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Distinct sedimentary processes reflected in the isotopic signatures of dolomitic concretions in the Miocene Pohang Basin (southwestern East Sea)
Journal of Asian Earth Sciences 29 (2007) 939–946 www.elsevier.com/locate/jaes
Distinct sedimentary processes reXected in the isotopic signatures of dolomitic concretions in the Miocene Pohang Basin (southwestern East Sea) Boo-Keun Khim a, Kyung Sik Woo b,¤, Young Kwan Sohn c a
Division of Earth Enviromental System, Pusan National University, Busan 609-735, Republic of Korea b Department of Geology, Kangwon National University, Chuncheon 200-701, Republic of Korea c Department of Earth and Environmental Sciences, Gyeongsang National University, Jinju 660-701, Republic of Korea Received 31 January 2005; received in revised form 4 October 2005; accepted 16 December 2005
Abstract Dolomitic concretions in diatomaceous hemipelagic sediments of the Miocene Pohang Basin in the southwestern East Sea (Sea of Japan) preserve distinct signals of two independent sedimentary processes, which controlled the extents of isotopic compositions. Variable 18O (¡9.1‰ to +0.7‰) and high 13C (+3.1‰ to +17.9‰) values suggest that the concretions formed in the methanogenic zone with alteration of the residual mid-Miocene seawater by volcanogenic sediments. Remarkable 18O and 13C values show a strong linear relationship, indicating that distinctly independent depositional processes operated during the formation of the concretions. The degree of methanogenesis was enhanced during rapid hemipelagic sedimentation of organic-rich particles, resulting in higher 13C values, and the eVect of volcaniclastics was diluted, maintaining the original properties of ambient mid-Miocene seawater. In contrast, lower 18O and 87 Sr/86Sr values characterize the eVect of volcaniclastic sediments that were transported by intermittent gravity Xows and interacted with mid-Miocene seawater. The input of volcaniclastic sediment probably degraded the role of methanogenesis by lowering the contents of organic matter and thereby decreased the 13C values within the concretions. Isotopic signals recorded within the concretions highlight understanding of the depositional environment and evolution of the pore-water chemistry. © 2006 Elsevier Ltd. All rights reserved. Keywords: Carbonate concretion; Stable isotope; Strontium isotope; Depositional processes; Miocene; Korea
1. Introduction The geochemistry of carbonate concretions in sedimentary deposits has provided a direct and principal signature for tracing diagenetic evolution of pore-water chemistry within sediments during shallow burial. Numerous studies have shown that the stable isotopic compositions of carbonate concretions can be utilized to interpret the diagenetic environments and their origins (e.g., Irwin et al., 1977; Irwin, 1980; Gautier and Claypool, 1984; Curtis et al., 1986), including the ion pathways for carbonate cements,
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microbial activity, and ambient temperature of formation. Carbonate concretions usually show core-to-margin chemical changes (Irwin, 1980; Gautier, 1982; Curtis et al., 1986), such that a radial chemical change from the concretion center to the margin is explained in terms of progressive (or temporal) variations in pore-water composition (Mozley, 1996; Raiswell and Fisher, 2000; Raiswell et al., 2002). Although some carbonate concretions have 18O values compatible with precipitation at shallow burial depths from unaltered seawater (i.e., 18O values almost identical to that of seawater at the time of precipitation), most concretions have 18O values far from those of seawater (Lawrence et al., 1975; Coleman and Raiswell, 1981; Scotchman, 1991; Mozely and Burns, 1993). For example, the most 18Oenriched dolomites from the Miocene Tripoli Formation
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(Italy) are attributed to intense evaporative condition (Kelts and McKenzie, 1982), and the anomalous 18 O-enrichment of deep-sea carbonates is attributed to destabilization of gas hydrates (Matsumoto, 1989). In contrast, several explanations have been proposed for the low 18O values of carbonate concretions in marine siliciclastic deposits (Morad et al., 1996), including (1) the eVect of meteoric water; (2) the oxidation of organic matter in the sulphate-reduction zone; (3) the interaction between seawater and sediments, particularly with reactive volcanic materials; (4) the upwelling of hot Xuids to near-surface; (5) the recrystallization or replacement of early carbonate cements during deep burial; and (6) carbonate precipitation during progressive burial as a result of the increasing temperature with depth. Pore-waters in marine siliciclastic sediments are altered systematically, resulting in changes in carbon isotopic compositions due to varying pathways of bacterially mediated decomposition of organic matter (Gautier and Claypool, 1984; Nelson and Lawrence, 1984; Curtis et al., 1986; Mozely and Burns, 1993). The extent of 12C incorporation into carbonate depends on the amount and reactivity of the organic matter, the depth of the suboxic zone beneath the sediment-seawater interface, and the degree of bioturbation. The 13C signatures of concretions formed in the sulfatereduction zone are inXuenced by dissolved carbon derived from the oxidation of organic matter, resulting in the negative values far from those of seawater (McArthur et al., 1986). In contrast, microbial methanogenesis in marine sediments causes the progressive enrichment of 13C in residual pore-waters, resulting in positive 13C values in carbonate concretions (Kelts and McKenzie, 1982; Gautier and Claypool, 1984; Nelson and Lawrence, 1984). Irwin et al. (1977) suggested a burial model to interpret diVerent diagenetic environments of concretion growth with unique 13C values based on the source of carbon for carbonate concretions. This study presents a peculiar case of isotopic variation in dolomitic concretions in the Miocene Pohang Basin in the southwestern East Sea (Sea of Japan; Fig. 1) that shows a co-varied pattern of 18O and 13C values within concretions. The data suggest that the isotopic compositions of the concretions were inXuenced by two independent sedimentary processes, i.e., the continuous hemipelagic input of organic-rich particles and the intermittent input of volcaniclastic sediments by mass Xows. Our results emphasize the utilization of isotopic geochemistry of concretions to help interpret the character of depositional processes and trace the evolution of ambient pore-water chemistry. 2. Geologic setting The Miocene Pohang Basin (Fig. 1A) in the southwestern East Sea is one of several Tertiary basins along the southeastern coast of Korea that were produced in association with back-arc spreading of the East Sea mostly during the Early Miocene (»23–17 Ma; Ingle, 1992; Jolivet et al., 1994). Non-marine to shallow marine sedimentation
occurred together with extensive volcanism during this period (Son et al., 2005). At about 17 Ma, the Pohang area began to subside rapidly, from inner neritic (<50 m deep) to middle-to-lower bathyal depths (>500 m deep; Ingle, 1992; Kim, 1999). Several fan deltas began to form along the western basin margin (Hwang et al., 1995; Sohn et al., 2001; Sohn and Son, 2004). As a result, the sedimentary succession, named the Yeonil Group, exhibits the intercalation of deltaic sediments and hemipelagic mudstones (Choe and Chough, 1988; Chough et al., 1990; Hwang and Chough, 1990; Hwang et al., 1995), is more than 1 km thick, and contains abundant fossil faunas of late Early to Middle Miocene age. Palaeontologic ages based on molluscs, foraminifers, nannofossils, and dinoXagellate cysts are generally between 17 and 11 Ma (Kim, 1990; Yi and Yun, 1995; Bak et al., 1996). The lower part of the Yeonil Group was deposited in a non-marine and shallow-water environment (Yoon, 1975). In contrast, the benthic foraminiferal assemblage indicates that the upper part of group was deposited in upper to lower bathyal environment (Shin, 1981). According to the fan-delta system, the Pohang Basin was composed of four discrete depositional environments, including fan delta, prodelta, slope apron, and basin plain. Recent sedimentological studies (Sohn et al., 2001; Sohn and Son, 2004) reveal that the Pohang Basin Wlls can be divided into several stratal packages or sequences bounded by laterally persistent stratigraphic and/or structural discontinuities (Fig. 1B). These include (1) a non-marine to shallowmarine, early syn-rift sequence (S1 in Fig. 1B) that formed prior to the major subsidence event in the Pohang area at about 17 Ma, (2) the marine Gilbert-type delta sequence (S2 in Fig. 1B), which represents a rift climax in the Pohang Basin, and (3) the overlying deep-marine strata, which represent post-rift sequences that accumulated under the inXuence of eustatic sea-level changes during the Middle Miocene (S3–S5 in Fig. 1B). Carbonate concretions in the Pohang Basin occur either in the early syn-rift, shallow-marine deposits (mostly sandstones) or in the post-rift deposits (mostly hemipelagic mudstones). These concretions, composed of calcite, dolomite, or a combination of the two, have variable sizes and morphologies (spheroidal, elliptical to irregular). The post-rift deposits, in which the carbonate concretions discussed in this paper are contained, consist of alternating conglomerates/sandstones and mudstones, suggestive of continuous hemipelagic sedimentation occasionally interrupted by sediment gravity Xows during the formation of the concretions. The clastic sediments were derived from the Cretaceous to Tertiary basement rocks, which consist of abundant volcanic rocks (dacite, rhyolite, and tuV) as well as sedimentary and plutonic rocks (Fig. 1A; Hong et al., 1998). The mudstone composition also changes from dark brown bentonitic mudstones in the lower part to siliceous or diatomaceous mudstones in the upper part of the post-rift sequences (Noh and Woo, 1997).
