Major Mid-Late Holocene Cooling in the East China Sea Revealed by an Alkenone Sea Surface Temperature Record

ZHAO Meixun, DING Ling XING Lei QIAO Shuqing, and YANG Zuosheng

1) Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, P. R. China

2) Institute of Marine Organic Geochemistry, Ocean University of China, Qingdao 266100, P. R. China

3) Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, P. R. China

4) College of Marine Geoscience, Ocean University of China, Qingdao 266100, P. R. China

Major Mid-Late Holocene Cooling in the East China Sea Revealed by an Alkenone Sea Surface Temperature Record

ZHAO Meixun1),2),*, DING Ling1),2), XING Lei1),2), QIAO Shuqing3),4), and YANG Zuosheng4)

1) Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, Qingdao 266100, P. R. China

2) Institute of Marine Organic Geochemistry, Ocean University of China, Qingdao 266100, P. R. China

3) Key Laboratory of Marine Sedimentology and Environmental Geology, First Institute of Oceanography, State Oceanic Administration, Qingdao 266061, P. R. China

4) College of Marine Geoscience, Ocean University of China, Qingdao 266100, P. R. China

sea surface temperature; alkenone; East China Sea; 4 ka; ITCZ

1 Introduction

Terrestrial and marine records have revealed that the Holocene climate has experienced both long- and short-term fluctuations with superimposed spatial characteristics (Mayewski et al., 2004; Wanner et al., 2008). For example, during the mid-Holocene, SST increased for the western Pacific warm pool but decreased for the equatorial East Pacific cold tongue (Koutavas et al., 2006), and the increased zonal SST gradient was typical of strong La Niña conditions. Since the mid-Holocene, the zonal SST gradient decreased, indicating a change to a typical modern condition characterized by enhanced El Niño/Southern Oscillation (ENSO). One short-term Holocene climate event, the ca. 4 ka, has received much attention recently (Staubwasser et al., 2003; Staubwasser and Weiss, 2006), partly because it might have caused the abrupt collapse of many agriculture-based societies and civilizations (Wu and Liu, 2004; Drysdale et al., 2006), especially via droughts at lower latitudes. Proposed mechanisms for the mid-late Holocene climate changes and the 4 ka event include the reduced monsoon precipitation caused by the southward migration of the Northern Hemisphere summer position of the Intertropical Convergence Zone (ITCZ) in response to the precessional cycle (Haug et al., 2001), and/or changes in ENSO patterns (Koutavas et al., 2006). However, questions remain about the apparent asynchroneity of the 4 ka event recorded in different records, and the abruptness of changes which is incompatible with a gradual change in the position of the ITCZ. Additional records, especially from marine environments, are needed for improved cross-correlation, determination of phase relationships among different records, and constraining mechanisms of both centennial and millennial scale Holocene climate changes.

Only a few deep-sea records have been obtained todocument the marine responses of the 4 ka event (Jian et al., 2000; Koutavas et al., 2006). Marginal sea sediments are especially suitable for such reconstructions because the climate signal there is often amplified since marginal seas are more sensitive to coupled atmosphere-ocean forcing (Wang et al., 1999; Wang et al., 2011). Here we report a Holocene SST record from the continental shelf region of the East China Sea based on alkenone analysis of Core B3. Our record reveals a major cooling during the mid- to late-Holocene transition, approximately coeval with the 4 ka event. We focus on the mechanism of this event, by correlating it with other relevant records from the region.

2 Oceanographic Setting

Fig.1 A map of China and the West Pacific marginal seas showing the locations of both the terrestrial and marine proxy records discussed in the text (Fig.1a). A detailed map for the Yellow Sea and the East China Sea is shown in Fig.1b with the marine core locations of B3, 255, B-3GC and MD982195 (abbreviated as MD95 in Fig.1b) indicated by filled squares. The surface currents during the winter season are also shown (modified after Su (2005) and Wang et al. (2011)). YSCC: Yellow Sea Coastal Current; YSWC: Yellow Sea Warm Current; TWWC: Taiwan Warm Current; KCC: Korea Coastal Current. The stippled areas labeled M1, M2 and M3 are the distal mud areas.

3 Materials and Methods

The Holocene age model for Core B3 was constrained by six accelerator mass spectrometry (AMS)14C dates of mixtures of benthic foraminiferal species (Fig.2), since no sufficient amount could be picked up from any single species. The AMS14C dates showed neither age reversals, nor detectable gaps for the last 8.0 kyr. The AMS14C dates were converted to calendar ages by CALIB 4.3 with a 400-year reservoir age correction. The age model, by linear interpolation between individual AMS14C dates, has an error of ±60 years, and yields average sedimentation rate of 23 cm kyr-1. Thus, the alkenone analysis sampling resulted in an average time resolution of about 250 years.

