Igisu et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-018-0242-1(2018) 5:85

Progress in Earth andPlanetary Science

RESEARCH ARTICLE

Open Access

FTIR microspectroscopy of carbonaceous
matter in ~ 3.5 Ga seafloor hydrothermal
deposits in the North Pole area, Western
Australia
Motoko Igisu1* , Yuichiro Ueno1,2,3 and Ken Takai1,3

Abstract
Carbonaceous matter in ~ 3.5 Ga hydrothermal vein deposits from the Dresser Formation, Western Australia,
was analyzed using Fourier transform infrared (FTIR) microspectroscopy. Based on the spectroscopy, thecarbonaceous matter was mainly composed of disordered aromatic structures, with minor aliphatic C–H functionalgroups. Spatially resolved analysis supports that the aliphatic C–H signatures are derived from kerogenous macromolecule and not from free bitumen or other artificial sources. The intensity ratios of the asymmetric aliphatic CH3
to the asymmetric aliphatic CH2 (R3/2 value) in the carbonaceous clots range from 0.22 to 0.51. Thermal alteration
may increase or not change the R3/2 value of organic matter, as suggested by previous experiments, but it isunlikely to be the cause of the substantially lower R3/2 values when compared with those of primary organic matter.
In particular, the low R3/2 values (< ~ 0.4) suggest that the carbonaceous matter mainly contains aliphatic
C–H bonds derived from bacterial cells. The carbonaceous clots may have been possibly produced by abiotic
reaction such as Fischer-Tropsch-type (FTT) synthesis. However, the organic matter source only producedby the FTT synthesis is inconsistent with the R3/2 values for the analyzed carbonaceous clots. The results
obtained by combining these spectroscopic features of the carbonaceous clots together with the previously
reported isotopic features may possibly suggest that both bacteria and archaea were colonized in the~ 3.5 Ga Dresser hydrothermal system.
Keywords: FTIR microspectroscopy, Carbonaceous matter, Dresser formation, Archean

Introduction
Hydrothermal systems have been considered as theorigin of life (Holm and Andersson 1998) and the locations of the earliest evolution of life and metabolism(Nisbet 1995). The geological study of ancient hydrothermal systems is crucial to understanding the potential
role of hydrothermal systems as the earliest ecosystems.
One of Earth’s oldest seafloor hydrothermal deposits
occurs in the ~ 3.5 Ga Dresser Formation in Western
Australia (Fig. 1). In this formation, prominent
silica-barite veins containing carbonaceous matter* Correspondence: igisu@jamstec.go.jp
1
Department of Subsurface Geobiological Analysis and Research, Japan
Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15
Natsushima-cho, Yokosuka 237-0061, Japan
Full list of author information is available at the end of the article

ubiquitously penetrate pillowed basaltic greenstones(Ueno et al. 2004; Van Kranendonk 2006; Van Kranendonk
et al. 2008). The carbonaceous matter in the veinsafford an important means of understanding howorganic matter was synthesized in the Earth’s earlyhydrothermal systems, although the biological andabiological processes cannot be fully distinguished.
The occurrence of biological activity in the Dresser
Formation has been inferred from the observation of putative stromatolites (Walter et al. 1980; Van Kranendonk
2006; Van Kranendonk et al. 2008), morphologicallypreserved putative microfossils (Ueno et al. 2001), microbially induced sedimentary structures (Noffke et al. 2013),
and chemical fossils such as 13C-depleted organic matterand methane (Ueno et al. 2001, 2004, 2006) and 34Sdepleted pyrites with characteristic Δ33S (Ueno et al. 2008;© The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0
International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, andreproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to
the Creative Commons license, and indicate if changes were made.

Igisu et al. Progress in Earth and Planetary Science(2018) 5:85

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Fig. 1 Geologic map of the Dresser Formation in the North Pole area. The localities of the silica vein samples 96NP452 and T-58 are shown(modified from Kitajima et al. 2001)

Shen et al. 2009). Sulfur isotopic signatures also suggestthat microbial sulfate reduction was involved in pyrite formation (Ueno et al. 2008; Shen et al. 2009). In addition, ancienthydrothermal fluid inclusions contain 13C-depleted methane,suggesting the presence of methanogenic archaea (Ueno etal. 2006), although it is arguable if those methane were produced by metal-catalyzed abiotic reactions (Sherwood- Lollar and McCollom 2006). The 13C-depletion of organic
matter indicates the predominance of autotrophic carbon
fixation (Ueno et al. 2001, 2004). The organic matter has beencharacterized by Raman microspectroscopy, FTIR spectroscopy, solid-state 13C nuclear magnetic resonance, andpyrolysis analysis (e.g., Ueno et al. 2001; Derenne et al. 2008;
Duda et al. 2018). The results of these organic geochemical
analyses of isolated insoluble organic matter reveal that theorganic matter is dominated by aromatic structures withaliphatic hydrocarbon moieties. In addition, the pyrolysis
fragments of the organic matter comprise long-chain aliphatic hydrocarbons and thiophenes, which are likely related
to the existence of microbes including sulfate-reducing bacteria (Derenne et al. 2008; Duda et al. 2018).