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Fig. 1. (A) Geologic map of the Pohang Basin and adjacent area modiWed after Sohn and Son (2004). The basement rocks around the Pohang Basin consist dominantly of Cretaceous to Tertiary volcanic rocks and volcanogenic sedimentary rocks. (B) Enlarged map of the study area in the Pohang Basin, showing the sampling locations of the carbonate concretions. The Pohang basin Wll is dominated by hemipelagic mudstones, dipping gently eastward, except for the fan-delta conglomerates along the western basin margin. Characteristics of the basin Wlls, modiWed after Sohn et al. (2001), are summarized in the inset box.
3. Materials and methods Carbonate concretions were collected mostly from the central parts of the Pohang Basin where thick sequences
of hemipelagic mudstones (siliceous mudstones or diatomites) crop out (Fig. 1B). The mudstones here are mostly composed of diatoms and other siliceous organisms (Noh and Woo, 1997) and belong to the topmost sequence of
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Sohn et al. (2001) (S5; Fig. 1B). The mineralogy of the concretions determined by X-ray diVractometer (XRD; Rigaku D/max-2200A) is dolomite. For isotope analyses, carbonate powders were collected at a discrete interval from the slabs using a JENSEN micromill, tracking an approximate cross-proWle from the top to bottom of the concretions. The stable oxygen and carbon isotope composition was measured by an isotope ratio mass spectrometer (VG Prism II). The analytical error of carbon and oxygen isotopes is §0.1‰. All isotopic values are referred relative to the PDB standard. The analysis of 87Sr/86Sr ratio was done using a Thermal Ionization Mass Spectrometer (VG Sector 5430 multi-collector mass spectrometer). Measured 87Sr/86Sr values were normalized internally against 86Sr/88Sr D 0.1194. The average of the measured 87Sr/86Sr ratios was 0.710238 § 0.000017 (n D 19). The error of the measured ratios is §0.00002. All analyses in our study were conducted at the Korea Basic Science Institute. 4. Results and discussion 4.1. Stable isotope variations within dolomitic concretions in Miocene Pohang Basin The dolomitic concretions always occur with their long axes aligned almost parallel to the bedding planes, and all are commonly observed as continuous dolomite beds or sometimes as individual ellipsoid or coalesced ellipsoids within the diatomaceous mudstones (Fig. 2A and B). Most concretions are a few tens of centimeters to one meter in height and from a few tens of centimeters to several tens of meters in length. These concretions are composed of dolomicrite to dolomicrospar, sometimes preserving the diatom relics and other microfossils (Fig. 2C; Noh and Woo, 1997). The stable oxygen and carbon isotope values and strontium isotope ratios of concretions are listed in Table 1. Two examples of specimens Ph4-2 and Ph10, representing the isotopic trend from the central part towards the top and bottom (Fig. 3), show good co-variance between 18O and 13C values within concretions. The degree of variation of 18O and 13C values is diVerent for diVerent sampling spots, but co-variance between 18O and 13C values is obviously characteristic. The isotopic variations show that the top-to-bottom proWle from the central part of the concretions is unusually asymmetric, dissimilar to the generally assumed concentric growth (Mozley, 1996; Raiswell and Fisher, 2000; Raiswell et al., 2002). In spite of the asymmetric pattern of isotopic variations, the concretions might have formed through radial growth. The 18O values of concretions show a wide range of variation from the relatively high 18O values (¡1‰) likely to reXect the unaltered mid-Miocene seawater to relatively depleted 18O values of about ¡6‰ (Fig. 3). Such low 18O values may be due mainly to the alteration of pore-water chemistry through continuous reaction between residual
Fig. 2. (A) Outcrop view of dolomitic concretions preserved in the diatomaceous mudstone (site Ph4-1). (B) Outcrop view of another dolomitic concretion (site Ph5). (C) Thin-section photomicrograph of dolomitic concretion. Note the diatom fragment partially replaced by dolomite.
seawater and volcanogenic sediment during the formation of the concretions (Lawrence et al., 1975; Morad et al., 1996). The 13C values of these dolomitic concretions are very high, reaching more than +9‰ (Fig. 3). Such high 13C values may well indicate that the formation of the concretions occurred within the methanogenic zone (Irwin et al., 1977; Curtis et al., 1986). The methanogenic zone was changed rapidly from the sulfate-reducing zone because of the continuous supply of abundant organic matter in the hemipelagic sedimentary environment (Coleman and Raiswell, 1981; Gautier and Claypool, 1984).
