Pliocene Coal Geochemistry: Palaeovegetation and Palaeoclimate Distribution in Berau Basin, Indonesia

We investigated the distributions and concentrations of plant-derived aliphatic-aromatic biomarkers in the Pliocene coal seam from the Pliocene Sajau Formation (CSF) in Berau Basin, Indonesia. In all coal samples, the presence of these components is abundant and varied, reflecting the diversity of paleoflora species and ancient climate variations during coal sedimentation. The distribution of aromatic hydrocarbons was detected, including 1,2,7-Trimethylnaphthalene (TMN), 1,6,7-TMN, and 1,2,5-TMN, cadalene, retene, 1-methylphenanthrene (M.P.), 3-MP, Phenantherene, 1,2,-dimethylphenanthrene (DMP), 2,6-DMP, 2,7-DMP, 6-isopropyl-1-isohexyl-2-methylnaphthalene, and other polycyclic aromatic hydrocarbons (PAHs)—saturated biomarkers including oleanane, des- A-oleanane, bicadinanes, and C29 ααα 20R sterane. The distribution of higher-plant biomarkers indicated that gymnosperms relatively dominated the Lower part of CSF. In contrast, the upper part of CSF showed an angiosperm-dominated higher plant terrigenous organic matter. The most charred coal in the Lower part of CSF is predominantly composed of combustion-derived PAHs, such as Benzo[a] anthracene, fluoranthene, and benzofluoranthenes. We show the cold-warm-hot climate conditions in the Pliocene in the Berau Basin by clear evidence of the distribution of biomarkers and spore-pollen.

Introduction

Biomarkers (biological markers), commonly called compound biological markers, are molecular fossils in sediments, rocks, and petroleum that only come from biosynthesis, have a specific origin, and are present in sediments [1]; only slight changes in the structure of compounds from living organisms, changes due to pressure and temperature during geological processes occur.

The Earth’s climate is experiencing gradual cooling due to the steady expansion of the Antarctic ice sheet and the large accumulation of ice in the Northern Hemisphere, as evidenced by the ð18O marine benthic foraminifera record during the Upper Miocene and Pliocene [2]. Likewise, in SE Asia, climatic conditions have controlled the spread of plants and the expansion of tropical forests. The Oligocene and early Miocene were moisture deficient in most areas, with an ever-wet rainforest climate first spreading around 20 Ma in the early Miocene, after which they occurred repeatedly [3]. Assessing variations in palaeoclimate and palaevegetational; can be done by studying the floral compositions, including palynological records [4], [5] and composition of biomarkers in sediment [6], [7]. We used the various types of biomarkers and their derivatives found in various higher plants to identify taxa. For instance, information on conifer species can be identified by diterpenoids [8], [9], while triterpenoids and oleanane-type bicadinanes come from the angiosperms [10]–[12]. This paper studies coal samples from the Early Pliocene Sajau Formation in the Berau Basin. For the first time, we comprehensively use all the biomarker composition data of the palynological record, the composition of macerals, and trace elements in the coals of the Sajau Formation to study the paleoclimate during periods of Late Miocene-Pliocene.

Methods

Solvent Extraction, Fractionation, Gas Chromato graphy-Mass Spectrometry

A total of 7 samples of coal seam A to L from the Sajau Formation coal were washed with deionized water and ethanol to remove all impurities on the sample’s surface. Next, we ground the samples to a fine size into a 200 mesh; extraction was carried out with dichloromethane as a solvent for three days using a Soxhlet apparatus.

The n-hexane fills in the solvent to obtain the soluble maltene fraction. Maltenes are separated into aliphatic hydrocarbons and aromatic hydrocarbons (elution with n-hexane: dichloromethane, 1:2, v:v) by column chromatography. We used GC-MS equipment to determine the biomarker composition of aliphatic and aromatic fractions of the coal extracts, to identify aliphatic and aromatic fraction biomarkers based on m/z number, specific fragmentation, retention time, mass spectrum, and comparison of previous studies published by the previous study.

Palynological Analysis

Preparation of palynological analysis through various stages, cleaning of dirt on the sample surface with deionized. To remove salts and carbonates, we added 10% HCl. We removed the silicates by adding 40% hydrogen fluoride. Heavy minerals disappear by adding ZnCl₂. After that, the liquid was removed and given an acetolysis solution (a mixture of (CH₃COO)₂O with concentrated H₂SO₄, a ratio of 9:1), mixed, and then heated in a water bath for 30 minutes in an acid chamber. A nylon mesh filter is used to filter the liquid with palynological residue and neutralize it with distilled water. Finally, an Olympus binocular microscope with 400× magnification identifies spore-pollen.

Results

Biomarker Composition

Aliphatic Biomarker

The n-alkanes, branched and acyclic isoprenoids, hopanes, and steranes are the aliphatic compounds that are distributed in coal samples (Table I, Fig. 1).

