Origin of methane and heavier hydrocarbons entrapped within Miocene methane-seep carbonates from central Japan

Accepted Manuscript Origin of methane and heavier hydrocarbons entrapped within Miocene methane-seep carbonates from central Japan

Yusuke Miyajima, Akira Ijiri, Akira Miyake, Takashi Hasegawa PII: DOI: Reference:

S0009-2541(18)30454-6 doi:10.1016/j.chemgeo.2018.09.014 CHEMGE 18907

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

17 May 2018 2 July 2018 10 September 2018

Please cite this article as: Yusuke Miyajima, Akira Ijiri, Akira Miyake, Takashi Hasegawa , Origin of methane and heavier hydrocarbons entrapped within Miocene methane-seep carbonates from central Japan. Chemge (2018), doi:10.1016/j.chemgeo.2018.09.014

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ACCEPTED MANUSCRIPT

Origin of methane and heavier hydrocarbons entrapped within

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Miocene methane-seep carbonates from central Japan

Oiwakecho,

Kitashirakawa,

Sakyo-ku,

b

606-8502,

Japan

Research Fellow of the Japan Society for the Promotion of Science, Japan

Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and

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c

Kyoto

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([email protected])

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Department of Geology and Mineralogy, Graduate School of Science, Kyoto University,

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a

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Yusuke Miyajimaa,b,1,*, Akira Ijiric,*, Akira Miyakea, and Takashi Hasegawad

Technology (JAMSTEC), 200 Monobe Otsu, Nankoku City, Kochi 783-8502, Japan

Department of Earth Sciences, Faculty of Geosciences and Civil Engineering, Institute

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d

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([email protected])

of Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan 1

Present address: Geochemical Research Center, Graduate School of Science, The

University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan *

Corresponding authors: Geochemical Research Center, Graduate School of Science,

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The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan; and Geomicrobiology Group, Kochi Institute for Core Sample Research, Japan Agency for

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Marine-Earth Science and Technology (JAMSTEC), 200 Monobe Otsu, Nankoku City, Kochi 783-8502, Japan. addresses:

[email protected]

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E-mail

Miyajima),

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[email protected] (A. Ijiri)

(Y.

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Abstract

We examined the carbon isotopic and molecular compositions of residual gases

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within Miocene methane-derived carbonates collected in Japan. Methane, ethane, and propane were extracted by acid digestion of powdered carbonates. The isotopic and

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molecular compositions of the extracted hydrocarbons are inconsistent with

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conventional thermogenic and microbial gases. Despite a range of δ13C values from −67‰ to −38‰ (relative to Vienna Pee Dee Belemnite (VPDB)), the liberated hydrocarbons yielded consistently low methane to ethane + propane ratios (2–30). The extracted ethane and propane yielded anomalous δ13C values as low as −84‰, lower than those of the coexisting methane. The ethane and propane were most likely produced through thermal cracking of organic compounds preserved within the seep

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carbonates during burial. The observed unusual isotopic trends may be explained by the mixing of two thermogenic gas components with different carbon isotopic and

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molecular compositions. Nevertheless, a positive correlation between δ13C values of methane and relatively immature carbonates at one study site (Nakanomata) indicates

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that while methane was oxidized to bicarbonate from which carbonates precipitated, it

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was preserved within the host carbonate cements. Such a scenario indicates that the

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residual methane at least partly originates from the Miocene seep fluid. Smaller

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amounts of methane were also released during heating and crushing of chipped samples. The results suggest that methane was entrapped mainly within intracrystal inclusions,

individual crystal.

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which is supported by the observation of abundant nanometer-scale voids in an

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isotopes

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Keywords: Carbonates, Hydrocarbon seep, Methane, Ethane, Propane, Carbon stable

1. Introduction

Methane is the simplest known organic molecule, is a potent greenhouse gas, and forms the dominant component of natural gases. Submarine methane seeps are characterized by elevated concentrations of methane, as well as other heavier

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hydrocarbons (e.g., Pape et al., 2010, 2014; Toki et al., 2007, 2012). At seeps, methane originates either from thermal degradation of organic matter or from microbial

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methanogenesis. Thermogenic methane production occurs at depth within sediments, typically at temperatures of >80°C, whereas microbial methanogenesis generally occurs

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at shallower depths (e.g., Claypool and Kvenvolden, 1983; Quigley and Mackenzie,

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1988; Whiticar, 1999). Constraining the origin of methane at seeps thus provides

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insights into subsurface biogeochemical processes and the migration of seep fluids. The

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origin of methane at modern seeps has been investigated through the analysis of stable carbon and hydrogen isotopic compositions of methane, as well as the ratio of methane

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to other heavier hydrocarbons (e.g., Pape et al., 2010, 2014; Toki et al., 2012). Methane seeps have commonly occurred along continental margins throughout the Phanerozoic

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(Campbell, 2006; Judd and Hovland, 2007). However, we currently lack a method to

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directly constrain the origin of methane contained in ancient seep fluids. Previous studies have instead used the regional geological background or the carbon isotopic composition of authigenic methane-seep carbonates and lipid biomarkers to infer the origin of ancient methane (e.g., Agirrezabala et al., 2013; Gómez-Pérez, 2003; Himmler et al., 2015; Niemann and Elvert, 2008; Peckmann et al., 2001). Although strongly 13

C-depleted isotopic signatures of seep carbonates and lipid biomarkers can allow to

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identify a microbial source of methane (e.g., Himmler et al., 2015), these approaches often produce ambiguous results.

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The extraction of methane and other hydrocarbon gases from ancient methane-seep carbonates or other sediments may help to constrain the origin of seep fluids and the

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subsurface biogeochemical processes that operated in the geological past. Various

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methods have been used to liberate hydrocarbon gases from clastic sediments and

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sedimentary rocks (Abrams, 2005). Methane likely adsorbs onto clay minerals in

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sediments through physical adsorption, and can be liberated by heating (Sugimoto et al., 2003; Toki et al., 2007). Ijiri et al. (2009) reported that acid treatment could be used to

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liberate methane and ethane from authigenic carbonate concretions recovered from deep-sea sub-seafloor sediments. These concretions yielded methane concentrations that

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were two orders of magnitude greater than the bulk sediment (Ijiri et al., 2009).

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Methane concentration was also observed to increase with increasing carbonate content of the bulk sediments (Ijiri et al., 2009). Methane liberated from the concretions yielded stable carbon isotopic compositions identical to, or lower than, methane contained in the surrounding bulk sediments. It was thus concluded that methane in the carbonate concretions and that in the surrounding sediments shared the same origin (Ijiri et al., 2009). Brekke et al. (1997) also proposed that gaseous hydrocarbons liberated by acid

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digestion of sediment are cogenetic with fine-grained carbonate cements, which were formed via microbial degradation of organic matter. Their results implied that

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authigenic carbonates play a key role in the storage of methane in sediments. Ijiri et al. (2009) suggested that methane was stored in the carbonates through simple physical

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adsorption.

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These previous studies indicate that methane contained in seep fluids may be

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preserved within authigenic methane-seep carbonates, which form via an alkalinity

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increase induced by the anaerobic oxidation of methane (AOM) (e.g., Peckmann and Thiel, 2004; Ritger et al., 1987). Recently, Blumenberg et al. (2018) used acid treatment

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to extract low concentrations (<257 ppb) of methane and heavier hydrocarbons from modern seep carbonates. Their results showed some offset of a few to ~10 per mil

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between the δ13C value of the carbonate-entrapped methane and that of the

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corresponding seep methane. Blumenberg et al. (2018) also found that the molecular (C1/(C2 + C3)) ratio of the extracted gases and the carbon isotopic composition of ethane and propane were distinct and not related to the seep gas. Morales et al. (2017) examined gas inclusions in Mesozoic glendonite, which pseudomorphs ikaite (CaCO3‧ 6H2O) and represents an authigenic precipitate related to AOM. They crushed the glendonite samples to liberate methane and heavier hydrocarbons, which yielded

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carbon isotopic signatures consistent with a thermogenic origin. As only a few attempts have been made to extract residual methane and other gases from ancient methane-seep

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carbonates (Ijiri, 2003), the storage mechanisms and modification processes of residual gases within seep carbonates remain unclear.