4 Results and Discussions

4.1 SST and Productivity Variations of the Last 8 kyr

The record for Core B3 (Fig.2b) reveals an annual SST change of about 6℃ during the last 8 kyr, with the highest SST occurring at 5.6 ka (24.7℃) and the lowest SST occurring at 0.4 ka (18.6℃). Although the record is of relatively low resolution, it can be broadly divided into three intervals. The first interval shows an increasing trend from 8.0 ka (22.1℃) to 5.6 ka (24.7℃), including some oscillations. The second interval from 5.6 ka to 3.8 ka is characterized by a major cooling, with SST reaching a minimum of 19.2℃ by 3.8 ka. Due to the limited sampling resolution (250 years), the record seems to suggest that the cooling was gradual, with a first decrease starting at 5.6 ka, followed by a decrease of almost 3℃ from 4.8 ka (22.1℃) to 3.8 ka (19.2℃). During the third interval (the last 3.8 kyr), SST oscillated between 18.6℃ and 21.9℃, within which the most obvious feature is an SST minimum of 18.6℃ at 0.4 ka. Due to the low sampling resolution, our SST record could not be used to evaluate whether the 4 ka cooling was an abrupt event or a gradual change. The core F10-B SST record also reveals a major cooling from 6 ka to 5 ka, followed by small and gradual cooling which reaches a broad low SST interval around 4 ka (Xing et al., 2013).

The alkenone content (Fig.2a) is used as a proxy for haptophyte productivity, and it can be characterized by two intervals. During the warm interval from 8.0 ka to 5.6 ka, the average haptophyte productivity was lower and the productivity variability was also lower. For the last 5 kyr when SST was lower, the average haptophyte productivity was higher and productivity oscillations also increased. This record is broadly consistent with the F10-B productivity record, although the latter revealed a major late Holocene increase partly due to higher sampling resolution (Yuan et al., 2013). Although SST and alkenone content appear to be inversely correlated, an X-Y plot (not shown) reveals no significant correlation between these two proxies.

Fig.2 K' U37 SST and alkenone content records for Core B3 and comparison with other regional climate records. (a) Alkenone content (ng g-1, dry sediment weight); (b) The B3 K' U37 SST record (solid line) and the Holocene part of the K' U37 SST record for MD982195 (Ijiri et al., 2005), re-calculated using the Müller equation (dashed line); (c) The P. obliquiloculata percentage in Core B-3GC (solid line) and Core 255 (dashed line) from the Okinawa Trough (Jian et al., 2000); (d) The δ18O (solid line) and growth rate (dashed line) records for Dongge Stalagmite D4 (Dykoski et al., 2005); (e) The five-point averaged TOC content record of core GH99a from Daihai Lake of Inner Mongolia (Xiao et al., 2006); (f) The redness record of Qinghai Lake sediment (Ji et al., 2005); (g) The summer solar insolation (June, July, August) at 30°N. The depth and corresponding 14C ages for Core B3 are shown on the top. The shaded vertical bar indicates the interval of minimum SST in Core B3 near the 4 ka event.

4.2 Comparison with Regional Climate Records

Our SST record can be compared with other relevant marine and terrestrial climate records which have revealed the mid-late Holocene climate change and especially the 4 ka event, in order to put our record in a regional and global context and to shed some light on relevant mechanisms (Fig.2). The Pulleniatina obliquiloculata percentage in Cores B-3GC and 255 (Figs.1b and 2c) from the Okinawa Trough is a proxy for the strength of the Kuroshio Current, and the Pulleniatina minimum event at 4.6 to 2.7 ka suggests a weakened Kuroshio influence and a stronger winter monsoon (Jian et al., 2000). The overall similarity between our SST record and the P. obliquiloculata percentage record and the coincidence of our SST minimum with the Pulleniatina minimum event suggest that the strength of the Kuroshio Current could be a major factor controlling B3 SST, probably through its shelf branches, the TWWC and the YSWC. Thus, the weakened Kuroshio influence could be partially responsible for the mid-late Holocene cooling in Core B3.

