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Journal of Oceanography, Vol. 53, pp. 1 to 7. 1997 Age Determination of Marine Sediments in the Western North Pacific by Aspartic Acid Chronology NAOMI HARADA1, NOBUHIKO HANDA2*, TADAMICHI OBA3, HIROMI MATSUOKA4, KATSUNORI KIMOTO5 and MASASHI KUSAKABE 1 1Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237, Japan for Hydrospheric-Atmospheric Sciences, Nagoya Univ., Furo-cho, Chikusa-ku, Nagoya 464-01, Japan 3Graduate School of Environmental Earth Science, Hokkaido Univ., Kita 10, Nishi 5, Kita-ku, Sapporo 060, Japan 4Department of Geology, Faculty of Science, Kochi Univ., 2-5-1 Akebono-cho, Kouchi 950-21, Japan 5Ocean Research Institute, Univ. of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo 164, Japan 2Institute (Received 10 November 1995; in revised form 11 April 1996; accepted 1 May 1996) The ages of fossil planktonic foraminifera, Pulleniatina obliquiloculata, in sediments (core 3bPC) from the western North Pacific were determined by aspartic acid chronology, which uses the racemization reaction rate constant of aspartic acid (kAsp). Aspartic acid racemization-based ages (Asp ages) ranged from 7,600 yrBP at the surface, to 307,000 yrBP at a depth of 352.9 cm in the sediments. This sediment core was also dated by the glacial-interglacial fluctuation of δ 18O chronology, and the ages determined by both chronologies were compared. The ages derived from aspartic acid chronology and δ 18O stratigraphy were more or less consistent, but there appeared to be some differences in age estimates between these two dating methods at some depths within the core. In the core top sediments, the likely cause for the age discrepancy could be the loss of the surface sediment during sampling of the core. At depths of 66.3 and 139 cm within the core, Asp ages indicated reduced sedimentation rates during ca. 60,000–80,000 yrBP and ca. 140,000– 190,000 yrBP. The maximum age differences in both chronologies are 33,000 yr and 46,600 yr during each of these periods. These anomalous reductions in sedimentation rates occurring during these periods could possibly be related to some geological events, such as an increased dissolution effect of the calcium carbonate in the western North Pacific. Another possible reason for these age differences could be the unreliability in δ 18O ages of core 3bPC as they were estimated by δ 18O ages of another core, 3aPC. 1. Introduction On a geological time scale, the time interval between 105 and 106 yrBP is very important in palaeoceanographic studies since it covers the glacial and interglacial periods, during which time the marine environment changed drastically. It is true that precise age determination of geological samples is an absolute prerequisite for explaining the paleoenvironment. Yet there are no reliable direct dating methods that can be applied to marine sediments in the age range from 1 × 105 to 1 × 106 yrBP. Even though the 230Th234 U method can be used for dating this period, the method is not very useful for marine sediment samples as they contain only small concentrations of Th and U, and U concentrations and isotopic composition in the foraminifera do not behave as closed systems (Henderson and O’Nions, 1995). Considerable efforts have therefore been directed at determining the ages of marine sediments by amino acid chronology (e.g., Kvenvolden et al., 1970, 1973; Bada et al., 1970; Bada and Schroeder, 1972). Of several amino acids associated with marine fossils, L-amino acids are exclusively produced by living organisms, except for a certain group of organisms (Meister, 1965; Lee and Bada, 1977). However, after death and burial of the organisms, L-amino acids slowly convert to D-amino acids, until equimolar ratios of the amino acid isomers are attained over the geological time scale. This racemization process proceeds as a first-order kinetic reaction, at a constant rate with time, on condition that the surrounding temperature is kept invariable. As a consequence, the relationship between D/L value and racemization reaction time t can be expressed *Present address: Aichi Prefectural University, 3-28 Takada-cho, Mizuho-ku, Nagoya 467, Japan. 