A millennial scale planktonic foraminifer record of the mid-pleistocene climate transition from the northern South China Sea

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1 Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) A millennial scale planktonic foraminifer record of the mid-pleistocene climate transition from the northern South China Sea Fan Zheng a, Qianyu Li b,c, *, Baohua Li d, Muhong Chen a, Xia Tu a, Jun Tian b, Zhimin Jian b a South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 53, China b State Key Laboratory of Marine Geology, Tongji University, Shanghai 292, China c School of Earth and Environmental Sciences, University of Adelaide, SA 5, Australia d Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, Nanjing 28, China Received 4 October 24; received in revised form 2 March 25; accepted 2 April 25 Abstract A high resolution record (~ yr) of planktonic foraminifers from ODP Site 44 in the northern South China Sea reveals rapid and strongly variable climatic changes during the mid-pleistocene transition period. The abundance of warm water species decreased from an average of 6% in marine isotope stage (MIS) 29 to b4% at MIS 22, followed by a steady increase in cool water species toward younger intervals. Many deep dwelling, warm water species decreased to a minimum during MIS 22 and remained extremely rare or even became absent in younger glacial intervals, indicating stepwise sea surface cooling in the region. Estimated SSTs show large fluctuations mostly at glacial interglacial transitions. A maximum winter temperature difference of 8C (7 28 8C) across MIS 23/22 boundary likely corresponded to a major growth of boreal ice sheets across the MPT center at.9 myr, coupled with a strengthened winter monsoon over East Asia. The MPT event not only led to a better correlation between changes in species abundances and glacial interglacial cycles but also a more constrained thermocline that shoaled considerably during subsequent glacial periods. The oxygen isotope record and the abundance of shallow water species display power spectra closely in pace with the 4, and, years cyclicities. A lower coherence over these cyclicities between deep-water dwelling species and the planktonic d 8 O, a shoaled thermocline, and more pisitive glacial d 8 O together suggest disturbances of surface and subsurface waters by intensified winter monsoons over the last..5 myr in the South China Sea. D 25 Elsevier B.V. All rights reserved. Keywords: Mid-Pleistocene; Climate transition; Planktonic foraminifera; Glacial cycles; South China Sea; ODP Site 44 * Corresponding author. State Key Laboratory of Marine Geology, Tongji University, Shanghai 292, China. Fax: addresses: qli@mail.tongji.edu.cn, qianyu.li@adelaide.edu.au (Q. Li). 3-82/$ - see front matter D 25 Elsevier B.V. All rights reserved. doi:.6/j.palaeo

2 35 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) Introduction The mid-pleistocene climate transition, or MPT, is marked by a gradual change in the dominant climate periodicity from 4, to, years at about.9 myr, corresponding to the marine isotope stages (MIS) 23/22 boundary (Prell, 982; Berger et al., 993). This change has been well documented with isotopic and faunal climate proxies from many localities (Raymo et al., 997; Schmieder et al., 2; Wang et al., 2), and has been related to a major growth of ice sheets and a reduction in CO 2 in the northern hemisphere (Ruddiman et al., 989; Berger et al., 993; Shackleton, 2), or alternatively an increase in the zonal SST gradient across the equatorial Pacific (Garidel- Thoron et al., 25). Schmieder et al. (2) studied the magnetic susceptibility record from the deep South Atlantic and found three phases or events associated with the MPT: an initial phase at ~92, years with a carbonate accumulation minimum, an interim stage from 92, to 54, years, and a Table Position of marine isotope stages (MIS) and other events at Site 44 based on planktonic isotopic data of Bühring et al. (24) MIS Depth mcd at base 3 yr at base FO G. ruber pink LO Stilostomella Microtektites FO=first occurrence; LO=last occurrence. 25 o 2 o 5 o o 5 o N o Red Indochina South China Pearl 7954 Sea 43 PRM 46 China m Borneo South 7957 m MD97242 Sulu Sea Bashi Strait 769 Luzon 5 o E o 5 o 2 o 25 o Fig.. Location of ODP Site 44 and other coring sites in the South China Sea. Site 44 is close to the Pear River mouth (PRM). terminal event at 54 53, years marked by a conspicuous diatom buildup. Why the climate transition from 4, to, years cycles occured in the middle Pleistocene and how this transition was mediated still remain to be known. It was suggested by Rutherford and D Hondt (2) and Wang et al. (23) that the mechanism triggering this transition may include a tropical forcing. Studies of high resolution records are needed to shed some light on the enigma, but high quality data on a millennial or finer time resolution from the tropics and subtropics are limited because well-preserved, continuous sections with a high sedimentation rate are rare. The Ocean Drilling Program (ODP) Leg 84 in the South China Sea (SCS) in 999 recovered some expanded late Neogene sequences with minimal dissolution because of the greater sill depth compared to other localities of the tropical western Pacific (Wang et al., 2). We address the climate change issue during the MPT in this high-resolution study of planktonic foraminifers from ODP Site 44 from the subtropical northern SCS, a site with the highest sedimentation rate ever recovered in the western Pacific region. The aim of our study was to document the planktonic foraminiferal response to the MPT on a

