Organic matter in Archean hydrothermal cherts may provide an
important archive for molecular traces of the 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). The Magadi
cherts contain low organic carbon (< 0.4 wt %) that occurs in
the form of finely dispersed clots, layers, or encapsulated within microscopic
carbonate rhombs. Both extractable (bitumen) and non-extractable organic
matter (kerogen) were analyzed. The bitumens contain immature “biolipids”
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 “geolipids” such
as
Organic matter trapped in Archean cherts is of utmost relevance for the reconstruction of the earliest microbial processes on Earth, but its origin is only poorly constrained. Diagenesis and metamorphic processes have been obliterating the original organic matter over billions of years and complicate its interpretation. Many of the Archean cherts are associated with hydrothermal settings, including shallow marine and terrestrial environments (e.g., Brasier et al., 2002; Duda et al., 2016, 2018; Djokic et al., 2017; Hickman-Lewis et al., 2018). In such environments, organic compounds may rapidly decompose due to elevated temperature and pressure conditions (Hawkes et al., 2015, 2016; Rossel et al., 2017) and may also be redistributed via hydrothermal cycling in the form of bitumen (e.g., Weston and Woolhouse, 1987; Clifton et al., 1990; Leif and Simoneit, 1995) or kerogen (Duda et al., 2018). The interpretation of organic signatures in Archean hydrothermal cherts therefore requires detailed knowledge on the preservation, alteration, and distribution pathways of organic matter in such environments. Some of these aspects can be studied in modern analogs.
Archean cherts generally originate from chemical precipitation or replacement processes of silica rather than biogenic precipitation by silicifying organisms (e.g., Sugitani et al., 2002; van den Boorn et al., 2007). Siliceous sediments associated with chemical precipitation are rare on the modern Earth but can be found in some hot spring or shallow lacustrine environments. Important sites include the Taupo Volcanic Zone (New Zealand; Campbell et al., 2003), the Geysir hot spring area (Iceland; Jones et al., 2007; Jones and Renaut, 2010), the El Tatio geothermal field (Chile; Jones and Renaut, 1997; Nicolau et al., 2014), and the East African Rift system (Kenya; Renaut et al, 2002). Among the latter, the alkaline Lake Magadi is of particular interest, as it represents an analog for Archean hydrothermal chert environments (Eugster and Jones, 1968; Pirajno and Van Kranendonk, 2005; Brenna, 2016).
Lake Magadi is located in the lowermost depression
of the East African Rift valley (south Kenya; ca. 1
Lake Magadi has strongly been influenced by changes in the local climate and
tectonics (Owen et al., 2019). Today the Magadi basin represents an
evaporation pan with a closed hydrological cycle (i.e., no outflow) that is
only recharged by ephemeral runoff and hydrothermal springs (ca. 28 to 86
A variety of cherts from Lake Magadi and its surroundings contain microbial structures (Behr, 2002; Behr and Röhricht, 2000; Brenna, 2016). In particular, the Green Bed cherts are associated with fingerprints of microbial activity such as stromatolites and silicified cyanobacteria cells (Behr and Röhricht, 2000). Organic matter archived in these cherts potentially encodes important information of geobiological value but has not been characterized so far.
Our study is focused on the origin, alteration, and preservation of the
organic matter in Pleistocene hydrothermal cherts from Lake Magadi. It is
aimed at assessing syndepositional and early diagenetic hydrothermal effects
on the organic compounds to support the interpretation of organic matter in
early Earth hydrothermal deposits. We essentially consider these
Cherts from the Pleistocene High Magadi Beds (LM-1692–1695) and Green Beds
(LM-1696–1699) were sampled from different surficial outcrops around the
present Lake Magadi (area of the Pleistocene Lake Magadi;
Röhricht, 1999). A modern siliceous sinter from the Great Geysir, Iceland
(IC-1700; 64
Petrographic observation was performed on thin sections using a Zeiss SteREO
Discovery.V8 stereomicroscope connected to an AxioCam MRc5 5-megapixel
camera (transmitted and reflected light) and with a Leica DMLP microscope
coupled to a Kappa Zelos-655C camera (polarized light). Chert fragments
(sputtered with Au-Pd, 7.3 nm, for 120 s) were furthermore investigated using
a LEO 1530 Gemini scanning electron microscope (SEM) coupled with an Oxford
INCA X-act energy dispersive X-ray spectrometer (EDX). Contents of organic
carbon (C
All materials used for biomarker preparation were heated to 500
The extraction residues were decalcified with HCl (37 %, 1 d,
20
HyPy is an open-system pyrolysis technique for studying the molecular kerogen composition (Love et al., 1995). It involves the gentle release of kerogen-bound compounds through progressive heating under a high-pressure hydrogen atmosphere (150 bar) and in the presence of a sulfided molybdenum catalyst (ammonium dioxydithiomolybdate). HyPy has been demonstrated to be very sensitive and to leave the stereochemistry of released compounds largely intact (e.g., Love et al., 1995, 1997; Bishop et al., 1998; Meredith et al., 2014).