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The present study shows that methane may be entrapped within ancient

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methane-seep carbonate crystals. The storage mechanism of methane within seep

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carbonates is inferred using various extraction methods, and the origin of the extracted

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gases is revealed through analysis of their stable carbon isotopic and molecular

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compositions.

2. Material

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Carbonate rock samples were collected from the Tortonian and Messinian sections

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of the Nodani Formation in Joetsu, central Japan (Nakanomata site: 37°05ʹ50ʺN, 138°09ʹ22ʺE) and the Serravallian section of the Bessho Formation in Matsumoto, central Japan (Anazawa/Akanuda site: 36°19ʹ25ʺN, 138°00ʹ34ʺE) (Fig. 1). The Nodani and Bessho formations are composed of marine clastic sediments deposited in the paleo-Japan Sea. The Japan Sea is a back-arc basin that opened in the Miocene (Burdigalian to Langhian), during which rift basins formed and were filled with

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volcanic and pyroclastic rocks (e.g., Iijima and Tada, 1990; Jolivet and Tamaki, 1992). The volcanics are overlain by a >5000-m-thick succession of Miocene–Pleistocene

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clastic marine sediments. The volcanic rocks and organic-rich sediments in this region act as present-day oil and gas source and reservoir rocks (Kikuchi et al., 1991; Okui et

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al., 2008). Seepage of thermogenic and microbial methane has been reported from

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several areas of the seafloor along the eastern margin of the Japan Sea (Matsumoto et al.,

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2009).

carbonates,

based

on

their

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The examined lithologies have been interpreted as ancient methane-seep paleontological,

petrographic,

and

geochemical

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characteristics including highly negative δ13C values of the carbonates (as low as ~−40‰; Miyajima et al., 2016, 2017; Nobuhara, 2010). At Nakanomata, the carbonate

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samples comprise microcrystalline and acicular aragonite, with grain sizes of up to a

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few tens of micrometers (Fig. 2). In contrast, the Anazawa/Akanuda carbonates were subjected to late diagenetic recrystallization and are composed of microcrystalline calcite (micrite) and larger sparry calcite crystals (a few hundreds of microns to >1 mm in diameter). In addition, sedimentary carbonate concretions were also collected from the Nodani Formation. These concretions are associated with vesicomyid bivalve (Calyptogena pacifica) fossils, which are commonly observed at modern methane seeps.

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However, the carbon isotopic composition of the concretions (δ13C > −25‰) indicates a mixture of different bicarbonate sources such as sulfate reduction of sedimentary

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organic matter and seawater dissolved inorganic carbon, as well as methane oxidation (AOM). Along with the lack of diagnostic biomarkers for AOM-related microbes in the

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concretions, it cannot be ruled out that the concretions are unrelated to AOM at seeps (Y.

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Miyajima, unpublished data). Maturity-related biomarker parameters indicate that

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organic matter in the Nakanomata seep carbonates and sedimentary concretions attained

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a lower thermal maturity (vitrinite reflectance Ro of ~0.6%) than the Anazawa/Akanuda carbonates (Ro > 0.8%) (for details of the maturity assessment, see Supplementary

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Material).

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3. Analytical methods

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To extract residual gases from seep carbonate samples, we performed three experiments: 1) acid digestion; 2) heating; and 3) crushing. Acid digestion was conducted on all samples, whereas heating and crushing experiments were performed only on Nakanomata seep carbonates.

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3.1. Acid digestion experiment For the acid digestion experiment, a hand-held rotary micromill was used to

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acquire powders from slabs or blocks of carbonate samples. Some samples were prepared by grinding crushed chips using a tungsten mortar and pestle. Samples were

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collected from early diagenetic carbonate phases, i.e., microcrystalline matrix and

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void-filling cements (acicular aragonite and sparry calcite). Hydrocarbon gases were

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extracted from carbonates using the method described by Ijiri et al. (2009). The powders

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(~50–100 mg; ~1–30 μm) were placed in glass vials (inner volume = 5 cm3), which were sealed with a butyl rubber septum and aluminum cap. The vials were evacuated,

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and orthophosphoric acid (~0.5 mL) was added to dissolve the carbonate minerals. After complete digestion of the carbonates at 50°C, a gas-tight syringe was used to extract 10

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mL of liberated gas, which was then injected through a silicone rubber septum into our

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analytical system at the Kochi Core Center (KCC), Nankoku, Japan. The analytical system was modified from the methods reported by Ijiri et al. (2003) and Tsunogai et al. (2002) to allow online analysis of the carbon isotopic ratio of hydrocarbon gases. This system consists of a gas drier (Magnesium perchlorate), a CO2-trapping port with Ascarite II, and one silico steel coiled trap (30-mm-long column packed with Porapak-Q). Most of the water vapor and CO2 in the gas sample was removed at the gas

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drier and CO2-trapping port. Then, the dried gas sample was collected in the coiled trap and held at liquid N2 temperature (−198°C). After the complete collection of gas sample,

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the two-position 6-port valve was turned and the concentrated gases in the trap were released by removing liquid N2 and heating the trap. The released gases were processed

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by gas chromatographic separation in a Thermo Scientific Trace GC using a 2.0 m × 1.0

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mm i.d. SHINCARBON-ST 80/100 column (Micropacked St. Shinwa Chemical

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Industries Ltd.) at a helium flow rate of 2 mL/min. The GC oven temperature was first

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held at 60°C for 17 min, increased to 280°C at 40°C/min, and then kept at 280°C for 40 min. The eluted methane, ethane, propane, i-butane, and n-butane were quantitatively

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converted to CO2 by passing through a 930°C combustion (CuO/Pt catalyzer) that carried them directly into a Finnigan Delta Plus XP isotope-ratio-monitoring mass

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spectrometer (IRMS). Extracted hydrocarbons were measured as nmol/g, assuming that

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all gases originated from the carbonate. Hydrocarbon concentrations were calculated by comparing measured

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CO2 concentrations with those of a standard gas containing

0.513% methane, 0.492% ethane, 0.490% propane, and 0.502%

n-butane.

Concentrations of i-butane are not reported in this study as i-butane can be liberated from the butyl rubber septum during acid treatment. Carbon stable isotopic compositions are reported as per-mil differences between the sample and VPDB

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standard using conventional delta notation (δ13C). Detection limits for the isotope ratio measurements were ~0.7 nmol for methane, ~0.1 nmol for ethane, and ~0.2 nmol for

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propane and butanes. Standard deviations from repeated analyses of the standard gas were <0.2‰.

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Carbon stable isotopic compositions of the carbonate powders were measured

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using a Thermo Scientific Kiel III/MAT253 IRMS at KCC and a Thermo Scientific

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GasBench II/Delta V Advantage IRMS at the Laboratory of Evolution of Earth

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Environment, Kanazawa University (LEEKU), Kanazawa, Japan. To produce CO2, ~50–70 and ~300–500 μg powders were reacted with orthophosphoric acid in glass

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vials at 70°C, within the online Kiel III and GasBench II carbonate devices, respectively. Standard deviations during replicate analyses of NBS19 and working standards LSVEC

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and JLs-1 by GasBench II/Delta V Advantage were better than 0.06‰ and 0.12‰ for

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δ13C and δ18O, respectively. Standard deviations during replicate analyses of NBS19 and working standard ANU-m2 by Kiel III/MAT253 were better than 0.07‰ and 0.08‰ for δ13C and δ18O, respectively.

