Isotopic signals in two morphotypes of Globigerinoides ruber (white) from the South China Sea: implications for monsoon climate change during the last glacial cycle

Palaeogeography, Palaeoclimatology, Palaeoecology 161 (2000) 381–394 www.elsevier.nl/locate/palaeo

Isotopic signals in two morphotypes of Globigerinoides ruber (white) from the South China Sea: implications for monsoon climate change during the last glacial cycle Luejiang Wang †, * Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, 060-0810 Japan Received 2 November 1998; accepted for publication 14 March 2000

Abstract Based on the study of surface and downcore sediment samples from the South China Sea (SCS), two morphotypes of Globigerinoides ruber (white) have been differentiated by taxonomic criteria at species and/or subspecies level, and also by their stable isotopic compositions of oxygen and carbon. The two morphotypes, G. ruber sensu stricto (s.s.) and G. ruber sensu lato (s.l.), represent two groups with different depth habitats, with G. ruber s.s. living in the upper 30 m of the water column and G. ruber s.l. living at depths below 30 m. The different depth habitats are indicated by the comparison between the present oceanographic data and isotopic signals of the two morphotypes in the surface sediment from the SCS. Application of this information to a hemipelagic sediment core in the northern SCS shows that drastic changes existed in the morphotypes isotopic records during the last glacial cycle. The isotope difference between the two morphotypes diminished during d18O stage 2, especially during the last glacial maximum. It is inferred that this is the result of intensified monsoon winds, which caused mixing of the upper layer of the water column. During stage 2 the proportion of G. ruber s.l. increased, which is interpreted as a result of the increased productivity due to mixing of the subsurface nutrient-rich water into the surface layer. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Globigerinoides ruber morphotypes; isotope geochemistry; last glacial maximum; micropaleontology; paleomonsoon climate; South China Sea

1. Introduction Taxonomic ambiguities are among the most common and severe problems in paleontology and hinder our understanding of the evolutionary processes. In particular, they cause problems in paleo* Corresponding author: Michael Sarnthein, Institut fu¨r Geowissenschaften, Universita¨t Kiel, D-24098 Kiel, Germany. Tel.: +49-431-880-2882; fax: +49-431-880-4376. E-mail address: [email protected] (M. Sarnthein) † Died on 29 October 1999 in an accident while diving for corals off Hainan.

ceanographic applications and interpretation of micropaleontological (e.g. Malmgren and Kennett, 1977) and geochemical (e.g. Keigwin and Jones, 1989) data of fossil records. One species of planktonic foraminifera (PF ) widely used in paleoceanographic and paleoclimatic studies, Globigerinoides ruber (d’Orbigny), has provided a good example for such taxonomic ambiguities. This species has both color (pink and white) and morphological variants, both of which have distinctive stable isotope signals (Deuser and Ross, 1989; Robbins and Healy-Williams, 1991).

0031-0182/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 3 1 -0 1 8 2 ( 0 0 ) 0 0 09 4 - 8

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A number of morphotype variants of Globigerinoides ruber have been recognized and even been classified at species level. For example, Globigerinoides elongatus (d’Orbigny, 1826) stands for forms with a tightly coiled trochospire, Globigerinoides pyramidalis (van den Broeck, 1876) for forms with a high trochospire, and Globigerinoides cyclostomus (Galloway and Wissler, 1927) for forms with a compact, small test and a small, round aperture. Based on the phylogeny of the Cenozoic PF, Kennett and Srinivasan (1983) have suggested that these morphotypes are phenotypic variants of the same species, G. ruber, which is partially supported by ecological studies of the living faunal distribution (Hecht, 1974; Be´ and Tolderlund, 1971; Be´ and Hutson, 1977). Before the 1990s, most ecological studies were focused on the white and pink variants of the species, especially concerning the difference in isotopic composition (e.g. Deuser and Ross, 1989), and their geographical and stratigraphical distribution patterns (e.g. Thompson et al., 1979; Keigwin and Jones, 1989). A comprehensive study based on Fourier-series analysis, amino-acid biochemistry, and isotope geochemistry has been carried out for Globigerinoides ruber morphotypes in 16 coretop samples from the tropical oceans (Robbins and Healy-Williams, 1991). In their study, six morphotypes were defined by biometrics based on shell elongation. This treatment of morphology is regardless of the criteria in PF taxonomy, and hence these morphologic groups are the result of arbitrary grouping solely depending on the shape of the test outline. Moreover, the color variants (pink and white) of this species were not differentiated. Hence, this arbitrary morphotype classification could not be related to the shellbiogeochemistry of the morphotypes (Robbins and Healy-Williams, 1991). On the other hand, the fact that a difference in shell-biogeochemistry does exist in morphotypes of several PF species has also been demonstrated by recent studies (Bijima et al., 1998; Darling et al., 1999). Although PF taxonomy is arbitrary to some extent, it is still the basis in present routine work of micropaleontology, i.e. identifying species and phenotypes under microscopes. Ideally, the mor-

