Short Articles
Bull. Chem. Soc. Jpn., 77, 1029–1030 (2004) 1029
The most useful amino acid for paleo-temperature studies is
aspartic acid, because its racemization rate is the fastest of any
stable amino acid. Becaucse the diagenetic process may prog-
ress with depth; the D/L ratios were determined and used as in-
dicators of the extent of organic matter alteration. Of particular
interest is the racemization reaction of aspartic acid, which can
be written as
Evidence of the Hypsithermal
Verified Using the Racemization
Rate Constant of Amino Acids:
An Estimation of Paleo-Ground
Temperatures
5
k1
L-aspartic acid ꢀ D-aspartic acid
ð1Þ
k2
where k1 and k2 are first-order rate constants for the intercon-
version of L- and D-enantiomers of aspartic acid. The first-order
rate constant for the interconversion of aspartic acid (kASP) D-
and L-enantiomers can be calculated from the following equa-
tion,
Yoshinori Takano, Junya Kudo,1
ꢀ
1
1
Takeo Kaneko, Kensei Kobayashi,
2
Yoji Ishikawa, and Katsumi Marumo
Institute for Marine Resources and Environment,
National Institute of Advanced Industrial Science and
Technology (AIST), AIST Central 7, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8567
ln½ð1 þ D=LÞ=ð1 ꢁ D=LÞꢂ ꢁ ln½ð1 þ D=LÞ=ð1 ꢁ D=LÞꢂ
t
t¼0
¼
2 kASP t
ð2Þ
ꢃ ꢃ
where t is the age in years, and D/L is the enantiomeric ratio of
aspartic acid in the sediment. ln½ð1 þ D=LÞ=ð1 ꢁ D=LÞꢂt¼0 is the
initial value of the reaction. When age relationships can be es-
tablished on a firm independent basis, the D/L ratios can be a
guide to the paleo-temperature. Bada and McDonald (1995)
calculated the preservation possibility of biomolecular chirality
1Department of Chemistry and Biotechnology,
Yokohama National University, 79-5 Tokiwadai,
Hodogaya-ku, Yokohama 240-8501
2Bio-Environmental Engineering Department,
Technical Research Institute, Obayashi Corporation,
6
of amino acids in simulated past biota. In the racemization re-
4
-640 Shimokiyoto, Kiyose-shi, Tokyo 204-8558
action, the rate constant (kASP) maintained for various ground
temperatures was estimated in a simulation of wet sedimentary
organics. The minimum temperature at which the ‘‘wet’’ condi-
tions could exist was assumed to be 252 K (ꢁ21 C). Assuming
Received November 21, 2003; E-mail: takano.yoshinori@aist.
go.jp
ꢄ
an accurate calculation, the relationship between the rate con-
stant (kASP) and the ground temperature profile was established.
In the present study the paleo-ground temperatures were de-
duced from the aspartic acid racemization reaction for a terres-
trial sediment core during the mid-Holocene period, collected
at Rikubetsu, Hokkaido, Japan. The present report provides ge-
ochemical evidence of a past warmer period, called ‘‘the Hyp-
sithermal’’.7
The kinetics of a racemization reaction for aspartic acid
in boreal terrestrial sediment core samples was investigated in
detail to determine the rate constant (kASP) and the difference
of the paleo-ground temperatures. The Arrhenius plot be-
tween the rate constant (kASP) and the literature data of corre-
sponding temperatures is defined as
Log kASP ¼ 16:1 ꢁ 5820=T
Core samples of boreal terrestrial sediments from depths of
0–200 cm were analyzed. The drilling site was located at
3 28 0 N, 143 44 5 E, near Rikubetsu, which is one of
5
4
where T is the absolute temperature (K). From the beginning
of the Holocene, estimated annual ground temperatures in-
creased to reach the warmest period, after which a slight de-
crease was observed. The present findings are geologically
consistent with the global transgression period called ‘‘the
Hypsithermal’’.
ꢄ
0
00
ꢄ
0 00
the coldest cities in Japan, and is located near the center of
8
Hokkaido. In addition, the site is situated in a slight marshy
area that is seasonally frozen to a depth of 80 cm, and is covered
with ice during the winter. The altitude of the drilling site was
ꢄ
9
2
ment core samples were analyzed for age using C radiocarbon
07 m, and the annual average temperature is 5.8 C. The sedi-
1
4
8
Only the L-form of amino acids are usually found in the pro-
teins of living organisms (except for glycine, which does not
have a D- or L-form). However, over long periods of geological
time, the L-amino acids undergo slow racemization, producing
dating by an accelerator mass spectrometric system (AMS).
The pretreatment10 of the core samples and the separation of
D- and L-aspartic acid enantiomers were carried out as de-
8
scribed in preliminary report. The determination of D- and L-
1
the D-form of amino acids. The racemization reaction depends
on time and temperature; thus, if one variable is known, the re-
aspartic acid was achieved using an RP-HPLC system, which
was composed of high-performance liquid chromatograph
pumps (TOSOH CCPM II), a reversed-phase column (YMC-
pack Pro C18 4.6 mm i.d. ꢅ 250 mm), a pre-column derivati-
zation system with OPA and N-AcCys, and a TOSOH UV
8020 detector.
2
action can be used to calculate the other. Pioneering studies
have shown that the racemization reaction of amino acids can
be used to estimate the paleo-temperatures. In the present
2
study, amino acid studies of the apparent rate constants, pa-
leo-thermometry models, paleo-ground temperature models,
and chronological implications for the Holocene period were
systematically reviewed.3
An Arrhenius plot of the Log kASP versus 1=T (K 1) for tem-
ꢁ
6
peratures, based on literature data, is shown in Fig. 1. The data
were fitted by the least squares method to give:
,4
Published on the web May 6, 2004; DOI 10.1246/bcsj.77.1029