Organic geochemistry of deep sea sediments from the Nankai Trough and the Southwest Japan forearc region

Earth's largest reactive carbon pool, marine sedimentary organic matter, becomes increasingly recalcitrant during burial, making it almost inaccessible as a substrate for microorganisms, and thereby limiting metabolic activity in the deep biosphere. Because elevated temperature acting over geological time leads to the massive thermal breakdown of the organic matter into volatiles, including petroleum, the question arises whether microorganisms can directly utilize these maturation products as a substrate. While migrated thermogenic fluids are known to sustain microbial consortia in shallow sediments, an in situ coupling of abiotic generation and microbial utilization has not been demonstrated. Here we show, using a combination of basin modelling, kinetic modelling, geomicrobiology and biogeochemistry, that microorganisms inhabit the active generation zone in the Nankai Trough, offshore Japan. Three sites from ODP Leg 190 have been evaluated, namely 1173, 1174 and 1177, drilled in nearly undeformed Quaternary and Tertiary sedimentary sequences seaward of the Nankai Trough itself. Paleotemperatures were reconstructed based on subsidence profiles, compaction modelling, present-day heat flow, downhole temperature measurements and organic maturity parameters. Today's heat flow distribution can be considered mainly conductive, and is extremely high in places, reaching 180 mW/m**2. The kinetic parameters describing total hydrocarbon generation, determined by laboratory pyrolysis experiments, were utilized by the model in order to predict the timing of generation in time and space. The model predicts that the onset of present day generation lies between 300 and 500 m below sea floor (5100-5300 m below mean sea level), depending on well location. In the case of Site 1174, 5-10% conversion has taken place by a present day temperature of ca. 85 °C. Predictions were largely validated by on-site hydrocarbon gas measurements. Viable organisms in the same depth range have been proven using 14C-radiolabelled substrates for methanogenesis, bacterial cell counts and intact phospholipids. Altogether, these results point to an overlap of abiotic thermal degradation reactions going on in the same part of the sedimentary column as where a deep biosphere exists. The organic matter preserved in Nankai Trough sediments is of the type that generates putative feedstocks for microbial activity, namely oxygenated compounds and hydrocarbons. Furthermore, the rates of thermal degradation calculated from the kinetic model closely resemble rates of respiration and electron donor consumption independently measured in other deep biosphere environments. We deduce that abiotically driven degradation reactions have provided substrates for microbial activity in deep sediments at this convergent continental margin.

Analysis of ferromanganiferous sedimentary rocks from Timor

Seven manganese nodules, eight ferromanganiferous shales from the Cretaceous Wai Bua Formation of Timor, and a pelagic limestone with four ferromanganese enriched layers from the Middle Eocene of Timor have been analysed. The nodules are compared with modern deep-sea nodules, and the ferromanganiferous shales are contrasted with relatively shallow marine manganiferous shales. The conclusion is reached that these rocks from Timor were probably deposited in a bathypelagic environment. There is a total absence of any indication that volcanic material has contributed to these deposits. The chemical composition of the ferromanganiferous rocks are discussed and some indications of biogenic influences are noted. The Middle Eocene pelagic limestone is compared with a similar modern sediment described from the Easter Island Rise in the Pacific.

SHRIMP zircon geochronology and geochemistry of Phanerozoic granitoids in southeastern Korea

Phanerozoic granitoids are widespread in the Korean Peninsula and form a part of the East Asian Cordilleran-type granitoid belt extending from southeastern China to Far East Russia. Here we present SHRIMP zircon U-Pb ages and geochemical and Nd isotopic compositions of Late Paleozoic to Early Jurassic granitoid plutons in the northern Gyeongsang basin, southeastern Korea; namely the Jangsari, Yeongdeok, Yeonghae, and Satkatbong plutons. The granite and associated gabbroic rocks from the Jangsari pluton were coeval and respectively dated at 257.3 ± 2.0 Ma and 255.7 ± 1.4 Ma. This result represents the first finding of a Late Paleozoic pluton in South Korea. Three granite samples from the Yeongdeok pluton yielded a slightly younger age span ranging from 252.9 ± 2.5 Ma to 246.7 ± 2.1 Ma. Two diorite samples from the Yeonghae pluton gave much younger ages of 195.1 ± 1.9 Ma and 196.3 ± 1.6 Ma. An Early Jurassic age of 192.4 ± 1.6 Ma was also obtained from a diorite sample from the Satkatbong pluton. The mineral assemblage and Al2O3/(Na2O + K2O) versus Al2O3/(CaO + Na2O + K2O) relationship indicate that all the analyzed plutons are subduction zone granitoids. Enrichments in large-ion-lithophile-elements and depletions in high-field-strength-elements of these plutons are also concordant with geochemical characteristics typical for the subduction zone magma. The presence of Late Permian to Early Triassic arc system is in contrast with the conventional idea that the arc magmatism along the continental margin of the Korean Peninsula has commenced from Early Jurassic after the termination of Triassic collisional orogenesis. The epsilon-Nd(t) values of the granitoid plutons are consistently positive (2.4-4.6), suggesting that crustal residence time of the basement beneath the Gyeongsang basin is relatively short. Moreover, the reevaluation of previously-published data reveals that geochemical compositions of the Yeongdeok pluton are compatible with those of high-silica adakites; La/Yb = 37.5-114.6, Sr/Y = 138.2-214.0, SiO2 = 62.9-72.0 wt. %, Al2O3 = 15.5-17.0 wt. %, Sr = 562-1173 ppm, MgO = 0.4-1.6 wt. %, Y = 3-6 ppm, Yb = 0.18-0.45 ppm, and Eu/Eu* = 0.92-1.31. The occurrence of adakites in southeastern Korea, and presumably in the Hida belt of central-western Japan, is indicative of a hot subduction regime developing at least partly along the East Asian continental margin during the Permian-Triassic transition period.

Sedimentological, petrographical and biostratigraphical characteristics of ODP Leg 190 volcanic ashes and siliceous claystones

Leg 190 was the first of a two-leg program across the Nankai accretionary prism and Trough, offshore Japan, aiming to evaluate existing models for prism evolution and to constrain syntectonic sedimentation, deformation styles, mechanical properties, and prism hydrology (Moore, Taira, Klaus, et al., 2001; Moore et al., 2001). More than 400 volcanic ash and siliceous claystone (altered ash) layers were penetrated and sampled during drilling of the six sites from two transects across the accretionary prism (Sites 1173-1178). In sites from the subducting Shikoku Basin (Sites 1173 and 1177) and in the trench axis (Site 1174), recognition of ash layers and diagenetically altered ashes was initially important in defining major lithostratigraphic units. However, it is clear that understanding the diagenesis of the volcanic ashes has considerable implications for prism evolution, mechanical properties, prism hydrology, geochemistry, and fluid flow in the accretionary prism and associated subducting sediments (cf. Masuda et al., 1996, doi 10.1346/CCMN.1996.0440402). Particle size, chemical composition, temperature, depth of burial, and time are all thought to be factors that may affect volcanic ash diagenesis and preservation (Kuramoto et al., 1992, doi:10.2973/odp.proc.sr.127128-2.235.1992; Underwood et al., 1993, doi:10.2973/odp.proc.sr.131.137.1993). The overall aim of this research is to evaluate factors influencing volcanic ash diagenesis in the Nankai Trough area. This data report presents just the results of the sedimentological and petrographic analysis of the volcanic ashes and siliceous claystones from Sites 1173, 1174, and 1177. It is anticipated that when the results of additional geochemical analysis of these lithologies is available a more meaningful evaluation of factors influencing volcanic ash alteration will be possible.