Download Age Determination of Marine Sediments in the Western North Pacific

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. I. Goes for his advice and discussion to improved
the manuscript. This study was supported by “Past Global
Changes” in International Geosphere-Biosphere Programme
(IGBP) of Japan.
References
Bada, J. L. and R. A. Schroeder (1972): Racemization of isoleucine in calcareous marine sediments: Kinetics and mechanism.
Earth Planet. Sci. Lett., 15, 1–11.
Bada, J. L. and R. A. Schroeder (1975): Amino acid racemization
reactions and their geochemical implications.
Naturwissenschaften, 62, 71–79.
Bada, J. L., B. P. Luyendyk and J. B. Maynard (1970): Marine
sediments: Dating by the racemization of amino acids. Science,
170, 730–732.
Belluomini, G. and L. Delitala (1988): Amino acid racemization
dating of Quaternary deposits of Central and Southern Italy.
Org. Geochem., 13, 735–740.
Harada, N. and N. Handa (1995): Amino acid chronology in the
fossil planktonic foraminifera, Pulleniatina obliquiloculata
from Pacific Ocean. Geophys. Res. Lett., 22, 2353–2356.
Henderson, G. M. and R. K. O’Nions (1995): 234U/238U ratios in
Quaternary planktonic foraminifera. Geochim. Cosmochim.
Acta, 59, 4685–4694.
Kvenvolden, K. A., E. Peterson and F. S. Brown (1970): Racemization of amino acids in sediments from Saanich Inlet,
British Columbia. Science, 169, 1079–1082.
Kvenvolden, K. A., E. Peterson, J. Wehmiller and P. E. Hare
(1973): Racemization of amino acids in marine sediments
determined by gas chromatography. Geochim. Cosmochim.
Acta, 37, 2215–2225.
Lee, C. and J. L. Bada (1977): Dissolved amino acids in the
equatorial Pacific, the Sargasso Sea, and Biscayne Bay. Limnol.
Oceanogr., 22, 502–510.
Martinson, D. G., N. G. Pisias, J. D. Hays, J. Imbrie, T. C. Moore
and N. J. Shackleton (1987): Age dating and the orbital theory
of the ice ages: Development of a high-resolution 0 to 300,000year chronostratigraphy. Quat. Res., 27, 1–29.
Matsuoka, H., K. Kimoto and S. Yamaguchi (1994): The change
of preservation condition of calcium carbonate in pelagic
sediments from north western Pacific. Monthly “Kaiyo”, 26,
425–428 (in Japanese).
Meister, A. (1965): Biochemistry of Amino Acids, 2nd edition.
Academic Press, N.Y., 1084 pp.
Nakatsuka, T., N. Harada, E. Matsumoto, N. Handa, T. Oba, M.
Ikehara, H. Matsuoka and K. Kimoto (1995): Glacial-interglacial migration of an upwelling field in the western equatorial
Pacific recorded by sediment 15N/14N, Geophys. Res. Lett., 22,
2525–2528.
Nishimura, A., K. Ikehara, N. Ioka and T. Yamazaki (1993):
Sedimentation record of the western Calorine Basin and ocean
circulation. Gekkan “Kaiyo”, 276, 350–355 (in Japanese).
Oba, T., M. Murayama, H. Matsuoka, T. Okamoto, S. Morita, T.
Suzuki and S. Tsukawaki (1993): Preliminary report of the
Hakuho-Maru Cruise KH-92-1, ed. by J. Segawa, Ocean
Research Institute, Univ. of Tokyo, pp. 147–214.
Oldfield, F. (1991): Environmental magnetism—a personal perspective. Quat. Sci. Rev., 10, 73–85.
Pedersen, T. F. (1983): Increased productivity in the eastern
equatorial Pacific during the last glacial maximum (19,000 to
14,000 yr B.P.). Geology, 11, 16–19.
Price, B. A. (1988): Equatorial Pacific sediments: Studies on
amino acid, organic matter, and manganese deposition. Ph.D.
Dissert., Univ. Calif., San Diego, La Jolla, Calif., 364 pp.
Smith, G. G., K. M. Williams and D. M. Wonnacott (1978):
Factors affecting the rate of racemization of amino acids and
their significance to geochronology. J. Org. Chem., 43, 1–5.
Williams, K. M. and G. G. Smith (1977): A critical evaluation of
the application of amino acid racemization to geochronology
and geothermometry. Origins of Life, 8, 91–144.
Wu, G., M. K. Yasuda and W. H. Berger (1991): Late Pleistocene
carbonate stratigraphy on Ontong-Java Plateau in the western
equatorial Pacific. Mar. Geol., 99, 135–150.
Age Determination of Marine Sediments in the Western North Pacific by Aspartic Acid Chronology
7