Jian-ming Gong, Jing Liao, Yu-xi Zhang,*, Jie Liang, Jian-wen Chen, Nuzhat Khan,Sye Waseem Haier

a Qingdao Institute of Marine Geology, China Geological Survey, Qingdao 266071, China

b Laboratory for Marine Mineral Resources, Pilot National Laboratory for Marine Science and Technology, Qingdao 266071, China

c Shandong University of Science and Technology, Qingdao 266071, China

d National Institute of Oceanography, ST-47, Block-1, Clifton, Karachi-75600, Pakistan

Keywords:Characteristics of major and trace elements Natural gas hydrates Marine algae Total organic carbon Makran Accretionary Prism NGH exploration trial engineering Arabian Sea Pakistan

ABSTRACT To accurately identify the natural gas hydrates (NGH) in the sea area of the Makran Accretionary Prism,Pakistan, this paper presents the testing and analysis of major and trace elements in sediment samples taken from two stations (S2 and S3) in the area by the China Geological Survey. As shown by testing results, all major elements are slightly different in content between the two stations except SiO2 and CaO.This also applies to the trace elements that include Sr and Ba primarily and Cr, Ni and Zn secondarily. It can be concluded in this study that the tectonic setting of the Makran Accretionary Prism is dominated by oceanic island arc and that provenance of the Makran Accretionary Prism is dominated by felsic igneous provenance, which is at the initial weathering stage and mainly consists of granodiorite. Besides terrigenous detritus, there are sediments possibly originating from Makran-Bela Ophiolite from the northwestern part and Murray Ridge igneous rocks from the southeastern part. The V/Cr, Ni/Co, and V/(V+Ni) ratios indicate that sediments of the two stations are in an oxidation-suboxidation environment.However, the authors infer that the sedimentary environment of the sediments 3.0 m below the seafloor tends to be gradually transformed into a reduction environment by comparison with the Qiongdongnan Basin in the South China Sea where NGH has been discovered. The sediments in the Makran Accretionary Prism are rich in organic matter, with total organic carbon (TOC) content greater than 1%. According to comprehensive research, the organic matter in the sediments mainly originates from marine algae and has high TOC content, which is favorable for the formation of NGH.

1. Overview of regional geology

The Makran Accretionary Prism in the northwestern part of the Indian Ocean was formed during the Arabian Plate subducted beneath the Eurasian Plate, having the characteristics such as low subduction angle (<3°), large sedimentary thicknesses (more than 7000 m), high sedimentary rate (0.2 − >1 mm/a) (Rad UV et al., 2000; Liao J et al., 2019; Pang KN et al., 2020), complex fault structure,high convergence rate (42 mm/a on average), and sufficient gas supply (Kopp C et al., 2000; Smith GL et al., 2014; Fig. 1).Therefore, it serves as an ideal place for the enrichment of natural gas hydrates (NGH) (Barbero E et al., 2020; Gong JM et al., 2020).

Since the 1990s, research institutes at home and abroad have carried out multiple expeditions of marine geological surveys in the Makran Accretionary Prism, aiming to identify the tectonics, cold springs, NGH, and their environmental effects in the area. As a result, they discovered bottom simulating reflectors (BSRs) which main indicators used to identify the presence of NGH and determined that there possibly exist free gas under the BSRs (Liu B et al., 2020;Smith GL et al., 2014; Gong JM et al., 2016, 2018a; Zhang Z et al., 2020). Especially during the Expedition M74/3 jointly conducted by the University of Hamburg and the University of Bremen in Germany in 2007, NGH samples were collected from the sediments of two stations. In 2018, the Qingdao Institute of Marine Geology jointly conducted a geochemical and geophysical survey aimed at NGH in the Makran Accretionary Prism. During this survey, geophysical data of 1507 km (Fig. 1) and sediment samples of five stations were collected, including the samples of cold spring organisms and carbonate crust acquired at Station S4(Fig. 2). This indicates that there may be rich NGH resources in the Makran Accretionary Prism.