B.-K. Khim et al. / Journal of Asian Earth Sciences 29 (2007) 939–946 Table 1 Summary of oxygen, carbon and strontium isotope data measured from the carbonate concretions Sample code
13C
18O
87
Ph1 Ph2 Ph4-1 (U2-2) Ph4-1 (U2-1) Ph4-1 (U1-5) Ph4-1 (U1-3) Ph4-1 (U1-2) Ph4-1 (U1-1) Ph4-1 (c) Ph4-2 (C-U3) Ph4-2 (C-U2) Ph4-2 (C-U1) Ph4-2 (C) Ph4-2 (C-D1) Ph4-2 (C-D2) Ph4-2 (D1) Ph4-2 (D2-1) Ph4-2 (D2-2) Ph4-2 (D2-3) Ph4-2 (D2-4) Ph4-2 (D2-5) Ph4-2 (D2-6) Ph4-2 (D2S) Ph4-2 (D3) Ph 5 Ph6-1 Ph6-3 Ph7-1 Ph7-2 Ph8 Ph9 Ph10 (U2-5) Ph10 (U2-4) Ph10 (U2-3) Ph10 (U2-2) Ph10 (U2-1) Ph10 (U1-3) Ph10 (U1-2) Ph10 (U1-1) Ph10 (C2) Ph10 (C1) Ph10 (D1-1) Ph10 (D1-2) Ph10 (D1-3) Ph10 (D1-5)
12.4 10.2 3.1 5.8 9.9 10.5 10.7 10.9 10.3 16.1 16.0 15.8 16.3 15.2 14.0 16.2 14.7 15.5 15.5 15.4 15.4 15.6 15.4 14.8 14.9 12.7 17.9 13.0 8.8 8.6 13.6 14.3 14.4 15.1 14.9 15.4 15.5 15.5 16.5 15.9 16.5 12.0 14.2 13.6 9.1
¡4.3 ¡7.8 ¡6.1 ¡7.3 ¡5.6 ¡3.7 ¡5.2 ¡5.2 ¡3.7 ¡2.2 ¡2.2 ¡2.3 ¡1.5 ¡2.4 ¡3.0 ¡2.0 ¡4.1 ¡2.3 ¡2.4 ¡2.3 ¡2.5 ¡2.4 ¡2.4 ¡3.6 ¡3.4 ¡4.7 ¡1.2 ¡3.1 ¡6.1 ¡6.0 ¡3.6 ¡0.8 ¡2.5 ¡0.2 ¡2.1 ¡1.8 ¡0.2 ¡1.7 0.6 ¡1.5 0.7 ¡5.5 ¡2.4 ¡2.7 ¡4.8
ND 0.70805 0.70820 0.70834 ND 0.70868 0.70863 ND 0.70869 ND 0.70882 ND 0.70877 ND ND 0.70869 0.70877 ND ND ND 0.70872 ND ND ND 0.70872 ND 0.70871 ND 0.70867 0.70850 ND 0.70875 0.70871 0.70869 ND ND 0.70874 ND 0.70887 ND 0.70875 0.70872 ND 0.70871 0.70881
Sr/86Sr
The isotopic proWles of 18O and 13C values show the asymmetric pattern from the center towards the top and bottom (Fig. 3). The concretion formation is likely to start from shallow burial depth where the pore-water retains the original seawater 18O values in the methanogenic zone. However, the remarkable pattern of isotopic variation clearly indicates that the diagenetic pore-water chemistry operates in a diVerent way radially, away from the central part of concretions (Raiswell et al., 2002). In other words, although the pore-water chemistry, which controls the precipitation of the concretion, is inXuenced by a variety of factors, the degree of chemical change is not uniform relative to the center of concretions. Such a peculiar trend has not been reported yet from concretions elsewhere. This iso-
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topic trend distinctly suggests that the carbonate precipitation occurred in a completely closed system, from which the ambient pore-water chemistry was reXected in the original geochemical signatures of the concretions (Woo and Khim, 2006). 4.2. Relationship between 13C and 18O values of dolomitic concretions The mineralogy of concretions is occasionally determined by the rate of organic-carbon oxidation. Calcite precipitation sometimes stops before the completion of sulfate-reduction and the initiation of methanogenesis (Kastner and Bake1r, 1983; Gautier and Claypool, 1984; Nelson and Lawrence, 1984). In general, precipitation of dolomite begins at the time when the sulfate ions in seawater are depleted by bacteria in the sulfate-reducing zone. Thus, the amount of organic matter provided into the diagenetic zone may be a major factor in determining the state of precipitation favorable for calcite or dolomite. In the central part of the Pohang Basin, the widespread diatomaceous mudstones with high contents of organic matter are enough to cause dolomite precipitation under methanogenic conditions (Hwang et al., 1995; Sohn et al., 2001). A remarkable feature of all dolomitic concretions in the basin plain of the Pohang Basin is a strong linear relationship between 18O and 13C values (Fig. 4A). The 18O and 13C values seem to be widely scattered. However, individual concretion shows an eVective linear relationship. In particular, the slopes of the regression are likely to limit two types, although the number of within-sampling is not great. Such a strong linear relationship between 18O and 13C values indicates that these isotope values are controlled at least by the same process. Based on the isotope proWles of concretions, the variation of 18O values in a range between ¡9.1‰ and +0.7‰ is mainly attributed to reaction with volcanogenic sediments that alters the mid-Miocene ambient seawater (Lawrence et al., 1975; Morad et al., 1996). The high 13C values of these concretions indicate that the concretions formed within the methanogenic zone (Irwin et al., 1977; Curtis et al., 1986), and the wide variation of 13C values between +3.1‰ and +17.9‰ indicates that the 13C values were inXuenced by the availability of 13C atoms that depend on the degree of methanogenesis controlled by the supply of organic matter. Fig. 4B provides more evidence of the low 18O values attributed to diagenetic alteration by volcaniclastic sediments. The relationship between 18O values and 87Sr/ 86 Sr ratios of concretions shows a good linear trend at lower values. The 87Sr/86Sr ratio of diagenetic dolomite can provide a clue as to whether mid-Miocene seawater retained its original composition in pore-water at the time of concretion formation, or rather altered residual seawater aVected the precipitation of concretions (e.g., Lawrence et al., 1979). In general, the mid-Miocene 87Sr/86Sr ratio of paleoseawater is approximately 0.7086–0.7088. Thus, the dolomitic concretions must provide a similar ratio if the
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Fig. 3. (A) Isotopic variation of 18O and 13C values of concretion Ph4-2 from top to bottom. The height of the concretion analyzed is about 20 cm. (B) Isotopic variation of 18O and 13C values of concretion Ph10 from top to bottom. The height of the concretion analyzed is about 30 cm. The dotted line marks the approximate center of the concretion.
dolomites were precipitated from the ambient mid-Miocene seawater. The relatively lower 87Sr/86Sr ratios seem to indi1cate that the modiWed seawater interacted with nearby volcanogenic sediments and was responsible for the formation of the concretions (Lawrence et al., 1979). Thus, the eVect of interaction between pore-water and volcaniclastic sediments causes the decrease of 18O values and 87Sr/86Sr ratios as found in DSDP (Deep Sea Drilling Project) studies (Lawrence et al., 1975, 1979). 4.3. Two independent depositional processes recorded in dolomitic concretions Carbonate concretions for this study were collected from the topmost sequence (S5 in Fig. 1B) of the Miocene Pohang basin Wlls. This sequence consists of stratiWed conglomerates and sandstones in the lower part, which were deposited in submarine fans and high-gradient deltas and developed after the exhumation of the Doumsan Gilberttype delta (Sohn et al., 2001). The coarse-grained deposits are, in turn, overlain by about 250 m thick siliceous mudstones. These mudstones are composed mainly of opalCT (Garrison et al., 1979), are light and yellowish-green in color, and easily distinguished from the dark brown bentonitic mudstones of the underlying sequences. The mudstone contains abundant biogenic grains such as diatoms, foraminifera and silicoXagellates (Shin, 1981), resembling the porcelanite of the Monterey Formation in California and the diatomaceous sediment in the East Sea, which was
deposited during the middle to late miocene (Ingle, 1992). Mollusk fragments and plant debris are also abundant. Sequence stratigraphic analysis (Sohn et al., 2001) and paleontological studies (Shin, 1981; Kim, 1990; Bak et al., 1996; Kim, 1999) suggest that the siliceous mudstones were deposited during the late Middle Miocene when global sea level was falling (Haq et al., 1988) and the Pohang Basin began to be uplifted because of tectonic inversion (Ingle, 1992; Jolivet et al., 1994). In spite of falling relative sea level during the late Middle Miocene, prevalent hemipelagic to pelagic sedimentation of terrigenous and biogenous materials suggests low suspension fallout by particle settling in the Pohang Basin because of the termination of coarse-grained sediments supplied through alluvial feeder systems at the western basin margin. The siliceous mudstones that accumulated during this stage are thus interpreted to have had high organic matter contents and facilitated the formation of dolomitic concretions under methanogenic conditions during shallow burial. These concretions are characterized by high 13C values and relatively uniform 18O values and 87 Sr/86Sr ratios (Fig. 4). The anomalously low 18O values and 87Sr/86Sr ratios, combined with low 13C values (Fig. 4), can be explained by occasional volcaniclastic-rich sediment gravity Xows into the basin center and subsequent alteration of seawater. A variety of Cretaceous to Tertiary volcanic rocks and volcanogenic sedimentary rocks surround the Pohang Basin, some of which protrude above the Miocene strata as basement highs (Fig. 1A). It is very likely that the volcanogenic materials were
