O and 87Sr/86Sr values of different calcite growth zones further confirmed episodic fluid flow. Pyrite δ34S values down to −49.9‰V‐CDT, together with systematically increased δ34S from crystal core to rim, suggest formation following microbial sulfate reduction under semi‐closed conditions. Assemblages involving MSR‐related pyrite generally have Devonian to Permian Rb/Sr ages, indicating an association to extension‐related fracturing and fluid mixing during foreland‐basin formation linked to Caledonian orogeny in the northwest. An assemblage with an age of 301 ± 7 Ma is potentially related to Oslo Rift extension, whereas the Neo‐Proterozoic ages relate to post‐Sveconorwegian extensional tectonics. Remnants of short‐chained fatty acids in the youngest calcite coatings further indicate a biogenic origin, while the absence of organic molecules in older calcite is in line with thermal degradation, potentially related to heating during Caledonian foreland basin burial.
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...
The title suggests more than the contents of the full article, since: 1) Cyanobacteria, Algae, Higher plants, ciliates, fungi and many bacteria and Archaea present in the Pleistocene setting were not present during the "early Earth", i.e. the (early) Archean. . ., Author's response: We agree in that the organic matter sources of modern and Archean organic matter were certainly not identical. We therefore will use a more cautious wording of the title, avoiding the term 'analog' in respect to organic matter.
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