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Thermal Stability of Amino Acids in Biogenic Sediments and Aqueous Solutions at Seafloor Hydrothermal Temperatures

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Thermal Stability of Amino Acids in Biogenic Sediments and Aqueous Solutions at Seafloor Hydrothermal Temperatures Geochemical Journal, Vol. 43, pp. 331 to 341, 2009 Thermal stability of amino acids in biogenic sediments and aqueous solutions at seafloor hydrothermal temperatures MIHO ITO,1* KYOKO YAMAOKA,2 HARUE MASUDA,1 HODAKA KAWAHATA1 and LALLAN P. G UPTA3 1Department of Geosciences, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan 2Graduate School of Frontier Sciences and Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164-8639, Japan 3Kochi Institute for Core Sample Research (JAMSTEC), B200 Monobe, Nankoku, Kochi 783-8502, Japan (Received December 3, 2008; Accepted March 16, 2009) Siliceous ooze was reacted with NaCl solution at temperatures of 100–250°C to evaluate the effect of the minera- logical and chemical properties of host sediments on the thermal stability of amino acids (AAs). Results were compared with those previously reported from calcareous ooze. The siliceous ooze preserved more AAs than did the calcareous ooze, and the solution with the siliceous ooze preserved the AAs for a longer time than did the solution with the calcareous ooze. When siliceous ooze was reacted under hydrothermal conditions, the AAs were released easily and were more stable in alkaline solution than in the NaCl solution. Solubility of silica was greater in alkaline solution than in NaCl solution, and an excess of hydroxyl ion and/or carbonate species in the alkaline solution underwent exchange more frequently with AAs in the siliceous ooze than in the NaCl solution. The thermal stability of neutral AAs was enhanced significantly in alkaline solution at temperatures higher than 200°C. When montmorillonite and saponite were heated in NaCl solution with a known amount of AAs at 250°C, some AA concentrations increased, probably due to negatively charged AAs binding to cations in the clay minerals. The results suggest that the AAs are dissolved into the solution from the sediments primary via ion exchange, and that polymerization of silica that included AAs in its framework stabilized AAs in siliceous ooze. Keywords: hydrothermal system, siliceous ooze, NaCl solution, clay mineral, seafloor Previous hydrothermal experiments examining the sta- INTRODUCTION bility and synthesis of pure AAs have yielded contradic- The seafloor hydrothermal system is a region where tory results about the decomposition and synthesis of AAs life may have originated and evolved (Holm and at temperatures greater than 100°C (reviewed by Holm Andersson, 1995). Collecting samples without contami- and Andersson, 2005). According to Yanagawa and nation from seafloor hydrothermal venting fields located Kobayashi (1992), AAs could be synthesized after 1.5 several thousands meters below sea level is difficult, hours in hydrothermal solution that contained therefore laboratory-based hydrothermal experiments are Fe(NH4)2(SO4)2, MnCl2, ZnCl2, CuCl2, CaCl2, BaCl2, and ° performed to simulate ideal hydrothermal conditions with- NH4C at 325 C and 200 atm under methane and nitrogen out concern about contamination. However, since amino gas. Oligomers of glycine were formed when glycine so- acids (AAs) are fundamental organic compounds neces- lution was heated at 200–350°C for 2 minutes in a sary for life, controversy has surrounded laboratory-based supercritical water flow reactor (Islam et al., 2003). Ac- hydrothermal experiments simulating biological activity cording to Cox and Seward (2007a, b), aspartic acid, under hydrothermal conditions (Bernhardt et al., 1984; alanine and glycine in aqueous solutions exhibited highly White, 1984; Miller and Bada, 1988; Qian et al., 1993; complex reactions, such as decarboxylation, polymeriza- Marshall, 1994; Bada et al., 1994; Andersson and Holm, tion and cyclization, when the solutions were heated at 2000). 120–170°C in a custom-built spectrophotometric reaction cell. In contrast, aqueous AAs were decomposed at tem- peratures greater than 240°C (Bada et al., 1994), and AAs were unstable under seafloor hydrothermal conditions *Corresponding author (e-mail: [email protected]) based on hydrothermal experiments at high temperature *Present address: Osaka Training Center, Advantec Co., Ltd., 303 Saito and pressure (250°C and 265 bar; Bernhardt et al., 1984; Bio Hills Center, 7-7-18 Saito Asagi, Ibaraki, Osaka 567-0085, Japan. Miller and Bada, 1988). Terashima (1991) demonstrated Copyright © 2009 by The Geochemical Society of Japan. that decomposition of acidic AAs (aspartic and glutamic 331 acids) in carbonates and muddy sediments obeyed first- assemblages and solution chemistry on the thermal sta- order kinetics at 180 and 220°C, and that the rate at 220°C bility of AAs in seafloor hydrothermal systems. was three-fold greater than that at 180°C. Qian et al. (1993) showed that the decomposition rates of glycine, MATERIALS AND METHODS alanine, and glutamic acid obeyed first-order kinetics in aqueous solutions