New findings on palynofacies characteristics of semi-enclosed deep-sea environments in the East Sea over 2\xa0million years

Yongmi Kim,Chang-Pyo Jun,Gil Young Kim,Eunmi Lee,Sangheon Yi

doi:10.1038/s41598-020-73493-3

Abstract

Phytoclasts in the form of plant debris in terrestrial sediments can be transported by water to distant areas because they are lighter than inorganic particles. The semi-enclosed East Sea, which is connected by narrow straits to other seas, is adjacent to continental shelves that are the source area of terrestrial sediment flowing into the East Sea. These shelves alternated repeatedly between terrestrial and marine environments as a result of eustatic sea-level changes during the Late Quaternary. Palynofacies analyses of the IODP Exp. 346 U1430 core, located in the Eastern South Korea Plateau (ESKP) of the East Sea, have revealed changes in the size and concentration of phytoclasts associated with glacial–interglacial cycles. These changes are generally negatively correlated with the global sea-level curve, and their anti-phase cycles with high amplitude are clearly evident during the last ca. 750 ka with the geotectonic stabilization period. In particular, several coarse-grained phytoclasts were observed during the glacial period, including the Last Glacial Maximum (LGM). These findings suggest that the concentration and size of phytoclasts flowing into the East Sea were influenced by changes in the distance of the source area, depending on the water depth of the strait and nearby shelves owing to sea-level changes in tandem with glacial–interglacial cycles and geotectonic events.

Highlights

Phytoclasts are plant-derived organic matter comprising clay- to fine sand-sized particles, excluding palynomorphs and amorphous organic materials
The U1430 continuous sediment core of almost 258 m was obtained from the Eastern South Korea Plateau (ESKP) by the IODP Expedition 346 project; the present study focuses on the top 56 m of this core
Shipboard-measured natural gamma radiation (NGR) data from the IODP Expedition 346 may be used to estimate the K, U and Th content, and the K content quantified from NGR spectra may be used to estimate dust input variability and aridity without clay mineral a nalysis[36]

Summary

Introduction

Phytoclasts are plant-derived organic matter comprising clay- to fine sand-sized particles, excluding palynomorphs and amorphous (structureless) organic materials. The southern sea shelf of the KP is composed of a narrow shelf less than 100 m deep connected to the Seomjin and Nakdong river mouths These continental shelves were completely exposed during the Last Glacial Maximum (LGM) when the eustatic sea level[21] fell by a maximum of 140 m, and they alternated repeatedly between terrestrial and marine environments in response to eustatic sea-level changes during the Late Quaternary. The ESKP is an ideal study area in which to investigate marine environmental changes based on sediment characteristics and organic matter supplies in response to fluctuations in sea level The reason for this is that the ESKP is influenced by the third branch of the TWC, which carries terrestrial sediments from nearby continental shelves and flows through the KS into the East S ea[24,25] (Fig. 1a). The various marine environments and tectonic conditions that influence palynofacies characteristics of sediment flowing into the ESKP are discussed

Methods

Results

Conclusion