photype variants should be based on both taxonomic criteria and shell-biogeochemistry. In this paper, we present a study on two morphotype variants of Globigerinoides ruber (white), namely G. ruber sensu stricto (s.s) and G. ruber sensu lato (s.l.), based on PF taxonomic criteria. Stable isotopic signals of these two morphotypes are presented for information on their ecological niche. An example of a downcore study in the South China Sea (SCS ) demonstrates how the isotopic time series of the two morphotypes can be applied to study the paleoceanographic and paleoclimatic variations, especially the East Asian Monsoon climate change during the last glacial cycle.

2. Materials and methods 2.1. Sediment samples The present study is based on both surface and downcore sediment sample sets that were obtained during the SONNE Cruise 95 in 1994 to the SCS (Sarnthein et al., 1994a). The surface sediment samples were taken from the uppermost 1 cm of the undisturbed surface of giant spade box cores and represent the genuine sediment surface ( Fig. 1). The high sedimentation rate (about 5– 50 cm/ka) in these cores (Sarnthein et al., 1994b) ensures that our surface sediment samples comprise a time slice of less than the last 200 years. Table 1 lists the 23 sites where samples contain enough specimens of both morphotypes for isotopic measurement to enable comparison between the two morphotype variants of Globigerinoides ruber. The downcore sample set is taken from gravity core 17939-2 (19°58.2∞N, 117°27.3∞E, from 2474 m water depth, with a core length of 12.74 m) ( Fig. 1). Samples of about 1 cm in diameter were taken at a spacing of 10 cm. 2.2. Micropaleontology Sample processing followed the standard technique for PF. The wet bulk samples were soaked in distilled water for 1 day before washing over a sieve with 63 mm mesh size. The residuals were

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Fig. 1. Site locations of the surface sediment samples (squares) and the piston core 17939-2 (dot). Arrows indicate the northeastern winter monsoon winds. The summer monsoon winds are in the opposite direction. The 100 m isobath is added to represent the approximate paleo-coastline during the last glacial maximum (LGM ) low sea level stand.

dried at 40°C in an oven, and then dry sieved to obtain size fractions of 63–150 mm, 150–315 mm, 315–400 mm, and >400 mm. The size fraction of 315–400 mm was used for morphotype ratio analysis of Globigerinoides ruber (white) as well as for picking specimens for stable isotope analysis. 2.3. Stable isotope and accelerator mass spectrometry (AMS) 14C analyses Stable oxygen and carbon isotopes were measured on samples composed of 15–20 specimens

of either of the two morphotypes of Globigerinoides ruber (white) in the 315–400 mm size fraction. Using a needle, all tests were crushed chamber by chamber under the microscope, repeatedly washed in ethanol (99.8%) in an ultrasonic bath, and dried at 40°C. Samples for AMS 14C dating were obtained from Globigerinoides sacculifer specimens, with each sample amounting to 5–12 mg. The samples were cleaned (without crushing the tests) in an ultrasonic bath with distilled water. Stable isotopes and AMS 14C ages were measured at the Leibniz Laboratory of Kiel University

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Table 1 Isotopic composition of the Globigerinoides ruber (d’Orbigny) (white) morphotypes from the surface sediment samples from the SCS (Grothmann, 1996) Station

17928 17929 17930 17931 17932 17933 17941 17942 17943 17944 17949 17950 17951 17954 17955 17957 17958 17959 17960 17961 17963 17964 17965

Latitude (°N )

18.27 20.68 20.33 20.10 19.95 19.53 21.52 19.33 18.95 18.66 17.35 16.09 16.29 14.80 14.12 10.90 11.62 11.14 10.12 8.51 6.17 6.16 6.16

Longitude (°E )

119.75 115.70 115.78 115.96 116.04 116.23 118.48 113.20 113.55 113.64 115.17 112.90 113.41 111.53 112.18 115.30 115.08 115.29 115.56 112.33 112.67 112.21 112.55

Water depth (m)

2486 371 629 1005 1365 1972 2201 329 917 1219 2195 1868 2304 1517 2404 2197 2581 1957 1707 1795 1233 1556 889

Globigerinoides ruber s.s.

