Biogeosciences, 15, 1535–1548, 2018
https://doi.org/10.5194/bg-15-1535-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
Ideas and perspectives: hydrothermally driven redistribution and
sequestration of early Archaean biomass – the “hydrothermal
pump hypothesis”
Jan-Peter Duda1,2, Volker Thiel1, Thorsten Bauersachs3, Helge Mißbach1,4, Manuel Reinhardt1,4, Nadine Schäfer1,2,
Martin J. Van Kranendonk5,6,7, and Joachim Reitner1,2
1Department of Geobiology, Geoscience Centre, Georg-August-Universität Göttingen, Goldschmidtstraße 3,
37077 Göttingen, Germany
2“Origin of Life” Group, Göttingen Academy of Sciences and Humanities, Theaterstraße 7, 37073 Göttingen, Germany
3Department of Organic Geochemistry, Institute of Geosciences, Christian-Albrechts-Universität Kiel,
Ludewig-Meyn-Straße 10, 24118 Kiel, Germany
4Department Planets and Comets, Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3,
37077 Göttingen, Germany
5Australian Centre for Astrobiology, University of New South Wales, Kensington, New South Wales 2052, Australia
6School of Biological, Earth and Environmental Sciences, University of New South Wales, Kensington,
New South Wales 2052, Australia
7Australian Research Council Centre of Excellence for Core to Crust Fluid Systems, School of Biological, Earth and
Environmental Sciences, University of New South Wales, Kensington, New South Wales 2052, Australia
Correspondence: Jan-Peter Duda (jan-peter.duda@geo.uni-goettingen.de)
Received: 1 December 2017 – Discussion started: 5 December 2017
Revised: 6 February 2018 – Accepted: 8 February 2018 – Published: 15 March 2018
Abstract. Archaean hydrothermal chert veins commonly
contain abundant organic carbon of uncertain origin (abi-
otic vs. biotic). In this study, we analysed kerogen contained
in a hydrothermal chert vein from the ca. 3.5 Ga Dresser
Formation (Pilbara Craton, Western Australia). Catalytic hy-
dropyrolysis (HyPy) of this kerogen yielded n-alkanes up to
n-C22, with a sharp decrease in abundance beyond n-C18.
This distribution (≤ n-C18) is very similar to that observed
in HyPy products of recent bacterial biomass, which was
used as reference material, whereas it differs markedly from
the unimodal distribution of abiotic compounds experimen-
tally formed via Fischer–Tropsch-type synthesis. We there-
fore propose that the organic matter in the Archaean chert
veins has a primarily microbial origin. The microbially de-
rived organic matter accumulated in anoxic aquatic (sur-
face and/or subsurface) environments and was then assimi-
lated, redistributed and sequestered by the hydrothermal flu-
ids (“hydrothermal pump hypothesis”).
1 Introduction
Extensive hydrothermal chert vein systems containing abun-
dant organic carbon are a unique phenomenon of early Ar-
chaean successions worldwide (Lindsay et al., 2005; Van
Kranendonk, 2006; Hofmann, 2011). A dense stockwork
of several hundred kerogen-rich hydrothermal chert veins
that penetrate footwall pillowed komatiitic basalts of the
ca. 3.5 Ga Dresser Formation (Pilbara Craton, Western Aus-
tralia; Fig. 1) are up to 2 km deep by 25 m wide (Hickman,
1973, 1983; Nijman et al., 1999; Van Kranendonk and Pira-
jno, 2004; Lindsay et al., 2005; Van Kranendonk, 2006; Van
Kranendonk et al., 2008) (Fig. S1 in the Supplement). De-
pleted stable carbon isotope signatures (δ13C) of bulk kero-
gens (−38.1 to −24.3 ‰) and of organic microstructures
(−33.6 to −25.7 ‰) in these hydrothermal chert veins, as
well as remnants of what appear to be microbial remains,
are consistent with a biological origin of the organic matter
(Ueno et al., 2001, 2004; Glikson et al., 2008; Pinti et al.,
2009; Morag et al., 2016). Problematically, however, simi-
Published by Copernicus Publications on behalf of the European Geosciences Union.
1536 J.-P. Duda et al.: The “hydrothermal pump hypothesis”
larly depleted δ13C values (partly down to ca.−36 ‰ relative
to the initial substrate) can also be formed through abiotic
processes, for instance via Fischer–Tropsch-type synthesis
(McCollom et al., 1999; McCollom and Seewald, 2006), and
putative microbial remains are not always reliable (Schopf,
1993; Brasier et al., 2002, 2005; Schopf et al., 2002; Bower
et al., 2016).
Organic biomarkers can help to trace life and biological
processes through deep time and add important information
on the origin of the organic matter, even in very old sedi-
mentary rocks (Brocks and Summons, 2003; Summons and
Hallmann, 2014). In Archaean rocks, however, molecular fin-
gerprints in the conventionally analysed bitumen (i.e. the ex-
tractable portion of organic matter) are commonly blurred
by thermal maturation and/or ancient or modern contamina-
tion (Brocks et al., 2008; Gérard et al., 2009; Brocks, 2011;
French et al., 2015). In contrast, the non-extractable portion
of organic matter, known as kerogen, tends to be less affected
by thermal maturation and contamination and is considered
to be syngenetic with the host rock (Love et al., 1995; Brocks
et al., 2003b; Marshall et al., 2007; Lockhart et al., 2008).
Catalytic hydropyrolysis (HyPy) is a powerful tool for sen-
sitively releasing kerogen-bound hydrocarbon moieties with
little structural alteration (Love et al., 1995). HyPy has been
successfully applied to Archaean kerogens in rocks from
the Pilbara Craton, liberating syngenetic organic compounds
consistent with a biogenic origin (Brocks et al., 2003b; Mar-
shall et al., 2007). However, this technique has not yet been
used on kerogens contained in hydrothermal chert veins of
this age.
Here, we present the results of analyses of kerogen em-
bedded in a freshly exposed hydrothermal chert vein of the
ca. 3.5 Ga Dresser Formation. Our analyses include field and
petrographic observations, Raman spectroscopy and organic
geochemistry (δ13CTOC; kerogen-bound molecules via HyPy
followed by gas chromatography – mass spectrometry (GC-
MS) and gas chromatography – combustion – isotope ratio
mass spectrometry (GC-C-IRMS)). To further constrain po-
tential sources of the Dresser kerogen, we additionally ap-
plied HyPy on (i) excessively pre-extracted cyanobacterial
biomass and (ii) produced abiotic organic matter via Fischer–
Tropsch-type synthesis using a hydrothermal reactor. Results
of these investigations suggest that the Dresser kerogen has
a primarily microbial origin. We hypothesize that biomass-
derived organic compounds accumulated in anoxic aquatic
environments and were then redistributed and sequestered
by subsurface hydrothermal fluids (“hydrothermal pump hy-
pothesis”).