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2. Sampling and testing

The stations S2and S3are located in the Pakistan sea area of the Makran Accretionary Prism (Fig. 1), with a water depth of 2850 m and 1549 m, respectively. Samples were taken at a length of 3.4 m and 3.82 m, respectively at S2and S3using gravity samplers (800 kg). The core of S2is generally comprised of grayish-black silty clay, which has a high content of organic matter and bears a small number of bioclastics. Meanwhile, S2produces a rotten egg odor at a horizon of 245−330 cm. S3has uniform lithology, with no apparent bedding. The core of S3generally consists of grayish-green silty clay, interbedded with a small number of white, fine-grained bioclastics and blocky, grayish-black clay at some horizon (Fig. 3).

Sediment samples were taken from the cores of S2and S3in the core warehouse of the Qingdao Institute of Marine Geology, with a sampling interval of 5 cm in the upper part,10 cm in the middle part, and 20 cm in the lower part of the cores according to extraction depth of pore water. As a result,19 and 25 sediment samples were obtained from the cores of S2and S3, respectively. These sediments were sent to the Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, and the Qingdao Institute of Marine Geology respectively for testing. The former was responsible for testing the samples for the content of total organic carbon (TOC), total nitrogen (TN),total sulfur (TS), and carbonate using an infrared CS230 Carbon/Sulfur Determinator with testing errors of less than 5%. The latter was in charge of testing the samples for major and trace elements using an X-ray fluorescence spectrometer Axios PW4400 and a Thermo Scientific XSeries 2 ICP/MS with testing errors of less than 5%. The analytical and testing results are shown in Table 1 and Table 2.

Fig. 1. Geotectonic location of the Makran Accretionary Prism, the study area, and sampling stations (Gong JM et al., 2018b).

Fig. 2. Cold spring organisms and carbonate crust at Station S4 in the Makran Accretionary Prism, and a seismic profile across S4 (see Fig. 1 for its location). a‒cold spring biota; b‒mussel and paragenetic polychaetes; c‒suspected tube worms; d‒carbonate crust; e‒seismic profile across site S4; BSR‒bottom-simulating reflector.

Fig. 3. Core photos and a seismic profile across S3 in the Makran Accretionary Prism, Pakistan. (a‒the core of S2; b‒the core of S3; c‒seismic profile across site S3; BSR‒bottom-simulating reflector).

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3. Testing results

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3.1. Characteristics of major elements

As shown in Table 1, the average content of major elements in the sediments of stations S2and S3is 38.03% for SiO2, 13.13% for Al2O3, 0.63% for TiO2, 5.68% for TFe2O3,0.06% for MnO, 9.54% for CaO, 2.39% for K2O, 0.12% for P2O5, 3.74% for MgO, and 2.16% for Na2O.

The major elements in the sediments of S2and S3tend to vary consistently except for SiO2, MnO, and CaO. The content of SiO2and MnO in the sediments of S2is higher than that of S3(40.69% and 0.07%vs.36.01% and 0.05%), while the content of CaO in the sediments of S3is higher than that of S2(11.04%vs.7.55%). The enrichment factors of SiO2,MnO, P2O5, and Na2O are all smaller than 0.85, indicating relative depletion compared to the upper continental crust. In contrast, the enrichment factors of TiO2, Al2O3, TFe2O3, and K2O vary in a range of 0.85−1.15 and the enrichment factors of CaO and MgO are all higher than 1.50, indicating relative enrichment compared to the upper continental crust (Fig. 4).

3.2. Characteristics of trace elements

Trace elements are more stable than major elements, thus becoming one of the important means to explore environmental change and provenance. Different trace elements and their associations (or ratios) reflect different sedimentary environments and serve as comprehensive manifestations and favorable indicators of the internal genesis and environmental information of geological events (Jin BF,2003; Algeo TJ et al., 2012).