Many terrestrial records from China have documented mid-late Holocene summer monsoon decreases. For example, the Dongge stalagmite δ18O and growth rate records (Figs.1a and 2d) show a significant and abrupt decrease in the summer monsoon strength at about 3.6 ka (Dykoski et al., 2005); the TOC content of Daihai Lake (Fig.2e) in north-central China (Fig.1a) suggests that the summer monsoon intensity began to decrease around 5.3 ka, and reached a minimum around 3.3 ka (Xiao et al., 2006); the sediment redness from Qinghai Lake (Figs.1a and 2f) shows an abrupt decrease at ca. 4.2 ka, marking a sudden decrease of the summer monsoon intensity (Ji et al., 2005). The apparent asynchronous changes in the monsoon records could be partly explained by the uncertainties of the chronologies of the different records, which would be resolved with more records and (or) better dating. Another explanation would be spatial variations in the changes in Holocene Asian monsoon intensity (An et al., 2000; He et al., 2004). Because it is well established that the East Asia summer and winter monsoons have an anti-phase relationship (Yancheva et al., 2007), it could be inferred from these summer monsoon records that the winter monsoon intensity increased significantly during the mid-late Holocene.

The beginning of the mid-late Holocene SST decrease in Core B3 is almost synchronous with the major δ18O change in Dongge record at ca. 5.6 ka, but leads the major decrease in the Daihai Lake TOC occurring at 4.8 ka. The Lake Qinghai redness record shows slightly different patterns, with values beginning to decrease at ca. 6 ka and large oscillations between 6 and 4 ka. For the 4 ka event, the B3 SST minimum is coincident, within age uncertainties, with the Dongge stalagmite growth rate minimum and with the Lake Qinghai redness minimum. However, the Dongge δ18O maximum and the Daihai Lake TOC minimum occurred later. These correlations suggest that mid-late Holocene climate changes have affected both Asian monsoon and the East China Sea SST. In addition, the decrease in the summer monsoon intensity and the increase in winter monsoon intensity might have initiated the modern East China Sea-Yellow Sea circulation system (Kim and Kucera, 2000; Xiang et al., 2008), which contributed significantly to the about 5℃ drop in the B3 and a smaller SST drop in F10-B during the transition from the early Holocene to the mid-late Holocene.

4.3 Possible Causes of SST Changes in Core B3 During the Holocene

We propose that the initiation or strengthening of theshelf circulation system could be the amplifier. The early Holocene circulation pattern in the Yellow Sea and the East China Sea was different from that in the late Holocene, as the YSCC would be weak due to the northerly position of the ITCZ and weaker winter monsoons. Without the interaction between the YSWC and the YSCC, the eddy circulation/cold front around the B3 area would not exist or be weak. Thus, SST at site B3 was higher than and was comparable with SST at Core MD982195 (Fig.2b) on a similar latitude in the Okinawa Trough (Ijiri et al., 2005). During the mid-late Holocene climate transition, the southward migration of the ITCZ and the increase of the winter monsoon intensity might have resulted in a stronger YSCC to establish the modern circulation system, which initiated or strengthened the eddy circulation/cold front that resulted in lower SST for the eddy area compared with non-eddy regions. As a result, the B3 SST decreased 3-4℃ from 5 ka to 3 ka, while the open ocean SST at MD982195 (Ijiri et al., 2005) actually increased slightly during the interval. Since ca. 4 ka and with the establishment of the Yellow Sea and East China Sea shelf circulation system, the SST difference between MD982195 and B3 has been 4-5℃. This proposed mechanism is also in accord with the modern observation that the East China Sea shelf eddy is stronger during El Niño years (Chen et al., 2004) when the ITCZ is further south. In addition, stronger upwelling/cold front since the mid-late Holocene has also resulted in higher surface productivity, as indicated by higher alkenone content in B3.

5 Concluding Remarks and Implications

SST records from more locations in the East China Sea and the Yellow Sea would provide a new approach to reconstruct the history of circulation changes in these marginal seas, which has important biological and economical implications. Today, the eddy-induced upwelling is an important factor causing the higher productivity in the Yellow Sea and the East China Sea, which in turn supports an important fishery economy for the surrounding countries. With global warming, it is likely that the ITCZ would migrate northward and the winter monsoon might weaken, which would reduce the YSCC flow and weaken the eddy circulation/cold front in the Yellow Sea and the East China Sea.

Acknowledgements

We thank Dr. Y. Saito and Dr. J. Liu for providing the AMS14C dates. This research was supported by the National Basic Research Program of China (973 Program 2010CB428901) and by the Natural Science Foundation of China (Grant Nos. 41221004 and 41020164005). This is MCTL Contribution #70.

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(Edited by Ji Dechun)

(Received April 1, 2014; revised May 15, 2014; accepted June 4, 2014)

© Ocean University of China, Science Press and Springer-Verlag Berlin Heidelberg 2014

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E-mail: maxzhao@ouc.edu.cn