Full text of DOI:10.1007/BF02907077

Vol. 44 No. 10
SCIENCE IN CHINA (Series D)
October 2001
An abrupt cooling event early in the last interglacial in the
northern South China Sea
TU Xia (涂 霞)1, ZHENG Fan (郑 范)1, WANG Jiliang (王吉良)2, CAI Huimei
(蔡慧梅)1, WANG Pinxian (汪品先)2, C. Bühring3 & M. Sarnthein3
1. The South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China;
2. Key Laboratory of Marine Geology, Ministry of Education, Tongji University, Shanghai 200092, China;
3. Institut für Geowissenschaften, Universität Kiel, Germany
Correspondence should be addressed to Zheng Fan (email: zfh@scsio.ac.cn)
Received May 28, 2001
Abstract The high-resolution quantitative analysis of the planktonic foraminifera and the δ18O re-
cords of the section between 96.49—137.6 mcd at ODP Site 1144 on the continental slope of
northern South China Sea reveals an abrupt cooling event of sea surface temperature (SST) dur-
ing the last interglacial (MIS 5.5, i.e. 5e). The dropping range of the winter SST may come to 7.5℃
corresponding to 1.2‰ of the δ18O value of sea surface water. This event is comparable with those
discovered in the west Europe and the northern Atlantic Ocean, but expressed in a more intensive
way. It is inferred that this event may have been induced by middle- to low-latitude processes
rather than by polar ice sheet change. Since the Kuroshio-index species Pulleniatina obliquilocu-
lata displayed the most distinct change at the event, it may also be related to the paleocean-
ographic change of the low-latitude area in the western Pacific Ocean. This event can be consid-
ered as one of “Younger Dryas-style coolings” and is indicative of climate variability of the last in-
terglacial stage.
Keywords: northern South China Sea, last interglacial period, sudden cooling event, MIS 5.5, climate variability.
From one of the ice cores of the Greenland ice sheet at the site GRIP, quick climate changes
were first discovered during marine isotope stage (MIS) 5e in the last interglacial period, but were
not found at the site GISP[1]. It has become an important problem for paleoclimate studies on
whether the climate was stable during the last interglacial period. Its importance should not be
underestimated since the climate record during the former interglacial period is helpful for
forecasting the future climate of the current interglacial. There are only limited number of sections
that can be used for correlation since a high-resolution stratigraphic record is required. The
deep-sea sediments have particular relevance to this study, an example is a sudden cooling event
lasting less than four hundred years during MIS 5e recently discovered at ODP Site 658[2] from
the northern Atlantic Ocean. It is essential to the evaluation of the stability of climate during
interglacial period to find out whether similar events are also recorded in other oceans. The
sediment high sedimentation rate sequences taken from the South China Sea (SCS) during the
ODP Leg 184 have provided ideal research material for tracing the sudden change of climate at
stage 5e. The present paper addresses the climate stability during interglacial periods in the north-

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ern SCS on the basis of deep-sea records with high-resolution stratigraphy.
Material and methods
1
The samples used for this study were collected from the section between 96.49—137.6 mcd
of Site 1144 (20°3.18′N, 117°25.14′E) on the northern continental slope of the SCS under 2037
m of water depth. The time span ranges from MIS 6 to the middle of MIS 5. A total of 345 sam-
ples was taken at a 10 cm interval in most cases, corresponding to a time resolution of about 160
years in average. All the samples were processed following the standard method of micropaleon-
tology. After quartering the residue, planktonic foraminifera were identified and counted from the
coarse fraction over 154 µm. From each sample 300—500 tests of planktonic foraminifera were
identified and counted, and the census data were processed using transfer function for paleocean-
ography. Furthermore, the planktonic foraminiferal species Globigerinoides ruber from the upper
section above 122 mcd were analyzed for oxygen and carbon isotopes at the Isotope Laboratory of
the Kiel University, Germany.
2
Results and discussion
2.1 Stratigraphy
Site ODP 1144 provides ideal material for studies on abrupt climatic changes during the last
interglacial because of its highest sedimentation rate known up-to-now in the SCS and the rich and
well-preserved foraminiferal fauna. The holes with 519 mcd of penetration have reached only
about 1 million years in record[3]. According to the studies in the Indo-Pacific region by Thomp-
son et al.[4], the last occurrence of pink-pigmented G. ruber corresponds to MIS 5e and is dated to
120 ka. At ODP Site 1144, pink-pigmented G. ruber occurs in samples below 110.33 mcd, making
up 10%—15% and sometimes up to 30%—40% in abundance of this species. Therefore, the depth
of 110.33 mcd is assigned to MIS 5.5. The δ18O of planktonic foraminifera G. ruber is now under
analysis. Although only the upper part above 121.73 mcd had been measured when this paper was
prepared, it is already clear that the analyzed upper section has reached MIS 5c (i.e. MIS 5.3), and
the boundary between MIS 5 and MIS 6 is located at the depth of 113.5 mcd. The δ18O below the
boundary increases obviously and reaches its maximum at the depth of 120 mcd, corresponding to
MIS 6.2 (fig. 1). According to the δ18O chronology by Martinson et al.[5], the ages of MIS 5.3 and
MIS 6.2 are 96 ka and 135 ka respectively. Therefore, the age of the bottom of the section selected
for this study is about 150 kaBP.
2.2 Paleotemperature record of surface sea water
The results of planktonic foraminiferal analysis show that the abundances of tropical and
subtropical warm-water species, such as Globigerinoides sacculifer, Globorotalia menardii, and
Pulleniatina obliquiloculata, are relatively high during MIS 5, whereas those of cool and temper-
ate water species, such as Neogloboquadrina pachyderma, Globigerina bulloides and Globoro-

