High- and low-latitude forcing on the south Yellow Sea surface water temperature variations during the Holocene

Global and Planetary Change 182 (2019) 103025

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Research article

High- and low-latitude forcing on the south Yellow Sea surface water temperature variations during the Holocene

T

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Yonghao Jiaa,b, Da-Wei Lia,b, , Meng Yua,b, Xiaochen Zhaoa,b, Rong Xiangc, Guangxue Lid, ⁎ Hailong Zhanga, Meixun Zhaoa,b, a Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education/Institute for Advanced Ocean Study, Ocean University of China, Qingdao 266100, China b Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266237, China c Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China d College of Marine Geosciences, Key Laboratory of Submarine Science and Prospecting Techniques, Ministry of Education, Ocean University of China, Qingdao 266100, China

A R T I C LE I N FO

A B S T R A C T

Keywords: South Yellow Sea Sea surface temperature Holocene Centennial-scale East Asian winter monsoon

Sea surface temperature (SST) of the modern Yellow Sea (YS) is sensitive to forcing by both the East Asian monsoon and the North Pacific western boundary current, however, detailed mechanisms for explaining the Holocene SST evolution remains to be understood. Here we present two decadal sampling resolution alkenone SST records from the south YS spanning the last 8.9 kyr to understand forcing mechanisms on different timescales. The results show that the two sites from the south YS experienced SST variations on millennial timescale, displaying mean high values of 16.5 °C (A03-B) and 15.2 °C (YS01) from 7.7 to 5.9 ka, followed by low values of 16.4 °C (A03-B) and 14.8 °C (YS01) from 5.9 to 2.9 ka, then by high values of 16.9 °C (A03-B) and 15.5 °C (YS01) from 2.9 to 0.6 ka. After the entrance of the YS Warm Current at about 7.5 ka, evolution of the south YS SSTs paralleled the pattern of the main Kuroshio Current intensity, i.e., low (high) SST coeval with weak (strong) Kuroshio Current, suggesting that millennial-scale SST variations in the south YS were primarily controlled by the Kuroshio Current. Superimposing on the millennial variations, seven abrupt centennial-scale cold events were observed during the Holocene, with SST oscillations broadly coeval with those of North Atlantic ice-rafted detritus and East Asian winter monsoon rather than SSTs from the Okinawa Tough and Indo-Pacific warm pool. Hence, we propose that the East Asian winter monsoon was the top-down conveyor transmitting North Atlantic cooling signals to the south YS on centennial timescale. Thus, our study reveals both high- and low-latitude climate forcing on the evolution of the south YS SST during the Holocene, with low-latitude forcing on millennial-scale and high-latitude forcing on centennial-scale.

1. Introduction The Holocene climate experienced a long-term cooling trend from the Holocene Thermal Maximum towards the late Holocene as indicated by records from Greenland ice δ18O and global surface temperature stacks (Mayewski et al., 2004; Marcott et al., 2013). Imposed on this cooling trend, however, the ice-rafted records from the North Atlantic sediments clearly indicated a number of abrupt centennialscale cooling events, i.e., Bond events, during the Holocene (Bond, 1997; Bond et al., 2001). And hereafter, synchronous temperature decreases responding to these cooling events had been widely reported during the Holocene, in particular from the northern hemisphere high-

latitude regions (Risebrobakken et al., 2003; Mayewski et al., 2004; Viau et al., 2006; Wanner et al., 2011; Marsicek et al., 2018). In addition, responses to the North Atlantic cooling events had also been observed from the East Asian monsoon records (Wang et al., 2005; Yu et al., 2011; An et al., 2012; Wang et al., 2012; Hao et al., 2017; Kang et al., 2018) and paleo-environmental archives from the western Pacific marginal-sea and open-ocean regions (Jian et al., 2000; Sun et al., 2005; Isono et al., 2009; Kubota et al., 2010; Wang et al., 2011; Kim and Lim, 2014; Ruan et al., 2015). Rapid total solar irradiance (TSI) variability had been proposed as the external driver on the rapid Holocene climate changes, and the North Atlantic thermohaline circulation acted as a climate amplifier

⁎ Corresponding authors at: Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education/Institute for Advanced Ocean Study, Ocean University of China, Qingdao 266100, China. E-mail addresses: [email protected] (D.-W. Li), [email protected] (M. Zhao).