The average content of the trace elements Sr, Ba, Cr, Ni,Zn, Cu, Co, V, Ga, Zr, Y, and Pb in the sediments of stations S2and S3are 355.9×10−6, 347.1×10−6, 126.2×10−6, 102.1×10−6, 90.2×10−6, 36.8×10−6, 27.5×10−6, 86.9×10−6, 14.0×10−6,144.4×10−6, 51.5×10−6and 18.2×10−6, respectively (Table 2).Compared with the upper continental crust, elements Sr, Cr,Ni, Zn, Cu, Co, and Y are relatively enriched, while Ba, Ga,and Zr are relatively depleted (Fig. 5). Meanwhile, the sediments of S2have lower Sr content and higher V content compared with the sediments of S3.

4. Discussion

4.1. Tectonic setting and provenance

4.1.1. Tectonic setting

The metallogenic geological conditions of NGH are closely related to the tectonic setting of NGH and the nature of the sediment source. Meanwhile, provenance analysis aims to determine the location, nature, and tectonic setting of the source region of sediments (Xiong YQ et al., 2006; Deng XG et al., 2006). The tectonic setting discrimination diagram of(K2O/Na2O)-SiO2(Fig. 6a; Roser BP et al., 1986) shows that the sediments of the two stations have a low content of quartzand mainly fall within oceanic island arc areas, which are consistent with the island arc environment evolving from detritus of felsic intrusive rocks that are determined according to tectonic setting discrimination diagram of (SiO2/Al2O3)-(K2O/Na2O) (Fig. 6b; Maynard JB et al., 1982).

Table 1. Content of major elements in sediments of stations S2 and S3 in the Makran Accretionary Prism, Pakistan.

Table 2. Content of trace elements in sediments of stations S2 and S3 in the Makran Accretionary Prism, Pakistan.

4.1.2. Nature of provenance

Inactive trace elements in sedimentary rocks play an important role in determining the nature of sediment sources since they are mainly controlled by the nature of parent rocks in provenance. The trace elements of the two stations were standardized using original mantle data. It can be seen from the standardized curves (Fig. 7) that elements of the two stations tend to vary consistently with the elements in the upper continental crust. Therefore, the original materials in the samples should originate from the upper continental crust.

Generally, the Al2O3/TiO2ratios are smaller than 14 for sediments originating from mafic volcanic rocks, while they vary in a range of 18−26 for sediments from felsic volcanic rocks. The Al2O3/TiO2ratios of the sediments from the study area range between 19.8 and 21.4, with an average of 20.7,indicating that the sediment provenance is more likely to be felsic volcanic rocks. To further infer the nature of the provenance, discrimination functions were determined using major elements of S2and S3according to the following formulas (Roser BP et al., 1988):

F1= −1.733TiO2+ 0.607Al2O3+ 0.76Fe2O3(total) −1.5MgO + 0.616CaO + 0.509Na2O − 1.224K2O − 9.09;

F2= 0.445TiO2+ 0.07Al2O3− 0.25Fe2O3(total) −1.142MgO + 0.438CaO + 0.475Na2O + 1.426K2O − 6.861.

Fig. 4. Standardized curves of major elements in the Makran Accretionary Prism, Pakistan (standard element content of the upper continental crust from).

Fig. 5. Standardized curves of trace elements in the Makran Accretionary Prism, Pakistan (standard element content of the upper continental crust from).

Fig. 6. Tectonic setting discrimination diagrams of the Makran Accretionary Prism, Pakistan. a‒K2O/Na2O-SiO2 diagram (after Roser BP et al., 1986); b‒SiO2/Al2O3-K2O/Na2O diagram (after Maynard JB et al., 1982). PM‒passive continental margin; ACM‒active continental margin;ARC‒oceanic island arc margin; A1‒island arc of basaltic and andesitic detritus; A2‒island arc evolving from the felsic intrusive rock.