3.2. Heating experiment For the heating and crushing experiments, chipped samples (~200–3000 mg; ~1–8

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mm) were prepared by mechanically crushing early diagenetic carbonate phases within the Nakanomata carbonates (i.e., microcrystalline aragonite and acicular aragonite crystal aggregates). The chips were placed in the bottom of crusher devices referred to

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as “S” and “L” (with inner diameters of 8 mm and 15 mm, respectively), which are

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composed of Pyrex glass and are described by Uemura et al. (2016). The crusher

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devices were then heated under vacuum at 90°C for 2 h (S) and 70°C for >4 h (L), using

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a mantle heater regulated by a temperature controller. The concentration of the released

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gas was monitored using a manometer. Large samples (>2000 mg) were placed in glass vials with inner volumes of 100 mL and kept under vacuum at 70°C overnight in an

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oven. The vials were then connected to the extraction system instead of the crusher devices. The released gases were collected at −198°C (using liquid N2) in a U-shaped

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trap containing silica gel (Supplementary Fig. 1). The trap was then heated to ~90°C

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(using hot water) and the liberated gases were introduced into the GC–IRMS in helium gas. We used the same analytical system and GC–IRMS instruments as in the acid digestion experiment, except that we did not use the CO2-trapping port to analyze the isotopic composition of CO2 and that the GC oven was held at 60°C for 20 min. It has been reported that methane does not form by pyrolysis when fresh sediment is heated at 121°C for 60 min using an autoclave in a laboratory (Sugimoto et al., 2003).

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3.3. Crushing experiment

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After heating and evacuation of the line, the chipped samples in the crusher devices were crushed under vacuum at 90°C (S) or 70°C (L). Heating continued during crushing

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to minimize the possible adsorption of gases onto newly created surfaces. Crushing was

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performed by rotating the grip at the end of a threaded valve stem connected to the

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crushers and pressing a glass rod onto the samples (Uemura et al., 2016). A filter was

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installed between the crusher and the line to prevent contamination by crushed particles (Supplementary Fig. 1). The released gases were collected in the trap and analyzed

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using GC–IRMS as described above. Because the crusher devices are too small to crush large volume samples, large samples heated in glass vials (100 mL) were divided into

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several fractions and crushed separately using the crusher device L. The evolved gases

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were collected in the trap and analyzed in total. Following the experiment, the crushed samples were removed from the crusher, powdered, and then dissolved using phosphoric acid in glass vials (inner volume = 5 cm3). The liberated gases were then analyzed by GC–IRMS. Large samples (>2000 mg) were first divided into two glass vials with inner volumes of 100 cm3.

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3.4. Stable isotope analyses of total organic carbon To determine the source of hydrocarbons extracted from the seep carbonates, we

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measured the carbon stable isotopic composition of total organic carbon (TOC) within the carbonate. The carbonates were completely dissolved through reaction with 5N HCl

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for 24 h at room temperature. Acid was then removed by repeated centrifugation and

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washing with deionized water. After freeze-drying, the residues (~1–4 mg, depending on

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TOC content) were analyzed using a Thermo Quest NA2500NCS elemental analyzer

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(EA) connected to a Thermo Scientific Delta V Advantage IRMS at LEEKU. Samples were oxidized at 1000°C to produce CO2 gas. Concentrations of TOC (in wt%) were 44

CO2 peak areas with those of working standard

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estimated by comparing measured

L-alanine (LAL) containing 50 μgC/5 μL. Carbon isotopic compositions are reported in

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δ notation (δ13C‰ relative to VPDB). Standard deviations during replicate analyses of

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the working standard LAL and international standard ANU sucrose were better than 0.06‰ and 0.11‰, respectively.

3.5. Electron microscope observations Carbonate samples were coated with platinum and observed using a HITACHI TM 3000 scanning electron microscope (SEM) at the Department of Geology and

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Mineralogy, Kyoto University (DGMKU), Japan. To perform scanning transmission electron microscopy (STEM), we used a FEI Helios NanoLab G3 CX focused ion beam (FIB) to produce an ultrathin section from a carbon-coated thin section of seep

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carbonate at DGMKU. A small area (~0.3–0.4 × 20 μm) of acicular crystals (~10 μm

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wide) was coated with Pt and cut to a depth of ~5–10 μm using a Ga+ ion gun. The

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obtained ultrathin section was then observed along an orientation perpendicular to the

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growth direction of the aciculae using a JEOL JEM-2100F transmission electron

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microscope (TEM) at DGMKU. To detect compositional contrast within aragonite crystals, we obtained annular dark-field (ADF) STEM images at 200 kV using an

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annular STEM detector within the TEM.

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4. Results

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4.1. Hydrocarbons extracted during acid digestion Methane and other heavier hydrocarbons were successfully extracted from the carbonate samples during acid digestion. Alkane concentrations and stable carbon isotopic compositions of hydrocarbon gases extracted from the carbonates are listed in Table 1. Methane was extracted from all samples, yielding concentrations of 87–402 nmol/g at Nakanomata and 114–417 nmol/g at Anazawa/Akanuda. The extracted gases

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yielded methane to ethane plus propane ratios (C1/(C2 + C3)) of 4–30 at Nakanomata (n = 24) and 2–5 at Anazawa/Akanuda (n = 10) (Fig. 3A). Sedimentary concretions from

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the Nodani Formation yielded methane concentrations of 119–237 nmol/g and C1/(C2 + C3) ratios of 7–11 (n = 5). Nakanomata carbonates and sedimentary concretions yielded

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ethane and propane concentrations of 6–33 and 3–12 nmol/g, respectively. Carbonates

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from Anazawa/Akanuda yielded higher concentrations of ethane and propane (19–163

(<3

nmol/g),

whereas

larger

amounts

were

liberated

from

the

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n-butane

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and 8–85 nmol/g, respectively). Nakanomata samples yielded small concentrations of

Anazawa/Akanuda samples (3–18 nmol/g).

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Methane extracted from the carbonates yielded a large range of δ13C values of −61‰ to −40‰ at Nakanomata and −67‰ to −38‰ at Anazawa/Akanuda (Fig. 3A).

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Methane liberated from void-filling acicular aragonite and sparry calcite typically

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yielded lower δ13C values than the microcrystalline matrix of the carbonates. For samples from Nakanomata, the carbon isotopic compositions of methane and carbonate powders from which the methane was liberated yield a significant positive correlation (r = 0.64, p < 0.05) (Fig. 4A). In contrast, samples from Anazawa/Akanuda yield no correlation between δ13C of methane and carbonate (r = 0.02) (Fig. 4B). Methane liberated from sedimentary concretions yielded δ13C values of −45‰ to −35‰.

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Liberated ethane and propane yielded δ13C values of −78‰ to −30‰ and −84‰ to −29‰, respectively, lower or higher than that of coexisting methane (Table 1; Figs. 3B, C, and 5). δ13C values of n-butane range from −50‰ to −41‰, higher than coexisting

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propane (Table 1 and Fig. 5). Comparison of δ13C values of the coexisting hydrocarbons

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revealed increases in δ13C with increasing carbon number, i.e., δ13Cmethane (δ13C1) <

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δ13Cethane (δ13C2) < δ13Cpropane (δ13C3) for some data; however, we also observed isotopic

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“reversals” such as δ13C1 > δ13C2 and δ13C2 > δ13C3 (Fig. 5).

4.2. Hydrocarbons extracted through heating

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During heating of the chipped carbonate samples, gas concentrations increased continuously (Supplementary Fig. 2), and the rate of this increase reduced over time.

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The produced gases contain methane (<2 nmol/g) and CO2 (42–519 nmol/g) (n = 7;

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Table 2 and Fig. 6). Ethane and propane were detected in trace amounts during heating. The low concentrations of methane restricted the measurement of carbon isotopic compositions.

4.3. Hydrocarbons extracted through crushing Crushing of the carbonate chips liberated methane and CO2, as well as small

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amounts of ethane and propane (<1 nmol/g; Table 3). The C1/(C2 + C3) ratio of hydrocarbons extracted by crushing ranges from 28 to 45 (n = 4). Crushing yielded

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higher concentrations of methane (4–10 nmol/g) than heating (Tables 2 and 3; Fig. 6). In contrast, less CO2 (30–193 nmol/g) was liberated during crushing than during heating.

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δ13C of the liberated methane and CO2 range from −55‰ to −49‰ and −41‰ to −25‰,

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respectively (Table 3; for discussion of CO2 liberated during heating and crushing

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experiments, see Supplementary Material). The small concentrations of ethane and

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propane limited isotopic measurements; however, one sample yielded an ethane δ13C value of −46‰.

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Acid digestion of the crushed carbonate samples released further methane (163–286 nmol/g) and heavier hydrocarbons (Table 4 and Fig. 6). δ13C values of

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methane released during acid digestion range from −64‰ to −55‰, similar to or lower

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than those of methane released during crushing (Tables 3 and 4). Ethane liberated during acid digestion yielded a lower δ13C value than ethane released during crushing.