Chemical analyses of Archean organic matter have provided limited but important information about the physiological and phylogenetic characteristics of the source
organic compounds (e.g., Hayes 1994; Brocks et al. 1999).
Although the observation of morphologically preserved
microfossils is a direct evidence of the existence of life, theidentification of microfossils based on only morphologicalanalysis is often insufficient (e.g., Brasier et al. 2002;
Schopf et al. 2002). The chemical analysis of carbonaceous
matter is necessary for understanding its origin andcharacteristics. However, the bulk analysis of isolated organic matter generally involves the potential risk of
post-depositional and experimental contamination of theorganic matter (e.g., Brocks et al. 1999; Derenne et al. 2008;
Rasmussen et al. 2008; Bourbin et al. 2012). In situ analysis
is a more reliable technique that avoids such contamination
problems and extracts chemical signatures specific to individual microstructures in petrographic thin sections (e.g.,
House et al. 2000, 2013; Kudryavtsev et al. 2001; Ueno et al.
2001; Schopf et al. 2002, 2005; Igisu et al. 2006, 2009; vanZuilen et al. 2007; Wacey et al. 2011; Lepot et al. 2013;

Igisu et al. Progress in Earth and Planetary Science(2018) 5:85

Williford et al. 2013). FTIR microspectroscopy has beenused to analyze Proterozoic organic-walled acritarchs andbacterial fossils, as well as Phanerozoic miospores (e.g.,
Arouri et al. 1999, 2000; Marshall et al. 2005; Igisu et al.
2006, 2009; Steemans et al. 2010; Qu et al. 2015). Thesestudies demonstrated the usefulness of IR signatures forcharacterizing organic-walled microfossils, including thosewith unknown affinity such as acritarchs. However, FTIR
microspectroscopy has not yet been applied to Archeancarbonaceous matter in petrographic thin sections. FTIR
spectroscopy has been performed on isolated Archean carbonaceous matter, and reported the presence of aliphatic
C-H stretching absorptions (Derenne et al. 2008). This previous attempt did not employ the in situ microscopic
method. It has been suggested that isolated and powderedsamples may be affected by contamination from volatile hydrocarbons in laboratory air (e.g., Salisbury and Walter
1989) and/or from other aliphatic C-H bearing components
during sample preparation and storage (e.g., Kebukawa et
al. 2009). Therefore, the data from powdered samples
would need to be interpreted with caution.
In this paper, we first report the FTIR microspectroscopic
features of early Archean carbonaceous matter obtained
from two ~ 3.5 Ga silica veins in the Dresser Formation in
the North Pole area of Western Australia. The IR signatures
are then compared with the characteristics of a variety of
biologically and abiotically produced organic compounds,
and the possible origins of the carbonaceous clots are
discussed.

Methods/Experimental
Samples

We analyzed two samples taken from the silica veins(96NP452 and T-58) that intrude on the basaltic greenstones of the Dresser Formation in the North Pole areaof Western Australia. The sampling localities of thesilica veins are shown in Fig. 1. Detailed descriptions of
the localities and samples can be found in the works of
Ueno et al. (2001, 2004, 2006). The Dresser Formationconsists of pillowed basaltic greenstones and chert beds.
In the North Pole area, these rocks have only undergone
low-grade metamorphism below their greenschist facies(Buick and Dunlop 1990; Kitajima et al. 2001; Ueno etal. 2004). The lowest chert unit of the formation isintercalated with several barite beds (Buick and Dunlop
1990), and numerous silica veins intrude the greenstones
below the chert/barite beds. The silica veins are 0.3–20 m
in width and generally > 100 m in length (Ueno et al.
2001). Zircon U-Pb dating yielded two distinct age groupsof 3458 ± 2 Ma (Thorpe et al. 1992a) and 3660 ± 52 Ma(Kitajima et al. 2008) of the felsic unit that overlies theDresser Formation. A model lead age of 3490 Ma was
obtained for galena taken from the Dresser Formation

Page 3 of 13(Thorpe et al. 1992b), and this may represent the actual
depositional age of the formation.
The first doubly polished petrographic thin section
sample was made from sample 96NP452 with a thickness of approximately 60 μm. The preparation process
was as described by Igisu et al. (2006). We used alumina
paste to polish only one side of the thin section toremove interference fringes. To avoid organic contamination during sample preparation, the sample was attached to a glass slide using only a carbon-based
dissoluble adhesive (Aron Alpha). The adhesive was used
to remove the doubly polished petrographic thin section
from the glass slide by immersing in acetone. Petrographic investigation of the thin section indicated thatthe silica vein sample was mainly composed of microcrystalline quartz (< ~ 10 μm), carbonaceous matter,
sulfides, and carbonates (see for example Fig. 2g, h). The
filamentous structures described by Ueno et al. (2001)
were not observed in the doubly polished thin section.
The second set of doubly polished thin section samples
was made from sample T-58 (Ueno et al. 2006) with a thickness of approximately 100 μm. A petrographic examinationof the sample revealed three distinctive fabrics, which were
denoted by F1–F3 (Fig. 2i). F1 was characterized bycoarse-grained quartz (> 1 mm) with abundant fluid inclusions. No carbonaceous matter was observed in F1. F2 comprised microcrystalline quartz chalcedony. Black aggregatesin F2 were carbonaceous matter (see Fig. 2i, j) and ion oxides
such as hematite. The center of the carbonaceous clot (whitepart in Fig. 2j) was quartz. F3 consisted mainly of microcrystalline quartz (< ~ 10 μm) and contained carbonaceous matter and ion oxides such as hematite (Fig. 2i). The analyzed
carbonaceous clots in the sample were preserved in a microcrystalline quartz matrix or microcrystalline quartz chalcedony, but not in the secondary quartz veins (Fig. 2).
FTIR microspectroscopy