3 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) millennial time scale and to analyze the climatic impacts of the changing strength of glacial interglacial cycles and east Asian monsoons for the past..5 myr. 2. Material and methods ODP Site 44 is located at 283.8V N, V E in the northern South China Sea (SCS), at a water depth of ~237 m, which is above the sill depth of the Bashi Strait (26 m) (Fig. ). Three holes were drilled to a composite depth of 59.9 mcd (meter composite depth), and the oldest sediment recovered was of earliest Pleistocene age. Sedimentation rates at this site are extremely high: 93 m/myr for the last.3 myr and about 39 m/myr between.3 and. myr because of its vicinity to the Pear River mouth (Wang et al., 2). With excellent core recovery (%), this thick sediment succession is ideal for high-resolution studies of fine-scale paleoclimatic and paleoceanographic changes in the region. The hemipelagic sediments are characterized by fine-grained, clay-sized terrigenous material, quartz silt, calcareous nannofossils and foraminifers, with frequent black biron sulfideq mottling and pyrite. At 386 mcd is a thin layer consisting entirely of microtectite grains (Wang et al., 2; this study). A total of 475 samples from 3.82 to 58. mcd of Site 44, taken mainly from Cores 44A and 44B as a spliced section (see also Bühring et al., 24), were used in this study. Sample spacing varies from 3 cm between 3.82 and mcd, to 5 to 7 cm between and 58. mcd. Three short intervals lack samples due mainly to contamination: mcd, mcd and mcd. On G. ruber δ 8 O( ) abundance/g FDX warm/cold FO G. ruber (pink) 5 6 depth (mcd) LO Stilostomella microtektites Pulleniatina D S coarse fraction % fragmentation % undecided Fig. 2. Oxygen isotopes (Bühring et al., 24), coarse fraction N.63 mm, planktonic foraminifer abundance, fragmentation and foraminifer dissolution index, and warm/cool species ratio for the interval 3 to 58 mcd at Site 44. Marine isotope stages (MIS) 4 to 29 are labeled, and glacial stages are shaded.

4 352 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) the basis of isotopic stratigraphy, the time resolution of these samples varies from to years, or ~ years on average. Samples, about 2 cc each, were processed using standard technique (Wang et al., 2). Residues N.63 mm were collected and separated into two fractions using a.5 mm sieve. Over 4 planktonic foraminifers from a manageable split of the N.5 mm fraction were sorted and identified, and the percentage abundances of various species were calculated and graphed. We followed the species taxonomy of Kennett and Srinivasan (983), Bolli and Saunders (985) and Hemleben et al. (989). Ecological preferences of species to different climate belts or faunal provinces and to different water layers, as summarized by Bé (977) and Hemleben et al. (989), were used for grouping species into faunal groups. The relative abundances of shallow water species that mainly live in the mixed layer and deepwater species that mainly live within and below the thermocline were used to estimate thermocline depth changes (Pflaumann and Jian, 999; Jian et al., 2). Fragments of planktonic foraminifers were also counted. Fragmentation evaluation, based on 8 fragments equal one complete test, was applied as an index of carbonate dissolution in bottom water (Le and Shackleton, 992). The results are viewed as a dissolution reference rather than a fixed index because of the informal nature of the method. Sea surface temperature (SST) was estimated using the transfer function FP-2E of Thompson (98) applied to planktonic foraminifer census data (see Appendix A). We also tested other methods including the modern analog technique (MAT) (Prell, 985) and SIMMAX-28 (Pflaumann and Jian, 999). Spectral analyses of planktonic d 8 O and several species abundances were performed to determine their coherence on major orbital bands. The software package warm species % 4 7 G. sacculifer % 2 4 Gr. tumida % Gr. menardii % depth (mcd) G. ruber % 5 G. conglobatus % 4 G. menardii/g. inflata Fig. 3. Percentage abundance of warm water species at Site 44 decreases up-section, with sudden changes across some glacial/interglacial boundaries.

