Abstract
The grass subfamily Pooideae originated in a temperate niche during the late Cretaceous; it is the largest Poaceae subfamily, consisting of almost 4,000 species, which are distributed worldwide. Pooideae responses to climate changes at different time scales, and different ecological zones are thus important in understanding Poaceae evolutionary processes and their relationship with climate change. In the study described in this article, we reconstructed Pooideae variability during the early Holocene, as inferred by a phytolith sequence from the Lower Yangtze in subtropical China. The phytolith assemblage was marked by three increases in Pooideae phytoliths, dated to ca 8.4–8.0, 7.8–7.6, and 7.4–7.2 ka BP (before present, 1950 AD), with each representing pronounced increases in Pooideae extent and distribution. All these increases were within age ranges that agreed well with the timing of weak Asian Monsoon events, at 8.2, 7.7, and 7.3 ka BP. The first Pooideae flourishing period in subtropical China was the most significant, lasting for approximately four centuries and being characterized by a double peak, which equated with an event at 8.2 ka. This suggested that cold and/or dry conditions—which occurred over a period of several hundred years and were linked to weakening of the Asian monsoon—probably caused Pooideae to flourish in the Lower Yangtze region. Comparison of two diagnostic trapezoid phytolith types—namely wavy and wavy narrow—which showed different changes between ca 8.4 and 8.0 ka BP, suggested that they responded differently to the climate change represented by the 8.2 ka event. Our phytolith records have provided not only new data clarifying the detailed Pooideae response to the 8.2 ka event but also a reliable index for past cold climates in subtropical China.
Highlights
The 8.2 ka BP event was a most pronounced and abrupt cooling event, disrupting the relatively stable warm Holocene climate, as clearly shown in Greenland ice-core records (Alley et al, 1997; Thomas et al, 2007)
(Sallun et al, 2012; dos Santos-Fischer et al, 2018; Voarintsoa et al, 2019). This global 8.2 ka event has been the focus of considerable research and has been well-documented in all research associated with cold events that occurred during the early Holocene
According to the age–depth model generated by the Bacon software (Blaauw and Christen, 2011), the sedimentation rates shown in the HG01 core ranged from 0.4 to 2.2 cm per year (Figure 2)
Summary
The 8.2 ka BP event was a most pronounced and abrupt cooling event, disrupting the relatively stable warm Holocene climate, as clearly shown in Greenland ice-core records (Alley et al, 1997; Thomas et al, 2007). Based on annual layer counting chronology, Greenland ice-cores and monsoon region stalagmite records have shown that the 8.2 ka event lasted for 150–160 years, and was characterized by a double peak structure, with the most dramatic cooling of approximately 1–2 C mainly concentrated between 8.2 and 8.1 ka BP (Alley and Agustsdottir, 2005; Rohling and Palike, 2005; Boch et al, 2009; Cheng et al, 2009; Liu et al, 2013; Morrill et al, 2013; Liu et al, 2015; Allan et al, 2018) Such a centennial cold event has been regarded as a global climatic reversal during the early Holocene (Mayewski et al, 2004; Wanner et al, 2011); it was not restricted to Greenland (Alley and Agustsdottir, 2005) and other high-latitude northern hemisphere regions (Daley et al, 2011; Eddudottir et al, 2018), and occurred in many low- to mid-latitude regions (Wu et al, 2012; Dixit et al, 2014; Schemmel et al, 2016; Peckover et al, 2019), and even in some areas of the southern hemisphere. There are still questions concerning this event, such as its impact on plant communities worldwide, and more research into diverse plant responses to the 8.2 ka abrupt cold event is needed to delineate how near-modern and future plant communities respond to this type of perturbation
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