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Response of upper ocean structure to the initiation of the North Hemisphere glaciation in the South China Sea
Palaeogeography, Palaeoclimatology, Palaeoecology 196 (2003) 305^318 www.elsevier.com/locate/palaeo
Response of upper ocean structure to the initiation of the North Hemisphere glaciation in the South China Sea Baoqi Huang a;b; , Xingrong Cheng b , Zhimin Jian b , Pinxian Wang b a
Department of Geography, College of Environmental Sciences, Peking University, Beijing 100871, PR China b Laboratory of Marine Geology, Tongji University, Shanghai 200092, PR China Received 25 January 2002; accepted 14 April 2003
Abstract Planktonic foraminifer as well as oxygen and carbon isotopic records from Ocean Drilling Project Core 1146, located at the northern continental slope of the South China Sea, were used to study the response of the upper water column structure to the formation and progressive intensification of the Northern Hemisphere glaciation during the Late Pliocene (3.2^2.0 Ma). Variations in the relative abundance of the Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, and Globorotalia inflata groups, Globorotalia crassaformis and Neogloboquadrina pachyderma showed that sea surface temperatures gradually decreased, coinciding with heavier oxygen isotope values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma. After 2.7 Ma, the relative abundance of mixed-layer species decreased, while that of thermocline dwellers, dominated by high productivity species, increased. vN13 C variations among G. sacculifer, Pulleniatina obliquiloculata and Cibicidoides wuellerstorfi suggest that the mixed-layer depth and vertical exchange was enhanced with the strenghtening of the East Asian winter monsoon, which in turn is associated with the progressive intensification of the Northern Hemisphere glaciation. < 2003 Elsevier Science B.V. All rights reserved. Keywords: Northern Hemisphere glaciation; oxygen and carbon isotopes; planktonic foraminifera; South China Sea; upper water column structure
1. Introduction The Late Pliocene (2.0^3.2 million years ago) is an interval of great interest for understanding the Earth’s climate system. It was a time of progressive global cooling, resulting in the initiation of extensive Northern Hemisphere glaciation (Shackleton et al., 1995; Maslin et al., 1996; Hil-
* Corresponding author. Tel.: +86-10-62756814. E-mail address: [email protected] (B. Huang).
gen, 1991), though glaciation is suggested to have begun in the Late Miocene (Spezzaferri, 1998) or the Late Miocene^Early Pliocene (Jansen et al., 1990). Formation and expansion of the glaciation signi¢cantly in£uenced deep ocean circulation (Sikes et al., 1991; Raymo et al., 1992; Ishman, 1996; Tiedemann and Franz, 1997), surface ocean circulation, the structure of the upper water column (Dowsett et al., 1996; Cannariato and Ravelo, 1997; Chaisson and Ravelo, 2000), and atmosphere circulation, such as the East Asian winter monsoon. Intensi¢cation of Northern Hemisphere
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glaciation would produce a strong atmospheric e¡ect (Ding et al., 1992). The general trend of Pliocene climate has been summarized by Raymo et al. (1992). However, variations in upper water column structure, which is a critical part of the thermal and nutrient exchange between the atmosphere and the ocean, is still poorly known for many ocean regions, including the South China Sea (SCS). The SCS is located in the western equatorial Paci¢c between the Western Paci¢c Warm Pool and East Asian landmass. Its surface circulation pattern is driven by the East Asian monsoon, which in turn is formed by seasonal pressure differences between the ocean and the continent. In summer, driven by the southwestern wind, the surface water from the tropical Indian Ocean £ows into the SCS and then northward into the Paci¢c Ocean, mostly through the Bashi Strait. In winter, the northeastern wind drives the tropical and subtropical Paci¢c waters together with the cooler water of coastal currents into the SCS through the Bashi Strait, and then across the Sunda Shelf into the Indian Ocean (Wang et al., 1995). The upper water column structure of the northern SCS exhibits a distinct seasonal character (Liu et al., 2000; Wang et al., 2001). Modern monthly mean mixed-layer depth from the northern slope is about 20^30 m in summer; however, in winter, the mixed-layer depth is about 40^80 m (NOAA, 1994). During the Last Glacial Maximum the SCS changed into a semi-enclosed basin, as a result of s 100 m sea-level drop (Wang, 1990; Wang and Wang, 1990). This led to a pronounced di¡erence in surface circulation that must have changed the upper structure of the water column. Previous paleoceanographic studies of the SCS focused mostly on carbonate cycles (Miao et al., 1994; Wang et al., 1995), sea surface temperature (SST) (Wang and Wang, 1990; Wang et al., 1995; P£aumann and Jian, 1999), deep-water masses (Miao and Thunell, 1996; Jian and Wang, 1997), surface ocean structure (Huang et al., 2002), and surface productivity (Thunell et al., 1992) for the Late Quaternary. Relatively little has been published about Pliocene paleoceanography.
Planktonic foraminifera are the most common marine microfossils used in paleoceanographic reconstructions. Ecological studies of planktonic foraminifera show that di¡erent species dominate di¡erent layers of the upper water column. Therefore, their relative abundance in the past has been used to reconstruct hydrographic conditions of the upper ocean (Be¤, 1977; Hemleben et al., 1988; Ravelo et al., 1990; Chen, 1994; Von Mu«cke and Oberha«nsli, 1999). Recently, the difference between stable isotope values of planktonic foraminiferal species inhabiting the mixed layer and the thermocline was applied to indicate changes in the upper water column structure (Chaisson and Ravelo, 1997; Farrell et al., 1995; Jian et al., 2000). In this paper, we investigate changes in the relative abundance of planktonic foraminifera and their oxygen isotope values for the Late Pliocene, in order to better understand the response of the upper ocean to the formation and progressive intensi¢cation of the Northern Hemisphere glaciation.
2. Materials and methods Ocean Drilling Project (ODP) Site 1146 in the SCS is located on the northern continental slope of the SCS (19‡27.40PN, 116‡16,37PE) (Fig. 1) at a water depth of 2092 m (Wang et al., 2000), above the sill depth of the Bashi Strait and the present lysocline. Three holes were drilled at the site with the deepest one reaching 642.25 meter composite depth (mcd), and having a base age of approximately 19 Ma (Wang et al., 2000). The recovered sediments are mainly hemipelagic muds and silts. In this study, we focus on the 196.3^241.7 mcd interval, with ages between 2.0 and 3.2 Ma, that are composed of greenish gray nannofossil clay. A total of 111 samples were examined, giving a sampling resolution of about 10 ka. Approximately 10 cm3 of sediment samples were disaggregated by soaking in tap water for several days until they were dispersed completely. Then, the samples were wet sieved over a 63-Wm screen and dried in an oven under 60‡C. The coarse fractions ( s 154 Wm) were split into subsamples containing about 250 planktonic forami-
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Fig. 1. The South China Sea and location of ODP Core 1146. White arrow represents the direction of the East Asian monsoon, and the black ones represent surface current directions of the SCS. (A) Winter. (B) Summer.
nifera tests, which were then picked, counted, and identi¢ed following the taxonomies of Kennett and Srinivasan (1983) and Saito et al. (1981). Many extinct Pliocene species, the immediate ancestors of modern species, are assumed to have had similar ecological niches as their descendants (Chaisson and Ravelo, 1997). This is con¢rmed by isotopic measurements that suggest that some extinct species occupied depth habitats similar to those of closely related modern species (Keller, 1985). Therefore, planktonic foraminiferal oxygen isotope (N18 O) values obtained from deep-sea sediments can denote the overlying hydrography (Ravelo and Fairbanks, 1992). Given these relationships, extant and extinct species have been combined in faunal groups to produce continuous faunal records (Dowsett and Poore, 1990). In order to use these Pliocene species appropriately and compare their relative depth habit with the extant species, we selected 18 di¡erent species in two samples (148 mcd, 1.2 Ma; and 200.2 mcd, 2.1 Ma) to analyze their oxygen and carbon stable isotope composition. Each foraminiferal shell diameter was individually measured. Following previous studies (Chaisson and Ravelo, 1997; Keller, 1985; Dowsett and Poore, 1990; Wang, 2001), we group the taxa into ¢ve groups.