It can be seen from the discrimination diagram (Fig. 8a)that the samples mainly fall in the intermediate igneous provenance and felsic igneous provenance. As shown in the Ti/Zr-Co/Y correlation diagram (Fig. 8b), S2is close to andesite, so it may be produced from the mixture of acidic and intermediate magma; S3is close to TTG, indicating that granodiorite may serve as the provenance of this station.

According to the results of correlation analysis of major elements from S2and S3(Table 3, Table 4), there is a strong positive correlation between Al2O3and TiO2, Fe2O3, and K2O for S2and between Al2O3and SiO2, TiO2, Fe2O3, K2O, and MgO for S3. This indicates that Al originated from the same source with Ti, Fe, and K in S2, while Al shares the same origin with Si, Ti, Fe, K, and Mg in S3, i.e., the terrestrial source related to clay minerals. Meanwhile, the correlation between Al and Mg and Si differs greatly between the two stations, indicating that Mg and Si of the two stations have different provenance. Moreover, there is a clear negative correlation between Al2O3and CaO of the two stations,indicating that Ca and Al have different provenance in the two stations. Indeed, Ca is a characteristic element of marine biogenic sediments. Al2O3and K2O of the two stations are slightly depleted compared to the upper continental crust,indicating low migration during the weathering of potash feldspar. As shown in the A-CN-K diagram of sediment provenance of the Makran Accretionary Prism (Fig. 9), the chemical index of alteration (CIA) in the study area is mostly less than 60, with the weathering trend close to the granodiorite trend line. This suggests that the provenance of the Makran Accretionary Prism is still at the initial weathering stage (Xu XT et al., 2018).

Fig. 7. Standardized curves of trace elements in the Makran Accretionary Prism, Pakistan (primitive mantle data from Taylor SR and Mclennan SM, 1985).

Fig. 8. Discrimination diagrams of sediment provenance of the Makran Accretionary Prism, Pakistan. a‒F1-F2 diagram (after Roser BP et al.,1988); b‒Ti/Zr-Co/Y diagram.

Table 3. Correlation coefficients of major elements in sediments of S2 in the Makran Accretionary Prism, Pakistan.

Table 4. Correlation coefficients of major elements in sediments of S3 in the Makran Accretionary Prism, Pakistan.

Fig. 9. A-CN-K diagram of sediment provenance of the Makran Accretionary Prism, Pakistan.

Table 5. Comparison of content of trace elements Cr, Zn, Ni,Cu, and Zr in the S2 and S3 and the continental shelf of the Indus Fan.

In addition, the content of trace elements Cr, Zn, Ni, Cu,and Zr of the two stations is significantly higher than that of the surface sediments in the continental shelf of the Indus Fan(Table 5). This also indicates that, besides the terrigenous clastic sediments, there are sediments from other provenance in the study area. The Makran-Bela Ophiolite and MurrayRidge igneous rocks are characterized by an occurrence of basalt and basic ultramafic rocks (Khan AA et al., 2019;Burgath KP et al., 2002; Ghose NC et al., 2014). Meanwhile,ultramafic rocks tend to have a high content of Cr, Co, and Ni and basalts tend to have a high content of Cu (Zahid M et al.,2015). Therefore, the surface sediments in the Makran Accretionary Prism possibly contain the Makran-Bela Ophiolite from the northwestern part and Murray Ridge igneous rocks from the southeastern part.

Table 6.Comparison of average content of partial major elements in sediments of the Makran Accretionary Prism and the neritic and continental sediments in China.