https://doi.org/10.1016/j.gloplacha.2019.103025 Received 7 November 2018; Received in revised form 26 August 2019; Accepted 26 August 2019 Available online 27 August 2019 0921-8181/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Modern oceanographic setting. (a) Location of sites A03-B ( ) and YS01 ( ) and other sites (●) mentioned in this study, on a map of winter sea surface temperature (0 m) derived from the World Ocean Atlas 2013 (http://www.nodc.noaa.gov/OC5/woa13/woa13data.html). EAWM, East Asian Winter monsoon; KC, Kuroshio Current. (b) Regional ocean circulation and mud sediment deposition areas (dark gray shaded area represents the CYSM, central Yellow Sea mud; light gray shaded areas represent other mud depositions) in the Yellow Sea and East China Sea. The currents include: KC, Kuroshio Current; YSWC, Yellow Sea Warm Current; SCC, Shandong Peninsula Coastal Current; KCC, Korean Coastal Current; YSCC, Yellow Sea Coastal Current. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Holocene, the EAWM had been reported to dominate the variations of the YSWC intensity (Ge et al., 2014), sediment grain-size fractions (Hu et al., 2012) and marine biomarker burial fluxes (Wu et al., 2016; Zhao et al., 2013) in the south YS. Whereas other studies had revealed that the KC controlled millennial-scale variations of YSWC (Kim and Kucera, 2000; Li et al., 2009), and the Pacific El Niño-Southern Oscillation state modulated the delivery of terrestrial lignin into the south YS (Gong et al., 2017; Hao et al., 2017). While at centennial timescale, the delivery of terrestrial sediment and lignin to the south YS was controlled by the evolution of EAWM (Hao et al., 2017; Hu et al., 2012). K′ U37 and TEX86 sea surface temperature (SST) records from the YS also displayed both millennial- and centennial-scale variations during the mid-late Holocene. These centennial-scale SST variations had been attributed to direct forcing either from the EAWM and solar activity changes (Wang et al., 2011; Nan et al., 2017a), or from the modulation of the KC (Nan et al., 2017b; Zhang et al., 2019); whereas the millennial-scale SST variations were attributed to sea level (Nan et al., 2017a), the EAWM (Ge et al., 2014), or the YSWC changes (Nan et al., 2017a; Wu et al., 2016; Zhang et al., 2019). However, the resolution of most published records was too low to reveal the detailed spatiotemporal SST variations and forcing mechanisms in the YS. Therefore, SST records revealing centennial variability during the Holocene are still needed to study mechanisms of YS SST evolution, especially to distinguish the controlling factors on different time scales. K′ In this study, we present high-resolution U37 SST records from two sediment cores (YS01 and A03-B) taken from the CYSM at a 35.5°N section to investigate the SST evolution during the Holocene. Our results revealed both millennial (> 1.5 kyr) and centennial (> 200 yr) SST variabilities during the Holocene. Our focus was on mechanisms driving the south YS SST variations and teleconnection between the south YS, the North Atlantic and tropical Pacific climate changes.

and conveyor (Bond, 1997; Bond et al., 2001; Chapman and Shackleton, 2000; Mayewski et al., 2004; Hall et al., 2004; Renssen et al., 2006; Hong et al., 2009; Wanner et al., 2011; Kang et al., 2018). In this hypothesis, the North Atlantic cooling signal was transmitted to the western Pacific region through the thermohaline circulation or the East Asian monsoon system (Mayewski et al., 2004; Hong et al., 2009; Mischke and Zhang, 2010; Wanner et al., 2011; Sun et al., 2011; Kang et al., 2018). However, other studies revealed that occurrence of the low- and mid-latitude cooling evens were not strictly in phase with the North Atlantic cold events and had different evolution periodicities (e.g., Wang et al., 2012; Khider et al., 2014; Sagawa et al., 2014; Ruan et al., 2015; Nan et al., 2017a); Therefore, other hypotheses for explaining the Holocene centennial climate oscillations had been proposed, including internal stochastic oscillations arising from the Earth climate system (Alley et al., 2001; Darby et al., 2012), and TSI variations with amplification (and transmitting) by the El Niño-Southern Oscillation (Moy et al., 2002; Emile-Geay et al., 2007). The Yellow Sea (YS) is a shallow marginal sea in the northwestern Pacific and is surrounded by China and the Korean Peninsula, which indirectly connects it with the open Pacific Ocean via the East China Sea. The modern YS environment is sensitive to both high-latitude forcing via the East Asian winter monsoon (EAWM) and low-latitude forcing by the open Pacific primarily through the Kuroshio Current (KC). The YS is characterized by high productivity and high sediment accumulation rates of up to 2.7 mm yr−1 in the central Yellow Sea mud (CYSM) area (Alexander et al., 1991). Hence, the YS provides the opportunity for generating high-resolution paleoclimate records to investigate the response of the YS environment to different climate forcing, e.g., TSI, EAWM, El Niño-Southern Oscillation, sea level, and KC, on both millennial and centennial timescales during the Holocene. Previous studies had revealed that the YS circulations, including the Yellow Sea Warm Current (YSWC), had not been established before about 7.5 ka due to low sea-level condition and shallow YS water depth (Kong and Park, 2007; Diekmann et al., 2008; Xiang et al., 2008; Li et al., 2009; Xu et al., 2014). On millennial-scale during the mid-late

2. Regional setting The YS covers an area of about 3.8 × 105 km2, with a maximum 2

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(version 2.3.4; Blaauw and Christen, 2011). Based on the model results, the AMS 14C datum at 1133 cm of core YS01 is considered an outlier. The AMS 14C dates indicate continuous sedimentation at both sites (Fig. 2). For core A03-B, the top and bottom sample ages are 37 yr and 8.8 ka, respectively, resulting in sedimentation rate between 26.4 cm kyr−1 and 45.4 cm kyr−1. For the composed core YS01, the core top age is 0.6 ka, which was the result of sediment loss during sample collection, and the age of the bottom sample was 8.9 ka; sedimentation rates for YS01 core ranged from 81.7 cm kyr−1 to 217.0 cm kyr−1. However, gravity core top ages might also be affected by physical disturbance during sampling (probable several centimeters on the core top), leading to older bias. Samples from core A03-B were taken at 1-cm intervals and the average sample-resolution is ca. 30 yr; YS01 samples were taken at 2-cm intervals yielding a temporal resolution of ca. 15 yr. Analysis of alkenones was carried out following previous studies (Li et al., 2013; Wu et al., 2016). Freeze-dried sediments were extracted by CH2Cl2/CH3OH (3:1, v/v) 4 times by ultrasonication for 15 min. The extracts were then hydrolyzed with 6% KOH in CH3OH, and the neutral lipids were extracted with hexane and then separated into apolar and polar fractions using silica-gel chromatography. The polar lipid fraction (containing alkenones) was eluted with CH2Cl2/CH3OH (95:5, v/v) and then derivatized using N,O-bis (trimethylsily) trifluoroacetamide (BSTFA). C37 alkenones were determined by gas chromatograph (Agilent 6890 A) with an FID detector and a HP-1 column K′ (50 m × 0.32 μm × 0.17 μm). The U37 index is calculated based on Eq. (1) and SST is calculated using a south YS local calibration (Eq. (2)) provided by Tao et al. (2012).

