Summer monsoon-induced costal upwelling variability documented by sea surface temperature changes in the Min-Zhe mud area over the last two millennia

Abstract

The observed spatial variability of Holocene sea surface temperature (SST) changes over the Chinese marginal seas highlights that reconstructing regional SST changes could help with understanding Asian monsoon variability and ocean current evolutions. SST in certain upwelling regions along the Chinese marginal seas is noticeably influenced by cooling effects associated with monsoon-induced coastal upwelling. Previous studies have suggested that when SST changes in upwelling regions are inconsistent with global/open ocean SST, SST and water column stratification can largely document the strength of monsoon-induced upwelling/mixing effect and thus EASM variability. Yet, the influence of upwelling on SST changes along the coastal regions of the East China Sea during the late Holocene remains unclear. The Min-Zhe mud area is distributed along the coast of Fujian and Zhejiang provinces, south of the Yangtze River estuary. This region is characterized by coastal upwelling controlled by the Asian summer monsoon and the Taiwan Warm Current. Here we present alkenone-based SST record (U37K'-SST) from core T08-A (28° 30.233' N, 122° 28.346' E, water depth 64 m, core length 219 cm), located at upwelling regions of Min-Zhe mud area, over the past two millennia. The core chronology suggests a basal age of ca. 0 A.D., and the average deposition rate is 0.10 cm per year. The sediment core was sampled at intervals of 1 cm, and we have analyzed long-chain alkenones in all 219 samples. Our U37K'-SST record ranges from ca. 24.7~27.7 ℃ during the past two millennia. Notably, the SST record exhibits lower values during the Medieval Warm Period(MWP, ca. 750~1350 A.D., ca. 26.5 ℃ in average) and the Current Warm Period (since ca. 1750 A.D., ca. 26.5 ℃ in average) compared to the Little Ice Age(LIA, ca. 1350~1750 A.D., ca. 26.9 ℃, in average), which is opposite to Northern Hemisphere temperature and SST changes in the open oceans, but consistent with SST records from upwelling regions of the northern South China Sea. Meanwhile, our SST record shows relatively high values during periods of low solar irradiance over the LIA. Our results suggest that monsoon-induced upwelling substantially modulated SST changes at coastal upwelling regions of the East China Sea, and thus enhanced Asian summer monsoon circulation and the Taiwan Warm Current during the MWP. Our result is supported by SST record from upwelling regions of the northern South China Sea coast, stalagmite oxygen isotope record from Monsoonal region, and mean grain size of > 63 μm contents from core T08-A. By comparing U37K'-SST and TEX86-based temperature records from upwelling regions of the Chinese Marginal seas, we suggest that paired alkenone-based SST and glycerol dialkyl glycerol tetraether-based temperature records could collectively indicate the strength of monsoon-induced upwelling/mixing effect and thus EASM variability. Findings in this study underscore the potential of SST records from upwelling regions in the Chinese marginal seas to reconstruct Asian summer monsoon variability.

中国边缘海的海表温度（ＳＳＴ）记录在全新世不同时间尺度上出现了显著的空间差异，区域ＳＳＴ的重建和对比研究对了解亚洲季风及洋流系统的演化历史具有关键意义。中国边缘海上升流区的ＳＳＴ受到上升流冷却效应的调节，但夏季上升流盛衰对晚全新世东海沿岸海域ＳＳＴ变化的影响尚不明确。本研究基于闽浙泥质区长度为２１９ｃｍ的Ｔ０８Ａ沉积柱的高分辨率长链烯酮不饱和度指标（ＵＫ′\３７）记录，重建了过去两千年以来的ＳＳＴ变化。结果显示，闽浙泥质区的ＳＳＴ在中世纪暖期和现代暖期低于小冰期，与东海非上升流区的ＳＳＴ变化和北半球温度变化相反，但与南海北部上升流区的ＳＳＴ变化一致；同时，ＳＳＴ记录在小冰期太阳辐射较低的两个时段出现高值。这种出现于沿岸上升流区的ＳＳＴ倒转现象指示了闽浙泥质区过去两千年以来的ＳＳＴ变化受到东海夏季沿岸上升流强度变化的调控；东海夏季沿岸上升流的盛衰主要受到亚洲夏季风和台湾暖流强度变化的共同影响。因此，该ＳＳＴ记录指示了夏季沿岸上升流在中世纪暖期增强、小冰期则减弱，进而可以反映亚洲夏季风和台湾暖流的演化历史。本结果显示了中国边缘海沿岸上升流区的ＳＳＴ记录指示晚全新世亚洲夏季风强度变化的潜力。