Our experiments were conducted with a HyPy device from Strata Technology
Ltd. (Nottingham, UK), following existing protocols (e.g., Love et al.,
1995; Brocks et al., 2003b; Marshall et al., 2007; Duda et al., 2018). In
brief, between 1 and 10 mg of pre-extracted kerogen (3
Molecular fractions were analyzed using a Thermo Trace 1310 gas
chromatograph (GC) coupled to a Thermo TSQ Quantum Ultra triple quadrupole
mass spectrometer (MS). The GC was equipped with a fused silica capillary
column (Phenomenex Zebron ZB-5MS, 30 m length, 250
Compound-specific stable carbon isotope ratios (
Raman spectroscopy was conducted on sample slices (thickness ca. 1 cm) and
isolated kerogen flakes (see Sect. 2.2). At least 10 measurements were
conducted per sample in order to evaluate internal variation and identify
potential outliers. The measurements were performed with a WITec alpha300
instrument. Spectra (scan range 100–4000 rel. cm
Most of the Lake Magadi cherts studied reveal a dense silica matrix, except
LM-1692 and LM-1693, which show microscopic pores of < 50
Petrographic characteristics of Lake Magadi cherts.
The samples show C
Geochemical bulk data (C, N, S).
The organic matter occurs either layered (up to 0.5 mm; Fig. 1a, h) or
finely dispersed in the form of small clots in the chert matrix (< 20
Raman spectra of isolated kerogen particles show a broad D band centered at
ca. 1354 cm
Environmental and maturity parameters from biomarker analysis
(GC-MS) and Raman spectroscopy. The C
Raman spectroscopy of kerogen isolated from Green Bed chert sample
LM-1697.
Sample LM-1697 exhibited a second kerogen population with D and G bands
centered at 1357 and 1577 cm
The GC-MS chromatograms from bitumens of High Magadi Bed cherts (Fig. 3a–d), Green Bed cherts (Fig. 3e, f), and a modern siliceous sinter from
the Great Geysir (Iceland; Fig. S1i–j in the Supplement) show a variety of organic compounds. The
most noticeable compound classes in all samples are
Odd-to-even predominances (OEPs; Scalan and Smith, 1970) in bitumens and kerogens.
OEPn
Total ion chromatograms (TICs; 10–65 min) of the derivatized
bitumens (alcohols were measured as trimethylsilyl ethers, carboxylic acids
as methyl esters) from the High Magadi Bed cherts
Partial GC-MS ion chromatograms (10–60 min) of the derivatized
alcohol/ketone (F2; alcohols were measured as trimethylsilyl ethers) and
polar (F3; carboxylic acids were measured as methyl esters) fractions from
bitumen of the High Magadi Bed chert LM-1694. Alkanoic acids (
Mean
Partial GC-MS ion chromatograms (10–55 min) of the hydrocarbon (F1) and derivatized alcohol/ketone fraction (F2; alcohols were measured as
trimethylsilyl ethers) from bitumen of the Green Bed chert LM-1698.
Medium-chain (
Glycerol monoethers (1-
Additionally, functionalized sesqui- and diterpenoids are always present, and
traces of C
The samples furthermore contain traces of 17
All samples contain low amounts of (monomethyl-) phenanthrenes, while
anthracene is only observed in IC-1700. The methylphenanthrene indices
(MPI-1, following Radke and Welte, 1983) vary between 0.48 and 1.02, resulting
in calculated vitrinite reflectances (
Results from the low-temperature pyrolysates (up to 330
Partial GC-MS ion chromatograms (
Kerogen-bound
The regular acyclic isoprenoids phytane (Ph) and
2,6,10,14,18-pentamethylicosane (PMI
All kerogen pyrolysates except LM-1694 contain (mono- and dimethylated)
phenanthrenes, anthracene, and various four- and five-ring PAHs. MPI-1 ranges
from 0.89 to 1.69, corresponding to
The studied Lake Magadi cherts are of Pleistocene age and have not been
buried. This is in good accordance with several molecular characteristics of
the bitumens that suggest an immature nature of the organic matter. These
features include the OEP29 of
A similar maturity offset is also reflected in bulk and molecular kerogen
characteristics. A low thermal maturity is for instance indicated by low
Raman-derived
Such offsets between different thermal maturity parameters are typically
related to an emplacement of organic material from another source (e.g.,
modern endoliths; e.g., Golubic et al., 1981; Hallmann et al., 2015). Most
of the Lake Magadi cherts studied reveal a dense silica matrix, but a few
samples (LM-1692, LM-1693) indeed show small pores that would allow for such
emplacements. However, a recent emplacement is unlikely for the following
reasons:
The analyzed samples did not show any viable microbial colonization (e.g.,
biofilms or endolith borings). No carbonaceous microbial remains were discovered via SEM coupled to EDX and
all detected microfossils are silicified (see Fig. 1e). High-temperature HyPy products of kerogens matched up with functionalized
moieties in their corresponding bitumens (e.g., C The Interior-versus-exterior experiments on LM-1692 and LM-1695 revealed similar
concentrations for medium-chain
Consequently, both the rather immature and the thermally altered organic matter can be considered syngenetic to the Pleistocene cherts.