water depth of ca. 100 m and an average water depth of 44 m (Qin et al., 1989). Today, the south YS has an annual SST range of 15.0–18.0 °C from north to south. Under the influence of monsoon wind reversal, the SST and surface current system in the YS display distinctive seasonal variations. In the summer, the water column is highly stratified due to the net heat gain from atmosphere, resulting in higher SST ranging from 23.0 to 24.5 °C (Wei et al., 2010) and weak surface circulation (dashed yellow lines in Fig. 1b) (Lie and Cho, 2016). During the cold seasons of winter and spring, the YS water column is vertically homogeneous due to strong mixing caused by the strong wind. Two YS coastal currents, i.e., the Korean Coastal Current and Yellow Sea Coastal Current, carry cold and low salinity water southward; meanwhile, the YSWC, a branch of the KC, transports warm and saline water northward along the western slope of the south Yellow Sea trough (Yuan and Hsueh, 2010; Liu et al., 2015; Lie and Cho, 2016). The south YS SST is influenced by the EAWM through direct surface cooling and by the northern extension of the warm YSWC (Moon et al., 2009; Xu et al., 2009; Yuan and Hsueh, 2010; Lie and Cho, 2016), leading to a steeper north (ca. 8.0 °C)-south (ca. 12.0 °C) SST gradient over the south YS. The winter basin-wide cyclonic gyre, caused by the YSWC and the Yellow Sea Coastal Current, has resulted in the formation of the CYSM (Park et al., 2000; Yang et al., 2003). Sediments in the CYSM were primarily originated from the riverine input, marine production and erosion from the old Yellow River Delta (Zhao et al., 1990; Park et al., 2000; Wei et al., 2003; Yang et al., 2003; Koo et al., 2018). The modern sediment accumulation rates in the CYSM fall in the range of 0.3 to 2.7 mm yr−1 (Alexander et al., 1991). Hence, the CYSM is an ideal place for finding high-resolution sedimentary records to investigate centennial-scale environmental changes during the Holocene (e.g., He et al., 2014; Hao et al., 2017; Nan et al., 2017a).

K′ U37 = [C37:2]/([C37:2] + [C37:3])

(1)

K′ SST (°C) = (U37 + 0.350)/0.059

(2)

where [C37:2] and [C37:3] indicated the C37 alkenones containing 2 and K′ 3 double bonds respectively. For U37 analysis, the error associated with instrumental analysis is 0.005 (1σ) and replicated measurements of parallel samples at the same depth of a core revealed a standard deviation of 0.012 (1σ). Hence, the accumulative analysis error in our data is 0.013, equivalent to 0.2 °C using Eq. (2). The south YS and the global calibrations demonstrated that the uncertainties of reconstructed absolute SSTs are 0.4 °C and 1.5 °C, respectively, which are 2–7 times of our analysis precision (0.2 °C). In this study, our aim is to discuss the relative SST variations rather than absolute temperature values, and the error in reconstructing relative SST is constrained by the analytical precision. As shown in Fig. 3, the amplitudes of temperature changes of both millennial and continental cold events are larger than 0.2 °C. Hence these events are robust.

3. Material and methods A gravity core A03-B (123°38′E, 35°28′N, water depth: 77.6 m, core length: 294 cm), with undisturbed sediments on top, was retrieved from the northern CYSM by R/V Dongfanghong II in 2011 (see Fig. 1). Another gravity core YS01-C (core length: 369 cm) and a piston core YS01A (core length: 3010 cm) were drilled in the same site from the western edge of CYSM by R/V K407 in 2006, and the YS01 core (122°29′E, 35°31′N, water depth: 58.5 m, core length: 1133 cm) referred in this study was established by combining the 0–369 cm of YS01-C and 385–1133 cm of the YS01-A. As described in Wang et al. (2014), for core YS01, the sediment between 0 and 1112 cm is characterized by homogeneous dark gray clayey silt and changed to silty clay between 1112 and 1133 cm (Fig. 2b). Similar to YS01 core, homogeneous clayey silt occurred between 0 and 271 cm in core A03-B, and the 271–301 cm section contains silty clay or sandy silt (Fig. 2a). Due to the lack of planktic foraminifera, benthic foraminifera were used to obtain AMS 14C data that have been used to constrain sedimentation rates in the YS (Xiang et al., 2008; Zhong et al., 2018). The chronologies of the A03-B and YS01 were established based on AMS 14C dating of mixed benthic foraminifera (Table 1 and Fig. 2). Benthic foraminiferal shells of Ammonia ketienziensis and Hanzawaia nipponica, which lives on sediment surface or within sediment shallower than 2 cm beneath seafloor (Xu et al., 2017), were picked for AMS 14C analysis. The AMS 14C ages of 7 samples for A03-B core were measured at the Peking University. The AMS 14C ages of 13 samples from core YS01 were measured at the Woods Hole Oceanographic Institution and Beta Analytic Radiocarbon Dating Laboratory, of which 9 ages have been published (Wang et al., 2014; Yang et al., 2018) and 4 ages are new at 123.0 cm, 320.0 cm, 600.0 cm, 854.0 cm (Fig. 2b, Table 1). AMS 14C dates were calibrated to calendar ages (BP, relative to 1950 CE) with the Marine 13 radiocarbon age calibration curve (Reimer et al., 2013). A regional marine reservoir age, ΔR = −128 ± 35 yr, was used in our calibration following Hao et al. (2017) and Zhong et al. (2018). Age models of these two cores were established using the Bacon software