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by mass Xows along with hemipelagic diatomites. In particular, dolomitic concretions in the diatomaceous hemipelagic sediments of the Pohang Basin preserve two distinct isotopic signals reXecting independent sedimentary processes. Remarkably, the co-varied pattern of 18O and 13C values with a strong linear relationship are attributed to distinctly independent depositional processes that operated during the formation of the concretions. Rapid hemipelagic sedimentation of organic-rich particles not only enhanced the degree of methanogenesis, resulting in higher 13C values, but also diluted the eVect of volcaniclastics, thus maintaining the original properties of ambient midMiocene seawater. Conversely, the lower 18O and 87Sr/ 86 Sr values indicate the eVect of volcaniclastic sediments that were transported by intermittent gravity Xows and interacted with the mid-Miocene seawater. The input of volcaniclastic sediment might have lessened the degree of methanogenesis by lowering the organic matter content and thereby decreased the 13C values within the concretions. Therefore, isotopic signals within the concretions reXect the depositional environment in addition to the evolution of pore-water chemistry. In addition, it may be possible to determine the diagenetic condition of formation for concretions based on a simple and unrelated dataset within a large depositional basin. The Miocene Pohang Basin, located in the western rim of the Circum-PaciWc, is characterized by a thick diatomaceous sedimentary sequence containing diverse dolomitic concretions. The counterpart of this siliceous Tertiary deposit is the Middle Miocene Monterey Formation in California, which was an important open marine depositional environment dominated by diatomaceous organic matter. Thus, more diverse approaches to similar conditions in the Miocene Pohang Basin may shed light on the late Neogene paleoceanographic evolution of the East Sea during global cooling. Fig. 4. (A) Biplot of 18O and 13C values of the individual dolomitic concretions. (B) Biplot of 18O and 87Sr/86Sr of the individual dolomitic concretions.
fed intermittently by sediment gravity Xows from around the Pohang Basin, although feeder systems at the western basin margin may have ceased to act as major conduits of sediment supply into the basin. Intermittent occurrence of these gravity Xows transported the volcanogenic sediments towards the basin plain, causing alteration of original seawater in terms of 18O values and 87Sr/86Sr ratios. Thus, the low 18O values and 87Sr/86Sr ratios corresponding to low 13C values during the formation of concretions clearly indicate the eVect of independent depositional processes. 5. Conclusions Carbonate concretions are preserved in sedimentary rocks of the Miocene Pohang Basin in the southwestern East Sea, which consists of fan-delta sediments deposited
Acknowledgements Dr. K.S. Lee (KBSI) was greatly appreciated on his isotopic analyses. This study was conducted by the grant from Korea Research Foundation (KRF-2000-DP0434). Critical review by Dr. Mieke De Craen and Dr. George R. Dix helps improve the data interpretation. References Bak, Y., Lee, J.D., Yun, H., 1996. Middle Miocene radiolarians from the Duho Formation in the Pohang Basin, Korea. Journal of Paleontological Society of Korea 12, 225–261. Choe, M.Y., Chough, S.K., 1988. The Hunghae Formation, SE Korea: Miocene debris aprons in a back-arc intraslope basin. Sedimentology 35, 239–256. Chough, S.K., Hwang, I.G., Choe, M.Y., 1990. The Miocene Doumsan fandelta, southeast Korea: a composite fan-delta system in back-arc margin. Journal of Sedimentary Petrology 60, 445–455. Coleman, M.L., Raiswell, R., 1981. Carbon, oxygen and sulphur isotope variations in concretions from the Upper Lias of NE England. Geochimica et Cosmochimica Acta 45, 329–340.
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Curtis, C.D., Coleman, M.L., Love, L.G., 1986. Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions. Geochimica et Cosmochimica Acta 50, 2321–2334. Garrison, R.E., Mack, L.E., Lee, Y.G., Chun, H.Y., 1979. Petrology, sedimentology and diagenesis of Miocene diatomaceous and opal-CT mudstone in the Pohang Area, Korea. Journal of Geological Society of Korea 15, 230–252. Gautier, D.L., 1982. Siderite concretions: indicators of early diagenesis in the Gammon Shale (Cretaceous). Journal of Sedimentary Petrology 52, 859–871. Gautier, D.L., Claypool, G.E., 1984. Interpretation of methanic diagenesis in ancient sediments by analogy with process in modern diagenetic environments. In: Mcdonald, D.A., Surdam, R.C. (Eds.), Clastic Diagenesis. Am. Ass. Petrol. Geol. Mem. 37, 111–123. Haq, B.U., Hardenbol, J., Vail, P.R., 1988. Mesozoic and Cenozoic chronostratigraphy and eustatic cycles. In: Wilgus, C.K. (Ed.), Sea-Level Changes: An Integrated Approach. SEPM Special Publ. 42, Tulsa, pp. 71–108. Hong, S.W., Chough, S.K., Hwang, I.G., 1998. Provenance of coarsegrained detritus in fan-delta systems, Miocene Pohang Basin, SE Korea: implications for boundary fault movements. Geosciences Journal 2, 46–58. Hwang, I.G., Chough, S.K., 1990. The Miocene Chunbuk Formation, southeastern Korea: marine Gilbert fan-delta system. Spec. Publ. Inter. Ass. Sed. 10, 235–254. Hwang, I.G., Chough, S.K., Hong, S.W., Choe, M.Y., 1995. Controls and evolution of fan delta systems in the Miocene Pohang Basin, SE Korea. Sedimentary Geology 98, 147–179. Ingle, J., 1992. Subsidence of the Japan Sea: stratigraphic evidence from ODP sites and onshore sections. In: Tamaki, K., Suyehiro, K., Allan, J., McWilliams, M. (Eds.), Proc. ODP Sci. Res. 127/128, Washington, DC, pp. 1197–1218. Irwin, H., 1980. Early diagenetic carbonate precipitation and pore Xuid migration in the Kimmeridge of Dorset, England. Sedimentology 27, 577–591. Irwin, H., Curtis, C., Coleman, M., 1977. Isotopic evidence for the source of diagenetic carbonate during burial of organic-rich sediments. Nature 269, 209–213. Jolivet, L., Tamaki, K., Fournier, M., 1994. Japan Sea, opening history and mechanism: a synthesis. Journal of Geophysical Research 99, 22237–22259. Kastner, M., Baker, P.A., 1983. A New Insight into the Origin of Sedimentary Dolomites. 1982/1983 Yearbook of Science and Technology. McGraw Hill. pp. 408. Kelts, K., McKenzie, J., 1982. A comparison of anoxic dolomite from deep-sea sediments: Quaternary Gulf of California and Messinian Tripoli Formation of Sicily. In: Garrison, R.E., Kastner, M., Zenger, D.H. (Eds.), Dolomites of the Monterery Formation and other organic-rich units. SEPM PaciWc Sect. Spec. Publ. 41, pp. 19–28. Kim, W.H., 1990. SigniWcance of early to middle Miocene planktonic foraminiferal biostratigraphy of the E-core in the Pohang Basin, Korea. Journal of Paleontological Society of Korea 6, 144–164. Kim, J.M., 1999. Early Neogene biochemostratigraphy of Pohang Basin: a paleoceanographic response to the early opening of the Sea of Japan (East Sea). Marine Micropaleontology 36, 269–290. Lawrence, J.R., Gieskes, J.M., Broecker, W.S., 1975. Oxygen isotope and cation composition of DSDP pore water and the alteration of Layer II basalts. Earth and Planetary Science Letters 27, 1–10.