Globigerinoides ruber s.l.

d18O (‰)

error (±‰)

d13C (‰)

error (±‰)

d18O (‰)

error (±‰)

d13C (‰)

error (±‰)

−2.83 −2.53 −2.61 −2.51 −2.60 −2.73 −2.73 −2.81 −2.98 −2.48 −3.01 −3.00 −2.91 −3.00 −2.91 −3.04 −2.84 −3.00 −3.14 −2.97 −3.24 −2.99 −3.24

0.03 0.03 0.03 0.03 0.06 0.02 0.04 0.01 0.01 0.04 0.03 0.02 0.03 0.03 0.02 0.02 0.02 0.02 0.05 0.03 0.04 0.02 0.03

1.58 1.35 1.34 1.29 1.14 1.14 1.12 1.41 1.63 1.44 1.31 1.55 1.43 1.43 1.17 1.27 1.22 1.27 1.34 1.31 1.28 0.86 1.08

0.02 0.02 0.01 0.02 0.03 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.01 0.02 0.02 0.01

−2.35 −2.35 −2.53 −2.30 −2.79 −2.42 −2.39 −2.33 −2.57 −2.37 −2.38 −2.73 −2.63 −2.97 −2.85 −2.55 −2.77 −3.11 −2.93 −3.00 −2.78 −2.99 −3.02

0.02 0.02 0.02 0.03 0.04 0.03 0.04 0.03 0.03 0.01 0.02 0.02 0.02 0.03 0.02 0.04 0.02 0.02 0.02 0.01 0.04 0.04 0.03

0.69 1.17 1.04 1.27 1.10 0.92 1.13 1.15 1.16 1.24 0.87 1.14 1.08 1.04 0.89 0.92 −0.08 1.09 0.96 1.03 0.76 0.75 0.93

0.02 0.01 0.02 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.04 0.01 0.02 0.02 0.03 0.03 0.03 0.02 0.02 0.01 0.01 0.02 0.01

following standard procedures (Ganssen and Sarnthein, 1983; Nadeau et al., 1997; Schleicher et al., 1998). The external errors of stable isotope analyses are ±0.08‰ and ±0.06‰ for d18O and d13C respectively. The average zero background of AMS 14C dating is 0.3% 14C (equal to about 46,000 ka BP). Standard errors including the background radioactivity were calculated per sample batch. Details of the AMS 14C dating and age calibration can be found in Wang et al. (1999).

3. Definition of the morphotype variants of Globigerinoides ruber In this paper, Globigerinoides ruber morphotype variants are defined according to the PF taxonomic criteria, such as characteristics of chamber shape, shell outline lobation, and shape/position of aper-

tures, etc. Since the morphotype variants should be based on both taxonomic criteria and shellbiogeochemistry, stable isotopes of carbon and oxygen served as biogeochemical parameters in this study. Moreover, in the Indo-Pacific region, pink Globigerinoides ruber disappeared at stage 5e, about 120 ka BP ( Thompson et al., 1979). Therefore, our study deals only with the white variants. Based on the basic description in Kennett and Srinivasan (1983), the two morphotypes are defined as follows. (1) Globigerinoides ruber s.s. (Plate I: 1–4): test medium trochospire with three spherical chambers in the final whorl, increasing moderately in size, symmetrical over the previous sutures; sutures radial, distinctively depressed; surface coarsely perforate; umbilicus narrow; primary aperture interiomarginal, umbilical, with a wide, high-arched

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PLATE I

Representatives of the two morphotype variants of Globigerinoides ruber (d’Orbigny) (white) from the surface and downcore sediment samples in the SCS. Horizontal bar below each figure indicates the 100 mm scale. 1–4. Globigerinoides ruber s.s. 5–9. Globigerinoides ruber s.l.

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opening bordered by a rim, symmetric over the previous suture; two wide, high-arched supplementary suture apertures situated opposite sutures of previous chambers. (2) Globigerinoides ruber s.l. (Plate I: 5–9): test medium to high trochospire with three compressed subspherical chambers in the final whorl, increasing moderately to rapidly in size, asymmetrical over the previous sutures; sutures radial, slightly to medium flushed; surface coarsely to medium densely perforate; umbilicus narrow; primary aperture interiomarginal, umbilical, with a small round or medium arched opening bordered by a rim, sometimes asymmetric over the earlier suture; two small to medium size, round supplementary suture apertures situated opposite sutures of previous chambers. Initially, the Globigerinoides ruber s.l. group was differentiated into tests with low and high trochospires. However, as these two sub-groups did not show significant differences in their isotopic signal, they were lumped again into one group. In general, the Globigerinoides ruber s.l. corresponds to the more compact and higher trochospiral form previously described as Globigerinoides elongatus (d’Orbigny, 1826), Globigerinoides pyramidalis