FTIR microspectroscopic analysis was conducted on the
two thin section samples using an automated XYZ stage
set in an FTIR microspectrometer (JASCO FTIR6200 +IRT7000) with a single-point mercury cadmium telluridedetector. The thin section was placed over a hole in thesample holder (Nakashima et al. 1989) and measured as
described below.
A reference background spectrum was first measured
away from the mounted sample (i.e., in air). A transmission IR spectrum of the sample was then measured. The
IR spectrum was described in terms of the IR absorbance, which is a function of the wavenumber (cm−1):
Absorbance ¼ − log10 ðT =T 0 Þð1Þ

where T and T0 denote the transmitted intensity of theinfrared light at each wavenumber for the sample and air

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Igisu et al. Progress in Earth and Planetary Science

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b

a

c

h
d

e

f

Fig. 2 Optical photomicrographs of the analyzed thin sections. a–g Plane-polarized light photomicrographs of the thin section 96NP452. b
Composite image of the photomicrographs; the dashed rectangle in b corresponds to the area shown in Fig. 3a. f Enlarged view of the dashedrectangle in e. h Cross-polarized light photomicrograph of the same area in g. i, j Plane-polarized light photomicrographs of the thin section
T-58. i Composite image of the photomicrographs. j Enlarged view of the dashed rectangle in i. The locations (50 × 50 μm2 dotted squares) of
the 12 clots with satisfactory signal/noise ratios and their calculated R3/2 ratios (1024 scans) are shown in a–d, f, g, jbackground, respectively. To search for organic signatures,the IR spectra of about 3500 individual carbonaceous clots
were first obtained by collecting 64 scans per spot withina spectral range of 4000–1000 cm−1 with a 4 cm−1 spectral
resolution. Among the ~ 3500 examined spots, 47 had an
IR absorbance of ~ 2925 cm−1, which indicated excess

analytical error for the adopted 64-scan acquisition mode.
These spots were therefore measured again by collecting
1024 scans per spot within a spectral range of 4000–1000
cm−1 with a 4 cm−1 spectral resolution. This was done toobtain the IR spectra with a better signal-to-noise ratio.
Each spectrum was obtained from a spot of 50 × 50 μm2

Igisu et al. Progress in Earth and Planetary Science(2018) 5:85

aPage 5 of 13bcFig. 3 An example of the optical photomicrographs and transmission IR spectra of the analyzed carbonaceous clots. a Optical photomicrograph.
The squares indicate the analyzed areas (50 × 50 μm2). The IR spectra of the two analyzed areas (thick-bordered squares A and B) are shown inb and c. b Transmission IR spectra (64 scans) of the carbonaceous clots with (a) and without (b) aliphatic CH moieties. One scale unit on thevertical axis corresponds to an absorbance of 0.2. The IR spectrum in b is representative of those obtained from the white squares in a (without
aliphatic CH bonds). c An enlarged view of the IR spectra in b within 3000–2800 cm−1. The range is indicated by the dotted rectangle in b.
The arrow represents the IR band at ~ 2925 cm−1. One scale unit on the vertical axis corresponds to an absorbance of 0.02

(Figs. 2 and 3a). Duplicate measurements were performedon the 47 spots in the 1024-scan acquisition mode. The
analytical errors were determined from the duplicate measurements at the same location as the background
spectrum and were found to be smaller than 0.003 absorbance units in the 3000–2800 cm−1 range for the 64-scanacquisition mode and smaller than 0.0007 absorbanceunits for the 1024-scan acquisition mode. All the spectral
data were processed using Spectra Manager (JASCO).
Structural characterization of carbonaceous clots

In the aliphatic C–H stretching region (3000–2800cm−1), the ~ 2925 cm−1 band is due to two asymmetricstretching groups, namely, aliphatic CH2 and aromatic
CH3 (Jones and Sandorfy 1956; Painter et al. 1981). Theband is therefore considered as being the resultant ofthese two groups. Because the aliphatic CH2 and aromatic CH3 groups have symmetric stretching bands withdifferent peak positions at ~ 2850 and ~ 2870 cm− 1,
respectively (Bellamy 1954; Jones and Sandorfy 1956),
the contribution of the aliphatic CH2 groups can be
estimated by elucidating the relationship between theintensities at ~ 2925 cm−1 and ~ 2850 cm−1. According
to Lambert-Beer’s Law:
Absorbance ¼ ε  c  d;ð2Þwhere ε is the molar absorption coefficient, c is theconcentration, and d is the sample thickness. Based on
Eq. (2), the absorbance ratio between the asymmetricCH2 and symmetric aliphatic CH2 is converted into theratio of their molar absorption coefficients as follows:
Abss =Absa ¼ εs =εa ;ð3Þwhere Abss and Absa are respectively the absorbances(peak heights) of the symmetric aliphatic CH2 andasymmetric aliphatic CH2, and εs and εa are the corresponding molar absorption coefficients. Thus, if thelinear correlation between the peak heights at ~ 2925
and ~ 2850 cm−1 is obtained, the ~ 2925 cm−1 band
would be observed to be mainly due to the aliphatic
CH2, and less attributable to the aromatic CH3 group.
The IR signatures of the aliphatic CH3 and CH2 ratios
generally reflect the degree of branching and the chain
length of the aliphatic hydrocarbon moiety (e.g., Lin andRitz 1993; Coates 2000). To evaluate the relative lengthand degree of branching of the aliphatic chain in the
carbonaceous clots, we used the aliphatic CH3/CH2
absorbance ratio (R3/2):
R3=2 ¼ AbsCH3 =AbsCH2 ;ð4Þ

where AbsCH3 and AbsCH2 respectively denote the peakheights of the asymmetric stretching bands of thealiphatic CH3 (end-methyl, ~ 2960 cm−1) and CH2(chain-methylene, ~ 2925 cm−1) after linear baseline correction (Igisu et al. 2009). For a more precise determination of R3/2, we used only the data for the absorbancesat ~ 2960 and 2925 cm−1, which were more than twice
the analytical error.
Raman microspectroscopy