4. Results The SST of core A03-B ranged from 14.5 to 18.2 °C with an average value of 16.5 °C over the last 8.8 kyr (Fig. 3a). SST experienced large fluctuations from 8.8 to ca. 7.5 ka, with high values of 15.9–18.2 °C from 8.8 to 8.4 ka, and low values of 14.5–15.4 °C from 8.3 to 7.9 ka. From 7.8 to 2.9 ka, SST varied in a narrow range between 15.8 °C and 17.2 °C, and it increased to high values at ca. 2.9 ka until the rapid decreasing trend over the last 0.5 kyr (Fig. 3a). The SST of core YS01 ranged from 13.2 to 16.4 °C with an average value of 15.1 °C over the last 8.9 kyr (Fig. 3b). After an initial rapid increase, the SST fluctuated between 13.6 °C and 16.3 °C during 8.8–6.0 ka with a significant cooling event at ca. 6.9 ka. Then the SST remained relatively low with an average value of 14.9 °C during 5.9–2.9 ka. Similar to A03-B core, SST from core YS01 also increased rapidly at ca. 2.9 ka and experienced relatively high values from 14.9 to 16.4 °C between 2.9 and 0.6 ka (Fig. 3b). In general, these two highresolution SST records from the south YS experienced similar variations on millennial timescales during the Holocene, and the averaged SST for A03-B core (16.5 °C) is 1.4 °C higher than that of YS01 core (15.1 °C) 3

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Fig. 2. Age model and schematics of the lithology for A03-B and YS01 cores. In both (a) and (b), black lines refer to the mean age and the gray dotted lines denote the 95% confidence interval (gray shadow), i.e., two sigma (2σ) age range. The schematic diagram for core A03-B was provided by Xiang R. (unpublished data) and for core YS01 was drew according the lithology description by Wang et al. (2014).

potential bias beyond temperature. Detailed discussions were carried out as follows.

during the last 8.8 kyr. K′ When using the global calibration (SST (°C) = (U37 − 0.044)/0.033; Müller et al., 1998), calculated SST for core A03-B ranged from 14.0 to 20.6 °C with an average value of 17.5 °C over the last 8.8 kyr (supplementary data). For core YS01, SST ranged from 11.7 to 17.4 °C with an average value of 15.1 °C over the last 8.9 kyr (supplementary data). Compared with results using the south YS calibration, SST values calculated by the global calibration have larger fluctuations in both cores, and higher (same) average SST value for core A03-B (YS01). As discussed in Tao et al. (2012), responses of haptophytes to temperature might be different between the south YS and the open ocean; hence SST values calculated by the south YS calibration were used in the following discussion.

K′ 5.1. Potential bias on the U37 index

5.1.1. Degradation Sedimentary C37 alkenone content is sensitive to degradation with preferential diagenetic loss of tri-unsaturated (C37:3) alkenone (Freeman and Wakeham, 1992). However, laboratory- and field-studies K′ provide no evidence of obvious alternation on U37 during degradation (Sikes et al., 1991; Teece et al., 1998). In a down-core analysis from the Madeira Abyssal Plain, C37 alkenone contents were reduced by 1–2 orders of magnitude in the oxidized parts of the turbidites compared to K′ the unoxidized parts, whereas only a small increase in the U37 values (equivalent to < 0.5 °C) was observed (Huguet et al., 2009). Considering the sedimentation rates of our cores are one to two orders high K′ than that in the open ocean, the change of U37 during diagenesis should be minimal in our cores.

5. Discussion K′ Robust use of U37 depends on understanding the relationship between sediment alkenone composition and overlying SST, as well as

Table 1 AMS 14C ages and calendar ages of Cores A03-B, YS01-C and YS01-A (based on mixed benthic foraminifera). 14

Sediment core

Core depth (cm)

Dating material

AMS

A03-B

4 51 98 148 189 240 279 23a 123b 220a 320b 415.5a 518a 600b 693.3a 780a 854b 878.1a 981a 1132.7a

Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Shell fragments and mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera Mixed benthic foraminifera

505 ± 20 1975 ± 20 3095 ± 20 4065 ± 20 4725 ± 20 6070 ± 25 7580 ± 25 1210 ± 25 2090 ± 25 2710 ± 25 3010 ± 35 3230 ± 30 3740 ± 45 4300 ± 30 4950 ± 35 5180 ± 45 5530 ± 30 6190 ± 35 6800 ± 55 9820 ± 40

YS01-C

YS01-A

a b

Data from Wang et al. (2014) and Yang et al. (2018). New data measured in this study. 4

C age BP (yr) BP)

Calendar age (cal yr BP)

2σ error bars (cal yr BP)

148 1690 3038 4290 5192 6651 8133 863 1748 2403 2893 3334 3963 4594 5307 5798 6329 6631 7503 9238

−219–426 1532–1838 2876–3202 4107–4453 5007–5353 6393–6868 7877–8335 729–990 1589–1873 2252–2539 2777–3016 3212–3497 3812–4140 4440–4758 5083–5450 5633–5978 6121–6589 6403–6813 7334–7767 8665–9908

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K′ K′ Fig. 3. Comparison of SST records from the south YS with other paleo-records during the Holocene. (a) U37 SST record from core A03-B (this study). (b) U37 SST K′ K′ record from core YS01 (this study). (c) U37 SST record from core ZY1 (Wu et al., 2016). (d) U37 SST record from core ZY3 (Wu et al., 2016). (e) The enrichment factor K′ (EF) of total mercury from site KX12–3 (Lim et al., 2017). (f) The abundance (%) of P. obliquiloculatata from site B-3GC (Jian et al., 2000). (g) U37 SST from the ODP site 1202B (Ruan et al., 2015). (h) Middle-high north hemisphere (90°N to 30°N) temperature stack (Marcott et al., 2013). (i) Anomaly of total solar irradiance, ΔTSI (Steinhilber et al., 2009). The AMS 14C age-control points for south YS cores were labeled as triangles above each record.

in coastal regions due to lower salinity, therefore, potential bias caused by lateral sediment transportation to the CYSM should be negligible.