i.e. : (1) the Globigerinoides ruber group which includes G. ruber, G. obliquus, and G. extremus; (2) the Globorotalia menardii group which includes G. menardii, G. multicamerata, G. miocenica, and G. limbata; (3) the Globigerinoides sacculifer group which includes G. sacculifer and G. altispira ; (4) the Neogloboquadrina group which includes N. dutertrei, N. humerosa, and N. acostaensis; and (5) the Globorotalia in£ata group which includes G. in£ata and G. crassaformis. The relative (%) abundance of these groups is used to trace the variations in SST, and to reconstruct the upper ocean conditions for the 2.0^ 3.2-Ma time interval in the SCS. Eight to eleven specimens of Globigerinoides sacculifer, without a saclike ¢nal chamber, and Pulleniatina obliquiloculata were picked from the 350^425-Wm size fraction of each sample for isotopic analysis. G. sacculifer and P. obliquiloculata were chosen to represent mixed-layer conditions (106 samples) and the thermocline (55 samples), respectively (Ravelo and Fairbanks, 1992; Jian et al., 2000). Cibicidoides wuellerstor¢ ( s 315 Wm), a benthic foraminiferal species considered to faithfully record bottom water carbon isotope chemistry (Shackleton and Hall, 1984), was analyzed to re£ect bottom water conditions. In samples in
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which C. wuellerstor¢ was absent we selected Uvigerina peregrina or Oridorsalis umbonatus (103 samples). Di¡erences in isotopic composition among the three benthic species were corrected following Shackleton et al. (1995). All isotopic analyses were made at the Laboratory of Marine Geology of Tongji University, using a Finnigan MAT 252 mass spectrometer. Analytical precision was U 0.07x PeeDee Belemnite (PDB) for N18 O and U 0.04x PDB for N13 C, which was regularly checked with a Chinese national carbonate standard (GBW04405) and international standard NBS19. Conversion to the international PDB scale was performed using NBS19 and NBS18 standards.
ratory, Brown University. Biostratigraphic control was given by the last occurrence (LO) of Dentoglobigerina altispira at 239.2 mcd (3.09 Ma; Berggren et al., 1995), and the Pulleniatina S^D coiling change at 266.8 mcd (3.95 Ma; Berggren et al., 1995; Wang et al., 2000). According to these sources, an age model for the upper 273.08 mcd of the core was determined, and the 196.3^241.7-mcd interval was dated in about 2.0^ 3.2 Ma. In addition, a detailed age model for the 196.3^241.7-mcd interval is constructed by correlating the record of N18 OCibicidoides wuellerstorfi of ODP Core 1146 with the N18 O record of ODP Core 846 from the Eastern Equatorial Paci¢c (Shackleton et al., 1995) (Fig. 2).
2.1. Age model 3. Results For the sequence of ODP Core 1146, oxygen isotope data from the uppermost 163 mcd (1.5 Ma) were measured and orbitally tuned by Clemens (2001) in the Benedum Stable Isotope Labo-
3.1. Isotope analyses The vertical distributions of N18 O in di¡erent
Fig. 2. Comparison of oxygen isotopic records of Cibicidoides wuellerstor¢ for ODP Core 1146, collected in the SCS (this study), and ODP Core 846, collected in south of the Galapagos Islands (Shackleton et al., 1995), for the 2.0^3.2-Ma interval. Numbers represent oxygen isotope stages.
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Fig. 3. Oxygen isotopes of selected planktonic foraminiferal species for 1.2 and 2.1 Ma (this study), the Holocene, and 3.2 Ma (Wang, 2001) from ODP Core 1146. Planktonic foraminifera are grouped into three assemblages: mixed-layer, thermocline, and deep-layer species. ‘wo’ and ‘w’ represent Globigerinoides sacculifer, without and with a saclike, respectively.
species show an analogous pattern for the Holocene, 2.1, 1.2 and 3.2 Ma (Fig. 3), suggesting that the relative vertical position of di¡erent planktonic foraminifera species in the water column has been nearly constant since 3.2 Ma, except for G. conglobatus, Globigerina calida, and Globigerina eaquilateralis. N18 O values ranged from 33.5x to 2.5x in 1.2-Ma and 2.1-Ma samples, that appear
as three distinct groups in the water column, separated by two boundaries at 31.3x and 30.2x. Following Dowsett et al. (1996), three groups of planktonic foraminifera are de¢ned: mixed-layer, thermocline, and deep-water species. In the Holocene, Globigerinoides ruber and Globigerinoides sacculifer, and Globorotalia menardii and Neogloboquadrina dutertrei are the main fau-
Table 1 Planktonic foraminifera species list (after Dowsett and Poore, 1990; Wang, 2001, and this study) Mixed-layer species
Thermocline species
Deep-layer species
Globigerinoides ruber Globigerinoides obliquus Globigerinoides extremus Globigerinoides sacculifer Globigerinoides bulloides Globigerinoides ¢stulosus Globigerina rubescens Globigerinita glutinata Dentoglobigerina altispira
Pulleniatina obliquiloculata Globigerina calida Globigerina eaquilateralis Globorotalia menardii Globorotalia limbata Globorotalia miocenica Globorotalia multicamerata Globorotalia margaritae Globorotalia tumida Spheroidina dehiscens Neogloboquadrina dutertrei Neogloboquadrina humerosa Neogloboquadrina acostaensis
Globorotalia Globorotalia Globorotalia Globorotalia
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scitula in£ata crassaformis truncatulinoides
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nal elements of the mixed-layer and thermocline groups, respectively (Ravelo and Fairbanks, 1992). For the Pliocene the mixed-layer assemblage also included Globigerinoides altispira, Globigerinoides obliquus, Globigerinoides extremus, and Globigerinoides ¢stulosus, whereas Neogloboquadrina acostaensis, Neogloboquadrina humerosa, Globorotalia limbata, Globorotalia miocenica, Globorotalia multicamerata, and Globorotalia margaritae were added to the thermocline assemblage. The deep-water group was composed of Globoro-
talia crassaformis, Globorotalia in£ata, Globorotalia scitula, and Globorotalia truncatulinoides (Table 1). Oxygen isotope values for Cibicidoides wuellerstor¢ ranged from 2.3x to 3.6x, steadily increasing from 3.2 to 2.0 Ma, and showing heavy values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma (Fig. 2). The N18 O record of Pulleniatina obliquiloculata gradually increased since 3.2 Ma varying between 30.1x and 31.4x without displaying any abrupt decrease, except at 2.8 Ma (Fig. 4).
Fig. 4. Oxygen and carbon isotopes of Globigerinoides sacculifer, Pulleniatina obliquiloculata and Cibicidoides wuellerstor¢ for ODP Core 1146. Gray bars indicate distinct glacial periods.
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PALAEO 3143 9-7-03 Fig. 5. Oxygen isotope curves of Cibicidoides wuellerstor¢ and relative (%) abundance of Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, and Globorotalia in£ata groups, and Neogloboquadrina pachyderma at ODP Core 1146. Gray bars indicate distinct glacial periods.
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Unlike P. obliquiloculata and C. wuellerstor¢, the N18 O curve of Globigerinoides sacculifer changed from 30.9x to 32.3x, and did not exhibit a trend from 3.2 to 2.0 Ma nor the distinct heavy values at 2.8, 2.72, 2.6, 2.16, and 2.08 Ma, with the exception of 2.5 Ma (Fig. 4). In contrast to oxygen isotopes, changes in the carbon isotopes of Cibicidoides wuellerstor¢, Pulleniatina obliquiloculata and Globigerinoides sacculifer, showed a di¡erent pattern. The N13 C values of G. sacculifer did not show a signi¢cant drop, changing between 1.9x and 0.6x. However, since 3.2 Ma, the N13 C values of C. wuellerstor¢ decreased from 0.4x to 30.7x. Before 2.5 Ma, the N13 C di¡erence between P. obliquiloculata and
C. wuellerstor¢ was small, but after that the di¡erence increased. The N13 C di¡erence between G. sacculifer and P. obliquiloculata was larger before 2.5 Ma, whereas it became smaller after that time (Fig. 4). 3.2. SST changes Globigerinoides sacculifer and Globigerinoides ruber are shallow dwelling tropical^subtropical species within the mixed layer, whereas Globorotalia menardii mainly inhabits the thermocline layer in tropical and subtropical regions (e.g. Fairbanks et al., 1982). Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, Glo-
Fig. 6. Oxygen isotope record of Cibicidoides wuellerstor¢ and relative (%) abundance of high productivity, mixed-layer, and thermocline species and the mixed-layer/thermocline species ratio for ODP Core 1146. Gray bars indicate distinct glacial periods.