4.4. Carbon isotopic compositions of TOC TOC contents of the examined carbonates were estimated to be ~0.3 wt% (Nakanomata), ~0.1 wt% (Anazawa/Akanuda), and ~0.4 wt% (sedimentary concretion),

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which yielded δ13C values of −33‰, −39‰, and −25‰, respectively.

STEM

observations

of

acicular

aragonite

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4.5. STEM observations crystals

revealed

abundant

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nanometer-scale pores along grain boundaries. Aragonite needles are ~10 μm wide and

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comprise bundles of <1 μm-wide sub-crystals, with nanopores distributed along

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sub-crystal boundaries (Fig. 7). The nanopores are sub-circular to polygonal in shape

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and range from <50 to ~300 nm in diameter.

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5. Discussion

5.1. Storage of methane in methane-seep carbonates

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Similar to clastic sediments, carbonate rocks may store gases through: 1)

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entrapment in intercrystalline pore spaces; 2) physical adsorption onto the surfaces of carbonates, other minerals, and organic matter; and 3) inclusion within carbonate crystals (Abrams, 2005; Ijiri et al., 2009). Acid digestion of powdered (~1–30 μm) carbonates can liberate gases adsorbed onto surfaces of carbonates or organic matter and entrapped within carbonate crystals (e.g., Brekke et al., 1997; Knies et al., 2004; Whiticar et al., 1994). Heating of chipped (~1–8 mm) carbonates can desorb gases

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physically adsorbed onto the surfaces of carbonates, clay minerals, and organic matter through the action of weak intermolecular forces, such as van der Waals forces

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(Sugimoto et al., 2003; Toki et al., 2007). Crushing has previously been used to extract intracrystalline gases (e.g., Ueno et al., 2006); however, as our samples comprise

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microcrystalline aragonite (up to a few tens of micrometers in diameter; Fig. 2),

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crushing can rather release gases from intercrystalline pores. Furthermore, as the

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samples were heated during crushing, it is also possible that gases were thermally

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desorbed from crystal surfaces that were exposed during the crushing. The heating and crushing experiments liberated much less methane than acid

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digestion (Tables 2–4; Fig. 6). This may indicate that methane was primarily entrapped within micrometer-scale crystals, with only minor amounts in intercrystalline pores or

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adsorbed onto crystal surfaces. Methane extracted during crushing (intercrystalline

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methane) yielded higher C1/(C2 + C3) ratios and δ13C values than that extracted through acid digestion (intracrystalline methane) (Tables 3 and 4). Therefore, the intra- and intercrystalline methane likely have distinct origins. Likewise, ethane released during crushing was enriched in

13

C relative to that

extracted through acid digestion (Tables 3 and 4; sample “nkm13-40m”). Intra- and intercrystalline ethane may therefore also have distinct origins. We infer that the

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depletion in

13

C in intracrystalline gases is due to the presence of hydrocarbons that

were generated from

13

C-depleted organic matter preserved within carbonate crystals

PT

(see section 5.2). Methane is likely contained primarily within nanometer-scale pores, such as those

RI

observed along boundaries between aragonite sub-crystals (Fig. 7). Methane in such

SC

small inclusions is difficult to liberate through heating or crushing, although it may be

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sufficiently liberated through complete dissolution of carbonate crystals. Further

MA

research is required to determine whether methane is contained within such nanopores.

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5.2. Origin of residual methane and heavier hydrocarbons Microbial gas is characterized by δ13C values of methane ranging from −110‰ to

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−50‰ (relative to VPDB) and methane to ethane + propane (C1/(C2 + C3)) ratios greater

AC

than 1000. Thermogenic gas typically yields higher methane δ13C values ranging from −50‰ to −20‰ and C1/(C2 + C3) ratios lower than 50 (Bernard et al., 1976; Whiticar, 1999). According to the classification of natural gas by Bernard et al. (1976) and Whiticar (1999), the δ13C values and C1/(C2 + C3) ratios of methane and other hydrocarbons extracted by acid digestion of the examined samples are mostly consistent with neither thermogenic nor microbial origins (Fig. 3A). Gases with methane δ13C

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values greater than −50‰ and C1/(C2 + C3) ratios lower than 50, such as those extracted from sedimentary concretions from the Nodani Formation, indicate a thermogenic origin. However, hydrocarbons liberated from the seep carbonates typically yielded δ13C

PT

values lower than −50‰ and C1/(C2 + C3) ratios lower than 1000, which cannot be

RI

classified using the conventional scheme. Such isotopic and molecular compositions

SC

may be partially attributed to a mixture of thermogenic and microbial gases. Preferential

NU

adsorption or encapsulation of high-molecular-weight hydrocarbons may also have

MA

contributed to the observed low C1/(C2 + C3) ratios (Blumenberg et al., 2018; Cheng and Huang, 2004; Ijiri et al., 2009).

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Hydrocarbons that yield a thermogenic signature (δ13Cmethane > −50‰, C1/(C2 + C3) < 50) may have been adsorbed onto carbonates during burial. Secondary adsorption of

CE

thermogenic gas is consistent with the observations that the host sediments presently act

AC

as natural gas source and reservoir rocks (Monzawa et al., 2006; Okui et al., 2008), and that modern seep carbonates yield much lower concentrations of methane than the analyzed samples (Blumenberg et al., 2018). However, such a hypothesis is not plausible in this case as methane and heavier hydrocarbons were only liberated in trace amounts during heating of the Nakanomata carbonates. As discussed in section 5.1, this result suggests that the concentration of hydrocarbons adsorbed onto crystal surfaces is

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insignificant. δ13C values of ethane and propane coexisting with the inferred thermogenic methane are generally lower than those of natural gas from the study area

PT

(Igari, 1999; Sakata, 1991; Waseda et al., 2002). Therefore, hydrocarbons extracted from our samples likely have distinct origins from the modern-day thermogenic gas

RI

generated and/or preserved in the host sediments.

SC

Liberated gases with δ13C values lower than −50‰ likely had a microbial origin.

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However, it is also possible that thermal cracking of 13C-depleted organic matter within

MA

the seep carbonates could have generated 13C-depleted thermogenic gas, as discussed in detail below. Nevertheless, it is noteworthy that there is a significant positive correlation

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between δ13C values of the liberated methane and the relatively immature carbonates at Nakanomata (Fig. 4A). Acicular aragonite yielded relatively low δ13Cmethane and

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δ13Ccarbonate values relative to microcrystalline aragonite. This correlation implies that

AC

while methane within Miocene seep fluids was oxidized to bicarbonate from which carbonates precipitated, it was preserved within the host carbonate cements. In contrast, no correlation is observed between δ13C values of methane and carbonates at Anazawa/Akanuda (Fig. 4B). This may be partly attributed to the dissolution and re-precipitation of carbonates during recrystallization, which resulted in the resetting of their isotopic compositions. It is thus suggested that methane entrapped in seep

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carbonates might preserve the original signature of seep methane at a certain, early stage of maturity and diagenesis. Hydrocarbons heavier than methane extracted during acid digestion yielded low

PT

δ13C values (>−84‰), as well as values typical of thermogenic gas (δ13C > ~−40‰),

RI

indicating derivation from multiple sources (Fig. 3B and C). Microbial ethane and

SC

propane with δ13C values as low as −70‰ have been reported in natural environments,

NU

but they are generally <1% of methane (Bernard et al., 2013; Oremland et al., 1988;

MA

Vogel et al., 1982; Waseda and Didyk, 1995; Whiticar, 1999). A microbial origin for ethane and propane is thus inconsistent with the observed C1/(C2 + C3) ratios (<100)

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(Fig. 3A).

The large amount of ethane and propane liberated from the seep carbonates (Fig.