5 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) ARAND developed by Philip Howell of Brown University was used for spectral analyses. Planktonic d 8 O data obtained on Globigerinoides ruber were provided by Bühring et al. (24). Other species were not measured for d 8 O in this study. 3. Results 3.. Age model and (bio)stratigraphic events Planktonic d 8 O stratigraphy (Bühring et al., 24) indicate that the studied interval between 3 and mcd spans marine oxygen isotope stages (MIS) 4 29 and older, representing continuous sedimentation for, at least the past..5 myr (Table ). The center of the MPT (.9 myr) falls at ~48 mcd, corresponding to the MIS 22/23 boundary. Abundant microtektites at ~386 mcd represent a major meteorite impact event close to the Brunhes/Matuyama boundary at.78 myr as reported widely from the Indo-Pacific region (Zhao et al., 999; Koeberl and Glass, 2) (Fig. 2). The interval below mcd has no age control points though the foraminifer assemblage suggests an earliest Pleistocene age (see below). Important foraminifer datum levels that relate to the geochronologic framework of Site 44 include:. The first occurrence (FO) of G. ruber pink form at 33 mcd, in the lower part of MIS 4. This datum bears an age of ~.55 myr, about.3 myr older than its consistent occurrence level dated previously at.42 myr (Wang et al., 2; Li et al., 25). 2. The last occurrence (LO) of the benthic foraminifer Stilostomella spp. at ~356 mcd in the lower part of MIS 7 that is ~.69 myr, or ~7, years older than the.62 myr record from other tropical Indo- Pacific regions (Schönfeld, 996). 3. At.5 mcd, the coiling direction of Pulleniatina obliquiloculata changs from sinistral to dex warm/cold G. inflata % G. falconensis % 5 N. pachyderma % depth (mcd) cold species % 5 G. bulloides % 35 7 N. dutertrei % Fig. 4. Abundance of cool/cold water species increases up-section, with peaks ~7% at MIS 22 and at MIS22/23 transition.

6 354 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) tral. This probably marks the last coiling change event in this taxon thus suggesting.6 myr age (Kennett and Srinivasan, 983; Qin, G.X., personal communication, 22). This finding has two implications: () the bottom part studied at Site 44 (59 mcd) may be ~.7 myr old or older (Bühring et al., 24), and/or (2) a hiatus lasting up to.6 myr (between. and.6 myr) could be present near the mcd level. Immediately above this level, at mcd, the LO of small nannofloral Gephyrocapsa acme bears an age of about. myr (Wang et al., 2). 4. An age older than. myr for the bottom part of Site 44 is also supported by the occurrence of Neogloboquadrina humerosa, a late Miocene to early Pleistocene planktonic foraminifer (8.5.3 myr) found mainly below mcd. Therefore, we interpret the interval below mcd as bolder than MIS 29Q pending further chronostratigraphic precision Abundance variations of planktonic foraminifer species The preservation of planktonic foraminifers in most samples is good, although 5 % or more fragmented tests show some dissolution. As in previous studies (Le and Shackleton, 992; Wang et al., 999; Xu et al., 25) more fragments are found in interglacial (maximum 6 8%) than in glacial intervals (mostly 5%) indicating a "Pacific Type" carbonate dissolution pattern. There are exceptions, however, with up to 3 4% fragmented tests in some glacial intervals (Fig. 2). Absolute abundances vary from b 3 to N4 specimens/g of dried sample (Fig. 2). More species and specimens are recorded in interglacial than in glacial intervals, although the absolute abundance may increase to 3 specimens/g at some levels in glacial MIS 6 and MIS 22 due to sudden increases of cool water forms, especially Globorotalia inflata and Neogloboquadrina pachyderma. G. aequilateralis % Sphaeroidinella % Gr. truncatulinoides % depth (mcd) Orbulina % deep water species % Pulleniatina % Gr. tumida % Fig. 5. Abundance profiles of Orbulina, Globigerinella, and the deep-dwelling taxa Pulleniatina, Sphaeroidinella and Globorotalia.

7 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) The planktonic foraminifer assemblage is typical of the subtropical faunal province (Bé, 977), and is dominated by Globigerinoides (G. ruber and G. sacculifer), Globorotalia (warm water G. menardii and cool water G. inflata), and Neogloboquadrina (N. dutertrei). There is a major shift in the distribution of warm and cool groupings at ~42 mcd, with warm species averaging N5% below (Fig. 3) and cool water specimens increasing to 35% or more (Fig. 4) above this level. Fluctuations are evident in all groupings, mainly responding to the alternation of glacial interglacial cycles, particularly above ~4 mcd (Figs. 3 5) Sea surface temperature estimates The SST estimates using the transfer function FP- 2E show relatively stable summer SSTs of C over the studied interval (Fig. 6). Prior to MIS 23, both summer and winter SSTs were relatively high and stable, indicating a relatively warm and less variable climate. Since MIS23, however, the winter SST decreased and varied more strongly, with a largest amplitude change from C occurring at the center of the MPT, about.9 myr (Fig. 6). The unusually low winter SST at the time caused a maximum of over 8C differences between summer and winter SSTs (Fig. 7). These variations contrasts sharply with the steady record of ~26 8C at core MD97-24 from the central Western Pacific Warm Pool (WPWP) (282V N, 4846V E, water depth 2547 m) (Fig. 8) (Garidel- Thoron et al., 25). SST variations across other glacial interglacial boundaries at ODP 44 appear to be much less dramatic, although SST estimations using the FP-2E method may only provide a general trend rather than a truly quantitative record. The SIMMAX-28 results as shown also in Fig. 7, however, often underestimate the glacial cooling, and G. ruber δ 8 O( ) winter SST FP-2E summer SST SST SIMMAX-28 SST similarity 5 o C FO G. ruber (pink) winter summer 5 6 Age ( 3 yr) LO Stilostomella microtektites Pulleniatina D S o C o C Fig. 6. Time plot of d 8 O, estimated winter and summer SST and their differences (DSST) using the FP-2E method. Also shown are results using the SIMMAX-28 method, showing rapid shifts to extremes due to a lack of mid to high latitude analogs.