(van den Broeck, 1876), and Globigerinoides cyclostomus (Galloway and Wissler, 1927), which largely corresponds to the end members 2, 3, and 4 described by Robbins and Healy-Williams (1991). Robbins and Healy-Williams (1991) mentioned that only their end member 1 and 6 dominate in the western Pacific. However, our observations in the SCS show that the two morphotypes described above, Globigerinoides ruber s.s. and G. ruber s.l. (Plate I ), are dominant. Compared with the classification of Robbins and Healy-Williams (1991), these two morphotypes basically correspond to (1) the end members 1, 5, and 6 for G. ruber s.s. with end member 6 as the dominant variant, and to (2) the end members 2, 3, and 4 for G. ruber s.l. with end member 3 as the dominant variant, in both the surface and downcore samples in the SCS.

4. Results and discussion 4.1. Isotopic signal in the surface sediment samples Stable oxygen and carbon isotope analyses have been carried out on sample pairs of Globigerinoides

Fig. 2. Comparison of the stable isotope signals of the two morphotypes, Globigerinoides ruber s.s. and G. ruber s.l., in the surface sediment samples. (A) oxygen isotope; (B) carbon isotope. The 1:1 correlation line and the linear regression line are added for reference. Note that in panel (A) most data points are in the upper-left part of the panel, whereas in panel (B) most data points locate in the lower-right part of the panel.

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ruber s.s. and G. ruber s.l. in 23 surface sediment samples from the SCS (Table 1; Grothmann, 1996). The results are shown in Fig. 2. It is clear that the two morphotypes represents two distinct groups of specimens of Globigerinoides ruber. In d18O (Fig. 2A), most of the G. ruber s.l. show heavier d18O values than G. ruber s.s. On the contrary, in d13C ( Fig. 2B), most of the G. ruber s.l. show lighter d13C values than G. ruber s.s. The mean differences between G. ruber s.l. and G. ruber s.s. are 0.21±0.21‰ and −0.28±0.29‰ for d18O and d13C respectively. Although the standard errors of the mean values are relatively large, an a-test of the null-hypothesis with assumed mean values of zero for both d18O and d13C resulted in t values of 2.141 and 5.622 for d18O and d13C respectively. These are greater than the critical t value of 1.717 with 22 degrees of freedom (i.e. 23 minus 1) and at the 5% (a= 0.05) level. Thus, the null-hypothesis of mean values not greater than zero is rejected. Hence, the mean differences in d18O and d13C between the two morphotypes of G. ruber are statistically significant. Three possible reasons that could explain this phenomenon in the SCS are: (1) a seasonal preference; (2) different water depth habitats; (3) a vital effect that differs between the two G. ruber morphotype variants. In the first scenario, if the Globigerinoides ruber s.l. lives in the cold season, as suggested by the heavy d18O values, it is difficult to find a general reason for the lighter d13C value of G. ruber s.l. relative to G. ruber s.s. Based on the sediment trap study published by Wiesner et al. (1996) (one trap located in the northern SCS at 18.47°N, 116.02°E, and the other in the middle of the SCS at 14.6°N, 115.10°E), the carbonate flux maxima occur from September to October 1997 and from February to March 1998, i.e. during the inter-monsoon periods. The PF are dominated by G. ruber and Globigerinoides sacculifer, as the seasonal rainfall contrast in the SCS and Southern China is characterized by a rainy summer (July to August) and a dry winter. Because G. ruber tests are mainly undertaken in late spring and late autumn, the seasonal river-discharge influence on their isotope composition may not be that large. Furthermore,