5.1.2. Lateral transportation Another possible bias on our SST is that fine sediments transported from warmer/colder waters. Since the south YS calibration equation is established by correlating modern SST with surface sediments which were accumulated over decadal timescale, the effects of lateral sediment transport have been accounted for by the calibration. In addition, K′ given that U37 values of surface sediments from the south YS resembles overlying SSTs (Tao et al., 2012), and haptophyte growth was inhibited

5.1.3. Water depth and salinity changes The C37 alkenone source algae, i.e., haptophyte, cannot thrive in freshwater as well as low salinity sea water, and C37 alkenones were not detected in shallow coastal south YS (water depth < 37 m, Tao et al., K′ 2012). However, U37 in south YS surface sediments with water depth 5

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changes. In addition, the reconstructed air temperature from middlelatitude regions of China also displayed a similar evolution trend (Wang et al., 2001; Zheng et al., 2017; Marsicek et al., 2018). However, south YS SSTs were relatively stable during the early-middle Holocene with a slightly warming shift during the last 2.9 kyr (Fig. 3a–d), which is inconsistent with the evolution of northern hemisphere temperature (Fig. 3h), the total solar activity (represented by ΔTSI in Fig. 3i), and the China air temperature changes (Wang et al., 2001; Marcott et al., 2013; Zheng et al., 2017). This suggests that the long-term evolution of the south YS SST was not likely controlled by the EAWM which could transmits north high-latitude climate signals to East Asia regions including the YS. Instead, the Holocene south YS SST evolution might be a unique regional feature that was tightly connected with the western Pacific environmental changes. In addition to surface direct heating, modern south YS SST is largely controlled by the YS circulation system, i.e., cold coastal currents and warm YSWC (Liu et al., 2015). The two sites in this study lie in the path of northward YSWC intrusion, and thus the SST records were affected by the evolution of the KC. Previous studies suggested that the YSWC entered the YS when the sea level reached the present position at about 7.5 ka (Kim and Kucera, 2000; Liu et al., 2004; Xiang et al., 2008). After the formation of the modern south YS warm current system, the YSWC intensity/variability as well as the YS SST had been tightly connected with that of the KC. The abundance of P. obliquiloculatata (Jian et al., 2000) and the sediment mercury (Hg) enrichment factor (Lim et al., 2017) from the Okinawa Trough had been used to reconstruct the intensity and dynamics of KC. These records revealed that the KC intrusion/intensity into the Okinawa Trough was strong since 9.3 ka, weakened between ca. 5.3–2.9 ka and strengthened again during the last 2.9 kyr (Fig. 3e, f). Correspondingly, the south YS SST was high during 7.7–5.9 ka, but low between 5.9 and 2.9 ka, and high 2.9–0.6 ka (Fig. 3a–d). Thus, the parallel variations of south YS SSTs and KC records suggest a tight connection between the intensity of the KC and south YS SST during the last 7.7 kyr. Although there are no independent YSWC records during the mid-late Holocene, these coherent variations between south YS SST and the KC suggest that the weakening of the KC during stage II reduced the YSWC and hence decreased SST in the south YS. Our higher-resolution regional results provide strong evidence supporting previous suggestions that the millennial variations of YS SST were dominated by the KC through the YSWC during the Holocene (Wang et al., 2011; Zhao et al., 2013; Nan et al., 2017a; Zhang et al., 2019). In summary, during the last 7.7 kyr, the millennial-scale SST variation from the south YS was a regional feature forced by the evolution of the KC.

range of 39–90 m displayed good linear relationship with SST (Tao et al., 2012). Given that sea level was 20 m lower than modern at ca. 8.8 ka (Li et al., 2014), and taking into consideration of the thickness of deposited sediments, the paleo-water-depths for YS01 and A03-B are estimated to about 50 m and 60 m, respectively, at 8.8 ka. By analogue, it is reasonable to postulate that our sites still lied in the depth range for K′ linear U37 -SST calibration, although no paleo-salinity data are available. 5.1.4. Seasonality The C37 alkenones are mainly produced by Emiliania huxleyi and Gephyrocapsa oceanica (Volkman et al., 1995; Conte et al., 1998). Modern surveys revealed that Emiliana huxleyi and Gephyrocapsa oceanica are the dominant haptophyte species in the south YS, and there is similar biomass of these two species in both summer and winter seasons (Sun et al., 2014). A regional study of the YS revealed that the best K′ linear correlation was observed between surface sediment U37 values and mean annual SSTs rather than seasonal SSTs (Tao et al., 2012). K′ Hence, U37 has been applied to reconstruct annual mean SST for the south YS (e.g., Xing et al., 2012; Wu et al., 2016; Nan et al., 2017b; K′ Yuan et al., 2018). For our cores, the core top U37 -SST is 15.8 °C using the south YS calibration in core A03-B, which is close to modern mean K′ annual SST (16.4 °C) in this region. Thus, the U37 SSTs from these two cores are interpreted as annual mean SSTs in this study. As discussed in Zhang et al. (2019), summer SST displayed uniform values in the south YS, and the spatial feature of the YS annual SST resembles that of winter K′ SST. Therefore, the variations of U37 SST was explained by SST changes in the winter season (Tao et al., 2012; Zhang et al., 2019). 5.2. Validation of centennial SST variations For an appropriate interpretation of paleo-records, the reliability of temporal resolution is important. According to the AMS 14C data, continuous sedimentation occurred in both cores without age reversal and/or significant changes in sedimentation rates. The average deposition rate of YS01 and A03-B cores are 1.37 mm yr−1 and 0.34 mm yr−1, resulting a resolution of 7.3 yr per cm and 29.1 yr per cm, respectively. According to 210Pb down-core profiles in the CYSM (Zhao and Li, 1991), surface sediment mixing depth is < 2 cm, leading to an age uncertainty < 15 yr for YS01 and 60 yr for A03-B. The time span of the centennial SST variations discussed in this study is larger than 200 yr, hence it is reasonable to conclude that the decadal and centennial SST variations from these two cores are robust. 5.3. Kuroshio current modulated millennial SST variations of the south YS