at 100–220°C, and that those increased Materials exponentially at increasing temperatures. The starting material was siliceous ooze sampled from The main sources of AAs in present seafloor sediments the deepest part (43.5–44.7 mbsf) of a sediment core are thought to be calcareous and siliceous biogenic de- (MD01-2409-30) collected offshore Shimokita (41°N, bris, from which AAs decompose during early diagenesis 141°E) during the IMAGES (International Marine Glo- (Henrichs and Farrington, 1987; Kawahata and Ishizuka, bal Change Study) cruise in 2001. The ooze was freeze- 1993). For example, biogenic opal and calcium carbon- dried and manually ground to fine powder using an agate ates occupied 85% of the mass of sinking materials col- pestle and mortar. X-ray diffractometry (Rigaku lected in sediment traps at 1000 m in the southern part of Geigerflex RAD-IA) revealed that the most abundant the Western Pacific ocean along 170°W (Honjo et al., mineral involved silica minerals (mostly quartz and mi- 2000). The AAs found in seafloor hydrothermal solution nor biogenic opallin silica), with moderate amounts of must have a biological origin based on the D/L ratio calcite and minor amounts of smectite and illite. (Horiuchi et al., 2004). Although the major portion of AAs Montmorillonite (JCSS-3102, Mikawa, Japan) and in natural oceanic sediments are considered to be of bio- synthesized saponite (JCSS-3501, Kunimine Industries genic origin, migration paths and thermal stability of those Co., Ltd.), distributed by the Clay Science Society of Ja- AAs during later diagenesis and hydrothermal alteration pan, were used for the other experiments. The are not well understood. montmorillonite was ground and sieved with a no. 300 In a previous experiment (Ito et al., 2006), the ther- mesh (45-µm sieve pore size) and pre-heated in a muffle mal stability of AAs was examined by reacting calcare- oven at 400°C to remove organic matter while maintain- ous ooze, a common organic matter-rich sediment in the ing mineral structure since this mineral structure changed ocean, with artificial seawater at temperatures between at 600°C. X-ray diffractometry confirmed that the min- 100 and 300°C. The results indicated that the AAs could eral structure was unchanged during the heating, how- not survive nor be synthesized in solution at temperatures ever, the AAs could not be completely decomposed (Fig. greater than 250°C; a temperature of about 150°C was 8). Saponite, synthesized by Kunimine Industries Co., Ltd. optimal for long-term stability of AAs. We also reported and containing a small amount of AAs (Fig. 8), was used that a small amount of AAs remained in the reacted ooze without pretreatment. after heating at temperatures greater than 250°C for 240 hr, suggesting that minor clay minerals could protect AAs Hydrothermal experiments from decomposition by adsorption. Naidja and Huang The siliceous ooze was reacted with 3.5% NaCl solu- (1994) observed that aspartic acid was adsorbed on natu- tion, which contained no detectable amount of AAs, un- ral Ca-montmorillonite surfaces within 2 hours at 25°C der two different experimental conditions as previously and that the d001 spacing increased due to the formation described by Ito et al. (2006). of aspartate-montmorillonite complexes. These complexes In one series of experiments, 5 g of powdered sample were maintained after heating at 170°C for 2 hours. Such was mixed with 150 ml NaCl solution in a titanium ves- studies indicate the importance of clay or other silicate sel (160 cm3). The vessel was closed tightly after flush- minerals to preserve AAs in hydrothermal systems. ing with argon gas, and heated in a mantle heater at 100– In the present study, the effect of host sediment on the 200°C for 240 hr. While maintaining constant tempera- stability of AAs with a biological origin was investigated ture, aliquots of the solution were sampled at predeter- under hydrothermal conditions. First, siliceous ooze was mined time intervals. The samples were filtered through reacted with NaCl solution under the same experimental a polytetrafluoroethylene (PTFE) membrane filter, and conditions as reported by Ito et al. (2006). The experi- stored in a glass ampoule at –20°C until analysis. After mental result examining the effect of solution pH reported completing the experiment, residual sediment in the ves- by Yamaoka et al. (2007) was combined with the results sel was freeze-dried and ground to fine powder. of this study. Second, montmorillonite and saponite were Two experiments at 150°C (150°C-1 and 150°C-2) reacted with NaCl solution containing AAs at 250°C un- were conducted to estimate the reproducibility of AA con- der similar conditions. Montmorillonite and saponite be- centrations, since this temperature was critical for the long to the smectite group, which is one of clay mineral thermal stability of amino acids in solution (Ito et al., groups having large cation exchange capacity (CEC). 2006). Figure 1 shows the amount of dissolved total Results are discussed in terms of the effect of mineral hydrolyzable amino acids (THAA; nmol) after comple- 332 M.
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