A laser Raman micro-spectrometer (Nanophoton
RAMANtouch) was used to characterize the carbonaceous
clots. Each sample was exposed to a green laser (532 nm
wavelength) for a period of 5 s at a laser power densityof 1 × 105 W/cm2. Raman spectra for wavenumbers within~ 2670–110 cm−1 were acquired with a 600 g/mm grating(spectral resolution of ~ 4 cm−1). A × 100 objective lens(NA = 0.9) was used to achieve Raman analysis spatial

Igisu et al. Progress in Earth and Planetary Science(2018) 5:85

resolution of < 1 μm, and the incident laser power was estimated to be ~ 0.4 mW at the sample surface. We only analyzed the carbonaceous matter embedded within the rocks
below the surface of the thin section to avoid the effect ofpolishing, which can induce deformation of carbonaceous
matter during sample preparation and thus possibly induce
artificial modification of the Raman spectroscopic feature(Pasteris 1989). Prior to sample analyses, wavenumber calibration was determined by comparison with the standard
silicon spectra (520 cm−1). All the spectral data were processed using RAMAN viewer (Nanophoton).
Raman spectroscopy has been used as a tool for providing a metamorphic grade indicator of carbonaceous
matter in geological samples (e.g., Yui et al. 1996; Beyssac et al. 2002; Rahl et al. 2005; Kouketsu et al., 2014).
Several indicators have been proposed to estimate peakmetamorphic temperatures by using Raman spectral parameters: peak position, intensity ratio, and full width at
half maximum (FWHM) of D band and G band. We
chose the D/G band intensity ratio (I1350/I1600) for estimation of the degree of thermal alteration of the carbonaceous matter in order to compare with the previous
spectral data of the Proterozoic samples (Qu et al. 2015).
The analytical errors for the Raman spectral features
were determined by taking duplicate measurements
within one area analyzed by FTIR microspectroscopy.

Results and discussion
IR spectra

Figures 3 and 4 show examples of the infrared transmission spectra of the carbonaceous clots in the silica veins.
We measured more than 3500 locations in the carbonaceous clots in the thin sections, and only 12 clots had
absorbances of ~ 2960, ~ 2925, or ~ 2850 cm−1, which
are more than twice the analytical error. The obtained
absorbances of ~ 2960, ~ 2925, and ~ 2850 cm−1 aresummarized in Table 1. The spectrum shown by line B
in Fig. 3 was the most common feature in the analyzed
carbonaceous clots. This is quite similar to that of thequartz matrix (black line in Fig. 4a, b1, c1), and suchdata were not used. Instead, some carbonaceous clots
show spectral patterns such as A in Fig. 3 (with a peakat ~ 2925 cm−1). The detailed peak assignments of thespectra are explained below.
The IR spectrum of the adhesive used to prepare thesamples is shown in Fig. 4a, b1, c1. The bands at 2988,
2944, 2908, and 2876 cm−1 are due to C–H stretching vibrations. The 1757 cm−1 band is due to ester C=O. The
1450 and 1375 cm−1 bands are due to C–H bending of
CH2 + CH3 and CH3 groups, respectively. The bands at
1260 and 1180 cm−1 are due to C–N and C–O, respectively. In the 3000–2800 cm−1 region, the most intense
band for the adhesive is observed at 2988 cm−1 (shownby a dotted arrow in Fig. 4b1). Based on the 4 cm−1

Page 6 of 13

wavenumber resolution of the IR spectrometer, thebands of the carbonaceous clots (~ 2960, ~ 2925, and/or~ 2850 cm−1) are considered to be different from the IRbands of the adhesive. The adhesive used to attach thethin section to the glass has been almost completely
washed by acetone. The difference between the absorbances at ~ 2960, ~ 2925, and ~ 2850 cm−1 of individual
analyzed spots (Table 1) would reflect the amounts ofcarbonaceous matter and/or its chemical structure. The
saturated bands (1400–1000 cm−1) and seven bands(1995, 1870, 1793, 1684, 1610, 1525, and 1492 cm− 1) in the
figures are due to the Si–O bonds of quartz (Nakashima etal. 1995; Ito and Nakashima 2002; Igisu et al. 2006, 2009;
Qu et al. 2015). The IR spectrum around 2300 cm−1 is notshown because this range is affected by absorption by CO2
in the air. In the region of lower wavenumbers of less than2500 cm−1, the many intense bands of the quartz matrix
mask the small signal from the organic matter (Figs. 3 and
4). Hence, we focused on the IR bands with wavenumbersof > 2500 cm−1.
The contribution of the aliphatic CH2 group to the intensity in the ~ 2925 cm−1 band was examined by comparison with those in the ~ 2925 and ~ 2850 cm−1 band.
Figure 5 shows the relationship between the intensities
in the ~ 2925 and ~ 2850 cm−1 bands. The strong correlation between these two bands suggests that they are
due to the same component(s) and that the ~ 2925 cm−1
band of the carbonaceous clots is mostly due to thealiphatic CH2 and is less attributable to the aromatic
CH3 groups. These results indicate that the Dresser
carbonaceous clots contain small amounts of aliphatics.
Raman spectral data