5.4. North Atlantic forcing on the centennial SST variations of the south YS For temporal variations, in general, SST records from A03-B and YS01 displayed similar variations on millennial timescales during the past 7.7 kyr (Fig. 3a, b), and three stages were divided as follows (Fig. 3): the stage III (7.7–5.9 ka) and II (5.9–2.9 ka) experienced relatively high and low SST mean values, respectively, and were characterized by relatively large-amplitude rapid fluctuations (Decadal timescale); the stage I (2.9–0 ka) experienced relatively high SST and small oscillations. In addition, these millennial variations are broadly in-phase with published high-resolution SST from site ZY1 over the last 6.0 ka (Fig. 3c) and site ZY3 over the last 7.2 kyr (Fig. 3d), suggesting at least a regional pattern of SST variations during the mid-late Holocene. Compared with global/regional temperature records, SST evolution from the south YS displayed distinctively different patterns during the Holocene. By compiling published sea surface and air temperatures, Marcott et al. (2013) had provided the global and northern hemisphere temperature stacks for the Holocene. Both stacks indicate that the surface temperature experienced early-Holocene warmth period and a gradual cooling through mid-late Holocene (Fig. 3h), which is consist with decreasing trends of obliquity and total insolation since 9.0 ka (Marcott et al., 2013), suggesting orbital forcing on Holocene climate

Superimposing on the millennial-scale variations, several centennial-scale cold events were observed from the south YS SST records (Fig. 3a, b). To better reveal the centennial-scale changes during the Holocene, low-frequency variability was removed by subtracting the 1500-yr moving average according to Isono et al. (2009), and therefore detrended south YS SSTs from A03-B and YS01 (Fig. 4a, b) were obtained. As displayed in Fig. 4, a series of rapid cooling events, with amplitude of 0.5–1.0 °C, were observed centered around 8.2 ka, 7.1 ka, 5.8 ka, 4.3 ka, 3.1 ka, 1.5 ka and 0.3 ka, respectively, on the detrended SST records. The SST records from site A03-B and YS01 show an obvious SST drop at 8.4–7.9 ka (Fig. 3a, b, 4a and b), coincident with the “8.2 ka” event given the uncertainties of the age models, which had been originally reported from the Greenland ice records (Alley et al., 1997). The “8.2 ka” event has also been widely observed in the northern hemisphere (Wanner et al., 2011), including in the YS and East China Sea (Wu et al., 2016; Nan et al., 2017a; Yuan et al., 2018). The “8.2 ka” event was trigged by a catastrophic meltwater discharge from the Laurentide area to the North Atlantic (Barber et al., 1999; Broecker, 6

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Fig. 4. Holocene centennial-scale variations of south YS SST and related climate records. (a) Detrended SST from core A03 (this study); (b) Detrended SST from core YS01 (this study); (c) Detrended North Atlantic Hematite-stained grains (HSG) from site MC52-VM29–191 (Bond et al., 2001), with Bond event numbers from 0 to 5b labeled. (d) Detrended total solar irradiance, ΔTSI (Steinhilber et al., 2009) with lower (higher) values representing weak (strong) solar activity. (e) Detrended Lake K′ Qinghai Westerlies index, WI (An et al., 2012). (f) Detrended lignin index, Σ8, from site N02 (Hao et al., 2017). (g) Detrended U37 SST record from ODP site 1202B (Ruan et al., 2015). (h) Detrended Mg/Ca-SST record from site MD98–2181 (Khider et al., 2014). All records were detrended by cut-of 1500 years moving average; bold solid lines represent 200-yr running means. Vertical bars indicate cold events. The AMS 14C age-control points were labeled as triangles above each record.

2007; Sun et al., 2011; Huang et al., 2011; Kong et al., 2014; Wen et al., 2016). Since 7.5 ka, six centennial-scale cold events occurred in the south YS SST, paced at the intervals of 1.1–1.6 kyr (Fig. 4a, b). With the uncertainty of our chronology, these cold events are broadly coeval with the centennial-scale increases of the North Atlantic hematitestained grains (indicator of ice-rafted detritus), i.e., Bond events (Fig. 4d). This suggests that the south YS SST drop occurred

2003; Matero et al., 2017), which further led to the slowdown of the Atlantic thermohaline circulation resulting in a significant cooling around the North Atlantic regions (Alley and Agustsdottir, 2005). The appearance of the “8.2 ka” cooling event in the China marginal seas suggest that there existed a teleconnection between North Atlantic and the northwestern Pacific marginal seas through the EAWM, as both proxy-based and simulation studies showed that the EAWM significantly strengthened during the “8.2 ka” event (Yancheva et al., 7

