Imprints of millennial-scale events during the MIS3 revealed by stalagmite δ¹³C records in China

During the last glacial period, a series of millennial-scale abrupt climatic events, including Heinrich events and Dansgaard-Oeschger events, exerted influential and profound impacts on the global climate systems. Due to advantages of high resolution, multiple proxies and ²³⁰Th dating methods, Chinese stalagmite δ¹⁸O records reveal distinct teleconnections between the climates in the northern high latitudes and the Asian monsoon domain. Generally, during cold Stadials in the northern high latitudes, Asian summer monsoon was weak and stalagmite δ¹⁸O values shifted positively, and during warm Interstadials, Asian summer monsoon was strong and stalagmite δ¹⁸O values shifted negatively. However, accompanied with the wide application comes a hot debate on the interpretation of stalagmite δ¹⁸O. It is suggested that Chinese stalagmite δ¹⁸O could possibly reflect Asian summer monsoon which is related with the average monsoonal intensity or the overall moisture transport to China, but could not merely represent local precipitation changes. For instance, during Stadials, under the influence of weak Asian summer monsoon, precipitation in southern China might increase, indicating inconsistent changes of "rainfall" and "wind". Climates in the monsoon marginal regions, namely northern China, are found in consistent behavior in terms of "rainfall" and “wind” changes. Besides, calcite δ¹³C is also potential for the reconstruction of paleoclimatology and paleo-environment, thus, to some extent, could compensate the shortage of calcite δ¹⁸O which lacks changing signals of local environment.

Yangquan City in Shanxi Province is located at the Loess Plateau and the northern edge of the Asian monsoon. Multi-proxy records induced from stalagmites in this region can provide us a better understanding of the "wind" and "rainfall" aspects of the monsoonal climate. At an elevation of 1,200 m above sea level, Lianhua Cave (38°10′N, 113°43′E) is developed in the Ordovician limestone, with a narrow entrance and passages. Relative humidity in the inner cave reaches 98%-100%, and the temperature in July reaches 11°C, close to the mean annual ground temperature. Average annual precipitation is 515 mm (AD 1970-AD 2000; recorded in a meteorological station of Yangquan, 20 km from the cave). Sample LH36 is candle-shaped, 206 mm in length and 80–110 mm in diameter. After halved and polished, we find clear growth layers in the sample and it is composed of milky-white and yellowish calcite. Alternating changes of the petrography and brown weathered layers are observed at the depth of 33-35 mm and 145-150 mm, indicating two growth hiatuses. Considering the hiatuses, we use the depth section of 37-145 mm for this study, which grew in the (Marine Isotope Stage) MIS3. On the polished profile, we drill 109 samples with a 0.3 mm-diameter carbide dental bur at 1 mm intervals, 50 μg each, for stable isotope analyses. Measurements are carried out by the usage of a Finnigan-MAT 253 mass spectrometer equipped with an automated Kiel Carbonate Device at College of Geography Science, Nanjing Normal University. The analytical errors are better than ± 0.06‰ for δ¹⁸O and ± 0.05‰ for δ¹³C. Eight powder samples for ²³⁰Th dating are drilled with 0.9 mm-diameter carbide dental bur, 70-130 mg each. Chemical procedures and U-Th isotopic measurement are performed on a multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS), Thermo Finnigan NEPTUNE, at the High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, and at the Isotope Laboratory, College of Geography Science, Nanjing Normal University. Uncertainties in isotopic data and dates are relative to AD 1950, and are given at the 2σ level.

Based on 8 ²³⁰Th dates and 109 sets of stable isotope data of LH36, we obtain a paleoclimate record with an average resolution of 120 years from 54.5 to 41.1 ka BP during the MIS3. Both Hendy test and Replication test indicate an equilibrium fractionation of isotopes during the stalagmite deposition. Comparison with other four independently-dated, high-resolution stalagmite δ¹³C records between 29°N and 41°N in the Asian monsoon region shows that the stalagmite δ¹³C records from different caves have good reproducibility during the overlapped growth period. We suggest that speleothem δ¹³C effectively indicates soil CO₂ production in the overlying area of the cave, reflecting changes in the cave’s external environment and Asian summer monsoon. Five millennial-scale Asian summer monsoon intensification events correspond to the Dansgaard-Oeschger (DO) 10-14 cycles recorded in the Greenland ice core within dating errors, and the two weak monsoon processes are closely related to the Heinrich Event 5 and Heinrich Event 5a in the North Atlantic. The spatial consistency of stalagmite δ¹³C records in China suggests that the Asian summer monsoon and the related regional ecological environment fluctuations sensitively respond to climate changes at northern high latitudes through sea-air coupling on the millennial timescale.