The carbonaceous clots in sample 96NP452 have disordered carbonaceous structures, which were observed as~ 1600 cm−1 (graphite, G) and ~ 1350 cm−1 (disordered,D) bands in the Raman spectra (Fig. 6). Their Raman
spectral features are shown in Table 1 and are similar toeach other for this sample. They are also consistent withthe previously reported data (Ueno et al. 2001, 2004)
and those of carbonaceous matter from lower greenschist facies metasediments (Yui et al. 1996). In addition,
it is inconsistent with the carbonaceous matter withdifferent thermal alteration phases observed in ~ 3.46 Ga
hydrothermal Apex Chert (Olcott-Marshall et al. 2012;
Sforna et al. 2014). These Raman and IR spectral features of the carbonaceous clots suggest that the clots
mainly consist of randomly arranged layered polyaromatic domains with minor aliphatic C–H bonds. On theother hand, the carbonaceous clot in sample T-58 does
not show clear G or D bands. Instead, it shows a sloping
baseline (Fig. 6), possibly suggesting fluorescence produced by hydrogen-rich saturated aliphatic hydrocarbons(Marshall et al. 2005). However, this appears inconsistentIgisu et al. Progress in Earth and Planetary Sciencea(2018) 5:85

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b6

c6

b7

c7

b1

c1

b8

c8

b2

c2

b9

c9

b3

c3

b10

c10

b4

c4

b11

c11

b5

c5

b12

c12

Fig. 4 The transmission IR absorbance spectra (1024 scans) of the carbonaceous clots. a Examples of the IR spectra of the carbonaceous clots with aliphatic
CH2 and CH3 bonds (blue line: spot #6 in Table 1), only aliphatic CH2 bond (green line: spot #12 in Table 1), and no CH2 bond (red line: spot #13 in Table 1)
together with quartz matrix (black line). IR transmission-reflection spectrum of the carbon-based dissolvable adhesive used in this study on an aluminum foil isshown by the brown line (modified from Igisu et al. 2006). One scale unit on the vertical axis corresponds to an absorbance of 0.2. (b) An enlarged view of the
IR spectra within 3000–2800 cm−1. The range for b1 is indicated by the dotted rectangle in a. Spot numbers (#1, #2, etc.) are shown in the upper right in eachfigure. One scale unit on the vertical axis corresponds to an absorbance of 0.02 in b1 and b4–b12, and 0.1 in b2 and b3. c The IR spectra after linear baseline
correction of the spectra in b within 3000–2800 cm−1. Spot numbers (#1, #2, etc.) are shown in the upper right in each figure. One scale unit on the verticalaxis corresponds to an absorbance of 0.002 in c1 and c12, and 0.001 in c2–c11. The spectra of the adhesive are re-scaled to a quarter in a, one-tenth in b1,
and one-hundredth in c1 for clarity

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Table 1 IR and Raman parameters for analyzed carbonaceous clots
Sample#Spot#

Microphotograph IR

Raman

Abs at ~ 2960
cm−1

Abs at ~ 2925
cm−1

Abs at ~ 2850
cm−1

R3/2

I1350/
I1600

A1350/A1600

FWHM1350 FWHM1600 N

96NP452 #1

Figure 2a0.0024 ± 0.0003

0.0062 ± 0.0005

0.0043 ± 0.0000

0.38 ±
0.02

1.54 ±
0.07

2.21 ±
0.06

65.0 ± 4.9

45.1 ± 3.027#2

Figure 2a0.0019 ± 0.0001

0.0050 ± 0.0002

0.0018 ± 0.0001

0.39 ±
0.00

1.46 ±
0.07

2.24 ±
0.07

67.6 ± 4.3

44.2 ± 2.928#3

Figure 2b0.0025 ± 0.0000

0.0088 ± 0.0010

0.0046 ± 0.0007

0.29 ±
0.03

1.45 ±
0.08

2.24 ±
0.05

68.3 ± 5.1

44.2 ± 2.2

27#4

Figure 2b0.0051 ± 0.0003

0.0100 ± 0.0005

0.0055 ± 0.0000

0.51 ±
0.01

1.46 ±
0.06

2.23 ±
0.03

67.1 ± 2.9

43.8 ± 1.3

27#5

Figure 2b0.0025 ± 0.0001

0.0070 ± 0.0006

0.0039 ± 0.0008

0.36 ±
0.05

1.52 ±
0.08

2.22 ±
0.08

63.2 ± 4.3

43.2 ± 3.1

27#6

Figure 2c

0.0017 ± 0.0000

0.0052 ± 0.0001

0.0026 ± 0.0001

0.34 ±
0.01

1.39 ±
0.07

2.20 ±
0.08

70.8 ± 5.5

44.8 ± 3.027#7

Figure 2d

0.0016 ± 0.0004

0.0066 ± 0.0005

0.0034 ± 0.0004

0.24 ±
0.04

1.47 ±
0.08

2.24 ±
0.04

65.1 ± 3.1

42.6 ± 1.2

27#8

Figure 2f

0.0018 ± 0.0005

0.0073 ± 0.0009

0.0040 ± 0.0005

0.26 ±
0.10

1.48 ±
0.06

2.25 ±
0.06

66.3 ± 2.7

43.6 ± 2.2

30*

#9

Figure 2f

0.0014 ± 0.0001

0.0038 ± 0.0001

0.0024 ± 0.0000

0.39 ±
0.05

1.48 ±
0.06

2.25 ±
0.06

66.3 ± 2.7

43.6 ± 2.2

30*

#10

Figure 2f

0.0021 ± 0.0003

0.0094 ± 0.0001

0.0048 ± 0.0002

0.22 ±
0.03

1.46 ±
0.04

2.26 ±
0.04

66.7 ± 1.6

42.9 ± 0.9

30

#11

Figure 2g

0.0026 ± 0.0002

0.0050 ± 0.0003

0.0024 ± 0.0002

0.51 ±
0.00

1.49 ±
0.08

2.25 ±
0.03

65.8 ± 3.9

43.4 ± 1.4

28

#12

n.d.