8 356 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) G. ruber δ 8 O( ) SST cold species warm vs. cold thermocline 4 8% m LGM value Holocene value Age ( 3 yr) o C 5 % 35 7% shallow, warm deep species Fig. 7. Time plot of d 8 O, DSST using the FP-2E method, abundance and ratio of warm/cold water species and deep-dwelling species, and estimated thermocline depth changes. Arrows indicate major steps in thermocline shoaling. Mean d 8 O values for the Holocene and last glacial maximum are also indicated. their low sensitivity to paleotemperature changes could be due to the lack of suitable analogues from temperate to subpolar regions in the calibration data set (Steinke et al., 2). Therefore, we mainly refer to the FP-2E estimates in our discussion Spectral analysis results The spectral analysis reveals the orbital relationship between planktonic d 8 O and 4 main planktonic species (Fig. 9) for the entire studied interval. The pre-.9 myr section is too short at Site 44 to be analyzed independently. The 4, years obliquity band is dominated by P. obliquiloculata and G. sacculifer, while the, years eccentricity band (varying between 95, years) characterizes the records of G. ruber, G. menardii and P. obliquiloculata. The lack of the, years band in G. sacculifer was probably due to the disturbance by monsoons, as discussed below. 4. Discussion 4.. Comparison between the northern and southern South China Sea records Our results indicate that planktonic foraminifer responses to glacial interglacial cycles were mainly by changes in species abundances, with strongest fluctuations cross the mid-pleistocene transition at about.9 myr (Figs. 2 5). More warm-water species occurred before this time boundary, whereas cold to cool water species increased substantially in younger

9 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) Age ( 3 yr) G. ruber δ 8 O( ) SST( o C) ODP 44 MD97-24 MPT MD Fig. 8. The d 8 O records from ODP Site 44 (Bühring et al., 24) and MD97-24 (central western Pacific warm pool, Garidel- Thoron et al., 25) plotted together to show higher d 8 O values from Site 44. The Mg/ Ca-derived SST curve from MD97-24 reveals a stable ~26 8C during MIS 23 and MIS 22, which is similar to the estimates for the major glacial MIS 6 at this central WPWP site. intervals. In parallel to this general trend, shallow or mixed layer species drop to only ~ 5% during and after MIS 22 (Fig. 2). Planktonic foraminifers in Core 7957 from the southern SCS (853.9V N, 588.3V E, water depth 295 m) also show rapid decreases in winter sea surface temperature from ~27 to 25 8C and changes in thermocline depth from 22 to ~7 m at.9 myr (Jian et al., 2). In Core 7957, a decrease in the abundances of tropical radiolarian species close to.9 myr was probably related to a southward shift of the North Equatorial Current likely induced by variations in the northern trade wind system during the MPT (Wang and Abelmann, 22). At ODP Site 43 in the southern SCS (982.72V N, 387.V E, water depth 2772 m), planktonic foraminifer changes are also in pace with a progressive MPT. The most remarkable feature, however, is that the abundance of P. obliquiloculata became reversed from high in interglacial intervals before the MPT to high in glacial intervals after the MPT (Xu et al., 25). As P. obliquiloculata prefers high salinity and is closely associated with the warm saline Kuroshio Current (Li et al., 997; Xu et al., 25), its high abundance in glacial intervals after the MPT may signal a more saline southern SCS when sea level was up to 2 m lower than today, the basin was semi-enclosed, and evaporation was high (Wang, 999; Wang and Wang, 99). However, P. obliquiloculata remains its high abundance in interglacials after the MPT in northern SCS localities including Site 44 (Fig. 5), probably signaling a continuous influence of the west Pacific water through the Bashi Strait ( 26 m) during all glacial and interglacial periods (Li et al., 24) Glacial interglacial contrasts during the mid-pleistocene transition A good correlation exists between planktonic foraminifer changes and the oxygen isotopic record on the long-term time scale (Figs. 2 5). If the isotope record chiefly reflects the waning and waxing of polar ice sheets, the planktonic foraminifer results reported here must have mainly resulted from temperature changes over glacial interglacial cycles. Prior to.9 myr, below 42 mcd, however, this relationship is unclear for many species. Abundant warm water species occur below 42 mcd, indicating a much warmer climate regime before the mid-pleistocene transition. This warm climate regime, however, abruptly changed with average winter SST decreasing from ~26 8C in early MIS 29 to ~22 8C in MIS 23 (Fig. 6). Accompanying this there was an intensification of the original relatively weak glacial interglacial contrasts and small summer winter SST differences. The largest magnitudes in winter SST fluctuations from 7.5 to 28 8C occurred at about.9 myr, at the center of the MPT (Figs. 6 and 7). The unusually low winter SST at the time caused a maximum of ~ 8C differences between summer and winter SST, the highest DSST value for the whole section (Fig. 7). These large SST drops in the SCS coincided with a period of major ice sheet expansion on the northern hemisphere (Ruddiman et al., 989; Berger et al., 993; Shackleton, 2), suggest-

