Terrestrial paleoclimate changes in Neogene

    Stalagmite, red earth and ice cores are the three significant archives for investigations on global change. Due to extremely low lipid concentration, less were reported in microbial lipid biomarkers in these sediments, though organic biomarkers have great potential for break-through inferences regarding actual climate reconstructions as well as ecological responses to climate. The MGP conducted the pioneering study on the geolipids preserved in the three significant archives. Its pioneering survey confirmed the ecological and paleoclimatic significance of geolipids preserved in stalagmite, recognized as ‘the only in depth investigation’, two of ‘the three significant stalagmite biomarker studies published prior to 2006’, and the ‘breakthrough’ investigation. Its pioneering work also demonstrated the paleoclimatic significance of trace geolipids preserved in red earth and ice cores.

    Coincident with these pioneering studies, the MGP developed a series of new proxies for specific paleoclimate reconstruction. Organic proxies, in particular microbial lipid proxies, are commonly used to evaluate temperature variation in paleoclimate studies. Few proxies, however, are available for deciphering changes in paleohydrology or hydroclimate, leading to the very limited knowledge on the relationship between paleotemperature and paleohydroclimate within a broad region in Earth history. The MGP’ study is an important contribution to filling this knowledge gap; the MGP group proposed 5 proxies of hydroclimate, in particular the ratio in abundance of archaeal isoprenoid GDGTs (glycerol dialkyl glycerol tetraethers) to bacterial branched GDGTs, Ri/b, as the aridity indicator. The Ri/b proxy was confirmed by someone else, and recognized as ‘a valuable proxy’. The MGP further proposed the accumulation rate of aerobic biomarkers, hopanoids in peatlands, as the indicator of water table and thus the paleohydrology proxies. Using this proxy, the MGP could demonstrate for the first time the spatiotemporal relationship between paleohydrological conditions and the ancient civilization throughout the whole Holocene in Middle Yangtze region. This work is recognized as ‘a successful attempt in using geobiological approaches to address major archaeological questions’. The MGP proposed some other proxies for paleohydrology including microbially-modulated average aromatic ring of triterpenes (AAR = [triaromatic%] ×3 + [tetraaromatic%] ×4 + [pentaaromatic%] ×5) value, the difference in oxygen isotope composition of two spatially separated cave sediments, the environmental magnetism parameter ARM/SIRM and trace elements in individual stalagmites.

     These novel proxies the MGP proposed enable the reconstruction of regional hydroclimate since the last deglaciation in central China. The MGP rebuilt decadal resolution storm records in the eastern Asian monsoon area since 8.6 ky on the basis of environmental magnetism parameter in stalagmites. Variance of storms in central China was found to exhibit close correlation with El Niño−Southern Oscillation activity at millennial and centennial time scales, and also occur on a significant 500-y cycle related to periodic solar activity. In particular, by using multiple proxies, the MGP found the dominant mode of summer rainfall variability over East Asia is characterized by a meridional “tripole” pattern of precipitation,with decreased rainfall over northeastern and southeastern China and increased rainfall over central eastern China during the YD event. This supports the “jet transition hypothesis”; the spatial distribution of monsoonal rainfall is affected by the south-north displacement of the westerlies relative to the Tibetan Plateau. This important work is recognized to ‘makes several important contributions to our understanding of the past and future of the EASM’.

    Establishment of the hydroclimate change in Central China further led to the discoveries of carbon cycles under particular climate conditions. New proxies led to the findings of some intervals of prominent peatland drying occurred during the mid-Holocene. Synchronous with these drier conditions, leaf wax δ13C values show large negative excursions, with the utilization of CO2 respired from the peatland subsurface for plant photosynthesis being a possible mechanism. Concurrently, bacterially derived hopane δ13C values suggest the occurrence of enhanced methane oxidation during the drier periods. Furthermore, using the fatty acids extracted from stalagmite, the MGP could find that heterotrophic soil respiration was highly substrate selective, indicating that NPP outpaced decomposition and the catchment behaved as a carbon sink. Thus, the MGP provide palaeoclimate evidence that subtropical soils in a warmer/wetter climate acted as a sink for organic carbon, and thus as a negative climate feedback, during warmer climatic phases.