CE

3A) indicates either a decrease in the relative content of methane due to oxidation

AC

(Whiticar and Faber, 1986), a thermogenic origin for the high-molecular-weight hydrocarbons, or a combination of these processes. It is also possible that ethane and propane were preferentially encapsulated into the carbonates through unknown mechanisms (Blumenberg et al., 2018; Cheng and Huang, 2004; Ijiri et al., 2009). However, the methane oxidation and preferential encapsulation of higher hydrocarbons cannot explain the highly negative δ13C values of the ethane and propane. Calculations

25

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using the formulae of Berner and Faber (1996) indicate that the δ13C values of ethane and propane extracted from the carbonates can be explained if they have undergone thermogenic generation from a source with δ13C values lower than −30‰ (as low as

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~−80‰; Fig. 3B and C). One candidate for this gas source is the TOC of the carbonates.

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Concentrations of hydrocarbons liberated from the carbonates (Table 1) are equivalent

SC

at most to 1/1000 of TOC (~83–333 μmol/g). The hydrocarbon concentrations are not

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unreasonable even if we assume that they were generated from TOC, according to the

MA

published results of laboratory pyrolysis experiments (Andresen et al., 1995). δ13CTOC values of modern and ancient methane-seep carbonates are generally low due to the

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input of methane-derived 13C-depleted carbon into the biomass produced at seeps (e.g., Smrzka et al., 2016). δ13CTOC values from the Nakanomata and Anazawa/Akanuda

CE

carbonates (<−30‰) are lower than that from the sedimentary concretion (~−25‰),

AC

although they are not low enough to explain the δ13C values as low as ~−80‰ of ethane and propane. Another candidate for the source of the gas is

13

C-depleted compounds

such as lipid biomarkers produced by methane-oxidizing archaea contained within the carbonates, which can yield δ13C values as low as −140‰ (e.g., Chevalier et al., 2014; Hinrichs

et

al.,

1999).

A lipid

biomarker

for

methane-oxidizing

archaea

pentamethylicosane (PMI) extracted from the Nakanomata and Anazawa/Akanuda

26

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carbonates yielded compound-specific δ13C values around −100‰ (details will be reported elsewhere). It should also be noted that in modern methane-seep sediments, low-molecular-weight fatty acids such as acetate, propionate, and butyrate are known to

13

C-depleted fatty acids are likely to represent degradation products of

RI

These

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yield low δ13C values as low as −85‰ (e.g., Heuer et al., 2006; Yoshinaga et al., 2015).

SC

AOM-related biomass (Yoshinaga et al., 2015). Such low-molecular-weight fatty acids

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are also potential precursors of gaseous hydrocarbons in the presence of water and

MA

mineral buffers at elevated temperatures (Seewald, 2001). Although it is unrevealed whether 13C-depleted acetate and other low-molecular-weight compounds are present in

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the examined seep carbonates, it is highly likely that thermal cracking of archaeal lipids and low-molecular-weight fatty acids during burial and thermal maturation may have

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generated the observed 13C-depleted ethane and propane (Ijiri, 2003).

AC

Hydrocarbon generation by thermal cracking within carbonates is possible in thermally mature samples such as the Anazawa/Akanuda carbonates (Ro > 0.8%). The occurrence of such processes within low maturity samples, such as those from the Nodani Formation (Nakanomata carbonates and sedimentary concretions, Ro = ~0.6%), is controversial. These carbonates are unlikely to have been subjected to temperatures of >100°C, as indicated by the preservation of diagenetically unstable aragonite and the

27

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positive δ18O values of the carbonates, both of which suggest a lack of thermal alteration (Miyajima et al., 2016). We infer that thermal cracking at relatively low

PT

temperatures produced small amounts of 13C-depleted ethane and propane, whereas the bulk organic matter remained relatively immature. Although main gas generation

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generally occurs at Ro > 1.0%, thermogenic gas generation at temperatures lower than

SC

62°C (before oil window) was also demonstrated (Rowe and Muehlenbachs, 1999).

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The distribution of carbon isotopes among hydrocarbons is dependent on mixing of

MA

hydrocarbons from different sources as well as their individual origins (e.g., Chung et al., 1988; Jenden et al., 1993; Tilley et al., 2011). δ13C values of thermogenic

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hydrocarbons linearly increase with increasing carbon number, that is, δ13Cmethane (δ13C1) < δ13Cethane (δ13C2) < δ13Cpropane (δ13C3). This isotopic trend results from the kinetic 12

C–12C bond is easier to break than a

12

C–13C bond in

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isotope effect, in which a

AC

kerogen molecules (Chung et al., 1988). A linear fit can be obtained when δ13C values of cogenetic hydrocarbons are plotted as a function of their inverse carbon number (referred to as the “Natural gas plot”). Some of hydrocarbons extracted from the examined carbonates display linear increases in δ13C values with increasing carbon number, although the majority of them does not show such trends particularly at Nakanomata (Fig. 5). The δ13C value of the source for the thermogenic hydrocarbons

28

ACCEPTED MANUSCRIPT

can be estimated through extrapolation of the linear isotopic trend to the y-intercept in the Natural gas plot (Chung et al., 1988; Igari, 1999; Pohlman et al., 2005). When applied to Fig. 5, δ13C values for the source (y-intercepts) of the hydrocarbons

PT

exhibiting the linear trends are close to the δ13CTOC value of the carbonates. This result

RI

supports the hypothesis that hydrocarbons within the examined carbonates at least partly

SC

originate from the thermal cracking of organic matter preserved within the carbonates.

NU

Hydrocarbons from the sedimentary concretions display isotopic and molecular

MA

signatures typical of thermogenic gases (Figs 3A and 5C). As the secondary adsorption of thermogenic gases during burial seems insignificant, thermogenic gases within the

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sedimentary concretions may also have been generated through thermal cracking of TOC (or more likely low-molecular-weight compounds such as acetate and propionate)

CE

within the concretions (Fig. 5C).

AC

Hydrocarbons released from the Nakanomata and Anazawa/Akanuda seep carbonates mostly display isotopic “reversals”, which are unusual for thermogenic and microbial hydrocarbons. The “reverse” trends were found between methane and ethane (δ13C1 > δ13C2) and ethane and propane (δ13C2 > δ13C3) (Fig. 5). Such “reverse” isotopic trends are attributed either to the abiotic generation of hydrocarbons through polymerization reactions or to the mixing of hydrocarbon gases of different origins.

29

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Abiogenic hydrocarbons commonly occur in hydrothermal systems and other settings characterized by extreme temperatures and pressures (Des Marais et al., 1981;

PT

Sherwood Lollar et al., 2002; Suda et al., 2017; Yuen et al., 1984), and therefore unlikely for the origin of hydrocarbons within methane-seep carbonates. Isotopic

RI

reversals such as δ13C1 > δ13C2 and δ13C2 > δ13C3 were shown to result from the mixing

SC

of thermogenic hydrocarbons with different relative amounts of C2+, generated at

NU

various temperatures both in pyrolysis experiments and natural gases (Des Marais et al.,

MA

1988; Jenden et al., 1993). Partial or full isotopic reversals have also been reported from high maturity shale gases (Ro > ~1.5%), which are the mixture of primary gas produced

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through thermal cracking of kerogen with secondary gas formed by cracking of oil or wet gas components (Burruss and Laughrey, 2010; Tilley et al., 2011; Xia et al., 2013;

CE

Zumberge et al., 2012).