0.0042 ± 0.0001

0.0015 ± 0.0002

1.40 ±
0.06

2.24 ±
0.03

69.6 ± 2.5

43.6 ± 0.9

27

#13

n.d.

n.d.

n.d.

1.36 ±
0.13

2.00 ±
0.37

77.9 ± 8.8

52.4 ±
12.7

27

0.0021 ± 0.0000

0.0056 ± 0.0004

0.0025 ± 0.0001

T-58

#14

Figure 2j

0.38 ±
0.02

Absorbances at ~ 2960 cm−1, ~ 2925 cm−1, and 2850 cm−1 and R3/2 values of the analyzed carbonaceous matter. Errors show standard deviations. n.d. not detected*The same data are shown because the analyzed area for FTIR microspectroscopy of the clots were overlappedwith the IR data that the absorbance of aliphatic CH2 inthe carbonaceous clots in sample T-58 falls within therange for those in sample 96NP452 (Fig. 5). The inconsistency between the Raman and IR data of the carbonaceous clots in sample T-58 and sample 96NP452 maybe due to difference in the amounts of organic matter.
Raman spectral data suggest that the analyzed carbonaceous
clots at least in sample 96NP452 are moderately graphitizedby metamorphism, supporting indigenous origin in
ca. 3.5 Ga rocks rather than contaminations.
Spatial distributions of aliphatic C–H groups

The R3/2 values of the carbonaceous clots range from
0.22 ± 0.03 to 0.51 ± 0.01 (Fig. 7 and Table 1). In sample
96NP452, some of the carbonaceous clots containing
aliphatic C–H signatures were located relatively close(< 100 μm) to the secondary quartz veins (Fig. 2b, c),while others were far (> 100 μm) from the veins (Fig. 2e,g). Both low (~ 0.3) and high (~ 0.5) R3/2 values wereobserved within one carbonaceous clot (Fig. 2f ) and also

along one secondary quartz vein (Fig. 2b). In sample
T-58, one carbonaceous clot with aliphatic C–H signatures was observed in the quartz chalcedony (F2; Fig. 2i),while the other carbonaceous clots in the quartz
chalcedony and microcrystalline quartz did not have aliphatic C–H signatures. These results indicate the absenceof an apparent distribution pattern among the carbonaceous clots with and without aliphatic C–H signatures. In
addition, there was significant variation among the R3/2
values within a given sample, and even within a given
carbonaceous clot. The aliphatic C–H signatures of the
carbonaceous clots are spatially heterogeneous on a micrometer to millimeter scale.
Origin of the carbonaceous matter

Post-depositional thermal alteration of the primary organic matter may have altered the R3/2 values. The
varying R3/2 values of the carbonaceous clots may also
be explained by a mixture of different sources of
biological organic compounds and/or abioticallyIgisu et al. Progress in Earth and Planetary Science(2018) 5:85

Fig. 5 Relationship between the intensities of the carbonaceous
clots at ~ 2925 cm−1 and ~ 2850 cm−1. The ~ 2925 cm−1 band is due
to asymmetric stretching vibrations of the aliphatic CH2 andaromatic CH3, and the ~ 2850 cm−1 band is due to symmetricstretching vibration of the aliphatic CH2. The yellow diamonds
indicate the data for sample 96NP452, while the green diamondindicates that for sample T-58. The error bars indicate the standard
deviations of the duplicate measurements

produced compounds. The possible occurrence of anaerobic chemoautotrophic microorganisms was suggestedby the carbon isotopic signatures of the carbonaceous
matter in the silica veins in the Dresser Formation(Ueno et al. 2004). The carbon isotopic signatures provide the potential chemical evidence of microbial activity in the environment at the time, but do not

Fig. 6 Examples of Raman spectra of the carbonaceous clots. The
spectra marked as CH2, CH2 only, and no CH2 were obtained fromsample 96NP452 (CH2: both aliphatic CH2 and CH3 bonds wereobserved in micro-FTIR measurements; CH2 only: CH2 bond wasobserved but CH3 was not; no CH2: neither CH2 nor CH3 wereobserved). Spot numbers (#3, #12, and #13) corresponding toTable 1 are also shown. The band around 464 cm−1 is due to thequartz matrix of the silica veins

Page 9 of 13

necessarily offer clues for distinguishing whether the
products are derived from the microbial functions of
the bacterial and/or the archaeal populations. The existence of sulfate-reducing bacteria has been proposedby the pyrolysis fragments of the organic matter(Derenne et al. 2008). However, the potential risk of
post-depositional and experimental contamination has
been found in the pyrolysis analysis of the isolated
Archean organic matter (e.g., Bourbin et al. 2012).
The presence of ancient methanogenic archaea has
been inferred from the fluid inclusions in the silica
veins containing 13C-depleted methane (Ueno et al.
2006), although the abiotic origin of the methane has
also been proposed (Sherwood-Lollar and McCollom2006). We will discuss the origin of the varying R3/2
values for the carbonaceous clots in the following
order: (1) abiological origin, (2) effect of thermal alteration, and (3) biological origin.

