A Holocene paleotemperature record based on radiolaria from the northern Okinawa Trough (East China Sea)

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Quaternary International 183 (2008) 115–122

A Holocene paleotemperature record based on radiolaria from the northern Okinawa Trough (East China Sea) Chang Fengming, Li Tiegang, Zhuang Lihua, Yan Jun Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China Available online 29 December 2006

Abstract Using a radiolarian-based transfer function, mean annual sea surface temperature (SST) and seasonal temperature range are reconstructed through the last 10,500 yrs in the northern Okinawa Trough. Down-core SST estimates reveal that throughout the Holocene the changes of mean annual SST display a three-step trend: (i) an early Holocene continuous warming between 10,500 and 8500 yr BP which ends up with a abrupt cooling at about 8200 yr BP; (ii) a relatively stable middle Holocene with high SST that lasted until 3200 yr BP; and (iii) a late-Holocene distinct SST decline between 3200 and 500 yr BP. This pattern is in agreement with the ice core and terrestrial paleoclimatic records in the Chinese continent and other regions of the world. Five cooling events with abrupt mean annual SST drops, which occur at 300–600, 1400, 3100, 4600–5100 and 8200 yr BP, are recognized during the last 10,500 yrs. Comparison of our results with records of GISP2 ice core and marine sediment in North Atlantic region suggests these cooling events are strongly coupled, which implies a possible significant climatic correlation between high- and low-latitude areas. r 2007 Elsevier Ltd and INQUA. All rights reserved.

1. Introduction The Holocene is the most recent period in the geological record. When viewed in a long-term perspective, the Holocene climate is commonly thought to be relatively stable compared to the high-amplitude climate oscillations of the last glacial (e.g. Dansgaard et al., 1993; Grootes et al., 1993; Schulz et al., 1998). However, over the last decade, globally distributed multi-proxy paleoclimatic data sets indicate that Holocene climate is more variable than previously thought. Evidence of centennial- to millennialscale climate variability during the Holocene has been well documented in marine (e.g. Bond et al., 1997, 1999; Mayewski et al., 1997; Bianchi and McCave, 1999; Giraudeau et al., 2000; Andersen et al., 2004) and terrestrial records (e.g. Denton and Karle´n, 1973; O’Brien et al., 1995; Campbell et al., 1998; deMenocal et al., 2000; Nesje et al., 2000). Although the periodicity of Holocene climate oscillations is still disputable, there is no doubt that marked and abrupt climate variations have been occurred Corresponding author. Tel.: +86 532 82898522; fax: +86 532 82898526. E-mail address: [email protected] (C. Fengming).

during the Holocene. Until quite recently, the cause of Holocene climate fluctuations and its extent were still elusive, and discussion about the mechanisms of Holocene climate changes mainly focused on the northern Atlantic region and its adjacent continents for the relative scarcity of high-resolution records from the tropical and midlatitude regions. However, recent studies suggest that the low-latitude areas, especially the tropical Pacific, also have played an important role in short and abrupt paleoclimate fluctuations (e.g. David, 2002; Koutavas et al., 2002; Stott et al., 2002; Philander and Fedorov, 2003; Rodgers et al., 2003). To seek a more comprehensive view of climate variability during the Holocene, high-resolution records from the mid- and low-latitudes are indispensable. The East China Sea (ECS) is a typical marginal sea, which is connected to the western North Pacific by a curved back arc basin of the Okinawa Trough. Along the western edge of the Okinawa Trough, the famous western boundary current known as the Kuroshio Current runs northeastward (Fig. 1). In the northern Okinawa Trough, the Kuroshio Current turns southeast and then returns to the western North Pacific through the Tokara Strait. But, a branch named the Tsushima warm current (TWC) flows continuously northward into the Sea of Japan. On the