Lawrence, J.R., Drever, J.I., Anderson, T.F., Bruecker, H.K., 1979. Importance of alteration of volcanic material in the sediments of Deep Sea Drilling Site 323: chemistry, 18O/16O and 87Sr/86Sr. Geochimica et Cosmochimica Acta 43, 573–588. Matsumoto, R., 1989. Isotopically heavy oxygen containing siderite derived from the decomposition of methane hydrate. Geology 17, 707–710. McArthur, J.M., Benmore, R.A., Coleman, M.L., Soldi, C., Yeh, H.W., O’Brien, G.W., 1986. Stable isotopic characterization of francolite formation. Earth and Planetary Science Letters 77, 20–34. Morad, S., De Ros, L.F., Al-Aasm, I.S., 1996. Origin of low 18O, precompactional ferroan carbonates in the marine Sto Formation (Middle Jurassic), oVshore NW Norway. Marine Petroleum Geology 13, 263–276. Mozely, P.S., Burns, S.J., 1993. Oxygen and carbon isotopic composition of marine carbonate concretions: an overview. Journal of Sedimentary Petrology 63, 73–83. Mozley, P.S., 1996. The internal structure of carbonate concretions in mudrocks: a critical evaluation of the conventional concentric model of concretion growth. Sedimentary Geology 103, 85–91. Nelson, C.S., Lawrence, M.F., 1984. Methane-derived high-Mg calcite submarine cement in Holocene nodules from the Fraser Delta, British Columbia, Canada. Sedimentology 31, 645–654. Noh, J.H., Woo, K.S., 1997. Occurrence, mineral facies, and formation of carbonate concretion from the Yeonil Group. Journal of Geological Society of Korea 33, 210–219. Raiswell, R., Fisher, Q.J., 2000. Mudrock-hosted carbonate concretions: a review of growth mechanisms and their inXuence on chemical and isotopic composition. Journal of Geological Society of London 157, 239–251. Raiswell, R., Bottrell, S.H., Dean, S.P., Marshall, J.D., Carr, A., HatWeld, D., 2002. Isotopic constraints on growth conditions of multiphase calcite-pyrite-barite concretions in Carboniferous mudstones. Sedimentology 49, 237–254. Scotchman, I.C., 1991. The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England. Sedimentology 38, 79–106. Shin, I.C., 1981. Foraminiferal studies on the Neogene Tertiary in the Pohang Basin, Korea. M.S. Thesis, Seoul National University, 76 pp. Sohn, Y.K., Son, M., 2004. Synrift stratigraphic geometry in a transfer zone coarse-grained delta complex, Miocene Pohang Basin, SE Korea. Sedimentology 51, 1387–1408. Sohn, Y.K., Rhee, C.W., Shon, H., 2001. Revised stratigraphy and reinterpretation of the Miocene Pohang basinWll, SE Korea: sequence development in response to tectonism and eustasy in a back-arc basin margin. Sedimentary Geology 143, 265–285. Son, M., Kim, I.S., Sohn, Y.K., 2005. Evolution of the Miocene Waup Basin, SE Korea, in response to dextral shear along the southwestern margin of the East Sea (Sea of Japan). Journal of Asian Earth Sciences,
Distinct sedimentary processes reXected in the isotopic signatures of dolomitic concretions in the Miocene Pohang Basin (southwestern East Sea) Boo-Keun Khim a, Kyung Sik Woo b,¤, Young Kwan Sohn c a
Division of Earth Enviromental System, Pusan National University, Busan 609-735, Republic of Korea b Department of Geology, Kangwon National University, Chuncheon 200-701, Republic of Korea c Department of Earth and Environmental Sciences, Gyeongsang National University, Jinju 660-701, Republic of Korea Received 31 January 2005; received in revised form 4 October 2005; accepted 16 December 2005
Abstract Dolomitic concretions in diatomaceous hemipelagic sediments of the Miocene Pohang Basin in the southwestern East Sea (Sea of Japan) preserve distinct signals of two independent sedimentary processes, which controlled the extents of isotopic compositions. Variable 18O (¡9.1‰ to +0.7‰) and high 13C (+3.1‰ to +17.9‰) values suggest that the concretions formed in the methanogenic zone with alteration of the residual mid-Miocene seawater by volcanogenic sediments. Remarkable 18O and 13C values show a strong linear relationship, indicating that distinctly independent depositional processes operated during the formation of the concretions. The degree of methanogenesis was enhanced during rapid hemipelagic sedimentation of organic-rich particles, resulting in higher 13C values, and the eVect of volcaniclastics was diluted, maintaining the original properties of ambient mid-Miocene seawater. In contrast, lower 18O and 87 Sr/86Sr values characterize the eVect of volcaniclastic sediments that were transported by intermittent gravity Xows and interacted with mid-Miocene seawater. The input of volcaniclastic sediment probably degraded the role of methanogenesis by lowering the contents of organic matter and thereby decreased the 13C values within the concretions. Isotopic signals recorded within the concretions highlight understanding of the depositional environment and evolution of the pore-water chemistry. © 2006 Elsevier Ltd. All rights reserved. Keywords: Carbonate concretion; Stable isotope; Strontium isotope; Depositional processes; Miocene; Korea
1. Introduction The geochemistry of carbonate concretions in sedimentary deposits has provided a direct and principal signature for tracing diagenetic evolution of pore-water chemistry within sediments during shallow burial. Numerous studies have shown that the stable isotopic compositions of carbonate concretions can be utilized to interpret the diagenetic environments and their origins (e.g., Irwin et al., 1977; Irwin, 1980; Gautier and Claypool, 1984; Curtis et al., 1986), including the ion pathways for carbonate cements,
*
Corresponding author. Fax: +82 33 244 8556. E-mail address: [email protected] (K.S. Woo).