2 Material and methods
2.1 Sample preparation
A fresh decimetre-sized sample of a Dresser chert vein
was obtained from a recent cut wall of the abandoned
Dresser Mine in the Pilbara Craton, Western Australia (GPS:
21◦09′04.13′′ S; 119◦26′15.21′′ E; Figs. 1, S1d; for geologi-
cal maps, see Hickman, 1983; Van Kranendonk, 1999; Hick-
man and Van Kranendonk, 2012). The external surfaces
(ca. 1–2 cm) of the sample block were removed using an
acetone-cleaned rock saw and then used for the prepara-
tion of thin sections. The surfaces of the resulting inner
block were extensively rinsed with acetone and then removed
(ca. 1–2 cm) with an acetone-cleaned high precision saw
(Buehler; Isomet 1000, Germany). The surfaces of the result-
ing sample were again rinsed with acetone and then crushed
and powdered using a carefully acetone-cleaned pebble mill
(Retsch MM 301, Germany).
2.2 Petrography and Raman spectroscopy
Petrographic analysis was conducted using a Zeiss SteREO
Discovery.V8 stereomicroscope (transmitted and reflected
light) linked to an AxioCam MRc5 5-megapixel camera.
Raman spectra were recorded using a Horiba Jobin Yvon
LabRam-HR 800 UV spectrometer (focal length of 800 mm)
attached to an Olympus BX41 microscope. For excitation
an Argon ion laser (Melles Griot IMA 106020B0S) with a
laser strength of 20 mW was used. The laser beam was fo-
cused onto the sample using an Olympus MPlane 100× ob-
jective with a numerical aperture of 0.9 and dispersed by a
600 Lmm−1 grating on a charge-couple device (CCD) de-
tector with 1024× 256 pixels. This yielded a spectral reso-
lution of < 2 cm−1 per pixel. Data were acquired over 10 to
30 s for a spectral range of 100–4000 cm−1. The spectrom-
eter was calibrated by using a silicon standard with a major
peak at 520.4 cm−1. All spectra were recorded and processed
using the LabSpec™ database (version 5.19.17; Jobin Yvon,
Villeneuve d’Ascq, France).
2.3 Raman-derived H /C data
H /C values were calculated based on integrated
peak intensities of Raman spectra using the formula
H /C= 0.871 · ID5/(IG+D2)− 0.0508 (Ferralis et al., 2016).
The peaks were fitted in the LabSpec™ software (see
Sect. 2.2) using the Gauss/Lorentz function.
2.4 Molecular analysis of the Dresser kerogen
All materials used for preparation were heated to 500 ◦C for
3 h and/or extensively rinsed with acetone prior to sample
contact. A laboratory blank was prepared and analysed in
parallel to monitor laboratory contaminations.
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J.-P. Duda et al.: The “hydrothermal pump hypothesis” 1537
Figure 1. Location of the study site in Western Australia. The hydrothermal chert vein analysed occurs in a recent cut wall of the abandoned
Dresser Mine close to Marble Bar.
We applied catalytic hydropyrolysis (HyPy) to release
molecules from the Dresser kerogen and pre-extracted
biomass of the heterocystous cyanobacterium Anabaena
cylindrica SAG 1403-2 following previously published pro-
tocols (Snape et al., 1989; Love et al., 1995, 2005). HyPy
allows the breaking of covalent bonds by progressive heat-
ing under high hydrogen pressure (150 bar). The released
products are immediately removed from the hot zone by a
constant hydrogen flow and trapped downstream on clean,
combusted silica powder cooled with dry ice (Meredith et
al., 2004). All hydropyrolysates were eluted from the silica
trap with high-purity dichloromethane (DCM), desulfurized
overnight using activated copper and subjected to gas chro-
matography – mass spectrometry (GC-MS). Before all ex-
periments, the empty HyPy system was heated (ambient tem-
perature to 520 ◦C, held for 30 min) to remove any residual
molecules.
Isolation of the Dresser kerogen followed standard proce-
dures (cf. Brocks et al., 2003b). 57 g of powdered sample was
first demineralized with hydrochloric acid (24 h) and then hy-
drofluoric acid (11 days). The purified organic matter was
then exhaustively extracted using three excess volumes of
DCM and n-hexane, respectively (×3), ultrasonic swelling
in pyridine (2× 20 min at 80 ◦C), and ultrasonic extraction
with methanol, DCM (×3) and DCM /methanol (1/1; v/v).
The kerogen was subsequently extracted with n-hexane un-
til no more compounds were detected via GC-MS. The pure
kerogen was used for HyPy following an experimental proto-
col optimized for the analysis of Archaean kerogens (Brocks
et al., 2003b; Marshall et al., 2007).
In order to monitor potential contamination, we applied
HyPy to blanks before and after the kerogen run. The Dresser
kerogen (131.06 mg) and blanks were sequentially heated in
the presence of a sulfided molybdenum catalyst (10 wt %)
and under a constant hydrogen flow (5 dm3 min−1) using
a two-step approach (cf. Brocks et al., 2003b; Marshall et
al., 2007; Fig. S2). The low-temperature step included heat-
ing from ambient temperature to 250 ◦C (300 ◦Cmin−1) and
then to 330 ◦C (8 ◦Cmin−1) to release any molecules that
were strongly adsorbed to the kerogen and not accessible
to solvent extraction. The silica powder was subsequently
recovered from the product trap, and the trap was refilled
with clean, combusted silica powder for the following high-
temperature step. This step included heating from ambient
temperature to 520 ◦C (8 ◦Cmin−1) to release molecules co-
valently bound to the kerogen.