4.2. Sedimentary environment

4.2.1. Bioclastic-rich fine-grained sediments

Based on the analytical results of major elements, the content of SiO2and CaO of the sediments from S2and S3is lower than that of neritic sediments in China and higher than that of continental sediments in China (Table 6). This indicates that organisms are relatively active in the Makran Accretionary Prism, contributing to a high abundance of organic matter (average TOC: 0.67%). Meanwhile, Al2O3mainly occurs in fine-grained sediments. Since the Al2O3content of the sediments from the two stations is higher than that of both neritic and continental sediments in China, the sediments in the study area are clay materials with smaller particle sizes.

Sr is mainly enriched in coarse-grained calcareous bioclastic sediments and is deemed to be a marker of biogenic sediments. Therefore, Sr tends to be classified as a biophile element in geochemical research of marine sediments (Zhu LM et al., 2007). The testing results of trace elements (Table 2)show that the Sr content of S3is higher than that of S2,indicating that S3is richer in bioclastic sediments than S2and that the particle size of the sediments at S3with shallow water may be greater than that of the sediments at S2with deep water (Khan AA et al., 2019).

Table 7. Ratio Range of redox-sensitive elements.

Therefore, the surface sediments of the two stations have small particle sizes and are generally rich in bioclastics, which is favorable for the enrichment of organic matter.

4.2.2. A trend towards reduction environment

Petrogeochemical characteristics serve as one of the critical means to determine the sedimentary environment. A series of quantitative indicator systems have been established to determine the redox state of sea-water in recent years, in which redox-sensitive elements such as U, V, and Ni are adopted to analyze the paleo-marine environment. Studies have demonstrated that V/Cr, Ni/Co, and V/(V+Ni) ratios can be used as parameters to determine paleo-marine redox environment (Table 7).

For S2and S3, the V/Cr ratio varies in a range of 0.65‒0.79 and 0.53‒0.74, respectively, less than 2; the Ni/Co ratio varies in a range of 3.43‒3.82 and 3.47‒4.46,respectively, less than 5, and the V/(V+Ni) ratio varies in a range of 0.42‒0.49 and 0.41‒0.46, respectively, greater than 0.4 (Fig. 10). All these indicate that the surface sediments of the two stations are in an oxidation-suboxidation environment.

Although the V/Cr ratio of the two stations is less than 2,it tends to gradually increase 3.0 m below the seafloor (Fig. 10).By comparison with the V/Cr ratio of the stations CL-1 and CL-2 in the Qiongdongnan Basin of the South China Sea (Fig.10; Deng YN et al., 2017), it can be concluded that the sedimentary environment of the sediments 3.0 m below the seafloor tend to be transformed into reduction environment at the two stations in the Makran Accretionary Prism, with the transformation rate of S3higher than that of S2.

In summary, the geochemical characteristics of trace elements of S2and S3show that the sediments 3.0 m below the seafloor in the Makran Accretionary Prism are possible in a reduction environment. Such a sedimentary environment is favorable for the formation and occurrence of NGH.

4.3. Algae with a high abundance of organic matter

Fig. 10. V/Cr, Ni/Co, and V/(V＋Ni) ratios of S2 and S3 and V/Cr ratio of CL-1 and CL-2 in Qiongdongnan Basin of the South China Sea varying with depth (Deng YN et al., 2017).

The TOC content of S2and S3tends to decline with depth(Fig. 11). In detail, the TOC content of S2declines with depth above 175 cm and slightly increases below 175 cm, with a varied range of 0.36%−0.83% and an average of 0.62%.Meanwhile, the TOC content of S3apparently declines with depth above 220 cm and slightly increases below 220 cm,with a varied range of 0.31%‒1.31% and an average of 0.75%. Therefore, the TOC content of S3is higher than that of S2. The TN content of the two stations tends to decline with depth. For S2, the TOC/TN ratio is 2.1−5.5, with an average of 3.3, and theδ13C values of TOC range between −22.12‰and −19.68‰, with an average of −20.77‰. For S3, the TOC/TN ratio is 2.8‒6.1, with an average of 4.0, which is slightly higher than that of S2. Meanwhile, theδ13C values of the TOC of S3range between −22.29‰ and −18.93‰, with an average of −20.56‰. According to relevant studies, the TOC/TN ratio of the organic matter in fresh algae with rich proteins is 3‒8, while that of the organic matter in higher terrestrial plants is greater than 20 (Sun TT et al., 2018; Zhang BD et al., 2015). Since the average TOC/TN ratio of the study area is 3.3‒4.0, it can be inferred that the organic matter in the study area mainly includes algae.