1367-9120/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2005.12.007
microbial activity, and ambient temperature of formation. Carbonate concretions usually show core-to-margin chemical changes (Irwin, 1980; Gautier, 1982; Curtis et al., 1986), such that a radial chemical change from the concretion center to the margin is explained in terms of progressive (or temporal) variations in pore-water composition (Mozley, 1996; Raiswell and Fisher, 2000; Raiswell et al., 2002). Although some carbonate concretions have 18O values compatible with precipitation at shallow burial depths from unaltered seawater (i.e., 18O values almost identical to that of seawater at the time of precipitation), most concretions have 18O values far from those of seawater (Lawrence et al., 1975; Coleman and Raiswell, 1981; Scotchman, 1991; Mozely and Burns, 1993). For example, the most 18Oenriched dolomites from the Miocene Tripoli Formation
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(Italy) are attributed to intense evaporative condition (Kelts and McKenzie, 1982), and the anomalous 18 O-enrichment of deep-sea carbonates is attributed to destabilization of gas hydrates (Matsumoto, 1989). In contrast, several explanations have been proposed for the low 18O values of carbonate concretions in marine siliciclastic deposits (Morad et al., 1996), including (1) the eVect of meteoric water; (2) the oxidation of organic matter in the sulphate-reduction zone; (3) the interaction between seawater and sediments, particularly with reactive volcanic materials; (4) the upwelling of hot Xuids to near-surface; (5) the recrystallization or replacement of early carbonate cements during deep burial; and (6) carbonate precipitation during progressive burial as a result of the increasing temperature with depth. Pore-waters in marine siliciclastic sediments are altered systematically, resulting in changes in carbon isotopic compositions due to varying pathways of bacterially mediated decomposition of organic matter (Gautier and Claypool, 1984; Nelson and Lawrence, 1984; Curtis et al., 1986; Mozely and Burns, 1993). The extent of 12C incorporation into carbonate depends on the amount and reactivity of the organic matter, the depth of the suboxic zone beneath the sediment-seawater interface, and the degree of bioturbation. The 13C signatures of concretions formed in the sulfatereduction zone are inXuenced by dissolved carbon derived from the oxidation of organic matter, resulting in the negative values far from those of seawater (McArthur et al., 1986). In contrast, microbial methanogenesis in marine sediments causes the progressive enrichment of 13C in residual pore-waters, resulting in positive 13C values in carbonate concretions (Kelts and McKenzie, 1982; Gautier and Claypool, 1984; Nelson and Lawrence, 1984). Irwin et al. (1977) suggested a burial model to interpret diVerent diagenetic environments of concretion growth with unique 13C values based on the source of carbon for carbonate concretions. This study presents a peculiar case of isotopic variation in dolomitic concretions in the Miocene Pohang Basin in the southwestern East Sea (Sea of Japan; Fig. 1) that shows a co-varied pattern of 18O and 13C values within concretions. The data suggest that the isotopic compositions of the concretions were inXuenced by two independent sedimentary processes, i.e., the continuous hemipelagic input of organic-rich particles and the intermittent input of volcaniclastic sediments by mass Xows. Our results emphasize the utilization of isotopic geochemistry of concretions to help interpret the character of depositional processes and trace the evolution of ambient pore-water chemistry. 2. Geologic setting The Miocene Pohang Basin (Fig. 1A) in the southwestern East Sea is one of several Tertiary basins along the southeastern coast of Korea that were produced in association with back-arc spreading of the East Sea mostly during the Early Miocene (»23–17 Ma; Ingle, 1992; Jolivet et al., 1994). Non-marine to shallow marine sedimentation
occurred together with extensive volcanism during this period (Son et al., 2005). At about 17 Ma, the Pohang area began to subside rapidly, from inner neritic (<50 m deep) to middle-to-lower bathyal depths (>500 m deep; Ingle, 1992; Kim, 1999). Several fan deltas began to form along the western basin margin (Hwang et al., 1995; Sohn et al., 2001; Sohn and Son, 2004). As a result, the sedimentary succession, named the Yeonil Group, exhibits the intercalation of deltaic sediments and hemipelagic mudstones (Choe and Chough, 1988; Chough et al., 1990; Hwang and Chough, 1990; Hwang et al., 1995), is more than 1 km thick, and contains abundant fossil faunas of late Early to Middle Miocene age. Palaeontologic ages based on molluscs, foraminifers, nannofossils, and dinoXagellate cysts are generally between 17 and 11 Ma (Kim, 1990; Yi and Yun, 1995; Bak et al., 1996). The lower part of the Yeonil Group was deposited in a non-marine and shallow-water environment (Yoon, 1975). In contrast, the benthic foraminiferal assemblage indicates that the upper part of group was deposited in upper to lower bathyal environment (Shin, 1981). According to the fan-delta system, the Pohang Basin was composed of four discrete depositional environments, including fan delta, prodelta, slope apron, and basin plain. Recent sedimentological studies (Sohn et al., 2001; Sohn and Son, 2004) reveal that the Pohang Basin Wlls can be divided into several stratal packages or sequences bounded by laterally persistent stratigraphic and/or structural discontinuities (Fig. 1B). These include (1) a non-marine to shallowmarine, early syn-rift sequence (S1 in Fig. 1B) that formed prior to the major subsidence event in the Pohang area at about 17 Ma, (2) the marine Gilbert-type delta sequence (S2 in Fig. 1B), which represents a rift climax in the Pohang Basin, and (3) the overlying deep-marine strata, which represent post-rift sequences that accumulated under the inXuence of eustatic sea-level changes during the Middle Miocene (S3–S5 in Fig. 1B). Carbonate concretions in the Pohang Basin occur either in the early syn-rift, shallow-marine deposits (mostly sandstones) or in the post-rift deposits (mostly hemipelagic mudstones). These concretions, composed of calcite, dolomite, or a combination of the two, have variable sizes and morphologies (spheroidal, elliptical to irregular). The post-rift deposits, in which the carbonate concretions discussed in this paper are contained, consist of alternating conglomerates/sandstones and mudstones, suggestive of continuous hemipelagic sedimentation occasionally interrupted by sediment gravity Xows during the formation of the concretions. The clastic sediments were derived from the Cretaceous to Tertiary basement rocks, which consist of abundant volcanic rocks (dacite, rhyolite, and tuV) as well as sedimentary and plutonic rocks (Fig. 1A; Hong et al., 1998). The mudstone composition also changes from dark brown bentonitic mudstones in the lower part to siliceous or diatomaceous mudstones in the upper part of the post-rift sequences (Noh and Woo, 1997).
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Fig. 1. (A) Geologic map of the Pohang Basin and adjacent area modiWed after Sohn and Son (2004). The basement rocks around the Pohang Basin consist dominantly of Cretaceous to Tertiary volcanic rocks and volcanogenic sedimentary rocks. (B) Enlarged map of the study area in the Pohang Basin, showing the sampling locations of the carbonate concretions. The Pohang basin Wll is dominated by hemipelagic mudstones, dipping gently eastward, except for the fan-delta conglomerates along the western basin margin. Characteristics of the basin Wlls, modiWed after Sohn et al. (2001), are summarized in the inset box.
3. Materials and methods Carbonate concretions were collected mostly from the central parts of the Pohang Basin where thick sequences
of hemipelagic mudstones (siliceous mudstones or diatomites) crop out (Fig. 1B). The mudstones here are mostly composed of diatoms and other siliceous organisms (Noh and Woo, 1997) and belong to the topmost sequence of
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Sohn et al. (2001) (S5; Fig. 1B). The mineralogy of the concretions determined by X-ray diVractometer (XRD; Rigaku D/max-2200A) is dolomite. For isotope analyses, carbonate powders were collected at a discrete interval from the slabs using a JENSEN micromill, tracking an approximate cross-proWle from the top to bottom of the concretions. The stable oxygen and carbon isotope composition was measured by an isotope ratio mass spectrometer (VG Prism II). The analytical error of carbon and oxygen isotopes is §0.1‰. All isotopic values are referred relative to the PDB standard. The analysis of 87Sr/86Sr ratio was done using a Thermal Ionization Mass Spectrometer (VG Sector 5430 multi-collector mass spectrometer). Measured 87Sr/86Sr values were normalized internally against 86Sr/88Sr D 0.1194. The average of the measured 87Sr/86Sr ratios was 0.710238 § 0.000017 (n D 19). The error of the measured ratios is §0.00002. All analyses in our study were conducted at the Korea Basic Science Institute. 4. Results and discussion 4.1. Stable isotope variations within dolomitic concretions in Miocene Pohang Basin The dolomitic concretions always occur with their long axes aligned almost parallel to the bedding planes, and all are commonly observed as continuous dolomite beds or sometimes as individual ellipsoid or coalesced ellipsoids within the diatomaceous mudstones (Fig. 2A and B). Most concretions are a few tens of centimeters to one meter in height and from a few tens of centimeters to several tens of meters in length. These concretions are composed of dolomicrite to dolomicrospar, sometimes preserving the diatom relics and other microfossils (Fig. 2C; Noh and Woo, 1997). The stable oxygen and carbon isotope values and strontium isotope ratios of concretions are listed in Table 1. Two examples of specimens Ph4-2 and Ph10, representing the isotopic trend from the central part towards the top and bottom (Fig. 3), show good co-variance between 18O and 13C values within concretions. The degree of variation of 18O and 13C values is diVerent for diVerent sampling spots, but co-variance between 18O and 13C values is obviously characteristic. The isotopic variations show that the top-to-bottom proWle from the central part of the concretions is unusually asymmetric, dissimilar to the generally assumed concentric growth (Mozley, 1996; Raiswell and Fisher, 2000; Raiswell et al., 2002). In spite of the asymmetric pattern of isotopic variations, the concretions might have formed through radial growth. The 18O values of concretions show a wide range of variation from the relatively high 18O values (¡1‰) likely to reXect the unaltered mid-Miocene seawater to relatively depleted 18O values of about ¡6‰ (Fig. 3). Such low 18O values may be due mainly to the alteration of pore-water chemistry through continuous reaction between residual
Fig. 2. (A) Outcrop view of dolomitic concretions preserved in the diatomaceous mudstone (site Ph4-1). (B) Outcrop view of another dolomitic concretion (site Ph5). (C) Thin-section photomicrograph of dolomitic concretion. Note the diatom fragment partially replaced by dolomite.
