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Understanding the origin and evolutionary history of drought events is of great significance, providing critical insight into future hydrological conditions under the changing climate. Due to the scarcity of drought proxies from northern China, the occurrence and underlying mechanisms of the drought events remains enigmatic on longer timescales. Here we utilize microbial lipid proxies to reconstruct significant drought events over the last four ice age terminations in the southernmost section (Weinan section) of the Chinese Loess Plateau. The abundance of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) relative to bacterial branched GDGTs, measured by R i/b and BIT indices, is diagnostic of enhanced drought conditions. The R i/b (and BIT) indices are stable and low (high) throughout most of the loess section spanning the last 350 thousand years, but they do exhibit sharp transient peaks (valleys) during the intervals associated with the four ice age terminations, and especially Terminations II and IV. These enhanced drought events are, non-intuitively, associated with a significant decrease in the relative abundance of C 4 plants, inferred by a decrease in the carbon isotope composition of bulk organic matter. Although the microbial records show some consistency with the Weinan grain size profiles, indicative of Eastern Asian winter monsoon variability, they also show some apparent difference. In fact, some features of the microbial records exhibit strong similarities with marine sediment planktonic foraminiferal δ 13 C records from the western Pacific warm pool, which reflect ENSO-like changes during glacial terminations. Therefore, enhanced droughts immediately before the interglacial warming in northern China could be explained, at least in part, by teleconnections in tropical ocean–atmosphere circulation via shifts in the Intertropical Convergence Zone (ITCZ) and associated Jet Stream over the Asian continent. According to our microbial biomarker data, these enhanced megadroughts are apparently different, both in terms of severity and causal mechanism, from the more commonly discussed dry conditions observed during glacial periods. [ABSTRACT FROM AUTHOR]

The recently proposed global and local calibrations for the mean annual air temperature (MAT) reconstruction on the basis of 5- and 6-methyl brGDGTs have rarely been applied to the Chinese LPS yet, leaving the applicability of these calibrations unclear. Here, we used the improved chromatography method to analyze 198 loess-paleosol samples from the Weinan section in the southern CLP for the past 350 kyr. The 6-methyl brGDGTs comprise a major proportion of total brGDGTs, pointing to alkaline conditions for most soil samples from the Weinan LPS. The decoupled profile variation of MBT′ and MBT′ suggests their response to different enviromental factors, possibly soil pH (or soil moisture) and temperature, respectively. This discrimination further corraborates that temperature and monsoonal precipitation were not in phase during the last four deglaciations on the CLP. Temperature estimates for the six calibrations tested show similar trends but remarkably differ in amplitudes. The soil moisture appears to affect the global MBT′/CBT calibration, the global MAT calibration (a calibration based on the multiple linear regression) and the Chinese local SSM (Stepwise Selection Method) calibration, resulting in a significant underestimation of late Holocene temperature. In contrast, the dry climate has no effect on the global MBT′ calibration. Of the six calibrations, only the Chinese local SSM calibration and global MAT calibration produce temperature variation amplitude over the past 350 ka that is consistent with other independent proxy data. The recently proposed local calibrations based on 5- and 6-methyl brGDGTs for North China yield the amplitude of temperature changes that is much larger than the results determined by other approaches. All the six calibrations have their own weakness in the MAT reconstruction, due partly to the inconsistency between the brGDGT distribution in the Weinan LPS and the modern soils used to establish these calibrations. [ABSTRACT FROM AUTHOR]

Abstract Floral communities in terrestrial habitats such as steppes are highly sensitive to regional climate changes. Here, we generated carbon and nitrogen isotope profiles for soil organic matter (SOM) in the Weinan loess-paleosol section in order to investigate changes in floral communities on the Chinese Loess Plateau (CLP) and their relationship to climate variation since 75 ka. δ13C SOM at Weinan ranges from −23.6 to −18.0‰ and δ15N SOM ranges from +3.4 to +5.5‰. Modeling based on the δ13C SOM record suggests that the proportion of C4 plants (%C4) has varied from lower during cold intervals (with minima of 24–32% at 73.1 ka, 20.9 ka (LGM), and 0.6 ka) to higher during warm intervals (with maxima of 62–64% at 53.0 ka and 8.2 ka). The δ13C SOM and δ15N SOM profiles are strongly correlated (r = +0.82; p(α) <0.001), suggesting that the N-isotope composition of SOM was also largely controlled by inputs of C4 versus C3 plants, consistent with known patterns of N-isotope variation among modern plants. The %C4 profile exhibits significant covariation with reconstructed mean annual temperatures (MAT) (r = +0.42; p(α) <0.001), mean annual precipitation (MAP) (r = +0.42; p(α) <0.001), and summer monsoon intensity (r = +0.50; p(α) <0.001), suggesting that paleovegetation was controlled by each of these climate variables to a degree. Comparing the Weinan record to SOM profiles across the CLP region, we identified three zones of varying monsoonal influence, with the East Asian Summer Monsoon (EASM) exerting the strongest influence on the southeastern margin of the CLP, and the East Asian Winter Monsoon (EAWM) greater influence on its northwestern margin. This paper provides new insights into deciphering paleovegetation patterns and their links to paleoclimate change on the basis of paired C- and N-isotope records. Highlights • Analysis of δ13C and δ15N of soil organics in Weinan section, Chinese Loess Plateau • δ13C SOM and δ15N SOM profiles exhibit strong correspondence since 75 ka • Both C- and N-isotope compositions controlled mainly by inputs of C3 vs C4 plants • Paleovegetation type strongly influenced by summer monsoon intensity [ABSTRACT FROM AUTHOR]