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several lines of evidence show an increased seasonality in the northern SCS during the last glacial time ( Wang and Wang, 1990; Miao and Thunell, 1994; Pelejero et al., 1999; Wang et al., 1999). If the d18O difference reflects the seasonality of the sea surface temperature, an increase in the d18O difference can be expected between the two morphotypes during the last glacial cold period. However, this is not the case for the downcore record (see discussion below). Both Dd18O and Dd13C between the two morphotypes show similar values in the glacial (0.31‰ and 0.11‰) and Holocene (0.33‰ and 0.09‰) times, and diminished to 0.14‰ and 0.02‰ in the later part, the last glacial maximum (18.5–21 cal. ka BP). These lines of evidence exclude a different seasonal preference of the two G. ruber morphotypes in the SCS as the reason to explain the difference in their isotopic signals. On the other hand, the second scenario of different depth habitats seems to fit the data quite well. Taking Globigerinoides ruber s.l. as the morphotype living in the subsurface waters, possibly immediately below the surface low salinity layer, the d18O of G. ruber s.l. should be heavy and its d13C should be light because of the high nutrient level and hence more 12C. This is the case as shown in Fig. 2. The different magnitudes of the isotope differences at various sites must be explained in terms of differences in the structure of the upper layer of the water column. Ideally, a correlation based on the depth of mixed layer or the temperature and/or salinity difference between the surface and subsurface layer should be made. However, no synoptic oceanographic data sets or hydrographic maps are available for this purpose. The Levitus and Boyer (1994) data set of 1°×1°-grid average smooths out any significant, small-scale variations in the hydrographic characteristics in the SCS. However, based on the vertical temperature decrease from the current hydrographic data set (Levitus and Boyer, 1994) and the in situ CTD profiles (Haupt et al., 1994), an estimate of 30 m for the boundary between the two morphotypes seems quite reasonable. CTD sites at 17935 (18.53°N, 116.32°E) and 17947 (18.28°N, 116.02°E ) in the northern SCS show about a 2– 3°C temperature decrease and a 0.20–0.25 psu

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salinity increase from the surface water (0–30 m average) to the sub-surface water (30–50 m average). These changes in temperature and salinity correspond to a difference of about 0.4‰ in the d18O signals [using the d18O and temperature equation of O’Neil et al. (1969), Shackleton (1974), and Duplessy et al. (1991)], which is similar to the maximum d18O differences between the two morphotypes of G. ruber (s.l. and s.s.), e.g. at the northwesterly sites of 17942 and 17943 near the Pearl River mouth where the d18O difference is 0.41‰ and 0.38‰ respectively. Recent published d13C data of the dissolved inorganic carbon in the SCS (in April, June, and October, 1995) indicate d13C values of 0.13–0.15‰ for the surface waters. The d13C values decrease by about 0.6–0.7‰ to about 0.7–0.8‰ at 100 m depth and remain low around −0.1–+0.3‰ from 500 to 3500 m (Lin et al., 1999). Hence the average decrease of 0.28‰ in the upper 30–50 m is quite reasonable for the present data set from the surface sediments of the SCS. Moreover, other evidence, such as the calculated equilibrium CaCO d18O 3 and d13C of the surface and sub-surface waters (Fairbanks et al., 1982), does support the interpretation of the Globigerinoides ruber morphotype isotope differences in the SCS as due to different depth habitats of the surface dweller of G. ruber s.s. and the sub-surface dweller of G. ruber s.l. The third possible explanation is that a differential vital effect might have created the isotope differences, especially in the d13C values. However, as the Dd13C and Dd18O between the Globigerinoides ruber s.s. and G. ruber s.l. are not constant over the last glacial cycle (see Section 4.2) the vital effect is probably not one of the main reasons for the isotope differences, in particular because we are dealing with morphotypes of the same species. To investigate a possible grouping within Globigerinoides ruber s.l., i.e. the high and low trochospire forms, 22 sample pairs were selected for stable isotope analysis from core 17939-2 in the northern SCS (depth ranges from 570 to 1070 cm downcore). The isotope data are shown in Fig. 3 and listed in Table 2. The correlation shown in Fig. 3 demonstrates that no systematic d18O and d13C deviation exists between the two

Fig. 3. Comparison of the stable isotope signals of the low and high trochospiral forms in the Globigerinoides ruber s.l. (A) Oxygen isotope; (B) carbon isotope. The 1:1 correlation line and the linear regression line are added for reference. Note that in both panels (A) and (B) the data points are scattered around 1:1 line.

sub-groups. This indicates that, within G. ruber s.l., specimens with high trochospires and low trochospires live in the same ecological niche and have the same isotopic fingerprints.

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Table 2 Comparison of stable isotope values between high and low trochospiral morphotype variants of Globigerinoides ruber s.l 17939-2 depth (cm) 100 510 590 620 630 670 680 690 700 710 720 730 760 780 830 870 890 900 960 1000 1020 1070

High trochospiral Globigerinoides ruber s.l.

