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Microbial Biomarker Records in Tibetan Peats: Monsoon Variability and its Impact on Methane Biogeochemistry

Final Report Summary - TIBETMETH (Microbial Biomarker Records in Tibetan Peats: Monsoon Variability and its Impact on Methane Biogeochemistry)

(1) The project’s objectives
We have achieved all of the key project objectives with respect to biomarker analyses from the Hani peats in Northeast China:
1) We quantified a suite of archaeal and bacterial lipids from Hani peats using GC-MS and HPLC-MS. The important lipids include archaeol (a proxy for methanogen biomass), acyclic GDGT (as a potential methanogen tracer), other isoprenoid and branched GDGTs (exploring the relative variations of acyclic GDGT relative to the total GDGTs). The distributions of archaeol revealed low methanogens correspond with the decreased East Asian summer monsoon precipitation in Hani peats from Northeast China, which is consistent with the results we concluded in Hongyuan peats in Tibetan Plateau. In addition, a decrease in archaeal biomass relative to bacteria occurred from the early Holocene to late Holocene. We also quantified a range of other lipid biomarkers that supplemented these interpretations.
2) To facilitate more interpretation, we have compiled records of the Asian monsoon systems, especially with respect to temperature changes from the south to the north in East Asian regions. The MBT/CBT temperature proxies based on brGDGTs indicate larger temperature variations in Northeast China than other Asian monsoon regions during the last deglaciation.
(2) Main results
The brGDGT temperature proxies showed the large reconstructed temperature difference (~10°C) between the Oldest Dryas and the present-day in Noetheast China is similar to the difference recorded by mean sea-surface temperatures (SSTs) in the region of the Oyashio and Kuroshio Currents off Japan (about 10-15 °C colder than today). The two-step warming of ~5-16 °C from the last deglaciation to the mid-Holocene in the Hani peat also agrees with SST change in this region over the same interval and in the North Atlantic Ocean. However, these changes are much larger than the mean annual temperature anomaly of ~3-5°C between stadial and interstadial phases recorded at Lake Suigetsu on the Sea of Japan coast. They are also larger than the 6-7 °C warming recorded by MBT'-CBT in the Loess Plateau Mangshan sequence and the Yuanbao loess. These smaller temperature changes are consistent with those recorded in Northern Hemisphere temperature stacks, which generally yield Last Glacial temperatures ~3°C lower than those of the Holocene. SST reconstructions from regional ocean basins suggests that warming trends are smallest at low latitudes (1-3°C) and higher at higher latitudes (3-6°C), with very large temperature differences between the Last Glacial and the Holocene restricted to a few local areas and the Greenland ice core. Therefore, compared with the low-mid latitude Oceans and other East Asian monsoon regions, the reconstructed MATpeat change at Hani of ~5-16°C is large, but it is similar to changes recorded at relatively high northern latitudes.
(3) Conclusions
The results indicate that air temperature increased from ~ -4 °C at the Oldest Dryas to a maximum of ~12 °C in the Holocene Optimum, a much greater amplitude of change than estimates for the low-mid latitude Oceans and other East Asian monsoon regions. We attribute this to a high degree of sensitivity to large-scale atmospheric circulation changes in the Hani Peat and more widely in Northeastern China. In particular, stronger westerlies during glacial times resulted in colder air being delivered to NE China via the East Asian Winter Monsoon. Such an explanation provides further evidence for teleconnections between the North Atlantic and Asian climate, mediated via the intensity of the Westerlies. Consistent with this, millennial temperature oscillations in Northeast China are also correlated with temperature variations in the North Atlantic and Greenland, suggesting a common forcing, and also of greater magnitude than observed in S China. We conclude that variations in the intensity of the cold northern westerlies impact the Asian Monsoon System, with a particularly strong impact on the northernmost parts of the monsoon-influenced area.
(4) Their potential impact and use and any socio-economic impact of the project
Our results have shown how more continental regions influenced by the westerlies and East Asian Monsoon from northeast China based on the microbial lipid proxies. During the glacial intervals, temperature decreased while the westerlies strengthened. Our results provide strong evidence for previous researchers’ inferences. Previous researchers have speculated that the Atlantic meridional overturning circulation influenced the Asian via the westerlies during the glacial-interglacial periods. Here, we show that this is exactly what happened to this peatland in Northeast China. Our new brGDGTs- derived temperature records give us the opportunity to better understand the driving forces of deglacial warming in the Asian monsoon systems. Our study has wider implication for how these systems work. The temperature characteristics provide key insight for understanding abrupt climate changes influenced by the westerlies in Asia during the last deglaciation and presents great potential to predict the interaction between Asian monsoon and westerlies changes linked to global warming in the future.