Although the Cretaceous is considered to have been a period with a warm-hot, greenhouse climate (Huber et al., 2002; O'Brien et al., 2017), Earth's climate during the Cretaceous underwent significant changes from a very warm mid-Cretaceous greenhouse climate to a long-term cooling period during the latest Cretaceous (Dhondt and Arthur, 1996; Linnert et al., 2018).

During the late Campanian–Maastrichtian, several major biotic events occurred, and a variable global greenhouse climate was established (Barrera, 1994; Macleod, 1994; Nordt et al., 2003; Wilf et al., 2003; Voigt et al., 2010; Jung et al., 2012). The Campanian–Maastrichtian Boundary Event (CMBE) represents a cooling phase registered in the δ¹⁸O values extracted from shells of planktonic and benthic foraminifera (Barrera, 1994). It was characterized by a negative δ¹³C_carb excursion in bulk carbonates (Voigt et al., 2010), lasting ~1.6 Myr in the Campanian–Maastrichtian transition (ca. 72.1–70.5 Ma; Jung et al., 2012). The simultaneous biotic changes included the extinction of inoceramid bivalves at high latitudes (Johnson and Kauffman, 1990; Macleod and Huber, 1996), widespread changes of phytoplankton communities and expansion of cool water taxa (Linnert et al., 2016; Dameron et al., 2017), and the diversity of angiosperms declined abruptly in North American (Johnson, 1992).

The Mid-Maastrichtian Event (MME) (ca. 69 Ma according to Frank et al., 2005) was first recognized because of the global extinction of inoceramid bivalves (MacLeod, 1994). The MME was marked by high atmospheric pCO₂ on land (Wilf et al., 2003), an increase in ocean temperatures (Jung et al., 2013), and a positive carbon isotope excursion (Voigt et al., 2012). This warming trend has been found in the western Central Pacific (Jung et al., 2013) and South Atlantic (Li and Keller, 1999). The biotic changes included an abrupt disappearance of rudist reefs (Johnson and Kauffman, 1990), the extinction of a long-ranging, widespread group of bottom-dwelling inoceramids at low latitudes (Macleod, 1994; Macleod and Huber, 1996; Bralower et al., 2002; Dameron et al., 2017), and an increase in the diversity and abundance of ammonites in Antarctica (Witts et al., 2015).

The latest Maastrichtian warming event (which took place during the last 450–110 kyr before the K/Pg boundary, Cretaceous/Paleogene boundary, according to Li and Keller, 1999) has been detected in marine and terrestrial systems, based on studies of oxygen isotopes of foraminifera in the South Atlantic (Li and Keller, 1999), leaf margin analysis used to estimate temperatures in North Dakota, USA (Wilf et al., 2003), and the estimation of atmospheric CO₂ from pedogenic carbonate nodules in Texas, USA (Nordt et al., 2003). During this warming period, the planktonic foraminifera underwent a part of extinction, with the proliferation of opportunist disaster species and dwarfism (Abramovich and Keller, 2003; Abramovich et al., 2010), the major acme of the warm-water species of calcareous nannofossils were found in the Tropical Pacific, with a lowering of fertility levels and a drop of species richness (Thibault and Gardin, 2010), and the land plants in North Dakota became abundant and diversified (Wilf and Johnson, 2004).

However, few studies have focused on the response of aquatic biotas to these significant global climate changes during the Late Cretaceous, and no terrestrial biotic response has previously been reported in the CMBE and MME due to the lack of continuous non-marine sequences rich in fossils.

Dr. LI Sha, Prof. WANG Qifei and Prof. ZHANG Haichun, from the ‘Modern terrestrial ecosystem origin and early evolution’ at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS), collaborated with Dr. Josep Sanjuan, from American University of Beirut-AUB, and Prof. WAN Xiaoqiao, from China University of Geosciences, Beijing, studied the response of the lacustrine flora in East Asia to global climate changes across the K/Pg boundary. This research was recently published on Global and Planetary Change.

The global climate during the latest Cretaceous became variable, with several global warming and cooling trends in a context of a greenhouse Earth. The responses of marine ecosystems to these climate events are relatively well known worldwide; however, lacustrine responses are poorly known due to the less frequent and discontinuous terrestrial fossil records.

The relative changes in charophyte diversity in continuous lacustrine sedimentary sequences from two basins in East Asia, i.e., Songliao and Jiaolai, are considered here for the first time to establish their correlation with global climate changes during the Late Cretaceous and K/Pg boundary, compared with the well-studied south European Ibero-Armorican Island.

Lacustrine deposits correlated with the Campanian–Maastrichtian Boundary Event (CMBE), related to a long cooling period provided a relatively low diversity in East Asia. In contrast, we detected a diverse charophyte flora in lacustrine deposits correlated with the Mid-Maastrichtian Event (MME) in East Asia in a global warming trend. A higher charophyte diversity was further found in the End-Cretaceous global warming event due to speciation under the background of latest Maastrichtian warming event (LMWE).

During the LMWE, Characeae species such as Microchara cristata reduced its size significantly due to environmental stress probably related to the Deccan volcanism. On the other hand, a general tendency of increasing the gyrogonite size in new Characeae taxa has been detected in populations extracted from lower Danian deposits, probably related to stable palaeoenvironmental conditions in a global cooling context.

This study represents the first attempt to correlate the response of the charophyte flora to global climate changes in permanent lacustrine systems during the three main Late Cretaceous–early Danian climatic turnovers.

This study was jointly funded by the National Natural Science Foundation of China, and Strategic Priority Pre-Research Program (B) of the Chinese Academy of Sciences.

Reference: Li, S., Sanjuan, J., Wang, Q.F., Zhang, H.C., Wan, X.Q., 2021. Response of the lacustrine flora in East Asia to global climate changes across the K/Pg boundary. Global and Planetary Change, 197, 103400. https://doi.org/10.1016/j.gloplacha.2020.103400.

Fig. 1. Stratigraphic log of the SK-1(N) borehole showing the position (modified after Li et al., 2019) and diversity of charophytes correlated with temperatures calculated based on paleosol carbonate clumped isotopes (Zhang et al., 2018), oxygen isotopes of benthic and planktonic foraminifera (Li and Keller, 1999), leaf-margin analysis (Wilf et al., 2003) and paleosol carbonate oxygen isotopes (Nordt et al., 2003). Abbreviations: CMBE, Campanian–Maastrichtian Boundary Event; MME: Mid-Maastrichtian Event; LMWE: latest Maastrichtian warming event.

Fig. 2. Comprehensive terrestrial responses of the charophyte flora, the palynological flora, seed cones and the ostracod fauna to global climate events, such as speciation and the dwarfing effect, and size changes on a large time scale.

Contact:  

LIU Yun, Propagandist

Email: yunliu@nigpas.ac.cn 

Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences  

Nanjing, Jiangsu 210008, China