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borotalia in£ata, and Neogloboquadrina pachyderma are sensitive to seasonal changes in SST (Chen et al., 2000; P£aumann and Jian, 1999). The percentage abundances of the G. sacculifer and G. ruber groups decreased from nearly 40% to 7% and 52% to 7%, respectively, and that of the G. menardii group decreased from 23% to 2% for the 3.2^2.0-Ma interval (Fig. 5), roughly coinciding with glacial^interglacial £uctuations. Signi¢cant drops in the percentage abundances of the G. ruber, G. sacculifer, and G. menardii groups occurred at 2.5, 2.08 and 2.2 Ma, respectively (Fig. 5). In contrast to the percentage variations of the tropical^subtropical species, the polar^subpolar species Globorotalia in£ata group (including G. crassaformis) and Neogloboquadrina pachyderma display the opposite trend (Fig. 5). Before 2.5 Ma, the percentage abundance of G. in£ata remained below 5%, but increased afterwards to a maximum of 14%. Although the abundance of N. pachyderma remained below 3%, as a polar species it is sensitive to temperature changes in the subtropical regions. After 2.5 Ma, the abundance of N. pachyderma increased, especially at 2.1 Ma (Fig. 5). Variations in the relative abundance of planktonic foraminifera implied a gradual cooling process since 3.2 Ma (Fig. 5). 3.3. Variations in mixed-layer and thermocline dwellers The relative abundance of thermocline dwelling species increased from below 30% to over 50% during the 3.2^2.0-Ma interval (Fig. 6). When Cibicidoides wuellerstor¢ exhibits heavier N18 O values, the abundance of thermocline species is generally higher. In contrast, the abundance of mixed-layer dwellers gradually decreased from nearly 77% to 20%, with drops occurring during glacial periods (Fig. 6). A change in the planktonic foraminiferal assemblage took place at 2.7 Ma. Before 2.7 Ma, mixed-layer species dominated the planktonic assemblage, more than 3 times the abundance of thermocline species. However, after 2.7 Ma, thermocline species increased quickly and the ratio of mixed-layer to thermocline species became close to 1 (Fig. 6).
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4. Discussion 4.1. Variations in upper ocean structure 4.1.1. SST variations Global SST reconstructions for the Middle Pliocene (2.85^3.15 Ma) show that modern conditions in low latitude regions changed little compared to modern times, but SST changed signi¢cantly in the middle and high latitudes of both hemispheres (Dowsett et al., 1996). In southwest Africa, temperatures declined by about 10‡C since 3.2 Ma (Marlow et al., 2000), and in the low latitude western Paci¢c, a Middle Pliocene cool event took place during the 3.1^2.7-Ma interval, with an abrupt and irreversible SST drop at 2.2 Ma (Wang, 1994). In the present study, warm water species, Globigerinoides ruber, Globigerinoides sacculifer, and Globorotalia menardii dominated the planktonic foraminiferal fauna, indicating relatively higher SST before 2.7 Ma. After this time, the percentage abundances of the G. ruber, G. sacculifer and G. menardii groups gradually decreased, whereas the Globorotalia in£ata group and Neogloboquadrina pachyderma increased. Abrupt cooling events coincided with the N18 O records of these species, indicating a close link between SST change in the SCS and the development of global ice sheets in the Northern Hemisphere (Shackleton et al., 1990; Maslin et al., 1998). Maslin et al. (1998) suggested that the long term cooling led to three key steps in the glaciation of the Northern Hemisphere, in ODP Core 1146 variations in oxygen isotope and planktonic foraminiferal assemblages indicate that glaciation was signi¢cant in the SCS at 2.7 Ma. Present day SST in the SCS is controlled by the East Asian monsoon and shows a conspicuous seasonality, with winter SST much lower than the summer value (Chen et al., 1985). During the Last Glacial Maximum, when the winter East Asian monsoon intensi¢ed, SST of the SCS was much lower than present (Wang and Wang, 1990). Therefore, we suggest that the expansion of the Northern Hemisphere glaciation intensi¢ed the East Asian monsoon system, particularly the winter monsoon (Ding et al., 1992), resulting in decreasing SST in the SCS.
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4.1.2. Mixed-layer and thermocline dwelling species variations In the eastern Paci¢c, upwelling induces a shoaling of the depth of the thermocline. When the thermocline shoals in the photic zone, the abundance of thermocline dwelling species increases relative to mixed-layer species (Leckie, 1989; Ravelo et al., 1990; Ravelo and Fairbanks, 1992). After 4.5 Ma, in the eastern Paci¢c (ODP Core 846), mixed-layer species decreased markedly, suggesting a shoaling of the thermocline. However at ODP Core 806, located on the Ontong Java Plateau in the western Paci¢c, the re-
verse was observed (Chaisson and Ravelo, 1997, 2000). Before 2.7 Ma, the relative abundance of mixed-layer dwelling species of ODP Core 1146 far outnumbers the thermocline dwellers, but after 2.7 Ma, the mixed-layer species declined while thermocline dwellers increased. The ratio of mixed-layer species to thermocline species was close to 1, indicating little imbalance between the two assemblages. At ODP Core 1146, no upwelling exists during the modern summer or winter, which would shoal the thermocline and increase the abundance of thermocline species. In contrast, when a stronger East Asian winter mon-
Fig. 7. Oxygen isotope record of Cibicidoides wuellerstor¢ and vN18 O among Globigerinoides sacculifer, Pulleniatina obliquiloculata and C. wuellerstor¢, against time for ODP Core 1146. Numbers indicate glacial stages.
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soon prevails over the SCS, SSTs are much lower than in summer, and the thermocline depth is deeper in winter (Liu et al., 2000). The strong East Asian winter monsoon transports large quantities of aeolian dust from land, thus enriching the surface with nutrients of the northern SCS (Wiesner et al., 1996). This in turn, will favor the blooming of high productivity species, such as Neogloboquadrina dutertrei. The abundance of high productivity species at ODP Core 1146 gradually increased after 2.7 Ma, and changed parallel to the relative abundance of thermocline species (Fig. 6). This suggests that after 2.7 Ma, varia-
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tions in mixed-layer and thermocline species were controlled by nutrients and SST, which in turn were a¡ected by changes in the East Asian monsoon. The N18 O gradient between species living in different layers will be equivalent to the vertical temperature di¡erence in the upper water column (Berger et al., 1978). Globigerinoides sacculifer, Pulleniatina obliquiloculata, Cibicidoides wuellerstor¢ inhabit the mixed-layer, the thermocline and the sea£oor, respectively. Therefore, their isotopic di¡erences imply variations in the vertical ventilation of the upper water column. Changes
Fig. 8. Oxygen isotope record of Cibicidoides wuellerstor¢ and vN13 C among Globigerinoides sacculifer, Pulleniatina obliquiloculata and C. wuellerstor¢. Gray bars indicate distinct glacial periods.
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in vN18 OP C (the gradient between P. obliquiloculata and C. wuellerstor¢) and vN18 OGs C (the gradient between G. sacculifer and C. wuellerstor¢) range from 33.5x to 35.5x, increasing during glacial periods (Fig. 7). The di¡erences indicate that the variations in N18 O values for C. wuellerstor¢ were larger than for G. sacculifer and P. obliquiloculata. The oxygen isotopic di¡erence between the mixed layer and the thermocline (vN18 OGs P ) species was small before 2.2 Ma, but increased after that time. Changes in vN18 OGs P were mostly caused by changes in the N18 O values of Pulleniatina obliquiloculata (Fig. 4). Isotopic data alone are ambiguous as to whether this outcome was the result of subsurface cooling or of a shoaling of the depth of the thermocline. Faunal variations at ODP Site 1146 show that after 2.5 Ma, particularly after 2.16 Ma, polar^ subpolar species increased, whereas tropical^subtropical species decreased. This suggests that changes in the vN18 OGs P values were caused by subsurface cooling. The carbon isotope (N13 C) gradient between different water layers results from the vertical exchange of nutrients in the ocean and the global carbon budget. After 2.8 Ma, vN13 CP C shows a signi¢cant increase, however, the vN13 CGs P values decreased after this time (Fig. 8). These conspicuous changes resulted from the strengthening of East Asian winter monsoon, which in turn supplied more nutrients to a more uniform upper water column due to vertical exchanges.