seawater and volcanogenic sediment during the formation of the concretions (Lawrence et al., 1975; Morad et al., 1996). The 13C values of these dolomitic concretions are very high, reaching more than +9‰ (Fig. 3). Such high 13C values may well indicate that the formation of the concretions occurred within the methanogenic zone (Irwin et al., 1977; Curtis et al., 1986). The methanogenic zone was changed rapidly from the sulfate-reducing zone because of the continuous supply of abundant organic matter in the hemipelagic sedimentary environment (Coleman and Raiswell, 1981; Gautier and Claypool, 1984).
B.-K. Khim et al. / Journal of Asian Earth Sciences 29 (2007) 939–946 Table 1 Summary of oxygen, carbon and strontium isotope data measured from the carbonate concretions Sample code
13C
18O
87
Ph1 Ph2 Ph4-1 (U2-2) Ph4-1 (U2-1) Ph4-1 (U1-5) Ph4-1 (U1-3) Ph4-1 (U1-2) Ph4-1 (U1-1) Ph4-1 (c) Ph4-2 (C-U3) Ph4-2 (C-U2) Ph4-2 (C-U1) Ph4-2 (C) Ph4-2 (C-D1) Ph4-2 (C-D2) Ph4-2 (D1) Ph4-2 (D2-1) Ph4-2 (D2-2) Ph4-2 (D2-3) Ph4-2 (D2-4) Ph4-2 (D2-5) Ph4-2 (D2-6) Ph4-2 (D2S) Ph4-2 (D3) Ph 5 Ph6-1 Ph6-3 Ph7-1 Ph7-2 Ph8 Ph9 Ph10 (U2-5) Ph10 (U2-4) Ph10 (U2-3) Ph10 (U2-2) Ph10 (U2-1) Ph10 (U1-3) Ph10 (U1-2) Ph10 (U1-1) Ph10 (C2) Ph10 (C1) Ph10 (D1-1) Ph10 (D1-2) Ph10 (D1-3) Ph10 (D1-5)
12.4 10.2 3.1 5.8 9.9 10.5 10.7 10.9 10.3 16.1 16.0 15.8 16.3 15.2 14.0 16.2 14.7 15.5 15.5 15.4 15.4 15.6 15.4 14.8 14.9 12.7 17.9 13.0 8.8 8.6 13.6 14.3 14.4 15.1 14.9 15.4 15.5 15.5 16.5 15.9 16.5 12.0 14.2 13.6 9.1
¡4.3 ¡7.8 ¡6.1 ¡7.3 ¡5.6 ¡3.7 ¡5.2 ¡5.2 ¡3.7 ¡2.2 ¡2.2 ¡2.3 ¡1.5 ¡2.4 ¡3.0 ¡2.0 ¡4.1 ¡2.3 ¡2.4 ¡2.3 ¡2.5 ¡2.4 ¡2.4 ¡3.6 ¡3.4 ¡4.7 ¡1.2 ¡3.1 ¡6.1 ¡6.0 ¡3.6 ¡0.8 ¡2.5 ¡0.2 ¡2.1 ¡1.8 ¡0.2 ¡1.7 0.6 ¡1.5 0.7 ¡5.5 ¡2.4 ¡2.7 ¡4.8
ND 0.70805 0.70820 0.70834 ND 0.70868 0.70863 ND 0.70869 ND 0.70882 ND 0.70877 ND ND 0.70869 0.70877 ND ND ND 0.70872 ND ND ND 0.70872 ND 0.70871 ND 0.70867 0.70850 ND 0.70875 0.70871 0.70869 ND ND 0.70874 ND 0.70887 ND 0.70875 0.70872 ND 0.70871 0.70881
Sr/86Sr
The isotopic proWles of 18O and 13C values show the asymmetric pattern from the center towards the top and bottom (Fig. 3). The concretion formation is likely to start from shallow burial depth where the pore-water retains the original seawater 18O values in the methanogenic zone. However, the remarkable pattern of isotopic variation clearly indicates that the diagenetic pore-water chemistry operates in a diVerent way radially, away from the central part of concretions (Raiswell et al., 2002). In other words, although the pore-water chemistry, which controls the precipitation of the concretion, is inXuenced by a variety of factors, the degree of chemical change is not uniform relative to the center of concretions. Such a peculiar trend has not been reported yet from concretions elsewhere. This iso-
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topic trend distinctly suggests that the carbonate precipitation occurred in a completely closed system, from which the ambient pore-water chemistry was reXected in the original geochemical signatures of the concretions (Woo and Khim, 2006). 4.2. Relationship between 13C and 18O values of dolomitic concretions The mineralogy of concretions is occasionally determined by the rate of organic-carbon oxidation. Calcite precipitation sometimes stops before the completion of sulfate-reduction and the initiation of methanogenesis (Kastner and Bake1r, 1983; Gautier and Claypool, 1984; Nelson and Lawrence, 1984). In general, precipitation of dolomite begins at the time when the sulfate ions in seawater are depleted by bacteria in the sulfate-reducing zone. Thus, the amount of organic matter provided into the diagenetic zone may be a major factor in determining the state of precipitation favorable for calcite or dolomite. In the central part of the Pohang Basin, the widespread diatomaceous mudstones with high contents of organic matter are enough to cause dolomite precipitation under methanogenic conditions (Hwang et al., 1995; Sohn et al., 2001). A remarkable feature of all dolomitic concretions in the basin plain of the Pohang Basin is a strong linear relationship between 18O and 13C values (Fig. 4A). The 18O and 13C values seem to be widely scattered. However, individual concretion shows an eVective linear relationship. In particular, the slopes of the regression are likely to limit two types, although the number of within-sampling is not great. Such a strong linear relationship between 18O and 13C values indicates that these isotope values are controlled at least by the same process. Based on the isotope proWles of concretions, the variation of 18O values in a range between ¡9.1‰ and +0.7‰ is mainly attributed to reaction with volcanogenic sediments that alters the mid-Miocene ambient seawater (Lawrence et al., 1975; Morad et al., 1996). The high 13C values of these concretions indicate that the concretions formed within the methanogenic zone (Irwin et al., 1977; Curtis et al., 1986), and the wide variation of 13C values between +3.1‰ and +17.9‰ indicates that the 13C values were inXuenced by the availability of 13C atoms that depend on the degree of methanogenesis controlled by the supply of organic matter. Fig. 4B provides more evidence of the low 18O values attributed to diagenetic alteration by volcaniclastic sediments. The relationship between 18O values and 87Sr/ 86 Sr ratios of concretions shows a good linear trend at lower values. The 87Sr/86Sr ratio of diagenetic dolomite can provide a clue as to whether mid-Miocene seawater retained its original composition in pore-water at the time of concretion formation, or rather altered residual seawater aVected the precipitation of concretions (e.g., Lawrence et al., 1979). In general, the mid-Miocene 87Sr/86Sr ratio of paleoseawater is approximately 0.7086–0.7088. Thus, the dolomitic concretions must provide a similar ratio if the
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Fig. 3. (A) Isotopic variation of 18O and 13C values of concretion Ph4-2 from top to bottom. The height of the concretion analyzed is about 20 cm. (B) Isotopic variation of 18O and 13C values of concretion Ph10 from top to bottom. The height of the concretion analyzed is about 30 cm. The dotted line marks the approximate center of the concretion.