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116

50

N 35°

KOREA

50

C

TW

KYUSHU

200

CDW

30°

EAST CHINA SEA CHINA

DOC082

Ku ro sh io Cu rre A nt TR OU GH

CCC

2. Material and methods 2.1. Material The study material is one gravity-core (DOC082; 5.2 m long), which was recovered from the western slope of the northern Okinawa Trough (29113.930 N, 128108.530 E) at a water depth of 1128 m (Fig. 1). It is mainly composed of light gray clayey silt or silty clay over the upper 390 cm containing no visible turbidites or volcanic ash layers. However, in the section between 390 and 520 cm, it contains some volcanic glass shards and volcanic clasts probably indicating a few disturbances. For this study, only the uppermost 390 cm of the core has been used to carry out the isotope and radiolarian analyses, with sampling at 5 cm intervals.

OK

IN

AW

2.2. Methods

TSF

00

20

25° WESTERN NORTH PACIFIC

120°

125°

130°E

Fig. 1. Map of the East China Sea showing the general circulation pattern and the location of core DOC082 (redrawn after Lie and Cho, 2002). All bathymetric contours are in meters. CDW ¼ Changjiang diluted water, TWC ¼ Tsushima warm current, CCC ¼ China coastal current, TSF ¼ Taiwan Strait flow.

other hand, the northwestern continental shelf of the ECS is dominated by freshwater discharged from the Chinese continental rivers. Thus, the northern Okinawa Trough is influenced not only by surface water masses derived from the Kuroshio but also the freshwater from the Chinese continent. Over this study area, the average SST is about 27–29 1C in summer and 9–23 1C in winter (Tseng et al., 2000; Lie and Cho, 2002; Lee and Chao, 2003). So, sedimentation from the northern Okinawa Trough generally provides both terrestrial and ocean signals, and the area is potentially more sensitive to climatic perturbations than the open ocean. In addition, because of the large discharge of terrigenous materials from the Yangtze River, a very high sedimentation rate is recorded in the region. Thus, it gives us a good opportunity to analyze paleoclimatic changes and land–ocean linkages with a relatively higher resolution. However, high-resolution studies on the Holocene environmental changes are relatively rare in the northern Okinawa Trough. This paper presents a reconstruction of Holocene sea surface temperature (SST) variations by studying radiolarian assemblages in a sediment core from this region. And, the reconstruction is further compared with paleoclimatic records from the Chinese continent and other parts of the world in order to improve understanding of global-scale climatic changes during the Holocene period.

The chronology is based on six AMS 14C dates on planktonic foraminifera Neogloboquadrina dutertrei, which were measured at the NOSAMS Facility, WHOI, USA (Table 1). The 14C dates are calibrated to calendar (cal.) ages by employing the calibration program CALIB 4.3 (Stuiver and Reimer, 1993; Stuiver et al., 1998), and a marine reservoir age correction of 400 yrs (Bard, 1988) is used in the calibration. In the six AMS 14C age dates, the datum at the layer of 390–400 cm gives an inverted age. The record of lithology shows that the layer is composed of light gray volcanic grass with a few volcanic clasts. This layer is probably related to disturbance and was not used when constructing the age model. Thus, the entire time scale of the core DOC082 is constructed excluding the disturbed layers (390–400 and 425–430 cm) from the sequence by linear interpolation between the residual five 14 C dates. Then, this age model is applied to the uppermost 390 cm of the core. Based on the age model, this part of the core represents a sediment record of the last 10,500 yrs and sampling intervals of 5 cm resulted in an average time resolution of 135 yrs.

Table 1 AMS 14C and calendar year ages for core DOC082 Sample depth (cm)

AMS 14C age (14C yr BP)

Calibrated 14C age (cal. yr BP)a,b

Calibrated 1s age range (cal. yr BP)

105–115 160–170 210–220 260–270 390–400 500–510

2880730 35207650 5410740 6262790 14,8207190 12,440780

2695 3388 5754 6712 17,158c 13,933

2709–2669 4237–2689 5853–5720 6794–6618 17,490–16,838 14,085–13,700

a14 C ages were corrected for an oceanic reservoir effect of 400 years (Bard, 1988). b14 C ages were converted to calendar ages using the calibration program CALIB4.3 (Stuiver and Reimer, 1993; Stuiver et al., 1998). c The age date was deleted from the age model.