N18 O values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma. Before 2.7 Ma, the relative abundance of mixed-layer species for outnumbered the thermocline species, but after 2.7 Ma, mixed-layer species declined while thermocline dwellers, dominated by high productivity species, increased. Variations in mixed-layer and thermocline species were not induced by upwelling but by a strengthened East Asian winter monsoon, which decreased SST and enriched surface nutrients in the northern SCS. The vN13 C data among Globigerinoides sacculifer, Pulleniatina obliquiloculata and Cibicidoides wuellerstor¢ suggest that the mixed layer became deeper and vertical exchanges increased with the strengthening of the East Asian winter monsoon being the result of progressive intensi¢cation of the Northern Hemisphere glaciation.
5. Conclusions
References
The N18 O values of planktonic species from samples at 1.2 and 2.2 Ma suggest that little changed in the distribution of the planktonic foraminifera in the upper water column. Following Wang (2001) and Dowsett and Poore (1990), planktonic foraminifera are grouped into mixedlayer, thermocline, and deep-water species for the 3.2^2.0 Ma period. Benthic foraminiferal oxygen isotopes and percentage abundances of planktonic foraminiferal species suggest that SST decreased from 3.2 to 2.0 Ma with conspicuous excursions to heavier
Be¤, A.W.H., 1977. An ecological, zoogeographical and taxonomic review of recent planktonic foraminifera. In: Ramsay, A.T.S. (Ed.), Oceanic Micropaleontology. Academic Press, London, pp. 1^100. Berggren, W.A., Kent, D.V., Swisher, C.C. III, Aubry, M.-P., 1995. A revised Cenozoic geochronology and chronostratigraphy. In: Berggren, W.A., Kent, D.V., Aubry, M.-P., Hardenbol, J. (Eds.), Geochronology: Time Scale and Global Stratigraphic Correlation. Spec. Publ.-Soc. Econ. Paleontol. Miner. (Soc. Sediment, Geol.) 54, 129^212. Berger, W.H., Killingley, J.S., Vincent, E., 1978. Glacial^Holocene transition in deep-sea carbonates: Box Core ERDC92, west equatorial Paci¢c. Oceanol. Acta 1, 203^216. Cannariato, K.G., Ravelo, A.C., 1997. Pliocene^Pleistocene
Acknowledgements We thank the Ocean Drilling Program and the Scienti¢c Party and crew of Leg 184 for excellent samples, Mr. Dingyuan Fang and Ms. Peifen Xia for their assistance in the isotopic analyses, and Dr. Steve Nathan as well as S.J. Kim for improving the English text of this paper and their constructive comments. This work was supported by the National Natural Science Foundation of China (Grants 49999560 and 2000078502).
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B. Huang et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 196 (2003) 305^318 evolution of eastern tropical Paci¢c surface water circulation and thermocline depth. Paleoceanography 12, 805^820. Chaisson, W.P., Ravelo, A.C., 1997. Changes in upper watercolumn structure at Site 925, late Miocene^Pleistocene: Planktonic foraminifer assemblage and isotopic evidence. In: Shackleton, N.J., Curry, W.B., Richter, C., Bradower, T.J. (Eds.), Proc. ODP, Sci. Results 154, pp. 255^268. Chaisson, W.P., Ravelo, A.C., 2000. Pliocene development of the east^west hydrographic gradient in the equatorial Pacific. Paleoceanography 15, 497^505. Chen, M.-T., 1994. Estimating thermocline from planktonic foraminifer faunal data: The development of paleoecological transfer functions for reconstructing low-latitude Paci¢c upper-layer conditions. J. Geol. Soc. China 37, 443^474. Chen, S., Chen, T., Xu, X., Chen, Z., Sui, S., 1985. The Vast South China Sea (in Chinese). China Science Press, Beijing, 218 pp. Chen, R.H., Jian, Z.M., Zheng, Y.L., Cheng, J.F., 2000. Seasonal variations of the planktonic foraminiferal £ux in the Central South China Sea (in Chinese with English abstract). J. Tongji Univ. 28, 73^77. Clemens, S.C., 2001. Isotope stratigraphy and isotope proxy comparison Site: 1146. In: Wang, P., Prell, W.L. (Eds.), ODP Leg 184 Post-Cruise Meeting and Asian Monsoon Symposium, 6.7. Ding, Z.L., Rutter, N.W., Han, J.T., Liu, T.S., 1992. A coupled environmental system formed at about 2.5 Ma over eastern Asia. Palaeogeogr. Palaeoclimatol. Palaeoecol. 94, 223^242. Dowsett, H.J., Poore, R.Z., 1990. A new planktonic foraminifer transfer function for estimating Pliocene^Holocene paleoceanographic conditions in the North Atlantic. Mar. Micropaleontol. 16, 1^23. Dowsett, H., Barron, J., Poore, R., 1996. Middle Pliocene sea surface temperatures: A global reconstruction. Mar. Micropaleontol. 27, 13^27. Fairbanks, R.G., Sverdlove, M., Free, R., Wiebe, P.H., Be¤, A.W.H., 1982. Vertical distribution and isotopic fractionation of living planktonic foraminifera from the Panama Basin. Nature 298, 841^844. Farrell, J.W., Murray, D.W., Mckenna, V.S., Ravelo, A.C., 1995. Upper ocean temperature and nutrient contrasts inferred from Pleistocene planktonic foraminifer N18 O and N13 C in the eastern Equatorial Paci¢c. In: Pisias, N.G., May, L.A., Janecek, T.R., Palmer-Julson, A., van Andel, T.H. (Eds.), Proc. ODP, Sci. Results 138, pp. 289^311. Hemleben, C., Spindler, M., Anderson, O.R., 1988. Modern Planktonic Foraminifera. Springer, New York, 363 pp. Hilgen, F.J., 1991. Extension of the astronomically calibrated (polarity) timescale to the Miocene/Pliocene boundary. Earth Planet. Sci. Lett. 107, 349^368. Huang, B.Q., Jian, Z.M., Cheng, X.R., Wang, P.X., 2002. Foraminiferal responses to upwelling variations in the South China Sea over the last 220 000 years. Mar. Micropaleontol. 47, 1^15Q in the References. Ishman, S.E., 1996. A benthic foraminiferal record of middle
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to late Pliocene (3.15^2.85 Ma) deep water change in the North Atlantic. Mar. Micropaleontol. 27, 165^180. Jansen, E., SjXholm, J., Bleil, U., Erichsen, J.A., 1990. Neogene and Pleistocene glaciations in the Northern hemisphere and late Miocene^Pliocene global ice volume £uctuations: Evidence from the Norwegian Sea. In: Bleil, U., Thiede, J. (Eds.), Geological History of the Polar Oceans: Arctic versus Antarctic, pp. 677^705. Jian, Z., Wang, L., 1997. Late Quaternary benthic foraminifera and deep-water paleoceanography in the South China Sea. Mar. Micropaleontol. 32, 127^154. Jian, Z., Wang, P., Saito, Y., Wang, J., P£aumann, U., Oba, T., Cheng, X., 2000. Holocene variability of the Kuroshio Current in the Okinawa Trough, northwestern Paci¢c Ocean. Earth Planet. Sci. Lett. 184, 305^319. Keller, G., 1985. Depth strati¢cation of planktonic foraminifers in the Miocene ocean. In: Kennett, J.P. (Ed.), The Miocene Ocean: Paleoceanography and Biogeography. Mem. Geol. Soc. Am. 163, pp. 177^196. Kennett, J.P., Srinivasan, M.S., 1983. Neogene Planktonic Foraminifera. Hutchinson Ross Public Company, 244 pp. Leckie, R.M., 1989. An oceanographic model for the early evolutionary history of planktonic foraminifera. Palaeogeogr. Palaeoclimatol. Palaeoecol. 73, 107^138. Liu, Q.Y., Yang, H.J., Wang, Q., 2000. Dynamic characteristics of seasonal thermocline in the deep sea region of the South China Sea. Chin. J. Oceanol. Limnol. 18, 104^109. Marlow, J.R., Lange, C.B., Wefer, G., Rosell-Mele¤, A., 2000. Upwelling intensi¢cation as part of the Pliocene^Pleistocene climate transition. Science 290, 2288^2291. Maslin, M.A., Haug, G.H., Sarnthein, M., Tiedmann, R., 1996. The progressive intensi¢cation of northern hemisphere glaciation as seen from the North Paci¢c. Geol. Rundsch. 