Archaeol and extended archaeol appear in all High Magadi Bed and two Green
Bed chert bitumens (LM-1696 and LM-1699), and their molecular fossils are
important contributors to the corresponding kerogens. While archaeol is a
common constituent of euryarchaeal lipids (e.g., Koga, 1993; Pancost et al.,
2011; Dawson et al., 2012; Villanueva et al., 2014), extended archaeol is
restricted to alkaliphilic and non-alkaliphilic haloarchaea (e.g., De Rosa et al.,
1982; Teixidor et al., 1993; Dawson et al., 2012) and, in traces, to some
methanogens (e.g., Grant et al., 1985; Becker et al., 2016). Archaeol and
extended archaeol were also found in various halophilic archaea from the recent
Lake Magadi (e.g.,
Cyanobacterial contribution to primary production is directly evidenced by
6Me-C
The monoethers found in the High Magadi Bed chert bitumens occur in various
bacteria and are particularly prevalent in sulfate reducers (e.g., Yang et
al., 2015; Vinçon-Laugier et al., 2016, and references therein). Given
the hydrothermally influenced setting, the broad variety of these compounds
in the Magadi cherts (C
All archaeal lipids in bitumens show an enrichment in
Tetrahymanol is typically produced by ciliates (Mallory et al., 1963; Harvey and McManus et al., 1991) but may also originate from few bacteria (e.g., Kleemann et al., 1990; Banta et al., 2015), ferns (Zander et al., 1969), and fungi (Kemp et al., 1984). It is furthermore associated with alkaline environments (e.g., ten Haven et al., 1989; Thiel et al., 1997), which is well in line with the evaporative setting of Lake Magadi.
The presence of only small amounts of typical algal sterols (cholesterol and
sitosterol; cf., Taipale et al., 2016) in the Lake Magadi cherts indicates
minor contributions from these primary producers. Long-chain alkanoic acids
and alkan-1-ols with an OEP29 of
The
In all Lake Magadi cherts a narrow, bell-shaped pattern of
Such hydrothermal processes may also yield compounds through the in situ cracking
of macromolecular organic matter from the cherts (e.g., alkanes and hopanes;
see Sect. 3.2.2). However, temperatures of hydrothermal waters from present
springs at Lake Magadi are not higher than 86
Alternatively, organic matter from older lake sediments (Oloronga Beds) may
have been penetrated by hot fluids, resulting in the formation of
hydrothermal petroleum, a process known from other hydrothermal environments
(e.g., Clifton et al., 1990; Weston and Woolhouse, 1987; Czochanska et al.,
1986; Leif and Simoneit, 1995). This is in good accordance with the early to
peak oil window maturity of some bitumen compounds as for example indicated by the
MPI-1 ratios (
Hydrothermal petroleum generation may furthermore be supported by the
unimodal distribution patterns of medium-chain
The relatively low abundance of PAHs in the bitumens may indicate low
formation temperatures of hydrothermal petroleum (cf., Simoneit, 1984;
Simoneit et al., 1987; Clifton et al., 1990). This could be due to a shallow
sedimentary source which is well in line with the geological situation at
Lake Magadi. The Oloronga Beds (maximal thickness of 45 m; Behr, 2002) are
the oldest sediments in the young rift basin (ca. 7 Ma; Baker, 1958, 1986;
geothermal gradient of ca. 200
Hydrothermal activity may not only have impacted the bitumens. Kerogen from
LM-1697 shows highly mature graphitic particles (Raman-based
We propose that the graphite was produced at depth through the
hydrothermally mediated alteration (cf., Luque et al., 2009) of the
surrounding trachyte and/or by mineral-templated growth (cf., van Zuilen et
al., 2012) during hydrothermal circulation of bitumen-rich fluids. The
hydrothermal fluids may then have transported graphite particles into the
lake. Likewise, thermally altered macromolecular particles from older
lake sediments may have also been introduced by hydrothermal fluids which
would explain the elevated mean Raman temperatures of LM-1692–1693 and
LM-1696 kerogens (Raman-based
The occurrence of thermally mature organic components in the studied materials is therefore most likely due to syndepositional hydrothermal processes and reflects an environmental signature.