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talia inflata are relatively low. All the species above change to an opposite direction during MIS 6
(fig. 2), indicating the features of SST variations in glacial/interglacial cycles.
Fig. 1. Downcore variations of SST based on transfer function, oxygen isotope, G. sacculifer/G. ruber ratio and G. menardii/G.
inflata ratio from MIS 5.3 to MIS 6 at ODP Site 1144, northern South China Sea (Shaded bar indicates the abrupt cooling event
within MIS 5.5).
The winter and summer paleo-SST has been estimated using paleoecology transfer function
FP-12E[6] and planktonic foraminiferal census data from Site 1144. The resulting curves fit well
with the δ18O record, as well as the ratios of G. menardii/G. inflata and G. sacculifer/G. ruber. As
seen from fig. 1, the paleo-SST changes are mainly observed in winter time, with the ranges of
SST changes from 18—24℃ in winter and 27—29℃ in summer during MIS 6, but 17—27℃
and 27—30℃ respectively during MIS 5. The SST changes between MIS 5 and 6 are estimated
as 7℃ for winter and 2℃ for summer. The SST variations, especially in winter, far exceed those
in the southern part of the SCS, and even greater comparing with those in the western Pacific
Ocean[7].
As indicated in figs. 1 and 2, abrupt changes with different amplitudes are observed not only
in the δ18O curve of planktonic foraminifera, but also in percentage of planktonic foraminifera and
paleotemperature curves. It is of interest to note the frequent SST variations occurring during the
glacial MIS 6, but variations with enhanced amplitudes occur in MIS 5. For example, the SST
during MIS 5.4 was as low as in the glacial period, and the difference with the warmest value
during MIS 5.3 may come to 6℃.

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Fig. 2. Downcore variations of some planktonic foraminiferal species from MIS 5.3 to MIS 6 at ODP Site 1144, northern South
China Sea (Shaded bar indicates the abrupt cooling event within MIS 5.5).
2.3 The cooling event within MIS 5.5
The oxygen isotopic curve of MIS 5.5 shows an obvious double-peak feature where δ18O is
as light as −2.5‰, separated by valley with δ18O of −1.25‰. The early peak is located at 112 mcd
and the later one at 110 mcd. The δ18O of the early peak is slightly lighter than the later one, the
base of which just corresponds to the last appearance of Globigerinoides ruber (pink) dated to
approximately 120 ka BP (late MIS 5.51). According to SPECMAP oxygen isotopic stratigraphy[5],
the two peaks are assigned to MIS 5.53 and MIS 5.51, respectively (fig. 1), thus the older peak to
123.5 ka. As seen from fig. 1, the relative abundances of the tropical warm water species of
planktonic foraminifera P. obliquiloculata and G. menardii in MIS 5.53 are higher than those in
MIS 5.51, whereas the percentages of cool water species G. inflata and G. bulloides are lower in
MIS 5.53 than in 5.51. Moreover, the downcore variations in G. sacculifer/G. ruber ratio display
the same double-peak feature, and again a higher value in MIS 5.53 than in 5.51, implying a
higher SST at 5.53. All this agrees well with oxygen isotopes at MIS 5.53, but is at odds with the
records in the open ocean where MIS 5.51 experienced the highest warming within the last inter-
glacial, and is also not supported by oxygen isotope records from other sites within the SCS. A
δ18O peak at MIS 5.51 clearly lighter than at 5.53 is observed at the nearby ODP Site 1145
(19°35′N, 117°37′E; personal communication by K. McIntyre, 2001), and at Cores 17954
(14°48′N, 111°32′E) and 17955 (14°07′N, 112°11′E)1)[8]. It indicates that the main part of
1) Heilig, Stephanie, Paläo-Ozeanographie vor Vietnam im Wandel von Glazial zu Interglazial, Diplomarbeit, Universität
zu Kiel, 1996, 57 S.