1 Copyright Oceanographic Society of Japan. Keywords: ⋅ Amino acid chronology, ⋅ racemization reaction, ⋅ marine sediment, ⋅ fossil foraminifera, ⋅ radiocarbon. by the following equation (e.g. Bada and Schroeder, 1975; Williams and Smith, 1977; Smith et al., 1978; Belluomini and Delitala, 1988): ln[(1 + D/L)/(1 – D/L)]t – ln[(1 + D/L)/(1 – D/L)]t=0 = 2 kt (1) where k is the rate constant of the racemization reaction. The age of marine sediment t can be calculated on the basis of the observed D/L value of the amino acid using Eq. (1), if k has already been determined. Therefore, it is first necessary to determine values of k using samples whose ages are already known. In this study, we estimated the foraminiferal age in the sediment core 3bPC from the western North Pacific over the past 300,000 yr using two kAsp (k value of aspartic acid) values. These k Asp values were obtained from P. obliquiloculata in the sediment core 2PC from the western equatorial Pacific (Harada and Handa, 1995). The foraminiferal sample was also dated by δ18O chronology. A comparison of Asp ages with δ18O ages indicated that both ages were approximately consistent. However, differences in ages were found at some depths within the sediment core. We report the age difference between these chronologies, with some discussion on the possible reasons for these discrepancies. 2. Materials and Methods 2.1 Foraminiferal sample for aspartic acid analysis A piston core (3bPC), 984 cm long, was collected from the western North Pacific (8°01.07′ N, 139°38.53′ E, 2831 m water depth) during the R/V Hakuho-Maru cruise KH-921 (Fig. 1). The sediment core was cut into 2.5 cm thick sections down to a depth of 353 cm. Sub-samples of about 7 g of wet sediment were suspended in distilled water and sieved with a screen (>250 µm). Bulk foraminiferal samples retained on the screen were then dried at room temperature. From the air dried foraminiferal assemblage, specimens of P. obliquiloculata were separated under an optical microscope. About 30 mg of P. obliquiloculata specimens were broken into small pieces with a mortar and pestle and then cleaned by ultrasonication (10 min) to remove clay particles from the samples. A calculated amount of 2 M HCl, which results in a 10% loss of total foraminiferal weight, was added to the samples in order to eliminate organic matter absorbed on the surface of the foraminifera. 2.2 Aspartic acid analysis Foraminiferal test samples (ca. 20 mg) cleaned with HCl were hydrolyzed with 6 M HCl for 22 hr at 105°C in Teflon coated-cap tubes. Hydrolyzed samples were passed through a cation exchange resin column (Dowex 50W-X8, 50–100 mesh, H+ form) to remove foreign materials. Fig. 1. Sampling location of the piston cores collected during the R/V Hakuho-Maru cruise (KH-90-3, 2PC (䊉) and KH-92-1, 3bPC and 3aPC (䊏)). 2 N. Harada et al. Bulk amino acids eluted from the column were dried with a rotary evaporator in order to reduce the sample volume and were then converted to their N-trifluoloacetyl isopropyl ester derivatives. Aspartic acid enantiomers were analyzed with a Shimadzu GC-7A gas chromotograph equipped with a Chirasil-D-Val fused silica column (0.25 mm I.d. × 25 m long) and FID to determine the D-isomer/Lisomer ratio of respective aspartic acids. Analytical errors, estimated by replicate measurements, were in the range of 2–5%. 2.3 δ18O Oxygen isotopic ratio data was measured using a mass spectrometer on the planktonic foraminifera Globigerinoides sacculifer, which had test sizes ranging from 300 to 355 µm in the core 3aPC (8°00.9′ N, 139°38.4′ E, water depth 2830 m), collected at the same site of core 3bPC. The results are reported in per mil deviations relative to the PDB standard. The measurement accuracy of δ18O was ±0.04‰. 3. Age Determination 3.1 k value of Asp The kAsp values utilized in this study have been obtained from the P. obliquiloculata in the piston core sample (2PC, Fig. 1) collected from the western equatorial Pacific (Harada and Handa, 1995). The two kAsp values were calculated as 0.94 × 10–5 yr–1 (the regression coefficient is significant at 99% confidence level by f-test) for the present to 25,000 yrBP, and 0.99 × 10–6 yr–1 (the regression coefficient is significant at 99% confidence level by f-test) for ages between 25,000 to 330,000 yrBP, respectively. For actual age determinations, either 0.94 × 10–5 yr–1 or 0.99 × 10–6 yr–1 was selected, depending on the D/L value of each sample (the inflection point is at 0.24). The scatter from the regression lines of each ln[(1 + D/L)/(1 – D/L)]t was assessed as a magnitude of the deviation as a percent-wise error. The average error value was calculated as 4.5%. 