Low trochospiral Globigerinoides ruber s.l.

d18O (‰)

d13C (‰)

d18O (‰)

d13C (‰)

−2.25 −0.65 −1.06 −0.99 −1.05 −0.93 −0.98 −1.11 −0.96 −1.08 −1.04 −0.86 −0.89 −0.85 −0.73 −0.87 −0.82 −0.67 −0.46 −0.79 −0.70 −0.99

1.25 0.95 1.01 1.08 0.97 0.94 0.77 0.98 0.9 0.98 0.83 0.98 1.09 0.90 1.09 1.00 1.13 1.01 0.84 0.88 0.91 1.21

−2.54 −1.19 −0.88 −1.12 −1.34 −1.06 −1.21 −1.01 −0.68 −0.94 −1.21 −1.09 −0.98 −0.88 −0.65 −0.66 −0.87 −0.89 −0.63 −0.91 −0.49 −1.28

1.45 0.92 0.96 1.06 1.08 0.85 0.98 0.96 0.75 0.85 0.98 0.93 1.04 0.89 0.98 1.02 1.07 1.19 1.03 1.14 1.19 1.18

4.2. Downcore variations for the last glacial cycle Oxygen and carbon isotopes of the two morphotypes of Globigerinoides ruber have been investigated for downcore variations in core 17939-2 from the northern continental slope of the SCS (Fig. 1). A shallow Parasound echo-profile at the site shows an undisturbed sediment structure (Sarnthein et al., 1994a). AMS 14C dating also indicates the continuous sedimentation for the last 42,000 years (see Fig. 5). The age–depth curve in Fig. 4 shows a sedimentation rate of 81.2 cm/ka for the last glacial maximum and early deglaciation, and 25.8 cm/ka for the post-glacial time. To complete the chronostratigraphy, the d18O minimum (stage 3.13) at 1210 cm [~38.3 cal. ka BP, Fig. 5, d18O curves in panel (b)] is assumed to correlate with the Dansgaard–Oeschger event 8, which results from a correlation based on compari-

Fig. 4. Age–depth plot of core 17939-2. Crosses are AMS 14Cdated levels with calibrated ages [see table 3 in Wang et al. (1999) for details]. The square is the ice-core analog age with Dansgaard–Oeschger event 8 correlated to the stage 3.13 (~38.3 cal. ka BP). Straight dashed line is linear interpolation. The solid curve represents a smooth-spline fit of the age levels.

son between a high resolution downcore record at the neighboring site 17940 and the GISP 2 icecore records (Grootes et al., 1993; Grootes and Stuiver, 1997; Wang et al., 1999). Thus, the average sedimentation rate for stage 3 is about 20.8 cm/ka. The age model is further constrained by connecting the dated and the analog age levels by a smoothspline fit (Stein, 1985) in order to avoid the artificial extremes in sedimentation rate induced by the possible errors in AMS 14C-dated ages ( Fig. 4). The stratigraphic pattern of the d18O curves of both morphotypes clearly shows the last glacial cycle [Fig. 5, curves in panel (b)]: from stage 3, stage 2 between 1070 and 620 cm (30.3– 18.5 cal. ka BP), through the deglaciation into the Holocene at 240 cm (~11.6 cal. ka BP). The d18O values are heaviest at about −1.0‰ during stage 2. Compared with the LGM (gray bar), Holocene samples are much lighter at about −2.5‰. A further broad minimum of heavy d18O values corresponds to the Oldest Dryas period between 590 and 420 cm (18.2–16.6) followed by the

Fig. 5. Isotope records of the two morphotypes of Globigerinoides ruber for the last 42,000 years. (a) d18O curves of Neogloboquadrina dutertrei; (b) d18O curves of G. ruber s.s. (squares) and G. ruber s.l. (circles); (c) d13C curves of G. ruber s.s. (squares) and G. ruber s.l. (circles); (d) d18O difference between G. ruber s.l. and G. ruber s.s.; (e) d13C difference between G. ruber s.s. and G. ruber s.l.; (f ) ratios of G. ruber s.l. in the total population of G. ruber from the 315–400 mm size fraction. Note: thick lines in (d), (e), and (f ) represent three-point running averages of the original data to reveal general trends in each record. YD: Younger Dryas; B/A: Bølling/Allerød; OD: Oldest Dryas; LGM: last glacial maximum [according to d18O records of G. ruber in panel (b)].

