石 油 技 術 協 会 誌 第60巻 第1号 (平 成7年1月) JOURNAL OF THE JAPANESE ASSOCIATION FOR PETROLEUM TECHNOLOGY VOL. 60, NO. 1 (Jan., 1995) 論 文 ・報 告 Molecular geochemical approach to the Paleoceano graphicassessment of Neogene sediments of Yashima area, Akita Basin, Japan* Masanobu Yamamoto** and Yoshio Watanabe** (Received July 28, 1994; accepted November 9, 1994) Abstract: Specific source marker compounds were analyzed for solvent extracts from the Onnagawa, Funakawa and Tentokuji Formations, Yashima, Akita, NE Japan. Relative abundances of higher plant markers, n-C27 alkane, n-C28 alkanoic acid and ƒÖ-hydroxy n-C22 alkanoic acid; to algal derived compounds increase upward through the Funakawa Formation, indicating the rapid increase of land plant input associated with the increase of detrital matter during the deposition of the Funakawa Formation. Dinosteranes as specific compounds of dinoflagellates are more abundant in the lower part of the Onnagawa and the Funakawa Formations than in the middle and upper parts of the Onnagawa and the Tentokuji Formations. C25 highly branched isoprenoid (HBI) alkane as a specific compound of diatoms is abundant in the Onnagawa Formation, while less abundant in the Funakawa and Tentokuji Formations. The negative behavior of C25 HBI alkane/n-C21 alkane and dinosterane/4-desmethylsterane ratios through the Onnagawa Formation probably reflects the changes in primary producers likely due to the changes of nutrient silica concentration of surface water. The high dinosterane/4- desmethylsterane ratio in the Tentokuji Formation seems to reflect the increase of coastal dinoflagellates under the influence of riverine water inflow. An increase of hopane/sterane ratio in the Funakawa Formation probably reflects the increase of aerobic bacterial activity during early diagenesis resulted from bottom water oxygenation in this period. type of primary producers and the rate of 1. Introduction nutrient supply, and "the degree and type of As the first step to assess paleoceanographic anoxia" which could be reflected in associa settings where petroleum souce rocks were tionsof macro and micro benthic organisms. deposited, it is important to evaluate "primary Recently molecular geochemistry is applied to productivity" which is closely related to the paleoenvironmental assessment in various geo logicalsettings. Although such application has * 平 成6年6月9日, 平 成6年 石 油 技 術 協 会 春 季 講 演 会 地 質 ・ some difficulties due to its unconfirmed prem 探鉱部門 シンポジウム 「我が国 にお ける石油根源岩 の堆積環 iseon precursor-product relationships, the or 境 」 に て 講 演 This paper was presented at the 1994 JAPT Geology ganicmolecule is undoubtedly a powerful tool and Exploration Symposium entitled "Depositional to provide valuable information on variable environments of petroleum source rocks in Japan", held aspects of paleoceanography such as primary in Nagaoka, Niigata, Japan, June 9,1994. ** 地 質 調 査 所 燃 料 資 源 部 Fuel Resources Department, Geo producers (e.g., Shimizu et al., 1976; Volkman logicalSurvey of Japan et al., 1992), paleotemperature (e.g., Brassell et Copyright (C) 1995, JAPT al., 1986), paleosalinity (de Leeuw and Sinninghe 28 Molecular approach to Paleoceanographic assessment of Neogene sediments Damste, 1990) and bottom water redox the Wakkanai Formation (equivalent of the potential (Peters and Moldowan, 1991). Onnagawa and Funakawa Formations) in the The lipid compositions in the sediments are Tenpoku Basin, Hokkaido, based on the distri controlled mainly by (1) input of terrigenous butionin the diversity of diatom assemblages, organic matter, (2) primary producers, (3) al total organic carbon contents and sedimentary terationby heterotrophic organisms and input structures in the basin. Geological Survey of of bacterial lipids and (4) thermal diagenesis. If Japan Group suggested a drastic change of we can trace abundances of organic com ocean circulation pattern of the paleo-Japan poundsspecific to higher plants, algae and bac Sea from stagnant to highly circulated condi teriaas source indicators, it will enable us to tionsat ca. 9 Ma based on the results of syn assess and evaluate each paleoceanographic theticapproach from inorganic geochemistry factor and analyze the relationships among (Watanabe et al., 1994), molecular geochemis them. try(Yamamoto and Watanabe, 1994) and sed The Onnagawa, Funakawa and Tentokuji imentology(Wa-tanabe, 1994). Formations in the Yashima area, Akita Basin, The results of our application of molecular were selected as a subject of our case study, geochemistry to paleoceanographic reconst because (1) they, particularly the Onnagawa ructionof Japanese Neogene sediments have Formation, are important as possible source been published partly in Yamamoto and Wa- rocks in the Akita oil-producing basin, (2) the tanabe (1994) and Yamamoto et al. (1994). De similarity of primary producers in the Neogene tailedmolecular distributions in various bound to Recent ones enables us to use an analogy forms in those sediments are reported