2.5 Molecular analysis of pre-extracted cyanobacterial
biomass (Anabaena cylindrica)
We used cell material of the heterocystous cyanobacterium
Anabaena cylindrica strain SAG 1403-2 as it fulfils all
criteria for reference material (availability, well character-
ized, etc.). The material was obtained from the Culture
Collection of Algae at the Georg-August-Universität Göt-
tingen (Germany) and grown at the Christian-Albrechts-
Universität Kiel (Germany). The axenic batch culture was
maintained in 250 mL of BG110 medium free of com-
bined nitrogen sources (Rippka et al., 1979) at 29 ◦C. A
light : dark regime of 14 h : 10 h with a photon flux density
of 135 µmol photonsm−2 s−1 was provided by a white flu-
orescent light bulb. At the end of the logarithmic growth
phase, cells were harvested by centrifugation, and they
were lyophilized thereafter. Subsequently, an aliquot of the
freeze-dried biomass was exhaustively extracted following
the methodology described by Bauersachs et al. (2014).
HyPy was performed following an experimental proto-
col optimized for biomass applications (Love et al., 2005).
Briefly, the material was heated in the presence of a sul-
fided molybdenum catalyst (1.5 times the weight of the bac-
terial extraction residue) and under a constant hydrogen flow
(6 dm3 min−1). The HyPy program included heating from
ambient temperature to 260 ◦C (300 ◦Cmin−1) and then to
500 ◦C (8 ◦Cmin−1).
2.6 Fischer–Tropsch-type synthesis of organic matter
under hydrothermal conditions
Fischer–Tropsch-type reactions were carried out based on
McCollom et al. (1999). A mixture of 2.5 g oxalic acid (dihy-
drate, suprapur®, Merck KGaA), 1 g montmorillonite (K10,
Sigma Aldrich; pre-extracted with DCM; ×4) and ca. 11 mL
ultrapure water was heated to 175 ◦C for 66 to 74 h in a
sealed Morey-type stainless steel autoclave. The autoclave
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1538 J.-P. Duda et al.: The “hydrothermal pump hypothesis”
was rapidly (≤ 15 min) cooled to room temperature with
compressed air.
Fluid and solid phases were collected and extracted with
DCM (×4). The montmorillonite was removed by centrifu-
gation and filtration with silica powder and sea sand (which
were combusted prior to use). The obtained extracts were
then concentrated by rotary evaporation (40 ◦C, 670 mbar)
and subjected to GC-MS analysis. Analytical blank exper-
iments were carried out with all reactants to keep track of
contamination.
2.7 Gas chromatography – mass spectrometry
GC-MS analysis of HyPy products was carried out with a
Thermo Scientific Trace 1310 GC coupled to a Thermo Sci-
entific Quantum XLS Ultra MS. The GC instrument was
equipped with a capillary column (Phenomenex Zebron ZB-
5, 30 m, 0.25 µm film thickness, 0.25 mm inner diameter).
Fractions were injected into a splitless injector and trans-
ferred to the GC column at 270 ◦C. He was used as carrier
gas with a constant flow rate of 1.5 mLmin−1. The GC oven
temperature was held isothermal at 80 ◦C for 1 min and then
ramped to 310 ◦C at 5 ◦Cmin−1, at which it was kept for
20 min. Electron ionization mass spectra were recorded in
full-scan mode at an electron energy of 70 eV with a mass
range of m/z 50–600 and scan time of 0.42 s.
2.8 Polyaromatic hydrocarbon ratios
Polyaromatic hydrocarbons (PAHs) are organic compounds
consisting of multiple fused benzene rings. Methylation and
isomerization characteristics of various GC-amenable PAHs
are altered during maturation and thus provide a measure for
assessing the thermal maturity of organic matter (Killops and
Killops, 2005; Peters et al., 2005). The following PAH matu-
rity parameters were used in this study:
1. methylnaphthalene ratio (MNR)= 2-MN / 1-MN
(Radke et al., 1984),
2. methylphenanthrene index (MPI-I)= 1.5 · (2-MP+ 3-
MP) / (P+ 1-MP+ 9-MP) (Radke and Welte, 1983),
and
3. computed vitrinite reflectance [Rc (MPI-I)]= 0.7 ·MPI-
1+ 0.22 (according to P /MP> 1; Boreham et al.,
1988).
MN stands for methylnaphthalene, P for phenanthrene and
MP for methylphenanthrene.
2.9 Total organic carbon (TOC) and δ13C analyses
(TOC and compound specific)
TOC, δ13CTOC and compound-specific δ13C analyses were
conducted at the Centre for Stable Isotope Research and
Analysis (KOSI) at the Georg-August-Universität Göttingen,
Germany. Stable carbon isotope data are expressed as delta
values relative to the Vienna Pee Dee Belemnite (VPDB) ref-
erence standard.
The TOC content and δ13CTOC values were determined
in duplicate using an elemental analyser (NA-2500 CE-
Instruments) coupled to an isotope ratio mass spectrometer
(Finnigan MAT Delta plus). Ca. 100 mg of powdered and
homogenized whole rock material were analysed in each
run. For internal calibration an acetanilide standard was used
(δ13C=−29.6 ‰; SD= 0.1 ‰). TOC content measurements
showed a mean deviation of 0.1 wt%. The average δ13CTOC
value had a standard deviation of 0.3 ‰.
Compound-specific δ13C analyses were conducted with a
Trace GC coupled to a Delta Plus isotope ratio mass spec-
trometer (IRMS) via a combustion interface (all Thermo Sci-
entific). The combustion reactor contained CuO, Ni and Pt
and was operated at 940 ◦C. The GC was equipped with two
serially coupled capillary columns (Agilent DB-5 and DB-
1; each 30 m, 0.25 µm film thickness, 0.25 mm inner diame-
ter). Fractions were injected into a splitless injector and trans-
ferred to the GC column at 290 ◦C. The carrier gas was He at
a flow rate of 1.2 mLmin−1. The GC oven temperature pro-
gram was identical to the one used for GC-MS analysis (see
above). CO2 with a known δ13C value was used for inter-
nal calibration. Instrument precision was checked using lab-
oratory standards. Standard deviations of duplicate measure-
ments were better than 1.7 ‰.
3 Results
The studied hydrothermal chert vein is hosted in komatiitic
pillow basalt that has undergone severe hydrothermal acid–
sulfate alteration, producing a kaolinite–illite–quartz mineral
assemblage (Van Kranendonk and Pirajno, 2004; Van Kra-
nendonk, 2006) (Fig. S1). The sampled vein crops out in a
recent cut wall of the abandoned Dresser Mine (Fig. S1) and
consists of a dense chert (microquartz) matrix of deep black
colour that contains kerogen and local concentrations of fresh
(unweathered) pyrite crystals (Fig. 2a, b). There is no field
and/or petrographic evidence for fluid-flow events that post-
date the initial vein formation (brecciation textures, etc.).