85, 452^465. Maslin, M.A., Li, X.S., Louter, M.-F., Berger, A., 1998. The contribution of orbital forcing to the progressive intensi¢cation of Northern Hemisphere glaciation. Quat. Sci. Rev. 17, 411^426. Miao, Q., Thunell, R.C., Anderson, D.M., 1994. Glacial^Holocene carbonate dissolution and sea surface temperatures in the South China and Sulu seas. Paleoceanography 9, 269^ 290. Miao, Q., Thunell, R.C., 1996. Late Pleistocene^Holocene distribution of deep-sea benthic foraminifera in the South China Sea and Sulu Sea: Paleoceanographic implications. J. Foraminif. Res. 26, 9^23. National Oceanic and Atmospheric Administration (NOAA), 1994. World Ocean Atlas 1994. National Oceanographic Data Center, NOAA, Washington, DC (www.nodc.noaa. gov/general/getdata.html). P£aumann, U., Jian, Z., 1999. Modern distribution patterns of planktonic foraminifera in the South China Sea and west Paci¢c: A new transfer technique to estimate regional seasurface temperatures. Mar. Geol. 156, 41^83. Ravelo, A.C., Fairbanks, R.G., Philander, G., 1990. Reconstructing tropical Atlantic hydrography using planktonic fo-
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raminifera and an ocean model. Paleoceanography 5, 409^ 431. Ravelo, A.C., Fairbanks, R.G., 1992. Oxygen isotopic composition of multiple species of planktonic foraminifera: Recorders of the modern photic temperature gradient. Paleoceanography 7, 815^831. Raymo, M.E., Hodell, D., Jansen, E., 1992. Response of deep ocean circulation to initiation of northern hemisphere glaciations (3^2 Ma). Paleoceanography 7, 645^672. Saito, T., Thompson, P., Breger, R., 1981. System Index of Recent and Pleistocene Planktonic Foraminifera. University of Tokyo Press, Tokyo, 199 pp. Shackleton, N.J., Berger, A., Peltier, W.R., 1990. An alternative astronomical calibration of the lower Pleistocene time scale based on ODP Site 677. Trans. R. Soc. Edinb. Earth Sci. 81, 251^261. Shackleton, N.J., Hall, M.A., Pate, D., 1995. Pliocene stable isotope stratigraphy of ODP Site 846. In: Mayer, L., Pisias, N., Janecek, T., et al. (Eds.), Proc. ODP, Sci. Res. 138, pp. 337^355. Shackleton, N.J., Hall, M.A., 1984. Oxygen and carbon isotope stratigraphy of Deep Sea Drilling Project Hole 552 A: Plio^Pleistocene glacial history. In: Roberts, D.G., Backman, J., Morton, A.C., et al. (Eds.), Initial Rep. Deep Sea Drill. Proj. 81, pp. 599^610. Sikes, E.L., Keigwin, L.D., Currey, W.B., 1991. Pliocene paleoceanography: Circulation and oceanography changes associated with the 2.4 Ma glacial event. Paleoceanography 6, 245^257. Spezzaferri, S., 1998. Planktonic foraminifer biostratigraphy and paleoenvironmental implications of Leg 152 Sites (east Greenland margin). In: Saunders, A.D., Larsen, H.C., Wise, S.W. Jr. (Eds.), Proc. ODP, Sci. Res. 152, pp. 161^ 189. Thunell, R.C., Miao, Q., Calvert, S.E., Pedersen, T.F., 1992. Glacial^Holocene biogenic sedimentation patterns in the South China Sea: Productivity variations and surface water CO2 . Paleoceanography 7, 143^162.
Tiedemann, R., Franz, S.O., 1997. Deep-water circulation, chemistry, and terrigenous sediment supply in the Equatorial Atlantic during the Pliocene, 3.3^2.6 Ma and 5^4.5 Ma. In: Shackleton, N.J., Currey, W.B., Richter, C., Bralower, T.J. (Eds.), Proc. ODP, Sci. Res. 154, pp. 299^318. Von Mu«cke, S.K., Oberha«nsli, H., 1999. The distribution of living planktonic foraminifera in relation to Southeast Atlantic oceanography. In: Fischer, G., Wefer, G. (Eds.), Use of Proxies in Paleoceanography: Examples from the South Atlantic. Springer, Berlin, Part 1, pp. 91^115. Wang, D., Du, Y., Shi, P., 2001. Evidence for thermocline ventilation in the South China Sea in winter. Chin. Sci. Bull. 9, 779^782. Wang, J.L., 2001. Planktonic Foraminiferal Assemblages and Paleoceanography During the Last 18 Ma (in Chinese with English abstract). Ph.D. Thesis, Tongji Univ., Shanghai. Wang, P.X., 1990. The Ice-Age China Sea ^ research results and problems. In: Wang, P., Lao, Q., He, Q. (Eds.), Proc. 1st Int. Conf. Asian Mar. Geol. China Ocean Press, Beijing, pp. 181^197. Wang, L., Wang, P., 1990. Late Quaternary paleoceanography of the South China Sea: Glacial^interglacial contrasts in an enclosed basin. Paleoceanography 5, 77^90. Wang, L., 1994. Sea surface temperature history of the low latitude western Paci¢c during the last 5.3 million years. Palaeogeogr. Palaeoclimatol. Palaeoecol. 108, 379^436. Wang, P.X., Prell, W.L., Blum, P., 2000. Proceedings of the Ocean Drilling Program, Init. Rep. 184, CD-ROM Contents Site 1146, pp. 1^82. Wang, P.X., Wang, L.J., Bian, Y.H., Jian, Z.M., 1995. Late Quaternary paleoceanography of the South China Sea: Surface circulation and carbonate cycles. Mar. Geol. 127, 145^ 165. Wiesner, M.G., Zheng, L., Wong, H.K., Wang, Y., Chen, W., 1996. Fluxes of particulate matter in the South China Sea. In: Ittekkot, V., Scho«fer, P., Honjo, S., Depetris, P.J. (Eds.), Particle Flux in the Ocean. Wiley, London, pp. 293^ 312.
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Response of upper ocean structure to the initiation of the North Hemisphere glaciation in the South China Sea Baoqi Huang a;b; , Xingrong Cheng b , Zhimin Jian b , Pinxian Wang b a
Department of Geography, College of Environmental Sciences, Peking University, Beijing 100871, PR China b Laboratory of Marine Geology, Tongji University, Shanghai 200092, PR China Received 25 January 2002; accepted 14 April 2003
Abstract Planktonic foraminifer as well as oxygen and carbon isotopic records from Ocean Drilling Project Core 1146, located at the northern continental slope of the South China Sea, were used to study the response of the upper water column structure to the formation and progressive intensification of the Northern Hemisphere glaciation during the Late Pliocene (3.2^2.0 Ma). Variations in the relative abundance of the Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, and Globorotalia inflata groups, Globorotalia crassaformis and Neogloboquadrina pachyderma showed that sea surface temperatures gradually decreased, coinciding with heavier oxygen isotope values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma. After 2.7 Ma, the relative abundance of mixed-layer species decreased, while that of thermocline dwellers, dominated by high productivity species, increased. vN13 C variations among G. sacculifer, Pulleniatina obliquiloculata and Cibicidoides wuellerstorfi suggest that the mixed-layer depth and vertical exchange was enhanced with the strenghtening of the East Asian winter monsoon, which in turn is associated with the progressive intensification of the Northern Hemisphere glaciation. < 2003 Elsevier Science B.V. All rights reserved. Keywords: Northern Hemisphere glaciation; oxygen and carbon isotopes; planktonic foraminifera; South China Sea; upper water column structure
1. Introduction The Late Pliocene (2.0^3.2 million years ago) is an interval of great interest for understanding the Earth’s climate system. It was a time of progressive global cooling, resulting in the initiation of extensive Northern Hemisphere glaciation (Shackleton et al., 1995; Maslin et al., 1996; Hil-
* Corresponding author. Tel.: +86-10-62756814. E-mail address: [email protected] (B. Huang).