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reduced (North Atlantic ice-rafted detritus increased), weakening the North Atlantic thermohaline circulation (Bond et al., 2001). The slowdown of the North Atlantic thermohaline circulation resulted in a colder winter and increased meridional temperature gradient in the northern hemisphere, which further strengthened the mid-latitude westerly winds leading to the intensification of the EAWM (Yancheva et al., 2007; Sun et al., 2011; Zheng et al., 2014; Kang et al., 2018). As a result, the south YS SST decreased in response to the intensified EAWM during each Bond event (Figs. 4, 5). Therefore, during the mid-late Holocene, the correlations between EAWM/westerly, North Atlantic ice-rafted debris, and south YS SST records suggest that the south YS SST centennial variations were the response to the North Atlantic climate variability transmitted by the EAWM (Fig. 5). It has been proposed that the Holocene centennial-climatic variability is also sensitive to total solar irradiance (e.g. Bond et al., 2001; Wanner et al., 2011; Darby et al., 2012; Kobashi et al., 2013; Sagawa et al., 2014; Kang et al., 2018); Accordingly, the south YS SST minima were correlated with ΔTSI minima (lower solar activity) on centennial scales with the uncertainty of our chronology (Fig. 4a–c). However, the total solar irradiance changes by themselves are too weak to directly driving significant ocean temperature changes (Bond et al., 2001; Foukal et al., 2006; Gray et al., 2010; Khider et al., 2014). As proposed by Bond et al. (2001), the “paired intervals of reduced solar irradiance and increases in North Atlantic ice drift” suggested that the North Atlantic thermohaline circulation might be the “mechanism for amplifying the solar signals and transmitting them globally”. Thus, the centennial-scale south YS SSTs cooling (0.5–1.0 °C) might be the indirect response to reduced solar irradiance (external forcing) which was amplified/transported by the Earth's internal feedbacks, e.g., the North Atlantic thermohaline circulation and the EAWM. As discussed in Section 5.3, Holocene millennial-scale south YS SST evolution was modulated by the intensity of KC, thus it is also necessarily to consider whether centennial-scale south YS SST cold events were also influenced by low-latitude ocean changes. The Indo-Pacific Warm Pool (IPWP) sustains the largest reservoir of surface ocean heat content, plays a critical role in the global climate, e.g., its tight connection with the El Niño-South Oscillation. Using planktonic foraminiferal distribution and δ18O records from the Okinawa Trough, Jian et al. (2000) had reported centennial-scale oscillations of the KC intensity during the Holocene (Jian et al., 2000). However, Jian's (2000) record is of lower resolution (> 130 yr) and is not suitable to make point-to-point comparison with North Atlantic ice-rafted detritus. K′ Instead, we used detrended high-resolution (ca. 40 yr) U37 SST record from ODP site 1202B in the south Okinawa Trough (Ruan et al., 2015), and a high-resolution (ca. 28 yr) Ma/Ca SST record (Globigerinoides ruber) from core MD98–2181 in the IPWP (Khider et al., 2014). As displayed in Fig. 4h, detrended MD98–2181 SST showed 9 centennial SST cold events centered around 8.8 ka, 7.6 ka, 6.8 ka, 6.0 ka, 4.9 ka, 3.7 ka, 2.8 ka, 1.7 and 0.6 ka, however, without a discernible “8.2 ka” event. Occurrences of theses IPWP cold events were not synchronous with the south YS SST cold events as indicated by vertical bars in Fig. 4. As proposed by Khider et al. (2014), the Holocene centennial-scale SST variability in the IPWP did not correlate with either ΔTSI or North Atlantic ice-rafted debris (Fig. 4), and the internal forcing, i.e., internal stochastic modes of ocean/atm interactions, within the climate system is the most plausible source for generating centennial-scale variability in IPWP SST. In addition, SST from ODP site 1202B displayed higher frequent oscillations, e.g., 14 cold events occurred during the last 9.0 kyr, and none of them corresponded to Bond events except the event at ca. 1.5 ka, which is coincident with Bond event 1. Hence, the low-latitude oceanic forcing is not the candidate driver generating the YS SST centennial variations after 9.0 ka. Alternatively, we speculate that the Holocene centennial-scale SST variations of ODP site 1202B might be modulated by the interaction of both IPWP and EAWM, hence generating rapid SST variability on about half-period of both low- and highlatitude climate cycles (Fig. 4f). In summary, our results suggest that

synchronously with reduced North Atlantic Deep Water formation. Previous studies suggest that there exists teleconnection between the EAWM and the North Atlantic ice-rafted detritus events during the midlate Holocene, with the EAWM acting as a bridge transmitting climate signals from the high-latitude North Atlantic to the mid- and low-latitude regions (Sun et al., 2011; Yu et al., 2011; An et al., 2012). To investigate this crossing-Eurasian teleconnection and to explore mechanisms of centennial-scale south YS SST variations, two published proxies are used to represent the evolution of the EAWM during the mid-late Holocene. One is a dust flux record from Lake Qinghai used to represent the intensity of north hemisphere westerlies (Fig. 4e), which plays a critical role in transmitting the North Atlantic cooling signals to the East Asian monsoon regions (An et al., 2012); another is a lignin index from the south YS (Fig. 4f), which indicates the “hydrodynamic sorting effect” that driven by the EAWM and was used as an indicator of local EAWM (Hao et al., 2017). For comparison, both dust flux and lignin records were detrended using the same method for treating south YS SSTs. As displayed in Fig. 4, the detrended south YS SSTs generally resemble the variations of detrended Westerly and EAWM records and North Atlantic ice-rafted detritus over the last 7.5 kyr, indicating that decreased south YS SST occurred coincidently with increased North Atlantic ice-rafted detritus and strengthened EAWM and Westerly intensities. Therefore, by analogue to mechanism explaining the south YS SST cooling during the “8.2 ka” event, we postulated that the centennial-scale south YS SST variability was directly generated by the variation of the EAWM during the last 7.5 kyr. Summarized in Fig. 5 is the proposed climate teleconnection between the North Atlantic and the south YS, as well as a mechanism of how these high-latitude climate signals affect the China marginal sea SSTs. During the Bond event, the North Atlantic Deep Water formation

