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The variability of the East Asian monsoon (EAM) during the Holocene exhibits significant regional response, and its evolution needs further discussion. A well-dated, high-resolution sea-surface temperature record based on long-chain unsaturated alkenones, grain-size data, and clay mineral assemblages from the South Yellow Sea sediment is presented to investigate the sedimentary provenance and reconstruct the EAM over the past 9.5 ka. The results show that the sediments are most likely supplied by the Huanghe. The evolution of the East Asian summer monsoon (EASM) can be divided into three periods: strong and relatively stable conditions during 9.5–7.0 ka, weakened conditions during 7.0–1.5 ka, and strengthened conditions during 1.5–0 ka. The East Asian winter monsoon (EAWM) has experienced five periods: weakened conditions during 9.5–6.7 ka, weak and relatively stable conditions during 6.7–5.6 ka, strong and relatively stable conditions during 5.6–2.6 ka, strengthened conditions during 2.6–1.5 ka, and weak and stable conditions during 1.5–0 ka. Moreover, in-phase correlation was found between the EAWM and EASM at the orbital time scale in response to orbital-driven solar insolation, but out-of-phase correlation at a centennial time scale is predominantly associated with solar activity.

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Investigating organic compounds in marine sediments can potentially unlock a wealth of new information in these climate archives. Here we present pilot study results of organic geochemical features of long-chain n-alkanes and alkenones and individual carbon isotope ratios of long-chain n-alkanes from a newly collected, approximately 8-meter long, located in the far reaches of the Pacific sector of the Southern Ocean. We analyzed a suite of organic compounds in the core. The results show abundant long-chain n-alkanes (C29-C35) with predominant odd-over-even carbon preference, suggesting an origin of terrestrial higher plant waxes via long-range transport of dust, possibly from Australia and New Zealand. The δ13C values of the C31 n-alkane range from -29.4 to -24.8‰, in which the higher δ13C values suggest more contributions from C4 plant waxes. In the analysis, we found that the mid-chain n-alkanes (C23-C25) have a small odd-over-even carbon preference, indicating that they were derived from marine non-diatom pelagic phytoplankton and microalgae and terrestrial sources. Furthermore, the C26 and C28 with lower δ13C values (~ -34‰) indicate an origin from marine chemoautotrophic bacteria. We found that the abundances of tetra-unsaturated alkenones (C37:4) in this Southern Ocean sediment core ranges from 11-37%, perhaps a marker of low sea surface temperature (SST). The results of this study strongly indicate that the δ13C values of long-chain n-alkanes and index are potentially useful to reconstruct the detailed history of C3/C4 plants and SST change in the higher latitudes of the Southern Ocean.

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