gen, 1991), though glaciation is suggested to have begun in the Late Miocene (Spezzaferri, 1998) or the Late Miocene^Early Pliocene (Jansen et al., 1990). Formation and expansion of the glaciation signi¢cantly in£uenced deep ocean circulation (Sikes et al., 1991; Raymo et al., 1992; Ishman, 1996; Tiedemann and Franz, 1997), surface ocean circulation, the structure of the upper water column (Dowsett et al., 1996; Cannariato and Ravelo, 1997; Chaisson and Ravelo, 2000), and atmosphere circulation, such as the East Asian winter monsoon. Intensi¢cation of Northern Hemisphere
0031-0182 / 03 / $ ^ see front matter < 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0031-0182(03)00458-9
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glaciation would produce a strong atmospheric e¡ect (Ding et al., 1992). The general trend of Pliocene climate has been summarized by Raymo et al. (1992). However, variations in upper water column structure, which is a critical part of the thermal and nutrient exchange between the atmosphere and the ocean, is still poorly known for many ocean regions, including the South China Sea (SCS). The SCS is located in the western equatorial Paci¢c between the Western Paci¢c Warm Pool and East Asian landmass. Its surface circulation pattern is driven by the East Asian monsoon, which in turn is formed by seasonal pressure differences between the ocean and the continent. In summer, driven by the southwestern wind, the surface water from the tropical Indian Ocean £ows into the SCS and then northward into the Paci¢c Ocean, mostly through the Bashi Strait. In winter, the northeastern wind drives the tropical and subtropical Paci¢c waters together with the cooler water of coastal currents into the SCS through the Bashi Strait, and then across the Sunda Shelf into the Indian Ocean (Wang et al., 1995). The upper water column structure of the northern SCS exhibits a distinct seasonal character (Liu et al., 2000; Wang et al., 2001). Modern monthly mean mixed-layer depth from the northern slope is about 20^30 m in summer; however, in winter, the mixed-layer depth is about 40^80 m (NOAA, 1994). During the Last Glacial Maximum the SCS changed into a semi-enclosed basin, as a result of s 100 m sea-level drop (Wang, 1990; Wang and Wang, 1990). This led to a pronounced di¡erence in surface circulation that must have changed the upper structure of the water column. Previous paleoceanographic studies of the SCS focused mostly on carbonate cycles (Miao et al., 1994; Wang et al., 1995), sea surface temperature (SST) (Wang and Wang, 1990; Wang et al., 1995; P£aumann and Jian, 1999), deep-water masses (Miao and Thunell, 1996; Jian and Wang, 1997), surface ocean structure (Huang et al., 2002), and surface productivity (Thunell et al., 1992) for the Late Quaternary. Relatively little has been published about Pliocene paleoceanography.
Planktonic foraminifera are the most common marine microfossils used in paleoceanographic reconstructions. Ecological studies of planktonic foraminifera show that di¡erent species dominate di¡erent layers of the upper water column. Therefore, their relative abundance in the past has been used to reconstruct hydrographic conditions of the upper ocean (Be¤, 1977; Hemleben et al., 1988; Ravelo et al., 1990; Chen, 1994; Von Mu«cke and Oberha«nsli, 1999). Recently, the difference between stable isotope values of planktonic foraminiferal species inhabiting the mixed layer and the thermocline was applied to indicate changes in the upper water column structure (Chaisson and Ravelo, 1997; Farrell et al., 1995; Jian et al., 2000). In this paper, we investigate changes in the relative abundance of planktonic foraminifera and their oxygen isotope values for the Late Pliocene, in order to better understand the response of the upper ocean to the formation and progressive intensi¢cation of the Northern Hemisphere glaciation.
2. Materials and methods Ocean Drilling Project (ODP) Site 1146 in the SCS is located on the northern continental slope of the SCS (19‡27.40PN, 116‡16,37PE) (Fig. 1) at a water depth of 2092 m (Wang et al., 2000), above the sill depth of the Bashi Strait and the present lysocline. Three holes were drilled at the site with the deepest one reaching 642.25 meter composite depth (mcd), and having a base age of approximately 19 Ma (Wang et al., 2000). The recovered sediments are mainly hemipelagic muds and silts. In this study, we focus on the 196.3^241.7 mcd interval, with ages between 2.0 and 3.2 Ma, that are composed of greenish gray nannofossil clay. A total of 111 samples were examined, giving a sampling resolution of about 10 ka. Approximately 10 cm3 of sediment samples were disaggregated by soaking in tap water for several days until they were dispersed completely. Then, the samples were wet sieved over a 63-Wm screen and dried in an oven under 60‡C. The coarse fractions ( s 154 Wm) were split into subsamples containing about 250 planktonic forami-
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307
Fig. 1. The South China Sea and location of ODP Core 1146. White arrow represents the direction of the East Asian monsoon, and the black ones represent surface current directions of the SCS. (A) Winter. (B) Summer.
nifera tests, which were then picked, counted, and identi¢ed following the taxonomies of Kennett and Srinivasan (1983) and Saito et al. (1981). Many extinct Pliocene species, the immediate ancestors of modern species, are assumed to have had similar ecological niches as their descendants (Chaisson and Ravelo, 1997). This is con¢rmed by isotopic measurements that suggest that some extinct species occupied depth habitats similar to those of closely related modern species (Keller, 1985). Therefore, planktonic foraminiferal oxygen isotope (N18 O) values obtained from deep-sea sediments can denote the overlying hydrography (Ravelo and Fairbanks, 1992). Given these relationships, extant and extinct species have been combined in faunal groups to produce continuous faunal records (Dowsett and Poore, 1990). In order to use these Pliocene species appropriately and compare their relative depth habit with the extant species, we selected 18 di¡erent species in two samples (148 mcd, 1.2 Ma; and 200.2 mcd, 2.1 Ma) to analyze their oxygen and carbon stable isotope composition. Each foraminiferal shell diameter was individually measured. Following previous studies (Chaisson and Ravelo, 1997; Keller, 1985; Dowsett and Poore, 1990; Wang, 2001), we group the taxa into ¢ve groups.
i.e. : (1) the Globigerinoides ruber group which includes G. ruber, G. obliquus, and G. extremus; (2) the Globorotalia menardii group which includes G. menardii, G. multicamerata, G. miocenica, and G. limbata; (3) the Globigerinoides sacculifer group which includes G. sacculifer and G. altispira ; (4) the Neogloboquadrina group which includes N. dutertrei, N. humerosa, and N. acostaensis; and (5) the Globorotalia in£ata group which includes G. in£ata and G. crassaformis. The relative (%) abundance of these groups is used to trace the variations in SST, and to reconstruct the upper ocean conditions for the 2.0^ 3.2-Ma time interval in the SCS. Eight to eleven specimens of Globigerinoides sacculifer, without a saclike ¢nal chamber, and Pulleniatina obliquiloculata were picked from the 350^425-Wm size fraction of each sample for isotopic analysis. G. sacculifer and P. obliquiloculata were chosen to represent mixed-layer conditions (106 samples) and the thermocline (55 samples), respectively (Ravelo and Fairbanks, 1992; Jian et al., 2000). Cibicidoides wuellerstor¢ ( s 315 Wm), a benthic foraminiferal species considered to faithfully record bottom water carbon isotope chemistry (Shackleton and Hall, 1984), was analyzed to re£ect bottom water conditions. In samples in
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which C. wuellerstor¢ was absent we selected Uvigerina peregrina or Oridorsalis umbonatus (103 samples). Di¡erences in isotopic composition among the three benthic species were corrected following Shackleton et al. (1995). All isotopic analyses were made at the Laboratory of Marine Geology of Tongji University, using a Finnigan MAT 252 mass spectrometer. Analytical precision was U 0.07x PeeDee Belemnite (PDB) for N18 O and U 0.04x PDB for N13 C, which was regularly checked with a Chinese national carbonate standard (GBW04405) and international standard NBS19. Conversion to the international PDB scale was performed using NBS19 and NBS18 standards.
ratory, Brown University. Biostratigraphic control was given by the last occurrence (LO) of Dentoglobigerina altispira at 239.2 mcd (3.09 Ma; Berggren et al., 1995), and the Pulleniatina S^D coiling change at 266.8 mcd (3.95 Ma; Berggren et al., 1995; Wang et al., 2000). According to these sources, an age model for the upper 273.08 mcd of the core was determined, and the 196.3^241.7-mcd interval was dated in about 2.0^ 3.2 Ma. In addition, a detailed age model for the 196.3^241.7-mcd interval is constructed by correlating the record of N18 OCibicidoides wuellerstorfi of ODP Core 1146 with the N18 O record of ODP Core 846 from the Eastern Equatorial Paci¢c (Shackleton et al., 1995) (Fig. 2).
