Organic signatures in Pleistocene cherts from Lake Magadi (Kenya), analogs for early Earth hydrothermal deposits

Abstract

<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) – important analogs for Archean cherts. The Magadi cherts contain low organic carbon (< 0.4 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 – 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>

Microbial biomarkers reveal a hydrothermally active landscape at Olduvai Gorge at the dawn of the Acheulean, 1.7 Ma

“…5B). A similar phenomenon has been observed at the Pleistocene cherts from Lake Magadi, Kenya, where the biomarker profile showing a co-occurrence of fresh and mature organic matter, MAGEs, tetrahymanol, and alkanols has been interpreted as the result of the presence of a hydrothermal setting (47).…”

Earth’s oldest tsunami deposit? Early Archaean high‐energy sediments in the ca 3.48 Ga Dresser Formation (Pilbara, Western Australia)

“…The presence of abundant chert beds in the studied succession (Figures 2 and 5) and elsewhere throughout the Palaeoarchaean successions of the Pilbara and Kaapvaal cratons (Ledevin, 2019; Van Kranendonk, 2006) indicates that Palaeoarchaean submarine sedimentation occurred under high SiO 2 (and bicarbonate) concentrations of seawater (Maliva et al, 2005; Siever, 1992). Highly SiO 2 enriched water is known from today's Lake Magadi in Kenya—a potential analogue for some Archaean hydrothermal environments (Reinhardt et al, 2019)—where evaporation of lake water causes the formation of brines with densities of up to 1.4 g/cm 3 (Eugster, 1970). In the Palaeoarchaean, active venting of silica‐rich hydrothermal fluids was pervasive (Hofmann & Harris, 2008; Van Kranendonk, 2006), and perhaps particularly so in the restricted Dresser environment, resulting in even higher SiO 2 concentrations (cf.…”

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

“…Author's response: Most Raman geothermometers, including those mentioned in this referee comment, focus on temperatures above 150 • C, so we feel that they cannot be usefully applied here. Schito et al (2017) appear to be the only authors attempting Raman thermometry below 100 • C.…”

“…The most important ones are the Ramanbased determination of H/C-ratio by Ferralis et al (2016, Carbon, 108, 440-449) and the D1-peak-based geothermometer of Kouketsu et al (2014, Island Arc, 23, 33-50).Author's response: Most Raman geothermometers, including those mentioned in this referee comment, focus on temperatures above 150 • C, so we feel that they cannot be usefully applied here. Schito et al (2017) appear to be the only authors attempting Raman thermometry below 100 • C.Changes planned: CoD-values (R2) for the fittings and a word of caution (see point 2 above) will be added to the manuscript. The signal-to-noise ratio has been addressed under point 1 (see above).…”