Since the Miocene (~12 Ma), the global temperature has gradually decreased, reaching 2.6 Ma, and a large ice sheet has formed in the northern hemisphere. Prior to this, during the Pliocene Warm Period, the Arctic region experienced several brief ice sheet development events. Theoretically, once an ice sheet is formed, it increases the albedo and leads to further cooling, a positive feedback mechanism that causes the climate system to rapidly enter an ice age. However, the Pliocene ice sheet development was short-lived, and the climate system did not enter the glacial period of large-scale ice sheet development, and the reasons for this have been highly debated.
In response to the above scientific problems, Yang Shiling, a researcher from the Institute of Geology and Geophysics, Chinese Academy of Sciences, systematically collected 24 calcium carbonate nodule samples from Holocene loess profiles (Fig. 1a), established a loess calcium nodule cluster isotope (∆47) thermometer (Fig. 1b, Fig. 1c), and determined the respiration CO2 concentration in paleosoil. Based on this, the evolution history of temperature and atmospheric CO2 concentration since 7.2 Ma was reconstructed.
Fig.1 Distribution of loess study sites in northern China (a), Holocene calcium∆47 temperature (T∆47; black line) and modern annual mean temperature (gray line) distribution (b), correlation between T∆47 temperature and modern annual average temperature (c)
The results of temperature reconstruction (Fig. 2) show that the Loess Plateau has shown fluctuating cooling since the Late Miocene, which is consistent with the temperature change of the tropical ocean, especially the Western Pacific Warm Pool, indicating that the East Asian monsoon system is an important link between the West Pacific Warm Pool and East Asia. The reconstruction results of atmospheric CO2 concentration (Fig. 2) show that the atmospheric CO2 concentration fluctuates between 150–325 ppm during the 7.24–4.3 Ma and 2.6–0 Ma periods, and rises to above 500 ppmv in stages during the 4.3–2.6 Ma period.
It is worth noting that the phased increase in atmospheric CO2 concentration during the period of 4.3–2.6 Ma corresponds to a phased decline in the δ13C value of foraminifera at global sea level and in the deep benthic (Figure 3), which is manifested by a decrease in atmospheric CO2 concentration and a decrease in the δ13C value of foraminifera at 4.3–4.0 Ma, 3.7–3.3 Ma, and 3.1–2.6 Ma, while at 4.0–3.7 Ma and 3.3- The sea level of 3.1 Ma rose slightly, the atmospheric CO2 concentration remained stable or slightly decreased, and the δ13C value of foraminifera remained relatively stable.
The above results reveal the negative feedback mechanism of global cooling since the Late Miocene: global cooling leads to the development of ice sheets in the northern hemisphere, the decline of sea level, the exposure of the continental shelf, and the oxidation of a large amount of organic matter (rich in 12C) stored on the continental shelf, thereby releasing a large amount of CO2, which makes the ocean carbon isotope negative, the climate warms up, and the sea level rises slightly; , officially opening the Quaternary Ice Age.
Fig.2 Annual mean temperature (a) and atmospheric CO2 concentration (b; blue curve is ice core CO2 record), tropical sea surface temperature (Herbert et al., 2016) (c), and sea surface temperature in the Western Pacific warm pool (Zhang et al., 2014) (d) reconstructed from the Lingtai loess-red clay sequence
图3 上新世全球海平面变化(Miller et al., 2020)(a)、重建大气CO2浓度(b)、底栖有孔虫碳同位素记录(Westerhold et al., 2020)(c)以及灵台重建年均温(红线)和北半球高纬海表温度(Herbert et al., 2016;蓝线)(d)
研究成果发表于国际学术期刊Global and Planetary Change（杨石岭*，王永达，黄晓芳，孙敏敏，汉景泰，王旭，陈祚伶，张师豪，姜文英，唐自华，顾兆炎，熊尚发，丁仲礼. Pliocene CO2 rise due to sea-level fall as a mechanism for the delayed ice age [J]. Global and Planetary Change, 2024, 236: 104431. DOI: 10.1016/j.gloplacha.2024.104431.）。 该研究受国家重点研发计划项目（2022YFF0800800）、国家自然学科基金（42107472，41725010）和中国科学院地质与地球物理研究所自主部署项目（IGGCAS-201905）联合资助。
Editor: Fu Shixu (East China Normal University)
Proofreading: Wan Peng