2.1. Age model 3. Results For the sequence of ODP Core 1146, oxygen isotope data from the uppermost 163 mcd (1.5 Ma) were measured and orbitally tuned by Clemens (2001) in the Benedum Stable Isotope Labo-
3.1. Isotope analyses The vertical distributions of N18 O in di¡erent
Fig. 2. Comparison of oxygen isotopic records of Cibicidoides wuellerstor¢ for ODP Core 1146, collected in the SCS (this study), and ODP Core 846, collected in south of the Galapagos Islands (Shackleton et al., 1995), for the 2.0^3.2-Ma interval. Numbers represent oxygen isotope stages.
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309
Fig. 3. Oxygen isotopes of selected planktonic foraminiferal species for 1.2 and 2.1 Ma (this study), the Holocene, and 3.2 Ma (Wang, 2001) from ODP Core 1146. Planktonic foraminifera are grouped into three assemblages: mixed-layer, thermocline, and deep-layer species. ‘wo’ and ‘w’ represent Globigerinoides sacculifer, without and with a saclike, respectively.
species show an analogous pattern for the Holocene, 2.1, 1.2 and 3.2 Ma (Fig. 3), suggesting that the relative vertical position of di¡erent planktonic foraminifera species in the water column has been nearly constant since 3.2 Ma, except for G. conglobatus, Globigerina calida, and Globigerina eaquilateralis. N18 O values ranged from 33.5x to 2.5x in 1.2-Ma and 2.1-Ma samples, that appear
as three distinct groups in the water column, separated by two boundaries at 31.3x and 30.2x. Following Dowsett et al. (1996), three groups of planktonic foraminifera are de¢ned: mixed-layer, thermocline, and deep-water species. In the Holocene, Globigerinoides ruber and Globigerinoides sacculifer, and Globorotalia menardii and Neogloboquadrina dutertrei are the main fau-
Table 1 Planktonic foraminifera species list (after Dowsett and Poore, 1990; Wang, 2001, and this study) Mixed-layer species
Thermocline species
Deep-layer species
Globigerinoides ruber Globigerinoides obliquus Globigerinoides extremus Globigerinoides sacculifer Globigerinoides bulloides Globigerinoides ¢stulosus Globigerina rubescens Globigerinita glutinata Dentoglobigerina altispira
Pulleniatina obliquiloculata Globigerina calida Globigerina eaquilateralis Globorotalia menardii Globorotalia limbata Globorotalia miocenica Globorotalia multicamerata Globorotalia margaritae Globorotalia tumida Spheroidina dehiscens Neogloboquadrina dutertrei Neogloboquadrina humerosa Neogloboquadrina acostaensis
Globorotalia Globorotalia Globorotalia Globorotalia
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scitula in£ata crassaformis truncatulinoides
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nal elements of the mixed-layer and thermocline groups, respectively (Ravelo and Fairbanks, 1992). For the Pliocene the mixed-layer assemblage also included Globigerinoides altispira, Globigerinoides obliquus, Globigerinoides extremus, and Globigerinoides ¢stulosus, whereas Neogloboquadrina acostaensis, Neogloboquadrina humerosa, Globorotalia limbata, Globorotalia miocenica, Globorotalia multicamerata, and Globorotalia margaritae were added to the thermocline assemblage. The deep-water group was composed of Globoro-
talia crassaformis, Globorotalia in£ata, Globorotalia scitula, and Globorotalia truncatulinoides (Table 1). Oxygen isotope values for Cibicidoides wuellerstor¢ ranged from 2.3x to 3.6x, steadily increasing from 3.2 to 2.0 Ma, and showing heavy values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma (Fig. 2). The N18 O record of Pulleniatina obliquiloculata gradually increased since 3.2 Ma varying between 30.1x and 31.4x without displaying any abrupt decrease, except at 2.8 Ma (Fig. 4).
Fig. 4. Oxygen and carbon isotopes of Globigerinoides sacculifer, Pulleniatina obliquiloculata and Cibicidoides wuellerstor¢ for ODP Core 1146. Gray bars indicate distinct glacial periods.
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PALAEO 3143 9-7-03 Fig. 5. Oxygen isotope curves of Cibicidoides wuellerstor¢ and relative (%) abundance of Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, and Globorotalia in£ata groups, and Neogloboquadrina pachyderma at ODP Core 1146. Gray bars indicate distinct glacial periods.
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Unlike P. obliquiloculata and C. wuellerstor¢, the N18 O curve of Globigerinoides sacculifer changed from 30.9x to 32.3x, and did not exhibit a trend from 3.2 to 2.0 Ma nor the distinct heavy values at 2.8, 2.72, 2.6, 2.16, and 2.08 Ma, with the exception of 2.5 Ma (Fig. 4). In contrast to oxygen isotopes, changes in the carbon isotopes of Cibicidoides wuellerstor¢, Pulleniatina obliquiloculata and Globigerinoides sacculifer, showed a di¡erent pattern. The N13 C values of G. sacculifer did not show a signi¢cant drop, changing between 1.9x and 0.6x. However, since 3.2 Ma, the N13 C values of C. wuellerstor¢ decreased from 0.4x to 30.7x. Before 2.5 Ma, the N13 C di¡erence between P. obliquiloculata and
C. wuellerstor¢ was small, but after that the di¡erence increased. The N13 C di¡erence between G. sacculifer and P. obliquiloculata was larger before 2.5 Ma, whereas it became smaller after that time (Fig. 4). 3.2. SST changes Globigerinoides sacculifer and Globigerinoides ruber are shallow dwelling tropical^subtropical species within the mixed layer, whereas Globorotalia menardii mainly inhabits the thermocline layer in tropical and subtropical regions (e.g. Fairbanks et al., 1982). Globigerinoides ruber, Globigerinoides sacculifer, Globorotalia menardii, Glo-
Fig. 6. Oxygen isotope record of Cibicidoides wuellerstor¢ and relative (%) abundance of high productivity, mixed-layer, and thermocline species and the mixed-layer/thermocline species ratio for ODP Core 1146. Gray bars indicate distinct glacial periods.
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borotalia in£ata, and Neogloboquadrina pachyderma are sensitive to seasonal changes in SST (Chen et al., 2000; P£aumann and Jian, 1999). The percentage abundances of the G. sacculifer and G. ruber groups decreased from nearly 40% to 7% and 52% to 7%, respectively, and that of the G. menardii group decreased from 23% to 2% for the 3.2^2.0-Ma interval (Fig. 5), roughly coinciding with glacial^interglacial £uctuations. Signi¢cant drops in the percentage abundances of the G. ruber, G. sacculifer, and G. menardii groups occurred at 2.5, 2.08 and 2.2 Ma, respectively (Fig. 5). In contrast to the percentage variations of the tropical^subtropical species, the polar^subpolar species Globorotalia in£ata group (including G. crassaformis) and Neogloboquadrina pachyderma display the opposite trend (Fig. 5). Before 2.5 Ma, the percentage abundance of G. in£ata remained below 5%, but increased afterwards to a maximum of 14%. Although the abundance of N. pachyderma remained below 3%, as a polar species it is sensitive to temperature changes in the subtropical regions. After 2.5 Ma, the abundance of N. pachyderma increased, especially at 2.1 Ma (Fig. 5). Variations in the relative abundance of planktonic foraminifera implied a gradual cooling process since 3.2 Ma (Fig. 5). 3.3. Variations in mixed-layer and thermocline dwellers The relative abundance of thermocline dwelling species increased from below 30% to over 50% during the 3.2^2.0-Ma interval (Fig. 6). When Cibicidoides wuellerstor¢ exhibits heavier N18 O values, the abundance of thermocline species is generally higher. In contrast, the abundance of mixed-layer dwellers gradually decreased from nearly 77% to 20%, with drops occurring during glacial periods (Fig. 6). A change in the planktonic foraminiferal assemblage took place at 2.7 Ma. Before 2.7 Ma, mixed-layer species dominated the planktonic assemblage, more than 3 times the abundance of thermocline species. However, after 2.7 Ma, thermocline species increased quickly and the ratio of mixed-layer to thermocline species became close to 1 (Fig. 6).