in Ya from the modern environment, (3) the im mamoto(submitted). In this paper, the authors maturityof these formations preserves more report the stratigraphic variations of selected valuable information of functionality of mole specific source markers in the Onnagawa, Fu cules,and (4) abundant geological information nakawaand Tentokuji Formations of the Ya is available on this section. shimaarea, Akita Basin, NE Japan, and discuss Recent research has made clearer the paleo the changes in terrigenous input, the primary ceanographicsetting of the Neogene back-arc producers in the surface seawater and the bac basin where the Onnagawa, Funakawa and terialactivity during and after the deposition. Tentokuji Formations and its equivalents were The sedimentological, geochemical and pa deposited. Iijima et al. (1988) and Iijima and leontologicalinformations on the studied area Tada (1990) compiled paleobathymetric maps are available from Taguchi et al. (1969), Kano of the basin showing that the basin was a silled et al. (1981), Tsuji et al. (1991), Waseda et al. basin whose sill depth was probably upper (1991) and Yamamoto and Watanabe (1994). In bathyal (-150 to -500 meters). Based on the the Yashima area, the detrital content in cyclic changes in mass accumulation rates to creasesupward from the bottom of the Onna biogenic silica and detrital matter in the Onna gawaFormation to the Tentokuji Formation, gawaFormation, Tada (1991) proposed a and the maturity of organic matter in these model whereby the intrusion of oxygen mini formations ranges from 0.09 to 0.62 of ƒÀƒÀ/ mumzone water from the Pacific Ocean via the total C30 hopane ratio, which are equivalent to sill to the basin caused by a sea level rise about 0.3-0.4% R0 (Yamamoto and Watanabe, resulted in bottom water anoxia in the silled 1994). During the middle Miocene period, the basin. Fukusawa (1992) suggested a seasonal Yashima area was located at the western slope wind induced upwelling model similar to that of a glauconite-topped small bank at almost found in the recent Gulf of California (van center of the NNE-SSW trending silled basin Andel, 1966) as the oceanographic model for situated on the western side of the northern 石 油 技 術協 会 誌 60巻1号 (1995) Masanobu Yamamoto•EYoshio Watanabe 29 Honshu (Figs. 1 and 2). The rock samples were crushed and milled to a particle size of about 200 mesh. More than 1kg sample was crushed at the same time and homogenized in order to eliminate the centimeter and decimeter scale heterogeneity of the sample. The description of the samples is shown in Yamamoto and Wata nabe(1994). The analytical methods for functionalized lipids are shown in Yamamoto (submitted). The analysis for hydrocarbons was carried out in the following way. The samples were extracted by ultrasonifica tionwith benzene/methanol (7/3 v/v) for 15 minutes three times. An aliquot of solvent extract was chromatographed by thin layer chromatography with silica gel (Whatman, PLKCI8F 20•~20CM,1000,ƒÊm) and hexane as a development solvent. The band of Rf 0.7-1.0 was scraped off from the plate and re-extracted with a mixture of benzene/methanol (1/1 v/v). Gas chromatography was carried out using a Hewlett Packard 5890A+5970B gas chromatog raphy-mass spectrometry (GC/MS) system Fig. 1 Map showing locations of area studied and with a splitless injection and a capillary the middle Miocene paleoceanography of column coated with Ultra-1 (25m, i.d. 0.20mm northern Honshu. The paleobathymetric 0.33ƒÊm thick). Samples was dissolved in ben information is from Iijima et al. (1988). S= zeneand then injected into the splitless injec shallow (0 to -150 meters), UB=upper bathyal (-150 to -500 meters) and MB= tionsystem at 300•Ž with helium carrier gas. middle bathyal (-500 to -2,000 meters). The oven temperature was programmed from 60•Ž to 160•Ž at 20•Ž/min., from 160•Ž to 300•Ž Honshu Arc (Fig. 1 ; Iijima et al., 1988). The at 4•Ž/min. and then isothermal at 300•Ž in 20 paleodepth of the Onnagawa Formation in the minutes. The mass spectrometer was run in the Yashima area was middle bathyal (-500 to - full scan ion monitoring mode (m/z 50-550) 2,000 meters) except for its uppermost horizon and the selected ion monitoring mode (SIM; which contains upper bathyal (-150 to -500 m/z 99, 177, 191, 205, 217 and 231). Electron meters) assemblages of benthic foraminifera impact spectra were obtained at 70eV. (Tsuji et al., 1991). Assignment of peaks was carried out by comparison with standards and literature, e.g., 2. Samples and methods Philp (1985) and Peters and Moldowan (1993). Eighteen outcrop rock samples in total were Relative abundance of the compounds was taken from the Onnagawa, Funakawa and Ten- measured in peak areas in the fragrnentograms tokujiPlioceneFormations frommiddle Miocene to of m/z 99 for acyclic alkanes, m/z 191 for agesexposed inthe Yashima areain the triterpanes, m/z 217 for 4-desmethylsteranes Akita oil-producing Neogene basin, northern and m/z 231 for 4-methylsteranes.