Petrographic analysis and Raman spectroscopy revealed
that kerogen (D bands at 1353 cm−1, G bands at 1602 cm−1)
is embedded in the chert matrix (SiO2 band at 464 cm−1) (see
Fig. 2c, for a representative Raman spectrum). The organic
matter occurs as small clots (< 50 µm) of variable shape. Ra-
man spectra of the organic matter exhibit relatively wide D
and G bands (82 and 55 cm−1 width, respectively) (Fig. 2c).
The total organic carbon (TOC) content is 0.2 wt%, and the
δ13CTOC value is −32.8± 0.3 ‰ (Table 1). Raman-derived
H /C ratios range between 0.03 and 0.14.
HyPy was applied to the isolated Dresser kerogen, as well
as to preceding and subsequent analytical blanks (combusted
sea sand). Hydropyrolysates of the preceding blank con-
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J.-P. Duda et al.: The “hydrothermal pump hypothesis” 1539
Table 1. Stable carbon isotopic composition (δ13C) of the total organic carbon (TOC) and n-alkanes released from high-temperature HyPy
of the Dresser kerogen.
n-C12 n-C13 n-C14 n-C15 n-C16 n-C17 n-C18 n-C12−18 TOC
(mean)
δ13C −30.3 −33.3 −32.7 −31.1 −29.4 −31.2 −31.7 −31.4 −32.8
SD 0.5 1.4 0.2 1.7 0.3 0.8 0.1 1.2 0.3
Table 2. Maturity indices (based on aromatic hydrocarbons) of
the Dresser kerogen. MP/P: methylphenathrene/phenanthrene
ratio; MPI-I: methylphenanthrene index (1.5 · (2-MP+ 3-
MP)/(P+ 1-MP+ 9-MP); Radke and Welte, 1983); P/MP:
phenanthrene/methylphenathrene ratio; Rc (MPI-I): computed
vitrinite reflectance (0.7 ·MPI-1+ 0.22, according to P/MP> 1;
Boreham et al., 1988); MNR: methylnaphthalene ratio (2-MN/1-
MN; Radke et al., 1984).
MP/P MPI-I P/MP Rc MNR
(MPI-I)
0.33 0.23 3.01 2.87 2.52
tained a series of n-alkanes ≤ n-C24, with maximum abun-
dances at n-C15 (Figs. 3, S2–S4). Sulfur (only after low-
temperature HyPy), traces of siloxanes and a phenol (Figs. 3,
S2, S4) were also present. The blank runs also contained
traces of aromatic hydrocarbons (Fig. S5).
Low-temperature HyPy of the Dresser kerogen produced
traces of C14−18 n-alkanes, with a maximum at n-C15
(Fig. S3). However, these compounds were significantly
less abundant than those released during high-temperature
HyPy (see below). Other compounds observed in the low-
temperature step pyrolysate were elemental sulfur, phenols,
phthalic acid, siloxanes and traces of aromatic hydrocarbons
(Figs. S2, S4–S5).
High-temperature HyPy of the Dresser kerogen yielded
n-alkanes ranging from n-C11 to n-C22 with a no-
table decrease (“step”) in the abundance of homologues
above n-C18 (Figs. 3b, S2, S3), which remained vir-
tually unaffected by blank subtraction (Fig. 3c). Apart
from that step, no carbon number preference is evident.
The n-alkanes have δ13C values ranging from −29.4 to
−33.3 ‰ (mean −31.4± 1.2 ‰; Fig. 3b; Table 1). The hy-
dropyrolysate also contained isomeric mixtures of C12−18
monomethylalkanes (Fig. S3) and a variety of aromatic
hydrocarbons, including (dimethyl-, methyl-)naphthalene(s),
(methyl-)biphenyl(s), (methyl-)acenaphthene(s), dibenzofu-
ran and (methyl-)phenanthrene(s) (Figs. 3b, c, S4–S6). Bi-
ologically diagnostic hydrocarbons such as hopanoids or
steroids were absent.
HyPy treatment of excessively pre-extracted biomass of
the heterocystous cyanobacterium Anabaena cylindrica SAG
1403-2 yielded a variety of organic compounds, but also in-
cluded n-alkanes with a clear restriction in carbon number to
homologues ≤ n-C18 (Fig. 3d). In contrast, our experimen-
tal synthesis of abiotic n-alkanes through Fischer–Tropsch-
type reactions under hydrothermal conditions produced a
unimodal distribution of homologues ranging from n-C12 to
n-C41 without any carbon number preference (Fig. 3e).
4 Discussion
4.1 Maturity of the Dresser kerogen
The organic record of Archaean rocks is commonly af-
fected by thermal maturation (Brocks et al., 2008; Brocks,
2011; French et al., 2015). The kerogen within the anal-
ysed Dresser sample is thermally mature and structurally
disordered, as evidenced by relatively wide D and G bands
in the Raman spectra (see Fig. 2c for a representative Ra-
man spectrum). This is also supported by the absence of
S1 bands at 2450 cm−1, high R1 ratios (0.98–1.05) and low
FWHM-D1 values (68.64–76.12), consistent with prehnite–
pumpellyite to lower greenschist metamorphism at a temper-
ature of ca. 300 ◦C (cf. Yui et al., 1996; Delarue et al., 2016).
All of these observations are well in line with published Ra-
man spectra of Archaean organic matter from the same re-
gion (Ueno et al., 2001; Marshall et al., 2007; Delarue et
al., 2016) and the general thermal history of the host rock
(regional prehnite–pumpellyite to lower greenschist meta-
morphism; Hickman, 1975, 1983, 2012; Terabayashi et al.,
2003).
Fitting of D5 peaks can be difficult for Raman spec-
tra of highly mature organic matter (Ferralis et al., 2016).
Therefore, the H /C ratios calculated herein (0.03–0.14)
should be treated with caution. However, the Raman-based
H /C ratios and the methylphenanthrene / phenanthrene
value (MP /P= 0.33; Table 2) of the Dresser kerogen are
in good accordance with data reported from more mature
kerogens from the same region (ca. 3.4 Ga Strelley Pool
Formation: 0.08–0.14 and 0.24–0.37, respectively; Marshall
et al., 2007). The low methylphenanthrene index (MPI-
I= 0.23) and high phenanthrene /methylphenanthrene index
(P /MP= 3.01) result in a computed vitrinite reflectance (Rc
(MPI-I)) of 2.87 (Table 2), indicating a thermal maturity
far beyond the oil-generative stage (Radke and Welte, 1983;
Boreham et al., 1988). However, it has to be considered that
the MPI-I is potentially affected by (de-)methylation reac-
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1540 J.-P. Duda et al.: The “hydrothermal pump hypothesis”
Figure 2. Petrographic observations on the hydrothermal chert vein.