Table I. Aliphatic Hydrocarbon Biomarker

Sample no	Pr/Ph	CPI	HVI	Ts/(Ts + Tm)	Ol/H	% aa 20 Sterane			H/S
						C27	C28	C29
SG-19/A	1.38	5.20	0.85	0.42	0.38	16.51	28.91	54.58	5.07
S003/B	1.49	4.86	0.93	0.39	0.27	10.24	4.79	89.47	10.56
S3/C	1.44	5.10	0.64	0.40	0.32	11.25	26.23	62.52	8.12
SG-1/D	2.18	5.26	0.94	0.41	0.26	16.10	28.02	55.88	11.62
SJ-12/E	2.82	4.24	0.91	0.36	0.24	11.62	10.24	78.14	14.28
S7/K	2.42	3.12	0.92	0.34	0.18	15.28	28.20	56.52	10.26
S8/L	3.22	3.48	0.88	0.34	0.16	17.23	28.24	54.53	9.75

Fig. 1. Representative partial m/z 191, and 217 mass chromatograms of the aliphatic fractions from representative samples in the Sajau coals, showing the distributions of terpanes (upper figure) and steranes and diasteranes (lower figure) from SG-19/A, and S-003. Ts = 18α(H) 21β(H)-22,29,30-trisnorneohopane; Tm = 18α(H) 21β(H)-22,29,30-trisnorhopane (Ts); OL= Oleanane.

The distribution of n-alkanes dominantly by odd-numbered long-chain n-alkanes (>nC25), having a CPI value of the coal samples, is mostly >3.0. Pr/Ph values ranged from 1.38–3.22 (average 1.86) in coal (Table I). C19–C29 tricyclic terpanes dominated Tricyclic terpane in all coals. The hopane distribution shows a relatively equal distribution across all examples. The Tm/Ts ratio ranged from 0.34–0.42. Coals also identified the oleanane, its derivatives (Fig. 1), and lupane. The oleanane/C30 αβ Hopana ratio range is low, between 0.16–0.32.

Aromatic Biomarker

The GC-MS analysis was carried out on samples of the aromatic fraction using several MIC chromatograms with different ions, and the ions consisted of ions m/z 156, m/z 170, m/z 178, m/z 192, m/z 231, and m/z 253. The distribution of aromatic hydrocarbons was detected, the dominance of methylphrenanthere (MN.) dimethyl-naphthalenes (DMP) and, trimethylnaphthalenes (TMN), and other polycyclic aromatic hydrocarbons (PAHs), such as retene, and cadinene (Fig. 2). Table II tabulated the concentration of aromatic biomarker.

Fig. 2. Representative partial m/z 156, m/z 170, m/z 178, and 192 mass chromatograms of the aromatic fraction of Sajau coals, showing the distributions of 1 = 2,6-dimethyl naphthalene, 8 = 1,3,7-trimethyl naphthalene, 17 = phenanthrene, 18 = 3-Methyl Phenanthrene from SG-19/A.

Table II. Aromatic Biomarker

No/Seam	Methyphenantrene (M.P.)/m/z 178; 192				Dimethyl naphtalene (DMN); m/z 156		Trimethyl naphtalene (DMN); m/z 170			Retene
	P	1-MP	9-MP	3-MP	1,7-DMP	1,2-DMP	1,2,5-TMN	1,2,7-TMN	1,3,6-TMN
SG-19/A	19.81	25.4	1.09	8.16	6.94	14.63	8.39	7.69	2.56	0.14
S003/B	11.22	19.21	0.99	7.14	6.48	12.26	8.01	7.12	4.02	0.12
S3/C	16.36	15.10	1.14	8.12	7.47	11.51	8.14	7.24	4.11	0.14
SG-1/D	13.46	15.26	1.61	8.14	8.49	11.70	9.01	6.11	5.12	0.12
SJ-12/E	9.16	14.24	1.72	7.85	7.47	9.28	5.24	5.23	5.94	0.09
S7/K	10.80	8.22	1.70	6.18	5.45	10.31	4.12	5.11	5.24	0.09
S8/L	9.86	9.08	1.84	7.28	6.02	9.25	5.24	5.02	4.86	0.08

Concentrations of compounds 1,2.5-TMN, 1,2,7-TMN, 1-MP, and 1,7-DMP in coal in the lower-central Sajau Formation (coal seam A–E) are relatively higher than the coal in the upper part of this formation (coal seam K–L). Likewise, concentrations of retene and cadinane are higher in the lower-middle part of the Sajau Formation.

Spore-Pollen Composition

The distribution of pollen-spore in coals was tabulated in Table III.

