Abstract
Coastal regions of the northern South China Sea (SCS) strongly interact with the Asian monsoon circulation (AMC). Thus, variations of sea surface temperature (SST) here are newly suggested to document AMC changes in an effective manner, but additional physical parameters of oceanic conditions, probably also in relation to the AMC system, remain poorly understood. In this study, we analyzed glycerol dialkyl glycerol tetraethers (GDGTs) from a well-dated sediment core YJ, retrieved at the northern SCS coast, to further scrutinize the intrinsic response of water column to winter AMC strength. It shows that within the time frame of past ∼1,000 years, the tetraether index of lipids with 86 carbon atoms (TEX86) and published alkenone (U37K′) temperature records together confirm a reduced thermal gradient during the Little Ice Age (LIA), in comparison to that during the Medieval Climate Anomaly (MCA). Considering concurrent variations of the branched and isoprenoid tetraether (BIT) and the ratio of archaeol to caldarchaeol (ACE), for example, with decreased values (<∼0.3) for the former and relatively high values for the latter at the LIA, indicative of stratification and salinity changes, respectively, these multiple lines of evidence thereby call for well mixing of onsite water at site YJ correspondingly. Our results suggest that winter AMC strength is a critical factor for mixing subsurface waters and modifying thermal/saline conditions at the northern SCS coasts through the last millennium and also, perhaps, on longer timescales.
Highlights
The Asian monsoon circulation (AMC), as triggered by large-scale thermal contrast between ocean and land, characterizes a seasonal reversal of prevailing wind directions
Together with the existing measurements of the alkenone unsaturation index (UK37′, see definition in Prahl et al, 1988) and hopane components, as earlier reported by Zhang et al (2019), this study aimed to investigate the hydrological properties of the onsite water column
Throughout the past millennium, archaeol to caldarchaeol (ACE) values appear to be relatively high during the Little Ice Age (LIA), especially at its onset, as compared to the Medieval Climate Anomaly (MCA) (Figure 3A)
Summary
The Asian monsoon circulation (AMC), as triggered by large-scale thermal contrast between ocean and land, characterizes a seasonal reversal of prevailing wind directions In the summertime, it carries an enormous amount of moisture from the Indian and Pacific Oceans toward southern and northeastern Asia, and, exerts a considerable influence over the water cycle and the terrestrial ecosystem (Wang et al, 2017; Zhang et al, 2017). It carries an enormous amount of moisture from the Indian and Pacific Oceans toward southern and northeastern Asia, and, exerts a considerable influence over the water cycle and the terrestrial ecosystem (Wang et al, 2017; Zhang et al, 2017) In this regard, much attention has been drawn until now to explore summer AMC variability and the physical mechanism(s) from seasonal to orbital timescales At Huguangyan Maar Lake, winter AMC intensity, as inferred from diatom assemblages (Wang et al, 2012) and magnetic susceptibility (Yancheva et al, 2007), respectively, presents controversial temporal features during the Holocene (since ∼11,700 years ago before present, “yr BP” hereafter)
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