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Stable oxygen isotope ratio was measured on a SIRA mass spectrometer at the Godwin Institute for Quaternary Research, University of Cambridge, UK. Analyses were carried out on samples of about 30 well-preserved planktonic foraminifera specimens of Neogloboquadrina dutertrei picked from the 300 to 355 mm fractions. Analytical precision of laboratory standards is better than 70.08%. Calibration to VPDB used the NBS19 standard. Samples for radiolarian analysis were prepared following the method proposed by Roelofs and Pisias (1986) and Welling and Pisias (1995). Mesh size used for the sieving was 45 mm. Qualitative and quantitative analyses were performed at 800  and 250  magnifications using a Carl Zeiss JENA microscope. Counts and identifications were carried out on permanent slides (random mounted with Canada balsam), and several traverses across each cover slip were examined. At least 300 radiolarian specimens were counted for samples with plentiful skeletons. For samples with sparse radiolarian skeletons, all specimens were counted in the slides. At least two cover slips per sample were scanned in this way, and between 18 and 780 specimens were counted for each sample. In this work, for most samples the counting numbers are more than 300 specimens. Only two are less than 200 specimens at the layer of 380 cm (170 specimens) and 387.5 cm (18 specimens). The radiolarian-based transfer function constructed by Pisias et al. (1997) is adopted to estimate the mean annual SST and annual SST range. The transfer function was developed in the Pacific Ocean with a standard error of 1.6 1C for both mean annual SST and annual SST range. Application of the radiolarian-based transfer function to sediment records from the northeast Pacific and the eastern N .dutertrei δ18O (‰ , PDB) 0

-0.5

-1

-1.5

117

equatorial Pacific showed excellent concordance with alkenone-UK 37 and foraminifera-based estimates, suggesting that these new equations can be utilized over a wide range of oceanographic settings (Pisias et al., 1997). 3. Results and discussion 3.1. Compositional characters of radiolarian assemblage The abundance and species number of radiolarians in the core DOC082 are relatively high. A total of 130 species belonging to two orders of Spumellaria and Nassellaria were identified and counted. Both Spumellaria and Nassellaria are important in the radiolarian fauna, which present 66 and 64 species, respectively. The 10 species most commonly encountered in the core DOC082 are, given in the order of percentage concentration, as follows: Tetrapyle octacantha, Lithomelissa thoracites, Tholospyris sp., Botryocyrtis scutum, Ommatartus tetrathalamus tetrathalamus, Phorticium clevei, Peromelissa phalacra, Lithomelissa setosa, Hexapyle spp. and Stylodictya multispina. Most of these same taxa have previously been reported in the Indian Ocean (Johnson and Nigrini, 1980) and the western Pacific (Boltovskoy, 1987). 3.2. Paleotemperature records Throughout the Holocene, estimated mean annual SST ranges from 23.5 to 25.6 1C over a range of 2.1 1C, and shows a significant general trend over the last 10,500 yrs (Fig. 2). During the early Holocene between 10,500 and 8500 cal. yr BP, the mean annual SST displays a general increasing trend from 23.5 to 25.2 1C, interrupted by an Seasonal temperature range (˚C)

-2

4

6

8

10

0 1000 2000

Cal. age ( yr BP)

3000 4000 5000 6000 7000 8000 9000 10000 11000 23.5 24.0 24.5 25.0 25.5 26.0 Mean annual SST (˚C) Fig. 2. Time-series records of planktonic foraminifera N. dutertrei d18O, mean annual sea surface temperature and seasonal temperature range estimated using radiolarian-based transfer function in core DOC082 from the northern Okinawa Trough.