(a, b) Thin section photographs (a transmitted light; b reflected
light) showing kerogen (brownish colours) and pyrite (arrows, black
colours in a, bright colours in b) embedded within a fine-grained
chert matrix. Note that the pyrite crystals (arrows) are excellently
preserved and show no evidence of oxidation. (c) Representative
Raman spectrum of kerogen (D band at 1353 cm−1 and G band at
1602 cm−1) present in the hydrothermal chert vein. Note the wide D
and G bands (82 and 55 cm−1 width, respectively), pointing to ther-
mally mature and structurally disordered kerogen, and the absence
of an S1 band (at 2450 cm−1), consistent with prehnite–pumpellyite
to lower greenschist metamorphosis and a temperature of ca. 300 ◦C
(cf. Yui et al., 1996; Delarue et al., 2016).
tions (Brocks et al., 2003a). The calculated methylnaphtha-
lene ratio (MNR= 2.52; Table 2) corresponds to a somewhat
lower mean vitrinite reflectance of ca. 1.5, which is again
in line with a post-oil window maturity (cf. Radke et al.,
1984). The mismatches between single aromatic maturity pa-
rameters are negligible, as these indices have limited appli-
cation for highly mature Archaean organic matter (Brocks
et al., 2003a). The putative offset between the indices and
the metamorphic overprint indicated by Raman data is most
likely due to the protection of kerogen-bound moieties even
under elevated thermal stress (Love et al., 1995; Lockhart et
al., 2008). Furthermore, it has been shown that kerogen iso-
lated from the Strelley Pool Formation also contains larger
PAH clusters which are not GC-amenable (> 10–15 rings;
Marshall et al., 2007). Consequently, it can be anticipated
that the compounds detected in the HyPy pyrolysate of the
Dresser kerogen represent only a small fraction of the bulk
macromolecular organic matter.
The distribution of monomethylalkanes≤ n-C18 (Figs. S3,
S4) released from the Dresser kerogen resembles high-
temperature HyPy products from the Strelley Pool kerogen
(Marshall et al., 2007; their Fig. 15). Such isomeric mixtures
are typically formed during thermal cracking of alkyl moi-
eties (Kissin, 1987) and are in good agreement with the esti-
mated maturity range. Methylated aromatics such as methyl-
naphthalenes and methylphenanthrenes have also been ob-
served in other hydropyrolysates from Archaean kerogens
that experienced low-grade metamorphism (Brocks et al.,
2003b; Marshall et al., 2007; French et al., 2015). In all of
these cases, the degree of alkylation varied with the exact
thermal alteration of the respective kerogens (Marshall et al.,
2007; French et al., 2015).
4.2 Syngeneity of the Dresser kerogen-derived
compounds
The kerogen of the Dresser Formation exclusively occurs in
the form of fluffy aggregates and clots embedded within a
very dense chert matrix that is, once solidified, highly im-
permeable to fluids. The depositional age of the formation is
constrained to 3481±3.6 Ma (Van Kranendonk et al., 2008),
and the investigated chert vein shows no evidence for dis-
ruption by post-depositional hydrothermal fluids. This has
also been described for other hydrothermal chert veins of the
Dresser Formation, where the kerogen has been interpreted
as being syngenetic (i.e. formed prior to host rock lithifica-
tion; Ueno et al., 2001, 2004; Morag et al., 2016). Further-
more, the maturity of the embedded kerogen is in good ac-
cordance with the thermal history of the host rock. An intro-
duction of solid macromolecular organic matter from strati-
graphically younger units in this region during later fluid-
flow phases, as proposed for the younger Apex chert (Olcott-
Marshall et al., 2014), can therefore be excluded.
As the bitumen fractions of Precambrian rocks are eas-
ily biased by the incorporation of contaminants during later
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J.-P. Duda et al.: The “hydrothermal pump hypothesis” 1541
Figure 3. Total ion currents (a–c; on the same scale) and ion chromatograms selective for alkanes (d–e; m/z 85). (a) High-temperature
HyPy (330–520 ◦C) product of analytical blank (combusted sea sand) obtained prior to HyPy of the Dresser kerogen. n-Alkanes in the
range of n-C14 to n-C24 (maxima at n-C15) represent HyPy background contamination. (b) High-temperature HyPy (330–520 ◦C) product
of the Dresser kerogen. Note the sharp decrease in abundance of n-alkanes beyond n-C18 (see arrows). δ13C values of n-C12 to n-C28
show a high similarity to the δ13CTOC value (−32.8± 0.3 ‰), further confirming syngeneity. (c) Blank subtraction (b minus a) showing
that contaminants have no major impact on the n-alkane pattern yielded during the high-temperature HyPy step of the Dresser kerogen (b).
(d) HyPy products of cell material of the heterocystous cyanobacterium Anabaena cylindrica. Note the sharp decrease in abundance of
n-alkanes beyond n-C18 (see arrows), similar to the n-alkane distribution of the Dresser kerogen (b). (e) Products of experimental Fischer–
Tropsch-type synthesis under hydrothermal conditions; abiogenic n-alkanes show a unimodal distribution that is distinctly different from
the Dresser kerogen. Black dots: n-alkanes (numbers refer to carbon chain lengths); N: naphthalene; MN: methylnaphthalenes; BiPh: 1,1′-
biphenyl; DMN: dimethylnaphthalenes; DBF: dibenzofuran; MAN: methylacenaphthene; P: phenanthrene; crosses: siloxanes (GC column
or septum bleeding).
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1542 J.-P. Duda et al.: The “hydrothermal pump hypothesis”
stages of rock history (Brocks et al., 2008; Brocks, 2011;
French et al., 2015), studies have increasingly focussed on
kerogen-bound compounds that are more likely to be syn-
genetic to the host rock (Love et al., 1995; Brocks et al.,
2003b; Marshall et al., 2007; French et al., 2015). Potential
volatile organic contaminants adhering to the kerogen are re-
moved through excessive extraction and a thermal desorption
step (∼ 350 ◦C) prior to high-temperature HyPy (550 ◦C;
cf. Brocks et al., 2003b; Marshall et al., 2007). The recur-
rence of few n-alkanes in the range of n-C14 to n-C24 (max-
imum at n-C15) in high-temperature HyPy blank runs ob-
tained immediately before and after the actual sample run
(Figs. 3a, S2, S3) indicates minor background contamination
during pyrolysis, with a source most likely within the HyPy
system. However, these contaminants do not significantly af-
fect the n-alkane pattern yielded by high-temperature HyPy
of the Dresser kerogen, as evidenced by blank subtraction
(Fig. 3c).