dolomites were precipitated from the ambient mid-Miocene seawater. The relatively lower 87Sr/86Sr ratios seem to indi1cate that the modiWed seawater interacted with nearby volcanogenic sediments and was responsible for the formation of the concretions (Lawrence et al., 1979). Thus, the eVect of interaction between pore-water and volcaniclastic sediments causes the decrease of 18O values and 87Sr/86Sr ratios as found in DSDP (Deep Sea Drilling Project) studies (Lawrence et al., 1975, 1979). 4.3. Two independent depositional processes recorded in dolomitic concretions Carbonate concretions for this study were collected from the topmost sequence (S5 in Fig. 1B) of the Miocene Pohang basin Wlls. This sequence consists of stratiWed conglomerates and sandstones in the lower part, which were deposited in submarine fans and high-gradient deltas and developed after the exhumation of the Doumsan Gilberttype delta (Sohn et al., 2001). The coarse-grained deposits are, in turn, overlain by about 250 m thick siliceous mudstones. These mudstones are composed mainly of opalCT (Garrison et al., 1979), are light and yellowish-green in color, and easily distinguished from the dark brown bentonitic mudstones of the underlying sequences. The mudstone contains abundant biogenic grains such as diatoms, foraminifera and silicoXagellates (Shin, 1981), resembling the porcelanite of the Monterey Formation in California and the diatomaceous sediment in the East Sea, which was
deposited during the middle to late miocene (Ingle, 1992). Mollusk fragments and plant debris are also abundant. Sequence stratigraphic analysis (Sohn et al., 2001) and paleontological studies (Shin, 1981; Kim, 1990; Bak et al., 1996; Kim, 1999) suggest that the siliceous mudstones were deposited during the late Middle Miocene when global sea level was falling (Haq et al., 1988) and the Pohang Basin began to be uplifted because of tectonic inversion (Ingle, 1992; Jolivet et al., 1994). In spite of falling relative sea level during the late Middle Miocene, prevalent hemipelagic to pelagic sedimentation of terrigenous and biogenous materials suggests low suspension fallout by particle settling in the Pohang Basin because of the termination of coarse-grained sediments supplied through alluvial feeder systems at the western basin margin. The siliceous mudstones that accumulated during this stage are thus interpreted to have had high organic matter contents and facilitated the formation of dolomitic concretions under methanogenic conditions during shallow burial. These concretions are characterized by high 13C values and relatively uniform 18O values and 87 Sr/86Sr ratios (Fig. 4). The anomalously low 18O values and 87Sr/86Sr ratios, combined with low 13C values (Fig. 4), can be explained by occasional volcaniclastic-rich sediment gravity Xows into the basin center and subsequent alteration of seawater. A variety of Cretaceous to Tertiary volcanic rocks and volcanogenic sedimentary rocks surround the Pohang Basin, some of which protrude above the Miocene strata as basement highs (Fig. 1A). It is very likely that the volcanogenic materials were
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by mass Xows along with hemipelagic diatomites. In particular, dolomitic concretions in the diatomaceous hemipelagic sediments of the Pohang Basin preserve two distinct isotopic signals reXecting independent sedimentary processes. Remarkably, the co-varied pattern of 18O and 13C values with a strong linear relationship are attributed to distinctly independent depositional processes that operated during the formation of the concretions. Rapid hemipelagic sedimentation of organic-rich particles not only enhanced the degree of methanogenesis, resulting in higher 13C values, but also diluted the eVect of volcaniclastics, thus maintaining the original properties of ambient midMiocene seawater. Conversely, the lower 18O and 87Sr/ 86 Sr values indicate the eVect of volcaniclastic sediments that were transported by intermittent gravity Xows and interacted with the mid-Miocene seawater. The input of volcaniclastic sediment might have lessened the degree of methanogenesis by lowering the organic matter content and thereby decreased the 13C values within the concretions. Therefore, isotopic signals within the concretions reXect the depositional environment in addition to the evolution of pore-water chemistry. In addition, it may be possible to determine the diagenetic condition of formation for concretions based on a simple and unrelated dataset within a large depositional basin. The Miocene Pohang Basin, located in the western rim of the Circum-PaciWc, is characterized by a thick diatomaceous sedimentary sequence containing diverse dolomitic concretions. The counterpart of this siliceous Tertiary deposit is the Middle Miocene Monterey Formation in California, which was an important open marine depositional environment dominated by diatomaceous organic matter. Thus, more diverse approaches to similar conditions in the Miocene Pohang Basin may shed light on the late Neogene paleoceanographic evolution of the East Sea during global cooling. Fig. 4. (A) Biplot of 18O and 13C values of the individual dolomitic concretions. (B) Biplot of 18O and 87Sr/86Sr of the individual dolomitic concretions.
fed intermittently by sediment gravity Xows from around the Pohang Basin, although feeder systems at the western basin margin may have ceased to act as major conduits of sediment supply into the basin. Intermittent occurrence of these gravity Xows transported the volcanogenic sediments towards the basin plain, causing alteration of original seawater in terms of 18O values and 87Sr/86Sr ratios. Thus, the low 18O values and 87Sr/86Sr ratios corresponding to low 13C values during the formation of concretions clearly indicate the eVect of independent depositional processes. 5. Conclusions Carbonate concretions are preserved in sedimentary rocks of the Miocene Pohang Basin in the southwestern East Sea, which consists of fan-delta sediments deposited
Acknowledgements Dr. K.S. Lee (KBSI) was greatly appreciated on his isotopic analyses. This study was conducted by the grant from Korea Research Foundation (KRF-2000-DP0434). Critical review by Dr. Mieke De Craen and Dr. George R. Dix helps improve the data interpretation. References Bak, Y., Lee, J.D., Yun, H., 1996. Middle Miocene radiolarians from the Duho Formation in the Pohang Basin, Korea. Journal of Paleontological Society of Korea 12, 225–261. Choe, M.Y., Chough, S.K., 1988. The Hunghae Formation, SE Korea: Miocene debris aprons in a back-arc intraslope basin. Sedimentology 35, 239–256. Chough, S.K., Hwang, I.G., Choe, M.Y., 1990. The Miocene Doumsan fandelta, southeast Korea: a composite fan-delta system in back-arc margin. Journal of Sedimentary Petrology 60, 445–455. Coleman, M.L., Raiswell, R., 1981. Carbon, oxygen and sulphur isotope variations in concretions from the Upper Lias of NE England. Geochimica et Cosmochimica Acta 45, 329–340.
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