Table III. The Composition of Spore-Pollen

No/Seam	Gymnosperm (%)	Angiosperm (%)	Pteridophyte (%)	AP	NAP
SG-19/A	55	40	5	63	37
S003/B	42	41	7	59	41
S3/C	39	11	40	57	43
SG-1/D	35	5	60	52	48
SJ-12/E	50	8	42	65	35
S7/K	40	10	40	70	30
S8/L	45	4	51	60	40

The distribution of spore-pollen in coals of the Upper Miocene-Pliocene (3.5–2.6 Ma) in the study area indicates a change in the overall interest of the sediment. During the coal deposition of the lower Sajau Formation (Seam A–B), there was a decrease in Angiosperm content, followed by an increase in Pteridophyte. In contrast, the Gymnosperm content is relatively stable. During sedimentation of the C–E coal seam (middle part of the Sajau Formation), the Gymnosperm and Pteridophyte are more dominant compared to the Angiosperm. Likewise, finally, the sedimentation of the upper part of the Sajau Formation (K–L coal seam), Gymnosperm, and Pteridophyite still dominate. Pinus, Alnus, and other temperate species were decreasing, and Gymnosperm was stable during the sedimentation in the Upper Sajau Formation.

Discussion

The n-alkanes distribution and isoprenoid compounds in coal, such as the dominance of long-chain fatty acids (C27–C31), the high value of the Carbon Preference Index (CPI) above 1.0 indicates the high contribution of terrestrial vascular plants [13], [14]. Likewise, Hydrocarbon vegetation independent (HVI) with a value > 1 reflects the dominated the terrestrial O.M. derived (such that if HVI > 1), trees have dominated. Pr/Ph ranged from 1.38–2.09 (mean 2.1) in coal. It is compatible with the relatively abundant terrestrial input in the more oxidized environment of the Sajau formation.

Distribution of hopane was similar in all samples in coal with C27 C31 hopane as the main compound, and homohopanes decreased steadily from C31 to C35, originating from aquatic algae associated with a small quantity of aquatic/terrestrial higher plant inputs. In all coal samples, Oleanane was detected, along with its derivatives, such as 18-alpha-oleanan-3.beta-ol, Friedoolean-6-ene, and Friedoolean-3-one, indicating that the angiosperms were a plant source. The association of Oleanana supports the above statement, where these biomarkers are derived from an ancestor (bendain, amyrin, and other pentacyclic triterpanes) [1], [2], where these biomarkers exclusively in angiosperms [3]. In addition, the abundance of oleanane relative to C30 hopane indicates a high contribution of angiosperms to sedimentary rocks [15]–[17].

Based on the higher concentration of 1,2,5-TMN and 1,2,7-TMN, it may sourced from aromatization products of angiosperm [18], but it can be from gymnosperms [19]. Meanwhile, detecting 1-MP compound 1,7-DMP indicated retene decomposition, but it could also be found in the conifer tree of abietane-type diterpenoids [20]. Retene reflects its source from abietane-type diterpenoids that occur ubiquitously in conifers [21] but can also from algae [22]. At the same time, cadinane is produced from vascular plants [23].

The biomarkers’ characteristic distribution reflects a more significant number of predominantly angiosperm inputs during the forming A–E seam coal in the lower-middle Sajau Formation. Due to more rainfall in this section, these conditions form under a warmer and more humid environment. On the other hand, the K–L seam coals are located at the top of the hotter climatic conditions.

During the deposition of the lower Sajau Formation, an increase in the content of angiosperms and ferns was found in the A–C seam rocks, as well as a relatively greater abundance of conifers compared to the middle Sajau and Upper Sajau formations (seams D–F and K–L). Temperate Alnus and Pinus are found in small quantities in the Upper Sajau Formation, while higher in the Lower Sajau to Middle Sajau Formations. This condition causes climate change in the Lower Sajau section, where the climate is hotter in the Upper Sajau compared to warmer and more humid conditions in the Lower Sajau Formation, which changes to become hotter in the Middle Sajau and Upper Sajau.

Conclusion

During the Pliocene period, coal in the Middle Sajau to Upper Sajau Formations received little angiosperm input compared to coal in the Lower Sajau Formation. The high content of higher plant aliphatic markers such as oleanane, bicadinanes, C29, and 20R sterane indicates this.

During the Lower to Middle Pliocene period, a warmer and more humid climate influenced the sedimentation process in Lower Sajau Formation to the Middle Formation in the Berau Basin. They are characterized by the large number of Pinus and Alnus species found.

Changes in climate conditions from warm to hotter gradually occurred from the Lower Pliocene-Upper Pliocene, characterized by the high content of aromatic gymnosperm biomarkers in the coal of the Lower Sajau Formation compared to the Upper Sajau Formation.

Apart from being caused by low annual rainfall, the change in climate from warmer to warmer during the Lower Pliocene to Upper Pliocene period was also caused by changes in flora.

These biomarkers and spore/pollen data showed a cold, warmer climate from the Lower to Middle and Upper Pliocene.