Contamination of the sample can be further deciphered by
the presence of polar additives or hydrocarbons that are not
consistent with the thermal history of the host rock. Plastic-
derived branched alkanes with quaternary carbon centres
(BAQCs), a common contaminant in Precambrian rock sam-
ples (Brocks et al., 2008), have not been detected (Fig. S7).
Traces of functionalized plasticizers (phenols and phthalic
acid; Figs. S2, S6, S8) are unlikely to survive (or result
from) HyPy treatment. These compounds are therefore con-
sidered as background contamination introduced during sam-
ple preparation and analysis after HyPy. The observed silox-
anes (Fig. 3, S2) probably originate from the GC column
or septum bleeding and are unlikely to be contained in the
sample. All of these compounds occur only in low or trace
abundances and can be clearly distinguished from the ancient
aliphatic and aromatic hydrocarbons contained in the Dresser
kerogen.
Contamination by (sub-)recent endoliths can be excluded
as sample surfaces have been carefully removed and there
is no petrographic indication for borings or fissures con-
taining recent organic material (Fig. 2). HyPy of untreated
or extracted biomass would yield a variety of acyclic and
cyclic biomarkers (cf. Love et al., 2005). However, high-
temperature HyPy of the Dresser kerogen almost exclusively
yielded n-alkanes, minor amounts of monomethylalkanes
and various aromatic hydrocarbons (Figs. 3b, c, S2–S6),
while hopanoids or steroids were absent (Figs. S8, S9). This
is in good agreement with the maturity of the Dresser kero-
gen and previous HyPy studies of Archaean rocks (Brocks et
al., 2003b; Marshall et al., 2007; French et al., 2015).
Accidental contamination of the kerogen by mono-, di-
and triglycerides (e.g. dust, skin surface lipids) can also
be ruled out as HyPy treatment of these compounds typ-
ically results in n-alkane distributions with a distinct pre-
dominance of n-C16 and n-C18 homologues, corresponding
to the n-C16 and n-C18 fatty acid precursors (Craig et al.,
2004; Love et al., 2005; unpublished data from our own ob-
servations). At the same time, the observed distribution of
kerogen-derived n-alkanes, with a distinct decrease beyond
n-C18 (Fig. 3b, c), is notably similar to high-temperature
HyPy products of kerogens isolated from the ca. 3.4 Ga Strel-
ley Pool Formation of the Pilbara Craton (Marshall et al.,
2007; their Fig. 14). Marshall and co-workers considered
these n-alkanes unlikely to be contaminants as they were
released only in the high-temperature HyPy step. Further-
more, the stable carbon isotopic composition of n-alkanes in
the Dresser high-temperature pyrolysate (−29.4 to−33.3 ‰;
mean −31.4± 1.2 ‰) is very similar to the δ13CTOC signa-
ture of the sample (−32.8± 0.3 ‰; Figs. 3, S10; Table 1),
indicating that these compounds were generated from the
kerogen. Hence, we consider the compounds released from
the Dresser kerogen during high-temperature HyPy (Fig. 3b,
c) as syngenetic.
High-temperature HyPy of the Dresser kerogen yielded a
variety of aromatic hydrocarbons, which are orders of mag-
nitudes lower or absent in all other pyrolysates (Figs. 3,
S2, S4–S6). It also produced significantly higher amounts
of n-alkanes than the low-temperature step, and these fur-
ther showed a distinct distribution pattern (i.e. a step ≤ n-
C18; Figs. 3, S2, S3). The lack of even low quantities of
n-alkenes that typically accompany bond cleavage in kero-
gen pyrolysis was also observed by Marshall et al. (2007)
in their HyPy analysis of cherts from the Strelley Pool For-
mation. As a possible explanation for the lack of n-alkenes,
these authors suggested that the n-alkanes were not cracked
from the kerogen but were rather trapped in closed microp-
ores until the organic host matrix was pyrolytically disrupted.
Alternatively, however, unsaturated cleavage products may
have been immediately reduced during HyPy by the steadily
available hydrogen and catalysts. Indeed, it has been demon-
strated that double bonds in linear alkyl chains are efficiently
hydrogenated during HyPy, even in the case of pre-extracted
microbial biomass (e.g. Love et al., 2005; our Fig. 3d). We
consider both as plausible scenarios to explain the absence of
n-alkenes in the Dresser chert hydropyrolysate.
4.3 Origin of the Dresser kerogen: hydrothermal
vs. biological origin
The δ13CTOC value of −32.8± 0.3 ‰ is consistent with car-
bon fixation by photo- or chemoautotrophs (cf. Schidlowski,
2001). However, organic compounds exhibiting similar 13C
depletions (partly down to ca. −36 ‰ relative to the initial
substrate) could also be formed abiotically during the serpen-
tinization of ultramafic rocks (Fischer–Tropsch-type synthe-
sis; McCollom et al., 1999; McCollom and Seewald, 2006;
Proskurowski et al., 2008). A further constraint on the origin
of the Dresser kerogen is provided by the distinct decrease
in n-alkane abundance beyond n-C18 observed in the high-
temperature HyPy pyrolysate (Fig. 3b, c). This distribution
resembles HyPy products of pre-extracted recent cyanobac-
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J.-P. Duda et al.: The “hydrothermal pump hypothesis” 1543
terial biomass, which also shows a very pronounced restric-
tion in carbon number to homologues ≤ n-C18 (Fig. 3d).
The pre-extracted cyanobacterial cell material and the abi-
otically produced n-alkanes studied as reference samples ex-
perienced no thermal maturation. It can nevertheless be ex-
pected that burial, and thus heating, of Anabaena biomass
(Fig. 3d) would initially liberate lower n-alkane homologues
from their predominant n-alkyl moieties while, up to a cer-
tain point, retaining the distinct step at n-C18. Experimental
maturation of an immature kerogen revealed the preservation
of distinct alkyl-chain length preferences even after 100 days
at 300 ◦C (Mißbach et al., 2016; e.g. a step beyond n-C31
in their Fig. 2). Further maturation would ultimately lead to
a unimodal distribution of short-chain n-alkanes and erase
the biologically inherited pattern (cf. Mißbach et al., 2016).