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abrupt drop at about 8200 cal. yr BP. The middle Holocene reveals relatively high and stable mean annual SST, ranging between 24 and 25.6 1C with an average of 24.7 1C, which lasts until 3200 cal. yr BP. However, this stable period is followed by a highly variable interval with distinctly lower mean annual SST during the last 3100 cal. yr BP. Between 3100 and 500 cal. yr BP, the mean annual SST are in the range of 23.6–24.8 1C with an average of 24.3 1C, which is 0.3 1C cooler than the period between 10,500 and 3200 cal. yr BP. In addition, the radiolarian SSTs also show marked and abrupt variations throughout last 10,500 yrs. Several of sudden drops of mean annual SST occur at 300–600, 1400, 3100, 4600–5100 and 8200 cal. yr BP. In this work, the seasonal temperature range was defined as the difference between the maximum and minimum seasonal temperatures (Pisias et al., 1997). The estimates of seasonal temperature range vary from 4.7 to 9.1 1C without any pronounced general trend (Fig. 2). Generally, moderate seasonal temperature contrasts (averaging about 6.8 1C) are noticed from 10,500 to 6300 cal. yr BP. Relatively high and stable seasonal temperature range with an average of 8.3 1C is recorded between 6200 and 3300 cal. yr BP. The seasonal temperature contrasts are lower with a high frequency of variation during the last 3200 cal. yr BP. 3.3. General pattern of Holocene SST changes The pattern of mean annual SST estimated for the northern Okinawa Trough displays three main phases: (i) early Holocene continuous warming, (ii) relatively stable and high SST middle Holocene, and (iii) late Holocene, with distinctly lower and highly variable SST (Fig. 2). This general pattern corresponds to the marine and terrestrial paleoclimatic records in the Chinese continent and other regions of the word. There are noticeable similarities between the northern Okinawa Trough radiolarian-based SST results and the paleoclimate records on the Chinese continent. At first, the oxygen isotope data of the Guliya (Yao et al., 1995; Thompson et al, 1997) and Dunde ice core (Yao and Thompson, 1992a, b; Yao et al., 1992) from the QinghaiTibetan Plateau of China indicate continuously warming early Holocene climate and a middle Holocene temperature maximum followed by unstable and many short-term colder stages during the last 3000 yrs. Secondly, Marked long-term Holocene climate changes recorded by several pollen sequences in China (Shi, 1992; He et al., 2003) are also largely comparable with our observations. In addition, other paleoclimatic proxies in different parts of China, such as sea and lake level fluctuations (Han and Meng, 1987; Wang, 1990b; Zhao et al., 1991; Shi, 1992) and peat sediments (Liu et al., 1997), also indicate a similar threestep climate trend throughout the Holocene. And, the general feature of our SST reconstruction is in agreement with other marine and continental paleoclimate studies in different parts of the world. A typical early