10 358 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) ka Power of G. ruber ka 6 4 Power of G. sacculifer Power of δ 8 O 5 Power of δ 2 O 5 4 ka 4 4 ka Coherency.4.4 Coherency ka ka 7 4 ka Power of G. menardii 2 5 Power of P. obliquiloculata 5 Power of δ 8 O 6 Power of δ 8 O 4 ka ka Coherency Coherency Frequency (/ka) Frequency (/ka) Fig. 9. Spectral analyses of four common planktonic species, showing spectrum powers and coherence with d 8 O at ODP Site 44. BW=.8672; 8% coherency= ing a climate teleconnection between low and high latitudes. After this winter temperature maximum, the mean winter SST at ODP Site 44 increased only slightly, from 2 to 23 8C (Fig. 7). Contrary to these strong fluctuations in the SCS, the Mg/ Ca-derived SST from the open western Pacific was stable at about 26 8C during MIS 23 and MIS 22 at IMAGES core MD97-24 (Fig. 8) (Garidel-Thoron et al., 25). It is not clear whether the high temperature contrast in our record represents an amplified signal or the methods mentioned, including the FP-2E, d 8 O and Mg/ Ca ratios, have been biased. Skinner and Elderfield (25) recommended that the d 8 O and Mg/ Ca methods also need some independent tuning. Glacial to interglacial transitions of planktonic foraminfer assemblages at ODP Site 44 had been rapid on a millennial scale. Sudden jumps in the ratios between the total warm and cold water species groups and between G. menardii and G. inflata indicate rapid transitions across glacial/interglacial boundaries (Fig. 4). These rapid changes from one climate mode to another continued into younger periods, although the overall climate had shifted after the MPT to much cooler conditions as shown by more abundant coolwater species. The cooler climate regime started affecting planktonic foraminifers more obviously during glacial periods. It is noteworthy, however, that climate deteriorations after the MPT on the millennial scale began early even in the later part of interglacials and continued across interglacial/glacial boundaries. For instance, cool to cold water species such as G. bulloides, G. pachyderma and G. inflata increased their abundance in the later part of MIS 2, 9 and 7 although the increases were neither always simulta-

11 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) neous nor of a similar amplitude due likely to a variable strengthening of the winter monsoon (Fig. 4). Declines in G. bulloides and G. pachyderma during some glacial periods including MIS 2, however, cannot be considered as representing a weakening winter monsoon because their abundance was simply suppressed by the more abundant G. inflata. These changes in the planktonic foraminifer response to interglacial glacial transitions mimic the more familiar, latest Pleistocene pattern from MIS 5 to 2 as reported from other localities of the SCS (Wang and Wang, 99; Wang et al., 999; Jian et al., 2) or from other parts of the globe, marking the typical, year climate cycles (Shackleton, 987, 2; Jouzel et al., 22). It is not surprising, therefore, that such typical late Pleistocene interglacial to pleniglacial to glacial transitions first started ~.9 myr ago at the turning point of the MPT because the faunal and d 8 O changes were both simultaneously affected by the establishment of the, year eccentricity cycle (Berger et al., 993; Raymo et al., 997) with distinct asymmetric glacial and deglacial patterns (Tziperman and Gildor, 23). Finer scale faunal changes from glacial to interglacial cycles were more rapid from.9 to ~.7 myr, between MIS 22 and 7, as exemplified by the warm/cool species ratio (Fig. 4). The strongest post-mpt glaciation during MIS 6 decreased all tropical subtropical species but encouraged an exceptionally high accumulation of cool-water species at the ODP Site 44 locality. The impact of this glaciation continued into the subsequent interglacial MIS 5 and glacial MIS 4 periods with minimal recovery of warm water species (Fig. 4) and a relative steady mean winter SST (Fig. 7). Glacial d 8 O values at Site 44 were consistently higher than in the open western Pacific record (Fig. 8), suggesting the influence of local climate factors including the East Asian winter monsoon Upper water stratification and thermocline depth Planktonic foraminifer species are good indicators of upper water stratification because they are found mainly living in specific layers of the world ocean. G. ruber is a typical surface water species living in the upper 5 m of the water column, while globorotaliid forms are more frequent in deeper waters with relative heavy d 8 O(Fairbanks et al., 982; Hemleben et al., 989). Surface water species increase in abundance when the thermocline deepens, and vice versa for deepwater dwellers including species of Globorotalia, Pulleniatina and Sphaeroidinella (Anderson and Ravelo, 997; Jian et al., 2). Fig. 5 shows the percentage abundance of the deepwater dwelling group, plus some ubiquitous low- to mid-latitude species such as Globigerinella aequilateralis and Orbulina spp. The total abundances of the deepwater dwelling group fluctuate between 3-5% throughout the studied interval, suggesting that the upper water structure in the northern SCS was relatively stable during the middle Pleistocene. The abundance variations of these deep dwellers in relation to glacial interglacial cycles, however, were not great until MIS 23; only then did they exhibit a relatively closer relationship with isotopic fluctuations (Fig. 5). The increases in P. obliquiloculata and G. menardii groups during interglacials at and after MIS 22 corresponds to the development of a well-constrained thermocline that shoaled during the MPT period. The reconstructed thermocline changes (Fig. 7) show strong fluctuations in the MIS 22-9 interval, from about 2 to ~6 m. Cautions should be exercised, however, when interpreting these results from faunal percentage data because they may only convey generalised trends rather than a precise signal. Nevertheless, these estimated thermocline fluctuations may still suggest paleoceanographic conditions with a shoaled thermocline caused through a mixing of the surface and subsurface waters by an intensified East Asian winter monsoon. Similarly, the decline of S. dehiscens after MIS 23 and the decrease of G. tumida and other deepdwelling species to almost zero after MIS 22 may have resulted from a less stratified upper water column with a weaker thermocline because of constant strong winter monsoon winds in a cool climate regime (Fig. 7) Faunal proxies of an astronomically forced monsoon climate The MPT marks the transition in dominance from 4, to, years cyclicity, shaping the Quaternary climate pattern into two modes (Prell, 982; Berger et al., 993; Raymo et al., 997; Schmieder et al., 2; Wang et al., 2). Orbital forcing is also reflected in individual planktonic foraminifer species as shown in Fig. 9. A prominent eccentricity response