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MIS 5.51 is missing at ODP Site 1144, and a hiatus of several thousand years occurs just below
the (truncated) upper peak in δ18O curve, resulting in the conspicuous absence of the warmest time
interval. The extreme values of MIS 5.53 may also present a sort of climatic overshoot phenome-
non along with glacial termination II.
When the δ 18O and planktonic foraminiferal records are compared with the transfer function
paleo-SST curves, an abrupt cooling event within MIS 5.5 becomes striking (fig. 1). The δ 18O
values increased and the content of the cool-warm water species G. inflata came to the highest in
the period between the two peaks, i.e. stage 5.52 (fig. 2), with the winter SST ranging from 19 to
20℃ in contrast to 25—26.5℃ at MIS 5.51 and 5.53. In other words, MIS 5.52 witnessed a dras-
tic cooling about 7.5℃ in winter and about 1.5℃ in summer, and the surface water δ 18O be-
comes 1.2‰ heavier. The range of variation is almost comparable with that of glacial / interglacial
cycles (fig. 1).
Actually, the MIS 5.52 cooling event was reported many years ago, such as in the alkenon
paleo-SST and benthic oxygen isotope records from ODP Site 658 (20°45′N, 18°35′E), northern
Atlantic Ocean[2, 9,10]. The event was ascribed to a Younger Dryas-style cooling which occurred at
each termination during the last 650000 years[11]. Similar coolings within MIS 5.5 have been dis-
covered in many other areas, such as the sudden drop event of pollen percentage of non-arboreal
in the Maar lake of central France [12]. The finding from ODP Site 1144 in the SCS is in accor-
dance with the records above, though the nature and cause of this event are still unclear. It merits
our attention that the response of δ13C of benthonic foraminifera to this event is far stronger than
that of δ18O in cores at ODP Site 658, and the range of variation in surface water δ 18O at ODP
1144 is also much greater than that recorded by the bottom water δ 18O at ODP 658, probably as a
result of the strong decrease in SST, hence indicating an increase in local sea surface salinity (SSS)
and a decrease in summer precipitation. All this infers that this event was probably brought about
by middle- to low-latitude processes rather than by ice-sheet expansion. Among the species of
planktonic foraminifera, the most distinct change is the sudden drop of P. obliquiloculata abun-
dance from more than 30% to about 10% at the cooling event (fig. 2). Since this is a species Ku-
roshio-index species in the northwest Pacific Ocean[13], the sudden decline of relative abundance
of P. obliquiloculata might indicate a decreasing influence of the Kuroshio in this area[14], result-
ing from a significant change in low-latitudinal paleoceanography, possibly sea-level induced.
3
Conclusions
The results of planktonic foraminiferal analyses in cores from ODP Site 1144 show great
changes in SST and SSS in the northern SCS from MIS 6 to 5, as well as within MIS 5.5. The en-
hanced SST variations and increased seasonality resulted presumably from ocean current driven
by winter monsoon winds, leading to the advection of water masses from temperate zone similar
to that during glacial periods. A cooling event occurred in the stage MIS 5.52 during the last inter-

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glacial period, when SST suddenly dropped about 7℃ and δ 18O increased by 1.2‰. This cooling
is comparable with Younger Dryas-style event recorded in the Atlantic Ocean and elsewhere, but
is more intensively expressed in the northern SCS[15,16]. This event may have possibly been in-
duced by middle- to low-latitude processes rather than by ice sheet changes, and provides one
more evidence for climate variability during the last interglacial.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No.
4999560). Dr. Jian Zhimin is acknowledged for his assistance in completing the final version of the manuscript.
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