Holocene continuous warming is widely recorded by a variety of archives from many regions of the world. Despite diachroneity, the middle Holocene is often referred to as the Holocene Thermal Optimum on the global scale. The middle Holocene relatively stable and high SSTs possibly corresponds to this period. Similar late Holocene cooling trends have been noticed in different areas of the world. Borehole temperature measurements in the Greenland Ice Sheet (Dahl-Jensen et al., 1998), glacier advances and retreats in the northeast Atlantic area (Lubinski et al., 1999) and pollen sequences documenting vegetational changes in Europe (Cheddadi et al., 1998) provide evidence of a cooling late Holocene. Since 3000 yr BP, continuous cooling in the southern Okhotsk Sea is also indicated by pollen data (Kawahata et al., 2003). The radiolarian-based mean annual SST pattern may depict a general pattern of the Holocene climate, at least in the region of the northern Okinawa Trough. Planktonic foraminifera d18O concentrations have been interpreted as a temperature signal. However, in core DOC082, the planktonic d18O shows a rather flat and higher value from 10,500 to 1200 cal. yr BP and a marked decrease after about 1000 cal. yr BP, which is significantly different to the SST results (Fig. 2). In this study, the d18O was measured on the planktonic foraminifera species of Neogloboquadrina dutertrei. The salinity change of the surface water also could influence the d18O records of foraminiferal shells. Thus, the discrepancy between the d18O data and SST results may be explained as follows: (1) the study site could be influenced by coastal freshwater (Xiang et al., 2001; Lie and Cho, 2002), and larger salinity changes might have occurred in the Holocene. Thus, the d18O should not be only regarded as a temperature signal; (2) Neogloboquadrina dutertrei is a deep-water dwelling species, and its d18O records represent the more stable, long-term mean oceanic conditions over a greater depth range. The radiolarian-based SST reconstructions predominantly reflect the mean annual temperature in the shallower surface water. 3.4. Holocene SST variation Recent studies have shown that the Holocene climate is characterized by many abrupt changes (e.g., Bond et al., 1997; deMenocal et al., 2000; Arz et al., 2001). A series of cooling events is observed in the radiolarian mean annual SST record throughout the last 10,500 yrs (Fig. 3). The first occurs at 8200 cal. yr BP, which seems to be the same as the ‘‘8200 year event’’ reported from different parts of the world (e.g., Alley et al., 1997; Hu et al., 1999; Spooner et al., 2002; Matthew et al., 2004). This SST result is comparable with the records of ice cores (Yao et al., 1992; Wang et al., 2002), lake sediments (e.g., Jin et al., 2003) in the Chinese continent, and sea-level changes in the northern South China Sea continental shelf (Yim et al., 2006). Therefore, this may provide evidence that the ‘‘8200 year event’’ occurs in this mid-latitude marine area.

ARTICLE IN PRESS DOC082 Mean annual SST (°C)

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119

26.0 25.5 25.0 24.5 24.0 23.5

GISP2 δ18O (‰, SMOW)

-34.5

-35.0

-35.5

-36.0 0

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000 Cal. age (yr BP)

Fig. 3. Comparison of the Holocene mean annual SST fluctuations recorded in the core DOC082 from the northern Okinawa Trough with GISP2 d18O (%). The gray vertical bars mark the abrupt cooling events at the northern Okinawa Trough. The curve of GISP2 d18O is a result of 5 points FFT smoothing of GISP2 d18O data (Stuiver et al., 1995; Stuiver et al., 1997).

However, this event is not so pronounced in the radiolarian SST record as in other archives worldwide. This necessitates more study to understand the response to the ‘‘8200 year event’’ in the Okinawa Trough. The second cooling event in the radiolarian record took place at 4600–5100 cal. yr BP. In China, this event is also found in the d18O record of peat cellulose (Hong et al., 1997). The d18O data from the Dunde ice core also show two lower values (Yao et al., 1992), which indicate cold climate and may be the same event as depicted here. A cooling period or some cold peaks also are documented at about 4000 cal. yr BP in pollen sequences from different regions of China (Wang, 1990a; Shi et al., 1992; Shi, 1993). In addition, between 4800 and 4500 cal. yr BP a cold event lasting about 300 yrs has also been recognized in the Alps (Baroni and Orombelli, 1996). This cooling event is ubiquitous, at least in China. The third, and the most marked, cooling event in the radiolarian mean annual SST record comes at about 3100 cal. yr BP. This event is also shown in the d18O records of the Guliya ice core (Yao et al., 1995). A variety of archives in China, such as pollen and charcoal records (Shi et al., 1992), glacier advance and retreat (Xu and Yao, 1991), and archeological sites (Zhu et al., 1998), imply that a remarkable and abrupt environment transition occurred around 3000 cal. yr BP, with a distinct climate cooling in the Chinese continent. Furthermore, the paleotemperature curve established by Zhu (1973) also depicts an obvious