12 36 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) (~, years) is shown by G. ruber and P. obliquiloculata. The obliquity (39, 4, years) and precession (9, 23, years) cycles are also strongly expressed by planktonic species especially P. obliquiloculata but are not well constrained in comparison to the d 8 O record. For G. sacculifer, however, the, years cycle is missing and the 4, years cycle is weak. These results suggest that () faunal responses to orbital forcing climatic changes differ in both frequencies and amplitude, and (2) local factors especially the monsoons may have played a crucial role in the low coherencies between the faunal and the d 8 O records (Chen and Huang, 998; Wang et al., 999). Other factors such as nutrients discharges from the Pearl River during sea level lowstands when the River mouth was more proximal to the studied site may have been important. Any river influence, however, is believed to have been low because coarse fraction sediment data only show small variations especially after the MPT (Fig. 2). Pollen results from Site 44 also suggest a very limited extent of exposed continental shelf before MIS 6 (Sun et al., 23). Stronger winter monsoon during glacials, particularly since MIS 6 (Sun et al., 23), likely reduced rainfall and runoff in the SCS region. Surface circulation in the South China Sea is presently controlled by the east Asian monsoon. Many studies of late Pleistocene marine records from the region demonstrate that a stronger winter monsoon developed during glacial times and a stronger summer monsoon during interglacial periods, likely due to a global ice volume forcing (Wang and Wang, 99; Wang et al., 999; Chen and Huang, 998; Chen et al., 23; Wei et al., 23). A decrease in atmospheric carbon dioxide concentration or a change in the carbon reservoir has been considered as the main trigger of this global cooling (Berger et al., 999; Shackleton, 2; Wang et al., 23). However, stable sea surface temperature found in the WPWP over the past.75 million years has led Garidel-Thoron et al. (25) to suggest that zonal SST gradient changes across the Pacific, rather than CO 2, may have had a significant influence on the mid-pleistocene climate transition. The monsoon climate is closely related to the development of the WPWP in both space and time, and this climate pattern is found amplified from marginal seas especially the SCS (Wang, 999; Wang et al., 2). Wei et al. (23) attributed the more negative d 8 O values of core MD97242 from the central eastern part of the SCS than those of ODP Site 769 from the Sulu Sea to high precipitation driven by intensified summer monsoons. It should be noted, however, that d 8 O values from sites along a central belt appear to be consistently lower than those from other localities. This belt approximately corresponds to the W-E chain of reefs that separates the southern from the northern SCS. For example, Holocene and last glacial maximum d 8 O values in cores 7954 and 7956 from the west are similar to those in MD97242 from the east, but are lower by ~.3.6x than those in cores from the north and the south (Wang et al., 999). Core 794 (7823.V E, 287.V N, water depth 727 m) is located on the northern slope of the SCS, in the vicinity of ODP Site 44. The average Holocene and LGM d 8 O values are respectively,.2x and ~ 2.5x from core 794 (Wang et al., 999), and ~.x and 2.5x from Site 44 (Bühring et al., 24). Measured by the same laboratory at Kiel University, Germany, these comparable results can then be used as references to determine the strength of variations in glaciations and winter monsoons during the MPT. At Site 44, d 8 O values higher than.x are recorded in six glacial periods between MIS 26 and MIS 6, with the highest.7x in both MIS 22 and MIS 6 (Fig. 7). The d 8 O values for the intermittent interglacial periods are generally between 2.3 and 2.5x. All this indicates that climate conditions between.97 and.62 myr were extremely cold, with SST of about to 3 8C lower than the LGM using the function of.25x d 8 O for 8C temperature change for the region (Wang and Wang, 99; Wang et al., 999). The glacial interglacial SST differences in the late Pleistocene SCS were estimated to have been about 3 6 8C in earlier studies (Wang and Wang, 99; Wang et al., 999). A drop of SST of up to 9 8C during the MPT as compared to the Holocene is supported also by paleotemperature estimates using the FP-2E method (Fig. 7). High d 8 O values, however, not only result from global cooling due to ice sheet expansion but also from intensified upwelling due to the winter monsoon, although separation of these signals using the d 8 O record alone is difficult. The fact that d 8 O values in the four glacial periods, MIS 22, 2, 8 and 6, are all higher than the LGM

