Elsevier

Marine Geology

Volume 430, December 2020, 106365
Marine Geology

Pollen records of orbitally modulated variation in East Asian winter monsoon intensity and freshwater inflow to the Ulleung Basin of East Sea, South Korea, during the last glacial period

https://doi.org/10.1016/j.margeo.2020.106365Get rights and content

Highlights

  • Paleovegetation changes in the East Sea during the last glacial period were reconstructed.

  • Changes of herbaceous pollen abundance correspond with fluctuations of freshwater inflow into the East Sea.

  • Bisaccate pollen was used as an indicator of East Asian winter monsoon intensity.

Abstract

Most pollen analysis in Korea has focused on the Holocene Epoch, because pre-last glacial maximum terrestrial archives suitable for palynological study in the Korean Peninsula are limited. Thus, to verify the responses of vegetation to climate changes over the orbital timescales, analysis of pollen in marine sediments is needed. Here, we present the pollen record of hemipelagic sediments of core DD09-ST21 obtained from the northern Ulleung Basin in the East Sea. The palynofloral assemblages were dominated by warm temperate broadleaved trees and boreal conifers during interglacial and glacial periods, respectively. We also demonstrated that bisaccate pollen and herbaceous pollen in the palynological sequence can be used as indicators of the variation in the East Asian winter monsoon (EAWM) intensity during the last glacial period and freshwater inflow into the East Sea, respectively. During glacial periods, the EAWM intensified in response to weakening of the summer insolation at 60° N, while freshwater influx into the East Sea decreased due to a decrease in precipitation.

Introduction

Pollen and spores are transported from terrestrial vegetation to marine sediments by wind and fluvial and oceanic currents (Traverse, 2007). The transport processes often bias the deposition of some pollen taxa in marine sediments from the surrounding terrestrial vegetation due to their dispersal ability (Heusser, 1988; Hooghiemstra et al., 2006; Mudie and McCarthy, 2006; Dai et al., 2014). Careful consideration of marine pollen records provides opportunities to understand the history of transportation, such as the wind regime and oceanic currents. For example, pine pollen and fern spores have been used as indicators of wind intensity and river runoff, respectively (Sun et al., 2003; Chen et al., 2017; Li et al., 2017). The backswamp, intervalley, and fluvial sediments deposited during the late Pleistocene are not available in Korea because they have been eroded (Kim, 2001) or undergone pedogenic processes during the last glacial period, especially the last glacial maximum (LGM; Park and Kang, 2001). Most of the Quaternary terrestrial sediments (e.g., wetlands or lakes) on the Korean Peninsula formed during the Holocene. Therefore, a palynological approach to marine sediment analysis is necessary to understand vegetation dynamics in response to climate changes on the orbital time scale.

Many studies have attempted to reconstruct the late Quaternary paleoenvironmental changes of the East Sea by focusing on the vertical circulation of the water column based on geochemistry (Khim et al., 2012; Minoura et al., 2012; Yao et al., 2012), biomarkers (Lee, K.E. et al., 2008; Yu et al., 2009; Xing et al., 2011), sedimentary facies (Watanabe et al., 2007), and microfossils (Oba et al., 1991; Ryu et al., 2005; Usami et al., 2013). It is still unclear how the intensity of the East Asian winter monsoon (EAWM) and freshwater influx to the East Sea changed during the last glacial period. In this study, we conducted pollen analysis using core DD09-ST21, recovered from the northern Ulleung Basin in the East Sea, to reconstruct vegetation changes on the orbital time scale. It assesses boreal conifers and herbs pollen signals from hemipelagic sediments as indices to reconstruct variations of EAWM intensity and freshwater influx into the East Sea.

Section snippets

Physiographic and oceanographic setting

The East Sea, which is composed of three basins (Ulleung Basin, Yamato Basin, and Japan Basin), is a semi-closed marginal sea surrounded by the eastern Asian continent and Japanese Islands. It connects to the East China Sea, Pacific Ocean, and Sea of Okhotsk through four shallow straits: the Korea Strait (sill depth = 140 m), Tsugaru Strait (130 m), Soya Strait (55 m), and Tartar Strait (12 m) (Fig. 1). The Tsushima Warm Current flows into the East Sea via the Korea Strait and forms the surface

Materials and methods

A 3.6-m-long piston core, DD09-ST21 (37° 16′ 50.3“ N, 131° 40’ 34.9” E; 2135 m water depth), was taken from the northern Ulleung Basin in the East Sea during a cruise of the R/V Ieodo of the Korea Ocean Research and Development Institute in 2009 (Fig. 1). The core is comprised mostly of mud and silt, except for the uppermost part which consists of sand-sized tephra grains (Fig. 2). A total of 40 subsamples were taken at 8-cm intervals from the core and prepared using standard palynological

Chronology

The age of core DD09-ST21 was previously established by Suh et al. (2013) based on AMS 14C dating, as 11,410 ± 60, 35,930 ± 360, and > 43,500 conventional yr BP at 42, 157, and 297 cm, respectively. The previous age model was refined by adding two tephra layers, detected at 7 and 304 cm. Compared to the tephra layers already known on the basis of morphology, chemical composition, and the stratigraphic position of glass shards, the tephra layers at 7 and 304 cm were identified as U-Oki and Aso-4

Vegetation responses to climate changes during the last glacial period

Our marine pollen record appears to faithfully reflect global climatic changes occurring during the Late Quaternary (e.g., glacial–interglacial cycles). For example, warm temperate broadleaved trees have high abundance during interglacial periods, while cold-tolerant boreal conifers increase during glacial periods (Fig. 4).

From 103.5 to 89.5 ka, the dominance of conifers such as Abies, Picea, and Pinus is indicative of a prolonged cold and dry climate, which is also reflected in the low

Conclusions

  • (1)

    Ninety varieties of pollen, spores, and NPPs were encountered in core DD09-ST21, obtained from Pleistocene hemipelagic sediments of the northern Ulleung Basin, East Sea. Overall, the pollen assemblage is dominated by trees, with a relatively low abundance of herbaceous pollen and spores. Four PAZs with two subzones were established; their boundaries are similar to those of MISs.

  • (2)

    The palynological record provides information on the variability of the paleoclimate and paleoenvironment during the

Acknowledgments

This study was supported by a Basic Research Project (GP2020-003) of the Korea Institute of Geoscience and Mineral Resources funded by the Ministry of Science and ICT, Korea, and in part by a project on sustainable research and development of Dokdo (PG51650) funded by the Ministry of Maritime Affairs and Fisheries.

Declaration of Competing Interest

None.

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