3.2 δ18O age δ18O values were measured in only one piston core, 3aPC. The δ18O ages of 3aPC sediments were determined by comparing the peaks of δ18O distribution pattern with the typical peaks of the standard δ18O curve (Martinson et al., 1987). We then obtained the δ18O age estimates of 3bPC based on that of 3aPC, using a method that requires a comparison of the magnetic susceptibility both cores. Oldfield (1991) described that the magnetic susceptibility of the marine sediment periodically synchrony with glacialinterglacial changes, such as δ18O variation. In addition, the Fig. 2. Correlation of the δ18 O curve of 3aPC and the magnetic susceptibility curves of 3aPC and 3bPC. Age Determination of Marine Sediments in the Western North Pacific by Aspartic Acid Chronology 3 glacial-interglacial change appearing in magnetic susceptibility profile of marine sediments would be caused by the variations of autochthonous calcium carbonate fluxes and lithogenic material supply during the glacial-interglacial periods (e.g. Nishimura et al., 1993). Thus, a comparison of magnetic susceptibility between 3aPC and 3bPC could be beneficial as a means of determining the δ18O age of 3bPC (Fig. 2). The magnetic susceptibility data utilized in this study were provided by Oba et al. (1993). Fig. 3. Vertical profile of aspartic acid isomers ratio (D/L) in P. obliquiloculata from 3bPC. 4. Results The downcore profile of the D/L value of Asp in P. obliquiloculata from 3bPC is shown in Fig. 3. The D/L value of Asp increased continuously with increasing depth of the sediment; however, a low value of D/L was observed at the 1.3 cm depth. Table 1 lists the Asp ages calculated using the two kAsp values. The Asp ages of the sediment samples ranged from 7,600 yrBP at 1.3 cm depth to 306,800 yrBP at 352.9 cm depth. The sedimentation rate of this core was estimated to be 0.5–1.5 cm/103 yr except for the surface sample. Change of sedimentation rates were conspicuously evident at depth intervals of approximately 60–70 cm and 135–150 cm in the core. The downcore profile of δ18O from core 3aPC and the magnetic susceptibility from cores 3aPC and 3bPC are shown in Fig. 2. Correlative lines indicate the relationship between the timing of the peaks in the δ18O and the magnetic susceptibility curves. These lines are the control points for determining the δ18O ages of 3bPC. A comparison of magnetic susceptibility curves between 3aPC and 3bPC indicates that the core-top data of 3bPC were clearly missing, which is possibly due to the loss of the surface sediments during sampling of the sediment core. Estimated δ18O ages for 3bPC are shown in Table 1. These values ranged from 19,900 yrBP at 1.3 cm depth to 270,000 yrBP at 280.3 cm. Due to lack of comparable δ18O data, Asp age of the sediment sample collected at 352.9 cm was not compared with the δ18O age. Comparisons of the ages determined by the Asp chronology and δ18O variation are shown in Fig. 4 and Table 1. Table 1. Ages calculated by Asp racemization reaction chronology and glacial-interglacial variation of δ18 O. Depth (cm) Asp D/L value Asp age (×10 3 yrBP) δ 18 O age (×10 3 yrBP) Difference* (×10 3 ) 1.3 11.3 21.3 36.3 51.3 66.3 80.4 95.0 109.7 124.3 139.0 202.4 226.8 280.3 352.9 0.140 0.253 0.263 0.278 0.291 0.331 0.328 0.337 0.352 0.363 0.396 0.405 0.413 0.439 0.494 7.6 21.2 32.0 48.3 62.6 107.4 104.0 114.2 131.4 144.1 183.1 193.9 203.6 235.7 306.8 19.9 28.2 36.7 49.2 61.7 74.4 85.9 100.3 124.7 130.9 136.4 194.6 223.3 270.0 — –12.3 –7.0 –4.7 –0.9 0.9 33.0 18.1 13.9 6.7 13.2 46.6 –0.7 –19.7 –34.3 — *Difference = Asp age – δ18O age. 4 N. Harada et al. Fig. 4. Comparison of the ages observed in P. obliquiloculata between the aspartic acid chronology (䊉) and δ18 O (䊊). The hatched areas indicate the glacial periods. The results indicate that the Asp ages are generally identical with those estimated by δ18O variation in the whole sediment core, with some exceptions at 1.3 cm, 66.3 cm, and 139 cm depths. At 1.3 cm depth, the Asp age and δ18O age were calculated as 7,600 yrBP and 19,900 yrBP, respectively, and the age difference being 12,300 yr. At 66.3 cm, Asp age and δ18O age were estimated as 107,400 yrBP and 74,400 