The palynology of the First Hard Clay Layer (late Pleistocene) from the Yangtze delta, China

Abstract

This paper reports on palynomorphs of environmentally diagnostic significance that were recovered from the First Hard Clay Layer, a widely distributed stratigraphic unit of late Pleistocene age in the Yangtze delta and adjacent areas in China. The palynomorphs represent the Last Glacial Maximum (LGM) and provide the first palynological evidence for reconstructing the sedimentary environment. The high abundance of the algal palynomorph Concentricystes indicates that the Hard Clay Layer is a freshwater deposit. Dinoflagellate cysts occur in low abundance suggesting that the area had minor marine influence. Spore and pollen assemblages show that the regional upland vegetation of the Yangtze delta and adjacent areas was a broadleaved and coniferous mixed forest with meadows present in the riverine wetlands and on the floodplains. The forest taxa are indicative of a temperate climate and show that the LGM was not a continuously cold and dry period, but was punctuated by relatively warm and relatively wet periods. The palynomorph assemblages indicate that the First Hard Clay Layer in the Yangtze delta and adjacent areas is of lacustrine-fluvial origin, rather than aeolian origin as some researchers have suggested.

Introduction

The Yangtze delta supports one of the most populous regions in the world. Few places on the planet have seen such sustained human impact on landscape and ecosystems as eastern China, yet little systematic work has been done on the long-term evolution of the relationship between environment and human impact in the region. Numerous Neolithic cultural sites have been excavated on the delta, with ages ranging from 7000 to 2000 yr BP (Wu, 1988), and these provide a basis to investigate interactions between environments and human activities. Reconstruction of the history of environmental changes and landscape evolution during the Holocene and latest Pleistocene provides essential background knowledge, and is highly desirable, for a better understanding of the human–environment relationship.

The evolutionary history of the Yangtze delta has been subjected to a substantial research effort. The present Yangtze delta covers an area of approximately 52,000 km², about 23,000 km² of which is subaerial and about 29,000 km² is subaqueous (Li et al., 1986). The topography of the Yangtze delta plain is subdued and is mainly about 2 m above mean sea level in the central delta plain, rising to 3–5 m on the periphery of the delta plain (Chen and Wang, 1999). Numerous shallow borings have revealed that the modern Yangtze delta began accreting after postglacial sea level reached its mid-Holocene highstand 7000 yrs ago (Yan and Hong, 1987, Chen and Stanley, 1993, Stanley and Warne, 1994, Li et al., 2001). Over the past 3000 yrs the high sea-level stand has gradually retreated, as it approached present-day level (Yang and Xie, 1984, Yan and Hong, 1987, Zhao et al., 1994). Deceleration of sea level rise by the mid-Holocene initiated fluvial sedimentation along the periphery of the delta plain (Stanley and Chen, 1996). Human occupation spread along with the propagation of the delta.

Extensive drilling has also revealed a detailed stratigraphy for the late Pleistocene–Holocene sediments in the Yangtze delta, which allows for reconstruction of the sedimentary and environmental history of the region (e.g. Chen and Stanley, 1993, Li and Wang, 1998). Throughout the Yangtze delta and adjacent areas, the Subei plain, the Hangjiahu plain, the Ningshao plain and the East China Sea shelf, there exist at least two layers of dark green to yellowish-brown stiff clay, each around 1–10 m thick, in the late Quaternary strata. These are locally known as Stiff Clay in Shanghai (Li and Wang, 1998) or Old Clay in the north of Zhejiang province (Yan and Hong, 1987). These clay layers are widespread and laterally correlative, and are therefore stratigraphical markers. The uppermost hard clay layer is known as the First Hard Clay Layer (Li et al., 1986, Li and Wang, 1998, Zheng et al., 1999). Based on stratigraphical correlation, the age of the First Hard Clay Layer has been assigned to be latest Pleistocene. ¹⁴C dating has demonstrated that it was formed during the period of 25,000–12,000 yr BP (Min and Wang, 1979, Li et al., 1986, Sun and Wu, 1987, Zheng et al., 1999, Cai et al., 2001). At this time, global sea level fell to − 150 m, and the shelf of the East China Sea was entirely exposed (Zhu, 1981, Geng, 1981). The origin of the First Hard Clay Layer and its sedimentary environment has been controversial. The major argument centers on whether the layer has an ‘aqueous’ or an ‘aeolian’ origin. Some researchers believe that the First Hard Clay Layer was formed as a result of successive deposition and continuing pedogenesis of flood plain material, when the Yangtze River was deeply incised during the LGM lowstand (Li and Wang, 1998, Chen and Li, 1998a, Chen and Li, 1998b). This would imply that the climate of the delta area was moderately cool and relatively dry, but not devoid of running water and swamps. By contrast, Zheng et al. (1999) argue that the First Hard Clay Layer was of aeolian origin, having been transported by the northwest wind, mechanisms similar to that which formed the loess deposits in central China. If this is the case, the First Hard Clay Layer could be regarded as a continuation of the aeolian deposits which spread in the western hilly areas of the Yangtze delta. These researchers believe that aeolian loess deposition occurred not only over the Yangtze delta and the coastal plain, but also stretched onto the continental shelf of the East China Sea during the LGM, when the shelf was exposed and experiencing desertification (Yu and Liu, 1995). The formation of the First Hard Clay Layer by this means would have occurred under a cold and dry climate (Zheng et al., 1999).

Arguments about the possible desertification of the East China Sea shelf during the LGM is beyond the scope of this paper, but the origin of the First Hard Clay Layer holds the key to understanding the environment of both the Yangtze delta and the continental shelf. One of the reasons for the controversy about the origin of the First Hard Clay Layer has been the lack of fossil evidence of diagnostic significance. Many researchers have tried routine pollen extraction techniques (following Moore et al., 1991) which have been unsuccessful in yielding significantly large palynomorph assemblages (Liu et al., 1995, Li and Wang, 1998, Zheng et al., 1999). In this study, we followed the method of Sun et al. (1999) that combines acid treatment with sieving to extract and concentrate pollen and spores. This method has enabled the extraction of relatively large amounts of pollen, spores and algae from the First Hard Clay Layer, and allowed for the recognition of distinctive palynomorph assemblages. On the basis of these assemblages the vegetation history of the Yangtze delta and adjacent areas during the LGM, and the sedimentary environment in which the First Hard Clay Layer was formed, are determined.