AC

The previous studies mentioned above indicate that mixing between two thermogenic gas components with distinct C1/(C2 + C3) ratios (relative amounts of ethane and propane) and carbon isotopic compositions can produce reverse isotopic trends. Figure 8 indicates that the molecular and isotopic compositions of the residual methane, ethane, and propane extracted from the carbonates may be explained by mixing between: 1) a relatively 13C- and light hydrocarbon-enriched gas; and 2) a highly

30

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13

C-depleted gas enriched in heavy hydrocarbons. This model assumes that there has

been no molecular and isotopic fractionation of the hydrocarbons during encapsulation

PT

into the carbonates. It is unlikely that the 13C depletion of hydrocarbons was caused by fractionation processes that result in the enrichment of higher hydrocarbons such as

RI

preferential encapsulation into carbonates or microbial oxidation. Hydrocarbons

SC

liberated from sedimentary concretions of the Nodani Formation may represent the C-enriched endmember, with a C1/(C2 + C3) ratio of 8. The 13C-depleted endmember

NU

13

MA

can be determined as follows. Assuming a δ13C value of −80‰ for the source (e.g., biomarkers of microbes utilizing methane-derived carbon and low-molecular-weight

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fatty acids; Fig. 3B and C), the carbon isotopic compositions of hydrocarbons generated from the source can be calculated using the equations of Berner and Faber (1996)

CE

(Supplementary Table 1). The results of closed- and open-system pyrolysis experiments

AC

on algal kerogens under various temperatures can be used to model hydrocarbon concentrations (Andresen et al., 1995; Berner et al., 1995). For our data, the C1/(C2 + C3) ratio of the 13C-depleted endmember generated in a closed or open system is 3 or 1, respectively (Supplementary Table 1). Mixing between the two thermogenic-gas endmembers described above predicts isotopic reversals among methane, ethane, and propane, which are dependent on the mixing ratio (Fig. 9; for details of the mixing

31

ACCEPTED MANUSCRIPT

models, see Supplementary Material). Only a small addition of the

13

C-depleted

endmember could produce the observed reverse isotopic trend. An extreme reverse trend

PT

(δ13C1 > δ13C2 > δ13C3) can be generated if the 13C-depleted endmember is produced in a closed system (Fig. 9A). Tilley et al. (2011) suggested that isotopic reversals occur only

13

C-enriched methane, with little or no gas lost during

SC

of oil mixes with mature

RI

in closed systems, such as shales where 12C-enriched ethane generated through cracking

NU

maturation. Pyrolysis experiments on source rocks and coal indicate that closed systems

MA

generate a greater proportion of propane than open systems at the same level of maturity (Andresen et al., 1995; Takahashi and Suzuki, 2017; Takahashi et al., 2014). The

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isotopic reversals observed in the Nakanomata seep carbonates (Fig. 5A) may therefore have resulted from the accumulation of secondary gases enriched in ethane and propane

CE

within closed to semi-closed carbonate cements. The difference in C1/(C2 + C3) between

AC

the two endmembers may have resulted from a difference in the timing of gas generation and/or in the composition of the source organic matter (e.g., kerogen vs. bitumen).

The mixing models in Fig. 9 also indicate that the isotopic reversal becomes insignificant when the proportion of the

13

C-depleted endmember is high. Thermal

cracking within seep carbonates proceeds with increasing thermal stress, resulting in an

32

ACCEPTED MANUSCRIPT

increased proportion of

13

C-depleted hydrocarbons. Such processes may have occurred

in the Anazawa/Akanuda seep carbonates, which evolved to a higher thermal maturity.

PT

The partial reverse and “normal” isotopic trends observed in the Anazawa/Akanuda carbonates (Fig. 5B) can thus be attributed to a high proportion of the

13

C-depleted

RI

endmember. Alternatively, the isotopic trends at this site could also have resulted from

SC

the mixing of the two endmembers in an open system during dissolution and

13

C-depleted endmember in the Nakanomata and

MA

the composition of the

NU

recrystallization of carbonate cements (Fig. 9B). Figure 8 also suggests a difference in

Anazawa/Akanuda gases.

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Based on the above discussion, we infer that the residual hydrocarbons (especially ethane and propane) extracted from the methane-seep carbonates contain secondary

CE

gases produced during thermal cracking of organic compounds within the carbonates.

AC

The secondary production of residual gases during burial of ancient seep carbonates is supported by much lower concentrations of methane and higher hydrocarbons liberated from modern seep carbonates (Blumenberg et al., 2018). It is also remarkable that concentrations of residual hydrocarbons increase with increasing age or maturity of carbonates (Table 1; Y. Miyajima, unpublished data). Morales et al. (2017) reported that propane extracted from Mesozoic glendonites lacked a biodegradation signature, and

33

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therefore does not indicate slow incorporation of hydrocarbons during recrystallization of the carbonates. However, the results of Blumenberg et al. (2018) and this study pose

PT

an alternative possibility to their findings that thermogenic hydrocarbons were generated in situ within the Mesozoic carbonates during thermal maturation.

RI

Nevertheless, several lines of evidence suggest that methane extracted from the

SC

Nakanomata seep carbonates could at least partially have been derived from the original

NU

Miocene seep fluid. Firstly, δ13C values of the methane liberated at Nakanomata display

MA

a smaller variation compared with those of ethane and propane (Fig. 5A), which cannot be explained by two-component mixing alone (Fig. 9). Secondly, hydrocarbons within

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the Nakanomata samples yield C1/(C2 + C3) ratios as high as 30 (mean = 11), which cannot be explained by the mixing of thermogenic hydrocarbons (Fig. 3A). Although

CE

the concentrations and isotopic signatures of ethane and propane at Nakanomata are

AC

consistent with the two-component mixing trend (Fig. 8B and C), methane contents deviate significantly from the mixing line (Fig. 8A). These observations may be explained by the presence of a microbial methane component within the Nakanomata carbonates, having δ13C values of ~−60‰ to −50‰ and containing negligible amounts of ethane and propane. The positive correlation between the extracted methane and the host carbonate cements (Fig. 4A) is indicative of the presence of microbial methane

34

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within the seep fluid. If the methane extracted from Nakanomata seep carbonates represents that of the

PT

original Miocene seep fluid, it can provide insights into subsurface biogeochemical processes at the Miocene seep site. The microbial origin of the extracted methane

RI

indicates that it was produced in the shallow subsurface by methanogenic archaea.

SC

Although the organic-rich sediment that hosts the seep carbonates (Nodani Formation)

NU

is today a source and reservoir for thermogenic hydrocarbons, it likely acted as a locus

MA

of microbial methane production in the Miocene.

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6. Conclusions

This study revealed that three distinct methods (acid digestion, heating, and

CE

crushing) can be used to extract and analyze residual gases from ancient methane-seep

AC

carbonates. The residual hydrocarbons extracted from seep carbonates can largely be attributed to thermogenic gases generated in situ within the carbonates from inter- and intracrystalline organic compounds during burial. Nevertheless, hydrocarbons extracted from the relatively immature Miocene Nakanomata seep carbonates possibly contain a significant amount of “paleo-methane” originating from the ancient seep fluid. The use of various gas extraction methods helps to reveal whether the residual

35

ACCEPTED MANUSCRIPT

gases are entrapped within intercrystalline pore spaces or crystal inclusions within the carbonates. Another possibility is that the residual gases were adsorbed onto the

PT

surfaces of carbonate crystals, other minerals, or organic particles. The residual methane at Nakanomata was likely entrapped largely within intracrystalline inclusions. Such

RI

inclusions occur on a nanometer scale and are distributed along boundaries of <1 μm

SC

sub-crystals, as observed by STEM for the first time in this study. However, future

NU

research is required to confirm the existence of methane within the nanopores. The

MA

entrapment of methane during carbonate precipitation induced by AOM may explain the positive correlation between the δ13C values of the methane and its host carbonate.

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Based on the results obtained from Nakanomata, we conclude that acid digestion is by far the most effective method to extract the original signature of seep methane from, if it

CE

is preserved in, immature ancient methane-seep carbonates.

AC

Residual gases within ancient seep carbonates may be influenced by the thermal maturation of the host sediments, as well as by experimental procedures. Even if the methane was partially derived from the seep fluids, its isotopic composition may be altered through methane oxidation. This may partly contribute to the spread of the δ13C values of methane extracted from the seep carbonates. Therefore, in assessing the origin of methane at ancient seeps, analytical results for residual gases should be carefully

36

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interpreted and complemented by other data such as biomarkers. Comparing the carbon isotopic signatures of residual methane and host carbonate cements, as well as analyzing

PT

intracrystalline inclusions and nanopores such as those observed in this study would aid in the detection of original methane isotopic signatures. Carbonate petrography also

RI

gives important information, because recrystallization of carbonates can prevent the

SC

preservation of or modify the isotopic and molecular signatures of residual gases. This

NU

study also showed that thermal cracking of organic compounds such as

MA

low-molecular-weight fatty acids in seep carbonates potentially results in the production of ethane and propane with very negative δ13C values, even in relatively immature

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samples. The gas wetness and isotopic signatures of higher hydrocarbons in ancient

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seep carbonates thus cannot be used to know the origin of seep gas in the past.