yrBP, respectively. The age difference between these two methods was approximately 33,000 yr. Also at 139 cm, Asp age and δ18O age were calculated as 183,100 yrBP and 136,400 yrBP, respectively, and the age difference was 46,600 yr. 5. Discussion 5.1 The surface layer of the core 3bPC The age difference between Asp chronology and δ18O chronology at 1.3 cm was 12,300 yr, which is a remarkably large value for core top sedimentary age. It is possible that the loss of ca. 30 cm length of the surface layers of core 3bPC during sampling could have caused the significant difference in ages dated by Asp chronology and δ18O chronology. Based on the δ18O age of 3aPC, the age of the whole sediment from the surface to a depth of 30 cm in 3bPC was estimated to be ca. 20,000 yr. Thus it is most likely that we took 19,900 yrBP for the age of the core top of 3bPC. On the other hand, Asp chronology gave 7,600 yrBP for the core top age. It is improbable that the entire surface sediment was completely lost during sampling. We believe that some portion of the surface sediment could have been left to give 7,600 yrBP as the Asp age. 5.2 The middle to the bottom layers of core 3bPC In general, almost identical ages were obtained by Asp chronology and δ18O chronology in the sediments from the layers below the surface. However, significant differences in the ages estimated by these two methods were also obvious at the depths of 66.3 and 139 cm. Anomalies in Asp age corresponding to the depths of 66.3 cm and 139 cm were 33,000 during 60,000–80,000 yrBP and 46,600 yr during 140,000–190,000 yrBP, respectively (Fig. 4). It is believed that such anomalies in Asp age are probably due to some geological events which occurred during these time periods. According to Nakatsuka et al. (1995), the coring site of 3bPC was located in an area where upwelling of sea water had occurred due to a cyclic circulation system composed of the North Equatorial Current (NEC), Mindanao Current (MC) and North Equatorial Counter Current (NECC). Such cyclonic current systems exist even at present, and they control the upwelling activity at 3bPC site. Nakatsuka et al. (1995) reported that the center of the upwelling zone located at the boundary of NEC and NECC could have shifted, moving northwards during the glacial and southwards during the interglacial period. These shifts could have resulted in similar shifts in high productivity areas (Nakatsuka et al., 1995). On the basis of the 15N/14N variation recorded in core 3bPC sediments, Nakatsuka et al. (1995) indicated that the upwelling activity at the 3bPC site was higher during the intervening period from the interglacial to the glacial period and during the glacial period as compared to the interglacial period. Pedersen (1983) and Price (1988) also reported that surface primary production increased in the equatorial Pacific during the glacial period. According to Wu et al. (1991), enhanced surface production causes an increase in organic matter flux to the sea floor sediments. It was postulated by Wu et al. (1991) that degradation of organic matter at the sediment-water interface could reduce the pH in this region and increase carbonate dissolution there. Consequently, it can be assumed that Asp age anomalies observed at the depths of ca. 60–70 cm and ca. 135–150 cm were mainly due to the decrease in the sedimentation rate caused by increased calcium carbonate dissolution. This is also supported by Matsuoka et al. (1994) who reported that the dissolution rate of calcium carbonate in 3bPC was high in these periods, as indicated by the extensive decrease in the content of perfect scales of nannofossil, Calcidiscus leptoporus and tests of planktonic foraminifera, Groborotalia menardii. This result suggests that it may be possible to assess the dissolution effect in sediments on the basis of anomalies in Asp ages. Detailed mechanisms, explaining why the δ18O method did not, but Asp chronology could detect the reduction in sedimentation rate during 60,000–80,000 yrBP and 140,000– 190,000 yrBP are rather unclear at this moment. However, it is possible that there would be a difference of sensitivity for detecting the geological event between Asp chronology and δ18O chronology. δ18O chronology could be less sensi- Age Determination of Marine Sediments in the Western North Pacific by Aspartic Acid Chronology 5 tive