cooling event at 3000 cal. yr BP, which is quite coincident with the SST reconstruction result. There was a worldwide climatic cooling between 3300 and 2400 cal. yr BP, known as ‘‘the third Neoglaciation’’ in Europe, and during this period a prominent glacier advance took place in the northern Hemisphere (Denton and Karle´n, 1973). These coincidences suggest the cooling event at 3100 cal. yr BP is possibly a response of the northern Okinawa Trough to the Holocene climatic transition. The next cooling episode revealed by the radiolarian data is recorded at 1400 cal. yr BP. This cooling event also can be recognized in the d18O record of Dunde ice core (Yao and Thompson, 1992a). Decade-scale analyses of the d18O data in the Guliya ice core show a period of drier and colder climate around 750 AD (Shi et al., 1992). In addition, the peat d18O and humification records from Hongyuan region of China also indicate three cold episodes at 950 AD–450 AD (Xu et al., 2002; Wang et al., 2003). However, this cool spike has, to our knowledge, not been widely reported in other regions of the world, which may suggest a regional climate signal. The last minor cooling event recognized in the radiolarian record is dated between 600 and 300 cal. yr BP. This interval is also found in the d18O record of the Dunde ice core (Yao and Thompson, 1992a), the Guliya ice core (Yao et al., 1995) and peat cellulose (Hong et al., 1997) from northeastern China, and is depicted in the paleotemperature curve established by Zhu (1973). Moreover, this event

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parallels approximately the century-scale period of anomalously cold, dry conditions between the 15th and late 19th centuries, which is referred to as ‘‘The Little Ice Age (LIA)’’ in most Northern Hemisphere paleoclimate reconstructions. At present, many studies suggest that the LIA was a global-scale event. However, because of the low time resolution in the top part of core DOC082, more highresolution materials are needed to find out the detailed record of the LIA in the northern Okinawa Trough. Interestingly, the radiolarian SST data reveals some similarities with the GISP2 d18O record (Stuiver et al., 1995; Stuiver et al., 1997). These cooling events are comparable in time to some lower isotopic peak values in the GISP2 ice core (Fig. 3). Furthermore, these cooling events are almost simultaneous with the ice-rafted debris events recorded in the North Atlantic sediments, with peaks at 1400, 2800, 4200, 5900, 8100 and 9400 cal. yr BP (Bond et al., 1997), except for the last event at 600–300 cal. yr BP and at 5900 cal. yr BP. A sequence of Holocene events of Kuroshio Current strength at 600, 1700, 3300, 4600, 5900, 8100, 9400 cal. yr BP have been reported by Jian et al. (2000), which is also consistent with the records from the North Atlantic region. So, if these Holocene events can be confirmed by further studies in the Okinawa Trough, the correspondence among the records in the two different regions possibly imply a climate teleconnection between high- and low-latitude areas. 4. Conclusions Radiolarian-based paleotemperature data from a marine sediment core in the northern Okinawa Trough show a significant general climate trend during the last 10,500 yrs. Throughout the last 10,500 yrs, the changes of mean annual SST display a three-step trend: (i) an early Holocene continuous warming between 10,500 and 8500 yr BP which ends up with a abrupt cooling at about 8200 yr BP; (ii) a relatively stable middle Holocene with high SST that lasts until 3200 yr BP; and (iii) a lateHolocene distinct SST decline which takes place between 3200 and 500 yr BP. This trend coincides with the paleoclimatic records in the Chinese continent and other regions of the word. Several cooling events are depicted in the radiolarian SST record, which occur at 300–600, 1400, 3100, 4600–5100 and 8200 cal. yr BP. The paleotemperature fluctuations in the northern Okinawa Trough are almost comparable in time to the records of the Greenland ice core and sediments from the North Atlantic realm. This implies a possible significant climate correlation between high- and low-latitude areas. Acknowledgements We especially thank Prof. Xin-Ke Yu for revision and suggestions on the early manuscript. Dr. Qi-Yuan Cao is also thanked for helpful discussions on the earlier draft of

this manuscript. The two anonymous reviewers are thanked for their very helpful comments. This work is supported by the National Natural Science Foundations of China (Grant 40506015 and 90411014). It is also supported by the Pilot Project of the National Knowledge Innovation Program of the Chinese Academy of Science (Project No. KZCX3-SW-233).

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