In contrast, abiotically synthesized extractable organic com-
pounds show a unimodal homologue distribution from the
beginning (Fig. 3e) and will retain it, while thermal mat-
uration would gradually shift the n-alkane pattern towards
shorter homologues. It can therefore be expected that or-
ganic compounds cleaved from an abiotic “Fischer–Tropsch-
kerogen” – whose existence has not been proven yet – would
also exhibit a unimodal distribution. Consequently, the dis-
tinctive distribution of n-alkanes released from the Dresser
kerogen (i.e. the step ≤ n-C18; Figs. 3b, c) can be regarded
as a molecular fingerprint relating to a biosynthetic origin of
the organic matter.
Highly 13C-depleted methane in primary fluid inclu-
sions in hydrothermal chert veins of the Dresser Forma-
tion (δ13C< 56 ‰) was taken as evidence for biological
methanogenesis and thus the presence of Archaea (Ueno
et al., 2006). Our δ13CTOC value (−32.8± 0.3 ‰; Table 1;
Fig. S10) would generally be consistent with both bacte-
rial and archaeal sources (cf. Schidlowski, 2001). Archaea
only synthesize isoprene-based compounds (Koga and Morii,
2007; Matsumi et al., 2011). Straight-chain (acetyl-based)
hydrocarbon moieties such as fatty acids – the potential
precursors of the kerogen-derived n-alkanes – are to cur-
rent knowledge being formed only by Bacteria and Eukarya,
where they typically function as constituents of membranes
or storage lipids (cf. Erwin, 1973; Kaneda, 1991). The forma-
tion of these lipids is tightly controlled by different biosyn-
thetic pathways resulting in characteristic chain-length dis-
tributions. In bacterial lipids, carbon chain lengths typically
do not extend above n-C18 (cf. Kaneda, 1991). Consequently,
given the lack of convincing evidence for the presence of Eu-
karya as early as 3.5 Ga (cf. Parfrey et al., 2011; Knoll, 2014;
French et al., 2015), the most plausible source of kerogen-
occluded n-alkanes in the Dresser hydrothermal chert is Bac-
teria. Strongly reducing conditions during the deposition of
the Dresser Formation are indicated by widespread pyrite,
Fe-rich carbonates and trace element signatures (Van Kra-
nendonk et al., 2003, 2008). Potential microbial sources for
the Dresser kerogen therefore may have included anoxygenic
photoautotrophic, chemoautotrophic and heterotrophic mi-
croorganisms.
4.4 The “hydrothermal pump hypothesis”
Our results strongly support a biological origin of the kero-
gen found in the early Archaean hydrothermal chert veins
of the Dresser Formation. We explain this finding by the
redistribution and sequestration of microbial organic matter
that may have formed in a variety of Dresser environments
through hydrothermal circulation (proposed herein as the
“hydrothermal pump hypothesis”; Fig. 4). Higher geothermal
gradients prior to the onset of modern-type plate tectonics
at ca. 3.2–3.0 Ga (Smithies et al., 2005; Shirey and Richard-
son, 2011) were possibly important drivers of early Archaean
hydrothermal systems. In fact, the Dresser Formation was
formed in a volcanic caldera environment affected by strong
hydrothermal circulation, where voluminous fluid circulation
locally caused intense acid–sulfate alteration of basalts and
the formation of a dense hydrothermal vein swarm (Nijman
et al., 1999; Van Kranendonk and Pirajno, 2004; Van Kra-
nendonk, 2006; Van Kranendonk et al., 2008; Harris et al.,
2009). In addition to the high crustal heat flow, the absence of
thick sedimentary cover may have facilitated the intrusion of
seawater into the hydrothermal system. The associated large-
scale assimilation of particulate and dissolved organic mat-
ter and its transport and alteration by hydrothermal fluids
(Fig. 4) therefore appears to be a plausible mechanism that
may, at least partly, explain the high amounts of kerogen in
early Archaean hydrothermal veins.
The hydrothermal pump hypothesis requires a source of
organic matter during the deposition of the Dresser For-
mation (Fig. 4). Whereas contributions from extraterres-
trial sources, as well as from Fischer–Tropsch-type synthe-
sis linked to the serpentinization of ultramafic rocks, cannot
be excluded, our results indicate a primarily biological origin
for the kerogen contained in the chert veins (Fig. 4). The in-
ferred biogenicity is also in line with the consistent δ13C off-
set between bulk kerogens (ca.−20 to−30 ‰) and carbonate
(ca. ±2 ‰) in Archaean rocks (Hayes, 1983; Schidlowski,
2001). Prokaryotic primary producers and heterotrophs may
have flourished in microbial mats (Dresser stromatolites;
Walter et al., 1980; Van Kranendonk, 2006, 2011; Philippot
et al., 2007; Van Kranendonk et al., 2008), the water column
(planktic “marine snow”; Brasier et al., 2006; Blake et al.,
2010) and even hot springs on land (Djokic et al., 2017). An-
other biological source for the ancient organic matter could
have been chemoautotrophs and heterotrophs thriving in sub-
surface environments, such as basalts (Banerjee et al., 2007;
Furnes et al., 2008) and hydrothermal vent systems (Shen et
al., 2001; Ueno et al., 2001, 2004, 2006; Pinti et al., 2009;
Morag et al., 2016) (Fig. 4). All of these systems are not mu-
tually exclusive and the largely anoxic conditions would have
encouraged a high steady-state abundance of organic matter
in the aquatic environment (Fig. 4).
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1544 J.-P. Duda et al.: The “hydrothermal pump hypothesis”
Figure 4. The “hydrothermal pump hypothesis”. Organic matter was predominantly biologically produced and heterotrophically processed by
Bacteria and, possibly, Archaea. Additionally, Fischer–Tropsch-type synthesis of organic matter linked to the serpentinization of ultramafic
rocks (McCollom et al., 1999; McCollom and Seewald, 2006) may have occurred locally. Primary producers (chemoautotrophs, anoxygenic
photoautotrophs) and heterotrophs may have flourished in surface waters (planktic “marine snow”), at the water–rock interface (microbial
mats and/or biofilms) and in cryptic environments (e.g. within basalts and hydrothermal vent systems). After accumulating in different
Dresser environments, the organic matter was redistributed and sequestered in veins by hydrothermal fluids.