Abiological origin

It is known that an FTT synthesis under specific hydrothermal conditions (e.g., in ultramafic rock-associatedhydrothermal systems) may produce organic compounds(e.g., Yanagawa and Kobayashi 1992; Holm and Andersson
1998; McCollom et al. 1999; McCollom and Seewald2006). The organic products of the FTT synthesis aremainly methane and normal alkanes with only a small
amount of branched hydrocarbons because the relative
abundance of these hydrocarbons decreases with increasing carbon number in the alkyl chains (McCollom andSeewald 2006, 2007). McCollom and Seewald (2006) reported that isobutane was the only branched product of
FTT synthesis, but the yield of isobutane (~ 0.02 mmol/kg)
was much lower than those of the normal alkanes (e.g.,
0.64 mmol/kg for n-butane). Therefore, it is supposed thatmixing of the organic matter produced by the synthesis(mainly the short-chain products) increases the R3/2 value.
Although we did not directly determine the carbon numberof the aliphatic components in the carbonaceous clots, theR3/2 values for the normal C10 alkane are estimated tobe ~ 0.6 (Fig. 7a; data from Igisu et al. 2009). The relative
abundance of the saturated > C10 hydrocarbons producedby the FTT synthesis has been shown to be several ordersof magnitude lower than those of the saturated < C6hydrocarbons (McCollom and Seewald 2006). Thus, itappears to be difficult to explain the observed R3/2 values of
the carbonaceous clots (0.22–0.51) at this stage based solely
on abiotically produced organic matter. Moreover, it isunlikely that the FTT synthesis of organic compounds
occurs during the formation of the silica veins. This isbecause native Fe–Ni minerals and other preferred catalysts
for FTT reactions were not observed in the thin sections of
the present study.

long straight chainprokaryotes
A Extant& n-alkanes
1.4

C Carbonaceous matter

B Thermal changes of prokaryotic cells
non-embedded cellsilica-embedded cell
membrane
1.4
protein
lipid

1.2

1.0

1.0
n-C10

0.8

Page 10 of 13

1.2

n-C18

1.0

bacterial cell

1.2(2018) 5:85

1.4
bacterial lipid
archaealcellarchaeal lipid

R3/2 value

short/branched chain

Igisu et al. Progress in Earth and Planetary Science

600

0.8

0.8
442

0.6

0.6

0.6

0.4

0.4

0.4

0.2

0.2

0.2
0.0

Extantprokaryotes alkanes(this study)
96NP452(Igisu et al., 2009)
Bitter Springs filament
Bitter Springs coccoid
Gunflint filament & coccoid
Gunflint CM(Qu et al., 2015)
Wumishan CM

0.0

0.0
100

200

300

400

0

0.5

low structuralorder

T [oC]

1
1.5
2I1350/I1600 high structuralorder

Fig. 7 R3/2 values of the extant prokaryotes, experimentally maturated cyanobacteria, and the carbonaceous clots. a R3/2 values of the extant
bacterial and archaeal cells and lipids (data from Igisu et al. 2012) together with the standard n-alkane samples (n-C10 and n-C18) (data from Igisu
et al. 2009). The R3/2 values for the representative extant untreated bacterial and archaeal cells range from 0.59 ± 0.01 to 0.70 ± 0.01 and from 0.86
± 0.04 to 0.95 ± 0.02, and those for bacterial and archaeal lipids range from 0.31 ± 0.09 to 0.41 ± 0.01 and from 0.51 ± 0.05 to 0.67 ± 0.08,
respectively. b Results of the heating experiments for the non-embedded and silica-embedded cyanobacterial cells (modified from Igisu et al.
2009, 2018). The R3/2 values for the heated samples are plotted against the heating temperature. All the experimental run products of the silicaembedded samples were obtained after heating for 600 min except the one indicated by a number (the sample marked was heated for 442 min).
Others were obtained after heating for 300 min except the one indicated by a number (the sample marked was heated for 600 min). c R3/2 valuesof the carbonaceous clots obtained from the Dresser Formation together with the Proterozoic prokaryotic fossils and carbonaceous matter (CM)in stromatolite. The error bars for the carbonaceous clots indicate the standard deviations of the duplicate measurements. The data for Bitter
Springs and Gunflint samples were obtained from Igisu et al. (2009), and that for Wumishan sample was obtained from Qu et al. (2015)

Effect of thermal alteration

Thermal alteration may increase or not change the R3/2
value of organic matter. This indicates the preferential
loss of the methylene group and/or formation of
branched CH3 in the aliphatic structures during the degradation of the organic matter, as revealed by FTIR measurements of coal (Wang and Griffiths 1985; Ibarra et al.
1996), and heating experiments performed on kerogens(Robin et al. 1977; Huang and Otten 1998) and extant
cyanobacteria (Igisu et al. 2009, 2018). Recently, a kinetic
approach applied to experimental maturation of cyanobacteria revealed that decreasing rate constants for aliphatic CH3 were generally similar to or lower thanaliphatic CH2 at 250–450 °C under both non-embedded
and silica-embedded conditions. This suggests that aliphatic CH3 groups degraded more slowly than aliphatic
CH2 groups (Igisu et al. 2018). The metamorphic gradeof basaltic greenstones surrounding the silica veins isgenerally below the greenschist facies (< 350 °C; Kitajima
et al. 2001). Thermal alteration may therefore explain
the varying R3/2 values, but is unlikely to be the cause of
the substantially lower R3/2 values when compared withthose of primary organic matter.
In addition, the coexistence of carbonaceous matter withdifferent degrees of thermal alteration may cause variationin the R3/2 value and the presence/absence of aliphatic
C–H signatures in the carbonaceous clots. There is thepossibility of contribution of some molecules and/or somefunctional groups, which are very resistant to heat even if
they suffered from the same heating processes, to the carbonaceous clots. Assuming that some parts of the molecules and groups were graphitized but some parts were not,there may be differences in the R3/2 values and Raman
spectral features observed in the carbonaceous clots insamples 96NP452 and T-58. Such heterogeneity was reported in Phanerozoic eukaryotic fossils (Qu et al. 2015).
Biological origin