The organic carbon isotope values (δ13C) of typical marine organisms and typical terrestrial C3plants are −22‰ ‒−19‰ and −28‰ ‒ −26‰, respectively and the averageδ13C values of terrestrial C4plants are −14‰ (Volvoikar SP et al.2014; Jiang SY et al., 2005). Therefore, the TOC/TN ratio can be used to determine whether sediments are from algae or terrestrial plants, whileδ13C values can be used to discriminate between the organic matter of terrestrial and marine sediments and to determine the plant types of terrestrial organic matter (Wu DD et al., 2017; Deng YN et al.2017; Cheng J et al., 2019). The plot of TOC/TN ratio andδ13C values of S2and S3(Fig. 12) shows that organic matter of the two stations mainly originates from marine algae.

Fig. 11. TN, TOC, TOC/TN, and δ13CTOC vs. depth at stations S2 and S3.

Fig. 12. Relationship of TOC/TN and δ13C of sediments from stations S2 and S3.

Therefore, the organic matter in the sediments of the Makran Accretionary Prism mainly originates from marine algae, with the TOC content slightly higher than that of the Shenhu Area, the South China Sea. This reflects favorable conditions for the formation of NGH.

4.4. Implications for NGH

It can be inferred from the aforementioned analysis that the sedimentary environment of the sediments 3.0 m below the seafloor tends to be transformed into a reduction environment at S2and S3in the Makran Accretionary Prism.However, no anomalous enrichment of redox-sensitive elements (i.e., V, Ni, and Zn) is visible and the TOC content increases to some extent. All these indicate that the transformation to a reduction environment is not caused by organic matter decomposition but possibly by anaerobic oxidation of methane. Sr is enriched 3.0 m below the seafloor at S2and 1.5−2.5 m below the seafloor at S3. Meanwhile,aragonite is a Sr-rich mineral and forms in the areas suffering methane leakage. All these indicate that methane leakage occurs below the sampling horizon. In combination with the cold spring organisms and carbonate crust discovered and the data of widely distributed BSRs, enhanced reflections, and blank reflection, it can be concluded that the study area boasts widely distributed NGH, sufficient gas supply, and great resource potential.

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5. Conclusions

(i) The tectonic setting of the Makran Accretionary Prism is dominated by an oceanic island arc. The provenance of the Makran Accretionary Prism mainly includes felsic igneous provenance, which is at the initial weathering stage and mainly consists of granodiorite.

(ii) The sediments 3.0 m below the seafloor in the Makran Accretionary Prism may be in a relative reduction environment and have a small particle size. They are conducive to the preservation and enrichment of organic matter.

(iii) The organic matter in the sediments of the Makran Accretionary Prism mainly originates from marine algae, with high TOC content. This reflects favorable conditions for the formation of NGH.

CRediT authorship contribution statement

Jian-ming Gong conceived of the presented idea. Jing Liao and Yu-xi Zhang drew all the figures. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgment

The authors would like to extend sincere appreciation to all workers in the “Shiyan3” Vessel, Chinese Academy of Sciences for their assistance in field sampling for this study.Thanks also go to the Lanzhou Center for Oil and Gas Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, and the Qingdao Institute of Marine Geology for their support in sample testing and analysis. This work was funded by the projects of the National Natural Science Foundation of China (91858208, 42076069) and the project of China Geological Survey (DD20190581). Also, the authors acknowledge anonymous reviewers and editors for their suggestions about this manuscript.