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4. Discussion 4.1. Variations in upper ocean structure 4.1.1. SST variations Global SST reconstructions for the Middle Pliocene (2.85^3.15 Ma) show that modern conditions in low latitude regions changed little compared to modern times, but SST changed signi¢cantly in the middle and high latitudes of both hemispheres (Dowsett et al., 1996). In southwest Africa, temperatures declined by about 10‡C since 3.2 Ma (Marlow et al., 2000), and in the low latitude western Paci¢c, a Middle Pliocene cool event took place during the 3.1^2.7-Ma interval, with an abrupt and irreversible SST drop at 2.2 Ma (Wang, 1994). In the present study, warm water species, Globigerinoides ruber, Globigerinoides sacculifer, and Globorotalia menardii dominated the planktonic foraminiferal fauna, indicating relatively higher SST before 2.7 Ma. After this time, the percentage abundances of the G. ruber, G. sacculifer and G. menardii groups gradually decreased, whereas the Globorotalia in£ata group and Neogloboquadrina pachyderma increased. Abrupt cooling events coincided with the N18 O records of these species, indicating a close link between SST change in the SCS and the development of global ice sheets in the Northern Hemisphere (Shackleton et al., 1990; Maslin et al., 1998). Maslin et al. (1998) suggested that the long term cooling led to three key steps in the glaciation of the Northern Hemisphere, in ODP Core 1146 variations in oxygen isotope and planktonic foraminiferal assemblages indicate that glaciation was signi¢cant in the SCS at 2.7 Ma. Present day SST in the SCS is controlled by the East Asian monsoon and shows a conspicuous seasonality, with winter SST much lower than the summer value (Chen et al., 1985). During the Last Glacial Maximum, when the winter East Asian monsoon intensi¢ed, SST of the SCS was much lower than present (Wang and Wang, 1990). Therefore, we suggest that the expansion of the Northern Hemisphere glaciation intensi¢ed the East Asian monsoon system, particularly the winter monsoon (Ding et al., 1992), resulting in decreasing SST in the SCS.
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4.1.2. Mixed-layer and thermocline dwelling species variations In the eastern Paci¢c, upwelling induces a shoaling of the depth of the thermocline. When the thermocline shoals in the photic zone, the abundance of thermocline dwelling species increases relative to mixed-layer species (Leckie, 1989; Ravelo et al., 1990; Ravelo and Fairbanks, 1992). After 4.5 Ma, in the eastern Paci¢c (ODP Core 846), mixed-layer species decreased markedly, suggesting a shoaling of the thermocline. However at ODP Core 806, located on the Ontong Java Plateau in the western Paci¢c, the re-
verse was observed (Chaisson and Ravelo, 1997, 2000). Before 2.7 Ma, the relative abundance of mixed-layer dwelling species of ODP Core 1146 far outnumbers the thermocline dwellers, but after 2.7 Ma, the mixed-layer species declined while thermocline dwellers increased. The ratio of mixed-layer species to thermocline species was close to 1, indicating little imbalance between the two assemblages. At ODP Core 1146, no upwelling exists during the modern summer or winter, which would shoal the thermocline and increase the abundance of thermocline species. In contrast, when a stronger East Asian winter mon-
Fig. 7. Oxygen isotope record of Cibicidoides wuellerstor¢ and vN18 O among Globigerinoides sacculifer, Pulleniatina obliquiloculata and C. wuellerstor¢, against time for ODP Core 1146. Numbers indicate glacial stages.
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soon prevails over the SCS, SSTs are much lower than in summer, and the thermocline depth is deeper in winter (Liu et al., 2000). The strong East Asian winter monsoon transports large quantities of aeolian dust from land, thus enriching the surface with nutrients of the northern SCS (Wiesner et al., 1996). This in turn, will favor the blooming of high productivity species, such as Neogloboquadrina dutertrei. The abundance of high productivity species at ODP Core 1146 gradually increased after 2.7 Ma, and changed parallel to the relative abundance of thermocline species (Fig. 6). This suggests that after 2.7 Ma, varia-
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tions in mixed-layer and thermocline species were controlled by nutrients and SST, which in turn were a¡ected by changes in the East Asian monsoon. The N18 O gradient between species living in different layers will be equivalent to the vertical temperature di¡erence in the upper water column (Berger et al., 1978). Globigerinoides sacculifer, Pulleniatina obliquiloculata, Cibicidoides wuellerstor¢ inhabit the mixed-layer, the thermocline and the sea£oor, respectively. Therefore, their isotopic di¡erences imply variations in the vertical ventilation of the upper water column. Changes
Fig. 8. Oxygen isotope record of Cibicidoides wuellerstor¢ and vN13 C among Globigerinoides sacculifer, Pulleniatina obliquiloculata and C. wuellerstor¢. Gray bars indicate distinct glacial periods.
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in vN18 OP C (the gradient between P. obliquiloculata and C. wuellerstor¢) and vN18 OGs C (the gradient between G. sacculifer and C. wuellerstor¢) range from 33.5x to 35.5x, increasing during glacial periods (Fig. 7). The di¡erences indicate that the variations in N18 O values for C. wuellerstor¢ were larger than for G. sacculifer and P. obliquiloculata. The oxygen isotopic di¡erence between the mixed layer and the thermocline (vN18 OGs P ) species was small before 2.2 Ma, but increased after that time. Changes in vN18 OGs P were mostly caused by changes in the N18 O values of Pulleniatina obliquiloculata (Fig. 4). Isotopic data alone are ambiguous as to whether this outcome was the result of subsurface cooling or of a shoaling of the depth of the thermocline. Faunal variations at ODP Site 1146 show that after 2.5 Ma, particularly after 2.16 Ma, polar^ subpolar species increased, whereas tropical^subtropical species decreased. This suggests that changes in the vN18 OGs P values were caused by subsurface cooling. The carbon isotope (N13 C) gradient between different water layers results from the vertical exchange of nutrients in the ocean and the global carbon budget. After 2.8 Ma, vN13 CP C shows a signi¢cant increase, however, the vN13 CGs P values decreased after this time (Fig. 8). These conspicuous changes resulted from the strengthening of East Asian winter monsoon, which in turn supplied more nutrients to a more uniform upper water column due to vertical exchanges.
N18 O values at 2.8, 2.72, 2.6, 2.5, 2.16, and 2.08 Ma. Before 2.7 Ma, the relative abundance of mixed-layer species for outnumbered the thermocline species, but after 2.7 Ma, mixed-layer species declined while thermocline dwellers, dominated by high productivity species, increased. Variations in mixed-layer and thermocline species were not induced by upwelling but by a strengthened East Asian winter monsoon, which decreased SST and enriched surface nutrients in the northern SCS. The vN13 C data among Globigerinoides sacculifer, Pulleniatina obliquiloculata and Cibicidoides wuellerstor¢ suggest that the mixed layer became deeper and vertical exchanges increased with the strengthening of the East Asian winter monsoon being the result of progressive intensi¢cation of the Northern Hemisphere glaciation.
5. Conclusions
References
The N18 O values of planktonic species from samples at 1.2 and 2.2 Ma suggest that little changed in the distribution of the planktonic foraminifera in the upper water column. Following Wang (2001) and Dowsett and Poore (1990), planktonic foraminifera are grouped into mixedlayer, thermocline, and deep-water species for the 3.2^2.0 Ma period. Benthic foraminiferal oxygen isotopes and percentage abundances of planktonic foraminiferal species suggest that SST decreased from 3.2 to 2.0 Ma with conspicuous excursions to heavier
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Acknowledgements We thank the Ocean Drilling Program and the Scienti¢c Party and crew of Leg 184 for excellent samples, Mr. Dingyuan Fang and Ms. Peifen Xia for their assistance in the isotopic analyses, and Dr. Steve Nathan as well as S.J. Kim for improving the English text of this paper and their constructive comments. This work was supported by the National Natural Science Foundation of China (Grants 49999560 and 2000078502).
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