The R3/2 values for the carbonaceous clots in the present
study (0.22–0.51) generally fall within the range for extant bacterial and archaeal cellular compounds (Fig. 7a,c). The observed low R3/2 values below ~ 0.4 cannot be
explained by the degraded cellular compounds otherthan degraded bacterial lipids, possibly representing therelatively bacteria-dominated fractions of the microbialcommunities. On the other hand, there are carbonaceous clots with higher R3/2 values (> ~ 0.4). Such carbonaceous clots may be caused by thermal alteration oforganic matter with lower R3/2 values or be derived fromthe archaea-dominated assemblages as well as

Igisu et al. Progress in Earth and Planetary Science(2018) 5:85

compounds with short-chain aliphatic components. Sucha possible contribution of aliphatic components to geomacromolecules by living organisms without resistantaliphatic precursors (e.g., algaenan) has been suggested
by Gupta et al. (2007a, 2007b, 2014).
A comparison of the R3/2 values of the carbonaceous
clots with those of other Proterozoic prokaryotic fossils
and carbonaceous matter in the stromatolites (~ 830 Ma
Bitter Springs Group, ~ 1500 Ma Wumishan Group, and~ 1900 Ma Gunflint Formation) reveals that the carbonaceous clots have similar R3/2 values as other Proterozoic
samples (Fig. 7c), while the Raman spectra of the carbonaceous clots show a more graphitized nature than those of
the Proterozoic microfossils (i.e., narrower bandwidth ofespecially the D band and higher D/G band intensity
ratio). This suggests the possible interpretation of theprokaryotic origin of the carbonaceous clots, assumingthat the R3/2 value either increases or remains unchanged
during the thermal degradation processes.
Given that the aliphatic moieties in the carbonaceous
clots might indicate the presence of cellular organic
aggregates of the prokaryotic population in the ancient
microbial communities, the IR spectroscopic features of
the carbonaceous clots likely suggest a contribution
from the bacterial lipids in the ancient microbial communities associated with the hydrothermal activities.
Our results thus afford additional important insights into
the indigenous microbial communities in the ~ 3.5 Ga
Dresser seafloor hydrothermal system that consists of not
only archaeal components, but also significant bacterial
populations. This supports the previous pyrolysis analysisof the isolated organic matter, suggesting microbial
origin of the organic matter (Derenne et al. 2008;
Duda et al. 2018).

Conclusions
FTIR microspectroscopic analysis was conducted on carbonaceous clots taken from two silica veins in the ~ 3.5
Ga Dresser Formation. We found the following:
1. The chemical structure of the carbonaceous clots
comprises randomly arranged layered polyaromatic
domains with very minor aliphatic straight chain
moieties.
2. It appears unlikely at this stage that the observed
R3/2 values of the carbonaceous clots and themineral assemblages in the analyzed samples do notsupport the only abiogenic origin of the
carbonaceous clots.
3. Thermal alteration may increase or not change theR3/2 value of organic matter. Therefore, it may
explain the varying R3/2 values, but is unlikely to be
the cause of the substantially lower R3/2 values

Page 11 of 13

when compared with those of primary organic
matter.
4. The R3/2 values of the carbonaceous clots (0.22–0.51) generally fall within the range of the values forextant prokaryotic cellular components. In
particular, the low R3/2 values (< ~ 0.4) could haveoriginated from bacterial lipid, assuming that theR3/2 value would either increase or remainunchanged during the thermal degradation
processes. This suggests the preservation ofremnants of the bacterial population, mainly as
aliphatic C–H groups, in the macromolecular
structure of the analyzed carbonaceous matter.
The new FTIR microspectroscopic data of the carbonaceous clots combined with the previous studies based on
their isotopic features (e.g., Ueno et al. 2001, 2004, 2006)
may possibly suggest that both bacteria and archaea werecolonized in the ~ 3.5 Ga Dresser hydrothermal system.
Abbreviations
FTIR: Fourier transform infrared (spectroscopy); FTT: Fischer-Tropsch-type;
Ga: Giga-annum; IR: Infrared

Acknowledgements
We would like to thank S. Maruyama, Y. Isozaki, M. Terabayashi, Y. Kato, K.
Okamoto, T. Ota, T. Kabashima, K. Kitajima, and K. Shimizu for their assistancein the field work; A. Thorne, K. J. McNamara, and A. H. Hickman for the field
collaboration; and Editage (www.editage.jp) for English language editing.
This work was supported by JSPS KAKENHI (24·7955, JP15J40157).

Funding
This work was supported by JSPS KAKENHI (24·7955, JP15J40157).

Availability of data and materials
Please contact the corresponding author regarding data requests.

Authors’ contributions
MI and YU proposed and designed the study. YU conducted the fieldwork
and sampling. MI carried out the measurements, analyzed the data, anddrafted the manuscript. YU and KT collaborated with the corresponding
author in the interpretation of the data and the construction of themanuscript. All authors read and approved the final manuscript.

Competing interests
The authors declare that they have no competing interests.

Publisher’s NoteSpringer Nature remains neutral with regard to jurisdictional claims inpublished maps and institutional affiliations.
Author details
1
Department of Subsurface Geobiological Analysis and Research, Japan
Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15
Natsushima-cho, Yokosuka 237-0061, Japan. 2Department of Earth andPlanetary Sciences, Tokyo Institute of Technology, 2-12-1 O-okayama,
Meguro-ku, Tokyo 152-8551, Japan. 3Earth-Life Science Institute, Tokyo
Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551,
Japan.

Igisu et al. Progress in Earth and Planetary Science(2018) 5:85

Received: 12 October 2017 Accepted: 15 November 2018

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