Fig. 5. A schematic illustration of climate forcing from both high- and lowlatitude regions on different timescale south YS SST changes during the Holocene. THC: thermohaline circulation; EAWM: East Asian Winter monsoon; YSCW: Yellow Sea Warm Current. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 8

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the centennial-scale south YS SST variation was a direct response to high-latitude forcing transmitted by the EAWM; and this provides additional support for the hypothesis that solar irradiance variation was the external driver generating centennial climate changes in high- and mid-latitude regions during the Holocene (Bond et al., 2001).

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6. Conclusions Two high-resolution SST records over the last 8.9 kyr revealed that the south YS SST variations were controlled by both high- and lowlatitude climate forcing but on different time scales. On millennial timescale, the general pattern of south YS SST records paralleled the intensity of the KC records during the last 7.7 kyr, suggesting that millennial scale south YS SST variation was primarily controlled by the intensity of KC via the transmission by the YSWC. On centennial-scale, the south YS SST experienced 7 cold events centered around 8.2 ka, 7.1 ka, 5.8 ka, 4.3 ka, 3.1 ka, 1.5 ka and 0.3 ka during the last 8.9 kyr, resembling that of North Atlantic ice-rafted debris and EAWM. We speculate that the centennial cooling events observed in the south YS SSTs were the responses to North Atlantic cool events, and the EAWM is the top-down conveyor transmitting the high-latitude North Atlantic cooling signals to the south YS. Our result provides further support to the hypothesis that solar activity was the external driver on the Holocene climate variability with the North Atlantic thermohaline circulation as a climate amplifier. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. U1706219, 41630966, 41876076), the “111” Project (No. B13030). This is MCTL contribution #184. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.gloplacha.2019.103025. References Tao, S., Xing, L., Luo, X., Wei, H., Liu, Y., Zhao, M., 2012. Alkenone distribution in surface K′ sediments of the southern Yellow Sea and implications for the U37 thermometer. GeoMar. Lett. 32, 61–71. Alexander, C.R., DeMaster, D.J., Nittrouer, C.A., 1991. Sediment accumulation in a modern epicontinental-shelf setting: The Yellow Sea. Mar. Geol. 98, 51–72. Alley, R., Agustsdottir, A., 2005. The 8k event: cause and consequences of a major Holocene abrupt climate change. Quat. Sci. Rev. 24, 1123–1149. Alley, R.B., Mayewski, P.A., Sowers, T., Stuiver, M., Taylor, K.C., Clark, P.U., 1997. Holocene climatic instability: a prominent, widespread event 8200 yr ago. Geology 25, 483–486. Alley, R.B., Anandakrishnan, S., Jung, P., 2001. Stochastic resonance in the North Atlantic. Paleoceanography 16, 190–198. An, Z., Colman, S.M., Zhou, W., Li, X., Brown, E.T., Jull, A.J., Cai, Y., Huang, Y., Lu, X., Chang, H., Song, Y., Sun, Y., Xu, H., Liu, W., Jin, Z., Liu, X., Cheng, P., Liu, Y., Ai, L., Li, X., Liu, X., Yan, L., Shi, Z., Wang, X., Wu, F., Qiang, X., Dong, J., Lu, F., Xu, X., 2012. Interplay between the Westerlies and Asian monsoon recorded in Lake Qinghai sediments since 32 ka. Sci. Rep. 2, 619. Barber, D.C., Dyke, A., Hillaire-Marcel, C., Jennings, A.E., Andrews, J.T., Kerwin, M.W., Bilodeau, G., McNeely, R., Southon, J., Morehead, M.D., Gagnon, J.M., 1999. Forcing of the cold event of 8,200 years ago by catastrophic drainage of Laurentide lakes. Nature 400, 344–348. Blaauw, M., Christen, J.A., 2011. Flexible paleoclimate age-depth models using an autoregressive gamma process. Bayesian Anal. 6, 457–474. Bond, G., 1997. A pervasive millennial-scale cycle in North Atlantic holocene and glacial climates. Science 278, 1257–1266. Bond, G., Kromer, B., Beer, J., Muscheler, R., Evans, M.N., Showers, W., Hoffmann, S., Lotti-Bond, R., Hajdas, I., Bonani, G., 2001. Persistent solar influence on North Atlantic climate during the Holocene. Science 294, 2130–2136. Broecker, W.S., 2003. Does the trigger for abrupt climate change reside in the ocean or in the atmosphere? Science 300, 1519–1522. Chapman, M.R., Shackleton, N.J., 2000. Evidence of 550-year and 1000-year cyclicities in North Atlantic circulation patterns during the Holocene. Holocene 10, 287–291. Conte, M.H., Thompson, A., Lesley, D., Harris, R.P., 1998. Genetic and physiological influences on the alkenone/alkenoate versus growth temperature relationship in Emiliania huxleyi and Gephyrocapsa Oceanica. Geochim. Cosmochim. Acta 62, 51–68.

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