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Bølling/Allerød between 400 and 320 cm (16.2– 13.4 cal. ka BP). The Younger Dryas ( YD) shows a double-peak heavy d18O event (similar to the YD event in GISP2 ice core d18O, Grootes et al., 1993) in the d18O curves of the two morphotypes in this core between 275 and 315 cm (11.6– 13.2 cal. ka BP). The Early Holocene summer monsoon maximum (210–240 cm, 9.2– 10.2 cal. ka BP) shows a plateau of medium-high values ( light d18O values averaging at −2.70‰ compared with the average Holocene d18O level of −2.54‰ in the d18O of Globigerinoides ruber s.s.). This event is better recorded in the d18O of G. ruber s.s. than that of G. ruber s.l., indicating the relatively strong signal of a salinity drop in the uppermost surface water due to an excess river runoff by increased summer monsoon rains over South China, as well as increased precipitation in the SCS. The riverine clay content in this core shows a pronounced broad peak between 190 and 260 cm (8.5–11.0 cal. ka BP) ( Wang et al., 1999). The pronounced broad minimum in d13C at this time indicates high nutrient levels, which probably result from increased riverine terrigenous nutrient input [Fig. 5, curves in panel (c)] together with the low d13C of dissolved inorganic carbon in the freshwater input. In general, d13C was lower during the last glacial period than during stage 3 and the middle-to-late Holocene. This may indicate a high productivity by increased nutrient content in stage 2 due to the mixing of subsurface and surface water by the intensified winter monsoon, as well as the increased wind-borne terrigenous nutrient input. The low d13C level might have also been partly the result of the estuarine circulation of the glacial isolated SCS with several big rivers discharging freshwater. Probably, an Amazon-like paleoriver system developed on the emerged Sundaland. This may also be one of the main reasons for the decreased d13C level during the deglacial periods between 18.5 and 11.0 cal. ka BP. The same d13C and d18O patterns of the last glacial cycle are more clearly demonstrated in the high-resolution records at the neighboring site 17940. For example, the early Holocene summer monsoon maximum is characterized by a broad minimum of low d13C and a prominent plateau of low d18O values plus the high riverine clay content

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after Termination Ib, between ~4.5 and ~6.5 m, corresponding to ~8.5 to ~11.5 cal. ka BP ( Wang et al., 1999). The short-term variations of the stable isotopes in both morphotypes of Globigerinoides ruber largely reflect the surface water above the thermocline. This is evidenced by the d18O record of Neogloboquadrina dutertrei in the same core [Fig. 5, curve (a)]. Studies on living PF indicate that this species lives below 50 m and under the thermocline (e.g. Jones, 1967; Fairbanks et al., 1982; Hemleben et al., 1989). The contrast of d18 O between glacial and Holocene is ~1.23‰ [Fig. 5, curve (a)], which is identical to the global ice-effect value of 1.1–1.3‰ (e.g. Fairbanks and Matthew, 1978; Labeyrie et al., 1987; Fairbanks, 1989, 1990). This evidence shows that there is no significant change in the thermocline at the site of 17939-2 over the past 42,000 years. Hence, the large short-term isotope variations in both morphotypes of G. ruber reflect variations of temperature and salinity in the surface water of the upper 50 m and above the thermocline. In both d18O and d13C records, the most striking feature is the changing difference between the two morphotypes through time [Fig. 5, curves (d) and (e)]. The three-point running average of Dd18O (s.l. and s.s.) amounts to 0.2–0.5‰, i.e. low d18O values of Globigerinoides ruber s.s. relative to G. ruber s.l. [Fig. 5, curve (d )]. Similarly, the Dd13C (s.s. and s.l.) amounts to 0.1–0.4‰, i.e. high d13C values of G. ruber s.s. relative to G. ruber s.l. [Fig. 5, curve (e)]. However, in both records there is a common feature that during stage 2, especially during the LGM, these differences diminished, with Dd18O and Dd13C reaching levels around 0‰. Accordingly, during the LGM (810–610 cm, 21.1–18.4 cal. ka BP) the oxygen isotope differences are reduced to an average of 0.14‰, which is optically more clear from the curves in panel (b) of Fig. 5, where the two morphotype d18O values converge. On the other hand, the average Dd18O values further upcore (0–610 cm, 0– 18.4 cal. ka BP) and below (810–1250 cm, 21.1– 41.3 cal. ka BP) are 0.31‰ and 0.33‰ respectively. This difference is especially pronounced in the case of Dd13C [Fig. 5, curve (e)]. During the LGM, Dd13C is reduced to an average of 0.02‰, whereas