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Acknowledgments

We are grateful to Takao Ubukata (Kyoto University, Japan) for his critical comments on an early draft of this article. We are also grateful to Robert G. Jenkins and Akiko S. Goto (Kanazawa University, Japan) for their insight and constructive discussions. Akemi Imajo (Japan Agency for Marine-Earth Science and Technology, JAMSTEC) courteously helped in residual gas analyses. Yukako Nabeshima, Yuki

37

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Fujimura, and Masafumi Murayama (Kochi University, Japan) are thanked for their permission and generous help in the use of their instrument for carbon and oxygen

PT

isotopic analyses. Akira Tsuchiyama (Kyoto University) is acknowledged for providing permission to use FIB and TEM apparatus. Daichi Maeyama (Hokkaido University,

RI

Japan, now at Japan Petroleum Exploration Co., Ltd., JAPEX) and Noriyuki Suzuki

SC

(Hokkaido University) provided valuable information and discussions on residual gases

NU

within seep carbonates. We acknowledge the staff at Stallard Scientific Editing for

MA

improving the English in the manuscript. Editorial work by M.E. Böttcher and thorough and insightful comments by M. Blumenberg and an anonymous reviewer greatly

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improved the manuscript. This work was supported by the Japan Society for the Promotion of Science (JSPS) [Grant-in-Aid (KAKENHI), grant numbers 15J01158,

AC

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26287128, 17H01871, 15H05695, 26247086, 26610168, and 16K13897].

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Figure captions

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Fig. 1. Location of the study sites in central Japan. The Nakanomata seep carbonates were collected as float blocks from the upper Miocene Nodani Formation. The

RI

Anazawa/Akanuda samples were collected from large carbonate bodies hosted in the

SC

middle Miocene Bessho Formation. For detailed geological information and

NU

paleontological, petrographic, and geochemical descriptions of the carbonates, see

MA

Miyajima et al. (2016, 2017).

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Fig. 2. SEM images of Miocene methane-seep carbonates from Nakanomata. A: Matrix comprising dominantly microcrystalline aragonite. B: Void-filling cement comprising

AC

CE

bundles of acicular aragonite crystals. Scale bars = 10 μm.

Fig. 3. Isotopic and compositional data for hydrocarbons extracted from Miocene seep carbonates and sedimentary concretions through acid digestion. (A) “Bernard diagram” (Bernard et al., 1976) showing the carbon stable isotopic compositions of methane and ratios of methane to ethane + propane. Shaded areas indicate typical ranges for microbial and thermogenic gases (Whiticar, 1999). Stars indicate hypothetical

52

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endmember values of

13

C-depleted thermogenic gases generated through closed- and

open-system pyrolysis, which are used in Figs 8 and 9 and Supplementary Table 1 (see 13

C-depleted gases and the average

PT

text). Dashed lines indicate mixing between

composition of gas extracted from the sedimentary concretions. (B, C) Cross plots of

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δ13C for methane and ethane (B) and ethane and propane (C). Solid, dashed, and dotted

SC

lines indicate the δ13C values expected for thermal cracking of source organic matter

NU

with δ13C values of between −80‰ and −30‰, as a function of vitrinite reflectance

MA

(%Ro; 0.5% to 2.5%; calculated after Berner and Faber, 1996).

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Fig. 4. Relationship between the carbon isotopic compositions of methane and carbonate from which the methane was extracted through acid digestion. r values are the

CE

correlation coefficients. (A) Nakanomata. (B) Anazawa/Akanuda. The key for symbols

AC

with respect to the study sites is the same as that in Fig. 3.

Fig. 5. “Natural gas plots” showing carbon stable isotopic compositions versus inverse carbon numbers (1/n) for methane (C1) to n-butane (C4) extracted from carbonates through acid digestion (Chung et al., 1988). Dashed lines are linear regressions through the data showing isotopic trends typical of thermogenic gas (δ13C1 < δ13C2 < δ13C3), the

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extrapolated intercepts of which indicate the δ13C values of the source organic matter. For comparison, δ13C values of total organic carbon (TOC) within the carbonates are

PT

indicated by filled squares. (A) Nakanomata. (B) Anazawa/Akanuda. (C) Sedimentary concretions. The key for symbols with respect to the study sites is the same as that in

SC

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Fig. 3.

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Fig. 6. Concentrations of methane extracted through heating, crushing, and acid

MA

digestion of the Nakanomata carbonate samples. Note the logarithmic scale.

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Fig. 7. ADF–STEM image of an acicular aragonite bundle within a Nakanomata seep carbonate. Abundant nanometer-scale pores (visible as dark spots) occur along

CE

sub-crystal boundaries. View direction is oriented normal to the growth direction of the

AC

aciculae. Scale bar = 500 nm.

Fig. 8. Comparison of the observed molecular and isotopic compositions of gases extracted through acid digestion of carbonates with two-component mixing models. (A) Carbon isotopic composition of methane versus methane to ethane ratio. (B) Carbon isotopic composition of ethane versus ethane to propane ratio. (C) Carbon isotopic

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composition of propane versus ethane to propane ratio. Stars indicate hypothetical endmember values of

13

C-depleted thermogenic gases generated by closed- and

13

C-depleted gases and the average composition

SC

of gas extracted from the sedimentary concretions.

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Dashed lines indicate mixing between

PT

open-system pyrolysis, which are used in Fig. 9 and Supplementary Table 1 (see text).

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Fig. 9. Carbon stable isotopic compositions versus inverse carbon number (1/n) for

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methane (C1) to propane (C3), calculated using mixing models for hydrocarbons of differing origins. (A, B) Two-component mixing between 13C-enriched and 13C-depleted

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thermogenic gases generated from a source with a δ13C value of −80‰ at Ro of 0.5% in a closed (A) and open (B) system. Mean molecular and isotopic compositions of

CE

hydrocarbons liberated from sedimentary concretions were used to represent the 13

AC

C-enriched endmember. The molecular compositions of

13

C-depleted endmembers in

(A) and (B) are based on Andresen et al. (1995) and Berner et al. (1995), respectively. The isotopic compositions of

13

C-depleted endmembers were calculated after Berner

and Faber (1996). The proportion of the

13

C-depleted component is indicated in the

color scale. For details of the mixing models, see the main text and Supplementary Material.

55

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Site

Sample

Phase

C1 content

C2 content

C3 content

C4 content

C1/(C2 + C3 )

δ13C1

δ13C2

δ13C3

δ13C4

δ13CCaCO3

Anazawa/Akanuda Anazawa/Akanuda

an0006 an0007

m sp

262 188

105 46

45 23

12 7

2 3

−67.2 −58.4

−61.9 −74.0

−53.4 −67.6

−49.5 n.a.

−39.9 −45.0

Anazawa/Akanuda Anazawa/Akanuda Anazawa/Akanuda Anazawa/Akanuda Anazawa/Akanuda Anazawa/Akanuda

an0008 an0009 an0010 ak0103 ak02 ak05

m sp + m sp m m m

227 183 114 170 417 217

88 26 19 65 163 94

35 12 8 33 85 46

10 n.d. 3 9 18 11

2 5 4 2 2 2

−62.8 −38.1 −46.7 −53.9 −57.5 −52.5

I R

−53.2 −49.1 −54.3 −51.4 −56.9 −55.3

−44.6 −40.7 −44.3 −46.6 −49.7 −47.3

−40.5 n.d. n.a. n.a. −48.8 n.a.

−36.5 −41.7 −40.9 −36.5 −29.3 −29.1

Anazawa/Akanuda Anazawa/Akanuda Nakanomata Nakanomata Nakanomata Nakanomata

an0011 an0012 nk1301 nk1302 nk1309 nk1312

m sp ac + m m ac + m m

198 370 99 109 103 286

71 91 12 8 16 33

29 40 7 4 11 12

8 9 1 tr n.d. n.d.

2 3 5 9 4 6

−63.1 −66.1 −60.7 −53.1 −56.2 −42.4

−57.4 −71.9 −70.2 −47.7 −60.2 −31.8

−49.0 −65.0 −82.4 n.a. n.a. −30.7

−43.6 n.a. n.a. n.a. n.d. n.d.