13 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) record at Site 44 have no match in high latitude records of either the north Atlantic (Ruddiman et al., 989; Raymo et al., 24) or south Atlantic (Schmieder et al., 2), indicating that the Site 44 d 8 O record must have preserved signals of both global ice volume and local monsoons. The planktonic d 8 O record by Garidel-Thoron et al. (25) from the central WPWP shows much lower glacial values, from.x in MIS 22 to.4x in MIS 2 to.5x in MIS 8, or about.4.7x lower than the Site 44 record (Fig. 8). A cooler SCS during glacial periods likely resembled the present winter climate conditions controlled by a cold tongue of C SST over the entire SCS due to intensified winter monsoon winds (Liu et al., 24). The upward movement of the thermocline from ~5 to b7 m water depths occurred in steps after.9 myr, apparently in pace with glacial interglacial cycles (Fig. 7). A shoaled thermocline depth may have accounted for the prolific growth of subsurface species to abundance peaks of ~7% at MIS 23 and MIS 22, but their sharp decline during subsequent glacial intervals should have been driven by stronger winter monsoons that weakened the upper water stratification through upwelling, in addition to a marked surface water cooling (Fig. 7). We would expect a closer coherent relationship between deeper water dwellers and the d 8 O record if their distributions were indeed only glacially driven. However, the subsurface to thermocline dwellers such as G. menardii (over the 4, years band) and P. obliquiloculata (over the, years band) show a weaker coherence with the d 8 O signal than their mixed layer counterparts including G. ruber, indicating the disturbance of surface subsurface waters by monsoons rather than a sole effect by low glacial SST (Fig. 9). The consistently higher glacial d 8 O values than their counterparts from the central WPWP (Fig. 8) also suggest a local monsoon influence. Therefore, the coherence between planktonic d 8 O and subsurface species abundance has potential to become a new marine time proxy of East Asian monsoons (Li et al., 24). A strongly fluctuating thermocline and extreme SST, as indicated by both planktonic foraminifer faunal and d 8 O variations, characterize the most important MPT period between.9 and.6 myr. A strengthened winter monsoon during this period was probably the main factor triggering cool-water invasion and some physical disturbances that distorted the upper water structure and subsequently affected planktonic foraminifer abundances and the planktonic d 8 O signal recorded at ODP Site Conclusions Planktonic foraminifer responses to the mid-pleistocene climatic transition documented in 475 samples from 3 58 mcd at ODP Site 44 in the northern South China Sea, provide the first millennial record with resolution of ~ yr for the western Pacific. The abundance fluctuations of major species provide a detailed record of climatic conditions over MIS 29 4, between. and.5 myr. Abundant warm water species typified by Globigerinoides occur in the lower part of the section, with 6% average abundance from MIS 29 and older intervals. Their abundances decrease to b4% during MIS 22 and younger glacial periods. In contrast, cool and cold water species including Globigerina bulloides, Neogloboquadrina pachyderma and Globorotalia inflata increase from b2% prior to MIS 23 to N35% in MIS 5 and 4. The deep dwelling, warm water species Sphaeroidinella dehiscens decreased to a minimum during MIS 22 and remained extremely rare, -4%, throughout the upper part of the section. These planktonic foraminifer changes across the MIS 22/23 boundary mark the major turning point of the mid-pleistocene transition at.9 myr. A well stratified upper water column and a stepwise shoaling thermocline fluctuating closely with glacial-interglacial cycles after.9 myr are indicated by deep dwelling planktonic foraminifers. The subsequent severe glacial coolings, however, almost completely eliminated several deep dwelling, warm water species including Globorotalia tumida (especially at MIS 6). Paleo-SST estimated using transfer function FP-2E and other methods shows smaller changes in summer and winter temperature and their differences before the MPT. The estimated SST was 29 8C for summer and C for winter before the MPT, but changed to C and 2 8C after the MPT, respectively. A maximum winter temperature difference of 8C (7-28 8C) was estimated for MIS 23/22 transition. Such large SST variations differ from climate records of the open western Pacific and other regions,