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the Dd18O values vary around average values of 0.09‰ and 0.31‰ above and below respectively. In the case of Dd18O, the record shows an immediate increase in the values after 18.4 cal. ka BP (610 cm), whereas the Dd13C remained relatively low until 16.6 cal. ka BP (420 cm) where Dd18O drops again to a minimum around 0‰. Another pronounced feature in the Dd13C record is the low average level of 0.02‰ during the YD period, and Dd18O also shows a decrease from the Bølling/Allerød maximum [Fig. 5, curves (d) and (e)]. This pattern in the isotopes is paralleled by changes in the Globigerinoides ruber morphotype ratio [Fig. 5, curve (f )]. Although the ratio is based on the size fraction of 315–400 mm, we assume that it reflects the total population of this species to some extent. The most striking feature is the high G. ruber s.l. percentage during stage 2, which also shows a progressive increase during the LGM. Around the YD event level, G. ruber s.l. shows a further broad plateau of medium-high values in relative abundance [Fig. 5, curve (f )]. From the above data, we can infer various paleoceanographic changes in the northern SCS during the last 42,000 years. During most of this period, a well-stratified upper water column structure prevailed in the northern SCS, which, as today, is caused by the river runoff of the Pearl River ( Wang et al., 1999). However, during stage 2, and especially during the LGM, this structure was weakened, as evidenced by the diminishing Dd18O and Dd13C values [Fig. 5, curves (d ) and (e)]. This may be an indication of decreased fluvial runoff and increased winter monsoon winds, which were exceptionally strong during the LGM. The intensified monsoon winds possibly caused strong mixing of the upper layer of the water column, bringing subsurface water to the surface, which resulted in the higher production of Globigerinoides ruber s.l. and reduced the isotope difference between the two morphotypes. In the meantime, the summer monsoon shows evidence of the onset of deglacial increase as early as about 21 cal. ka BP ( Wang and Sarnthein, 1999), which coincides with the highest relative abundance of Globigerinoides ruber s.l. and the lowest carbon isotope difference between the

two morphotypes at 830–810 cm (21.1– 21.3 cal. ka BP) in core 17939-2. As G. ruber is a perennial species with highest production in late spring and late autumn in the SCS, increases in both winter and summer monsoon wind would have resulted in the maximum mixing of the upper layer of the water column. Based on our record, it seems that both continental aridity and the winter monsoon maximum during the LGM might be the main reason for the diminishing of the isotope difference between the two morphotypes of G. ruber.

5. Conclusions Based on the study of two morphotype variants of Globigerinoides ruber from surface and downcore sediments from the South China Sea, several conclusions can be drawn: (1) Two morphotypes can been differentiated, based on taxonomic criteria at species and/or subspecies level. Globigerinoides ruber s.s. refers to specimens with spherical chambers sitting symmetrically over previous sutures with a wide higharched aperture over the suture; G. ruber s.l. refers to more compact tests with compressed chambers sitting asymmetrically over the previous sutures, with a round or medium-arched and relatively small aperture over the suture. (2) The differentiation of the two morphotypes, Globigerinoides ruber s.s. and G. ruber s.l., is substantiated by the isotopic signal obtained from the surface sediment samples. Stable isotope difference are statistically significant between G. ruber s.l. and G. ruber s.s., being about 0.21±0.21‰ and −0.28±0.29‰ for d18O and d13C respectively. This suggests a different depth habitat for these two morphotypes, with G. ruber s.s. living in the upper 30 m of the water column and G. ruber s.l. living at depths below 30 m. (3) Downcore variations in stable isotopes of Globigerinoides ruber and Neogloboquadrina dutertrei show drastic changes in water column structure above the thermocline during the last glacial cycle. The isotope difference between the two morphotypes of G. ruber diminished during d18O stage 2, especially during the LGM. It is inferred

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that this is the result of continental aridity and intensified winter monsoon winds, which caused the mixing of the upper layer of the water column above the thermocline. The morphotype ratio also indicates that stage 2 was a time of increased proportion of G. ruber s.l.

Acknowledgements I sincerely thank H. Erlenkeuser, K. Kibling, M. Nadeau, and M. Schleicher ( Kiel University), A. Ijiri and M. Shimamura (Hokkaido University) for their laboratory assistance and the Kiel working group of M. Sarnthein for numerous fruitful discussions and the unpublished data of A. Grothmann (1996). Special thanks are due to T. Pletsch and U. Schuldt for their help in scanning electronic microscopy. Thanks also go to Captain Bruns and the crew on R/V SONNE Cruise 95 and, especially, to N. Mu¨hlhan and W. Rehder for technical assistance in retrieving the undisturbed sediment cores. I gratefully acknowledge the German Ministry for Education and Research (BMBF ) and the Deutsche Forschungsgemeinschaft (DFG) for their support of the SONNE Cruise 95 to the SCS and the subsequent scientific evaluation. I would also like to express my sincere thanks to J. Bijma, D. Kroon, and J.-C. Duplessy for their effort in reviewing the manuscript. For part of the data acquisition and evaluation, the Japanese Ministry of Education is also acknowledged for the financial support of the Promotion Project No. 10740242.

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