The visual appearance of organic matter in the Pleistocene Lake Magadi cherts (i.e., in clots, layers, or carbonate rhombs) and its thermal heterogeneity even within given samples is, to some extent, similar to findings from Archean cherts (see Ueno et al., 2004; Allwood et al., 2006; Tice and Lowe, 2006; Glikson et al., 2008; Morag et al., 2016). In the Archean record, varying organic matter characteristics are commonly related to metamorphic processes that significantly postdated chert formation (e.g., Ueno et al., 2004; Tice and Lowe, 2006; Olcott Marshall et al., 2012; Sforna et al., 2014; Morag et al., 2016). Variations in kerogen maturity in the ca. 3.5 Ga old Apex chert (Pilbara Craton, Western Australia) could, for instance, reflect younger hydrothermal alteration events that were entirely unrelated to the original formation of the host rock (Olcott Marshall et al., 2012; Sforna et al., 2014). Our results demonstrate that organic matter of very different nature and maturity may already be enclosed into chert precipitates a priori (i.e., during the initial formation of the deposit, prior to potential metamorphic processes), which is largely driven by hydrothermal circulation. A similar syndepositional, hydrothermally driven mixing of different organic components has been proposed for a variety of Archean chert environments (Allwood et al., 2006; Glikson et al., 2008; Morag et al., 2016; Duda et al., 2018). Our results highlight that thermal heterogeneities of Archean organic matter may indeed reflect syndepositional hydrothermal activity rather than post-depositional metamorphism in some cases (if the maturity is not significantly lower than the estimated peak metamorphic temperature of the host rock).
In addition, our kerogen data indicate that archaeal lipid biomarkers
(C
The fact that the C
The depositional record of Lake Magadi (Kenya) contains Pleistocene cherts
with different maturity fractions of organic matter, a feature similar to
Archean cherts from the Pilbara Craton (Western Australia) and the Barberton
Greenstone Belt (South Africa). We found that a significant portion of the
bitumens (extractable) and kerogens (non-extractable) in these cherts is
thermally immature and contains biomarkers of various prokaryotic
microorganisms (e.g., thermophilic cyanobacteria, sulfate reducers, and
haloarchaea), in line with an evaporitic hydrothermal environment. At the
same time, both the bitumens and kerogens also exhibit a thermally mature
fraction. We explain this apparent offset between different maturity
parameters (immature vs. mature) as a result of syndepositional hydrothermal
alteration (e.g., defunctionalization, pre-maturation) and redistribution of
organic matter in the environment. These processes include hydrothermal
petroleum expulsion in underlying sedimentary units (Oloronga Beds) and the
fluid-based introduction of the resulting cracking products and mature
macromolecules into the lake. Our findings aid in the interpretation of
heterogeneous organic signatures in Archean rocks which may, in cases,
reflect original environmental conditions rather than post-depositional
metamorphism or contamination. In addition, the preservation of archaeal
lipid biomarkers (C
The original samples are stored at the Geoscience Centre, University of Göttingen, Germany. All data are available upon request.
The supplement related to this article is available online at:
MR, VT, WG, JR, and JPD designed the study. MR and JR conducted petrographic
analyses. MR conducted organic-geochemical analyses and catalytic
hydropyrolysis (HyPy). WG and MR performed Raman spectroscopy. CH and MR
conducted
The authors declare that they have no conflict of interest.
We are grateful to Marcel van der Meer, Jan de Leeuw, and two anonymous reviewers for their thoughtful comments. Gernot Arp, Wolfgang Dröse, Jens Dyckmans, Axel Hackmann, Dorothea Hause-Reitner, Helge Mißbach, Andreas Reimer, and Birgit Röring are thanked for scientific and technical support. We furthermore thank Andrea Schito for providing Raman fitting parameters.
This research has been supported by the International Max Planck Research School (IMPRS) for Solar System Science at the University of Göttingen, the Deutsche Forschungsgemeinschaft (grant no. DU 1450/3-1), the Deutsche Forschungsgemeinschaft (grant no. DU 1450/4-1), the Göttingen Academy of Sciences and Humanities, and the Deutsche Zentrum für Luft- und Raumfahrt (grant no. 50QX1401). The article processing charges for this open-access publication were covered by the Max Planck Society.
This paper was edited by Marcel van der Meer and reviewed by Jan de Leeuw and two anonymous referees.