in detecting a geological event such as dissolution effect of calcium carbonate, in contrast to Asp chronology. A problem possibly exists in our comparison of the magnetic susceptibility and the determination of δ18O for 3aPC and 3bPC. Because δ18O values for 3bPC were not measured, the δ18O ages for 3bPC were indirectly evaluated from the δ18O curve of 3aPC with the comparison of the magnetic susceptibility curves between 3aPC and 3bPC. If the shapes of the magnetic susceptibility variation curves for these two sediment cores were similar, the matching of the peaks in the magnetic susceptibility curves for the two sediment cores could have been effortless, with a small margin of error. It appeared that 3aPC and 3bPC were under the different sedimentation conditions, and the thickness of the sedimentary layer of 3bPC relatively stretched and contracted in the whole core, in contrast with 3aPC. Thus, for 3aPC and 3bPC, it was difficult to match the magnetic susceptibility due to the inconsistency in the pattern of both curves. Furthermore, there were two steps involved in determining δ18O ages of 3bPC. One involved a comparison between the δ18O curve of 3aPC and its magnetic susceptibility. The second step involved comparing the magnetic susceptibility between 3aPC and 3bPC. These steps could have introduced errors in determining the exact δ18 O age, and thus could have been responsible for the large age differences between both chronologies. 6. Conclusions In this study, Asp ages for P. obliquiloculata in 3bPC from the western North Pacific were evaluated by using two kAsp values (Harada and Handa, 1995), and were compared with ages estimated by δ18O variation through the glacialinterglacial changes. Asp ages of P. obliquiloculata ranged from 7,600 yrBP at the surface sediment (1.3 cm depth) to 307,000 yrBP at the bottom sediment (352.9 cm depth). While the ages of both Asp chronology and δ18O variation are in approximate agreement with each other, significant age differences between Asp chronology and δ18O chronology were observed at depths of 1.3 cm, 66.3 cm, and 139 cm. The age difference at the top 1.3 cm depth could have been caused by the loss of the surface sediment during coring, although same amounts of amino acid in the surface sediment could have been left in the sample of 1.3 cm. This indicates that estimation of the real age by δ18O method can be complicated by this factor, especially in the case of the Holocene interval. Asp chronology, in contrast, is one of the most practical dating methods of the sediment since it is not easily affected by geological effects such as erosion, dissolution, and hiatus. At the depths of 66.3 cm and 139 cm, the large age differences in both chronologies were 33,000 yr during 60,000–80,000 yrBP and 46,600 yr during 140,000–190,000 yrBP, respectively. During the age intervals of ca. 60,000– 80,000 yrBP and ca. 140,000–190,000 yrBP, upwelling 6 N. Harada et al. activity was stronger than during other geological periods in areas surrounding 3bPC and this could have resulted in poor preservation of calcium carbonate. These changes in preservation of calcium carbonate were clearly evident from the sedimentation rates calculated from Asp ages, which showed anomalous decreases during these periods. Other problems also exist. The δ18O ages of 3bPC were indirectly determined by correlation with δ18O age of 3aPC. Therefore, it is believed that the mixture of some effects as dissolution of calcium carbonate, and the difficulty of accurate and complete matching of magnetic susceptibility profiles between core 3aPC and core 3bPC, could mainly be responsible for generating serious differences in Asp and δ18O ages. We believe that direct estimations of δ18O in P. obliquiloculata, within 3bPC would provide a much more accurate comparison between the Asp and δ18O ages; and more better agreement between the Asp and δ18O ages could be brought by the direct comparison in both chronologies for core 3bPC. Acknowledgements The authors wish to express their thanks to the captain, crews, and scientists participated in KH-90-3 and KH-92-1 cruises of R/V Hakuho-Maru, Univ. of Tokyo, for sampling of the piston core sediments. We thank Drs. H. Okada and S. Morita for magnetic susceptibility measurements. We also thank Dr. J. 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