−40.2 −41.8 −39.0 −31.3 −36.0 −21.3

Nakanomata Nakanomata Nakanomata Nakanomata Nakanomata Nakanomata

nk1315 nk1321 nk1329 nk1343 nk1344 nk1345

m ac + m ac m m ac

A

120 155 144 402 102 134

11 9 15 16 8 18

5 4 8 9 4 9

n.d. 1 tr 3 tr tr

8 13 6 16 9 5

−49.1 −50.4 −59.5 −57.3 −52.8 −53.7

−46.0 −44.6 −73.5 −74.7 −48.0 −55.1

n.a. n.a. −83.6 −83.7 n.a. n.a.

n.d. n.a. n.a. n.a. n.a. n.a.

−34.9 −26.4 −37.9 −25.7 −33.4 −42.6

Nakanomata Nakanomata

nk1346 nk1347

m ac

271 148

19 14

8 6

tr 1

10 8

−40.0 −53.1

−30.2 −56.1

−32.0 −74.5

n.a. n.a.

−15.4 −40.7

T P E

D E

C C

N A

C S U

M

56

T P

ACCEPTED MANUSCRIPT Nakanomata

nk1348

m

216

12

5

tr

12

−48.9

−42.3

−47.4

n.a.

−32.9

Nakanomata Nakanomata Nakanomata Nakanomata Nakanomata Nakanomata

nk1349 nk1350 nk1351 nk1352 nk1353 nk1354

ac m m m ac + m m

87 141 155 215 181 226

8 10 10 12 6 14

4 5 5 5 3 6

n.d. n.d. n.d. n.d. n.d. n.d.

8 9 10 13 20 11

−49.2 −51.7 −50.2 −48.5 −54.4 −51.2

−38.4 −49.0 −44.5 −42.6 −57.8 −58.5

n.a. n.a. n.a. −46.8 n.a. −72.2

n.d. n.d. n.d. n.d. n.d. n.d.

−25.8 −31.5 −32.8 −27.0 −26.0 −30.6

Nakanomata Nakanomata Nakanomata Nakanomata Nakanomata Sedimentary concretion

nk1355 nk1356 nk1357 nk1358 nk1359 nk1403

m ac m ac ac m

104 156 118 175 166 119

7 15 7 8 5 12

4 7 3 5 n.d. 4

n.d. 1 n.d. n.d. n.d. n.d.

9 7 11 14 30 7

−54.7 −57.6 −50.0 −57.7 −50.7 −35.1

I R

−60.0 −62.8 −44.5 −78.0 −44.1 −30.8

n.a. −77.7 n.a. n.a. n.d. n.a.

n.d. n.a. n.d. n.d. n.d. n.d.

−33.1 −34.8 −29.3 −27.8 −29.3 n.a.

Sedimentary concretion Sedimentary concretion Sedimentary concretion Sedimentary concretion

nk1407 nk1408 nk1401 nk14p

m m m m

135 135 152 237

13 13 16 16

5 5 6 6

8 8 7 11

−37.9 −40.4 −39.2 −44.5

−31.1 −30.4 −32.1 −30.5

n.a. −29.0 −31.7 −28.9

n.d. n.d. n.d. n.d.

n.a. n.a. n.a. n.a.

T P E

D E

N A

C S U

M

n.d. n.d. n.d. n.d.

T P

Concentrations are in nmol/g. δ13C values are in ‰ relative to VPDB. C1, methane; C2, ethane; C3, propane; C4, n-butane; ac, acicular aragonite or calcite; m, microcrystalline aragonite or calcite; sp, sparry calcite; n.a., not analyzed due to low concentration; n.c., not calculated; n.d., not detected; tr, trace (<1).

C C

A

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Table 2 Crushing device

Phase

C1 content

CO2 content

C2 content

C3 content

C1/(C2 + C3)

nk1342 nkm13-40ac nk13 nkm13-40ac nkm13-40m nkm13-15ac

S S S L L L

ac ac m ac m ac

2 2 2 tr tr tr

466 265 490 42 300 81

n.a. n.a. n.a. tr n.a. n.a.

n.a. n.a. n.a. tr n.a. n.a.

n.a. n.a. n.a. 21 n.a. n.a.

nkm13-15m

L

m

tr

519

n.a.

Sample

M

n.a.

δ13CCO2

δ13C2

δ13C3

n.a. n.a. n.a. n.a. n.a. n.a.

−11.3 −12.2 −13.1 −13.1 −8.6 −8.8

n.a. n.a. n.a. n.a. n.a. n.a.

n.a. n.a. n.a. n.a. n.a. n.a.

n.a.

−6.8

n.a.

n.a.

T P

I R

C S U

N A n.a.

δ13C1

Concentrations are in nmol/g. δ13C values are in ‰ relative to VPDB. C1, methane; C2, ethane; C3, propane; ac, acicular aragonite; m, microcrystalline aragonite; n.a., not analyzed due to low concentration; tr, trace (<1).

D E

T P E

C C

A

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Table 3 Crushing device

Phase

C1 content

CO2 content

C2 content

C3 content

C1/(C2 + C3)

nk1342 nkm13-40ac nk13 nkm13-40ac nkm13-40m nkm13-15ac

S S S L L L

ac ac m ac m ac

10 4 4 6 7 5

193 41 66 30 109 40

n.a. n.a. n.a. tr tr tr

n.a. n.a. n.a. tr tr tr

n.a. n.a. n.a. 29 45 29

nkm13-15m

L

m

6

145

tr

Sample

M

tr

28

δ13CCO2

δ13C2

δ13C3

−53.9 −49.9 −55.1 −48.5 −52.4 −52.3

−25.0 −26.8 −25.2 −28.3 −40.5 −35.6

n.a. n.a. n.a. n.a. −46.3 n.a.

n.a. n.a. n.a. n.a. n.a. n.a.

−49.5

−34.7

n.a.

n.a.

T P

I R

C S U

N A

δ13C1

Concentrations are in nmol/g. δ13C values are in ‰ relative to VPDB. Abbreviations are the same as Table 2.

D E

T P E

C C

A

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Table 4 Crushing device

Phase

C1 content

C2 content

C3 content

C1/(C2 + C3)

nk1342 nkm13-40ac nk13 nkm13-40ac nkm13-40m nkm13-15ac

S S S L L L

ac ac m ac m ac

247 163 235 265 * 286 * 248

8 23 11 33 * 14 * 23

4 12 5 16 * 7 * 12

21 5 14 5 * 14 * 7

nkm13-15m

L

m

*

*

Sample

217

15

N A

*

M

7

*

10

δ13C2

δ13C3

−56.4 −63.5 −55.8 −59.5 * −55.7 * −58.7

−61.3 −81.4 −59.9 −72.5 * −56.8 * −70.0

n.a. −96.1 n.a. −84.4 * −69.9 * −85.7

T P

I R

C S U

δ13C1

−54.8

*

−56.2

*

−69.7

*

Concentrations are in nmol/g. δ13C values are in ‰ relative to VPDB. Abbreviations are the same as Table 2. *Crushed samples were divided into two vials for acid digestion, and the average values of the two measurements are shown.

D E

T P E

C C

A

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Table captions

PT

Table 1. Concentrations and carbon stable isotopic compositions of hydrocarbons extracted from Miocene seep carbonates and sedimentary concretions during acid

SC

RI

digestion experiments.

NU

Table 2. Concentrations and carbon stable isotopic compositions of hydrocarbons and

MA

carbon dioxide extracted through heating of Nakanomata carbonate samples.

PT E

D

Table 3. Concentrations and carbon stable isotopic compositions of hydrocarbons and

CE

carbon dioxide extracted through crushing of Nakanomata carbonate samples.

AC

Table 4. Concentrations and carbon stable isotopic compositions of hydrocarbons extracted through acid digestion of crushed Nakanomata samples.

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ACCEPTED MANUSCRIPT

Highlights

RI

PT

Hydrocarbon gases were extracted from 1 Miocene methane-seep carbonates in Japan. Most hydrocarbons are thermogenic gases produced from organic matter in carbonates. The original seep methane could partly be preserved in immature carbonate

AC

CE

PT E

D

MA

NU

SC

crystals.

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Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9