Asian Winter Monsoon Imprint on Holocene SST Changes at the Northern Coast of the South China Sea

Abstract

Independent inference of the Asian winter monsoon (AWM), albeit achieved at several sparse sites, has reached no consensus for its variability through the Holocene. A sediment core from the northern coast of the South China Sea (SCS) was utilized to analyze organic biomarkers at (bi‐)decadal resolution, unveiling how SCS oceanic conditions fingerprint the AWM signal. Generally, alkenone sea surface temperature (SST) record, resembling the temporal structures of integrated tropical SST over the past 7,500 years, shows abnormal cool temperatures (up to ~4 °C) during the Little Ice Age and between ~1,200 and 2,500 years BP, when windborne terrigenous hopane compounds experienced considerable increases superimposed on a general increasing trend. Our results, together with augmented SST gradient between the SCS coast and open ocean, consistently suggest AWM strengthening toward the late Holocene. An intensified AWM during cold intervals like the Little Ice Age would have provided strong positive feedback to enhance coastal cooling.

Comparison of the U<sub>37</sub><sup>K<sup>′</sup></sup>, LDI, TEX<sub>86</sub><sup>H</sup>, and RI-OH temperature proxies in sediments from the northern shelf of the South China Sea

“…6a); however, they lie outside of the 95 % prediction band using the global TEX 86 -RI relationship (Fig. 6b, Zhang et al, 2016) but, with the exception of two samples, within the 95 % prediction of a shallow-water TEX 86 -RI re-lationship ( Fig. 6b, Jia et al, 2017).…”

“…In marine sediments, the iGDGT composition may sometimes be impacted or even controlled by nonthermal factors, e.g., sources of iGDGTs other than Thaumarchaeota (Zhang et al, 2016). Several indices, e.g., the MI , BIT (Hopmans et al, 2004), the [2] / [Cren] ratio (Weijers et al, 2011), and the RI (Zhang et al, 2016), have been developed to assess these impacts. Relatively low MI values (≤ 0.25) were observed at most sites in our study accompanied by low [2] / [Cren] ratios (0.07-0.15; Fig.…”

“…These values may suggest little input of iGDGTs from archaea involved in methane cycling that are typically characterized by high MI values (> 0.3) or substantially elevated [2] / [Cren] ratios (> 0.2; Weijers et al, 2011;Zhang et al, 2016). The exception was the slope sample (LD-21), showing a slightly higher MI value (0.31) and a higher [2] / [Cren] ratio of 0.25, which could suggest some contributions from archaea involved in methane cycling.…”

Early Holocene weakening and mid- to late Holocene strengthening of the East Asian winter monsoon

“…Zheng et al, 2014), changes in the proportion of mobile desert dunes in northern and northeastern China ( Fig. 3E; Xu et al, 2020), the U k′ (sea-surface temperature proxy) record from the northern coast of the South China Sea (Zhang et al, 2019), and lake MGS records from arid northwestern China (An et al, 2011). The discrepancies in EAWM variation details between these studies are primarily due to chronological uncertainty, proxy sensitivity, and sediment resolution.…”

“…are well understood, mostly based on loess records from the Chinese Loess Plateau (CLP) (Ding et al, 1995;Guo et al, 2002;Sun et al, 2012). Variations in Holocene EAWM intensity on multiple time scales have also been investigated using records including marine sediments (Huang et al, 2011;Zheng et al, 2014;Zhang et al, 2019), lake sediments (An et al, 2011;Wang et al, 2012), loess deposits (Stevens et al, 2008;Yang and Ding, 2014;Kang et al, 2018), and climate modeling (Wen et al, 2016). However, these reconstructed EAWM variations are inconsistent at a sub-orbital scale, such as weakening throughout the Holocene (Huang et al, 2011;Wen et al, 2016) or weakening in the early Holocene and strengthening in the mid-to late Holocene (Yang and Ding, 2014;Zheng et al, 2014).…”