Dissolved organic matter (DOM) in modern seawater may
resist decomposition over millennial timescales (Druffel and
Griffin, 2015). In recent hydrothermal fields, however, or-
ganic matter becomes thermally altered and redistributed (Si-
moneit, 1993; Delacour et al., 2008; Konn et al., 2009). Lab-
oratory experiments using marine DOM indicate that ther-
mal alteration already occurs at temperatures > 68–100 ◦C
and efficient removal of organic molecules at 212–401 ◦C
(Hawkes et al., 2015, 2016). It has been argued, however,
that such DOM removal may also be due to transformation
into immiscible material through, for example, condensation
(Castello et al., 2014) and/or defunctionalization reactions
(Hawkes et al., 2016). These processes, however, are as yet
poorly understood. In the Dresser Formation, hydrothermal
temperatures ranged from ca. 300 ◦C at depth to 120 ◦C near
the palaeosurface, causing propylitic (ca. 250–350 ◦C) and
argillic (including advanced argillic: ca. 100–200 ◦C) alter-
ation of the host rocks (Van Kranendonk and Pirajno, 2004;
Van Kranendonk et al., 2008; Harris et al., 2009). Given this
variety of thermal regimes, and the generally anoxic nature of
early Archaean seawater (e.g. Van Kranendonk et al., 2003,
2008; Li et al., 2013), it is likely that some of the organic sub-
stances underwent in situ alteration, but not complete oxida-
tion, during hydrothermal circulation. The entrained organ-
ics would have been trapped in the chert that instantaneously
precipitated from the ascending hydrothermal fluids due to
subsurface cooling (cf. Van Kranendonk, 2006).
In summary, the hydrothermal pump hypothesis proposed
here (Fig. 4) includes (i) a net build-up of organic matter in
different Dresser environments under largely anoxic condi-
tions, (ii) a large-scale assimilation of particulate and dis-
solved organic matter from various biological sources and its
subsurface transport and alteration by hydrothermal fluids,
and (iii) its sequestration within hydrothermal chert veins as
kerogen. This model explains the presence of abundant or-
ganic carbon in early Archaean hydrothermal veins, as well
as its morphological, structural and isotopic variability ob-
served in the Dresser hydrothermal chert veins (Ueno et al.,
2001, 2004; Pinti et al., 2009; Morag et al., 2016). It does not,
however, help to pinpoint the formation pathways of distinct
carbonaceous structures, as for instance those preserved in
the Dresser Formation (Glikson et al., 2008) or the younger
Apex chert (e.g. Schopf, 1993; Brasier et al., 2002, 2005;
Schopf et al., 2002; Bower et al., 2016). Further work is nec-
essary to test whether consistent molecular and compound-
specific isotopic patterns can be generated from a larger set
of Archaean kerogens.
5 Conclusions
Kerogen embedded in a hydrothermal chert vein from the
ca. 3.5 Ga Dresser Formation (Pilbara Craton, Western Aus-
tralia) is syngenetic. A biological origin is inferred from
the presence of short-chain n-alkanes in high-temperature
HyPy pyrolysates, showing a sharp decrease in homologue
abundance beyond n-C18. HyPy products of pre-extracted re-
cent bacterial biomass exhibited a similar restriction to car-
bon chain lengths ≤ n-C18, whereas abiotic compounds ex-
perimentally formed via Fischer–Tropsch-type synthesis ex-
hibited a unimodal distribution. A biological interpretation
for Dresser organics is further consistent with the δ13CTOC
value (−32.8± 0.3 ‰) and the stable carbon isotopic com-
position of n-alkanes in the Dresser high-temperature py-
rolysate (−29.4 to −33.3 ‰; mean −31.4± 1.2 ‰). Based
on these observations, we propose that the original organic
matter was primarily biologically synthesized. We hypothe-
size that microbially derived organic matter accumulating in
anoxic aquatic (surface and/or subsurface) environments was
Biogeosciences, 15, 1535–1548, 2018 www.biogeosciences.net/15/1535/2018/
J.-P. Duda et al.: The “hydrothermal pump hypothesis” 1545
assimilated, redistributed and sequestered by hydrothermal
fluids (“hydrothermal pump hypothesis”).
Data availability. Data are available upon request.
The Supplement related to this article is available online
at https://doi.org/10.5194/bg-15-1535-2018-supplement.
Author contributions. JPD, JR and MJVK conducted the field work
and designed the study. JR conducted petrographic analyses. NS
performed Raman spectroscopic analyses. MR and JPD conducted
pyrolysis experiments. HM conducted Fischer–Tropsch-type syn-
thesis. TB prepared cyanobacterial cell material. JPD, HM, MR and
VT performed biomarker analyses. JPD wrote the manuscript. All
authors discussed the results and provided input to the manuscript.
Competing interests. The authors declare that they have no conflict
of interest.
Acknowledgements. This work was financially supported by the
Deutsche Forschungsgemeinschaft (grant Du 1450/3-1, DFG
Priority Programme 1833 “Building a Habitable Earth”, to
Jan-Peter Duda and Joachim Reitner; grant Th 713/11-1 to
Volker Thiel), the Courant Research Centre of the Georg-August-
Universität Göttingen (DFG, German Excellence Program), the
Göttingen Academy of Sciences and Humanities (to Jan-Peter Duda
and Joachim Reitner), the International Max Planck Research
School for Solar System Science at the Georg-August-Universität
Göttingen (to Manuel Reinhardt and Helge Mißbach) and the
ARC Centre of Excellence for Core to Crust Fluid Systems (Mar-
tin J. Van Kranendonk). We thank Martin Blumenberg, Cornelia
Conradt, Wolfgang Dröse, Jens Dyckmans, Axel Hackmann, Merve
Öztoprak and Burkhard C. Schmidt for scientific and technical sup-
port. Josh Rochelmeier is thanked for assistance during sample ex-
traction of A. cylindrica. We are indebted to Malcolm Walter and
Mark A. van Zuilen for helpful comments on the manuscript and
Jack Middelburg for editorial handling. This is publication num-
ber 5 of the Early Life Research Group (Department of Geobiol-
ogy, Georg-August-Universität Göttingen; Göttingen Academy of
Sciences and Humanities) and contribution 980 from the ARC Cen-
tre of Excellence for Core to Crust Fluid Systems.
We acknowledge support by the German Research Foundation
and the Open Access Publication Funds of Georg-August-
Universität Göttingen.
This open-access publication was funded
by the University of Göttingen.
Edited by: Jack Middelburg
Reviewed by: Mark van Zuilen and Malcolm Walter
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