14 362 F. Zheng et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 223 (25) suggesting the influence of local climate factors mainly the winter monsoon. The faunal responses to glacial-interglacial climate changes on a millennial time scale were rapid with marked shifts in species abundance, but were gradual and progressive on a longer time scale. Surface dwelling species exhibit abundance changes closely coherent with the d 8 O record over the 4, yr obliquity and, yr eccentricity bands. Increases in the abundance of deepwater planktonic foraminifers, a shoaled thermocline depth, high planktonic d 8 O values, and a poor coherence between subsurface species and d 8 O over astronomical bands together indicate a strengthening of the East Asian winter monsoon during the mid-pleistocene transition in the western marginal Pacific. Acknowledgments This research used samples and data provided by the Ocean Drilling Program (ODP). ODP is sponsored by the U.S. National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. Funding for this research was provided by the National Natural Science Foundation of China (Grant ), National Key Basic Research Fund of China (Grant G278), Chinese Academy of Sciences (Knowledge Innovation Project No. KZCX3-SW- 22), and Australian Research Council. Zhifei Liu provided the base map for Fig.. The manuscript was reviewed by Ann Holbourn and an anonymous reviewer, whose comments greatly improved the final presentation of this paper. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:.6/ j.palaeo References Anderson, D.J., Ravelo, A.C., 997. Tropical Pacific ocean thermocline depth reconstructions for the last glacial maximum. Paleoceanography 2, Bé, A.W.H., 977. An ecological, zoogeographical and taxonomic review of recent planktonic foraminifera. In: Ramsay, A.T.S. (Ed.), Oceanic Micropaleontology, vol.. Academic Press, London, pp.. Berger, W.H., Bickert, T., Jansen, E., Wefer, G., Yashuda, M., 993. The central mystery of the Quaternary Ice Age. Oceanus 36, Berger, A., Li, X.S., Loutre, M.F., 999. Modelling northern hemisphere ice volume over the last 3 Ma. Quaternary Science Reviews 8,. Bolli, H.M., Saunders, J.B., 985. Oligocene to Holocene lower latitude planktonic foraminifera. In: Bolli, H.M., Saunders, J.B., Perch-Nielsen, K. (Eds.), Plankton Stratigraphy. Cambridge University Press, Cambridge, pp Bühring, C., Sarnthein, M., Erlenkeuser, H., 24. Toward a highresolution stable isotope stratigraphy of the last. million years: site 44, South China Sea. Proceedings of Ocean Drilling Program, Scientific Results 84, 29 (online). Chen, M.-T., Huang, C.-Y., 998. Ice-volume forcing of winter monsoon climate in the South China Sea. Paleoceanography 3, Chen, M.-T., Shiau, L.-J., Yu, P.-S., Chiu, T.-C., Chen, Y.-G., Wei, K.-Y., 23. -year records of carbonate, organic carbon, and foraminiferal sea-surface temperature from the southeastern South China Sea (near Palawan Island). Palaeogeography, Palaeoclimatology, Palaeoecology 97, 3 3. Fairbanks, R.G., Sverdlove, M., Free, R., Wiebe, P.H., Bé, A.W.H., 982. Vertical distribution and isotopic fractionation of living planktonic foraminifera from the Panama Basin. Nature 298, de Garidel-Thoron, T., Rosenthal, Y., Bassinot, F., Beaufort, L., 25. Stable sea surface temperatures in the western Pacific warm pool over the past.75 million years. Nature 433, Hemleben, C., Spindler, M., Anderson, O.R., 989. Modern planktonic foraminifera. Springer-Verlag, New York. Jian, Z., Wang, P., Chen, M., Li, B., Zhao, Q., Buhring, C., Laj, C., Lin, H., Pflaumann, U., Bian, Y., Wang, R., Cheng, X., 2. Foraminiferal responses to major Pleistocene paleoceanographic changes in the southern South China Sea. Paleoceanography 5, Jouzel, J., Hoffmann, G., Parrenin, F., Waelbroeck, C., 22. Atmospheric oxygen 8 and sea-level changes. Quaternary Science Reviews 2, Kennett, J.P., Srinivasan, M.S., 983. Neogene Planktonic Foraminifera: A Phylogenic Atlas. Hutchinson Ross Publishing Company, New York, pp Koeberl, C., Glass, B.P., 2. Tektites and the age paradox in mid- Pleistocene China. Science 289, 57. Le, J., Shackleton, N.J., 992. Carbonate dissolution fluctuations in the western equatorial Pacific during the late Quaternary. Paleoceanography 7, Li, B., Jian, Z., Wang, P., 997. Pulleniatina obliquiloculata as a paleoceanographic indicator in the southern Okinawa Trough during the last 2, years. Marine Micropaleontology 32,

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